A simplified homodyned optical setup

Abstract

A device for performing dual-heterodyned (or dual-homodyned) transient grating spectroscopy (TGS) using a single probe and reference beam is disclosed. A probe laser beam is generated with equal amplitudes for two orthogonal linear polarization components. The probe laser beam is passed through a half-wave plate to cause a phase difference of 180 degrees between the two orthogonal linear polarization components, and is caused to diffract from a transient grating on a target sample. The diffracted beam is heterodyned with a reference beam and split using a polarizing beam splitter to generate two polarization signals. The two polarization signals correspond to two heterodyned signals corresponding to heterodyne phases of 0 degrees and 180 degrees. Enabling the simultaneous measurement of two heterodyne signals using a single probe beam eliminates the complications associated with employing two separate beams, such as the need for individual calibration.

Description

PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCTA SIMPLIFIED HOMODYNED OPTICAL SETUPCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent document claims priority to and benefits of U.S. Provisional Patent Application No. 63 / 752,564, titled “A SIMPLIFIED HETERODYNED OPTICAL SETUP” and filed on January 31, 2025. The entire contents of the aforementioned patent application are incorporated by reference as part of the disclosure of this patent document.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This his invention was made with government support under DE-SC0022528 awarded by the U.S. Department of Energy. The government has certain rights in the invention.TECHNICAL FIELD

[0003] Disclosed herein are methods, devices, and systems relating to techniques for transient grating spectroscopy (TGS), and more specifically, dual-heterodyned optical signal techniques in TGS.BACKGROUND

[0004] Transient grating spectroscopy (TGS) is a laser-based measurement technique used to characterize material properties such as thermal diffusivity and acoustic wave velocity. In TGS, a short-pulse "pump" laser is used to produce a grating interference pattern on a sample surface, while a continuous-wave "probe" laser is used to monitor the transient response to the induced grating. The measured signal can be improved by heterodyning the diffracted beam with a reference beam. The reference beam and the diffracted beam can be derived from the same probe laser source and combined to amplify the modifications in the diffracted beam caused by the transient grating. The signal can be further isolated by performing background subtraction. Background subtraction can be performed using two separate heterodyned signals corresponding to different phase differences between the probe and reference beams.1185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1A shows a top-down view of an example optical apparatus for performing dualhomodyne TGS using a polarization scheme, according to embodiments of the present technology.

[0006] FIG. IB depicts the spatial layout of laser beams described with respect to FIG. 1A, according to some embodiments of the present technology.

[0007] FIG. 2A shows a top-down view of an example optical apparatus for performing dualhomodyne TGS using a polarization scheme and coplanar laser beams, according to embodiments of the present technology.

[0008] FIG. 2B depicts the spatial layout of laser beams described with respect to FIG. 2A, according to some embodiments of the present technology.

[0009] FIG. 3 is a flow diagram illustrating an example method of a signal indicative of a physical property of a target using a transient grating spectroscopy (TGS) technique, according to embodiments of the present technology.

[0010] FIG. 4 shows an example of a return signal for a tungsten sample measured on a first photodetector and a second photodetector using the optical apparatus of FIG. 1A, according to embodiments of the present technology.

[0011] FIG. 5 shows example TGS data for a tungsten sample illustrating the return signal at different phase differences between the reference beam and the diffracted probe beam, according to embodiments of the present technology.

[0012] FIG. 6 shows TGS data and fitted curves demonstrating that the background- subtracted measurement signal can be used to determine properties of the target, according to embodiment of the present technology

[0013] FIG. 7A is a top-down schematic view of a di-homodyne TGS setup utilizing two separate probe beams.

[0014] FIG. 7B depicts the spatial layout of laser beam described with respect to FIG. 7A.

[0015] FIG. 8 is schematic top-down view of an optical setup for TGS that utilizes a quarterwave plate and polarized beam splitter to redirect the probe beam after diffraction from the sample surface.DETAILED DESCRIPTION

[0016] The transient grating spectroscopy (TGS) technique is a well-documented non- 2185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT destructive, near surface technique that has been shown to accurately extract thermal diffusivity and material elastic parameters. A short pulse "pump" laser is used to produce a grating interference pattern on a sample surface, while a continuous wave "probe" laser is used to monitor the transient response to the induced grating. This laser-induced grating is composed of a physical displacement as well as a change in thermo-reflectance. In addition, the elasticity of the material lattice responds to the physical displacement and launches counter-propagating surface acoustic waves (SAWs). Hence, the TGS signal may contain thermal diffusivity and / or elastic parameter information.

[0017] Previous implementations have utilized a "4f" optical arrangement to homodyne the TGS signal by coherent interference of the diffracted signal relative to a phase shifted reference signal. Previously, this has been referred to as heterodyning but is referred to herein as homodyning, since the diffracted signal and the reference signal originate from the same beam, only differing in phase and not frequency. In addition, the diffraction signal can be increased by implementing a neutral-density filter (NDF) to reduce the reflected intensity of the reference signal. The intensity of the first order diffraction (e.g., the signal of interest) is typically a small fraction of the zeroth order diffraction (e.g., the reflected reference beam). By reducing the reference intensity with the NDF and using the full probe beam intensity to produce the first order diffraction signal, the intensity of the combined, homodyned signal can be maximized without damaging the photodetector.

[0018] The TGS technique can include implementing a method of taking two measurements n radians out of phase relative to each other to obtain the so-called "amplitude grating" or "phase grating" signal. In addition to removing the background signal, this method can isolate the purely thermal decay (thermoreflectance) or the combination thermal (thermo-reflectance and displacement) and oscillatory (displacement) decay signal, respectively.

[0019] A di-homodyne TGS setup (also referred to herein as a dual-homodyne TGS setup) can be implemented to produce time-resolved TGS data by measuring both 7r-out-of-phase-homodyned signals at the same time. Previous mono-homodyned systems required moving a phase adjuster (PA) between measurements to acquire both measurements Trout of phase, significantly slowing data acquisition. Further discussion of this dual-homodyned setup utilizing two probe beams is presented below with respect to FIGS. 7 and 8. Additionally, optical setups have been3185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT demonstrated that include a mono-homodyne setup that reduces the optical complexity from 3D to a 2D, planar arrangement. Instead of spatially separating the probe beam off-plane from the pump laser, the probe is asymmetrically positioned at an angle off normal from the grating phase matrix (GPM). Due to space limitations and the need to spatially separate probe beams, the use of di-homodyning appears to be impractical in this planar geometry. Fortunately, as will be shown subsequently, the polarization scheme described in this work can easily be used to produce a dihomodyne, planar TGS system.

