CN110579280A - Vortex wave measuring system and method based on terahertz time-domain spectroscopy technology - Google Patents
Vortex wave measuring system and method based on terahertz time-domain spectroscopy technology Download PDFInfo
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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
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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
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Abstract
The invention discloses a vortex wave measuring system based on a terahertz time-domain spectroscopy technology, which comprises: the terahertz wave scanning 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 inputs a modulation frequency signal of the spatial light modulator and is used for phase-locking amplifying signals of different vortex topology charge components, and time domain waveform scanning of the vortex topology charge components 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. The vortex wave measuring system and method provided by the invention are suitable for the terahertz time-domain spectroscopy technology, and compared with the traditional pinhole limitation and CCD observation method, the system and method provided by the invention 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 system and a method for measuring vortex waves based on a terahertz time-domain spectroscopy technology.
Background
Terahertz waves are a general term for electromagnetic waves of a specific frequency band, and the spectrum of the terahertz waves is located between microwaves and infrared rays and is in the transition stage from macroscopic electronics to microscopic photonics. Generally, it refers to an electromagnetic wave having a frequency range of 0.3 to 3THz, but an electromagnetic wave in the vicinity of 0.1 to 10THz is sometimes called a terahertz wave (1THz 10)12Hz)。
the terahertz time-domain spectroscopy technology is a new technology of a terahertz generation and detection method based on ultrafast optoelectronics, and compared with a traditional Fourier infrared transform spectrometer, the terahertz time-domain spectroscopy technology can provide not only amplitude information of a spectrum, but also corresponding phase information, and provides a good foundation for research and development in the terahertz technology field.
vortex beams are a type of beam that has an equal phase plane in a spiral shape. Because the intensity coherence caused by the uncertainty of the phase information of the spiral wave front is cancelled, the phase at the center of the vortex light beam cannot be determined, so that a phase singularity exists at the center, and the interface light intensity is distributed in a ring shape. 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)
where exp (il θ) is a phase term, θ is a space azimuth, L is an angular quantum number, also called a topological charge number or a topological charge number, and theoretically may be any integer. Photons of a vortex beam with a topological charge number L carry orbital angular momentum of(is a reduced planck constant). Thus, the vortex topological charge number is related to the photon orbital angular momentum.
Generally, the vortex light beam detection adopts an interference method, the light beam to be detected is interfered with plane waves or spherical waves, and the vortex topological charge number of the light to be detected is judged according to interference patterns. This method is often used for detecting continuous light, is not highly sensitive, and is difficult to quantitatively distinguish components by interferometry when a plane wave component or a multi-topology charge component is present in a light beam. In the terahertz technology, for a terahertz time-domain spectroscopy system, the conventional method for measuring the topological charge number is to limit point-by-point scanning of a wave surface by using a small hole, but the method is very time-consuming. Alternatively, a CCD can be used to directly observe the wave front, but the signal-to-noise ratio is low. And both of the two modes are not suitable for multi-topology load beams, and the beam components of each topology load cannot be analyzed.
Disclosure of Invention
Based on the above, there is a need to provide a system and a method capable of analyzing vortex waves of a plurality of different topological charge components of a vortex terahertz pulse, and having a fast measurement speed and a high accuracy.
The embodiment of the invention provides a system for measuring vortex topological charge state based on terahertz time-domain spectroscopy technology, which comprises:
A femtosecond laser for providing far infrared femtosecond laser pulses;
The first beam splitter is used for splitting the far infrared femtosecond laser pulse into a pump light pulse and a detection light pulse;
the generating module is arranged on a pumping light pulse light path and 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 optical path and is used for scanning and prolonging the flight time of a detection light pulse, and the sensor is arranged at the light path convergence part of the modulated vortex terahertz pulse and the detection light pulse and is used for detecting a time domain signal of a modulation wave surface of the vortex terahertz pulse and converting the time domain signal into an electric signal for outputting;
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, 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 an electric signal, and the reference input end of the phase-locked amplifier is electrically connected with the controller and is 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 a vortex terahertz pulse time-domain signal with the topological charge of L.
