CN114252653A - Ultrafast imaging device and method thereof - Google Patents
Ultrafast imaging device and method thereof Download PDFInfo
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Abstract
The invention provides an ultrafast imaging device and method thereof, the ultrafast imaging device includes: the system comprises an ultrafast laser for generating a probe laser and a pump laser, wherein the pump laser is used for exciting a sample; a field emission system having a photocathode excited by a detection laser to emit pulsed photoelectrons or thermally emitted thermal field electrons; and an illumination system, an imaging system and a detector; further comprising: a first extraction electrode having a positive potential of a first voltage for separating pulsed photoelectrons from the photocathode; the second extraction electrode has a positive potential of a second voltage and is used for driving pulsed light electrons to move towards the acceleration system in an accelerated way and focus the pulsed light electrons; the acceleration system is used for accelerating the pulsed photoelectrons to a third voltage. The device and the method further improve the spatial and time resolution capability of the ultrafast transmission electron microscope and simplify the complexity of light path adjustment.
Description
Technical Field
The invention relates to the field of transmission electron microscopes, in particular to an ultrafast imaging device and a method thereof.
Background
With the development of materials, physics, chemistry and the field of bioscience medicine, there is a great demand for transmission electron microscopy in the dynamic process research of materials in another dimension-time resolution, that is, it is required to observe transient states in a sufficiently short time (such as nanoseconds, even femtoseconds).
The ultrafast imaging technology based on the ultrafast laser and the transmission electron microscope has the advantages that a plurality of experimental technologies (in-situ, high and low temperature, electric field and magnetic field) can be integrated, and the dynamic behavior of the physical state can be researched under high spatial resolution, energy resolution and time resolution. Observing transient states and behaviors (e.g., chemical reactions, structural deformations or phase changes, etc.) is key to understanding many of the basic behaviors in chemistry, biology, physics, and materials science. In the chemical field, a key issue is how to understand the kinetics and reaction mechanisms of chemical reactions, such as the variation of different active sites of a catalyst. In physics and material science, research on the dynamic processes of various phase transition behaviors, such as structural phase transition, metal-insulator transition and competition of different phases under external field conditions, is the basis for understanding the mechanisms of various physical properties of materials. In biology and medicine, the research on the structures of different biomolecules, such as cells, proteins and the like, is beneficial to understanding the main functions of the biomolecules in life, and can greatly promote the development of modern medicine and biology.
The ultrafast transmission electron microscope is a product of organic combination of ultrafast laser (time-resolved) and electron microscope (spatial-resolved) technologies, and has become an important new direction for the development of electron microscopes. The ultrafast transmission electron microscope platform is established on the basis of a modern electron microscope platform, time resolution is introduced through an ultrafast laser system, pulse laser emitted by an ultrafast laser is divided into two beams through a spectroscope, wherein one beam is used as a detection pulse and is focused on a cathode of an electron gun to generate an ultrashort electron pulse after frequency doubling and frequency tripling; and the other beam is used as pumping laser, is guided into a sample chamber in a transmission electron microscope through a delay light path after single frequency or frequency multiplication, and is finally focused on the surface of the sample for exciting an ultrafast process, and the dynamic image research of a high-space-time-resolution object state structure can be realized by adopting a pumping detection technology in combination with the high time resolution of the ultrafast laser and the high space resolution of the transmission electron microscope.
The key technology of the ultrafast electron microscope is how to realize the control of the ultrafast laser to the cathode electron source, the performance of the ultrafast electron microscope is greatly influenced by the cathode electron source as with the performance of the traditional transmission electron microscope, for the thermal emission cathode with larger diameter of the cathode filament, no matter in a single pulse mode or a continuous pulse mode, the optical excitation can generate enough electrons, the photoelectron beam spot is generally more than dozens of microns, the brightness and the spatial coherence of photoelectrons are greatly limited, the inventor successfully realizes the time resolution function on the transmission electron microscope of the first generation thermal emission cathode, the technical blank of the domestic ultrafast electron microscope is filled, but due to the influence of the coherence, the stability and the spherical aberration, the application of the high resolution of the ultrafast electron microscope, the coherent electron diffraction and the electronic holography is greatly limited.
Disclosure of Invention
In view of the above technical problems in the prior art, a first aspect of the present invention provides an ultrafast imaging apparatus, comprising: an ultrafast laser for generating a probe laser and a pump laser, wherein the pump laser is used for exciting a sample; a field emission system having a photocathode excited by the detection laser to emit pulsed photoelectrons or thermally emitted thermal field electrons; and an illumination system, an imaging system and a detector; characterized in that the field emission system further comprises:
a first extraction electrode having a positive potential of a first voltage for separating the pulsed photoelectrons from the photocathode;
the second extraction electrode has a positive potential of a second voltage and is used for driving the pulsed light electrons to move towards the acceleration system in an accelerated mode and focus the pulsed light electrons;
wherein the acceleration system is configured to accelerate the pulsed photo-electrons to a third voltage.
Preferably, the first voltage is in the range of 0kV to 4.5 kV; the second voltage ranges from 5kV to 8.5 kV; and said third voltage is in the range of 80kV-200kV, preferably 80kV,120kV,160kV or 200 kV.
Preferably, the field emission lighting system further comprises a laser introducing port arranged between the field emission lighting system and the illumination lighting system and a regulating rod matched with the laser introducing port, the laser introducing port is a cavity with a top wall, a bottom wall and a side wall, the side wall is provided with a reflector adjusting opening, the regulating rod is provided with a reflector at one end of the regulating rod, the reflector and a part of the regulating rod are sealed in the cavity of the laser introducing port through the reflector adjusting opening, and the regulating rod can translate or rotate.
