A method and system for characterizing local heat dissipation dynamics based on infrared photothermal microscopy

Abstract

This invention discloses a method and system for characterizing local heat dissipation dynamics based on infrared photothermal microscopy. This system, based on scanning probe microscopy (SPM), achieves sensitive detection of infrared absorption without requiring any interferometer-related optical components. A microsphere with both infrared absorption and Raman thermometry functions is manipulated using a scanning probe and sequentially brought into contact with different test areas on a substrate to construct a heat source-medium-heat sink microscopic thermal heterojunction. A modulated infrared pump beam and a visible detector beam are synthesized into a coaxial beam to selectively heat the microsphere, causing periodic temperature changes. The scattered light modulation signal is collected and obtained through lock-in amplification and demodulation to acquire the photothermal signal. By combining the quantitative relationship between the peak position of Raman spectral characteristic peaks and temperature, the relative differences in heat conduction mechanisms in each region are calculated and inverted, achieving non-destructive characterization of local heat conduction dynamics at the micrometer scale.

Description

Technical Field

[0001] This invention relates to the field of micron-scale local heat conduction technology, and in particular to a method and system for characterizing local heat dissipation dynamics based on infrared photothermal microscopy. Background Technology

[0002] The infrared (IR) band covers the fundamental vibrational region of most molecules, providing "fingerprint" information reflecting chemical bond types and molecular structural characteristics. However, traditional Fourier transform infrared (FTIR) microscopy is limited by the diffraction limit, with a spatial resolution of approximately 3-30 µm. Meanwhile, the development of two-dimensional materials and on-chip devices has made the problems of thermal conduction and dissipation at the micro- and nano-scale increasingly prominent. Existing thermal conductivity measurement techniques (such as suspended micro-hot stages and time-domain thermal reflectometry) are complex to prepare and require stringent conditions, making it difficult to directly compare the differences in thermal dissipation of different structures on-chip using a controllable localized heat source. While near-field techniques such as scanning near-field optical microscopy (s-SNOM) and photothermal induced resonance (PTIR) have pushed the resolution to approximately 20 nm, they are complex systems, probe shallow surfaces, and have limited ability to characterize thick samples and localized thermal conduction processes. Taking the mid-infrared (MIR) band as an example, mid-infrared photothermal microscopy (MIP) converts infrared absorption into visible light scattering or phase modulation, eliminating the need for infrared detectors and interferometric demodulation. This can improve the resolution to the micrometer level, making it an important pathway between the far-field (µm level) and near-field (approximately 20 nm level).

[0003] In summary, existing infrared photothermal microscopy schemes mostly focus on chemical imaging and lack quantitative calibration of the relationship between photothermal signals and sample physical temperature rise. Especially under on-site conditions, it is difficult to directly map the photothermal signal intensity into quantifiable thermal physical variables, thus failing to meet the quantitative characterization requirements of micro- and nano-scale thermal properties. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, the technical problem this invention aims to solve is to propose a method and system for characterizing local heat dissipation dynamics based on infrared photothermal microscopy. It also proposes a method that utilizes a probe to manipulate the same probe microsphere as a controllable and repositionable local heat source, combining Raman spectroscopy calculations and inversions to obtain the differences in relative heat conduction mechanisms in different regions, thus achieving non-destructive characterization of local heat conduction dynamics at the micrometer scale. Furthermore, an infrared photothermal microscopy spectroscopy system based on a scanning probe microscope (SPM) is constructed, enabling sensitive detection of infrared absorption without requiring any optical components related to interferometers.

[0005] To achieve this objective, the present invention adopts the following technical solution: This invention provides a method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy, comprising the following steps: S00: Prepare a substrate having at least two distinct test regions; S10: Using a scanning probe to manipulate the same microsphere with infrared absorption characteristics, the microsphere is brought into contact with different test areas in sequence; S20: When the microsphere is located in each test area, perform the following measurement procedure: First, the modulated infrared pump beam and the visible detection beam are combined into a coaxial beam. The microsphere is then irradiated with the coaxial beam, causing the microsphere to undergo periodic temperature changes due to infrared absorption. The scattered light modulation signal caused by the periodic temperature changes is then collected. Finally, the scattered light modulation signal is amplified and demodulated using a lock-in method to obtain the photothermal signal intensity that reflects the change in the refractive index gradient of the microsphere. S30: Before and during the measurement process, the Raman spectrum of the microsphere is collected, and the absolute temperature change of the microsphere under a given infrared pumping condition is calibrated by using the predetermined quantitative relationship between the peak position of the Raman characteristic peak and the temperature. S40: Based on the differences in photothermal signal intensity exhibited by the microspheres in different test areas, quantitatively compare and characterize the local heat dissipation dynamics of the different test areas.

[0006] A local heat dissipation dynamics characterization system based on infrared photothermal microscopy, used to implement the local heat dissipation dynamics characterization method based on infrared photothermal microscopy as described above, includes the following modules: The scanning probe microscope module features a movable scanning probe for manipulating and repositioning infrared-absorbing microspheres to different test areas of the sample. An infrared pump light source is used to generate a modulated infrared beam to selectively heat the microspheres; Visible light detection light source, used to generate a detection beam; The beam combining and focusing optical path includes a wavelength-selective reflective / transmittable sheet and a focusing element, used to combine and collinearly focus an infrared pump beam and a visible probe beam onto a microsphere on the sample; the wavelength-selective reflective / transmittable sheet is highly transparent to visible light and highly reflective to infrared light; the focusing element is an off-axis parabolic mirror. The signal acquisition module includes a photodetector, which is used to collect the visible probe light scattered by the microsphere and convert it into an electrical signal; A lock-in amplifier, synchronized with the modulation signal of the infrared pump light source, is used to demodulate the photothermal signal from the electrical signal, and the demodulation frequency is synchronized with the modulation frequency of the infrared pump light. The Raman spectroscopy acquisition module is used to acquire the Raman spectrum of the microspheres in situ for temperature calibration. The Raman spectroscopy acquisition module shares the visible light detection source and part of the optical path with the signal acquisition module, and the scattered light is introduced into the Raman spectrometer or photodetector through a switchable flip mirror; The control and data processing module is used to collect signals, perform noise reduction and optimization processing, and then calculate the temperature of the microspheres and analyze the heat dissipation characteristics of different regions based on the photothermal signals and Raman spectrum peak shift data.

[0007] The beneficial effects of this invention are as follows: (1) This case utilizes a self-built photothermal system to further extract the difference in thermal conductivity of the substrate in contact with the sample by measuring the linear variation characteristics of the photothermal signal with the incident IR power; thereby achieving high-precision detection of the infrared absorption spectrum of the standard sample PS microspheres, evaluating the system's performance, and obtaining a full width at half maximum (FWHM) of only 5.096 cm. -1 The signal-to-noise ratio can reach up to 76, and the system does not require interference demodulation. The time to acquire the spectrum at a single wavenumber point is on the order of hundreds of milliseconds, thus achieving high-fidelity and high-sensitivity detection. (2) Using the SPM probe to manipulate the same probe microsphere, it was pushed sequentially to different substrates / different structural regions of the sample to be tested (such as two-dimensional materials with different layers, different metal substrates, etc.). Combined with Raman pyrography, the correspondence between the infrared vibrational relaxation process and temperature change was established in the MIP, and the linear temperature coefficient κ of the Raman peak was calculated. With the help of the precise manipulation capability of the SPM probe on the sample surface, a two-dimensional material thermal heterostructure based on the same PS microsphere was constructed. The heat dissipation dynamics in the structure were locally characterized by numerical simulation and physical model. (3) This case demonstrates the linear mapping between MIP intensity, MIR optical power, and PS temperature by calibrating the temperature coefficient based on Raman spectroscopy. Within a linear range that does not damage the sample, the temperature change of the PS microspheres can be adjusted by controlling the MIR pump power, and the local temperature rise can be accurately measured by using the photothermal signal intensity and the calibrated temperature coefficient. This calibration process means that photothermal technology can not only be used for qualitative chemical information measurement, such as infrared absorption spectroscopy, but also has the potential to become an effective tool for thermal measurement. This controllable photothermal modulation provides a new experimental approach for conducting micro-nano scale heat transport research using MIP technology; (4) This study focuses on layer number control for heat conduction, using PS microspheres as local heat sources. It successfully observed the difference in total thermal conductivity of two-dimensional materials caused by variations in the thickness of WS2, and discussed the changing trend of local heat dissipation dynamics. Compared with existing methods for measuring the thermal conductivity of two-dimensional materials, this method has significant advantages: it eliminates the need for fabricating suspended micro-thermal stages and the use of ultrafast laser systems required for time-domain thermal reflectometry (TDTR). Furthermore, this method can directly construct precise local heat sources on-chip and be used for measuring thermal processes within the device. Attached Figure Description

