A continuous scanning half-wave sampling system and method for multidimensional coherent spectroscopy

Abstract

The present invention discloses a continuous scanning half-wave sampling system for multidimensional coherent spectroscopy, comprising a continuous laser, wherein the continuous laser output by the continuous laser is combined with the femtosecond laser pulse output by an ultrashort femtosecond laser pulse generator, and then the continuous laser is modulated and delayed respectively to obtain a multidimensional coherent femtosecond laser pulse sequence for exciting the fluorescence signal generated by the sample and converting it into a first electrical signal, and the obtained continuous laser with different modulation frequencies is converted into a second electrical signal; the total mixing signal obtained by mixing the multi-channel modulated signals is used as a reference to demodulate the second electrical signal, and the first electrical signal is demodulated with the second electrical signal at the same time. The present invention also discloses a continuous scanning half-wave sampling method for multidimensional coherent spectroscopy, which uses the half-wave sampling method to accurately calibrate the optical time delay to obtain a complex-phase multidimensional coherent spectrum. The sampling step of the present invention breaks through the minimum sampling step limit of traditional step acquisition and significantly improves the sampling bandwidth.

Description

Technical Field

[0001] The present invention belongs to the technical field of multidimensional coherent spectroscopy, and in particular relates to a continuous scanning half-wave sampling system for multidimensional coherent spectroscopy, and also relates to a continuous scanning half-wave sampling method for multidimensional coherent spectroscopy. Background Art

[0002] Multidimensional coherent spectroscopy is a nonlinear ultrafast spectroscopy technique based on the principle of quantum coherence. Since Tanimura and Mukamel first extended the concept of two-dimensional nuclear magnetic resonance spectroscopy from microwave wavelengths to the optical wavelength range in 1993, multidimensional coherent spectroscopy has become an important tool for studying the microstructure and dynamics of matter. This technique utilizes nested optical interferometers to generate a sequence of femtosecond laser pulses with precisely controllable optical time delays. This pulse sequence is manipulated to interact with matter in a specific time sequence, stimulating third-order nonlinear polarization. The polarized radiation signals at the corresponding optical time delays are then collected to construct a multidimensional spectral data matrix in the time domain. This data is then subjected to a multidimensional Fourier transform to obtain a multidimensional coherent spectrum in the frequency domain. Compared to one-dimensional spectroscopy, multidimensional coherent spectroscopy can expand overlapping and crowded one-dimensional spectra into multiple dimensions, facilitating the resolution of complex processes such as uniform and inhomogeneous broadening, probing many-body correlations, decoupling coherent coupling of quantum states, and tracking quantum excitation pathways. These advantages have attracted widespread attention in fields such as single atoms and molecules, quantum dots (wells), novel two-dimensional semiconductor materials, and chemical and biological macromolecules.

[0003] One of the keys to achieving multidimensional coherent spectroscopy is to maintain high phase stability for a long time. Compared with one-dimensional spectroscopy, the acquisition time of multidimensional spectroscopy increases exponentially, which makes it quite challenging to maintain high phase stability during long-term acquisition. For this reason, a variety of experimental schemes have emerged, among which the active feedback phase-locked non-collinear scheme and the pulse spatiotemporal shaping non-collinear scheme are the two most classic technical schemes. They usually use a grating spectrometer to replace the step-by-step scanning of an optical time axis, thereby effectively shortening the acquisition time of multidimensional coherent spectroscopy. However, due to the limitations of specific nonlinear processes and limited spatial resolution, the above schemes are not suitable for the study of complex quantum systems such as high-order nonlinear processes and small-sized quantum materials. A new type of phase-modulated phase-locked multidimensional coherent spectroscopy technology developed recently has attracted widespread attention because it can detect multiple nonlinear processes and has diffraction-limited spatial resolution. However, since this scheme requires step-scanning multiple optical time delays in the time domain to obtain multidimensional coherent spectra, it not only increases the acquisition time of the time-domain multidimensional spectral data matrix, but also poses a challenge to maintaining phase stability for a long time, seriously affecting the data quality and experimental efficiency of multidimensional coherent spectroscopy. In addition, according to the sampling bandwidth formula (Where, is the signal bandwidth, The sampling bandwidth is limited by the optical delay sampling step size. Multidimensional coherent spectroscopy typically uses optical delay step acquisition. However, its minimum sampling step size is limited by the mechanical structure of the optical delay line, resulting in a limited sampling bandwidth and difficulty in applying it to broadband signal acquisition. This also causes signal undersampling, which in turn affects the correct interpretation of multidimensional coherent spectroscopy signals. Summary of the Invention

[0004] The purpose of the present invention is to address the above-mentioned problems existing in the prior art and to provide a continuous scanning half-wave sampling system for multidimensional coherent spectroscopy, and also to provide a continuous scanning half-wave sampling method for multidimensional coherent spectroscopy.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy includes a continuous laser. The continuous laser output by the continuous laser is combined with the ultrashort femtosecond laser pulse output by an ultrashort femtosecond laser pulse generator and then split to obtain four collimated lights. Each collimated light passes through a corresponding acousto-optic modulation unit and an optical delay line to generate a corresponding modulated collimated light. Each acousto-optic modulation unit is driven by a corresponding modulation signal. All modulation signals are output by a direct digital frequency synthesis signal generator.

[0007] Four-way modulated collimated photosynthetic beam is used to output multi-dimensional coherent femtosecond laser pulse sequences and continuous lasers with different modulation frequencies through a D-shaped reflector.

[0008] A multi-dimensional coherent femtosecond laser pulse sequence excites the sample to generate a fluorescence signal, which is converted into a first electrical signal by a first photodetector; a continuous laser with different modulation frequencies is converted into a second electrical signal by a second photodetector;

[0009] The direct digital frequency synthesis signal generator also outputs four modulation signals to the mixing unit to generate a total mixing signal. The frequency of the total mixing signal is the same as the modulation frequency carried by the fluorescence signal.

