Time-resolved spectroscopy based direct measurement system and method for light speed in transparent medium

Abstract

The application discloses a kind of transparent medium light speed direct measurement system and method based on time-resolved spectroscopy, including femtosecond laser transient absorption spectroscopy measurement subsystem and light speed calculation subsystem, the femtosecond laser transient absorption spectroscopy measurement subsystem is used to test the transient absorption spectroscopy before and after joining the transparent medium to be measured in optical path, the light speed calculation subsystem specifically includes: first rising edge acquisition unit, for extracting the rising edge corresponding time point of the transient absorption spectroscopy before joining the transparent medium to be measured, as first time point;Second rising edge acquisition unit, for extracting the rising edge corresponding time point of the transient absorption spectroscopy after joining the transparent medium to be measured, as second time point;Delay acquisition unit, for calculating the difference of first time point and second time point, as the delay caused after joining the transparent medium to be measured;Light speed calculation unit, for calculating the light speed in the transparent medium to be measured according to delay according to the following formula.The application can directly measure light speed, applicable to various transparent medium.

Description

Technical Field

[0001] This invention relates to light speed measurement technology, and more particularly to a system and method for direct measurement of light speed in a transparent medium based on time-resolved spectroscopy. Background Technology

[0002] The speed of light in a vacuum is a fundamental parameter in physics, and its precise measurement has made significant contributions to the advancement of modern science and technology. According to Einstein's theory of relativity and centuries of experimental measurements by scientists, the speed of light in a vacuum is invariant and a constant.

[0003] In contrast, the speed of light in a medium is typically lower than that in a vacuum, which determines the photoelectric properties of the medium. However, it is difficult to measure directly. Current technologies usually employ indirect methods such as obtaining the refractive index through interfacial light reflection or measuring the phase difference through optical interferometry. Therefore, these methods have stringent requirements regarding the smoothness and thickness of the sample surface, limiting their widespread applicability. Summary of the Invention

[0004] Purpose of the invention: This invention addresses the problems existing in the prior art by providing a widely applicable system and method for direct measurement of the speed of light in transparent media based on time-resolved spectroscopy.

[0005] Technical Solution: The time-resolved spectroscopy-based direct light speed measurement system for transparent media of the present invention includes a femtosecond laser transient absorption spectroscopy measurement subsystem and a light speed calculation subsystem. The femtosecond laser transient absorption spectroscopy measurement subsystem is used to test the transient absorption spectrum before and after adding the transparent medium to be tested into the optical path. The light speed calculation subsystem specifically includes:

[0006] The first rising edge acquisition unit is used to extract the time point corresponding to the rising edge of the transient absorption spectrum before the transparent medium to be tested is added, and use it as the first time point.

[0007] The second rising edge acquisition unit is used to extract the time point corresponding to the rising edge of the transient absorption spectrum after the addition of the transparent medium to be tested, and use it as the second time point.

[0008] The delay acquisition unit is used to calculate the difference between the first time point and the second time point, which is taken as the delay caused by the addition of the transparent medium to be tested;

[0009] The speed of light calculation unit is used to calculate the speed of light in the transparent medium under test according to the following formula based on the delay:

[0010]

[0011] In the formula, c is the speed of light in a vacuum, and n airΔt is the optical refractive index of air, Δt is the time delay caused by adding the transparent medium to be tested, and d is the distance between the inner walls of the cuvette containing the transparent medium to be tested.

[0012] Furthermore, the femtosecond laser transient absorption spectroscopy measurement subsystem specifically includes a laser, a beam splitter, a movable delay device, a first mirror assembly, a frequency doubling crystal, a chopper, a second mirror assembly, a microscope objective, a semiconductor thin film (such as MoS2, MoSe2, and Si thin films), a photodetector, a lock-in amplifier, and a spectral calculation module. The beam splitter is located behind the laser's output end and is used to split the laser output beam into a probe beam and a pump beam. The first mirror assembly is used to redirect the propagation direction of the probe beam, thereby enabling the probe beam to reach the microscope objective's input end via the delay device. The second reflector assembly is used to redirect the propagation direction of the pump beam, thereby guiding the pump beam sequentially through a frequency doubling crystal and a chopper to the light input end of the microscope objective. The semiconductor thin film is located at the focal point of the microscope objective, and the photodetector is located behind the light input end of the microscope objective. During measurement, the photodetector is used to detect the intensity value of the reflected beam from the semiconductor thin film before and after the addition of the transparent medium to be measured in the pump beam / detector beam optical path. The lock-in amplifier is used to demodulate the intensity value of the reflected beam, and the spectral calculation module is used to calculate the transient absorption spectrum before and after the addition of the transparent medium to be measured from the signal demodulated by the lock-in amplifier.

