Optical system and photocurrent regulation method

Abstract

The application discloses an optical system and a photocurrent regulation method. The optical system comprises a light source, a beam splitting unit, a first beam returning mirror group, a second beam returning mirror group, a beam combining unit and a semiconductor device. The application sets the contact potential barrier between the first electrode and the functional layer in the semiconductor device to be different from the contact potential barrier between the second electrode and the functional layer, so that there is a potential energy difference between the first electrode and the second electrode, a bias electric field between the electrodes is constructed, a semiconductor device is excited by combining the interference of the two femtosecond laser pulses, the light intensity of the combined interference pulse is changed by adjusting the transition between the interference cancellation and the interference of the two femtosecond laser pulses, the diffusion and drift competition mechanism of the photo-generated carriers is regulated, and the size and direction of the photocurrent are effectively and superfast regulated.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor optical technology, specifically relating to an optical system and a photocurrent control method. Background Technology

[0002] Currently, high-performance computing chips typically operate at frequencies in the gigahertz (GHz) range. With the rapid development of fields such as artificial intelligence, big data processing, and high-speed communication, the demand for chip computing power is growing exponentially. In traditional integrated circuit architectures, one of the core approaches to improving chip computing speed is to increase its clock frequency, i.e., shortening the signal processing cycle by increasing the oscillation frequency of the electric field in the circuit. From a physical mechanism perspective, although higher clock frequencies can be achieved by increasing the electric field frequency, when the photon energy corresponding to the electromagnetic field approaches or exceeds the threshold energy for electrons to cross potential barriers or interband transitions, the device operation will inevitably be limited by the intrinsic electron excitation process of the material, thus restricting the upper limit of the frequency of photocurrent modulation via electromagnetic fields. Therefore, there is an urgent need to develop a novel optical modulation mechanism to generate higher frequency photocurrents to meet the potential demand for higher computing speeds in future computing chips.

[0003] In recent years, there have been two main types of techniques for generating petahertz (PHz) photocurrents using femtosecond laser pulses: one is the carrier envelope phase (CEP) modulation technique for few-period femtosecond pulses, and the other is quantum interference technique using two-color femtosecond laser pulses. The former, by modulating the CEP of a few-period femtosecond laser pulse, can change the magnitude and direction of the photoelectric field integral within the pulse. Taking silicon dioxide (SiO2) as an example, a strong laser pulse excites photogenerated carriers, while a weaker laser pulse drives photoelectron drift. By changing the CEP of the weaker pulse, the magnitude and direction of the PHz photocurrent can be controlled. The latter, on the other hand, simultaneously excites carriers in a semiconductor using fundamental and harmonic light. Due to the different electron transition paths, quantum interference occurs at the microscopic scale using these different transition paths, generating a PHz photocurrent. By changing the phase difference between the fundamental and harmonic light, the magnitude and direction of the photocurrent can be effectively controlled. In semiconductors, this quantum interference effect leads to unequal excitation of electrons with opposite velocities, changing the phase difference and polarization direction between two light pulses. This can control the asymmetry of the population momentum of conduction band photoelectrons, thereby enabling the modulation of the amplitude and direction of PHz photocurrent.

[0004] Based on the two excitation mechanisms mentioned above, related studies have observed PHz photocurrents in direct bandgap semiconductors gallium arsenide (GaAs), indirect bandgap semiconductors silicon (Si) and germanium (Ge), as well as graphene. However, industrializing these excitation mechanisms still faces several major challenges: the CEP technology for controlling femtosecond laser pulses has a high technical threshold, making miniaturization and integration difficult; quantum interference of photogenerated carrier wave packets requires high-quality semiconductor materials; and two-color pulse excitation is selective for the material's bandgap, making it difficult to apply universally to most semiconductor materials.

[0005] Therefore, in order to address the above-mentioned technical problems, it is necessary to provide an optical system and a photocurrent control method. Summary of the Invention

[0006] The purpose of this invention is to provide an optical system and a photocurrent control method that can achieve ultrafast control of the magnitude and direction of the photocurrent.

