Cavity external optical coupling system applied to ultrahigh vacuum cavity of scanning tunneling microscope

Abstract

The invention provides a cavity external optical coupling system applied to an ultrahigh vacuum cavity of a scanning tunneling microscope. A femtosecond laser generates femtosecond pulse light; the first beam splitter splits the femtosecond pulse light to obtain a first light beam and a first residual light beam; the second beam splitter splits the first residual light beam into a second light beam and a second residual light beam; the third beam splitter splits the second residual light beam into a third light beam and a fourth light beam; the first light path module is used for processing the first light beam as first sampling light; the second light path module is used for processing the second light beam; the third light path module is used for processing the third light beam as second sampling light; the terahertz generation module is used for generating terahertz pulses according to the second light beam and the fourth light beam; the first parabolic mirror unit is used for focusing terahertz pulses to the electro-optical crystal; the electro-optical crystal is used for detecting THz light; the second parabolic mirror unit is used for focusing and emitting the terahertz pulse; and the gold reflector is used for guiding the terahertz pulse to the tunnel junction of the scanning tunneling microscope.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of microscopy, in particular to the light coupling technology in the light coupling scanning tunneling microscope, more particularly to a cavity light coupling system applied to the scanning tunneling microscope ultra-high vacuum cavity. BACKGROUND

[0002] The light coupling scanning tunneling microscope (STM) is a powerful means to explore the interaction between materials and light at the microscale. Among them, the THz-STM uses THz band pulsed light (or continuous light), which has small photo-thermal effect and can be regarded as an ultrafast transient bias, and has attracted much attention in recent years. Focusing invisible and low-energy THz light to the tunnel junction of the ultra-high vacuum low-temperature STM is a major difficulty in this technology. The current realization scheme of THz light focusing mainly includes two kinds: one is to use a convex lens to focus, and the other is to use a parabolic mirror to focus.

[0003] The convex lens scheme has lower focusing requirements. As long as the paraxial condition is met, the incident parallel light can be well focused at the focal point. Since the incident light direction is collinear with the focal point, the adjustment of the convex lens is relatively convenient, and it is easy to install inside the ultra-high vacuum to be as close as possible to the STM tunnel junction to achieve a larger numerical aperture. However, since the refractive index of the convex lens is significantly different for different wavelengths of light, the focusing state of the convex lens often needs to be recalibrated when switching light sources, and the pulse width of the pulsed light is inevitably affected. For THz pulses, which are invisible and have a wavelength far from the visible reference light, it is more inconvenient. The parabolic mirror scheme avoids this problem in the convex lens scheme. Since the focusing condition and the focal point position of the parabolic mirror depend almost entirely on the geometric characteristics of the mirror surface and are hardly dependent on the material refractive index, the parabolic mirror has good universality for different wavelengths of light. However, the parabolic mirror has the disadvantage that its focusing condition is relatively strict. Since the incident light and the focal point of the parabolic mirror are not collinear, the adjustment steps are more complicated, requiring adjustment of the three-dimensional translational degrees of freedom, as well as adjustment of the azimuth and elevation rotational degrees of freedom. This requires a five-dimensional mirror frame, but a five-dimensional mirror frame is often bulky, and the space near the microscope scanning head is usually cramped. Installing such a five-dimensional mirror frame inside the ultra-high vacuum will place a heavy burden on the cooling and damping of the scanning head, which is not conducive to system stability. In addition, due to the excessive degrees of freedom, it is difficult to ensure that the focal point of the parabolic mirror coincides well with the tunnel junction during in-cavity adjustment. In order to solve the problem of adjustment difficulty, some commercial products try to fix the position of the parabolic mirror after adjusting it outside the cavity, and then install it together with the scanning head into the ultra-high vacuum cavity. However, every time the needle tip is changed, the sample is changed, or the scanning head is changed in temperature, the tunnel junction position will inevitably shift, which will greatly reduce the focusing effect of the parabolic mirror. Another approach is to adjust the parabolic mirror outside the cavity, which will reduce the adjustment difficulty. However, since the STM requires ultra-high vacuum and low temperature, the cavity and dewar are often designed to be large, and if the parabolic mirror is placed outside the cavity, it will be too far away from the tunnel junction, which is not conducive to focusing and collection.

[0004] Therefore, there is a need for a new design of an ultra-high vacuum low-temperature STM that is compatible with the optical requirements to improve experimental efficiency. It should be noted that the background art is only used to introduce relevant information of the present application, in order to help understand the technical solutions of the present application, but it does not mean that the relevant information must be prior art. In the absence of evidence that the relevant information has been disclosed before the filing date of the present application, the relevant information should not be considered as prior art. SUMMARY

[0005] Therefore, the purpose of the present application is to overcome the defects of the prior art, and to provide a new cavity external light coupling system applied to an ultra-high vacuum cavity of a scanning tunneling microscope.

