CN110579280B - Vortex wave measurement system and method based on terahertz time-domain spectroscopy technology - Google Patents
Vortex wave measurement system and method based on terahertz time-domain spectroscopy technology Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/02—Details
- G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
- G01J3/0229—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using masks, aperture plates, spatial light modulators or spatial filters, e.g. reflective filters
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- G—PHYSICS
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- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
- G01J3/433—Modulation spectrometry; Derivative spectrometry
- G01J3/4338—Frequency modulated spectrometry
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
Abstract
The invention discloses a vortex wave measuring system based on terahertz time-domain spectroscopy technology, which comprises: the device comprises a femtosecond laser, a first beam splitter, a generation module, a modulation module, a detection module and a control module, wherein the modulation module comprises a spatial light modulator and is used for periodically modulating vortex terahertz pulses, the control module comprises a phase-locked amplifier, a reference input end of the phase-locked amplifier is used for inputting modulation frequency signals of the spatial light modulator and is used for phase-locking and amplifying signals of different vortex topology charge components, and time domain waveform scanning of each vortex topology charge component is completed through a scanning delay line of the detection module. The invention also discloses a vortex wave measuring method based on the terahertz time-domain spectroscopy technology. Compared with the traditional aperture limitation and CCD observation method, the vortex wave measuring system and method provided by the invention are suitable for terahertz time-domain spectroscopy technology and have the effects of high speed, high precision and simplicity and convenience in operation.
Description
Technical Field
The invention relates to a terahertz technology, in particular to a vortex wave measurement system and method based on a terahertz time-domain spectroscopy technology.
Background
Terahertz waves are a general term for electromagnetic waves in a specific frequency band, and the spectrum of the terahertz waves is between microwaves and infrared rays and is in a transition stage from macroscopic electronics to microscopic photonics. Electromagnetic waves having a frequency range of 0.3 to 3THz are generally referred to, but electromagnetic waves in the vicinity of 0.1 to 10THz are sometimes also referred to as terahertz waves (1 thz=10 12 Hz)。
The terahertz time-domain spectroscopy technology is a novel technology based on a terahertz generation and detection method of ultra-fast photoelectron, and compared with a traditional Fourier infrared transformation spectrometer, the terahertz time-domain spectroscopy technology can provide not only spectrum amplitude information, but also corresponding phase information, and provides a good foundation for research and development of the terahertz technical field.
Vortex beams are a type of beam with an equal phase of a spiral. Because the intensity coherence caused by uncertainty of the spiral wave front phase information is cancelled, the phase at the center of the vortex beam cannot be determined, so that a phase singular point exists at the center, and the interface light intensity is distributed annularly. An important feature of a vortex beam is that the photons have orbital angular momentum. The vortex beam propagating along the Z axis can be expressed in cylindrical coordinates as:
E(r,θ,z,t)＝E0(r,θ,z)exp(-iLθ)exp(-ikz)exp(iωt)
wherein exp (il theta) is a phase term, theta is an attitude angle, L is an angular quantum number, which is also called a topological charge number or a topological charge number, and any integer can be theoretically taken. The photons of the vortex beam with topological charge number L carry orbital angular momentum of(Is a reduced planck constant). Thus, the vortex topology charge number is related to the photon orbital angular momentum.
In general, vortex beam detection adopts an interference method, a beam to be detected interferes with plane waves or spherical waves, and the vortex topology charge number of the beam to be detected is judged according to interference patterns. This method is mostly used for detecting continuous light, has low sensitivity, and when plane wave components or multi-topology charge components exist in the light beam, it is difficult for the interferometry to quantitatively distinguish the components. In the terahertz technology, aiming at a terahertz time-domain spectrum system, the current method for measuring the topological charge number is to utilize small holes to limit point-by-point scanning of wave surfaces, but the method is very time-consuming. Alternatively, a CCD can be used to directly observe the wavefront, but the signal-to-noise ratio is low. And both the two methods are not suitable for multi-topology charge beams, and the beam components of each topology charge cannot be analyzed.
Disclosure of Invention
Based on the above, it is necessary to provide a system and a method capable of analyzing vortex waves of a plurality of different topological charge components of vortex terahertz pulses, and having high measurement speed and high precision.
