Method and device for regulating and controlling coherence process of perovskite nanocrystal excitons through magnetic field

Abstract

The invention belongs to the technical field of perovskite nanocrystals, and particularly relates to a method and device for regulating and controlling the coherence process of perovskite nanocrystal excitons through a magnetic field. The method comprises the steps that incident light beams of pump light and detection white light pass through perovskite nanocrystals at low temperature, a magnetic field is applied in the light propagation direction, the linear polarization eigenstate in the perovskite nanocrystals is converted into the elliptic polarization eigenstate through the magnetic field, and the elliptic polarization eigenstate approaches the circular polarization eigenstate along with increase of the magnetic field; the beat frequency dynamic learning of the perovskite nanocrystal under circular polarized light pumping-detection is gradually weakened; under the pumping-detection of linearly polarized light, the beat frequency dynamic learning of the perovskite nanocrystal is gradually enhanced; the eigenstate and beat frequency dynamics of the perovskite nanocrystal are regulated and controlled by adjusting the magnetic field intensity. According to the method, the band edge exciton eigenstate of the perovskite nanocrystal is regulated and controlled by changing the magnetic field intensity, and the change of the band edge exciton eigenstate can cause the change of the coherent state dynamics process, so that the coherent state dynamics regulation and control of the perovskite nanocrystal can be realized by regulating the magnetic field intensity.

Description

Technical Field

[0001] The present invention belongs to the technical field of perovskite nanocrystals (NCs), and in particular relates to a method and a device for regulating the exciton coherence process of perovskite nanocrystals through a magnetic field. Background Art

[0002] Semiconductor quantum dots are important building blocks of modern quantum information technology. On the one hand, they can generate high-purity single photons for photon-based quantum information processing; on the other hand, they can also serve as carriers of qubits based on carrier or exciton spins. However, traditional epitaxially grown quantum dots often require expensive and complex high-temperature and vacuum environments to achieve preparation, and the morphology of the quantum dots themselves is difficult to precisely control. Colloidal semiconductor quantum dot materials in the field of chemistry can be prepared at low cost and on a large scale through a wealth of solution chemistry methods, and can achieve precise control of morphology and size through reaction kinetics and thermodynamics, so they may be used as a low-cost solution for quantum information technology. Although the technical route based on traditional II-VI group (such as CdSe, ZnSe, etc.) colloidal quantum dots has been initially explored, its smaller size brings a larger (specific) surface area and a small transition dipole moment, which brings great difficulties to the lifetime of quantum coherence and the control of quantum states.

[0003] Compared with the above-mentioned traditional colloidal quantum dots, the emerging perovskite colloidal quantum dots have the advantages of defect tolerance, transition dipole maximum, and long optical coherence time; and have made many advances in the field of quantum information technology, such as superfluorescence radiation, coherent single-photon emission, stable band-edge exciton coherence, and carrier spin manipulation at room temperature. These great advances are closely related to the fine structure of the band-edge excitons of perovskite quantum dots. The valence band edge is a relatively pure spin 1 / 2 state, while the conduction band edge is a state of 1 / 2 of the total angular momentum split by spin-orbit coupling. Considering the electron-hole exchange effect, the band-edge structure of perovskite colloidal quantum dots is composed of bright triplet states and dark singlet states; and the reduction of the symmetry of both the crystal contour and the lattice structure will lead to changes in the transition (polarization) selection law of the bright state, making it possible to measure the coherent behavior between these bright states using polarization spectroscopy. In the current study, single-particle fluorescence spectroscopy at liquid helium temperature clearly reveals that external fields (such as electric fields, magnetic fields, etc.) can effectively regulate the position of the fine structure of band-edge excitons, but the regulation of their coherent behavior by external fields remains unknown. Especially at high temperatures, due to the broadening of colloidal quantum dots, this frequency domain measurement method can hardly even provide information on the fine structure of band-edge excitons, further increasing the difficulty of revealing the regulation of these fine structures under external fields. Although previous magneto-optical spectroscopy measurements of II-VI and III-V epitaxially grown single quantum dots have demonstrated the possibility of external field regulation of the coherent behavior of band-edge excitons, the fine structure of band-edge excitons of this type of quantum dots has only two bright states, which is significantly different from perovskite quantum dots (three). These differences need to be reconsidered and established when establishing the method. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to provide a method and device for regulating the coherence process of perovskite nanocrystal excitons through a magnetic field, so as to solve the above-mentioned technical problem of regulating the coherence process between excitons of lead-based perovskite nanocrystals.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] On one hand, the present invention provides a method for regulating the exciton coherence process of perovskite nanocrystals by magnetic field, characterized in that an incident light beam including pump light and detection white light passes through the perovskite nanocrystals at low temperature, and a magnetic field of 0 to 7T is applied in the light propagation direction, the magnetic field converts the linearly polarized eigenstate in the perovskite nanocrystals at low temperature into an elliptically polarized eigenstate, and approaches the circularly polarized eigenstate as the magnetic field increases; under circularly polarized light pump-probe, the beat frequency dynamics of the perovskite nanocrystals will gradually weaken; and under linearly polarized light pump-probe, the beat frequency dynamics of the perovskite nanocrystals will gradually increase; by adjusting the magnetic field intensity, the eigenstate and beat frequency dynamics of the perovskite nanocrystals are regulated.

