A modular collimated femtosecond transient magneto-optic Kerr measurement device

Abstract

The utility model discloses a kind of modularization collimation's femtosecond transient magneto-optical kerr measurement device, including total spectrometer system, spectrometer subsystem and sample detection subsystem;The total spectrometer system includes femtosecond laser;The spectrometer subsystem includes broadband half-wave plate beam splitter, pumping light path system and probe light path system;The pumping light path system includes motorized delay line and optical chopper;The probe light path system includes variable diaphragm, first convex lens and BBO crystal;The sample detection subsystem includes sample environment system, signal probe light path system and data acquisition and control system;Sample environment system includes the sample stage and deep space camera of being arranged at the magnetic field center of electromagnet;The signal probe light path system includes third ultrafast mirror, Wollaston prism and balanced detector;Data acquisition and control system includes lock-in amplifier and processor.The femtosecond transient magneto-optical kerr measurement device of the utility model can improve measurement efficiency and improve measurement accuracy.

Description

Technical Field

[0001] This invention belongs to the field of magneto-optical measurement technology, specifically relating to a femtosecond transient magneto-optical Kerr measurement device with modular collimation. Background Technology

[0002] The Time-Resolved Magneto-Optical Kerr Effect (TR-MOKE) is a core experimental method for probing the ultrafast magnetic dynamics of materials using femtosecond laser pump-probe technology. The basic principle is as follows: a strong femtosecond laser beam (pump beam) excites the sample, causing an instantaneous change in its magnetization state; a weaker femtosecond laser beam with a precise time delay (probe beam) is reflected from the sample surface. By measuring the change in its polarization state (Kerr rotation angle), the magnetic state of the sample at different moments after excitation can be detected. By continuously changing the time delay between the pump and probe beams, the entire ultrafast evolution of the material's magnetization state can be tracked.

[0003] Existing TR-MOKE systems are typically built on open optical platforms, which has the following significant drawbacks and limitations: (1) Optical path collimation is extremely difficult and time-consuming; Pump light and probe light need to travel a long distance in free space and finally be precisely focused on a point on the sample at a very small angle, within a micrometer. When the sample is placed in a special environment (such as a cryostat or a strong magnetic field electromagnet), due to factors such as window and space limitations, it is extremely difficult to precisely couple the external light path with the sample, requiring experienced researchers to spend a lot of time on repeated adjustments. (2) Poor system stability; Open optical paths are very sensitive to environmental vibrations and temperature changes. Any slight disturbance can cause the optical path to deviate, thus affecting the accuracy and repeatability of the measurement results and requiring frequent recalibration. (3) Low degree of modularity and poor flexibility; Changing the sample environment device (e.g., from a room temperature electromagnet to a cryogenic unit) usually means that the entire detection optical path needs to be redesigned and collimated on a large scale, resulting in low experimental efficiency. (4) Limited improvement in signal-to-noise ratio; In complex sample environments, wavefront distortion or stray reflections may be introduced by optical components such as windows and lenses, which can affect the quality of the final detection signal and reduce the signal-to-noise ratio. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a modular collimated femtosecond transient magneto-optical Kerr measurement device, which can improve measurement efficiency and accuracy.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is: A modular collimated femtosecond transient magneto-optical Kerr measurement device includes a total beam splitting system, a beam splitting subsystem, and a sample detection subsystem, wherein... The total beam splitting system includes a femtosecond laser, which is used to generate femtosecond laser pulses with a center wavelength of 800 nm; The beam splitter system includes a broadband half-wave plate beam splitter, a pump optical path system, and a probe optical path system, wherein... The broadband half-wave plate beam splitter is used to split the femtosecond laser pulse into pump light and probe light; The pump optical path system includes an electric delay line and an optical chopper, wherein the pump light sequentially enters the electric delay line, the optical chopper and the first modular collimation and lifting filter subsystem; the pump light coming out of the first modular collimation and lifting filter subsystem is reflected by multiple first ultrafast mirrors and then irradiates the sample to be tested. The detection optical path system includes a variable aperture, a first convex lens, and a BBO crystal. The detection light sequentially enters the variable aperture, the first convex lens, the BBO crystal, and the second modular collimation and lifting filter subsystem. The detection light exiting the second modular collimation and lifting filter subsystem is reflected by multiple second ultrafast mirrors and then irradiates the sample to be detected. The sample detection subsystem includes a sample environment system, a signal detection optical path system, and a data acquisition and control system, wherein... The sample environment system includes a sample stage and a deep-space camera set at the center of the magnetic field of the electromagnet, wherein the sample to be tested is placed on the sample stage; the deep-space camera is used to capture image data of the magnetic domain changes of the sample and transmit the image data to the data acquisition and control system. The signal detection optical path system includes a third ultrafast reflector, a Wollaston prism, and a balanced detector. The detection light reflected from the sample surface passes sequentially through the third ultrafast reflector and the Wollaston prism before being received by the balanced detector. The balanced detector outputs a differential signal to the data acquisition and control system. The data acquisition and control system includes a lock-in amplifier and a processor. The lock-in amplifier is used to receive the differential signal output by the balanced detector, extract the Kerr rotation signal and transmit it to the processor. The processor is also used to receive image data acquired by the deep space camera.