The embodiment of the invention provides a vortex topology state of charge measurement system based on terahertz time-domain spectroscopy technology, which comprises the following steps:
the femtosecond laser is used for providing far infrared femtosecond laser pulses;
the first beam splitter is used for splitting the far infrared femtosecond laser pulse into a pumping light pulse and a detection light pulse;
the generation module is arranged on the pumping light pulse optical path and is used for generating vortex terahertz pulses;
the modulation module comprises a spatial light modulator and is used for periodically modulating the wave surface of the vortex terahertz pulse;
the detection module comprises a scanning delay line and a sensor, wherein the scanning delay line is arranged on a detection light pulse light path and used for scanning and prolonging the flight time of detection light pulses, and the sensor is arranged at a light path aggregation position of the modulated vortex terahertz pulses and the detection light pulses and used for detecting time domain signals of modulation wave surfaces of the vortex terahertz pulses and converting the time domain signals into electric signals to be output;
the control module comprises a controller and a phase-locked amplifier, wherein the controller is electrically connected with the spatial light modulator and used for controlling the modulation frequency of the spatial light modulator, the signal input end of the phase-locked amplifier is electrically connected with the signal output end of the sensor and used for receiving an electric signal, and the reference input end of the phase-locked amplifier is electrically connected with the controller and used for setting the reference frequency of the phase-locked amplifier;
when the reference frequency of the phase-locked amplifier is set to be L times of the modulation frequency of the controller, the output of the phase-locked amplifier is the vortex terahertz pulse time-domain signal with the topology load of L.
Preferably, the spatial light modulator is a thin plate, the thin plate is made of an opaque material of terahertz light, a notch is arranged at the center of the thin plate, terahertz pulses are normally incident to the thin plate, the center of the thin plate coincides with the beam center of the terahertz pulses, the thin plate can rotate around a central shaft, and the rotation frequency of the thin plate is controlled by the controller.
Preferably, the generating module comprises a beam control element, a terahertz source device, a filter and an S-wave plate, wherein the pump light pulse is converged on the terahertz source device through the beam control element, the terahertz source device is excited by the pump light pulse to generate terahertz pulses, the filter is used for filtering residual pump light, and the S-wave plate is used for modulating the terahertz pulses to form vortex terahertz pulses.
Preferably, the modulation module further comprises a terahertz beam shrinking element, which is used for shrinking the vortex terahertz pulse beam so that the whole vortex terahertz pulse wave surface falls into the spatial light modulator.
Preferably, the sensor comprises an electro-optic crystal, a 1/4 wave plate, a Wollaston prism, a light diode detector and a circuit component, wherein the detection light pulse passes through the electro-optic crystal modulated by vortex terahertz pulse, then passes through the 1/4 wave plate and the Wollaston prism to obtain two beams of light with mutually perpendicular polarization directions, and the two beams of light are detected by the light diode detector and are differentially output through the circuit component.
The embodiment of the invention also provides a method for measuring the vortex topology state of charge based on the terahertz time-domain spectroscopy technology, which comprises the following steps:
s1, generating far infrared femtosecond laser pulses by a femtosecond laser;
s2, splitting the far infrared femtosecond laser pulse into a pumping light pulse and a detection light pulse by a first beam splitter;
s3, the generation module receives the pumping light pulse light and generates vortex terahertz pulses;
s4, periodically modulating the wave surface of the vortex terahertz pulse by a modulation module;
s5, the detection module comprises a scanning delay line and a sensor, wherein the scanning delay line scans and prolongs the flight time of the detection light pulse, and the sensor detects the time domain signal of the modulation wave surface of the vortex terahertz pulse and converts the time domain signal into an electric signal;
s6, the control module comprises a controller and a phase-locked amplifier, the modulation frequency of the spatial light modulator is controlled through the controller, and the electric signal is received through the phase-locked amplifier;
and S7, setting the reference frequency of the phase-locked amplifier to be L times of the modulation frequency of the controller, so that the phase-locked amplifier outputs vortex terahertz pulse time-domain signals with the topological charge of L.
Preferably, the generating module includes a first beam control element, a terahertz source device, a filter and an S-wave plate, the beam control element converges the pump light pulse on the terahertz source device, the pump light pulse excites the terahertz source device and generates a terahertz pulse, the filter filters the remaining pump light, and the S-wave plate modulates the terahertz pulse into a vortex terahertz pulse.
