CN111650404B - Light-induced STM dynamic response detection system and method - Google Patents
Light-induced STM dynamic response detection system and method Download PDFInfo
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- CN111650404B CN111650404B CN202010541346.8A CN202010541346A CN111650404B CN 111650404 B CN111650404 B CN 111650404B CN 202010541346 A CN202010541346 A CN 202010541346A CN 111650404 B CN111650404 B CN 111650404B
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Abstract
The application discloses a light-induced STM dynamic response detection system and a method, wherein a pulse selector triggers a frequency divider to generate a step signal; the first light beam reflection device is provided with a micro-motion piezoelectric linear displacement platform which is driven by a step signal to carry out step position change; the light beam converging device converges the probe light reflected by the first light beam reflecting device and the pump light reflected by the second light beam reflecting device to an experimental material positioned below the STM probe; the second light beam reflection device is provided with a long-range linear displacement platform for adjusting the time difference between the detection light and the time when the pump light reaches the material; the phase-locked amplifier takes the step signal as a reference signal to perform phase-locked amplification processing on the needle point tunnel current signal so as to determine an STM dynamic response result; the modulation frequency of the micro-motion piezoelectric linear displacement platform is one half of the frequency of the pulse selector, and the phase between the micro-motion piezoelectric linear displacement platform and the pulse selector is locked and adjustable. The technical scheme can realize reliable detection of STM dynamic response.
Description
Technical Field
The invention relates to the technical field of spectra, in particular to a light-induced STM dynamic response detection system and method.
Background
Since the appearance of a Scanning Tunneling Microscope (STM) in 1981, the fields of nano material science, micro-nano structure devices, novel nano design and the like which take the research of the electronic state structure of the material with the atomic scale as the leading factor are greatly improved, and a powerful means is provided for the human understanding of the electronic state and the quantization effect of the nano scale. The technique utilizes quantum tunneling effect, by applying a variable bias to a probe at a position several angstroms in height on the surface of a semiconductor or metal material, when the potential of the probe matches the energy level or band of the material atoms, electrons will cross the vacuum barrier and tunnel to the energy level of the material atoms to form a tunnel current. The tunnel current and the height of the needle tip from the surface of the material form an exponential decay relationship, so that super atomic scale spatial resolution capability is obtained. This resolving power provides useful information for humans to understand and master single molecules, nanostructures, and low dimensional materials. However, it is not possible to obtain information on the steady state or the sub-dynamic state, and it is not possible to fully understand the microscopic behavior of the material. The constitutional units (atoms, molecules, ions and groups) of all material systems are in dynamic processes when being disturbed or functionalized, and the dynamics of electronic states of the constitutional units are the real responses of devices and materials, and also determine important factors of material characteristics, so that the time-resolved test obtained on the STM becomes an important technical exploration.
Limited to the electronic device response time, time resolution of only microsecond order can be achieved with electrically modulated time resolution STM. In order to obtain the micro-structural response of nanosecond, picosecond and femtosecond time domain, the laser pumping detection technology is combined with the STM, the initial technology is to generate an ultra-fast electric signal pulse by utilizing a laser induction electrode, and is an improvement on electric modulation essentially, and the micro dynamic change of a material is not actively induced by light. In 2010, Japanese researchers proposed an optical pumping-detection STM design scheme and published a corresponding series of semiconductor material dynamic STM test results, FIG. 1 shows a technical design path adopted by the researchers, and other later researchers basically follow the experimental design path. This is the current popular test technique for optical pump-probe STMs. The basic design idea of the technology is to adopt two femtosecond laser pulse beams which are synchronously output, the output frequencies of two paths of pulse sequences are the same, one path of laser pulse sequence has mode hopping, and j pulse intervals can be selected for mode hopping. The two laser beams pass through a displacement delayer to adjust the time interval Td of reaching the probe tip sample, and the time interval can be continuously adjusted, so that the influence of the two pulse time intervals Td on the tunneling current of the STM probe can be used for understanding the dynamic process of material atoms on the change of the light response time. However, the extraction of the STM tunneling current is to select the value difference between a certain time and a time after a delay (j/f) time on a response curve of a real dynamic process as an STM dynamic response signal at the certain time, generally, the delay time is at least ten and several ns, so that if the electronic state dynamic response time of a material is in a nanometer range, the obtained time-resolved STM signal curve is a dynamic change of the time difference, and a real dynamic process response curve cannot be obtained, as shown in fig. 2, the material response curve is actually the curve a in fig. 2, and the change curve of Δ I moving along with Td is provided by the technology. Furthermore, the time constant given by such a curve, such as the lifetime of the response, is not of physical significance. It can only be used as an auxiliary verification for other testing technologies, and cannot be quantified.