[0010] The first phase-locked amplifier demodulates the second electrical signal with the total mixing signal as a reference, and an orthogonal component of the demodulated second demodulated electrical signal is collected and stored in real time by the synchronous data acquisition unit. The second phase-locked amplifier demodulates the first electrical signal with the second electrical signal as a reference to obtain the first demodulated electrical signal, and the two orthogonal components of the first demodulated electrical signal are synchronously collected and stored by the synchronous data acquisition unit. The sampling unit samples the corresponding data points of the two orthogonal components of the first demodulated electrical signal from the synchronous data acquisition unit based on the zero crossing point of the second demodulated electrical signal.

[0011] As described above, the ultrashort femtosecond laser pulse generating device includes a laser seed source. The seed laser generated by the laser seed source is amplified by the nonlinear crystal parameters in the optical parametric amplifier to generate ultrashort femtosecond laser pulses that resonate with the atomic line transition of the sample.

[0012] As mentioned above, there is a spatial offset between the two collimated beams of ultrashort femtosecond laser pulse and continuous laser in the direction perpendicular to the horizontal plane.

[0013] As described above, the ultrashort femtosecond laser pulse and the continuous laser are combined and then split by the first beam splitter to obtain a first combined beam and a second combined beam; the first combined beam is split into a first collimated beam and a second collimated beam at the second beam splitter; the second combined beam passes through the second optical delay line and is then split by the third beam splitter to obtain a third collimated beam and a fourth collimated beam;

[0014] The first collimated light passes through the first optical delay line and the first acousto-optic modulation unit in sequence, and outputs a first modulated collimated light;

[0015] The second collimated light passes through the second acousto-optic modulation unit to output a second modulated collimated light;

[0016] The third collimated light passes through the third acousto-optic modulation unit to output a third modulated collimated light;

[0017] The fourth collimated light passes through the third optical delay line and the fourth acousto-optic modulation unit in sequence, and outputs a fourth modulated collimated light;

[0018] The fourth beam splitter combines the first modulated collimated light and the second modulated collimated light into a first modulated combined beam; the fifth beam splitter combines the third modulated collimated light and the fourth modulated collimated light into a second modulated combined beam; the first modulated combined beam and the second modulated combined beam are combined by the sixth beam splitter to obtain a total modulated combined beam; the D-shaped reflector separates the multidimensional coherent femtosecond laser pulse sequence and the continuous laser light with different modulation frequencies in the total modulated combined beam;

[0019] A multi-dimensional coherent femtosecond laser pulse sequence is focused onto the sample by the first lens; the fluorescence signal spontaneously emitted by the sample is collected by the second lens and focused onto the first photodetector; the first photodetector converts the fluorescence signal into a first electrical signal; and continuous laser light carrying different modulation frequencies is converted into a second electrical signal by the second photodetector.

[0020] The modulation frequencies of the modulation signals corresponding to the above-mentioned acousto-optic modulation units are different from each other.

[0021] A continuous scanning half-wave sampling method for multidimensional coherent spectroscopy, using the continuous scanning half-wave sampling system for multidimensional coherent spectroscopy as described above, comprises the following steps:

[0022] Step 1: Building a continuous scanning half-wave sampling system for multidimensional coherent spectroscopy;

[0023] Step 2: starting the optical delay line to perform continuous scanning of the optical time delay;

[0024] The first lock-in amplifier continuously demodulates the second electrical signal, and the second lock-in amplifier synchronously and continuously demodulates the first electrical signal; the synchronous data acquisition unit synchronously records one orthogonal component of the second demodulated electrical signal and two orthogonal components of the first demodulated electrical signal; the sampling unit samples corresponding data points of the two orthogonal components of the first demodulated electrical signal from the synchronous data acquisition unit based on the zero-crossing point of the second demodulated electrical signal, and uses the optical period of the second demodulated electrical signal to determine the optical time delay of each sampling point of the first demodulated electrical signal.

[0025] The continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy as described above further includes the following steps:

[0026] Step 3: constructing a time domain spectrum data matrix using the sampling points of the two orthogonal components of the first demodulated electrical signal and the corresponding optical time delays;

[0027] Step 4: Fourier transform the time domain spectrum data matrix into the frequency domain to obtain a complex phase multidimensional coherent spectrum.

[0028] The continuous scanning of the optical time delay in step 2 specifically includes the acquisition process of the two-dimensional coherent spectrum:

[0029] Step 2.1, fixing the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scanning through the optical time delay between the third collimated light and the fourth collimated light;

[0030] Step 2.2, stepping the optical time delay between the first collimated light and the second collimated light, and scanning through the optical time delay between the third collimated light and the fourth collimated light;

[0031] Step 2.3: Return to step 2.2 until the entire optical time delay range between the first collimated light and the second collimated light is traversed.

[0032] The continuous scanning of the optical time delay in step 2 mentioned above includes the acquisition process of the three-dimensional coherent spectrum:

[0033] Step 2.1, fixing the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scanning through the optical time delay between the third collimated light and the fourth collimated light;

[0034] Step 2.2, stepping the optical time delay between the first collimated light and the second collimated light, and scanning through the optical time delay between the third collimated light and the fourth collimated light;

[0035] Step 2.3, return to step 2.2 until the entire optical time delay range between the first collimated light and the second collimated light is traversed, and then execute step 2.4;

[0036] Step 2.4: Step the optical time delay between the second collimated light and the third collimated light, and return to step 2.1 until the entire optical time delay range between the second collimated light and the third collimated light is traversed and controlled.

[0037] As described above, the sampling moment of each sampling point of the first demodulated electrical signal is the moment when the amplitude of the second demodulated electrical signal is zero, and the optical time delay of the sampling point is the time interval between the sampling moment and time zero, and time zero is the moment when the interference intensity of the first demodulated electrical signal is the strongest.