[0006] Preferably, both the total beam splitting system and the beam splitting subsystem are equipped with a fourth ultrafast mirror for changing the propagation direction of the pump light and the probe light.

[0007] Preferably, there are two sets of first convex lenses, which are located on both sides of the BBO crystal. The probe light from the variable aperture passes through one set of first convex lenses and enters the BBO crystal. The probe light from the BBO crystal passes through the other set of first convex lenses and enters the second modular collimation and lifting filter subsystem.

[0008] Preferably, the sample detection subsystem is further provided with a second convex lens, through which the probe light from the second ultrafast reflector passes and illuminates the sample to be detected; the probe light reflected from the sample surface passes through the second convex lens and enters the third ultrafast reflector.

[0009] Preferably, the first modular collimation and lifting filter subsystem adopts a double elliptical mirror structure, wherein the installation angle of the elliptical mirror in the double elliptical mirror structure is 45 degrees and the working wavelength is 750-1100nm.

[0010] Preferably, the second modular collimation and lifting filter subsystem adopts a double elliptical mirror structure, wherein the elliptical mirror in the double elliptical mirror structure is installed at an angle of 45 degrees and has a working wavelength of 400-750nm.

[0011] Preferably, the processor is a control computer.

[0012] Preferably, the first ultrafast reflector and the fourth ultrafast reflector operate at wavelengths of 700–930 nm and have a reflectivity greater than 99%.

[0013] Preferably, the second and third ultrafast reflectors operate at wavelengths of 335–445 nm and have a reflectivity greater than 99%.

[0014] Preferably, the focal length of the first convex lens and the second convex lens is 500mm.

[0015] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. The femtosecond transient magneto-optical Kerr measurement device of this invention solves the problems of difficult and time-consuming coupling between the optical path and the sample environment in traditional methods; through modular design, it enables rapid replacement and access to different sample environments (such as low temperature and strong magnetic field), thereby improving experimental efficiency; 2. The femtosecond transient magneto-optical Kerr measurement device of this utility model also enhances the mechanical stability and anti-interference ability of the entire system. It adopts a modular collimation lifting and filtering subsystem design to ensure the reliability of long-term measurement.

[0016] 3. The femtosecond transient magneto-optical Kerr measurement device of this invention improves the signal-to-noise ratio and accuracy of the measurement by optimizing the detection optical path. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the modular collimated femtosecond transient magneto-optical Kerr measurement device of this utility model.

[0018] Figure 2 This is a structural diagram of the first modular collimation and lifting filter subsystem and the second modular collimation and lifting filter subsystem.

[0019] In the diagram: 1-Femtosecond laser; 2-Broadband flat beam splitter; 3-Fourth ultrafast mirror; 4-Broadband flat beam splitter; 5-Electrically powered delay line; 6-Optical chopper; 7-Variable aperture; 8-First convex lens; 9-BBO crystal; 10-Second ultrafast mirror; 11-Wollaston prism; 12-Balance detector; 13-Sample holder; 14-Electromagnet; 15-Control computer; 16-Deep space camera; 17-Second modular collimation and lifting filter subsystem; 18-First modular collimation and lifting filter subsystem; 19-Second convex lens; 20-Third ultrafast mirror; 21-Elliptical mirror; 22-First ultrafast mirror. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0021] See Figures 1-2 The modular collimated femtosecond transient magneto-optical Kerr measurement device of this invention includes a total spectral system, a spectral subsystem, and a sample detection subsystem.

[0022] See Figures 1-2 The total beam splitting system includes a femtosecond laser 1 and a broadband half-wave plate beam splitter 2. The femtosecond laser 1 is used to generate femtosecond laser pulses with a center wavelength of 800 nm. The broadband half-wave plate beam splitter 2 is used to better utilize the laser source (whose refracted light is used in other experiments) to improve the utilization rate of the laser source.

