An enhanced photoacoustic conversion composite material and an excitation method and system thereof

Abstract

The application discloses an enhanced photoacoustic conversion composite material and an excitation method and system thereof, and belongs to the technical field of photoacoustic conversion, wherein the composite material comprises a phase change absorption layer and a thermal expansion layer arranged on the phase change absorption layer; a second pulsed laser is emitted to irradiate the composite material at the same time when a first continuous laser is turned off, so as to generate photoacoustic waves; the excitation method comprises the following steps: S1, the composite material is irradiated by the first continuous laser; S2, the reflected light signal of the composite material to the first continuous laser is collected in real time; S3, the first continuous laser is turned off, and the second pulsed laser is triggered to irradiate the composite material synchronously; and S4, the volume expansion of the phase change absorption layer after phase change and the increase of the thermal conductivity after phase change drive the thermal expansion layer to expand, so that photoacoustic waves are generated. The application has a relatively simple structure, can form a point-like, planar or arrayed photoacoustic conversion unit according to application requirements, and two effects are synergistically acted, so that the photoacoustic signal far stronger than the traditional thermoelastic mechanism is generated.

Description

Technical Field

[0001] This invention belongs to the field of photoacoustic transduction technology, specifically relating to an enhanced photoacoustic conversion composite material and its excitation method and excitation system. Background Technology

[0002] The photoacoustic effect refers to the physical process by which a material absorbs light energy and converts it into sound energy through nonradiative relaxation, temperature changes, and the resulting volume changes. Photoacoustic transduction features non-contact excitation, fast response speed, and ease of miniaturization and arraying, and can be applied in fields such as laser ultrasound, non-destructive testing, medical imaging, microscale sound sources, and sound field calibration.

[0003] Existing photoacoustic conversion materials typically generate sound waves through thermoelastic expansion. Specifically, the light-absorbing material absorbs incident pulsed or modulated light, resulting in a transient temperature rise. This temperature rise causes thermal expansion of the material, forming a transient pressure wave under certain thermal and stress constraints. The intensity of this type of photoacoustic signal is usually related to factors such as the material's light absorption capacity, thermal expansion properties, heat capacity, thermal conductivity, Grüneisen parameter, and the pulse width and energy density of the incident light.

[0004] To improve photoacoustic conversion efficiency, existing technologies typically employ methods such as increasing the light absorption rate of materials, introducing materials with high thermal expansion coefficients, constructing composite structures of light-absorbing materials and polymers, or optimizing material thickness. For example, a light-absorbing layer can be used to enhance light energy absorption, and the large thermal expansion coefficient of the polymer layer can be used to convert temperature changes into mechanical deformation. However, these technical solutions mainly rely on the continuous temperature rise and conventional thermoelastic expansion generated after the material absorbs light energy. Their output sound pressure typically varies continuously with the absorbed energy and temperature rise, and the photoacoustic conversion capability is still limited by the inherent thermophysical parameters of the material and the thermal diffusion process.

[0005] When it is necessary to further increase the photoacoustic signal intensity, the incident laser energy density usually needs to be increased. However, excessively high laser energy density may cause local overheating, ablation, interface delamination, or performance degradation of the material, which is detrimental to the stable and repetitive operation of the photoacoustic conversion material. Therefore, how to enhance the transient volumetric response and thermomechanical response by utilizing the rapid changes in the internal physical properties of the material without excessively increasing the peak energy of the pulsed laser is a problem that needs to be solved to improve the photoacoustic conversion performance.

[0006] On the other hand, phase change materials can undergo structural phase transitions under external energy excitation, accompanied by physical processes such as latent heat absorption or release, changes in thermal conductivity, and adjustments in crystal structure. Vanadium dioxide (VO2) is a typical strongly correlated oxide material that undergoes a first-order metal-insulator phase transition around 68°C, characterized by the following features:

[0007] 1. Accompanied by a significant latent heat process;

[0008] 2. Sudden changes occur in physical properties such as thermal conductivity and electrical conductivity;

[0009] 3. The crystal structure rearranges, causing a change in volume;

[0010] In existing technologies, VO2 is mainly used in optical modulation, smart windows, thermal management, and storage devices. Summary of the Invention

[0011] The technical problem solved by this invention is to provide an enhanced photoacoustic conversion composite material and its excitation method and excitation system, so as to achieve high-efficiency, nonlinear enhanced photoacoustic conversion.