Preferably, the side wall of the laser introducing port further has a first laser introducing window and an ion pump interface for externally connecting to a vacuum ion pump, and the top wall and the bottom wall are oppositely provided with a first opening and a second opening for allowing the probing laser and/or the pulsed light electrons to pass through.
Preferably, the mirror is provided with a plurality of parallel slits of different widths in the range of 0.7mm to 1.0 mm.
Preferably, the laser introducing port is made of a non-magnetic stainless steel material or a tungsten bronze material, and the first laser introducing window comprises fused silica glass plated with an ultraviolet antireflection film.
Preferably, the ultrafast imaging apparatus further includes a beam splitter, first and second laser frequency conversion elements, first and second focusing lenses, first and second laser position monitoring devices, and a retarder, wherein:
the ultrafast laser outputs laser, the laser is divided into two beams by the beam splitter, one beam of the laser generates detection laser by the first laser frequency conversion element, the detection laser enters the laser leading-in port by the first focusing lens, and the first laser position monitoring equipment is used for monitoring the deviation of the spot position of the detection laser incident to the photocathode; and the other laser beam generates pump laser through the second laser frequency conversion element, and the second laser position monitoring equipment is used for monitoring the deviation of the spot position of the pump laser incident on the sample.
A second aspect of the present invention provides a method for the aforementioned ultrafast imaging apparatus, comprising:
(1) precisely irradiating the detection laser on the photocathode and generating the pulse photoelectrons, wherein the pulse photoelectrons irradiate the sample and obtain diffraction or microscopic image information;
(2) irradiating the pump laser and the pulsed photoelectrons at the same position on the sample and generating an ultrafast process;
(3) determining action time zero points of the pulsed photoelectrons and the pump laser and determining a spatial coincidence point of the action time zero points according to the electron energy loss spectrum of the ultrafast process;
(4) and obtaining at least one piece of diffraction or microscopic image information of different time delays between the pump laser and the detection laser according to the determined time zero point and the determined spatial coincidence point so as to obtain a diffraction or microscopic image in the ultrafast process.
Preferably, step (1) further comprises:
(1.1) determining the photocathode luminescence optical path by causing the photocathode to emit thermal field electrons;
(1.2) adjusting the detection laser to irradiate the photocathode according to the photocathode light-emitting path;
(1.3) precisely irradiating the photo cathode with the detection laser.
Preferably, step (2) further comprises:
(2.1) determining the luminescence path of the sample according to the emitted fluorescence by using a fluorescent material as the sample;
(2.2) adjusting the pump laser to irradiate the sample according to the sample luminescence route;
(2.3) precisely irradiating the sample with the pump laser.
Preferably, the step (3) of determining the zero point of the action time of the pulsed photoelectrons and the pump laser according to the electron energy loss spectrum of the ultrafast process includes:
(3.1) taking a multi-walled carbon nanotube as the sample, exciting the multi-walled carbon nanotube to expand radially by using the pump laser in a diffraction mode, and preliminarily determining the action time zero point and the space coincidence point of the pulse photoelectrons and the pump laser by judging the position and the starting time of the sample expanded radially;
and (3.2) determining a zero loss peak and a plurality of satellite peaks at integral multiples of photon energy on two sides of the zero loss peak through an electron energy loss spectrum by utilizing a near-field effect generated on the surface of the sample by the laser, and determining the time corresponding to the zero loss peak as a time zero point, wherein the satellite peaks only appear on two sides of the time zero point and have duration equal to the convolution of the pulse width of the pulsed light electron and the pulse width of the pump laser.
The ultrafast imaging device and the ultrafast imaging method further improve the space and time resolution capability of the ultrafast transmission electron microscope and simplify the complexity of light path adjustment.
Drawings
Embodiments of the invention are further described below with reference to the accompanying drawings, in which:
FIG. 1 is a schematic diagram of a prior art ultrafast imaging apparatus;
FIG. 2 is a schematic structural diagram of an ultrafast imaging device in accordance with a preferred embodiment of the present invention;
FIGS. 3 and 4 depict schematic perspective views of the laser port of introduction and the adjustment rod, according to the present invention;
fig. 5 shows a topography and a diffraction picture, an electron energy loss spectrum and an electroholographic result thereof taken by the ultrafast imaging apparatus according to fig. 2 in a thermal field emission mode. Wherein a is a morphology of a standard gold standard sample, b is electron diffraction of single-crystal gold, c is an electron energy loss spectrum, and d is an electron hologram of Fe nano-particles;
fig. 6 shows a photographing result and measured information in the light emission mode of the ultrafast imaging apparatus according to fig. 2.
Fig. 7 illustrates experimentally obtained data of steps of calculating a time zero point according to the ultrafast imaging apparatus of fig. 2.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail by embodiments with reference to the accompanying drawings.
Fig. 1 shows an ultrafast imaging apparatus in the prior art, wherein an ultrafast laser 1 generates laser light and is divided into two beams by a beam splitter 3, a first beam of laser light generates detection laser light by a first laser frequency conversion element 4, is focused by a first focusing lens 9, enters a reflector 17 of an electron gun 26 from a laser introducing window 13, is reflected by the reflector and irradiates a photocathode 19 and excites pulsed photoelectrons, and the pulsed photoelectrons are accelerated by an acceleration system 20 and are focused by an illumination system 22 and irradiate a sample 23 in a sample chamber 27; another laser beam is generated by the second laser frequency conversion element 5 to be pump laser, delayed by the retarder 8 and focused by the second focusing lens 10, and is incident from the laser introducing window 14 to the reflecting mirror 18 of the sample chamber 27 and irradiates the surface of the sample 23 on which the detection laser is focused, wherein a laser position monitoring device 16 is further arranged between the second focusing lens 10 and the laser introducing window 14 and is used for adjusting the optical path of the pump laser. The pump laser excites the surface of the tested sample 23 to generate an ultrafast process, the detection laser detects the process and the detector 25 receives and analyzes signals of the tested sample 23 such as microscopy, diffraction and the like; the ultrafast imaging device further comprises an external vacuum device.
