Optical heating device, optical heating system, and optical heating methods using the same

Abstract

The present invention is related to an optical heating device (10) for heating a sample (20) by absorption of light (L) from a coherent light source (30). The optical heating device (10) comprises a sample holder (40) adapted to place the at least partly reflecting sample surface (22) in the light path (L) from the coherent light source (30). The optical heating device (10) further comprises reflector optics (50), preferably concave reflector optics (50), placed along the light path (L) reflected by the sample surface (22). Adjustment means (60) are adapted to adjust the distance (d) between the sample surface (22) and the reflector optics (50) along the optical axis (OA) of the reflector optics (50), preferably along the rotational symmetry axis of the concave reflector optics (50), such that the reflector optics (50) and the sample surface (22) form an optical resonator, and preferably form a hemispherical optical cavity.

Description

[0001] Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0002] 1

[0003] Optical Heating Device, Optical Heating System, and Optical Heating Methods using the same

[0004] BACKGROUND

[0005] Precise temperature control of solid samples in ultra-high vacuum (UHV) environments is a critical requirement for many scientific and technological applications, particularly in surface science, condensed matter physics, and materials research. Typical examples are controlled gas-surface interactions or synthesis of defined surface structures using methods like molecular beam epitaxy or atomic layer deposition. Experiments often require UHV conditions where the absence of gas molecules minimizes unwanted interactions with the sample surface, allowing for highly controlled studies of surface phenomena such as adsorption, desorption, catalysis, and thin-film growth. However, accurately controlling and stabilizing the temperature of samples in UHV without disturbing other sensitive measurements remains a significant technical challenge.

[0006] Current approaches for sample heating in vacuum systems mainly use resistive, electronbombardment or radiative heating. All these methods involve hot filaments (typically tungsten), which are necessarily at much higher temperatures than the actual sample. As a result, the ultra-high vacuum often degrades due to degassing of the filaments or nearby components that are also heated, leading to impurities being deposited on the sample surface. In addition, these heating methods may produce charged particles (electrons and positively charged ions) that lead to background when sensitive detection techniques employing, for example, microchannel plate (MCP) detectors, are used. In molecular beam epitaxy (MBE) where large oxygen pressures are used, hot filaments will rapidly oxidize and fail.

[0007] Optical heating using lasers has the potential to be a much cleaner solution, since the laser radiation can be directed in a controlled way to heat only the sample, which can then be raised to a temperature higher than all other parts housed within the UHV chamber. This can provide a better overall vacuum, as outgassing from other materials can be avoided. Laser-based Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0008] 2 heating in an MBE apparatus has been previously reported; using a fiber-coupled 300 Watt Nd:YAG laser operating at 1064 nm temperatures up to 1700K could be achieved. CO2 laserheating has also been used in pulsed laser and thin film deposition experiments, but the temperature range was limited, and the use of CO2 laser radiation required infrared windows as they are not compatible with optical fibers.

[0009] Laser-heating systems have been commercialized employing either CO2 or diode lasers. While these commercial systems show a significant improvement of vacuum conditions and rapid heating rates of even several 100 K / s, they require high power lasers with several 100W or even kW output power, making them significantly more expensive than classical electronic heating approaches. CO2 lasers are further inconvenient since they are incompatible with fiber optics. Moreover, these products work poorly for highly reflective samples like metals.

[0010] Surface science instruments require excellent vacuum to ensure surface cleanliness throughout the measurement time. They also require control of sample temperature, both to clean the surface of contaminants and to control reaction rates at the surface (e.g. for molecular beam epitaxy or velocity-resolved kinetics experiments). Optical heating using lasers can deliver energy directly to the sample and avoid the use of hot filaments, which may lead to outgassed molecules, electrons and ions being present in the vacuum chamber. Filaments may even disintegrate under reacting conditions. Despite its advantages, optical heating is rarely used as high-power lasers are considered expensive and unreliable and, more fundamentally, light does not efficiently heat highly reflective samples like metals.

[0011] SUMMARY

[0012] It is thus an object of the present invention to provide an optical heating device, an optical heating system, and optical heating methods using the same, by which a sample can be optically heated in an efficient and a cost reduced way. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0013] 3

[0014] This object is solved by the subject-matter of the independent claims. Advantageous embodiments and refinements of the present invention are described in the respective subclaims.

[0015] According to an embodiment of the present invention, an optical heating device for heating a sample by absorption of light from a coherent light source may comprise a sample holder adapted to place the at least partly reflecting sample surface in the light path from the coherent light source, reflector optics, preferably concave reflector optics, placed along the light path reflected by the sample surface; and adjustment means adapted to adjust the distance (d) between the sample surface and the reflector optics along the optical axis of the reflector optics, preferably along the rotational symmetry axis of the concave reflector optics, such that the reflector optics and the sample surface form an optical resonator, and preferably form a hemispherical optical cavity.

[0016] According to another embodiment of the present invention, an optical heating device for heating a sample by means of absorption of light from a light source may comprise a sample holder adapted to place the sample surface in the light path from the light source, and a reflector optics placed, along the reflected light path behind the sample surface, wherein the reflector optics forms with the sample surface an optical cavity.

[0017] According to another embodiment of the present invention, an optical heating device for heating a sample by absorption of light from a coherent light source may comprise reflector optics, preferably concave reflector optics, placed along a light path from the light source and reflected by a sample surface of the sample, and adjustment means adapted to adjust the distance between the sample surface and the reflector optics along the optical axis of the reflector optics, preferably along the rotational symmetry axis of the concave reflector optics, such that the reflector optics and the sample surface form an optical resonator, and preferably form a hemispherical optical cavity. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0018] 4

[0019] According to another embodiment of the present invention, an optical heating device for heating a sample by absorption of light from a coherent light source may comprise reflector optics, preferably concave reflector optics, placed along a light path from the light source and reflected by a sample surface of the sample, wherein the reflector optics and the sample surface form an optical resonator.

[0020] According to another embodiment of the present invention, a radiation heating device for heating a sample by means of absorption of electromagnetic radiation from a radiation source may comprise a sample holder adapted to place the sample surface in the radiation path from the radiation source, and a reflector optics placed, along the reflected light path, behind the sample surface, wherein the reflector optics forms with the sample surface an radiation cavity.

[0021] According to another embodiment of the present invention, a radiation heating device for heating a sample by means of absorption of electromagnetic radiation from a radiation source may comprise a sample holder adapted to place the sample into the radiation path from the radiation source, and two reflector optics placed, along the radiation path behind the sample, wherein the two reflector optics form an radiation cavity for heating the sample within the radiation cavity.

[0022] Herein, a distance between the sample surface and the concave reflector optics along the rotational symmetry axis of the concave reflector optics may be smaller than or equal to the radius of curvature, ROC, of the concave reflector optics, such that the concave reflector optics and the sample surface form a hemispherical optical cavity.

[0023] According to another embodiment, the light from the coherent light source may pass a hole in the reflector optics through the optical axis of the reflector optics.

[0024] Herein, a light cone exiting the hole may have a maximum opening angle greater than ±5° or has a maximum opening angle greater then ±10°, or has a maximum opening angle smaller then ±15°, or has a maximum opening angle smaller then ±20°. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0025] 5

[0026] According to another embodiment, the diameter of the hole may be in a range between 1 mm and 5 mm, or between 1.5 mm and 4.5 mm, or between 2 mm and 4 mm, or between 2.5 mm and 3.5 mm.

[0027] According to another embodiment, the diameter of the hole may be smaller than 10 mm, or smaller than 5 mm, or smaller than 4 mm, or smaller than 3 mm, or smaller than 2 mm, or smaller than 1 mm, and greater than 0.2 mm.

[0028] Preferably, the optical heating device may further comprise a fiber port adapted to couple out the light guided by a fiber optics from the light source to the optical heating device, wherein the fiber port comprises a fiber end placed in the hole.

[0029] According to another embodiment, a fiber exit surface of the fiber end in the hole may be flush with the reflecting surface of the reflector optics.

[0030] Herein, the adjustment means may be adapted to adjust the distance (d) to fulfill % ROC < d < 1 ROC, such that the concave reflector optics and the sample surface form a hemispherical optical cavity and back reflections of light into the fiber end are reduced.

[0031] According to another embodiment, the inner diameter of the hole may be equal to the outer diameter of the fiber port.

[0032] Preferably, the light coupled out at the fiber port may have a maximum opening angle defined by the numerical aperture, NA, of the fiber optics.

[0033] Preferably, the fiber port may comprise no collimation optics and may be not adapted to collimate the light coupled out at the fiber port, wherein a light cone is emitted by the fiber exit surface at the fiber end into the concave reflector optics.

[0034] According to another embodiment, the adjustment means may be further adapted to tilt the sample surface or to displace the sample surface in a direction perpendicular to the optical axis of the reflector optics such that the reflector optics and the sample surface form an optical cavity. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0035] 6

[0036] Herein, the reflector optics may have a reflectivity of at least 90%, or may have a reflectivity of at least 95%, or may have a reflectivity of at least 98%, or may have a reflectivity of at least 99%, or may have a reflectivity of at least 99.5%, or may have a reflectivity of at least 99.8%, or may have a reflectivity of at least 99.9%.

[0037] According to another embodiment, the reflector optics may comprise a polished metal mirror or a coated optical mirror with protected metallic or dielectric coating.

[0038] The sample holder may comprise two stainless steel arms mounted to a manipulator unit for gripping the sample and for placing the sample surface to face the light from the light source.

[0039] Preferably, the optical heating may further comprise an optics mount providing an exchangeable mount for concave reflector optics each having a different radius of curvature, ROC.