[0038] Compared with the existing technology, it has the following beneficial effects:

[0039] Based on the phase modulation phase-locked multi-dimensional coherent spectroscopy technology, this invention designs and introduces a continuous scanning half-wave sampling system: using continuous scanning optical time delay to replace the traditional step acquisition of one optical time axis, effectively shortening the spectrum measurement time. While shortening the acquisition time, it also reduces the difficulty of maintaining phase stability; at the same time, using the half-wave sampling method to accurately calibrate the optical time delay, the half-wavelength sampling step size breaks through the minimum sampling step size limit of traditional step acquisition, significantly improving the sampling bandwidth. This invention achieves a 387 THz bandwidth ( Wavelength range, This method allows for rapid acquisition of multidimensional coherent spectra (with wavelengths of 100 nm) within 3 hours. Compared to conventional optical time-delay stepping methods (75 THz bandwidth, 12-hour measurement), this method significantly improves the acquisition rate and sampling bandwidth of multidimensional coherent spectroscopy. Furthermore, this continuous scanning half-wave sampling system simply adds modules such as continuous scanning optical time delay and half-wave sampling to the existing multidimensional coherent spectroscopy system, simplifying the experimental setup. Furthermore, the present invention can be extended to other areas of Fourier transform spectroscopy, thus also encompassing Fourier ultrafast spectroscopy technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a schematic diagram of the structure of a continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy proposed by the present invention.

[0041] Figure 2 Schematic diagram of half-wave sampling of continuous lasers with different modulation frequencies according to embodiment 2 of the present invention, (a) shows the variation of the amplitude of an orthogonal component of the second demodulated electrical signal with a modulation frequency of 14 kHz with the optical time delay, and (b) shows the amplitude of Rb87 The amplitude of an orthogonal component in the first demodulated electrical signal corresponding to the atom changes with the optical time delay.

[0042] Figure 3 is Rb of Example 2 of the present invention 87 Schematic diagram of atomic complex phase one-dimensional coherent spectroscopy.

[0043] Figure 4 is Rb of Example 2 of the present invention 87 Schematic diagram of atomic complex phase multidimensional coherent spectroscopy.

[0044] Among them, 100 is a laser generation module; 101 is a continuous laser; 102 is a laser seed source; 103 is an optical parametric amplifier; 200 is a multi-dimensional coherent spectroscopy module; 201 is a first beam splitter; 202 is a second beam splitter; 203 is a third beam splitter; 204 is a fourth beam splitter; 205 is a fifth beam splitter; 206 is a sixth beam splitter; 207 is a first optical delay line; 208 is a second optical delay line; 209 is a third optical delay line; 210 is a first acousto-optic modulation unit; 211 is a second acousto-optic modulation unit; 212 is a third acousto-optic modulation unit; 213 is a fourth acousto-optic modulation unit Unit; 214—D-shaped reflector; 215—first lens; 216—sample; 217—second lens; 218—first photodetector; 219—second photodetector; 220—direct digital frequency synthesis signal generator; 300—half-wave sampling demodulation module; 301—first mixer; 302—second mixer; 303—third mixer; 304—first phase-locked amplifier; 305—second phase-locked amplifier; 306—synchronous data acquisition unit; 307—first acquisition channel; 308—second acquisition channel; 309—third acquisition channel; 310—fourth acquisition channel. DETAILED DESCRIPTION

[0045] In order to facilitate those skilled in the art to understand and implement the present invention, the present invention is further described in detail below with reference to the embodiments. The embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0046] Example 1

[0047] A continuous scanning half-wave sampling system for multidimensional coherent spectroscopy, such as Figure 1 As shown. In this embodiment, Rb of the phase modulation phase locking scheme is 87 Taking atomic complex phase multidimensional coherent spectroscopy as an example, it is not limited to the Rb phase modulation phase locking scheme. 87Atomic complex phase multidimensional coherent spectroscopy. Its purpose is to utilize this continuously scanning half-wave sampling system to achieve fast broadband acquisition of multidimensional coherent spectra, improve the spectral data acquisition rate and sampling bandwidth, and reduce the difficulty of maintaining long-term phase stability. The principle is to replace the traditional step acquisition method with a continuously scanning optical time delay, and combine it with a half-wave sampling method to accurately calibrate the optical time delay, breaking the minimum sampling step limit, achieving fast broadband multidimensional coherent spectroscopy acquisition, and significantly improving the acquisition rate and signal bandwidth.

[0048] The present invention provides a continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy, comprising: a laser generating module 100, a multi-dimensional coherent spectroscopy module 200, and a half-wave sampling demodulation module 300;

[0049] The laser generation module 100 is used to generate collimated ultrashort femtosecond laser pulses and continuous lasers, with the continuous laser serving as a reference laser; the multidimensional coherent spectroscopy module 200 is used to generate a multidimensional coherent femtosecond laser pulse sequence with precisely controllable optical time delay and a continuous laser carrying different modulation frequencies; the half-wave sampling and demodulation module 300 is used to demodulate the first electrical signal corresponding to the fluorescence signal radiated by the sample 216, and to perform half-wave sampling on the second electrical signal corresponding to the synchronously collected continuous laser carrying different modulation frequencies, so as to accurately calibrate the optical time delay of the fluorescence signal radiated by the sample 216 and realize the acquisition of fast broadband multidimensional coherent spectra.

[0050] In the embodiment of the present invention, the laser generation module 100 includes: a continuous laser 101, a laser seed source 102, and an optical parametric amplifier 103;

[0051] The multi-dimensional coherent spectroscopy module 200 includes: a first beam splitter 201, a second beam splitter 202, a third beam splitter 203, a fourth beam splitter 204, a fifth beam splitter 205, a sixth beam splitter 206, a first optical delay line 207, a second optical delay line 208, a third optical delay line 209, a first acousto-optic modulation unit 210, a second acousto-optic modulation unit 211, a third acousto-optic modulation unit 212, a fourth acousto-optic modulation unit 213, a D-shaped reflector 214, a first lens 215, a sample 216, a second lens 217, a first photodetector 218, a second photodetector 219, and a direct digital frequency synthesis signal generator 220;

[0052] The half-wave sampling demodulation module 300 includes: a first mixer 301, a second mixer 302, a third mixer 303, a first lock-in amplifier 304, a second lock-in amplifier 305, and a synchronous data acquisition unit 306. The connection relationship between each component is as follows:

[0053] (1) In the laser generating module 100,

[0054] The continuous laser 101 generates continuous laser light. At the same time, the ultrashort femtosecond laser pulse generating device outputs ultrashort femtosecond laser pulses that resonate with the atomic line transition of the sample 216. The ultrashort femtosecond laser pulses and the continuous laser light are both collimated lights, and there is a spatial offset in the direction perpendicular to the horizontal plane, and they are input into the multi-dimensional coherent spectroscopy module 200 together.

