Angle-resolved luminescence spectroscopy system and method

Abstract

According to various embodiments, an angle-resolved luminescence spectroscopy system is provided. The system includes an optical arrangement including a first reflective member configured to focus a laser beam for exciting a sample and collect an electromagnetic radiation beam emitted from the sample upon excitation; and a second reflective member configured to correct non-concentricity in the beam; a collimator configured to collimate the beam; a stage module configured to receive the collimated beam; and an interferometry module in optical communication with the stage module. The stage module includes a lens arrangement configured to convert radial coordinates of the collimated beam into axial coordinates; and a translation stage operable to adjust the lens arrangement for repeatably selecting the polar angles to be transmitted towards the interferometry module. According to further embodiments, an angle-resolved luminescence spectroscopy method, and an optical arrangement and a stage module for facilitating angle-resolved luminescence spectroscopy are also provided.

Description

ANGLE-RESOLVED LUMINESCENCE SPECTROSCOPY SYSTEM AND METHODCross-Reference To Related Application

[0001] This application claims the benefit of priority of Singapore patent application No. 10202500134X, filed 16 January 2025, the content of it being hereby incorporated by reference in its entirety for all purposes.Technical Field

[0002] Various embodiments relate to an angle-resolved luminescence spectroscopy system and an angle-resolved luminescence spectroscopy method. Various embodiments also relate to an optical arrangement and a stage module for facilitating angle-resolved luminescence spectroscopy.Background

[0003] Low band gap semiconductors are of strategic interest because of applications for emission and detection of infrared light. Even today, known mid-infrared (MIR) semiconductor materials are significantly rare, and so there is an active hunt for novel MIR semiconductors, e.g. two-dimensional semiconductors, and study their optoelectronic properties.

[0004] K-space-resolved luminescence spectroscopy (KLS) is a measurement technique that combines luminescence spectroscopy with K-space or momentum-space mapping to provide a view of the distribution of luminescent states in momentum space.

[0005] It is commonly used in, e.g. the characterization of the band structure of photonic crystals, photoexcited states and coupled systems of both. Rather than measuring a real-space hyperspectral image in (x, y, A) space, KLS instead measures a K- (or momentum-, or Fourier-) space dispersion in (fcr, fcy, A) space. The data thus obtained is the photonic equivalent of the ubiquitous electronic band structure for solids.

[0006] The KLS technique essentially involves selecting a small range of emission angles out of the whole emission cone and characterizing that selection with a spectrometer. Home-built setups for microscopic KLS in the visible and near-infrared (NIR) regions have been reported from many labs worldwide.

[0007] However, an equivalent capability in the mid-infrared (MIR) region has not been reported, so the application of low bandgap semiconductors in the above-mentioned areas lags. The lack of microscopic KLS in MIR may be due to the challenges associated with MIR optics. Obtaining KLS data for MIR semiconductors (in other words, microscopic mid-infrared K-resolved luminescence spectroscopy (MIRKLS)) presents at least the following challenges:• Microscopic samples'. Newly synthesized solid samples and photonic crystal / cavity structures are often microscopic in size, so an optical microscope of some form is required. Meanwhile, commercial Fourier transform infrared spectroscopy (FTIR) instruments and variable-angle ‘add-ons’ are limited to large samples and reflectance measurements only.• Visible-to-MIR broadband achromaticity. The microscope must handle visible light for sample imaging and possible photoexcitation, while simultaneously transmitting the MIR luminescence signal for spectroscopy.• No obscuration'. Broadband achromatic objectives are commercially available and are based on the reflective Schwarzschild / Cassegrain design. However, this design features a central obscuration that blocks the central emission cone from the sample. For KLS, this implies an unacceptable gap in the data. Therefore, a different approach to MIR luminescence collection is required.• Low luminescence. MIR luminescence is typically very weak and the detectivity of available MIR detectors is relatively poor, so signal-to-noise ratio is usually poor. A detection scheme to improve signal strength is required.• Angle resolution'. The essence of KLS is emission cone angle resolution. A solution for a photon-efficient method of selecting a cone angle for MIR does not exist. For example, FTIR instruments and variable-angle ‘add-ons’ are unable to measure angles near zero degree.

[0008] Due to the combination of the abovementioned challenges, a microscope for mid-IR KLS (MIRKLS) did not apparently exist in prior art.

[0009] Thus, there is a need for a mid-infrared K-space luminescence spectroscopy microscope system that can measure angle dependent mid-infrared spectra (particularly, with wavelength ranging from about 3-15 gm) from microscopic samples, involving applications in e.g. thermal radiation, chemical fingerprinting and gas / atmosphere monitoring, while addressing at least the problems discussed above.Summary

[0010] According to an embodiment, an angle-resolved luminescence spectroscopy system is provided. The system includes an optical arrangement including a first reflective member configured to focus a laser beam for exciting a sample and collect an electromagnetic radiation beam emitted from the sample upon excitation; and a second reflective member configured to correct non-concentricity in the electromagnetic radiation beam, the non-concentricity being caused by the first reflective member; a collimator configured to collimate the electromagnetic radiation beam from the optical arrangement; a stage module configured to receive the collimated electromagnetic radiation beam from the collimator; and an interferometry module in optical communication with the stage module. The stage module includes a lens arrangement configured to convert radial coordinates of the collimated electromagnetic radiation beam into axial coordinates, wherein the radial coordinates are to be converted from polar angles of the electromagnetic radiation beam emitted from the sample upon excitation, and the axial coordinates correspond to positions along an optical axis of the collimated electromagnetic radiation beam; and a translation stage operable to adjust the lens arrangement for repeatably selecting the polar angles to be transmitted towards the interferometry module. The interferometry module is configured to measure a luminescence spectrum of the sample based on at least the selected polar angles.

[0011] According to an embodiment, an optical arrangement for facilitating angle-resolved luminescence spectroscopy is provided. The optical arrangement includes a first reflective member configured to focus a laser beam for exciting a sample and collect anelectromagnetic radiation beam emitted from the sample upon excitation; and a second reflective member configured to correct non-concentricity in the electromagnetic radiation beam, the non-concentricity being caused by the first reflective member.

