A multi-cycle terahertz pulse generation and detection system and method

Abstract

The application discloses a multi-period terahertz pulse generation and detection system and method, relates to the technical field of terahertz science, and generates pulse replication of laser pulse generation light through a multi-period terahertz pulse generation light path, excites a spin terahertz emitter, utilizes fast linear response to generate a pulse train containing multiple sub-pulses with adjustable time-domain intervals, adjusts the relative delay inside the replication unit of the multi-period terahertz pulse generation light path, changes the sub-pulse interval, flexibly modulates the number of periods of the final terahertz waveform, obtains a multi-period terahertz pulse, and the multi-period narrow-band terahertz pulse contains multiple complete oscillation periods in the time domain and corresponds to a terahertz electromagnetic wave pulse with extremely narrow spectral width in the frequency domain. The physical limitation of "device typing, i.e. frequency fixing" is fundamentally broken, the time-domain form of the terahertz pulse is flexibly regulated, and the tunability and flexibility of the multi-period terahertz pulse generation and detection system are improved.

Description

Technical Field

[0001] This application relates to the technical field of terahertz science, and more specifically, to a multi-cycle terahertz pulse generation and detection system and method. Background Technology

[0002] Multi-period narrowband terahertz pulses refer to terahertz electromagnetic wave pulses that contain multiple complete oscillation cycles in the time domain and correspond to an extremely narrow spectral width in the frequency domain. Multi-period narrowband terahertz pulses play a crucial role in cutting-edge fields such as high-order harmonic generation, ultrafast electron acceleration, next-generation high-speed wireless communication, and high-resolution terahertz spectroscopy.

[0003] Currently, the main methods for obtaining multi-cycle narrowband terahertz pulses in existing multi-cycle terahertz pulse systems include: chirped pulse beat frequency technology, optical rectification technology based on periodically polarized nonlinear crystals (such as periodically polarized lithium niobate (PPLN)), terahertz free-electron lasers, and cascading bandpass filter devices after broadband terahertz sources.

[0004] However, in chirped pulse beat frequency technology, due to the complexity of the optical path and the extremely high requirements for spatial alignment accuracy, the number of terahertz pulse periods generated is limited by the modulation depth of the incident spectrum, resulting in poor tunability. Furthermore, based on periodically polarized nonlinear crystals and physical filter devices, which are limited by fixed fabrication processes, the center frequency of the device is difficult to change once it is formed, resulting in a lack of system flexibility.

[0005] Therefore, improving the tunability and flexibility of multi-cycle terahertz pulse generation and detection systems is a problem that this application urgently needs to solve. Summary of the Invention

[0006] In view of this, this application discloses a multi-cycle terahertz pulse generation and detection system and method, which aims to achieve flexible control of the time domain morphology of terahertz pulses and improve the tunability and flexibility of the multi-cycle terahertz pulse generation and detection system.

[0007] To achieve the above objectives, the disclosed technical solution is as follows:

[0008] The first aspect of this application discloses a multi-cycle terahertz pulse generation and detection system, wherein the system is sequentially configured with a femtosecond laser source 1, a non-polarizing flat beam splitter 2, a multi-cycle terahertz pulse generation optical path, a terahertz collection module, and an electro-optic detection optical path; the multi-cycle terahertz pulse generation optical path is provided with a spin terahertz emitter 18.

[0009] The unpolarized flat beam splitter 2 is used to split the laser pulse output by the femtosecond laser source into a generating light and a probe light;

[0010] The multi-cycle terahertz pulse generation optical path is used to cascade pulse replication of the generated light to obtain a pulse train containing multiple sub-pulses, and to excite the spin terahertz emitter 18 to process the pulse train to obtain a multi-cycle terahertz pulse.

[0011] The terahertz collection module is used to guide the multi-cycle terahertz pulses to the electro-optic detection optical path;

[0012] The electro-optic detection optical path is used to adjust the time delay of the detection optical path and reconstruct the waveform of the multi-cycle terahertz pulse based on the adjusted detection light.

[0013] Optionally, the femtosecond laser source has a single-pulse energy of 9 millijoules and a wavelength of 800 nanometers.