Preferably, the modulation module further comprises a second beam control element, and the second beam control element controls focusing or collimation of the vortex terahertz pulse.
Preferably, the sensor comprises an electro-optic crystal, a 1/4 wave plate, a Wollaston prism, a light diode detector and a circuit component, wherein the detection light pulse passes through the electro-optic crystal modulated by vortex terahertz pulse, passes through the 1/4 wave plate and the Wollaston prism to obtain two beams of light with mutually perpendicular polarization directions, and is detected by the light diode detector and differentially output through the circuit component.
According to the vortex topology state-of-charge measuring system and method of the terahertz time-domain spectroscopy technology, the spatial light modulator of the polar coordinate graph is used for modulating the wave surface of the vortex terahertz pulse, and the phase-locked separation and amplification principle of the phase-locked amplifier is matched, so that the relevant signals are separated from the modulated vortex terahertz pulse, namely the vortex topology state-of-charge time-domain signals related to the harmonic frequency of the spatial light modulation frequency by the reference frequency. Such a system can measure any form of vortex topology charge component, including its amplitude and phase information, and its accuracy is not affected by the detector, but rather is determined by the design shape of the spatial light modulator, its measurement speed is much greater than that of the conventional aperture limiting method, and is more accurate.
Drawings
FIG. 1 is a schematic diagram of a system module according to an embodiment of the present invention;
FIG. 2 is a schematic view of an optical path structure according to an embodiment of the present invention;
FIG. 3 is a schematic plan view of a spatial light modulator according to an embodiment of the present invention;
fig. 4 is a flow chart of a measurement method according to an embodiment of the present invention.
The reference numerals in the figures denote:
1. a laser; 2. a first beam splitter; 3. detecting the light pulse; 4. a pump light pulse; 5. vortex terahertz pulse; 6. the modulated vortex terahertz pulse; 7. a far infrared femtosecond laser pulse; 13. a notch; 20. a generating module; 30. a modulation module; 40. a detection module; 50. a control module; 31. a first parabolic mirror; 32. a second parabolic mirror; 33. a spatial light modulator; 34. a sample; 41. scanning a delay line; 42. a sensor.
Detailed Description
It should be noted that, without conflict, the embodiments of the present invention and features of the embodiments may be combined with each other.
Referring to fig. 1, fig. 1 is a schematic diagram of a system module according to a first embodiment of the invention. The first embodiment of the invention provides a vortex topology state of charge measurement system based on terahertz time-domain spectroscopy, which comprises: a femtosecond laser 1 for providing a far infrared femtosecond laser pulse 7; the first beam splitter 2 is used for splitting the far infrared femtosecond laser pulse into a pumping light pulse 4 and a detection light pulse 3; a generating module 20 for generating a vortex terahertz pulse 5; the modulation module 30 is used for periodically modulating the wave surface of the vortex terahertz pulse 5; a detection module 40 for scanning and extending the time of flight of the detection light pulses 3; the control module 50 is used for controlling the modulation frequency of the spatial light modulator, and further modulating the light pulse.
Wherein the laser pulse time width of the infrared femtosecond laser pulse 7 is equal to the laser pulse time width of the femtosecond (10 -15 s) magnitude, repetition frequency of 80MHz, peak power of 2 6 W. The infrared femtosecond laser pulse 7 has strong enough peak power, and forms a pumping light pulse 4 and a detection light pulse 3 after being split by the first beam splitter 2. Pulse of pump light4 excitation of terahertz light by the generation module 20, ultra-short pulse width in the order of femtoseconds is compared with picosecond (10 -12 s) magnitude terahertz pulse width, can completely measure the time domain waveform of terahertz pulse, and is an indispensable part of terahertz time-domain spectroscopy technology.
The first beam splitter 2 splits the far infrared femtosecond laser pulse into a pump light pulse 4 and a detection light pulse 3, wherein the pump light pulse 4 is used for generating terahertz light, and the detection light pulse 3 is used for detection. And when the pump light pulse 4 and the probe light pulse 3 are converged in the probe module 40, the optical path lengths of the pump light pulse 4 and the probe light pulse 3, that is, the flight time of the two pulses is required to be strictly equal by the system. The generating module 20 is disposed on the optical path of the pump light pulse 4 to respond to the pump light pulse 4 and generate the vortex terahertz pulse 5.
