IR EMMITTER WITH MODULAR EMISSION LEVEL BASED ON METAMATERIALS

Description

[0001] The invention relates to a modulatable infrared emitter comprising a heating element, a planar base element, a dielectric intermediate layer, and a planar cover element, which is a structured metamaterial, and an actuator, wherein the actuator is configured for relative movement of the cover element and the base element between a first and second position in order to modulate the intensity of the infrared emitter's emission. The invention further relates to manufacturing methods for the infrared emitter, methods for the modulated emission of infrared red radiation by means of the infrared emitter, and preferred uses of the infrared emitter. A system comprising the infrared emitter and a control device for regulating the actuator is also preferably part of the invention. Background and state of the art

[0002] Modulatable infrared emitters (IR emitters) are relevant for a wide variety of spectroscopy applications. In particular, gas spectroscopy is frequently performed using infrared radiation. Electromagnetic radiation in the infrared range induces vibrations in the gas molecules at specific frequencies or wavelengths, which can be detected by absorption lines in the spectrum.

[0003] For example, environmental sensing relies heavily on optical or spectroscopic systems that operate in the mid-infrared range – i.e., at wavelengths from 2 µm to 10 µm – because this is where the infrared bands of many important substances such as carbon dioxide or methane are located.

[0004] Photoacoustic spectroscopy is frequently used, employing intensity-modulated infrared radiation with frequencies in the absorption spectrum of a molecule to be detected in a gas. If this molecule is present in the beam path, modulated absorption occurs, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. These heating and cooling processes cause expansion and contraction of the gas, generating sound waves at the modulation frequency. These sound waves can be measured using acoustic detectors (microphones) or flow sensors.

[0005] Photoacoustic spectroscopy allows the detection of minute concentrations of gases and has a wide range of applications. One example is the detection of CO₂, which plays a role in research and climate control. The concentration of, for example, exhaust gases in the air can also be measured in this way. Military applications are also relevant, where even the smallest concentrations of poison gas can be detected.

[0006] Various emitters are used as radiation sources for the aforementioned applications, each with its own advantages and disadvantages. For example, narrowband infrared laser sources can be used. These allow for high radiation intensities and can be modulated at high frequencies with standard components, e.g., for photoacoustic spectroscopy. However, the setup is complex and expensive. In particular, a corresponding number of lasers are required for the detection of different molecules.

[0007] It is also known to provide narrowband thermal emitters based on optical nanostructures. Inoue et al. 2015 disclose a number of different approaches ([8]). Referring to Landy et al. 2008, the use of metamaterials for selective infrared emitters is discussed (

[13] ). For example, the provision of a single-band and dual-band IR emitter using metamaterials is disclosed. This allows wavelength-specific transmission with high emissivity and Q-factors to be ensured. In contrast to the laser sources described above, high-frequency modulation of the emitted infrared radiation is not possible.

[0008] Thermal, broadband emitters are also well-known. These offer the advantage of a broad spectrum and often low cost. However, the modulation frequency of these emitters is limited, and direct modulation by varying the current supply is slow due to thermal time constants, significantly reducing the component's lifespan. Slow modulation often results in measurements with a poor signal-to-noise ratio due to the inherent noise of the detection components. External modulation using rotating chopper wheels is faster, but the setup is complex and not as compact and robust as would be desirable for many applications. Furthermore, the modulation bandwidths are limited, and varying the chopper's rotational speed is cumbersome due to inertia.

[0009] At the same time, there is a strong interest in miniaturizing these IR sensor elements in order to enable their integration into as many technical applications as possible.

[0010] Sensors and spectrometers based on the photoacoustic effect are best suited for miniaturization, as this is a method of absorption spectroscopy and therefore requires significantly shorter optical path lengths for a sufficient signal-to-noise ratio, in stark contrast to transmission spectroscopy methods [7]. However, this also necessitates the availability of miniaturized infrared emitters, which must achieve the highest possible intensity—as this directly increases the measurement signal—and be modulated at the highest possible frequencies. Emitter modulation is essential for the photoacoustic effect to occur; otherwise, no acoustic signal will be generated in the medium under investigation.In parallel, a higher modulation frequency of the emitter means both an increase in the operating speed of the photoacoustic sensor and an improvement in the signal-to-noise ratio (SNR) [7].

[0011] To date, mainly miniaturized thermal IR emitters, so-called microhotplates [5,6], are used, which operate at temperatures of several hundred degrees and achieve modulation frequencies of only a few Hz. Although the spectra of such thermal emitters can be positively influenced by the use of plasmonic structures [8-10], the fundamental problem of high temperatures and low modulation frequencies remains and cannot be solved within the scope of the existing technology.

[0012] Microsystems technology is now used in many application areas for the production of miniaturized, mechanical-electronic devices. The microsystems that can be produced in this way (English: microsystems) are called microsystems or microcontrollers. microelectromechanical system, MEMS (short for MEMS) are very compact (micrometer range) while offering excellent functionality and ever-decreasing manufacturing costs. For example, fast and compact comb drives are described as MEMS actuators in DE 10 2017 206 183 A1.

[0013] A combination of micro- / nanoelectromechanical systems with metamaterials is also being proposed, which holds enormous potential for the development of novel, tunable devices. The possible application areas range from diverse fields such as communication and THz technology to medical diagnostics [1-4].

[0014] Liu et al. 2017 reported a modulation of the spectral emission of surfaces that can be used for infrared camouflage or friend-or-foe identification (

[19] ). Using a metamaterial, the detectable emission is modified over a range corresponding to a temperature change of 20°C. In combination with MEMS actuators, this can generate dynamic infrared patterns on surfaces to evade infrared camera detection or to enable marking of the objects. The use of MEMS technology based on metamaterials to modulate a heatable IR emitter suitable for application in photoacoustic spectroscopy is unknown in the prior art. Object of the invention

[0015] The object of the invention is to provide a modulatable infrared emitter and a method for generating modulated infrared radiation, which do not exhibit the disadvantages of the prior art. In particular, it was an object of the invention to provide a high-frequency and variably modulatable infrared emitter which is simultaneously characterized by a simple, cost-effective, and compact design. Summary of the invention

[0016] The problem is solved by the features of independent claim 1. Preferred embodiments of the invention are described in the dependent claims.

[0017] The actuator of the modulatable infrared emitter enables particularly fast and simple modulation of the emitted infrared radiation intensity. Unlike known intensity modulations of infrared emitters achieved by varying the current supply, the modulation according to the invention is not limited by thermal time constants. Instead, MEMS actuators, for example, can be used to achieve modulation frequencies well above 100 Hz up to 100 kHz. Such modulation frequencies are particularly advantageous for photoacoustic spectroscopy. However, the modulatable infrared emitter is also suitable for any application requiring fast and reliable modulation of infrared radiation.

[0018] As explained in detail below, the modulation of infrared radiation emission is based on exploiting the properties of the structured metamaterial of the cover element. In the second position, the cover element made of the structured metamaterial is preferably located at a distance, limited only by the dielectric intermediate layer, as close as possible and with the greatest possible overlap above the base element. In this second (resonant) position, infrared radiation can couple to the layer system consisting of the cover element, dielectric intermediate layer, and base element, so that at a preferred infrared resonance wavelength, a particularly high emissivity occurs, or infrared emission with high intensity takes place.

[0019] In the first position, resonant coupling preferably does not occur, so that the IR emitter exhibits a significantly reduced emissivity at the resonant wavelength, or rather, infrared emission occurs with a significantly reduced intensity. With respect to the resonant wavelength, the intensity of the infrared radiation emission can be modulated quickly and reliably by switching the position between a resonant state (second position) and a non-resonant state (first position) mediated by the actuator.

[0020] For example, the actuator can be configured to lower the vertical distance between the top and bottom elements from a first, non-resonant position to a second, resonant position. It is also possible for the actuator to shift the bottom and top elements horizontally relative to each other, with resonant emission occurring in the second position if there is sufficient overlap. This fast and reliable modulation capability clearly distinguishes the IR emitter from known prior art infrared emitters.

[0021] For the purposes of the invention, a modulatable infrared emitter is a device that emits electromagnetic radiation. This radiation preferably has a wavelength range in the infrared (IR) range, in particular between approximately 700 nanometers (nm) and 1 millimeter (mm). The corresponding frequency of the emitted radiation can be in the range between approximately 300 gigahertz (GHz) and 400 terahertz (THz). The spectrum can also preferably be represented using the wavenumber m⁻¹ or cm⁻¹, as is customary in the field of spectroscopy. A person skilled in the art knows how to convert to these units.

[0022] The spectrum is specifically selected to correspond to the emitter's preferred application area, namely infrared spectroscopy and, in particular, photoacoustic spectroscopy. Vibrational excitation of the gas molecules to be spectroscopically and / or detected is especially preferred, with the excitation corresponding to a preferred spectral range depending on the gas molecules. For example, a spectral range of approximately 2.4 micrometers (µm) is suitable for exciting CO₂ molecules. Particularly preferred wavelength ranges for the infrared radiation are 700 nm to 10 µm, preferably 1 to 10 µm, and most preferably 2 µm to 10 µm.

[0023] The IR emitter preferably emits a beam oriented in a preferred direction in the form of a straight line. The beam's orientation is preferably determined by the surface normal to the base element, the dielectric intermediate layer, and the cover element, which are excited to infrared radiation by the heating element. In the following, the term "beam" is intended to describe the preferably focused portion of the radiation emitted by the emitter along its preferred beam direction, with the regions of greatest intensity along this direction defining the beam. Intensity is preferably defined as areal power density and preferably has the unit watts per square meter, or abbreviated W / m².