[0020] In the existing state of the art, this simultaneous dual-homodyned collection typically requires aligning two completely separate laser beams (separate in space with different mirrors and optical components). That is, the existing techniques have two laser beams (e.g., two probe beams) that need to be physically separated (e.g. the beams are separated by a few millimeters in height in order to use a mirror to direct one beam along a different path to a separate detector). Each beam must be individually aligned to focus onto the grating phase mask at the correct position and angle, and separate phase adjusters control the phase of each beam independently. Each beam must be separately focused on the target when a new target is introduced. This configuration introduces alignment complexity and can result in ambiguity in the relative phase shift between the two homodyne signals. Because the two probe beams travel through different optical components along different paths, they can additionally accumulate different background signals that reduce the quality of background subtraction.

[0021] Disclosed herein are embodiments that solve the above problems by allowing two homodyne signals to be measured from a single probe beam. Some embodiments of the present technology use the linear polarization states of a laser beam to carry two signals within a single laser beam. For example, a probe beam and a reference beam can be generated that each have an equal amplitude in their respective s and p polarization components. The s and p polarization can be defined with respect to the surface of the target T and / or the polarizing beam splitter (PBS) that separates the two polarizations of the probe beam. A phase difference can be introduced between the s and p polarizations of the probe beam (e.g., using a half-wave plate), and each polarization component can be treated as an independent probe beam. In particular, the s polarization components of the probe and reference beams can be treated as a probe-reference pair (e.g., with a 0 degree phase difference) while the p polarization components of the probe and reference beams4185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT can be treated as a second, independent probe-reference beam pair (e.g., with a 180 degree phase difference). The diffracted probe beam and reflected reference beam can be combined into a single homodyned beam, and then split into two beams corresponding to the two polarizations (e.g., using a polarizing beam splitter) to measure the two different homodyned signals corresponding to the two different phase differences.

[0022] Some embodiments of the disclosed technology can be used to achieve, among other things, superior background subtraction over existing techniques since the single beam does not have to travel along a different optical path. Instead of separately focusing and aligning two probe beams that are significantly spatially separated, the disclosed technique enables the use of the same beam, the same optics, and virtually the same beam path. The resulting combined homodyned beam can be split using a beam-splitting cube (e.g., polarizing beam splitter) and sent to two individual photodetectors, where the signal from the photodetectors can be subtracted to perform background subtraction and reduce environmental and systematic noise.Optical Apparatus

[0023] FIG. 1A shows a top-down view of an example optical apparatus 100 for performing dual-homodyne TGS using a polarization scheme, according to embodiments of the present technology. The polarization scheme implemented in the optical apparatus 100 can eliminate the need for a second spatially separated probing laser beam. In FIG. 1A, numbers are used to differentiate when the same type of optical component is used multiple times and the abbreviations shown therein correspond to the following components:

[0024] L - Lens (focuses light for single wavelength of laser light)

[0025] AC - Achromat lens (focuses light for multiple wavelengths of laser light)

[0026] M - Mirror (redirects laser light)

[0027] OC - Optical chopper (periodically blocks a laser beam to create a periodic beam)

[0028] GPM - Grating phase mask (also known as diffraction grating or phase mask, splits incoming laser light into many diffraction orders)

[0029] PA - Phase adjuster (changes relative phase of laser beam)

[0030] NDF - Neutral-density filter (attenuates all wavelengths of laser light equally)

[0031] NF - Notch filter (blocks all but single wavelength of laser light)

[0032] HW - Half-wave plate (changes polarization for single wavelength of laser light)5185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT

[0033] PBS - Polarizing beam splitter

[0034] APD - Avalanche photodiode (laser detector)

[0035] T - Target surface

[0036] f - focal length (e.g., of the achromat lenses)

[0037] In the optical apparatus 100, a pump laser source generates a pump beam 102. The pump laser source can be configured to generate pulsed laser beams, with pulses having a certain duration and certain period between pulses. The pump beam 102 is held at a constant height as a mirror Ml redirects the beam to be centered on a pair of achromat lenses (AC1 and AC2). The pump beam 102 is then focused (e.g., by the lens LI) onto the GPM, which can be configured to focus most of the light into the first-order diffraction beams (m = ±1). Beam blockers (black squares) are used to remove all diffracted beams except the first-order diffraction beams, while noting that there are many other diffraction orders not shown in FIG. 1 A. The first-order diffraction beams enter the pair of achromat lenses AC1 and AC2 as a first pump beam 104 and a second pump beam 106. The two pump beams 104, 106, are then focused onto the target T to create an interference pattern that generates the transient gradient.

[0038] The optical apparatus 100 includes a probe laser source that generates a probe beam 110. The probe beam 110 can be spatially separated from the pump beam 102 (e.g., the probe beam 110 can be at a higher or lower height). In the example optical apparatus 100, the probe beam 110 is generated at a lower height than the pump beam 102. The probe beam 110 can pass through an optical chopper OC. The optical apparatus can include a first half-wave plate HW1 on an optical path of the probe beam 110 to ensure that two mutually orthogonal polarization components of the probe beam 110 have an equal intensity (e.g., an equal amplitude). The probe beam 110 can be focused (e.g., by L2 and Ml) onto the GPM at the same point as the LI focused pump beam, resulting in the probe beam 110 shifting from below to above the height of the pump beam 102. The GPM splits the probe beam 110 into a signal beam 114 and a reference beam 112. Traversing the GPM, the four beams (e.g., first pump beam 104, second pump beam 106, signal beam 114, and reference beam 112) can be configured to originate at the focus of the first achromat lens AC1 (e.g., with a focal length of approximately 150 mm), and can be vertically separated and parallelized between AC1 and AC2 (e.g., being directed onto AC1 at an angle corresponding to the focus of AC1 such that AC1 redirects the beam to be parallel to an optical axis of AC1). A6185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT neutral-density filter NDF can be placed to attenuate the reference beam 112 and configured to prevent damage to the photodetectors APD1 and APD2. In some embodiments, the neutral-density filter NDF can be configured to be at an angle with respect to the optical path of the reference beam 112 to shift the phase of the reference beam 112 as well as attenuate it.