[0013] Furthermore, the demodulation method of the lock-in amplifier corresponds to the modulation method of the chopper.

[0014] Furthermore, the first reflector assembly specifically includes a first reflector, a second reflector, and a third reflector arranged sequentially according to the direction of light propagation, ultimately enabling the probe beam to reach the light input end of the microscope objective.

[0015] Furthermore, the second reflecting objective lens assembly specifically includes a fourth reflecting mirror and a fifth reflecting mirror. The fourth reflecting mirror is located behind the pump beam emitted from the beam splitter, and the fifth reflecting mirror is located behind the fourth reflecting mirror and reflects the pump beam to the light input end of the microscope objective lens.

[0016] Furthermore, the movable delay device includes a displacement platform and an optical signal delay unit located on the displacement platform.

[0017] Furthermore, the microscope objective causes the spot focal points of the pump beam and the probe beam to coincide on the surface of the semiconductor thin film, and causes the pump beam and the probe beam to be perpendicularly incident and perpendicularly emitted.

[0018] This invention also provides a method for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy. This method is implemented based on the system described above and specifically includes:

[0019] Step S1: Use a femtosecond laser transient absorption spectroscopy measurement subsystem to test the transient absorption spectrum before and after adding the transparent medium to be tested into the optical path;

[0020] Step S2: Extract the time point corresponding to the rising edge of the transient absorption spectrum before adding the transparent medium to be tested, and take it as the first time point;

[0021] Step S3: Extract the time point corresponding to the rising edge of the transient absorption spectrum after adding the transparent medium to be tested, and use it as the second time point;

[0022] Step S4: Calculate the difference between the first time point and the second time point as the delay caused by adding the transparent medium to be tested;

[0023] Step S5: Calculate the speed of light in the transparent medium under test according to the following formula based on the delay:

[0024]

[0025] In the formula, c is the speed of light in a vacuum, and n air Δt is the optical refractive index of air, Δt is the time delay caused by adding the transparent medium to be tested, and d is the distance between the inner walls of the cuvette containing the transparent medium to be tested.

[0026] Furthermore, the transparent medium to be tested is specifically placed in the pump beam path or probe beam of the femtosecond laser transient absorption spectroscopy measurement subsystem.

[0027] Furthermore, if the transparent medium to be tested is placed in the pump beam optical path, specifically behind the frequency doubling crystal; or if the transparent medium to be tested is placed in the probe beam optical path, specifically behind the delay device.

[0028] Compared with the prior art, the beneficial effects of this invention are as follows: This invention proposes a system and method for direct measurement of the speed of light in transparent media based on time-resolved spectroscopy. By measuring the time required for a femtosecond laser pulse to penetrate the medium, the propagation speed of the femtosecond laser pulse in the medium can be obtained. This invention can directly measure the speed of light and has universal reference significance for the accurate characterization and measurement of the basic physical properties of the medium. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the femtosecond laser transient absorption spectroscopy measurement subsystem;

[0030] Figure 2 This is a schematic diagram of the transient absorption spectra before and after the addition of the transparent medium;

[0031] Figure 3 The speed of light was measured in aqueous solutions of glycerol with different volume fractions. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] This embodiment provides a direct measurement system for the speed of light in a transparent medium based on time-resolved spectroscopy, including a femtosecond laser transient absorption spectroscopy measurement subsystem and a speed of light calculation subsystem. The femtosecond laser transient absorption spectroscopy measurement subsystem is used to test the transient absorption spectrum before and after the transparent medium to be tested is added to the optical path, such as... Figure 1 As shown, the system specifically includes a laser 1, a beam splitter 2, a movable time delay device 3, a first mirror assembly (first mirror 4-1, second mirror 4-2, and third mirror 4-3), a frequency doubling crystal 5, a chopper 6, a second mirror assembly (fourth mirror 7-1 and fifth mirror 7-2), a microscope objective 8, a semiconductor thin film (using mechanically exfoliated few-layer MoSe2 film as an example) 9, a photodetector 10, a lock-in amplifier 11, and a spectral calculation module 12. Each component is described below.