[0024] The design of the modulatable infrared emitter defines a preferred emission direction of an infrared beam along the surface normal of the layer structure on the heating element. However, additional components, such as lenses, can be integrated into the emitter or attached externally to focus or collimate the beam. A person skilled in the art knows how to shape the emission profile of the radiation source by designing the IR emitter and using other components to achieve a desired beam profile and direction. The modulatable IR emitter can preferably function without additional lenses or be a system comprising a radiation source and at least one lens for collimating the beam.

[0025] The emitter is modulatable, meaning that the intensity of the emitted radiation, preferably the beam intensity, can be controlled and varied over time. The modulation should preferably produce a temporal change in intensity as a measurable quantity. This means, for example, that the difference in intensity over time between the weakest intensity measured within the measurement period and the strongest intensity measured within the same period is greater than the sensitivity of a device typically used for measuring or determining intensity for the radiation spectrum and application. Preferably, the difference is significantly greater than a factor of 2, more preferably 4, 6, or 8, between the strongest and weakest adjustable intensity. The modulation of the beam intensity is particularly preferably performed for one or more predetermined resonant wavelengths.A modulatable infrared emitter has a wide range of applications. Particularly relevant are all forms of infrared spectroscopy, and especially photoacoustic spectroscopy.

[0026] To generate infrared radiation, thermal energy is provided in the form of a heating element. A micro-heating element is particularly preferred. A micro-heating element is preferably understood to be a heating element with dimensions on the order of micrometers (µm). The heating element comprises a heatable layer of a conductive material which produces Joule heat when an electric current flows through it. The heat produced preferably exhibits a dependence on the ohmic resistance of the element and the square of the current or on the square of the applied voltage and the inverse ohmic resistance, depending on whether a current or a voltage source is used.

[0027] In an equilibrium state, the heat produced is equal to the heat losses due to conduction, convection, and thermal radiation (synonyms: infrared radiation) emitted at the outer interfaces of the current-carrying, heatable layer. As is known to those skilled in the art, the heat produced causes, among other things, thermal radiation, particularly through the thermal motion of particles, which results, for example, in the acceleration of charge carriers and / or oscillating dipole moments. Thus, infrared radiation can be selectively generated by a current-carrying, heatable layer. The heatable layer is preferably made of metal, for example, tungsten or platinum. By applying a suitable voltage and the resulting current flow, Joule heating and thus ultimately infrared radiation is generated.

[0028] The radiation spectrum of a heated body can preferably be approximated by Planck's radiation law, where the differences between a real heatable layer and a black body are known to those skilled in the art, for example, the emissivity or the actual deviation from thermal equilibrium of the body. Despite these deviations, the generated spectrum and its intensity are essentially described by the temperature and the radiating surface area according to Planck's radiation law.

[0029] Thus, a person skilled in the art can achieve a preferred spectrum with a preferred intensity distribution through targeted design of the micro-heating element. In addition to the material and geometric design of the heating element, the supplied electrical energy and the magnitude of the heat losses of the heating element, besides thermal radiation, are particularly important for this purpose. The magnitude of these heat losses is determined, for example, by the thermal conductivity between the heating element and the adjacent materials and / or fluids, as well as their heat capacity and the size of the interface(s).

[0030] The micro-heating element is preferably at least partially freestanding and allows, for example, thermal expansion within the IR emitter due to significant temperature changes as well as translational movements. Partially freestanding means that it is not at least partially connected to other elements of the emitter at the interfaces by force and / or form-fit, and therefore has one degree of freedom of movement in a direction essentially perpendicular to the interface.

[0031] The infrared emitter according to the invention is characterized in that a planar base element, a dielectric intermediate layer and a planar cover element made of a metamaterial are present on the heating element.

[0032] "Planar" preferably refers to a dimensioning where the thickness of the base element or cover element is significantly less than its length and / or width. For example, the term "planar" can mean a thickness of less than 1500 nm or preferably less than 500 nm, the elements having a length or width in the range of hundreds of micrometers or several millimeters, such that a thickness-to-length or width ratio of the base element or cover element of more than 1:10, more than 1:50, or more than 1:100 is preferably given.

[0033] The shape of the base element, the dielectric intermediate layer, or the cover element can be, for example, rectangular, square, or circular. Preferably, however, the base element, dielectric intermediate layer, or cover element have essentially the same shape and essentially the same surface area.

[0034] As explained in detail below, the layer structure on the heating element, consisting of a base element, a dielectric intermediate layer and a cover element, which can be moved by the actuator, significantly influences the intensity of the emitted infrared radiation.

[0035] While in a second position there is a resonant emission of infrared radiation (the emissivity with respect to a resonance wavelength is particularly high), a first relative positioning of the base element and the cover element is chosen so that there is no resonant IR emission or only a significantly reduced IR emission.

[0036] The modulation of the emissivity by the actuator is thus based on an electromagnetic resonance for the emission of infrared radiation, which occurs in the second relative position of the cover element and the base element, in contrast to the first position. For this purpose, it is preferred that in the second position the cover element made of the structured metamaterial is located at the closest possible distance to the base element, limited only by the dielectric intermediate layer, with the greatest possible degree of overlap.

[0037] In the second position, the IR emitter is preferably characterized by a structure in which the basic element made of a conductive material is separated from a covering element made of a structured metamaterial essentially only by the dielectric intermediate layer, and wherein the covering element covers the surface of the basic element to a preferably maximum extent.

[0038] With a suitable choice of metamaterial, such a layer structure consisting of a base element, a dielectric intermediate layer and a cover element preferably forms a metamaterial perfect absorber or at least an absorber which metamaterial perfect absorber very comes close. metamaterial perfect absorber is characterized by the fact that incident electromagnetic radiation is almost completely absorbed, i.e., has an absorption of almost 1 with respect to at least one resonance wavelength.

[0039] Due to Kirchhoff's radiation law, such a layered structure advantageously functions as a nearly perfect IR emitter in the context of the IR emitter structure according to the invention. Kirchhoff's radiation law states that for all bodies in thermal equilibrium at a given temperature, the ratio between emission and absorption A for radiation of the same wavelength is constant and equal in magnitude to the specific emission of the black body at that temperature. In other words, a material with a high absorptivity for a specific wavelength will also exhibit a high emissivity for that wavelength.

[0040] The cover element made of metamaterial, the dielectric layer, and the base element can be selected such that resonant absorption occurs in the second position at at least one preferred infrared wavelength. In this position, the IR emitter will resonantly emit the thermal energy supplied by the heating element at at least that resonant wavelength. The first position can advantageously be selected such that resonance does not occur and the IR emitter exhibits only a low emissivity over a broad infrared spectrum.

[0041] With regard to the resonant wavelength, the emissivity can be modulated quickly and reliably by switching the actuator between a resonant state (second position) and a non-resonant state (first position). As explained in more detail below, MEMS actuators, for example, make it possible to achieve modulation frequencies of several kHz. The combination of a movable top element made of a structured metamaterial and a base element that can be heated by a heating element thus allows for the creation of a high-frequency IR emitter with a high modulation depth.

[0042] This results in an excellent signal-to-noise ratio at high speeds, which is advantageous for applications in photoacoustic spectroscopy. While the speed of known modulatable IR emitters is limited by the modulation frequency of the IR emitter, with the described high-frequency modulatable IR emitter, the physical upper limit is preferably determined only by the relaxation times of the electrons in the gas under investigation.

[0043] The selection of one or more resonant wavelengths can be achieved by appropriately choosing a structured metamaterial.

[0044] For the purposes of the invention, the terms "structured metamaterial" or "metamaterial" are preferably used synonymously and denote an artificially produced structure whose permeability to electric and magnetic fields, i.e., its electric permittivity (ε(w)) or magnetic permeability (µ(w)), can be adjusted in a frequency-dependent and application-oriented manner. In particular, structured metamaterials can thus exhibit electromagnetic properties that are not found in natural materials.

[0045] For example, it was already shown in 1968 that metamaterials can achieve a negative refractive index

[11] .

[0046] Preferably, the structured metamaterial is formed from an array of periodic unit cells whose dimensions are smaller than the wavelength of the electromagnetic radiation that the metamaterial is intended to influence. A "unit cell" or "unit cell" is preferably understood, analogously to known unit cells of a crystal, as the smallest geometric unit of the structured metamaterial, which repeats periodically in the two-dimensional plane of the metamaterial. Preferably, a two-dimensional, periodic arrangement of all unit cells forms the surface of the structured metamaterial.

[0047] Highly conductive metals, such as gold, silver, or copper, are particularly suitable materials for the metamaterial, as they are used, for example, in the production of metals such as copper, silver, or copper. split-ring resonators can be arranged

[12] .

[0048] Such resonators are particularly suitable for the formation of a metamaterial perfect absorbers. For example, as from Landy et al. shown, a metamaterial perfect absorber They are formed from two metallic layers separated by a dielectric intermediate layer. The upper metallic layer can contain several so-called electric ring resonators (EER) while the lower metal layer is formed by a homogeneous strip

[13] . In such a setup, electrical coupling with the upper metal layer and magnetic coupling with antiparallel currents in both layers and a Lorentz-like magnetic response occur. In the original setup, resonance in the microwave range is achieved. By appropriately dimensioning the unit cell, resonance of the absorptions in the infrared range can also be achieved

[14] .