Preferably, the spatial light modulator is a sheet made of an opaque material of terahertz light, a notch is formed in the center of the sheet, the terahertz pulse is normally incident on the sheet, the center of the sheet is overlapped with the center of a light beam of the terahertz pulse, the sheet can rotate around a central shaft, and the rotation frequency of the sheet is controlled by the controller.
Preferably, the generation module comprises a beam control element, a terahertz source device, a filter and an S-wave plate, the pumping light pulse is converged on the terahertz source device through the beam control element, the terahertz source device is excited by the pumping light pulse to generate terahertz pulses, the filter is used for filtering residual pumping light, and the S-wave plate is used for modulating the terahertz pulses to enable the terahertz pulses to become vortex terahertz pulses.
Preferably, the modulation module further comprises a terahertz beam-shrinking element for shrinking the vortex terahertz pulse so that the whole wave surface of the vortex terahertz pulse falls into the spatial light modulator.
Preferably, the sensor comprises an electro-optical crystal, an 1/4 wave plate, a Wollaston prism, a photodiode detector and a circuit assembly, wherein detection light pulses pass through the electro-optical crystal modulated by vortex terahertz pulses, pass through a 1/4 wave plate and the Wollaston prism to obtain two beams of light with mutually vertical polarization directions, and are detected by the photodiode detector and differentially output through the circuit assembly.
The embodiment of the invention also provides a method for measuring the vortex topological charge state 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 pump light pulse and a detection light pulse by a first beam splitter;
S3, a generating module receives the pump 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, the scanning delay line scans and prolongs the flight time of the detection light pulse, and the sensor detects a time domain signal of a 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 controls the modulation frequency of the spatial light modulator, and the phase-locked amplifier receives the electric signal;
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 topological charge of L.
Preferably, the generation 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 out 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 the focusing or collimation of the vortex terahertz pulses.
Preferably, the sensor comprises an electro-optical crystal, an 1/4 wave plate, a Wollaston prism, a photodiode detector and a circuit assembly, wherein 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 vertical polarization directions, and the two beams of light are detected by the photodiode detector and differentially output through the circuit assembly.
according to the vortex topological charge state measuring system and method of the terahertz time-domain spectroscopy technology, the wave surface of vortex terahertz pulses is modulated by using the spatial light modulator with polar coordinate graphs, and relevant signals, namely time-domain signals of vortex topological charge states related to the harmonic times of the spatial light modulation frequency, are separated from the modulated vortex terahertz pulses by matching with the phase-locked separation and amplification principle of a phase-locked amplifier. The system can measure vortex topological charge components of any shape, including amplitude and phase information, the precision of the system is not influenced by a detector, but is determined by the design shape of the spatial light modulator, and the measurement speed of the system is far higher than that of the traditional pinhole limiting method and is more accurate.
Drawings
FIG. 1 is a system block diagram of an embodiment of the invention;
FIG. 2 is a schematic diagram of an optical path structure according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a planar structure 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 invention.
The reference numbers in the figures denote:
1. A laser; 2. a first beam splitter; 3. detecting a light pulse; 4. pumping light pulses; 5. vortex terahertz pulses; 6. modulated vortex terahertz pulses; 7. far infrared femtosecond laser pulse; 13. a notch; 20. a generation 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 the embodiments and features of the embodiments may be combined with each other without conflict.
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 system for measuring vortex topological charge state based on terahertz time-domain spectroscopy technology, which comprises: a femtosecond laser 1 for providing far infrared femtosecond laser pulses 7; the first beam splitter 2 is used for splitting the far infrared femtosecond laser pulse into a pump light pulse 4 and a detection light pulse 3; a generation module 20 for generating vortex terahertz pulses 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 pulse 3; and a control module 50 for controlling the modulation frequency of the spatial light modulator to modulate the light pulse.