[0007] Femtosecond pulse laser is used as pump light and probe light to improve the dynamic resolution in the time domain; the pump light is a resonant monochromatic femtosecond pulse that only excites the band-edge bright-state excitons of the perovskite nanocrystals; the probe light is a supercontinuum white light femtosecond pulse, and the wavelength covers the band-edge energy level of the perovskite nanocrystals and 515 to 1000 nm; the angle between the pump light and the probe light is less than 10°, and both are polarized light, including circularly polarized light and linearly polarized light.

[0008] Another aspect of the present invention provides a device for implementing the method of regulating the perovskite nanocrystal exciton coherence process by magnetic field as described above, the device comprising:

[0009] A femtosecond pulse generation system, used for obtaining an incident light beam, wherein the incident light beam is a femtosecond pulse laser including pump light and detection white light;

[0010] The sample chamber is provided with an incident window and an exit window. The incident light beam generated by the femtosecond pulse generation system enters through the incident window and then passes through the perovskite nanocrystal and then exits through the exit window.

[0011] A sample stage is arranged in the sample chamber, and the sample stage is used to carry the perovskite nanocrystal;

[0012] A cooling system, used to provide a low temperature environment for the sample chamber;

[0013] A superconducting magnet is arranged around the sample stage, and applies a magnetic field to the perovskite nanocrystal along the light propagation direction;

[0014] The data acquisition system is used to receive the laser beam emitted from the exit window of the sample chamber and record the intensity according to the wavelength of the laser beam.

[0015] The femtosecond pulse generating system comprises a laser, a beam splitter, a pump light generating optical component and a detection light generating optical component, wherein the laser is used to generate a femtosecond pulse laser, and the femtosecond pulse laser is divided into two laser beams by the beam splitter, the first laser beam is incident on the pump light generating optical component to generate pump light, and the second laser beam is incident on the detection light generating optical component to generate detection light.

[0016] The pump light generating optical component comprises an optical parametric amplifier, a chopper, an attenuation plate I, a wave plate group I and a telephoto lens I which are sequentially arranged along the optical path, wherein the optical parametric amplifier is used to obtain femtosecond light of a resonant excitation wavelength; the femtosecond light obtains the required polarization after passing through the chopper, the attenuation plate I and the wave plate group I, and then is focused into the sample chamber through the telephoto lens I.

[0017] The pump light generating optical component also includes a telescope group, a reflector I and a reflector II, wherein the telescope group is arranged between the optical parametric amplifier and the chopper for expanding the beam of femtosecond light; the telescope group includes two plano-convex lenses arranged in sequence along the optical path; the reflector I and the reflector II are respectively arranged on both sides of the attenuation plate I along the optical path for steering the optical path.

[0018] The detection light generating optical component comprises a mechanical delay stage, an attenuation plate II, a short-focus lens, a YAG crystal, a wave plate group II and a long-focus lens II which are sequentially arranged along the optical path, wherein the mechanical delay stage is used to control the optical path difference between the pump light and the detection light; after the second laser beam is attenuated by the attenuation plate II, it is focused on the YAG crystal through the short-focus lens, the YAG crystal excites supercontinuum white light in the visible-near infrared band, then obtains the required polarization through the wave plate group II, and then is focused into the sample chamber through the long-focus lens II.

[0019] The detection light generating optical component also includes a reflector III, an off-axis parabolic mirror and a reflector IV, wherein the reflector III is arranged between the mechanical delay stage and the attenuation plate II, and the off-axis parabolic mirror and the reflector IV are arranged along the optical path between the YAG crystal and the wave plate group II for optical path steering.