[0040] According to the present invention, the ratio PH / PL between the light power absorbed by the sample PH and the light power of the light source PL of a formed stable optical cavity may be greater than 50%, or greater than 60%, or greater than 70%, or greater than 80%, the reflectivity of the reflector optics being greater than 95%, or being greater than 99%, or being greater than 99.5%, independently of the reflectivity of the sample surface.

[0041] According to the present invention, the ratio PH / PL between the light power absorbed by the sample PH and the light power of the light source PL of a formed stable optical cavity may be greater than 80%, the reflectivity of the reflector optics being greater than 95%, or being greater than 99%, or being greater than 99.5%, independently of the laser wavelength of the light source.

[0042] According to an embodiment, an optical heating system may be provided, which comprises the optical heating device of the present invention, and further comprises a light source adapted to emit light; sample temperature sensing means adapted to measure the temperature of the sample; and a control unit adapted to control the light output power of the Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0043] 7 light source and to monitor the sample temperature measured by the temperature sensing means.

[0044] Preferably, the control unit may be adapted to control the adjustment means to ramp up / down the distance between the sample surface and the reflector optics and detect an optimized distance having a maximized cavity enhanced heating effect by detecting a maximized sample temperature at a constant light output power.

[0045] According to another embodiment, the optical heating system may further comprise a light detector to detect light escaping from the optical cavity, wherein the control unit may be adapted to control the adjustment means to ramp up / down the distance between the sample surface and the reflector optics and detect an optimized distance having a maximized cavity enhanced heating effect by detecting minimized escaped light at a constant light output power.

[0046] Herein, the optimized distance may be greater than 0.5 ROC, or greater than 0.6 ROC, or greater than 0.7 ROC, or greater than 0.8 ROC, or smaller than 0.9 ROC, or smaller than 1 ROC, ROC being the radius of curvature of the reflector optics.

[0047] Preferably, the control unit may comprise sample temperature control means adapted to stabilize a set sample temperature by a closed-loop control of the light output power of the light source in reaction to the actual sample temperature measured by the temperature sensing means.

[0048] The maximum power of the light source may be smaller than 1 kW, or smaller than 500 W, or smaller than 200W, or smaller than 100W, or smaller than 50W, or smaller than 30W, or greater than 20 W, or greater than 10 W.

[0049] The light source may be a diode laser.

[0050] Preferably, the optical heating system may further comprise fiber optics adapted to guide the light from the light source to the optical heating device. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0051] 8

[0052] According to an embodiment, a method for heating a sample by means of absorption of light from a light source using the optical heating device may comprise the step of adjusting the distance between the sample surface and the reflector optics along the optical axis of the reflector optics such that the reflector optics and the sample surface form an optical cavity.

[0053] According to an embodiment, a method for heating a sample by means of absorption of light from a light source using the optical heating system may comprise the steps of controlling the light output power of the light source and monitoring the sample temperature measured by the temperature sensing means.

[0054] According to another embodiment, the optical heating method may further comprise the step of ramping up / down the distance between the sample surface and the reflector optics and detecting an optimized distance having a maximized cavity enhanced heating effect at a constant light output power.

[0055] Preferably, the optimized distance may be greater than 0.5 ROC, or greater than 0.6 ROC, or greater than 0.7 ROC, or greater than 0.8 ROC, or smaller than 0.9 ROC, or smaller than 1 ROC, ROC being the radius of curvature of the concave reflector optics.

[0056] The optical heating method may further comprise the step of stabilizing a set sample temperature by a closed-loop control of the light output power of the light source in reaction to the actual sample temperature measured by the temperature sensing means.

[0057] According to an embodiment, a data processing system may comprise means for carrying out the steps of the method for heating a sample by means of absorption of light from a light source using the optical heating system according to the present invention.

[0058] According to an embodiment, a computer program product may comprise instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method for heating a sample by means of absorption of light from a light source using the optical heating system according to the present invention. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0059] 9

[0060] According to an embodiment, a computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the steps of the method for heating a sample by means of absorption of light from a light source using the optical heating system according to the present invention.

[0061] Those skilled in the art will recognize additional features and advantages upon reading the following detailed description and on viewing the accompanying drawings.

[0062] BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the invention and together with the description serve to explain principles of the invention. Other embodiments of the invention and intended advantages will be readily appreciated as they become better understood by reference to the following detailed description.

[0064] FIG. 1 is a schematic view of an optical heating device in accordance with an embodiment of the present invention.

[0065] FIG. 2A and 2B are schematic perspective views of an optical heating device having an open design in accordance with an embodiment of the present invention.

[0066] FIG. 3A and 3B are schematic perspective views of an optical heating device having a capsulated design in accordance with an embodiment of the present invention.

[0067] FIG. 4A and 4B are a schematic perspective and a cross-sectional view of an optical heating device having a capsulated design and further comprising a linear motor as an adjustment means in accordance with an embodiment of the present invention. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0068] 10

[0069] FIG. 5A and 5B are schematic perspective views of an optical heating device having a capsulated design and a cage design, respectively, in accordance with embodiments of the present invention.

[0070] FIG. 6A to 6D are schematic views of the optical heating device without using fiber optics and adaptive reflector optics of the optical heating device according to further embodiments of the present invention.

[0071] FIG. 7 shows ray tracing simulations of the optical heating device for 20 reflections off the sample at different distances between reflector optics and sample surface.

[0072] FIG. 8A and 8B show a diagram and a table for estimation of the maximum laser power that will be absorbed by different sample materials upon usage of a reflector optics for cavity enhancement having a reflectivity of 92.0% and a 455nm diode laser source.

[0073] FIG. 9A and 9B show a diagram and a table for estimation of the maximum laser power that will be absorbed by different sample materials upon usage of a reflector optics for cavity enhancement having a reflectivity of 99.87% and a 455nm diode laser source.

[0074] FIG. 10A and 10B show a diagram and a table for estimation of the maximum laser power that will be absorbed by different sample materials upon usage of a reflector optics for cavity enhancement having a reflectivity of 99.75% and a 1064nm laser source.

[0075] FIG. 11 A and 11 B show diagrams illustrating blackbody emission at different temperatures and compensation for the heat loss at a target temperature of 1300 K by cavity enhanced heating using different reflector optics having different reflectivity in dependence of the wavelength of the coherent light source.

[0076] FIG. 12 shows a schematic view of an optical heating system in accordance with an embodiment of the present invention.

[0077] FIG. 13 shows a schematic view of an optical heating system in accordance with another embodiment of the present invention. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0078] 11

[0079] FIG. 14 shows a diagram for illustrating surface temperatures as a function of laser power for cavity-enhanced laser heating of a sample of an optical heating system in accordance with an embodiment of the present invention.

[0080] FIG. 15 shows a diagram for illustrating performance of the optical heating system for heating of a sample from room temperature to 1173K using an optical heating system in accordance with an embodiment of the present invention.

[0081] FIG. 16 shows a diagram for illustrating performance data as shown in FIG. 15 but extended over a period of 3 hours using an optical heating system in accordance with an embodiment of the present invention.

[0082] FIG. 17 shows a diagram for illustrating an example for heating and cooling a sample to several temperatures between 300-1300K using an optical heating system in accordance with an embodiment of the present invention.

[0083] FIG. 18 shows a process chart of a method for heating a sample by means of absorption of light from a light source using the optical heating device according to embodiments of the present invention.

[0084] FIG. 19 shows a process chart of a method for heating a sample by means of absorption of light from a light source using the optical heating system according to embodiments of the present invention.

[0085] FIG. 20 shows a schematic block diagram of a data processing system, a computer program product, and a computer-readable medium. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0086] 12

[0087] DETAILED DESCRIPTION

[0088] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which are shown by way of illustrations specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized, and structural or logical changes may be made without departing from the scope of the present invention. For example, features illustrated or described for one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the present invention includes such modifications and variations. The examples are described using specific language which should not be construed as limiting the scope of the appending claims. The drawings are not scaled and are for illustrative purposes only. For clarity, the same elements have been designated by corresponding references in the different drawings if not stated otherwise.

[0089] First, the basic physical concepts are presented needed to understand sample heating in an ultra-high vacuum environment. The predominant design parameter is the effective heating power that can be delivered to the sample, Ph. For radiative laser heating, this is given by:

[0090] Ph= (l - fi(A)) x Ptor0) (1) where R(A) is the reflectivity of the sample at the laser wavelength, A, and PLaserW 'sthe laser power. Note that the reflectivity is usually temperature dependent as well as dependent on angle of incidence (AOI). To reach a specified sample temperature, Ts, the heating power needs to balance the temperature-dependent heat losses, which arise from heat conduction and radiation. The effect of heat conduction can be limited by a careful design of the sample holder. Using Tungsten wires, as illustrated in Fig. 2A, to thermally isolate the sample from its surroundings, the expression for conductive heat loss is given by Eq. 2: Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0091] 13 where KW= 175 W / (m K) is the thermal heat conductivity of tungsten, L is the length of a tungsten wire between the sample and the sample mount, Awis the cross-section of the tungsten wires, and Tsand Tmountare the sample and mount temperature, respectively. The factor of four arises since the sample has four wire connections to the mount shown in Fig. 2A. Note that conductive heat losses scale approximately linearly with sample temperature.

[0092] Radiative (black body) heat loss scales with the 4thpower in sample Temperature and is given by the Stefan-Boltzmann law:

[0093] Pbb(Ts) = a x Asx Ts4x e (3) where a = 5.670 10'8W / (m2K4) is the Stefan-Boltzmann constant, Asis the surface area and e is the emissivity of the sample. In Eq. 3, the emissivity is critically important and illustrative emissivity data is shown in Table 1.