[0055] In this embodiment, the ultrashort femtosecond laser pulse generator includes a laser seed source 102. The laser seed source 102 generates a seed laser. The seed laser is parametrically amplified by a nonlinear crystal in an optical parametric amplifier 103 to generate an ultrashort femtosecond laser pulse that resonates with the atomic line transition of the sample 216. The specific configuration of the continuous laser 101 and the ultrashort femtosecond laser pulse generator is as follows: the continuous laser 101 generates a 773 nm wavelength continuous laser, which resonates with the sample 216 (in this embodiment, the sample 216 is Rb 87 The wavelength of the 794.979 nm fluorescence signal of the D1 line transition radiation of the atom is only 21.979 nm different, which can effectively offset the Rb 87 The atomic fluorescence signal phase fluctuates. At the same time, the laser seed source 102 outputs a seed laser with a wavelength of 1030 nm. The seed laser is amplified by the nonlinear crystal parameter in the optical parametric amplifier 103 to generate a signal that is similar to the sample 216 (i.e., Rb 87 An ultrashort femtosecond laser pulse with a central wavelength of 780 nm and a pulse width of 65 fs (femtoseconds) resonates with the D1 line transition of atom (e.g., atom). This ultrashort femtosecond laser pulse and continuous laser light are input into the multidimensional coherent spectroscopy module 200 together. In this embodiment, the ultrashort femtosecond laser pulse and continuous laser light are spatially offset in a direction perpendicular to the horizontal plane to facilitate subsequent beam splitting using a D-shaped reflector 214.

[0056] (2) In the multi-dimensional coherent spectroscopy module 200,

[0057] A. Four collimated beams are obtained from ultrashort femtosecond laser pulses and continuous lasers, and the four collimated beams are modulated and optically time-delayed.

[0058] Ultrashort femtosecond laser pulses are combined with continuous laser light and then split to obtain four collimated light beams. Each collimated light beam is modulated by a corresponding acousto-optic modulation unit, and an optical time delay is obtained by a corresponding optical delay line to obtain a corresponding modulated collimated light beam. At the same time, each acousto-optic modulation unit is driven by a corresponding modulation signal. All modulation signals are output by a direct digital frequency synthesis signal generator 220. The modulation frequencies of the modulation signals corresponding to the acousto-optic modulation units are different from each other. The specific settings of this embodiment are as follows:

[0059] The optical delay line of this embodiment includes a first optical delay line 207, a second optical delay line 208, and a third optical delay line 209; the acousto-optic modulation unit includes a first acousto-optic modulation unit 210, a second acousto-optic modulation unit 211, a third acousto-optic modulation unit 212, and a fourth acousto-optic modulation unit 213;

[0060] The first beam splitter 201 combines the ultrashort femtosecond laser pulse output by the laser generation module 100 with the continuous laser beam to obtain a total combined beam, and splits the obtained total combined beam including the ultrashort femtosecond laser pulse and the continuous laser beam into a first combined beam and a second combined beam, and the transmission directions of the first combined beam and the second combined beam are perpendicular to each other; the first combined beam is split into a first collimated beam and a second collimated beam at the second beam splitter 202; after the second combined beam passes through the second optical delay line 208, it is split by the third beam splitter 203 to obtain a third collimated beam and a fourth collimated beam; the second optical delay line 208 is used to precisely control the optical time delay between the second collimated beam and the third collimated beam;

[0061] The first collimated light passes through the first optical delay line 207 and the first acousto-optic modulation unit 210 in sequence, and outputs the first modulated collimated light. The first optical delay line 207 is used to precisely control the optical time delay between the first collimated light and the second collimated light.

[0062] The second collimated light passes through the second acousto-optic modulation unit 211 to output second modulated collimated light;

[0063] The third collimated light passes through the third acousto-optic modulation unit 212 to output third modulated collimated light;

[0064] The fourth collimated light passes through the third optical delay line 209 and the fourth acousto-optic modulation unit 213 in sequence, and outputs a fourth modulated collimated light. The third optical delay line 209 is used to precisely control the optical time delay between the third collimated light and the fourth collimated light.

[0065] In this embodiment, the direct digital frequency synthesis signal generator 220 includes four output channels, each of which outputs a modulation signal, that is, the direct digital frequency synthesis signal generator 220 outputs a first channel modulation signal, a second channel modulation signal, a third channel modulation signal, and a fourth channel modulation signal, and each channel modulation signal is divided into two paths. The first branch of the first channel modulation signal is input into the first acousto-optic modulation unit 210, the first branch of the second channel modulation signal is input into the second acousto-optic modulation unit 211, the first branch of the third channel modulation signal is input into the third acousto-optic modulation unit 212, and the first branch of the fourth channel modulation signal is input into the fourth acousto-optic modulation unit 213; in addition, the second branch of the first channel modulation signal, the second branch of the second channel modulation signal, the second branch of the third channel modulation signal, and the second branch of the fourth channel modulation signal are simultaneously input into the half-wave sampling demodulation module 300;

[0066] The first acousto-optic modulation unit 210, the second acousto-optic modulation unit 211, the third acousto-optic modulation unit 212, and the fourth acousto-optic modulation unit 213 are each composed of two convex lenses with a focal length of 200 mm and an acousto-optic modulator. The first acousto-optic modulation unit 210 assigns a first modulation frequency to the first collimated light, the second acousto-optic modulation unit 211 assigns a second modulation frequency to the second collimated light, the third acousto-optic modulation unit 212 assigns a third modulation frequency to the third collimated light, and the fourth acousto-optic modulation unit 213 assigns a fourth modulation frequency to the fourth collimated light:

[0067] The first collimated light is focused by the first convex lens of the first acousto-optic modulation unit 210 onto the acousto-optic modulator of the first acousto-optic modulation unit 210. The acousto-optic modulator of the first acousto-optic modulation unit 210 is driven by the first branch of the first channel modulation signal output by the direct digital frequency synthesis signal generator 220 to generate first-order diffracted light with a modulation frequency of 80.000 MHz. Then, the second convex lens of the first acousto-optic modulation unit 210 shapes the first-order diffracted light with a modulation frequency of 80.000 MHz into a collimated first modulated collimated light.