Before the above-described design of the detection structure of fig. 1, there is also a class of designs: firstly, the time-resolved STM is integrally operated in an air environment, so that the STM probe current is influenced by multiple factors, such as ionization of air ions and gas molecules, scattering of electrons and gas molecules and the like, and background signals and noise are induced on the probe current. Even if the design device is transplanted into a vacuum cavity, the design inevitably causes uncertainty of system phase caused by pulse sine modulation mode locking. The sine modulation mode locking is an excellent design under the condition of steady-state continuous laser output, and can effectively improve the signal-to-noise ratio. Unfortunately, in the optical pumping-detection time-resolved STM test, a femtosecond pulse laser is required, and the duration of a single pulse is only dozens of femtoseconds (10)-15s). In the case of a specific repetition frequency of the laser pulses, the actual signal does not change continuously in sinusoidal time but only at several points on a sinusoidal periodic modulation time curve, most of the time points being blank. The phase of these points in time on the sinusoid is not necessarily effectively locked, that is, a point on one cycle and a point on the next cycle are not on one phase, as shown in figure 3, which leads to signal instability and noise enhancement due to phase drift, particularly with respect to the tunneling current of the STM tip, which directly determines that reliable information cannot be obtained.
Disclosure of Invention
In view of this, the present invention provides a system and a method for detecting an STM dynamic response by light induction, which can reliably detect the STM dynamic response. The specific scheme is as follows:
in a first aspect, the application discloses a light-induced STM dynamic response detection system, which comprises a femtosecond laser, a pulse selector, a beam splitter, a first light beam reflection device, a second light beam reflection device, a light beam convergence device, an STM probe, a needle point current detection device, a phase-locked amplifier, a frequency divider and a long-range linear displacement platform; wherein the content of the first and second substances,
the pulse selector is used for performing pulse selection on the femtosecond pulse laser emitted by the femtosecond laser to emit corresponding femtosecond pulse laser, and sending a trigger signal to the frequency divider to trigger the frequency divider to generate a step signal;
the beam splitter is used for generating detection light and pumping light based on the femtosecond pulse laser emitted by the pulse selector;
the first light beam reflection device comprises a micro-motion piezoelectric linear displacement table which is connected with the frequency divider and performs step-type position change under the drive of the step-type signal, and the micro-motion piezoelectric linear displacement table is used for reflecting the detection light emitted by the beam splitter through a first retro-reflector arranged on the micro-motion piezoelectric linear displacement table;
the second light beam reflection device is used for reflecting the pump light emitted by the beam splitter;
the light beam converging device is used for converging the detection light reflected by the first light beam reflecting device and the pump light reflected by the second light beam reflecting device to an experimental material positioned below the STM probe;
the long-range linear displacement platform is arranged on the second light beam reflection device and is used for adjusting and controlling the time difference between the detection light and the time difference between the pump light and the experimental material;
the needle point current detection device is used for detecting a needle point tunnel current signal of the STM probe;
the phase-locked amplifier is respectively connected with the frequency divider and the needle point current detection device and is used for performing phase-locked amplification processing on the needle point tunnel current signal by taking the step signal as a reference signal and transmitting the obtained phase-locked amplified signal to a computer system so that the computer system can determine an STM dynamic response result according to the phase-locked amplified signal;
and the modulation frequency of the micro-motion piezoelectric linear displacement platform is half of the frequency of the pulse selector, and the phase between the micro-motion piezoelectric linear displacement platform and the pulse selector is locked and adjustable.
Optionally, the wavelength of the femtosecond laser is tunable between 690 and 1020 nanometers, the output phase of the frequency divider is tunable between 0 and 180 degrees, and the displacement amplitude of the micro-motion piezoelectric linear displacement stage is tunable between 1 and 100 micrometers.
Optionally, the step-type signal generated by the frequency divider is a signal for driving the micro-motion piezoelectric linear displacement stage to stay at each displacement endpoint for a preset time length in the step-type position variation process.
Optionally, the step-up signal generated by the frequency divider is a signal for controlling the position of the first retro-reflector at the displacement end point when the first retro-reflector acts on the probe light emitted from the beam splitter.
Optionally, the second light beam reflecting device includes:
the frequency doubling crystal is used for doubling the frequency of the pump light emitted by the beam splitter to obtain frequency doubled light;
the filtering module is used for filtering long-wave band components in the frequency doubling light emitted by the frequency doubling crystal to obtain filtered laser;
the second retro-reflector is used for reflecting the filtered laser light to obtain a corresponding reflected light beam, and the reflected light beam is transmitted to the light beam converging device through the broadband half-wave plate.
Optionally, the frequency doubling crystal is a barium metaborate crystal, and the filtering module includes a band-pass filter or a glan prism.
Optionally, the light beam converging device includes:
the dichroscope is used for transmitting the detection light and reflecting the pumping light so as to enable the detection light and the pumping light to be coaxially superposed;
plane mirror and concave surface speculum, wherein, the plane mirror is used for reflecting the probe light and the pump light behind the coaxial coincidence extremely the concave surface speculum, then passes through the concave surface speculum assembles probe light and pump light to lieing in experimental material under the STM probe.
Optionally, one or more groups of reflectors are further disposed in a position area opposite to the long-range linear displacement stage, so as to control light to go back and forth between the long-range linear displacement stage and the one or more groups of reflectors.
Optionally, the beam splitter is made of fused quartz material.