[0049] The expert is familiar with various preferred forms of metamaterials and structures for metamaterial perfect absorbers,which are suitable for generating resonant absorption at a desired wavelength

[15] . For example, a hexagonal array of circular structures can also be used as a metamaterial above a homogeneous base plate

[16] . Rectangular or cross-shaped resonators can also be used for a metamaterial perfect absorber are preferably used in accordance with the invention.

[0050] As explained in detail below, the structured metamaterial, the dielectric intermediate layer and the basic element are preferably designed such that resonant emission occurs at a resonant wavelength in the infrared range, preferably in a wavelength range of 1 µm to 10 µm, preferably 2 µm to 10 µm.

[0051] In addition to the preferred embodiments described for a cover element consisting of a metamaterial, base element, and a dielectric intermediate layer, the person skilled in the art can also find other suitable structures, dimensions, and / or materials based on numerical simulation. The electromagnetic theory and systems of equations for numerical solution are known to them

[15] .

[0052] The layer system consisting of the base element, dielectric intermediate layer and top element is preferably selected in such a way that a particularly high emissivity is achieved in the second (resonant) position for a desired resonance wavelength.

[0053] For a metamaterial perfect absorberThe energy balance can be summarized as follows: T(ω) + R(ω) + A(ω) = 1, where T is the transmission, R is the reflection, and A is the absorption. The parameters of the cover element, the base element, and the dielectric intermediate layer are preferably chosen such that A(ω) is maximized in the second position, which, according to Kirchhoff's law, also results in resonant emission at the frequency ω and maximizes the emissivity.

[0054] The emissivity of a body primarily indicates how much radiation it emits compared to an ideal heat radiator, a black body. In the second position, with a nearly perfect absorber on the heating element, a particularly high emissivity of theoretically up to 1 can be achieved.

[0055] In one example, the modulatable infrared emitter is characterized in that the unit cell of the structured metamaterial comprises a resonator which is shaped by struts of the conductive material, wherein the resonator preferably has the shape of a split ring resonators (SRR), electric ring resonator (ERR), a cross, a square, a circle, a hexagon and / or combinations of these shapes.

[0056] A resonator preferably refers to a structure made of struts of a conductive material that can couple to an electric field. In particular, the aforementioned forms of resonators in the form of... split ring resonators (SRR), electric ring resonator(ERR), crosses, squares, circles, and / or hexagons are particularly suitable for coupling to the electric field of infrared radiation. However, other shapes of electrical resonators may also be preferred to achieve the desired resonant emission. Such shapes are known to those skilled in the art, for example, from US 2013 / 0314765 A1, where the skilled in the art knows how to adjust the dimensions to achieve resonance at a desired wavelength, preferably in the range of 1 µm to 10 µm, more preferably 2 µm to 10 µm.

[0057] In a preferred embodiment, the unit cells form a two-dimensional periodic lattice, wherein the lattice angle is between 60° and 120°, preferably 90°, and the two lattice constants are between 5% and 40%, preferably 10% and 25% of a resonance wavelength, wherein the resonance wavelength is selected from a range between 1 µm to 10 µm, preferably 2 µm to 10 µm.

[0058] The preferred dimensioning of the unit cells is therefore based on the desired resonance wavelength, at which a high emissivity is present and emission occurs with particularly high intensity.

[0059] For the particularly preferred resonance wavelengths in a range between 2 µm and 10 µm, preferred lattice constants lie in a range between 100 nm and 4 µm, particularly preferably 200 nm and 2.5 µm. Intermediate ranges from the aforementioned ranges may also be preferred, such as 100 nm to 200 nm, 200 nm to 300 nm, 300 nm to 400 nm, 400 nm to 500 nm, 600 nm to 700 nm, 700 nm to 800 nm, 800 nm to 900 nm, 900 nm to 1000 nm, 1000 nm to 1100 nm, 1100 nm to 1200 nm, 1300 nm to 1400 nm, 1400 nm to 1500 nm, 1600 nm to 1700 nm, 1700 nm to 1800 nm, 1800 nm to 1900 nm, 1900 nm to 2000 nm, 2000 nm to 2100 nm, 2100 nm to 2200 nm nm, 2200 nm to 2300 nm, 2300 nm to 2400 nm, 2400 to 2500 nm, 2600 to 2700 nm, 2700 to 2800 nm, 2800 nm to 2900 nm, 2900 nm to 3000 nm, 3000 to 3100 nm, 3100 to 3200nm, 3300 to 3400nm, 3400nm to 3500nm, 3600nm to 3700nm, 3700nm to 3800nm, 3800nm ​​to 3900nm, 3900nm to 4000nm.A person skilled in the art will recognize that the aforementioned range limits can also be combined to obtain further preferred ranges, such as 200 nm to 800 nm, 1000 nm to 1500 µm or 1400 nm to 2500 nm.

[0060] The terms lattice angle and lattice constant are preferably understood to be the usual lattice parameters for describing a (two-dimensional) lattice. As is well known, a two-dimensional lattice can be generated by periodically translating a unit cell by the same distance in two specific spatial directions, i.e., along two lattice vectors. The lattice angle preferably corresponds to the angle between the two lattice vectors, while the lattice constants denote the length of unit cells in the two dimensions and the distance between two unit cells along the translation direction, respectively.

[0061] The aforementioned grating angles and grating constants prove to be particularly advantageous for generating electromagnetic resonant emission of IR radiation at wavelengths in the range of 1 µm to 10 µm, preferably 2 µm to 10 µm. The embodiment is therefore particularly suitable for use in infrared spectroscopy, especially for photoacoustic spectroscopy.

[0062] The choice of lattice constant preferably depends on the desired resonance wavelength. For preferred resonance wavelengths in a range between 2 µm and 3 µm, for example, it is preferable to select lattice constants from a range between 100 nm and 1.2 µm, particularly preferably between 200 nm and 750 nm.

[0063] It is particularly preferred that both lattice constants are essentially the same, so that a particularly high emissivity can be achieved at a specific resonance wavelength.

[0064] However, it may also be preferable to select different lattice constants in the different spatial directions. This advantageously makes it possible to provide a modulatable IR emitter that, in its second (resonant) position, exhibits IR radiation with a high emissivity over a range of at least two resonance wavelengths. This allows the modulatable IR emitter to advantageously provide IR radiation simultaneously for the photoacoustic spectroscopy of two or more gases. A first resonance wavelength can, for example, be approximately 2.4 µm to detect CO₂, while a second resonance wavelength can be approximately 3 µm to detect, for example, methane.In particular for spectroscopic applications, such as photoacoustic spectroscopy for the detection of gas molecules, the use of a metamaterial in an IR emitter according to the invention opens up diverse application possibilities with high economic relevance.

[0065] In a preferred embodiment, the cover element is made of a metal, particularly preferably gold, silver, aluminium, tungsten, molybdenum, titanium and / or copper.

[0066] In a further preferred embodiment, the basic element is made of a metal, particularly preferably gold, silver, aluminium, tungsten, molybdenum, titanium and / or copper.

[0067] Particularly preferred are the cover element and the base element made of a material which has essentially the same electrical conductivity.

[0068] Terms such as "essentially", "approximately", "about", "approximately", etc. preferably describe a tolerance range of less than ± 20%, preferably less than ± 10%, even more preferably less than ± 5%, and particularly less than ± 1%. Statements using "essentially", "approximately", "about", etc., always disclose and include the exact value stated.

[0069] In another preferred embodiment, the base element and the cover element are made of the same material.

[0070] In a further preferred embodiment, the dielectric intermediate layer is made of a material selected from a group comprising aluminium nitride, silicon nitride, aluminium oxide, silicon oxide, titanium dioxide (TiO 2 ) and / or tantalum oxide (Ta 2 O 5 ).

[0071] The aforementioned selection of materials for the base element, the dielectric intermediate layer, and the top element in the form of a structured metamaterial involves particularly careful matching, preferably with especially good magnetic coupling, so that high electromagnetic resonances and IR emissions are achieved in the second position. Furthermore, the materials are characterized by high resistance to corrosion, oxidation, and mechanical influences, thus ensuring a durable and robust IR emitter.

[0072] In a preferred embodiment, the base element is a continuous conductive layer. This reduces lossy reflections and achieves a particularly good impedance match between the cover element and the base element.

[0073] In another preferred embodiment, the base element, the dielectric intermediate layer, and the top element are made of CMOS-compatible materials. This allows the modulatable IR emitter to be manufactured on a single chip using CMOS interconnection. The potential integration of CMOS technology thus enables the provision of highly miniaturized and powerful modulatable IR emitters. Furthermore, CMOS technology facilitates cost-effective mass production, ensuring a commercially attractive product. Preferred materials and manufacturing steps for using CMOS technology to produce MEMS devices are known to those skilled in the art, for example, from Qu et al.

[17] .

[0074] In another preferred embodiment, the cover element has a layer thickness between 100 nm and 1500 nm. Preferred layer thicknesses lie within the aforementioned range, although intermediate ranges may also be preferred, such as 100 nm to 200 nm, 200 nm to 300 nm, 300 nm to 400 nm, 400 nm to 500 nm, 600 nm to 700 nm, 700 nm to 800 nm, 800 nm to 900 nm, 900 nm to 1000 nm, 1000 nm to 1100 nm, 1100 nm to 1200 nm, 1300 nm to 1400 nm, or even 1400 nm to 1500 nm. A person skilled in the art will recognize that the aforementioned range limits can also be combined to obtain further preferred ranges, such as 100 nm to 500 nm, 400 nm to 1000 nm, or even 800 nm to 1500 nm.