[0012] According to an embodiment, a stage module for facilitating angle-resolved luminescence spectroscopy is provided. The stage module includes a lens arrangement configured to convert radial coordinates of a collimated electromagnetic radiation beam into axial coordinates, wherein the radial coordinates are to be converted from polar angles of an electromagnetic radiation source from which the collimated electromagnetic radiation beam originated, and the axial coordinates correspond to positions along an optical axis of the collimated electromagnetic radiation beam; and a translation stage operable to adjust the lens arrangement for repeatably selecting the polar angles to be transmitted towards an interferometry module.

[0013] According to an embodiment, an angle-resolved luminescence spectroscopy method is provided. The method includes via a first reflective member, focusing a laser beam to excite a sample and collecting an electromagnetic radiation beam emitted from the excited sample; correcting, by a second reflective member, non-concentricity in the electromagnetic radiation beam, the non-concentricity being caused by the first reflective member; collimating the electromagnetic radiation beam; converting radial coordinates of the collimated electromagnetic radiation beam into axial coordinates, wherein the radial coordinates are converted from polar angles of the electromagnetic radiation beam emitted from the excited sample, and the axial coordinates correspond to positions along an optical axis of the collimated electromagnetic radiation beam; selecting the polar angles; and measuring a luminescence spectrum of the sample based on at least the selected polar angles.Brief Description of the Drawings

[0014] In the drawings, like reference characters generally refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description,various embodiments of the invention are described with reference to the following drawings, in which:

[0015] FIG. 1 shows a schematic view of an angle-resolved luminescence spectroscopy system, according to various embodiments.

[0016] FIG. 2 shows a flow chart illustrating an angle-resolved luminescence spectroscopy method, according to various embodiments.

[0017] FIG. 3 shows a schematic view illustrating an emission cone from the photoluminescence of a sample that is pumped by a focused laser spot, according to a prior art example.

[0018] FIG. 4 shows a schematic view of the MIRKLS optical design, according to an example.

[0019] FIG. 5 shows schematic illustrations exemplifying the commonly reported back focal plan technique for angle-resolved spectroscopy (left) and a K space circle (right), according to a prior art example.

[0020] FIG. 6 shows a schematic view of point luminescence illustrating the main purpose of MIRKLS to measure the luminescence spectra for different polar angles, according to an example.

[0021] FIG. 7 shows a simplified schematic view of the MIRKLS angle resolution stage with rays from a collimated beam, illustrating how polar angles are isolated, according to an example.

[0022] FIG. 8 shows a simplified schematic view of a ‘pizza slice’ aperture, according to an example.

[0023] FIG. 9 shows a photograph of an off-axis parabolic mirror (OAPM) objective overhanging a breadboard, according to an example.

[0024] FIG. 10 shows (left) a plot illustrating calculated polar angle contours in the luminescence beam profile as collected by the OAPM objective; and (right) a plot illustrating after correction by a second identical OAPM and re-collimation by a Ge aspheric lens.

[0025] FIG. 11 shows a simplified line schematic view of the angle-resolution stage, including thermal images of MIR beam as different angles are selected, according to various examples.

[0026] FIG. 12 shows a schematic view of an alternative design for an angle-resolution stage based on three axicons, according to an example.

[0027] FIG. 13 shows an image of a 20 lines / mm Ronchi ruling as captured by the MIRKLS white light microscope, and the same image after software curvature correction, according to an example.

[0028] FIG. 14A shows a plot illustrating polar angle dependent MIR emission spectra from a quantum cascade laser chip (nominal wavelength 7.95 pm), operated just below lasing threshold, according to an example.

[0029] FIG. 14B shows normalized spectra of FIG. 14A.Detailed Description

[0030] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0031] Embodiments described in the context of one of the methods or devices are analogously valid for the other methods or devices. Similarly, embodiments described in the context of a method are analogously valid for a device, and vice versa.

[0032] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0033] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0034] In the context of various embodiments, the phrase “at least substantially” may include “exactly” and a reasonable variance.

[0035] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

[0036] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0037] As used herein, the phrase of the form of “at least one of A or B” may include A or B or both A and B. Correspondingly, the phrase of the form of “at least one of A or B or C”, or including further listed items, may include any and all combinations of one or more of the associated listed items.

[0038] As used herein, the expression “configured to” may mean “constructed to” or “arranged to”.

[0039] Various embodiments provide a mid-infrared K-space luminescence spectroscopy microscope system. The system may be designed to measure the K-space resolved luminescence spectroscopy in mid-infrared range with microscopic capability for small samples. The system may be capable of performing one or more of the followings: image and locate microscopic samples like a regular microscope, focus an excitation laser beam onto the microscopic samples, measure the mid-infrared luminescence spectra from the microscopic samples, and isolate different emission angles to obtain luminescence spectra that cover a map in K-space.

[0040] FIG. 1 shows a schematic view / representation of an angle-resolved luminescence spectroscopy system 100, according to various embodiments. The system 100 includes an optical arrangement 102 including a first reflective member 104 configured to focus a laser beam for exciting a sample and collect an electromagnetic radiation beam emitted from the sample upon excitation; and a second reflective member 106 configured to correct nonconcentricity in the electromagnetic radiation beam, the non-concentricity being caused by the first reflective member 104. The first reflective member 104 and the second reflective member 106 are in optical communication with each other, as denoted by line 120. The system 100 further includes a collimator 108 configured to collimate the electromagnetic radiation beam from the optical arrangement 102; a stage module 110 configured to receive the collimated electromagnetic radiation beam from the collimator 108; and aninterferometry module 116 in optical communication with the stage module 110, as denoted by line 128. The collimator 108 is in optical communication with the optical arrangement 102 and the stage module 110, respectively denoted by lines 122, 124. The stage module 110 includes a lens arrangement 112 configured to convert radial coordinates of the collimated electromagnetic radiation beam into axial coordinates, wherein the radial coordinates are to be converted from polar angles of the electromagnetic radiation beam emitted from the sample upon excitation, and the axial coordinates correspond to positions along an optical axis of the collimated electromagnetic radiation beam; and a translation stage 114 operable to adjust the lens arrangement 112 for repeatably selecting the polar angles to be transmitted towards the interferometry module 116. The translation stage 114 is in communication with the lens arrangement 112, as denoted by line 126 The interferometry module 116 is configured to measure a luminescence spectrum of the sample based on at least the selected polar angles.