[0094] The optical heating scheme of the present invention has been tested with a highly reflective (low emissivity) platinum sample. Emissivity may depend strongly on the surface quality. For example, emissivity values varying from 0.037 to 0.21 have been reported for different platinum surface grades. Moreover, the emissivity can show a significant temperature dependence, for example for Si. As a rule of thumb, surfaces with high reflectivity, e.g. polished metal surfaces, have a low emissivity, which leads to lower radiative heat loss.

[0095] Of course, such surfaces also absorb poorly; making laser heating inefficient. This data highlights the nature of any radiative heating approach: high reflectivity (low emissivity) suppresses the power available for heating, but, on the other hand, helps in achieving the maximum sample temperature since radiative heat losses scale with emissivity. In general, radiative heat losses dominate at high temperature due to the physical T4scaling. Put more concretely, laser heating of graphite yields higher heating rates, whereas laser heated shiny metal samples achieve higher temperatures. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0096] 14

[0097] Table 1 below shows literature values for emissivity factors and reflectivity at different wavelength for a selection of metal and semiconductor materials. Reflectivity values are given for 0° angle of incidence (AOI). Note that the emissivity of silicon has a strong temperature dependence.

[0098] FIG. 1 is a schematic view of an optical heating device 10 in accordance with an embodiment of the present invention. FIG. 2A and 2B are schematic perspective views of the optical heating device 10 having an open design with respect to the sample 20 in accordance with an embodiment of the present invention. FIG. 3A to 3B are schematic perspective and cross- sectional views of the optical heating device 10 having a capsulated design in accordance with an embodiment of the present invention. FIG. 4A to 4B are schematic perspective and cross-sectional views of the optical heating device 10 having a linear motor as adjustment means in accordance with an embodiment of the present invention. FIG. 5A and 5B are schematic perspective views of the optical heating device 10 having a capsulated design and a cage design, respectively.

[0099] The optical heating device 10 is adapted to heat a sample 20 by absorption of light L from a coherent light source 30. The sample 20 may be a metal or semiconductor or insulator (e.g., chosen from a material as shown in Table 1 above), preferably a sample 20 with high reflectivity which is difficult to heat using the conventional optical heating methods. The sample 20 should be defined to have an at least partly reflecting sample surface 22 having a reflectivity Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0100] 15 of smaller than 90%, or smaller than 95%, or smaller than 98%, or smaller than 99%, and to be not adapted or at least not intended (by e.g. being not cooled) to be used as an optical mirror in a known optical laser resonator.

[0101] The coherent light source 30 may be a diode laser. The coherent light source 30 may be a solid-state laser. The coherent light source 30 may be a compact engraving laser having a maximum laser power of lower than 40W, or lower than 30W, or lower than 20W. The coherent light source 30 may be a neodym laser (e.g. Nd:YAG, Nd:YLF) at a wavelength of 1064nm. The coherent light source 30 may be a diode laser emitting at a wavelength of 455 nm. The coherent light source 30 may be a laser emitting at a wavelength between 400 nm and 1200 nm. The coherent light source 30 may be a laser emitting at a wavelength of 455 nm. The coherent light source 30 may be a laser emitting at a wavelength of 1064 nm. The maximum power of the coherent light source 30 may be smaller than 1 kW, or smaller than 500 W, or smaller than 200W, or smaller than 100W, or smaller than 50W, or smaller than 30W, or greater than 20 W, or greater than 10 W, or greater than 1 W.

[0102] While radiative heating can also be accomplished with non-coherent broadband light sources like heated filaments or light bulbs, a laser-based approach offers the important advantage that optical heating with coherent light is not bound to the thermodynamical law that the temperature of the heated sample cannot be higher than the temperature of the incoherently radiating heating source (you cannot light a fire with a giant magnifying glass and moon light). In other words, the temperature of the sample is not limited to the temperature of the heating source when using optical heating using a coherent light source. Furthermore, a laser-based approach offers the additional advantage that one can choose the wavelength to enhance absorption by the sample. Wavelength dependent reflectivity data of different sample materials is also shown in Table 1 above. Moreover, narrow band laser radiation can easily be blocked by optical filter or rapidly pulsed off (gated) if the light disturbs other sensitive measurements. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0103] 16

[0104] Some commercial systems use a CO2 laser operating in the mid-infrared as they offer high average powers at moderate costs. However, most metals exhibit high reflectivity at 10.6pm (AOI = 0°), meaning that this laser heats them poorly. CO2 lasers are, furthermore, incompatible with optical fibers, limiting the flexibility of the sample mount and position inside the chamber. Fiber optics can be used with the output of Nd:YAG lasers. However, most metals still exhibit only weak absorption at 1064 nm. These lasers also tend to be rather expensive. Most materials, including metals, absorb short-wavelength light more strongly, hence, high power 455nm diode laser have become commonly used for material processing. Compact engraving lasers exhibit optical output powers with several tens of Watts and are commercially available at surprisingly low costs. In addition, standard stepped-index optical fibers can transmit up to 1 kW of 455nm radiation. A laser heating system 100 according to an advantageous embodiment may comprise as a coherent light source 30 a compact 455 nm diode laser. The diode laser may be, for example, an Atomstack M150 laser with a specified optical output power of 30-33W, which is a commercial engraving laser head that is available at low-costs of lower than EUR 1000.00. After removing of the standard focusing optics, the maximum measured output power can be coupled into stepped-index fiber using, for example, a SMA fiber port (Thorlabs, NA=0.3).

[0105] As further shown in FIG. 1 to 4B, the optical heating device 10 comprises a sample holder 40 adapted to place the at least partly reflecting sample surface 22 in the light path L from the coherent light source 30. The sample holder 40 may comprise two stainless steel arms 42a, 42b. These arms 42a, 42b may be mounted to a manipulator unit 44 for gripping the sample 20 and for placing the sample surface 22 to face the light L from the coherent light source 30, as shown for example, in Fig. 2A. The manipulator 44 may be realized as an UHV manipulator HPT 2 of the company Vacuum Generators Ltd. As a sample 20, a Pt(111) crystal sample (MaTeck GmbH, 010 mm x 2 mm, front-surface polished) may be provided, which may be machined to provide two grooves 24 (with a size in the range of 0.5 mm) on opposite edges, as illustrated, for example, in FIG. 2A to 4B. This allows mounting the sample 20 to the Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0106] 17 stainless-steel arms 42a, 42b via two straight tungsten wires 46a, 46b as shown in Fig. 2A and 3B, having, for example, a diameter of 0.4mm and a length of 25 mm.

[0107] The optical heating device 10 further comprises reflector optics 50 as shown in Fig.1 and in more detail, in Figs. 6A to 6D, which will be explained in detail later. The reflector optics 50 may comprise preferably concave reflector optics, placed along the light path L being at least partly reflected by the sample surface 22 of the sample 20.

[0108] The reflector optics 50 may comprise a reflecting surface 52, which has a reflectivity of at least 90%, or has a reflectivity of at least 95%, or has a reflectivity of at least 98%, or has a reflectivity of at least 99%, or has a reflectivity of at least 99.5%, or has a reflectivity of at least 99.8%, or has a reflectivity of at least 99.9%.

[0109] As can be seen from Table 1 above, the reflectivity of different materials depends on the light wavelength. Thus, for the purpose of the present invention, the claimed reflectivity values are defined to be reflectivity of the reflector optics 50 or the sample surface 22 at the emitting wavelength (for example 455 nm or 1064 nm) of the coherent light source 30. For example, the reflector optics 50 may also absorb part of the incident laser radiation. It may be water cooled by a copper capillary 54 (FIG. 2B), which may have a diameter of 3 mm, and maintains the reflector optics 50 at room temperature when using maximum laser power. The sample surface 22 preferably has a lower reflectivity than a reflecting surface 52 of the reflector optics 50. The difference between the reflectivity of the reflecting surface 52 of the reflector optics 50 and the sample surface 22 may be higher than 5%, or may be higher than 10%, or may be higher than 15%.

[0110] The optical heating device 10 may further comprise an optics mount 56 as shown in Fig. 1 and FIG. 4A and 4B, for example. The optics mount 56 may provide an exchangeable mount for the concave reflector optics 50 each having a different radius of curvature (ROC).

[0111] In addition, the optical heating device 10 may comprise adjustment means 60 adapted to adjust the distance d between the sample surface 22 and the reflector optics 50 along the Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0112] 18 optical axis OA of the reflector optics 50, preferably along the rotational symmetry axis OA of the concave reflector optics 50, such that the reflecting surface 52 of the reflector optics 50 and the sample surface 22 form an optical resonator, and preferably form a hemispherical optical cavity. The distance d will be defined below, when discussing a stable optical cavity condition as shown in FIG. 7 (panel (a) and (b)). The adjustment means 60 may be further adapted to tilt the sample surface 22 or to displace the sample surface 22 in a direction perpendicular to the optical axis OA of the reflector optics 50 such that the reflector optics 50 and the sample surface 22 form an optical cavity.

[0113] The adjustment means 60 may be adapted to ensure a stable optical cavity condition between the sample 20 and the reflecting surface 52 of the reflector optics 50 only. The adjustment means 60 may therefore have a very simple structure and can be even omitted in case a recurrent heating process for similar samples 20 in a continuous production line is performed, wherein the adjustment means 60 may be washers or spacers placed between a connection, preferably a screw connection, between the reflector optics 50 and the sample holder 40. In this case, the adjustment means 60 may comprise no movable parts but may be adapted to adjust the distance d only once in a run-in process of a starting production line.

