A method for improving signal-to-noise ratio of femtosecond transient absorption spectrum

Abstract

The application discloses a method for improving signal-to-noise ratio of femtosecond transient absorption spectrum, which comprises a femtosecond transient absorption spectrometer with an inner wall having an optical absorption coating, and the femtosecond transient absorption spectrometer is further internally provided with a low-dispersion laser focusing assembly and a micro-nano reflection assembly in front of a sample table, the micro-nano reflection assembly comprises a reflection structure and a light transmission area, the reflection structure is not completely perpendicular to an incident light path, so that the light path reflected by the reflection structure can irradiate the optical absorption coating; the low-dispersion laser focusing assembly focuses supercontinuum white light and provides dispersion to the micro-nano reflection assembly, and the micro-nano reflection assembly selectively reflects according to the dispersion effect to adjust the spatial distribution of light of different wavelengths near a focal point. The application adjusts the transmission rate of the probe white light by the micro-nano reflection assembly, and does not cause the pulse width of the femtosecond pulse probe white light to be widened. Under the premise of maintaining the maximum time resolution of the femtosecond transient absorption spectrum, the signal-to-noise ratio of the femtosecond transient absorption spectrum is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of femtosecond transient absorption spectrometers, and particularly to a method for improving the signal-to-noise ratio of femtosecond transient absorption spectrometers. Background Technology

[0002] Transient absorption spectroscopy is a powerful tool for studying the structure of excited-state energy levels and the relaxation processes of excited states. It can reveal the dynamics of changes in absorption signals from excited-state molecules with extremely short retention times and low concentrations. Femtosecond transient absorption spectroscopy, based on femtosecond lasers, is one of the most important methods for studying ultrafast dynamics. It combines femtosecond time-resolved pump-probe technology with absorption spectroscopy, and by analyzing the dynamics of changes in absorption spectra, it can not only obtain information on transitions between excited-state energy levels of a single substance, but also study energy transfer and electron transfer processes between different substances. It has important applications in physics, chemistry, materials science, biology, and medicine.

[0003] Since no signal detector currently achieves femtosecond-level time resolution, femtosecond transient absorption spectroscopy employs a pump-probe technique. A single femtosecond laser beam is split into two paths. One path acts as the pump light, causing the sample to transition to an excited state; the other path generates supercontinuum white light, which serves as the probe light to detect the absorption spectral properties of the excited sample molecules at a specific time delay. This time delay can be adjusted by the optical path difference between the two paths. By recording the absorption spectra at different time delays, the absorption spectrum of the excited state of the sample as a function of time can be obtained. Further processing and analysis yield the decay dynamics information of all energy levels during the radiative energy relaxation process from the high-energy excited state to the low-energy ground state.

[0004] The signal intensity of the femtosecond transient absorption spectrum is determined by the quality of the probe light. The uniformity of intensity across different wavelengths of the white light used as the probe light significantly impacts the measured signal-to-noise ratio. In femtosecond transient absorption spectroscopy, the probe white light is generated by focusing a monochromatic femtosecond pulsed laser within a nonlinear medium, through nonlinear optical effects. This is known as supercontinuum white light. The absorption spectrum measurement in femtosecond transient absorption spectroscopy uses this supercontinuum white light as the white light source. Because the intensity of the supercontinuum white light spectrum in its original wavelength band (i.e., the band centered on the wavelength of the monochromatic femtosecond pulsed laser and its surrounding bands) is much higher than in other bands, the original wavelength band must be filtered out before testing.

[0005] The temporal resolution of femtosecond transient absorption spectra is determined by the pulse width of the femtosecond laser and the magnitude of dispersion caused by internal instrument components. Introducing highly dispersive elements significantly reduces the test temporal resolution (e.g., introducing highly dispersive elements such as gratings or lenses at the sample front end will reduce temporal resolution). Due to this limitation, filters are currently generally used for spectral shaping of the probe light. When the test band is green, red, or infrared, the original band is generally located in the long-wavelength or short-wavelength direction of the test band, with no overlap. Long-pass or short-pass filters can be used to filter out the original band without affecting the intensity of the white light in the test band. However, when the test band is ultraviolet or blue (350-530nm), the original band overlaps with the test band. Notch filters or band-stop filters are typically used to filter out the original band. This method significantly weakens the intensity of some test bands, thereby reducing the signal-to-noise ratio of the full-spectrum femtosecond transient absorption detection.