[0034] Laser 1 is specifically a femtosecond laser with a repetition frequency of 80MHz, an output femtosecond laser pulse width of approximately 100fs, and a wavelength of 800nm.

[0035] Beam splitter 2 is located behind the optical output end of laser 1 and is used to split the output beam of laser 1 into two beams, namely the probe beam and the pump beam.

[0036] The delay device 3 is located behind the output end of the probe beam of the beam splitter 2. It includes a displacement platform and an optical signal delay device located on the displacement platform. The optical signal delay device can be an optical delay line or other devices to delay the probe beam. Different delay values ​​can be achieved by moving the position of the displacement platform.

[0037] The first reflector assembly is used to redirect the propagation direction of the probe beam, thereby ensuring that the probe beam reaches the light input end of the microscope objective after passing through the delay device 3. Specifically, it includes a first reflector 4-1, a second reflector 4-2, and a third reflector 4-3 arranged sequentially according to the light path propagation direction, ultimately ensuring that the probe beam reaches the light input end of the microscope objective 8. Those skilled in the art will understand that in other embodiments, the first reflector assembly can also be configured with other numbers and positions of reflectors, as long as the probe beam reaches the light input end of the microscope objective after passing through the delay device 3.

[0038] The frequency doubling crystal 5 is specifically a barium metaborate crystal, used to achieve frequency doubling of the pump beam to obtain a 400nm pump beam.

[0039] Chopper 6 is used to achieve pump beam modulation, even if the pump beam exhibits periodic changes between lit and unlit states, with a chopper modulation frequency of 4 kHz.

[0040] The second set of mirrors is used to redirect the propagation direction of the pump beam, thereby guiding the pump beam sequentially through the frequency doubling crystal and the chopper to the light input end of the microscope objective. Specifically, it includes a fourth mirror 7-1 and a fifth mirror 7-2. The fourth mirror 7-1 is located behind the pump beam emitted from the beam splitter 2, and the fifth mirror 7-2 is located behind the chopper 6, reflecting the pump beam to the light input end of the microscope objective 8.

[0041] The microscope objective 8 focuses the pump beam and probe beam onto the surface of the semiconductor thin film 9. The pump beam and probe beam are incident perpendicularly on the surface of the semiconductor thin film 9, and the beams reflected from the surface of the semiconductor thin film 9 are also emitted perpendicularly from the output end to the input end of the microscope objective 8. The powers of the pump light and the probe light are 80uW and 30uW, respectively.

[0042] The semiconductor thin film 9 is located at the focal point of the microscope objective 8. For example, a few-layer MoSe2 film can be prepared as the semiconductor thin film 9 using K9 glass as a substrate through mechanical exfoliation. When the pump beam is focused onto the semiconductor thin film 9, a transient absorption process occurs, exciting electrons in the semiconductor thin film 9 from the valence band to the conduction band. Simultaneously, due to the delay of the probe beam, it will illuminate the semiconductor thin film 9 after the pump beam. Since the conduction band is filled with excited-state electrons, the reflectivity of the semiconductor thin film 9 to the probe beam changes. The reflected probe beam is collected again by the microscope objective 8 and delivered to the photodetector 10.

[0043] The photodetector 10, model 2107, detects the intensity of the reflected beam emitted by the microscope objective 8 and converts it into an electrical signal.

[0044] The lock-in amplifier 11, model SR830, is used to demodulate the chopper and can extract a high signal-to-noise ratio signal from the original signal.