[0012] Technical Solution: To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0013] An enhanced photoacoustic conversion composite material includes a phase change absorption layer and a thermal expansion layer disposed on the phase change absorption layer. When the composite material is in operation, it is irradiated by a first continuous laser. The first continuous laser enters from the thermal expansion layer, penetrates the thermal expansion layer, and is absorbed by the phase change absorption layer. After absorbing the first continuous laser, the temperature of the phase change absorption layer rises. The first continuous laser is then turned off. Simultaneously, a second pulsed laser is emitted to irradiate the composite material. Within the pulse width of the second pulsed laser, the phase change absorption layer completes a phase change and continues to absorb laser energy within the pulse width of the second pulsed laser, causing its temperature to rise. Upon completion of the phase change, the phase change absorption layer expands in volume. The increased thermal conductivity of the phase change absorption layer after the phase change causes heat to be conducted to the thermal expansion layer, triggering its thermal expansion. The superimposed volume expansion of the phase change absorption layer generates photoacoustic waves.

[0014] Preferably, the composite material further includes a substrate, the phase change absorption layer is located on the substrate, the phase change absorption layer is a vanadium dioxide thin film, and the thermal expansion layer is an organosilicon material or an elastomer material.

[0015] Preferably, the phase change absorption layer has a thickness of d1, where 50nm ≤ d1 ≤ 200nm, and the thermal expansion layer has a thickness of d2, where 200nm ≤ d2 ≤ 10μm.

[0016] The present invention also provides a method for exciting an enhanced photoacoustic conversion composite material, which is used to excite the above-mentioned enhanced photoacoustic conversion composite material, comprising the following steps:

[0017] S1. The composite material is irradiated with a first continuous laser to locally preheat the phase change absorption layer, so that its temperature gradually rises and falls below the phase change temperature.

[0018] S2. Real-time acquisition of the reflected light signal of the composite material to the first continuous laser, and determination of whether the phase change absorption layer has entered the first state based on the change of the reflectivity R(t) of the first continuous laser;

[0019] S3. When it is determined that the phase change absorption layer has entered the first state, the first continuous laser is turned off, and the second pulse laser is simultaneously triggered to irradiate the composite material.

[0020] S4. After the second pulsed laser is absorbed by the phase change absorption layer, the phase change absorption layer crosses the phase change temperature and completes the phase change. The phase change absorption layer continues to be heated within the pulse width of the second pulsed laser. The volume expansion caused by the phase change of the phase change absorption layer and the increase in thermal conductivity after the phase change drive the thermal expansion layer to expand, generating photoacoustic waves.

[0021] Preferably, the wavelength of the first continuous laser is λ1, 400nm≤λ1≤800nm, the wavelength of the second pulsed laser is λ2, 400nm≤λ2≤800nm, and the pulse width of the second pulsed laser is T, 5ns≤T≤100ns.

[0022] Preferably, in step S2, the determination of whether the phase transition absorption layer has entered the first state based on the first continuous laser reflectivity R(t) is made in the following manner:

[0023] When the first continuous laser reflectivity R(t) satisfies R1≤R(t)≤R2, it is determined that the phase change absorption layer has entered the first state. R1 is the lower limit of the reflectivity of the phase change absorption layer in the first state, and R2 is the upper limit of the reflectivity of the phase change absorption layer in the first state.

[0024] Preferably, in step S2, determining whether the phase transition absorption layer has entered the first state includes the following steps:

[0025] Step T1: Set the reflectivity threshold range [R1, R2], and the phase change absorption layer corresponding to the reflectivity threshold range enters the first state;

[0026] Step T2: Real-time reflectivity R(t) of the composite material to the first continuous laser is collected, and it is determined whether R(t) enters the reflectivity threshold interval [R1, R2].

[0027] Step T3: If R(t) falls within the reflectivity threshold interval [R1, R2], the phase change absorption layer is determined to have entered the first state.

[0028] The present invention also provides an enhanced photoacoustic conversion composite material excitation system for realizing the above-mentioned enhanced photoacoustic conversion composite material excitation method, comprising:

[0029] The first laser module is used to output the first continuous laser beam;

[0030] The second laser module is used to output the second pulse laser;

[0031] The reflection detection module is used to acquire the incident light intensity I0 of the first continuous laser and the reflected light intensity I of the composite material to the first continuous laser. r ;

[0032] The control module is used to calculate the reflectivity of the first continuous laser in real time based on the signal emitted by the reflection detection module, determine whether the phase change absorption layer has entered the first state based on the reflectivity, and control the first laser module to turn off the first continuous laser and trigger the second laser module to output the second pulse laser when the phase change absorption layer enters the first state.

[0033] Preferably, the reflection detection module includes a beam sampling mirror and a photodetector. The beam sampling mirror is used to separate the detection light from the first continuous laser, and the photodetector is used to convert the detection light and the reflected light from the composite material into electrical signals. The control module includes a data acquisition card and a host computer. The data acquisition card is connected to the reflection detection module to receive signals and transmit the signals to the host computer. The host computer controls the operation of the first laser module and the second laser module through the data acquisition card according to a preset threshold rule.

[0034] Beneficial effects: Compared with the prior art, the present invention has the following advantages.