[0006] The object of the present application is achieved by the following technical solutions:

[0007] The application provides a cavity outer light coupling system applied to a scanning tunneling microscope ultra-high vacuum cavity, which is used for realizing light coupling outside the ultra-high vacuum cavity and focusing at a tunnel junction of the ultra-high vacuum cavity, and the system comprises a femtosecond laser, a first beam splitter, a second beam splitter, a third beam splitter, a first optical path module, a second optical path module, a third optical path module, a terahertz generation module, a first parabolic mirror unit, an electro-optic crystal, a second parabolic mirror unit and a gold mirror, wherein the femtosecond laser is used for generating original femtosecond pulse light of 1030 nm; the first beam splitter is used for splitting the original femtosecond pulse light to obtain a first light beam and a first residual light beam; the second beam splitter is used for splitting the first residual light beam to obtain a second light beam and a second residual light beam; the third beam splitter is used for splitting the second residual light beam to obtain a third light beam and a fourth light beam, and the fourth light beam is directly transmitted to the terahertz generation module; the first optical path module is used for reflecting, frequency doubling and time delay processing of the first light beam, and the first light beam processed by the first optical path module is used as first sampling light; the second optical path module is used for reflecting and time delay processing of the second light beam, and the second light beam processed by the second optical path module is transmitted to the terahertz generation module; the third optical path module is used for reflecting and time delay processing of the third light beam, and the third light beam processed by the third optical path module is used as second sampling light; the terahertz generation module is used for generating a first terahertz pulse and a second terahertz pulse based on the second light beam and the fourth light beam respectively; the first parabolic mirror unit comprises a plurality of parabolic mirrors and is used for focusing the first terahertz pulse or the second terahertz pulse at the electro-optic crystal respectively; the electro-optic crystal is used for detecting THz light; the second parabolic mirror unit comprises a plurality of parabolic mirrors and is used for focusing the first terahertz pulse or the second terahertz pulse out respectively; and the gold mirror is used for guiding the focused first terahertz pulse or the second terahertz pulse to the scanning tunneling microscope tunnel junction.

[0008] Preferably, the first beam splitter is a beam splitter with an energy ratio of 1:1; the second beam splitter is a beam splitter with an energy ratio of 1:1; and the third beam splitter is a beam splitter with an energy ratio of 92:8, wherein the light beam with smaller energy after splitting by the third beam splitter is the third light beam.

[0009] Preferably, a first wave plate, a first mirror and a second mirror are sequentially arranged between the first beam splitter and the second beam splitter; a third mirror and a fourth mirror are sequentially arranged between the second beam splitter and the third beam splitter; wherein the first wave plate is used for optimizing grating diffraction efficiency, and the first mirror, the second mirror, the third mirror and the fourth mirror are used for adjusting the optical path of the light beam.

[0010] Preferably, the system further includes a flip-up mirror disposed in the exit paths of the first and third beams for selecting the first or third beam as the sampling light by means of a flip angle.

[0011] Preferably, the first optical path module includes a fifth reflector, an optical parametric amplifier, a sixth reflector, a seventh reflector, a first delay stage, and an eighth reflector arranged sequentially, wherein: the fifth reflector is used to reflect the first beam emitted from the first beam splitter into the optical parametric amplifier; the optical parametric amplifier is used to perform frequency doubling and tuning processing on the first beam; the sixth and seventh reflectors are used to reflect the first beam after frequency doubling and tuning processing and input it into the first delay stage; the first delay stage is used to delay the incoming first beam to adjust the optical path difference and send it into the eighth reflector; the eighth reflector is used to send the delayed first beam into the flip-up reflector.

[0012] Preferably, the second optical path module includes a ninth reflector, a second delay stage, a tenth reflector, and an eleventh reflector arranged sequentially, wherein: the ninth reflector is used to reflect the second beam split by the second beam splitter into the second delay stage; the second delay stage is used to delay the incident second beam to adjust the optical path difference and send it into the tenth reflector; the tenth reflector is used to send the incident second beam into the eleventh reflector; and the eleventh reflector is used to send the second beam into the THz generation module at a preset angle with the first beam.

[0013] Preferably, the third optical path module includes a twelfth reflector, a third delay stage, and a thirteenth reflector arranged in sequence, wherein: the twelfth reflector is used to reflect the third beam emitted by the third beam splitter into the third delay stage; the third delay stage is used to delay the incoming third beam to adjust the optical path difference and send it into the thirteenth reflector; the thirteenth reflector is used to send the incident third beam into the flip-up reflector.

[0014] Preferably, the terahertz generation module includes a fourteenth reflector, a chopper, a fifteenth reflector, a sixteenth reflector, a grating, a second waveplate, a first convex lens, and a nonlinear crystal arranged sequentially, wherein: the fourteenth reflector is used to reflect the second or third beam into the chopper; the chopper is used to modulate the light intensity to filter part of the laser and improve the laser resolution; the fifteenth and sixteenth reflectors are used to reflect the second or third beam after the chopper into the grating; the second waveplate is used to optimize the THz generation efficiency of the nonlinear crystal, and it is a half-waveplate; the first convex lens is used to focus the second or third beam onto the nonlinear crystal and make it meet the matching condition for THz tilted wavefront generation.

[0015] Preferably, the system further includes a third waveplate for focusing the first or second sampling light onto the electro-optic crystal.

[0016] Preferably, the system further comprises a third convex lens, a third wave plate, a Wollaston prism, and a photodetector, wherein: the third convex lens is used for focusing the selected sampling light onto the photodetector; the third wave plate is used for converting the sampling light into circularly polarized light; the Wollaston prism is used for splitting the s-polarized component and the p-polarized component of the circularly polarized light; and the photodetector is used for detecting the difference between the light intensities of the s-polarized component and the p-polarized component to detect the THz intensity waveform.