Fig. 2 is a schematic structural diagram of an ultrafast imaging apparatus 200 according to an embodiment of the present invention. For inducing a transmission electron mirror field emission cathode with pulsed laser, an ultrafast imaging device 200 comprising:
(1) the ultrafast laser system comprises an ultrafast laser 201, a beam splitter 203, a first laser frequency conversion element 207, a first focusing lens 209, a first three-dimensional electric control displacement table 210 for bearing the first focusing lens, and first laser position monitoring equipment 213; and a second laser frequency conversion element 204, a retarder 206, a second focusing lens 215, a second three-dimensional electric control displacement table 214 carrying the second focusing lens, and a second laser position monitoring device 218. The ultrafast laser system is used for converting laser generated by the ultrafast laser 201 into detection laser and pumping laser with predetermined characteristics.
According to one embodiment of the present invention, the ultrafast laser 201 is a tunable femtosecond laser, the wavelength of which is tunable between 210nm and 16um, the output power is greater than 10W, the repetition frequency is tunable between 1-1MHz, the maximum pulse energy is greater than 400uJ, and the pulse width is tunable between 190fs-10 ps. The beam splitter 203 is a semi-transparent semi-reflective dielectric film beam splitter; the first laser frequency conversion element 207 and the second laser frequency conversion element 204 adopt BBO crystals to realize the process of frequency doubling, frequency tripling or optical parametric amplification through phase matching; the retarder 206 comprises an electrically controlled displacement stage with a precision of 1 μm and/or a stroke of 1m and a hollow retroreflector; the first and second laser position monitoring devices 213 and 218 include beam sampling mirrors 212 and 216, respectively, and position sensitive detectors 211 and 217, respectively, on which a portion of laser light divided from the pump laser light is irradiated. The beam sampling optic may be a beam splitter. The position sensitive detector may be a CCD, more preferably the position detector may be a direct electron detector and an energy filtering system.
(2) A field emission system 232, which includes a photocathode 221, a suppressor 222, a first extractor 223, a second extractor 224, an acceleration system 225, and a variable limiting diaphragm 226, for converting the detection laser into pulsed photoelectrons and accelerating to a specified voltage.
(3) The laser introduction port 233, which is in the shape of a cavity, includes a first laser introduction window 219, a mirror 227, a first through hole 2331, and a second through hole (to be described in detail with reference to fig. 3). The laser port 233 also has a mirror adjustment opening through its wall and its position and angle can be adjusted by an external adjustment lever 2335 (described in detail in connection with fig. 3). The photocathode 221, the suppressor 222, the first extractor 223, the second extractor 224, the accelerator 225, the variable limiting diaphragm 226 and the reflector 227 of the laser introducing port 233 of the field emission system 232 are used to cooperate with each other to form a complete optical path for the passage of the detection laser and the pulsed photoelectrons excited thereby. Wherein the reflecting mirror 227 is a plane reflecting mirror which is a metal Mo with a polished surface.
(4) A microscope system 234 including an illumination system 228, an imaging system 229 and a projection system 230 and a sample chamber 235. Wherein the illumination system 228, the imaging system 229 and the projection system 230 are used to irradiate pulsed photoelectrons onto the sample and generate an enlarged microscopic image and diffraction information; the sample chamber 235 includes a second laser introduction window 220, a second reflecting mirror 237 for placing and precisely irradiating the pumping laser onto the sample to be measured. Wherein field emission system 232 and microscope system 234 may be an integrated whole.
(5) And a detector 231 for receiving and analyzing signals of the sample to be tested, such as microscopic signals and diffraction signals.
(6) Vacuum apparatus (not shown in FIG. 2) for maintaining a high vacuum in the field emission system, the laser port, the light emitting cathode, the microscope system 234 and its sample chamber 235.
Fig. 3 illustrates a perspective view of the laser introduction port 233 and fig. 4 illustrates a perspective view of the adjustment lever 2335, according to an embodiment of the present invention. Wherein the laser introduction port 233 shown in fig. 3 is a substantially hollow cylindrical cavity, and the adjustment lever 2335 shown in fig. 4 is used to dispose the mirror 227 disposed at one end thereof inside the cavity of the laser introduction port 233 and to seal it; the other end of the adjustment lever 2335 has a grip portion to easily adjust the rotation angle of the reflecting mirror 227 from the outside of the laser introduction port 233 and the depth along the adjustment lever 2335 to protrude into the inside of the laser introduction port 233.
Specifically, the cavity of the laser introduction port 233 has a substantially circular top wall 2332 and a bottom wall disposed opposite thereto, and a side wall enclosed between the top wall 2332 and the bottom wall. Wherein the first through hole 2331 is located in the center of the top wall 2332; top wall 2332 and first through-hole 2331 are placed in sealed communication with field emission system 32 using a rubber gasket. The second through hole disposed at the center of the bottom wall corresponds to the first through hole 2331 at the center of the top wall 2332; the bottom wall and the second through hole are arranged in sealing communication with the microscope system 234 using a rubber ring. The first laser light incidence window 219, the mirror adjustment port 2333, and the ion pump interface 2334 are respectively disposed on the side walls of the laser light introduction port 233. The first laser introducing window 219 is transparent to ultraviolet light, and may be made of ultraviolet fused silica glass coated with an ultraviolet antireflection film, wherein the fused silica glass has a thickness of 5mm and a diameter of 25 mm. The ion pump interface 2334 is a knife-edge flange, which may be of the type CF35, for example, and is used to externally connect a high-speed ion pump to further increase the vacuum in the photocathode 221 and the laser entrance port 233, and to reduce the space charge effect caused by the pulsed photoelectrons during the transmission process.