In a second aspect, the present application discloses a method for detecting a light-induced STM dynamic response, which is applied to the light-induced STM dynamic response detection system disclosed above, and includes:
pulse selection is carried out on the femtosecond pulse laser emitted by the femtosecond laser through a pulse selector so as to emit corresponding femtosecond pulse laser, and a trigger signal is sent to a frequency divider so as to trigger the frequency divider to generate a step signal;
generating probe light and pump light by a beam splitter based on the femtosecond pulse laser emitted by the pulse selector;
reflecting the detection light emitted by the beam splitter by a first retro-reflector of a micro-motion piezoelectric linear displacement platform arranged on a first light beam reflection device, wherein the micro-motion piezoelectric linear displacement platform is driven by the step signal to perform step position change;
reflecting the pump light emitted by the beam splitter by a second light beam reflecting device;
converging the detection light reflected by the first light beam reflecting device and the pump light reflected by the second light beam reflecting device to an experimental material positioned below the STM probe through a light beam converging device;
detecting a needle point tunnel current signal of the STM probe through a needle point current detection device;
the step type signal is used as a reference signal through a phase-locked amplifier which is respectively connected with the frequency divider and the needle point current detection device, the needle point tunnel current signal is subjected to phase-locked amplification processing, and the obtained phase-locked amplified signal is transmitted to a computer system, so that the computer system can determine an STM dynamic response result according to the phase-locked amplified signal;
and the modulation frequency of the micro-motion piezoelectric linear displacement platform is one half of the frequency of the pulse selector, the phase between the micro-motion piezoelectric linear displacement platform and the pulse selector is locked and adjustable, and the second light beam reflection device is also provided with a long-range linear displacement platform for adjusting and controlling the time difference between the detection light and the pump light reaching the experimental material.
In the application, the light-induced STM dynamic response detection system comprises a femtosecond laser, a pulse selector, a beam splitter, a first light beam reflection device, a second light beam reflection device, a light beam convergence device, an STM probe, a needle point current detection device, a phase-locked amplifier, a frequency divider and a long-range linear displacement platform; the pulse selector is used for performing pulse selection on the femtosecond pulse laser emitted by the femtosecond laser to emit corresponding femtosecond pulse laser, and sending a trigger signal to the frequency divider to trigger the frequency divider to generate a step signal; the beam splitter is used for generating detection light and pumping light based on the femtosecond pulse laser emitted by the pulse selector; the first light beam reflection device comprises a micro-motion piezoelectric linear displacement table which is connected with the frequency divider and performs step-type position change under the drive of the step-type signal, and the micro-motion piezoelectric linear displacement table is used for reflecting the detection light emitted by the beam splitter through a first retro-reflector arranged on the micro-motion piezoelectric linear displacement table; the second light beam reflection device is used for reflecting the pump light emitted by the beam splitter; the light beam converging device is used for converging the detection light reflected by the first light beam reflecting device and the pump light reflected by the second light beam reflecting device to an experimental material positioned below the STM probe; the long-range linear displacement platform is arranged on the second light beam reflection device and is used for adjusting and controlling the time difference between the detection light and the time difference between the pump light and the experimental material; the needle point current detection device is used for detecting a needle point tunnel current signal of the STM probe; the phase-locked amplifier is respectively connected with the frequency divider and the needle point current detection device and is used for performing phase-locked amplification processing on the needle point tunnel current signal by taking the step signal as a reference signal and transmitting the obtained phase-locked amplified signal to a computer system so that the computer system can determine an STM dynamic response result according to the phase-locked amplified signal; and the modulation frequency of the micro-motion piezoelectric linear displacement platform is half of the frequency of the pulse selector, and the phase between the micro-motion piezoelectric linear displacement platform and the pulse selector is locked and adjustable.
Therefore, the technical scheme of the application is that a pulse space optical path modulation phase locking technology based on single laser is adopted, the modulation frequency of the micro-motion piezoelectric linear displacement table is fixed to be half of the frequency of a laser pulse selector, and discrete point phase floating caused by random modulation frequency is eliminated. The phase of the micro-motion piezoelectric linear displacement platform is locked with the phase of the pulse selector, and simultaneously, the optimal adjustment is ensured, the response signal is enhanced, meanwhile, the noise background intensity is inhibited, the technical scheme can effectively solve the problems of detection and amplification of extremely weak dynamic signals generated in the process of electron state transient response of STM needle point tunnel current on the atoms of the optical pumping-detection time resolution inducing material, meanwhile, the modulation technology based on the micro-motion piezoelectric linear displacement platform in the application is utilized to realize the effect of controlling the differential micro-variable of a time axis through displacement adjustment, so that the first-order differential operation of tunnel current can be obtained, the responsivity of slowly-varying and steady-state signals is inhibited, and the long-range linear displacement table arranged on the second light beam reflection device realizes the adjustment and control of the time difference of the detection light and the pump light reaching the experimental material. In conclusion, the STM dynamic response detection method and device can achieve reliable detection of STM dynamic response.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
FIG. 1 is a schematic diagram of a prior art optical pumping-detection STM;
FIG. 2 is a diagram illustrating the difference between a response curve based on the prior art and an actual response curve;
FIG. 3 is a schematic diagram illustrating the effect of phase drift;
