Dual-optical-path Z-scanning nonlinear optical measurement device and method

Abstract

The invention provides a double-optical-path Z-scanning nonlinear optical measurement device and method, and belongs to the technical field of nonlinear optical measurement. The device comprises a pulse laser used for emitting ultra-short laser pulses; the beam splitter BS1 is used for splitting the ultrashort laser pulse into probe light and pump light; the detection light is focused on a sample to be detected through the lens L1; the pump light is focused on a sample to be detected through a lens L2; and the beam splitter BS2 is used for splitting the detection light penetrating through the sample to be detected into two beams, and the two beams are respectively conveyed to the hole opening detector D1 and the hole closing detector D2. By adopting the double optical paths, the problems that the temperature of a sample is possibly increased due to long-time laser irradiation, so that a heat effect is caused, and laser scattering is caused by unevenness of the surface of the sample or irregularity of an internal structure can be effectively solved.

Description

Technical Field

[0001] The invention belongs to the technical field of nonlinear optical measurement, and in particular relates to a dual-light path Z scanning nonlinear optical measurement device and method. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Z-scan technology is a method to characterize the nonlinear optical properties of materials by measuring the changes in light intensity caused by the sample at the focus of a Gaussian beam. In the experiment, the sample moves along the optical axis through the focus of the laser beam. Due to the nonlinear effect, the sample changes the focusing characteristics of the beam, resulting in changes in the far-field beam diameter and intensity. By analyzing these changes, the nonlinear refractive index and nonlinear absorption coefficient of the material can be obtained. Specifically, the nonlinear response of the sample will cause the light beam to self-focus or self-defocus at the focus, forming characteristic peak-valley curves on the detector. The shape and amplitude of these curves are directly related to the nonlinear parameters of the material.

[0004] When using the Z-scan technique to measure the nonlinear optical properties of a material, the inventors discovered the following technical problems:

[0005] (1) The experimental results are affected by the photothermal effect. When the sample has significant linear absorption, the photothermal effect may cause the local temperature of the sample to rise after absorbing laser energy. The temperature change caused by sample absorption can change the refractive index of the material, thereby affecting the propagation of the light beam and thus affecting the accuracy of the experimental results. Therefore, a strong photothermal effect may mislead the Z-scan experimental results.

[0006] (2) Experimental results are affected by scattering. Z-scan technology relies on measuring the changes in the light beam after it passes through the sample. If the sample has highly scattering characteristics, the propagation of the light beam may be disturbed, resulting in inaccurate measurement results. Nonlinear scattering effects in scattering media may cause changes in the intensity of the light beam, and these changes may be mistaken for nonlinear refraction or nonlinear absorption caused by the material.

[0007] (3) Weak changes are difficult to measure accurately. When the nonlinear effect of the sample is weak, the resulting change in beam intensity may be close to the noise level, making it difficult to distinguish between true nonlinear signals and noise. In addition, the system noise in the experiment, the sensitivity of the detector, the stability of the laser beam, and the inhomogeneity of the sample itself may affect the accuracy of the measurement results. In materials with weak nonlinear effects, because the Z-scan technique relies on detecting changes in beam intensity, this may be difficult to measure accurately in the experiment. Summary of the invention

[0008] In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides a dual-light path Z-scanning nonlinear optical measurement device and method.

[0009] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0010] A first aspect of the present invention provides a dual-light path Z-scanning nonlinear optical measurement device;

[0011] A dual-optical path Z-scan nonlinear optical measurement device, comprising:

[0012] Pulsed lasers, used to emit ultrashort laser pulses;

[0013] The beam splitter BS1 is used to split the ultrashort laser pulse into a detection light and a pump light; the detection light is focused onto the sample to be tested through the lens L1; the pump light is focused onto the sample to be tested through the lens L2;

[0014] The beam splitter BS2 is used to split the detection light after passing through the sample to be tested into a first detection light and a second detection light. The first detection light is received by the closed-hole detector D2, and the second detection light is received by the open-hole detector D1.

[0015] As a further technical solution, the pulse laser adopts a linearly polarized dual-frequency Nd:YAG pulse laser.

[0016] As a further technical solution, the sample to be tested can move along the z-axis.

[0017] As a further technical solution, a reflector M1 and a reflector M2 are sequentially arranged between the beam splitter BS1 and the lens L2.

[0018] As a further technical solution, a lens L3 is further arranged between the aperture detector D1 and the beam splitter BS2.