[0020] A silver mirror is provided under the sample stage. The pump light and the detection light are incident on the silver mirror at an angle less than 10°. The silver mirror is used to reflect the pump light and the detection light entering from the incident window onto the sample stage. The pump light passes through the perovskite nanocrystal along a direction with an angle less than 10° with the magnetic field direction, and the detection light passes through the perovskite nanocrystal along a direction parallel to the magnetic field.

[0021] The cooling system includes a liquid storage tank, a liquid helium tank and a compressor, wherein the liquid storage tank and the liquid helium tank are arranged in the sample bin, the liquid storage tank is arranged below the sample stage, and the liquid storage tank is connected to the sample stage through red copper to cool the perovskite nanocrystals on the sample stage; the sample stage is made of non-magnetic metal; the liquid storage tank is connected to the sample stage and the superconducting magnet, the compressor is arranged outside the sample bin, and is connected to the liquid storage tank and the liquid helium tank through a circulation pipeline to form a circulation cooling pipeline.

[0022] The advantages and beneficial effects of the present invention are: the present invention provides a method for regulating the coherent process of perovskite nanocrystal excitons by means of a magnetic field, wherein the eigenstates of band-edge excitons of perovskite nanocrystals can be regulated by changing the intensity of the magnetic field, and since changes in the eigenstates of band-edge excitons will cause changes in the coherent state dynamics process, the coherent state dynamics of perovskite nanocrystals (NCs) can be regulated by adjusting the intensity of the magnetic field.

[0023] The present invention uses a steady magnetic field and ultrafast transient absorption spectroscopy (TA) to characterize the red-edge photoabsorption of CsPbI3NCs (cesium lead iodine, a type of perovskite nanocrystal) at 250K under different magnetic fields, proving that the energy gap of the in-plane excitons with edge polarization under the magnetic field expands and transforms to a circular polarization state due to the contribution of Zeeman energy, which significantly shortens the beat frequency oscillation period of NCs, while the apparent depolarization lifetime does not change significantly. This method of regulating the coherent dynamics of perovskite NCs excitons by magnetic field provides a new means for regulating excitons in spintronic devices in the future. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the working principle of the device for regulating the coherent process of perovskite nanocrystal excitons through a magnetic field according to the present invention;

[0025] Figure 2 It is a sample chamber structure diagram of a device for regulating the perovskite nanocrystal exciton coherence process by a magnetic field according to the present invention;

[0026] Figure 3 is a reflectivity curve diagram of the silver mirror in the sample chamber to incident light of different polarizations and wavelengths in an embodiment of the present invention;

[0027] Figure 4 is a morphology diagram of the CsPbI3NCs sample in an embodiment of the present invention;

[0028] Figure 5 is the steady-state absorption spectrum of the CSPbI3NCs sample in the embodiment of the present invention;

[0029] Figure 6 (a)-(b) are schematic diagrams of the dynamic measurement results of band edge excitons at a wavelength of 684 nm under different magnetic fields and polarizations in an embodiment of the present invention;

[0030] Figure 7 It is a two-dimensional grayscale image of the signal under different magnetic fields and polarizations in the embodiment of the present invention.

[0031] In the figure: 1-laser, 2-beam splitter, 3-optical parametric amplifier, 4-chopper, 5-reflector I, 6-attenuation plate I, 7-reflector II, 8-wave plate group I, 9-telephoto lens I, 10-mechanical delay stage, 11-reflector III, 12-attenuation plate II, 13-short-focus lens, 14-YAG crystal, 15-off-axis parabolic mirror, 16-reflector IV, 17-wave plate group II, 18-telephoto lens II, 19-silver mirror, 20-superconducting magnet, 21-magnetic flux lines, 22-sample, 23-sample chamber, 24-liquid storage tank, 25-sample stage, 26-incident beam, 27-incident window, 28-exit window, 29-liquid helium tank, 30-compressor. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] On one hand, the present invention provides a method for regulating the exciton coherence process of perovskite nanocrystals by magnetic field. A magnetic field of 0 to 7 T is applied in the direction of light propagation. The magnetic field converts the linearly polarized eigenstate (a certain energy value can be obtained under the action of an energy operator, and this state is the eigenstate; linear polarization or circular polarization is the polarization property of this state) in the perovskite nanocrystals at low temperature into an elliptically polarized eigenstate, and approaches the circularly polarized eigenstate as the magnetic field increases; under circularly polarized light pumping-probing, the beat frequency dynamics of the perovskite nanocrystals will gradually weaken; and under linearly polarized light pumping-probing, the beat frequency dynamics of the perovskite nanocrystals will gradually increase; by adjusting the magnetic field intensity, the eigenstate and beat frequency dynamics of the perovskite nanocrystals are regulated.