[0008] Figure 1 This is a system optical path diagram of infrared photothermal microscopy provided in a specific embodiment of the present invention, which includes a spectrometer, an infrared laser, a visible laser, a mirror, indium tin oxide (ITO), zinc selenide (ZnSe), a flip mirror, a detector, a lock-in amplifier, and a scanning probe microscope (SPM). Figure 2 This is the optical system signal connection diagram (PD: photodetector, FG: function generator) provided in Embodiment 1 of the specific implementation of the present invention. Figure 3 The images show the time-response curve (left) and energy level diagram (right) of the photothermal process provided in Embodiment 1 of this invention. For vibrational states, g is the ground state; Figure 4 The simulated microsphere cross-sectional temperature distribution diagram provided in a specific embodiment of the present invention includes: (a) Numerical simulation results of temperature distribution obtained under 4 mW infrared excitation, where Temp. represents temperature.

[0009] (b) Schematic diagram of the time relationship between the mid-infrared pump pulse, the temperature rise of the PS microspheres, and the volume change. The period of the infrared light source is T, and the pulse width is τ; Figure 5 This is a diagram of MIP signal variation provided in a specific embodiment of the present invention, wherein (a) is a schematic diagram of the time evolution of the MIP signal, and (b) is a schematic diagram of the physical model of the quasi-steady-state MIP process; Figure 6 This is a schematic diagram illustrating the variation of MIP intensity with modulation frequency according to a specific embodiment of the present invention, wherein (a) shows the linear relationship between MIP intensity and visible light (633 nm) detection power, and (b) shows the relationship between MIP intensity and 1494 cm⁻¹. -1 The linear relationship of pump power, (c) is the photothermal spectral response at different modulation / pulse frequencies, and (d) is the response at a specific vibration mode (1496 cm⁻¹). -1 At point ), the trend of MIP intensity changing with modulation frequency; Figure 7 This is a schematic diagram of the probe at different positions provided in a specific embodiment of the present invention, wherein (a) shows the relative position of the SPM probe and the PS microsphere, and (b) and (c) show the manipulation of the probe to move the PS sphere to different substrates. The dashed box in the figure marks the same PS sphere. Figure 8 The simulated temperature gradient and energy flow direction (black arrow) are provided in a specific embodiment of the present invention. Figure 9 Here is a schematic diagram of heat flux change provided in a specific embodiment of the present invention: (a) is a color diagram showing the magnitude of heat flux, and the arrow indicates the direction of the heat flux vector; (b) is a schematic diagram of the heat transfer path during the vibration relaxation of the PS microsphere. Figure 10 This is the relationship between the integral heat flux and the number of layers in WS2 provided in a specific embodiment of the present invention; Figure 11 This is a schematic diagram of the photothermal absorption spectrum relationship of PS microspheres provided in a specific embodiment of the present invention, wherein (a) is the MIP response under a 100 kHz infrared pulse, (b) is the energy absorbed by the molecule released through thermal dissipation, (c) is the spectral fidelity, the FTIR spectrum of the PS microsphere (top) and the normalized MIP spectrum measured on a 5 µm PS microsphere (bottom), and (d) is the spectral resolution of the MIP. Figure 12 This is the PS benzene ring vibration mode (approximately 1000 cm) provided in a specific embodiment of the present invention. -1 Raman intensity distribution diagrams of the sample are shown in (a) optical microscope image, (b) calibration curve of PS Raman peak as a function of temperature, (c) Raman spectra at different infrared powers, showing characteristic vibrational modes at ~1000 cm-1, and (d) linear relationship between the position of 1001 cm-1 Raman peak and infrared power. Figure 13 This is a schematic diagram of probe manipulation provided in a specific embodiment of the present invention, wherein (a) is a schematic diagram of SPM probe manipulation, and (b) is an optical image of multilayer WS2 (top), wherein the 1L, 2L, 4L, and 5L regions are marked by dashed lines; the illustration is a schematic diagram of the corresponding layer outline; Figure 14 This is a schematic diagram showing the variation of MIP intensity of PS microspheres with MIR power in a specific embodiment of the present invention, wherein (a) shows the relationship between MIP intensity of PS microspheres on 2LWS2 and MIR power, and (b) shows the relationship between MIP intensity of PS microspheres on 5LWS2 and MIR power. Figure 15 This is a schematic diagram of the normalized photothermal response of the same PS microsphere on different layers of WS2 provided in a specific embodiment of the present invention. Detailed Implementation

[0010] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0011] Existing infrared photothermal microscopy schemes mostly focus on chemical imaging and lack quantitative calibration of the relationship between photothermal signals and sample physical temperature rise. Especially under on-sheet in-situ conditions, it is difficult to directly map the photothermal signal intensity into quantifiable thermal physical variables, thus failing to meet the quantitative characterization requirements of micro- and nano-scale thermal properties. This can be summarized in the following three aspects: (1) Traditional methods make it difficult to directly compare the differences in heat dissipation of different structures on the chip, and lack in-situ temperature calibration methods; (2) Traditional thermal conductivity measurement techniques are complex to prepare and require harsh conditions, making it impossible to perform local thermal characterization in situ on the device; (3) Traditional near-field techniques (such as s-SNOM) are complex systems, and traditional FTIR is limited by diffraction.

[0012] Based on the above, this invention provides a method and system for characterizing local heat dissipation dynamics based on infrared photothermal microscopy. The main concept integrates the chemical recognition capability of infrared photothermal spectroscopy (MIP), the use of a probe-manipulated microsphere as a movable local heat source, and Raman spectroscopy temperature measurement onto a single SPM platform. Specifically, firstly, it utilizes a probe to manipulate the same probe microsphere as a controllable and repositionable local heat source and temperature measurement, establishing a quantitative bridge between photothermal signals and temperature; secondly, it constructs a standardized microscopic thermal measurement model (heat source-medium-heat sink model) on-chip based on the SPM probe, avoiding complex suspended micro-heat stages and other special sample preparation methods; and thirdly, it constructs an interferometer-free photothermal-Raman hybrid system, achieving highly sensitive detection and temperature calibration with lower cost and complexity. In practice, a single microsphere is simultaneously used as a movable local heat source and a Raman temperature probe, combined with SPM manipulation to construct a standardized microscopic thermal measurement model. This method maps photothermal signal intensity into quantifiable thermal physical variables at the micro- and nano-scale, thus enabling quantitative characterization of thermal properties at the micro- and nano-scale. The following description covers both the characterization methods and the system.