[0006] Femtosecond transient absorption spectroscopy is based on nonlinear effects and requires extremely high instrument precision. Due to the influence of component displacement caused by minute vibrations, the positions of internal components need to be fine-tuned and reset periodically. These minute displacements of internal optical components significantly affect the spectral shape of supercontinuum white light. In other words, the supercontinuum white light spectrum changes to some extent after each adjustment. Using filters with fixed transmittance makes it difficult to achieve optimal shaping of the supercontinuum white light spectrum. Therefore, improvements are needed to the existing probe light shaping methods for femtosecond transient absorption spectrometers, specifically tailored to the characteristics of ultrafast spectroscopy. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for improving the signal-to-noise ratio of femtosecond transient absorption spectra. This method aims to reduce the intensity difference of supercontinuous white light at different wavelengths without reducing the temporal resolution of femtosecond transient absorption spectra, thereby achieving high signal-to-noise ratio testing across the entire spectrum.

[0008] The above-mentioned technical objective of the present invention is achieved through the following technical solution: a method for improving the signal-to-noise ratio of femtosecond transient absorption spectrometry, comprising a femtosecond transient absorption spectrometer for generating supercontinuous white light, wherein the inner wall of the femtosecond transient absorption spectrometer is provided with a light-absorbing coating.

[0009] The femtosecond transient absorption spectrometer also has a built-in low-dispersion laser focusing component and a micro-nano reflection component located in front of the sample stage. The micro-nano reflection component includes a reflection structure and a light-transmitting area. The reflection structure is not completely perpendicular to the incident light path, so that the light path reflected by the reflection structure can illuminate the inner wall of the femtosecond transient absorption spectrometer and be absorbed by the light-absorbing coating.

[0010] The low-dispersion laser focusing component focuses the supercontinuous white light and provides dispersion to the micro-nano reflection component. The micro-nano reflection component selectively reflects light according to the dispersion effect to adjust the spatial distribution of light of different wavelengths near the focal point, and adjusts the intensity difference of the supercontinuous white light to 0.2 < X < 1.0. The X value is calculated as: minimum light intensity / maximum light intensity in the 370nm-680nm wavelength range.

[0011] The above technical solution aims to reduce the intensity difference of supercontinuous white light at different wavelengths without reducing the temporal resolution of the femtosecond transient absorption spectrum, thereby achieving high signal-to-noise ratio testing across the entire spectrum. On one hand, it avoids introducing high-dispersion elements (such as gratings, lenses, and filters) in front of the sample stage by using low-dispersion elements (concave mirrors) and micro / nano reflective components to ensure that the temporal resolution of the femtosecond transient absorption spectrum is not reduced. On the other hand, it utilizes the dispersion effect of low-dispersion elements to balance the light intensity of each band through micro / nano reflective components. High-intensity bands are reflected more, while low-intensity bands are reflected less or not at all. This light intensity is then absorbed by the light-absorbing coating on the inner wall of the spectrometer. By reducing the intensity difference of different wavelengths of the supercontinuous white light to 0.2 < X < 1.0, the signal-to-noise ratio is improved.

[0012] Furthermore, the femtosecond transient absorption spectrometer includes a femtosecond laser. The femtosecond light emitted by the femtosecond laser passes sequentially through a frequency doubling device and a convex lens and is focused onto a nonlinear medium to generate supercontinuous white light. The spectral intensity of the supercontinuous white light is centered at 515 nm.

[0013] Furthermore, the low-dispersion laser focusing component includes a front concave mirror group and a rear concave mirror group located on both sides of the micro-nano reflective component, wherein the front concave mirror group is located between the nonlinear medium and the micro-nano reflective component;

[0014] The concave mirror group is used to focus supercontinuous white light and provide dispersion to the micro-nano reflective components.

[0015] The concave mirror assembly is used to restore the supercontinuous white light, after the intensity difference has been adjusted, into parallel light and converge it onto the sample stage.

[0016] Furthermore, the micro-nano reflective component is disposed at the focal point of the concave mirror assembly and in front of and behind it, and the focal point is a focal point with a wavelength of 515nm.