In other embodiments, the generating module 20 may include a beam control element, a terahertz source device, a filter and an S-wave plate, where the pump light pulses 4 are converged on the terahertz source device by the beam control element, the terahertz source device is excited by the pump light pulses 4 to generate terahertz pulses, the filter is used for filtering out the remaining pump light pulses 4, and the S-wave plate is used for modulating the terahertz pulses into vortex terahertz pulses 5. The terahertz source device can be a photoconductive antenna or an electro-optic crystal.
Referring to fig. 2 together, fig. 2 is a schematic view of an optical path structure according to a second embodiment of the present invention. The modulation module 30 is used for periodically modulating the wave surface of the vortex terahertz pulse 5. As shown in fig. 2, the modulation module 30 includes a spatial light modulator 33, a first parabolic mirror 31, a second parabolic mirror 32, and a sample 34. The first parabolic mirror 31 and the second parabolic mirror 32 are used to control the wave surface of the terahertz light, such as focusing or collimating the vortex terahertz pulse so that the wave surface of the terahertz pulse falls within the modulation range of the spatial light modulator 33, or focusing so that the terahertz pulse sufficiently acts with the sample 34.
Since the wavelength of the terahertz wave is much smaller than the size of the laboratory optics, for example, the wavelength of the terahertz wave of 1THz is 0.3mm, and the size of the optics is generally between 3 and 5cm, the terahertz wave can be controlled as a normal light wave, for example, a parabolic mirror with a thin metal layer coated on the surface can be selected as a light beam control element, as shown in fig. 2, a first parabolic mirror 31 and a second parabolic mirror 32, wherein the first parabolic mirror 31 focuses the terahertz pulse, so that the terahertz pulse 5 penetrates the spatial light modulator 33 and is converged at the sample 34. After interaction with the sample 34, the terahertz pulse 5 is refocused by the second parabolic mirror 32 and converged at the sensor 42.
In other embodiments, the modulation module 30 may include a terahertz lens or other beam control element to modulate the terahertz wave plane to change the beam shape. As shown in fig. 2, after the vortex terahertz pulse 5 acts on the sample 34, the components of the vortex topology charges in the light beam change, and these changes, including the change of the time domain waveform and the change of the phase of the components of the vortex topology charges, can be detected by the measurement system provided by the embodiment of the present invention, and the terahertz optical parameters of the sample 34 can be obtained by the Kramer-Kronig relation.
As shown in fig. 2, the detection module 40 comprises a scanning delay line 41 and a sensor 42, wherein the scanning delay line 41 is arranged on the optical path of the detection light pulse 3 for scanning and extending the time of flight of the detection light pulse 3. Since the pump light pulse 4 passes through the modulation module and interacts with the sample 34, its time of flight to reach the sensor 42 is long, and in order to enable the probe light pulse 3 to reach the sensor 42 simultaneously with the pump light pulse 4, it is necessary to lengthen the optical path of the probe light pulse and increase the time of flight of the pulse light. In addition, based on the coherence principle of the terahertz spectrum technology, in order to measure the whole time domain electric field of the generated terahertz pulse, the detection light pulse 3 needs to be controlled by a stepping motor or other modes on the original basis, so that the flight time of the detection light pulse 3 is increased or reduced in a scanning manner, and the terahertz pulse time domain waveform can be completely detected. The sensor 42 is disposed at the optical path aggregation position of the modulated vortex terahertz pulse 6 and the detection light pulse 3, and is used for detecting the time domain signal of the modulated vortex terahertz pulse 6, converting the time domain signal into an electrical signal and outputting the electrical signal to the control module 50.
In other embodiments, the sensor 42 may preferably include an electro-optic crystal, a 1/4 wave plate, a Wollaston prism, a photodiode detector, and circuit components. The detection light pulse 3 electro-optical crystal passes through a 1/4 wave plate and a Wollaston prism to obtain two beams of light with mutually perpendicular polarization directions, and the two beams of light are detected by a light-emitting diode detector and differentially output through a circuit component. In other embodiments, the sensor 42 may include a photoconductive antenna, a 1/4 wave plate, a Wollaston prism, a photodiode detector, and circuit components.
