Femtosecond-level time resolution light response single molecule signal detection method and device

Abstract

The invention relates to the technical field of signal testing, in particular to a femtosecond-level time resolution light response single molecule signal detection method and device, and the method comprises the following steps: a pumping pulse light path passes through a series of reflectors, a broadband optical parameter amplifier, a first diaphragm, a chopper, a first attenuation sheet and a second beam combining and splitting sheet, and then enters a leading-in optical fiber; the light path of the probe light passes through a time delayer, a series of reflectors, a second diaphragm, a white light generation module and a second beam combining and splitting sheet and then enters a guide-in optical fiber; the two beams of light enter a multifunctional sample rod and irradiate a graphene single-molecule device connected with a single light response molecule after passing through a polaroid; a single light response molecule is excited after being irradiated by the pumping pulse light path, and then is detected through the detection light path. According to the method and the device provided by the invention, the detection of a single light response molecule weak signal with femtosecond-level time resolution can be realized, and the signal-to-noise ratio of an electrical signal measured by a comprehensive physical property measurement system can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of signal testing, and in particular to a method and device for detecting single-molecule signals with light response at a femtosecond time resolution. Background Art

[0002] Studying individual molecules provides researchers with unprecedented resolution to observe and probe molecular behavior. Traditional chemical research is usually at the ensemble level, that is, measuring the average behavior of a large group of molecules, which often overwhelms the unique properties of individual molecules. When the spatial scale is refined to the scale of a single molecule, researchers can directly observe the physical and chemical properties of individual molecules, which helps to reveal the nature of processes such as molecular interactions, reaction dynamics, conformational changes, and electron transfer.

[0003] The study of single molecules requires the use of single-molecule electrical devices. Currently, a commonly used single-molecule electrical device is to connect a chemically designed single molecule with amino groups at both ends to a graphene electrode through an amide covalent bond, thereby constructing a stable graphene-molecule-graphene single-molecule junction. Since the signal of a single molecule is very weak and easily interfered by environmental noise, the signal testing conditions for a single molecule are very strict. Among them, the comprehensive physical property measurement system (PPMS) can provide an electrical testing environment with ultra-low temperature and strong magnetic field. Therefore, PPMS is often required for testing in the study of single-molecule electronics, which has important application value.

[0004] PPMS was developed by Quantum Design in the United States and is widely used in fields such as materials science, condensed matter physics and nanotechnology. The main component of PPMS is that it integrates a variety of physical property measurement methods such as fully automatic thermal, electrical and magnetic properties on a finely controlled ultra-low temperature and strong magnetic field platform. In addition, PPMS also provides some extended function options to meet the different research needs of researchers. This highly integrated test instrument has been recognized by scientists around the world in the past decade, so in the field of single-molecule electronics, PPMS has become a research tool.

[0005] However, due to the limitation of the instrument response time resolution in electrical testing, the test on PPMS only meets the nanosecond time scale. However, for a single molecule, the time scale of multiple processes such as molecular configuration changes, electron transfer, and energy level transitions usually occurs on the picosecond or even femtosecond scale, so the existing PPMS testing technology cannot test the above processes of a single molecule. In addition, since the photoinduced current generated by a single photoresponsive molecule when excited by light is usually in the order of several picoamperes, it is a weak signal, and the noise in conventional testing is at the nanoampere level, which will completely submerge the photoresponsive signal, it is difficult to test the photoinduced current generated by a single photoresponsive molecule. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a method and device for detecting light-responsive single-molecule signals with femtosecond time resolution, which can realize the detection of weak signals of single light-responsive molecules with femtosecond time resolution.

[0007] The present invention is achieved through the following technical solutions: The method for detecting single-molecule signals with light response at a femtosecond time resolution comprises the following steps: S1: The laser emitted from the femtosecond laser is reflected by the first reflector to the first beam combiner and splitter. 50% of the light is transmitted to form the pump pulse optical path, and 50% of the light is reflected to form the detection light path. S2: The pump pulse light path is reflected by the second reflector to the broadband optical parametric amplifier, then reflected by the third reflector to enter the first aperture for beam positioning, then emitted to the tunable first attenuator after passing through the chopper with a fixed frequency, and then emitted into the introduction optical fiber through the fourth reflector and the second beam combining and splitting plate respectively; S3: The detection light is reflected in the time delay device to achieve femtosecond delay, and then reflected by the fifth and sixth reflectors to enter the second aperture for beam positioning, and then passes through the white light generation module to generate continuous detection white light, and then passes through the seventh reflector and the second beam combiner and splitter to be injected into the introduction optical fiber; S4: The pump pulse optical path and the detection light optical path enter the multifunctional sample rod placed in the test cavity of the comprehensive physical property measurement system, and are polarized by the polarizer in the multifunctional sample rod to generate left-handed or right-handed circularly polarized light, which is then irradiated onto the graphene single-molecule device connected to a single photoresponsive molecule to be detected and placed on the sample carrier of the multifunctional sample rod. The graphene single-molecule device connected to a single photoresponsive molecule to be detected is connected to the electrode column of the sample carrier through a copper wire and a conductive silver glue; S5: A single photoresponsive molecule is excited after being irradiated by the pump pulse optical path, and then detected by the detection light optical path, and reflected by the signal acquisition unit and displayed on the computer.

[0008] Optimally, the femtosecond laser is a titanium sapphire femtosecond laser.

[0009] Optimally, the white light generating module comprises a first convex lens, a tunable second attenuation plate, a Yag white light crystal, a second convex lens and a filter which are arranged in sequence.

[0010] Furthermore, the multifunctional sample rod includes a sample rod body, an introduction optical fiber interface, a clamp, a polarizer, a sample carrier and a twisted wire, wherein the introduction optical fiber interface is fixedly installed on the top of the sample rod body and connected to the introduction optical fiber, the clamp is fixedly installed in the sample rod body and located below the introduction optical fiber interface, the polarizer is installed on the clamp, the sample carrier is installed in the sample rod body and located below the clamp, a graphene single-molecule device connected to a single light-responsive molecule to be detected is fixedly placed on the sample carrier, and the twisted wire is connected to a gold electrode and a signal acquisition unit of the graphene single-molecule device connected to a single light-responsive molecule to be detected.