[0041] In other words, with the system 100, by mapping radial coordinates to axial coordinates (which are values along a z-axis that is substantially parallel to the optical axis of the collimated electromagnetic radiation beam), a particular polar angle (or radius) may be selected in a mechanically convenient manner, e g. by a pinhole along the z-axis, and a translation stage that moves in the z-direction. With this radial-to-axial mapping, the effects of an adjustable annular aperture may be realized. As appreciated, such adjustable annular aperture is not a readily available component and, in fact, may not even exist in the current market.

[0042] In the context of various embodiments, the expression “correct non-concentricity” may refer to perform adjustments to achieve concentricity.

[0043] In various embodiments, the collimator 108 may include an aspheric lens. The aspheric lens may be any aspheric collimating lens that is transparent to the radiation to be measured. For example, the collimator may include a Ge aspheric lens.

[0044] The interferometry module 116 may include an interferometer configured to perform interference on the selected polar angles to generate a resultant beam, and a detector configured to detect the resultant beam. The resultant beam may be insensitive to deviations caused by the translation stage 114. Generally, the detector may be a photodetector that is responsive to the electromagnetic radiation beam emitted from thesample. For example, the detector may include a mercury cadmium telluride (HgCdTe) photovoltaic detector.

[0045] The phrase “insensitive to” may mean independent of, or unaffected by.

[0046] The interferometry module 116 may further include a lock-in amplifier coupled to the detector, the lock-in amplifier configured to reduce noise in an electrical signal based on the detected resultant beam. A value of the noise-reduced (cleaned) electrical signal at the lock-in amplifier may then be logged into a computer. As this signal value varies with the interferometer optical path difference, a plurality of signal values may be collected and the computer may generate a plot of the signal value vs optical path difference, and this would be an interference pattern. The computer may then calculate the luminescence spectrum from the interference pattern.

[0047] As an alternative, the interferometry module 116 may include two interferometers. The first interferometer may be for the luminescence beam from the sample, and this beam may be selected for polar angle. The second interferometer may be for a reference laser of known wavelength. The interference fringes of a reference beam from the reference laser may inform the actual position of both interferometers.

[0048] In various embodiments, the system 100 may further include an excitation source configured to generate the laser beam.

[0049] The system 100 may further include a light source in optical communication with the first reflective member 104. The light source may be configured to emit light to the sample. For example, the light source may include a white light emitting diode.

[0050] Various embodiments provide an optical arrangement (e g. 102) for facilitating angle-resolved luminescence spectroscopy.

[0051] For the system 100 and / or the optical arrangement 102, the first reflective member 104 may be further configured to focus light from a light source to illuminate the sample and collect the light reflected from the sample. The optical arrangement 105 may further include a third reflective member configured to reduce image distortion in the light reflected from the sample and relay a virtual image of the sample to a microscope objective, the image distortion being caused by the first reflective member 104.

[0052] The first reflective member 104 and the second reflective member 106 may include identical or substantially identical off-axis parabolic mirrors.

[0053] The third reflective member may include an off-axis parabolic mirror that is identical or substantially identical to the first reflective member 104 and / or the second reflective member 106.

[0054] Various embodiments provide a stage module (e g. 110) for facilitating angle-resolved luminescence spectroscopy.

[0055] For the system 100 and / or the stage module 102, the lens arrangement 112 may include at least one axicon lens configured to transform the collimated electromagnetic radiation beam into a distribution of concentric rings, each concentric ring being associated with a radial coordinate that corresponds to a polar angle. The lens arrangement 112 may further include an aperture arranged along the optical axis. The aperture and the at least one axicon lens may be configured to move relative to each other to allow a selected polar angle to be transmitted through the aperture towards the interferometry module 116.

[0056] The lens arrangement 112 may further include a recollimating axicon lens configured to collimate the selected polar angle. The aperture may be arranged between the at least one axicon lens and the recollimating axicon lens.

[0057] The lens arrangement 112 may further include a focusing element configured to converge the collimated polar angle to a single point towards the interferometry module 116. The recollimating axicon lens may be arranged between the aperture and the focusing element.

[0058] For example, the aperture may include a pinhole or a fibre tip. The aperture may be 1 mm or less to transmit one polar angle.

[0059] The translation stage 114 may include a motorized translation stage.

[0060] In various embodiments, the system 100 may further include a slit aperture configured to rotate about the optical axis for selecting an azimuthal angle of the electromagnetic radiation beam emitted from the sample upon excitation. The slit aperture may extend radially along a cross-sectional plane substantially perpendicular to the optical axis.

[0061] In various embodiments, the stage module 110 may further include a slit aperture configured to rotate about the optical axis for selecting an azimuthal angle of the electromagnetic radiation source. The slit aperture may extend radially along a cross-sectional plane substantially perpendicular to the optical axis.

[0062] The azimuthal angle to be selected may be from 0° to 360°.

[0063] The slit aperture may have a shape of a circular sector. For example, the circular sector may cover an angle of 5° to 20°. It should be appreciated that small angles give higher resolution at the expense of signal strength.

[0064] FIG. 2 shows a flow chart illustrating an angle-resolved luminescence spectroscopy method 200, according to various embodiments. As seen in FIG. 2, at Step 204, a laser beam is focused, via a first reflective member (e.g. 104 of FIG. 1), to excite a sample and an electromagnetic radiation beam emitted from the excited sample is collected. At Step 206, non-concentricity in the electromagnetic radiation beam is corrected by a second reflective member (e.g. 106 of FIG. 1), the non-concentricity being caused by the first reflective member 104. At Step 208, the electromagnetic radiation beam is collimated. At Step 212, radial coordinates of the collimated electromagnetic radiation beam are converted into axial coordinates. The radial coordinates are converted from polar angles of the electromagnetic radiation beam emitted from the excited sample, and the axial coordinates correspond to positions along an optical axis of the collimated electromagnetic radiation beam. At Step 214, the polar angles are selected. At Step 216, a luminescence spectrum of the sample is measured based on at least the selected polar angles.

[0065] For example, the selected polar angles may range from 0° to 37.1°.