As shown in fig. 4, the adjustment rod 2335 has a reflecting mirror 227 at one end thereof, wherein the reflecting mirror 227 is for being disposed inside the cavity of the laser introduction port 233, and the adjustment rod 2335 has a substantially straight rod shape of which one end is connected to the reflecting mirror 227 by a stainless steel clip and an air-permeable screw and the other end has a grip end. The adjustment rod 2335 is sized in diameter to pass right through the mirror adjustment opening 2333 so that it can be sealingly attached to the mirror adjustment opening 2333 after the insertion laser introduction port 233 therethrough. The mirror 227 is arranged to have a plurality of slits 2271 of different widths arranged in parallel for the passage of the electron beam, and the optical path of the detection laser light and the precise angle of the irradiation photocathode can be precisely adjusted by adjusting the tilt angle and the forward-backward translational position (with respect to the mirror adjustment port 2333) of the adjustment lever 2335.
According to a preferred embodiment of the present invention, the first through hole 2331 and the second through hole of the laser introduction port 233, the first laser incidence window 219, the mirror adjustment port 2333, and the ion pump interface 2334 are all circular interfaces, preferably KF flange interfaces. The diameter of the first laser incident window 219 may be between 16mm and 100mm, and the material thereof may be selected from other materials having a relatively high light transmittance (e.g. greater than 95%) at a wavelength of 210-500nm, and the shape of the window may also be other shapes, such as a square shape; the adjustment lever 2335 may be manually adjusted or electrically adjusted. The diameter of the laser introduction port 233 may be consistent with the diameter of a cylindrical field emission transmission electron microscope housing, and the height thereof is between 5cm and 16cm, preferably 9cm, but the size is not a limitation of the present invention.
The principle of cooperation between the components of the ultrafast imaging apparatus 200 will be described in detail with reference to fig. 2.
Femtosecond or nanosecond laser output by the ultrafast laser 201 passes through the beam splitter 203 along the optical path 202 to generate two beams of laser, one beam is detection laser and is incident to the photocathode 221 through the optical path 208 to generate photoelectrons, and the other beam is pumping laser and is incident to the sample chamber 235 through the optical path 205 to excite a sample. Ultrafast laser 201 may generate a series of optical pulses having a defined pulse width and pulse interval.
Wherein the detection laser is generated (typically ultraviolet laser) by a first laser frequency conversion element 207 (typically a frequency tripling device). The detection laser is sequentially focused by a first focusing lens 209 disposed on the first three-dimensional electric control displacement stage 210, passes through the first laser position monitoring system 213, enters the cavity through a first laser introducing window 219 (or an optical window) of the laser introducing port 233, is reflected by a reflector 227, passes through a first through hole 331 of the laser introducing port 233, and is incident on a photocathode 221 to generate photoelectrons.
The first focusing lens 209 may be a plano-convex quartz glass lens coated with an ultraviolet antireflection film, and the focal length of the first focusing lens 209 is selected according to actual needs, and is generally selected to be 500 mm. The first three-dimensional electric control displacement table 210 can be assembled by using three motor-controlled one-dimensional electric control displacement tables and is used for adjusting the position of the first focusing lens 209, so that the focal position of the detection laser focused by the first focusing lens 209 can be adjusted. And a first laser position monitoring system 213 for monitoring and controlling the position drift of the spot of the detection laser incident on the photo cathode 221. The first laser position monitoring system 213 is composed of a beam sampling lens 212 and a position sensitive detector 211, the beam sampling lens 212 divides the detection laser 28 from the main optical path into a small part of laser to irradiate on the position sensitive detector 211, the position sensitive detector 211 can reflect the deviation of the light spot position by detecting the deviation of the beam splitting position, and the real-time correction of the light spot position at the photocathode 221 can be realized by feedback adjusting the optical element at the front end of the optical path. In this embodiment, in order to control the pulsed photoelectrons excited by the probe laser incident on the photocathode 221 to achieve high polymerization degree and prevent the excessive scattering angle, a photocathode 221 smaller than the prior art is used, which has a thermal field emission filament with a diameter ranging from 10nm to 5um, and more preferably from 100nm to 2um, the filament may be ZrO/W (100), W (100)/ZrO, W (100), W (110), W (310), and the photocathode 221 can emit light in situ and at the same time. The pulse width of the pulse photoelectron is 190fs-10ps, the single pulse dose is 1-106 electron numbers, and the electron energy is 80 keV-200 keV; the smaller size of the photocathode places higher demands on the accuracy of the laser irradiation position, and the first laser position monitoring system 213 facilitates effective control of the light path accuracy, and at the same time, corrects the deviation of the excitation signal intensity and position at the sample observation position due to the drift of the spot position over time due to the instability of the optical system.