[0011] Optimally, the twisted wires are twisted pairs.

[0012] Furthermore, the signal acquisition unit includes a pre-current amplifier and a phase-locked amplifier. The pre-current amplifier is used to amplify the detection information transmitted by the twisted wire and transmit it to the phase-locked amplifier. The phase-locked amplifier is used to extract components with the same frequency and phase correlation as the reference signal and transmit them to the computer.

[0013] Furthermore, the method of extracting the component with the same frequency and phase correlation with the reference signal by the phase-locked amplifier is as follows: D1: The lock-in amplifier expresses the input signal as an AC signal superimposed on a DC bias according to equation (1): (1); in: represents the lock-in amplifier input signal, represents the average current generated by the graphene single-molecule device connected to a single photoresponsive molecule to be detected, represents the photocurrent generated by the graphene single-molecule device connected to a single photoresponsive molecule to be detected, represents the reference frequency generated by the lock-in amplifier, which is equal to the chopper frequency, Indicates time, represents the initial phase of the lock-in amplifier, represents random noise; D2: The lock-in amplifier multiplies the input signal and the reference signal synchronously and integrates them, and extracts the signal component in phase with the reference frequency according to equation (2): (2); in: represents the extracted signal component that is in phase with the reference frequency, represents the integration time period; D3: The extracted signal that is in phase with the reference frequency is obtained by sum-difference product calculation (3): (3); D4: Fix the initial phase of the phase-locked amplifier, lock the photoelectric signal submerged in the noise at the reference frequency, amplify it, and then transmit it to the computer for reading.

[0014] Optimally, in step D4, the initial phase of the phase-locked amplifier is fixed to 0.

[0015] Optimally, the wavelength range of the continuous detection white light generated by the white light generation module in step S3 is 500 nanometers to 950 nanometers.

[0016] A device for detecting single-molecule signals with light response at a femtosecond time resolution is used to perform any of the above-mentioned methods for detecting single-molecule signals with light response at a femtosecond time resolution, comprising a femtosecond laser, a first reflector, a first beam-combining beam-splitting plate, a pump pulse optical path, a detection light path, a second beam-combining beam-splitting plate, an introduction optical fiber, a comprehensive physical property measurement system, a multifunctional sample rod, a signal acquisition unit and a computer. The femtosecond laser, the first reflector and the first beam-combining beam-splitting plate are coupled in sequence. The pump pulse optical path comprises a second reflector, a broadband optical parametric amplifier, a third reflector, a first aperture, a chopper with a fixed frequency, a tunable first attenuation plate and a fourth reflector coupled in sequence. The detection light path comprises a time delay device, a fifth reflector, a sixth reflector, a second aperture, a white light generation module and a seventh reflector coupled in sequence. The white light generation module comprises a first convex lens, a tunable second attenuation plate, a Yag white light crystal, a second convex lens and a filter coupled in sequence. The pump pulse optical path is connected to the optical path of the detector. The detection light path enters the introduction optical fiber after being combined by the second beam combining and splitting plate. The multifunctional sample rod is placed in the test cavity of the comprehensive physical property measurement system. The multifunctional sample rod comprises a sample rod body, an introduction optical fiber interface, a clamp, a polarizer, a sample carrier and a twisted pair. The introduction optical fiber interface is fixedly installed on the top of the sample rod body and connected to the introduction optical fiber. The clamp is fixedly installed in the sample rod body and located below the introduction optical fiber interface. The polarizer is installed on the clamp. The sample carrier is installed in the sample rod body and located below the clamp. The graphene single-molecule device connected to a single light-responsive molecule to be detected is fixedly placed on the sample carrier and connected to the sample carrier electrode column through a copper wire and a conductive silver glue. The twisted wire is connected to the gold electrode of the graphene single-molecule device connected to a single light-responsive molecule to be detected and a signal acquisition unit. The signal acquisition unit comprises a pre-current amplifier and a phase-locked amplifier connected to each other. The current amplifier is connected to the twisted pair, and the phase-locked amplifier is connected to a computer.

[0017] Beneficial effects of the invention: The method and device for detecting single-molecule signals with light response at a femtosecond time resolution provided by the present invention have the following advantages: 1. The detection device, which is composed of a comprehensive physical property measurement system and a femtosecond laser, combines the advantages of optical and electrical detection technologies. It can not only control a single molecule by multimodal means such as optical field and electric field, but also improve the time resolution to the femtosecond level. It can monitor the femtosecond-level photoresponse dynamics of a single molecule, and can use a time delay device introduced into the optical path to tune the optical path, so that the photoresponse process of different time scales can be flexibly detected; 2. The pump pulse frequency is modulated by a chopper, and the chopper frequency is output as a reference frequency by a phase-locked amplifier. Combined with the phase-sensitive detection of the phase-locked amplifier, the weak signal detection of a single photoresponsive molecule can be realized; 3. By independently designing a multifunctional sample holder, processing a fixture on the sample stage of the multifunctional sample holder, and installing a polarizer, it can be easily and quickly used to switch femtosecond ultrafast light pulses with different polarization modes, thereby converting the introduced linearly polarized ultrafast laser into left-handed or right-handed circularly polarized light, and can conduct light response research related to single-molecule chiral spin; 4. Choosing twisted pair cables for current transmission can reduce the noise of the electrical signals measured by the comprehensive physical property measurement system, reducing the original nanoampere level noise to the hundred-picoampere level, thereby improving the signal-to-noise ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the process of the present invention.

[0019] Figure 2 It is a schematic diagram of the device of the present invention.

[0020] Figure 3 It is a schematic diagram of the assembly structure of the multifunctional sample rod of the present invention.