[0034] Furthermore, femtosecond pulse laser is used as pump light and probe light to improve the dynamic resolution in the time domain; the pump light is a resonant monochromatic femtosecond pulse, which only excites the bright-state excitons at the band edge of the perovskite nanocrystal; the pump light passes through the sample at an angle less than 10° to the direction of the magnetic field; the probe light is a supercontinuum white light femtosecond pulse, and the probe light covers the band edge energy level and higher energy levels (515-1000nm) of the perovskite nanocrystal; the angle between the pump light and the probe light is less than 10°, and both are polarized light, including circularly polarized light and linearly polarized light.

[0035] The present invention provides a method for regulating the coherent process of perovskite nanocrystal excitons by means of a magnetic field. The eigenstates of band-edge excitons of perovskite nanocrystals can be regulated by changing the intensity of the magnetic field. Since changes in the eigenstates of band-edge excitons will lead to changes in the coherent state dynamics process, the coherent state dynamics of perovskite nanocrystals (NCs) can be regulated by adjusting the intensity of the magnetic field.

[0036] See also Figure 1 , Figure 2As shown, the present invention provides a device for regulating the coherent process of perovskite nanocrystal excitons by magnetic field, the device comprising a femtosecond pulse generating system, a sample chamber 23, a sample stage 25, a cooling system, a superconducting magnet 20 and a data acquisition system, wherein the femtosecond pulse generating system is used to obtain an incident light beam 26, the incident light beam 26 is a femtosecond pulse laser including pump light and detection white light; the sample chamber 23 is provided with an incident window 27 and an exit window 28, the incident light beam 26 generated by the femtosecond pulse generating system enters the incident window 27 through the incident window 28 The light enters the sample chamber 23, passes through the perovskite nanocrystal and then is emitted from the exit window 28; the sample stage 25 is arranged in the sample chamber 23, and the sample stage 25 is used to carry the perovskite nanocrystal; the cooling system is used to provide a low temperature environment for the sample chamber 23; the superconducting magnet 20 is arranged around the sample stage 25, and the magnetic flux lines 21 generated by the superconducting magnet 20 pass through the perovskite nanocrystal, and the magnetic flux lines near the sample are parallel to the direction of light propagation; the data acquisition system is used to receive the laser beam emitted from the exit window 28 of the sample chamber 23, and record the intensity according to the wavelength of the laser beam.

[0037] See also Figure 1 As shown, in an embodiment of the present invention, a femtosecond pulse generating system includes a laser 1, a beam splitter 2, a pump light generating optical component and a detection light generating optical component, wherein the laser 1 is used to generate a femtosecond pulse laser, and the femtosecond pulse laser is divided into two laser beams by the beam splitter 2, the first laser beam is incident on the pump light generating optical component to generate pump light; the second laser beam is incident on the detection light generating optical component to generate detection light.

[0038] In an embodiment of the present invention, the pump light generating optical component includes an optical parametric amplifier 3, a chopper 4, an attenuator Ⅰ6, a wave plate group Ⅰ8 and a telephoto lens Ⅰ arranged in sequence along the optical path, wherein the optical parametric amplifier 3 is used to obtain femtosecond light of a resonant excitation wavelength; the pump light is generated by the optical parametric amplifier 3 (OPA) in a frequency combination manner, and only half of the pulses are allowed to pass through the chopper 4; the femtosecond light obtains the required polarization after passing through the attenuator Ⅰ6 and the wave plate group Ⅰ8, and then is focused into the sample chamber 23 through the telephoto lens Ⅰ. Specifically, the wave plate group Ⅰ8 includes a wire grid, a λ / 2 wave plate and a λ / 4 wave plate arranged in sequence along the optical path, and the polarization conversion is achieved by adjusting the angles of the λ / 2 wave plate and the λ / 4 wave plate.

