Method and apparatus for performing a measurement on a sample that depends on the excitation intensity of a light source

Abstract

Method for performing a measurement on a sample that depends on the excitation intensity of a light source, - wherein the light source is chosen such that it can generate light pulses; wherein the method comprises the following steps: - Irradiating the sample with a first plurality (300) of light pulses from the light source in a first time interval; - Performing the measurement on the sample; - at least one repetition of the irradiation and measurement with a second plurality (330) of light pulses from the light source in a second time interval; - where the first and second time intervals are of equal length; and - wherein a reduction in the excitation intensity of the light source is effected by omitting light pulses (320) in the second plural (330) of light pulses compared to the first plural (300) of light pulses or in the first plural of light pulses compared to the second plural of light pulses.

Description

Field of invention

[0001] Intensity-dependent measurements, such as of luminescence intensity or other spectroscopically observable parameters, are an important component in the characterization of materials, such as semiconductors, molecules, or nanoparticles, especially those exhibiting photon upconversion. Different sample excitation intensities allow the observation of various parameters and processes, such as charge carrier mobility, the number of defects, or—as in upconversion materials—different transitions and population states.

[0002] In the up-conversion of light, the up-conversion materials absorb at least two photons of a longer wavelength, for example from the visible or near-infrared range, and then emit one photon of a shorter wavelength, for example from the near-ultraviolet spectral range.

[0003] Up-conversion nanoparticles are used, for example, in 3D printers. The UV light emitted by the up-conversion nanoparticles after irradiation triggers photopolymerization to harden the materials used.

[0004] A precise characterization of the intensity dependence of light absorption and emission in these materials is crucial for many applications. This requires the highest possible measurement accuracy without interfering influences. State of the art

[0005] Intensity-dependent measurements can be generated in an obvious way by varying the intensity of the light source. However, this changes not only the intensity but also many other parameters and fundamental characteristics of the light source, such as the beam profile, pulse width, and wavelength of the emitted light. The measurement results therefore exhibit not only an intensity-dependent component but also a multitude of other components. These latter components must be filtered out in complex characterization steps, which generally leads to corresponding uncertainties and measurement errors.

[0006] With lasers, for example, the input power at the laser head can be varied using a suitable laser driver, such as by varying the applied current. However, this often results not only in changes to the laser's output power, but also in changes to laser-characteristic properties such as pulse width, pulse shape, and wavelength. Furthermore, changes in the laser's output power due to changes in the applied current are non-linear and can also result in power jumps.

[0007] It is well known that the power of an excitation light source is therefore often reduced by using attenuation filters. For example, a neutral density glass filter can be inserted into the excitation beam path. The excitation power is thereby reduced by a fixed value, usually by an order of magnitude, without altering the characteristics of the excitation light source. The disadvantage of this method is that the excitation power can only be reduced in fixed, predetermined steps, and no precise statements can be made about the actual attenuation. This is because, even with neutral density glass filters, the attenuation depends on the wavelength of the attenuated light. Therefore, the filter must be characterized again in elaborate series of measurements for each measurement.

[0008] German patent DE 10 2005 027 896 A1 describes a measuring arrangement with a pulsed laser for the optical measurement of a sample. The repetition rate of the pulsed laser is varied to optimize the photon yield from the sample. Task

[0009] The object of the invention is therefore to provide a teaching with which intensity-dependent measurements on samples can be reliably carried out. Solution

[0010] This problem is solved by the subject matter of the independent claims. Advantageous embodiments of the subject matter of the independent claims are identified in the dependent claims. The wording of all claims is hereby incorporated by reference into this description.

[0011] The use of the singular should not exclude the plural, and the same applies in reverse, unless otherwise stated.

[0012] The following section describes individual process steps in more detail. In a preferred embodiment of the invention, these steps are carried out in the specified order. However, the steps need not necessarily be performed in the specified order, and the process described may also include further, unmentioned steps.