[0021] Figure 4 It is a schematic diagram of the partially enlarged structure of part B of the multifunctional sample rod of the present invention.

[0022] Figure 5 It is a comparison diagram of background noise in electrical testing of the multifunctional sample rod of the present invention.

[0023] Figure 6 This is a current-voltage image of a graphene single-molecule device connected to a single photoresponsive molecule of the present invention when no femtosecond laser is applied.

[0024] Figure 7 This is a current-voltage image of the graphene single-molecule device connected to a single photoresponsive molecule of the present invention when femtosecond laser irradiation is applied and the time delay line is 100 femtoseconds.

[0025] In the figure: 1, femtosecond laser; 2, first reflector; 3, first beam combiner and splitter; 4, second reflector; 5, broadband optical parametric amplifier; 6, third reflector; 7, first aperture; 8, chopper with fixed frequency; 9, tunable first attenuator; 10, fourth reflector; 11, time delay; 12, computer; 13, fifth reflector; 14, sixth reflector; 15, second aperture; 16, first convex lens; 17, tunable second attenuator Plate; 18, Yag white light crystal; 19, second convex lens; 20, filter; 21, seventh reflector; 22, second beam combiner and splitter; 23, lead-in optical fiber; 24, multifunctional sample rod; 24-1, lead-in optical fiber interface; 24-2, sample rod body; 24-3, twisted pair; 24-4, polarizer; 24-5, fixture; 24-6, sample carrier; 25, comprehensive physical property measurement system; 26, pre-current amplifier; 27, phase-locked amplifier. DETAILED DESCRIPTION

[0026] A method for detecting single-molecule signals with light response at a femtosecond time resolution comprises the following steps, and its flow chart is as follows: Figure 1 As shown: S1: The laser emitted from the femtosecond laser is reflected by the first reflector to the first beam combiner and splitter. 50% of the light is transmitted to form the pump pulse optical path, and 50% of the light is reflected to form the detection light path. Specifically, the femtosecond laser may preferably be a titanium sapphire femtosecond laser. Further, the titanium sapphire femtosecond laser may preferably be the PHAROS series titanium sapphire femtosecond laser produced by Light Conversion, which has high pulse energy and high average power in the millijoule level, generates a laser wavelength of 1030 nanometers, and the laser repetition rate is usually 100 kHz, and the laser pulse width can be less than 100 femtoseconds.

[0027] S2: The pump pulse light path is reflected by the second reflector to the broadband optical parametric amplifier, and then reflected by the third reflector to enter the first aperture for beam positioning, and then emitted to the tunable first attenuator after passing through the chopper with a fixed frequency, and then respectively emitted into the introduction optical fiber through the fourth reflector and the second beam combining and splitting plate; Specifically, the broadband optical parametric amplifier may preferably be an ORPHEUS series colinear optical parametric amplifier produced by Light Conversion, which is used to convert the generated femtosecond laser into pump pulses of other wavelengths and has a tunable function.

[0028] The 1030-nanometer femtosecond laser output by the titanium sapphire femtosecond laser passes through the first beam combiner and the second reflector and enters the broadband optical parametric amplifier, where it can be frequency-doubled to obtain a pump pulse of 325 to 1000 nanometers.

[0029] The chopper can preferably be a chopper manufactured by THORLABS, which can block the interval pulse light of the ultrafast laser with a repetition rate of 100 kHz, and the final frequency is the frequency set by the chopper, forming pump detection relative to the detection light.

[0030] S3: The detection light is reflected in the time delay device to achieve femtosecond delay, and then reflected by the fifth and sixth reflectors to enter the second aperture for beam positioning, and then passes through the white light generation module to generate continuous detection white light, and then passes through the seventh reflector and the second beam combiner and splitter to be injected into the introduction optical fiber; The time delay here can preferably be a time delay manufactured by THORLABS, which is composed of a high-precision motor platform and multiple platform internal reflectors. The femtosecond light output by the titanium sapphire femtosecond laser is reflected multiple times by the time delay to set the optical path. The moving range of the high-precision motor platform is 0 to 300 nanometers, and the femtosecond laser can be repeated 8 times in the high-precision motor platform. According to the speed of light of 3×108 m / s, the delay time of the time delay for the detection light is 0 to 8 nanoseconds.

[0031] Since the time delay device can achieve femtosecond optical path delay through a high-precision motor platform, the interval between the pump pulse and the detection pulse can reach the femtosecond level, so the time resolution of the test can be increased to the femtosecond level. In addition, the tunability of the time delay device can make the interval between the pump pulse and the detection pulse reach up to 8 nanoseconds, so that the light response dynamics process of different time scales can be detected.

[0032] Optimally, the white light generating module comprises a first convex lens, a tunable second attenuation plate, a Yag white light crystal, a second convex lens and a filter which are arranged in sequence.

[0033] The Yag white light crystal here is about 13 mm thick and can convert the incident 1030-nanometer femtosecond laser into detection white light with a specific wavelength range of 500 nanometers to 950 nanometers.

[0034] S4: The pump pulse optical path and the detection light optical path enter the multifunctional sample rod placed in the test cavity of the comprehensive physical property measurement system, and are polarized by the polarizer in the multifunctional sample rod to generate left-handed or right-handed circularly polarized light, which is then irradiated onto the graphene single-molecule device connected to a single photoresponsive molecule to be detected and placed on the sample carrier of the multifunctional sample rod. The graphene single-molecule device connected to a single photoresponsive molecule to be detected is connected to the electrode column of the sample carrier through a copper wire and a conductive silver glue; The multifunctional sample holder is placed in the test cavity of the comprehensive physical property measurement system, and the pump pulse optical path and the detection light optical path enter the multifunctional sample holder. The comprehensive physical property measurement system can provide the ultra-low temperature and strong magnetic field experimental environment required for single molecule testing. It is equipped with a dilution refrigerator, the lowest temperature of which is about 50 millikelvin degrees, and the strongest magnetic field can reach ±9 Tesla.

