Optical system, construction method and photocurrent control method

Abstract

The present invention discloses an optical system, a construction method and a photocurrent control method. The optical system realizes the generation, splitting and combining of dual light beams by frequency doubling a femtosecond laser light beam, and realizes adjustable relative delay and relative polarization of the dual light beams in the optical path. By utilizing the principle of dual-beam quantum interference, femtosecond laser light sources of different wavelengths can be selectively utilized to efficiently realize the generation of ultrafast photocurrent on most semiconductor samples, and the size and direction of the ultrafast photocurrent can be controlled by the relative delay and relative polarization of the dual light beams.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductor optical technology, and specifically relates to an optical system for generating adjustable photocurrent on a semiconductor, as well as a construction method of the optical system and a photocurrent control method. Background Art

[0002] Most current semiconductor devices use purely electrical means to control their response. If the current is switched on and off faster in the device, the performance of the device is considered to be better. High electron mobility transistors (HEMTs) operate at a switching rate of ~1THz. If the current is controlled by purely optical means, the switching rate can be increased to more than 100THz, which can greatly improve the sensitivity of the device response. At the same time, there is no need for electrode control. The position where the current is generated can be flexibly controlled by the position reached by the light beam, and the application range is wider.

[0003] In order to explore new applications in the next generation of electronics and optoelectronics, it is required to generate ultrafast photocurrents with smaller time scales in semiconductors. However, the interaction between light and semiconductor materials is relatively weak, and it is necessary to enhance and control ultrafast photocurrents at extreme scales to improve the response rate of semiconductor devices.

[0004] Related studies have used two-color pulsed lasers to measure the voltage drop between two metal contacts of a direct bandgap semiconductor gallium arsenide (GaAs) sample. Similar phenomena have also been observed in indirect bandgap semiconductors silicon (Si) and germanium (Ge) as well as graphene, with the resulting current drop time being at least on the order of picoseconds.

[0005] The current induced by ultrashort pulses appears virtually instantaneously, as electrons are directly excited into a state with non-zero velocity, without the need for them to be accelerated by an external field. In this case, the time scale is determined by the light-matter interaction, which is usually limited by the pulse duration, and even ultrafast photocurrents on the order of femtoseconds can be achieved. Typically, current measurement techniques rely on the detection of voltage drops or surrounding magnetic fields, which require external probes that limit spatial resolution. In contrast, the all-photocurrent detection scheme will be limited only by the laser beam, which can be focused on an arbitrary point with microscopic dimensions. In addition, the time resolution will not be limited by electronic devices, and ultrafast current dynamics of carriers can be studied using femtosecond pulses.

[0006] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention and should not be regarded as an acknowledgment or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art. Summary of the invention

[0007] The object of the present invention is to provide an optical system for generating an adjustable photocurrent on a semiconductor, a method for constructing the optical system, and a method for controlling the photocurrent. The optical system can perform double-beam quantum interference through fundamental frequency light and its frequency-doubled light, and adjust the delay by changing the optical path difference of the double beams, and / or, by adjusting the relative polarization of the double beams, the size and direction of the generated photocurrent can be controlled. At the same time, the magnitude of the generated photocurrent can be two orders of magnitude larger than the same-frequency light interference in the prior art, and the generated current signal is easier to observe.

[0008] In order to achieve the above object, a technical solution provided by a specific embodiment of the present invention is as follows:

[0009] An optical system for generating an adjustable photocurrent on a semiconductor, comprising:

[0010] A light source, used to generate femtosecond laser light, wherein the femtosecond laser light is fundamental frequency light;

[0011] A frequency doubling unit is located in the optical path generated by the light source, and is used to double the frequency of the femtosecond laser. The frequency doubling unit can be controlled to move and rotate to adjust its position and angle in the optical path;

[0012] A beam splitting unit is located on the light path generated by the light source and downstream of the frequency doubling unit, and is used to reflect the frequency doubled light generated by the frequency doubling unit and transmit the fundamental frequency light from the frequency doubling unit;

[0013] A first beam return mirror group is located on the optical path of the frequency-doubled light reflected by the beam splitting unit, and the first beam return mirror group can be controlled to move to adjust the optical path of the frequency-doubled light;

[0014] A second beam return mirror group is located on the optical path of the fundamental frequency light transmitted by the beam splitting unit, and the second beam return mirror group can be controlled to move to adjust the optical path of the fundamental frequency light;

[0015] A dispersion compensation unit, located on the optical path of the fundamental frequency light transmitted by the beam splitting unit and downstream of the second beam return mirror group, the dispersion compensation unit is used to compensate for the dispersion introduced by the beam splitting unit;

[0016] A polarization unit, located on the optical path of the fundamental frequency light transmitted by the beam splitting unit and downstream of the dispersion compensation unit, wherein the polarization unit can be controlled to rotate to adjust the polarization direction of the fundamental frequency light;

[0017] A beam combining unit, used for reflecting the doubled frequency light from the first beam returning mirror group and transmitting the fundamental frequency light from the polarization unit, so as to combine the fundamental frequency light and the doubled frequency light;

[0018] The femtosecond laser is frequency-doubled by the frequency-doubling unit to form fundamental frequency light and frequency-doubled light. The fundamental frequency light is transmitted by the beam splitting unit, passes through the second beam return mirror group, the dispersion compensation unit and the polarization unit, and is incident on the beam combining unit. The frequency-doubled light is reflected by the beam splitting unit, passes through the first beam return mirror group, and is incident on the beam combining unit. The fundamental frequency light and the frequency-doubled light are combined by the beam combining unit to form quantum interference, and are incident on the semiconductor sample to generate photocurrent.

[0019] The optical system has an initial state, in which the optical paths of the fundamental frequency light and the doubled frequency light in the combined light beam are equal, and the polarization directions of the fundamental frequency light and the doubled frequency light in the combined light beam are the same;

[0020] The magnitude of the photocurrent generated by the optical system is adjusted at least by adjusting the movement of the second beam return mirror group and / or the rotation of the polarization unit.

[0021] In one or more embodiments of the present invention, the optical system further comprises:

[0022] a pulse modulation unit, located on the optical path of the combined light beam combined by the beam combining unit, the pulse modulation unit being used to pulse modulate the combined light beam, output and amplify the modulation frequency; and

[0023] The detection unit is at least electrically connected to the pulse modulation unit, the second beam return mirror group, the polarization unit and the semiconductor sample. The detection unit is used to control and receive the modulation frequency of the pulse modulation unit, receive the voltage signal or current signal on the semiconductor sample, and control the movement or rotation of the second beam return mirror group and the polarization unit.

[0024] In one or more embodiments of the present invention, the frequency doubling unit comprises a frequency doubling crystal, and the frequency doubling crystal is arranged on the first movable translation stage; and / or,

[0025] The beam splitting unit includes a first dichroic mirror, and the beam combining unit includes a second dichroic mirror; and / or,

[0026] The first beam return mirror group includes a plurality of reflectors, and the plurality of reflectors are arranged on the second movable displacement stage; and / or,

[0027] The second beam return mirror group includes a hollow roof prism reflector, and the hollow roof prism reflector is arranged on a third movable displacement stage, and the third movable displacement stage can be controlled to move in micrometer level; and / or,

[0028] The dispersion compensation unit comprises a chirped reflector and an adjustable wedge pair arranged in sequence along the optical path of the fundamental frequency light; and / or,

[0029] The polarization unit comprises a half-wave plate, and the half-wave plate is arranged on a fourth movable displacement stage, and the fourth movable displacement stage can be controlled to rotate; and / or,

[0030] The pulse modulation unit includes a chopper and a lock-in amplifier, and the chopper is electrically connected to the lock-in amplifier.

[0031] In one or more embodiments of the present invention, the optical system further comprises a focusing unit, which is disposed upstream of the frequency doubling unit and is used to focus the fundamental frequency light incident to the frequency doubling unit, and is disposed upstream of the semiconductor sample and is used to focus the fundamental frequency light and the frequency doubling light incident to the semiconductor sample;

[0032] The collimating unit is arranged downstream of the frequency doubling unit and is used for collimating the fundamental frequency light and the frequency doubling light emitted from the frequency doubling unit.

