Integrated strong-field terahertz pumping optical Faraday detection equipment

Abstract

The invention relates to the technical field of strong-field terahertz detection, in particular to integrated strong-field terahertz pumping optical Faraday detection equipment which comprises a laser source, a beam splitter, a terahertz strong source generation module, a collection module, a delay module, a Py detector and a detection module. Laser generated by the laser source is divided into pump light and detection light through the beam splitter, the pump light is focused on a Py detector of the detection module through the terahertz strong source generation module and the collection module, and the detection light is focused on the Py detector of the detection module through the delay module. The detection light output by the Py detector passes through a detection module to obtain a terahertz time-domain spectral waveform; according to the invention, distortion of terahertz wave detection can be reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of strong-field terahertz detection, and in particular to an integrated strong-field terahertz pumped optical Faraday detection device. Background Art

[0002] Terahertz waves, located in the electromagnetic spectrum between microwaves and infrared light, possess unique frequency and temporal characteristics. As a new type of low-photon energy, high-field light source, combined with technologies such as optical pump-terahertz detection, terahertz pump-terahertz detection, and terahertz time-domain spectroscopy (THz-TDS), have demonstrated significant application value in materials science, biomedicine, and communications. With the rapid development of high-field terahertz light sources, it has become possible to generate high-quality, high-stability, high-field THz light sources with single-pulse energies exceeding millijoules or even tens of millijoules, and peak electric fields exceeding MV / cm.

[0003] Compared to the rapid breakthroughs in high-field terahertz light sources, the development of high-field terahertz detection technology has lagged behind. Currently, there are two main approaches: electronics and photonics. Electronics methods, such as photoconductive antennas, utilize femtosecond lasers to excite photogenerated carriers in semiconductors. The terahertz field drives their motion, generating transient currents. After amplification, these currents are converted into electrical signals, which, combined with scanning, can reconstruct the terahertz field waveform. Photonics methods, exemplified by electro-optical sampling, utilize the terahertz field to induce changes in the refractive index of the detection crystal, resulting in changes in the polarization state of the sampling light, thereby indirectly acquiring the intensity and phase information of the terahertz pulse.

[0004] While existing electronic and photonic terahertz detection methods can meet certain needs, they both have significant limitations in strong field environments. In electronic methods, when photoconductive antennas detect strong-field terahertz pulses, the high density of photogenerated carriers creates a reverse electric field, which weakens the incident terahertz field and causes nonlinear saturation of the signal amplitude, making it unable to accurately reflect the actual field strength. Furthermore, excessively high electric fields can cause irreversible damage to the device, seriously affecting detection efficiency and lifespan. Although electro-optical sampling technology is widely used in fields such as terahertz radiation detection, spectroscopy, and imaging, it is also limited by its own material properties. When the applied terahertz electric field intensity is too high, the refractive index change of commonly used electro-optical crystals (such as ZnTe) exceeds their linear response range, resulting in overrotation, which in turn triggers nonlinear distortion of the detection signal and makes accurate measurement of the terahertz pulse electric field vector difficult. In summary, it is essential to explore how to detect terahertz waves without distortion under strong terahertz field conditions. Summary of the Invention

[0005] In view of the above problems, the present invention provides an integrated strong-field terahertz pumped optical Faraday detection device, which solves the technical problem of how to detect terahertz waves without distortion in the prior art.

[0006] The present invention provides an integrated strong-field terahertz pumped optical Faraday detection device, comprising a laser source, a beam splitter, a terahertz strong source generation module, a collection module, a delay module, a Py detector, and a detection module;

[0007] The terahertz strong source generation module includes a chopper, a grating, a gold mirror, a first plano-convex lens, a first half-wave plate, a second plano-convex lens and a lithium niobate crystal arranged in sequence on the optical path;

[0008] The collecting module comprises a first reflecting mirror, a first off-axis parabolic mirror, a second off-axis parabolic mirror, a third off-axis parabolic mirror, and a fourth off-axis parabolic mirror, which are sequentially arranged on the optical path;

[0009] The delay module includes a delay line, a second reflector, and a third reflector sequentially arranged on the optical path;

[0010] The detection module includes a fourth reflector, a second half-wave plate, a Wollaston prism, and a balanced detector arranged in sequence on the optical path;

[0011] The laser generated by the laser source is divided into pump light and detection light by a beam splitter. The pump light is focused on the Py detector of the detection module through the terahertz strong source generation module and the collection module. The detection light is focused on the Py detector of the detection module through the delay module. The detection light output by the Py detector passes through the detection module to obtain the terahertz time-domain spectrum waveform.