[0017] Compared with the prior art, the advantages of the present application are as follows: the present application can very efficiently realize better THz focusing, the photocurrent can reach the order of 100 pA under the condition of a repetition frequency of 1 MHz, corresponding to 1000 electrons being stably tunneling under the driving of each pulse, stable photocurrent signals can be observed in various materials, and higher time resolution and spatial resolution can be realized. The parabolic mirror is installed outside the cavity in the present application, thereby overcoming the disadvantage of difficult adjustment while exerting the advantage of wavelength universality. The device is easy to operate, has high signal-to-noise ratio, and has superior performance, and has wide upgrading space and application prospect. Most of the existing technologies place the convex lens and the parabolic mirror used for focusing in the cavity to be as close as possible to the scanning head. The convex lens has a limited wave band, and can affect the THz waveform; and the parabolic mirror is complex to adjust, and is not easy to adjust in the cavity. Therefore, the existing THz-STM system is relatively complex to operate, or is not convenient for integrating comprehensive experimental technologies across wave bands. BRIEF DESCRIPTION OF DRAWINGS

[0018] The embodiments of the present application are further described below with reference to the drawings, in which:

[0019] Figure 1 FIG. 1 is a schematic diagram of a light path of a cavity outside scanning tunneling microscope ultra-high vacuum cavity according to an embodiment of the present application;

[0020] Figure 2 FIG. 4 is a schematic diagram of THz photocurrent rectification spectrum signals under different currents according to an embodiment of the present application;

[0021] Figure 3 FIG. 5 is a schematic diagram of time resolution of THz driving photocurrent according to an embodiment of the present application;

[0022] Figure 4 FIG. 6 is a schematic diagram of spatial resolution of THz driving photocurrent according to an embodiment of the present application;

[0023] Figure 5 FIG. 7 is a schematic diagram of a result of observing a material by using the system according to an embodiment of the present application. DETAILED DESCRIPTION

[0024] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0025] As described in the background, the existing THz focusing schemes all have some problems, which can be summarized as follows: first, in the existing convex lens focusing scheme, the convex lens needs to be continuously calibrated for different wavelengths, which will affect the pulse width of the pulsed light, and it will be very inconvenient for invisible THz light; second, in the existing parabolic mirror focusing scheme, the five-dimensional mirror frame will affect the cooling and damping of the scanning head, which is not conducive to the stability of the system, and cannot well guarantee the coincidence with the tunnel junction; third, in the existing scheme of focusing outside the cavity and then moving into the cavity, it cannot guarantee the accurate coincidence with the tunnel junction after the shift; fourth, in the existing scheme of directly focusing the tunnel junction outside the cavity, due to the large size of the system, the distance is far, which is not conducive to focusing and collecting.

[0026] Researchers have designed a more compact cavity and scanning head for THz-STM than traditional ultra-high vacuum STM, which greatly shortens the distance from the light coupling window to the STM scanning head, and can well adapt to the cavity outside focusing scheme, however, the existing cavity outside THz focusing scheme cannot well adapt to this cavity, therefore, the present application proposes a new cavity outside THz focusing scheme.

[0027] In order to better understand the present application, first introduce the basic principle of scanning tunneling microscope. The cooling of STM is realized by a liquid helium-free low-temperature insert rod, the temperature at the bottom of the insert rod can reach 2.5K, the cavity vacuum can reach 3x10 -10Pa. The scanning head is directly suspended at the bottom of the cryostat stem by springs and copper rods to achieve the lowest temperature of the sample. The scanning head is wrapped by a cold shield consisting of a layer of stator and a layer of rotor fixed on the cryostat stem. The corresponding windows are opened only when the sample is transferred and the light is on to isolate the external thermal radiation and electromagnetic radiation noise and ensure the stability of the scanning head. The stem is fixed on a four-dimensional displacement table, which enables the scanning head to translate or change the inclination angle with the stem, further facilitating light coupling. The entire cavity is located on two active damping modules, which can actively compensate for vibrations in the xyz three dimensions to reduce mechanical noise from the building or other parts of the floor. In order to facilitate light coupling, the area near the tunnel junction is designed to be relatively open. All lines are connected to the electrodes on the scanning head using coaxial cables with shielding layers, and the shielding layers are wrapped around the cryostat stem to reduce heat leakage and are directly grounded through the cavity. All high-voltage lines related to the scanning tube and coarse approximation are introduced to the outside of the vacuum chamber through a 10-pin aviation plug. Bias and current signal lines are separately connected to the outside of the vacuum chamber through a BNC interface to avoid crosstalk from other lines. A well-grounded preamplifier outside the chamber converts the tunneling current signal into a voltage signal with an amplification factor of 1x109V / A. The signal lines and high-voltage lines outside the chamber are all connected to the corresponding interfaces of the commercial STM controller through cables with shielding layers. The vacuum chamber is separately connected to the ground prepared by the laboratory to shield the electromagnetic radiation and power noise in the environment. The coarse approximation uses a Pan-type structure composed of three groups of piezoelectric ceramic pieces bonded into motor legs to ensure the stability of the scanning head. Part of the eddy current damping magnet is reduced, and the volume is greatly reduced, so that the cavity volume is also very small. We can adjust the focusing parabolic mirror outside the chamber, and the center distance of the tunnel junction from the window is only 4 cm.

[0028] Due to the compact cavity, the distance from the light coupling window to the STM scanning head is shortened, providing an advantage for external focusing of the cavity.