[0033] In one or more embodiments of the present invention, the focusing unit and the collimating unit both include parabolic reflectors; and / or,

[0034] The focusing unit includes a parabolic reflective silver mirror and / or a parabolic reflective aluminum mirror, and the collimating units include a parabolic reflective aluminum mirror.

[0035] In one or more embodiments of the present invention, the optical system further includes an attenuation unit, which is disposed on the optical path of the fundamental frequency light transmitted by the beam splitting unit and upstream of the second beam return mirror group, and the attenuation unit is used to adjust the intensity of the fundamental frequency light.

[0036] In one or more embodiments of the present invention, the optical system further includes a plurality of reflectors disposed on the optical path of the fundamental frequency light, the optical path of the frequency-doubled light, and the optical path of the combined light, so as to control the optical path so that the final combined light is incident on the semiconductor sample.

[0037] In one or more embodiments of the present invention, the plurality of reflectors include a reflective silver mirror and a reflective aluminum mirror;

[0038] The reflective silver mirror is arranged on the optical path through which the fundamental frequency light passes;

[0039] The reflective aluminum mirror is arranged on the optical path through which the frequency-doubled light and the combined light pass.

[0040] A method for constructing an optical system, wherein the optical system is used to generate an adjustable photocurrent on a semiconductor, comprising:

[0041] Determine the semiconductor sample and select a corresponding femtosecond laser light source according to the bandgap width of the semiconductor sample;

[0042] Select corresponding optical elements of a frequency doubling unit, a beam splitting unit, a first beam return mirror group, a second beam return mirror group, a dispersion compensation unit, a polarization unit, and a beam combining unit to construct an optical path, so that the femtosecond laser energy generated by the light source can form a dual beam of fundamental frequency light and frequency doubling light after being frequency doubling by the frequency doubling unit, wherein the fundamental frequency light is transmitted by the beam splitting unit and then incident on the beam combining unit through the second beam return mirror group, the dispersion compensation unit, and the polarization unit in sequence; the frequency doubling light is reflected by the beam splitting unit and then incident on the beam combining unit through the first beam return mirror group;

[0043] Selecting a corresponding pulse modulation unit and a detection unit, the pulse modulation unit performs pulse modulation on the combined light of the beam combining unit and outputs a modulation frequency, the detection unit controls, receives and amplifies the modulation frequency of the pulse modulation unit, receives a voltage signal or a current signal on the semiconductor sample, and controls the movement or rotation of the second beam return mirror group and the polarization unit;

[0044] Adjusting the displacement of the first beam return mirror group so that the optical path lengths of the fundamental frequency light and the double frequency light are approximately equal;

[0045] Adjusting the rotation angle of the polarization unit so that the polarization directions of the fundamental frequency light and the double frequency light are parallel;

[0046] Adjusting the displacement of the second beam return mirror group to make the optical path lengths of the fundamental frequency light and the double frequency light equal;

[0047] Determine the thickness and refractive index of the optical element through which the fundamental frequency light passes, calculate the dispersion that needs to be compensated by the dispersion compensation unit, and perform dispersion compensation; and

[0048] Adjust the modulation frequency of the appropriate pulse modulation unit and feed it back to the detection unit.

[0049] In one or more embodiments of the present invention, adjusting the displacement of the first beam return mirror group so that the optical path lengths of the fundamental frequency light and the doubled frequency light are substantially equal comprises:

[0050] The polarization unit, the pulse modulation unit and the detection unit are removed, and a second frequency doubling unit, a filter unit and a detection lens are sequentially arranged downstream of the optical path of the beam combining unit;

[0051] The fundamental frequency light and the doubled frequency light are focused to the second doubled frequency unit after passing through the beam combining unit. After the fundamental frequency light is doubled again, it is incident to the filter unit with the same polarization direction as the original doubled frequency light to filter out the fundamental frequency light.

[0052] Manually adjust the movement of the first beam return mirror group until interference fringes of the doubled frequency light are observed through the detection lens.

[0053] In one or more embodiments of the present invention, adjusting the rotation angle of the polarization unit so that the polarization directions of the fundamental frequency light and the doubled frequency light are parallel includes:

[0054] The second frequency doubling unit, the filter unit and the detection lens are removed, a polarization unit is arranged between the dispersion compensation unit and the beam combining unit, and a pulse modulation unit and a detection unit are arranged in sequence downstream of the optical path of the beam combining unit;

[0055] Polarizers and spectrometers are used to determine the polarization directions of the fundamental frequency light and the double frequency light. At the same time, the rotation angle of the polarization unit is adjusted so that the polarization directions of the fundamental frequency light and the double frequency light remain consistent before beam combination.

[0056] In one or more embodiments of the present invention, adjusting the displacement of the second beam return mirror group to make the optical path lengths of the fundamental frequency light and the doubled frequency light equal includes:

[0057] The second beam return mirror group is controlled and adjusted to move in micrometer level according to the current signal detected by the detection unit until the detected current signal reaches the maximum.

[0058] In one or more embodiments of the present invention, determining the thickness and refractive index of the optical element through which the fundamental frequency light passes, calculating the dispersion required to be compensated by the dispersion compensation unit, and performing dispersion compensation includes:

[0059] Calculate the thickness of the optical element through which the fundamental frequency light passes and the refractive index of the optical element, calculate the optical path of the fundamental frequency light, and then calculate the number of reflections of the optical element in the dispersion compensation unit by determining the group delay generated by the fused silica of the corresponding thickness for each reflection compensation, wherein the dispersion compensation unit includes a chirped reflector and an adjustable wedge pair, and the number of reflections of the optical element in the dispersion compensation unit is the number of reflections of the chirped reflector;

[0060] Select a number of reflections greater than or equal to the calculated number for dispersion compensation;

[0061] When a reflection number greater than the calculated number is selected for dispersion compensation, a dispersion compensation transition will occur, and the dispersion is increased by adjusting the thickness of the adjustable wedge pair to perform dispersion compensation regulation.

[0062] A photocurrent control method, comprising:

[0063] Providing the above optical system, the optical system is used to generate an adjustable photocurrent on a semiconductor;

[0064] In the initial state of the optical system, controlling the second beam return mirror group to move relative to the optical path to adjust the magnitude and direction of the generated photocurrent;

[0065] In the initial state of the optical system, the polarization unit is controlled to rotate the polarization direction to adjust the magnitude and direction of the generated photocurrent.

[0066] Compared with the prior art, the optical system, construction method and photocurrent control method of the present invention generate photocurrent through double-beam quantum interference of fundamental frequency light and its frequency doubled light, and adjust the delay by changing the optical path difference of the double beams, and / or, by adjusting the relative polarization of the double beams, the size and direction of the photocurrent can be controlled. At the same time, the magnitude of the photocurrent generated is two orders of magnitude larger than that of the same-frequency light interference in the prior art, and the generated current signal is easier to observe.

[0067] The present invention realizes the generation, splitting and combining of dual light beams by doubling the frequency of a femtosecond laser light beam, and realizes adjustable relative delay and relative polarization of the dual light beams in the optical path. By utilizing the principle of dual-beam quantum interference, femtosecond laser light sources of different wavelengths can be selectively used to efficiently realize the generation of ultrafast photocurrent on most semiconductors, and the size and direction of the ultrafast photocurrent can be regulated by the relative delay and relative polarization of the dual light beams.

[0068] The optical system, construction method and photocurrent control method of the present invention can adjust not only the delay but also the polarization by using the second harmonic experiment of the fundamental frequency light and its doubled frequency. By changing multiple factors in a single way, the influence of different factors on the size and direction of the photocurrent can be studied.

[0069] The optical system, construction method and photocurrent control method of the present invention can accurately eliminate the dispersion caused by light passing through the lens and prevent pulse broadening. At the same time, quantitative changes in polarization direction are introduced to further explore the influence of the polarization of the two beams of light on the photocurrent.