In this embodiment, the control module 50 includes a controller and a lock-in amplifier, where the controller is electrically connected to the spatial light modulator 33 and is used to control the modulation frequency of the spatial light modulator 33, the signal input end of the lock-in amplifier is electrically connected to the signal output end of the sensor 42 and is used to receive the electrical signal, and the reference input end of the lock-in amplifier is electrically connected to the controller and is used to set the reference frequency of the lock-in amplifier;
when the controller controls the spatial light modulator 33 to rotate at the frequency f, the wavefront of the terahertz pulse containing various vortex components is modulated, the detected signal is a partial wavefront signal of the terahertz pulse transmitted through the notch of the disk of the spatial light modulator, and the position of the notch is periodically moved due to the rotation of the spatial light modulator, and the detected wavefront signal is also periodically changed accordingly. When the reference frequency of the phase-locked amplifier is set as the modulating frequency f of the controller, after the phase-locked amplifier performs phase locking on the waveform of the signal end, the output waveform is a signal which appears in the input signal at the frequency f and is a plane wave signal with vortex of zero; when the reference frequency of the phase-locked amplifier is set to be L times of the modulation frequency f of the controller, after the phase-locked amplifier performs phase locking on the waveform of the signal end, the output waveform is a signal which appears in f.L frequency in the input signal, and as the topological charge phase of the vortex beam is distributed according to the polar coordinate period, the partial signal is the terahertz pulse vortex signal with the topological charge number of L, and the time domain waveform of all topological charge components in the vortex terahertz pulse can be obtained by matching with the scanning detection of the delay scanning line 41 as long as the numerical value of L is changed, and the phase information can also be obtained. The slope of the phase curve of the vortex wave of the topological charge number L is related to the topological charge sign, and the slope is positive and indicates that the topological charge is positive, and is negative otherwise.
Referring to fig. 3, fig. 3 is a schematic plan view of a spatial light modulator according to a second embodiment of the present invention. As shown in fig. 3, the spatial light modulator 33 is a circular sheet.
Such modulator flakes are made of an opaque material, such as metal, for terahertz light, with metallic aluminum in this embodiment. Since the equiphase of the vortex beam is in a three-dimensional spiral structure, and the phase of the vortex beam changes according to polar coordinates in the cross section of the beam, a notch 13 is arranged at the center of the slice edge for extracting the beam signals of the same topological load, and the notch 13 is symmetrical about the polar coordinates. In this embodiment, two sheets are used, each sheet has a semicircular hollow, the two sheets are hinged at the center, one of the sheets is rotated, and the opening angle of the spatial light modulator is changed, so that notch angle adjustment can be performed to adapt to different requirements of light intensity and measurement accuracy. In this embodiment, the notch angle may be set to 30 degrees, so as to detect the vortex optical time domain waveform of the low-order topological charge number, and when detecting the vortex optical of the high-order topological charge, the notch angle needs to be set to a smaller value, which depends on the detected topological charge number. Since the vortex terahertz pulse 5 shown in fig. 2 is normally incident to the spatial light modulator sheet, the sheet center is required to coincide with the beam center of the terahertz pulse for measurement accuracy. During measurement, the size of a notch angle is selected, the spatial light modulator is fixed on a support, the spatial light modulator is controlled by a controller to rotate at uniform speed around the central axis of the disc, and the rotation frequency is set by the controller. In other embodiments, the spatial light modulator may be other shapes.
The test of the embodiment of the invention does not respond to plane wave or spherical wave components in terahertz pulse light, and when the center of the space light modulator sheet is not coincident with vortex light singular points, or the terahertz pulse light intensity is asymmetric to the singular point distribution, a background signal exists in a measurement result.