[0017] Furthermore, the micro-nano reflective component includes a micro-nano reflective sheet, the center of which is provided with a reflective structure, the thickness of which ranges from 10μm to 3000μm;

[0018] By using a three-dimensional micro-displacement stage, the focal position of the 515nm wavelength is precisely positioned to illuminate the center of the reflective structure of the micro-nano reflective sheet. The center of the reflective structure of the micro-nano reflective sheet is not completely perpendicular to the incident light path, so that the light path reflected through the center of the reflective structure can illuminate the inner wall of the femtosecond transient absorption spectrometer.

[0019] Furthermore, the micro-nano reflective assembly also includes two overlapping micro-nano spectral intensity shaping plates. Each micro-nano spectral intensity shaping plate includes a fan-shaped transmission region 1, an annular transmission region 2, and a reflective structure. Within the fan-shaped region 1, two reflective blocks are symmetrically arranged at the center, with a gap between them. The focal point of a 515nm wavelength can pass through this gap and precisely illuminate the reflective structure of the micro-nano reflective plate. The reflective area of ​​the annular region 2 is smaller than the reflective area of ​​the fan-shaped region 1. The light path reflected from the reflective structure of the micro-nano spectral intensity shaping plate illuminates the inner wall of the femtosecond transient absorption spectrometer.

[0020] The micro-nano spectral shaping sheet is located between the micro-nano reflective sheet and the concave mirror assembly.

[0021] Furthermore, the spacing between the micro / nano spectral shaping sheet and the micro / nano reflective sheet is less than 1 mm, in order to adjust the transmittance of the corresponding band near the 515 nm wavelength focal point.

[0022] Furthermore, the micro / nano spectral shaping sheet has a light-absorbing surface on the side near the concave mirror assembly;

[0023] When the micro-nano spectral shaping sheet is located between the micro-nano reflective sheet and the concave mirror group, the light path reflected from the center of the reflective structure of the micro-nano reflective sheet can directly illuminate the light-absorbing surface of the micro-nano spectral shaping sheet.

[0024] Furthermore, both the front concave mirror group and the rear concave mirror group include at least two concave mirrors, each of which is an off-axis parabolic reflector.

[0025] Furthermore, the micro / nano spectral shaping sheet and the micro / nano reflective sheet are both made of metal, semiconductor, coated quartz sheet, coated glass sheet, etc. through micro / nano processing.

[0026] In summary, the present invention has the following beneficial effects:

[0027] 1. The use of reflective components for filtering does not cause the pulse width of the femtosecond laser to broaden.

[0028] 2. By adjusting the relative position of the micro-nano reflective components and the focal point through a three-dimensional micro-displacement stage, the transmittance of white light in any band can be flexibly adjusted, enabling precise shaping of the supercontinuous white light spectrum.

[0029] 3. Effectively reduces the transmittance of high-intensity bands of supercontinuous white light, and can effectively broaden existing measurement bands by co-tuning with the gradual attenuation of supercontinuous white light.

[0030] 4. Reducing the use of filters can reduce the dispersion caused by femtosecond lasers in transparent media.

[0031] 5. It can effectively improve the signal-to-noise ratio of femtosecond transient absorption spectra while maintaining the maximum time resolution of femtosecond transient absorption spectra.

[0032] 6. It has good compatibility with existing equipment and can be directly integrated into the instrument without changing the instrument's optical path settings. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the signal-to-noise ratio enhancement device in Embodiment 1 of the present invention.

[0034] Figure 2 This is the supercontinuum white light spectrum in Embodiment 1 of the present invention without any filtering.

[0035] Figure 3 This is a schematic diagram illustrating how the focal point of light of different wavelengths differs due to the dispersion effect.

[0036] Figure 4 This is the supercontinuous white light cone-angle radiation obtained by CCD measurement.

[0037] Figure 5 This is a magnified structural diagram of the 1-5C1 and 1-5C2 micro / nano reflective sheets.

[0038] Figure 6 In Embodiment 1 of the present invention, the supercontinuum white light spectra were measured when (a) the 100μm reflective structure and (b) the 800μm reflective structure were located at the supercontinuum white light focusing position. (c) is a comparison of the transmission spectra measured by spectrometer 2-6 when the attenuation intensity of the 1-7 graded adjustable attenuator remains unchanged, with and without the 1-5C1 placed on the 800μm reflective structure.