[0039] The optical apparatus 100 can include a second half-wave plate HW2 (e.g., disposed between the achromat lenses AC1 and AC2). The second half-wave plate HW2 can be configured to create a phase difference between the two orthogonal linear polarizations of the signal beam 114 (e.g., the s polarization component and the p polarization component). For example, the second half- wave plate HW2 can be configured such that the polarized light travels a longer optical path length along the slow axis by an additional half wavelength relative to the light traveling along the fast axis. In some embodiments, the second half-wave plate HW2 can be configured to receive the signal beam 114 and generate a processed signal beam 116 by introducing a phase difference (e.g., 180 degrees) between the two polarization components. The second half-wave plate HW2 can thus create a phase difference (e.g.. a 180 degree phase difference) between one polarization of the processed signal beam 116 and the corresponding polarization of the reference beam 112. The optical apparatus 100 can include a phase adjuster PA configured to shift the phase of the processed signal beam 116 (e.g., with respect to the reference beam 112). The PA can be configured to apply a phase shift equally to both linear polarizations of the processed signal beam 116. In some embodiments, the optical apparatus 100 can be configured with a PA in the optical path of the reference beam 112 (e.g., applying a phase shift to the reference beam 112). In some embodiments, the PA can be configured to apply a phase shift of 90 degrees (e.g., TT / 2 radians).

[0040] The optical apparatus 100 is configured to focus, using the second achromat lens AC2, the processed signal beam 116 and the reference beam 112 onto the target T. The target T can have a transient grating on the surface, caused by the interfering pump beams 104, 106. The processed signal beam 116 can diffract and / or reflect from the transient grating, generating a diffracted signal beam directed into the second achromat lens AC2. For example, the processed signal beam 116 can diffract from the transient grating to generate a diffraction pattern, and the diffracted signal beam can correspond to the first-order diffraction (e.g., m = +1 or m = - 1 diffraction) of the diffraction pattern. The diffracted signal beam can be directed into the second achromat lens AC2 for further processing. Additionally, the reference beam 112 can diffract and / or reflect from the7185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT transient grating, creating a reflected reference beam directed into the second achromat lens AC2. For example, the reference beam 112 can diffract and / or reflect from the transient grating to generate a diffraction pattern, and the reflected reference beam can correspond to the zeroth-order diffraction (e.g., m = 0 diffraction) of the diffraction pattern. The reflected reference beam can be directed into the second achromat lens AC2 for further processing. The second half-wave plate HW2 can ensure that the relative phase differences A(f>R,Dbetween the collinear reflected reference beam and the diffracted signal beam will add to approximately 180 degrees between the two linear polarizations (e.g., the s and p polarizations). For example, the relative phase difference between the s polarization components for the reflected reference beam and diffracted signal beam, A< >,D, and the relative phase difference between the p polarization components for the reflected reference beam and diffracted signal beam, <pp’D, can satisfy (>s’D+ ^(pp0= 180°.

[0041] In some embodiments, the diffracted signal beam and the reflected reference beam are combined into a result signal 120. For example, the reference beam 112 and the processed signal beam 116 can be directed at the transient grating on the target T such that the first-order diffraction generated by the processed signal beam 116 is collinear with (e.g.. overlapping with and traveling in the same direction as) the zeroth-order diffraction generated by the reference beam 112. The result signal 120 can be directed into the second achromat lens AC2 for further processing.

[0042] FIG. IB depicts the spatial layout of laser beams described with respect to FIG. 1A, according to some embodiments of the present technology. FIG. IB depicts the incoming (e.g., toward the target T) and outgoing laser beams with respect to the second achromat lens AC2 (i.e., as viewed from the right side of FIG. 1 A). The second achromat lens AC2 focuses the beams (e.g., the pump beams 104, 106, the reference beam 112, and the processed signal beam 116) onto the target T, and focuses the returning laser beams (e.g., the result signal 120). The 4f configuration ensures the diffracted signal beam is collinear with the reflected reference beam, generating the resulting TGS signals, such as the return signal 120.

[0043] The optical apparatus 100 includes a mirror M2 to redirect the result signal 120 through a focusing lens L3 and a notch filter NF to attenuate any stray pump light into a light-tight box to individual avalanche photodiodes APD1 and APD2. The result signal 120 is directed through a polarizing beam splitter PBS that separates the result signal 120 into a first polarization signal 122 and a second polarization signal 124, corresponding to the first and second polarization8185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT components, respectively, of the result signal 120. The first and second polarization signals 122 and 124 can correspond to two homodyned signals of different phase differences. For example, the first and second polarization signals 122 and 124 can correspond to homodyned signals with phase differences (e.g., between their respective signal beam and reference beam) of 0 degrees and 180 degrees, respectively. Alternately, the first and second polarization signals 122 and 124 can correspond to homodyned signals with phase differences of 90 degrees and 270 degrees, respectively. Using the same focusing lens L3 ensures the single beam, composed of two polarization-dependent TGS signals, will reach each avalanche photodiode having traveled almost the exact same optical path, at least up to the PBS and the intended shift within the second halfwave plate HW2.

[0044] The optical geometry used in the dual-homodyne setup, such as in the optical apparatus 100, could be used in other laser-based techniques that physically separate in space two different probe beams (e.g., optical setups with two probe beams vertically separated and each with corresponding reference beams and result TGS signals). Whereas the TGS setup described in this example uses a short pulsed "pump" laser to create a disturbance on a sample, there are other methods to induce surface changes on a sample that can likewise be measured with a "probe" laser as disclosed herein.

[0045] FIG. 2 A shows a top-down view of an example optical apparatus 200 for performing dual-homodyne TGS using a polarization scheme and coplanar laser beams, according to embodiments of the present technology. FIG. 2A uses similar labels to FIG. 1A, where analogous labels represent analogous (but not necessarily identical) elements. As illustrated in FIG. 2A, the probe beam is directed to a grating phase mask GPM at an angle such that the signal beam and reference beam are positioned beside the pump beams, allowing the signal and reference beams to be at the same height as the pump beams (e.g., such that the pump beams, reference beam, signal beam, and result signal are mutually coplanar). Beam blocks are shown for the m = 0 diffraction order and the ignored reflection, while noting there are many other diffraction orders not shown. The coplanar embodiment of FIG. 2A uses a larger grating (e.g., of the GPM) than in FIG. 1A, and this is proportional to the probing depth. For space conservation, the phase adjuster PA has been removed and the neutral-density filter (NDF) is tilted to shift the reference beam phase as well as attenuate it. The location of the second half-wave plate HWP2 can be placed on either the9185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT reference or probe beams and still maintain the relative phase shift between the components of the TGS signal. Considering space limitations, HWP2 is placed on the outermost beam (i.e. the uppermost path).

[0046] FIG. 2B depicts the spatial layout of laser beams described with respect to FIG. 2A, according to some embodiments of the present technology. FIG. 2B depicts the incoming (e.g., toward the target T) and outgoing laser beams with respect to the second achromat lens AC2 (i.e., as viewed from the right side of FIG. 2A). The second achromat lens AC2 focuses the beams (e.g., the pump beams, the reference beam, and the signal beam) onto the target T, and focuses the returning laser beams (e.g.. the result signal). The 4f configuration ensures the diffracted signal beam is collinear with the reflected reference beam, generating the resulting TGS signals, such as the return signal.