[0070] The optical system, construction method and photocurrent control method of the present invention configure corresponding silver mirrors or aluminum mirrors for the reflector, focusing unit and collimation unit according to the type of light beam generated, so that the intensity loss of fundamental frequency light and frequency doubled light is minimized, and the distance from the frequency doubled unit to the focusing unit and the collimation unit and the angle of the frequency doubled unit are adjustable so that the generation efficiency of the frequency doubled light is maximized.

[0071] The optical system, construction method and photocurrent control method of the present invention use two dichroic mirrors to improve the separation efficiency of fundamental frequency light and doubled frequency light, without adding filters, and when combining beams, only the reversible principle of optical path is needed to combine the beams more conveniently.

[0072] The optical system, construction method and photocurrent control method of the present invention, the use of chirped reflectors and the introduction of adjustable wedge pairs can compensate for the dispersion inevitably introduced by the dichroic mirror, making the effects of relative delay and polarization on photocurrent most realistic.

[0073] The optical system, construction method and photocurrent control method of the present invention realize a method for determining that the optical paths of the doubled frequency light and the fundamental frequency light are equal.

[0074] The optical system, construction method and photocurrent control method of the present invention can not only study the effect of the relative delay of two light beams on the generation of ultrafast photocurrent, but also use this optical path to study the effect of the relative polarization of two light beams on ultrafast photocurrent.

[0075] The optical system, construction method and photocurrent control method of the present invention can achieve the generation of ultrafast photocurrents on different semiconductors and study the effects of different semiconductor materials on the photocurrent generation efficiency by matching the light source wavelength and the semiconductor bandgap width.

[0076] The optical system, construction method and photocurrent control method of the present invention utilize the principle of double-beam quantum interference to generate ultrafast photocurrent, which can greatly improve the intensity of ultrafast photocurrent. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0078] Figure 1 Schematic diagram of the optical path of the optical system in Example 1 of the present invention;

[0079] Figure 2 This is a flow chart of a method for building an optical system in Embodiment 1 of the present invention;

[0080] Figure 3 A schematic diagram of a light path in a method for constructing an optical system in Embodiment 1 of the present invention;

[0081] Figure 4 This is a flow chart of the photocurrent control method of the optical system in Example 1 of the present invention. DETAILED DESCRIPTION

[0082] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0083] In the prior art, the only method that studies the effect of photocurrent changes with the optical path length of two beams of light at the same frequency is to modulate the optical path length between the two beams of light at the same frequency. However, the magnitude of the current generated by this method is too small and needs to be amplified jointly by a preamplifier and a phase-locked amplifier, which makes the optical system complicated.

[0084] Based on the interference of two laser beams of different colors (different frequencies), which are coherently superimposed on a solid (usually a semiconductor), much higher peak intensities than continuous wave lasers can be achieved by concentrating the energy in pulses with a duration of picoseconds or femtoseconds.

[0085] By simultaneously stimulating the same electronic transition in a semiconductor with two harmonically related light beams, quantum interference control can occur by regulating the relative phase and polarization between the two light waves, thereby generating ultrafast photocurrent and regulating its size and direction, which is a nonlinear optical process.

[0086] Taking the 780nm fundamental wave and its second harmonic as examples, their wavelengths and energies differ by two times. For semiconductors such as gallium nitride, the bandgap is 3.4ev. The light field generated by the fundamental wave ω can make the valence band electrons absorb three photons at the same time and transition to the conduction band. The second harmonic 2ω makes the valence band electrons absorb two photons and excite to the conduction band, which correspond to three-photon absorption and two-photon absorption, respectively. Considered separately, the three-photon absorption of a high-energy light beam and the two-photon absorption of another light beam only lead to an increase in the number of excited charge carriers. If the two beams are superimposed, the two absorption paths will interfere. In semiconductors, this quantum interference ultimately leads to unequal excitation of electrons with opposite speeds. The relative phase between the two light waves and the polarization can control the asymmetry of the momentum of the conduction band population, thereby driving the current through the semiconductor and controlling the amplitude and direction of the generated photocurrent.

[0087] As an all-optical process, by superimposing a laser beam with its second harmonic, current can be optically generated without the need for an externally applied voltage, while quantum interference allows the injection of current at any point on a semiconductor with a microstructure. By controlling the polarization and optical phase of the two pulses, direction-resolved information about electron transport can be determined, and the amplitude and direction of the current can be regulated.

[0088] Based on this, the present invention provides an optical system for generating adjustable photocurrent on a semiconductor, a method for constructing the optical system, and a method for controlling the photocurrent. The optical system realizes the generation, splitting and combining of dual light beams by doubling the frequency of a femtosecond laser beam, and realizes adjustable relative delay and relative polarization of the dual light beams in the optical path. By utilizing the principle of dual-beam quantum interference, femtosecond laser light sources of different wavelengths can be selectively used to efficiently generate ultrafast photocurrent on most semiconductors, and the size and direction of the ultrafast photocurrent can be controlled by the relative delay and relative polarization of the dual light beams.

[0089] An optical system in an embodiment of the present invention is used to generate an adjustable photocurrent on a semiconductor, and the optical system includes the following components:

[0090] The light source is used to generate femtosecond laser, which is fundamental frequency light.

[0091] A frequency doubling unit is located in the optical path generated by the light source. The frequency doubling unit is used to double the frequency of the femtosecond laser. The frequency doubling unit can be controlled to move and rotate to adjust its position and angle in the optical path.

[0092] A beam splitting unit is located on the light path generated by the light source and downstream of the frequency doubling unit, and the beam splitting unit is used to reflect the frequency doubled light generated by the frequency doubling unit and transmit the fundamental frequency light from the frequency doubling unit;

[0093] A first beam return mirror group is located on the optical path of the frequency-doubled light reflected by the beam splitting unit, and the first beam return mirror group can be controlled to move to adjust the optical path of the frequency-doubled light;

[0094] A second beam return mirror group is located on the optical path of the fundamental frequency light transmitted by the beam splitting unit, and the second beam return mirror group can be controlled to move to adjust the optical path of the fundamental frequency light;

[0095] A dispersion compensation unit is located on the optical path of the fundamental frequency light transmitted by the beam splitting unit and downstream of the second beam return mirror group, and the dispersion compensation unit is used to compensate for the dispersion introduced by the beam splitting unit;

[0096] A polarization unit is located on the optical path of the fundamental frequency light transmitted by the beam splitting unit and downstream of the dispersion compensation unit, and the polarization unit can be controlled to rotate to adjust the polarization direction of the fundamental frequency light;

[0097] A beam combining unit is used to reflect the double frequency light from the first beam return mirror group and transmit the fundamental frequency light from the polarization unit, so as to combine the fundamental frequency light and the double frequency light;

[0098] a pulse modulation unit, located on the optical path of the combined light beam combined by the beam combining unit, and used for pulse modulating the combined light beam, outputting the pulse modulation unit, and amplifying the modulation frequency; and

[0099] The detection unit is at least electrically connected to the pulse modulation unit, the second beam return mirror group, the polarization unit and the semiconductor sample. The detection unit is used to control, receive and amplify the modulation frequency of the pulse modulation unit, receive the voltage signal or current signal on the semiconductor sample, and control the movement or rotation of the second beam return mirror group and the polarization unit.

[0100] After the femtosecond laser is frequency-doubled by the frequency-doubling unit, it forms fundamental frequency light and frequency-doubled light. After the fundamental frequency light is transmitted by the beam splitting unit, it passes through the second beam return mirror group, the dispersion compensation unit and the polarization unit, and then enters the beam combining unit; after the frequency-doubled light is reflected by the beam splitting unit, it passes through the first beam return mirror group and then enters the beam combining unit; after the fundamental frequency light and the frequency-doubled light are combined by the beam combining unit, they form quantum interference, and after being modulated by the pulse modulation unit, they are incident on the semiconductor sample to generate photocurrent, which is monitored by the detection unit;

[0101] The optical system has an initial state, in which the optical paths of the fundamental frequency light and the doubled frequency light in the combined light beam are equal, and the polarization directions of the fundamental frequency light and the doubled frequency light in the combined light beam are the same;

[0102] The magnitude of the photocurrent generated by the optical system is adjusted at least by adjusting the movement of the second beam return mirror group and / or the rotation of the polarization unit.