[0039] Figure 7 This is a schematic diagram of the signal-to-noise ratio enhancement device in Embodiment 2 of the present invention.

[0040] Figure 8 This is a schematic diagram of the structure of the 1-5A and 1-5B micro / nano spectral intensity shaping sheets.

[0041] Figure 9 This diagram illustrates how the transmittance of different wavelength bands can be adjusted by varying the overlap of micro / nano spectral intensity shaping plates. The transmittance values ​​marked in the diagram represent the transmittance of sector 1.

[0042] Figure 10(a) and (c) are the supercontinuum white light spectra and their detection signal-to-noise ratios obtained in Embodiment 2 of the present invention. Figure 10 (b) and (d) show the final supercontinuum white light spectrum and its detection signal-to-noise ratio obtained in Comparative Example 1.

[0043] Figure 11 (a) is the instrument response function (I RF) measured in Example 2 of the present invention, and (b) is the instrument response function measured in Comparative Example 2.

[0044] Figure 12 This is a schematic diagram of the measuring device used in Comparative Example 2.

[0045] Figure 13 The supercontinuum white light spectrum under the combination of three filter angles is shown in Comparative Example 2.

[0046] Figure reference numerals: 1-1 convex lens; 1-2 nonlinear medium; 1-3, 1-4, 1-6, 1-8 concave mirrors; 1-5A, 1-5B micro / nano spectral intensity shaping plates; 1-5C1 micro / nano reflector; 1-7 graded adjustable attenuator; 1-9 sample stage; 1-10, 1-13 reflectors; 1-11, 1-12 filters; 2-1 femtosecond laser; 2-2 frequency doubling device; 2-3, 2-4 convex lenses; 2-5 optical fiber; 2-6 spectrometer. Detailed Implementation

[0047] The present invention will be further described in detail below with reference to the accompanying drawings.

[0048] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0049] Due to the dispersion effect, light of different wavelengths will have slightly different focal lengths when converged by the same lens. That is, light of different wavelengths will have different focal points in space (e.g., light of different wavelengths converges at different locations). Figure 3 Furthermore, after a nonlinear medium is filamentated by a monochromatic femtosecond pulsed laser, the emitted supercontinuous white light propagates in the form of concentric rainbow-like rings (e.g., Figure 4 This phenomenon is called cone-angle radiation. Therefore, after supercontinuum white light is focused by the same lens, the divergence angles of different wavelengths of light also differ. This application utilizes a specially designed micro / nano reflector and a micro / nano spectral intensity shaping plate to selectively transmit or reflect light near the focal point, thereby adjusting the transmittance of different wavelengths of supercontinuum white light. The structural design of the micro / nano reflector and the micro / nano spectral intensity shaping plate must consider the size of the focused spot and the range of filtered wavelengths.

[0050] Example 1:

[0051] Figure 1 This is a schematic diagram of the signal-to-noise ratio enhancement device in Example 1. The 1030nm femtosecond light emitted by the 2-1 femtosecond laser is converted to a 515nm femtosecond laser by the 2-2 frequency doubling device, and then focused by the 1-1 convex lens onto the 1-2 nonlinear medium. The resulting supercontinuum white light is focused by the 1-3 and 1-4 concave mirrors, reflected as parallel light by the 1-6 concave mirror, and after the light intensity is adjusted to the minimum by the 1-7 gradually adjustable attenuator, it is converged by the 1-8 concave mirror onto the 1-9 sample stage. The transmitted supercontinuum white light is then converged by the 2-3 and 2-4 convex lens group to the fiber optic port 2-5, and finally reaches the 2-6 spectrometer. The spectrometer detects that the supercontinuum white light spectrum at this time is as follows: Figure 2 As shown. The spectrum is unfiltered, and its intensity distribution is centered at 515 nm with a full width at half maximum (FWHM) of 5 nm. From Figure 2 It can be seen that the spectral intensity is higher when the wavelength is closer to 515nm.