[0013] 1. Firstly, a method for characterizing localized heat dissipation dynamics based on infrared photothermal microscopy is proposed. This method uses a probe to manipulate the same test microsphere as a controllable and repositionable localized heat source. Combined with Raman spectroscopy calculations and inversion, the relative differences in heat conduction mechanisms in different regions are obtained, achieving non-destructive characterization of localized heat conduction dynamics at the micrometer scale. Specifically, it includes the following steps: S00: Prepare a substrate having at least two distinct test areas; use conventional micro / nano fabrication or material preparation to ensure that the test areas have a suitable surface morphology for microsphere contact.

[0014] S10: Manipulate the same microsphere with infrared absorption characteristics using a scanning probe. Preferably, the microsphere is a polymer microsphere. Polymer microspheres are chosen because, compared to rigid inorganic particles, they possess a certain degree of viscoelasticity, allowing for a more flexible and conforming contact with the surface of the two-dimensional material under applied force, effectively reducing and stabilizing interfacial thermal resistance. Further, polymer microspheres (such as polystyrene) are rich in CH and other groups, exhibiting a strong and broad absorption band in the mid-infrared region, making them easily and efficiently heated by infrared pump light. Simultaneously, the Raman spectrum of polymers has sharp skeletal vibration peaks (such as benzene ring breathing vibrations, CC stretching, etc.), with peak positions sensitive to temperature and unlikely to overlap with the signals of the two-dimensional material, making them ideal intrinsic temperature probes. The shift in Raman peak position reflects the temperature of the microsphere after absorbing infrared light, laying a good foundation for subsequent steps.

[0015] Next, the diameter range of the microspheres is selected based on the size of the area to be measured and the optical resolution. Small sizes (nanometer-scale) can detect extremely small areas, but the visible light scattering cross section is small, resulting in a low signal-to-noise ratio. Large sizes (micrometer-scale) provide strong signals, but they average out the differences in submicrometer structures. Flexible selection based on the size of the area to be measured allows for optimal matching of resolution and sensitivity. This selection is also considered in conjunction with the system's optical resolution limit.

[0016] The microspheres are brought into contact with different test areas in sequence; the test areas are two-dimensional material sheets with different numbers of layers. By setting up different layers of regions arranged in sequence on the same substrate, they can share the same microspheres, substrate heat sink, and environmental conditions. Subsequently, through self-comparison of the same microsphere in each test area, the complex factors such as the heat capacity and absorptivity of the microspheres cancel each other out in the difference, so that the weak difference in out-of-plane thermal conductivity can be detected with high sensitivity.

[0017] The microspheres, two-dimensional material sheets, and substrate construct a microscopic thermal heterostructure with a "heat source-medium-heat sink" configuration. In this configuration, the microspheres act as a local heat source, the two-dimensional material sheets as a heat medium, and the substrate as a heat sink. Heat is generated within the microspheres, flows through the microsphere-two-dimensional material sheet interface, passes through the two-dimensional material sheet, then through the two-dimensional material-substrate interface, and finally dissipates into the heat sink substrate. Furthermore, by comparing the differences in photothermal signal intensity of the microspheres on two-dimensional material sheets with different numbers of layers, the relative differences in thermal conduction mechanisms in each test region and the thermal conduction characteristics of the two-dimensional material sheet heterostructure are characterized.

[0018] In step S10, manipulating the same microsphere using a scanning probe effectively eliminates individual differences between microspheres, ensuring consistency in the contact state of the microsphere and achieving repeatable and non-destructive manipulation. Specifically, "manipulating the same microsphere with infrared absorption characteristics using a scanning probe" involves controlling the probe of the scanning probe microscope to perform insertion, withdrawal, and lateral movement operations, sequentially pushing the same microsphere to the target measurement area and ensuring consistent contact between the microsphere and each measurement area. This achieves self-reference comparison measurement, with all measurement areas being detected by the same microsphere, thus better utilizing the differences in the thermal properties of the measurement areas to reflect other signal differences (such as differences in photothermal signals measured by lock-in amplifiers, absolute temperature rise calibrated by Raman spectroscopy, and background signal noise).

[0019] S20: When the microsphere is located in each test area, perform the following measurement procedure: First, the modulated infrared pump beam and the visible detector beam are combined into a coaxial beam. This coaxial beam is then used to irradiate the microsphere. Specifically, the modulated infrared pump beam irradiates the microsphere, causing periodic temperature changes due to infrared absorption. The visible detector beam then irradiates the microsphere and collects the scattered light modulation signal caused by these periodic temperature changes. More specifically, a specific wavelength (such as a mid-infrared quantum cascade laser) is selected to match the strong absorption peak of the microsphere material. Intensity modulation is achieved through an acousto-optic modulator, generating a periodic heat source. In this case, the wavelength of the infrared pump beam is tuned to the characteristic infrared absorption peak of the microsphere.

[0020] Simultaneously, the visible laser (e.g., 532 nm / 633 nm) is focused onto the microsphere, collecting scattered light. Temperature changes in the microsphere modulate its scattering intensity through thermo-optical effects (refractive index change dn / dT) and thermal expansion. This modulation signal is synchronized with the temperature oscillation of the microsphere. Furthermore, the visible probe beam is a continuous laser; compared to pulsed probe beams, continuous lasers avoid complex time gating.

[0021] Building upon the above, to achieve perfect spatial overlap and optimal signal collection of the infrared pump light and the visible probe light, whose wavelengths differ by several times, on a micro-nano scale microsphere, the infrared pump beam and the visible probe beam are further combined into a coaxial beam using a wavelength-selective reflector / transmitter. This coaxial beam is then focused onto the sample region by an off-axis parabolic mirror (OAP). The coaxial beam combined by the wavelength-selective reflector / transmitter (similar to a dichroic mirror) serves as the probe beam. In this way, all the energy of the pump light and all the sensitivity of the probe light are concentrated on the same spatial point on the microsphere, achieving spatial overlap between pumping and detection.

[0022] Finally, the scattered light modulation signal is amplified and demodulated using a lock-in method to obtain the photothermal signal intensity that reflects the change in the refractive index gradient of the microsphere. Lock-in amplification and demodulation can efficiently extract the scattered light changes that are synchronized with the infrared modulation frequency, thereby obtaining a photothermal signal with a high signal-to-noise ratio.

[0023] S30: Before and during the measurement process, the Raman spectrum of the microsphere is acquired. Using a pre-determined quantitative relationship between the peak position of the Raman characteristic peak and temperature, the absolute temperature change of the microsphere under a given infrared pumping condition is calibrated. The absolute temperature calibration of the Raman spectrum includes: Pre-calibration: The quantitative relationship between the Raman characteristic peaks and temperature is pre-determined as follows: the microspheres are placed in a temperature-controlled cavity, and their Raman spectra are acquired at multiple temperature points. Linear fitting is performed on the selected characteristic peak positions to obtain the Raman temperature coefficient κ. The fitted coefficient κ is as follows:

[0024] in, This represents the shift (redshift) of the Raman peak. The change in temperature , The Raman temperature coefficient κ reflects only the intrinsic lattice vibration anharmonicity of the microsphere material.

[0025] Real-time calibration is performed by selecting the Raman characteristic peak of the microsphere molecular framework; using the redshift of this peak position as the infrared pump power increases, and combined with the Raman temperature coefficient κ, the absolute temperature rise of the microsphere at each pump power is calculated in real time.

[0026] To ensure that the temperature scale (Raman) and the measured quantity (photothermal signal) are acquired under identical physical conditions, thus establishing a mapping relationship, in steps S20 and S30, the Raman spectrum acquisition and the photothermal signal acquisition are performed in situ at the same optical system and the same microsphere location. Since the acquisition is synchronous or quasi-synchronous under the same microsphere, the same location, and the same pump-probe light field, the Raman peak position and the photothermal signal intensity are correlated under consistent physical conditions. The scattered signal is then directed to the photodetector and the Raman spectrometer respectively via an optical path switching device. This allows for rapid completion of Raman spectrum acquisition and photothermal signal acquisition, or alternating acquisition of both, within the same contact period when the microsphere remains in the area to be measured.