[0114] In addition, although not shown, the adjustment means 60 may be a robot arm holding a laser processing (welding / cutting / sintering) head, wherein the reflector optics 60 is mounted as a collar around the laser processing nozzle of the laser processing head such that reflected laser light from the keyhole or cutting area or sintering area of the processed material usually made of metal can be reflected back again to the material, and forming an optical cavity with the reflector optics 50. In this case, a sample holder 40 would be not necessary since the “sample” 20 may be a part of a car like a door part being machined by a laser processing head. In this case, the “sample holder” may be rather interpreted as a conveyor band transporting a complete car through an automotive production line. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0115] 19

[0116] Furthermore, the optical heating device may also be used in a thermal annealing process for semiconductor manufacturing lines, wherein the reflector optics 50 may have diameters in the range of 0.5 to 2 m and the sample holder 40 may be a conveyor band, on which semiconductor devices are processed by defined temperature steps. Since semiconductor production processes are very sensitive if it comes to temperature budgets, the present invention may provide a very advantageous optical heating device for these processes. In such applications, the laser processing should not be restricted to take part in an UHV environment.

[0117] It shall be emphasized that the embodiments shown in FIG. 2A to 5B and discussed herein are setups in a research environment. However, the present invention should not be restricted to the same because the embodiments of the present invention can be applied to a broad field of heating applications, even including microwave ovens using the cavity enhanced heating effect for efficiently heating food. Furthermore, the “sample” 20 may be a side wall of a cooking or heating chamber, which is optically heated in accordance with the principles of the present invention by a laser having a maximum laser power smaller than 10W.

[0118] As can be seen from FIG. 2B, the adjustment means 60 as indicated by an arrow may comprise a threaded coupling (not shown in FIG. 2B) adapted to bring the stainless-steel arms 42a, 42b in engagement with the reflector optics 50. It is preferable that the adjustment means 60 is adapted to move the reflector optics 50 relative to the sample holder 40 while the sample 20 is fixed within the sample holder 40 and not movable relative to the sample holder 40.

[0119] However, the adjustment means 60 explained above with regard to FIG. 2B are just an example. The adjustment means 60 may be also realized, for example, by nano-positioners of the company attocube as illustrated in FIG. 4A and 4B. These linear stages offer high precision, compact design and are available for UHV environments and even low-temperature environments. Furthermore, the adjustment means 60 may be realized as a Picomotor™ of the company Newport or a PiezoMike actuator of the company Physik Instrumente. These Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0120] 20 linear motors consist of fine threaded screws that are driven by two piezos according to the inertia drive (stick-slip) principle. These piezo linear actuators are available in non-magnetic, vacuum and ultra-high vacuum compatible versions. They offer the same exceptional precision, small step sizes, and set-and-forget stability as the standard Picomotor actuators. Ultra-High Vacuum Picomotor™ Actuators with Kapton wires can be used in high radiation applications, as well as for VUV / EUV applications.

[0121] For example, in FIG. 4A, the upper side 62 of the adjustment means being a linear nanopositioner may be connected to the sample holder 40 and translatable lower side 64 of the nano-positioner may be in contact with a connection bracket 66. The connection bracket 66 in turn may be fixedly connected to the optics mount 56. The linear motor between the first and second element 64, 66 (not shown in FIG. 4A and 4B) may move the first element 62 and the second element 66 relative to each other in a linear direction to adjust the distance d between the sample surface 22 and the reflective surface 52 of the reflector optics 50. This direction may be parallel to the optical axis OA of the reflector optics 50. The sample holder 40 may comprise a tubular unit 41 , into which a corresponding tubular part 57 of the optics mount 56 may be telescopically inserted such that a telescopically movement between the sample holder 40 and the reflector optics 50 (fixedly connected to the optics mount 58) along the optical axis OA of the reflector optics 50 may be achieved.

[0122] Furthermore, various linear motors for vacuum applications as, for example, Linear, Rotation, Elevation Stages and Accessories of the company PI may be used to act as adjustment means 60. Since the requirement for forming a stable optical cavity in the sense of the present invention usually needs an adjustment precision in a mm range, there are no major technical problems in varying the distance from the sample surface 22 to the reflector optics 50 in an ultra-high vacuum. And even if the precision requirements should be much higher, there would be components such as the nano-positioner from the company Attocube mentioned above. These examples are, however, non-limiting, and further configurations for the adjustment means 60 are possible. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0123] 21

[0124] As further shown in Fig. 1 to 4B, the light L from the coherent light source 30 may pass a hole 70 in the reflector optics 50 through the optical axis OA of the reflector optics 50. A light cone L exiting the hole 70 may have a maximum opening angle greater than ±5°, or may have a maximum opening angle greater then ±10°, or may have a maximum opening angle smaller then ±15°, or may have a maximum opening angle smaller then ±20°. The diameter of the hole 70 may be in a range between 1 mm and 5 mm, or may be between 1.5 mm and 4.5 mm, or may be between 2 mm and 4 mm, or may be between 2.5 mm and 3.5 mm. The diameter of the hole 70 may be smaller than 10 mm, or may be smaller than 5 mm, or may be smaller than 4 mm, or may be smaller than 3 mm, or may be smaller than 2 mm, or may be smaller than 1 mm, and may be greater than 0.2 mm.

[0125] According to a preferred embodiment as shown in FIG. 1 to FIG. 5B, a fiber optics 80 may be provided, which may be adapted to guide the light L from the light source 30 to the optical heating device 10. In this regard, the optical heating device 10 may further comprise a fiber port 90 adapted to couple out the light L guided by the fiber optics 80 from the light source 30 to the optical heating device 10, wherein the fiber port 90 may comprise a fiber end 92 placed in the hole 70 as shown in Fig. 1 , 2B 3B, 4A and 4B. A fiber exit surface 94 of the fiber end 92 in the hole 70 may be flush with the reflecting surface 52 of the reflector optics 50. However, the fiber end 92 may also protrude into or may be recessed from the reflecting optics 50 of the concave reflector optics 50 in a range of smaller than 10%, or smaller than 5%, or smaller than 1% of the radius of curvature ROC of the concave reflector optics 50.

[0126] The inner diameter of the hole 70 may be equal to the outer diameter of the fiber port 90. The light cone L coupled out at the fiber port 90 may have a maximum opening angle defined by the numerical aperture (NA) of the fiber optics 80. The fiber port 90 may comprise no collimation optics to collimate the light L coupled out at the fiber port 90. In detail, the fiber port 90 may comprise no collimation optics and may be not adapted to collimate the light L coupled out at the fiber port 90, wherein a light cone L is emitted by the fiber exit surface 94 at the fiber end 92 into the inner reflecting part of the concave reflector optics 50. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0127] 22

[0128] As can be seen from Fig. 2A to 4E, the hole 70, preferably having a diameter between 3 mm and 4 mm, or between 2 mm and 3 mm, or between 1 mm and 2 mm according to different embodiments, through the rotational symmetry axis OA of the reflector optics 50 may be provided. The fiber port 90, which may comprise a high temperature, vacuum compatible SMA905 stainless steel ferrule, may be mounted into the hole 70. Through the fiber port 90 the fiber optics 80 may be attached. The fiber optics 80 may be an UHV compatible polyimide- coated stepped-index multimode fiber. The fiber end 92 may be glued into the fiber port 90 with high temperature low outgassing epoxy (EPO-TEK® 353ND), which is also used to form an UHV optical feedthrough by sealing the fiber end 92 to a hole bored through a CF16 flange. The epoxy may be temperature stable up to 250°C. Loss-free transmission at 26W of laser radiation may be possible for fiber core diameters of the fiber optics 80 of >400pm. According to an embodiment, a 400pm multimode stepped-index fiber of the fiber optics 80 has a numerical aperture NA = 0.22, which results in a maximum opening angle of ±12.71° at the fiber exit surface 94. For example, the sample 20 may be in the radius of curvature (ROC) of a concave aluminum reflector 50 as shown in FIG. 1 , which’s center may hold the exit of a 400pm multimode fiber optics 80.

[0129] As can be seen from Fig. 3A to 5A, the optical heating device 10 may have a more encapsulated design than the previous open design as shown in FIG. 2A and 2B. This means that laser radiation from the coherent light source 30 may be better shielded, which increases safety. The optical heating device 10 has been integrated into a round body 12, forming a tube 12a surrounding the side portions of the optical cavity between the reflector optics 50 and the sample surface 22. According to an embodiment, a commercial concave mirror may be used as the reflector optics 50. According to an embodiment, a custom-made aluminum reflector 50 can be used. However, a higher reflectivity can be achieved with suitable mirrors. The round body 12 is made of a thermally conductive material such as aluminum to dissipate any heat loss. The sample 20 is held on two thin wires 46a, 46b, which may be connected to ceramic insulators 48. The two thin wires 46a, 46b are typically made of high melting pure Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0130] 23 material like tungsten or tantalum. This ensures electrical contact, but above all good thermal insulation of the sample 20, which minimizes heat conduction losses. A heat shield 14 preferably made of tantalum is to be mounted in front of the sample 20 to shield heat radiation from the heated sample 20 and to close the tube-shaped round body 12 on the side of the sample 20. This results in a better vacuum in the UHV chamber, as less radiant heat can heat up the chamber walls. Furthermore, radiant heat losses from the sample 20 can be reduced, which enables higher temperatures. The laser heating device 10 may be open to an UHV chamber and may be pumped through it. In addition, a light detector 130 may be provided to detect light escaping from the optical cavity, as will be explained in detail below when discussing the optical heating system 100 as shown in FIG. 12. However, as emphasized above, although the optical heating device 10 maximizes its heating effect in a vacuum environment due to non-existing convectional heat loss, the present invention shall not be restricted to this embodiment.