[0014] Optionally, the multi-cycle terahertz pulse generation optical path includes multiple pulse replication units, a chopper 15, and a beam expander connected in sequence;

[0015] The plurality of pulse replication units are used to replicate the generated light into pulses to obtain a pulse train containing multiple sub-pulses;

[0016] The chopper 15 is used to modulate the pulse train;

[0017] The beam expanding system is used to expand the modulated pulse train.

[0018] Optionally, the plurality of pulse replication units include a broadband half-wave plate 7, a broadband polarization beam splitter cube 8, a delay line 11, and a polarization beam splitter 12.

[0019] The broadband half-wave plate 7 is used to adjust the ratio of the laser pulse; wherein the ratio is the ratio of the p-polarization component to the s-polarization component.

[0020] The broadband polarization beam splitter cube 8 is used to separate the adjusted laser pulse into p-polarization and s-polarization components.

[0021] The delay line 11 is used to adjust the optical path of the s-polarization component; wherein, the delay line 11 is arranged in the optical path of the s-polarization component;

[0022] The polarization beam splitter 12 is used to combine the adjusted s-polarization component and the p-polarization component to form a pulse train containing multiple sub-pulses.

[0023] Optionally, the beam expanding system includes a concave lens 16 and a convex lens 17;

[0024] When the focal length difference between the convex lens 17 and the concave lens 16 is equal to the actual distance between the concave lens 16 and the convex lens 17, the concave lens 16 and the convex lens 17 are used to convert the divergence state of the modulated pulse train into a collimated state so that the expanded pulse train is directly incident on the spin terahertz transmitter 18.

[0025] Optionally, the spin terahertz transmitter 18 has a ferromagnetic / heavy metal / substrate structure, and the spin terahertz transmitter 18 is placed in an in-plane magnetic field, specifically for generating multi-cycle terahertz pulses.

[0026] Optionally, the broadband half-wave plate 7 is specifically used to make the ratio of the p-polarized component and the s-polarized component equal, so that the sub-pulse energies corresponding to the p-polarized component and the s-polarized component are equal.

[0027] Optionally, the delay line 11 is specifically used to change the optical path of the s-polarization component by adjusting the position of the linear displacement stage, thereby continuously adjusting the time delay between the sub-pulses corresponding to the p-polarization component and the s-polarization component.

[0028] Optionally, the electro-optic detection optical path includes a delay line 22, an electro-optic crystal 24, a broadband quarter-wave plate 25, a Wollaston prism 26, and a balanced photodetector 29;

[0029] The delay line 22 is used to delay the detection optical path in time, and to make the multi-period terahertz pulse and the time-delayed detection optical path collinear and enter the electro-optic crystal 24;

[0030] The electro-optic crystal 24 is used to birefractively the collinear multi-period terahertz pulse and the time-delayed detection optical path, so that the detection optical path is ellipticized.

[0031] The broadband quarter-wave plate 25 is used to convert the ellipsoidal probe optical path into first orthogonal linearly polarized light;

[0032] The Wollaston prism 26 is used to convert the ellipsoidal probe light path into a second orthogonal linearly polarized light;

[0033] The balanced photodetector 29 is used to detect the signals of the first orthogonal line-polarized light and the second orthogonal line-polarized light through a lock-in amplifier.

[0034] The second aspect of this application discloses a method for generating and detecting multi-cycle terahertz pulses, the method being applied to the multi-cycle terahertz pulse generation and detection system described in any one of the first aspects above, the method comprising:

[0035] The unpolarized flat beam splitter 2 splits the laser pulse output from the femtosecond laser source 1 into a generating light and a probe light.

[0036] The generated light is cascaded and pulsed through the multi-cycle terahertz pulse generation optical path to obtain a pulse train containing multiple sub-pulses, and the spin terahertz emitter (18) is excited to process the pulse train to obtain a multi-cycle terahertz pulse.

[0037] The multi-cycle terahertz pulses are guided to the electro-optic detection optical path by the terahertz collection module;

[0038] The time delay of the probe light is adjusted by the electro-optic probe light path, and the waveform of the multi-cycle terahertz pulse is reconstructed based on the adjusted probe light.