[0045] The spectral calculation module 12 uses the change in intensity of the probe laser light reflected with and without a pump beam as the transient absorption light signal. During measurement, the computer program controls the reciprocating motion of the displacement platform in the delay device 3 to obtain the transient absorption spectral signal at different times. A Gaussian integral is used to fit the rising edge of the transient absorption spectral signal to obtain the zero point, i.e., the first time point. Then, the transparent medium 13 to be tested is added, and the transient absorption spectrum after adding the transparent medium 13 is measured. A Gaussian integral is used to fit the rising edge of the transient absorption spectral signal to obtain the second time point. The transparent medium 13 to be tested is specifically placed in the pump beam path or the probe beam of the femtosecond laser transient absorption spectral measurement subsystem. If the transparent medium 13 to be tested is placed in the pump beam path, it is specifically placed behind the frequency doubling crystal 5; if the transparent medium 13 to be tested is placed in the probe beam path, it is specifically placed behind the delay device 3.

[0046] The light speed calculation subsystem specifically includes: a first rising edge acquisition unit, a second rising edge acquisition unit, a delay acquisition unit, and a light speed calculation unit. The first rising edge acquisition unit is used to extract the time point corresponding to the rising edge of the transient absorption spectrum before the addition of the transparent medium to be tested, as the first time point; the second rising edge acquisition unit is used to extract the time point corresponding to the rising edge of the transient absorption spectrum after the addition of the transparent medium to be tested, as the second time point; the delay acquisition unit is used to calculate the difference between the first time point and the second time point, as the delay Δt caused by the addition of the transparent medium to be tested, such as... Figure 2 As shown; the light speed calculation unit is used to calculate the speed of light in the transparent medium under test according to the following formula based on the delay:

[0047]

[0048] In the formula, c is the speed of light in a vacuum, and n air Δt is the optical refractive index of air, Δt is the time delay caused by adding the transparent medium to be tested, and d is the distance between the inner walls of the cuvette containing the transparent medium to be tested.

[0049] This embodiment also provides a method for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy. This method is implemented based on the above-mentioned system and specifically includes:

[0050] Step S1: Use a femtosecond laser transient absorption spectroscopy measurement subsystem to test the transient absorption spectrum before and after adding the transparent medium to be tested into the optical path;

[0051] Step S2: Extract the time point corresponding to the rising edge of the transient absorption spectrum before adding the transparent medium to be tested, and take it as the first time point;

[0052] Step S3: Extract the time point corresponding to the rising edge of the transient absorption spectrum after adding the transparent medium to be tested, and use it as the second time point;

[0053] Step S4: Calculate the difference between the first time point and the second time point as the delay caused by adding the transparent medium to be tested;

[0054] Step S5: Calculate the speed of light in the transparent medium under test according to the following formula based on the delay:

[0055]

[0056] In the formula, c is the speed of light in a vacuum, and n air Δt is the optical refractive index of air, Δt is the delay caused by adding the transparent medium to be tested, and d is the distance between the inner walls of the cuvette containing the transparent medium to be tested.

[0057] The invention was experimentally verified below. Six different volume fractions of glycerol and water solutions (0%, 20%, 40%, 60%, 80%, and 100%) were thoroughly mixed and used as the transparent medium to be tested. These solutions were placed in cuvettes with an inner wall spacing of d and then placed in the optical path for measurement, ensuring that the light beam was parallel. The final measured speed of light is as follows: Figure 3 The figure shows the propagation speed of light with a wavelength of 400 nm in aqueous glycerol solutions of different volume fractions. It can be seen that this invention can achieve direct measurement of the speed of light in different transparent media.

Claims

1. A system for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy, characterized in that: It includes a femtosecond laser transient absorption spectroscopy measurement subsystem and a light speed calculation subsystem. The femtosecond laser transient absorption spectroscopy measurement subsystem is used to test the transient absorption spectrum before and after adding the transparent medium to be tested into the optical path. The light speed calculation subsystem specifically includes: The first rising edge acquisition unit is used to extract the time point corresponding to the rising edge of the transient absorption spectrum before the transparent medium to be tested is added, and use it as the first time point. The second rising edge acquisition unit is used to extract the time point corresponding to the rising edge of the transient absorption spectrum after the addition of the transparent medium to be tested, and use it as the second time point. The delay acquisition unit is used to calculate the difference between the first time point and the second time point, which is taken as the delay caused by the addition of the transparent medium to be tested; The speed of light calculation unit is used to calculate the speed of light in the transparent medium under test according to the following formula based on the delay: , In the formula, c is the speed of light in a vacuum, and n air It is the optical refractive index of air. The delay caused by adding the transparent medium to be tested is d, which is the distance between the inner walls of the cuvette containing the transparent medium to be tested.