[0013] To solve the problem, a method is proposed for performing a measurement on a sample that depends on the excitation intensity of a light source.

[0014] The proposed method first requires a light source capable of generating light pulses. Typically, and indeed particularly well-suited to the method, the light pulses should be identical in wavelength, energy, and duration, except for unavoidable variations. The emission wavelength of the light source should match an absorption band of the sample.

[0015] The sample is irradiated with light pulses from this light source, specifically with an initial plurality of light pulses from the light source within a first time interval. The light source can emit light pulses continuously or in individual pulse sequences followed by pauses. The first time interval is freely selectable; it can be defined by the length of a pulse sequence, but it can also be defined independently.

[0016] After or during irradiation of the sample, the light emitted by the sample is detected. This can be done by counting individual emitted photons. These photons can originate from typical emission processes such as fluorescence or phosphorescence.

[0017] The irradiation and measurement of the sample is repeated at least once with a second set of light pulses from the light source in a second time interval. The second time interval is also freely selectable, with the restriction that the first and second time intervals must be of equal length.

[0018] The first time interval can also occur after the second time interval. The terms "first" and "second" time interval do not express a chronological order.

[0019] To perform intensity-dependent measurements on the sample, the excitation intensity of the light source is varied. This is achieved by omitting light pulses in the second majority of pulses compared to the first majority, or in the first majority of pulses compared to the second majority. Omitting individual pulses in a pulse train can be accomplished in various ways, for example, by blocking or transmitting the beam using an acousto-optic modulator. Alternatively, a laser can be used that electronically generates each individual laser pulse; in such a case, individual pulses can be omitted electronically.

[0020] Instead of excitation intensity, one can equally speak of excitation power. Light intensity is defined as the light power per unit area. Since the excitation energy is typically focused onto the sample (with a specific focal cross-sectional area) or the sample is located within a sample volume that is irradiated alone, any excitation power can be converted into an excitation intensity and vice versa. The intensity-dependent measurements then become power-dependent measurements. A variation in the excitation power of x% leads to a proportionally equal variation in the excitation intensity if the focal cross-sectional area remains unchanged. Therefore, both terms will be used synonymously in the following discussion.

[0021] Omitting pulses leads to a significantly more predictable variation in excitation power than ◯ the use of neutral gas filters, ◯ that varying the laser power by reducing the power supply to the laser, ◯ the change in energy per laser pulse or ◯ the change in pulse width.

[0022] All these methods typically lead to a partially undefined excitation of the sample and thus to an unverifiable measurement signal, which will distort the intensity-dependent measurement.

[0023] The sample is therefore irradiated with a different number of essentially identical light pulses of the same energy in the first and second time intervals of equal length. With respect to the time interval, the sample is thus exposed to different irradiation intensities.

[0024] The proposed method is therefore particularly suitable for measuring nonlinear dependencies of the sample signal on the excitation intensity. However, it can also be used simply to vary the excitation power, e.g., to measure processes that depend linearly on the excitation power.

[0025] In many cases, one wants to observe the lifetime of states or the transition rates between states, such as the fluorescence or phosphorescence lifetime of specific excitation states of the sample, for example, a dye molecule or semiconductor. In these cases, a pulsed excitation followed by a pause is recommended. In such cases, the first majority of light pulses from the light source in the initial time interval defines an initial excitation pulse sequence of a predetermined duration. After the initial excitation pulse sequence, the sample is no longer irradiated with any further light pulses from the light source for the duration of a detection phase. The measurement of the sample is performed during the detection phase. The detection phase thus defines the period in which, for example, a long-lived sample signal can be measured.

[0026] The detection phase is followed by at least one repetition of the sequence consisting of the excitation pulse sequence and the detection phase. It is not uncommon for there to be several tens of thousands of repetitions of the initial excitation pulse sequence with its respective detection phase.