FIG. 4 is a schematic structural diagram of a light-induced STM dynamic response detection system disclosed in the present application;
FIG. 5 is a schematic structural diagram of a particular light-induced STM dynamic response detection system disclosed herein;
FIG. 6 is a timing diagram of signals;
fig. 7 and 8 are schematic views of voltage driving modes of the piezoelectric ceramics disclosed in the present application;
fig. 9 is a flowchart of a method for detecting dynamic response of a light-induced STM disclosed in the present application.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 4, the embodiment of the present application discloses a light-induced STM dynamic response detection system, which includes a femtosecond laser 10, a pulse selector 11, a beam splitter 12, a first beam reflection apparatus 13, a second beam reflection apparatus 14, a beam converging apparatus 15, an STM probe 16, a tip current detection apparatus 17, a lock-in amplifier 18, a frequency divider 19, and a long-range linear displacement stage 20; wherein the content of the first and second substances,
the pulse selector 11 is configured to perform pulse selection on the femtosecond pulse laser emitted by the femtosecond laser 10 to emit corresponding femtosecond pulse laser, and send a trigger signal to the frequency divider 19 to trigger the frequency divider 19 to generate a step signal;
the beam splitter 12 is configured to generate probe light and pump light based on the femtosecond pulse laser emitted by the pulse selector 11;
the first light beam reflection device 13 comprises a micro-motion piezoelectric linear displacement stage which is connected with the frequency divider 19 and performs step-type position change under the drive of the step-type signal, and is used for reflecting the detection light emitted by the beam splitter 12 through a first retro-reflector arranged on the micro-motion piezoelectric linear displacement stage;
the second light beam reflecting device 14 is configured to reflect the pump light emitted by the beam splitter 12;
the light beam converging device 15 is used for converging the detection light reflected by the first light beam reflecting device 13 and the pump light reflected by the second light beam reflecting device 14 to an experimental material positioned below the STM probe 16;
the long-range linear displacement table 20 is arranged on the second light beam reflection device 14 and is used for adjusting and controlling the time difference between the probe light and the pump light reaching the experimental material;
the needle tip current detection device 17 is used for detecting a needle tip tunnel current signal of the STM probe 16;
the phase-locked amplifier 18 is respectively connected with the frequency divider 19 and the needle point current detection device 17, and is configured to perform phase-locked amplification processing on the needle point tunnel current signal with the step signal as a reference signal, and transmit an obtained phase-locked amplified signal to a computer system, so that the computer system determines an STM dynamic response result according to the phase-locked amplified signal;
and the modulation frequency of the micro-motion piezoelectric linear displacement platform is half of the frequency of the pulse selector 11, and the phase between the micro-motion piezoelectric linear displacement platform and the pulse selector 11 is locked and adjustable.
In this embodiment, after the femtosecond pulse laser of the femtosecond laser 10 is selected by the pulse selector 11, the formed frequency-reduced femtosecond pulse laser generates a long-wavelength-band laser with weak intensity and a short-wavelength-band laser with strong intensity through the beam splitter 12, so as to be respectively used as probe light and pump light; wherein, the probing light reachs the experimental material that is located STM probe 16 through beam converging device 15 after first beam reflection device 13 reflection, and pump light reachs the experimental material that is located STM probe 16 through beam converging device 15 after 14 reflections of second beam reflection device. The tip tunnel current signal of the STM probe 16 is then detected by the tip current detection means 17 and transmitted to the lock-in amplifier 18. In this embodiment, the pulse selector 11 sends a trigger signal to the frequency divider 19 to trigger the frequency divider 19 to generate a step signal, the micro-motion piezoelectric linear displacement stage on the first beam reflecting device 13 performs step position variation under the driving of the step signal, the micro-motion piezoelectric linear displacement stage is provided with a first retro-reflector for reflecting the probe light emitted from the beam splitter 12, and the lock-in amplifier 18 performs lock-in amplification processing by using the step signal as a reference signal. Moreover, in the embodiment, the modulation frequency of the micro-motion piezoelectric linear displacement platform is half of the frequency of the pulse selector 11, and the phase between the micro-motion piezoelectric linear displacement platform and the pulse selector 11 is locked and adjustable. In addition, in this embodiment, the long-range linear displacement stage 20 is disposed on the second light beam reflection device 14, and the long-range linear displacement stage 20 can adjust and control the time difference between the probe light and the pump light reaching the experimental material.
It can be seen from the above that, a single femtosecond laser is adopted in the light-induced STM dynamic response detection system of this embodiment, and the modulation frequency of the micro-motion piezoelectric linear displacement stage is set to be one half of the frequency of the pulse selector 11, which is favorable for eliminating the phase floating of discrete points caused by random modulation frequency, and in this embodiment, the phase between the micro-motion piezoelectric linear displacement stage and the pulse selector 11 is locked and adjustable, which is favorable for enhancing the response signal and suppressing the background intensity of noise. This embodiment can effectively solve STM needle point tunnel current through above-mentioned technical scheme and survey the detection and the amplification problem of the very weak little dynamic signal that takes place among the time domain transient response process on the time resolution inducing material atom at the optical pumping-detection, utilize above-mentioned modulation technique based on fine motion piezoelectricity linear displacement platform in this application simultaneously, realize the effect through displacement adjustment control time axis differential micro variable, can obtain tunnel current's first order differential operation from this, restrain slowly change and steady state signal's responsiveness, and set up the long-range linear displacement platform on second light beam reflection device through this application embodiment and realized the adjustment control to the time difference that probing light and pumping light arrived on the experimental materials. In summary, the embodiment of the application can realize reliable detection of STM dynamic response.