[0131] As can be seen from Fig. 5B, alternatively a cage design can be used for the optical heating device 10. The optical heating device 10 having a cage design basically has the same structure as that having an encapsulated design as shown in FIG. 3A to 5B. However, the round body 12 is modified to have horizontal and vertical slits 16 in the outer circumferential surface of the round body 12. This improves pumping efficiency between the reflector optics 50 and the sample 20 via the vacuum chamber. A better vacuum is achieved. However, more scattered light can of course escape again without further measures. However, this might be advantageous if the light detector 130 is provided in the vicinity of one of the horizontal and vertical slits 16, to detect light escaping from the optical cavity, as will be explained in detail below when discussing the optical heating system 100 as shown in FIG. 12.

[0132] In the following, further embodiments, and refinements of the reflector optics 50 will be discussed. As can be seen, for example, from FIG. 6A, the invention is not restricted to an embodiment using fiber optics 80 but the light L may be also adjusted to directly pass the hole 70. FIG. 6B to 6D are schematic views of exemplary embodiments for the reflector optics 50 Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0133] 24 of the optical heating device 10 of the present invention, which may be configured as adaptive reflector optics 50. In greater detail, the reflector optics 50 is not restricted to a concave reflector optics 50 having a fixed radius of curvature (ROC). To achieve reflector optics 50 having a variable radius of curvature (ROC), adaptive optics (AO) may be used.

[0134] As can be seen from FIG. 6B, the reflector optics 50 may be configured to optically generate or emulate reflecting surfaces 52a, 52b having a variable radius of curvature (ROC). For example, the reflector optics 50 using AO may comprise a selection of MEMS Deformable Mirrors (DM), Piezoelectric Deformable Mirrors, Shack-Hartmann Wavefront Sensors, and AO kit. Furthermore, ALPAO Deformable Modal Mirrors may be used to provide correction of common optical aberrations and thus providing a reflector optics 50 having a variable radius of curvature (ROC). However, the present invention is not restricted to these examples, and further solutions may be used for the reflector optics having AO. In addition, AO can be also formed by using classical optical components.

[0135] In the example shown in Fig. 6C two spherical mirrors 50a, 50b in a kind of mirror telescope may be used to generate a variable "effective" radius of curvature (ROC). Alternatively, as shown in Fig. 6D, a combination of concave mirror 50a and a concave lens 50b may be used, which would enable a linear setup. Furthermore, the reflector optics 50 shall not be restricted to a concave reflector optics 50 (although it is an advantageous embodiment of the present invention) but may also be realized as a hollow retroreflector mirror, which uses three orthogonal surfaces to reflect light back to the source at the same angle as the incident beam, regardless of changes in position or alignment, creating an inverted image.

[0136] FIG. 7 shows ray tracing simulations of the optical heating device 10 for 10 reflections off the sample 20 at different distances d between the reflector optics 50 and the sample surface 22. The laser radiation of the light source 30 may be fed to the sample 20 via a UHV compatible 400pm multimode fiber acting as the fiber optics 80 with an NA = 0.22. The fiber end 92 is mounted to the center of the concave aluminum reflector optics 50. The fiber optics 80 Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0137] 25 transmits the light L to the center of the reflector optics 50 with a radius of curvature ROC = 10 mm. The lines show rays exiting the fiber exit surface 94 within the maximum divergence angle given its NA. The rays impinge the sample 20 where they are partially reflected towards the reflector optics 50, which sends them back onto the sample surface 22 of the sample 20. Thus, to enhance absorption of the light radiation L of the coherent light source 30, emitting radiation at, for example, a wavelength of 455 nm, a polished concave Aluminum reflector acting as the reflector optics 50 may be placed behind the sample 20 in the direction of the light path L.

[0138] The panels in Fig. 7 show the three limiting cases for the relation between sample distance d and ROC: (a) d < ROC, (b) d = ROC and (c) d > ROC. Panel (b) of FIG. 7 shows the behavior for 10 reflections for a sample-reflector distance d = ROC. Note that this geometry corresponds to a hemispherical optical resonator. As shown in panel (a) of FIG. 7, by adhering to the stable cavity condition (d < ROC), loss of light from the cavity only arises due to absorption at the reflector optics 50 (for example an Al surface) and the sample surface 22 (for example a Pt surface). The lines show rays exiting the fiber exit surface 94 of the fiber end 92 of the fiber optics 80 over an angular range given by its numerical aperture (NA) and striking the backside of the Pt sample 20. The surface 22 of the Pt crystal sample 20 reflects 85% of the 455 nm light, whereas the aluminum reflector of the reflector optics 50 reflects 92% of light. This ensures that more light is absorbed by the Pt crystal and because the light cannot escape the cavity, a large fraction of the light is absorbed by the Pt sample 20. Thus, if distance d between the sample surface 22 of the sample 20 and the reflector optics 50 is smaller or equal its ROC, the rays do not leave the stable cavity.

[0139] Herein, the distance d shall be defined as the distance d between the sample surface 22 and the reflecting surface 52 (which could also be a theoretical or virtual reflecting surface, when adaptive optics is used) of the reflector optics 50 along the optical axis OA of the reflector optics 50. The optical axis OA of the reflector optics 50 is a well-defined term in optics and does not need further definition. For example, if the reflector optics 50 is a concave mirror, the Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0140] 26 distance d shall be the distance between the sample surface 22 of the sample 20 and the fiber exit surface 94 in the hole 70 being flush with the reflecting surface 52 of the reflector optics 50 along the rotational symmetry axis of the concave reflector optics 50. In case, no fiber optics 80 or a different structure of fiber optics 80 is used, the distance d shall be the distance between the sample surface 22 of the sample 20 and the surface of the hole 70 being flush with the reflecting surface 52 of the reflector optics 50 along the rotational symmetry axis of the concave reflector optics 50.

[0141] As shown in panel (c) of FIG. 7, for d> ROC, the cavity is instable, and light will escape. The escaped light may be detected by a light detector 130, to detect an instable optical cavity condition, as will be explained below about the optical heating system 100.

[0142] Eq. 1 as explained above is extended to account for the multi-pass effect. In this context, the effective heating power supplied to the sample 20 is given by: where N is the number of reflections and RRW is the reflectivity of the reflector at the laser wavelength.

[0143] As a result, it is preferable that the distance d between the sample surface 22 and the concave reflector optics 50 along the rotational symmetry axis of the concave reflector optics 50 is smaller than or equal to the ROC of the concave reflector optics 50, such that the concave reflector optics 50 and the sample surface 22 form a hemispherical optical cavity.

[0144] In case the fiber optics 80 is used such that the fiber end 92 is mounted within the hole 70, it is preferable that the adjustment means 60 are adapted to adjust the distance d to fulfill % ROC < d < 1 ROC, such that the concave reflector optics 50 and the sample surface 22 form a hemispherical optical cavity and back reflections of light into the fiber end are reduced or can be neglected. For example, for % ROC < d and a fiber surface exit area of (0.5 mm)2the amount of light reflected back into the fiber end 92 has been calculated to be lower than 1%. Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0145] 27

[0146] Fig. 8A shows the effective normalized laser heating power - given as a fraction of the laser output power - for the materials listed in Table 1 and an Al reflector with a reflectivity of 92%. For low reflectivity material samples 20 such as graphite or silicon, almost all laser light L is absorbed by the sample 20 within about four reflections. The flattening of the curve above this value reflects the fact that all laser power PL is absorbed by the sample 20 and the Al reflector of the reflector optics 50. The normalized heating power is given by the ratio of the reflectivity of Aluminum versus the sample 20. Except for Au, which absorbs well at 455 nm, metal samples 20 generally require 10-20 reflections to deplete the laser radiation. Furthermore, the normalized heating power at complete absorption is smaller - note that for Aluminum it is 0.5. The laser heating power PH is given as a fraction PH / PL of the laser output power PL.

[0147] For example, if the sample 20 of Pt is chosen (it exhibits representative metallic behavior), using the reflectivity values given in Table 1 for a platinum sample (Rpt=0.85) and an aluminum reflector (RAI=0.92) at 455nm, it is calculated that the Pt crystal absorbs 58% and the reflector absorbs 33% of the laser radiation L of the coherent light source 30 after twenty reflections. Consequently, the laser light absorption by the Pt crystal, and therefore the effective heating power, can be increased nearly four-fold from 15% to about 60%.

[0148] As can be seen from the curves of the mirrors in Figs. 8A and 8B, aluminium may not be an ideal choice for the reflector optics 50 due to its low reflectivity. However, aluminum is suitable material to build a full metal reflector due to its low costs, high heat conduction and good machining properties. It shall be noted that the curves shown in Fig. 8A are valid for full metal aluminium reflector as well as for mirrors consisting of a glass substrate (BK7, quartz) with an appropriate aluminium coating. In addition, the reflector optics 50 may comprise mirrors with a hole 70 for the fibre feed-through and / or mirrors with higher reflectivity. The reflector optics 50 may comprise a polished metal mirror or a coated optical mirror with protected metallic or dielectric coating. According to an embodiment of Fig. 8A, the reflector optics 50 may comprise a polished aluminium reflector, which optimally achieves a reflectivity of 92% at 455nm. This means that 8% of the power goes into the reflector for each pass. Logically, only a maximum Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0149] 28 absorption of 50% can be achieved for an aluminum sample 20. For other highly reflective samples such as platinum, the effective heating power is also significantly limited, but still considerably higher than with a single reflection.