[0007] To achieve the above objectives, an embodiment of the present invention provides the following technical solution:

[0008] An optical system comprising: a light source for generating femtosecond laser pulses; a beam splitter unit located on the propagation path of the femtosecond laser pulses for splitting the femtosecond laser pulses into a first femtosecond laser pulse and a second femtosecond laser pulse, the first femtosecond laser pulse and the second femtosecond laser pulse having the same frequency; a first beam return mirror group located on the propagation path of the first femtosecond laser pulse transmitted by the beam splitter unit; a second beam return mirror group located on the propagation path of the second femtosecond laser pulse reflected by the beam splitter unit; and a beam combiner unit for reflecting the first femtosecond laser pulse from the first beam return mirror group. The first femtosecond laser pulse is received and transmitted from the second femtosecond laser beam combiner unit to generate an interference pulse. A semiconductor device is located on the propagation path of the interference pulse generated by the beam combiner unit. The semiconductor device includes a functional layer and a first electrode and a second electrode located on the functional layer. The contact barrier between the first electrode and the functional layer is greater than the contact barrier between the second electrode and the functional layer. The semiconductor device generates a photocurrent under the illumination of the interference pulse. An optical path adjustment component is provided in the first and / or second beam combiner units.

[0009] In one embodiment, the optical system includes: a first state in which the first femtosecond laser pulse and the second femtosecond laser pulse interfere destructively, and the photocurrent flows from the second electrode to the first electrode; and a second state in which the first femtosecond laser pulse and the second femtosecond laser pulse interfere constructively, and the photocurrent flows from the first electrode to the second electrode.

[0010] In one embodiment, the frequency of the photocurrent is PHz.

[0011] In one embodiment, the optical system further includes a focusing unit located between the beam combining unit and the semiconductor device, for focusing the interference pulses generated by the beam combining unit onto a functional layer adjacent to the first electrode.

[0012] In one embodiment, the spot size of the interference pulse focused by the focusing unit is less than or equal to 1 μm.

[0013] In one embodiment, the optical system further includes a dispersion compensation unit located between the light source and the beam splitting unit. The dispersion compensation unit is used to compensate for the dispersion introduced during the propagation of the femtosecond laser pulse generated by the light source to the beam splitting unit. The dispersion compensation unit includes a chirped mirror pair and an adjustable wedge pair arranged sequentially along the propagation optical path of the femtosecond laser pulse.

[0014] In one embodiment, the dispersion compensation unit further includes a plurality of reflecting silver mirrors; and / or, the first return beam mirror group includes a plurality of reflecting silver mirrors; and / or, the second return beam mirror group includes a plurality of reflecting silver mirrors.

[0015] In one embodiment, the beam splitting unit is a first beam splitter, and the beam combining unit is a second beam splitter, wherein the first beam splitter and the second beam splitter are the same.

[0016] In one embodiment, the functional layer is any one of a gallium nitride layer, a gallium arsenide layer, or a graphene layer.

[0017] Another embodiment of the present invention provides the following technical solution:

[0018] A photocurrent modulation method includes the following steps: providing an optical system, the optical system being the one described above; adjusting the optical path difference between a first femtosecond laser pulse and a second femtosecond laser pulse to modulate the magnitude and direction of the generated photocurrent.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention sets the contact barrier between the first electrode and the functional layer in a semiconductor device to be different from the contact barrier between the second electrode and the functional layer, so that there is a potential energy difference between the first electrode and the second electrode, thereby constructing a bias electric field between the electrodes. Combined with the excitation of the semiconductor device by dual femtosecond laser pulse interference at the same frequency, the intensity of the interference pulse after beam combining is changed by adjusting the transition between the two femtosecond laser pulses in destructive and constructive interference, so as to control the diffusion and drift competition mechanism of photogenerated carriers and achieve ultrafast and effective control of the magnitude and direction of photocurrent. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the optical path of the optical system in Embodiment 1 of the present invention;

[0023] Figure 2 This is a flowchart of the optical system construction method in Embodiment 1 of the present invention;

[0024] Figure 3 This is a schematic diagram of the optical system in the first state in Embodiment 1 of the present invention;

[0025] Figure 4 This is a schematic diagram of the second state of the optical system in Embodiment 1 of the present invention.