[0012] Preferably, in the terahertz strong source generation module, the pump light passes through a chopper, enters a grating at a preset angle, is reflected by a gold mirror, passes through a first plano-convex lens, a first half-wave plate, and a second plano-convex lens in sequence, and then enters a lithium niobate crystal to generate a terahertz wave; the first plano-convex lens and the second plano-convex lens form a 4f imaging system.

[0013] Preferably, the detection light passes through the delay line, the second reflector, the third reflector in sequence, and then passes through the fourth off-axis parabolic mirror to be focused on the Py detector together with the pump light, and keeps vertical incidence on the Py detector.

[0014] Preferably, the Py detector is composed of a ferromagnetic film and a substrate, and the Py detector is prepared as follows:

[0015] The pressure of the vacuum main chamber was maintained at 1×10 -8 Torr below, the quartz substrate was heated in acetone at 60° C. for 10 minutes in a vacuum main chamber, then placed in isopropanol, and then dried to obtain a double-polished quartz substrate; a ferromagnetic film was grown on the double-polished quartz substrate by a magnetron sputtering method.

[0016] Preferably, the double-polished quartz substrate of the Py detector is 10 mm long, 10 mm wide, and 3 mm thick, and the thickness of the ferromagnetic film is 9 nm.

[0017] Preferably, the Py detector is placed on a three-dimensional translation stage through a mirror frame, and a permanent magnet is provided outside the mirror frame with a magnetic field strength of 500 mT.

[0018] Preferably, the detection light after passing through the Py detector passes through the fourth reflector, the second half-wave plate, the Wollaston prism and the balanced detector in sequence to generate a differential magnetization electric signal;

[0019] The waveform of the differential magnetization electric signal is used as the terahertz time-domain spectrum waveform.

[0020] Preferably, the terahertz magnetic field is calculated by the following expression,

[0021]

[0022] Where m is the normalized magnetization unit vector, γ is the electron gyromagnetic ratio, m0 is the equilibrium position of m, and B THz is the terahertz magnetic field.

[0023] Preferably, the maximum single pulse energy of the laser source is 7 mJ, the wavelength is 800 nm, and the repetition frequency is 1 kHz.

[0024] Compared with the prior art, the present invention has at least the following beneficial effects:

[0025] (1) The integrated high-field terahertz pumped optical Faraday detection device provided by the present invention is based on lithium niobate tilted wavefront technology and can realize a strong terahertz source with a wavelength of 800nm, an electric field strength greater than 300kV / cm, and a repetition frequency of 1kHz.

[0026] (2) The Py detector of the integrated high-field terahertz pumped optical Faraday detection device provided by the present invention can realize real-time extraction of terahertz waveforms. Even when detecting terahertz waveforms under high field strength, the shape distortion will not occur and the Py detector will not be easily damaged by heat.

[0027] (3) The Py ferromagnetic element of the integrated high-field terahertz pumped optical Faraday detection device provided by the present invention can improve the sensitivity to the sampling signal by flexibly changing the film thickness. To a certain extent, the thicker the ferromagnetic film, the stronger the detected signal amplitude.

[0028] (4) The Py ferromagnetic detector of the integrated strong-field terahertz pumped optical Faraday detection device provided by the present invention can be applied to different terahertz strong source systems, further expanding the detectable spectrum range and having great application potential.

[0029] (5) The preparation of the Py ferromagnetic detector in the integrated strong-field terahertz pumped optical Faraday detection device provided by the present invention does not require a complex and high-cost manufacturing process, overcoming the shortcomings of many existing solutions in terms of detection accuracy, waveform distortion, and high cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The drawings are only for purposes of illustrating particular embodiments and are not to be considered limiting of the invention.

[0031] Figure 1 Schematic diagram of the integrated high-field terahertz pumped optical Faraday detection device provided by the present invention.

[0032] Figure 2 This is a schematic diagram of the strong-field terahertz time-domain signal measured using a Py detector provided by the present invention.