[0047] In an example usage of the optical apparatus 100 and / or the optical apparatus 200, the probe beam 110 can be reduced to a ~ 10% duty cycle by the optical chopper OC and the pump laser 102 is pulsed at a 500 Hz repetition rate. The pulse of the pump laser 102 is centered within the time the probe is on. The first half-wave plate HW1 can be adjusted to produce nearly equal overlap of the measurements on the avalanche photodiodes APD1 and APD2. The second halfwave plate HW2 can be rotated to shift the relative phase between polarizations, and the phase adjuster PA can be tilted to shift both polarizations to the desired homodyne condition. The interference of the reference and diffracted signals for a relative phase shift 40 of 0 degrees and 180 degrees can be taken at the same time. Likewise, the signals for 90 degrees and 270 degrees (i.e., -90 degrees) can be taken together. The difference between TGS signals obtained at a phase shift of 0 and 180 gives the so-called "amplitude grating" while the difference between 90 and -90 gives the so-called "phase grating." These differences isolate the homodyned TGS signal and remove the time-varying background signal. The phase grating can be fitted to determine the thermal diffusivity (a) and the Rayleigh wave speed (c5j4M). SAW speeds can be calculated using Malischewsky's approximation.Method Flow

[0048] FIG. 3 is a flow diagram illustrating an example method 300 of a signal indicative of a physical property of a target using a TGS technique, according to embodiments of the present technology. The TGS technique can be a dual-homodyne technique that uses two homodyned10185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT signals to two different homodyne phases, where each homodyne phase corresponds to a phase difference between a signal beam and a reference beam that are homodyned into the homodyned signal. The TGS technique can implement a polarization technique, in which the two homodyned signals are carried by two orthogonal linear polarization components of a probe beam. In some embodiments, the method 300 is performed by components of the example optical apparatus 100 described in more detail with respect to FIG. 1A and / or by components of the example optical apparatus 200 described in more detail with respect to FIG. 2A. Likewise, embodiments can include different and / or additional operations or can perform operations in different orders.

[0049] At 302, the method 300 can include generating a probe beam comprising a first polarization component and a second polarization component. The first and second polarization components can correspond to orthogonal linear polarizations. For example, the first polarization component can correspond to an s polarization and the second polarization component can correspond to a p polarization component. The probe beam can be created by a probe beam source of an optical apparatus. In some embodiments, the probe beam is configured such that the probe beam has a substantially equal amplitude and / or intensity for each of the first and second polarization components. Configuring the beam to have a substantially equal amplitude for each of the first and second polarization components can include directing the probe beam through a birefringent material (e.g., a half-wave plate).

[0050] At 304. the method 300 can include introducing a phase difference between the first polarization component and the second polarization component to generate a processed probe beam. The processed probe beam can be generated by using a birefringent optical element (e.g., a half-wave plate) to introduce the phase difference between the first polarization component and the second polarization component. In some embodiments, the phase difference is substantially 90 degrees or 180 degrees. In some embodiments, the method 300 includes attenuating the processed probe beam. For example, the method 300 can include passing the probe beam and / or the processed probe beam through a neutral-density filter. In some embodiments, the method 300 includes directing the probe beam through a phase adjuster to apply a substantially equal phase adjustment to the first polarization component and the second polarization component of the probe beam and / or the processed probe beam.

[0051] At 306, the method 300 can include directing the processed probe beam to the target11185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT using a lens system. The lens system can be configured to cause the probe beam to be directed to a portion of the target containing a transient grating (e.g., created by an interference pattern of two pump beams). In some embodiments, the lens system is configured to additionally focus a pump laser beam onto the target to create a transient grating. For example, the method 300 can include generating (e.g., prior to directing the processed probe beam to the target) a pump laser beam, where the pump laser beam comprises a first pump beam and a second pump beam (e.g., created by directing the pump laser beam through a diffraction grating), and directing, using the lens system, the pump laser beam onto the target to create an interference pattern on a surface of the target and / or create a transient grating on the target.

[0052] In some embodiments, the lens system includes a first achromat lens and a second achromat lens. For example, the lens system can cause the processed probe beam to be directed at the target by redirecting, using the first achromat lens, the probe beam and / or the processed probe beam and redirecting, using the second achromat lens, the processed probe beam such that the processed probe beam is directed at the target. The target can be placed at the focus of the second achromat lens. In some embodiments, the birefringent optical element can be positioned between the first and second achromat lenses such that the processed probe beam is generated between the first and second achromat lenses.

[0053] At 308, the method 300 can include receiving (e.g., at the lens system) a return signal from the target. The return signal can correspond to a diffracted probe beam generated by the processed probe beam interacting with (e.g.. diffracting and / or reflecting from) the target (e.g., the surface of the target and / or a transient grating on the target).

[0054] In some embodiments, the return signal corresponds to a homodyned signal. For example, the method 300 can include generating a reference beam, directing the reference beam at the target (e.g., using the lens system), receiving a reflected reference signal generated by the reference beam interacting with the target, and causing the reflected reference signal to become homodyned with the diffracted probe beam to generate the return signal. In some embodiments, the method 300 includes attenuating the reference beam using a neutral-density filter. In some embodiments, the reference beam is generated from the probe beam. For example, the method 300 can include causing the probe beam to be directed to a beamsplitting optical element (e.g., a grating phase mask and / or diffraction grating) to generate the reference beam. The reference beam can12185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT correspond to a first-order diffraction beam (e.g.. an m=+l order diffraction) generated from directing the probe beam to the beamsplitting optical element. For example, the probe beam can be directed towards a diffraction grating configured to diffract the probe beam into a plurality of diffraction orders, where a first-order diffraction beam can be isolated as the reference beam and / or an additional first-order diffraction beam can be isolated as the probe beam (e.g., that continues into the lens system). The beamsplitting optical element can be positioned at a focal point of a first achromat lens (e.g., of the lens system) such that beams generated by the diffraction grating are redirected by the first achromat lens to be substantially parallel to each other.