[0035] 1. The phase change absorption layer is a vanadium dioxide thin film, and the thermal expansion layer is an organosilicon material or an elastomer material. After exceeding the phase change temperature, the sudden volume change of the vanadium dioxide thin film forms a transient mechanical stress source at the interface. At the same time, the thermal conductivity of the vanadium dioxide thin film increases rapidly, causing the thermal expansion layer to expand rapidly. This generates a significant thermo-mechanical response in a short time. The two effects work together to produce a photoacoustic signal that is much stronger than the traditional thermoelastic mechanism.

[0036] 2. The layered composite structure is composed of a phase change absorption layer and a thermal expansion layer. The structure is relatively simple and easy to prepare through thin film deposition, spin coating, coating or other micro-nano processing technology. It can also be formed into point, surface or arrayed photoacoustic transducer units according to application requirements.

[0037] 3. The phase change absorption layer is preheated by the first continuous laser, so that the phase change absorption layer is in a state close to the phase change temperature before the second pulse laser is incident. The second pulse laser does not need to undertake the entire heating process before the phase change, which helps to reduce the pulse laser energy required to trigger the phase change.

[0038] 4. By detecting the real-time reflectivity of the first continuous laser, the state of the phase change absorption layer can be determined. Compared with open-loop control based solely on the continuous laser irradiation time or preset heating power, the influence of changes in initial temperature, material batch, film thickness, and environmental conditions on the phase change triggering time can be reduced, thereby improving the consistency and repeatability of photoacoustic excitation.

[0039] 5. The preset heating process and the pulse excitation process are controlled separately, and the working parameters of the first continuous laser and the second pulse laser can be adjusted independently, thereby adjusting the phase transition triggering conditions, photoacoustic signal intensity and repetitive excitation period.

[0040] 6. The present invention forms a photoacoustic excitation process with threshold characteristics. When the phase change absorption layer reaches the pre-phase change state, the second pulse laser triggers the phase change and generates a composite thermomechanical response, which is beneficial to obtain a nonlinear photoacoustic output that is different from the conventional continuous thermoelastic response.

[0041] 7. This invention can be used in applications such as laser ultrasound, photoacoustic imaging, microscale sound source construction, ultrasonic transduction, non-destructive testing, and sound field calibration. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the composite material structure of Embodiment 1 of the present invention;

[0043] Figure 2 This is a graph showing the relationship between the normalized reflectivity of the first continuous laser and the temperature of the vanadium dioxide material.

[0044] Figure 3 This is a schematic diagram of the composite material irradiated by the first continuous laser and the second pulse laser.

[0045] Figure 4 This is a graph showing the relationship between the intensity of the second pulse laser and time, and the relationship between the temperature of the VO2 thin film and the temperature of the PDMS thin film and time in Example 1.

[0046] Figure 5 This is a comparison chart of the photoacoustic envelope signal and the normalized absorption coefficient of the composite material formed by VO2 thin film and the composite material formed by Cr thin film in Example 1 under the same laser irradiation conditions.

[0047] Figure 6 This is a system structure block diagram of Example 1;

[0048] In the figure: 1. Phase change absorption layer; 2. Thermal expansion layer; 3. Substrate; 4. First laser module; 5. Second laser module; 6. Photodetector; 7. Beam sampling mirror; 8. Water tank; 9. Light transmission window. Detailed Implementation

[0049] The present invention will be further illustrated below with reference to specific embodiments. These embodiments are implemented based on the technical solutions of the present invention, and it should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0050] Example 1

[0051] like Figure 1 As shown, an enhanced photoacoustic conversion composite material includes a phase change absorption layer 1, a thermal expansion layer 2, and a substrate 3. The phase change absorption layer 1 is located on the substrate 3, which is a material with low thermal conductivity (below 1.5 W / (m·K)), such as quartz glass. In this embodiment, quartz glass is used. The thickness of the substrate 3 is d3, where 1 mm ≤ d3, and in this embodiment, d3 = 5 mm. The phase change absorption layer 1 is a vanadium dioxide (VO2) thin film with a thickness of d1, where 50 nm ≤ d1 ≤ 200 nm, and in this embodiment, d1 = 150 nm. Its phase change temperature is 68 °C. The thermal expansion layer 2 is disposed on the phase change absorption layer 1 with a thickness of d2, where 200 nm ≤ d2 ≤ 10 μm. The thermal expansion layer 2 is composed of organosilicon materials or elastomer materials, such as PDMS (polydimethylsiloxane), butyl rubber, etc. In this embodiment, the thermal expansion layer 2 is a PDMS (polydimethylsiloxane) thin film with d2 = 1000 nm. After absorbing laser light, the phase change absorption layer 1 experiences a temperature rise. Once the temperature exceeds the phase change temperature (68°C), the phase change absorption layer 1 completes its phase change. During this phase change, the phase change absorption layer 1 undergoes volume expansion. Simultaneously, the phase change causes a jump in thermal conductivity in the high-temperature phase, leading to heat conduction from the phase change absorption layer 1 to the thermal expansion layer 2, which in turn causes thermal expansion. The volume expansion of the phase change absorption layer 1, combined with the thermal expansion of the thermal expansion layer 2, generates enhanced photoacoustic waves. The substrate 3 uses a material with low thermal conductivity to reduce heat leakage and improve the transient temperature rise and heat accumulation efficiency of the phase change absorption layer 1, making it easier for it to cross the threshold and complete the phase change within the pulsed laser's action time (especially when the VO2 film is thin and the pulse width is short).