[0033] Figure 1: 1-laser source, 2-beam splitter, 3-chopper, 4-grating, 5-gold mirror, 6-first plano-convex lens, 7-first half-wave plate, 8-second plano-convex lens, 9-lithium niobate crystal, 10-first reflector, 11-first off-axis parabolic mirror, 12-second off-axis parabolic mirror, 13-third off-axis parabolic mirror, 14-fourth off-axis parabolic mirror; 15-delay line, 16-second reflector, 17 third reflector, 18-Py detector, 19-fourth reflector, 20-second half-wave plate, 21-Wollaston prism, 22-balanced detector. DETAILED DESCRIPTION

[0034] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. In addition, the present invention can also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited by the specific embodiments disclosed below.

[0035] In order to illustrate the effectiveness of the method proposed by the present invention, the above technical solution of the present invention is described in detail below through a specific embodiment. Figure 1 As shown, an integrated strong-field terahertz pumped optical Faraday detection device is disclosed, including a laser source 1, a beam splitter 2, a terahertz strong source generation module, a collection module, a delay module, a Py detector 18 and a detection module.

[0036] In some embodiments, the maximum single pulse energy of the laser source 1 of the present invention is 7 mJ, the wavelength is 800 nm, and the repetition frequency is 1 kHz.

[0037] The terahertz strong source generation module includes a chopper 3, a grating 4, a gold mirror 5, a first plano-convex lens 6, a first half-wave plate 7, a second plano-convex lens 8 and a lithium niobate crystal 9 which are sequentially arranged on the optical path.

[0038] In some embodiments, the first plano-convex lens 6 and the second plano-convex lens 8 constitute a 4f imaging system.

[0039] By configuring a terahertz power source generation module, a chopper 3 is used for signal modulation to facilitate subsequent phase-locked amplification and detection. A grating 4 and a gold mirror 5 achieve beam shaping and minimize reflection losses. A first plano-convex lens 6 performs focusing. A first half-wave plate 7 adjusts the laser polarization state to adapt to the three-dimensional nonlinear effects of a lithium niobate crystal 9. The lithium niobate crystal 9, with its strong nonlinear response, is used for efficient terahertz radiation generation, improving the efficiency and stability of the terahertz power source.

[0040] The collecting module includes a first reflecting mirror 10, a first off-axis parabolic mirror 11, a second off-axis parabolic mirror 12, a third off-axis parabolic mirror 13, and a fourth off-axis parabolic mirror 14 which are sequentially arranged on the optical path.

[0041] The present invention combines a multi-stage reflector and off-axis parabolic mirror to efficiently collect and focus terahertz pulses in space, minimizing transmission loss and distortion. The off-axis parabolic mirror effectively avoids spherical aberration, improving focusing accuracy and efficiency, ensuring stable and efficient transmission of terahertz waves to downstream detectors, and facilitating the acquisition of highly sensitive terahertz time-domain signals.

[0042] The delay module includes a delay line 15 , a second reflector 16 , and a third reflector 17 , which are sequentially arranged on the optical path.

[0043] The detection module includes a fourth reflector 19 , a second half-wave plate 20 , a Wollaston prism 21 , and a balanced detector 22 , which are sequentially arranged on the optical path.

[0044] The detection module of the present invention uses a fourth reflector 19 to adjust the spatial path, and a second half-wave plate 20 to adjust the polarization to meet the beam-splitting requirements of a Wollaston prism 21. The Wollaston prism 21 separates the light beam into orthogonal polarization states to eliminate common-mode noise. A balanced detector 22 improves the system's signal-to-noise ratio, ultimately extracting the terahertz signal and enhancing the detection sensitivity and anti-interference capability of terahertz time-domain spectroscopy.

[0045] The laser generated by the laser source 1 is divided into pump light and detection light by the beam splitter 2. The pump light is focused on the Py detector 18 of the detection module through the terahertz strong source generation module and the collection module. The detection light is focused on the Py detector 18 of the detection module through the delay module. The detection light output by the Py detector 18 passes through the detection module to obtain the terahertz time domain spectrum waveform.

[0046] In some embodiments, the Py detector 18 includes a ferromagnetic film and a substrate. The ferromagnetic film is grown on a double-polished quartz substrate by magnetron sputtering.

[0047] In some embodiments, the Py detector 18 is manufactured at room temperature using an ultra-high vacuum magnetron sputtering system, the steps of which include: controlling the pressure of the vacuum main chamber to be maintained at 1×10 -8 Torr below, the quartz substrate was heated in acetone at 60° C. for 10 minutes, then placed in isopropyl alcohol, and then dried to prepare a double-polished quartz substrate; and a ferromagnetic film was grown on the double-polished quartz substrate by a magnetron sputtering method.