[0066] The method 200 may involve the same or like elements or components as those of the angle- resolved luminescence spectroscopy system 100 of FIG. 1, and as such, the same numerals are assigned and the like elements may be as described in the context of the angle-resolved luminescence spectroscopy system 100 of FIG. 1 , and therefore the corresponding descriptions are omitted here.

[0067] In various embodiments, measuring the luminescence spectrum of the sample at Step 216 may include performing interference on the selected polar angles to generate a resultant beam; and detecting the resultant beam. The resultant beam may be insensitive to deviations caused when selecting the polar angles.

[0068] The method 200 may further include determining an interference pattern based on the resultant beam, and extracting the luminescence spectrum from the interference pattern.

[0069] The method 200 may further include via the first reflective member 104, focusing light to illuminate the sample and collect the light reflected from the sample; via a thirdreflective member, reducing image distortion in the light reflected from the sample, the image distortion being caused by the first reflective member 104 and via the third reflective member, relaying a virtual image of the sample based on the light reflected from the sample to a microscope objective.

[0070] Converting the radial coordinates of the collimated electromagnetic radiation beam into axial coordinates at Step 212 may include transforming, by at least one axicon lens, the collimated electromagnetic radiation beam into a distribution of concentric rings, each concentric ring being associated with a radial coordinate that corresponds to a polar angle

[0071] Selecting the polar angles at Step 214 may include: providing an aperture arranged along the optical axis; and performing relative movement between the aperture and the at least one axicon lens to allow a selected polar angle to be transmitted through the aperture.

[0072] In various embodiments, the method 200 may further include collimating the selected polar angle; and converging the collimated polar angle to a single point towards an interferometry module (e.g. 116 of FIG. 1).

[0073] The method 200 may further include rotating a slit aperture about the optical axis to select an azimuthal angle of the electromagnetic radiation beam emitted from the excited sample. The slit aperture may extend radially along a cross-sectional plane substantially perpendicular to the optical axis.

[0074] The electromagnetic radiation beam may include a mid-infrared (MIR) beam. The luminescence spectrum may include a MIR spectrum, e.g. medium wavelength infrared (MWIR) or long wavelength infrared (LWIR).

[0075] The method 200 may include a K-space-resolved luminescence spectroscopy method.

[0076] The sample may include a microscopic-sized sample. For example, the dimension of the microscopic-sized sample may range from 10 pm to 100 pm.

[0077] While the method described above is illustrated and described as a series of steps or events, it will be appreciated that any ordering of such steps or events are not to be interpreted in a limiting sense. For example, some steps may occur in different orders and / or concurrently with other steps or events apart from those illustrated and / or described herein. In addition, not all illustrated steps may be required to implement one or moreaspects or embodiments described herein. Also, one or more of the steps depicted herein may be carried out in one or more separate acts and / or phases.

[0078] Applications may include mid- infrared to far-infrared K-space or momentum-space resolved luminescence spectroscopy measurement for condensed matter physics, materials study and device characterization, including the characterization of semiconductors, 2D semiconductors, photonic crystals, micro-nano-optical structures, metasurfaces, laser diodes, light emitting diodes, and so on.

[0079] Examples relating to the system and method for angle-resolved luminescence spectroscopy (e.g. 100 of FIG. 1, 200 of FIG. 2) will be described in detail below.K-resolved luminescence spectroscopy (KLS)

[0080] Regarding KLS, the ‘K’ in KLS refers to the momentum wavevector k, so K-resolved spectroscopy is also known as momentum-, Fourier- and angle-resolved spectroscopy. Accordingly, in the context of various embodiments described above, the system 100 (FIG. 1) and the method 200 (FIG. 2) may refer to MIR angle-resolved spectroscopy, MIR K-space spectroscopy, MIR K-space hyperspectral, Fourier plane spectroscopy, MIR momentum space spectroscopy.

[0081] When photoexcited states in a sample decay emit light, the emitted light typically travels in a broad range of directions. The energy (or frequency, or wavelength) spectrum of the emitted light may depend on its direction. This arises from anisotropy of the sample structure: the crystal structure of the semiconductor itself (e g. low-D material or liquid crystal), the heterostructure of the semiconductor (e.g. in light emitting diode, LED or laser), or perhaps a photonic crystal or cavity (e.g. in metalens or spectral filter).

[0082] FIG. 3 shows a schematic view 301 of an emission cone 305 from the photoluminescence of a sample 307 that is pumped by a focused laser spot (excitation beam 303). The polar angle 6 and azimuth angle <|) of the emission 305 are defined in FIG. 3.[ 00831 Measuring and visualizing this energy-momentum (E-k ) dispersion of the emission 305 reveals coupling between excited states and electromagnetic modes and is the primary data in the study of coupled phenomena such as polariton lasing, Bose-Einstein condensation, and cavity quantum electrodynamics.Microscopic mid-infrared K-resolved luminescence spectroscopy (MIRKLS) measurement apparatus design

[0084] FIG. 4 shows a schematic view of the MIRKLS optical design 400, according to an example The MIRKLS optical design 400 may include the same or like elements or components as those of the angle-resolved luminescence spectroscopy system 100 of FIG.1, and as such, the same ending numerals are assigned and the like elements may be as described in the context of the angle-resolved luminescence spectroscopy system 100 of FIG. 1 , and therefore the corresponding descriptions are omitted here.