[0052] This embodiment also provides a method for preparing an enhanced photoacoustic conversion composite material, which includes the following steps:

[0053] Step 1: Prepare a vanadium dioxide thin film on substrate 3 as phase change absorption layer 1;

[0054] In this embodiment, the substrate 3 is first prepared: a 5.0 mm thick quartz glass substrate 3 is selected, and ultrasonically cleaned in acetone, isopropanol (IPA), and deionized water for 5 min-10 min each. After removal, it is dried with nitrogen gas. Oxygen plasma is used to rinse for 30 s-120 s to improve the adhesion of the VO2 film on the substrate 3. Finally, it is baked on a 120℃ hot stage for 5 min-10 min to dehumidify and cooled to room temperature for later use.

[0055] Then, VO2 thin films are prepared: the cavity of the magnetron sputtering coating machine is pre-evacuated, and the preferred background vacuum of the cavity is <(5×10). -4 ) Pa; A metal V target is used to deposit a vanadium oxide film in an Ar (argon) or Ar / O2 (oxygen) mixed atmosphere; The thickness d1 of the VO2 film is controlled by sputtering power, gas flow rate and deposition time. In this embodiment, d1 = 150 nm. The film is annealed under controlled oxygen partial pressure (or vacuum / inert atmosphere + trace oxygen) to transform the film into the VO2 main phase.

[0056] In this embodiment, the phase transition characteristics of the VO2 thin film were also confirmed after its preparation:

[0057] (1) Structure and phase: XRD or Raman spectroscopy confirms the characteristic peak of VO2, and XPS is used to assess the chemical valence state if necessary;

[0058] (2) Phase transition temperature: Measure the resistance-temperature (RT) curve to confirm that there is a significant abrupt change and hysteresis loop around 68°C.

[0059] Step 2: Apply PDMS solution to the vanadium dioxide film by spin coating.

[0060] In this embodiment, a commercially available two-component PDMS solution (Dow Corning Sylgard 184) was used. The base adhesive and curing agent were mixed according to the manufacturer's recommended ratio, generally controlled within the range of 5:1-20:1. Hexane was added as a diluent and stirred thoroughly until homogeneous to reduce viscosity and improve spin coating / spin coating uniformity. The prepared solution was allowed to stand at room temperature for 10-30 minutes to reduce microbubbles. The PDMS solution was dropped onto the surface of the VO2 film and spin-coated according to the procedure of "low-speed spreading + high-speed thickness setting". The PDMS film thickness was controlled by the spin coating speed, time and dilution ratio.

[0061] Step 3: After the PDMS solution is cured, a PDMS film is formed as the thermal expansion layer 2. The PDMS solution is cured by heating, with a heating temperature of 60℃-80℃ and a curing time of more than 2 hours.

[0062] In this embodiment, the substrate 3 after steps 1 and 2 is placed in an oven or hot table at 60-80℃ for curing, and the curing time is greater than 2 hours to obtain the thermal expansion layer 2; after cooling to room temperature, the thickness d2 of the PDMS film is measured using a step meter or ellipsometry to be 1000nm.

[0063] This embodiment also provides a method for exciting an enhanced photoacoustic conversion composite material, which includes the following steps:

[0064] S1. The composite material is irradiated with a first continuous laser to locally preheat the vanadium dioxide film, so that its temperature gradually rises and falls below the phase transition temperature; the wavelength of the first continuous laser is λ1, 400nm≤λ1≤800nm, in this embodiment, λ1=532nm, the first continuous laser is incident from the thermal expansion layer 2, penetrates the thermal expansion layer 2 and is absorbed by the phase transition absorption layer 1.

[0065] In this embodiment, the composite material is placed in the water of the pool 8. The first continuous laser is incident on the composite material through the light-transmitting window 9. The first continuous laser passes through the thermal expansion layer 2 and then incident on the phase change absorption layer 1. The first continuous laser is a continuously output laser.

[0066] S2. Real-time acquisition of the reflected light signal of the composite material to the first continuous laser, and determination of whether the vanadium dioxide film has entered the first state based on the change of the reflectivity R(t) of the first continuous laser. The determination of whether the vanadium dioxide film has entered the first state is made by the following method:

[0067] When the first continuous laser reflectivity R(t) satisfies R1≤R(t)≤R2, it is determined that the vanadium dioxide thin film has entered the first state. R1 is the lower limit of the reflectivity of the phase change absorption layer 1 entering the first state, and R2 is the upper limit of the reflectivity of the phase change absorption layer 1 entering the first state.