[0026] Explanation of key figure labels:

[0027] 10-Light source, 20-Beam splitter unit, 311-Seventh reflecting silver mirror, 312-Eighth reflecting silver mirror, 313-Ninth reflecting silver mirror, 321-Tenth reflecting silver mirror, 322-Eleventh reflecting silver mirror, 323-Twelfth reflecting silver mirror, 33-Optical path adjustment component, 40-Beam combiner unit, 50-Semiconductor device, 601-Chirped mirror pair, 602-Adjustable wedge pair, 603-First reflecting silver mirror, 604-Second reflecting silver mirror, 605-Third reflecting silver mirror, 606-Fourth reflecting silver mirror, 607-Fifth reflecting silver mirror, 608-Sixth reflecting silver mirror, 70-Focusing unit, 801-Chopper, 802-Lock-in amplifier, 90-Detector unit. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solutions in this disclosure, the technical solutions in the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. Based on the embodiments in this disclosure, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this disclosure.

[0029] This invention discloses an optical system comprising: a light source for generating femtosecond laser pulses; a beam splitter unit located on the propagation optical path of the femtosecond laser pulses, for splitting the femtosecond laser pulses into a first femtosecond laser pulse and a second femtosecond laser pulse, wherein the first femtosecond laser pulse and the second femtosecond laser pulse have the same frequency; a first beam return mirror group located on the propagation optical path of the first femtosecond laser pulse transmitted by the beam splitter unit; a second beam return mirror group located on the propagation optical path of the second femtosecond laser pulse reflected by the beam splitter unit; a beam combiner unit for reflecting the first femtosecond laser pulse from the first beam return mirror group and transmitting the second femtosecond laser pulse from the second beam return mirror group, and combining the first femtosecond laser pulse and the second femtosecond laser pulse to generate an interference pulse; and a semiconductor device located on the propagation optical path of the interference pulse generated by the beam combiner unit, wherein the semiconductor device includes a functional layer and a first electrode and a second electrode located on the functional layer, wherein the contact barrier between the first electrode and the functional layer is greater than the contact barrier between the second electrode and the functional layer, and the semiconductor device generates a photocurrent under the irradiation of the interference pulse; wherein the first beam return mirror group and / or the second beam return mirror group are provided with an optical path adjustment component.

[0030] This invention also discloses a photocurrent modulation method, comprising the following steps:

[0031] An optical system is provided, which is the optical system described above;

[0032] The optical path difference between the first and second femtosecond laser pulses is adjusted to control the magnitude and direction of the generated photocurrent.

[0033] The present invention will be further illustrated below with specific examples.

[0034] Comparative Example 1:

[0035] The optical system in this comparative example includes a light source, a frequency doubling unit, a beam splitting unit, a beam combining unit, an optical path control unit, a polarization control unit, a semiconductor device, and a detection unit. The light source is used to generate two-color femtosecond laser pulses. In this comparative example, the femtosecond laser pulses are used by the frequency doubling unit to generate fundamental and frequency-doubled light. The two-color beams are separated by the beam splitting unit, and after independent optical path adjustment and polarization control, they are recombined and focused on the surface of the semiconductor device. Ultrafast photocurrent is generated using quantum interference effects. By adjusting the relative delay and relative polarization between the two beams, the magnitude and direction of the photocurrent are controlled.

[0036] However, in this comparative example, when generating photocurrent, it is necessary to use fundamental frequency light and frequency harmonic light to excite electrons through different transition paths, which imposes strict requirements on the band gap of semiconductor materials in semiconductor devices and has low universality.

[0037] Comparative Example 2:

[0038] The optical system in this comparative example includes a light source, a semiconductor device, and a detection unit. The light source can generate femtosecond laser pulses with controllable waveforms. The semiconductor device is a nanojunction sample composed of a metal layer, a dielectric layer, and a metal layer stacked in sequence, and the dielectric layer is selected as a silicon oxide layer. In this comparative example, a few-cycle laser pulse with a central wavelength of 750 nm, a pulse width of 4 fs, and a peak electric field intensity of 1.7 V / Å is focused on the silicon oxide layer between the two electrodes, and the polarization direction of the laser pulse is perpendicular to the interface between the metal layer and the dielectric layer, thereby generating a photocurrent. At the same time, by adjusting the CEP of the laser pulse, the control of the direction and amplitude of the photocurrent is achieved, and the photocurrent oscillates sinusoidally with the CEP. In addition, in this comparative example, two orthogonally polarized laser pulses are used to separately decouple the carrier injection and transport control, and by adjusting the relative delay of the double pulses, a sub-femtosecond-level switching of the photocurrent direction is achieved.