Referring to fig. 5, the embodiment of the present application discloses a specific light-induced STM dynamic response detection system, which includes a femtosecond laser 10, a pulse selector 11, a beam splitter 12, a first beam reflection apparatus 13, a second beam reflection apparatus 14, a beam converging apparatus 15, an STM probe 16, a tip current detection apparatus 17, a lock-in amplifier 18, a frequency divider 19, and a long-range linear displacement stage 20; wherein the content of the first and second substances,
the pulse selector 11 is configured to perform pulse selection on the femtosecond pulse laser emitted by the femtosecond laser 10 to emit corresponding femtosecond pulse laser, and send a trigger signal to the frequency divider 19 to trigger the frequency divider 19 to generate a step signal;
the beam splitter 12 is configured to generate probe light and pump light based on the femtosecond pulse laser emitted by the pulse selector 11;
the first light beam reflection device 13 comprises a micro-motion piezoelectric linear displacement stage 130 connected to the frequency divider 19 and driven by the step signal to perform step-like position change, and is used for reflecting the detection light emitted from the beam splitter 12 by a first retro-reflector 131 arranged on the micro-motion piezoelectric linear displacement stage 130;
the second light beam reflecting device 14 is configured to reflect the pump light emitted by the beam splitter 12;
the light beam converging device 15 is used for converging the detection light reflected by the first light beam reflecting device 13 and the pump light reflected by the second light beam reflecting device 14 to an experimental material positioned below the STM probe 16;
the long-range linear displacement table 20 is arranged on the second light beam reflection device 14 and is used for adjusting and controlling the time difference between the probe light and the pump light reaching the experimental material;
the needle tip current detection device 17 is used for detecting a needle tip tunnel current signal of the STM probe 16;
the phase-locked amplifier 18 is respectively connected with the frequency divider 19 and the needle point current detection device 17, and is configured to perform phase-locked amplification processing on the needle point tunnel current signal with the step signal as a reference signal, and transmit an obtained phase-locked amplified signal to a computer system, so that the computer system determines an STM dynamic response result according to the phase-locked amplified signal;
and the modulation frequency of the micro-motion piezoelectric linear displacement platform 130 is half of the frequency of the pulse selector 11, and the phase between the micro-motion piezoelectric linear displacement platform 130 and the pulse selector 11 is locked and adjustable.
It can be understood that the timing signal relationship among the pulse selector, the frequency divider and the micro-motion piezoelectric linear displacement stage in the present embodiment can be determined according to actual needs. For example, fig. 6 shows a timing relationship between signals where the frequency of the frequency divider is the frequency obtained by halving the frequency synchronization of the pulse selector, the phase modulation is from-90 to 90 degrees, and the signal trigger of the frequency divider may be a signal trigger based on a rising edge.
In this embodiment, the beam splitter 12 is specifically a non-equal-ratio beam splitter, and is configured to generate two perpendicular beams, where the beam intensity ratio may be 1:4, where weak light is used as the probe light and strong light is used as the pump light. And weak light as detection light is transmitted to a first retro-reflector 131 driven by the micro-motion piezoelectric linear displacement platform 130, after two adjacent pulses of a pulse sequence of the detection light are reflected by a synchronous locking micro-motion retro-reflector, the optical path forms a specific stroke difference, the stroke difference only depends on the extreme value of the peripheral driving voltage of the piezoelectric ceramic in the micro-motion piezoelectric linear displacement platform 130, the mode is irrelevant to the reciprocating motion process mode of the retro-reflector, and the position of the reflector is ensured to be at a displacement end point when the detection light pulse acts on the retro-reflector instead of the process of changing the stroke by adjusting the phase of the frequency divider 19.
In fig. 5, the wavelength of the femtosecond laser 10 is tunable between 690 and 1020 nm, the output phase of the frequency divider 19 is tunable between 0 and 180 degrees, and the displacement amplitude of the micro-motion piezoelectric linear displacement stage 130 is tunable between 1 and 100 μm.
As can be seen from the above, in this embodiment, the phase of the micro-motion piezoelectric linear displacement stage 130 and the phase of the pulse selector 11 are locked and simultaneously optimized, the phase adjustment range is from 0 to 180 degrees, the optimized phase can reach the maximum response signal, and the noise background intensity is suppressed. In addition, the displacement amplitude of the micro-motion piezoelectric linear displacement stage 130 in this embodiment is adjustable and controllable between 1 and 100 micrometers, that is, the time difference can be adjusted between 0.67fs and 67fs, so that an optimized adjustment is formed between the micro-motion piezoelectric linear displacement stage and the output amplification signal of the lock-in amplifier 18.
More specifically, the femtosecond laser 10 in this embodiment is tunable between 690 nm and 1020 nm, has a power of 1500mW, a pulse width of 35fs, a spectral width of about 60nm, and a repetition frequency of 82MHz, and is modulated at 500Hz to 1MHz by the pulse selector 11.
In this embodiment, the step-type signal generated by the frequency divider 19 is a signal for driving the micro-motion piezoelectric linear displacement stage to stay at each displacement endpoint for a preset time length in the step-type position variation process, so as to ensure that a corresponding complete pulse is detected within the preset time length. The preset duration is not longer than the pulse cycle time, and may be 0.5 to 100 microseconds, and complete pulse coverage at the upper and lower line points as shown in fig. 6 can be achieved through the position dwell operation, so that differential time floating caused by fluctuation of the output time interval of the laser pulse can be prevented. In addition, it can be understood that, in the embodiment, when the micro-motion piezoelectric linear displacement stage stops at the displacement end point, corresponding phase adjustment is required to be performed so as to detect a complete pulse.