[0085] The MIRKLS optical (measurement apparatus) design 400, may include the following.A) Three identical off-axis parabolic (OAP) mirrors:i. the first 404 acting as a reflective objective for focusing an excitation laser beam, collecting MIR emission and imaging the sample 407 (providing a parabolic mirror objective as denoted by dotted circle 402);ii. the second 406 for correcting non-concentricity in the MIR beam profile caused by the first OAP mirror (providing distortion correction as denoted by dotted circle 402’ in one part);iii. the third 406a acting as a white light sample image relay that also reduces sample image distortion (providing distortion correction as denoted by dotted circle 402’ in another part). The virtual image thus created is observed under a regular optical microscope. B) A step-scan Fourier transform spectrometer to measure MIR spectrum. Optical path difference of the interferometer 416 is referenced to the interferogram of the excitation laser light. The interferometer 416 may be described in similar context to the interferometry module 116 of FIG. 1.C) A lock-in amplifier to detect the MIR emission. MIR emission may be modulated at the lock-in frequency by:i. modulating the excitation laser in the case of photoluminescence, or ii. modulating the sample current or voltage in the case of electroluminescence, or iii. mechanically chopping the MIR beam directly.D) An angle-resolution stage 412, including:i. One or more axicon lenses 431 that converts the luminescence polar angle into a z-coordinate, where z is a distance along the optical axis;li. A small aperture 429, e g. a pinhole or fiber end, placed along the z-axis to transmit only a narrow range of polar angles, thus providing polar angle resolution; iii. A translation stage 414 to control the relative distance between the axicon 431 and aperture 429, and hence repeatably select for polar angles;iv. Optionally, a ‘pizza-slice’ aperture that is rotatable in the z axis, which selects a narrow range of azimuthal angles from the luminescence.The angle-resolution stage 412 may be described in similar context to the stage module 110 of FIG. 1. The small aperture 429 and the one or more axicon lenses 431 may be collectively described in similar context to the lens arrangement 112 of FIG. 1. The translation stage 414 may be described in similar context to the translation stage 114 of FIG. 1.

[0086] As seen in FIG. 4, the MIRKLS optical design 400 may be fitted on a 750 x 450 mm optical breadboard. A 532 nm pump laser beam 403 may be provided via the laser aperture 417 having a heat sink 419. White light 413 may be provided by a white LED 421. MIR luminescence beams 415 may travel toward the optical photodetectors (OPD) 423 and liquid nitrogen (LN) dewar Mercury Cadmium Telluride (MCT) detector 425.

[0087] The overarching concepts that drive the design 400 are as follows:• Microscope objective based on an off-axis parabolic mirror (OAPM): OAPMs 404, 406, 406a are achromatic, able to focus and collimate light from the visible to far- infrared range. Unlike typical reflective objectives used in infrared microscopes that have a central obscuration. The design 400 collimates a complete emission cone of the infrared luminescence. OAPMs 404, 406 may be described in similar context to the first reflective member 104 and the second reflective member 106 of FIG. 1, respectively.• Axicons 431 for angle resolution: A lens (or OAPM in this case) converts an emission polar angle into a radial coordinate in the collimated beam. The design 400 uses an axicon 431 to then convert the radial coordinate into a position along the optical axis. This makes it much more signal-efficient since a point detector 425would detect all luminescence from that polar angle. Azimuthal angle may be selected by a ‘pizza-slice’ aperture along the collimated MIR beam 415.• Step-scan Fourier transform spectroscopy: Since MIR luminescence is typically very weak, a lock-in detection scheme greatly reduces noise levels. Fourier transform spectroscopy is also a signal-efficient technique (Fellgetf s advantage). Therefore, to combine both techniques, it is necessary to configure the Fourier transform spectrometer as a step-scan configuration to give the lock-in amplifier the required integration times.Angle resolution

[0088] The most reported method of angle resolution involves imaging the back focal plane of the microscope objective, which is applicable to area emission. For angle-resolved spectroscopy, the back focal plane is imaged onto the entrance slit of a dispersive spectrometer, as shown in FIG. 5. FIG. 5 shows schematic illustrations exemplifying the commonly reported back focal plan technique for angle-resolved spectroscopy (left) and a K space circle (right), according to a prior art example.

[0089] The back focal plane technique utilizes only a narrow strip of emission and captures the emission spectrum over a range of kyvalues, at a constant kx. For many samples, such as those with planar microcavity sandwich structures, the emission pattern is radially symmetric, and the entrance slit wastes much of the emission.

[0090] In the design (e g. 400), the excitation pump laser (excitation beam) 603 is focused to a spot on the sample 607, so the resulting MIR luminescence may be considered as point emission, as illustrated in FIG. 6. This allows for a different method of angle resolution. FIG. 6 shows a schematic view 601 of point luminescence illustrating the main purpose of MIRKLS to measure the luminescence spectra (PL spectrum 1 605a, PL spectrum 2605b, PL spectrum 3 605c) for different polar angles.

[0091] The angle of emission from a point source may be characterized by two angles: a polar angle 0, and an azimuthal angle <|>. The polar angle 0i, 02, 03 describes respective concentric cones in the emitted beam, and once the emitted beam is collimated, each radial coordinate (or concentric ring) of the collimated beam profile corresponds to a polar angle 01, 02, 03.

[0092] To select one of these concentric rings, an axicon and a pinhole may be used. FIG.7 shows a simplified schematic view of the M1RKLS angle-resolution stage 412 with rays from a collimated beam, illustrating how polar angles are isolated, according to an example. The MTRKLS angle-resolution stage 412 includes two axicons 431, 431a, a pinhole / aperture 429 and a motorized translation stage (e g. 414 of FIG. 4) that moves a first axicon 431 along the optical axis. The axicon 431 transforms a radial coordinate in the collimated beam into an axial coordinate (or axial position) along the optical axis. By adjusting the distance, x between the first axicon 431 and the pinhole / aperture 429, a different polar angle is allowed to transmit toward the photodetector 423. The selected polar angle has a linear dependence on the distance, x. After the pinhole 429, a second axicon 431a may be used to re-collimate the ring, and a focusing element (e.g. a lens) 433 can be used to focus the ring onto a photodetector 423. This configuration allows a complete ring (i.e. all emission for a polar angle) to reach the photodetector 423 (or spectrometer), so the weak MIR luminescence is not lost / unused.

[0093] Since only a point detector is needed (rather than array detectors), this method is highly suited to Fourier transform spectroscopy. In the design 400, the interferometer 416 is placed after the recollimating axicon 431a for spectroscopy.

[0094] The uncertainty in polar angle selection depends on the pinhole diameter and the axicons’ refractive index and apex angle. Using a 1 mm pinhole and 160° axicons made of ZnSe, an uncertainty of about ±1.8° may be calculated. The range of measurable polar angles is determined by the off-axis parabolic mirror (OAPM) 404, 406, 406a, and is nominally 0- 30°. The range may however be extended to about 37°.