[0027] Once the sample has been measured at the excitation intensity in the first time interval, the same procedure is followed, mutatis mutandis, with a second set of light pulses in the same time interval, i.e., at a reduced excitation intensity. This can typically be repeated several tens of thousands of times. The result is two measurements at two different excitation intensities. The procedure can be continued analogously with further reduced excitation intensities by omitting additional light pulses.

[0028] When excitation is performed with such an excitation pulse sequence followed by a detection phase, it is logical to choose the duration of the first excitation pulse sequence as the initial time interval. The excitation power is then calculated using the following formula: Excitation power = Number of pulses * Energy per pulse / Duration of the first excitation pulse sequence

[0029] The excitation power is changed by altering the number of pulses in the respective excitation pulse sequence. The energy per pulse remains unchanged, as does the duration of the first excitation pulse sequence.

[0030] The considerations regarding the variation of the excitation power can be carried out analogously if one speaks not of the excitation power, but of the excitation energy hitting the sample per excitation pulse sequence. Excitation energy = number of pulses in the excitation pulse sequence * energy per pulse

[0031] Omitting individual light pulses leads to a reduction in the excitation energy acting on the sample.

[0032] The omission of light pulses can be specified, resulting in a similarly specified change in excitation without altering the characteristics of the excitation laser, causing power jumps, or resulting in excessively large, undefined steps in the power change. Accordingly, the excitation intensity only needs to be determined once for the sample. Subsequently, a change in excitation intensity can be achieved by modifying the excitation pulse sequence. In this way, specific excitation intensities can be set.

[0033] To measure the lifetime of the observed states of the sample as accurately as possible, the detection phase is adapted to the lifetime of the sample states. For example, to measure the lifetime of a state, the detection phase is chosen to be 3 to 10 times longer than the lifetime of that state. If the duration of the detection phase were made even longer, the measurement process could potentially take too long.

[0034] Furthermore, measurement accuracy increases if the excitation pulse sequence is as short as possible compared to both the lifetime of the sample state and the duration of the detection phase. To still excite the sample with sufficient maximum radiation energy, it is recommended that the duration of the detection phase be 1 to 3 times, 3 to 5 times, 5 to 10 times, 10 to 20 times, or 20 to 50 times the duration of the excitation pulse sequence, but preferably no longer. The excitation pulse sequence is correspondingly short compared to the detection phase. If the excitation pulse sequence is chosen to be too short, the power available for excitation generally decreases.

[0035] If the initial excitation pulse sequence consists of, for example, N light pulses, the excitation power can be varied with an accuracy of 1 / N. N can be between 10 and 100, or between 100 and 1,000, or between 1,000 and 10,000, or between 10,000 and 100,000, or between 100,000 and 1 million, or between 1 million and 10 million, depending on the application. In typical cases, N is equal to 1,000. The accuracy of the excitation power variation thus exceeds what can usually be achieved with alternative approaches.

[0036] The second set of light pulses from the light source in at least one second time interval defines a second excitation pulse sequence of a predetermined duration. There are various ways to reduce the number of light pulses in the second excitation pulse sequence compared to the number of light pulses in the first excitation pulse sequence. The simplest of these possibilities is to differentiate the second excitation pulse sequence from the first by omitting light pulses at the beginning and / or end.

[0037] Omitting light pulses at the beginning or end of the excitation pulse sequence simplifies many analyses of the recorded data, particularly when the first time interval consists of a sequence of the first excitation pulse sequence followed immediately by a first detection phase, and when the second time interval consists of a sequence of the second excitation pulse sequence followed immediately by a second detection phase. The first and second time intervals remain of equal length. That is, the sum of the duration of the excitation pulse sequence and the duration of the detection phase remains unchanged. Therefore, if the excitation pulse sequence is shortened, the detection phase is lengthened accordingly.

[0038] The excitation power then results, for example, from Excitation power = Number of pulses * Energy per pulse / Duration of the excitation pulse sequence + Duration of the detection phase where the denominator always remains constant.