[0035] Specifically, the multifunctional sample rod includes a sample rod body, an introduction optical fiber interface, a clamp, a polarizer, a sample carrier and a twisted wire. The introduction optical fiber interface is fixedly installed on the top of the sample rod body and connected to the introduction optical fiber. The clamp is fixedly installed in the sample rod body and is located below the introduction optical fiber interface. The polarizer is installed on the clamp. The sample carrier is installed in the sample rod body and is located below the clamp. The graphene single-molecule device connected to a single light-responsive molecule to be detected is fixedly placed on the sample carrier. The twisted wire is connected to the gold electrode of the graphene single-molecule device connected to a single light-responsive molecule to be detected and a signal acquisition unit.

[0036] Installing the polarizer required for the experiment on the fixture can change the polarization state of the femtosecond laser to left-handed circularly polarized light or right-handed circularly polarized light, thereby realizing the light response dynamics process related to the chiral spin of a single molecule to meet the experimental needs of different polarized light.

[0037] Optimized, twisted-pair cables can reduce noise from external interference.

[0038] The comparison of background noise of multifunctional sample rod electrical test is shown in the figure below. Figure 5 As shown in the figure, a is the background noise diagram of the electrical test of the multifunctional sample rod when the coaxial line is used, and b is the background noise diagram of the electrical test of the multifunctional sample rod when the twisted pair is used. By comparison, it is found that the background noise when the twisted pair current is tested is reduced from the nanoampere level to the hundred-picoampere level. This effectively improves the signal-to-noise ratio in the test of weak signals of single light-responsive molecules.

[0039] S5: A single photoresponsive molecule is excited after being irradiated by the pump pulse optical path, and then detected by the detection light optical path, and reflected by the signal acquisition unit and displayed on the computer.

[0040] By adjusting the time delay and combining it with the experimental environment of the comprehensive physical property measurement system itself, it is possible to test a single photoresponsive molecule with femtosecond resolution.

[0041] Specifically, a femtosecond laser is irradiated through a polarizer onto a graphene single-molecule device connected to a single photoresponsive molecule on a sample stage. Under the excitation of the pump pulse, the photoresponsive molecule will be excited to an excited state, and then after a certain time delay, the detection pulse is irradiated onto the photoresponsive molecule for detection. This time delay is determined by the time delay device and can be achieved in the femtosecond order. Therefore, the time resolution of the photoresponsive dynamics of a single photoresponsive molecule can reach the femtosecond level. Then, a single photoresponsive molecule generates a corresponding photoresponse, such as a photocurrent phenomenon, and the signal is transmitted to the signal acquisition unit through the multifunctional sample rod to collect the detection signal.

[0042] Furthermore, the signal acquisition unit includes a pre-current amplifier and a phase-locked amplifier. The pre-current amplifier is used to amplify the detection information transmitted by the twisted wire and transmit it to the phase-locked amplifier. The phase-locked amplifier is used to extract the components with the same frequency and phase correlation as the reference signal and transmit them to the computer.

[0043] Since the pump pulse optical path is equivalent to passing through a fixed-frequency chopper modulated by a phase-locked amplifier, the pump pulse excites a single photoresponsive molecule at this fixed frequency.

[0044] When the pump pulse is present, a single photoresponse molecule is excited, and an electrical signal with a photoresponse process can be obtained under the action of the detection pulse; when the pump pulse is blocked, a single photoresponse molecule is not excited, and an electrical signal without a photoresponse process is obtained. The lock-in amplifier uses phase-sensitive detection technology to extract only the components that are at the same frequency and phase-related as the reference signal, and other noise signals that are not at this frequency will be greatly suppressed.

[0045] Specifically, the method of extracting the component with the same frequency and phase correlation as the reference signal by the lock-in amplifier is as follows: D1: The lock-in amplifier expresses the input signal as an AC signal superimposed on a DC bias according to equation (1): (1); in: represents the lock-in amplifier input signal, represents the average current generated by the graphene single-molecule device connected to a single photoresponsive molecule to be detected, , is the current signal when the pump pulse exists, is the current signal when the pump pulse is blocked, represents the photocurrent generated by the graphene single-molecule device connected to a single photoresponsive molecule to be detected, , represents the reference frequency generated by the lock-in amplifier, which is equal to the chopper frequency, Indicates time, represents the initial phase of the lock-in amplifier, represents random noise, which includes noise components of unrelated reference frequencies such as thermal noise and electromagnetic interference; Since the pump pulses are periodically turned on and off, in the long run is a DC component, and the lock-in amplifier operates at a frequency The pump light is modulated, so the current signal will have an AC component of the same frequency. It is the useful signal that we hope to extract through the lock-in amplifier; Random noise and They are not of the same frequency and can therefore be effectively suppressed in phase lock detection.

[0046] D2: The lock-in amplifier multiplies the input signal and the reference signal synchronously and integrates them, and extracts the signal component in phase with the reference frequency according to equation (2): (2); in: represents the extracted signal component that is in phase with the reference frequency, represents the integration time period; D3: The extracted signal that is in phase with the reference frequency is obtained by sum-difference product calculation (3): (3); D4: Fix the initial phase of the lock-in amplifier, lock the photoelectric signal submerged in the noise at the reference frequency, amplify it, and transmit it to the computer for reading, so as to extract the useful signal to the maximum extent. The weak photoelectric signal that was originally submerged in the noise is locked to the reference frequency and amplified and read out.

[0047] Optimally, in step D4, the initial phase of the phase-locked amplifier is fixed to 0.

[0048] Specifically, the synthetic structure of a single photoresponsive molecule A is as follows: Specific synthesis method: Under nitrogen atmosphere, (102 mg, 0.43 mmol) was added to a benzene solution (10 mL) of A-Boc-OF (32 mg, 0.023 mmol). The mixture was stirred at room temperature for 6.5 h, and then the insoluble matter was removed by filtration under reduced pressure. The filtrate was concentrated and purified by column chromatography to obtain A-Boc.