[0039] Furthermore, the pump light generating optical component also includes a telescope group, a reflector I5 and a reflector II7, wherein the telescope group is arranged between the optical parametric amplifier 3 and the chopper 4 for expanding the beam of femtosecond light; the telescope group includes two plano-convex lenses arranged in sequence along the optical path; the reflector I5 and the reflector II7 are respectively arranged on both sides of the attenuation plate I6 along the optical path for steering the optical path.

[0040] In an embodiment of the present invention, the detection light generating optical component includes a mechanical delay stage 10, an attenuation plate II 12, a short-focus lens 13, a YAG crystal 14, a wave plate group II 17 and a long-focus lens II 18 arranged in sequence along the optical path, wherein the mechanical delay stage 10 is used to control the optical path difference between the pump light and the detection light; after the second laser beam is attenuated by the attenuation plate II 12, it is focused on the YAG crystal 14 through the short-focus lens 13, and the YAG crystal 14 (yttrium aluminum garnet) excites supercontinuum white light in the visible-near infrared band, and then obtains the required polarization through the wave plate group II 17, and then focuses into the sample chamber 23 through the long-focus lens II 18. The delay time between the two femtosecond pulses is controlled by controlling the optical path difference through the electric mechanical displacement stage 10. Specifically, the wave plate group II 17 includes a wire grid and a λ / 4 wave plate, and the polarization conversion is achieved by adjusting the angle of the λ / 4 wave plate.

[0041] Furthermore, the detection light generating optical component also includes a reflector III 11, an off-axis parabolic mirror 15 and a reflector IV 16, wherein the reflector III 11 is arranged between the mechanical delay stage 10 and the attenuation plate II 12, and the off-axis parabolic mirror 15 and the reflector IV 16 are arranged along the optical path between the YAG crystal 14 and the wave plate group II 17 for optical path steering.

[0042] See also Figure 2 As shown, in the embodiment of the present invention, a silver mirror 19 is embedded under the sample stage 25, and the pump light and the probe light are incident on the silver mirror 19 at an angle less than 10°. The silver mirror 19 is used to deflect the pump light and the probe light entering from the incident window 27 by 90° and then reflect them onto the sample stage 25. Specifically, the pump light passes through the perovskite nanocrystal along a direction with an angle less than 10° with the magnetic field direction, and the probe light passes through the perovskite nanocrystal along a direction parallel to the magnetic field.

[0043] Specifically, the sample chamber 23 is a hollow cylindrical structure, with the superconducting magnet 20 on the periphery. An incident window 27 is arranged on the side of the sample chamber 23, and an exit window 28 is arranged on the top.

[0044] See also Figure 2 As shown, in an embodiment of the present invention, the cooling system includes a liquid storage tank 24, a liquid helium tank 29 and a compressor 30, wherein the liquid storage tank 24 and the liquid helium tank 29 are arranged in the sample chamber 23, the liquid storage tank 24 is arranged below the sample stage 25, and the liquid storage tank 24 is connected to the sample stage 25 through copper to cool the perovskite nanocrystals on the sample stage 25; the sample stage 25 is made of non-magnetic metal, preferably copper. The liquid storage tank 24 is connected to the sample stage 25 and the superconducting magnet 20, and the compressor 30 is arranged outside the sample chamber 23, and is connected to the liquid storage tank 24 and the liquid helium tank 29 through a circulation pipeline to form a circulation cooling pipeline.

[0045] In an embodiment of the present invention, the data acquisition system includes a linear array CCD camera and a computer. After the detection light passes through the optical fiber, it is split by the grating. Light of different wavelengths enters different pixel points on the linear array CCD camera, and an electrical signal is obtained according to the intensity. After being converted into a digital signal, it is input into the computer for recording and storage. After the pump light is emitted from the sample chamber 23, it is focused by the lens and sent into the optical fiber, and then enters the linear array CCD camera.