[0039] However, the optical system in this comparative example has high requirements for the control of CEP, is sensitive to the parameters of the laser source, and the generated photocurrent signal is weak, and it needs to be amplified jointly by a preamplifier and a lock-in amplifier.

[0040] Comparative Example 3:

[0041] In this comparative example, a mechanically exfoliated molybdenum disulfide layer is used as the channel material, and gold electrodes are deposited on both sides to form a symmetric MoS2 / Au Schottky contact. By locally irradiating a femtosecond laser with a pulse width of 200 fs and a spot diameter of 20 μm on one of the contact areas, the Schottky barrier height on this side is permanently reduced, thereby breaking the original symmetry. After the femtosecond laser pulse, the semiconductor device type is changed from a photoconductor to a photodiode, and the rectification ratio is increased from 1 to more than 10 4 , and a short-circuit current of 37.2 nA and a maximum output electrical power of 1.67 nW are achieved under zero bias.

[0042] However, the direction of the photocurrent generated in this comparative example is fixed and the control of the direction cannot be achieved.

[0043] Example 1:

[0044] Refer Figure 1As shown, the optical system in this embodiment includes: a light source 10 for generating femtosecond laser pulses; a beam splitter 20 located on the propagation optical path of the femtosecond laser pulses, used to split the femtosecond laser pulses into a first femtosecond laser pulse and a second femtosecond laser pulse, the first femtosecond laser pulse and the second femtosecond laser pulse having the same frequency; a first beam return mirror group located on the propagation optical path of the first femtosecond laser pulse transmitted by the beam splitter 20; a second beam return mirror group located on the propagation optical path of the second femtosecond laser pulse reflected by the beam splitter 20; and a beam combiner 40 for reflecting the first femtosecond laser pulse from the first beam return mirror group. An optical pulse, and a second femtosecond laser pulse transmitted from the second beam-returning mirror group, are combined to generate an interference pulse; a semiconductor device 50 is located on the propagation path of the interference pulse generated by the beam-returning unit. The semiconductor device includes a functional layer and a first electrode and a second electrode located on the functional layer. The contact barrier between the first electrode and the functional layer is greater than the contact barrier between the second electrode and the functional layer. The semiconductor device 50 generates a photocurrent under the irradiation of the interference pulse; wherein, an optical path adjustment component 33 is provided in the first beam-returning mirror group and / or the second beam-returning mirror group.

[0045] In this embodiment, the center wavelength of the femtosecond laser pulse generated by the light source 10 is 780nm and the pulse width is 9fs.

[0046] Furthermore, the optical system also includes a dispersion compensation unit located between the light source 10 and the beam splitting unit 20, which is used to compensate for the dispersion introduced during the propagation of the femtosecond laser pulse generated by the light source to the beam splitting unit.

[0047] Specifically, the dispersion compensation unit in this embodiment includes several reflecting silver mirrors, as well as a pair of chirped mirrors 601 and an adjustable wedge pair 602 arranged sequentially along the propagation optical path of the femtosecond laser pulse, so as to effectively compensate for the dispersion of the femtosecond laser pulse by the optical path and air, and ensure that the interference of the first femtosecond laser pulse and the second femtosecond laser pulse has the best effect on the control of photocurrent.

[0048] More specifically, a first reflecting silver mirror 603 is provided between the light source 10 and the chirped mirror pair 601. A second reflecting silver mirror 604 and a third reflecting silver mirror 605 are sequentially provided between the chirped mirror pair 601 and the adjustable wedge pair 602 along the propagation optical path of the femtosecond laser pulse. A fourth reflecting silver mirror 606, a fifth reflecting silver mirror 607, and a sixth reflecting silver mirror 608 are sequentially provided between the adjustable wedge pair 602 and the beam splitting unit 20 along the propagation optical path of the femtosecond laser pulse. That is, in this embodiment, the femtosecond pulse laser generated by the light source 10 passes through the first reflecting silver mirror 603, the chirped mirror pair 601, the second reflecting silver mirror 604, the third reflecting silver mirror 605, the adjustable wedge pair 602, the fourth reflecting silver mirror 606, the fifth reflecting silver mirror 607, and the sixth reflecting silver mirror 608 in sequence before being incident on the beam splitting unit 20.