[0068] Specifically, the following steps are included:

[0069] Step T1: Set the reflectivity threshold range [R1, R2], and the vanadium dioxide thin film enters the first state corresponding to the reflectivity threshold range;

[0070] Step T2: Real-time reflectivity R(t) of the composite material to the first continuous laser is collected, and it is determined whether R(t) enters the reflectivity threshold interval [R1, R2].

[0071] Step T3: If R(t) falls within the reflectivity threshold interval [R1, R2], the vanadium dioxide thin film is determined to have entered the first state.

[0072] In this embodiment, the first state is the state in which the vanadium dioxide thin film is close to undergoing a phase transition. At this time, the temperature of the vanadium dioxide thin film is lower than the phase transition temperature (68°C) and greater than or equal to 66.5°C, and the vanadium dioxide thin film has not undergone a phase transition but is close to undergoing one (the first state corresponds to...). Figure 2 (In the suggested trigger region), the reflectivity of the vanadium dioxide thin film changes as it approaches the phase transition (e.g., Figure 2 As shown), reflectivity is calculated in real time. In the formula, Indicates the intensity of reflected light. The incident light intensity is represented and its real-time value is compared with the first threshold R1 and the second threshold R2 of the preset reflectivity threshold range. When R(t) reaches the preset reflectivity threshold range, it is determined that the vanadium dioxide film has entered the first state.

[0073] S3. When it is determined that the phase change absorption layer 1 enters the first state, the first continuous laser is turned off, and the second pulse laser is triggered to irradiate the composite material simultaneously. The wavelength of the second pulse laser is λ2, 400nm≤λ2≤800nm. In this embodiment, λ2=532nm. The pulse width of the second pulse laser is T, 5ns≤T≤100ns. In this embodiment, T=10ns. The turning off of the first continuous laser triggers the emission of the second pulse laser.

[0074] In this embodiment, the second pulsed laser also passes through the light-transmitting window 9 and is incident on the composite material. The second spot of the second pulsed laser irradiating the phase change absorption layer 1 is located within the range of the first spot of the first continuous laser irradiating the phase change absorption layer 1. When both the first and second spots are circular, the diameter of the first spot is greater than or equal to the diameter of the second spot (in this embodiment, the diameter of the first continuous laser spot is 10 mm, and the diameter of the second pulsed laser spot is 1 mm). Figure 3 As shown, the first continuous laser is slightly tilted, while the second pulsed laser is incident normally.

[0075] S4. After the second pulse laser is absorbed by the phase change absorption layer 1, the phase change absorption layer 1 crosses the phase change temperature and completes the phase change. The phase change absorption layer 1 continues to be heated within the pulse width of the second pulse laser. The volume expansion caused by the phase change of the phase change absorption layer 1 and the increase in thermal conductivity after the phase change jointly drive the thermal expansion layer 2 to generate enhanced photoacoustic waves.

[0076] In this embodiment, as Figure 4 As shown, the vanadium dioxide film crosses the phase transition threshold of 68°C and rapidly completes the phase transition under the action of the second pulse laser. Within the pulse width of the second pulse laser, the phase-transformed vanadium dioxide film is further heated by the second pulse laser, and the temperature rises. Subsequently, its volume transition and thermal conductivity transition jointly drive the PDMS film to undergo a steep thermomechanical response, thereby forming an enhanced transient photoacoustic wave in the water medium in front of the composite material. The ultrasonic wave generated by the photoacoustic conversion composite material is directly transmitted into the water.

[0077] In this embodiment, the first continuous laser beam serves as both heating light and reflected light from the phase transition absorption layer 1, used for phase transition state monitoring. In-situ pre-heating via continuous laser ensures accurate regional coverage and low thermal inertia. A phase transition excitation mechanism of "pre-heating—optical judgment—synchronous triggering" is constructed. During the continuous laser irradiation stage, direct photoacoustic wave output is not pursued; instead, a local area of ​​the phase transition absorption layer 1 is pushed to the pre-phase transition state. The reflectivity change of the phase transition absorption layer 1 to the first continuous laser beam is used to feedback the temperature change of the vanadium dioxide film. During the pulse triggering stage, a short pulse is used to cross the threshold at the optimal critical moment, causing the volume transition and thermal conductivity transition of vanadium dioxide to be released in a concentrated manner within a short time, thereby obtaining a steeper strain acceleration and a stronger transient acoustic source. Compared to simply relying on the overall temperature setting of the composite material (e.g., heating the entire composite material to a certain temperature via external heating), this invention relies on local real phase state signals, thus better compensating for drift caused by film thickness, heat dissipation boundaries, ambient temperature, and batch differences in composite material samples.