[0019] As a further technical solution, the lens L1, the lens L2, and the lens L3 are all convex lenses.

[0020] As a further technical solution, a small hole is further provided between the closed-hole detector D2 and the beam splitter BS2.

[0021] A second aspect of the present invention provides a dual-light path Z-scanning nonlinear optical measurement method.

[0022] A dual-light-path Z-scanning nonlinear optical measurement method, comprising:

[0023] The pulse laser emits an ultrashort laser pulse, which is split into a detection light and a pump light by a beam splitter BS1;

[0024] The detection light is focused onto the sample to be tested through lens L1; the pump light is reflected onto lens L2 through reflectors M1 and M2 in sequence, and lens L2 is used to focus the pump light onto the sample to be tested; the pump light is used to excite the sample, and the detection light is used to detect the nonlinear behavior generated by the pump light excitation;

[0025] After the detection light passes through the sample to be tested, the detection light is divided into a first detection light and a second detection light by a beam splitter BS2; the first detection light is transmitted to the closed-hole detector D2, and the second detection light is transmitted to the open-hole detector D1; the energies of the open-hole detector D1 and the closed-hole detector D2 are recorded to obtain the nonlinear absorption and refraction dynamics properties of the sample to be tested.

[0026] As a further technical solution, during the process of the first detection light being transmitted to the closed-hole detector D2, it is received by the closed-hole detector D2 through the small hole.

[0027] As a further technical solution, the second detection light is also focused by the lens L3 and then received by the aperture detector D1.

[0028] One or more of the above technical solutions have the following beneficial effects:

[0029] (1) The dual-optical design uses a pulsed laser to emit ultrashort laser pulses, which are then split into probe light and pump light by beam splitter BS1, allowing independent adjustment of pump light and probe light, and optimizing experimental parameters to minimize thermal effects. The device also reduces the time the laser interacts with the sample by using ultrashort pulse lasers and precise beam control, thereby reducing heat accumulation.

[0030] (2) Since the pump light and the detection light are independent, the detection light is not directly affected by the intensity fluctuation of the pump light, which reduces noise interference and improves the purity of the detection signal.

[0031] (3) In the traditional Z-scan experiment, only one beam of light passes through the sample, and the nonlinear effect is mainly concentrated near the focus of the beam. In the dual-path pump Z-scan experiment, the interaction area of ​​the two beams at the sample forms a smaller local area, and the nonlinear effect in this area is more concentrated and significant. In addition, since the pump light and the probe light are independent, the probe light is not directly affected by the intensity fluctuation of the pump light. The radius of the pump light at the sample is usually 2 to 3 times that of the probe light, and the angle between the two beams is very small. The probe light can evenly cover the sample area excited by the pump light. This reduces noise interference, making the detection results more uniform and reliable, thereby improving the overall accuracy of the measurement results.

[0032] (4) Traditional Z scanning technology requires a large sample displacement range (usually about 5 cm). The large displacement range reduces experimental efficiency and increases measurement time. Secondly, the precise positioning of the sample requires high precision and is easily affected by mechanical vibration or external interference, which leads to measurement errors. In addition, this large range of movement also poses higher challenges to the complexity and space requirements of the experimental equipment, which may affect the stability of the experiment and the reliability of the data.

[0033] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0035] Figure 1 This is a diagram of the overall device structure of the first embodiment.

[0036] Figure 2 The open pore data and closed pore data of zinc selenide measured by the device in the first embodiment are shown in FIG.

[0037] Figure 3 Figure 2. Open hole data and closed hole data measured by traditional single beam Z scanning. DETAILED DESCRIPTION

[0038] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0039] It should be noted that the terms used herein are for describing specific embodiments only and are not intended to be limiting of exemplary embodiments according to the present invention.

[0040] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0041] In traditional Z scanning technology, long-term laser irradiation may cause the sample temperature to rise, thereby causing thermal effects, which will change the optical properties of the sample and affect the accuracy of the experimental results; in addition, the unevenness of the sample surface or the irregularity of the internal structure will also interfere with the detector's accurate measurement of nonlinear signals. For some samples with weaker nonlinear effects, the changes in beam intensity caused by them are close to the noise level, making it difficult to distinguish between real nonlinear signals and noise. Based on the above problems, this application proposes a dual-path Z scanning nonlinear optical measurement device and method, which will be described in detail in conjunction with specific embodiments below.