Wherein the laser pulse time width of the infrared femtosecond laser pulse 7 is femtosecond (10)-15s) magnitude, repetition frequency of 80MHz, peak power of 26W is added. The infrared femtosecond laser pulse 7 has strong enough peak power, and forms a pump light pulse 4 and a detection light pulse 3 after being split by the first beam splitter 2. The pumping light pulse 4 excites the terahertz light by the generating module 20, and the ultrashort pulse width of the femtosecond order is compared with picosecond (10)-12s) magnitude terahertz pulse width, can completely measure the time domain waveform of the terahertz pulse, and is an indispensable part of the terahertz time domain spectrum 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 detection light pulse 3 pass through the aggregation in the detection module 40, the optical path lengths of the pump light pulse 4 and the detection light pulse 3, that is, the flight times of the two pulses are strictly equal to each other. And the generating module 20 is arranged on the optical path of the pump optical pulse 4 and is used for responding to the pump optical pulse 4 and generating the vortex terahertz pulse 5.
in other embodiments, the generation module 20 may include a beam control element, a terahertz source device, a filter and an S-wave plate, the pump light pulse 4 is converged to the terahertz source device through the beam control element, the terahertz source device is excited by the pump light pulse 4 to generate a terahertz pulse, the filter is configured to filter out the remaining pump light pulse 4, and the S-wave plate is configured to modulate the terahertz pulse to become the vortex terahertz pulse 5. The terahertz source device can be selected from a photoconductive antenna or an electro-optic crystal and the like.
referring to fig. 2, fig. 2 is a schematic diagram of an optical path structure according to a second embodiment of the present invention. And 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 for controlling 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 the terahertz pulse to sufficiently interact with the sample 34.
Because the wavelength of the terahertz wave is far smaller than the size of the laboratory optical device, for example, the wavelength of the terahertz wave of 1THz is 0.3mm, and the size of the general optical device is between 3 cm and 5cm, the terahertz wave can be controlled as a common light wave, for example, a parabolic mirror with a metal thin 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 are provided, wherein the first parabolic mirror 31 focuses the terahertz pulse, so that the terahertz pulse 5 penetrates through the spatial light modulator 33 and is converged at a sample 34. After the terahertz pulse 5 interacts with the sample 34, it is focused again by the second parabolic mirror 32 and focused at the sensor 42.
in other embodiments, the modulation module 30 may include a terahertz lens or other beam steering element to modulate the terahertz wave surface to change the beam shape. As shown in fig. 2, after the vortex terahertz pulse 5 acts on the sample 34, the vortex topological charge components in the beam change, and the measurement system provided by the embodiment of the present invention can detect the changes, including the change of the time-domain waveform and the change of the phase of each vortex topological charge component, and can obtain the terahertz optical parameters of the sample 34 through the Kramer-Kronig relational expression.
As shown in fig. 2, the detection module 40 includes a scanning delay line 41 and a sensor 42, wherein the scanning delay line 41 is disposed on the optical path of the detection light pulse 3 for scanning and extending the flight time of the detection light pulse 3. Since the pump light pulses 4 have a long flight time to reach the sensor 42 due to the interaction with the sample 34 and passing through the modulation module, the optical path of the probe light pulses needs to be extended to increase the flight time of the pulse light in order to allow the probe light pulses 3 to reach the sensor 42 simultaneously with the pump light pulses 4. In addition, based on the coherence principle of the terahertz spectrum technology, in order to measure the entire time domain electric field of the generated terahertz pulse, the detection optical pulse 3 is controlled by a stepping motor or other methods on the original basis, so that the flight time scanning type is increased or reduced, and the terahertz pulse time domain waveform can be completely detected. The sensor 42 is disposed at a light path convergence of the modulated vortex terahertz pulse 6 and the detection light pulse 3, and is configured to detect a time-domain signal of the modulated vortex terahertz pulse 6, convert the time-domain signal into an electrical signal, and output the electrical signal to the control module 50.
in other embodiments, the sensor 42 may preferably include an electro-optic crystal, 1/4 wave plates, a Wollaston prism, a photodiode detector, and circuit components. The detection light pulse 3 electro-optical crystal is processed by 1/4 wave plates and a Wollaston prism to obtain two beams of light with mutually vertical polarization directions, and the two beams of light are detected by a light diode detector and differentially output through a circuit assembly. In still other embodiments, sensor 42 may include a photoconductive antenna, 1/4 waveplates, a Wollaston prism, a photodiode detector, and circuit components.