[0029] To this end, the application provides a cavity external light coupling system applied to a scanning tunneling microscope ultra-high vacuum cavity, which is used for realizing light coupling outside the ultra-high vacuum cavity and focusing on the tunnel junction of the ultra-high vacuum cavity, and the system comprises a femtosecond laser, a first beam splitter, a second beam splitter, a third beam splitter, a first optical path module, a second optical path module, a third optical path module, a terahertz generation module, a first parabolic mirror unit, an electro-optic crystal, a second parabolic mirror unit and a gold mirror, wherein: the femtosecond laser is used for generating original femtosecond pulse light of 1030 nm; the first beam splitter is used for splitting the original femtosecond pulse laser to obtain a first light beam and a first remaining light beam, according to an embodiment of the application, the first beam splitter is a beam splitter with an energy ratio of 1:1; the second beam splitter is used for splitting the first remaining light beam to obtain a second light beam and a second remaining light beam, according to an embodiment of the application, the second beam splitter is a beam splitter with an energy ratio of 1:1; the third beam splitter is used for splitting the second remaining light beam to obtain a third light beam and a fourth light beam, the fourth light beam is directly transmitted to the terahertz generation module, according to an embodiment of the application, the third beam splitter is a beam splitter with an energy ratio of 92:8, wherein the light beam with less energy after splitting by the third beam splitter is the third light beam; the first optical path module is used for reflecting, frequency doubling and time delay processing of the first light beam to make the path to the electro-optic crystal consistent with other light beams but the optical path inconsistent, and the first light beam processed by the first optical path module is used as the first sampling light; the second optical path module is used for reflecting and time delay processing of the second light beam to make the path to the electro-optic crystal consistent with other light beams but the optical path inconsistent, and the second light beam processed by the second optical path module is transmitted to the terahertz generation module; the third optical path module is used for reflecting and time delay processing of the third light beam to make the path to the electro-optic crystal consistent with other light beams but the optical path inconsistent, and the third light beam processed by the third optical path module is used as the second sampling light; the terahertz generation module is used for generating a first terahertz pulse and a second terahertz pulse based on the second light beam and the fourth light beam respectively; the first parabolic mirror unit comprises a plurality of parabolic mirrors and is used for focusing the first terahertz pulse or the second terahertz pulse to the electro-optic crystal respectively; the electro-optic crystal is used for detecting THz light; the second parabolic mirror unit comprises a plurality of parabolic mirrors and is used for focusing the first terahertz pulse or the second terahertz pulse out respectively; and the gold mirror is used for guiding the focused first terahertz pulse or the second terahertz pulse to the tunnel junction of the scanning tunneling microscope.

[0030] According to an embodiment of the application, a first wave plate, a first mirror and a second mirror are sequentially arranged between the first beam splitter and the second beam splitter; and a third mirror and a fourth mirror are sequentially arranged between the second beam splitter and the third beam splitter. The first wave plate is used for optimizing the diffraction efficiency of the grating, and the first mirror, the second mirror, the third mirror and the fourth mirror are used for adjusting the optical path of the light beam.

[0031] According to one embodiment of the present application, the system further comprises a foldable mirror arranged on the first light beam and the third light beam exit path for selecting the first light beam or the third light beam as the sampling light by flipping the angle.

[0032] According to one embodiment of the present application, the first light path module comprises a fifth mirror, an optical parametric amplifier, a sixth mirror, a seventh mirror, a first delay stage, and an eighth mirror arranged in sequence, wherein: the fifth mirror is used for reflecting the first light beam split by the first beam splitter into the optical parametric amplifier; the optical parametric amplifier is used for frequency doubling and tuning processing of the first light beam; the sixth mirror and the seventh mirror are used for reflecting the first light beam after frequency doubling and tuning processing into the first delay stage; the first delay stage is used for delaying the first light beam to adjust the optical path difference and then entering the eighth mirror; and the eighth mirror is used for reflecting the first light beam after delay processing into the foldable mirror. The second light path module comprises a ninth mirror, a second delay stage, a tenth mirror, and an eleventh mirror arranged in sequence, wherein: the ninth mirror is used for reflecting the second light beam split by the second beam splitter into the second delay stage; the second delay stage is used for delaying the second light beam to adjust the optical path difference and then entering the tenth mirror; the tenth mirror is used for reflecting the second light beam into the eleventh mirror; and the eleventh mirror is used for reflecting the second light beam into the THz generation module at a preset angle with the first light beam. The third light path module comprises a twelfth mirror, a third delay stage, and a thirteenth mirror arranged in sequence, wherein: the twelfth mirror is used for reflecting the third light beam split by the third beam splitter into the third delay stage; the third delay stage is used for delaying the third light beam to adjust the optical path difference and then entering the thirteenth mirror; and the thirteenth mirror is used for reflecting the third light beam into the foldable mirror.

[0033] The THz generation module comprises a fourteenth mirror, a chopper, a fifteenth mirror, a sixteenth mirror, a grating, a second wave plate, a first convex lens, and a nonlinear crystal arranged in sequence, wherein: the fourteenth mirror is used for reflecting the second light beam or the third light beam into the chopper; the chopper is used for modulating the light intensity to filter part of the laser to improve the laser resolution; the fifteenth mirror and the sixteenth mirror are used for reflecting the second light beam or the third light beam after the chopper into the grating; the second wave plate is used for optimizing the THz generation efficiency of the nonlinear crystal, which is a half wave plate; and the first convex lens is used for focusing the second light beam or the third light beam onto the nonlinear crystal and satisfying the matching condition of THz tilted wavefront generation.

[0034] According to one embodiment of the present application, the system further comprises a third wave plate for focusing the first sampling light or the second sampling light onto the electro-optic crystal; the system further comprises a third convex lens, the third wave plate, the Wollaston prism, and a photodetector, wherein: the third convex lens is configured to focus the selected sampling light onto the photodetector; the third wave plate is configured to convert the sampling light into circularly polarized light; the Wollaston prism is configured to split the s-polarization component and the p-polarization component of the circularly polarized light; and the photodetector is configured to detect the difference between the light intensities of the s-polarization component and the p-polarization component to detect the THz intensity waveform.

[0035] For a more clear understanding of the present application, the application will now be described in detail with reference to the following drawings.