[0075] In another preferred embodiment, the dielectric intermediate layer has a layer thickness between 100 nm and 1500 nm. Preferred layer thicknesses lie within the aforementioned range, although intermediate ranges may also be preferred, such as 100 nm to 200 nm, 200 nm to 300 nm, 300 nm to 400 nm, 400 nm to 500 nm, 600 nm to 700 nm, 700 nm to 800 nm, 800 nm to 900 nm, 900 nm to 1000 nm, 1000 nm to 1100 nm, 1100 nm to 1200 nm, 1300 nm to 1400 nm, or even 1400 nm to 1500 nm. A person skilled in the art will recognize that the aforementioned range limits can also be combined to obtain further preferred ranges, such as 200 nm to 600 nm, 300 nm to 1000 nm, or even 700 nm to 1500 nm.

[0076] In another preferred embodiment, the base element has a layer thickness between 100 nm and 1500 nm. Preferred layer thicknesses lie within the aforementioned range, although intermediate ranges may also be preferred, such as 100 nm to 200 nm, 200 nm to 300 nm, 300 nm to 400 nm, 400 nm to 500 nm, 600 nm to 700 nm, 700 nm to 800 nm, 800 nm to 900 nm, 900 nm to 1000 nm, 1000 nm to 1100 nm, 1100 nm to 1200 nm, 1300 nm to 1400 nm, or even 1400 nm to 1500 nm. A person skilled in the art will recognize that the aforementioned range limits can also be combined to obtain further preferred ranges, such as 100 nm to 400 nm, 400 nm to 1200 nm, or even 700 nm to 1400 nm.

[0077] The aforementioned layer thicknesses are particularly suitable for achieving an almost [missing information] in the resonant second position. perfect metamaterial absorber to provide which, when heated by the heating element, forms a nearly perfect IR emitter.

[0078] The aforementioned preferred embodiments of the metamaterial structuring, as well as the layer thicknesses and choice of materials, are particularly well suited to ensuring high emissivity and high-intensity radiation in a (second) resonant position. While theoretically, by means of a nearly perfect metamaterial absorberWhile emissivities of up to 1 are possible, manufacturing tolerances or energy losses in practice lead to lower maximum emissivities in the resonant position. Advantageously, a high modulation depth can also be achieved with emissivities significantly less than 1 in the (second) resonant position. The modulation depth of the modulatable IR emitter is preferentially influenced by the difference in emissivity between the second and first positions. This means that the possible modulation depth of the IR emitter is determined not only by the maximum achievable emissivity in the second (resonant) position, but also by the preferentially lower emissivity in the first (non-resonant) position.

[0079] The inventors have recognized that the vertical distance between the base element and the top element, as well as their degree of overlap, are suitable for adjusting the emissivity in a first (resonant) position and in a second (non-resonant) position.

[0080] In a preferred embodiment, the actuator is therefore configured for a vertical translational movement of the cover element and / or base element along the emission direction of the infrared emitter, which changes the distance between the cover element and the base element. Preferably, the distance in the second position is smaller than in the first position.

[0081] Preferably, the top element and base element are arranged one above the other in two parallel planes. In a first position, the actuator preferably holds the top element and / or base element at a sufficiently vertical distance, which allows for electromagnetic resonance and the formation of a metamaterial perfect absorbersprevented. Only through a preferential reduction of the distance by a vertical translational movement, i.e., for example, lowering the cover element onto the dielectric intermediate layer, does a high emissivity and resonant radiation occur in a second position.

[0082] In a preferred embodiment, in the first position the cover element has a distance to the dielectric layer of at least 500 nm, preferably at least 1000 nm, and in the second position has a distance to the dielectric layer of at most 200 nm, preferably at most 50 nm, particularly preferably 0 nm.

[0083] The aforementioned distances have proven particularly advantageous in ensuring significantly reduced emission in the first position compared to the second position. A vertical distance of 0 nm between the cover element and the dielectric layer preferably denotes contact between the cover element and the dielectric layer. It is known to those skilled in the art that even with direct contact, an actual distance of more than 0 nm may exist due to roughness on the two contacting surfaces. Such deviations are accounted for in the disclosed embodiment.

[0084] The inventors also recognized that, in addition to varying the distance through a vertical translational movement, a relative horizontal translation of the top element and the base element can also be used to vary the emissivity and thus the intensity of the infrared radiation.

[0085] In a preferred embodiment, the actuator is configured for a horizontal translational movement of the cover element and / or the base element orthogonal to the emission direction of the infrared emitter, which changes the degree of overlap between the cover element and the base element. Preferably, the degree of overlap is lower in the first position than in the second position.

[0086] The degree of coverage is preferably understood to be the ratio of the area of ​​the base element over which a section of the cover element is located in the emission direction to the total area of ​​the base element.

[0087] A degree of coverage of 0% thus describes a state in which no section of the cover element made of structured metamaterial is located above the base element in the emission direction. At a degree of coverage of 0%, no resonant emission of IR radiation occurs.

[0088] A degree of coverage of 100%, on the other hand, denotes a relative positioning or state in which a section of the cover element made of structured metamaterial is located over the entire surface of the base element in the emission direction. At a degree of coverage of 100%, resonant radiation is emitted essentially over the entire surface of the base element. At a degree of coverage between 0 and 100%, resonant radiation preferably occurs only over the covered portion of the base element.

[0089] Advantageously, the emissivity or intensity of the emitted IR beam can thus be precisely modulated by a horizontal translational movement and a variation in the degree of coverage.

[0090] In a preferred embodiment of the invention, in the first position the cover element and base element have a degree of overlap of less than 40%, preferably less than 10%, particularly preferably 0%, while in the second position the cover element and base element have a degree of overlap of more than 40%, preferably more than 80%, particularly preferably 100%.

[0091] The aforementioned coverage levels can be implemented very well mechanically and ensure sufficient modulation depths of the IR emitter.

[0092] When the cover element is moved horizontally relative to the base element to change the degree of overlap, the actuator is preferably configured such that the vertical distance in the second position is preferably less than 200 nm, more preferably less than 100 nm, and more preferably 0 nm. For this purpose, a horizontal translation can be performed without an additional vertical translation. However, it may also be preferred that a horizontal translation is superimposed on a vertical translation.

[0093] In the context of the invention, "vertical" preferably refers to the emission direction of the IR emitter, which is defined by the surface normal to the top and bottom elements. "Horizontal," on the other hand, preferably refers to a direction in a plane parallel to the planes in which the top and bottom elements are arranged. Terms such as "superimposed," "overlapping," etc., refer to the vertical direction, i.e., the emission direction of the IR emitter. This direction may, but need not, coincide with a gravitational direction.

[0094] To ensure the relative movement of the base and top elements, an actuator is used. Preferably, the actuator converts an electrical control signal into a movement. This can be a MEMS actuator, which is, for example, an electrostatic actuator.

[0095] The ratio between the maximum and minimum intensity of the emitted IR radiation, adjustable by relative motion, can be called the extinction ratio. It can be directly determined from the quotient of maximum and minimum intensity and is preferably expressed directly by this quotient. However, it is also preferable for this ratio to be expressed on the logarithmic scale of decibels (dB), as is common practice, for example, in telecommunications. Those skilled in the art understand that the higher the extinction ratio, the greater the difference between the emissivity levels adjustable in the first and second positions.

[0096] The maximum modulation frequencies achievable at the desired modulation depths should preferably be at least 1 kilohertz (kHz), particularly preferably at least 10 kHz, more preferably at least 20 kHz, most preferably at least 30 kHz, and particularly preferably at least 100 kHz. For applications in photoacoustic spectroscopy, it is particularly preferred to achieve modulation frequencies in the audible and / or ultrasonic range. The modulation bandwidth over which the desired modulation depth is achieved preferably covers the entire frequency range from 0 Hz to the maximum modulation frequency.

[0097] The desired modulation can preferably be expressed in a corresponding temporal profile of the emitted radiation intensity. To determine the feasibility of such a desired temporal intensity profile, the modulation depth and the bandwidth over which this modulation depth is essentially achievable are particularly important. The resolution of an electronic control of the IR emitter is also preferably relevant for feasibility. It can be significant which different intensity levels between minimum and maximum intensity can be achieved at which frequency.

[0098] It is preferred that the IR emitter has an electrical control system that regulates the relative movement of the base element and the cover element caused by the at least one actuator. Such a system can be implemented, for example, via a control device. The desired spectra, intensities, and modulations can be set by means of this control system.

[0099] Control preferably means that electrical control signals are transmitted directly to the actuator and the heating element, resulting in the desired radiation characteristics. In particular, a specific temperature and / or a specific temperature profile can be set for the heating element. Furthermore, a specific modulation signal can be achieved through the relative movement triggered by the actuator (possibly in conjunction with a temperature profile). Typically, this is an analog signal generated by a control unit. This control unit, in turn, can preferably receive a suitable digital electronic signal, for example, from a control computer, which is then advantageously translated by the control unit into suitable control signals.