[0049] 1 H NMR (400 MHz, Chloroform- d) δ 8.15 (d, J = 7.0 Hz, 2H), 8.06 (d, J =7.0 Hz, 2H), 7.17 – 7.68 (m, 4H), 7.67 – 7.62 (m, 10H), 7.52 (d, J = 1.7 Hz,2H), 7.51 (d, J = 1.6 Hz, 2H), 7.48 (td, J = 6.8, 1.3 Hz, 2H), 7.42 (s, 2H), 7.37(s, 2H), 7.25 – 7.21 (m, 4H), 5.21 (t, J = 4.9 Hz, 2H), 3.18 (td, J = 5.7, 4.9Hz, 4H), 2.66 (tt, J = 7.7, 1.0 Hz, 4H), 1.80 (tt, J = 7.9, 5.6 Hz, 4H), 1.41 (s, 18H), 1.26 (s, 36H).

[0050] 13 C NMR (100 MHz, Chloroform- d ) δ 187.35, 156.50, 149.43, 148.12,146.06, 142.44, 139.73, 138.92, 135.19, 134.28, 132.52, 129.39, 129.14,127.66, 127.64, 126.43, 126.30, 125.60, 124.73, 124.49, 124.42, 123.61,123.56, 122.43, 79.55, 60.38, 40.22, 34.63, 33.59, 29.66, 29.29, 28.30.

[0051] Under nitrogen atmosphere, trifluoroacetic acid (22 μL, 0.29 mmol) was added to a dichloromethane solution (10 mL) of A-Boc (20 mg, 0.014 mmol). The mixture was stirred at room temperature and monitored by thin layer chromatography until the reaction was completed. The reaction was extracted with dichloromethane and sodium bicarbonate solution, and the organic phases were combined, concentrated, and recrystallized from petroleum ether to obtain a single photoresponsive molecule A.

[0052] <h2 style=";text-align:left;direction:ltr"> 1 <h2 style=";text-align:left;direction:ltr"> H NMR (400 MHz, Chloroform-<h2 style=";text-align:left;direction:ltr"> d <h2 style=";text-align:left;direction:ltr"> ) δ 8.15 (d,<h2 style=";text-align:left;direction:ltr"> J <h2 style=";text-align:left;direction:ltr"> = 7.0 Hz, 2H), 8.06 (d,<h2 style=";text-align:left;direction:ltr"> J <h2 style=";text-align:left;direction:ltr"> =7.0 Hz, 2H), 7.17 – 7.68 (m, 4H), 7.67 – 7.62 (m, 10H), 7.52 (d,<h2 style=";text-align:left;direction:ltr"> J <h2 style=";text-align:left;direction:ltr"> = 1.7 Hz,2H), 7.51 (d,<h2 style=";text-align:left;direction:ltr"> J <h2 style=";text-align:left;direction:ltr"> = 1.6 Hz, 2H), 7.48 (td,<h2 style=";text-align:left;direction:ltr"> J <h2 style=";text-align:left;direction:ltr"> = 6.8, 1.3 Hz, 2H), 7.42 (s, 2H), 7.37 (s, 2H), 7.25 – 7.21 (m, 4H), 2.77 (tt,<h2 style=";text-align:left;direction:ltr"> J <h2 style=";text-align:left;direction:ltr"> = 6.3, 5.4 Hz, 4H), 2.65 (tt,<h2 style=";text-align:left;direction:ltr"> J <h2 style=";text-align:left;direction:ltr"> = 7.7,1.0 Hz, 4H), 1.76 (tt,<h2 style=";text-align:left;direction:ltr"> J <h2 style=";text-align:left;direction:ltr"> = 7.5, 5.4 Hz, 4H), 1.59 (d,<h2 style=";text-align:left;direction:ltr"> J <h2 style=";text-align:left;direction:ltr"> = 12.6 Hz, 4H), 1.26 (s,36H)。<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0053] <h2 style=";text-align:left;direction:ltr"> 13 <h2 style=";text-align:left;direction:ltr"> C NMR (100 MHz, Chloroform-<h2 style=";text-align:left;direction:ltr"> d) δ 193.44, 193.36, 148.32, 148.29,146.18, 146.13, 146.08, 146.04, 144.46, 144.44, 141.75, 141.74, 140.15,140.14, 138.75, 138.73, 137.69, 137.65, 134.38, 134.33, 134.08, 134.02,132.06, 132.01, 129.34, 129.30, 129.26, 129.22, 128.86, 128.84, 33.91, 122.60, 122.58, 65.69, 65.65, 41.73,41.70, 34.66, 33.91, 33.10, 33.06, 29.34, 29.32, 29.30.

[0054] The structural formula of a single photoresponsive molecule A is as follows: The synthetic route structure of A-Boc-OF is as follows: Under nitrogen atmosphere, Ⅰ (161 mg, 0.2 mmol), Ⅱ 3',5'-di-tert-butyl-4'-hydroxy-[1,1'-biphenyl]-2-carbaldehyde (155 mg, 0.5 mmol), ammonium acetate (869 mg, 11.3 mmol) in acetic acid (3 mL) were stirred at 110°C for 2 days. The reaction mixture was cooled to room temperature and neutralized with aqueous ammonia. The precipitate was filtered and washed with hexane to obtain A-Boc-OF (a mixture of structural isomers).

[0055] 1 H NMR (400 MHz, Chloroform- d ) δ 8.08 (dd, J = 8.2, 1.3 Hz, 2H), 7.73 –7.62 (m, 14H), 7.58 (td,J = 8.0, 1.3 Hz, 2H), 7.54 – 7.48 (m, 6H), 7.29 – 7.19(m, 8H), 5.74 (s, 2H), 5.21 (t, J = 4.9 Hz, 2H), 3.18 (td, J = 5.7, 4.9 Hz, 4H),2.66 (tt, J = 7.7, 1.0 Hz, 4H), 1.80 (tt, J = 7.9, 5.6 Hz, 4H), 1.41 (s, 56H).