In this embodiment, the step signal generated by the frequency divider 19 is a signal for controlling the position of the first retro-reflector at the displacement end point when the first retro-reflector 131 acts on the probe light emitted from the beam splitter 12, thereby avoiding the probe light from being reflected during the course of the shift.
It is understood that the step signal in the present embodiment includes a periodic square wave signal or a sawtooth step signal.
In this embodiment, the second light beam reflection device 14 may specifically include:
the frequency doubling crystal 140 is used for frequency doubling the pump light emitted by the beam splitter 12 to obtain frequency doubled light;
a filtering module 141, configured to filter a long-wavelength band component in the frequency-doubled light emitted from the frequency-doubled crystal 140, so as to obtain filtered laser light;
a second retro-reflector 142 and a broadband half-wave plate 143, wherein the second retro-reflector 142 is configured to reflect the filtered laser light to obtain a corresponding reflected light beam, and the reflected light beam is transmitted to the light beam converging device 15 through the broadband half-wave plate 143.
The frequency doubling crystal 140 may specifically be a BBO crystal (i.e., a barium metaborate crystal), and the filtering module 141 includes an ultrafast pulse band-pass filter or a glan prism. It can be seen that, this embodiment can be with leading before pulse selector 11 of laser frequency multiplier, increases a band-pass filter or glan prism and separates fundamental frequency light and frequency doubling light, keeps the 1/2 wave plates in the light path, can reduce double-colored beam splitter through this design, only needs to use conventional ultrafast pulse beam splitter, reduce cost. In addition, the embodiment adopts tunable double-frequency laser, expands the laser band, can better meet the requirement on the broadband, and can cover 400-900nm of the system response spectrum.
In this embodiment, the light beam converging device 15 may specifically include:
a dichroic mirror 150 for transmitting the probe light and reflecting the pump light so that the probe light and the pump light are coaxially coincident;
It can be understood that the dichroic mirror 150 in this embodiment is specifically an ultrafast pulse dichroic mirror, and has the effects of transmitting a long wavelength band and a high reverse short wavelength band. Next, the plane mirror 151 in this embodiment is specifically an ultrafast broadband plane mirror. In addition, in the embodiment, since the concave reflector 152 is used for light convergence, the use of a dispersion converging element is avoided, and the pulse time broadening phenomenon caused by group velocity dispersion is favorably reduced.
In this embodiment, a long-range linear displacement stage 20 is disposed on the second light beam reflection device 14, and is used for adjusting and controlling a time difference between the probe light and the pump light reaching the experimental material. That is, in the present embodiment, the time difference between the two laser beams reaching the surface of the material is changed by the long-range linear displacement stage 20. In this embodiment, the stroke of the long-range linear displacement stage 20 may be set to 30cm, which is equivalent to a time range of 2ns, and if the time needs to be longer, one or more groups of reflectors may be added opposite to the long-range linear displacement stage 20, so as to realize multiple round trips of light between the long-range linear displacement stage 20 and the above-mentioned groups of reflectors, thereby realizing time expansion of 4ns, 8ns, 16ns, and the like. In the present embodiment, the long-range linear displacement stage 20 is kept stationary when the tip current is detected. It is understood that, in order to realize the control of the long-range linear displacement table 20, the present embodiment further needs to be provided with a displacement table controller 21 for controlling the movement of the long-range linear displacement table 20.
In this embodiment, the micro-motion piezoelectric linear displacement stage 130 and the long-range linear displacement stage 20 respectively control the time displacement of one laser beam pulse. The time delay change of one path of laser pulse sequence modulated by the long-range linear displacement does not affect the laser pulse time sequence of the optical path of the micro-motion piezoelectric linear displacement platform 130, and only the output phase of the frequency divider 19 is optimized, and at the position of the step end point, the laser pulse reaches the retro-reflector and then the phase is locked.
In this embodiment, different displacement stages are used to control corresponding paths of laser beams, so that the reference position of the short-range motion of the micro-motion piezoelectric linear displacement stage 130 is fixed, and the precision requirement on the displacer can be properly reduced.
Further, the beam splitter 12 is made of a fused quartz material with low dispersion, which is beneficial to reducing the pulse time broadening phenomenon caused by group velocity dispersion.
It should be further noted that, in this embodiment, the micro-motion piezoelectric linear displacement stage 130 needs to ensure that the position of the retro-reflector is changed in a step-like manner and is kept stable for a certain time at the end point of the step-like position, so that the time-varying mode of the voltage output by the driver driving the piezoelectric ceramic micro-motion stage during circuit design needs to be a step-like manner, and this embodiment may specifically adopt two modes as shown in fig. 7 and 8, where the voltage fluctuation at the two end points is less than the peak value of 10%.
In addition, it should be noted that the technical solution in this embodiment can be adapted to a high vacuum STM system, thereby avoiding unnecessary scattering and noise of the background, and can correspond to a time-resolved STM of material electronic states in atomic resolution order.