[0048] The Py detector of the present invention adopts the above preparation method and adopts a double-polished quartz substrate as a base plate. The quartz substrate has excellent transmittance performance for terahertz light and is very suitable for terahertz transmission applications of magnetic thin films.

[0049] In some embodiments, the double-polished quartz substrate of the Py detector 18 has a length of 10 mm, a width of 10 mm, a thickness of 3 mm, and a ferromagnetic film thickness of 9 nm.

[0050] In some embodiments, the Py detector 18 is placed on a three-dimensional translation stage through a mirror frame, which can achieve high-precision spatial positioning of the Py detector 18. A permanent magnet with a magnetic field strength of 500mT is added to the outside of the mirror frame, which can provide a stable magnetization field for the ferromagnetic film and ensure detection performance.

[0051] The optical path of the integrated high-field terahertz pumped optical Faraday detection device of the present invention is described in detail below.

[0052] The 800 nm laser output from the laser source is first split by beam splitter 2 into pump light and probe light. The pump light first passes through chopper 3 and then enters grating 4 at a specific angle. After reflection by gold mirror 5, it enters a 4f imaging system consisting of a first plano-convex lens 6 and a second plano-convex lens 8. It then enters lithium niobate crystal 9, where it radiates terahertz waves through optical rectification. A first half-wave plate 7 is used to generate polarization that matches the tilted wavefront. The radiated terahertz waves pass through first reflector 10 and enter a collection module consisting of a first off-axis parabolic mirror 11, a second off-axis parabolic mirror 12, a third off-axis parabolic mirror 13, and a fourth off-axis parabolic mirror 14.

[0053] The detection light passing through beam splitter 2 sequentially passes through a delay module consisting of a delay line 15, a second reflector 16, and a third reflector 17. It then passes through a fourth off-axis parabolic mirror 14 and is focused together with the pump light onto a Py detector 18, achieving spatiotemporal overlap and maintaining vertical incidence on the Py detector 18. When a strong terahertz magnetic field acts on the Py detector, the induced Zeeman torque causes the material's magnetization state to dynamically change. This change can then be indirectly sensed through the Faraday rotation effect, enabling the detection of the terahertz magnetic field component.

[0054] After passing through the Py detector 18, the probe light is detected by a fourth reflector 19, a second half-wave plate 20, a Wollaston prism 21, and a balanced detector 22. The Wollaston prism 21 splits the pump light into two beams of different polarizations, which enter the two probes of the balanced detector 22 to generate a differential signal. This allows the terahertz electric field intensity at each moment to be determined, allowing the complete terahertz time-domain spectrum to be plotted.

[0055] For the detection process, the magnetization dynamics process of detecting strong field terahertz by Py can be expressed by the Landau-Lifshitz-Gilbert equation (LLG equation):

[0056]

[0057] Where m = M / Ms is the magnetization unit vector normalized to the saturation magnetization Ms, γ is the electron gyromagnetic ratio, is the time, α is the Gilbert damping parameter, and the first term of the LLG equation describes the precession motion of the magnetic moment caused by the effective magnetic field. The effective field B eff It consists of four parts: external magnetic field B Ext , demagnetizing field B Dem and anisotropy field B Ani and the terahertz magnetic field B THz , as shown in formula (1-2),

[0058] B eff =(B Ext +B Dem +B Ani )+B THz (1-2)

[0059] The present invention simplifies the effective field to:

[0060] B eff =B0+B THz (1-3)

[0061] Since the static field B0 is equal to 0 at the equilibrium position, the terahertz magnetic field acts as a driving torque. Considering that the magnetization change m<<1, when the driving frequency (i.e., THz frequency) is much higher than the system eigenfrequency (FMR), the magnetic moment does not have time to establish a resonant response. Therefore, formula (1-1) can be rewritten as:

[0062]

[0063] Where m0 is the equilibrium position of m, and the time derivative of m is related to B THz The time derivative of the measured data is proportional to the THz magnetic field. When the Py detector detects a strong-field THz signal, the true THz time-domain signal can be obtained simply by taking the time derivative of the raw data. This is exploited by the present invention to detect THz time-domain waveforms.

[0064] Specifically, the differential magnetization electric signal obtained by the two probes of the balanced detector 22 is equivalent to time-differentiating the magnetization electric signal according to formula (1-4), and finally the waveform of the differential magnetization electric signal is used as the waveform of the terahertz time-domain spectrum.