[0100] It is particularly advantageous for large parts of the IR emitter to have dimensions in the micrometer range in order to enable a miniaturized design.

[0101] In a preferred embodiment, the heating element is a micro-heating element.

[0102] In a preferred embodiment, the actuator is a MEMS actuator, preferably selected from the group comprising electrostatic actuator, piezoelectric actuator, electromagnetic actuator and / or thermal actuator.

[0103] A MEMS actuator is preferably an actuator that is manufactured using standard microsystems technology methods and advantageously has dimensions on the order of micrometers. Such an actuator is particularly compact, robust, and requires little maintenance, and can be manufactured easily and cost-effectively. In particular, large portions of the emitter can be MEMS elements, i.e., elements with the preferred properties mentioned above, and can be manufactured in a single manufacturing step along with the MEMS actuator. It is desirable that the same substrate can be used for some of the manufacturing process. This simplifies and reduces the cost of production.

[0104] The actuators mentioned above are particularly well-suited for a large number of rapid translational movements and, due to their compact design, exhibit low energy consumption. Furthermore, the range of achievable translational speeds is very high due to their compact design, low inertia, and linear motion.

[0105] In another preferred embodiment of the modulatable infrared emitter, the MEMS actuator is an electrostatic actuator in the form of a comb drive based on a variation of the comb overlap and / or the comb spacing. MEMS comb drives are known from the prior art, e.g., from patent application DE 10 2017 206 183 A1. Depending on the embodiment, the comb overlap and / or the comb spacing can be varied. It has been shown that such MEMS comb drives, due to their dimensions and the movements they can generate, are particularly suitable for a preferred translational movement and a compact IR emitter.

[0106] In a further preferred embodiment, the actuator is coupled to the cover element and configured for a translational movement of the cover element relative to the base element. Translational movement refers in particular to a displacement of the cover element or base element. This displacement is preferably vertical along the emission direction. "Coupled" in this context means, in particular, that there is a direct mechanical connection between the cover element and at least one movable element of the actuator, such that a movement of the movable actuator element triggers a movement of the cover element in the desired direction.

[0107] The actuator and cover element can preferably be directly connected to each other. It may even be preferred that both the actuator and the cover element comprise the same substrate and / or are made from it. The coupling to the actuator can be not only mechanical, but also thermal and / or electrical.

[0108] However, it may also be preferable for the actuator to be coupled to the base element and configured for translational movement of the base element relative to the cover element. If the base element is rigidly connected to a dielectric layer, the actuator can also be connected to the base element indirectly via the dielectric intermediate layer.

[0109] As explained in more detail above, the modulation of IR radiation can be achieved, in particular, by changing the vertical distance between the top and bottom elements or their degree of overlap. In this respect, both elements are therefore suitable for translational movement.

[0110] However, it is particularly advantageous for the heating element, the base element, and the dielectric layer to be mechanically coupled, with the cover element being moved by means of the actuator. This allows for particularly efficient manufacturing of the IR emitter, and the low weight of the cover element also enables fast and precise translational movements.

[0111] In a preferred embodiment, a spacer frame is provided on the dielectric intermediate layer, the height of which defines the vertical distance between the dielectric intermediate layer and the cover element. Preferably, the spacer frame extends along the outer perimeter of the dielectric intermediate layer, with the inner circumference of the spacer frame being larger than the outer circumference of the planar cover element. The at least one actuator preferably connects the spacer frame to the cover element, such that, for example, in a first position, the cover element is held at the level of the upper end of the spacer frame, and the actuator is configured for a vertical downward movement of the cover element towards the dielectric intermediate layer.

[0112] In this embodiment, the heating element, the base element, and the dielectric intermediate layer are preferably stationary, with the relative movement between them and the cover element being realized by a translational movement of the cover element, and the movement being triggered by the actuator. In this case, a translational movement preferably refers to a vertical or horizontal displacement of the cover element.

[0113] The actuator and spacer frame can preferably be directly connected to each other. It may even be preferred that both the spacer frame and the actuator comprise the same substrate and / or are made from it.

[0114] In a preferred embodiment, the infrared emitter comprises at least four MEMS actuators, which are installed on the outer sides of the cover element and configured to simultaneously control the relative movement of the cover element and the base element between the first and second positions. The use of at least four MEMS actuators on the outer sides of the cover element ensures a particularly uniform and rapid translation of the cover element.

[0115] Particularly preferably, the multiple MEMS actuators can be attached to a spacer frame as described above, which is preferably applied as a border on the dielectric intermediate layer and whose height determines the vertical distance from the cover element to the dielectric intermediate layer in the first position.

[0116] The preferred number of MEMS actuators can be determined by the shape of the cover element. In the case of a flat, rectangular cover element and a correspondingly larger rectangular spacer frame, at least four MEMS actuators are preferred, with one MEMS actuator installed on each side. In the case of a hexagonal cover element, six MEMS actuators are preferably used, while in the case of a triangular shape, three MEMS actuators may also be preferred. Integer multiples of the respective number of MEMS actuators are also preferred, for example, to install two, three, or more MEMS actuators on each outer side of the cover element.

[0117] In another preferred embodiment, the infrared emitter has a housing in which the heating element, the base element, the dielectric intermediate layer, the cover element and the actuator are installed.

[0118] The housing can preferably be oriented towards the dimensions and shapes of the installed elements. It can also be preferred that the housing be significantly larger than the installed elements to improve the handling of the emitter and to create a robust device. For example, the heating element, actuator, base element, dielectric intermediate layer, and / or cover element can be MEMS elements and / or have dimensions in the (sub-)micrometer range, while the housing has dimensions in the centimeter range.

[0119] Preferably, the housing has a continuous outer surface and is closed on the inside. The heating element is installed inside the housing. This protects it from external influences and prevents the emission of IR radiation to the outside, except in the preferred emission direction.

[0120] It is preferred that the micro-heating element is not thermally insulated from the housing, but rather that non-radiative heat transfer from the heating element to the housing is possible, allowing heat to dissipate from the heating element. This allows, for example, a desired balance to be achieved between the heat generated by the current-carrying, heatable layer of conductive material and the heat emitted by the heating element to the surroundings, thus generating the desired radiative properties and / or achieving the desired modulation characteristics.

[0121] For example, it may be preferred that the housing and / or micro-heating element elements are made of the same material and that there is sufficient heat conduction between directly connected elements.

[0122] The housing may preferably have a heat sink for its own heat dissipation.

[0123] It may be desirable to thermally decouple certain elements, such as the cover element, from the housing. This preferably means that, by using at least one suitable material at the connection point or by a suitable design (for example, a small contact area and / or suitable thickness of the connection), the cover element does not heat up significantly.

[0124] A suitable material at the joint preferably covers the entire joint surface. Suitable materials are defined in particular by their thermal conductivity, expressed in watts per meter and Kelvin (W / m·K).

[0125] Preferred thermal conductivities at the connection point are less than 10 W / m·K, particularly preferably less than 1 W / m·K and particularly less than 0.1 W / m·K.

[0126] For example, an oxide layer may be preferred. To minimize the direct heat transfer between the micro-heating element and the aperture structure, it may be preferred that the housing be configured to generate an internal vacuum. A vacuum preferably means a pressure of less than 30 x 10³ Pascals (Pa), more preferably less than 100 Pa, and particularly 0.1 Pa or less. Configured means that the housing is designed to be sufficiently pressure-tight.

[0127] Due to the resonant emission behavior of the IR emitter in the second position, IR emission with narrower frequency spectra around one or more resonance wavelengths already occurs. Depending on the application of the IR emitter, e.g., in various spectroscopy methods, it may be desirable to utilize even narrower spectra. Furthermore, particularly in the first position, emission occurs, albeit with low intensity, over a broader frequency spectrum.

[0128] Frequency filters can be preferably used to select particularly narrowband spectra and to minimize IR emission outside the desired narrowband spectra. These are preferably integrated into a housing.

[0129] The filters used can advantageously have various filter properties; for example, bandpass filters, shortpass filters, longpass filters, notch filters, and any combination of these filters that achieves the desired spectral characteristics can be used. The frequencies or frequency ranges in which the filters operate can be selected as needed, depending on the application. A filter wheel, for instance, can be used, on which filters with different filter properties are mounted. The desired filter can then be selected mechanically by rotating the filter wheel. Preferably, the filter wheel can be rotated by an electric drive.

[0130] The use of a Fabry-Pérot filter is also conceivable. Such a filter can be used, for example, to select very narrow spectra. Preferably, the underlying Fabry-Pérot interferometer is tunable, for example, by adjusting the temperature or by mechanical modification. This allows for flexible selection of desired spectra.

[0131] Alternatively, suitable thin-film filters can be used. These are particularly easy to manufacture and very compact. Especially when the IR emitter is manufactured in an integrated design within a single production process, the production of such a thin-film filter can be easily incorporated into the process. This reduces costs.

[0132] A flexible combination of thin-film filters or a thin-film filter that can be tuned by, for example, a temperature-adjustable method is also advantageous.

[0133] Filters can also be used for other properties of IR radiation, e.g., polarization.

[0134] In a preferred embodiment, the heating element comprises a substrate on which at least one heatable layer of a conductive material is applied, and on which contacts for a current and / or voltage source are provided. Preferably, the heatable layer is a separate layer onto which the base element is applied. However, the heatable layer can also be formed by the base element itself.