[0150] As can be seen from Figs 9A and 9B, in general, it is therefore desirable to use a reflector optics 50 that can reflect almost 100% of the laser radiation. This is possible in the visible range, e.g. by using so-called dielectric mirrors. These mirrors work via a layer system based on interference and achieve reflectivity > 99.8%. One disadvantage is that the reflection range is limited, e.g. IR light (blackbody radiation) is not reflected. Reflector optics 50 comprising silver mirrors could offer an advantage here. As can be further seen from Fig. 8A, when using reflector optics 50 comprising an aluminum reflector, the effective heating power is still significantly limited for highly reflective samples 20. For a platinum sample 20, for example, 30% is not available as it is absorbed by the reflector optics 50. As can be seen from Fig. 9A, when using a reflector with R>99%, almost 100% of the heating power is available for almost all materials. Fig. 9B shows a cavity enhancement defined in the table as the ratio of absorbed laser power after one reflection and the absorbed laser power after 25 reflections.

[0151] As can be seen from Figs. 10A and 10B, an optical heating device 10 using a coherent light source having an emitting light wavelength of 1064nm requires significantly more reflections due to the low absorption by the sample 20, which of course places higher demands on the optical cavity. However, very high theoretical efficiencies can also be achieved here. However, the dependence on the mirror quality is even greater than with 455nm (as shown in Figs. 8A and 9A). The reflector optics 50 has thus preferably a reflectivity of at least 90%, or has a reflectivity of at least 95%, or has a reflectivity of at least 98%, or has a reflectivity of at least 99%, or has a reflectivity of at least 99.5%, or has a reflectivity of at least 99.8%, or has a reflectivity of at least 99.9%.

[0152] In view of the above described embodiments, the ratio PH / PL between the light power absorbed by the sample 20 (PH) and the light power of the coherent light source 30 (PL) of a formed stable optical cavity may be greater than 50%, or greater than 60%, or greater than Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0153] 29

[0154] 70%, or greater than 80%, the reflectivity of the reflector optics 50 being greater than 95%, or being greater than 99%, or being greater than 99.5%, independently of the reflectivity of the sample surface 22. In other words, due to the cavity enhancement of the optical heating device 10, also highly reflective samples 20 can be heated optically, since the laser power PL is reflected back to the sample 20 multiple times, enabling high absorption of light power and a high percentage of the ratio PH / L, not being dependent on the reflectivity of the sample surface 22.

[0155] The same principle is applicable if it comes to the wavelength of the coherent light source 30, since the dependence of reflectivity I absorption of light L by the sample surface 22 on the wavelength of the light L from the coherent light source 30 can be levelled out by the multiple reflections I absorptions (about 20 times up to 100 times) of the laser power PL by the sample 20. Thus, the ratio PH / PL between the light power absorbed by the sample 20 (PH) and the light power of the coherent light source 30 (PL) of a formed stable optical cavity may be greater than 80%, the reflectivity of the reflector optics being greater than 95%, or being greater than 99%, or being greater than 99.5%, independently of the laser wavelength of the coherent light source 30.

[0156] FIG. 11A and 11 B illustrate that the heating power PH required to heat a sample 20 to a certain temperature Ts must compensate for the heat loss at the target temperature (PH = Pioss — > T const.). At high temperatures Ts, the losses due to thermal radiation always dominate as they increase with the 4thpower in temperature (Stefan-Boltzmann law). In addition to good reflectivity for the laser radiation, it will therefore also be useful at high temperatures if the reflector optics 50 also reflects the thermal radiation of the sample 20. In this respect, reflector optics 50 comprising broadband silver mirrors can have an advantage over dielectric mirrors, as the latter only ever reflects a "narrow" wavelength range. A coating for a visible laser 30 will no longer show good reflectivity in the infrared range, in which the blackbody radiation of the hot sample is mainly emitted. Reflector optics 50 comprising silver mirrors generally have Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0157] 30 a slightly lower reflectivity than dielectric mirrors, but this is uniformly good up to the far infrared.

[0158] Fig. 12 shows an optical heating system 100 comprising the optical heating device 10. The optical heating system 100 may further comprise the coherent light source 30 adapted to emit light L. In addition, sample temperature sensing means 110 may be provided, which are adapted to measure the temperature Ts of the sample 20. A control unit 120 may be adapted to control the light output power PL of the coherent light source 30 and to monitor the sample temperature Ts of the sample 20 measured by the temperature sensing means 110.

[0159] The sample temperature sensing means 110 may comprise a thermometer, wherein the sample temperature may be measured via a bare type-K thermocouple pressed into a hole (having, for example, a diameter of 0.2 mm) on the side of the sample 20. Alternatively, the thermometer may be a pyrometer detecting the black body radiation from the sample 20. The reflector optics 50 preferably comprising an aluminum reflector, or a dielectric reflector, or a silver reflector, may be mounted behind the sample 20 (for example, a Pt(111) crystal) and may have a polished concave reflecting surface 52 with an radius of curvature (ROC) of 10 mm, for example. The optical heating system 100 may further comprise fiber optics 80 adapted to guide the light L from the light source 30 to the optical heating device 10.

[0160] The control unit 120 may comprise sample temperature control means 140 adapted to stabilize a set sample temperature by a closed-loop control of the light output power of the light source 30 in reaction to the actual sample temperature Ts measured by the temperature sensing means 110. The control unit 120 may be further adapted to control the adjustment means 60 to ramp up / down the distance d between the sample surface 22 and the reflector optics 50 and detect an optimized distance dopthaving a maximized cavity enhanced heating effect by detecting a maximized sample temperature at a constant light output power. The control unit 120 may thus detect the sample temperature Ts at different distances d at light output powers PL being lower than the intended heating light output power (for example lower than 50% of Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0161] 31 the final heating light output power, or lower than 20% of the final heating light output power, or lower than 10% of the final heating light output power), in order to find the best distance d at which the sample temperature Ts response to a constant low light output power PL is best.

[0162] However, in case the ROC of the reflector optics 50 and thus the stable optical cavity condition is known, the control unit 120 may also just calculate an optimized distance dopt(for example dopt = 0.9 ROC) and control the adjustment means 60 to adjust the distance d correspondingly.

[0163] The optical heating system 100 may further comprise a light detector 130 to detect light escaping from the optical cavity, wherein the control unit 120 is adapted to control the adjustment means 60 to ramp up / down the distance between the sample surface 22 and the reflector optics 50, and detect an optimized distance dopthaving a maximized cavity enhanced heating effect by detecting minimized escaped light at a constant light output power PH. The light detector 130 may be mounted in the vicinity of the optical cavity, to detect light escaping therefrom, as shown, for example, in FIG. 12, or in FIG. 5A and 5B. Herein, a minimum of light L escaping the optical cavity between the sample surface 22 and the reflector optics 50 indicates a stable optical cavity condition. Detecting an optimized distance dopthaving a maximized cavity enhanced heating effect by detecting minimized escaped light at a constant light output power PH may also comprise detecting a range of distance d, in which a stable optical cavity is achieved (for example, d<ROC or % ROC < d < 1 ROC as discussed above).

[0164] The optimized distance doptmay be greater than 0.5 ROC, or greater than 0.6 ROC, or greater than 0.7 ROC, or greater than 0.8 ROC, or smaller than 0.9 ROC, or smaller than 1 ROC, ROC being the radius of curvature of the reflector optics.

[0165] Fig. 13 shows a schematic drawing of the sample temperature control means 140 for laser heating control of the optical heating system 100. The sample temperature Ts may be controlled by a software with a graphical user interface, which reads the sample temperature Ts via a connected thermocouple module. A build-in software PID controller may calculate and submit the laser power set point (Pset) to a microcontroller, for example, an ESP32 Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0166] 32 microcontroller. The microcontroller may adjust the duty cycle of its pulse width modulation (PWM) output that controls the output power PL of the coherent light source 30, which may be a 455 nm diode laser. The laser light L may be coupled into the fiber optics 80, for example a multimode fiber, which transmits it to the sample 20. The laser output power PL may be controlled via the duty cycle of a 1 kHz PWM generated by the microcontroller. The laser output PL may be activated when the PWM signal is HIGH and turned off when the PWM signal is LOW. Temperature control may be accomplished via a LabView interface that reads the sample temperature via a USB thermocouple measurement device (National Instruments, USB-TC01) and compares the measured value to a set point. A software PID controller may compute the required change in laser power PL and sends the new laser power PL to the microcontroller, which changes the duty cycle of the PWM accordingly with e.g. 12-bit resolution. It is emphasized that it is also straightforward to implement temperature reading and PID control on the ESP32 and use external commands only to set the desired temperature.

[0167] Fig. 14 shows an evaluation of the capabilities of the cavity-enhanced laser-heater of the present invention. The sample temperature is set to values between 373K and 1373K and allowed the PID controlled laser power to stabilize. The dots in Fig. 14 show the Pt(111) temperature as function of 455nm laser power. The maximum set point of 1373 K requires a laser power of 25 W. At the maximum power of 26W, the sample reaches -1400K. The curve flattens with increasing temperature, due to the increasing rate of radiative heat loss. The dashed lines show calculation results applying equations (2), (3) and (4) to model the laser power dependent sample temperature using an effective emissivity of e=0.29 (eq. (3)) and adjusting the heat conductivity (eq. (2)) by a factor of 0.5 to match the data. This calibrated curve allows us to predict performance at higher temperatures (inset). Fig. 14 illustrates surface temperatures as a function of laser power for cavity-enhanced laser heating of a Pt(111) crystal. The figure compares reachable sample temperatures using a 455 nm diode laser with maximum output power of 26 W (dots) and results for a 532 nm DPSS laser with a Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0168] 33 maximum output power of 9 W (triangles). The dashed line shows a theoretical curve based on equations (2), (3) and (4) for Pt heating with 455 nm light. The triangles in Fig. 14 show measurements made with a 532 nm DPSS laser (Spectra Physics, Millenia eV) with a maximum optical output power of 9 W. Note that the Pt reflectivity at 532nm (Rs32=0.93) is already significantly higher than at 455nm (R45s=0.85); nevertheless, similar maximum temperatures are achieved at comparable laser powers. This shows that the cavity enhanced heating partially overcomes the problem of wavelength dependent heating efficiency.