[0055] In some embodiments, the probe beam and / or the processed probe beam comprises a reference beam and a signal beam. For example, the method 300 can include directing the probe beam to a beamsplitting optical element to generate the signal beam and the reference beam (e.g., that travel along separate optical paths). The method 300 can include generating the processed probe beam by introducing a phase difference between the first polarization component and the second polarization component of the signal beam and / or the reference beam to generate a processed probe beam (e.g., comprising a processed signal beam and the reference beam). The processed probe beam can then be directed at the target via the lens system such that the processed signal beam and the reference beam are directed at the target. The processed signal beam can be reflected and / or diffracted from a transient grating on the target to generate a diffracted signal beam, and the reference beam can be reflected and / or diffracted from the transient grating to generate a reflected reference beam. The diffracted signal beam and the reflected reference beam can be combined (e.g., directed at the target such that the diffracted signal beam and the reflected reference beam are collinear) to form a return signal that includes a homodyned combination of signals received from the target (e.g., such that the interference between the diffracted signal beam and the reflected reference beam forms a homodyne signal).

[0056] At 310. the method 300 can include splitting the return signal into a first polarization signal and a second polarization signal. The first and second polarization signals can correspond to the first and second polarization components, respectively. In some embodiments, splitting the return signal includes splitting the return signal using a polarizing beam splitter. In some embodiments, the method 300 includes directing the return signal through a notch filter configured to attenuate light at a wavelength of the pump laser source.13185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT

[0057] At 312, the method 300 can include detecting the first polarization signal at a first photodetector. The first photodetector can measure the first polarization signal and / or generate a first electrical signal. In some embodiments, the first photodetector is an avalanche photodiode.

[0058] At 314, the method 300 can include detecting the second polarization signal at a second photodetector. The second photodetector can measure the second polarization signal and / or generate a second electrical signal. In some embodiments, the second photodetector is an avalanche photodiode.

[0059] At 316, the method 300 can include performing a subtraction between the first electrical signal and the second electrical signal to obtain the signal indicative of the physical property of the target. The subtraction can be a background subtraction that removes background noise and / or systematic noise. In some embodiments, the physical property of the target is at least one of thermal diffusivity or surface acoustic wave speed.

[0060] It is contemplated that the operations or descriptions of FIG. 3 may be used with any other embodiment of this disclosure. In addition, the operations and descriptions described in relation to FIG. 3 may be done in alternative orders, in parallel, or multiple times to further the purposes of this disclosure. Furthermore, it should be noted that any of the devices or equipment discussed in relation to the other figures or otherwise disclosed herein could be used to perform one or more of the operations in FIG. 3.Experimental Results

[0061] FIG. 4 shows an example of TGS measurement results for a tungsten sample using the optical apparatus 100. Measurements utilizing the optical apparatus 100 depicted in FIG. 1A were taken on pristine polycrystalline samples of tungsten (W), titanium (Ti), and molybdenum (Mo) as well as single crystal silicon (Si) targets. These materials correspond to example targets on which transient grating spectroscopy (TGS) can be performed using the disclosed optical apparatus. All measurements were taken with the target T under vacuum to remove convective thermal loss due to Scholte waves. FIG. 4 shows the resulting TGS signal for W measured on APD1 and APD2 (units presented on left vertical axis) and the difference between the signals (units presented on right vertical axis). The TGS signal of interest occurs near t = 0 ps.

[0062] In order to limit additional heating during measurement, the probe beam 110 is reduced to a -10% duty cycle by the optical chopper OC and the pump pulse (500 Hz repetition rate) is14185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT centered within the time the probe beam 110 is on. The return signal 120 is measured by the first photodetector APD1 and the second photodetector APD2. In some embodiments, the first photodetector APD1 and the second photodetector APD2 comprise avalanche photodiodes (e.g., Hamamatsu C5658) with a lower frequency limit of 50 kHz, resulting in the differentiation of the step-like probe signal. The second birefringent optical element (e.g., the first half-wave plate HW1) can be adjusted to produce nearly equal overlap of the measurements on APD1 and APD2, here shown with a small offset to differentiate (left side of FIG. 4). The nearly flat difference between APD1 and APD2 (right side of FIG. 4) demonstrates that the intensity of each polarization component (e.g.. the first polarization component and the second polarization component) within the reference beam 112 are approximately equal when reaching the first photodetector APD1 and the second photodetector APD2. The return signal 120 of interest occurs near t = 0 ps.

[0063] FIG. 5 shows TGS data for a 6.00 pm grating on a tungsten sample, illustrating how the phase difference between the first polarization component and the second polarization component can be adjusted to obtain different measurement signals. The phase difference between the reference and diffracted probe beams in FIG. 5 correspond to the “amplitude grating” and “phase grating” conditions. The birefringent optical element (e.g., the second half-wave plate HW2) can be rotated to shift the relative phase between the first polarization component and the second polarization component, and the phase adjuster PA can be tilted to shift both polarization components to the desired homodyne condition. In FIG. 5, the interference of the reference beam 112 and the diffracted signal beam for a relative phase shift (d<p) of 0 and K (i.e., 0 degrees and 180 degrees) were taken at the same time. Likewise, the signals for 7i / 2 and -K / 2 (i.e., 90 degrees and -90 degrees) were taken together. The difference between the first polarization signal 122 and the second polarization signal 124 obtained at a phase shift of 0 and 7t gives the so-called "amplitude grating" while the difference between K / 2 and -n / gives the so-called "phase grating." These differences isolate the homodyned TGS signal and remove the time-varying background signal, corresponding to the background- subtracted measurement signal indicative of a property of the target as described with respect to the optical apparatus 100.

[0064] FIG. 6 shows TGS data and fitted curves demonstrating that the background- subtracted measurement signal can be used to determine properties of the target, such as thermal diffusivity (a) and surface acoustic wave (SAW) speed (cSyW), according to embodiment of the present15185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT technology. The phase grating signal can be fitted to determine the thermal diffusivity (a) and the Rayleigh wave speed (cSAW). FIG. 6(a) and FIG. 6(b) show the respective fits to the amplitude grating and phase grating using a pump-laser-induced grating wavelength (A) of 6.00 pm on a tungsten (W) sample surface. FIG. 6(c), FIG. 6(d), and FIG. 6(e) show the respective fits for the phase grating on titanium (Ti), molybdenum (Mo), and silicon (Si) samples, respectively.