[0027] After calibration in step S30: the same microsphere, under the same infrared pumping conditions, is contacted with different test areas. At this point, the final temperature change of the microsphere is... This depends on the balance between its own heat generation and heat dissipation to the test area. That is, step S40 involves a quantitative comparison of local heat dissipation characteristics: based on the differences in photothermal signal intensity exhibited by the microspheres in different test areas, the local heat dissipation dynamics of the different test areas are quantitatively compared and characterized. Two scenarios are possible: In areas with good heat dissipation, the heat from the microspheres is quickly conducted away, and the microspheres themselves... Lower; In areas with poor heat dissipation, heat accumulates in the microspheres, which themselves... It is high; By comparing the calibrated By using this value, we can quantitatively and directly compare the local heat dissipation capacity of different regions.

[0028] In step S40, the "quantitative comparison and characterization of the local heat dissipation dynamics of the different test areas" includes: extracting the slope of the photothermal signal intensity of the microsphere on different test areas as a function of infrared pump power, or calculating its steady-state temperature rise under the same pump power; wherein, a smaller signal slope or a lower steady-state temperature rise corresponds to a stronger heat dissipation capability in that area.

[0029] 2. A local heat dissipation dynamics characterization system based on infrared photothermal microscopy, used to implement the local heat dissipation dynamics characterization method based on infrared photothermal microscopy as described above, comprising the following modules: The scanning probe microscope module features a movable scanning probe for manipulating and repositioning infrared-absorbing microspheres to different test areas of the sample. An infrared pump light source is used to generate a modulated infrared beam to selectively heat the microspheres; Visible light detection light source, used to generate a detection beam; The beam combining and focusing optical path includes a wavelength-selective reflective / transmittable sheet and a focusing element, used to combine and collinearly focus an infrared pump beam and a visible probe beam onto a microsphere on the sample; the wavelength-selective reflective / transmittable sheet is highly transparent to visible light and highly reflective to infrared light; the focusing element is an off-axis parabolic mirror. The signal acquisition module includes a photodetector, which is used to collect the visible probe light scattered by the microsphere and convert it into an electrical signal; A lock-in amplifier, synchronized with the modulation signal of the infrared pump light source, is used to demodulate the photothermal signal from the electrical signal, and the demodulation frequency is synchronized with the modulation frequency of the infrared pump light. The Raman spectroscopy acquisition module is used to acquire the Raman spectrum of the microspheres in situ for temperature calibration. The Raman spectroscopy acquisition module shares the visible light detection source and part of the optical path with the signal acquisition module, and the scattered light is introduced into the Raman spectrometer or photodetector through a switchable flip mirror; The control and data processing module is used to collect signals, perform noise reduction and optimization processing, and then calculate the temperature of the microspheres and analyze the heat dissipation characteristics of different regions based on the photothermal signals and Raman spectrum peak shift data.

[0030] Therefore, Example 1 is proposed: 1. Sample preparation 1.1) Substrate: Gold film preparation was performed using an ultra-flat gold film prepared by reverse peeling. A cleaned and polished single-crystal silicon wafer or silicon nitride wafer was used as the substrate. A gold layer approximately 30 nm thick was deposited on the substrate surface using an electron beam evaporation apparatus, with the evaporation rate controlled at 0.1 Å / s. Photosensitive adhesive was then applied to the gold-deposited substrate, and a quartz window was placed on the adhesive before curing the adhesive under ultraviolet light. The quartz substrate was then peeled back from the silicon wafer using a scalpel blade to obtain an ultra-flat gold film surface. 1.2) Microsphere Samples: Polystyrene (PS) microspheres with a diameter of 5 µm were used. Commercially available PS microspheres were ultrasonically dispersed, diluted with ethanol / deionized water, and ultrasonicated for 1 min. The suspension was then drop-coated onto a silicon substrate covered with an approximately 30 nm gold film. After natural drying, residual solvent was gently purged with nitrogen. All microsphere samples below are referred to as PS samples. 1.3) Transfer and preparation of two-dimensional material sheets: WS2 obtained by mechanical exfoliation was selectively transferred to the above gold (Au) film substrate using a PDMS-assisted dry transfer method. PS microsphere dispersion was then drop-coated, and the film was naturally dried and purged with nitrogen.

[0031] 2. Optical System Setup The hardware system upon which the method described in this case relies is based on a commercially available scattering near-field scanning optical microscope system (s-SNOM, Neaspec / Attocube, Germany), with modifications made to its external optical path and signal link. The sample stage movement and scanning are accomplished by the scanning probe microscope (SPM) module integrated into the s-SNOM.

[0032] This case uses mid-infrared light as an example. In the excitation optical path, the mid-infrared pump source (IR laser) adopts a tunable quantum cascade laser (QCL) system, model MIRcat (Daylight Solutions). The QCL has a wide spectral coverage range, covering a variety of important molecular vibrational resonance modes. The detection source uses a 633 nm continuous visible laser (Vis laser) (CNI Co., China). First, the output power of the two lasers is measured and recorded. Then, by adjusting the reflector and the beam combining optical path, the spatial coupling between the mid-infrared pump light and the visible detection light is checked at the sample position to ensure good spatial overlap between the two beams.

[0033] like Figure 1As shown, the mid-infrared laser light output by the QCL is guided by a mirror assembly and reflected by an indium tin oxide (ITO) coating, while the 633 nm visible detector light passes through the ITO coating. ZnSe, a zinc selenide window, serves as both a mid-infrared transmission window and a visible scattering signal reflection coupling element in this system. ZnSe is a semi-reflective and semi-transmissive lens in both the visible and infrared bands; in this case, its role is to reflect the scattered light signal to the collection path without affecting the transmission of the incident light. Utilizing the high transmittance of visible light and the reflectivity of mid-infrared light of the ITO coating, the mid-infrared pump light and the 633 nm visible detector light are coaxially combined. The combined beam is finally focused onto the sample region by an off-axis parabolic mirror (OAP, NA = 0.24) in the s-SNOM architecture, thus achieving collinear excitation and detection of the infrared pump and visible detector.

[0034] The detection signal generated by sample scattering is converged by the collecting optical path, switched to the detector end by the flip mirror, and then... Figure 2 As shown, the signal is coupled into a high-speed photodetector (Thorlabs, PDA100A2) with preamplification for far-field detection. The electrical signal output from the photodetector (PD) is then input to the signal terminal (Sig. In) of a high-speed lock-in amplifier (Zurich Instruments, HF2LI) for demodulation. The lock-in amplifier operates in external reference mode, and its reference signal is synchronously output after being driven by a function generator (FG) and modulated by a QCL, and then connected to the reference terminal (Ref.) of the lock-in amplifier. This ensures strict synchronization between pump modulation and signal detection at the hardware level. The demodulated data transmission, acquisition, storage, and subsequent processing are implemented using an AFM and a computer (PC).

[0035] In this case, a self-built photothermal system was integrated with a spectrometer, enabling the acquisition of Raman spectra at the same spatial location as the photothermal signal. Temperature calibration experiments were performed using a commercial Raman spectrometer and a high-precision temperature-controlled cavity. Using the 638 nm laser of a commercial Raman microscope, the Raman spectrum of a single PS microsphere was first acquired and imaged. A 1001 cm⁻¹ section of the PS molecular framework was selected. -1 The ring breathing vibration mode at the location is used as the characteristic peak. (Technical effect) Figure 12 The image shows the two-dimensional distribution of the Raman peak intensity, which matches well with the outline (dashed line) of the optical micrograph of the microsphere, proving the accuracy of the Raman signal localization.