[0135] The substrate preferably forms the base of the heating element. The substrate can also at least partially comprise other elements of the IR emitter, such as the actuator, base element, dielectric intermediate layer, cover element, and / or housing elements. Advantageously, the substrate can be suitably shaped using established process steps, particularly those from semiconductor and / or microsystem manufacturing. A heatable layer made of a conductive material can then preferably be applied to or integrated into the substrate, e.g., by doping and / or coating. It is preferred that the heatable layer be contacted with an electrical energy source to establish an electrical contact.

[0136] In another preferred embodiment of the modulatable infrared emitter, the substrate is selected from a group comprising silicon, monocrystalline silicon,

[0137] Polysilicon, silicon dioxide, silicon carbide, silicon germanium, silicon nitride, nitride, germanium, carbon, gallium arsenide, gallium nitride, and / or indium phosphide. These materials are particularly easy and cost-effective to process in semiconductor and / or microsystem manufacturing and are also well-suited for mass production. Likewise, these materials are especially suitable for doping and / or coating to achieve the desired electrical, thermal, and / or radiation properties in specific areas.

[0138] In a further preferred embodiment of the modulatable infrared emitter, the conductive material for forming the heatable layer is selected from the group comprising platinum, tungsten, (doped) tin oxide, monocrystalline silicon, polysilicon, molybdenum, titanium, tantalum, titanium-tungsten alloy, metal silicide, aluminum, graphite, and / or copper. These materials exhibit the desired thermal, electrical, mechanical, and / or radiation properties and are also particularly easy and inexpensive to process.

[0139] An example of a manufacturing process for an infrared emitter as described above includes the following steps: Etching a substrate; depositing a conductive material onto the substrate to form a heatable layer and contacting the heatable layer; depositing a conductive material to form the base element; depositing a dielectric material to form a dielectric intermediate layer; depositing a conductive material to form a cover element and / or structuring the cover element as a metamaterial with periodically arranged unit cells, wherein preferably an etching and / or structuring process is selected from the group comprising dry etching, wet chemical etching and / or plasma etching, in particular reactive ion etching, reactive ion deep etching (Bosch process); and / or the deposition process is selected from the group comprising physical vapor deposition (PVD), in particular thermal evaporation, laser beam evaporation, arc evaporation, molecular beam epitaxy, sputtering, chemical vapor deposition (CVD) and / or atomic layer deposition (ALD).

[0140] One of the preferred materials mentioned above can be used as a substrate. During etching, a blank, for example a wafer, can be formed into the desired basic shape of a heating element. In the next steps, the conductive material for the heatable layer, the conductive material for forming a base element, and a conductive material for forming a top element can be deposited.

[0141] Should further structuring of the conductive material be desired, particularly for the cover element, this can be achieved, for example, through further etching processes. Similarly, additional material can be deposited or doping can be carried out using conventional methods.

[0142] To connect the heatable layer, suitable material such as copper, gold, and / or platinum can be deposited onto the conductive material using standard processes. Physical vapor deposition (PVD), chemical vapor deposition (CVD), or electrochemical deposition are preferred methods for this.

[0143] This method allows for the production of particularly fine structures with dimensions in the nanometer or micrometer range. These manufacturing steps have also proven highly effective and are standard procedures in semiconductor processing.

[0144] In a further preferred embodiment of the manufacturing process, an etching and / or a structuring is selected from the group comprising dry etching, wet chemical etching and / or plasma etching, in particular reactive ion etching, reactive ion deep etching (Bosch process); and / or the deposition is selected from the group comprising physical vapor deposition (PVD), in particular thermal evaporation, laser beam evaporation, arc evaporation, molecular beam epitaxy, sputtering, chemical vapor deposition (CVD) and / or atomic layer deposition (ALD).

[0145] These processes are particularly suitable for the fabrication of fine structures on the micrometer scale. In particular, the Bosch process enables the creation of very fine structures with a high aspect ratio, which are advantageous for a compact, efficient, and integrated design of IR emitter components.

[0146] In another aspect, the invention relates to a system encompassing a) a modulatable infrared emitter according to the invention or preferred embodiments thereof b) a control device, wherein the control device is configured to regulate the actuator for a relative movement of the top element and the base element between a first and second position in order to modulate the intensity of the emission of the infrared emitter.

[0147] The control unit preferably accepts an input and converts this input into suitable control signals. An input can be, for example, a desired spectrum, a desired resonant wavelength, a desired intensity, modulation depth, and / or modulation frequency. The control unit primarily generates corresponding analog electrical signals, which are then transmitted to the actuator and / or the micro-heating element to produce the desired IR radiation.

[0148] However, more complex signals can also serve as input, specifying a precise temporal amplitude profile of the outgoing IR radiation for a desired spectrum. The control unit then preferentially provides the appropriate control signals to generate the desired modulated IR radiation.

[0149] The control unit is specifically configured to regulate the actuator for the relative movement between the base element and the cover element between (at least) a first and (at least) a second position. For this purpose, electrical signals are generated that trigger the required translational movement of the at least one actuator.

[0150] Preferably, the control device includes a control loop, wherein a feedback mechanism can correct any discrepancy between the desired control and the actual movement of the actuator and / or heating of the heating element.

[0151] It may be preferable that the temperature profile of the heating element can also be controlled by the control device for additional, slow modulation of the IR radiation.

[0152] The system's control unit can be located externally or integrated on the IR emitter.

[0153] The control unit preferably includes a processor, for example a microprocessor. Other integrated circuits used for control in digital electronics can also be used.

[0154] The use of such a system, including a suitable control unit, can significantly simplify the desired use of the IR emitter. For example, suitable spectroscopy signals can be designed on a PC. The desired signals are then transmitted to the control unit via the input. This unit generates the control signals, ensuring a corresponding IR signal in high agreement with the theoretical specifications.

[0155] A control unit, particularly one integrated into the emitter, is very compact and easy to use. The control unit preferably has a suitable interface for input, allowing connection to, for example, a computer. It may also be desirable for data to be transmitted from the control unit to the input device via this interface, such as the current temperature of the heating element, the modulation frequency, or other status information.

[0156] In another preferred embodiment of the system, the control device is configured to regulate the temperature of the heatable areas of the micro-heating element, preferably in a range between 50 °C and 1000 °C.

[0157] Such a control device is preferably capable of supplying suitable electrical power to the heating element. In particular, the temperature should be adjustable with sufficient accuracy and / or be able to be kept constant. A control mechanism with a feedback loop can be used for this purpose. To measure the current temperature of the heating element, at least one temperature sensor can be integrated at a suitable location on the element.

[0158] In another preferred embodiment of the system, the control device is configured to regulate the actuator for an oscillating relative movement of the top element and the base element between a first and a second position, preferably achieving a modulation frequency of the emitted infrared radiation between 10 Hz and 100 kHz.

[0159] Preferably, the translational movement initiated by the actuator between a first and a second position is repeated regularly, resulting in an oscillation between the positions and a periodicity to the translational movement. At the end of the translational movement, the starting point of the movement should preferably be reached again, and the movement should be executed anew in the following period. The frequency of the oscillating movement preferably determines the resulting modulation frequency of the power intensity of the emitted infrared radiation.

[0160] It is also possible to continuously adjust the translation frequency, and thus the modulation frequency, within the limits of the electronic resolution and / or bandwidth of the control device and / or the actuator. This allows the modulation frequency to be varied, preferably over time.

[0161] Furthermore, it may be preferable to vary not only the translation frequency but also the translation amplitude within the range of motion of the actuator.

[0162] For example, the vertical distance between the top element and the dielectric intermediate layer, or the degree of overlap between the top element and the base element, can be varied. This allows the emissivity, and thus the modulation depth, to be adjusted over time at the selected positions.

[0163] This results in a system that allows for very flexible and efficient variation of the modulation frequency and modulation depth of the IR radiation.

[0164] In another preferred embodiment of the system, the control device is configured to regulate the actuator for an oscillating relative movement of the base element and the top element in such a way that a modulation frequency of the emitted infrared radiation between 10 Hz and 100 kHz, particularly preferably between 100 Hz and 20 kHz, is achieved.

[0165] For this purpose, it is particularly advantageous that all required components, such as control unit, actuator, etc., enable the required bandwidth.

[0166] The frequencies mentioned have proven particularly effective for preferred applications in the field of spectroscopy. In particular, they have proven especially suitable for use in photoacoustic spectroscopy, as they cover a wide range of acoustic frequencies, the generation of which is central to this spectroscopy method.

[0167] An example of a method for the modulated emission of infrared radiation includes the following steps: Providing a modulatable infrared emitter as described above; heating the heating element to emit infrared radiation; controlling the actuator for a relative movement of the top element and the base element between a first and second position to modulate the intensity of the emission of the infrared emitter.

[0168] The average person skilled in the art recognizes that the technical features, definitions and advantages of preferred embodiments of the IR emitter and system according to the invention also apply to the method.

[0169] Examples of the use of a modulatable infrared emitter according to the preceding description or a system according to the preceding description are photoacoustic spectroscopy and / or infrared spectroscopy.

[0170] A compact, durable IR emitter that can be modulated at high speed is of interest for a wide range of applications. Especially for photoacoustic spectroscopy, many applications are conceivable that do not take place in the laboratory and must function under everyday conditions. Examples include environmental sensing for the detection of (polluting) substances in ambient air or as a warning device for toxic gases, such as carbon monoxide.