[0103] The optical system also includes the following components:

[0104] A focusing unit is disposed upstream of the frequency doubling unit and is used to focus the fundamental frequency light incident to the frequency doubling unit, and is disposed upstream of the semiconductor sample and is used to focus the fundamental frequency light and the frequency doubling light incident to the semiconductor sample;

[0105] The collimating unit is arranged downstream of the frequency doubling unit and is used for collimating the fundamental frequency light and the frequency doubling light emitted from the frequency doubling unit.

[0106] The attenuation unit is arranged on the optical path of the fundamental frequency light transmitted by the beam splitting unit and is located upstream of the second beam return mirror group. The attenuation unit is used to adjust the intensity of the fundamental frequency light.

[0107] A plurality of reflectors are arranged on the optical path of the fundamental frequency light, the optical path of the frequency-doubled light, and the optical path of the combined light to adjust the optical path so that the final combined light is incident on the semiconductor sample.

[0108] The frequency doubling unit includes a frequency doubling crystal, which is arranged on a first movable displacement stage; the beam splitting unit includes a first dichroic mirror, and the beam combining unit includes a second dichroic mirror; the first beam return mirror group includes a plurality of reflectors, which are arranged on a second movable displacement stage; the second beam return mirror group includes a hollow roof prism reflector, which is arranged on a third movable displacement stage, and the third movable displacement stage can be controlled to move at the micron level; the dispersion compensation unit includes a plurality of reflectors arranged in sequence along the optical path of the fundamental frequency light. A chirped reflector and an adjustable wedge pair; a polarization unit includes a half-wave plate, which is arranged on a fourth movable displacement stage, the fourth movable displacement stage can be controlled to rotate, and the half-wave plate is a dispersion-free half-wave plate; a pulse modulation unit includes a chopper and a phase-locked amplifier, and the chopper is electrically connected to the phase-locked amplifier; a focusing unit and a collimating unit both include a parabolic reflector; the focusing unit includes a parabolic reflective silver mirror and / or a parabolic reflective aluminum mirror, and the collimating units both include a parabolic reflective aluminum mirror; an attenuation unit includes an attenuation plate, and the attenuation of the attenuation plate is adjustable. A plurality of reflectors include a reflective silver mirror and a reflective aluminum mirror; the reflective silver mirror is arranged on the optical path through which the fundamental frequency light passes; and the reflective aluminum mirror is arranged on the optical path through which the frequency-doubled light and the combined beam light pass.

[0109] like Figure 1As shown, the optical system includes a femtosecond laser light source 10, a first reflection silver mirror 210, a second reflection silver mirror 211, a third reflection silver mirror 212, a first parabolic reflection silver mirror 30, a BBO crystal 40, a first parabolic reflection aluminum mirror 51, a first reflection aluminum mirror 220, and a first dichroic mirror 61. The fundamental frequency light emitted by the femtosecond laser light source 10 passes through the first reflection silver mirror 210, the second reflection silver mirror 211, the third reflection silver mirror 212, the first parabolic reflection silver mirror 30, the BBO crystal 40, the first parabolic reflection aluminum mirror 51, and the first reflection aluminum mirror 220 in sequence and then enters the first dichroic mirror 61.

[0110] The optical system further includes a second reflecting aluminum mirror 221, a third reflecting aluminum mirror 222, a fourth reflecting aluminum mirror 223, a fifth reflecting aluminum mirror 224, a sixth reflecting aluminum mirror 225, and a seventh reflecting aluminum mirror 226 on the optical path of the doubled frequency light B. The doubled frequency light B reflected by the first dichroic mirror 61 passes through the second reflecting aluminum mirror 221, the third reflecting aluminum mirror 222, the fourth reflecting aluminum mirror 223, the fifth reflecting aluminum mirror 224, the sixth reflecting aluminum mirror 225, and the seventh reflecting aluminum mirror 226 in sequence, and then enters the second dichroic mirror 62. Among them, the first beam return mirror group includes the fourth reflecting aluminum mirror 223 and the fifth reflecting aluminum mirror 224.

[0111] The optical system further includes an attenuation plate 70, a hollow roof prism reflector 80, a fourth reflective silver mirror 213, a chirped reflector 91 and an adjustable wedge pair 92, a half-wave plate 100, and a fifth reflective silver mirror 214 on the optical path of the fundamental frequency light A. The fundamental frequency light A transmitted through the first dichroic mirror 61 sequentially passes through the attenuation plate 70, the hollow roof prism reflector 80, the fourth reflective silver mirror 213, the chirped reflector 91 and an adjustable wedge pair 92, the half-wave plate 100, and the fifth reflective silver mirror 214, and is incident on the second dichroic mirror 62.

[0112] The optical system further includes a second dichroic mirror 62, an eighth reflective aluminum mirror 227, a chopper 110, a ninth reflective aluminum mirror 228, a second parabolic reflective aluminum mirror 52, a semiconductor sample 120, a lock-in amplifier 130, and a detection unit 140 (computer). The combined light beams combined by the second dichroic mirror 62 are sequentially passed through the eighth reflective aluminum mirror 227, the chopper 110, the ninth reflective aluminum mirror 228, and the second parabolic reflective aluminum mirror 52, and then incident on the semiconductor sample 120, where quantum interference occurs and ultrafast photocurrent is generated. The lock-in amplifier 130 and the detection unit 140 (including a computer - LABVIEW software is used in this embodiment) are used to amplify and receive information from the chopper 110, and detect the magnitude and direction of the generated ultrafast photocurrent, so as to study the influence of the relative delay and relative polarization of the two light beams on the current.

[0113] Among them, the optical system has an initial state, in which the optical paths of the fundamental frequency light A and the doubled frequency light B in the combined light beam are equal, and the polarization directions of the fundamental frequency light A and the doubled frequency light B in the combined light beam are the same; the magnitude of the photocurrent generated by the optical system can be adjusted by adjusting the movement of the third movable displacement stage and the hollow roof prism reflector 80 thereon and / or the rotation of the fourth movable displacement stage and the half-wave plate 100 thereon.

[0114] like Figure 2 As shown, the present invention also provides a method for constructing the above optical system, comprising the following steps:

[0115] S1, determine the semiconductor sample and select the corresponding femtosecond laser light source according to the bandgap width of the semiconductor sample.

[0116] S2, select the corresponding optical elements of the frequency doubling unit, beam splitting unit, first beam return mirror group, second beam return mirror group, dispersion compensation unit, polarization unit, and beam combining unit to build the optical path. The requirements of the optical path are: the femtosecond laser energy generated by the light source is doubled by the frequency doubling unit to form a dual beam of fundamental frequency light and doubled frequency light, wherein the fundamental frequency light is transmitted by the beam splitting unit and then incident on the beam combining unit through the second beam return mirror group, the dispersion compensation unit, and the polarization unit in sequence; the doubled frequency light is reflected by the beam splitting unit and then incident on the beam combining unit through the first beam return mirror group.

[0117] S3, select the corresponding pulse modulation unit and detection unit, the pulse modulation unit performs pulse modulation on the combined light of the combining unit and outputs the modulation frequency, the detection unit controls, receives and amplifies the modulation frequency of the pulse modulation unit, receives the voltage signal or current signal on the semiconductor sample, and controls the movement or rotation of the second beam return mirror group and the polarization unit.

[0118] S4, adjusting the displacement of the first beam return mirror group so that the optical path lengths of the fundamental frequency light and the doubled frequency light are approximately equal.

[0119] S5, adjusting the rotation angle of the polarization unit so that the polarization directions of the fundamental frequency light and the doubled frequency light are parallel.

[0120] S6, adjusting the displacement of the second beam return mirror group to make the optical path lengths of the fundamental frequency light and the doubled frequency light equal.