[0042] Embodiment 1

[0043] This embodiment discloses a dual-light path Z-scanning nonlinear optical measurement device;

[0044] like Figure 1 As shown, a dual-path Z-scan nonlinear optical measurement device includes: a pulse laser for emitting ultrashort laser pulses; in this embodiment, a linearly polarized dual-frequency Nd:YAG pulse laser with a wavelength of 532nm, a frequency of 10Hz, and a pulse width of 20ps is used as a pump source, which can emit ultrashort laser pulses, is suitable for the requirements of Z-scan technology, can provide stable and efficient laser output, and can accurately measure the nonlinear optical properties of materials.

[0045] The pulse width of the linearly polarized dual-frequency Nd:YAG pulse laser is in the picosecond range. This pulse width can provide enough energy to effectively measure nonlinear effects while maintaining time resolution, which is necessary to capture and analyze rapid changes in nonlinear processes. The linearly polarized laser it emits can ensure that the polarization state of the laser remains consistent during the Z scan, which is very important for accurately measuring the nonlinear refractive index because nonlinear optical effects are often related to the polarization state. The Nd:YAG (neodymium-doped yttrium aluminum garnet) used in the pulse laser is a very common laser gain medium with high efficiency, high stability and good thermal conductivity. The Nd:YAG laser can provide high-power laser output and has high output efficiency at a wavelength of 1.064 microns, which is ideal for many nonlinear optical measurements because it has good transmittance and nonlinear response in many materials.

[0046] The ultrashort laser pulse is split into two beams by the beam splitter BS1, a stronger pump light is used to excite the sample, and the other weaker probe light is used to detect the nonlinear behavior generated by the pump light excitation. The dual optical path design allows independent adjustment of the pump light and the probe light, and the experimental parameters can be optimized to minimize thermal effects. The device also reduces the time of laser interaction with the sample by using ultrashort pulse lasers and precise beam control, thereby reducing heat accumulation. Since the pump light and the probe light are independent, the probe light is not directly affected by the intensity fluctuations of the pump light, which reduces noise interference and improves the purity of the detection signal. By using a highly sensitive detector and an optimized detection system, the device can capture nonlinear signals more accurately without being affected by scattered light.

[0047] The detection light is focused onto the sample to be measured through lens L1; the pump light is focused onto the sample to be measured through lens L2; ​​in the process of focusing onto the sample to be measured through lens L2, the pump light also passes through reflectors M1 and M2 in sequence, and the pump light transmission path is adjusted by reflectors M1 and M2. During the measurement process, the sample to be measured can move left and right along the z-axis. By adjusting the distance of the sample to be measured on the z-axis, the position of the focus point on the sample can be changed: in the Z scan, the sample moves along the optical axis (Z-axis) from near the focus to a position far from the focus. In this way, the nonlinear response of the sample under different light intensities can be studied, because the light intensity at the focus is the largest, and the light intensity will gradually decrease as the distance from the focus increases. The movement of the sample on the Z-axis will also change the transmittance of the sample. When the sample is near the focus, the transmittance of the sample will change due to changes in nonlinear absorption or nonlinear refraction. The nonlinear refractive index and nonlinear absorption coefficient of the sample can be obtained.

[0048] After the detection light passes through the sample to be tested, the detection light passing through the sample to be tested is further divided into the first detection light and the second detection light by the beam splitter BS2, and transmitted to the closed-hole detector D2 and the open-hole detector D1 respectively. The open-hole detector D1 and the closed-hole detector D2 can record the energy of the detectors D1 and D2, and are used to measure the energy of laser pulses. It works by absorbing laser energy and converting it into electrical signals, thereby measuring the energy of the laser and obtaining the nonlinear absorption and refraction dynamics of the sample to be tested.

[0049] A lens L3 is also provided between the aperture detector D1 and the beam splitter BS2, and the lens L3 is used to converge the divergent light rays together, or to focus the parallel light rays onto a focal point, so as to ensure that the aperture detector D1 can completely collect the energy of the light beam.

[0050] A small hole is also provided between the closed-hole detector D2 and the beam splitter BS2, and the first detection light is received by the closed-hole detector D2 after passing through the small hole.

[0051] In this embodiment, the lens L1 , the lens L2 , and the lens L3 are all convex lenses.