[0039] As can be seen from the above technical solution, this application discloses a multi-cycle terahertz pulse generation and detection system and method. The system replicates the generated laser pulse through a multi-cycle terahertz pulse generation optical path, exciting a spin terahertz emitter. Utilizing its fast linear response, it generates a pulse train containing multiple sub-pulses with adjustable time-domain intervals. By adjusting the relative delay within the replication unit of the multi-cycle terahertz pulse generation optical path, the sub-pulse interval can be changed, thereby flexibly modulating the number of cycles of the final terahertz waveform to obtain a multi-cycle terahertz pulse. This multi-cycle narrowband terahertz pulse contains multiple complete oscillation cycles in the time domain and corresponds to an extremely narrow spectral width terahertz electromagnetic wave pulse in the frequency domain. This fundamentally breaks through the physical limitation of "fixed device design, i.e., fixed frequency," enabling flexible control of the time-domain morphology of the terahertz pulse and improving the tunability and flexibility of the multi-cycle terahertz pulse generation and detection system. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of this application. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0041] Figure 1 This is a schematic diagram of the structure of a multi-cycle terahertz pulse generation and detection system disclosed in an embodiment of this application;

[0042] Figure 2 The following are typical time-domain waveforms and corresponding frequency-domain spectra of the multi-cycle terahertz pulses disclosed in the embodiments of this application;

[0043] Figure 3 This is an overall schematic diagram of a multi-cycle terahertz pulse generation and detection system disclosed in an embodiment of this application;

[0044] Figure 4This is a flowchart illustrating a multi-cycle terahertz pulse generation and detection method disclosed in an embodiment of this application. Detailed Implementation

[0045] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0046] In this application, the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0047] As can be seen from the background technology, existing multi-period terahertz pulse generation and detection systems for obtaining multi-period narrowband terahertz pulses have poor tunability due to the complexity of the optical path and the extremely high requirements for spatial alignment accuracy. The number of terahertz pulse periods generated is limited by the modulation depth of the incident spectrum. Furthermore, the periodic polarization nonlinear crystal and physical filter devices are limited by fixed fabrication processes, and once the device is formed, its center frequency is difficult to change, resulting in a lack of system flexibility.

[0048] To address the aforementioned issues, this application discloses a multi-cycle terahertz pulse generation and detection system and method. This scheme replicates the generated laser pulse using a multi-cycle terahertz pulse generation optical path, exciting a spin terahertz emitter. Utilizing its fast linear response, it generates a pulse train containing multiple sub-pulses with adjustable time-domain intervals. By adjusting the relative delay within the replication unit of the multi-cycle terahertz pulse generation optical path, the sub-pulse interval can be changed, thereby flexibly modulating the number of cycles of the final terahertz waveform to obtain a multi-cycle terahertz pulse. This multi-cycle narrowband terahertz pulse contains multiple complete oscillation cycles in the time domain and corresponds to an extremely narrow spectral width of terahertz electromagnetic wave pulse in the frequency domain. This fundamentally breaks through the physical limitation of "fixed device design, i.e., fixed frequency," achieving flexible control of the time-domain morphology of the terahertz pulse and improving the tunability and flexibility of the multi-cycle terahertz pulse generation and detection system. Specific implementation methods are described in detail in the following embodiments.

[0049] It should be noted that the multi-cycle terahertz pulse generation and detection system and method provided in this application relates to the technical fields of terahertz science, etc. The above is only an example and does not limit the application field of the multi-cycle terahertz pulse generation and detection system and method provided in this application.

[0050] refer to Figure 1 As shown, this application discloses a multi-cycle terahertz pulse generation and detection system. The multi-cycle terahertz pulse generation and detection system is configured with a femtosecond laser source 1, a non-polarized flat beam splitter 2, a multi-cycle terahertz pulse generation optical path, a terahertz collection module, and an electro-optic detection optical path in sequence. The multi-cycle terahertz pulse generation optical path is equipped with a spin terahertz emitter 18.

[0051] The data processing procedure between the femtosecond laser source 1, the unpolarized flat beam splitter 2, the multi-period terahertz pulse generation optical path, the terahertz collection module, and the electro-optic detection optical path is as follows:

[0052] The unpolarized flat beam splitter 2 is used to split the laser pulse output from the femtosecond laser source 1 into a generating light and a probe light; wherein, the single pulse energy of the femtosecond laser source 1 is 9 millijoules (mJ), the wavelength is 800 nanometers (nm), and the repetition frequency is 800 Hz.