[0078] When a laser irradiates a composite material, its photoacoustic enhancement mechanism can be divided into the following stages:

[0079] Phase 1: Energy Accumulation Stage Before Phase Transition

[0080] During the initial laser irradiation phase, the vanadium dioxide thin film absorbs laser energy. Since the vanadium dioxide thin film is close to the phase transition temperature, the absorbed energy preferentially participates in the pre-phase transition processes, including electronic structure rearrangement and lattice softening.

[0081] During this stage, the macroscopic temperature rise rate of the system is relatively slow, and the absorbed energy is mainly used for phase transition precursor processes such as electronic structure rearrangement and lattice softening, and is not immediately converted into significant macroscopic volume expansion.

[0082] Phase Two: Phase Transition Triggering and Rapid Completion Stage

[0083] When the vanadium dioxide thin film that has absorbed energy reaches the phase transition temperature, the vanadium dioxide completes the metal-insulator phase transition within a timescale of nanoseconds (less than 20 nanoseconds) or even shorter; this phase transition process has the following characteristics:

[0084] The rapid absorption of latent heat causes a sudden change in the energy distribution inside the material;

[0085] A significant jump in thermal conductivity: the thermal conductivity of vanadium dioxide after the phase change is 3-5 times higher than that before the phase change;

[0086] The volume change caused by crystal structure reconstruction is approximately 1%.

[0087] Phase 3: Interface Coupling and Sound Wave Generation

[0088] The aforementioned phase transition-induced thermal and mechanical changes are transferred to the thermal expansion layer through interfacial coupling between vanadium dioxide and PDMS:

[0089] The surge in thermal conductivity causes the stored energy to be released into the PDMS film in a very short time.

[0090] The abrupt change in the volume of vanadium dioxide creates a transient mechanical stress source at the interface;

[0091] PDMS (polydimethylsiloxane), as a low-modulus, high-thermal-expansion-coefficient material, exhibits a significant thermo-mechanical response in a short time.

[0092] The combined effects of these multiple factors create a transient sound source with high amplitude and a steep rising edge, thereby generating a photoacoustic signal that is far stronger than that of the traditional thermoelastic mechanism.

[0093] This invention clearly enhances the expansion from two physical processes: (1) the volume transition during the completion of the VO2 phase transition triggers expansion; (2) the increased thermal conductivity of VO2 after the phase transition leads to faster heat transfer to the PDMS, and the low thermal conductivity before the phase transition causes a large temperature difference at the VO2-PDMS boundary, triggering nonlinear expansion. The temperature changes of different material layers with the laser treatment time are as follows: Figure 5 As shown.

[0094] like Figure 1 , Figure 3 and Figure 6 As shown, this embodiment also provides an enhanced photoacoustic conversion composite material excitation system for realizing the above-mentioned enhanced photoacoustic conversion composite material excitation method, including a first laser module 4, a second laser module 5, a reflection detection module, and a control module.

[0095] The first laser module 4 is used to output a first continuous laser to locally preheat the phase change absorption layer 1 of the composite material; in this embodiment, the first laser module 4 adopts an existing continuous laser that can emit continuous laser.

[0096] The second laser module 5 is used to output a second pulsed laser so that the phase change absorption layer 1 crosses the phase change temperature and generates an enhanced transient photoacoustic wave; in this embodiment, the second laser module 5 adopts an existing pulsed laser that can emit pulsed lasers.

[0097] The first continuous laser and the second pulsed laser can irradiate the composite material using a common optical path or by using independent optical paths. When using a common optical path structure, the first continuous laser and the second pulsed laser can be coupled to the same incident optical path using a beam combiner; in this embodiment, as shown... Figure 3 As shown, the first continuous laser and the second pulsed laser use independent optical paths. The first continuous laser is slightly tilted, while the second pulsed laser is incident normally.

[0098] The reflection detection module is used to collect the incident light intensity of the first continuous laser beam. The intensity of reflected light from composite materials The reflection detection module includes a beam sampling mirror 7 and two photodetectors 6. The beam sampling mirror 7 is used to separate the detection light from the incident light path of the first continuous laser (a beam sampling mirror can also be used if the energy of the reflected light path is too high; in this embodiment, a beam sampling mirror is not used for the reflected light path). In this embodiment, two photodetectors 6 are provided. The photodetectors 6 are used to detect the intensity of the incident light and the intensity of the reflected light, and convert the intensity of the incident light and the intensity of the reflected light into electrical signals.