In general, in the cylindrical coordinate system, a beam of eddy current with a single frequency topological charge number L can be expressed as:
E(r,θ,z,t)＝E0(r,θ,z)exp(-iLθ)exp(-ikz)exp(iωt)
when there are multiple frequency components, each with a different topological charge number of components, the vortex beam can be expressed as:
E(r,θ,z,t)＝∫∫E 0 (r,θ,z)exp(-iLθ)exp(-ikz)exp(iωt)dLdω
since the value of θ ranges from 0 to 2π, L is a discrete integer value, i.e., L=0, 1, 2, 3, 4, … (L=0 represents the non-vortex light component in the beam), which can be regarded as a periodic function with 2π as a period
Based on the detection principle of terahertz time-domain spectroscopy, the vortex wave detection method provided by the invention can realize detection of vortex light components with specific topological charge number L, effectively filters other vortex light components or non-vortex light components, and adopts a simple time-lapse scanning mode, wherein the vortex light beam expression is as follows:
E(r,θ,z,t)＝∫E 0 (r,θ,z)exp(-iLθ+θ 0 (ω))exp(-ikz)exp(iωt)dω
here θ 0 And (ω) is a phase difference corresponding to the vortex component having a frequency ω and a topological charge number L. And carrying out Fourier transformation on the waveform to obtain different frequency components in the light beam with the topological charge number L.
Compared with the prior art, the technical scheme provided by the invention has the beneficial effects that:
the method can detect vortex light components with various topological charges in vortex terahertz pulses, is particularly aimed at complex light beams with plane waves, spherical wave components or vortex light components with multi-topological charges, can accurately and quantitatively give out the relative intensity and relative phase of various components, and has important application value for terahertz spectrum analysis and development of relevant basic researches.
Referring to fig. 1 and fig. 4 together, fig. 4 is a flowchart of a measurement method according to an embodiment of the invention. As shown in fig. 4, the present invention further provides a method for measuring vortex waves based on terahertz time-domain spectroscopy, which includes:
s1, generating far infrared femtosecond laser pulses by a femtosecond laser;
s2, splitting the far infrared femtosecond laser pulse into a pumping light pulse and a detection light pulse by a first beam splitter;
s3, the generation module receives the pumping light pulse light and generates vortex terahertz pulses;
s4, periodically modulating the wave surface of the vortex terahertz pulse by a modulation module;
s5, the detection module comprises a scanning delay line and a sensor, wherein the scanning delay line scans and prolongs the flight time of the detection light pulse, and the sensor detects the time domain signal of the modulation wave surface of the vortex terahertz pulse and converts the time domain signal into an electric signal;
s6, the control module comprises a controller and a phase-locked amplifier, the modulation frequency of the spatial light modulator is controlled through the controller, and the electric signal is received through the phase-locked amplifier;
and S7, setting the reference frequency of the phase-locked amplifier to be L times of the modulation frequency of the controller, so that the phase-locked amplifier outputs vortex terahertz pulse time-domain signals with the topological charge of L.
Wherein the laser pulse time width of the infrared femtosecond laser pulse 7 is equal to the laser pulse time width of the femtosecond (10 -15 s) magnitude, the repetition frequency is 80MHz, the peak power is 2MW. The infrared femtosecond laser pulse 7 has strong enough peak power, and forms a pumping light pulse 4 and a detection light pulse 3 after being split by the first beam splitter 2. The pump light pulse 4 excites terahertz light by the generation module 20, and the ultra-short pulse width in the femtosecond order is picoseconds (10 -12 s) of the magnitude of terahertz pulse width, and coherent detection of the electric field intensity of the terahertz pulse can be performed by using the ultra-short pulse.
The first beam splitter 2 splits the far infrared femtosecond laser pulse 7 into a pump light pulse 4 and a detection light pulse 3, the generation module receives the pump light pulse 4 and generates a vortex terahertz pulse 5, and the detection module 40 receives the detection light pulse 3 for detection. The detection module 40 aggregates the pump light pulse 4 and the detection light pulse 3, and the optical path lengths of the pump light pulse 4 and the detection light pulse 3, i.e. the flight time of the two pulses is required to be strictly equal by the system.
Referring to fig. 2 again, the modulation module 30 performs periodicity on the wave surface of the modulated vortex terahertz pulse 5. As shown in fig. 2, the modulation module 30 includes a spatial light modulator 33, a first parabolic mirror 31, a second parabolic mirror 32, and a sample 34. The wavefront of the terahertz light is controlled by the first parabolic mirror 31 and the second parabolic mirror 32, for example, the vortex terahertz pulse is focused or collimated so that the wavefront of the terahertz pulse falls within the modulation range of the spatial light modulator 33, or the terahertz pulse is sufficiently acted upon by the sample 34.