The photo cathode 221 of the field emission system 232, which is irradiated by the detection laser, generates pulsed photoelectrons that sequentially pass through the first extraction electrode 223, the second extraction electrode 224 and the acceleration system 225. Specifically, when the photo-cathode 221 is irradiated by the detection laser and generates pulsed photoelectrons in all directions, only the pulsed photoelectrons directed to the sample chamber 235 are needed, and the pulsed photoelectrons in the other directions may constitute a potential interference factor for the measurement result. The suppression electrode 222 is thus arranged around the illuminated tip area of the photo-cathode 221, which has a negative potential with respect to the photo-cathode 221, suppressing a larger fraction of the electrons with respect to the desired light path scattering angle outside the light path so that it does not disturb the measurement process on the sample. The first extraction electrode 223 and the second extraction electrode 224 shown in fig. 2 have positive potentials with respect to the photo cathode 221, the first extraction electrode 223 generates an "extraction voltage" for extracting pulsed photoelectrons, which generates a strong electric field to tunnel the pulsed photoelectrons out of the needle tip of the photo cathode 221, and the second extraction electrode 224 is used for accelerating and focusing electrons extracted from the first extraction electrode 223 so as to attract the generated pulsed photoelectrons to advance substantially along the desired optical path direction and enter the acceleration system 225. Wherein the first voltage regulating range of the first extraction pole 223 is 0kV-4.5kV, and the second voltage regulating range of the second extraction pole 224 is 5kV-8.5 kV. The pulsed photoelectrons that act through the first extraction electrode 223 are only one electron cluster or envelope with a large cross section and substantially along the desired optical path direction, and the precision is still not ideal, and the pulsed photoelectrons are more easily dispersed because the present embodiment introduces the laser introduction port 233 with a non-negligible size, which increases the distance that the pulsed photoelectrons travel from excitation to incidence on the sample surface. The combination of the second extraction electrode 224 and the first extraction electrode 223 can effectively control the focusing fineness of the pulsed photoelectrons advancing to the target sample and the electron throughput per unit time. After more than three passes of the suppressor and the first and second extractor, the screened out highly coherent electrons enter the acceleration system 225 and are accelerated to a predetermined third voltage, wherein the acceleration system has a high voltage characteristic, and the typical value of the third voltage may be one of 80kV to 200kV, preferably 80kV,120kV,160kV or 200 kV. The accelerated pulsed light electrons pass through the limiting diaphragm 226 and are output. Since this embodiment employs a smaller photocathode and outputs fewer pulsed light electrons, replacing the diaphragm in the prior art with the limiting diaphragm 226 set to a variable size can make its aperture larger (e.g., increased to a 3mm diameter) by adjusting its aperture change, and the electron throughput is higher, and even eliminate the diaphragm there. The imaging efficiency of photoelectrons can be improved.
Another part of laser output by the ultrafast laser 201 passes through the second frequency conversion device 24 to generate pulse laser with different wavelengths, so as to meet the requirements of excitation and research of different samples. The second frequency conversion device 24 can be a frequency doubling system, a frequency tripling system or an optical parametric amplification system, and can realize continuous adjustability of the laser wavelength from 210nm to 16 μm. The pump laser light generated by the second laser frequency conversion element 204 is introduced into a retarder 206 (other optical delay lines are also possible) along the optical path 205 such that the pump laser light and the probe laser light have a certain time delay.
Similar to the optical path that the detection laser travels, the pump laser output by the retarder 206 sequentially passes through the second focusing lens 215 disposed on the second three-dimensional electrically-controlled displacement stage 214, and then enters the electron microscope system 234 through the second laser position monitoring system 218 and the second laser introduction window 220 (or optical window).
The second laser introducing window 220 is transparent to visible light, and ultraviolet fused silica glass coated with a visible light antireflection film can be selected. The second focusing lens 215 can be a plano-convex quartz glass lens plated with a visible antireflection film, and the focal length of the lens 215 is selected according to actual needs, generally 500 mm. The second three-dimensional electric control displacement stage 214 can be assembled by using three motor-controlled one-dimensional electric control displacement stages, and is used for adjusting the position of the second focusing lens 215, so as to adjust the focal position of the pump laser focused by the second focusing lens 215. In addition, similar to the first laser position monitoring system 213, the second laser position monitoring system 218 includes a beam sampling optic 216 and a position sensitive detector 217. Which is used to correct for drift in spot position over time due to optical system instability, resulting in deviations in excitation signal intensity and position at the observed location of the sample.
The sample chamber comprises a second laser introduction window 220, a second mirror 237. After being focused by the second focusing lens 215, the pumping laser is reflected by the second reflecting mirror 237 and irradiates on the sample to be measured, so as to excite the sample to be measured to generate an ultrafast process.
The pulsed photoelectrons accelerated by the acceleration system 225 pass through the laser introduction port 233 again and enter the microscope system 234 through the second through hole thereof, are guided by the illumination system 228 to enter the sample chamber 235, are converged on the sample to be measured, and the pulsed photoelectrons interact with the sample to generate an image carrying characteristic information of the sample, and form diffraction, image and spectral signals at the detector 231 through the imaging system 230 and are recorded by the detector 231. The plurality of electromagnetic lenses constituting the illumination system 228 and the imaging system 230 have magnification, deflection, and translation functions. The detector 231 may be a photographic negative, an imaging plate, a scintillator CCD camera, a direct electron detection camera, an energy filtering system for obtaining an electron energy loss spectrum, and the like.
The top wall 2332 and bottom wall of the laser introduction port 233 shown in fig. 3 are in communication with the field emission system 232 and the microscope system 234, respectively, by hex set screws 2336. To ensure the sealing of the connection, a sealing ring is provided on the outside of the top wall 2332 and the bottom wall of the laser introduction port 233 to ensure the vacuum inside the whole.