[0065] Figure 2 The strong-field terahertz time-domain signal measured by the present invention is shown in Figure 1. (a) is the original signal detected by the Py detector, and (b) is the real terahertz signal after the original signal is time-inversely measured.

[0066] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. An integrated high-field terahertz pumped optical Faraday detection device, characterized in that: include: Laser source (1), beam splitter (2), terahertz strong source generation module, collection module, delay module, Py detector (18) and detection module; The terahertz strong source generation module comprises a chopper (3), a grating (4), a gold mirror (5), a first plano-convex lens (6), a first half-wave plate (7), a second plano-convex lens (8), and a lithium niobate crystal (9), which are sequentially arranged on an optical path; The collection module comprises a first reflector (10), a first off-axis parabolic mirror (11), a second off-axis parabolic mirror (12), a third off-axis parabolic mirror (13), and a fourth off-axis parabolic mirror (14) which are sequentially arranged on the optical path; The delay module comprises a delay line (15), a second reflector (16), and a third reflector (17) which are sequentially arranged on the optical path; The detection module comprises a fourth reflector (19), a second half-wave plate (20), a Wollaston prism (21), and a balanced detector (22) which are sequentially arranged on the optical path; The laser light generated by the laser source (1) is divided into pump light and detection light through a beam splitter (2); the pump light is focused on a Py detector (18) of the detection module through a terahertz strong source generation module and a collection module; the detection light is focused on the Py detector (18) of the detection module through a delay module; and the detection light output by the Py detector (18) passes through the detection module to obtain a terahertz time-domain spectrum waveform.

2. The integrated high-field terahertz pumped optical Faraday detection device according to claim 1, characterized in that: In the terahertz strong source generation module, pump light passes through a chopper (3), is incident on a grating (4) at a preset angle, is reflected by a gold mirror (5), sequentially passes through a first plano-convex lens (6), a first half-wave plate (7), and a second plano-convex lens (8), and is then incident on a lithium niobate crystal (9) to generate a terahertz wave; the first plano-convex lens (6) and the second plano-convex lens (8) form a 4f imaging system.

3. The integrated high-field terahertz pumped optical Faraday detection device according to claim 2, characterized in that: The detection light passes through the delay line (15), the second reflector (16), the third reflector (17) in sequence, and then passes through the fourth off-axis parabolic mirror (14) to be focused on the Py detector (18) together with the pump light, and is kept vertically incident on the Py detector (18).

4. The integrated high-field terahertz pumped optical Faraday detection device according to claim 3, characterized in that: The Py detector (18) is composed of a ferromagnetic film and a substrate. The preparation method of the Py detector (18) is as follows: The pressure of the vacuum main chamber was maintained at 1×10 -8 Torr below, the quartz substrate was heated in acetone at 60° C. for 10 minutes in a vacuum main chamber, then placed in isopropanol, and then dried to obtain a double-polished quartz substrate; a ferromagnetic film was grown on the double-polished quartz substrate by a magnetron sputtering method.

5. The integrated high-field terahertz pumped optical Faraday detection device according to claim 4, characterized in that: The double polished quartz substrate of the Py detector (18) is 10 mm long, 10 mm wide and 3 mm thick, and the thickness of the ferromagnetic film is 9 nm.

6. The integrated high-field terahertz pumped optical Faraday detection device according to claim 5, characterized in that: The Py detector (18) is placed on a three-dimensional translation stage through a mirror frame, and a permanent magnet is arranged outside the mirror frame, and the magnetic field strength is 500mT.

7. The integrated high-field terahertz pumped optical Faraday detection device according to claim 6, characterized in that: The detection light after passing through the Py detector (18) passes through the fourth reflector (19), the second half-wave plate (20), the Wollaston prism (21) and the balanced detector (22) to generate a differential magnetization electric signal; The waveform of the differential magnetization electric signal is used as the terahertz time-domain spectrum waveform.

8. The integrated high-field terahertz pumped optical Faraday detection device according to claim 7, characterized in that: The terahertz magnetic field is calculated by the following expression, Where m is the normalized magnetization unit vector, γ is the electron gyromagnetic ratio, m0 is the equilibrium position of m, and B THz is the terahertz magnetic field.

9. The integrated high-field terahertz pumped optical Faraday detection device according to claim 8, characterized in that: The maximum single pulse energy of the laser source (1) is 7 mJ, the wavelength is 800 nm, and the repetition frequency is 1 kHz.

Images