[0099] The control module is connected to the first laser module 4, the second laser module 5, and the reflection detection module, respectively. It receives the electrical signal output from the photodetector 6, calculates the real-time reflectivity R(t) of the first continuous laser, and determines whether the real-time reflectivity R(t) enters a preset reflectivity threshold range. When the real-time reflectivity R(t) enters the reflectivity threshold range, the control module controls the first laser module 4 to turn off the first continuous laser and triggers the second laser module 5 to output a second pulse laser. In this embodiment, the control module includes a data acquisition card and a host computer. The data acquisition card has an analog signal acquisition port and a digital input / output port. The data acquisition card converts the analog signal from the photodetector 6 into a digital signal and transmits it to the host computer. The host computer uses an existing computer and has a preset reflectivity calculation program and threshold determination rules. After the threshold judgment condition (R1≤R(t)≤R2) is reached, the data acquisition card outputs a shutdown signal to the first laser module 4 and a trigger signal to the second laser module 5 through the digital input / output port. Under the action of pulsed laser, the vanadium dioxide film crosses the phase transition threshold of 68℃ and quickly completes the phase transition. Subsequently, its volume transition and thermal conductivity transition jointly drive the PDMS to undergo a steep thermomechanical response, thereby forming an enhanced transient photoacoustic wave in the water medium in front of the composite material sample.

[0100] In this embodiment, a 100MHz ultrasonic transducer was placed in water in front of the composite material sample to detect ultrasonic waves and record the sound pressure waveform. For comparative verification, under the same laser conditions, the VO2 film was replaced with a control absorption layer of the same thickness, Cr (chromium), which did not undergo a phase transition. The photoacoustic signal intensity was normalized to the absorption coefficients of VO2 and Cr, as shown below. Figure 5 As shown, the photoacoustic signal of the VO2 phase change absorption layer system (the composite material in this embodiment) is significantly enhanced, approximately 4.1 times that of the Cr metal thin film.

[0101] Example 2

[0102] The difference from Example 1 is that substrate 3 is replaced with quartz (still with low thermal conductivity and better heat resistance). The cleaning and activation methods are the same as in Example 1. The VO2 film is prepared by PLD (pulsed laser deposition), and the VO2 film thickness d1 = 50 nm (preferably thin to reduce thermal inertia and improve rise time). The PDMS dilution ratio is increased (more diluted) to obtain a PDMS film with d2 of 200 nm-800 nm to reduce thermal inertia and improve the rise time of the response. Spin coating controls the film thickness. Curing, preheating, and laser conditions are the same as in Example 1. The thinner VO2 film and PDMS can reduce the thermal inertia of the film layer and shorten the characteristic time of temperature change and mechanical response, making it suitable for applications with high requirements for the rise time and high frequency components of photoacoustic signals.

[0103] This embodiment employs the same reflectance closed-loop determination method as in Embodiment 1. Considering the film thickness and quartz substrate conditions in this embodiment, the reflectance threshold range [R1, R2] is recalibrated, and the spot of the second pulsed laser is positioned within the preheated spot of the first continuous laser. The wavelengths of both the first continuous laser and the second pulsed laser are 532 nm, and the pulse width of the second pulsed laser is 5 ns to 20 ns. The remaining preparation steps, excitation steps, and detection methods are the same as in Embodiment 1.

[0104] Example 3

[0105] The difference from Example 1 is that this example uses a glass substrate, and the VO2 film thickness d1 is 200 nm to improve absorption capacity and energy carrying capacity. The PDMS film is formed into a 2μm-10μm thick film by blade coating or low-speed spin coating; the curing can be carried out in two stages (pre-curing at 60℃ and then fully curing at 80℃) to reduce residual solvent and stress inside the thick film, and the curing time is >2h.

[0106] This embodiment still employs the excitation method of pre-heating with a first continuous laser, determining the reflectivity threshold, and synchronously triggering with a second pulsed laser. The pulse width of the second pulsed laser can be set from 20 ns to 100 ns, and the single-pulse laser energy can be selected to have a higher energy output. The pulse energy and reflectivity threshold range are re-determined based on the thickness of the phase change absorption layer 1 and the thermal response of the thermal expansion layer 2. This structure has a large absorption volume and thermal expansion volume, making it suitable for applications requiring high sound pressure output or high laser energy tolerance.

[0107] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An enhanced photoacoustic conversion composite material, characterized in that, The composite material includes a phase change absorption layer (1) and a thermal expansion layer (2) disposed on the phase change absorption layer (1). When the composite material is working, it is irradiated by a first continuous laser. The first continuous laser is incident from the thermal expansion layer (2), penetrates the thermal expansion layer (2), and is absorbed by the phase change absorption layer (1). After the phase change absorption layer (1) absorbs the first continuous laser, its temperature rises and the first continuous laser is turned off. At the same time as the first continuous laser is turned off, a second pulse laser is emitted to irradiate the composite material. Within the pulse width of the second pulse laser, the phase change absorption layer (1) completes a phase change and continues to absorb laser energy within the pulse width of the second pulse laser, causing its temperature to rise. When the phase change absorption layer (1) completes the phase change, its volume expands. After the phase change is completed, the thermal conductivity of the phase change absorption layer (1) increases, causing the heat of the phase change absorption layer (1) to be conducted to the thermal expansion layer (2) and causing its thermal expansion. The superposition of the volume expansion of the phase change absorption layer (1) generates photoacoustic waves. The composite material also includes a substrate (3), the phase change absorption layer (1) is located on the substrate (3), the phase change absorption layer (1) is a vanadium dioxide film, and the thermal expansion layer (2) is a polydimethylsiloxane film or a butyl rubber film.