By providing the laser introduction port 233 and matching with the adjustment rod 2335 penetrating through it in a sealing manner, the ultrafast imaging apparatus 200 of the present embodiment can flexibly and accurately adjust the angle, the scattering angle luminous flux and the focus point of the detection laser incident to the photocathode by translating the adjustment rod back and forth and twisting it to rotate, for example, without disassembling each component under a high vacuum environment based on the field emission transmission electron microscope of the prior art, thereby reducing the complexity of the operation of the apparatus.
In the light emission mode, the photocathode 221 needs to reduce the filament temperature below 1400K to completely eliminate the thermal field electrons from being emitted, and then uses the photoelectric effect to excite pulsed light electrons by the detection laser for imaging. In the thermal field emission mode, the filament temperature of the photocathode 221 is adjusted to 300-1800K. The filament is heated to 1800K, a certain extraction electric field is applied to the filament, thermally excited electrons are extracted through the first extraction electrode 223, the focusing power of the electron beam is controlled through the second extraction electrode 224, and the screened electrons enter the imaging system with better focusing power as far as possible.
The second extraction electrode 224 in this embodiment can effectively control the electron clusters or envelopes to have smaller scattering angle or larger convergence angle, and the electron clusters or envelopes can effectively regulate the focusing fineness and the electron throughput per unit time when pulsed light electrons advance to a target sample by adjusting the respective voltage values in cooperation with the first extraction electrode 223.
The reflector 227 may also be a mirror of metal Al with a polished surface or a high reflectivity and high conductivity material plated with Mo, Al, etc.
The introducing port 233, the adjusting lever 2335, and the like are made of a nonmagnetic stainless steel material, but the present invention is not limited thereto, and these components may be tungsten bronze or a material which is nonmagnetic and non-magnetizable.
The field emission system 232 and the microscope system 234 are typically integrated together in a field emission transmission electron microscope and are commercially available. In this embodiment, the main body of the field emission transmission electron microscope is a 2100F field emission transmission electron microscope of JEOL corporation.
In addition, different field parameters can be properly selected according to the electron properties of the required pulsed light, and the range of the field parameters does not depart from the scope covered by the idea of the application.
Fig. 5 shows a topography and diffraction photograph, an electron energy loss spectrum and an electroholographic result thereof taken with the ultrafast imaging apparatus 200 according to fig. 2 in a thermal field emission mode. In FIG. 5, a is the high resolution of the gold standard with an image resolution of 0.23 nm; b is an electron diffraction pattern of single-crystal gold, c is an electron energy loss spectrum, and the resolution is about 1 eV; d is the electroholographic result of the iron nanoparticles. It can be seen that with the thermal field emission function in this example, the spatial resolution is also quite high.
Fig. 6 shows measured information of photographing various samples in the light emission mode according to the ultrafast imaging apparatus 200 of fig. 2. Wherein a is a light spot in a light emission mode, and the size of the light spot is 2 nm; b electron diffraction of gold single crystal; c is the magnetic domain structure of the PbFeO3 sample; d is a high resolution image of the graphite sample; e is the electron energy loss spectrum in the light emission mode; f and g are the electronic hologram and phase information of the iron nanowire, respectively. It can be seen that with this embodiment, the photoelectrons obtained have good electronic coherence. The spatial resolution of the light emission image can reach up to 0.34 nm.
Fig. 7 shows experimentally obtained data of steps of calculating a time zero point of the ultrafast imaging apparatus according to the embodiment of the present invention. Wherein (a) is the diffraction pattern of negative time delay (t-10 ps) and positive time delay (t-20 ps) and the difference between them; (b) is (100) the intensity decay of the diffraction ring as a function of time, (c) the electron energy loss spectrum at different time delays (which is obtained by the energy filtering system as described earlier). When electrons interact with photons, a series of satellite peaks appear at integral multiples of photon energy on both sides of the zero loss peak. The specific steps for calculating the time zero point are as follows:
first, a detection laser is precisely irradiated on the photocathode 221 and generates pulsed photoelectrons, which are irradiated on the sample, and diffraction or microscopic image information is obtained by the imaging system 230 and recorded by the detector 231. According to an embodiment of the present invention, the step of accurately irradiating the probe laser onto the photocathode 221 includes:
(1) determining a light emitting path of the photocathode 221 by causing the photocathode 221 to emit thermal field electrons;
(2) adjusting the detection laser to enter the field emission system 232 according to the light emitting path of the photocathode 221 and irradiate the photocathode 221 with light;
(3) according to the first laser position monitoring system 213, the position of the first focusing lens 209 is adjusted by adjusting the first three-dimensional electric control displacement stage 210, and the reflective mirror 227 is adjusted by the adjusting rod 2335, so that the detection laser passes through the first focusing lens 209 and precisely irradiates on the photocathode 221, and the pulsed photoelectrons can enter the sample chamber 235 through the slit of the reflective mirror 227.
Secondly, the pumping laser and the pulsed photoelectrons are precisely irradiated on the same position on the sample and an ultrafast process is generated. According to one embodiment of the present invention, the method for precisely irradiating the pump laser and the pulsed photoelectrons on the sample comprises the following steps:
(1) the sample is arranged as a fluorescent material, such as zinc sulfide, pulsed light electron beams are focused by the illumination system 228, and then the sample in the sample chamber 235 is irradiated, and the light emitting path of the sample is determined according to the fluorescence emitted by the pulsed light electrons. Green light emitted from the fluorescent material can be observed through the second laser introduction window 220; two limiting diaphragms are arranged on a light path through which the pump laser passes, so that the bright light emitted by the fluorescent material passes through the two limiting diaphragms;
(2) introducing a beam of pump laser into a sample chamber 235 to irradiate the sample to a region where the sample is gathered by a pulsed photon beam, and adjusting the pump laser to pass through two limiting diaphragms and enter the sample chamber 235 according to a sample light-emitting route;
(3) and placing a second focusing lens 215, and adjusting a second three-dimensional electric control displacement platform 214 to adjust the position of the second focusing lens 15, so that the pump laser passing through the second focusing lens 215 can still accurately irradiate the position of the sample through the limiting diaphragm.