[0049] Furthermore, in this embodiment, the first beam return mirror group includes several reflecting silver mirrors, and the second beam return mirror group includes several reflecting silver mirrors. Since the center wavelength of the femtosecond laser pulse generated by the light source in this embodiment is 780nm, and the reflectivity of the reflecting silver mirrors for light in the 650nm~1100nm wavelength range is higher than 95%, all mirrors in this embodiment are made of reflecting silver mirrors, which can reduce intensity loss during the propagation of the femtosecond laser pulse.

[0050] Specifically, in this embodiment, the first femtosecond laser pulse transmitted by the beam splitter 20 passes sequentially through the seventh reflecting silver mirror 311, the eighth reflecting silver mirror 312 and the ninth reflecting silver mirror 313 before entering the beam combiner 40; the second femtosecond laser pulse reflected by the beam splitter 20 passes sequentially through the tenth reflecting silver mirror 321, the eleventh reflecting silver mirror 322 and the twelfth reflecting silver mirror 323 before entering the beam combiner 40.

[0051] More specifically, in this embodiment, the optical path adjustment component 33 is an electrically driven stage, and the tenth reflecting silver mirror 321 and the eleventh reflecting silver mirror 322 are mounted on the electrically driven stage to adjust the optical path of the second femtosecond laser pulse. Of course, in other embodiments, the optical path adjustment component can also be set in the first beam return mirror group or simultaneously in the first and second beam return mirror groups, so as to control the optical path difference between the first and second femtosecond laser pulses.

[0052] Furthermore, the beam-splitting unit 20 is a first beam-splitting mirror, and the beam-combining unit 40 is a second beam-splitting mirror, with the first and second beam-splitting mirrors being identical. Therefore, the optical system in this embodiment does not require an additional neutral density filter to balance the intensity of the first and second femtosecond laser pulses and minimize optical path dispersion.

[0053] Furthermore, the optical system also includes a focusing unit 70, which is located between the beam combining unit 40 and the semiconductor device 50, and is used to focus the interference pulse generated by the beam combining unit 40 onto the functional layer adjacent to the first electrode.

[0054] Specifically, in this embodiment, the focusing unit 70 is an objective lens. The focusing unit enables the spot size of the interference pulse focused on the surface of the functional layer to be less than or equal to 1 μm, thereby achieving a higher power density and maximizing the carrier concentration excited in the functional layer.

[0055] Furthermore, the functional layer includes, but is not limited to, any one of gallium nitride, gallium arsenide, or graphene layers.

[0056] Specifically, in this embodiment, the functional layer is a gallium nitride layer. The bandgap of the gallium nitride layer is 3.4 eV. Since the central wavelength of the femtosecond laser pulse generated by the light source in this embodiment is 780 nm and the pulse width is 9 fs, the photon energy = 1.59 eV. Therefore, photo-generated carriers can be excited through the three-photon absorption mechanism, effectively improving the signal-to-noise ratio of the generated photocurrent.

[0057] In addition, the optical system in this embodiment further includes a pulse modulation unit and a detection unit. The pulse modulation unit includes a chopper 801 disposed between the beam combining unit 40 and the focusing unit 70 and a lock-in amplifier 802 electrically connected to the chopper 801. The lock-in amplifier 802 is electrically connected to the first electrode and the second electrode respectively. The detection unit 90 is a computer and is electrically connected to the lock-in amplifier 802. The lock-in amplifier receives the signal provided by the chopper as a reference frequency signal, and simultaneously detects and amplifies the photocurrent signal generated by the semiconductor device. The lock-in amplifier is connected to the computer and transmits the magnitude and direction of the processed photocurrent to the computer. Preferably, the lock-in amplifier in this embodiment is an SR830 lock-in amplifier.

[0058] Specifically, in this embodiment, the LabVIEW software is used to realize the synchronous control and data acquisition of the optical path adjustment component 33 and the lock-in amplifier 802, so as to record the current value every hundred-nanometer-level displacement.