[0056] 13 C NMR (100 MHz, Chloroform- d ) δ 156.50, 154.56, 149.76, 142.44,140.01, 139.94, 138.92, 138.69, 137.70, 137.53, 130.90, 130.85, 130.25,129.69, 129.39, 128.73, 128.46, 128.26, 127.64, 127.21, 127.12, 125.87,125.43, 79.55, 40.22, 34.74, 33.59, 30.29, 29.66, 28.30.

[0057] The synthetic route structure of Ⅰ is as follows: Under nitrogen atmosphere, 2,2′-(1,4-phenylene)bis(1-(4-bromophenyl))ethyl-1,2-dione (500 mg, 1 mmol), 4-tert-butylcarbamatephenylboronic acid pinacol ester (902 mg, 2.5 mmol), Pd(dppf)Cl2 (36 mg, 0.05 mmol), 1 mL of supersaturated potassium carbonate solution, and 10 mL of tetrahydrofuran solution were reacted at 80 °C overnight. The mixture was cooled to room temperature, extracted with dichloromethane, and the organic phases were combined, concentrated, and purified by column chromatography to obtain the above Ⅰ.

[0058] 1 H NMR (400 MHz, Chloroform- d ) δ 8.06 (s, 4H), 7.99 – 7.90 (m, 8H), 7.53 – 7.50 (m, 4H), 7.23 (dt,J = 8.2, 1.0 Hz, 4H), 5.21 (t, J = 4.9 Hz, 2H),3.18 (td, J = 5.7, 4.9 Hz, 4H), 2.66 (tt, J = 7.7, 1.0 Hz, 4H), 1.80 (tt, J = 7.9,5.6 Hz, 4H), 1.41 (s, 18H)。

[0059] 13 C NMR (100 MHz, Chloroform- d ) δ 193.28, 193.19, 193.12, 193.03,156.43, 156.39, 144.47, 144.46, 142.20, 142.19, 138.94, 138.92, 135.73,135.67, 134.03, 133.97, 130.56, 130.54, 130.53, 130.51, 130.06, 130.04,130.03, 130.01, 129.34, 129.30, 129.26, 129.22, 128.71, 128.66, 128.62,128.58, 127.44, 127.39, 127.34, 127.29, 79.58, 79.54, 40.40, 40.36, 33.59,30.02, 29.99, 28.32, 28.31。

[0060] Then, a graphene nanogap point electrode array was prepared. The specific steps are as follows: a single layer of graphene was obtained on a copper sheet by chemical vapor deposition, and then polymethyl methacrylate (PMMA) was spin-coated on the single layer of graphene at a speed of 4000 r / min. After the obtained PMMA-graphene-copper was placed in a ferric chloride solution for 4 hours, it was transferred to a clean 1 cm × 1 cm silicon wafer by a wet method to obtain a PMMA-graphene-silicon wafer. Then, the PMMA-graphene-silicon wafer was placed in boiling acetone to remove the PMMA on the surface of the graphene, and a graphene film attached to the silicon wafer was obtained. Then, photoresist was spin-coated on the silicon wafer with the graphene film attached at a speed of 4000 r / min, and strips were photoetched by an ultraviolet photolithography machine. After being fully immersed in a developer for 5 seconds, it was taken out and blown dry to obtain a silicon wafer with photoresist only above the middle graphene strip and no photoresist on the rest of the place. Then it is placed in an oxygen plasma etcher for etching to obtain a silicon wafer with graphene strips attached. After acetone debonding, the photoresist is repeatedly spin-coated again, the electrode is photolithographyed by a UV photolithography machine, and after development, a layer of gold film is evaporated by a thermal resistance evaporation coating machine. After acetone debonding, a gold electrode-graphene strip-silicon wafer is obtained.

[0061] Furthermore, an electron beam exposure machine was used to etch dotted lines on the gold electrode-graphene strip-silicon wafer, each dotted line was 150 nanometers long and 36 nanometers apart, and after development, it was placed in an oxygen plasma etcher for etching to obtain a graphene nanogap point electrode array.

[0062] A single photoresponsive molecule A is connected to the prepared graphene nanogap point electrode array through a stable amide bond to obtain a graphene single-molecule device of a single photoresponsive molecule A. The specific process is as follows: the graphene nanogap point electrode array is placed in a two-necked flask, 1-(3-dimethylaminopropyl)-3-2-ethylcarbodiimide hydrochloride and A molecules are added to the flask, the two-necked flask is sealed, and the gas is pumped out several times to place the sealed two-necked flask in a nitrogen environment. 10 mL of anhydrous pyridine is injected into the above two-necked flask with a syringe, and the reaction is carried out at room temperature for 48 hours to obtain a graphene single-molecule device connected to a single photoresponsive molecule.

[0063] Then, the Agilent B1500A semiconductor parameter analyzer and probe station can be used for testing. The gold electrode of the graphene single-molecule device connected to a single photoresponsive molecule is connected to the electrode column of the sample carrier through copper wire and conductive glue, and the multifunctional sample rod is placed in the test cavity of the comprehensive physical property measurement system. The current-voltage image of the graphene single-molecule device connected to a single photoresponsive molecule when no femtosecond laser is applied is as follows: Figure 6 ,As the voltage changes, the current value changes significantly, proving the effective connection of a single photoresponsive molecule.

[0064] Finally, turn on the switches of each instrument, set the time delay to 100 femtoseconds, and set the chopper frequency to 1 kHz through the phase-locked amplifier. Set the temperature of the comprehensive physical property measurement system to 2 degrees Kelvin, and test the current-voltage image of the graphene single-molecule device connected to a single photoresponsive molecule under this condition when femtosecond laser irradiation is applied and the time delay line is 100 femtoseconds. Figure 7 shown.

[0065] contrast Figure 6 and Figure 7 It was found that the current value of the graphene single-molecule device connected to a single A molecule at the same voltage increased significantly when femtosecond laser irradiation was applied and the time delay line was 100 femtoseconds, indicating that this method can achieve the detection of single-molecule photoresponse signals with femtosecond resolution.