And thirdly, searching an electronic pulse and laser pulse time zero point according to the electronic energy loss spectrum in the ultrafast process. The process mainly comprises the following steps:
(1) preliminarily determining ultrafast process spatial information of the sample: the XY position of the second three-dimensional electric control displacement platform 214 is adjusted in a step-by-step scanning mode, so that the pump laser can penetrate through the position of the sample, and is reflected for multiple times by the inner wall of the lens barrel of the imaging system 234 to irradiate the detector 231 to form a circular scattering light spot, and the position of the pump laser, which is far away from the sample, is relatively close;
(2) and accurately determining spatial information of the ultrafast process of the sample: the multi-walled carbon nanotube is used as a sample, and the position of the multi-walled carbon nanotube is further accurately determined through lattice expansion of the multi-walled carbon nanotube under the irradiation of pump laser; specifically, the X-axis (or Y-axis) position in the second three-dimensional displacement table 14 is scanned, the diffraction pattern of the multi-walled carbon nanotube at each X-axis position is recorded, the diffraction ring position can be obtained through radial integration, the diffraction ring position changes along with the adjustment of the X-axis position, which reflects that the pump laser generates different degrees of expansion when irradiating a sample at different positions, and the position where the position change is the maximum is the central position of the pump laser; wherein, the optical path difference between the pumping optical path and the detection optical path before entering the laser introducing window needs to be measured; measuring the respective optical paths of the pump laser to the sample and the detection laser to the photocathode; calculating the time required by the pulsed photoelectrons to reach the sample and converting the time into an optical path; readjusting the ultrafast imaging device optical path according to the obtained respective optical paths so that the final position of the time zero is located at the middle position of the retarder 8; and adjusting the Z-axis position of the second three-dimensional displacement table 14 to enable the expansion amplitude of the multi-wall carbon nano tube to be maximum, wherein the focus of the pump laser focused by the second focusing lens 15 is accurately located at the sample.
(3) Preliminarily determining the ultrafast process time zero point information of the sample: the fundamental properties of multi-walled carbon nanotubes are determined by the emission of electrons using thermal fields. The detection laser is adjusted to generate pulsed photoelectrons, and a diffraction image of the multi-walled carbon nanotube sample is obtained by the imaging system 30 and the detection system 31.
The time delay between the pump laser and the probe laser is changed, and diffraction information of the multi-walled carbon nanotube at two moments before and after the time zero point (for example, t is-10 picoseconds, t is +20 picoseconds, and the minus sign represents the time before the arrival of the pump laser) is respectively obtained.
And subtracting the diffraction image of which t is 20 picoseconds from the diffraction image of which t is-10 picoseconds to obtain the difference between the two diffraction images, wherein the difference reflects the expansion between the nanotube layers, similarly, a series of difference values corresponding to different time delays are calculated, the position with the maximum expansion is found to be a time zero point, and the resolution of the method to the time zero point is 2ps and still needs to be further accurately determined.
(4) And accurately determining the time zero point information of the ultrafast process of the sample: according to the time zero point determined in the previous step, in combination with the electron energy loss spectrum, searching an electron energy loss spectrum zero peak when electrons and photons interact and a series of satellite peaks at integral multiples of photon energy on two sides of the electron energy loss spectrum zero peak; wherein the satellite peak only appears near the time zero and has a short duration, which is approximately the convolution of the electronic pulse width and the laser pulse width; the zero peak of the electron energy loss spectrum determined by the method corresponds to an accurate time zero point, and the time resolution of the zero peak is less than 500 fs.
According to the determined time zero point, diffraction or microscopic image information of the sample corresponding to the time delay between different pump lasers and detection lasers can be accurately recorded; and combining the two to obtain a diffraction or microscopic image of the sample in an ultrafast process after being excited by the pump laser.
According to the method and the ultrafast imaging device, various ultrafast structure change processes of substances occurring on a femtosecond to nanosecond time scale and an atomic space scale can be observed, and the functions of time-resolved ultrafast electron diffraction and images, ultrafast electron energy loss spectroscopy, ultrafast electron holography and the like are realized.
It can realize in-situ laser excitation, ultrafast diffraction and microscopic image measurement of single crystal and polycrystal samples, the time resolution is less than or equal to 300fs,
the energy resolution is less than or equal to 2eV, and the method can be combined with various in-situ techniquesThe electric field, the magnetic field and the high and low temperatures are adjusted, and the temperature range of the sample is 10K-1200K.
The ultrafast transmission electron microscope system can finely test the ultrafast structure change process of a sample under different laser parameters and environmental temperatures, wherein the ultrafast structure change process comprises different excitation wavelengths, pulse widths, laser power, repetition frequency, sample temperature and the like, the collected signals comprise diffraction, microscopic images, energy loss spectrums and the like, and the ultrafast structure change process is analyzed by analyzing the position and the intensity of diffraction peaks, image contrast change and the like.
Although the above-mentioned specific method and steps for determining the zero point of the action time of the pulsed laser and the pump laser and obtaining the ultrafast process image of the sample according to the method are described by, for example, the ultrafast imaging apparatus 200 of the preferred embodiment of the present invention shown in fig. 2, the method is not limited to this, and similarly, the ultrafast imaging apparatus in the prior art shown in fig. 1 may be used in combination with an energy filtering system to obtain an electron energy loss spectrum, and then determine the zero peak and the satellite peak characteristics at both sides, and use the position of the zero peak as the zero point of action corresponding to the moment.