[0052] The 1-5C1 micro / nano reflector is positioned near the focal point between the concave mirrors 1-4 and 1-6. The magnified structure of the 1-5C1 micro / nano reflector is shown below. Figure 5 As shown, the center of the instrument features reflective structures of varying thicknesses. A three-dimensional micro-displacement stage is used to precisely position the focal point of the 515nm wavelength reflector at the center of the reflective structure of the 1-5C1 micro / nano reflector. Furthermore, the center of the reflective structure of the 1-5C1 micro / nano reflector is not perfectly perpendicular to the incident light path; it should be positioned at a certain angle so that the light reflected from the center of the reflective structure can pass through the gap between the 1-3 and 1-4 concave mirrors and illuminate the inner wall of the 2-6 femtosecond transient absorption spectrometer, which is made of a light-absorbing coating.

[0053] Adjusting the 1-7 graded attenuator allows for adjusting the light intensity until a sufficient signal can be observed on the spectrometer (2-6), with no wavelength exceeding the spectrometer's range across the entire spectrum. The bandwidth of the filter can be adjusted by changing the thickness of the effective reflective structure of the 1-5 C1 micro / nano reflector. Figure 6 a and 6b are the transmission spectra measured by the spectrometer 2-6 when the 100μm and 800μm reflective structures are located at the supercontinuum white light focusing position, respectively. The filter bandwidth of the 800μm reflective structure is greater than that of the 100μm reflective structure. Figure 6 c represents a comparison of the transmission spectra measured by a spectrometer (2-6) between an 800μm reflective structure with and without the 1-5C1 filter, when the attenuation intensity of the 1-7 graded adjustable attenuator remains constant. It should be noted that... Figure 6 The only difference in the test parameters between the 800μm reflective structures b and 6c is the attenuation intensity of the 1-7 graded adjustable attenuator.

[0054] In the above, this application sets four low-dispersion elements (1-3, 1-4, 1-6, and 1-8 concave mirrors) in the femtosecond transient absorption spectrometer. Among them, concave mirror 1-3 is used to concentrate the diffused white light into parallel light, concave mirror 1-4 is used to focus and provide dispersion to the micro-nano reflective components, concave mirror 1-6 is used to restore the light to parallel light, and concave mirror 1-8 is used to focus the light onto the sample.

[0055] It should be noted that the filtering effect of the center of the reflective structure of the 1-5C1 micro-nano reflective sheet in this application can attenuate light of a specific wavelength band to a very low level, while allowing light of other wavelength bands to pass through. In this invention, "filter bandwidth" refers to half the bandwidth of the attenuated wavelength band. This application uses a 1-5C1 micro-nano reflective sheet for filtering, which will not cause pulse width broadening of the femtosecond pulse laser.

[0056] Furthermore, the micro / nano reflective sheet used in this application is fabricated using micro / nano processes, which is existing technology. Due to the extremely high peak power and extremely high power density after focusing of femtosecond lasers, conventional absorption-type filter elements will deform or shift due to thermal effects. The micro / nano reflective sheet used in this application can effectively solve the thermal effect problem. Simultaneously, the specially shaped micro / nano reflective sheet can selectively transmit or block light in the space near the focal point, thereby adjusting the transmittance of different wavelengths of supercontinuous white light. The structural design of the micro / nano reflective sheet must be adapted to the size of the focused spot and the range of filtered wavelengths.

[0057] Example 2:

[0058] Figure 7 This is a schematic diagram of the signal-to-noise ratio enhancement device in Example 2. The 1030nm femtosecond light emitted by the femtosecond laser (2-1) is converted to a 515nm femtosecond laser by the frequency doubling device (2-2), and then focused by the convex lens (1-1) onto the nonlinear medium (1-2). The resulting supercontinuum white light is focused by concave mirrors (1-3, 1-4), reflected as parallel light by the concave mirror (1-6), and then its intensity is adjusted to the minimum by the gradually adjustable attenuator (1-7) before being focused onto the sample stage (1-9) by the concave mirror (1-8). The transmitted supercontinuum white light is then focused by the convex lens group (2-3, 2-4) at the fiber optic port (2-5), and finally reaches the spectrometer (2-6).

[0059] The 1-5C1 micro-nano reflector was positioned near the focal point between the concave mirrors 1-4 and 1-6. Using a three-dimensional micro-displacement stage, the focal point of the 515nm wavelength was positioned precisely to illuminate the 50μm reflective structure of the 1-5C1 micro-nano reflector. The graded attenuator 1-7 was adjusted to regulate the light intensity until a sufficient signal could be observed on the spectrometer 2-6, with no wavelength exceeding the spectrometer's range across the entire spectrum.