The detection module 40 comprises a scanning delay line 41 and a sensor 42, wherein the scanning delay line 41 scans and extends the time of flight of the detection light pulse 3. Since the pump light pulse 4 passes through the modulation module and interacts with the sample 34, its time of flight to reach the sensor 42 is long, and in order to enable the probe light pulse 3 to reach the sensor 42 simultaneously with the pump light pulse 4, it is necessary to lengthen the optical path of the probe light pulse and increase the time of flight of the pulse light. The sensor 42 is disposed at the optical path aggregation position of the modulated vortex terahertz pulse 6 and the detection light pulse 3, so as to detect the time domain signal of the modulated vortex terahertz pulse 6, and convert the time domain signal into an electrical signal to output to the control module 50.
As shown in fig. 2, the control module 50 includes a controller and a lock-in amplifier, the controller is electrically connected with the spatial light modulator 33 to control the modulation frequency of the spatial light modulator 33, a signal input end of the lock-in amplifier is electrically connected with a signal output end of the sensor 42 for receiving an electrical signal, and a reference input end of the lock-in amplifier is electrically connected with the controller for setting the reference frequency of the lock-in amplifier;
when the controller controls the spatial light modulator 33 to rotate at the frequency f, the wavefront of the terahertz pulse containing various vortex components is modulated, the detected signal is a partial wavefront signal of the terahertz pulse transmitted through the notch of the disk of the spatial light modulator, and the position of the notch is periodically moved due to the rotation of the spatial light modulator, and the detected wavefront signal is also periodically changed accordingly. When the reference frequency of the phase-locked amplifier is set as the modulating frequency f of the controller, after the phase-locked amplifier performs phase locking on the waveform of the signal end, the output waveform is a signal which appears in the input signal at the frequency f and is a plane wave signal with vortex of zero; when the reference frequency of the phase-locked amplifier is set to be L times of the modulation frequency f of the controller, after the phase-locked amplifier performs phase locking on the waveform of the signal end, the output waveform is a signal which appears in f.L frequency in the input signal, and as the topological charge phase of the vortex beam is distributed according to the polar coordinate period, the partial signal is the terahertz pulse vortex signal with the topological charge number of L, and the time domain waveform of all topological charge components in the vortex terahertz pulse can be obtained by matching with the scanning detection of the delay scanning line 41 as long as the numerical value of L is changed, and the phase information can also be obtained. The slope of the phase curve of the vortex wave of the topological charge number L is related to the topological charge sign, and the slope is positive and indicates that the topological charge is positive, and is negative otherwise.
While the preferred embodiment of the present invention has been described in detail, the present invention is not limited to the embodiments, and those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention, and these equivalent modifications or substitutions are included in the scope of the present invention as defined in the appended claims.
Claims (8)
1. The utility model provides a measurement system of vortex topology state of charge based on terahertz time-domain spectroscopy technique which characterized in that includes:
the femtosecond laser is used for providing far infrared femtosecond laser pulses;
the first beam splitter is used for splitting the far infrared femtosecond laser pulse into a pumping light pulse and a detection light pulse;
the generation module is arranged on the pump light pulse optical path and is used for generating vortex terahertz pulses;
the modulation module comprises a spatial light modulator, wherein the spatial light modulator is a sheet made of an opaque material of terahertz light, a notch is arranged in the center of the sheet, vortex terahertz pulses are normally incident to the sheet, the center of the sheet coincides with the beam center of the vortex terahertz pulses, and the sheet rotates around a central shaft at uniform speed and is used for periodically modulating the wave surface of the vortex terahertz pulses;
the detection module comprises a scanning delay line and a sensor, wherein the scanning delay line is arranged on the detection light pulse light path and is used for scanning and prolonging the flight time of the detection light pulse, and the sensor is arranged at the light path aggregation position of the modulated vortex terahertz pulse and the detection light pulse and is used for detecting the time domain signal of the modulation wave surface of the vortex terahertz pulse and converting the time domain signal into an electric signal for output;
the control module comprises a controller and a phase-locked amplifier, wherein the controller is electrically connected with the spatial light modulator and is used for controlling the modulation frequency of the spatial light modulator to enable the spatial light modulator to rotate at the modulation frequency, the signal input end of the phase-locked amplifier is electrically connected with the signal output end of the sensor and is used for receiving the electric signal, and the reference input end of the phase-locked amplifier is electrically connected with the controller so as to set the reference frequency of the phase-locked amplifier;
when the reference frequency of the phase-locked amplifier is set to be L times of the modulation frequency of the controller, the phase-locked amplifier outputs vortex terahertz pulse time-domain signals with the topology charge of L.