[0052] Combination Figure 3 , the device in this embodiment is used to measure the nonlinear optical properties of zinc selenide crystals. The open-hole Z scan of ZnSe shows a single valley curve with the zero position as the lowest point, while the closed-hole Z scan shows a curve shape with a peak first and a valley later. According to the Z-scan theory, these correspond to anti-saturation absorption and nonlinear negative refraction, respectively. According to the Z-scan formula, the third-order nonlinear absorption and refraction of ZnSe are fitted (solid line). The open-hole data graph and the closed-hole data graph respectively show the open-hole (absorption) and closed-hole (refraction) Z-scan curves when the incident peak light intensity is 41.5 microjoules. It can be seen from the two figures that zinc selenide crystals show anti-saturation absorption and self-defocusing curves. This shows that ZnSe materials have very good nonlinear optical effects in the picosecond time domain, which gives them good application prospects in optoelectronics and all-optical switches.

[0053] In the dual-path Z scan, the interaction area of ​​the pump light and the probe light at the sample forms a smaller local area, and the nonlinear effect in this area is more concentrated and significant. This concentrated nonlinear response enables the probe light to capture the nonlinear characteristics of the sample more effectively. At the same time, the probe light is not directly affected by the fluctuation of the pump light intensity, which reduces the measurement noise caused by the change of the pump light intensity, thereby obtaining more accurate open and closed hole data. Due to the concentration of nonlinear effects, the "valley" and "peak" features in the measurement results of open and closed holes are more obvious, and the characteristic values ​​(such as "valley" depth and "peak" height) are more significant, which helps to more accurately analyze the nonlinear absorption and refraction characteristics of the sample. If you want to quantify it, you can compare the valley peak value of the closed hole, combined with Figure 2 and Figure 3 , the traditional z-scan measurement znse is (1.4-0.6=0.8), while the dual-path z-scan is (1.8-0.2=1.6).

[0054] Since the interaction of the two beams at the sample forms a smaller local area, the nonlinear effect in this area is more concentrated and significant. This design means that changes in nonlinear effects can be captured within a smaller "z" coordinate range, thereby reducing the distance the sample has to move. Figure 3 , a conventional Z scan may require movement of 5 cm (the horizontal coordinate of a conventional Z scan data graph is approximately 70 mm) or more, while the device in this embodiment only requires approximately 1 cm (the horizontal coordinate of the data graph is 8 mm) to complete all measurements.

[0055] Embodiment 2

[0056] This embodiment discloses a dual-light path Z-scanning nonlinear optical measurement method;

[0057] like Figure 2 As shown, a dual-light path Z-scanning nonlinear optical measurement method comprises:

[0058] The pulse laser emits an ultrashort laser pulse, which is split into a detection light and a pump light by a beam splitter BS1;

[0059] The detection light is focused onto the sample to be tested through lens L1; the pump light is reflected onto lens L2 through reflectors M1 and M2 in sequence, and lens L2 is used to focus the pump light onto the sample to be tested; the pump light is used to excite the sample, and the detection light is used to detect the nonlinear behavior generated by the pump light excitation;

[0060] After the detection light passes through the sample to be tested, it is divided into a first detection light and a second detection light by the beam splitter BS2; the first detection light is transmitted to the closed-hole detector D2, and the second detection light is transmitted to the open-hole detector D1; the first detection light is received by the closed-hole detector D2 through the small hole, and the second detection light is received by the open-hole detector D1 after being focused by the lens L3. During the detection process of the sample to be tested, the radius of the pump light at the sample is usually 2-3 times that of the detection light. Considering that the angle between the two beams is very small, it can be considered that the detection light uniformly detects the area of ​​the sample excited by the pump light. By moving the position of the sample on the z-axis and recording the energy of the detectors D1 and D2 at the same time, the nonlinearity of the sample to be tested can be obtained at the same time.

[0061] Specifically, when the incident laser pulse is Gaussian light, the lateral electric field distribution on the front surface of the sample to be measured is:

[0062]

[0063] Where z is the distance between the sample to be measured and the pump beam waist, w 2 (z) = w0 2 (1+z 2 / z0 2 ) is the spot radius of the pump light at the sample to be tested, R(z)=z(1+z 2 / z0 2 ) is the wavefront curvature radius at position z, z0 = kw0 2 / 2 is the Rayleigh (diffraction) length, and k = 2π / λ is the wave vector. When the thickness of the sample to be measured satisfies L<<z0, the sample can be considered as a thin sample, that is, within the sample thickness range, the intensity and phase changes of the beam caused by diffraction and nonlinear refraction can be ignored. Under the slow-varying amplitude and thin sample approximation, the intensity I and phase φ changes of the incident light inside the sample can be written as:

[0064]

[0065] Among them, z' represents the propagation depth of light in the sample, α(I) = α0 + βI includes the linear absorption coefficient and the nonlinear absorption term, β is the nonlinear absorption coefficient, Δn(I) = γI, γ is the nonlinear refractive index. α(I e ) and Δn(I e ) represents the changes in the absorption coefficient and refractive index of the sample caused by the pump light.