[0060] Then, overlap and place the 1-5B1 and 1-5B2 micro / nano spectral intensity shaping sheets (or 1-5A1 and 1-5A2 micro / nano spectral intensity shaping sheets) in front of the 1-5C1 micro / nano reflective sheet. The magnified structure of the 1-5B1 and 1-5B2 micro / nano spectral intensity shaping sheets is as follows. Figure 8 As shown, the micro-nano reflector has a fan-shaped transmission area 1, an annular transmission area 2, and a reflective structure as shown in the cross-section. Two reflective blocks are symmetrically arranged at the center of the fan-shaped area 1, with a gap between them. The focal point of a 515nm wavelength can pass through this gap and precisely illuminate the 50μm reflective structure of the 1-5C1 micro-nano reflector. Each reflective block can be fan-shaped or triangular. The unique structure of the micro-nano spectral intensity shaping sheet of this application is specifically adapted to supercontinuous white light cone-angle radiation. The fan-shaped area 1 is mainly used to adjust the transmittance in the 470nm-540nm band. The annular area 2 is mainly used to adjust the transmittance in the 370nm-470nm and 540nm-680nm bands. Since the intensity of supercontinuous white light in the 370nm-470nm and 540nm-680nm bands is lower than that in the 470nm-540nm band, the reflective area of ​​the annular area 2 is smaller than that of the fan-shaped area 1. The light reflected by the reflective structure passes through the gap between the concave mirrors 1-3 and 1-4 and illuminates the inner wall of the 2-6 femtosecond transient absorption spectrometer, where it is absorbed.

[0061] The spacing between the preferred micro / nano spectral intensity shaping sheet (1-5B1, 1-5B2) and the 1-5C1 micro / nano reflective sheet is less than 1 mm, in order to adjust the transmittance of the corresponding band near the 515 nm wavelength focal point.

[0062] In addition, in this embodiment, light-absorbing surfaces can be provided on the side of the 1-5B1 and 1-5B2 micro-nano spectral shaping sheets near the 1-6 concave mirror, that is, the 1-5B1 and 1-5B2 micro-nano spectral shaping sheets have reflection and light absorption functions on both sides respectively.

[0063] When the 1-5B1 and 1-5B2 micro-nano spectral shaping sheets are located between the 1-5C1 micro-nano reflector and the 1-4 concave mirror, the light reflected from the center of the reflective structure of the 1-5C1 micro-nano reflector can directly illuminate the light-absorbing surface of the 1-5B1 and 1-5B2 micro-nano spectral shaping sheets.

[0064] like Figure 7 As shown, using a three-dimensional micro-displacement stage, the focal position of the 470nm-540nm wavelength range was precisely positioned to illuminate the center of the reflective structure of the overlapping 1-5B1 and 1-5B2 micro-nano spectral shaping plates. Adjusting the 1-7 gradually adjustable attenuator, the light intensity was adjusted until a sufficient signal could be observed on the spectrometer at 2-6, and no wavelength in the full spectrum exceeded the spectrometer's range. Figure 9By adjusting the overlap of the 1-5B1 and 1-5B2 micro-nano spectral shaping sheets, the transmittance in the 470nm-540nm wavelength band can be adjusted. Alternatively, a combination of 1-5A1 and 1-5A2 reflective sheets can also be used.

[0065] Based on the spectrometer test results, the positions and overlap of the 1-5B1 and 1-5B2 micro / nano spectral shaping plates were adjusted to minimize the intensity difference between different wavelengths of the supercontinuum white light (the intensity difference between different wavelengths was evaluated by the ratio of the lowest to the highest intensity in the 370-680nm band, with the ratio greater than 0.2 and less than 1.0). At this point, the supercontinuum white light spectrum is as follows: Figure 10 As shown in a. Under the blank control test, the ΔT / T values ​​in the 370nm-680nm band are all less than 3×10-5, indicating excellent detection signal-to-noise ratio.