[0039] If one wishes to maintain the duration of the second excitation pulse sequence compared to the first, it is advisable to achieve the desired reduction in the excitation intensity of the light source by omitting light pulses at regular intervals within the second set of pulses compared to the first set. For example, if a reduction of a fraction 1 / n is desired, one can omit every nth light pulse (which, apart from special mathematical cases and a generally acceptable error, also applies when N is not a multiple of n, especially as long as N is much larger than n).

[0040] Light sources that can emit light pulses with a repetition rate between 0.1 and 1 MHz, 1 and 10 MHz, 10 and 20 MHz, 20 and 50 MHz, 50 and 100 MHz, 100 and 200 MHz, or 200 and 2000 MHz are particularly suitable for the described method. Such light sources are generally known.

[0041] Another way to omit light pulses is to reduce the repetition rate of the light source's pulses in the second excitation pulse sequence compared to the repetition rate in the first excitation pulse sequence. This is achieved while maintaining the duration of the excitation pulse sequence and the energy per pulse. The reduced repetition rate means the sample is irradiated with fewer light pulses in the second excitation pulse sequence within the given duration. A suitable laser driver for this purpose is, for example, the Taiko PDL M1 from PicoQuanT GmbH, a company specializing in optoelectronic research and development, located at Rudower Chaussee 29, 12489 Berlin, Germany. See also the corresponding datasheet: https: / / www.picoquant.com / dl_datasheets / Taiko.pdf, last accessed on July 7, 2021.

[0042] Particularly advantageous for the method described here are light sources that can be electronically triggered to individually initiate each light pulse. Such light sources can omit individual pulses with appropriate electronic control. Programmable pulse generators are suitable for this purpose. These can achieve sampling rates up to the GHz range and therefore easily create pulse sequences with repetition rates of, for example, 100 MHz. A suitable programmable pulse generator for this purpose is, for example, the PPG 512 from the aforementioned company PicoQuanT GmbH. See also the corresponding datasheet https: / / www.picoquant.com / dl_datasheets / PPG512.pdf, last accessed on July 7, 2021.

[0043] The problem is further solved by a device set up to carry out the described procedure.

[0044] Such a device for performing a measurement on a sample that depends on the excitation intensity of a light source first requires a light source capable of generating light pulses. For the method to be practically feasible, the light source must be able to emit light pulses with a repetition rate between 0.1 and 1 MHz, or 1 and 10 MHz, or 10 and 20 MHz, or 20 and 50 MHz, or between 50 and 100 MHz, or between 100 and 200 MHz, or between 200 and 2000 MHz. Finally, the device requires a control system configured to execute the steps of the described method.

[0045] As a rule, the device requires a detector to detect light emitted by the sample.

[0046] Furthermore, the task is solved by a computer program comprising commands that cause the device just described to perform the process steps of the described procedure.

[0047] A computer-readable medium on which the computer program just described is stored also solves the task.

[0048] Further details and features will become apparent from the following description of a preferred embodiment in conjunction with the figures. The respective features can be implemented individually or in combination. The possibilities for solving the problem are not limited to this embodiment. For example, range specifications always include all intermediate values ​​(not explicitly stated) and all conceivable sub-intervals.

[0049] An exemplary embodiment is shown schematically in the figures. Identical reference numerals in the individual figures denote identical or functionally equivalent elements, or elements corresponding to each other with respect to their functions. Specifically, the figures show: Fig. Figure 1 shows a schematic representation of absorption, relaxation, energy transfer, and emission between different excitation levels in up-conversion nanoparticles doped with two lanthanides. However, this up-conversion process only occurs above an excitation intensity of at least 2.2 W / cm². 2 a; Fig. 2A an emission spectrum of thulium at an excitation intensity of approximately 2.2 W / cm² 2 ; Fig. 2B an emission spectrum of thulium at a second excitation intensity significantly above the value of 2.2 W / cm 2 ; Fig. 3A is an example of a repeating excitation pulse sequence, each followed by a detection phase at maximum excitation power; Fig. 3B the example according to Fig. 3A when individual excitation pulses are omitted at regular intervals within the excitation pulse sequence; Fig. 3C the example according to Fig. 3A when individual excitation pulses are omitted at the end of the excitation pulse sequence; Fig. 4A is an example of fluorescence that is linearly dependent on the excitation power; and Fig. 4B is an example of luminescence that is non-linearly dependent on the excitation power. Fig. 1