[0065] Table 1 shows thermal diffusivity (a) and SAW speed (cSAW) values obtained from fitted data with the optical apparatus 100 utilizing the polarization-based scheme compared to literature values. The thermal diffusivity and SAW speed values determined using the optical apparatus 100 are commensurate with the literature values for each material, where some values may disagree due to sample preparation and / or purity. SAW speeds were calculated using Malischewsky's approximation.Table 1: Thermal diffusivity and SAW speed values obtained from fitted data with the new polarization setup compared to literature values.Prior Works

[0066] FIG. 7A is a top-down schematic view of a di-homodyne TGS setup utilizing two separate probe beams. Under- or overlines are used to delineate the vertical position of the optical components with reference to the central height that has no lines below or above the labels. Double-lines are used to denote optical components shifted twice as much from the central line compared to the single-lines. A triple-line denotes an optical component that spans both single-and double-line heights. Here, the pump laser beam is held at a constant height as a mirror (Ml) redirects the beam to be centered on the achromats (AC1 and AC2). The probe beam begins at a lower height and passes through an optical chopper (OC), then is divided by a half-wave plate 16185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT (HWP) and a polarized beam splitter (PBS). One probe beam is sent directly to the lens L2, while the other is vertically lowered by mirrors M5-M7 and redirected to L2. Both probe beams exit L2, but at varied heights, and focus onto the grating phase mask (GPM) at the same point as the Ll-focused pump beam, resulting in both probe beams shifting from below to above the central height. Beam blockers (black squares) are used to remove all diffracted beams except the first order beams (m = ±1). Traversing the GPM, the six beams originate at the focus of AC1, but are vertically separated and parallelized between AC1 and AC2. The NDF is placed to attenuate both probe beams on the top path as the first phase adjuster (PAI) shifts the phase of both probe beams on the bottom path, where top / bottom refer to the schematic and not the vertical height. PA2 is mounted vertically higher to only shift the phase of the higher probe beam.

[0067] FIG. 7B depicts the incoming laser beams with respect to AC2 for the optical setup described with respect to FIG. 7A. AC2 focuses the crossed beams onto the target (T), as well as the returning laser beams. The 4f configuration ensures both the higher and lower probe beams have their own collinear reference and diffracted beams (i.e. TGS signals). A mirror (M2) redirects both TGS signals through a notch filter (NF) to attenuate any stray pump light into a light tight box to individual avalanche photodiodes (APDs).

[0068] FIG. 8 is schematic top-down view of an optical setup for TGS that utilizes a quarterwave plate (QWP) and polarized beam splitter (PBS 2) to redirect the probe beam after diffraction from the sample surface in a well-known optical isolator scheme. The previously described setups spatially separated the probe beam vertically while the setup of FIG. 8 utilizes the symmetry of the GPM to produce two spatially separated beams in-plane and two homodyned signals n out of phase with one another using the PA. Yet the symmetry of the setup of FIG. 8 prevents the use of a NDF. The utility of the setup of FIG. 8 is reduced by the optical isolation that makes the di-homodyning possible. Placement of the QWP between the AC2 and T reduces the possible applications that measure a sample from a distance. When shifting GPM to change the thermal grating wavelength, the PBS redirects the returning TGS signals at a different angle that requires additional re-alignment of mirrors (M2 and M3) to reach the detectors. In contrast, mounting a "pick-off" mirror on a linear translation stage perpendicular to the returning TGS signal in the middle of the 4f setup (i.e. M2 in FIG. 7A) provides simple redirection to the detectors.Examples17185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT

[0069] In some embodiments in accordance with the disclosed technology (example Al), an optical apparatus for performing transient grating spectroscopy (TGS) on a target includes: a probe laser source configured to generate a probe beam comprising a first polarization component and a second polarization component; a birefringent optical element positioned in an optical path of the probe beam and configured to generate a processed probe beam by introducing a phase difference between the first polarization component and the second polarization component; a lens system configured to direct the processed probe beam to the target and to receive a return signal from the target; a polarizing beam splitter configured to split the return signal into a first polarization signal and a second polarization signal; a first photodetector configured to receive the first polarization signal; and a second photodetector configured to receive the second polarization signal, wherein a combination of the first polarization signal and the second polarization signal provides a background-subtracted measurement signal indicative of a property of the target.

[0070] Example A2 includes the optical apparatus of example Al or any of examples A1-A30, wherein the phase difference is substantially 180 degrees or 90 degrees.

[0071] Example A3 includes the optical apparatus of example Al or any of examples A1-A30, wherein the birefringent optical element comprises a half- wave plate.

[0072] Example A4 includes the optical apparatus of example A3 or any of examples A1-A30, wherein the half-wave plate is configured to provide half-wave retardation at a wavelength of the probe beam.

[0073] Example A5 includes the optical apparatus of example Al or any of examples A1-A30, wherein the first polarization component and the second polarization component comprise orthogonal linear polarizations.

[0074] Example A6 includes the optical apparatus of example A5 or any of examples A1-A30, wherein the first polarization component is an s polarization and the second polarization component is a p polarization.

[0075] Example A7 includes the optical apparatus of example Al or any of examples A1-A30, wherein the lens system comprises a first achromat lens and a second achromat lens, and wherein the birefringent optical element is disposed between the first achromat lens and the second achromat lens.

[0076] Example A8 includes the optical apparatus of example A7 or any of examples A1-A30,18185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT wherein the return signal is received by at least one achromat lens of the lens system.

[0077] Example A9 includes the optical apparatus of example Al or any of examples A1-A30, comprising a phase adjuster configured to apply a substantially equal phase adjustment to the first polarization component and the second polarization component of the probe beam.

[0078] Example AID includes the optical apparatus of example Al or any of examples Al-A30, comprising a phase adjuster configured to apply a substantially equal phase adjustment to the first polarization component and the second polarization component of the processed probe beam.

[0079] Example All includes the optical apparatus of example Al or any of examples Al-A30, comprising a neutral-density filter configured to apply a substantially equal amplitude decrease to the first polarization component and the second polarization component of the probe beam and / or the processed probe beam.

[0080] Example A12 includes the optical apparatus of example Al or any of examples Al-A30, wherein: the birefringent optical element is a first birefringent optical element, and the optical apparatus comprises a second birefringent optical element disposed upstream of the birefringent optical element and configured to adjust relative intensities of the first polarization component and the second polarization component.

[0081] Example A13 includes the optical apparatus of example A12 or any of examples Al-A30, wherein the second birefringent optical element comprises a half- wave plate.

[0082] Example A 14 includes the optical apparatus of example Al or any of examples Al-A30, comprising a diffraction grating positioned in the optical path of the probe beam and configured to diffract the probe beam into a plurality of diffraction orders.

[0083] Example A15 includes the optical apparatus of example A14 or any of examples Al-A30, wherein the diffraction grating comprises a phase mask.

[0084] Example A16 includes the optical apparatus of example A 14 or any of examples Al-A30, wherein: the processed probe beam comprises a reference beam and a signal beam, each derived from one of the plurality of diffraction orders from the diffraction grating, and the return signal comprises an interference between the reference beam reflected from the target and a diffracted beam generated by interaction of the signal beam with a transient grating on the target.