[0095] To select or transmit an azimuthal angle, a ‘pizza slice’ aperture, that has an axis of rotation along the optical axis, may be used. FIG. 8 shows a simplified schematic view of the ‘pizza slice’ aperture 835, which selects azimuthal angles Rotating the ‘pizza slice’ (opening) allows different azimuthal angles to be selected.MIR collection and correction

[0096] Referring to FIG. 4, to meet the requirements for a visible-MIR achromatic microscope with no obscuration, an off-axis parabolic mirror (OAPM) with silver coating 404 as the microscope objective. In other words, the OAPM collects without centralobscuration. This is completely different from existing methods (e.g. back focal plane imaging, in commercial Fourier Transform Infrared (FTIR) add-ons, or commercial reflective objections that cannot collect central angles).

[0097] This OAPM objective 404 has a reflected focal length of 25.4 mm. FIG 9 shows a photograph 901 of the OAPM objective 404 overhanging a breadboard 939. The objective 404 and the accompanying elliptical plane mirror 406 are rigidly assembled on a manual translation stage 409, which provides focus adjustment with an iris 913. The assembly (denoted by a line denoting the parabolic mirror objective 402 and the distortion correction 402’) is designed such that it overhangs the breadboard 939 to provide as large as possible a working distance between itself and the sample, as seen from FIG. 9.

[0098] The OAPM objective 404 collects the MIR luminescence into a collimated beam, but also introduces a distortion in its radial distribution, i.e. concentric rings in the collimated beam do not correspond to emission polar angles. This distortion is illustrated in FIG. 10. FIG. 10 shows a simplified line schematic view of the parabolic mirror (microscope) objective / (distortion) correction 402, 402’, including (left) a plot illustrating calculated polar angle contours in the luminescence beam profile as collected by the OAPM objective 404 from the sample 907, showing non- concentricity of the constant polar angles (the contours are tighter at the bottom and spread out towards the top - the axicon-based angle selection stage would not be able to isolate a polar angle from this beam); and (right) a plot illustrating after correction by a second identical OAPM 406 and re-collimation by a Ge aspheric lens 435, the contours are restored to concentricity, although the optical axis and beam center no longer coincide.

[0099] To correct for the non-concentricity, the second identical OAPM 406 to reverse the distortion, which effectively creates a virtual image of the MIR emitter. The MIR beam is then re-collimated by the Ge aspheric lens 435 before being passed to the angle-resolution stage 412 and / or interferometer (e.g. 416 of FIG. 4).

[0100] Even after restoring concentricity of the MIR beam, the concentric rings are off-center from the beam axis. To use the axicon angle-resolution stage 412, the center of the rings - not the beam - must align with the central axis of the axicons 431, 431a and the pinhole 429. This asymmetry has the following effects when selecting polar angles: 1) acomplete ring is selectable only up to a polar angle of 22.6°; 2) the maximum selectable polar angle (albeit forming an incomplete ring) is 37.1°.

[0101] The combination of OAP mirrors provide for the collection of MIR emission and then the correction of polar angle distortion for angle resolution.

[0102] The first effect to calibrate the angle-resolution stage 412 may be considered. MIR emission from a quantum cascade laser is collected and collimated as described above, and the MIR beam is visualized using a thermal camera 441 placed just after the angle resolution stage 412, for every 2 mm of axicon translation.

[0103] FIG. 11 shows a simplified line schematic view of the angle-resolution stage 412, including thermal images of MIR beam as different angles are selected, according to various examples. The break in the ring corresponds directly to a particular calculated polar angle value as limited by the OAPM objective.

[0104] As shown in FIG. 11, at the -2 mm position, the ring may be seen to break, indicating correspondence with the calculated 22.6° polar angle. Based on this information, a calibration relation was derived as 0 = 20.7° - 0.96z, where 0 and z are the polar angle (in degrees) and axicon position (in mm).

[0105] The full travel of the translation stage 414 (z =±15 mm) limits the range of measurable polar angles to within 6.3° to 35.1°, with an uncertainty of ±1.8°. The ability to cover the narrow angles 0° - 6.3° may be done by moving the pinhole 429 nearer to the first axicon 431, or by using more axicons, as shown in FIG. 12 illustrating a schematic view of an alternative design for an angle-resolution stage 412’ based on three axicons 431’. A smaller uncertainty is also possible with a smaller pinhole, but at the cost of throughput.Optical imaging[0106| Distortion of the optical image caused by the OAPM objective is unavoidable because the OAPM objective collects and collimates light over a large area. The most serious distortion to correct for is a tilt in focal plane, which renders the image unusable. This focal plane tilt is mitigated with an identical OAPM, which relays a virtual image of the sample in front of the regular visible microscope objective.

[0107] However, the image as captured by the camera still has a curvature distortion, but this was corrected by software. To calibrate for this correction, a Ronchi ruling sample was imaged and the line features were aligned by shifting the rows of pixels, as shown in FIG.13 Thereafter, this pixel row shift was applied to all images More specifically, FIG. 13 shows an image of a 20 lines / mm Ronchi ruling as captured by the MIRKLS white light microscope, and the same image after software curvature correction, according to an example.Interferometer

[0108] Referring to FIG. 4, the interferometer 416 has two sides: the right side for the MIR beam 415, and the left side for the reference laser beam 403. During a measurement, the intensities (i.e. interferograms) of both beams 415, 403 are simultaneously recorded while the translation stage 414 makes discrete steps in the optical path difference.

[0109] In the MIR interferometer, the MIR beam 415 is split by a ZnSe beam splitter 437, and the reflected portion is directed by the moving retroreflector 439 onto a plane mirror 441, which causes the beam 415 to traverse the retroreflector 439 a second time. This optical arrangement makes alignment much easier since the beam’s position and direction are restored when it recombines with the other beam. Another major advantage of this arrangement is its insensitivity to deviations in the motion of the translation stage 414 (pitch, yaw, crosstalk).