[0036] 3. Signal generation principle (excluding two-dimensional material sheets) Taking the mid-infrared band as an example, the working mechanism of mid-infrared photothermal spectroscopy in signal acquisition is shown in Figure 3. When molecules in the sample absorb incident infrared (IR) photons, their vibrational state transitions from the ground state g to the corresponding infrared vibrational energy level. .

[0037] This absorption-excitation process occurs on a femtosecond to picosecond timescale. Subsequently, the excited molecules release energy into the surrounding medium through a series of ultrafast nonradiative relaxation pathways, generating localized thermal energy and causing the PS sample temperature to rise, which in turn leads to sample volume expansion. and refractive index ( The sample undergoes repeated heating and cooling processes under periodic pulse excitation, resulting in periodic temperature fluctuations and alternating volume expansion and contraction. Phase-locked demodulation of the probe light intensity reflects the temperature changes caused by volume expansion. The photothermal signal is acquired in the time domain and analyzed in the frequency domain.

[0038] Under periodic MIR (mid-infrared) pulse irradiation, the PS sample temperature exhibits a typical heating-cooling cycle over time. When the pulse is on (Phase I), continuous infrared absorption and relaxation processes cause a rapid increase in the local sample temperature. After the pulse is off (Phase II), the PS sample dissipates heat to the surrounding environment due to thermal diffusion, and the temperature gradually decays back to its initial state. These periodic temperature fluctuations further lead to periodic modulation of the refractive index and volume, thereby altering the scattering or phase of the visible probe light.

[0039] The temperature field and heat flux distribution of the PS sample during the photothermal process were calculated using finite element numerical simulation. To simplify the model and calculation, the air layer and interfacial thermal resistance were temporarily ignored. The wave optics and heat transfer modules of the commercial finite element simulation software COMSOL Multiphysics® were used to numerically simulate the temperature field and heat flux distribution. In the wave optics module, the substrate (gold thin film) was approximated as an ideal electrical conductor in the mid-infrared (MIR) band to simulate its near-perfect reflection characteristics. A substrate size of 10 mm was preferred. Taking a 10 mm diameter as an example, and with PS microspheres having a diameter of 5 µm, to balance reasonable boundary conditions and computational efficiency, the gold film is treated as a semi-infinite thickness medium in the model. The temperature of the gold film substrate is set to room temperature. Figure 4 The figure shows the simulated temperature distribution of the microsphere cross section. As can be seen from the figure, under the action of a pump laser pulse, the temperature inside the PS microsphere rises most significantly, and the heat is mainly confined to the internal region of the microsphere itself, with only a small portion of the heat being conducted to the substrate through the bottom contact point.

[0040] Mechanistically, the modulation of the scattered light intensity mainly originates from the changes in volume (V) and refractive index (n) caused by the temperature change of the PS sample. By adding the first-order terms of the expansion of the scattered light intensity relative to the microsphere radius r and refractive index n, the modulation amount of the scattered intensity satisfies the following formula.

[0041] When a pulsed MIR laser irradiates a PS microsphere (PS-MS) at t = 0, the absorbed energy is converted into heat through vibrational relaxation. The temperature of the microsphere gradually increases within the time interval 0 ≤ t ≤ τ, resulting in an increase in heat (temperature change) of ΔT. The microsphere then cools back to room temperature. This temperature rise causes thermoelastic expansion of the microsphere; for small perturbations, the relative volume change is proportional to the temperature rise, where α is the linear thermal expansion coefficient of the PS microsphere.

[0042] In the measurement, the MIP signal corresponds to the modulation component of the side-scattered probe field collected by the reflective objective lens, and is obtained through lock-in amplification and demodulation. As described below, the measured modulation is ultimately generated by... Through thermoelasticity (i.e., the expansion due to changes in the volume of microspheres) ) and thermal refraction ( n) Driven by both pathways.

[0043] The MIP signal strength I is proportional to the total volume expansion ΔV within the time window 0~τ, i.e. Based on this, an energy conservation principle is introduced. Within the time interval 0 to τ, the MIR pump light will transfer energy Q... MIR When injected into PS microspheres, a portion of the energy is transferred as heat through the interface between the microspheres and the metal substrate and dissipated to the heat sink (gold film). The remaining energy, Qabs, contributes to the temperature increase of the PS microspheres. Therefore:

[0044] The heat dissipation term Qdis consists of three parts: heat conduction, radiation, and convection. Based on the calculations above, the temperature gradient is largest at the interface between the PS microspheres and the substrate. Assuming that heat losses due to radiation and convection are negligible, Qdis is approximated as being entirely contributed by heat conduction. The gradient determines the sign, where The thermal conductivity of the substrate can be expressed in general form as:

[0045] If we denote the contact area between the microsphere and the substrate as S, then the total heat dissipation energy over the time interval (0~τ) can be written as:

[0046] Because the distance between the microspheres and the substrate is only on the order of micrometers ( The spatial gradient of the temperature field is mainly distributed along the x-direction, therefore it can be approximated as:

[0047] in, With incident MIR pump power Approximately proportional This represents the characteristic distance between the microsphere and the substrate, which can be taken as the radius of the PS microsphere. This approximation holds under quasi-steady-state heating conditions within the thermal diffusion depth range.

[0048] It can be derived as follows:

[0049]

[0050] In summary, as Figure 5 As shown, the conclusion is that the first derivative of the MIP intensity with respect to pump power decreases as the thermal conductivity of the substrate increases. In other words, by measuring the linear variation characteristics of the photothermal signal with incident IR power, the differences in thermal conductivity of the substrates in contact with the sample can be compared. This theoretical model provides a foundation for analyzing local heat dissipation dynamics using photothermal methods.

[0051] 4. Signal acquisition and testing In principle, when an IR pulse periodically modulates a sample, the induced photothermal signal will also change at the corresponding pump modulation frequency. Using commercially available PS microspheres as the sample, when the infrared pulse frequency was set to 100 kHz, a characteristic photothermal scattering signal at the corresponding frequency was observed on the surface. Notably, the signal at this frequency disappeared when the infrared beam was turned off, proving that the observed signal was indeed generated by IR excitation.

[0052] To verify the chemoselectivity of this method for PS microspheres, the mid-infrared pump was tuned to a wavelength of 1496 cm⁻¹ for PS. - ¹ Absorption peak. (1492-1496 cm⁻¹ in the following text) - ¹ These all refer to the test wavenumber near the characteristic absorption peak; the specific value may vary slightly depending on the experimental conditions. Infrared light was alternately turned on and off by controlling the shutter speed. The visible scattering signal from the sample surface exhibited a significant increase and decrease with the shutter speed, reflecting a typical IR light-induced photothermal triggering process. When the excitation wavenumber was adjusted to 1640 cm⁻¹... - ¹ (corresponding to the non-absorption region of PS) shows almost no response to the photothermal signal, and its behavior is similar to that when there is no infrared irradiation, indicating that the method has bond-selective excitation characteristics for infrared vibration molds.

[0053] At a pump power of 3.3 mW, the MIP signal intensity increases approximately linearly with the 633 nm probe power. Each 1 mW increase in probe power corresponds to an approximately 68% increase in MIP signal intensity. Within this power range, simply increasing the probe power proportionally amplifies the signal amplitude, and no photodamage or saturation effect was observed. When the probe power is fixed at 8 mW, the MIP signal also exhibits a linear increasing trend with the mid-infrared pump power; each 1 mW increase in pump power results in an approximately 234% increase in photothermal signal intensity, indicating that the MIP signal primarily originates from the heat generated during the infrared vibrational relaxation process.