[0050] Fig. Figure 1 shows in the left part some excited states of a triply positively charged ytterbium atom (Yb). 3+ ) and in the right part some excited states of a triply positively charged thulium atom Tm 3+ , both in a NaYF4:Yb 3+ / Tm 3+Up-conversion nanoparticles.

[0051] The solid horizontal lines represent different energy levels in which an electron of the outermost shell (valence shell) can be located.

[0052] The solid arrow 100 represents the absorption of a long-wavelength photon with a wavelength of 980 nm by the ytterbium ion, thereby removing an electron from the outermost shell. 2 F 7 / 2 level to the 2 F 5 / 2 The level is raised. The dashed arrow 110 represents the emission of a photon of the same wavelength upon returning to the ground state. 2 F 7 / 2 .

[0053] The same applies to the triply positively ionized thulium atom Tm 3+Here, the upward-pointing arrows indicate the absorption of energy, while the downward-pointing arrows predominantly indicate the emission of a photon. The short, downward-pointing, dashed arrows 120 represent a (vibrationally induced) relaxation from a slightly higher level to a slightly lower level. At the right edge of the figure are the different energy levels for electrons in the valence shell of Tm. 3+ The four arrows on the right, which represent emissions, indicate the wavelengths that belong to these emissions.

[0054] One can recognize in Fig. 1, that for the emission of a photon with one of the wavelengths specified in the right-hand part, one of the levels 1 G4, 1 D2 or 1 I6 must have been occupied. None of these levels can be reached by the absorption of a single long-wavelength photon.

[0055] The process for reaching the higher energy levels of the thulium ion is essentially as follows: The laser excites the ytterbium ion with light at a wavelength of 980 nm. This ytterbium ion transfers its energy to the thulium ion via non-radiative energy transfer, designated 1st. This process can be repeated after the same ytterbium ion or a different ytterbium ion has been (again) excited by light at a wavelength of 980 nm. These further possible non-radiative energy transfers are described in Fig. 1 with the reference numbers 2nd and 3rd. This allows the H, F, or G energy levels of the thulium ion to be reached. The transition from 1 The G4 energy level can be raised to the thulium ground state by emission of a photon with a wavelength of 476 nm.

[0056] Furthermore, the non-radiative energy transfers 130 occurring between different thulium ions, symbolized by the partly dashed and partly solid diamonds, can also lead to the attainment of energy level D. On the one hand, photons with wavelengths of 450 or 362 nm can be emitted from this energy level. On the other hand, a Fig. 1. Further energy transfer from the ytterbium ion, designated as 4th, to a P or P. 1 The I6 energy level of the thulium ion is reached. From the 1 The I6 energy level can be transformed into a transition to the I6 energy level by the emission of a photon with a wavelength of 345 nm. 3 F4 level of the thulium ion.

[0057] The in Fig. The ions shown in Figure 1 allow photons of longer wavelength and lower energy to be converted into photons of shorter wavelength and higher energy through multiple absorption. This process is called up-conversion, i.e., the conversion of long-wavelength photons of lower energy into photons of shorter wavelength and higher energy. Fig. 2A

[0058] Fig. 2A shows the luminescence spectrum of Tm 3+ The number of photons counted during the measurement period, depending on the wavelength, was determined by excitation with 11 mW of excitation power (measured as continuous power). The excitation wavelength was 975 nm. The focus diameter was approximately 100 µm. The focus cross-sectional area was therefore approximately 8,000 µm². The excitation intensity was thus approximately 140 W / cm². 2 (measured as continuous intensity).