[0110] The reference interferometer serves to record the actual optical path difference based on the known wavelength of the visible pump laser (532 nm). This is necessary because the position reported by the translation stage (Physik Instrument V-408) encoder is sufficiently erroneous to introduce sideband artifacts into the final spectrum.[oni] The interferometer 416 may be referred as a step scan interferometer configured to perform modulation by chopper, AC electroluminescence or modulated laser photoluminescence, wherein modulated MIR luminescence may be detected by lock-in amplifier at each step. In other words, at each step in optical path difference, the translation stage 414 holds its position, allowing time for MIR detection. The pump laser 417 is modulated at 10 kHz via a square wave voltage signal produced by a programmable data acquisition (DAQ) card (National Instruments PCIe-6343) This same square wave signalis fed as a frequency reference to a lock-in amplifier (Stanford Research 830). The MIR beam intensity is detected by a liquid nitrogen-cooled HgCdTe (MCT) photovoltaic detector 425 (Teledyne Judson J19D12-M163-R01M-60), which outputs its photocurrent to a transimpedance amplifier (Femto DHPCA) The voltage signal from the transimpedance amplifier is then demodulated by the lock-in amplifier, and the signal amplitude is sent to e g. a desktop computer for recording.Validation[0112J Quantum cascade lasers (QCL) with distributed feedback emit a single frequency of MIR light as determined by the built-in feedback grating in the QCL chip. As typical of any laser, losses occur due to spontaneous emission of off-frequency photons, which are then emitted at broader angles due to lack of coherence. This provides a simple use-case for characterizing semiconductor laser chips and for validating the angle-resolution stage design. By operating just below the lasing threshold of a QCL, both spontaneous and stimulated emissions may be observed and distinguished by their spectra: stimulated emission is spectrally sharp and directionally narrow; spontaneous emission is spectrally broader and directional wider.

[0113] FIG. 14A shows a plot illustrating polar angle dependent MIR emission spectra from a quantum cascade laser chip (nominal wavelength 7.95 pm), operated just below lasing threshold, according to an example. FIG. 14B shows normalized spectra of FIG.14 A.

[0114] The emission from a Thorlabs QD7950CM1 QCL chip (square wave modulated drive current with 450-500 mA peak-to-peak at 10 kHz) was characterized using MIRKLS. The emission spectra 1401, 1403, 1405, 1407 at polar angles 18°, 27°, 31° and 33°, respectively, are shown in FIGS. 14A and 14B Indeed, at a narrow polar angle of 18°, the spectrum is dominated by the sharp laser line at 0.155 eV (7.95 pm). As the polar angle increases, this laser line diminishes in intensity and the emission is predominantly broadband and LED-like (covering a range of 0.02 eV or 1.3 pm), corresponding to the spontaneous emission losses suffered by the QCL chip. Thus, the angle resolution function

[0115] The angle-resolved luminescence spectroscopy system 100 (e.g. the MIRKLS optical design 400) is capable of:• visualizing and locating micro-sized samples using a regular camera;• measuring the mid-IR luminescence spectrum while eliminating thermal background;• operating at different angles in the emission cone;• exciting the sample with a laser beam if necessary (photoluminescence).

[0116] The applications of the angle-resolved luminescence spectroscopy system 100 (e.g. the MIRKLS optical design 400) and the angle-resolved luminescence spectroscopy method 200 include but are not limited to mid-infrared to far-infrared angle resolved luminescence spectroscopy measurement; condensed matter physics in materials study and device characterization (e.g. for novel low band gap semiconductors, 2D semiconductors, laser diodes, LEDs); photonics (e g. involving micro-nano-optical structures, photonic crystals, metasurfaces); and micro-scale variable angle FTIR applications (e.g. selfassembled monolayers, coating thickness, surface contaminants).

[0117] The angle-resolved luminescence spectroscopy system 100 and the angle-resolved luminescence spectroscopy method 200 advantageously provide angle resolution for microscopic samples, luminescence (but adaptable for reflectance as well), and the axicon angle resolution stage design is applicable to any wavelength range.

[0118] Table 1 shows the technology benchmarking of the angle-resolved luminescence spectroscopy system 100 against some existing systems / methods.

[0119] Table 1

[0120] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

CLAIMS1. An angle-resolved luminescence spectroscopy system comprising:an optical arrangement comprising a first reflective member configured to focus a laser beam for exciting a sample and collect an electromagnetic radiation beam emitted from the sample upon excitation; and a second reflective member configured to correct non-concentricity in the electromagnetic radiation beam, the non-concentricity being caused by the first reflective member;a collimator configured to collimate the electromagnetic radiation beam from the optical arrangement;a stage module configured to receive the collimated electromagnetic radiation beam from the collimator; andan interferometry module in optical communication with the stage module, wherein the stage module comprises:a lens arrangement configured to convert radial coordinates of the collimated electromagnetic radiation beam into axial coordinates, wherein the radial coordinates are to be converted from polar angles of the electromagnetic radiation beam emitted from the sample upon excitation, and the axial coordinates correspond to positions along an optical axis of the collimated electromagnetic radiation beam; anda translation stage operable to adjust the lens arrangement for repeatably selecting the polar angles to be transmitted towards the interferometry module, andwherein the interferometry module is configured to measure a luminescence spectrum of the sample based on at least the selected polar angles.

2. The system as claimed in claim 1, wherein the collimator comprises an aspheric lens.

3. The system as claimed in claim 1 or 2, wherein the interferometry module comprises:an interferometer configured to perform interference on the selected polar angles to generate a resultant beam, anda detector configured to detect the resultant beam, wherein the resultant beam is insensitive to deviations caused by the translation stage.

4. The system as claimed in claim 3, wherein the detector comprises a mercury cadmium telluride (HgCdTe) photovoltaic detector.

5. The system as claimed in claim 3 or 4, wherein the interferometry module further comprises a lock-in amplifier coupled to the detector, the lock-in amplifier configured to reduce noise in an electrical signal based on the detected resultant beam6. The system as claimed in any one of claims 1 to 5 further comprising an excitation source configured to generate the laser beam.

7. The system as claimed in any one of claims 1 to 6 further comprising a light source in optical communication with the first reflective member, wherein the light source is configured to emit light to the sample.

8. The system as claimed in claim 7, wherein the light source comprises a white light emitting diode.

9. An optical arrangement for facilitating angle-resolved luminescence spectroscopy, the optical arrangement comprising:a first reflective member configured to focus a laser beam for exciting a sample and collect an electromagnetic radiation beam emitted from the sample upon excitation; and a second reflective member configured to correct non-concentricity in the electromagnetic radiation beam, the non-concentricity being caused by the first reflective member.