[0054] While keeping the mid-infrared pump pulse duty cycle constant, the pulse modulation frequency was swept from 20 kHz to 120 kHz. The peak intensity decreased with increasing frequency. That is, as the modulation frequency increased, the intensity of each infrared absorption peak decreased overall. This phenomenon can be understood as follows: with a fixed duty cycle, increasing the modulation frequency shortens the single-cycle thermal accumulation time, leading to a decrease in steady-state temperature rise and more rapid thermal relaxation, thereby weakening the MIP response.

[0055] 5. Calculation of temperature coefficient based on Raman spectroscopy Under the same optical path and conditions as described above, a photothermal experiment with variable pump power was conducted, and Raman temperature calibration measurements were performed simultaneously. The photothermal spectroscopy system used was based on a single-wavelength continuous 633 nm laser, which could also be used as the excitation source for Raman spectroscopy. Local temperature calibration could be achieved by utilizing the Raman peak position shift of PS microspheres, thereby establishing the correspondence between the infrared vibrational relaxation process and temperature change in the MIP method.

[0056] Temperature calibration experiments were performed using a commercial Raman spectrometer and a high-precision temperature-controlled chamber. Using the 638 nm laser of a commercial Raman microscope, the Raman spectra of individual PS microspheres were first acquired and imaged. The characteristic Raman peak positions for the PS microspheres are shown in the table below. The ring breathing vibrational mode at 1001 cm⁻¹ in the PS molecular backbone was selected as the characteristic peak.

[0057] Table 1 Raman vibration modes of PS microspheres

[0058] The probe microspheres were placed under a Raman microscope with a closed-loop temperature-controlled cavity, and the temperature was gradually increased (typically within a linear range from room temperature to below 65°C, avoiding the polymer glass transition temperature region, approximately 80-105°C). Raman spectra of the probe microspheres were collected at each temperature point, and the selected characteristic Raman peaks (approximately 1001 cm⁻¹ in the PS system) were analyzed. -1 The circulatory breathing model was fitted, its peak position was extracted, and a linear fit was performed on the peak position-temperature relationship: ν(T)=ν0+κ(T-T0) Where ν represents the peak position and κ represents the Raman temperature coefficient; Based on this, a variable pump power experiment was conducted on the same PS microsphere using a spectrometer. The Raman measurement conditions were as follows: a grating with a groove density of 300 grooves / mm was used, the single integration time was 30 s, and the number of integrations was 1, to further quantify the temperature rise behavior under infrared excitation.

[0059] 6. Controllable heat source with integrated SPM Leveraging the precise manipulation capabilities of the SPM probe on the sample surface, the aforementioned photothermal method is further extended to the study of the thermal conductivity properties of two-dimensional material heterostructures. Specifically, the SPM probe is first inserted into the sample surface once, and then lifted. In the withdrawn state, the distance between the probe tip and the sample surface is greater than 2 μm, which is less than the diameter of the PS microspheres used (5 μm). At this point, the PS microspheres can be pushed to the target position by manipulating the probe longitudinally and laterally through the SPM system, as shown in Figure 7. Using the PS microspheres as a controllable heat source, the SPM probe is used to push and place them on WS2 sheets (i.e., two-dimensional material sheets, hereinafter the same) with different numbers of layers, thus constructing a microscopic thermal heterostructure with a "heat source-medium-heat sink" configuration. Using the same PS microsphere for experiments can reduce signal fluctuations introduced by individual differences in commercially available PS microspheres. At the same time, since the same microsphere has a consistent detection interface and contact morphology and contact area with the substrate, the physical model can be simplified to a certain extent, making it easier to compare the influence of different dielectric layers on thermal conductivity.

[0060] Since the two-dimensional material exhibits almost no absorption in the selected MIR band, this strategy enables selective heating of PS microspheres. In this thermal heterostructure, the PS microspheres generate heat through vibrational absorption under mid-infrared laser irradiation, acting as a localized heat source. WS2 serves as the heat conduction medium, while the bottom metal substrate acts as a heat sink, continuously absorbing the conducted heat, thus functioning as a heat sink.

[0061] The heat transfer from PS microspheres to the Au substrate was obtained through finite element simulation, such as... Figure 8 The diagram illustrates the temperature gradient distribution at the PS microsphere-WS2-gold substrate interface. The black arrows indicate the direction and relative magnitude of heat flow, demonstrating the process of heat transfer from the PS microspheres to the Au substrate. It can be seen that heat mainly enters from the bottom of the PS microspheres into the WS2 sheet, and then is conducted through WS2 to the underlying metal substrate. The heat transfer to the substrate is driven by the temperature gradient at the interface, a fact verified by numerical simulation results. The length of the black arrows is proportional to the heat flux. The incident angles of both the pump and probe beams relative to the substrate normal are 60°.

[0062] Further heat flux simulations of WS2 with different thicknesses were conducted using the heat transfer module in COMSOL Multiphysics to simulate the heat flux through the WS2 material. Thermal conductivity k, heat flux h, and thermal resistance R were defined, and the thickness of a single WS2 layer was defined as 0.7 nm. Multilayer thicknesses were calculated accordingly. The in-plane thermal conductivity k was set. ip The out-of-plane thermal conductivity component k is 100 W / (m·K). op It is 2W / (m·K).

[0063] The heat source is primarily attributed to the vibrational relaxation of the PS-MS. Some heat is transferred to the WS2 layer beneath the PS microspheres (PS-MS). Due to the significant anisotropic thermal conductivity of WS2, the diffused heat will be transported via two paths: out-of-plane and in-plane along WS2. Here, the heat flux at the PS microsphere contact region on WS2 is integrated to quantify the in-plane and out-of-plane heat transported by the two-dimensional material. With increasing layer number, the out-of-plane thermal resistance R... op Increase, while in-plane thermal resistance R ip It remains unchanged.

[0064] as follows Figure 9 and 10 As shown, htotal represents the total heat flux, h ip and h op These represent the in-plane and out-of-plane components of the heat flux, respectively. Although the out-of-plane component h... op It has almost no response to layer thickness, and the in-plane component h ip This shows a linear dependence on the number of WS2 layers. Therefore, a thicker WS2 layer increases the total heat transferred, which helps the PS microspheres cool more quickly. The numerical simulations show good agreement with the experimental results above.

[0065] 7. Effects It mainly includes two major stages: 7.1) Stage I (Calibration Stage): High-precision detection of the infrared absorption spectrum of the standard PS microsphere sample. The system performance was evaluated, yielding a full width at half maximum (FWHM) of only 5.096 cm⁻¹. -1 The signal-to-noise ratio can reach up to 76, and the system does not require interference demodulation. The time to acquire the spectrum at a single wavenumber point is on the order of hundreds of milliseconds. Specifically, it can achieve high-fidelity and high-sensitivity detection at 1460-1660 cm. -1 The photothermal spectrum of the PS microspheres was obtained by scanning the infrared pump wavenumber within the range and compared with the conventional FTIR spectrum. The results show that the photothermal absorption spectrum of the PS microspheres measured by the MIP system is highly consistent with the FTIR spectrum; for example... Figure 11 As shown, it is clearly visible that the stretching vibration of the C=C bonds in the PS microspheres at 1496 cm⁻¹ is due to... -1and 1602 cm -1 The characteristic peak at [location missing]. This indicates that the photothermal detection method in this case can accurately detect the vibrational modes of molecules. Subsequently, at 1492 cm⁻¹ of PS... -1 Near the absorption peak, a finer scan was performed using a smaller step size, resulting in a full width at half maximum (FWHM) of only 5.096 cm⁻¹. -1 The signal-to-noise ratio reached a maximum of 76, validating the system's spectral resolution. Currently, the spatial resolution of the MIP system is approximately 1.3 µm, primarily determined by the 633 nm probe wavelength and the parabolic mirror with a numerical aperture (NA) of 0.24. Furthermore, the elimination of interference demodulation reduces the time required to acquire spectral signals at a single wavenumber point to the order of hundreds of milliseconds, significantly improving the spectral acquisition speed.