[0059] The pulse length was 80 ps at a repetition rate of 80 MHz. This corresponds to an energy of approximately 135 pJ (10^-12 joules) per pulse. The LDH-PC-980 diode laser from PicoQuanT GmbH was used as the light source. This diode laser triggers each pulse individually, i.e., independently of the other pulses, using its control electronics.

[0060] The emission band of Tm 3+ The emission band at 450 nm is barely visible. However, the emission band at 476 nm is discernible. Fig. 2B

[0061] Fig. 2B shows the luminescence spectrum of Tm 3+The results were again expressed as a count of photons depending on the wavelength, after excitation with 402 mW of excitation power (measured as continuous power). The excitation wavelength was 980 nm. The pulse duration was 5800 ps at a repetition rate of 80 MHz. This corresponds to an energy of approximately 5 nJ (10⁻⁹ joules) per pulse. The LDH-PC-980MB diode laser from the aforementioned company PicoQuanT was used as the light source. This diode laser also triggers each individual pulse using its control electronics.

[0062] In this opposite Fig. At approximately 2A, with an excitation power about 40 times higher, multiphoton absorption occurs. Consequently, the higher energy levels of the valence electrons of Tm are absorbed. 3+ Occupied, and corresponding emissions occur. The emission bands at 362 nm, 450 nm, and 476 nm are clearly visible. The signal, i.e., the number of photons counted, is compared to Fig. 2A is about 3 orders of magnitude stronger. Fig. 3A

[0063] Fig. Figure 3A schematically shows an initial excitation pulse sequence 300 followed by a detection phase 310 in which no further light pulses are emitted. The pattern is repeated several times depending on the application.

[0064] The example shown has 18 pulses in the excitation pulse sequence 300. If, for example, the LDH-PC-980 light source from the aforementioned company PicoQuanT is used with a repetition rate of 80 MHz, this corresponds to a pulse interval of 12.5 ns. The excitation pulse sequence 300 then lasts approximately 18 × 12.5 ns = 225 ns. If, for example, the detection phase 310 is chosen to be ten times as long as the excitation pulse sequence 300, the resulting detection phase is 2.25 µs. Together with the duration of the excitation pulse sequence 300, this results in a total duration of the repeating cycle of excitation pulse sequence and detection phase of 2.5 µs. This equates to 400,000 cycles per second. For example, if one wanted to measure for 10 seconds to obtain a sufficiently noise-free measurement signal, this would correspond to 4 million cycles of excitation pulse sequence 300 and detection phase 310. Fig. 3B

[0065] Fig. 3B shows the same cycle as in Fig. 3A, however, every sixth light pulse 320 was omitted during this second excitation pulse sequence 330, thus reducing the excitation power by 17%. Each excitation pulse sequence 330 is followed by a detection phase 340.

[0066] All other parameters of this example should be replaced with the parameters of the example according to Fig. 3A must match so that the difference between the two measurements lies solely in the omitted light pulses 320 and thus in the reduced excitation power. The measurement with the reduced excitation power using the second excitation pulse sequence 330 can, for example, also be performed using 4 million cycles. Fig. 3C

[0067] Fig. 3C shows the same cycle as in Fig. 3A, however, light pulses were omitted at the end of the excitation pulse sequence 350. This shortened the excitation pulse sequence by a time x. The subsequent detection phase 360 ​​was lengthened by the same time x, so that the sum of the durations of the excitation pulse sequence 350 and the detection phase 360 ​​remained unchanged.