[0046] Another aspect of the present invention provides a device for regulating the coherence process of perovskite nanocrystal excitons by magnetic field, and its working principle is:

[0047] The band edge structure of perovskite crystals is relatively simple. The energy band at the bottom of the conduction band is mainly an empty band obtained by the spin-orbit coupling splitting of the lead atom 6p orbital and the halogen atom p orbital. The band edge only has this empty band, and the energy difference with other conduction bands can reach hundreds of millielectron volts; the top of the valence band is mainly a full band contributed by the lead atom 6s orbital, and the energy difference between the band edge and other valence bands is also large. Perovskite nanocrystals consider quantum confinement on the basis of crystals, resulting in discontinuity of band edge energy levels. Therefore, the band edge exciton structure of perovskite nanocrystals is very simple, and only four exciton states obtained by the linear combination of two spin states of electrons and holes need to be considered. Among these four exciton states, only three are bright states allowed by the optical transition selection law, and the remaining dark states can have an energy difference of several millielectron volts with the bright states. Due to the decrease in lattice symmetry and morphology symmetry, the three bright states are split linear states in energy level. In the Faraday configuration, the two linear excitons in the normal plane of the light propagation direction are coupled, and the excited exciton state is the linear superposition of the two. The resulting quantum state is not an eigenstate, but will evolve over time. The evolution path is related to the Hamiltonian, that is, to the eigenstate of the nanocrystal. The applied magnetic field can couple the two linear states in the plane into two circular states. The more the state properties are biased toward circular polarization, the closer the dynamic behavior is to population relaxation.

[0048] Before starting the measurement, the perovskite nanocrystal sample solution is concentrated and then spin-coated on a cut glass slide. The glass slide used is pre-cleaned in an ultrasonic cleaner with isopropanol for 20 minutes, then irradiated with a UV irradiator for 20 minutes, and then naturally cooled. The nanocrystalline film sample obtained by spin coating is transferred to a sample chamber 23 with a superconducting magnet 20. The sample chamber 23 is closed, and nitrogen is sent in after evacuation, repeatedly to clean the chamber, and then the chamber is evacuated. Subsequently, helium is compressed to obtain superfluid helium, and then the superfluid helium is circulated in the pipeline, a part of which is used to maintain the low temperature of the superconducting coil of the superconducting magnet 20, and a part of which is filled into a liquid storage tank 24 in the sample chamber 23, and the liquid storage tank 24 is directly connected to the sample stage 25 through copper to cool the sample 22 on the sample stage 25. Specifically, the helium is first compressed into liquid helium by an external compressor 30 and sent into a liquid helium tank 29. The liquid helium tank 29 is connected to the liquid storage tank 24 in the sample chamber 23 and is cooled to a set temperature by heat transfer through a copper part. The set temperature in this embodiment is 250K. A portion of the heated and vaporized helium is condensed through the internal gas path, and the remaining portion is compressed into liquid helium again by the compressor 30. The sample 22 (perovskite nanocrystal) is in direct contact with the sample stage 25 for heat transfer. After the sample stage 25 reaches the set temperature, thermal equilibrium is continued for two hours to ensure that the sample 22 reaches the set temperature.

[0049] A femtosecond laser with a wavelength of 1030 nanometers is drawn out from the laser 1, and 75% of the laser is sent to the optical parametric amplifier 3 through the beam splitter 2. The optical parametric amplifier 3 is adjusted to obtain femtosecond light of the resonance excitation wavelength, and then the beam is expanded by a telescope system composed of two plano-convex lenses, and then the required polarization is obtained by the chopper 4, the attenuation plate Ⅰ6 and the wave plate group Ⅰ8. The other 25% of the femtosecond light passes through the mechanical delay stage 10 (time range 0-8ns) and is attenuated by the attenuation plate Ⅱ 12, and is focused on the YAG crystal 14 through the short-focus lens 13 to excite supercontinuum white light in the visible-near infrared band, and then is pulled into parallel light by the off-axis parabolic mirror 15, and the required polarization is obtained by the wave plate group Ⅱ 17. The two beams of light each pass through a telephoto lens in a nearly collinear form (the angle between the two beams of light is less than 10°) and intersect on the sample stage 25 through the window plate of the incident window 27 of the sample chamber 23. The optical waists of the two beams of light (the position with the smallest optical radius in the direction of beam propagation) are also in the plane of the sample stage 25, and the pump light spot is larger than the detection light spot.

[0050] After cooling the sample, a circularly polarized pump light passes through the sample 22 at a small angle, preferably at an angle less than 10° to the magnetic field direction M, to excite the sample 22 within the beam waist range; after a specified time, the detection light passing through the mechanical delay stage 10 detects the sample near the center of the pump beam waist with a smaller optical waist to avoid detecting samples at the edge of the beam waist that are not excited or have weak excitation intensity. The two beams of light will lose polarization after passing through the silver mirror 19. The reflectivity of different polarizations at each wavelength can be seen in Figure 3 shown.