[0068] The second collimated light is focused by the first convex lens of the second AOM unit 211 onto the AOM of the second AOM unit 211. The AOM of the second AOM unit 211 is driven by the first branch of the second channel modulation signal output by the DDS signal generator 220 to generate first-order diffracted light with a modulation frequency of 80.019 MHz. The second convex lens of the second AOM unit 211 shapes the first-order diffracted light with a modulation frequency of 80.019 MHz into collimated second modulated collimated light.

[0069] The third collimated light is focused by the first convex lens of the third AOM unit 212 onto the AOM of the third AOM unit 212. The AOM of the third AOM unit 212 is driven by the first branch of the third channel modulation signal output by the DDS signal generator 220 to generate first-order diffracted light with a modulation frequency of 80.104 MHz. The second convex lens of the third AOM unit 212 shapes the first-order diffracted light with a modulation frequency of 80.104 MHz into a collimated third modulated collimated light.

[0070] The fourth collimated light is focused by the first convex lens of the fourth acousto-optic modulation unit 213 onto the acousto-optic modulator of the fourth acousto-optic modulation unit 213. The acousto-optic modulator of the fourth acousto-optic modulation unit 213 is driven by the first branch of the fourth channel modulation signal output by the direct digital frequency synthesis signal generator 220 to generate first-order diffracted light with a modulation frequency of 80.109 MHz. The second convex lens of the fourth acousto-optic modulation unit 213 shapes the first-order diffracted light with a modulation frequency of 80.109 MHz into collimated fourth modulated collimated light.

[0071] In this embodiment, the modulation speed of the AOM unit at 80 MHz is much faster than the movement speed of the optical delay line. The AOM unit can be placed after the optical delay line without significantly interfering with the experimental results. Therefore, the AOM unit can be placed before or after the optical delay line.

[0072] B. Obtain multi-dimensional coherent femtosecond laser pulse sequences and continuous lasers carrying different modulation frequencies using four-way modulated collimated light.

[0073] The four-path modulated collimated light (the four-path modulated collimated light includes the first modulated collimated light, the second modulated collimated light, the third modulated collimated light, and the fourth modulated collimated light) are combined, and then a multi-dimensional coherent femtosecond laser pulse sequence and a continuous laser with different modulation frequencies are output through the D-shaped reflector 214. The specific configuration of this embodiment is as follows:

[0074] The fourth beam splitter 204 combines the first modulated collimated light and the second modulated collimated light into a first modulated combined light beam; the fifth beam splitter 205 combines the third modulated collimated light and the fourth modulated collimated light into a second modulated combined light beam; the first modulated combined light beam and the second modulated combined light beam are combined by the sixth beam splitter 206 to obtain a total modulated combined light beam consisting of a multi-dimensional coherent femtosecond laser pulse sequence carrying different modulation frequencies and a continuous laser beam carrying different modulation frequencies;

[0075] Subsequently, the D-shaped mirror 214 separates the multi-dimensional coherent femtosecond laser pulse sequence with spatial offset in the total modulated combined beam from the continuous laser light carrying different modulation frequencies.

[0076] C. A multi-dimensional coherent femtosecond laser pulse sequence excites the sample 216 to generate a fluorescence signal, and a photodetector is used to detect the continuous laser and the fluorescence signal carrying different modulation frequencies respectively.

[0077] A multi-dimensional coherent femtosecond laser pulse sequence excites the sample 216 to generate a fluorescence signal, which is converted into a first electrical signal by a first photodetector 218. Continuous laser light with different modulation frequencies is converted into a second electrical signal by a second photodetector 219. The specific configuration of this embodiment is as follows:

[0078] The multi-dimensional coherent femtosecond laser pulse sequence is reflected by the D-shaped reflector 214 and focused by the first lens 215 onto the sample 216. The fluorescence signal spontaneously emitted by the sample 216 is collected by the second lens 217 and focused onto the first photodetector 218. The first photodetector 218 converts the fluorescence signal into a first electrical signal and transmits it to the half-wave sampling and demodulation module 300. At the same time, continuous laser light carrying different modulation frequencies is transmitted through the non-reflective area of ​​the D-shaped reflector 214 to the second photodetector 219. The second photodetector 219 converts the continuous laser light carrying different modulation frequencies into a second electrical signal.

[0079] In this embodiment, the first lens 215 is a convex lens with a focal length of 50 mm, which is used to focus the multidimensional coherent femtosecond laser pulse sequence onto the sample 216; the sample 216 interacts with the multidimensional coherent femtosecond laser pulse sequence and radiates a fluorescence signal; the second lens 217 is a convex lens with a focal length of 60 mm, which is used to collect the fluorescence signal radiated by the sample 216 and focus the fluorescence signal onto the first photodetector 218; the first photodetector 218 is used to detect the fluorescence signal radiated by the sample 216 under the action of the multidimensional coherent femtosecond laser pulse sequence; the second photodetector 219 is used to detect continuous laser light carrying different modulation frequencies.

[0080] (3) Half-wave sampling demodulation module 300

[0081] The second branch of the modulation signal output by each output channel of the direct digital frequency synthesis signal generator 220 is input to the mixing unit to obtain a total mixed signal. The modulation signal input to the mixing unit by the same output channel of the direct digital frequency synthesis signal generator 220 is the same as the modulation signal corresponding to the acousto-optic modulation unit. The frequency of the obtained total mixed signal, the modulation frequency carried by the fluorescence signal, the modulation frequency carried by the first electrical signal, and the modulation frequency carried by the second electrical signal are all the same. The first branch of the total mixed signal and the second electrical signal are simultaneously input to the first lock-in amplifier 304. The first lock-in amplifier 304 uses the total mixed signal as a reference. The second electrical signal is demodulated to obtain a second demodulated electrical signal. An orthogonal component of the second demodulated electrical signal is split into two paths and simultaneously input into the synchronous data acquisition unit 306. The first electrical signal and the second branch of the second electrical signal are simultaneously input into the second phase-locked amplifier 305. The second phase-locked amplifier 305 demodulates the first electrical signal with the second electrical signal as a reference to obtain a first demodulated electrical signal. Both orthogonal components of the first demodulated electrical signal are input into the synchronous data acquisition unit 306. The sampling unit samples the corresponding data points of the two orthogonal components of the first demodulated electrical signal from the synchronous data acquisition unit 306 based on the zero crossing point of the second demodulated electrical signal. The specific configuration of this embodiment is as follows:

[0082] The mixing unit includes a first mixer 301, a second mixer 302 and a third mixer 303; the second branch of the first channel modulation signal output from the first channel of the direct digital frequency synthesis signal generator 220 and the second branch of the second channel modulation signal output from the second channel are mixed by the first mixer 301 to obtain a first beat frequency reference signal of 19 kHz, the second branch of the third channel modulation signal output from the third channel of the direct digital frequency synthesis signal generator 220 and the second branch of the fourth channel modulation signal output from the fourth channel are mixed by the second mixer 302 to obtain a second beat frequency reference signal of 5 kHz, the first beat frequency reference signal and the second beat frequency reference signal are mixed by the third mixer 303 to obtain a total mixed signal of 14 kHz, and the frequency of the total mixed signal is the same as Rb 87 The complex phase fluorescence signal radiated by the atoms carries the same modulation frequency;

[0083] The first lock-in amplifier 304 demodulates the second electrical signal corresponding to the continuous laser with different modulation frequencies using the total mixing signal as a reference; the second lock-in amplifier 305 demodulates the first electrical signal corresponding to the complex-phase fluorescence signal using the second electrical signal as a reference; the synchronous data acquisition unit 306 is provided with four synchronous data acquisition channels (i.e., a first acquisition channel 307, a second acquisition channel 308, a third acquisition channel 309, and a fourth acquisition channel 310), which are used to synchronously record the second demodulated electrical signal corresponding to the continuous laser with different modulation frequencies and the first demodulated electrical signal corresponding to the fluorescence signal, wherein the first acquisition channel 307 and the third acquisition channel 309 are both used to record one orthogonal component of the second demodulated electrical signal, and the second acquisition channel 308 and the fourth acquisition channel 310 are respectively used to synchronously record two orthogonal components of the first demodulated electrical signal.

[0084] Due to the velocity non-uniformity of the optical delay line, directly scanning and recording the first demodulated electrical signal obtained by demodulation by the second lock-in amplifier 305 results in varying degrees of phase error at each sampling moment. This hinders the direct determination of the optical time delay of the fluorescence signal spontaneously emitted by the sample 216 and affects the accurate interpretation of the fluorescence signal (which contains multi-dimensional coherent spectral information). Therefore, the second electrical signal corresponding to the continuous laser light with different modulation frequencies is demodulated using the total mixed signal generated by the modulation signals output by the four output channels of the direct digital frequency synthesis signal generator 220 to obtain a second demodulated electrical signal (the second demodulated electrical signal is the beat frequency electrical signal corresponding to the continuous laser light with different modulation frequencies). One of the two orthogonal components of the second demodulated electrical signal is used as a time marker. Based on the zero crossing point of the second demodulated electrical signal, the sampling unit samples the corresponding data points of the two orthogonal components of the first demodulated electrical signal from the synchronous data acquisition unit 306. Since the continuous laser and the ultrashort femtosecond laser pulse are transmitted synchronously in the optical path, the moment when the four pulses in the multidimensional coherent femtosecond laser pulse sequence coincide (when the four pulses in the multidimensional coherent femtosecond laser pulse sequence coincide, the interference intensity of the first demodulated electrical signal collected is the strongest) is taken as the time zero point, and the number of optical cycles from each sampling point in the demodulated second demodulated electrical signal to the time zero point is calculated, that is, half-wave sampling is performed on the second demodulated electrical signal and the first demodulated electrical signal ( ), the optical time delay at each sampling point is determined, thereby accurately obtaining the optical time delay obtained by the continuous laser and the ultrashort femtosecond laser pulse, and accurately calibrating the optical time delay of the fluorescence signal radiated by sample 216. Subsequently, a time-domain spectral data matrix is ​​constructed from the sampling points of the two orthogonal components of the first demodulated electrical signal and the corresponding optical time delays, thereby obtaining a complex phase multidimensional coherent spectrum. The specific method is described in Example 2.

[0085] Example 2

[0086] A continuous scanning half-wave sampling method for multidimensional coherent spectroscopy, using the continuous scanning half-wave sampling system for multidimensional coherent spectroscopy described in Example 1, specifically comprises the following steps:

[0087] Step 1: construct a continuous scanning half-wave sampling system for multidimensional coherent spectroscopy described in Example 1;

[0088] Step 2: Start the optical delay line to perform continuous scanning of the optical time delay;

[0089] For the acquisition of two-dimensional coherent spectra,

[0090] Step 2.1, fixing the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scanning through the optical time delay between the third collimated light and the fourth collimated light;

[0091] Step 2.2, stepping the optical time delay between the first collimated light and the second collimated light, and scanning through the optical time delay between the third collimated light and the fourth collimated light;

[0092] Step 2.3: Return to step 2.2 until the entire optical time delay range between the first collimated light and the second collimated light is traversed.

[0093] In this embodiment, for the acquisition of a two-dimensional coherent spectrum, the first optical delay line 207 and the second optical delay line 208 are first fixed, and after continuously scanning through the optical time delay range of the third optical delay line 209, the first optical delay line 207 is stepped, and then the optical time delay range of the third optical delay line 209 is continuously scanned through. The process of stepping the first optical delay line 207 and scanning through the optical time delay range of the third optical delay line 209 is repeated until the optical time delay range of the first optical delay line 207 is traversed, thereby acquiring a complex-phase two-dimensional coherent spectrum.