[0036] As Figure 1As shown, it shows the optical path diagram of the cavity external light coupling system of the application applied to the scanning tunneling microscope ultra-high vacuum cavity, the femtosecond laser outputs 1030nm femtosecond pulse, and is divided into four beams, which are respectively marked as ABCD in the figure, wherein C is the first light beam, B is the second light beam, D is the third light beam, and A is the fourth light beam, the first beam splitter (1) divides the femtosecond pulse into light beam C and first remaining light beam, the first remaining light beam is injected into the second beam splitter (5) after the first wave plate (2), the first mirror (3) and the second mirror (4), the second beam splitter (5) divides the first remaining light beam into light beam B and second remaining light beam, the second remaining light beam is injected into the third beam splitter (8) after the third mirror (6) and the fourth mirror (7), the third beam splitter (8) divides the second remaining light beam into light beam A and light beam D. Light beam C is injected into the optical parametric amplifier after the fifth mirror (29), and is injected into the first delay table (32) after frequency doubling and tuning of the optical parametric amplifier, reflected by the eighth mirror after coming out of the first delay table (32), and injected into the foldable mirror (5). Light beam B is reflected into the second delay table (18) by the ninth mirror, and is injected into the THz generation module after coming out of the second delay table, passing through the tenth mirror (19) and the eleventh mirror (20). Light beam D is injected into the third delay table (22) by the twelfth mirror (21), and is injected into the foldable mirror (5) after being reflected by the thirteenth mirror after coming out of the third delay table (22). Light beam A is directly injected into the THz generation module. The THz generation module adopts the tilted wavefront technology, which includes the fourteenth mirror (9), the chopper ①, the fifteenth mirror (10), the sixteenth mirror (11), the grating ②, the second wave plate (12), the nonlinear crystal ③ in turn. The THz pulse generated by the THz generation module is focused on the electro-optic crystal ④ after passing through the first parabolic mirror (14) and the second parabolic mirror (15), and the THz pulse passing through the electro-optic crystal ④ is focused on the gold mirror ⑦ by the third parabolic mirror (16), and the THz pulse is guided to the scanning tunneling microscope tunnel junction by the gold mirror ⑦. The selected one of light beam C and light beam D is focused on the electro-optic crystal ④ by the second convex lens (24). The gold mirror ⑦ can adjust the angle to inject the sampling light into the third convex lens (25) to focus on the photodetector (amplifier in the figure), and convert the circularly polarized light into circularly polarized light after passing through the third wave plate (26), and then use the Wollaston prism ⑥ to divide the s polarization component and the p polarization component of the circularly polarized light to facilitate the photodetector to detect the difference between the light intensity of the s polarization component and the p polarization component to detect the THz intensity waveform.

[0037] In the scheme of the present application, the femtosecond pulse light outputted by the laser is divided into four beams, and three delay tables are provided to adjust the time delay among the beams, and different delays can be achieved by adjusting the parameters of each delay table according to the required delay interval. In the present application, the tilted wavefront technology is used to generate THz pulses. Among the four beams, the first two beams AB pass through the tilted wavefront of the grating ②, and then are incident on the nonlinear crystal LiNbO3 ③ at a specific angle, so as to exactly meet the phase matching condition of THz generation, thereby generating THz pulses. The advantage of this method is that the generated THz energy is strong, which is more conducive to the generation of photocurrent signals; and the generated THz pulses and the infrared light are emitted along non-collinear directions, which is conducive to the collection of THz light. In the present application, the repetition frequency of the laser is 1 MHz, the maximum output power is 20 W, and the THz energy conversion efficiency can reach 0.05%. Each of the two infrared beams AB generates a THz pulse and is emitted along the same path. In order to prevent the two beams from interfering near the time zero point, the eleventh mirror (20) in the present application has an angle with the route of the beam A, so that the two infrared beams AB are diverged by a small angle (0.15°) to be incident on the LN crystal. Compared with other schemes using a Michelson-Morley interferometer to split THz, the advantage of this method is that the THz pulse does not need to pass through a special THz beam splitter mirror, so there is almost no influence on the waveform and energy loss after the THz is generated. Although the infrared light is split at a strength ratio of about 1:1, the generated THz intensity ratio reaches 4:1 or even more, and the THz pulse intensity is sufficient to generate a photocurrent signal.

[0038] The beam C is frequency-doubled by a commercial OPA (Optical Parameter Amplifier) and tuned after the pulse light, and is focused on the electro-optic crystal ZnTe (ZT) ④ as the sampling light of the THz waveform detection. In the present application, the balanced detection scheme is used, and the ZT crystal is cut along the <110> surface. After the sampling light passes through the ZT crystal, it is first focused by a convex lens (25), and then converted into circularly polarized light by a 1 / 4 glass (26). Then, the s-polarized component and the p-polarized component of the sampling light are split by a Wollaston prism ⑥, and the difference between the intensities of the two beams is detected. When there is no THz electric field and the sampling light reaches the ZT crystal at the same time, the intensities of the two beams are the same, and the system is in a "balanced" state. When there is a THz electric field and the sampling light reaches the ZT crystal at the same time, the THz electric field will change the refractive index of the ZT crystal, thereby affecting the polarization state of the sampling light, leading to the deviation of the intensities of the two beams from the "balanced" state. The difference between the intensities of the two beams is linearly related to the THz field strength, and therefore, by scanning the time delay between the THz pulse and the sampling light, the THz electric field intensity waveform can be obtained.

[0039] The beam D is directly co-focused with the THz beam on the ZT crystal as the optional sampling light (selecting C or D as the sampling beam by the foldable mirror 5), and also as the optional Pump light to the tunnel junction. In summary, all the four beams have achieved the full overlap in time and space at the ZT crystal (and the subsequent STM tunnel junction).