[0066] In addition, the current over time signal is tested, and the phase-locked amplifier will continuously extract the modulation component at the 1 kHz reference frequency through phase-sensitive detection, and the noise that is not in the same frequency and phase with this frequency will be suppressed, thus realizing the detection of weak signals of single-molecule light response. By changing the setting of the time delay device and repeating the above test, a light response kinetic curve with time resolution can be obtained, realizing the detection of weak signals of single-molecule light response with femtosecond resolution.

[0067] A femtosecond time resolution light response single molecule signal detection device is used to perform any of the above-mentioned femtosecond time resolution light response single molecule signal detection methods, and the device diagram is as shown in FIG. Figure 2 As shown, it includes a femtosecond laser 1, a first reflector 2, a first beam-combining and beam-splitting plate 3, a pump pulse optical path, a detection light optical path, a second beam-combining and beam-splitting plate 22, an introduction optical fiber 23, a comprehensive physical property measurement system 25, a multifunctional sample holder 24, a signal acquisition unit and a computer 12. The femtosecond laser, the first reflector, and the first beam-combining and beam-splitting plate are coupled in sequence, and the pump pulse optical path includes a second reflector 4, a broadband optical parametric amplifier 5, a third reflector 6, a first aperture 7, a chopper with a fixed frequency 8, a tunable first attenuator 9, and a fourth reflector. 10, the detection light path includes a time delay device 11, a fifth reflector 13, a sixth reflector 14, a second aperture 15, a white light generation module, and a seventh reflector 21 coupled in sequence, the white light generation module includes a first convex lens 16, a tunable second attenuation plate 17, a Yag white light crystal 18, a second convex lens 19, and a filter 20 coupled in sequence, the pump pulse light path and the detection light light path are combined through the second beam combining and splitting plate and then enter the introduction optical fiber, the multifunctional sample rod is placed in the test cavity of the comprehensive physical property measurement system, and the schematic diagram of the multifunctional sample rod structure is shown in FIG. Figure 3 , Figure 4As shown, the multifunctional sample rod includes a sample rod body 24-2, an introduction fiber interface 24-1, a clamp 24-5, a polarizer 24-4, a sample carrier 24-6 and a twisted pair 24-3. The introduction fiber interface is fixedly installed on the top of the sample rod body and connected to the introduction fiber. The clamp is fixedly installed in the sample rod body and is located below the introduction fiber interface. The polarizer is installed on the clamp. The sample carrier is installed in the sample rod body and is located below the clamp. The graphene single-molecule device connected to a single light-responsive molecule to be detected is fixedly placed on the sample carrier and connected to the sample carrier electrode column through a copper wire and a conductive silver glue. The twisted wire is connected to the gold electrode of the graphene single-molecule device connected to a single light-responsive molecule to be detected and a signal acquisition unit. The signal acquisition unit includes a pre-current amplifier 26 and a phase-locked amplifier 27 connected to each other. The current amplifier is connected to the twisted pair, and the phase-locked amplifier is connected to a computer.

[0068] The present invention utilizes a detection device composed of a comprehensive physical property measurement system and a femtosecond laser, and combines the advantages of optical and electrical detection technologies to control a single molecule by multimodal means such as optical field and electric field, thereby improving the time resolution to the femtosecond level and being able to monitor the femtosecond-level light response dynamics of a single molecule. The pump pulse frequency is modulated by a chopper, and the chopper frequency is output as the reference frequency through a phase-locked amplifier. Combined with the phase-sensitive detection of the phase-locked amplifier, the weak signal detection of a single photoresponsive molecule is realized. A multifunctional sample rod is independently designed and a polarizer is installed in the multifunctional sample rod. Femtosecond ultrafast light pulses with different polarization modes can be switched, thereby converting the introduced linearly polarized ultrafast laser into left- or right-handed circularly polarized light, which can be used for light response research related to the chiral spin of a single molecule. At the same time, the twisted pair selected for current transmission can reduce the noise of the electrical signal measured by the comprehensive physical property measurement system, reducing the original nanoampere level noise to the hundred-picoampere level, thereby improving the signal-to-noise ratio.

[0069] In summary, the method and device for detecting single-molecule light-responsive signals with femtosecond time resolution provided by the present invention can realize the detection of weak signals of single light-responsive molecules with femtosecond time resolution, and can improve the signal-to-noise ratio of electrical signals measured by a comprehensive physical property measurement system.

[0070] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for detecting single-molecule signals with light response at a femtosecond time resolution, characterized in that: The steps include: S1: The laser emitted from the femtosecond laser is reflected by the first reflector to the first beam combiner and splitter. 50% of the light is transmitted to form the pump pulse optical path, and 50% of the light is reflected to form the detection light path. S2: The pump pulse light path is reflected by the second reflector to the broadband optical parametric amplifier, then reflected by the third reflector to enter the first aperture for beam positioning, then emitted to the tunable first attenuator after passing through the chopper with a fixed frequency, and then emitted into the introduction optical fiber through the fourth reflector and the second beam combining and splitting plate respectively; S3: The detection light is reflected in the time delay device to achieve femtosecond delay, and then reflected by the fifth and sixth reflectors to enter the second aperture for beam positioning, and then passes through the white light generation module to generate continuous detection white light, and then passes through the seventh reflector and the second beam combiner and splitter to be injected into the introduction optical fiber; S4: The pump pulse optical path and the detection light optical path enter the multifunctional sample rod placed in the test cavity of the comprehensive physical property measurement system, and are polarized by the polarizer in the multifunctional sample rod to generate left-handed or right-handed circularly polarized light, which is then irradiated onto the graphene single-molecule device connected to a single photoresponsive molecule to be detected and placed on the sample carrier of the multifunctional sample rod. The graphene single-molecule device connected to a single photoresponsive molecule to be detected is connected to the electrode column of the sample carrier through a copper wire and a conductive silver glue; S5: A single photoresponsive molecule is excited after being irradiated by the pump pulse optical path, and then detected by the detection light optical path, and reflected by the signal acquisition unit and displayed on the computer.