[0023] See Figures 1-2 The beam splitter system includes a broadband half-wave plate beam splitter 4, a pump optical path system, and a probe optical path system, wherein... The broadband half-wave plate beam splitter 4 is used to split the femtosecond laser pulse into a pump beam with higher energy (e.g., 70%) and a probe beam with lower energy (e.g., 30%). The pump optical path system includes an electrically driven delay line 5 and an optical chopper 6. The pump light sequentially enters the electrically driven delay line 5, the optical chopper 6, and the first modular collimation and lifting filter subsystem 18. The pump light exiting the first modular collimation and lifting filter subsystem 18 is reflected by multiple first ultrafast mirrors 22 and then illuminates the sample to be tested. The electrically driven delay line 5 precisely controls the optical path difference between the pump light and the probe light, thereby achieving a time delay scan on the order of nanoseconds to picoseconds. The optical chopper 6 performs intensity modulation (e.g., from several hundred Hz to several kHz), which is crucial for subsequent signal extraction using a lock-in amplifier. The entire pump optical path is guided by multiple high-reflectivity dielectric film mirrors (e.g., the fourth ultrafast mirror 3), i.e., changing the direction of light propagation. The detection optical path system includes a variable aperture 7, a first convex lens 8, and a BBO crystal 9. The detection light sequentially enters the variable aperture 7, the first convex lens 8, the BBO crystal 9, and a second modular collimation and lifting filter subsystem 17. The detection light exiting the second modular collimation and lifting filter subsystem 17 is reflected by multiple second ultrafast reflectors 10 and then illuminates the sample to be tested. The variable aperture 7 is used to precisely control the aperture of the laser beam, filtering out the poor-quality edges of the laser spot and allowing only the most uniform and energy-concentrated central portion to pass through. It also blocks stray light that may exist in the optical path, thus facilitating subsequent focusing and frequency doubling processes. A high-quality, well-defined beam is provided; the first convex lens 8 is used to receive the shaped beam from the variable aperture 7 and focus it into a very small spot. This process greatly increases the power density (power per unit area) at the spot location. Generating a sufficiently high power density is a prerequisite for achieving nonlinear optical effects (such as frequency doubling in the next step); the BBO crystal 9 is a nonlinear optical crystal. When the high-power-density 800nm ​​probe light focused by the first convex lens 8 passes through the BBO crystal 9, a second harmonic conversion occurs, thereby generating a new laser beam with a wavelength of 400nm (twice the frequency of the original light).

[0024] In this embodiment, both the pump light and the probe light pass through a standardized "modular collimation-lifting and filtering subsystem" before entering the sample environment system and after exiting the sample environment system. This modular collimation-lifting and filtering subsystem is a pre-collimated, rigidly connected module that internally employs optical elements such as a double elliptical mirror 21 to ensure near-perpendicular incident and exit (e.g.,...). Figure 2The 45° mirror assembly shown in the figure precisely guides and guides horizontally propagating free-space beams into and out of the vertical optical path window of the sample environment system (such as the deep space camera 16). For different wavelengths, the modular collimation and lifting filter subsystem has different optimized versions. For example, the first modular collimation and lifting filter subsystem 18 is adapted to pump light of 750-1100nm; the second modular collimation and lifting filter subsystem 17 is adapted to probe light of 400-750nm.

[0025] See Figures 1-2 The sample detection subsystem includes a sample environment system, a signal detection optical path system, and a data acquisition and control system, wherein... The sample environment system includes a sample stage 13 and a deep space camera 16 set at the center of the magnetic field of the electromagnet. The sample to be tested is placed on the sample stage 13. The deep space camera 16 is used to clearly capture the magnetic domain changes of the sample under the measurement conditions of varying magnetic field (e.g., 0-2T) and transmit the captured image data to the data acquisition and control system. The signal detection optical path system includes a third ultrafast reflector 20, a Wollaston prism 11, and a balanced detector 12. The detection light reflected from the sample surface passes sequentially through the third ultrafast reflector 20 and the Wollaston prism 11 before being received by the balanced detector 12. The balanced detector 12 outputs a differential signal to the data acquisition and control system. The detection light reflected from the sample surface carries information about the magnetization state (Kerr rotation of the polarization plane). This detection light is reflected and shaped by the third ultrafast reflector 20, a specially designed artificial micro / nano structure reflective element used to precisely compensate for wavefront aberrations introduced by the sample environment window or to enhance polarization contrast. The detection light enters the Wollaston prism 11, which decomposes the detection light into two beams with mutually orthogonal polarization directions (p-polarized and s-polarized). These two beams are received by a pair of balanced detectors 12. The data acquisition and control system includes a lock-in amplifier and a processor. The lock-in amplifier is used to receive the differential signal output by the balanced detector 12, demodulate it with the frequency of the optical chopper 6 as a reference, thereby greatly suppressing noise, extracting the Kerr rotation signal and transmitting it to the processor. The processor is also used to receive image data acquired by the deep space camera 16, and the delay line control, data acquisition and processing of the entire system are all completed by the control computer 15.