[0040] The generated THz pulses are guided to the ZT crystal for waveform collection or focused to the tunnel junction inside the vacuum cavity by a series of gold mirrors 7 and off-axis parabolic mirrors. The part of the optical path after THz generation is entirely placed in a nitrogen atmosphere, ensuring the humidity below 10%, to prevent the influence of water vapor in the air on the THz waveform. The entire equipment is placed in a super-clean room, keeping the room temperature stable at 23°C, to ensure that the laser and the optical path work in a relatively constant temperature and humidity.

[0041] Other optical elements in the optical path: (3) (4) (6) (7) (9) (10) (11) (17) (19) (20) (21) (23) (29) (30) (31) (33) are Ag-coated ultrafast mirrors that can change the direction of the infrared light beam without introducing broadening and energy attenuation. (1) is a 1:1 energy ratio beam splitter used to split part of the infrared light as the input of the OPA. (5) is a 1:1 energy ratio beam splitter used to split part of the infrared light to generate a second THz pulse. (8) is a 92:8 energy ratio (transmission: reflection) beam splitter used to split part of the infrared light as the sampling light for THz waveform detection. (2) is an infrared half-wave plate used to optimize the diffraction efficiency of the grating. (12) is a half-wave plate used to optimize the THz crystal generation efficiency. (13) is a convex lens used to image the grating, thereby achieving the condition that the infrared pulse satisfies the phase matching condition of the tilted wave inside the LiNbO3 crystal. (14) is an Au-coated off-axis parabolic mirror used to convert the THz pulse into parallel light and change its propagation direction. (15) (16) are confocal Au-coated off-axis parabolic mirrors used to focus the THz pulse on the ZnTe crystal to detect the waveform, where (15) has a small hole on the back for the sampling light to pass through, so that the sampling light can also be co-focused on the ZnTe crystal. (18) is a delay table with a right-angle mirror used to adjust the optical path difference between the two THz pulses. (22) (32) are delay tables with right-angle mirrors used to adjust the optical path difference between the THz pulse and the 1030 nm sampling light, and the THz pulse and the OPA sampling light, respectively. (24) is a convex lens used to focus the sampling light on the ZnTe crystal. (25) is a convex lens used to focus the sampling light on the photodetector. (26) is a quarter-wave plate used to convert the sampling light into circularly polarized light (without THz electric field). (27) (28) are photodetectors used to detect the light intensity of the sampling light s and p, respectively.

[0042] The THz beam is focused from outside the cavity onto the tunnel junction through a long-focal-length parabolic mirror with a focal length of 200mm. A visible guide beam is used to simulate the THz optical path, and the gold mirror used for THz guidance and focusing is adjusted based on this. Observations from the THz camera show that the THz beam diameter is approximately 2mm after focusing. Once the THz beam is focused onto the tunnel junction, if the THz pulse has an electric field component perpendicular to the sample surface, a THz-driven tunneling current signal can be detected. By fine-tuning the long-focal-length parabolic mirror to maximize the photocurrent signal, it is considered that the THz beam is essentially focused near the tunnel junction.

[0043] For a tunnel junction, the equivalent bias voltage of the THz electric field can be considered as a quasi-static bias voltage. The tunneling current is generated within the order of fs, so the photocurrent direction will reverse within one THz period as the THz electric field direction reverses. However, for the entire circuit, the THz frequency is much larger than the circuit bandwidth and the preamplifier bandwidth (4kHz), so the controller can only detect the rectified signal of the photocurrent. The rectified signal is heavily dependent on the THz waveform. If the THz period is too long, or the electric field bias direction is not obvious, the rectified signal will be very small and difficult to detect. In addition, the generation of the rectified signal also depends on the nonlinearity of the tunnel junction. Since the THz is generated by the optical rectification effect of the LN crystal, if the IV characteristic of the tunnel junction is not nonlinear, there will be no rectified signal, and the photocurrent signal cannot be detected.

[0044] Furthermore, this rectified signal is mixed with the DC tunneling current. Therefore, a chopper can be applied in the optical path to modulate the light intensity. The chopper frequency is typically set between 500-1kHz, much lower than the laser repetition rate but higher than the photocurrent signal sampling frequency. The corresponding frequency components in the current signal are then demodulated using a lock-in amplifier to obtain the (rectified) photocurrent signal. The difference between the chopper phase and the photocurrent phase depends on the entire system, including the tip and tunnel junction states, the controller circuit response, and the phase delay introduced by the cable. The relaxation timescale of the photocurrent signal that the sample can influence is usually much smaller than the modulation period applied by the chopper; therefore, under constant external experimental conditions, the phase difference remains relatively stable. In the Pump-Probe experiment, the chopper modulates both beams simultaneously, resulting in a photocurrent signal that is the total rectified signal driven by both THz beams.

[0045] To verify the THz energy of this invention, the THz rectified spectrum signal acquired in constant current mode on Si(111) is as follows: Figure 2 As shown, the rectified spectrum signal is considered to reflect the information of the mirror charge state on the Si surface. It can be seen that the peak THz photocurrent signal near -3V exceeds 100pA.

[0046] Figure 3 The time resolution of THz driven photocurrent was demonstrated. Figure 4 It demonstrates the spatial resolution of THz driven photocurrent, from Figure 3 It can be seen that the full width at half maximum (FWHM) of the autocorrelation peak at time zero reaches 700 fs, indicating that the THz-STM of this invention can achieve sub-picosecond time resolution. Figure 4 The invention demonstrates spatial scanning imaging of THz photocurrent. Under constant-height mode and zero DC bias, every atom on the Si surface is clearly visible, indicating that the THz-STM of this invention can achieve atomic-level spatial resolution.