[0066] According to the formula of Z-scan theory:

[0067] α=α0+α2 eff I;

[0068] n=n0+n2 eff I;

[0069] The experimental data can be fitted.

[0070] In traditional Z-scan experiments, only one beam of light passes through the sample, and the nonlinear effect is mainly concentrated near the focus of the beam. In the dual-path pump Z-scan experiment, the interaction area of ​​the two beams of light at the sample forms a smaller local area, and the nonlinear effect in this area is more concentrated and significant. Moreover, since the pump light and the probe light are independent, the probe light is not directly affected by the intensity fluctuation of the pump light. The radius of the pump light at the sample is usually 2-3 times that of the probe light, and the angle between the two beams of light is very small, so the probe light can evenly cover the sample area excited by the pump light. This reduces noise interference, making the detection results more uniform and reliable, thereby improving the overall accuracy of the measurement results.

[0071] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A dual-path Z-scan nonlinear optical measurement device, characterized in that: include: Pulsed lasers, used to emit ultrashort laser pulses; A beam splitter BS1, used for splitting the ultrashort laser pulse into a probe light and a pump light; The detection light is focused onto the sample to be tested through lens L1; the pump light is focused onto the sample to be tested through lens L2; The beam splitter BS2 is used to split the detection light after passing through the sample to be tested into two beams, which are respectively transmitted to the open-aperture detector D1 and the closed-aperture detector D2.

2. A dual-light-path Z-scanning nonlinear optical measurement device as claimed in claim 1, characterized in that: The pulse laser adopts a linear polarization dual-frequency Nd:YAG pulse laser.

3. A dual-light-path Z-scanning nonlinear optical measurement device as claimed in claim 1, characterized in that: The sample to be tested can move along the z-axis.

4. A dual-light-path Z-scanning nonlinear optical measurement device as claimed in claim 1, characterized in that: A reflecting mirror M1 and a reflecting mirror M2 are arranged in sequence between the beam splitter BS1 and the lens L2.

5. A dual-light-path Z-scanning nonlinear optical measurement device as claimed in claim 1, characterized in that: A lens L3 is further arranged between the aperture detector D1 and the beam splitter BS2.

6. A dual-light-path Z-scanning nonlinear optical measurement device as claimed in claim 1 or 5, characterized in that: The lens L1, the lens L2 and the lens L3 are all convex lenses.

7. A dual-light-path Z-scanning nonlinear optical measurement device as claimed in claim 1, characterized in that: A small hole is also provided between the closed-hole detector D2 and the beam splitter BS2.

8. A dual-path Z-scan nonlinear optical measurement method, characterized in that: include: The pulse laser emits an ultrashort laser pulse, which is split into a detection light and a pump light by a beam splitter BS1; The detection light is focused onto the sample to be tested through lens L1; the pump light is reflected onto lens L2 through reflectors M1 and M2 in sequence, and lens L2 is used to focus the pump light onto the sample to be tested; the pump light is used to excite the sample, and the detection light is used to detect the nonlinear behavior generated by the pump light excitation; After the detection light passes through the sample to be tested, the detection light is divided into a first detection light and a second detection light by a beam splitter BS2; the first detection light is transmitted to the closed-hole detector D2, and the second detection light is transmitted to the open-hole detector D1; the energies of the open-hole detector D1 and the closed-hole detector D2 are recorded to obtain the nonlinear absorption and refraction dynamics properties of the sample to be tested.

9. A dual-light-path Z-scanning nonlinear optical measurement method as claimed in claim 8, characterized in that: When the first detection light is transmitted to the closed-hole detector D2, it is received by the closed-hole detector D2 through the small hole.

10. A dual-light-path Z-scanning nonlinear optical measurement method as claimed in claim 8, characterized in that: The second detection light is also focused by the lens L3 and then received by the aperture detector D1.

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