[0094] For the acquisition of three-dimensional coherent spectra, continuous scanning with optical time delay includes the following process:

[0095] Step 2.1, fixing the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scanning through the optical time delay between the third collimated light and the fourth collimated light;

[0096] Step 2.2, stepping the optical time delay between the first collimated light and the second collimated light, and scanning through the optical time delay between the third collimated light and the fourth collimated light;

[0097] Step 2.3, return to step 2.2 until the entire optical time delay range between the first collimated light and the second collimated light is traversed, and then execute step 2.4;

[0098] Step 2.4: Step the optical time delay between the second collimated light and the third collimated light, and return to step 2.1 until the entire optical time delay range between the second collimated light and the third collimated light is traversed.

[0099] In this embodiment, for the acquisition of three-dimensional coherent spectrum, the two-dimensional coherent spectrum acquisition process is first performed, and then the second optical delay line 208 is stepped. The above two-dimensional coherent spectrum acquisition process and the stepping of the second optical delay line 208 are repeated until the second optical delay line 208 is traversed.

[0100] During this process, the first lock-in amplifier 304 synchronously and continuously demodulates the second electrical signal corresponding to the continuous laser with different modulation frequencies, and the second lock-in amplifier 305 synchronously and continuously demodulates the first electrical signal corresponding to the fluorescence signal with different modulation frequencies; the synchronous data acquisition unit 306 synchronously collects and records an orthogonal component of the second demodulated electrical signal corresponding to the continuous laser with different modulation frequencies and two orthogonal components of the first demodulated electrical signal corresponding to the ultrashort femtosecond laser pulse; the sampling unit samples the corresponding data points of the two orthogonal components of the first demodulated electrical signal from the synchronous data acquisition unit 306 based on the zero crossing point of the second demodulated electrical signal, and determines the optical time delay of each sampling point using the optical period of the second demodulated electrical signal;

[0101] In this embodiment, the first lock-in amplifier 304 demodulates the second electrical signal with the total mixing signal as a reference to obtain two orthogonal components of the second demodulated electrical signal with a modulation frequency of 14 kHz, and uses one of the orthogonal components as a time calibration input to the first acquisition channel 307 and the third acquisition channel 309 of the synchronous data acquisition unit 306. In this embodiment, the amplitude of the second demodulated electrical signal corresponding to the continuous laser with different modulation frequencies changes with the optical time delay as shown in the following figure: Figure 2 As shown in (a); at the same time, the second lock-in amplifier 305 demodulates the first electrical signal with the second electrical signal as a reference to obtain two orthogonal components of the first demodulated electrical signal corresponding to the complex phase fluorescence signal. The two orthogonal components of the first demodulated electrical signal are respectively input into the second acquisition channel 308 and the fourth acquisition channel 310 of the synchronous data acquisition unit 306. In this embodiment, the amplitude of the first demodulated electrical signal corresponding to the complex phase fluorescence signal changes with the optical time delay as shown in FIG. Figure 2 As shown in (b);

[0102] Since the continuous laser and the ultrashort femtosecond laser pulse are transmitted synchronously in the optical path, the optical time delay of the fluorescence signal radiated by the sample 216 can be accurately calibrated by performing half-wave sampling on the second demodulated electrical signal recorded by the synchronous data acquisition unit 306, thereby effectively avoiding the phase error caused by the uneven linear velocity of the optical delay.

[0103] The sampling moment of the sampling point of the orthogonal component of the first demodulated electrical signal is the moment when the amplitude of the second demodulated electrical signal is zero, and the optical time delay of the sampling point is the time interval between the sampling moment and time zero. Time zero is defined as the moment when the four pulses in the multidimensional coherent femtosecond laser pulse sequence coincide, that is, the moment when the interference intensity of the first demodulated electrical signal is the strongest.

[0104] Step 3: constructing a time domain spectrum data matrix according to the sampling points of the two orthogonal components of the first demodulated electrical signal and the corresponding optical time delays;

[0105] The sampling interval of the sampling points in the time domain spectroscopy data matrix is ​​half of the continuous laser optical period, corresponding to a bandwidth of 387 THz, such as Figure 3 shown.

[0106] The phase-modulation phase-locked scheme of traditional optical time-delay step acquisition is limited by the optical delay line and cannot achieve a half-wavelength sampling step. It takes 12 hours to complete the acquisition of a multidimensional coherent spectrum with a 75 THz bandwidth and 1 THz resolution. The half-wave sampling used in the present invention breaks through the minimum sampling step limit of traditional step acquisition, significantly improves the sampling bandwidth, effectively avoids signal undersampling, and significantly improves the spectral acquisition rate, shortening the acquisition time of the time-domain spectral data matrix corresponding to a set of multidimensional coherent spectra with a 387 THz bandwidth and 0.1 THz resolution to 3 hours.

[0107] Step 4: Fourier transform the time domain spectrum data matrix into the frequency domain to finally obtain the complex phase multidimensional coherent spectrum. In this embodiment, Rb 87 The complex phase multidimensional coherent spectrum of atoms is as follows Figure 4 shown.

[0108] It should be noted that the embodiments described herein are merely illustrative of the spirit of the present invention. Persons skilled in the art may make various modifications, additions, or substitutions to the described embodiments without departing from the spirit of the present invention or exceeding the scope of the appended claims.

Claims

1. A continuous scanning half-wave sampling system for multidimensional coherent spectroscopy, characterized in that: The invention comprises a continuous laser (101), wherein the continuous laser output by the continuous laser (101) is combined with the ultrashort femtosecond laser pulse output by the ultrashort femtosecond laser pulse generator and then splits the beam to obtain four collimated lights, each collimated light passes through a corresponding acousto-optic modulation unit and an optical delay line to generate a corresponding modulated collimated light, each acousto-optic modulation unit is driven by a corresponding modulation signal, and all modulation signals are output by a direct digital frequency synthesis signal generator (220); Four-way modulated collimated photosynthetic beam is output through a D-shaped reflector (214) to output a multi-dimensional coherent femtosecond laser pulse sequence and a continuous laser with different modulation frequencies; A multi-dimensional coherent femtosecond laser pulse sequence excites a sample (216) to generate a fluorescence signal, which is converted into a first electrical signal via a first photodetector (218); a continuous laser with different modulation frequencies is converted into a second electrical signal via a second photodetector (219); The direct digital frequency synthesis signal generator (220) further outputs four modulation signals to the mixing unit to generate a total mixing signal, the frequency of the total mixing signal being the same as the modulation frequency carried by the fluorescence signal; The first lock-in amplifier (304) demodulates the second electrical signal with the total mixing signal as a reference, and an orthogonal component of the demodulated second demodulated electrical signal is collected and stored in real time by the synchronous data acquisition unit (306). The second lock-in amplifier (305) demodulates the first electrical signal with the second electrical signal as a reference to obtain a first demodulated electrical signal, and both orthogonal components of the first demodulated electrical signal are synchronously collected and stored by the synchronous data acquisition unit (306). The sampling unit samples corresponding data points of the two orthogonal components of the first demodulated electrical signal from the synchronous data acquisition unit (306) based on the zero crossing point of the second demodulated electrical signal.