[0047] In this invention, the generated THz pulse experiences a half-wave loss (phase inversion) each time it passes through a focal point or a node of a THz reflector. THz phase reversal can be achieved using a set of confocal TPX lenses with a focal length of 50mm. A THz linear grid polarizer allows for continuous adjustment of the THz polarization. Furthermore, continuous control of the THz carrier envelope phase can be achieved by combining a THz glass plate with a polarizer. The THz pulse first passes through a quarter-wave plate in the THz band, causing the phase to expand according to polarization; then it passes through a THz half-wave plate, causing the polarization of the desired phase to flip to the direction of the subsequent THz polarizer; finally, the THz polarizer absorbs pulses with other polarization directions (phases). Therefore, continuous phase control can be achieved by rotating the half-wave plate.

[0048] The above data demonstrates that the THz-STM of this invention possesses high signal strength and signal-to-noise ratio, as well as high temporal and spatial resolution. Furthermore, it enables various experimental techniques, including THz rectified spectroscopy, THz dual-pulse pump-probe, and THz scanning imaging, allowing for in-depth exploration of the THz response or dynamic characteristics of various samples from multiple perspectives. All of this relies on the precise and efficient focusing of the THz dual pulse, thanks to our external coupling scheme and the compatible compact ultra-high vacuum cavity and scanning head.

[0049] Since the parabolic mirror is used to converge the light spot in the present application, the infrared, visible, ultraviolet light, pulse or continuous light can be focused to the tunnel junction uniformly without the influence of the wavelength of the light, except for the THz pulse. In addition to the THz Pump-probe experiment, the device of the present application can theoretically implement the infrared Pump-THz probe, infrared Pump-infrared probe and other pulse light combination modes of different wavebands. In addition, the two-dimensional spectroscopy technology of more than two pulses into the sample can also be combined with the STM to detect the coherent evolution between different energy levels on a microscopic scale. This new technology is called Coherent Anti-stokes Raman Spectroscopy (CARS), which has just appeared internationally.

[0050] In terms of materials, since the THz pulse can be regarded as a transient bias applied to the tunnel junction, theoretically, any material that can be characterized by STM can be observed by THz-STM. As shown in GaGeTe, Ag(111) and the like, the THz photocurrent signal can be observed in these materials. Among them, the topography (first row) and the photocurrent image (second row) of GaGeTe (left) and Ag(111) (right) surfaces under constant current mode are shown. The non-uniformity of the THz photocurrent signal of GaGeTe is mainly caused by the change of the height of the needle tip during constant current scanning. Some defects on the Ag(111) surface are reflected as bright spots in the THz photocurrent, which is also caused by the approach of the height of the needle tip during scanning. Figure 5

[0051] The present application can efficiently realize good THz focusing. The photocurrent can reach the order of 100 pA under the condition of 1 MHz repetition frequency, corresponding to 1000 electrons driven by each pulse to stably tunnel. Stable photocurrent signals can be observed in various materials, and high time resolution and spatial resolution can be realized. The parabolic mirror is installed outside the cavity in the present application, which overcomes the disadvantage of difficult adjustment while taking advantage of its wavelength universality. The device is easy to operate, has high signal-to-noise ratio, superior performance, and has broad upgrading space and application prospect. Most of the existing technologies place the focusing convex lens and parabolic mirror in the cavity to be as close as possible to the scanning head. The convex lens has a limited waveband and will affect the THz waveform; the parabolic mirror is complex to adjust and is not easy to adjust in the cavity. Therefore, the existing THz-STM system is relatively complex to operate, or is not convenient for integrated cross-waveband comprehensive experimental technology.

[0052] ​It is to be understood that even though various embodiments can be described in conjunction with a specific order, the order is not limiting. For example, except as otherwise required by the disclosure, the steps can be performed in any order. Some steps can be performed in parallel. Furthermore, some steps can be performed by different entities than the ones identified in the above description.

[0053] The present application can be a system, a method, and / or a computer program product. The computer program product can include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present application.

[0054] The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium can be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves.

[0055] Embodiments of the application have been described above, with reference to specific embodiments. Any references to language such as "appliance," "device," "system," "method," or "computer program product" are used for convenience only, and do not limit the intended scope of such terms. Alterations and modifications to such alternatives will become apparent to the skilled practitioner. Any feature described above can be replaced by an alternative feature having the same or similar effect. The scope of protection is limited by the claims that follow this disclosure, which are that which is currently or will be eventually allowed and granted.