[0066] Figure 11 (a) shows the instrument response function (IRF) test results. IRF is used to quantify the temporal resolution of femtosecond transient absorption spectra. The IRF is found to be 0.204 ps through fitting. The existing femtosecond transient absorption spectroscopy apparatus has a structure similar to that in Comparative Example 1 below. In Comparative Example 1 below, the IRF is 0.216 ps. The difference between the two is within the normal measurement error range. It is evident that the signal-to-noise ratio enhancement device does not cause a decrease in the temporal resolution of femtosecond transient absorption spectroscopy testing.

[0067] Comparative Example 1:

[0068] Figure 12 This is a schematic diagram of the measurement device used in Comparative Example 1. The 1030nm femtosecond light emitted by the femtosecond laser (2-1) is converted to 515nm femtosecond laser light by the frequency doubling device (2-2), and then focused by the convex lens (1-1) onto the nonlinear medium (1-2). The resulting supercontinuum white light is reflected into parallel light by the concave mirror (1-3), reflected by (1-10), passes through the filters (1-11, 515nm notch filter, bandwidth 20nm), (1-12, 430nm-550nm band-stop filter), is reflected by (1-13), and then through the graded adjustable attenuator (1-7). Finally, it is focused by the concave mirror (1-8) onto the sample stage (1-9). The transmitted supercontinuum white light is then focused by the convex lens groups (2-3, 2-4) onto the fiber optic port (2-5), and finally reaches the spectrometer (2-6).

[0069] Adjusting the gradually adjustable attenuator (1-7) until the light intensity reaches (2-6) allows for sufficient signal observation on the spectrometer, with no wavelength exceeding the spectrometer's range. Spectrometer readings at this point show the supercontinuum white light spectrum as follows: Figure 10As shown in (b), under the blank control test, the ΔT / T value was less than 3×10⁻⁵ only in the 405nm-460nm, 491nm-525nm, and 562nm-664nm bands. The signal-to-noise ratio was poor in the 460nm-491nm and 525nm-562nm bands, making it difficult to achieve high signal-to-noise ratio testing across the entire band.

[0070] Figure 11 (b) shows the instrument response function test results. The IRF value is 0.204 ps, obtained through fitting.

[0071] Comparative Example 2:

[0072] Figure 12 This is a schematic diagram of the measurement device used in Comparative Example 2. The 1030nm femtosecond light emitted by the femtosecond laser (2-1) is converted to 515nm femtosecond laser light by the frequency doubling device (2-2), and then focused by the convex lens (1-1) onto the nonlinear medium (1-2). The resulting supercontinuum white light is reflected into parallel light by the concave mirror (1-3), reflected by 1-10, passed through filters (1-11, adjustable band-stop bidirectional filter), and (1-12, adjustable band-stop bidirectional filter), reflected by 1-13, and then by the graded adjustable attenuator (1-7). Finally, it is focused by the concave mirror (1-8) onto the sample stage (1-9). The transmitted supercontinuum white light is then focused by the convex lens groups (2-3, 2-4) onto the fiber optic port (2-5), and finally reaches the spectrometer (2-6).

[0073] Adjust the 1-7 gradually adjustable attenuator to adjust the light intensity until a sufficient signal can be observed on the spectrometer at 2-6, and the wavelength of any wavelength in the full spectrum does not exceed the spectrometer's range. By adjusting the angles of filters 1-11 and 1-12, the transmittance of the filters in different wavelength bands can be adjusted. Figure 13 The supercontinuum white light spectrum is shown under three random angle combinations. The light intensity varies greatly across different wavelengths, making it difficult to achieve a high signal-to-noise ratio across the entire wavelength range.

[0074] The specific embodiments are merely illustrative of the present invention and are not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to these embodiments without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A method for improving the signal-to-noise ratio of femtosecond transient absorption spectroscopy, comprising a femtosecond transient absorption spectrometer for generating supercontinuous white light, characterized in that: The inner wall of the femtosecond transient absorption spectrometer is provided with a light-absorbing coating. The femtosecond transient absorption spectrometer also has a built-in low-dispersion laser focusing component and a micro-nano reflection component located in front of the sample stage. The micro-nano reflection component includes a reflection structure and a light-transmitting area. The reflection structure is not completely perpendicular to the incident light path, so that the light path reflected by the reflection structure can illuminate the inner wall of the femtosecond transient absorption spectrometer and be absorbed by the light-absorbing coating. The low-dispersion laser focusing component focuses the supercontinuous white light and provides dispersion to the micro-nano reflection component. The micro-nano reflection component selectively reflects light according to the dispersion effect to adjust the spatial distribution of light of different wavelengths near the focal point, and adjusts the intensity difference of the supercontinuous white light to 0.2 < X < 1.