[0066] 7.2) Stage II (Measurement Stage): Using the SPM probe, the same probe microsphere was manipulated and sequentially pushed to different substrates / structural regions of the sample under test (such as two-dimensional materials with different layers, different metal substrates, etc.). Combined with Raman pyrography, the correspondence between the infrared vibrational relaxation process and temperature change was established in the MIP, and the linear temperature coefficient κ of the Raman peak was calculated. The temperature rise of the sample under 24 mW infrared pump was calibrated to be approximately 28.7°C. Various parameters affecting the signal during the detection process were optimized. With the precise manipulation capability of the SPM probe on the sample surface, a two-dimensional material thermal heterostructure based on the same PS microsphere was constructed. The thermal dissipation dynamics in the structure were locally characterized and the mechanism was explored by combining numerical simulation and physical model. 7.2.1) As Figure 12 As shown, d is the 1001 cm selected from c. -1 Relationship between Raman peak and IR power. When the MIR pump is tuned to the characteristic absorption peak of PS at 1496 cm⁻¹. -1 As the pump power is gradually increased, the PS microspheres reach a height of approximately 1000 cm³. -1 The Raman peaks exhibited a continuous monotonic redshift, and the peak position change showed an excellent linear relationship with the MIR power. Based on this linear relationship and combined with the previously calibrated Raman temperature coefficient κ, it was calculated that the temperature rise of the PS microspheres was approximately 28.7 °C under the highest 24 mW infrared pump. After the temperature increase, the absolute temperature of the PS microspheres remained below the glass transition temperature of polystyrene (approximately 80-105 °C), therefore no permanent deformation or degradation occurred. The above experiments demonstrated a linear mapping between MIP intensity, MIR optical power, and PS temperature. The results show that, within a linear range without causing sample damage, the temperature change of the PS microspheres can be adjusted by controlling the MIR pump power, and the local temperature rise can be accurately measured using the photothermal signal intensity and the calibrated temperature coefficient. This calibration process implies that photothermal technology can not only be used for qualitative chemical information measurements, such as infrared absorption spectroscopy, but also holds promise as an effective tool for thermal measurements. This controllable photothermal modulation provides a new experimental approach for conducting nanoscale heat transport research using MIP technology.

[0067] 7.2.2) Regarding the layer number modulation for heat conduction, in the two-dimensional material heat dissipation embodiment, the same polystyrene (PS) microsphere was moved to WS2 regions with different layers using a scanning probe microscope probe. Under the same mid-infrared pump power, the photothermal signal generated when the PS microsphere was located in a thinner WS2 region was stronger, while the photothermal signal generated when located in a thicker WS2 region was weaker. Further linear fitting of the photothermal signals under different pump powers revealed that as the number of WS2 layers increased, the slope of the photothermal signal response to the pump power decreased. This result indicates that a thicker WS2 region has a stronger overall heat dissipation capacity, which can more effectively conduct the heat generated by the PS microsphere to the gold film substrate heat sink, thereby reducing the temperature rise of the PS microsphere itself and weakening the photothermal signal. To analyze this phenomenon more quantitatively, such as Figures 13-15 As shown, linear fitting and normalization were performed on the photothermal signal-power curves of WS2 with different layer numbers. It can be seen that WS2 exhibits significant thermal conductivity anisotropy, with its in-plane thermal conductivity being significantly higher than its out-of-plane thermal conductivity. Furthermore, as the number of layers increases, interlayer phonon coupling strengthens, thereby improving its transplane thermal conductivity. Therefore, a significant decrease in the characteristic photothermal peak of PS is observed in the thicker WS2 region, reflecting an increase in its total out-of-plane thermal conductivity.

[0068] By utilizing PS microspheres as a local heat source, the transplanar thermal conductivity difference caused by variations in the thickness of WS2 was successfully observed, and the changing trend of local heat dissipation dynamics was discussed. Compared with existing methods for measuring the thermal conductivity of two-dimensional materials, this method has significant advantages: it eliminates the need for fabricating suspended micro-thermal stages and the use of ultrafast laser systems required for time-domain thermal reflectometry (TDTR). Furthermore, this method enables the direct construction of precise local heat sources on-chip for measuring internal thermal processes, demonstrating significant potential in the study of the thermal properties of two-dimensional materials and on-chip thermal management applications.

[0069] Example 2: (Microsphere / Heat Source Substitution) In practice, the microspheres are not limited to PS microspheres; other microspheres, particles, or thin film samples with characteristic infrared absorption peaks and capable of generating detectable photothermal responses under infrared excitation can also be used. The local heat source can also be PMMA microspheres, polymer microspheres, silica microspheres with surface-modified infrared absorption layers, functionalized microspheres containing specific molecular vibrational groups, carbon-based particles, metal nanostructure-modified microspheres, composite material microspheres, or other micro / nano particles capable of absorbing infrared light and undergoing photothermal conversion. The size of the microspheres can be selected from nanoscale, submicron-scale, or micron-scale microspheres based on the size of the area to be measured, optical resolution, and thermal diffusion length. As long as the microsphere can serve as a mobile local heat source and generate a stable and repeatable photothermal response under infrared excitation, it can be used in this application. WS2 can be replaced with MoS2, WSe2, MoSe2, graphene, h-BN, black phosphorus, two-dimensional heterojunctions, semiconductor thin films, metal thin films, dielectric thin films, polymer thin films, or on-chip multilayer materials. The region to be tested can be a region of different materials, different interfaces, different defects, different crystal orientations, different substrates, different heat sink structures, or a region in different states before and after the device is in operation.

[0070] Example 3: (Light source replacement) In this embodiment, a tunable quantum cascade laser (QCL) is used as the infrared pump source. In actual implementation, the infrared pump source can also be replaced with other light sources that can cover the absorption of the sample or analyte molecule.

[0071] For example, an optical parametric oscillator (OPO), a difference-frequency generation (DFG) infrared light source, a synchrotron radiation infrared light source, a tunable infrared laser, a narrow-linewidth infrared laser, or a modulated broadband infrared light source can be used. As long as the light source can excite the infrared vibrational absorption of the microspheres or the material under test at the target wavenumber and can be intensity-modulated, pulse-modulated, or externally triggered, the photothermal excitation function of this invention can be achieved. In this embodiment, a 633 nm continuous visible laser is used as the probe light. In actual implementation, the probe light wavelength is not limited to 633 nm. The probe light can be replaced with 485 nm, 532 nm, 638 nm, 660 nm, 785 nm, or other ultraviolet, visible, and near-infrared lasers. The selection principle for the probe light is: it should not significantly heat or damage the sample; it should be able to be scattered, reflected, or phase-modulated by the sample; and it should be effectively received by the selected detector. The probe light can be continuous or pulsed; it can be collinear with the infrared pump light or be incident in a non-collinear manner. Any light source that can detect thermal expansion, refractive index changes, or scattering intensity changes caused by infrared absorption can be used as the detection light source in this case.

[0072] Example 4: Alternative to Scanning Probe Platforms The commercially available scattering scanning near-field optical microscope system used in this embodiment, namely the s-SNOM system, is primarily used to provide a scanning probe microscope probe manipulation, sample positioning, sample scanning, and optical path integration platform. In actual implementation, the s-SNOM system is not the only option.

[0073] Any instrument possessing scanning probe or similar micro / nano probe manipulation capabilities, enabling the positioning, pushing, transferring, or repositioning of microspheres, can replace the s-SNOM system in this embodiment. For example, the scanning probe platform can be replaced by a conventional atomic force microscope (AFM), an AFM with an integrated optical interface, an AFM-Raman coupled system, an AFM-IR system, a scanning probe microscope (SPM), an optical microscopy system with micro / nano manipulators, or other micro / nano manipulation platforms with probe insertion, retraction, lateral movement, and sample scanning functions. Similarly, any instrument possessing the ability to move microparticles, such as an optical tweezers system, or an optical method capable of moving and detecting microparticles, is also within the scope of this patent.