[0121] S7, determining the thickness and refractive index of the optical element through which the fundamental frequency light passes, calculating the dispersion that needs to be compensated by the dispersion compensation unit, and performing dispersion compensation.

[0122] S8, adjusting the modulation frequency of the appropriate pulse modulation unit and feeding it back to the detection unit.

[0123] The step S4 of adjusting the displacement of the first beam return mirror group so that the optical path lengths of the fundamental frequency light and the doubled frequency light are substantially equal specifically includes:

[0124] The polarization unit, the pulse modulation unit and the detection unit are removed, and a second frequency doubling unit, a filter unit and a detection lens are sequentially arranged downstream of the optical path of the beam combining unit;

[0125] The fundamental frequency light and the doubled frequency light are focused to the second doubled frequency unit after passing through the beam combining unit. After the fundamental frequency light is doubled again, it is incident to the filter unit with the same polarization direction as the original doubled frequency light to filter out the fundamental frequency light.

[0126] Manually adjust the movement of the first beam return mirror group until interference fringes of the doubled frequency light are observed through the detection lens.

[0127] The step S5 of adjusting the rotation angle of the polarization unit so that the polarization directions of the fundamental frequency light and the double frequency light are parallel includes:

[0128] The second frequency doubling unit, the filter unit and the detection lens are removed, a polarization unit is arranged between the dispersion compensation unit and the beam combining unit, and a pulse modulation unit and a detection unit are arranged in sequence downstream of the optical path of the beam combining unit;

[0129] A spectrometer is used to determine the polarization directions of the fundamental frequency light and the frequency-doubled light. At the same time, the rotation angle of the polarization unit is adjusted so that the polarization directions of the fundamental frequency light and the frequency-doubled light remain consistent before beam combination.

[0130] The step S6 of adjusting the displacement of the second beam return mirror group to make the optical path lengths of the fundamental frequency light and the double frequency light equal specifically includes: controlling and adjusting the second beam return mirror group to move in micrometer level according to the current signal detected by the detection unit until the detected current signal is maximum.

[0131] Determining the thickness and refractive index of the optical element through which the fundamental frequency light passes, calculating the dispersion required to be compensated by the dispersion compensation unit, and performing dispersion compensation in step S7 specifically includes:

[0132] Calculate the thickness of the optical element through which the fundamental frequency light passes and the refractive index of the optical element, calculate the optical path of the fundamental frequency light, and then calculate the number of reflections of the optical element in the dispersion compensation unit by determining the group delay generated by the fused silica of the corresponding thickness for each reflection compensation, wherein the dispersion compensation unit includes a chirped reflector and an adjustable wedge pair, and the number of reflections of the optical element in the dispersion compensation unit is the number of reflections of the chirped reflector;

[0133] Select a number of reflections greater than or equal to the calculated number for dispersion compensation;

[0134] When a reflection number greater than the calculated number is selected for dispersion compensation, a dispersion compensation transition will occur, and the dispersion is increased by adjusting the thickness of the adjustable wedge pair to perform dispersion compensation regulation.

[0135] Specifically, Figure 1 and Figure 3 As shown, Figure 3The figure shows a schematic diagram of the optical path used to roughly control the optical path of the double beams in the method of building the optical system. Figure 3 Build the basic optical path as shown and make a rough adjustment of the optical path.

[0136] A femtosecond laser light source 10 is selected that can emit a central wavelength of 780nm, a fundamental wave of 780nm red light (fundamental frequency light A), and a second harmonic of 390nm purple light (double frequency light B) after the red light is doubled in frequency. The photon energy of the fundamental frequency light A is ω=1.59 ev, the photon energy of the doubled light B is 2 ω=3.18ev. / 3< ω< / 2,( =h / 2π, where h is Planck's constant, is the bandgap width of the semiconductor) corresponds to three-photon absorption. The light field generated by the fundamental wave ω can make the valence band electrons absorb three photons at the same time and transition to the conduction band; / 2<2 ω< , corresponding to two-photon absorption, the second harmonic 2ω causes the valence band electrons to absorb two electrons simultaneously and excite to the conduction band; similarly, if the corresponding photon energy is greater than , then the valence band electrons directly absorb a photon and transition to the conduction band, corresponding to single-photon absorption. The three-photon absorption of fundamental frequency light A and the two-photon absorption of double-frequency light B can respectively stimulate the increase in the number of charge carriers. When the two light beams are superimposed, quantum interference will occur in the two absorption paths, thereby generating a photocurrent on the semiconductor sample 120.

[0137] β-BBO crystal 40 is selected to double the frequency of red light. Since the silver mirror has a high reflectivity for the red light band and the aluminum mirror has a high reflectivity for the blue-violet light band, the light path through which the fundamental frequency light A (hereinafter referred to as red light) passes is uniformly reflected by the silver mirror, and the light path through which the doubled frequency light B (hereinafter referred to as violet light) passes and the combined beam light path are uniformly reflected by the aluminum mirror.

[0138] Place the BBO crystal 40 on the first movable displacement stage, fine-tune the BBO crystal 40 so that the red light is focused on the center of the BBO crystal 40, and fine-tune to focus to maximize the efficiency of second harmonic generation. At the same time, rotate the BBO crystal 40 and use a spectrometer to observe that the intensity of the blue-violet light is maximum. The focusing in front of the BBO crystal 40 and the collimation behind the BBO crystal 40 use the first parabolic reflective silver mirror 30 and the first parabolic reflective aluminum mirror 51 respectively instead of using a lens to avoid introducing dispersion.

[0139] The first dichroic mirror 61 can transmit the red light band and reflect the purple light band, with a reflection angle of 56°, thereby realizing the splitting of the two-color light. After the two beams of light pass through two adjustable displacement return mirror groups (the first return mirror group - the fourth reflective aluminum mirror 223 and the fifth reflective aluminum mirror 224, the second return mirror group - the hollow roof prism reflector 80), the optical path is adjustable, and then the reversible principle of the optical path is used to pass through the second dichroic mirror 62 again to achieve collimation and beam combining.

[0140] The second beam-returning mirror group is placed on the third movable displacement stage. The third movable displacement stage on which the second beam-returning mirror group is placed is controlled by the detection unit 140 (including a computer - LABVIEW software is used in this embodiment) and is capable of fine adjustment of the optical path at the micron level. That is, the third movable displacement stage on which the hollow roof prism reflector 80 is placed is a precision electric displacement platform, which can be controlled to achieve micron-level movement and change the red light optical path. The first beam-returning mirror group is placed on the second movable displacement stage. The second movable displacement stage on which the first beam-returning mirror group is placed is a manually controlled coarse adjustment of the optical path.

[0141] In order to find the delay zero point (i.e. the optical path lengths of the two beams are equal), after roughly estimating the optical path lengths of the two beams and placing the optical components, it is necessary to roughly adjust the optical path length of the purple light, such as Figure 3 As shown. The specific method is that after the double light beams pass through the second dichroic mirror 62, they are focused onto the second BBO crystal 150 through the second parabolic reflective aluminum mirror 52. Because the violet light band is not in the BBO crystal phase matching band, the violet light will directly pass through the second BBO crystal 150 without frequency doubling again, and the red light will be frequency doubling again through the second BBO crystal 150 to become violet light. A filter 160 is added after the second BBO crystal 150 to filter out the red light, so that the light after the filter 160 is the violet light obtained by the first frequency doubling and the violet light obtained by the second red light frequency doubling (the polarization directions of the two violet light beams are consistent). The violet light spot can be seen by using the CCD detection lens 170. The second movable displacement stage where the first beam return mirror group (violet light) is placed is roughly adjusted until the fringes after the interference of the two violet light beams of the same frequency are clearly seen. It can be considered that it has been roughly adjusted to the zero point of the optical path of the two beams of light, that is, in this state, the optical path of the red light and the violet light are roughly equal.