[0171] The high modulation frequencies of several kHz allow for significantly improved signal-to-noise ratios compared to direct electrical modulation of the heating element, thus enabling reliable measurements.

[0172] In another aspect, the invention relates to a photoacoustic spectroscope for the analysis of gas, comprising a modulatable infrared emitter as described above, a gas-fillable analysis volume, an acoustic detector, wherein the analysis volume is arranged between the infrared emitter and the sound detector, so that the infrared radiation emitted by the infrared emitter can be modulated for photoacoustic spectroscopy of the gas.

[0173] Those skilled in the art are familiar with how photoacoustic spectroscopy is performed and which components are used. The modulatable infrared emitter is preferably arranged and configured such that infrared radiation emitted by the infrared emitter strikes the gas in the analysis volume. If the modulated irradiation occurs with an infrared wavelength corresponding to the absorption spectrum of a molecule in the gas, modulated absorption takes place, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. According to the photoacoustic effect, the heating and cooling processes cause expansion and contraction of the gas, generating sound waves with the modulation frequency. These can then be measured by the sound detector. The power of the sound waves is preferably directly proportional to the concentration of the absorbing gas.Preferably, the sound detector or sound pressure detector can be a microphone. However, other structures that are passively set into vibration by sound waves, such as membranes or vibrating beams, can also be used as sound detectors.

[0174] The gas to be analyzed is contained within a gas-fillable analysis volume. In a preferred embodiment, this volume (or chamber) is at least partially or completely sealed off from the outside and contains the gas, or allows it to be introduced, for example, through a closable opening in the form of a closure and / or valve and / or through a supply line. Alternatively, it can be a completely sealed or closable volume or chamber that has at least one, preferably two, closable openings for introducing and / or venting the gas to be analyzed. This allows for very precise localization of the gas to be analyzed, particularly within a specific infrared radiation range.

[0175] The analysis volume can preferably also be at least partially open. This allows, in particular, the measurement and analysis of the composition of a gas atmosphere surrounding the spectrometer, against which the analysis volume is at least partially open. This is especially interesting for applications in the field of pollutant measurement, but also, for example, for military applications or counterterrorism, e.g., against a chemical weapons attack.

[0176] In this case, it is advantageous that the analysis volume is well defined, so that the infrared emitter, the analysis volume and the sound detector are arranged in such a way that the infrared radiation emitted by the infrared emitter can excite the gas in the analysis volume to form sound pressure waves, which can be measured with the help of the sound pressure detector.

[0177] The analysis volume is preferably located in the beam path of the infrared emitter. This preferably means that the intensity of the beam is substantially or at least partially directed onto the side of the analysis volume facing the emitter. Partially preferably means at least 40%, more preferably at least 50%, 60%, 70%, 80% or more.

[0178] In a preferred embodiment, the infrared emitter can be directed from the outside onto a preferred region of the analysis volume. If it is necessary to radiate through an outer wall of the volume to excite a gas inside, the outer wall is preferably essentially transparent to IR radiation, at least in this region. However, the infrared emitter can also be arranged inside the analysis volume.

[0179] The analysis volume is preferably fillable with gas. This means that even if the volume is at least partially sealed or closable, a preferably closable opening for filling is provided. A gas flow can also preferably be implemented by providing the analysis volume with, for example, an inlet and an outlet. Thus, either a continuous gas flow into the volume or a discontinuous gas flow can be achieved, in which, for example, the gas in the analysis volume can be filled or exchanged during a filling or gas exchange phase. During an analysis phase, the gas flow is preferably interrupted to allow photoacoustic spectroscopy to take place. If different gases are to be analyzed at different times, this can preferably be achieved using such a setup.A cleaning gas can preferably be introduced between two gases to be analyzed to remove any gas residues from the volume.

[0180] In an analysis volume that is at least partially open and thus preferably allows a permanent gas exchange with an environment, the analysis volume is filled by interaction with a gas atmosphere of the environment.

[0181] An analysis volume can preferably comprise a sample chamber and a reference chamber, which are connected or connectable via a connecting channel.

[0182] In the case of an embodiment of an analysis volume which has a sample chamber and a reference chamber, it may be preferred to have at least one acoustic detector in each chamber in order to measure separately in each chamber and thus be able to preferably subtract sources of interference, e.g. external sound pressure waves which do not originate from the IR radiation absorbed in the sample chamber, after the measurement.

[0183] It is also preferable for the infrared emitter to irradiate the sample chamber and not the reference chamber, and for a connecting channel containing an acoustic detector to exist between the sample chamber and the reference chamber. This embodiment is characterized by particularly precise photoacoustic spectroscopy, since, for example, sound from unwanted sound sources is excluded or not measured during the measurement and / or evaluation of the measurement. Preferably, the sample volume and a reference volume can have essentially the same dimensions to implement an accurate differential measurement method.

[0184] The sample volume and the reference volume can contain the same gas. It is also preferred that the sample volume and the reference volume contain different gases, with the reference volume containing a gas with known properties and the sample volume containing a gas to be analyzed. The use of two volumes and at least one sound pressure detector advantageously allows for improved elimination of error sources, such as unwanted sound waves, because these act on both volumes. The sound pressure detector arranged between the volumes preferably detects essentially only the sound pressure waves in the sample volume caused by IR radiation, which are relevant for photoacoustic spectroscopy, as a difference signal between the sample volume and the reference volume.

[0185] Due to the unique, compact, and high-frequency modulatable IR emitter, the entire setup for various photoacoustic spectroscope variants can be manufactured to be exceptionally compact and suitable for everyday use. The high modulation frequencies also enable a wide range of analysis possibilities, particularly improving the signal-to-noise ratio. For example, typical 1 / f noise in sound detectors such as microphones can be significantly reduced.

[0186] In a preferred embodiment, the photoacoustic spectroscope comprises at least two or more modulatable infrared emitters as described above, wherein the resonance wavelengths of the two or more modulatable infrared emitters differ. As explained above, the resonance wavelength can be adjusted, in particular by suitable structuring of the metamaterial or the periodically repeating unit cells, so that in the second position a high emission of infrared radiation occurs.

[0187] By using two or more modulatable infrared emitters with different resonant wavelengths, different molecules in the gas can be analyzed simultaneously. Since the two or more modulatable infrared emitters also have separate actuators, they can preferably be operated using different modulation frequencies, allowing the sound waves of the different molecules to be detected with exceptional precision. Due to its compact design, a miniaturized photoacoustic spectroscope can be provided, characterized by a high bandwidth of simultaneously detectable molecules with the highest accuracy. Detailed description

[0188] The invention will be explained in more detail below using examples and figures, without being limited to these. Brief description of the images

[0189] Figure 1shows a schematic representation of a cross-section of a preferred embodiment of the modulatable infrared emitter. Figure 2 shows a schematic 3D view of a preferred embodiment of the modulatable infrared emitter. Detailed description of the illustration

[0190] Figure 1 shows a schematic representation of a preferred embodiment of a modulatable infrared emitter 1 in cross-section. A layered structure comprising a planar base element rests on a heating element 11. 9, a dielectric intermediate layer 7 and a flat ceiling element 5 from a structured metamaterial. The heating element 11 can, for example, comprise a Si substrate on which a heatable layer of a conductive material is applied, on which contacts for a current and / or voltage source are located. However, it may also be preferred that the basic element 9It functions as a heatable layer and is contacted for this purpose. The basic element 9 and the deck element 5 are preferably made of a conductive material, especially preferably of a metal.

[0191] The cover element 5 is equipped with actuators 3 coupled, which is responsible for a vertical translational movement of the deck element 5 are configured in relation to the basic element and can control at least two positions.

[0192] In first position ( Fig. 1A ) the cover element 5 at a greater distance above the dielectric intermediate layer 7 held as in a second position ( Fig. 1B ).

[0193] For this purpose, a spacer frame is provided in the embodiment shown. 13 on the dielectric intermediate layer 7 before, which determines the vertical distance of the deck element 5to the dielectric intermediate layer 7 Defined in the first position. The spacing frame 13 This could be, for example, a nitride or oxide layer, which extends along the perimeter of the dielectric intermediate layer. 7 is applied at a defined height. The actuators 3 are both with the spacing frame 13, as well as with the deck element 5 coupled and configured to place the deck element in a first position 5 at the level of the upper end of the spacer frame 13 to hold, while in the second position a lowering of the deck element 5 to the dielectric intermediate layer 7 This will take place, preferably until contact is made.

[0194] The actuators 3 are preferably MEMS actuators, especially electrostatic MEMS actuators, wherein, for example, the cover element is activated by applying a voltage. 5It can be driven into a first or second position in a targeted manner.

[0195] Through the means of the actuators 3 controlled translational movement of the deck element 5 The intensity of the infrared emitter is modulated. 1.

[0196] During the operation of the infrared emitter 1 will the heating element 11 preferably controlled to a temperature in the range of 50° to 1000°. The resulting emitted infrared radiation depends on the distance at which the cover element is located. 5 from a structured metamaterial above the dielectric interlayer 7 or the basic element 9 is located.

[0197] In the Figure 1 B The second position shown is the deck element. 5at a distance of preferably less than 200 nm, particularly preferably at a distance corresponding to contact between the two elements. In this state, the cover element forms 5 from a structured metamaterial, the dielectric intermediate layer 7 and the basic element 9 on the heating element 11 preferably one metamaterial perfect absorber, This results in a particularly high emission coefficient for one or more preferred resonant wavelengths. In other words, an electromagnetic resonance occurs in the second position, which allows the coupling of infrared radiation to a specific resonant wavelength.