2. A continuous scanning half-wave sampling system for multidimensional coherent spectroscopy according to claim 1, characterized in that: The ultrashort femtosecond laser pulse generating device comprises a laser seed source (102). The seed laser generated by the laser seed source (102) is amplified by nonlinear crystal parameters in an optical parametric amplifier (103) to generate an ultrashort femtosecond laser pulse that resonates with the atomic line transition of a sample (216).

3. The continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy according to claim 1, characterized in that: There is a spatial offset between the two collimated beams of the ultrashort femtosecond laser pulse and the continuous laser in a direction perpendicular to the horizontal plane.

4. The continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy according to claim 1, characterized in that: The ultrashort femtosecond laser pulse and the continuous laser are combined and then split by the first beam splitter (201) to obtain a first combined light and a second combined light; the first combined light is split into a first collimated light and a second collimated light at the second beam splitter (202); the second combined light passes through the second optical delay line (208) and is then split by the third beam splitter (203) to obtain a third collimated light and a fourth collimated light; The first collimated light passes through the first optical delay line (207) and the first acousto-optic modulation unit (210) in sequence, and outputs the first modulated collimated light; The second collimated light passes through the second acousto-optic modulation unit (211), and outputs the second modulated collimated light; The third collimated light passes through the third acousto-optic modulation unit (212), and outputs the third modulated collimated light; The fourth collimated light passes through the third optical delay line (209) and the fourth acousto-optic modulation unit (213) in sequence, and outputs fourth modulated collimated light; The fourth beam splitter (204) combines the first modulated collimated light and the second modulated collimated light into a first modulated combined beam; the fifth beam splitter (205) combines the third modulated collimated light and the fourth modulated collimated light into a second modulated combined beam; the first modulated combined beam and the second modulated combined beam are combined through the sixth beam splitter (206) to obtain a total modulated combined beam; the D-shaped reflector (214) separates the multi-dimensional coherent femtosecond laser pulse sequence and the continuous laser light carrying different modulation frequencies in the total modulated combined beam; A multi-dimensional coherent femtosecond laser pulse sequence is focused onto a sample (216) via a first lens (215); a fluorescence signal spontaneously emitted by the sample (216) is collected by a second lens (217) and focused onto a first photodetector (218); the first photodetector (218) converts the fluorescence signal into a first electrical signal; and a continuous laser carrying different modulation frequencies is converted into a second electrical signal by a second photodetector (219).

5. The continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy according to claim 1, characterized in that: The modulation frequencies of the modulation signals corresponding to the acousto-optic modulation units are different from each other.

6. A continuous scanning half-wave sampling method for multidimensional coherent spectroscopy, utilizing the continuous scanning half-wave sampling system for multidimensional coherent spectroscopy according to claim 1, characterized in that: The steps include: Step 1: Building a continuous scanning half-wave sampling system for multidimensional coherent spectroscopy; Step 2: starting the optical delay line to perform continuous scanning of the optical time delay; The first lock-in amplifier (304) continuously demodulates the second electrical signal, and the second lock-in amplifier (305) synchronously and continuously demodulates the first electrical signal; the synchronous data acquisition unit (306) synchronously records an orthogonal component of the second demodulated electrical signal and two orthogonal components of the first demodulated electrical signal; The sampling unit samples corresponding data points of two orthogonal components of the first demodulated electrical signal from a synchronous data acquisition unit (306) based on the zero crossing point of the second demodulated electrical signal, and determines the optical time delay of each sampling point of the first demodulated electrical signal using the optical period of the second demodulated electrical signal.

7. A continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy according to claim 6, characterized in that: The following steps are also included: Step 3: constructing a time domain spectrum data matrix using the sampling points of the two orthogonal components of the first demodulated electrical signal and the corresponding optical time delays; Step 4: Fourier transform the time domain spectrum data matrix into the frequency domain to obtain a complex phase multidimensional coherent spectrum.

8. The continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy according to claim 6, characterized in that: The continuous scanning of optical time delay in step 2 specifically includes the acquisition process of two-dimensional coherent spectrum: Step 2.1, fixing the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scanning through the optical time delay between the third collimated light and the fourth collimated light; Step 2.2, stepping the optical time delay between the first collimated light and the second collimated light, and scanning through the optical time delay between the third collimated light and the fourth collimated light; Step 2.3: Return to step 2.2 until the entire optical time delay range between the first collimated light and the second collimated light is traversed.

9. The continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy according to claim 7, characterized in that: The continuous scanning of the optical time delay in step 2 includes the acquisition process of the three-dimensional coherent spectrum: Step 2.1, fixing the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scanning through the optical time delay between the third collimated light and the fourth collimated light; Step 2.2, stepping the optical time delay between the first collimated light and the second collimated light, and scanning through the optical time delay between the third collimated light and the fourth collimated light; Step 2.3, return to step 2.2 until the entire optical time delay range between the first collimated light and the second collimated light is traversed, and then execute step 2.4; Step 2.4: Step the optical time delay between the second collimated light and the third collimated light, and return to step 2.1 until the entire optical time delay range between the second collimated light and the third collimated light is traversed and controlled.

10. The continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy according to claim 6, characterized in that: The sampling moment of each sampling point of the first demodulated electrical signal is the moment when the amplitude of the second demodulated electrical signal is zero, the optical time delay of the sampling point is the time interval between the sampling moment and time zero, and time zero is the moment when the interference intensity of the first demodulated electrical signal is the strongest.

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