[0059] Furthermore, the optical system in this embodiment includes: a first state, where the first femtosecond laser pulse and the second femtosecond laser pulse interfere destructively, and the photocurrent flows from the second electrode to the first electrode; a second state, where the first femtosecond laser pulse and the second femtosecond laser pulse interfere constructively, and the photocurrent flows from the first electrode to the second electrode.

[0060] Refer Figure 2 As shown, the construction of the optical system in this embodiment includes the following steps:

[0061] S1. Fabricate a semiconductor device.

[0062] Specifically, in this embodiment, a gallium nitride layer is selected as the functional layer. Then, a first electrode and a second electrode are fabricated on the functional layer. Both the first electrode and the second electrode are gold electrodes. By controlling the thickness or growth temperature of the first electrode and the second electrode to be different, the contact potential barrier between the first electrode and the functional layer is made greater than the contact potential barrier between the second electrode and the functional layer, so that the electric potential energies corresponding to the first electrode and the second electrode form a potential difference, thereby constructing a bias electric field between the electrodes.

[0063] It is worth noting that in other embodiments, the first electrode and the second electrode can be prepared by using different metal materials to make the contact barrier between the first electrode and the functional layer greater than the contact barrier between the second electrode and the functional layer.

[0064] S2. Build an external test circuit.

[0065] Specifically, gold wires are soldered to the first and second electrodes, and then connected to the signal input of a lock-in amplifier via a Bayonet Neill-Concelman (BNC) connector to receive the photocurrent signal generated by the semiconductor device. The signal output of the lock-in amplifier is connected to the detection unit via an RS-232 serial cable to measure the photocurrent.

[0066] S3. Construct the optical system.

[0067] Specifically, an optical system is constructed by selecting appropriate light sources, beam splitting units, dispersion compensation units, first return mirror groups, second return mirror groups, beam combining units, pulse modulation units, and focusing units. The femtosecond laser pulses output from the light source first undergo dispersion pre-compensation through the dispersion compensation unit, then are split into first and second femtosecond laser pulses by the beam splitting unit. These pulses then pass through the first and second return mirror groups respectively, before reaching the beam combining unit. After being combined, the pulses are chopped by a chopper and focused onto the functional layer surface of the semiconductor device. Simultaneously, the second return mirror group contains an optical path adjustment component to adjust the optical path difference between the first and second femtosecond laser pulses.

[0068] S4. Data pre-collection.

[0069] Specifically, LabVIEW software was used to achieve synchronous control and data acquisition of the optical path adjustment component and the lock-in amplifier, recording the current value every hundred nanometers of displacement. By changing the optical path difference between the first and second femtosecond laser pulses, the relationship curve between the relative delay and the ultrafast photocurrent was obtained, thus revealing the regulation of the relative delay between the first and second femtosecond laser pulses on the magnitude and direction of the photocurrent.

[0070] S5. Calibrate the optical path zero point.

[0071] Specifically, the optical path lengths of the first and second femtosecond laser pulses are first calibrated using measuring tools. Then, the electric displacement stage is finely adjusted to make the optical path lengths of the first and second femtosecond laser pulses equal. Near the position where the optical path lengths of the first and second femtosecond laser pulses are equal (i.e., the optical path zero point), the combined beam spot exhibits obvious interference fringes, obtaining an interference pulse. When the brightness of the combined beam spot is at its maximum, it is the precise optical path zero point.

[0072] S6. Measure the pulse width.

[0073] Specifically, the optical path adjustment component is controlled by LabVIEW software to move near the optical path zero point, and the current signal output by the lock-in amplifier is recorded simultaneously, namely the autocorrelation current signal of the interference between the first and second femtosecond laser pulses. The pulse widths of the first and second femtosecond laser pulses can be obtained by fitting, and the pulse widths are much greater than 9 fs.

[0074] S7. Perform dispersion compensation.

[0075] The overall optical path dispersion is calculated based on the fitted pulse width, and the reflection count of the chirped mirror pair and the coordinate positions of the adjustable wedge pair are determined accordingly. By fine-tuning the coordinates of the adjustable wedge pair, the optical path dispersion is compensated to the greatest extent.

[0076] S8. Perform two-dimensional photocurrent imaging.