[0198] The vertical distance in the first position ( Figure 1A ) is chosen such that no resonant coupling can occur and the infrared emitter 1 has a low emission level.

[0199] The actuators 3Mediated transitions between a first (non-resonant) and a second (resonant) position thus modulate the intensity of the emitted infrared radiation. Put simply, the IR emitter 1 The first position is "off" and the second position is "on". Advantageously, MEMS actuators can be used to achieve high-frequency modulation between the two states in the kHz range, making the described infrared emitter particularly suitable for applications in infrared spectroscopy.

[0200] Figure 2 shows a schematic 3D top view of a preferred embodiment of a modulatable infrared emitter 1. Preferably, this is an embodiment of a modulatable infrared emitter. 1, how he in relation to the in Fig. 1 The cross-section shown was explained. As can be seen in the 3D view, the preferred infrared emitter comprises 1 four MEMS actuators3, which are located on the outer sides of the deck element 5 The four MEMS actuators are installed. 3 each on the spacer frame 13 and on the cover element 5 coupled and for a vertical relative movement of the deck element 5 from a first position ( Fig. 2A ) to a second position ( Fig. 2B ) configured. The use of the four MEMS actuators 3 allows for a particularly fast and reliable lowering of the deck element. 5 to a desired distance.

[0201] As with regard to Fig. 1 As explained, this is achieved by lowering the deck element vertically. 5From a structured metamaterial onto the (not shown) dielectric intermediate layer, resonant infrared radiation is emitted at one or more preferred resonance wavelengths. The schematic diagram shows the structuring of the metamaterial by rectangular unit cells. These serve primarily for illustration; as described above, various shapes and / or dimensions of the unit cells can be used to ensure effective resonance radiation in the second position. Reference symbol list

[0202] 1 Modulatable infrared emitter 3 Actuator 5 Cover element made of structured metamaterial 7 Dielectric intermediate layer 9 Base element made of a conductive material 11 Heating element 13 Spacer frame REFERENCES

[0203] [1] Liu et al.: Micromachined tunable metamaterials: a review, Journal of Optics 14, 114009, 2012. [2] Zheludev et al.: From metamaterials to metadevices, Nature Materials , Vol 11, 2012. [3] Tao et al.: MEMS Based Structurally Tunable Metamaterials at Terahertz Frequencies, J Infrared Milli Terahz Waves Vol 32, 580-595, 2011. [4] Turpin et al.: Reconfigurable and Tunable Metamaterials: A Review of the Theory and Applications, International Journal of Antennas and Propagation,Volume 2014, Article ID 429837, 2014. [5] Hildenbrand et al.: Micromachined Mid-Infrared Emitter for Fast Transient Temperature Operation for Optical Gas Sensing Systems. IEEE Sensors Journal, 10 (2), 2010. [6] Spannhake et al.: High-temperature MEMS Heater Platforms: Long-term Performance of Metal and Semiconductor Heater Materials, Sensors 2006, 6, 405-419. [7] Harald A. Beck, Anwendung der Photoakustischen Spektroskopie in der Prozess- und Umweltanalytik, Dissertation, TU München, 2003. [8] Pusch et al.: A highly efficient CMOS nanoplasmonic crystal enhanced slow-wave thermal emitter improves infrared gas-sensing devices, Nature Scientific Reports, 5:17451, 2015. [9] Lochbaum et al.: On-Chip Narrowband Thermal Emitter for Mid-IR Optical Gas Sensing, ACS Photonics, 4, 1371-1380, 2017.

[10] Tsai, et al.: Reflection and emission properties of an infrared emitter, Optics Express, Vol. 15, No. 22, 2007.

[11] V. G. Veselago, Sov. Phys. Usp. 1968, 10 , 509.

[12] J. B. Pendry , A. J. Holden , D. J. Robbins , W. J. Stewart , IEEE Trans. Microwave Theory Tech. 1999 , 47 , 2075.

[13] N. I. Landy , S. Sajuyigbe , J. J. Mock , D. R. Smith , W. J. Padilla , Phys. Rev. Lett. 2008 , 100 , 207402.

[14] X. Liu , T. Starr , A. F. Starr , W. J. Padilla , Phys. Rev. Lett. 2010 , 104 , 207403.

[15] Claire M. Watts , Xianliang Liu , and Willie J. Padilla. Metamaterial Electromagnetic Wave Absorbers, Adv. Mater. 2012. 24, OP98-OP120.

[16] I. Puscasu , W. L. Schaich , Appl. Phys. Lett. 2008 , 92 , 233102.

[17] Qu et al. Micromachines 2016, 7, 14; doi:10.3390 / mi7010014.

[18] T. Inoue, M. de Zoysa, T. Asano und S. Noda, Realization of narrowband thermal emission with optical nanostructures, Optica vol. 2, no. 2015.

[19] X: Liu and W. J. Padilla, Reconfigurable room temperature metamatenial infrared emitter, Optica, vol. 4, no. 4, 2017.

[20] EP3315929

[21] WO2017088071

[22] US2012170114

[23] US2017288125.

Claims

1. Photoacoustic spectroscope for analyzing gas, comprising - a modulatable infrared emitter (1) - an analysis volume that can be filled with gas and - a sound detector, wherein the analysis volume is arranged between the infrared emitter (1) and the sound detector, so that the infrared radiation emitted in a modulatable manner by the infrared emitter (1) can be used for photoacoustic spectroscopy of the gas and which is characterized in that the modulatable infrared emitter (1) comprises - a heating element (11) - a planar base element (9) made of a conductive material - a dielectric interlayer (7) - a planar cover element (5) made of a conductive material and - an actuator (3), wherein the cover element (5) is a structured metamaterial with periodically arranged unit cells and the actuator (3) is configured for relative movement of the cover element (5) and the base element (9) between a first and second position in order to modulate the intensity of the emission of the infrared emitter (1).

2. Photoacoustic spectroscope according to the preceding claim characterized in that the relative movement comprises a vertical translational movement of the cover element (5) and / or base element (9) along the emission direction of the infrared emitter (1), which changes the distance between the cover element (5) and the base element (9).

3. Photoacoustic spectroscope according to one of the preceding claims characterized in that the relative movement comprises a horizontal translational movement of the cover element (5) and / or base element (9) orthogonal to the emission direction of the infrared emitter (1), which changes the degree of overlap between the cover element (5) and the base element (9).

4. Photoacoustic spectroscope according to one or more of the preceding claims characterized in that the emissivity in the direction of the surface normal of the cover element (5) for at least one resonance wavelength, in a range of from 1 µm to 10 µm, is higher in the second position than in the first position by a factor of 2, preferably 4, more preferably 8, wherein it is preferred that the emissivity in the direction of the surface normal of the cover element (5) for at least one resonance wavelength in a range from 1 µm to 10 µm in the second position has a value of more than 0.7, preferably more than 0.8, 0.9, and in the first position, a value of less than 0.4, preferably less than 0.3, 0.2.

5. Photoacoustic spectroscope according to one or more of the preceding claims characterized in that the cover element (5) is made of a metal, particularly preferably gold, silver, aluminum, tungsten, molybdenum, titanium, and / or copper, and / or the base element (9) is a continuous conductive layer and is preferably made of the same conductive material as the cover element (5).

6. Photoacoustic spectroscope according to one or more of the preceding claims characterized in that the dielectric interlayer (7) is made of a material selected from the group consisting of aluminum nitride, silicon nitride, aluminum oxide, silicon oxide, titanium dioxide, and / or tantalum oxide.

7. Photoacoustic spectroscope according to one or more of the preceding claims characterized in that the infrared emitter (1) comprises at least four MEMS actuators, which are installed on the outer sides of the cover element (5) and are configured to control the relative movement of the cover element (5) and the base element (9) between the first and second position simultaneously.

8. Photoacoustic spectroscope according to one or more of the preceding claims characterized in that in the first position, the cover element (5) has a distance from the dielectric layer of at least 500 nm, preferably at least 1000 nm, and in the second position has a distance from the dielectric layer of at most 200 nm, preferably at most 50 nm, and particularly preferably 0 µm.

9. Photoacoustic spectroscope according to one or more of the preceding claims if dependent on claim 3, characterized in that in the first position, the cover element (5) and base element (9) have a degree of overlap of less than 40%, preferably less than 10%, and in the second position, the cover element (5) and base element (9) have a degree of overlap of more than 40%, preferably more than 10%, wherein the vertical distance between the cover element (5) and base element (9) in the second position is preferably less than 200 nm.

10. Photoacoustic spectroscope according to one or more of the preceding claims characterized in that the photoacoustic spectroscope comprises a control device, wherein the control device is configured to regulate the actuator for relative movement of the cover element (5) and the base element (9) between a first and second position in order to modulate the intensity of the emission of the infrared emitter (1).

11. Photoacoustic spectroscope according to one or more of the preceding claims characterized in that the control device is configured to regulate the actuator (3) for an oscillating relative movement of the cover element (5) and the base element (9) between a first and a second position, whereby a modulation frequency of the power intensity of the emitted infrared radiation is preferably achieved between 10 Hz and 100 kHz.

12. Photoacoustic spectroscope according to the previous claim characterized in that the sound detector is a microphone and the modulation frequency of the power intensity of the emitted infrared radiation is at least 1 kHz.