In this embodiment, the control module 50 includes a controller and a lock-in amplifier, the controller is electrically connected to the spatial light modulator 33 for controlling the modulation frequency of the spatial light modulator 33, a signal input terminal of the lock-in amplifier is electrically connected to a signal output terminal of the sensor 42 for receiving an electrical signal, a reference input terminal of the lock-in amplifier is electrically connected to the controller for setting a reference frequency of the lock-in amplifier;
When the controller controls the spatial light modulator 33 to rotate at the frequency f, the wave surface of the terahertz pulse containing various vortex components is modulated, the detected signal is a partial wave surface signal of the terahertz pulse penetrating through the notch of the disk of the spatial light modulator, and the detected wave surface signal is also periodically changed correspondingly because the notch position is periodically moved due to the rotation of the spatial light modulator. When the reference frequency of the phase-locked amplifier is set as the modulation frequency f of the controller, the phase-locked amplifier performs phase locking on the waveform of the signal end, and the output waveform is a signal which appears in the input signal in the frequency f and is a plane wave signal with zero vortex; when the reference frequency of the phase-locked amplifier is set to be L times of the modulation frequency f of the controller, the phase-locked amplifier phase-locks the waveform of the signal end, the output waveform is a signal which appears at f.L frequency in the input signal, the vortex light beam topological charge phase is distributed according to the polar coordinate period, the part of the signal is a terahertz pulse vortex signal with the topological charge number of L, the time domain waveforms 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 value of L is changed, and the phase information can also be obtained. The slope of the phase curve of the vortex wave with the topological charge number L is related to the sign of the topological charge, and the slope is positive to indicate that the topological charge is positive, and the slope 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 invention. As shown in fig. 3, the spatial light modulator 33 is a circular sheet.
such a modulator sheet is made of an opaque material for terahertz light, such as metal, and the present embodiment uses metallic aluminum. Because the equiphase surface of the vortex light beam is in a three-dimensional spiral structure, the phase of the vortex light beam changes according to the polar coordinate in the cross section of the light beam, a notch 13 is arranged at the center of the edge of the sheet in order to extract the light beam signal of the same topological load, and the notch 13 is symmetrical about the polar coordinate. In the embodiment, two sheets are used, each sheet is provided with a semicircular hollow part, the two sheets are connected in a twisted mode at the center, one sheet is rotated, the opening angle of the spatial light modulator is changed, and therefore the notch angle can be adjusted to meet different light intensity and measurement accuracy requirements. In this embodiment, the notch angle may be set to 30 degrees, and is used to detect the vortex optical time domain waveform of the low-order topological charge number, and when detecting the vortex optical time domain waveform of the high-order topological charge, the notch angle needs to be set to a smaller value, which is specifically determined by the detected topological charge number. Since the vortex terahertz pulse 5 shown in fig. 2 is normally incident to the spatial light modulator wafer, for measurement accuracy, the center of the wafer is required to coincide with the beam center of the terahertz pulse. During measurement, the angle of a notch is selected, the spatial light modulator is fixed on a support, the controller controls the spatial light modulator to rotate around the central axis of the disc at a uniform speed, 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 the terahertz pulse light, and when the center of the spatial light modulator slice is not coincident with the singular point of vortex light or the distribution of the terahertz pulse light intensity to the singular point is asymmetric, a background signal exists in the measurement result.
Generally, in a cylindrical coordinate system, a bundle of vortex rotation 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, the vortex beam can be expressed as:
E(r,θ,z,t)＝∫∫E0(r,θ,z)exp(-iLθ)exp(-ikz)exp(iωt)dLdω
since θ can be regarded as a periodic function with a period of 2 pi, with a range of 0 to 2 pi, L has discrete integer values, i.e., L ═ 0, 1, 2, 3, 4, … (L ═ 0 represents the non-vortex light component in the beam)
based on the detection principle of the terahertz time-domain spectroscopy technology, the vortex wave detection method provided by the invention can realize detection of vortex light components with specific topological charge number L, and effectively filters other vortex optical components or non-vortex optical components, and through a simple time-delay scanning, the vortex light beam expression is as follows:
E(r,θ,z,t)＝∫E0(r,θ,z)exp(-iLθ+θ0(ω))exp(-ikz)exp(iωt)dω
Where theta is0And (omega) is the phase difference corresponding to the vortex component with the frequency of omega and the topological charge number of L. The waveform is Fourier transformed to obtain different frequency components in the light beam with topological charge number L.