10. The system as claimed in any one of claims 1 to 6 or the optical arrangement as claimed in claim 9, wherein the first reflective member is further configured to focus light from a light source to illuminate the sample and collect the light reflected from the sample, and wherein the optical arrangement further comprises a third reflective member configured to reduce image distortion in the light reflected from the sample and relay a virtual image of the sample to a microscope objective, the image distortion being caused by the first reflective member.

11. The system as claimed in any one of claims 1 to 8 or the optical arrangement as claimed in claim 9, wherein the first reflective member and the second reflective member comprise identical off-axis parabolic mirrors.

12. The system or the optical arrangement as claimed in claim 10, wherein the third reflective member comprises an off-axis parabolic mirror that is identical to the first reflective member and the second reflective member.

13. A stage module for facilitating angle-resolved luminescence spectroscopy, the stage module comprising:a lens arrangement configured to convert radial coordinates of a collimated electromagnetic radiation beam into axial coordinates,wherein the radial coordinates are to be converted from polar angles of an electromagnetic radiation source from which the collimated electromagnetic radiation beam originated, andthe axial coordinates correspond to positions along an optical axis of the collimated electromagnetic radiation beam; anda translation stage operable to adjust the lens arrangement for repeatably selecting the polar angles to be transmitted towards an interferometry module.

14. The system as claimed in any one of claims 1 to 8 or the stage module as claimed in claim 13, wherein the lens arrangement comprises at least one axicon lens configured to transform the collimated electromagnetic radiation beam into a distribution of concentricrings, each concentric ring being associated with a radial coordinate that corresponds to a polar angle.

15. The system or the stage module as claimed in claim 14, wherein the lens arrangement further comprises an aperture arranged along the optical axis, wherein the aperture and the at least one axicon lens are configured to move relative to each other to allow a selected polar angle to be transmitted through the aperture towards the interferometry module.

16. The system or the stage module as claimed in claim 15, wherein the lens arrangement further comprises a recollimating axicon lens configured to collimate the selected polar angle, wherein the aperture is arranged between the at least one axicon lens and the recollimating axicon lens.

17. The system or the stage module as claimed in claim 16, wherein the lens arrangement further comprises a focusing element configured to converge the collimated polar angle to a single point towards the interferometry module, wherein the recollimating axicon lens is arranged between the aperture and the focusing element18. The system or the stage module as claimed in any one of claims 15 to 17, wherein the aperture comprises a pinhole or a fibre tip.

19. The system or the stage module as claimed in any one of claims 15 to 18, wherein the aperture is 1 mm or less.

20. The system as claimed in any one of claims 1 to 8 or any one of claims 14 to 19, or the stage module as claimed in any one of claims 13 to 19, wherein the translation stage comprises a motorized translation stage.

21. The system as claimed in any one of claims 1 to 8 or any one of claims 14 to 20 further comprising a slit aperture configured to rotate about the optical axis for selectingan azimuthal angle of the electromagnetic radiation beam emitted from the sample upon excitation, wherein the slit aperture extends radially along a cross-sectional plane substantially perpendicular to the optical axis.

22. The stage module as claimed in any one of claims 13 to 20 further comprising a slit aperture configured to rotate about the optical axis for selecting an azimuthal angle of the electromagnetic radiation source, wherein the slit aperture extends radially along a cross-sectional plane substantially perpendicular to the optical axis.

23. The system as claimed in claim 21 or the stage module as claimed in claim 22, wherein the slit aperture has a shape of a circular sector.

24. An angle-resolved luminescence spectroscopy method comprising:via a first reflective member, focusing a laser beam to excite a sample and collecting an electromagnetic radiation beam emitted from the excited sample;correcting, by a second reflective member, non-concentricity in the electromagnetic radiation beam, the non-concentricity being caused by the first reflective member;collimating the electromagnetic radiation beam;converting radial coordinates of the collimated electromagnetic radiation beam into axial coordinates, wherein the radial coordinates are converted from polar angles of the electromagnetic radiation beam emitted from the excited sample, and the axial coordinates correspond to positions along an optical axis of the collimated electromagnetic radiation beam;selecting the polar angles; andmeasuring a luminescence spectrum of the sample based on at least the selected polar angles.

25. The method as claimed in claim 24, wherein measuring the luminescence spectrum of the sample comprises performing interference on the selected polar angles to generate a resultant beam; and detecting the resultant beam, wherein the resultant beam is insensitive to deviations caused when selecting the polar angles26. The method as claimed in claim 25 further comprising determining an interference pattern based on the resultant beam, and extracting the luminescence spectrum from the interference pattern.

27. The method as claimed in any one of claims 24 to 26, wherein the selected polar angles range from 0° to 37.1°.

28. The method as claimed in any one of claims 24 to 27 further comprising:via the first reflective member, focusing light to illuminate the sample and collect the light reflected from the sample;via a third reflective member, reducing image distortion in the light reflected from the sample, the image distortion being caused by the first reflective member, and via the third reflective member, relaying a virtual image of the sample based on the light reflected from the sample to a microscope objective.

29. The method as claimed in any one of claims 24 to 28, wherein converting the radial coordinates of the collimated electromagnetic radiation beam into axial coordinates comprises transforming, by at least one axicon lens, the collimated electromagnetic radiation beam into a distribution of concentric rings, each concentric ring being associated with a radial coordinate that corresponds to a polar angle.

30. The method as claimed in claim 29, wherein selecting the polar angles comprises:providing an aperture arranged along the optical axis; andperforming relative movement between the aperture and the at least one axicon lens to allow a selected polar angle to be transmitted through the aperture.

31. The method as claimed in claims 30 further comprising:collimating the selected polar angle; andconverging the collimated polar angle to a single point towards an interferometry module.

32. The method as claimed in any one of claims 24 to 31 further comprising: rotating a slit aperture about the optical axis to select an azimuthal angle of the electromagnetic radiation beam emitted from the excited sample, wherein the slit aperture extends radially along a cross-sectional plane substantially perpendicular to the optical axis.

33. The method as claimed in any one of claims 24 to 32, wherein the electromagnetic radiation beam comprises a mid-infrared (MIR) beam, and the luminescence spectrum comprises a MIR spectrum.

34. The method as claimed in any one of claims 24 to 33 comprising a K-space-resolved luminescence spectroscopy method.

35. The method as claimed in any one of claims 24 to 34, wherein the sample comprises a microscopic-sized sample.

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