[0169] Fig. 15 shows the performance of the 455nm laser-heater for heating of a Pt(111) sample from room temperature to 1173K. The three panels show the temporal evolution of the heating rate (a), laser power (b) and sample temperature (c). The temperature set point (dashed) is changed from room temperature to 1173K after ~5s. The PID controller as shown in FIG. 13 quickly increases the laser power PH to the maximum of 26W within 5s. Simultaneously, the surface temperature increases. The heating rate reaches a peak of 32 K / s after ~10s and then decreases due to increasing heat losses at higher temperatures. The present invention also achieves good heating rates and long-term temperature stability. Herein, the maximum power change was limited to 5% per control cycle. The sample temperature Ts follows the increase of laser power PL with some minor delay and reaches the set point after only 45 s. As the sample 20 approaches the set temperature, the PID controller 140 starts to decrease the laser power PL, minimizing overshoot to only 7 K. The sample temperature Ts finally stabilizes after 60 s. A detailed analysis of the heating rate shows that it reaches a peak of 32 K / s at 15 s and decreases afterwards to 16 K / s at 40 s before PID controller 140 reduces the laser power PL. Over the entire heating ramp, we achieve an average heating rate of -22 K / s.

[0170] Fig. 16 shows the same data extended over a period of 3 hours. The Pt sample temperature Ts remains stable within ~0.2 K (standard deviation = 0.03 K) which is higher than the accuracy of type K thermocouples of ±1.5°C at the given temperature. At the same time, the reflector temperature (FIG. 16, panel (b)) remains at -300K applying minimal cooling through 3mm Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0171] 34 copper capillaries. The required laser power PL to stabilize the sample drops from 14W at early times to ~13W at later times indicating a moderate warm up of the sample holder 40.

[0172] To test the long-term stability, a temperature variation program ~8 hours per day for two weeks was performed. Fig. 17 shows an example over a course of 30min of operation for laser power (top panel) and temperature (bottom panel). The program varies the temperature set point randomly between 300-1300 K and stabilizes at each temperature for 1 min. Over the course of the two weeklong test period, any change in the performance of the system has been observed.

[0173] The present invention as described above show the performance quality of cavity-enhanced laser heating, which can clearly compete with other methods like resistive or electron bombardment heating and may have certain advantages, which are now discussed. The optical heating device 10 and the optical heating device 100 described above has the property that it heats only the sample 20, which will be the hottest element in the vacuum chamber. This is a simple and often overlooked disadvantage of conventional heating methods like resistive and electron bombardment heating, which require filaments that may be significantly hotter than the sample itself. These hot filaments are typically made of pure refractory metals like tungsten or tantalum; despite this, they may emit thermal electrons and ions, while radiatively heating surrounding objects within the vacuum chamber that invariably outgas and degrade vacuum quality. In addition, metal atoms will evaporate over time and contaminate surrounding parts.

[0174] The optical heating device 10 heats the sample 20 from behind and scattered light can be further reduced by appropriate design of the sample holder. Nevertheless, care should be taken for experiments requiring detection of weak light signals near 455 nm, or for ion detectors operating with blue phosphor screens (e.g. P47, which emit atthe 400-455nm). Even in those cases, the laser light could be suppressed with a narrow bandwidth optical notch filter. The optical cavity described above also allows lasers of other wavelengths to be employed without sacrificing heating efficiency. Consequently, avoiding possible problems associated Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0175] 35 with use of a P47 phosphor could be accomplished using an 808nm diode laser, which are common as a typical pump source for solid-state lasers and therefore available at high power and reasonable prices. A short-pass filter then allows blocking of the heating light without influencing detection sensitivity.

[0176] A technical point worth mentioning is that electrical heating usually requires the sample 20 to be electrically insulated from the rest of the experiment, e.g. the sample surface 22 needs to be biased for electron bombardment. This is not required for optical heating. Finally, it is noted that the use of fiber optics 80 coupling not only simplifies the design requirement for moving samples 20 within the vacuum chamber, it relaxes laser safety demands that would be more severe for an open beam path.

[0177] Fig. 18 shows a process chart of a method 1000 for heating a sample 20 by means of absorption of light L from a coherent light source 30 using the optical heating device 10 of the present invention. A method for heating a sample 20 by means of absorption of light L from a coherent light source 30 using the optical heating device 10 may comprise the step S1100 of adjusting the distance d between the sample surface 22 and the reflector optics 50 along the optical axis OA of the reflector optics 50 such that an optical cavity is formed (S1200) by the reflector optics 50 and the sample surface 22, as discussed in all detail above with regard to FIG. 1 to 17.

[0178] Fig. 19 shows a process chart of a method 2000 for heating a sample 20 by means of absorption of light L from a coherent light source 30 using the optical heating system 100 of the present invention. According to an embodiment, a method 2000 for heating a sample 20 by means of absorption of light L from a coherent light source 30 using the optical heating system 100 may comprise the step S2100 of controlling the light output power PL of the coherent light source 30, and the step S2200 of monitoring the sample temperature Ts measured by the temperature sensing means 110. The optical heating method 2000 may further comprise the step S2300 of ramping up / down the distance between the sample surface 22 and the reflector optics 50, and the step S2400 of detecting an optimized distance dopt Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0179] 36 having a maximized cavity enhanced heating effect at a constant light output power PL. The optical heating method 2000 may further comprise the step S2500 of stabilizing, by the sample temperature control means 140, a set sample temperature by a closed-loop control of the light output power PL of the coherent light source 30 in reaction to the actual sample temperature Ts measured by the temperature sensing means 110. The optimized distance doptmay be greater than 0.5 ROC, or greater than 0.6 ROC, or greater than 0.7 ROC, or greater than 0.8 ROC, or smaller than 0.9 ROC, or smaller than 1 ROC, ROC being the radius of curvature of the concave reflector optics 50.

[0180] FIG. 20 shows a schematic block diagram of a data processing system 200, a computer program product 300, and a computer-readable medium 400. According to the present invention, the data processing system 200 may be provided, comprising means for carrying out the steps of the above method 2000. According to the present invention, the computer program product 300 may be provided, comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the above method 2000. According to the present invention, the computer-readable medium 400 may be provided, comprising instructions which, when executed by a computer, cause the computer to carry out the steps of the above method 2000.

[0181] In summary, according to the present invention, a cavity-enhanced laser-based optical heater 10 may be provided that is applicable to highly reflective as well as absorbing samples 20. The gain from the cavity design is large for reflective sample 20 as the multi-pass effect of the stable cavity ensures that a large fraction of the light L is absorbed by the sample 20. The optical heating system 100 may use, for example, a single-crystal Pt(111) sample 20. Pt is one of the more challenging materials for laser heating as it exhibits high reflectivity even at 455 nm and it requires annealing temperatures for sample preparation as high as 1300 K. According to the present invention, a coherent light source preferably comprising a low-cost 455 nm diode laser delivering only 26 Watts to the sample 20 used in combination with a fiber optics 80 preferably comprising a standard stepped-index multimode fiber and a reflector Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO

[0182] 37 optics 50 comprising a concave aluminum reflector can heat the sample 20, for example a Pt crystal to 1400K. Higher temperatures could be reached with higher power lasers or with more elaborate radiation shielding of the sample 20.

[0183] According to an advantageous embodiment of present invention, a simple sample heater 10 based on a 455 nm diode laser 30 may be provided, whose heating efficiency is enhanced by an optical cavity formed of an Aluminum mirror 50 and the specular reflective surface 22 of the sample 20. With this setup, a 1-cm dia. x 2-mm thick Pt sample 20 could be heated to 1200 K in less than one minute, using 26 Watts of laser power PL directed to the sample 20 with a fiber optic 80. The maximum temperature Ts of 1400 K achievable at 26 Watts of laser could be reached within 2 min. Excellent programmable temperature control and long-term temperature stability may be achieved by the optical heating system 100. The entire system 100 may comprise components costing less than typical electron bombardment heaters (filament, DC power supply, high voltage power supply, electrical feedthroughs).

[0184] According to the present invention, a laser-based optical heater 10 may take advantage of a concave aluminum reflector 50 operating in combination with the flat sample surface 22 to form a hemispherical optical cavity. This gives rise through a multi-pass effect to achieving rapid laser power depletion between the sample 20 and the reflector 50, which effectively divides the laser power PL according to the relative reflectivity of the sample 20 and the aluminum mirror 50. This approach uses the heating power PH of a laser so efficiently that temperatures Ts of 1400K could be achieved for a highly reflective Pt crystal sample 20 using a commercial low-cost 455nm diode laser 30 (engraving laser, Atomstack M150, ca. 700€) with a maximum output power PL of only 26W. The laser radiation L is brought to the sample 20 by standard stepped-index optical fibers 80 (ca. 15€ per meter), providing a high degree of flexibility, similar to the use of cables in electrical heating systems.