Claims

1. An external optical coupling system for use in the ultra-high vacuum cavity of a scanning tunneling microscope, used to achieve optical coupling outside the ultra-high vacuum cavity and focus it at the tunnel junction of the ultra-high vacuum cavity, characterized in that, The system includes: a femtosecond laser, a first beam splitter, a second beam splitter, a third beam splitter, a first optical path module, a second optical path module, a third optical path module, a terahertz generation module, a first parabolic mirror unit, an electro-optic crystal, a second parabolic mirror unit, and a gold reflector, wherein... Femtosecond lasers are used to generate raw femtosecond pulses of light at 1030 nm. The first beam splitter is used to split the original femtosecond pulse laser beam to obtain a first beam and a first residual beam; The second beam splitter is used to split the first residual beam to obtain a second beam and a second residual beam; The third beam splitter is used to split the second remaining beam to obtain the third beam and the fourth beam. The fourth beam is directly transmitted to the terahertz generation module. The first optical path module is used to reflect, multiply, and delay the first beam, and uses the first beam processed by the first optical path module as the first sampling light. The second optical path module is used to reflect and delay the second beam, and then transmits the second beam processed by the second optical path module to the terahertz generation module. The third optical path module is used to reflect and delay the third beam, and the third beam processed by the third optical path module is used as the second sampling light. The terahertz generation module is used to generate a first terahertz pulse and a second terahertz pulse based on the second beam and the fourth beam, respectively. The first parabolic mirror unit includes multiple parabolic mirrors, which are used to focus the first terahertz pulse or the second terahertz pulse onto the electro-optic crystal, respectively. Electro-optic crystals are used to detect THz light; The second parabolic mirror unit includes multiple parabolic mirrors for focusing and emitting the first terahertz pulse or the second terahertz pulse, respectively. A gold mirror is used to guide the focused first or second terahertz pulse to the tunnel junction of the scanning tunneling microscope.

2. The external optical coupling system for use in ultra-high vacuum cavities according to claim 1, characterized in that: The first beam splitter is a beam splitter with an energy ratio of 1:1; The second beam splitter is a beam splitter with an energy ratio of 1:1; The third beam splitter is a beam splitter with an energy ratio of 92:

8. The beam with the lower energy among the beams split by the third beam splitter is the third beam.

3. The external optical coupling system for use in the ultra-high vacuum cavity of a scanning tunneling microscope according to claim 2, characterized in that: A first waveplate, a first reflector, and a second reflector are sequentially arranged between the first beam splitter and the second beam splitter. A third reflector and a fourth reflector are sequentially arranged between the second and third beam splitters; The first waveplate is used to optimize the diffraction efficiency of the grating, while the first, second, third, and fourth reflecting mirrors are used to adjust the optical path of the beam.

4. The external optical coupling system for use in ultra-high vacuum cavities according to claim 3, characterized in that, The system also includes a flip-up mirror disposed in the exit paths of the first and third beams for selecting the first or third beam as the sampling light by means of a flip angle.

5. The external optical coupling system for use in the ultra-high vacuum cavity of a scanning tunneling microscope according to claim 4, characterized in that, The first optical path module includes, in sequence, a fifth reflector, an optical parametric amplifier, a sixth reflector, a seventh reflector, a first delay stage, and an eighth reflector, wherein: The fifth mirror is used to reflect the first beam emitted by the first beam splitter into the optical parametric amplifier; An optical parametric amplifier is used to perform frequency doubling and tuning of the first beam; The sixth and seventh mirrors are used to reflect the first beam, after frequency doubling and tuning, into the first delay stage; The first delay stage is used to delay the incoming first beam to adjust the optical path difference and allow it to enter the eighth reflecting mirror; The eighth reflector is used to direct the delayed first beam of light into the flip-up reflector.

6. The external optical coupling system for use in the ultra-high vacuum cavity of a scanning tunneling microscope according to claim 5, characterized in that, The second optical path module includes a ninth reflector, a second delay stage, a tenth reflector, and an eleventh reflector arranged sequentially, wherein: The ninth mirror is used to reflect the second beam split by the second beam splitter into the second delay stage; The second delay stage is used to delay the incoming second beam to adjust the optical path difference and then send it into the tenth reflecting mirror; The tenth reflecting mirror is used to direct the incident second beam of light into the eleventh reflecting mirror; The eleventh reflector is used to direct the second beam into the THz generation module at a preset angle with the first beam.

7. The external optical coupling system for use in the ultra-high vacuum cavity of a scanning tunneling microscope according to claim 6, characterized in that, The third optical path module includes a twelfth reflector, a third delay stage, and a thirteenth reflector arranged sequentially, wherein: The twelfth reflecting mirror is used to reflect the third beam emitted by the third beam splitter into the third delay stage; The third delay stage is used to delay the incoming third beam to adjust the optical path difference and direct it into the thirteenth reflecting mirror; The thirteenth mirror is used to direct the incident third beam of light into the flip-up mirror.

8. The external optical coupling system for use in the ultra-high vacuum cavity of a scanning tunneling microscope according to claim 7, characterized in that, The terahertz generation module includes, in sequence, a fourteenth mirror, a chopper, a fifteenth mirror, a sixteenth mirror, a grating, a second waveplate, a first convex lens, and a nonlinear crystal, wherein: The fourteenth mirror is used to reflect the second or third beam into the chopper. A chopper is used to modulate the light intensity to filter out part of the laser and improve laser resolution; The fifteenth and sixteenth reflectors are used to reflect the second or third beam of light after the chopper into the grating. The second waveplate is used to optimize the THz generation efficiency of the nonlinear crystal; it is a half-waveplate. The first convex lens is used to focus the second or third beam onto the nonlinear crystal and make it meet the matching condition generated by the THz tilted wavefront.

9. The external optical coupling system for a high vacuum cavity in a scanning tunneling microscope according to claim 8, characterized in that, The system also includes a third waveplate for focusing the first or second sampling light onto the electro-optic crystal.

10. The external optical coupling system for a high-vacuum cavity in a scanning tunneling microscope according to claim 9, characterized in that, The system also includes a third convex lens, a third waveplate, a Wollaston prism, and a photodetector, wherein: The third convex lens is used to focus the selected sampling light onto the photodetector; The third waveplate is used to convert the sampled light into circularly polarized light; Wollaston prisms are used to split circularly polarized light into s-polarized and p-polarized components. Photodetectors are used to detect the intensity difference between the s-polarization component and the p-polarization component of light in order to detect the THz intensity waveform.

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