[0085] Example A 17 includes the optical apparatus of example A 16 or any of examples Al-19185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT A30, wherein the reference beam and the signal beam are directed to the target along separate optical paths.

[0086] Example Al 8 includes the optical apparatus of example A 17 or any of examples Al-A30, wherein the birefringent optical element is disposed in an optical path of the signal beam.

[0087] Example A19 includes the optical apparatus of example A 17 or any of examples Al-A30, wherein the birefringent optical element is disposed in an optical path of the reference beam.

[0088] Example A20 includes the optical apparatus of example A17 or any of examples Al-A30, wherein the optical apparatus comprises a neutral-density filter configured to apply a substantially equal amplitude decrease to the first polarization component and the second polarization component of the reference beam.

[0089] Example A21 includes the optical apparatus of example A 16 or any of examples Al-A30, comprising a phase adjuster configured to apply a substantially equal phase adjustment to the first polarization component and the second polarization component of the probe beam, wherein the phase adjuster is disposed in an optical path of the signal beam.

[0090] Example A22 includes the optical apparatus of example A16 or any of examples Al-A30, comprising a phase adjuster configured to apply a substantially equal phase adjustment to the first polarization component and the second polarization component of the probe beam, wherein the phase adjuster is disposed in an optical path of the reference beam.

[0091] Example A23 includes the optical apparatus of example A 16 or any of examples Al-A30, wherein the interference produces a heterodyne and / or homodyne signal.

[0092] Example A24 includes the optical apparatus of example Al or any of examples Al-A30, further comprising a pump laser source configured to induce a transient grating on the target.

[0093] Example A25 includes the optical apparatus of example A24 or any of examples Al-A30, wherein the pump laser source comprises a pulsed laser source.

[0094] Example A26 includes the optical apparatus of example A24 or any of examples Al-A30, wherein the lens system is configured to direct pump beams from the pump laser source to the target such that the pump beams interfere to form the transient grating.

[0095] Example A27 includes the optical apparatus of example A24 or any of examples Al-A30, comprising a notch filter disposed in an optical path of the return signal and configured to attenuate light at a wavelength of the pump laser source.20185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT

[0096] Example A28 includes the optical apparatus of example Al or any of examples Al-A30, wherein the property of the target comprises at least one of a thermal diffusivity or a surface acoustic wave speed.

[0097] Example A29 includes the optical apparatus of example Al or any of examples Al-A30, wherein the probe laser source, the birefringent optical element, the lens system, and the polarizing beam splitter are arranged in a planar optical configuration.

[0098] Example A30 includes the optical apparatus of example Al or any of examples Al-A30, wherein the first photodetector comprises an avalanche photodiode (APD).

[0099] In some embodiments in accordance with the disclosed technology (example Bl), a method for acquiring a signal indicative of a physical property of a target using a transient grating spectroscopy (TGS) technique includes: generating a probe beam comprising a first polarization component and a second polarization component; introducing a phase difference between the first polarization component and the second polarization component using a birefringent optical element to generate a processed probe beam; directing the processed probe beam to the target using a lens system; receiving a return signal from the target; splitting the return signal into a first polarization signal and a second polarization signal using a polarizing beam splitter; detecting the first polarization signal at a first photodetector to produce a first electrical signal; detecting the second polarization signal at a second photodetector to produce a second electrical signal; and performing a subtraction between the first electrical signal and the second electrical signal (e.g., subtracting measurements and / or digital representations of the first electrical signal and the second electrical signal, such as by using a computing system) to obtain the signal indicative of the physical property of the target.

[0100] Example B2 includes the method of example Bl or any of examples B1-B7, wherein the first polarization component corresponds to an s polarization and the second polarization component corresponds to a p polarization.

[0101] Example B3 includes the method of example Bl or any of examples B1-B7, wherein an amplitude of the first polarization component of the probe beam is substantially equal to an amplitude of the second polarization component of the probe beam.

[0102] Example B4 includes the method of example Bl or any of examples B1-B7, comprising: prior to directing the processed probe beam to the target, passing the processed probe21185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT beam through an optical phase adjustment element, wherein the optical phase adjustment element equally modifies phases of the first and second polarization components of the processed probe beam.

[0103] Example B5 includes the method of example B 1 or any of examples B 1-B7, comprising inducing a transient grating on the target with a pump laser prior to directing the processed probe beam to the target.

[0104] Example B6 includes the method of example B5 or any of examples B1-B7, wherein: the processed probe beam comprises a reference beam and a signal beam, and wherein the return signal comprises an interference between the reference beam reflected from the target and a diffracted beam generated by interaction of the signal beam with the transient grating on the target.

[0105] Example B7 includes the method of example B5 or any of examples B 1-B7, comprising generating, from the probe beam, a reference beam; and directing the reference beam to the target using the lens system, wherein the return signal comprises a heterodyned and / or homodyned combination of signals received from the target.

[0106] In some embodiments in accordance with the disclosed technology (example Cl), a method for acquiring an optical signal (e.g., electrical signal) of interest includes: causing first laser light comprising two polarizations to be incident upon a birefringent optical element configured to generate, from the first laser light, second laser light comprising a phase shift of 180 degrees between the two polarizations; causing the second laser light to be incident upon a beam splitter configured to split the second laser light into a first beam and a second beam; receiving the first beam at a first photodetector configured to produce a first optical signal (e.g., electrical signal) based on the first beam; receiving the second beam at a second photodetector configured to produce a second optical signal (e.g., electrical signal) based on the second beam; and performing background subtraction between the first optical signal and the second optical signal to obtain the optical signal of interest.

[0107] Example C2 includes the method of example Cl or any of examples C1-C5, wherein the first laser light comprises s and p polarizations.

[0108] Example C3 includes the method of example Cl or any of examples C1-C5, wherein the birefringent optical element is a half- wave plate matched to a wavelength of the first laser light.

[0109] Example C4 includes the method of example Cl or any of examples C1-C5, wherein22185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT each of the first beam and the second beam have passed through at least one mirror before being received at the first photodetector or the second photodetector.

[0110] Example C5 includes a computer program product comprising instructions, which, when the computer program product is executed by a computer, cause the computer to carry out the method of example Cl or any of examples C1-C5.