Claims

Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO38Claims1. An optical heating device (10) for heating a sample (20) by absorption of light (L) from a coherent light source (30), comprising: a sample holder (40) adapted to place the at least partly reflecting sample surface (22) in the light path (L) from the coherent light source (30), reflector optics (50), preferably concave reflector optics (50), placed along the light path (L) reflected by the sample surface (22); and adjustment means (60) adapted to adjust the distance (d) between the sample surface (22) and the reflector optics (50) along the optical axis (OA) of the reflector optics (50), preferably along the rotational symmetry axis of the concave reflector optics (50), such that the reflector optics (50) and the sample surface (22) form an optical resonator, and preferably form a hemispherical optical cavity.

2. The optical heating device (10) of claim 1, wherein a distance (d) between the sample surface (22) and the concave reflector optics (50) along the rotational symmetry axis (OA) of the concave reflector optics (50) is smaller than or equal to the radius of curvature, ROC, of the concave reflector optics (50), such that the concave reflector optics (50) and the sample surface (22) form a hemispherical optical cavity.

3. The optical heating device (10) of claims 1 or 2, wherein the light (L) from the coherent light source (30) passes a hole (70) in the reflector optics (50) through the optical axis (OA) of the reflector optics (50).

4. The optical heating device (10) of claim 3, wherein a light cone (L) exiting the hole (70) has a maximum opening angle greater than ±5°, or has a maximum opening angle greater then ±10°, or has a maximum opening angle smaller then ±15°, or has a maximum opening angle smaller then ±20°.

5. The optical heating device (10) of claims 3 or 4, wherein the diameter of the hole (70) is in a range between 1 mm and 5 mm, or between 1.5 mm and 4.5 mm, or between 2 mm and 4 mm, or between 2.5 mm and 3.5 mm.

6. The optical heating device (10) of any one of the claims 3 to 5, wherein the diameter of the hole (70) is smaller than 10 mm, or smaller than 5 mm, or smaller than 4 mm,Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO39 or smaller than 3 mm, or smaller than 2 mm, or smaller than 1 mm, and greater than 0.2 mm.

7. The optical heating device (10) of any one of claims 3 to 6, further comprising a fiber port (90) adapted to couple out the light (L) guided by a fiber optics (80) from the light source (30) to the optical heating device (10), wherein the fiber port (90) comprises a fiber end (92) placed in the hole (70).

8. The optical heating device (10) of claim 7, wherein a fiber exit surface (94) of the fiber end (92) in the hole (70) is flush with the reflecting surface (52) of the reflector optics (50).

9. The optical heating device (10) of claims 7 or 8, wherein the adjustment means (60) are adapted to adjust the distance (d) to fulfill % ROC < d < 1 ROC, such that the concave reflector optics (50) and the sample surface (22) form a hemispherical optical cavity and back reflections of light (L) into the fiber end (92) are reduced.

10. The optical heating device (10) of any one of the claims 7 to 9, wherein the inner diameter of the hole (70) is equal to the outer diameter of the fiber port (90).

11. The optical heating device (10) of any one of the claims 7 to 10, wherein the light (L) coupled out at the fiber port (90) has a maximum opening angle defined by the numerical aperture, NA, of the fiber optics (80).

12. The optical heating device (10) of any one of the claims 7 to 11 , wherein the fiber port (90) comprises no collimation optics for collimating the light (L) coupled out at the fiber port (90).

13. The optical heating device (10) of any one of the preceding claims, wherein the adjustment means (60) are further adapted to tilt the sample surface (22) or to displace the sample surface (22) in a direction perpendicular to the optical axis (OA) of the reflector optics (50) such that the reflector optics (50) and the sample surface (22) form an optical cavity.

14. The optical heating device (10) of any one of the preceding claims, wherein the reflector optics (50) has a reflectivity (R) of at least 90%, or has a reflectivity of at least 95%, or has a reflectivity of at least 98%, or has a reflectivity of at least 99%, or has a reflectivity of at least 99.5%, or has a reflectivity of at least 99.8%, or has a reflectivity of at least 99.9%.Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO4015. The optical heating device (10) of any one of the preceding claims, wherein the reflector optics (50) comprises a polished metal mirror or a coated optical mirror with protected metallic or dielectric coating.

16. The optical heating device (10) of any one of the preceding claims, wherein the sample holder (40) comprises two stainless steel arms (42a, 52b) mounted to a manipulator unit (44) for gripping the sample (20) and for placing the sample surface (22) to face the light (L) from the coherent light source (30).

17. The optical heating device (10) of any one of the preceding claims, further comprising an optics mount (56) providing an exchangeable mount for concave reflector optics (50) each having a different radius of curvature, ROC.

18. The optical heating device (10) of any one of the preceding claims, wherein the ratio PH / PL between the light power absorbed by the sample (PH) and the light power of the light source (PL) of a formed stable optical cavity is greater than 50%, or greater than 60%, or greater than 70%, or greater than 80%, the reflectivity of the reflector optics (50) being greater than 95%, or being greater than 99%, or being greater than 99.5%, independently of the reflectivity (Rsampie) of the sample surface (20).

19. The optical heating device (10) of any one of the preceding claims, wherein the ratio PH / PL between the light power absorbed by the sample (PH) and the light power of the light source (PL) of a formed stable optical cavity is greater than 80%, the reflectivity of the reflector optics being greater than 95%, or being greater than 99%, or being greater than 99.5%, independently of the wavelength of the coherent light source (30).

20. An optical heating system (100) comprising an optical heating device (10) of any one of the preceding claims, further comprising: a coherent light source (30) adapted to emit light (L); sample temperature sensing means (110) adapted to measure the temperature (Ts) of the sample (20); and a control unit (120) adapted to control the light output power (PL) of the coherent light source (30) and to monitor the sample temperature (Ts) measured by the temperature sensing means (110).

21. The optical heating system (100) of claim 20, wherein the control unit (120) is adapted toMax-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO41 control the adjustment means (60) to ramp up / down the distance (d) between the sample surface (22) and the reflector optics (50), and detect an optimized distance (dopt) having a maximized cavity enhanced heating effect by detecting a maximized sample temperature (Ts) at a constant light output power (PL).

22. The optical heating system (100) of claims 20 or 21 , further comprising a light detector (130) to detect light (L) escaping from the optical cavity, wherein the control unit (120) is adapted to control the adjustment means (60) to ramp up / down the distance (d) between the sample surface (22) and the reflector optics (50), and detect an optimized distance (dopt) having a maximized cavity enhanced heating effect by detecting minimized escaped light (L) at a constant light output power (PL).

23. The optical heating system (100) of any one of claims 20 to 22, wherein the optimized distance (dopt) is greater than 0.5 ROC, or greater than 0.6 ROC, or greater than 0.7 ROC, or greater than 0.8 ROC, or smaller than 0.9 ROC, or smaller than 1 ROC, ROC being the radius of curvature of the reflector optics (50).

24. The optical heating system (100) of any one of claims 20 to 23, wherein the control unit (120) comprises sample temperature control means (140) adapted to stabilize a set sample temperature by a closed-loop control of the light output power of the coherent light source (30) in reaction to the actual sample temperature (Ts) measured by the temperature sensing means (110).

25. The optical heating system (100) of any one of the claims 20 to 24, wherein the maximum light output power (PL) of the coherent light source (30) is smaller than 1 kW, or smaller than 500 W, or smaller than 200W, or smaller than 100W, or smaller than 50W, or smaller than 30W, or greater than 20 W, or greater than 10 W.

26. The optical heating system (100) of any one of the claims 20 to 25, wherein the coherent light source (30) is a diode laser.

27. The optical heating system (100) of any one of the claims 20 to 25, further comprising fiber optics (80) adapted to guide the light (L) from the coherent light source (30) to the optical heating device (10).Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. P79657WO4228. A method (1000) for heating a sample (20) by means of absorption of light (L) from a light source (30) using the optical heating device (10) of any one of claims 1 to 19, comprising the step of adjusting (S1100) the distance (d) between the sample surface (22) and the reflector optics (50) along the optical axis (OA) of the reflector optics (50) such that the reflector optics (50) and the sample surface (22) form an optical cavity.

29. A method (2000) for heating a sample (20) by means of absorption of light (L) from a coherent light source (30) using the optical heating system (100) of any one of the claims 20 to 27, comprising the steps of controlling (S2100) the light output power (PL) of the coherent light source (30), and monitoring (S2200) the sample temperature (Ts) measured by the temperature sensing means (110).

30. The optical heating method (2000) of claim 29, further comprising the step of ramping up / down (S2300) the distance (d) between the sample surface (22) and the reflector optics (50), and detecting (S2400) an optimized distance (dopt) having a maximized cavity enhanced heating effect at a constant light output power (PL).

31. The optical heating method (2000) of claims 29 or 30, wherein the optimized distance (dOpt) is greater than 0.5 ROC, or greater than 0.6 ROC, or greater than 0.7 ROC, or greater than 0.8 ROC, or smaller than 0.9 ROC, or smaller than 1 ROC, ROC being the radius of curvature of the concave reflector optics.

32. The optical heating method (2000) of any one of the claims 29 to 31 , further comprising the step of stabilizing (S2500) a set sample temperature by a closed-loop control of the light output power (PL) of the coherent light source (30) in reaction to the actual sample temperature (Ts) measured by the temperature sensing means (110).

33. A data processing system (200) comprising means for carrying out the steps of the method (2000) of any one of the claims 29 to 32.

34. A computer program product (300) comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method (2000) of any one of the claims 29 to 32.Max-Planck-Gesellschaft zur Fdrderung der Wissenschaften e.V. P79657WO35. A computer-readable medium (400) comprising instructions which, when executed by a computer, cause the computer to carry out the steps of the method (2000) of any one of the claims 29 to 32.

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