[0053] The multi-cycle terahertz pulse generation optical path is used to cascade pulse replication of the generated light to obtain a pulse train containing multiple sub-pulses, and to excite the spin terahertz emitter 18 to process the pulse train containing multiple sub-pulses to obtain a multi-cycle terahertz pulse; wherein, the spin terahertz emitter 18 has a ferromagnetic / heavy metal / substrate structure, preferably a CoFeB / Pt / quartz structure, and is placed in an in-plane magnetic field; in some embodiments, the structure of the spin terahertz emitter 18 can be replaced with other ferromagnetic / heavy metal combinations, such as Fe / Pt, Co / Pt, etc.;

[0054] The terahertz collection module is used to guide multi-cycle terahertz pulses to the electro-optic detection optical path; the terahertz collection module includes an off-axis parabolic mirror 19, an off-axis parabolic mirror 20, and an off-axis parabolic mirror 21; in some embodiments, the number of off-axis parabolic mirrors can be arbitrarily increased or decreased; the number of off-axis parabolic mirrors in this application is not specifically limited;

[0055] The electro-optic detection optical path is used to adjust the time delay of the detection light and reconstruct the waveform of the multi-cycle terahertz pulse based on the adjusted detection light. The specific waveform of the multi-cycle terahertz pulse is as follows: Figure 2 As shown. Figure 2 The typical time-domain waveform and corresponding frequency-domain spectrum of a multi-cycle terahertz pulse are shown.

[0056] Figure 2The horizontal axis of the upper part represents time (picoseconds (ps)), and the vertical axis represents signal strength (S (au)). Figure 2 The lower part has a horizontal axis representing frequency (terahertz (THz)) and a vertical axis representing signal strength (S (au)).

[0057] When the laser pulse output from femtosecond laser source 1 is incident on a ferromagnetic thin film sample, it can achieve multi-cycle terahertz pulse radiation, which is manifested in the time domain spectrum ( Figure 2 The periodic oscillations in the upper part, and the frequency domain spectrum ( Figure 2 The peak value appears in the lower part of the spectrum. A multi-cycle terahertz time pulse corresponds to a narrow-band terahertz source in the frequency domain. In contrast, a common single-cycle terahertz pulse consists of only one pulse, with a wide spectral coverage, typically 0.1 THz–3 THz. Furthermore, by adjusting the time interval of the preceding sub-pulses, the center frequency of this narrow-band terahertz source can be adjusted.

[0058] The specific structure and application of the multi-cycle terahertz pulse generation and detection system are as follows:

[0059] The multi-cycle terahertz pulse generation optical path includes multiple pulse replication units (such as pulse replication unit 3, pulse replication unit 4, and pulse replication unit 5) connected in sequence, a femtosecond laser mirror 14, a chopper 15, and a beam expander system.

[0060] It should be noted that pulse replication unit 3, pulse replication unit 4 and pulse replication unit 5 have the same structure; in some embodiments, the number of pulse replication units can be increased or decreased arbitrarily; the number of pulse replication units in this application is not specifically limited.

[0061] Pulse replication unit 3, pulse replication unit 4 and pulse replication unit 5 are used in sequence to replicate the generated light into pulses to obtain a pulse train containing multiple sub-pulses;

[0062] Chopper 15 is used to modulate pulse trains; specifically, chopper 15 can be used in conjunction with lock-in amplifiers for signal modulation, amplification, and detection.

[0063] The beam expander system is used to expand the modulated pulse train. The beam expander system includes a concave lens 16 and a convex lens 17. The focal length of the concave lens 16 is -f1, and the focal length of the convex lens 17 is f2. When the focal length difference (f2-f1) between the convex lens 17 and the concave lens 16 is equal to the actual distance between them, the concave lens 16 and the convex lens 17 are used to convert the divergent state of the modulated pulse train into a collimated state, so that the expanded pulse train is directly incident on the spin terahertz transmitter 18. It should be noted that when the collimated beam is incident on the concave lens 16 with a focal length of -f1, it becomes a divergent beam, and the beam diameter increases. Subsequently, it is incident on the plano-convex lens 17 with a focal length of f2. When the value of f2-f1 is equal to the distance between the two lenses (concave lens 16 and convex lens 17), the beam changes from a divergent state to a collimated state; the initial beam diameter increases by a factor of f2 / f1.