[0068] All other parameters of this example should be replaced with the parameters of the example according to Fig. 3A must match so that the difference between the two measurements lies solely in the omitted light pulses and thus in the reduced excitation power. The measurement with the reduced excitation power using the shortened excitation pulse sequence 350 can, for example, also be performed using 4 million cycles. Fig. 4A

[0069] Fig. Figure 4A shows the fluorescence intensity (normalized to 100%) as a function of the excitation power (also normalized to 100%). The example shown is the fluorescence intensity of the well-known dye Coumarin 6 in spectroscopic EtOH, which was excited at 451 nm using the LDH-PC-450 diode laser from the aforementioned company PicoQuanT with a repetition rate of 40 MHz and a pulse width of 60 ps. 100% of the excitation power corresponds to a continuous diode laser power of 10 mW. This corresponds to an energy of 250 pJ per pulse.

[0070] The excitation laser power was subsequently reduced successively, predominantly in 10 percentage point increments, by omitting light pulses. A linear dependence of the fluorescence intensity on the excitation power is observed. Fig. 4B

[0071] Fig. Figure 4B shows the luminescence intensity (again normalized to 100%) as a function of the excitation power (also normalized to 100%). The figure illustrates the luminescence intensity of up-conversion nanoparticles (UCNPs) in spectroscopic EtOH, which were excited at 980 nm using the LDH-PC-980MB diode laser from PicoQuanT at a repetition rate of 80 MHz and a pulse width of 5800 ps. 100% of the excitation power corresponds to a continuous diode laser power of 402 mW. This corresponds to an energy of 5 nJ per pulse. Measurements were taken of both the 450 nm and 474 nm emission bands.

[0072] The power of the excitation laser was subsequently reduced successively by omitting light pulses, predominantly in steps of approximately 12 percentage points. For comparison, in Fig. Figure 4B also shows a linear dependence on the excitation power.

[0073] A non-linear dependence of the UCNP luminescence intensity on the excitation power is observed. This non-linearity is even more pronounced in the 450 nm band than in the 474 nm band. Glossary Stimulus service

[0074] Excitation power refers to the light power with which the sample is irradiated. A portion of this power is absorbed by the sample. In a broader sense, excitation power is the power emitted by a light source used to irradiate the sample. In a narrower sense, excitation power is the light power that reaches the sample. Light

[0075] Light is a form of electromagnetic radiation. In a narrower sense, this refers only to the portions of the entire electromagnetic spectrum visible to the human eye. In a broader sense, electromagnetic waves of shorter wavelengths (ultraviolet) and longer wavelengths (infrared) are also included (see the German Wikipedia entry "Licht"). For the present patent application, wavelengths between 100 nm and 2.5 µm are typically relevant. light source

[0076] A light source is a device for generating light. In connection with the present invention, lasers, in particular diode lasers, are typically considered as light sources. Luminescence, fluorescence, phosphorescence

[0077] In luminescence, a physical system is excited to a higher state by externally supplied energy and emits light (including radiation outside the visible spectrum) as it transitions to its ground state by releasing photons. The term luminescence refers either to the process (the phenomenon) or to the emitted radiation. If the emission begins immediately after the absorption of energy, it is called fluorescence. If an excited intermediate state can "freeze" the energy for a certain period of time, it is called phosphorescence. (See the German Wikipedia entry "Lumineszenz") Reference sign 100 Absorption of a long-wavelength photon 110 Emission of a long-wavelength photon 120 Relaxation 130 energy transfers between thulium ions 1st Energy transfer between a ytterbium and a thulium ion 2nd energy transfer between a ytterbium and a thulium ion 3. Energy transfer between a ytterbium and a thulium ion 4th Energy transfer between a ytterbium and a thulium ion 300 first excitation pulse sequence 310 Detection phase 320 omitted light pulses 330 second excitation pulse sequence 340 Detection phase 350 excitation pulse sequence 360° detection phase