[0142] In the above scheme, first, when building the first beam return mirror group (purple light) and the second beam return mirror group (red light), you can use a ruler to measure the optical path at both ends to preliminarily determine the placement position of the second movable stage of the first beam return mirror group (purple light). After placement, use the above scheme to roughly adjust the second movable stage until the optical path of red light and purple light is roughly the same (based on the fringes after the interference of the two purple light beams). Verify that the optical path is interfering with the optical path. Figure 3 After the second BBO crystal has doubled the frequency by 150 times, the light that comes out should be twice-doubled violet light and fundamental red light, and the polarization directions of the twice-doubled violet light are consistent.

[0143] Notice, Figure 3 The half-wave plate 100 in the optical system of the present application is not provided, thereby ensuring that the polarization directions of the two beams of purple light are the same. The red light is then filtered out with the filter 160, and two divergent beams of collinear purple light are obtained. The second movable stage of the purple light is manually adjusted until obvious interference fringes just appear, that is, the optical path of the red light and the purple light after the first frequency doubling is roughly equal.

[0144] It should be noted that the optical path lengths of red light and violet light after passing through the BBO crystal are different, because the refractive index of the BBO crystal for the two beams of light is different. Therefore, the optical path length of the violet light after the second frequency doubling is not completely equal to the red light without the second frequency doubling. There will be a slight error between the two, but if obvious interference fringes can be seen, it must be near the zero point of the optical path length. That is, at this time, the optical path lengths of the red light and violet light without the second frequency doubling are actually roughly equal.

[0145] Based on this, after adjusting the position of the second movable stage so that the optical paths of the two beams are roughly equal, refer to Figure 1 As shown, the second BBO crystal 150, the filter 160 and the CCD detection lens 170 are removed, and the polarization unit, the chopper 110, the phase-locked amplifier 130 are placed and the detection unit 140 is connected.

[0146] After the second BBO crystal 150 is removed, the polarization directions of the red light and the purple light are changed. Due to the phase matching condition, the vibration directions of the red light and the purple light obtained after passing through the BBO crystal 40 are orthogonal. The polarization directions of the two beams of light are determined by using a polarizer and a spectrometer respectively, and the red light is rotated 45° through the achromatic half-wave plate 100 before the beam is combined to make the polarization directions of the two beams consistent.

[0147] After the polarization directions of the two beams of light (red light and purple light) are adjusted to be consistent, the optical path of the red light is finely adjusted to make the optical paths of the red light and the purple light completely equal. Specifically, the detection unit 140 (including a computer - LABVIEW software is used in this embodiment) controls the third movable stage to move at the micrometer level, and records the current at each displacement point, finds the point corresponding to the peak current (the point of maximum current) and moves the third movable stage to the point (the true optical path zero point), at which time it is considered that the optical paths of the two beams are completely consistent (experiments show that the current is maximum when the delay is 0).

[0148] Figure 1In the entire optical path, almost no dispersion is introduced in the purple light path, while the red light passes through the dichroic mirror and the attenuation plate twice (the intensity of the red light will be much greater than that of the purple light after being split by the dichroic mirror, and the attenuation of the attenuation plate can be adjusted. Because it is uncertain whether the size of the generated photocurrent is related to the relative intensity of the two beams of light, the attenuation plate is added to adjust the intensity of the red light. It is understandable that the relative intensity of the two beams of light can also be used as a factor to study the influence of the photocurrent size in the future), and dispersion is introduced. In order to better obtain the variation law of the ultrafast photocurrent, it is necessary to compensate for the introduced dispersion, so a dispersion compensation unit is continued to be set in the red light path.

[0149] The chirped reflector 91 in the dispersion compensation unit can correct the pulse broadening that occurs when the ultrashort pulse passes through the optical system. The optical path of the red light is calculated by calculating the thickness of the lens through which the red light passes and the refractive index of the medium, and then the number of reflections required by the chirped reflector 91 is calculated by determining the group delay generated by the corresponding thickness of fused silica for each reflection compensation.

[0150] The optical path length of the red light is calculated in order to calculate the number of times the chirped mirror reflects 91. For example, the chirped reflector used in the experiment can compensate for the dispersion caused by passing through 1.5 mm thick quartz with one reflection. Optical path length = medium refractive index × distance traveled by the light in the medium. Calculating the optical path length of the red light can estimate how many times the chirped mirror reflection 91 needs to reflect, which can be more or less. If the number of reflections is too much, it means that the pre-compensation dispersion is excessive, and the thickness of the adjustable wedge pair 92 can be increased to fully compensate.

[0151] In this embodiment, the corresponding number of reflections is selected and some thickness is pre-compensated, and then the thickness is adjusted by the adjustable wedge pair 92 to slightly increase the dispersion to achieve complete dispersion compensation. The number of reflections can be adjusted according to the angle of the chirped reflector pair 91, which is relatively simple. For example, if the dichroic mirror is 2 mm quartz twice, then the reflection three times pre-compensates 4.5 mm, and after passing through the dichroic mirror twice, a dispersion of 4 mm is generated, and there is about 0.5 mm left to fine-tune the adjustable wedge pair 92. In fact, the refractive index may change during the process, and the basis for the final complete dispersion compensation is that the intensity of the second harmonic observed by the spectrometer is the maximum, which is the complete dispersion compensation.

[0152] Next, the rotating blades of the chopper 110 are placed in the optical path. As the blades rotate, the light will periodically pass through and be blocked, thereby applying pulse modulation to the optical signal, and the modulation frequency depends on the rotation frequency of the blades. In this way, the light passing through the chopper 110 is fixed frequency light, and the modulation frequency can be output at the same time. The frequency of the chopper 110 is adjusted to a fixed appropriate value, and the signal of the optical chopper 110 is input into the phase-locked amplifier 130 for reference, which is used to amplify the measured voltage signal.

[0153] It is understandable that the lock-in amplifier is divided into two modes: voltage input and current input. If the resistance of the semiconductor sample 120 to be measured is known, voltage input can be selected, and the voltage divided by the resistance is the corresponding current, or the voltage-delay image can also reflect the change of the photocurrent. Voltage input is easier to operate. If the resistance of the selected semiconductor sample 120 is unknown, current input can also be selected.

[0154] Based on this, the optical system was built and debugged.

[0155] LABVIEW can control the micron-level movement and rotation of the second beam return mirror group and polarization unit.

[0156] Based on the zero point of the optical path of the purple light and the red light, the second beam return mirror group is controlled by LABVIEW to move in micrometer level to achieve the micrometer level change of the relative optical path of the red light and the purple light, and the detection unit 140 (including a computer - LABVIEW software is used in this embodiment) reads the voltage output of the phase-locked amplifier 130 (SR830) (the voltage at each position measured by the phase-locked amplifier 130 will be read by LABVIEW to make a regular image of the photocurrent change), and records the corresponding current at each moving position, so as to obtain the influence of the relative delay of the two beams on the current. At this time, it is necessary to ensure that the polarization direction of the red light and the purple light is the same.

[0157] Because the delay of 1fs corresponds to an optical path of 0.3 microns (speed × time), under the premise of equal optical paths, each movement of the hollow roof prism reflector 80° causes the optical path of the red light to change by 0.3 microns, and the delay is 1fs, thereby achieving a voltage reading every micron-level displacement. By changing the optical path difference of the two light beams, the relationship between the relative delay and the ultrafast photocurrent can be obtained to draw an image.

[0158] The optical path of the two beams of light is adjusted back to the zero position, and on the basis that the polarization directions of the two beams are parallel, LABVIEW is used to control the angle change of the fourth movable translation stage on which the half-wave plate 100 (polarization unit) is placed, and the detection unit 140 (including a computer - LABVIEW software is used in this embodiment) is used to read the voltage output of the phase-locked amplifier 130 (SR830) (the voltage at each angle measured by the phase-locked amplifier 130 will be read by LABVIEW to make an image of the law of photocurrent changes), and the corresponding current is recorded at each angle, thereby obtaining the influence of the relative polarization of the two beams of light on the current.

[0159] When the optical path is fixed, the LABVIEW software is used to control the rotation of the half-wave plate 100 to change the relative polarization of the double light beams, so that the voltage reading can be read once every specific rotation angle, the relationship between the relative polarization and the ultrafast photocurrent can be obtained, and an image can be drawn.