[0064] The spin terahertz transmitter 18 is used to generate multi-cycle terahertz pulses based on the expanded pulse train;

[0065] Each pulse replication unit includes a broadband half-wave plate 7, a broadband polarization beam splitter cube 8, a femtosecond laser mirror 9, a femtosecond laser mirror 10, a delay line 11, a polarization beam splitter 12, and a femtosecond laser mirror 13.

[0066] The femtosecond laser reflector 6 is used to change the propagation direction of the laser so that the incident pulse corresponding to the laser propagates towards the broadband half-wave plate 7. In some embodiments, the number of femtosecond laser reflectors can be increased or decreased arbitrarily. The number of femtosecond laser reflectors in this application is not specifically limited. The femtosecond laser reflectors in this solution are used to change the propagation direction of light, making the entire instrument more compact.

[0067] The broadband half-wave plate 7 is used to adjust the ratio of the laser pulses; wherein the ratio is the ratio of the p-polarized component to the s-polarized component; the broadband half-wave plate 7 is specifically used to make the ratio of the p-polarized component and the s-polarized component equal, by controlling the energy ratio of the two sub-pulses of the p-polarized component and the s-polarized component, so that the energy of the sub-pulses corresponding to the p-polarized component and the s-polarized component are equal; in some embodiments, the angle of the broadband half-wave plate 7 is adjusted to make the energies of the p and s components equal, thereby achieving equal energy of the two sub-pulses;

[0068] Broadband polarization beam splitter cube 8 is used to separate the modulated laser pulse into p-polarization and s-polarization components.

[0069] Delay line 11 is used to adjust the optical path of the s-polarization component. Delay line 11 is positioned along the optical path of the s-polarization component. Delay line 11 consists of a femtosecond laser mirror 30, a femtosecond laser mirror 31, and a linear displacement stage. Specifically, delay line 11 is used to change the optical path of the s-polarization component by adjusting the position of the linear displacement stage, thereby continuously adjusting the time delay between the two sub-pulses corresponding to the p-polarization component and the s-polarization component, achieving flexible control over the number of terahertz pulse cycles. In some implementations, adjusting the position of delay line 11 can change the time delay between sub-pulses, thereby controlling the periodic structure of the terahertz pulse.

[0070] The polarization beam splitter 12 is used to combine the adjusted s-polarization component and p-polarization component to form a pulse train containing multiple sub-pulses.

[0071] The specific structure and application of the electro-optic detection optical path are as follows:

[0072] The electro-optic detection optical path includes a delay line 22, an electro-optic crystal 24, a broadband quarter-wave plate 25, a Wollaston prism 26, and a balanced photodetector 29; the delay line 22 includes a femtosecond laser mirror 32 and a femtosecond laser mirror 33;

[0073] Delay line 22 is used to adjust the time delay of the probe light and to make the multi-cycle terahertz pulse and the time-delayed probe light path collinear and enter the electro-optic crystal 24 through the femtosecond laser reflector 23;

[0074] The electro-optic crystal 24 is used to birefractively refract the collinear multi-cycle terahertz pulse and the time-delayed probe light, causing the probe light to be ellipsometric; wherein, the electro-optic crystal 24 is made of zinc telluride (ZnTe) or potassium phosphide (GaP); in some embodiments, the electro-optic crystal 24 can be replaced with GaP, which is suitable for the detection of higher frequency terahertz waves.

[0075] A broadband quarter-wave plate 25 is used to convert the elliptically polarized probe light into first orthogonally linearly polarized light;

[0076] The Wollaston prism 26 is used to convert the ellipsometric probe light path into a second orthogonal linearly polarized light;

[0077] Both femtosecond laser mirror 27 and femtosecond laser mirror 28 are used to change the direction of light propagation;

[0078] The balanced photodetector 29 is used to detect signals from the first orthogonal line-polarized light and the second orthogonal line-polarized light via a lock-in amplifier.

[0079] To facilitate understanding of the execution process of a multi-cycle terahertz pulse generation and detection system, combined with Figure 3 To illustrate, we will provide examples. Figure 3A schematic diagram of the multi-cycle terahertz pulse generation and detection system is shown.