It can be seen that this embodiment has solved three key problems in the prior art: 1. in the prior art, time difference modulation (mode hopping) of pulse mode selection is adopted, but because the mode hopping time is specific and is modulated by large time difference, the obtained signals are response information mixed at different moments, the time accurate analysis of a dynamic process can not be carried out substantially, and the obtained dynamic process of time resolution STM and the electronic state of material atoms is not corresponding; 2. the sinusoidal displacement modulation time difference adopted by the prior art can not be locked on the phase when facing a pulse laser sequence, the capability of obtaining a reliable STM time resolution signal is greatly reduced, and accurate and real information is difficult to obtain. In the practical application process, the method is basically abandoned by recent users, and reports are only made more than ten years ago; 3. the existing technical scheme is only fixed on specific wavelength in the visible and near infrared spectral range, cannot expand the spectral band range of excitation induced laser, and cannot adapt to wide material systems.
Therefore, according to the technical scheme of the embodiment of the application, aiming at double-beam ultrafast pulse laser induced detection time resolution STM dynamic differential response, the problem of extraction of a discrete extremely weak signal phase locking technology is solved by inducing needle point current phase locking through pulse laser displacement, time axis differential micro-variables are controlled by adopting displacement adjustment, and time differential resolution delta t is controlled to reach femtosecond magnitude (10 t)-15s) and the variation range is controllable and adjustable, so that the difference delta I (t) of the needle point tunnel current on the time difference can be obtained, and the differential response delta I (t)/delta t of the dynamic variation of the tunnel current can be obtained by algorithm operation based on the time difference. The dynamic response time function can correspond to the microscopic dynamic process of the material, and due to phase locking, system noise generated by phase-locked signal floating caused by phase fluctuation of discrete points is avoided.
Further, referring to fig. 9, an embodiment of the present application further discloses a method for detecting a dynamic response of a light-induced STM correspondingly, which is applied to the system for detecting a dynamic response of a light-induced STM disclosed above, and includes:
step S11: pulse selection is carried out on the femtosecond pulse laser emitted by the femtosecond laser through a pulse selector so as to emit corresponding femtosecond pulse laser, and a trigger signal is sent to a frequency divider so as to trigger the frequency divider to generate a step signal;
step S12: generating probe light and pump light by a beam splitter based on the femtosecond pulse laser emitted by the pulse selector;
step S13: reflecting the detection light emitted by the beam splitter by a first retro-reflector of a micro-motion piezoelectric linear displacement platform arranged on a first light beam reflection device, wherein the micro-motion piezoelectric linear displacement platform is driven by the step signal to perform step position change;
step S14: reflecting the pump light emitted by the beam splitter by a second light beam reflecting device;
step S15: converging the detection light reflected by the first light beam reflecting device and the pump light reflected by the second light beam reflecting device to an experimental material positioned below the STM probe through a light beam converging device;
step S16: detecting a needle point tunnel current signal of the STM probe through a needle point current detection device;
step S17: the step type signal is used as a reference signal through a phase-locked amplifier which is respectively connected with the frequency divider and the needle point current detection device, the needle point tunnel current signal is subjected to phase-locked amplification processing, and the obtained phase-locked amplified signal is transmitted to a computer system, so that the computer system can determine an STM dynamic response result according to the phase-locked amplified signal;
and the modulation frequency of the micro-motion piezoelectric linear displacement platform is one half of the frequency of the pulse selector, the phase between the micro-motion piezoelectric linear displacement platform and the pulse selector is locked and adjustable, and the second light beam reflection device is also provided with a long-range linear displacement platform for adjusting and controlling the time difference between the detection light and the pump light reaching the experimental material.
For more specific implementation of the above steps, reference may be made to corresponding contents disclosed in the foregoing embodiments, and details are not repeated here.
Therefore, the technical scheme of the application is that a pulse space optical path modulation phase locking technology based on single laser is adopted, the modulation frequency of the micro-motion piezoelectric linear displacement table is fixed to be half of the frequency of a laser pulse selector, and discrete point phase floating caused by random modulation frequency is eliminated. The phase locking of the micromotion piezoelectric linear displacement platform and the phase locking of the pulse selector can be simultaneously ensured to be optimized and adjusted, response signals are favorably enhanced, and meanwhile, the background intensity of noise is inhibited. In conclusion, the STM dynamic response detection method and device can achieve reliable detection of STM dynamic response.
Finally, it should also be noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.
The system and the method for detecting the dynamic response of the light-induced STM provided by the invention are described in detail, specific examples are applied in the system to explain the principle and the implementation mode of the invention, and the description of the examples is only used for helping to understand the method and the core idea of the invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, there may be variations in the specific embodiments and the application scope, and in summary, the content of the present specification should not be construed as a limitation to the present invention.