Compared with the prior art, the technical scheme provided by the invention has the beneficial effects that:
The method can realize detection of vortex light components of various topological charge numbers in the vortex terahertz pulse, can accurately and quantitatively give relative intensity and relative phase of various components particularly for the existence of plane waves and spherical wave components or complex light beams containing multi-topological charge number vortex light components, and has important application value for development of terahertz spectrum analysis and related basic research.
Referring to fig. 1 and 4, 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 a vortex wave based on the 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 pump light pulse and a detection light pulse by a first beam splitter;
S3, a generating module receives the pump 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, the scanning delay line scans and prolongs the flight time of the detection light pulse, and the sensor detects a time domain signal of a 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 controls the modulation frequency of the spatial light modulator, and the phase-locked amplifier receives the electric signal;
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 topological charge of L.
wherein the laser pulse time width of the infrared femtosecond laser pulse 7 is femtosecond (10)-15s) magnitude, repetition frequency of 80MHz, peak power of 2 MW. The infrared femtosecond laser pulse 7 has strong enough peak power, is split by the first beam splitter 2 to form a pumppump light pulse 4, probe light pulse 3. The pumping light pulse 4 excites the terahertz light through the generation module 20, and the ultra-short pulse width of the femtosecond order is picosecond (10)-12s) magnitude terahertz pulse width, and coherent detection can be carried out on the terahertz pulse electric field intensity by utilizing the ultrashort 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 system, require that the flight times of the two pulses are strictly equal.
referring back to fig. 2, the modulation module 30 performs periodicity on the wave surface of the modulated vortex terahertz pulses 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 wave surface of the terahertz light is controlled by the first parabolic mirror 31 and the second parabolic mirror 32, for example, a vortex terahertz pulse is focused or collimated so that the wave surface of the terahertz pulse falls within the modulation range of the spatial light modulator 33, or the terahertz pulse is sufficiently acted on the sample 34 by focusing.
the detection module 40 comprises a scanning delay line 41 and a sensor 42, wherein the scanning delay line 41 scans and extends the flight time of the detection light pulse 3. Since the pump light pulses 4 have a long flight time to reach the sensor 42 due to the interaction with the sample 34 and passing through the modulation module, the optical path of the probe light pulses needs to be extended to increase the flight time of the pulse light in order to allow the probe light pulses 3 to reach the sensor 42 simultaneously with the pump light pulses 4. The sensor 42 is arranged at the light path convergence of the modulated vortex terahertz pulse 6 and the detection light pulse 3 to detect the time-domain signal of the modulated vortex terahertz pulse 6, convert the time-domain signal into an electric signal and output the electric signal 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 to the spatial light modulator 33 for controlling the modulation frequency of the spatial light modulator 33, a signal input terminal of the lock-in amplifier is electrically connected to a signal output terminal of the sensor 42 for receiving an electrical signal, and a reference input terminal of the lock-in amplifier is electrically connected to 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 wave surface of the terahertz pulse containing various vortex components is modulated, the detected signal is a partial wave surface signal of the terahertz pulse penetrating through the notch of the disk of the spatial light modulator, and the detected wave surface signal is also periodically changed correspondingly because the notch position is periodically moved due to the rotation of the spatial light modulator. When the reference frequency of the phase-locked amplifier is set as the modulation frequency f of the controller, the phase-locked amplifier performs phase locking on the waveform of the signal end, and the output waveform is a signal which appears in the input signal in the frequency f and is a plane wave signal with zero vortex; when the reference frequency of the phase-locked amplifier is set to be L times of the modulation frequency f of the controller, the phase-locked amplifier phase-locks the waveform of the signal end, the output waveform is a signal which appears at f.L frequency in the input signal, the vortex light beam topological charge phase is distributed according to the polar coordinate period, the part of the signal is a terahertz pulse vortex signal with the topological charge number of L, the time domain waveforms 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 value of L is changed, and the phase information can also be obtained. The slope of the phase curve of the vortex wave with the topological charge number L is related to the sign of the topological charge, and the slope is positive to indicate that the topological charge is positive, and the slope is negative otherwise.