2. The method for detecting single-molecule signals with light response at a femtosecond time resolution according to claim 1, characterized in that: The femtosecond laser is a titanium sapphire femtosecond laser.

3. The method for detecting single-molecule signals with light response at a femtosecond time resolution according to claim 1, characterized in that: The white light generation module comprises a first convex lens, a tunable second attenuation plate, a Yag white light crystal, a second convex lens and a filter which are arranged in sequence.

4. The method for detecting single-molecule signals with light response at a femtosecond time resolution according to claim 1, characterized in that: The multifunctional sample rod comprises a sample rod body, an introduction optical fiber interface, a clamp, a polarizer, a sample carrier and a twisted wire. The introduction optical fiber interface is fixedly installed on the top of the sample rod body and connected to the introduction optical fiber. The clamp is fixedly installed in the sample rod body and is located below the introduction optical fiber interface. The polarizer is installed on the clamp. The sample carrier is installed in the sample rod body and is located below the clamp. A graphene single-molecule device connected to a single light-responsive molecule to be detected is fixedly placed on the sample carrier. The twisted wire is connected to a gold electrode and a signal acquisition unit of the graphene single-molecule device connected to a single light-responsive molecule to be detected.

5. The method for detecting single-molecule signals with light response at a femtosecond time resolution according to claim 4, characterized in that: The twisted wire is a twisted pair wire.

6. The method for detecting single-molecule signals with light response at a femtosecond time resolution according to claim 4, characterized in that: The signal acquisition unit includes a pre-current amplifier and a phase-locked amplifier. The pre-current amplifier is used to amplify the detection information transmitted by the twisted wire and transmit it to the phase-locked amplifier. The phase-locked amplifier is used to extract the components with the same frequency and phase correlation as the reference signal and transmit them to the computer.

7. The method for detecting single-molecule signals with light response at a femtosecond time resolution according to claim 6, characterized in that: The method of extracting the component with the same frequency and phase correlation with the reference signal by the lock-in amplifier is as follows: D1: The lock-in amplifier expresses the input signal as an AC signal superimposed on a DC bias according to equation (1): （1）； in: represents the lock-in amplifier input signal, represents the average current generated by the graphene single-molecule device connected to a single photoresponsive molecule to be detected, represents the photocurrent generated by the graphene single-molecule device connected to a single photoresponsive molecule to be detected, represents the reference frequency generated by the lock-in amplifier, Indicates time, represents the initial phase of the lock-in amplifier, represents random noise; D2: The lock-in amplifier multiplies the input signal and the reference signal synchronously and integrates them, and extracts the signal component in phase with the reference frequency according to equation (2): （2）； in: represents the extracted signal component that is in phase with the reference frequency, represents the integration time period; D3: The extracted signal that is in phase with the reference frequency is obtained by sum-difference product calculation (3): （3）； D4: Fix the initial phase of the phase-locked amplifier, lock the photoelectric signal submerged in the noise at the reference frequency, amplify it, and then transmit it to the computer for reading.

8. The method for detecting single-molecule signals with light response at a femtosecond time resolution according to claim 7, characterized in that: In step D4, the initial phase of the lock-in amplifier is fixed to 0.

9. The method for detecting single-molecule signals with light response at a femtosecond time resolution according to claim 1, characterized in that: The wavelength range of the continuous detection white light generated by the white light generation module in step S3 is 500 nanometers to 950 nanometers.

10. A device for detecting single-molecule signals with light response at a femtosecond time resolution, for performing a method for detecting single-molecule signals with light response at a femtosecond time resolution as claimed in any one of claims 1 to 9, characterized in that: The invention comprises a femtosecond laser, a first reflector, a first beam-combining and beam-splitting plate, a pump pulse optical path, a detection light optical path, a second beam-combining and beam-splitting plate, an introduction optical fiber, a comprehensive physical property measurement system, a multifunctional sample rod, a signal acquisition unit and a computer. The femtosecond laser, the first reflector and the first beam-combining and beam-splitting plate are coupled in sequence. The pump pulse optical path comprises a second reflector, a broadband optical parametric amplifier, a third reflector, a first aperture, a chopper with a fixed frequency, a tunable first attenuation plate and a fourth reflector coupled in sequence. The detection light optical path comprises a time delay device, a fifth reflector, a sixth reflector, a second aperture, a white light generating module and a seventh reflector coupled in sequence. The white light generating module comprises a first convex lens, a tunable second attenuation plate, a Yag white light crystal, a second convex lens and a filter coupled in sequence. The pump pulse optical path and the detection light optical path are combined by the second beam-combining and beam-splitting plate and then enter the introduction optical fiber. The multifunctional sample rod comprises a first convex lens, a second convex lens and a filter coupled in sequence. The multifunctional sample rod is placed in a test cavity of a comprehensive physical property measurement system. The multifunctional sample rod comprises a sample rod body, an introduction optical fiber interface, a fixture, a polarizer, a sample carrier and a twisted pair. The introduction optical fiber interface is fixedly installed on the top of the sample rod body and connected to the introduction optical fiber. The fixture is fixedly installed in the sample rod body and located below the introduction optical fiber interface. The polarizer is installed on the fixture. The sample carrier is installed in the sample rod body and located below the fixture. A graphene single-molecule device connected to a single light-responsive molecule to be detected is fixedly placed on the sample carrier and connected to the sample carrier electrode column through a copper wire and a conductive silver glue. The twisted wire is connected to a gold electrode of the graphene single-molecule device connected to a single light-responsive molecule to be detected and a signal acquisition unit. The signal acquisition unit comprises a pre-current amplifier and a phase-locked amplifier connected to each other. The current amplifier is connected to the twisted pair, and the phase-locked amplifier is connected to a computer.

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