Claims (10)
1. A light-induced STM dynamic response detection system is characterized by comprising a femtosecond laser, a pulse selector, a beam splitter, a first light beam reflection device, a second light beam reflection device, a light beam convergence device, an STM probe, a needle point current detection device, a phase-locked amplifier, a frequency divider and a long-range linear displacement platform; wherein the content of the first and second substances,
the pulse selector is used for performing pulse selection on the femtosecond pulse laser emitted by the femtosecond laser to emit corresponding femtosecond pulse laser, and sending a trigger signal to the frequency divider to trigger the frequency divider to generate a step signal;
the beam splitter is used for generating detection light and pumping light based on the femtosecond pulse laser emitted by the pulse selector;
the first light beam reflection device comprises a micro-motion piezoelectric linear displacement platform which is connected with the frequency divider and performs step-type position change under the drive of the step-type signal, and the micro-motion piezoelectric linear displacement platform is used for reflecting two adjacent pulses in the detection light emitted by the beam splitter through a first retro-reflector arranged on the micro-motion piezoelectric linear displacement platform;
the second light beam reflection device is used for reflecting the pump light emitted by the beam splitter;
the light beam converging device is used for converging the detection light reflected by the first light beam reflecting device and the pump light reflected by the second light beam reflecting device to an experimental material positioned below the STM probe;
the long-range linear displacement platform is arranged on the second light beam reflection device and is used for adjusting and controlling the time difference between the detection light and the time difference between the pump light and the experimental material;
the needle point current detection device is used for detecting a needle point tunnel current signal of the STM probe;
the phase-locked amplifier is respectively connected with the frequency divider and the needle point current detection device and is used for performing phase-locked amplification processing on the needle point tunnel current signal by taking the step signal as a reference signal and transmitting the obtained phase-locked amplified signal to a computer system so that the computer system can determine an STM dynamic response result according to the phase-locked amplified signal;
and the modulation frequency of the micro-motion piezoelectric linear displacement platform is half of the frequency of the pulse selector, and the phase between the micro-motion piezoelectric linear displacement platform and the pulse selector is locked and adjustable.
2. The light induced STM dynamic response detection system of claim 1, wherein the femtosecond laser is tunable in wavelength between 690 and 1020 nanometers, the output phase of the frequency divider is tunable between 0 and 180 degrees, and the displacement amplitude of the micro-motion piezoelectric linear displacement stage is tunable between 1 and 100 micrometers.
3. The light-induced STM dynamic response detection system of claim 1, wherein the step signals generated by the frequency divider are signals for driving the micro-motion piezoelectric linear displacement stage to stay at each displacement end point for a preset duration during the step position change.
4. A light induced STM dynamic response detection system as claimed in claim 3 wherein the step signal generated by the frequency divider is a signal for controlling the first retroreflector position of the probe light exiting the beam splitter at the displacement end point when acted upon by the first retroreflector.
5. A light induced STM dynamic response detection system as claimed in claim 1 wherein the second beam reflection means comprises:
the frequency doubling crystal is used for doubling the frequency of the pump light emitted by the beam splitter to obtain frequency doubled light;
the filtering module is used for filtering long-wave band components in the frequency doubling light emitted by the frequency doubling crystal to obtain filtered laser;
the second retro-reflector is used for reflecting the filtered laser light to obtain a corresponding reflected light beam, and the reflected light beam is transmitted to the light beam converging device through the broadband half-wave plate.
6. The light-induced STM dynamic response detection system of claim 5, wherein the frequency doubling crystal is a barium metaborate crystal, and the filtering module comprises a band pass filter or a glan prism.
7. The light induced STM dynamic response detection system of claim 1, wherein the light beam converging device comprises:
the dichroscope is used for transmitting the detection light and reflecting the pumping light so as to enable the detection light and the pumping light to be coaxially superposed;
plane mirror and concave surface speculum, wherein, the plane mirror is used for reflecting the probe light and the pump light behind the coaxial coincidence extremely the concave surface speculum, then passes through the concave surface speculum assembles probe light and pump light to lieing in experimental material under the STM probe.
8. A light induced STM dynamic response detection system as claimed in claim 1 further comprising one or more sets of mirrors positioned in a region opposite the long-range linear translation stage to control light to travel back and forth between the long-range linear translation stage and the one or more sets of mirrors.
9. The light-induced STM dynamic response detection system of claim 1, wherein the beam splitter is a beam splitter made of fused silica material.
10. A method of detecting light induced STM dynamic response applied to a system of any of claims 1 to 9, comprising:
pulse selection is carried out on the femtosecond pulse laser emitted by the femtosecond laser through a pulse selector so as to emit corresponding femtosecond pulse laser, and a trigger signal is sent to a frequency divider so as to trigger the frequency divider to generate a step signal;
generating probe light and pump light by a beam splitter based on the femtosecond pulse laser emitted by the pulse selector;
reflecting two adjacent pulses in the detection light emitted by the beam splitter by a first retro-reflector of a micro-motion piezoelectric linear displacement platform arranged on a first light beam reflection device, wherein the micro-motion piezoelectric linear displacement platform is driven by the step signal to perform step position change;
reflecting the pump light emitted by the beam splitter by a second light beam reflecting device;
converging the detection light reflected by the first light beam reflecting device and the pump light reflected by the second light beam reflecting device to an experimental material positioned below the STM probe through a light beam converging device;
detecting a needle point tunnel current signal of the STM probe through a needle point current detection device;
the step type signal is used as a reference signal through a phase-locked amplifier which is respectively connected with the frequency divider and the needle point current detection device, the needle point tunnel current signal is subjected to phase-locked amplification processing, and the obtained phase-locked amplified signal is transmitted to a computer system, so that the computer system can determine an STM dynamic response result according to the phase-locked amplified signal;
and the modulation frequency of the micro-motion piezoelectric linear displacement platform is one half of the frequency of the pulse selector, the phase between the micro-motion piezoelectric linear displacement platform and the pulse selector is locked and adjustable, and the second light beam reflection device is also provided with a long-range linear displacement platform for adjusting and controlling the time difference between the detection light and the pump light reaching the experimental material.
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