while the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (9)
1. a vortex topology charge state measuring system based on a terahertz time-domain spectroscopy technology is characterized by comprising:
A femtosecond laser for providing far infrared femtosecond laser pulses;
The first beam splitter is used for splitting the far infrared femtosecond laser pulse into a pump light pulse and a detection light pulse;
The generating module is arranged on the pumping light pulse optical path and 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, the scanning delay line is arranged on the optical path of the detection optical pulse and is used for scanning and prolonging the flight time of the detection optical pulse, and the sensor is arranged at the optical path convergence part of the modulated vortex terahertz pulse and the detection optical pulse and is used for detecting a time domain signal of a modulation wave surface of the vortex terahertz pulse and converting the time domain signal into an electric signal for outputting;
The control module comprises a controller and a phase-locked amplifier, the controller is electrically connected with the spatial light modulator and is 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 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 a vortex terahertz pulse time domain signal with the topological charge of L.
2. The system for measuring the vortex topological charge state based on the terahertz time-domain spectroscopy is characterized in that the spatial light modulator is a sheet made of opaque material of terahertz light, a notch is formed in the center of the sheet, the vortex terahertz pulse is normally incident on the sheet, and the center of the sheet is coincided with the center of a light beam of the vortex terahertz pulse.
3. The system for measuring the vortex topological charge state based on the terahertz time-domain spectroscopy is characterized in that the generating module comprises a first beam control element, a terahertz source device, a filter plate and an S wave plate, the pump light pulse is converged on the terahertz source device through the first beam control element, the terahertz source device is excited by the pump light pulse to generate the terahertz pulse, the filter plate is used for filtering out residual pump light, and the S wave plate is used for modulating the terahertz pulse to enable the terahertz pulse to become the vortex terahertz pulse.
4. The system for measuring the vortex topological charge state based on the terahertz time-domain spectroscopy is characterized in that the modulation module further comprises a second beam control element for controlling the focusing or collimation of the vortex terahertz pulses.
5. The system for measuring the vortex topological charge state based on the terahertz time-domain spectroscopy is characterized in that the sensor comprises an electro-optic crystal, an 1/4 wave plate, a Wollaston prism, a photodiode detector and a circuit assembly, wherein the detection light pulse passes through the electro-optic crystal modulated by the vortex terahertz pulse, then passes through a 1/4 wave plate and the Wollaston prism to obtain two beams of light with mutually vertical polarization directions, and the two beams of light are detected by the photodiode detector and differentially output through the circuit assembly.
6. A method for measuring vortex topological charge state based on a terahertz time-domain spectroscopy technology is characterized by comprising the following steps:
S1, generating far infrared femtosecond laser pulses by a femtosecond laser;
s2, splitting the far infrared femtosecond laser pulse into a pump light pulse and a detection light pulse by a first beam splitter;
S3, a generating module receives the pump light pulse 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, the scanning delay line scans and prolongs the flight time of the detection light pulse, and the sensor detects a time domain signal of a 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 controls the modulation frequency of the spatial light modulator, and the phase-locked amplifier receives the electric signal;
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 topological charge of L.
7. the method for measuring the vortex topological charge state based on the terahertz time-domain spectroscopy is characterized in that the generating module comprises a first beam control element, a terahertz source device, a filter plate 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 the terahertz pulse, the filter plate filters out the residual pump light, and the S wave plate modulates the terahertz pulse into the vortex terahertz pulse.
8. The method for measuring vortex topological charge state based on terahertz time-domain spectroscopy according to claim 6, wherein the modulation module further comprises a second beam control element, and the second beam control element controls the focusing or collimation of the vortex terahertz pulses.
9. the method for measuring the vortex topological charge state based on the terahertz time-domain spectroscopy is characterized in that the sensor comprises an electro-optic crystal, an 1/4 wave plate, a Wollaston prism, a photodiode detector and a circuit assembly, wherein the detection light pulse passes through the electro-optic crystal modulated by the vortex terahertz pulse, then passes through a 1/4 wave plate and the Wollaston prism to obtain two beams of light with mutually vertical polarization directions, and the two beams of light are detected by the photodiode detector and differentially output through the circuit assembly.
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