[0051] Subsequently, the pump light is emitted from the rear window of the exit window 28, converged at the optical fiber port through a short-focus lens, and enters the linear array CCD camera to be collected.

[0052] Specifically, the mechanical delay table 10, the laser 1, the chopper 4 and the linear array CCD camera are connected through a timing card. The laser 1 sends an electrical pulse to the timing card, and then the timing card sends the pulse signal to the chopper 4 and the linear array CCD camera. The computer controls the above components through the timing card.

[0053] The size distribution and room temperature absorption spectra of the CsPbI3NCs samples used are shown in Figure 4 , Figure 5 Different magnetic fields are applied by the superconducting magnet 20, and then different polarization signals are subtracted (σ + σ - -σ + σ + , VV-VH) obtained by dynamic detection results and two-dimensional grayscale images of signals, see Figure 6 and Figure 7 As shown. It can be seen that at 0T, the result of circularly polarized excitation-probe shows a weak beat signal, which comes from the circularly polarized pump light exciting the superposition state of linearly polarized eigenstates. The evolution of the state between eigenstates leads to projection in the initialization direction; the result of linearly polarized excitation-probe also has a beat signal, which comes from the uncertainty of the dipole orientation of the linearly polarized eigenstate, so that the linearly polarized pump light can also excite the superposition state of eigenstates. At 7T, since the Zeeman energy dominates the energy gap between bright states, the angular momentum direction of the eigenstate is very close to the direction of light propagation. Therefore, one of the states excited by circularly polarized excitation-probe is dominant, and the dynamic behavior can be fitted with a single exponential decay; and since the linearly polarized pump excites the superposition state of circularly polarized eigenstates, a beat signal can be observed.

[0054] In the embodiment of the present invention, the lead-based perovskite quantum dot ensemble is used as a model, and the magnetic field is used to successfully achieve on-demand regulation of the coherence between these band edge fine structures; using polarization-resolved transient absorption spectroscopy, it is clearly seen that the coherent signal of the band edge fine structure changes from circular polarization to linear polarization under the modulation of the longitudinal magnetic field. This regulation method can not only regulate the band edge exciton state, but also provide a method for constructing a variety of novel quantum bits.

[0055] The present invention can regulate the band edge exciton eigenstate of perovskite nanocrystals by changing the magnetic field intensity, and the change of the band edge exciton eigenstate will cause the coherent state dynamic process to change. The red edge photoabsorption of CsPbI3NCs under different magnetic fields at 250K is characterized by using a steady magnetic field and ultrafast transient absorption spectroscopy (TA), proving that the energy gap of the in-plane excitons of the band edge linear polarization under the magnetic field expands and transforms to a circular polarization state due to the contribution of Zeeman energy, so that the beat frequency oscillation period of NCs is significantly shortened, while the apparent depolarization lifetime does not change significantly. The method and device for regulating the perovskite nanocrystal exciton coherence process by magnetic field, using a steady magnetic field to achieve perovskite nanocrystal carrier dynamics regulation, can regulate the exciton coherence dynamics of perovskite nanocrystals in a non-destructive manner, and the method has great guiding value and significance for the dynamics regulation in future spin electronics devices.

[0056] The above description is only an embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, expansion, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A method for regulating the exciton coherence process of perovskite nanocrystals by magnetic field, characterized in that: An incident light beam containing pump light and detection white light passes through the low-temperature perovskite nanocrystal, and a magnetic field of 0 to 7T is applied in the direction of light propagation. The magnetic field converts the linearly polarized eigenstate in the low-temperature perovskite nanocrystal into an elliptically polarized eigenstate, and approaches the circularly polarized eigenstate as the magnetic field increases. Under circularly polarized light pump-probe, the beat frequency dynamics of the perovskite nanocrystal will gradually weaken; while under linearly polarized light pump-probe, the beat frequency dynamics of the perovskite nanocrystal will gradually increase. By adjusting the magnetic field intensity, the eigenstate and beat frequency dynamics of the perovskite nanocrystal can be regulated.

2. The method for regulating the exciton coherence process of perovskite nanocrystals by magnetic field according to claim 1, characterized in that: Femtosecond pulse laser is used as pump light and probe light to improve the dynamic resolution in the time domain; the pump light is a resonant monochromatic femtosecond pulse that only excites the band-edge bright-state excitons of the perovskite nanocrystals; the probe light is a supercontinuum white light femtosecond pulse, and the wavelength covers the band-edge energy level of the perovskite nanocrystals and 515 to 1000 nm; the angle between the pump light and the probe light is less than 10°, and both are polarized light, including circularly polarized light and linearly polarized light.