[0077] Specifically, after dispersion compensation is completed, the optical path adjustment component is moved to the optical path zero point. The sample stage (i.e., the semiconductor device) is then moved at the nanometer level using LabVIEW software to scan a two-dimensional photocurrent image of the region near the first and second electrodes. Due to the potential energy difference between the first and second electrodes, there is a significant difference in photocurrent intensity near them. The area with weaker photocurrent is the high-potential electrode (the first electrode), while the area with stronger photocurrent is the low-potential electrode (the second electrode).

[0078] S9. Optimize focus position.

[0079] Based on the two-dimensional scanning results, the light spot focused by the objective lens is moved to the surface of the functional layer near the high-potential electrode, i.e., the first electrode.

[0080] S10. Perform photocurrent measurement and control.

[0081] Specifically, the optical path adjustment component is controlled by LabVIEW software to move at the level of hundreds of nanometers (corresponding to a time resolution of ~0.1 fs), and the photocurrent signal output by the lock-in amplifier is measured simultaneously to obtain the influence of the interference of the first femtosecond laser pulse and the second femtosecond laser pulse on the magnitude and direction of the photocurrent.

[0082] The photocurrent modulation method using the above-described optical system in this embodiment includes the following steps:

[0083] S1. Provide optical systems.

[0084] S2. Adjust the optical path difference between the first femtosecond laser pulse and the second femtosecond laser pulse to control the magnitude and direction of the generated photocurrent.

[0085] Specifically, by adjusting the optical path difference between the first femtosecond laser pulse and the second femtosecond laser pulse, the first femtosecond laser pulse and the second femtosecond laser pulse are switched between the destructive interference state and the constructive interference state, thereby controlling the intensity of the interference pulse irradiated onto the surface of the functional layer and regulating the competition mechanism between the diffusion and drift of photogenerated carriers, so that the optical system switches between the first state and the second state, generating a photocurrent with a frequency on the order of PHz.

[0086] In this embodiment, a gallium nitride (GaN) layer is used as an example. A first electrode and a second electrode with different contact barrier heights are grown on the surface of the GaN layer. The potential energy difference between the two electrodes forms a bias electric field, driving electron drift. An interference pulse is focused onto the GaN layer surface near the high-potential electrode using a focusing unit, thereby generating photogenerated carriers by multiphoton excitation of the GaN. Since the concentration of photogenerated carriers exhibits a Gaussian distribution on the GaN layer surface, the carrier drift direction is from the center of the light spot to the edge of the light spot. (See reference...) Figure 3 As shown, when the power of the interference pulse after beam combining is low (i.e., the first femtosecond laser pulse and the second femtosecond laser pulse interfere destructively), the generated photogenerated carriers are mainly driven by the bias electric field, drifting from the high potential energy electrode (first electrode) to the low potential energy electrode (second electrode), and the photocurrent flows from the second electrode to the first electrode. (See reference...) Figure 4 As shown, when the combined interference pulse power is high (i.e., the first and second femtosecond laser pulses interfere constructively), the generated photogenerated carriers mainly exhibit spatial diffusion. Since the photogenerated carriers are closer to the high-potential electrode (first electrode), most carriers diffuse to the high-potential electrode (first electrode), at which point the photocurrent direction reverses, flowing from the first electrode to the second electrode. Simultaneously, when the optical path control component moves at the hundred-nanometer level, the constructive and destructive interference conditions between the first and second femtosecond laser pulses change drastically, causing the light intensity of the interference pulses to oscillate rapidly. This allows the optical system to transition between the first and second states within a sub-femtosecond timescale, generating a photocurrent with a frequency of PHz.

[0087] Compared to existing technologies, the optical system in this embodiment does not rely on quantum interference and has no special requirements for the band gap of the functional layer in the semiconductor device, making it more widely applicable. Furthermore, the optical system in this embodiment can achieve photocurrent direction control without fine-tuning the CEP, making the optical system simpler and generating photocurrent intensities on the order of nA, resulting in higher signal strength. In addition, while achieving the generation and direction control of PHz-level photocurrent, the femtosecond laser pulse's manipulation of the contact barrier in the semiconductor device is reversible and does not permanently affect the material's steady-state properties.