2. The terahertz time-domain spectroscopy-based vortex topology state-of-charge measurement system according to claim 1, wherein the generation module comprises a first beam control element, a terahertz source device, a filter and an S-wave plate, the pump light pulses are converged on the terahertz source device through the first beam control element, the terahertz source device is excited by the pump light pulses to generate terahertz pulses, the filter is used for filtering residual pump light, and the S-wave plate is used for modulating the terahertz pulses to vortex terahertz pulses.
3. The terahertz time-domain spectroscopy-based vortex topology state of charge measurement system of claim 1, wherein the modulation module further comprises a second beam control element for controlling the vortex terahertz pulse to focus or collimate.
4. The terahertz time-domain spectroscopy technology-based vortex topology state of charge measurement system according to claim 1, wherein the sensor comprises an electro-optic crystal, a 1/4 wave plate, a Wollaston prism, a light diode detector and a circuit component, and the detection light pulse passes through the electro-optic crystal modulated by the vortex terahertz pulse, then passes through the 1/4 wave plate and the Wollaston prism to obtain two beams of light with mutually perpendicular polarization directions, and the two beams of light are detected by the light diode detector and differentially output through the circuit component.
5. The method for measuring the vortex topology state of charge based on the terahertz time-domain spectroscopy technology is characterized by comprising the following steps of:
s1, generating far infrared femtosecond laser pulses by a femtosecond laser;
s2, splitting the far infrared femtosecond laser pulse into a pumping light pulse and a detection light pulse by a first beam splitter;
s3, the generation module receives the pump light pulse and generates vortex terahertz pulse;
s4, the modulation module comprises a spatial light modulator, and the spatial light modulator periodically modulates the wave surface of the vortex terahertz pulse;
s5, the detection module comprises a scanning delay line and a sensor, wherein the scanning delay line scans and prolongs the flight time of the detection light pulse, and the sensor detects the time domain signal of the modulation wave surface of the vortex terahertz pulse and converts the time domain signal into an electric signal;
s6, the control module comprises a controller and a phase-locked amplifier, the controller is used for controlling the modulation frequency of the spatial light modulator, so that the spatial light modulator rotates at the modulation frequency, and the phase-locked amplifier is used for receiving the electric signal;
and S7, setting the reference frequency of the phase-locked amplifier to be L times of the modulation frequency of the controller, so that the phase-locked amplifier outputs vortex terahertz pulse time-domain signals with the topological charge of L.
6. The method for measuring the vortex topology state of charge based on the terahertz time-domain spectroscopy of claim 5, wherein the generation module comprises a first beam control element, a terahertz source device, a filter and an S-wave plate, the beam control element converges the pump light pulse to the terahertz source device, the pump light pulse excites the terahertz source device and generates terahertz pulses, the filter filters out the remaining pump light, and the S-wave plate modulates the terahertz pulses into vortex terahertz pulses.
7. The method for measuring the vortex topology state of charge based on the terahertz time-domain spectroscopy of claim 5, wherein the modulation module further comprises a second beam control element, and the second beam control element controls the vortex terahertz pulse focusing or collimation.
8. The method for measuring the vortex topology state of charge based on the terahertz time-domain spectroscopy of claim 5, wherein the sensor comprises an electro-optical crystal, a 1/4 wave plate, a Wollaston prism, a light diode detector and a circuit component, and the detection light pulse passes through the electro-optical crystal modulated by the vortex terahertz pulse, then passes through the 1/4 wave plate and the Wollaston prism to obtain two beams of light with mutually perpendicular polarization directions, and is detected by the light diode detector and differentially output by the circuit component.
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