Although the present invention has been described by way of preferred embodiments, the present invention is not limited to the embodiments described herein, and various changes and modifications may be made without departing from the scope of the present invention.
Claims (11)
1. An ultrafast imaging apparatus, comprising: an ultrafast laser for generating a probe laser and a pump laser, wherein the pump laser is used for exciting a sample; a field emission system having a photocathode excited by the detection laser to emit pulsed photoelectrons or thermally emitted thermal field electrons; and an illumination system, an imaging system and a detector;
characterized in that the field emission system further comprises:
a first extraction electrode having a positive potential of a first voltage for separating the pulsed photoelectrons from the photocathode;
the second extraction electrode has a positive potential of a second voltage and is used for driving the pulsed light electrons to move towards the acceleration system in an accelerated mode and focus the pulsed light electrons;
wherein the acceleration system is configured to accelerate the pulsed photo-electrons to a third voltage.
2. The ultrafast imaging apparatus of claim 1,
the first voltage is in the range of 0kV-4.5 kV;
the second voltage ranges from 5kV to 8.5 kV; and
the third voltage is in the range of 80kV-200kV, preferably 80kV,120kV,160kV or 200 kV.
3. The ultrafast imaging apparatus of claim 1, further comprising a laser introduction port disposed between the field emission system and the illumination system and an adjustment lever coupled thereto, the laser introduction port being a cavity having a top wall, a bottom wall and a side wall, the side wall having a mirror adjustment opening therein, and the adjustment lever having a mirror at one end thereof, a portion of the mirror and adjustment lever being sealed within the cavity of the laser introduction port by the mirror adjustment opening and the adjustment lever being translatable or rotatable.
4. The ultrafast imaging device according to claim 3, wherein said laser introducing port further has a first laser introducing window on a side wall thereof and an ion pump interface for externally connecting to a vacuum ion pump, and said top and bottom walls have a first opening and a second opening disposed opposite to each other for passing said probing laser and/or pulsed electrons.
5. An ultrafast imaging device as claimed in claim 3, wherein said mirror is provided with a plurality of parallel slits of different widths in the range of 0.7mm to 1.0 mm.
6. The ultrafast imaging apparatus of claim 3, wherein the laser introducing port is formed of a non-magnetic stainless material or a tungsten bronze material, and the first laser introducing window comprises fused silica glass coated with an ultraviolet antireflection film.
7. The ultrafast imaging apparatus of claim 1, further comprising a beam splitter, first and second laser frequency conversion elements, first and second focusing lenses, first and second laser position monitoring devices, and a retarder, wherein:
the ultrafast laser outputs laser, the laser is divided into two beams by the beam splitter, one beam of the laser generates detection laser by the first laser frequency conversion element, the detection laser enters the laser leading-in port by the first focusing lens, and the first laser position monitoring equipment is used for monitoring the deviation of the spot position of the detection laser incident to the photocathode; and the other laser beam generates pump laser through the second laser frequency conversion element, and the second laser position monitoring equipment is used for monitoring the deviation of the spot position of the pump laser incident on the sample.
8. A method for the ultrafast imaging apparatus of any one of claims 1 to 7, comprising:
(1) precisely irradiating the detection laser on the photocathode and generating the pulse photoelectrons, wherein the pulse photoelectrons irradiate the sample and obtain diffraction or microscopic image information;
(2) irradiating the pump laser and the pulsed photoelectrons at the same position on the sample and generating an ultrafast process;
(3) determining action time zero points of the pulsed photoelectrons and the pump laser and determining a spatial coincidence point of the action time zero points according to the electron energy loss spectrum of the ultrafast process;
(4) and obtaining at least one piece of diffraction or microscopic image information of different time delays between the pump laser and the detection laser according to the determined time zero point and the determined spatial coincidence point so as to obtain a diffraction or microscopic image in the ultrafast process.
9. The use method of the ultrafast imaging apparatus as claimed in claim 8, wherein the step (1) further comprises:
(1.1) determining the photocathode luminescence optical path by causing the photocathode to emit thermal field electrons;
(1.2) adjusting the detection laser to irradiate the photocathode according to the photocathode light-emitting path;
(1.3) precisely irradiating the photo cathode with the detection laser.
10. The use method of the ultrafast imaging apparatus as claimed in claim 8, wherein the step (2) further comprises:
(2.1) determining the luminescence path of the sample according to the emitted fluorescence by using a fluorescent material as the sample;
(2.2) adjusting the pump laser to irradiate the sample according to the sample luminescence route;
(2.3) precisely irradiating the sample with the pump laser.
11. The use method of the ultrafast imaging apparatus according to claim 8, wherein the step of determining the action time zero point of the pulsed photoelectrons and the pump laser according to the electron energy loss spectrum of the ultrafast process in step (3) comprises:
(3.1) taking a multi-walled carbon nanotube as the sample, exciting the multi-walled carbon nanotube to expand radially by using the pump laser in a diffraction mode, and preliminarily determining the action time zero point and the space coincidence point of the pulse photoelectrons and the pump laser by judging the position and the starting time of the sample expanded radially;
and (3.2) determining a zero loss peak and a plurality of satellite peaks at integral multiples of photon energy on two sides of the zero loss peak through an electron energy loss spectrum by utilizing a near-field effect generated on the surface of the sample by the laser, and determining the time corresponding to the zero loss peak as a time zero point, wherein the satellite peaks only appear on two sides of the time zero point and have duration equal to the convolution of the pulse width of the pulsed light electron and the pulse width of the pump laser.
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