[0080] For example, the single pulse output by the femtosecond laser source 1 passes through a series of cascaded pulse replication units 3, 4 and 5 in sequence;

[0081] The multi-stage cascaded pulse replication unit 3, pulse replication unit 4 and pulse replication unit 5 adjust the ratio of the p-polarization component to the s-polarization component of the single pulse through a broadband half-wave plate 7.

[0082] The broadband polarization beam splitter 8 completely separates the p-polarization and s-polarization components, generating two sub-pulses. The s-polarization component, after its optical path is adjusted by the delay line 11, is recombined with the p-polarization component through the polarization beam splitter 12, forming a pulse train containing two tunable sub-pulses. Each pulse is converted into a pulse train with up to eight micro-pulse structures after passing through three pulse replication units. Specifically, the initial laser polarization state is linearly polarized light, and the half-wave plate 7 can change the polarization angle of the linearly polarized light. When the polarization angle changes, the ratio of the p-polarization component to the s-polarization component also changes. The broadband polarization beam splitter 8 and the polarization beam splitter 12 can transmit p-light (p-light is light with its polarization direction parallel to the incident plane) and reflect s-light; therefore, the beam splitting ratio of transmitted and reflected light can be changed by adjusting the broadband half-wave plate 7. The p-beam, as the transmitted light, is incident on the polarization beam splitter 12 after passing through two fixed mirrors and is completely transmitted, with the optical path remaining unchanged. The s-beam, as the reflected light, has its optical path changed by the delay line 11, and is incident on the polarization beam splitter 12 at a 90-degree angle to the p-beam and is completely reflected. Thus, the s-beam and p-beam achieve beam combining. The optical path difference between the two beams can be adjusted by adjusting the delay line 11, ultimately resulting in a pulse train containing two sub-pulses, and the time delay between the two sub-pulses is adjustable.

[0083] A single pulse replication unit can double the number of sub-pulses; therefore, n cascaded pulse replication units can increase the number of sub-pulses to 2. n The number of pulses can be multiplied; taking a 3-pulse replication unit as an example, it can replicate one pulse into a maximum of 8 pulses.

[0084] After being modulated by chopper 15 and expanded by beam expansion system, the pulse train is incident on spin terahertz transmitter 18.

[0085] The spin terahertz emitter 18 has a ferromagnetic / heavy metal / substrate structure and is placed in an in-plane magnetic field. The spin terahertz emitter 18 generates multi-cycle terahertz pulses based on the modulated and expanded pulse train. The multi-cycle terahertz pulses are focused onto the electro-optic crystal 24 by the terahertz collection module.

[0086] Meanwhile, after the time delay is adjusted by the delay line 22, the probe light passes collinearly with the multi-period terahertz pulse through the electro-optic crystal 24, and is split into two beams of orthogonal polarization by the broadband quarter-wave plate 25 and the Wollaston prism 26. The beams are received by the balanced photodetector 29 and combined with the lock-in amplifier to achieve high signal-to-noise ratio detection of the signal.

[0087] Chopper 15 sinusoidally modulates the terahertz signal to a frequency of F. This chopping frequency F is transmitted to the lock-in amplifier via a trigger. The signal received by the balanced photodetector 29 is the probe light, which may contain stray light and also the terahertz signal modulated at frequency F. After receiving the signal from the photodetector, the lock-in amplifier retains only the signal at frequency F, thus filtering out stray signals and retaining and amplifying the terahertz signal, achieving high signal-to-noise ratio detection.

[0088] In some implementation schemes, the multi-cycle terahertz pulses of this scheme can be generated in the following manner:

[0089] The femtosecond laser source 1 is a femtosecond laser with a center wavelength of 800 nm, a single pulse energy of 9 mJ, and a repetition frequency of 800 Hz. The laser beam is split into a generation beam and a probe beam by an unpolarized flat beam splitter 2.

[0090] The generated light sequentially passes through multiple pulse replication units 3, 4, and 5. In each pulse replication unit, the laser beam enters the polarization beam splitter cube 8 after the polarization ratio is adjusted by the half-wave plate 7, and is split into two beams, p-polarized and s-polarized; the s-beam is combined with the p-beam at the polarization beam splitter 12 after the optical path is adjusted by the delay line 11.

[0091] After being cascaded in multiple stages, a pulse train of up to 8 micropulses is formed. This pulse train is modulated by chopper 15, expanded by concave lens 16 and convex lens 17, and then incident perpendicularly on spin terahertz transmitter 18.