[0026] See Figures 1-2 Both the total beam splitting system and the beam splitting subsystem are equipped with a fourth ultrafast reflector 3 for changing the propagation direction of the pump light and the probe light.

[0027] See Figures 1-2The first convex lens 8 consists of two sets, with the two sets of first convex lenses 8 located on both sides of the BBO crystal 9. The probe light from the variable aperture 7 passes through one set of first convex lenses 8 and enters the BBO crystal 9. The probe light from the BBO crystal 9 passes through the other set of first convex lenses 8 and enters the second modular collimation lifting filter subsystem 17.

[0028] See Figures 1-2 The sample detection subsystem is also equipped with a second convex lens 19. The detection light from the second ultrafast reflector 10 passes through the second convex lens 19 and then illuminates the sample to be detected. The detection light reflected from the sample surface passes through the second convex lens 19 and then enters the third ultrafast reflector 20.

[0029] See Figures 1-2 The first modular collimation and lifting filter subsystem 18 adopts a double elliptical mirror structure, wherein the elliptical mirror 21 in the double elliptical mirror structure is installed at an angle of 45 degrees and has a working wavelength of 750-1100nm; the second modular collimation and lifting filter subsystem 17 adopts a double elliptical mirror structure, wherein the elliptical mirror 21 in the double elliptical mirror structure is installed at an angle of 45 degrees and has a working wavelength of 400-750nm.

[0030] See Figures 1-2 The first ultrafast reflector 22 operates at a wavelength of 700–930 nm and has a reflectivity greater than 99%; the second ultrafast reflector 10 operates at a wavelength of 335–445 nm and has a reflectivity greater than 99%.

[0031] See Figures 1-2 The focal length of the first convex lens 8 and the second convex lens 19 is 500mm.

[0032] See Figures 1-2 The modular collimated femtosecond transient magneto-optical Kerr measurement device of this invention has the following advantages: 1. Greatly simplifies optical path collimation and improves experimental efficiency: This invention employs a modular collimation and lifting filter subsystem to transform the most complex and time-consuming coupling process between the free-space optical path and the sample environment into a standardized, plug-and-play modular interface. This eliminates the need for users to perform tedious adjustments to the optical path entering the cryostat / magnet; they only need to align the external optical path with the standard inlet of the modular collimation and lifting filter subsystem. This significantly shortens experimental preparation time and improves the efficiency and convenience of instrument use.

[0033] In other words, the modular collimation and lifting filter subsystem is a pre-calibrated, rigidly integrated optical module used to achieve rapid, stable, and precise beam introduction and extraction between the external free-space optical path and the internal sample environment system (such as a cryostat or electromagnet). At the same time, the system can achieve optical path isolation, ensuring that the uncertain reflected light generated by the components of the spectrophotometer system does not interfere with the final sample, and also facilitates sample replacement.

[0034] 2. Enhanced system stability and modularity: The modular collimation and lifting filter subsystem of this invention has a rigid structure and its internal optical path is pre-calibrated and fixed. Compared with an open optical path composed of multiple independent mirrors, its mechanical stability and resistance to environmental interference are significantly enhanced. This modular design in the modular collimation and lifting filter subsystem makes it simple and quick to change different types of sample environments (such as thermostats and magnets from different manufacturers), thereby enhancing the system's versatility and scalability.

[0035] 3. Potentially higher signal-to-noise ratio: The detection optical path of this invention innovatively uses an ultrafast reflector. Compared with traditional reflectors, the ultrafast reflector can achieve more complex optical field manipulation functions, such as compensating for wavefront distortion that may be introduced by the sample environment window, thereby improving the spot quality and polarization purity of the detection light, and ultimately helping to improve the measurement signal-to-noise ratio of the Kerr signal.

[0036] 4. Fully functional and widely applicable: This invention integrates a second harmonic generator (BBO crystal 9), allowing for the selection of pump light wavelength. Combined with a strong magnetic field environment, it enables the study of a wide range of ultrafast physical phenomena in various material systems, such as photo-induced demagnetization, spin precession, and magnetic phase transitions.