[0088] As can be seen from the above technical solution, the present invention has the following beneficial effects:

[0089] This invention sets the contact barrier between the first electrode and the functional layer in a semiconductor device to be different from the contact barrier between the second electrode and the functional layer, so that there is a potential energy difference between the first electrode and the second electrode, thereby constructing a bias electric field between the electrodes. Combined with the excitation of the semiconductor device by dual femtosecond laser pulse interference at the same frequency, the intensity of the interference pulse after beam combining is changed by adjusting the transition between the two femtosecond laser pulses in destructive and constructive interference, so as to control the diffusion and drift competition mechanism of photogenerated carriers and achieve ultrafast and effective control of the magnitude and direction of photocurrent.

[0090] It will be apparent to those skilled in the art that this disclosure is not limited to the details of the exemplary embodiments described above, and that this disclosure can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of this disclosure is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within this disclosure. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0091] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. An optical system, characterized in that, The optical system includes: The light source is used to generate femtosecond laser pulses; A beam splitting unit is located in the propagation optical path of the femtosecond laser pulse and is used to split the femtosecond laser pulse into a first femtosecond laser pulse and a second femtosecond laser pulse, wherein the first femtosecond laser pulse and the second femtosecond laser pulse have the same frequency. The first beam mirror group is located on the propagation optical path of the first femtosecond laser pulse transmitted by the beam splitter unit; The second beam mirror group is located on the propagation optical path of the second femtosecond laser pulse reflected by the beam splitter unit; The beam combining unit is used to reflect the first femtosecond laser pulse from the first return beam mirror group and transmit the second femtosecond laser pulse from the second return beam mirror group. After combining the first femtosecond laser pulse and the second femtosecond laser pulse, an interference pulse is generated. A semiconductor device is located in the propagation optical path of the interference pulse generated by the beam combining unit. The semiconductor device includes a functional layer and a first electrode and a second electrode located on the functional layer. The contact barrier between the first electrode and the functional layer is greater than the contact barrier between the second electrode and the functional layer. The semiconductor device generates a photocurrent under the irradiation of the interference pulse. The first and / or second beam return lens groups are equipped with optical path adjustment components.

2. The optical system according to claim 1, characterized in that, The optical system includes: In the first state, the first femtosecond laser pulse and the second femtosecond laser pulse interfere with each other and cancel each other out, and the photocurrent flows from the second electrode to the first electrode; In the second state, the first femtosecond laser pulse and the second femtosecond laser pulse interfere constructively, and the photocurrent flows from the first electrode to the second electrode.

3. The optical system according to claim 1, characterized in that, The frequency of the photocurrent is PHz.

4. The optical system according to claim 1, characterized in that, The optical system further includes a focusing unit located between the beam combining unit and the semiconductor device, which is used to focus the interference pulses generated by the beam combining unit onto the functional layer adjacent to the first electrode.

5. The optical system according to claim 4, characterized in that, The focusing unit focuses an interference pulse with a spot size of less than or equal to 1 μm.

6. The optical system according to claim 1, characterized in that, The optical system also includes a dispersion compensation unit located between the light source and the beam splitting unit. The dispersion compensation unit is used to compensate for the dispersion introduced during the propagation of the femtosecond laser pulse generated by the light source to the beam splitting unit. The dispersion compensation unit includes a pair of chirped mirrors and an adjustable wedge pair arranged sequentially along the propagation optical path of the femtosecond laser pulse.

7. The optical system according to claim 6, characterized in that, The dispersion compensation unit further includes several reflecting silver mirrors; and / or, The first beam-returning lens group includes a plurality of reflecting silver mirrors; and / or, The second beam mirror group includes several reflecting silver mirrors.

8. The optical system according to claim 1, characterized in that, The beam splitting unit is a first beam splitter, and the beam combining unit is a second beam splitter. The first beam splitter and the second beam splitter are the same.

9. The optical system according to claim 1, characterized in that, The functional layer is any one of gallium nitride, gallium arsenide, or graphene.

10. A photocurrent modulation method, characterized in that, The photocurrent modulation method includes the following steps: An optical system is provided, wherein the optical system is the optical system according to any one of claims 1 to 9; The optical path difference between the first and second femtosecond laser pulses is adjusted to control the magnitude and direction of the generated photocurrent.

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