[0074] In these alternative solutions, the scanning probe's role is not limited to imaging; it can also serve as a microsphere manipulation tool, used to sequentially move the same PS microsphere or other infrared-absorbing microspheres to different test areas. As long as the repeatable positioning of the same microsphere across multiple test areas can be maintained, and photothermal signal detection can be performed in conjunction with infrared pump light and visible probe light, the same or similar technical effects as in this case can be achieved.

[0075] Example 5: Alternative to Temperature Calibration Methods In this embodiment, temperature calibration is performed using the relationship between Raman peak position and temperature. Alternative methods include: Raman Stokes / anti-Stokes intensity ratio measurement, thermal reflectance measurement, infrared thermography, phase change material calibration, standard sample calibration with known thermal conductivity, finite element simulation inversion calibration, or a combination of multiple temperature measurement methods.

[0076] When Raman spectroscopy is used, the selected characteristic peaks can be chosen from other Raman characteristic peaks that have strong temperature sensitivity, stable peak shape, and high signal-to-noise ratio, depending on the selected microsphere material.

[0077] In summary, the key to this invention is not limited to specific models of s-SNOM systems, light sources, detectors, or microsphere materials, but rather to utilizing repositionable infrared-absorbing microspheres as localized heat sources. By using a platform with probe manipulation capabilities, the same microsphere is sequentially placed in different test areas, and the localized heat dissipation capability is characterized using infrared photothermal detection and temperature calibration methods. Therefore, any equivalent alternative scheme that can achieve the above-mentioned localized heat source construction, photothermal excitation, detection signal acquisition, and heat dissipation parameter analysis should fall within the scope of protection of this application. This invention is not limited to the specific embodiments disclosed herein; other embodiments falling within the scope of the claims of this application are also within the scope of protection of this invention.

Claims

1. A method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy, characterized in that, Includes the following steps: S00: Prepare a substrate having at least two distinct test regions; S10: Using a scanning probe to manipulate the same microsphere with infrared absorption characteristics, the microsphere is brought into contact with different test areas in sequence; S20: When the microsphere is located in each test area, perform the following measurement procedure: First, the modulated infrared pump beam and the visible detection beam are combined into a coaxial beam. The microsphere is then irradiated with the coaxial beam, causing the microsphere to undergo periodic temperature changes due to infrared absorption. The scattered light modulation signal caused by the periodic temperature changes is then collected. Finally, the scattered light modulation signal is amplified and demodulated using a lock-in method to obtain the photothermal signal intensity that reflects the volume expansion and refractive index gradient change of the microsphere. S30: Before and during the measurement process, the Raman spectrum of the microsphere is collected, and the absolute temperature change of the microsphere under a given infrared pumping condition is calibrated by using the predetermined quantitative relationship between the peak position of the Raman characteristic peak and the temperature. S40: Based on the differences in photothermal signal intensity exhibited by the microspheres in different test areas, quantitatively compare and characterize the local heat dissipation dynamics of the different test areas.

2. The method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy according to claim 1, characterized in that: In step S10, the microspheres are polymer microspheres, and the diameter range of the microspheres is selected according to the size of the area to be measured and the optical resolution; The region to be tested is a two-dimensional material sheet with different numbers of layers; the microspheres, the two-dimensional material sheet and the substrate form a microscopic thermal heterostructure with a "heat source-medium-heat sink" configuration, wherein the microspheres are used as a local heat source, the two-dimensional material sheet is used as a heat medium and the substrate is used as a heat sink. By comparing the differences in photothermal signal intensity of microspheres on two-dimensional material sheets with different layers, the relative differences in thermal conduction mechanisms of each test area and the thermal conduction characteristics of heterojunctions of two-dimensional material sheets can be characterized.

3. The method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy according to claim 1, characterized in that, In step S10, the "manipulating the same microsphere with infrared absorption characteristics using a scanning probe" specifically means: controlling the probe of the scanning probe microscope to perform insertion, withdrawal and lateral movement operations, so that the same microsphere is pushed to the target area to be measured in sequence, and the microsphere and each area to be measured form a consistent contact state.

4. The method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy according to claim 1, characterized in that, In step S20, the wavelength of the infrared pump beam is tuned to the characteristic infrared absorption peak of the microsphere, and the visible detection beam is a continuous laser.

5. The method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy according to claim 1, characterized in that, In steps S20 and S30, the Raman spectrum acquisition and the photothermal signal acquisition are performed in situ at the same location of the same microsphere using the same optical system, and the scattered signals are directed to the photodetector and the Raman spectrometer respectively through an optical path switching device.

6. The method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy according to claim 1, characterized in that, In step S20, the infrared pump beam and the visible detection beam are combined into a coaxial beam through a wavelength selective reflector / transmitter, and then focused onto the sample area by an off-axis parabolic mirror.

7. The method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy according to claim 1, characterized in that, In step S30, the quantitative relationship between the Raman characteristic peak and temperature is predetermined by placing the microsphere in a temperature-controlled cavity, collecting its Raman spectrum at multiple temperature points, and performing linear fitting on the selected characteristic peak position to obtain the Raman temperature coefficient κ.

8. The method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy according to claim 7, characterized in that, In step S30, the Raman characteristic peak of the microsphere molecular framework is selected; using the redshift of the peak position as the infrared pump power increases, and combined with the Raman temperature coefficient κ, the absolute temperature rise of the microsphere at each pump power is calculated in real time.

9. The method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy according to claim 1, characterized in that, In step S40, the "quantitative comparison and characterization of the local heat dissipation dynamics of the different test areas" includes: extracting the slope of the photothermal signal intensity of the microsphere on different test areas as a function of infrared pump power, or calculating its steady-state temperature rise under the same pump power; wherein, a smaller signal slope or a lower steady-state temperature rise corresponds to a stronger heat dissipation capability of the area.

10. A system for characterizing local heat dissipation dynamics based on infrared photothermal microscopy, used to implement the method for characterizing local heat dissipation dynamics based on infrared photothermal microscopy as described in any one of claims 1 to 9, characterized in that, Includes the following modules: The scanning probe microscope module has a movable scanning probe for manipulating and repositioning microspheres to different test areas of the sample. An infrared pump light source is used to generate a modulated infrared beam to selectively heat the microspheres; Visible light detection light source, used to generate a detection beam; The beam combining and focusing optical path includes a wavelength-selective reflective / transmittable sheet and a focusing element, used to combine and collinearly focus an infrared pump beam and a visible probe beam onto a microsphere on the sample; the wavelength-selective reflective / transmittable sheet is highly transparent to visible light and highly reflective to infrared light; the focusing element is an off-axis parabolic mirror. The signal acquisition module includes a photodetector, which is used to collect the visible probe light scattered by the microsphere and convert it into an electrical signal; A lock-in amplifier, synchronized with the modulation signal of the infrared pump light source, is used to demodulate the photothermal signal from the electrical signal, and the demodulation frequency is synchronized with the modulation frequency of the infrared pump light. The Raman spectroscopy acquisition module is used to acquire the Raman spectrum of the microspheres in situ for temperature calibration. The Raman spectroscopy acquisition module shares the visible light detection source and part of the optical path with the signal acquisition module, and the scattered light is introduced into the Raman spectrometer or photodetector through a switchable flip mirror; The control and data processing module is used to collect signals, perform noise reduction and optimization processing, and then calculate the temperature of the microspheres and analyze the heat dissipation characteristics of different regions based on the photothermal signals and Raman spectrum peak shift data.

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