3. A device for implementing the method of controlling the perovskite nanocrystal exciton coherence process by magnetic field according to claim 1 or 2, characterized in that: The device comprises: A femtosecond pulse generation system, used for obtaining an incident light beam, wherein the incident light beam is a femtosecond pulse laser including pump light and detection white light; The sample chamber is provided with an incident window and an exit window. The incident light beam generated by the femtosecond pulse generation system enters through the incident window and then passes through the perovskite nanocrystal and exits through the exit window. A sample stage is arranged in the sample chamber, and the sample stage is used to carry the perovskite nanocrystal; A cooling system, used to provide a low temperature environment for the sample chamber; A superconducting magnet is arranged around the sample stage, and applies a magnetic field to the perovskite nanocrystal along the light propagation direction; The data acquisition system is used to receive the laser beam emitted from the exit window of the sample chamber and record the intensity according to the wavelength of the laser beam.

4. The device according to claim 3, characterized in that The femtosecond pulse generating system comprises a laser, a beam splitter, a pump light generating optical component and a detection light generating optical component, wherein the laser is used to generate a femtosecond pulse laser, and the femtosecond pulse laser is divided into two laser beams by the beam splitter, the first laser beam is incident on the pump light generating optical component to generate pump light, and the second laser beam is incident on the detection light generating optical component to generate detection light.

5. The device according to claim 4, characterized in that The pump light generating optical component comprises an optical parametric amplifier, a chopper, an attenuation plate I, a wave plate group I and a telephoto lens I which are sequentially arranged along the optical path, wherein the optical parametric amplifier is used to obtain femtosecond light of a resonant excitation wavelength; the femtosecond light obtains the required polarization after passing through the chopper, the attenuation plate I and the wave plate group I, and then is focused into the sample chamber through the telephoto lens I.

6. The device according to claim 5, characterized in that The pump light generating optical component also includes a telescope group, a reflector I and a reflector II, wherein the telescope group is arranged between the optical parametric amplifier and the chopper for expanding the beam of femtosecond light; the telescope group includes two plano-convex lenses arranged in sequence along the optical path; the reflector I and the reflector II are respectively arranged on both sides of the attenuation plate I along the optical path for steering the optical path.

7. The device according to claim 4, characterized in that The detection light generating optical component comprises a mechanical delay stage, an attenuation plate II, a short-focus lens, a YAG crystal, a wave plate group II and a long-focus lens II which are sequentially arranged along the optical path, wherein the mechanical delay stage is used to control the optical path difference between the pump light and the detection light; after the second laser beam is attenuated by the attenuation plate II, it is focused on the YAG crystal through the short-focus lens, the YAG crystal excites supercontinuum white light in the visible-near infrared band, then obtains the required polarization through the wave plate group II, and then is focused into the sample chamber through the long-focus lens II.

8. The device according to claim 7, characterized in that The detection light generating optical component also includes a reflector III, an off-axis parabolic mirror and a reflector IV, wherein the reflector III is arranged between the mechanical delay stage and the attenuation plate II, and the off-axis parabolic mirror and the reflector IV are arranged along the optical path between the YAG crystal and the wave plate group II for optical path steering.

9. The device according to claim 3, characterized in that A silver mirror is provided under the sample stage. The pump light and the detection light are incident on the silver mirror at an angle less than 10°. The silver mirror is used to reflect the pump light and the detection light entering from the incident window onto the sample stage. The pump light passes through the perovskite nanocrystal along a direction with an angle less than 10° with the magnetic field direction, and the detection light passes through the perovskite nanocrystal along a direction parallel to the magnetic field.

10. The device according to claim 3, characterized in that The cooling system includes a liquid storage tank, a liquid helium tank and a compressor, wherein the liquid storage tank and the liquid helium tank are arranged in the sample bin, the liquid storage tank is arranged below the sample stage, and the liquid storage tank is connected to the sample stage through red copper to cool the perovskite nanocrystals on the sample stage; the sample stage is made of non-magnetic metal; the liquid storage tank is connected to the sample stage and the superconducting magnet, the compressor is arranged outside the sample bin, and is connected to the liquid storage tank and the liquid helium tank through a circulation pipeline to form a circulation cooling pipeline.

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