0. The X value is calculated as: minimum light intensity / maximum light intensity in the 370nm-680nm wavelength range. The low-dispersion laser focusing component includes a front concave mirror group and a rear concave mirror group located on both sides of the micro-nano reflective component, wherein the front concave mirror group is located between the nonlinear medium and the micro-nano reflective component; The concave mirror group is used to focus supercontinuous white light and provide dispersion to the micro-nano reflective components. The concave mirror group is used to restore the supercontinuous white light after the intensity difference adjustment is completed into parallel light and converge it onto the sample stage. The micro-nano reflective assembly includes a micro-nano reflective sheet, and the center of the micro-nano reflective sheet has a reflective structure, the thickness of which ranges from 10μm to 3000μm. Using a three-dimensional micro-displacement stage, the focal position of the 515nm wavelength is precisely irradiated at the center of the reflective structure of the micro-nano reflective sheet. The center of the reflective structure of the micro-nano reflective sheet is not completely perpendicular to the incident light path, so that the light path reflected through the center of the reflective structure can irradiate the inner wall of the femtosecond transient absorption spectrometer. The micro-nano reflective assembly further includes two overlapping micro-nano spectral intensity shaping plates. Each micro-nano spectral intensity shaping plate includes a sector-shaped transmission region 1, an annular transmission region 2, and a reflective structure. Within the sector-shaped region 1, two reflective blocks are symmetrically arranged at the center, with a gap between the two reflective blocks. The focal point of a 515nm wavelength can pass through this gap and precisely illuminate the reflective structure of the micro-nano reflective plate. The reflective area of ​​the annular region 2 is smaller than the reflective area of ​​the sector-shaped region 1. The light path reflected from the reflective structure of the micro-nano spectral intensity shaping plate illuminates the inner wall of the femtosecond transient absorption spectrometer. The micro-nano spectral shaping sheet is located between the micro-nano reflective sheet and the concave mirror assembly.

2. The method for improving the signal-to-noise ratio of femtosecond transient absorption spectroscopy according to claim 1, characterized in that: The femtosecond transient absorption spectrometer includes a femtosecond laser. The femtosecond light emitted by the femtosecond laser passes sequentially through a frequency doubling device and a convex lens and is focused onto a nonlinear medium to generate supercontinuous white light. The spectral intensity of the supercontinuous white light is centered at 515 nm.

3. The method for improving the signal-to-noise ratio of femtosecond transient absorption spectroscopy according to claim 1, characterized in that: The micro-nano reflective component is disposed at the focal point of the concave mirror assembly and in front of and behind it, and the focal point is a focal point with a wavelength of 515nm.

4. The method for improving the signal-to-noise ratio of femtosecond transient absorption spectroscopy according to claim 1, characterized in that: The spacing between the micro / nano spectral shaping sheet and the micro / nano reflective sheet is less than 1 mm, so as to adjust the transmittance of the corresponding band near the 515 nm wavelength focal point.

5. The method for improving the signal-to-noise ratio of femtosecond transient absorption spectroscopy according to claim 4, characterized in that: The micro-nano spectral shaping sheet has a light-absorbing surface on the side near the concave mirror assembly. When the micro-nano spectral shaping sheet is located between the micro-nano reflective sheet and the concave mirror group, the light path reflected from the center of the reflective structure of the micro-nano reflective sheet can directly illuminate the light-absorbing surface of the micro-nano spectral shaping sheet.

6. The method for improving the signal-to-noise ratio of a femtosecond transient absorption spectrum according to claim 5, characterized in that: Both the front concave mirror group and the rear concave mirror group include at least two concave mirrors, and each concave mirror is an off-axis parabolic reflector.

7. The method for improving the signal-to-noise ratio of a femtosecond transient absorption spectrum according to claim 6, characterized in that: Both the micro / nano spectral shaping sheet and the micro / nano reflective sheet are made of metal, semiconductor, coated quartz sheet, and coated glass sheet through micro / nano processing.

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