2. The enhanced photoacoustic conversion composite material according to claim 1, characterized in that, The phase change absorption layer (1) has a thickness of d1, 50nm≤d1≤200nm, and the thermal expansion layer (2) has a thickness of d2, 200nm≤d2≤10μm.

3. A method for exciting an enhanced photoacoustic conversion composite material, used to excite the enhanced photoacoustic conversion composite material according to any one of claims 1 to 2, characterized in that, Includes the following steps: S1. The composite material is irradiated with a first continuous laser to locally preheat the phase change absorption layer (1), so that its temperature gradually rises and falls below the phase change temperature; S2. Real-time acquisition of the reflected light signal of the composite material to the first continuous laser, and determination of whether the phase change absorption layer (1) has entered the first state based on the change of the reflectivity R(t) of the first continuous laser; S3. When it is determined that the phase change absorption layer (1) enters the first state, the first continuous laser is turned off and the second pulse laser is triggered to irradiate the composite material simultaneously. S4. After the second pulse laser is absorbed by the phase change absorption layer (1), the phase change absorption layer (1) crosses the phase change temperature and completes the phase change. The phase change absorption layer (1) continues to be heated within the pulse width of the second pulse laser. The volume expansion caused by the phase change of the phase change absorption layer (1) and the increase in thermal conductivity after the phase change drive the thermal expansion layer (2) to expand, generating photoacoustic waves.

4. The method for exciting reinforced photoacoustic conversion composite materials according to claim 3, characterized in that, The first continuous laser wavelength is λ1, 400nm≤λ1≤800nm, the second pulse laser wavelength is λ2, 400nm≤λ2≤800nm, and the pulse width of the second pulse laser is T, 5ns≤T≤100ns.

5. The method for exciting reinforced photoacoustic conversion composite materials according to claim 3, characterized in that, In S2, the determination of whether the phase transition absorption layer (1) has entered the first state based on the first continuous laser reflectivity R(t) is made in the following way: When the first continuous laser reflectivity R(t) satisfies R1≤R(t)≤R2, it is determined that the phase change absorption layer (1) enters the first state. R1 is the lower limit of the reflectivity of the phase change absorption layer (1) entering the first state, and R2 is the upper limit of the reflectivity of the phase change absorption layer (1) entering the first state.

6. The method for exciting reinforced photoacoustic conversion composite materials according to claim 3, characterized in that, In step S2, determining whether the phase change absorption layer (1) has entered the first state includes the following steps: Step T1: Set the reflectivity threshold range [R1, R2], and the phase change absorption layer (1) enters the first state corresponding to the reflectivity threshold range; Step T2: Real-time reflectivity R(t) of the composite material to the first continuous laser is collected, and it is determined whether R(t) enters the reflectivity threshold interval [R1, R2]. Step T3: If R(t) falls within the reflectivity threshold interval [R1, R2], it is determined that the phase change absorption layer (1) has entered the first state.

7. An enhanced photoacoustic conversion composite material excitation system for realizing the enhanced photoacoustic conversion composite material excitation method according to any one of claims 3 to 6, characterized in that, include: The first laser module (4) is used to output the first continuous laser; The second laser module (5) is used to output the second pulse laser; The reflection detection module is used to acquire the incident light intensity I0 of the first continuous laser and the reflected light intensity I of the composite material to the first continuous laser. r ; The control module is used to calculate the reflectivity of the first continuous laser in real time based on the signal emitted by the reflection detection module, and to determine whether the phase change absorption layer (1) has entered the first state based on the reflectivity. When the phase change absorption layer (1) enters the first state, the control module (4) is used to turn off the first continuous laser and trigger the second laser module (5) to output the second pulse laser.

8. The enhanced photoacoustic conversion composite material excitation system according to claim 7, characterized in that, The reflection detection module includes a beam sampling mirror (7) and a photodetector (6). The beam sampling mirror (7) is used to separate the detection light from the first continuous laser, and the photodetector (6) is used to convert the detection light and the reflected light of the composite material into an electrical signal. The control module includes a data acquisition card and a host computer. The data acquisition card is connected to the reflection detection module to receive signals and transmit the signals to the host computer. The host computer controls the operation of the first laser module (4) and the second laser module (5) through the data acquisition card according to a preset threshold rule.

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