[0092] The spin terahertz transmitter 18 generates multi-cycle terahertz pulses based on the modulated and expanded pulse train;

[0093] The multi-cycle terahertz pulse is focused onto the electro-optic crystal 24 by the terahertz collection module; the probe light is delayed by the delay line 22 and then passes collinearly with the terahertz pulse through the electro-optic crystal 24. After passing through the quarter-wave plate 25 and the Wollaston prism 26, it is split into orthogonally polarized light, which is received by the balanced photodetector 29 and combined with the lock-in amplifier to realize signal detection.

[0094] In this embodiment, a multi-cycle terahertz pulse generation optical path replicates the generated laser pulse, exciting a spin terahertz emitter. Its fast linear response generates a pulse train containing multiple sub-pulses with adjustable time-domain intervals. By adjusting the relative delay within the replication unit of the multi-cycle terahertz pulse generation optical path, the sub-pulse interval can be changed, thus flexibly modulating the number of cycles of the final terahertz waveform to obtain a multi-cycle terahertz pulse. This multi-cycle narrowband terahertz pulse contains multiple complete oscillation cycles in the time domain and corresponds to an extremely narrow spectral width terahertz electromagnetic wave pulse in the frequency domain. This fundamentally breaks through the physical limitation of "fixed device design, i.e., fixed frequency," enabling flexible control of the time-domain morphology of the terahertz pulse and improving the tunability and flexibility of the multi-cycle terahertz pulse generation and detection system. This scheme allows for flexible control of the time-domain morphology of the terahertz pulse and has advantages such as strong signal, high signal-to-noise ratio, and adjustable repetition frequency.

[0095] refer to Figure 4 As shown, this application discloses a method for generating and detecting multi-cycle terahertz pulses. This method is applied to the aforementioned multi-cycle terahertz pulse generation and detection system, and includes the following steps:

[0096] S401: The laser pulse output from the femtosecond laser source 1 is split into a generation light and a probe light by the unpolarized flat beam splitter 2;

[0097] S402: The generated light is cascaded and pulsed through the multi-cycle terahertz pulse generation optical path to obtain a pulse train containing multiple sub-pulses, and the spin terahertz emitter 18 is excited to process the pulse train to obtain a multi-cycle terahertz pulse.

[0098] S403: Guides multi-cycle terahertz pulses to the electro-optic detection optical path through the terahertz collection module;

[0099] S404: Adjusts the time delay of the probe light through the electro-optic probe light path, and reconstructs the waveform of the multi-cycle terahertz pulse based on the adjusted probe light.

[0100] The execution process and principle of S401-S404 are consistent with those of the above-mentioned multi-cycle terahertz pulse generation and detection system, and can be referred to accordingly. They will not be repeated here.

[0101] The beneficial effects of this method embodiment are the same as those of the above system embodiment, and can be referred to accordingly. They will not be repeated here.

[0102] For the foregoing method embodiments, in order to simplify the description, they are all described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, because according to this application, some steps can be performed in other orders or simultaneously. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily essential to this application.

[0103] It should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For apparatus embodiments, since they are basically similar to method embodiments, the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

[0104] The steps in the methods of the various embodiments of this application can be adjusted, combined, or deleted according to actual needs.

[0105] Finally, it should be noted that in this paper, relational terms such as first and second are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations.

[0106] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0107] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A multi-cycle terahertz pulse generation and detection system, characterized in that, The system is sequentially configured with a femtosecond laser source (1), a non-polarized flat beam splitter (2), a multi-period terahertz pulse generation optical path, a terahertz collection module, and an electro-optic detection optical path; the multi-period terahertz pulse generation optical path is equipped with a spin terahertz emitter (18). The unpolarized flat beam splitter (2) is used to split the laser pulse output by the femtosecond laser source into a generating light and a probe light; The multi-cycle terahertz pulse generation optical path is used to cascade pulse replication of the generated light to obtain a pulse train containing multiple sub-pulses, and to excite the spin terahertz emitter (18) to process the pulse train to obtain a multi-cycle terahertz pulse. The terahertz collection module is used to guide the multi-cycle terahertz pulses to the electro-optic detection optical path; The electro-optic detection optical path is used to adjust the time delay of the detection optical path and reconstruct the waveform of the multi-cycle terahertz pulse based on the adjusted detection light.