[0111] In some embodiments in accordance with the disclosed technology (example DI), a method for acquiring a transient grating spectroscopy (TGS) signal by diffracting a probe laser beam from a target material, the probe laser beam comprising two mutually orthogonal polarization components, the method includes: causing a first laser beam to be incident upon a beam splitter configured to split the first laser beam into a reference beam and the probe beam; causing either the reference beam or the probe beam to be incident upon a birefringent optical element, wherein the birefringent optical element is configured to cause the two mutually orthogonal polarization components of a light beam passing therethrough to receive a phase difference of approximately 180 degrees; causing the probe beam to be incident upon the target material to generate a diffracted probe beam; causing the reference beam to be incident upon the target material to generate a reflected reference beam; causing the diffracted probe beam and the reflected reference beam to be combined to generate a combined beam; causing the combined beam to be incident on a polarizing beam splitter configured to split the combined beam into a first measurement beam and a second measurement beam, wherein the first and second measurement beams respectively correspond to the two mutually orthogonal polarization components; receiving the first measurement beam at a first photodetector configured to produce a first optical signal based on the first measurement beam; receiving the second measurement beam at a second photodetector configured to produce a second optical signal based on the second measurement beam; and performing a subtraction between the first optical signal and the second optical signal to obtain a signal indicative of a physical property of the target material.

[0112] Example D2 includes the method of example DI or any of examples D1-D4, comprising: causing the probe beam to be incident on a phase adjuster, wherein the phase adjuster causes the probe beam to acquire a phase difference with respect to the reference beam.

[0113] Example D3 includes the method of example DI or any of examples D1-D4, wherein the probe beam, reference beam, diffracted probe beam, and reflected reference beam are mutually23185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT coplanar.

[0114] Example D4 includes the method of example DI or any of examples D1-D4, comprising: prior to causing the first laser beam to be incident on the beam splitter, causing the first laser beam to be incident upon a second birefringent optical element, wherein the second birefringent optical element is configured to ensure that the first laser beam has a substantially equal amplitude for the two mutually orthogonal polarization components.Conclusion

[0115] Implementations of the subject matter and the functional operations described in this patent document can be implemented in various systems, digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Implementations of the subject matter described in this specification can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a tangible and non-transitory computer readable medium for execution by, or to control the operation of, data processing apparatus. The computer readable medium can be a machine-readable storage device, a machine-readable storage substrate, a memory device, a composition of matter effecting a machine-readable propagated signal, or a combination of one or more of them. The term “data processing unit” or “data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them.

[0116] A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple24185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT coordinated files (e.g.. files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

[0117] The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

[0118] Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Computer readable media suitable for storing computer program instructions and data include all forms of nonvolatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

[0119] Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.25185396529.6

Claims

PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT CLAIMSWhat is claimed is:

1. An optical apparatus for performing transient grating spectroscopy (TGS) on a target, the optical apparatus comprising:a probe laser source configured to generate a probe beam comprising a first polarization component and a second polarization component;a birefringent optical element positioned in an optical path of the probe beam and configured to generate a processed probe beam by introducing a phase difference between the first polarization component and the second polarization component; a lens system configured to direct the processed probe beam to the target and to receive a return signal from the target;a polarizing beam splitter configured to split the return signal into a first polarization signal and a second polarization signal;a first photodetector configured to receive the first polarization signal; anda second photodetector configured to receive the second polarization signal, wherein a combination of the first polarization signal and the second polarization signal provides a background-subtracted measurement signal indicative of a property of the target.

2. The optical apparatus of claim 1, wherein the phase difference is substantially 180 degrees or 90 degrees.

3. The optical apparatus of claim 1, wherein the birefringent optical element comprises a half-wave plate.26185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT 4. The optical apparatus of claim 1, wherein the first polarization component and the second polarization component comprise orthogonal linear polarizations.

5. The optical apparatus of claim 1, wherein the lens system comprises a first achromat lens and a second achromat lens, and wherein the birefringent optical element is disposed between the first achromat lens and the second achromat lens.

6. The optical apparatus of claim 1, comprising a phase adjuster configured to apply a substantially equal phase adjustment to the first polarization component and the second polarization component of the processed probe beam.

7. The optical apparatus of claim 1, wherein:the birefringent optical element is a first birefringent optical element, andthe optical apparatus comprises a second birefringent optical element disposed upstream of the birefringent optical element and configured to adjust relative intensities of the first polarization component and the second polarization component.

8. The optical apparatus of claim 1, comprising a diffraction grating positioned in the optical path of the probe beam and configured to diffract the probe beam into a plurality of diffraction orders.

9. The optical apparatus of claim 8. wherein:the processed probe beam comprises a reference beam and a signal beam, each derived from one of the plurality of diffraction orders from the diffraction grating, and27185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT the return signal comprises an interference between the reference beam reflected from the target and a diffracted beam generated by interaction of the signal beam with a transient grating on the target.

10. The optical apparatus of claim 9, wherein the reference beam and the signal beam are directed to the target along separate optical paths.

11. The optical apparatus of claim 10, wherein the birefringent optical element is disposed in an optical path of the signal beam.

12. The optical apparatus of claim 9, wherein the interference produces a homodyne signal.

13. The optical apparatus of claim 1, further comprising a pump laser source configured to induce a transient grating on the target.

14. The optical apparatus of claim 13, wherein the lens system is configured to direct pump beams from the pump laser source to the target such that the pump beams interfere to form the transient grating.

15. The optical apparatus of claim 13, comprising a notch filter disposed in an optical path of the return signal and configured to attenuate light at a wavelength of the pump laser source.

16. The optical apparatus of claim 1, wherein the property of the target comprises at least one of a thermal diffusivity or a surface acoustic wave speed.28185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT17. The optical apparatus of claim 1, wherein the probe laser source, the birefringent optical element, the lens system, and the polarizing beam splitter are arranged in a planar optical configuration.

18. The optical apparatus of claim 1, wherein the first photodetector comprises an avalanche photodiode (APD).

19. A method for acquiring a signal indicative of a physical property of a target using a transient grating spectroscopy (TGS) technique, the method comprising:generating a probe beam comprising a first polarization component and a second polarization component;introducing a phase difference between the first polarization component and the second polarization component using a birefringent optical element to generate a processed probe beam;directing the processed probe beam to the target using a lens system;receiving a return signal from the target;splitting the return signal into a first polarization signal and a second polarization signal using a polarizing beam splitter;detecting the first polarization signal at a first photodetector to produce a first optical signal;detecting the second polarization signal at a second photodetector to produce a second electrical signal; andperforming a subtraction between the first electrical signal and the second electrical signal to obtain the signal indicative of the physical property of the target.29185396529.6PCT Application Attorney Docket No.: 009062.8572.WO00SD2025-079-2PCT 20. The method of claim 19, comprising:generating, from the probe beam, a reference beam; anddirecting the reference beam to the target using the lens system,wherein the return signal comprises a homodyned combination of signals received from the target.30185396529.6

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