2. The system for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy according to claim 1, characterized in that: The femtosecond laser transient absorption spectroscopy measurement subsystem specifically includes a laser, a beam splitter, a movable delay device, a first mirror assembly, a frequency doubling crystal, a chopper, a second mirror assembly, a microscope objective, a semiconductor thin film, a photodetector, a lock-in amplifier, and a spectral calculation module. The beam splitter is located behind the laser's output end and is used to split the laser output beam into a probe beam and a pump beam. The first mirror assembly is used to redirect the propagation direction of the probe beam, thereby ensuring that the probe beam reaches the microscope objective's input end after passing through the delay device. The second mirror assembly is used to redirect the pump beam... The propagation direction of the pump beam is determined, so that the pump beam passes sequentially through a frequency doubling crystal and a chopper to reach the light input end of the microscope objective. The semiconductor thin film is located at the focal point of the microscope objective, and the photodetector is located behind the light input end of the microscope objective. During measurement, the photodetector is used to detect the intensity value of the reflected beam from the semiconductor thin film before and after the addition of the transparent medium to be measured in the pump beam or the probe beam optical path. The lock-in amplifier is used to demodulate the intensity value of the reflected beam. The spectral calculation module is used to calculate the transient absorption spectrum before and after the addition of the transparent medium to be measured from the signal demodulated by the lock-in amplifier.

3. The system for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy according to claim 2, characterized in that: The demodulation method of the lock-in amplifier corresponds to the modulation method of the chopper.

4. The system for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy according to claim 2, characterized in that: The first reflector assembly specifically includes a first reflector, a second reflector, and a third reflector arranged sequentially according to the direction of light propagation, ultimately enabling the probe beam to reach the light input end of the microscope objective.

5. The system for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy according to claim 2, characterized in that: The second reflector assembly specifically includes a fourth reflector and a fifth reflector. The fourth reflector is located behind the pump beam emitted from the beam splitter, and the fifth reflector is located behind the fourth reflector and reflects the pump beam to the light input end of the microscope objective.

6. The system for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy according to claim 2, characterized in that: The movable delay device includes a displacement platform and an optical signal delay unit located on the displacement platform.

7. The system for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy according to claim 2, characterized in that: The microscope objective causes the focal points of the pump beam and the probe beam to coincide on the surface of the semiconductor thin film, and causes the pump beam and the probe beam to be incident and emitted perpendicularly.

8. A method for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy, characterized in that: This method is implemented based on the system described in any one of claims 1-7, and specifically includes: Step S1: Use a femtosecond laser transient absorption spectroscopy measurement subsystem to test the transient absorption spectrum before and after adding the transparent medium to be tested into the optical path; Step S2: Extract the time point corresponding to the rising edge of the transient absorption spectrum before adding the transparent medium to be tested, and take it as the first time point; Step S3: Extract the time point corresponding to the rising edge of the transient absorption spectrum after adding the transparent medium to be tested, and use it as the second time point; Step S4: Calculate the difference between the first time point and the second time point as the delay caused by adding the transparent medium to be tested; Step S5: Calculate the speed of light in the transparent medium under test according to the following formula based on the delay: , In the formula, c is the speed of light in a vacuum, and n air It is the optical refractive index of air. The delay caused by adding the transparent medium to be tested is d, which is the distance between the inner walls of the cuvette containing the transparent medium to be tested.

9. The method for direct measurement of the speed of light in a transparent medium based on time-resolved spectroscopy according to claim 8, characterized in that: If the transparent medium to be tested is placed in the pump beam optical path, specifically behind the frequency doubling crystal; if the transparent medium to be tested is placed in the probe beam optical path, specifically behind the delay device.

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