[0160] Through the above two methods, the regulation rules of the relative delay and relative polarization of the two beams of light on the size and direction of the photocurrent are obtained respectively. After obtaining the rules of the relative delay and relative polarization on the size and direction of the photocurrent, the ultrafast current of the required size and direction can be generated on the semiconductor sample 120, and the response of the semiconductor device can be further controlled. The law finally measured in the experiment is a regular oscillating waveform that gradually decreases from the center to both sides with a sinusoidal change.

[0161] It can be understood that, under the premise of equal optical path and the same polarization direction, the relationship between the relative intensity of the two beams of light and the photocurrent can be further studied by adjusting the attenuation degree of the attenuation plate 70, and an image can be drawn. Furthermore, an ultrafast current of the required size and direction can be generated on the semiconductor sample 120, and the response of the semiconductor device can be further controlled.

[0162] like Figure 4 As shown, the present invention also provides a method for photocurrent regulation using the above optical system, comprising the following steps:

[0163] S10, providing the above optical system, wherein the optical system is used to generate an adjustable photocurrent on a semiconductor;

[0164] S20, in the initial state of the optical system, controlling the second beam return mirror group to move relative to the optical path to adjust the magnitude and direction of the generated photocurrent;

[0165] S30, in the initial state of the optical system, controlling the polarization unit to rotate the polarization direction to adjust the magnitude and direction of the generated photocurrent.

[0166] Compared with the prior art, the optical system, construction method and photocurrent control method of the present invention generate photocurrent through double-beam quantum interference of fundamental frequency light and its frequency doubled light, and adjust the delay by changing the optical path difference of the double beams, and / or, by adjusting the relative polarization of the double beams, the size and direction of the photocurrent can be controlled. At the same time, the magnitude of the photocurrent generated is two orders of magnitude larger than that of the same-frequency light interference in the prior art, and the generated current signal is easier to observe.

[0167] The present invention realizes the generation, splitting and combining of dual light beams by doubling the frequency of a femtosecond laser light beam, and realizes adjustable relative delay and relative polarization of the dual light beams in the optical path. By utilizing the principle of dual-beam quantum interference, femtosecond laser light sources of different wavelengths can be selectively used to efficiently realize the generation of ultrafast photocurrent on most semiconductors, and the size and direction of the ultrafast photocurrent can be regulated by the relative delay and relative polarization of the dual light beams.

[0168] The optical system, construction method and photocurrent control method of the present invention can adjust not only the delay but also the polarization by using the second harmonic experiment of the fundamental frequency light and its doubled frequency. By changing multiple factors in a single way, the influence of different factors on the size and direction of the photocurrent can be studied.

[0169] The optical system, construction method and photocurrent control method of the present invention can accurately eliminate the dispersion caused by light passing through the lens and prevent pulse broadening. At the same time, quantitative changes in polarization direction are introduced to further explore the influence of the polarization of the two beams of light on the photocurrent.

[0170] The optical system, construction method and photocurrent control method of the present invention configure corresponding silver mirrors or aluminum mirrors for the reflector, focusing unit and collimation unit according to the type of light beam generated, so that the intensity loss of fundamental frequency light and frequency doubled light is minimized, and the distance from the frequency doubled unit to the focusing unit and the collimation unit and the angle of the frequency doubled unit are adjustable so that the generation efficiency of the frequency doubled light is maximized.

[0171] The optical system, construction method and photocurrent control method of the present invention use two dichroic mirrors to improve the separation efficiency of fundamental frequency light and doubled frequency light, without adding filters, and when combining beams, only the reversible principle of optical path is needed to combine the beams more conveniently.

[0172] The optical system, construction method and photocurrent control method of the present invention, the use of chirped reflectors and the introduction of adjustable wedge pairs can compensate for the dispersion inevitably introduced by the dichroic mirror, making the effects of relative delay and polarization on photocurrent most realistic.

[0173] The optical system, construction method and photocurrent control method of the present invention realize a method for determining that the optical paths of the doubled frequency light and the fundamental frequency light are equal.

[0174] The optical system, construction method and photocurrent control method of the present invention can not only study the effect of the relative delay of two light beams on the generation of ultrafast photocurrent, but also use this optical path to study the effect of the relative polarization of two light beams on ultrafast photocurrent.

[0175] The optical system, construction method and photocurrent control method of the present invention can achieve the generation of ultrafast photocurrents on different semiconductors and study the effects of different semiconductor materials on the photocurrent generation efficiency by matching the light source wavelength and the semiconductor bandgap width.

[0176] The optical system, construction method and photocurrent control method of the present invention utilize the principle of double-beam quantum interference to generate ultrafast photocurrent, which can greatly improve the intensity of ultrafast photocurrent.

[0177] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0178] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. An optical system for generating an adjustable photocurrent on a semiconductor, characterized in that: include: A light source, used to generate femtosecond laser light, wherein the femtosecond laser light is fundamental frequency light; A frequency doubling unit is located in the optical path generated by the light source, and is used to double the frequency of the femtosecond laser. The frequency doubling unit can be controlled to move and rotate to adjust its position and angle in the optical path; A beam splitting unit is located on the light path generated by the light source and downstream of the frequency doubling unit, and is used to reflect the frequency doubled light generated by the frequency doubling unit and transmit the fundamental frequency light from the frequency doubling unit; A first beam return mirror group is located on the optical path of the frequency-doubled light reflected by the beam splitting unit, and the first beam return mirror group can be controlled to move to adjust the optical path of the frequency-doubled light; A second beam return mirror group is located on the optical path of the fundamental frequency light transmitted by the beam splitting unit, and the second beam return mirror group can be controlled to move to adjust the optical path of the fundamental frequency light; A dispersion compensation unit, located on the optical path of the fundamental frequency light transmitted by the beam splitting unit and downstream of the second beam return mirror group, the dispersion compensation unit is used to compensate for the dispersion introduced by the beam splitting unit; A polarization unit, located on the optical path of the fundamental frequency light transmitted by the beam splitting unit and downstream of the dispersion compensation unit, wherein the polarization unit can be controlled to rotate to adjust the polarization direction of the fundamental frequency light; A beam combining unit is used to reflect the doubled frequency light from the first beam returning mirror group and transmit the fundamental frequency light from the polarization unit, so as to combine the fundamental frequency light and the doubled frequency light; the femtosecond laser is frequency-doubled by the frequency doubling unit to form the fundamental frequency light and the doubled frequency light; the fundamental frequency light is transmitted by the beam splitting unit, passes through the second beam returning mirror group, the dispersion compensation unit and the polarization unit, and then enters the beam combining unit; After the doubled frequency light is reflected by the beam splitting unit, it passes through the first beam return mirror group and is incident on the beam combining unit; after the fundamental frequency light and the doubled frequency light are combined by the beam combining unit to form quantum interference, they are incident on the semiconductor sample to generate photocurrent; The optical system has an initial state, in which the optical paths of the fundamental frequency light and the frequency doubled light in the combined light beam are equal, and the polarization directions of the fundamental frequency light and the frequency doubled light in the combined light beam are the same; The magnitude of the photocurrent generated by the optical system is adjusted at least by adjusting the movement of the second beam return mirror group and / or the rotation of the polarization unit; The dispersion compensation unit comprises a chirped reflector and an adjustable wedge pair arranged in sequence along the optical path of the fundamental frequency light; the optical path of the fundamental frequency light is calculated by calculating the thickness of the optical element through which the fundamental frequency light passes and the refractive index of the optical element, and then the number of reflections of the chirped reflector in the dispersion compensation unit is calculated by determining the group delay generated by the fused silica of the corresponding thickness of each reflection compensation; a number of reflections greater than or equal to the calculated number is selected for dispersion compensation; wherein, when a number of reflections greater than the calculated number is selected for dispersion compensation, a dispersion compensation transition occurs, and the dispersion is increased by adjusting the thickness of the adjustable wedge pair to perform dispersion compensation regulation.