[0037] The above are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above content. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A modular collimated femtosecond transient magneto-optical Kerr measurement device, characterized in that, It includes a total spectrophotometer system, a spectrophotometer subsystem, and a sample detection subsystem, among which, The total beam splitting system includes a femtosecond laser, which is used to generate femtosecond laser pulses with a center wavelength of 800 nm; The beam splitter system includes a broadband half-wave plate beam splitter, a pump optical path system, and a probe optical path system, wherein... The broadband half-wave plate beam splitter is used to split the femtosecond laser pulse into pump light and probe light; The pump optical path system includes an electric delay line and an optical chopper, wherein the pump light sequentially enters the electric delay line, the optical chopper and the first modular collimation and lifting filter subsystem; the pump light coming out of the first modular collimation and lifting filter subsystem is reflected by multiple first ultrafast mirrors and then irradiates the sample to be tested. The detection optical path system includes a variable aperture, a first convex lens, and a BBO crystal. The detection light sequentially enters the variable aperture, the first convex lens, the BBO crystal, and the second modular collimation and lifting filter subsystem. The detection light exiting the second modular collimation and lifting filter subsystem is reflected by multiple second ultrafast mirrors and then irradiates the sample to be detected. The sample detection subsystem includes a sample environment system, a signal detection optical path system, and a data acquisition and control system, wherein... The sample environment system includes a sample stage and a deep-space camera set at the center of the magnetic field of the electromagnet, wherein the sample to be tested is placed on the sample stage; the deep-space camera is used to capture image data of the magnetic domain changes of the sample and transmit the image data to the data acquisition and control system. The signal detection optical path system includes a third ultrafast reflector, a Wollaston prism, and a balanced detector. The detection light reflected from the sample surface passes sequentially through the third ultrafast reflector and the Wollaston prism before being received by the balanced detector. The balanced detector outputs a differential signal to the data acquisition and control system. The data acquisition and control system includes a lock-in amplifier and a processor. The lock-in amplifier is used to receive the differential signal output by the balanced detector, extract the Kerr rotation signal and transmit it to the processor. The processor is also used to receive image data acquired by the deep space camera.

2. The modular collimated femtosecond transient magneto-optical Kerr measurement device according to claim 1, characterized in that, Both the main beam splitting system and the beam splitting subsystem are equipped with a fourth ultrafast mirror for changing the propagation direction of the pump light and the probe light.

3. The modular collimated femtosecond transient magneto-optical Kerr measurement device according to claim 1, characterized in that, The first convex lens consists of two sets, with the two sets of first convex lenses located on opposite sides of the BBO crystal. The probe light from the variable aperture passes through one set of first convex lenses and enters the BBO crystal. The probe light from the BBO crystal passes through the other set of first convex lenses and enters the second modular collimation and lifting filter subsystem.

4. The modular collimated femtosecond transient magneto-optical Kerr measurement device according to claim 3, characterized in that, The sample detection subsystem is also equipped with a second convex lens. The probe light from the second ultrafast mirror passes through the second convex lens and then illuminates the sample to be detected. The probe light reflected from the sample surface passes through the second convex lens and then enters the third ultrafast mirror.

5. The modular collimated femtosecond transient magneto-optical Kerr measurement device according to claim 1, characterized in that, The first modular collimation and lifting filter subsystem adopts a double elliptical mirror structure. The installation angle of the elliptical mirror in the double elliptical mirror structure is 45 degrees, and the working wavelength is 750-1100nm.

6. The modular collimated femtosecond transient magneto-optical Kerr measurement device according to claim 1, characterized in that, The second modular collimation and lifting filter subsystem adopts a double elliptical mirror structure, wherein the elliptical mirror in the double elliptical mirror structure is installed at an angle of 45 degrees and has a working wavelength of 400-750nm.

7. The modular collimated femtosecond transient magneto-optical Kerr measurement device according to claim 1, characterized in that, The processor is a control computer.

8. The modular collimated femtosecond transient magneto-optical Kerr measurement device according to claim 2, characterized in that, The first and fourth ultrafast reflectors operate at wavelengths of 700–930 nm and have a reflectivity greater than 99%.

9. The modular collimated femtosecond transient magneto-optical Kerr measurement device according to claim 1, characterized in that, The second and third ultrafast reflectors operate at wavelengths of 335–445 nm and have a reflectivity greater than 99%.

10. The modular collimated femtosecond transient magneto-optical Kerr measurement device according to claim 4, characterized in that, The focal lengths of the first convex lens and the second convex lens are 500mm.

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