Claims

[1] Method for performing a measurement on a sample that depends on the excitation intensity of a light source, - wherein the light source is chosen such that it can generate light pulses; wherein the method comprises the following steps: - Irradiating the sample with a first plurality (300) of light pulses from the light source in a first time interval; - Performing the measurement on the sample; - at least one repetition of the irradiation and measurement with a second plurality (330) of light pulses from the light source in a second time interval; - where the first and second time intervals are of equal length; and - wherein a reduction in the excitation intensity of the light source is effected by omitting light pulses (320) in the second plural (330) of light pulses compared to the first plural (300) of light pulses or in the first plural of light pulses compared to the second plural of light pulses. [2] Method according to the preceding claim, characterized by , - that the first plurality of light pulses from the light source in the first time interval defines a first excitation pulse sequence (300) of a predetermined duration; - that after the first excitation pulse sequence, the sample is no longer irradiated with any further light pulses from the light source for the duration of a detection phase (310); and - that the measurement on the sample is carried out during the detection phase (310); and - that the detection phase (310) is followed by at least one repetition of the sequence of excitation pulse sequence (300) and detection phase (310). [3] Method according to the preceding claim, characterized by , that the duration of the detection phase (310) is 1 to 3 times or 3 to 5 times or 5 to 10 times or 10 to 20 times or 20 to 50 times the duration of the excitation pulse sequence (300). [4] Method according to claim 2 or 3, characterized by , that a number of light pulses in the excitation pulse sequence (300) is between 10 and 100 or between 100 and 1,000 or between 1,000 and 10,000 or between 10,000 and 100,000 or between 100,000 and 1 million or between 1 million and 10 million. [5] Method according to any one of claims 2 to 4, characterized by , - that the second plurality of light pulses from the light source in the second time interval defines a second excitation pulse sequence (330) of a predetermined duration; - that the second excitation pulse sequence (330) differs from the first excitation pulse sequence (300) in that light pulses are omitted at the beginning and / or at the end. [6] Method according to the immediately preceding claim, characterized by , - that the first time interval is formed from a sequence of first excitation pulse sequence (300) and an immediately following first detection phase (310); - that the second time interval is formed from a sequence of a second excitation pulse sequence (330) and an immediately following second detection phase (340); and - that the duration of the first and the duration of the second time interval are equal. [7] Method according to any one of the preceding claims, characterized by , that the reduction of the excitation intensity of the light source is caused by omitting light pulses (320) at regular intervals within the second majority (330) of light pulses compared to the first majority (300) of light pulses. [8] Method according to any one of the preceding claims, characterized by , that the light source emits light pulses with a repetition rate between 0.1 and 1 MHz or 1 and 10 MHz or between 10 and 20 MHz or between 20 and 50 MHz or between 50 and 100 MHz or between 100 and 200 MHz or between 200 and 2000 MHz. [9] Method according to any one of the preceding claims, characterized by , that the omission of light pulses is caused by a reduction in the repetition rate of the light source. [10] Method according to any one of the preceding claims, characterized by, that the light source is chosen in such a way that it can trigger each individual light pulse individually. [11] Apparatus set up for carrying out the method according to any one of claims 1 to 10. [12] Device for performing a measurement on a sample that depends on the excitation intensity of a light source, the device comprising the following components: - the light source, wherein the light source is designed in such a way that it generates light pulses; - wherein the light source is configured to emit light pulses with a repetition rate between 0.1 and 1 MHz or 1 and 10 MHz or 10 and 20 MHz or 20 and 50 MHz or between 50 and 100 MHz or between 100 and 200 MHz or between 200 and 2000 MHz; and - a control system configured such that the device performs the steps of the method according to any one of claims 1 to 10. [13] Device according to the immediately preceding claim, characterized by a detector for detecting light emitted by the sample. [14] Computer program comprising instructions that cause the device according to one of claims 11 to 13 to perform the method steps according to one of claims 1 to 10. [15] Computer-readable medium on which the computer program according to the immediately preceding claim is stored.

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