2. The optical system according to claim 1, characterized in that Also includes: A pulse modulation unit, located on the optical path of the combined light beam combined by the beam combining unit, and configured to pulse modulate and amplify the combined light beam and output a modulation frequency; as well as The detection unit is at least electrically connected to the pulse modulation unit, the second beam return mirror group, the polarization unit and the semiconductor sample. The detection unit is used to control and receive the modulation frequency of the pulse modulation unit, receive the voltage signal or current signal on the semiconductor sample, and control the movement or rotation of the second beam return mirror group and the polarization unit.

3. The optical system according to claim 2, characterized in that The frequency doubling unit comprises a frequency doubling crystal, and the frequency doubling crystal is arranged on the first movable translation stage; and / or, The beam splitting unit includes a first dichroic mirror, and the beam combining unit includes a second dichroic mirror; and / or, The first beam return mirror group includes a plurality of reflectors, and the plurality of reflectors are arranged on the second movable displacement stage; and / or, The second beam return mirror group includes a hollow roof prism reflector, and the hollow roof prism reflector is arranged on a third movable displacement stage, and the third movable displacement stage can be controlled to move in micrometer level; and / or, The polarization unit comprises a half-wave plate, and the half-wave plate is arranged on a fourth movable displacement stage, and the fourth movable displacement stage can be controlled to rotate; and / or, The pulse modulation unit includes a chopper and a lock-in amplifier, and the chopper is electrically connected to the lock-in amplifier.

4. The optical system according to claim 1, characterized in that It also includes a focusing unit, which is arranged upstream of the frequency doubling unit and is used to focus the fundamental frequency light incident to the frequency doubling unit, and is arranged upstream of the semiconductor sample and is used to focus the fundamental frequency light and the frequency doubling light incident to the semiconductor sample; The collimating unit is arranged downstream of the frequency doubling unit and is used for collimating the fundamental frequency light and the frequency doubling light emitted from the frequency doubling unit.

5. The optical system according to claim 4, characterized in that The focusing unit and the collimating unit both include parabolic reflectors; and / or, The focusing unit includes a parabolic reflective silver mirror and / or a parabolic reflective aluminum mirror, and the collimating unit includes a parabolic reflective aluminum mirror.

6. The optical system according to claim 1, characterized in that It also includes an attenuation unit, which is arranged on the optical path of the fundamental frequency light transmitted by the beam splitting unit and located upstream of the second beam return mirror group, and is used to adjust the intensity of the fundamental frequency light.

7. The optical system according to claim 1, characterized in that It also includes a plurality of reflectors, which are arranged on the optical path of the fundamental frequency light, the optical path of the doubled frequency light, and the optical path of the combined light, so as to control the optical path so that the final combined light is incident on the semiconductor sample.

8. The optical system according to claim 7, characterized in that The plurality of reflectors include a reflective silver mirror and a reflective aluminum mirror; The reflective silver mirror is arranged on the optical path through which the fundamental frequency light passes; The reflective aluminum mirror is arranged on the optical path through which the frequency-doubled light and the combined light pass.

9. A method for constructing an optical system for generating an adjustable photocurrent on a semiconductor, characterized in that: include: Determine the semiconductor sample and select a corresponding femtosecond laser light source according to the bandgap width of the semiconductor sample; Select corresponding optical elements of a frequency doubling unit, a beam splitting unit, a first beam return mirror group, a second beam return mirror group, a dispersion compensation unit, a polarization unit, and a beam combining unit to construct an optical path, so that the femtosecond laser energy generated by the light source can form a double beam of fundamental frequency light and frequency doubling light after being frequency doubling by the frequency doubling unit, wherein the fundamental frequency light is transmitted by the beam splitting unit and then incident on the beam combining unit through the second beam return mirror group, the dispersion compensation unit, and the polarization unit in sequence; the frequency doubling light is reflected by the beam splitting unit and then incident on the beam combining unit through the first beam return mirror group; wherein the dispersion compensation unit includes a chirped reflector and an adjustable wedge pair; Selecting a corresponding pulse modulation unit and a detection unit, the pulse modulation unit performs pulse modulation on the combined light of the beam combining unit and outputs a modulation frequency, the detection unit controls, receives and amplifies the modulation frequency of the pulse modulation unit, receives a voltage signal or a current signal on the semiconductor sample, and controls the movement or rotation of the second beam return mirror group and the polarization unit; Adjusting the displacement of the first beam return mirror group so that the optical path lengths of the fundamental frequency light and the double frequency light are approximately equal; Adjusting the rotation angle of the polarization unit so that the polarization directions of the fundamental frequency light and the double frequency light are parallel; Adjusting the displacement of the second beam return mirror group to make the optical path lengths of the fundamental frequency light and the double frequency light equal; Determine the thickness and refractive index of the optical element through which the fundamental frequency light passes, calculate the dispersion that needs to be compensated by the dispersion compensation unit, and perform dispersion compensation; Adjust the modulation frequency of the appropriate pulse modulation unit and feed it back to the detection unit; The step of determining the thickness and refractive index of the optical element through which the fundamental frequency light passes, calculating the dispersion required to be compensated by the dispersion compensation unit, and performing dispersion compensation comprises: Calculate the thickness of the optical element through which the fundamental frequency light passes and the refractive index of the optical element, calculate the optical path of the fundamental frequency light, and then calculate the number of reflections of the optical element in the dispersion compensation unit by determining the group delay generated by the fused silica of the corresponding thickness for each reflection compensation, wherein the number of reflections of the optical element in the dispersion compensation unit is the number of reflections of the chirped reflector; Select a number of reflections greater than or equal to the calculated number for dispersion compensation; When a reflection number greater than the calculated number is selected for dispersion compensation, a dispersion compensation transition will occur, and the dispersion is increased by adjusting the thickness of the adjustable wedge pair to perform dispersion compensation regulation.

10. The method for constructing an optical system according to claim 9, characterized in that: Adjusting the displacement of the first beam return mirror group so that the optical path lengths of the fundamental frequency light and the doubled frequency light are approximately equal includes: The polarization unit, the pulse modulation unit and the detection unit are removed, and a second frequency doubling unit, a filter unit and a detection lens are sequentially arranged downstream of the optical path of the beam combining unit; The fundamental frequency light and the doubled frequency light are focused to the second doubled frequency unit after passing through the beam combining unit. After the fundamental frequency light is doubled again, it is incident to the filter unit with the same polarization direction as the original doubled frequency light to filter out the fundamental frequency light. Manually adjust the movement of the first beam return mirror group until interference fringes of the doubled frequency light are observed through the detection lens.

11. The method for constructing an optical system according to claim 10, characterized in that: The rotation angle of the polarization unit is adjusted so that the polarization directions of the fundamental frequency light and the double frequency light are parallel, including: The second frequency doubling unit, the filter unit and the detection lens are removed, a polarization unit is arranged between the dispersion compensation unit and the beam combining unit, and a pulse modulation unit and a detection unit are arranged in sequence downstream of the optical path of the beam combining unit; Polarizers and spectrometers are used to determine the polarization directions of the fundamental frequency light and the double frequency light. At the same time, the rotation angle of the polarization unit is adjusted so that the polarization directions of the fundamental frequency light and the double frequency light remain consistent before beam combination.

12. The method for constructing an optical system according to claim 11, characterized in that: The displacement of the second beam return mirror group is adjusted to make the optical path lengths of the fundamental frequency light and the double frequency light equal, including: The second beam return mirror group is controlled and adjusted to move in micrometer level according to the current signal detected by the detection unit until the detected current signal is maximum.

13. A photocurrent control method, characterized in that: include: Providing an optical system as claimed in any one of claims 1 to 8, wherein the optical system is used to generate an adjustable photocurrent on a semiconductor; In the initial state of the optical system, controlling the second beam return mirror group to move relative to the optical path to adjust the magnitude and direction of the generated photocurrent; In the initial state of the optical system, the polarization unit is controlled to rotate the polarization direction to adjust the magnitude and direction of the generated photocurrent.

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