2. The system according to claim 1, characterized in that, The femtosecond laser source has a single-pulse energy of 9 millijoules and a wavelength of 800 nanometers.

3. The system according to claim 1, characterized in that, The multi-cycle terahertz pulse generation optical path includes multiple pulse replication units, a chopper (15), and a beam expander connected in sequence; The plurality of pulse replication units are used to replicate the generated light into pulses to obtain a pulse train containing multiple sub-pulses; The chopper (15) is used to modulate the pulse train; The beam expanding system is used to expand the modulated pulse train.

4. The system according to claim 3, characterized in that, The plurality of pulse replication units include a broadband half-wave plate (7), a broadband polarization beam splitter cube (8), a delay line (11), and a polarization beam splitter (12). The broadband half-wave plate (7) is used to adjust the ratio of the laser pulses; wherein the ratio is the ratio of the p-polarization component to the s-polarization component; The broadband polarization beam splitter (8) is used to separate the adjusted laser pulse into p-polarization components and s-polarization components. The delay line (11) is used to adjust the optical path of the s-polarization component; wherein, the delay line (11) is arranged in the optical path of the s-polarization component; The polarization beam splitter (12) is used to combine the adjusted s-polarization component and the p-polarization component to form a pulse train containing multiple sub-pulses.

5. The system according to claim 3, characterized in that, The beam expanding system includes a concave lens (16) and a convex lens (17). When the focal length difference between the convex lens (17) and the concave lens (16) is equal to the actual distance between the concave lens (16) and the convex lens (17), the concave lens (16) and the convex lens (17) are used to convert the divergent state of the modulated pulse train into a collimated state so that the expanded pulse train is directly incident on the spin terahertz transmitter (18).

6. The system according to claim 3, characterized in that, The spin terahertz transmitter (18) has a ferromagnetic / heavy metal / substrate structure and is placed in an in-plane magnetic field, specifically for generating multi-cycle terahertz pulses.

7. The system according to claim 4, characterized in that, The broadband half-wave plate (7) is specifically used to make the ratio of the p-polarized component and the s-polarized component equal, so that the sub-pulse energies corresponding to the p-polarized component and the s-polarized component are equal.

8. The system according to claim 4, characterized in that, The delay line (11) is specifically used to change the optical path of the s-polarization component by adjusting the position of the linear displacement stage, thereby continuously adjusting the time delay between the sub-pulses corresponding to the p-polarization component and the s-polarization component.

9. The system according to claim 1, characterized in that, The electro-optic detection optical path includes a delay line (22), an electro-optic crystal (24), a broadband quarter-wave plate (25), a Wollaston prism (26), and a balanced photodetector (29). The delay line (22) is used to delay the detection optical path in time, and to make the multi-period terahertz pulse and the time-delayed detection optical path collinear and enter the electro-optic crystal (24). The electro-optic crystal (24) is used to birefringent the collinear multi-period terahertz pulse and the time-delayed probe optical path, so that the probe optical path is ellipticized. The broadband quarter-wave plate (25) is used to convert the ellipsoidal probe optical path into first orthogonal linearly polarized light; The Wollaston prism (26) is used to convert the ellipsoidal probe light path into a second orthogonal linearly polarized light; The balanced photodetector (29) is used to detect the first orthogonal line-polarized light and the second orthogonal line-polarized light by means of a lock-in amplifier.

10. A method for generating and detecting multi-cycle terahertz pulses, characterized in that, The method is applied to the multi-cycle terahertz pulse generation and detection system according to any one of claims 1 to 9, and the method includes: The laser pulse output from the femtosecond laser source (1) is divided into a generating light and a probe light by the unpolarized flat beam splitter (2); The generated light is cascaded and pulsed through the multi-cycle terahertz pulse generation optical path to obtain a pulse train containing multiple sub-pulses, and the spin terahertz emitter (18) is excited to process the pulse train to obtain a multi-cycle terahertz pulse. The multi-cycle terahertz pulses are guided to the electro-optic detection optical path by the terahertz collection module; The time delay of the probe light is adjusted by the electro-optic probe light path, and the waveform of the multi-cycle terahertz pulse is reconstructed based on the adjusted probe light.

Images