CN113654462A - Method and device for monitoring detection light spot position of ultrafast electron microscope - Google Patents
Method and device for monitoring detection light spot position of ultrafast electron microscope Download PDFInfo
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Abstract
The invention relates to a device and a method for monitoring the position of a detection light spot of an ultrafast electron microscope, wherein the device comprises a femtosecond detection laser pulse, a reflector, a laser focusing lens, a beam splitter, a CCD camera and an optical flange window; after being reflected by a reflector, the femtosecond detection laser pulse is focused by a laser focusing lens, and focused laser is incident to a beam splitter and is divided into two beams; a beam of focused laser emitted by the beam splitter is incident into the CCD camera at a certain angle according to the beam splitting ratio to present a light spot image; and another beam of focused laser emitted by the beam splitter is used as a detection laser pulse and is emitted from the optical flange window to the tip of a photocathode filament of the electron gun of the ultrafast electron microscope, and the filament is excited to generate a detection photoelectron pulse. The invention provides a novel convenient way for quickly and accurately obtaining the real-time state of the photoelectron detection pulse generated by the probe laser spot acting on the tip of the filament.
Description
Technical Field
The invention relates to a method and a device for monitoring the detection spot position of an ultrafast electron microscope, and relates to the technical field of ultrafast electron microscope detection.
Background
An ultrafast electron microscope is an ultrafast imaging technology based on a pumping-detecting principle and combining the advantages of high spatial resolution of an electron microscope (a transmission electron microscope or a scanning electron microscope) and high temporal resolution of femtosecond laser, and can simultaneously have ultrahigh spatial and temporal resolution, which is not popular in recent years and becomes an important development direction of an electron microscopy characterization technology. Taking the ultrafast scanning electron microscope technology as an example, the ultrafast scanning electron microscope mainly depends on the pumping-detecting principle of femtosecond pulse laser, and combines the unique advantage that a high-energy electron beam emitted by a scanning electron microscope has short electron wavelength to realize scanning secondary electron imaging with ultrahigh time resolution, and is widely applied to the research of ultrafast carrier dynamics including various materials such as metals, photoelectric semiconductors, organic polymer films and the like. One of the core steps of the ultrafast scanning electron microscope technology is that femtosecond laser pulses in an ultraviolet band penetrate through an optical flange window pre-installed on an electron gun lens barrel of a scanning electron microscope, the tip of an electron gun photocathode is excited by an Einstein photoelectric effect to generate a photoelectron pulse sequence, pulse electrons enter the scanning electron microscope lens barrel in an accelerated mode, and after being converged by an electromagnetic lens and scanned and deflected, the pulse electrons reach the surface of a sample to excite secondary electrons and are collected by a secondary electron detector to perform time-resolved detection. However, since the electron gun itself has a very narrow space and the internal filament is very fine (especially, the radius of curvature of the tip of the field emission filament is in the order of hundreds of nanometers), the position where the detection laser is incident into the electron gun cannot be directly observed by naked eyes. Therefore, it is extremely difficult to focus the uv femtosecond laser to a diameter of only several tens of micrometers and to precisely irradiate a fine filament tip on the order of hundreds of nanometers.
In the existing research of ultrafast electron microscope, ultraviolet detection pulse laser is used to excite electron microscope photocathode to generate pulse photoelectrons, and the method and device mainly use red visible light which is emitted by the photocathode due to heating and directly reflects the filament contour and position to calibrate the light path in the conventional thermal emission or thermal field emission mode. Specifically, red light emitted by the filament sequentially passes through an optical flange window of the electron gun and a focusing lens placed in front of the optical flange window of the electron gun to enter a preset digital microscope or a long-focus camera, and a clear red filament image can be directly displayed on a display screen connected with the camera by finely adjusting the receiving angle and the focus of the digital microscope or the camera; then, according to the principle that two points determine a straight line, two diaphragms with adjustable aperture sizes are sequentially added between the digital microscope and the focusing lens, and the filament red light spots observed in the digital microscope are guaranteed to be uniformly zoomed by taking the center of the diaphragm as the center of a circle when the size of each diaphragm is changed, so that the optical path of the red light emitted by the filament entering the digital microscope through the optical flange and the focusing lens is determined. And finally, according to the reversibility principle of the light path, adjusting a reflector in the light path to enable the ultraviolet detection femtosecond laser to pass through the centers of the two positioning diaphragms, returning to the original path of the light path transmitted by the red light emitted by the filament and injecting the red light into the tip of the filament to generate pulse photoelectrons for detection and imaging. Although this method can excite the photocathode of the electron gun to generate pulsed photoelectrons, because the filament tip is too small, it is still very difficult to determine whether the laser is applied to the filament by only two common positioning diaphragms placed in the light path, and each calibration needs to spend a lot of time to finely adjust the position of the ultraviolet femtosecond laser irradiated on the filament tip by the mirror in the front light path in a blind scanning manner, so the process is rough, complicated, time-consuming and labor-consuming. Because the femtosecond laser light path is easily affected by external conditions such as temperature, humidity and mechanical vibration of the surrounding environment, the directivity of the light path is difficult to keep stable for a long time, and the reflector of the light path needs to be finely adjusted frequently to ensure the optimal pulsed light electron emission efficiency. Because there is no detector capable of directly presenting the specific situation of the incident detection laser spot, it is impossible to further judge whether the detection laser really acts on the tip of the filament all the time to realize the optimal excitation efficiency.
Therefore, in the conventional method for exciting an electron gun photocathode by using ultraviolet femtosecond pulse laser to generate pulse photoelectrons in the ultrafast electron microscopy technology, the difficult problems that the precise positioning cannot be realized and the precise position of the femtosecond detection laser acted on the electron gun photocathode tip cannot be visually monitored exist all the time. In order to obtain the optimal pulsed light electron emission efficiency and ensure that stable pulsed light electron emission is obtained for a long time, the development of a method and a device capable of directly monitoring the real-time position of an ultraviolet femtosecond pulse laser spot is urgently needed, so that an ultrafast electron microscope can stably run for a long time and obtain clear ultrafast dynamic information.
Disclosure of Invention
In view of the above problems, an object of the present invention is to provide a device for monitoring the position of a detection spot of an ultrafast electron microscope, which can realize the function of monitoring the position and shape of a femtosecond detection laser spot on an ultrafast scanning electron microscope or an ultrafast transmission electron microscope in real time, so as to obtain pulsed photoelectrons with optimal yield to detect the ultrafast carrier dynamics process after a sample is excited.
The second objective of the present invention is to provide a method for monitoring the position of the probe spot of the ultrafast electron microscope, and the method can be applied to monitoring the position of the pump spot of the ultrafast electron microscope.
In order to achieve the purpose, the invention adopts the following technical scheme:
in a first aspect, the invention provides a device for monitoring the position of a detection light spot of an ultrafast electron microscope, which comprises a femtosecond detection laser pulse, a reflector, a laser focusing lens, a beam splitter, a CCD camera and an optical flange window;
after being reflected by the reflector, the femtosecond detection laser pulse is focused by the laser focusing lens, and focused laser is incident to the beam splitter and is divided into two beams;
a beam of focused laser emitted by the beam splitter is incident into the CCD camera at a certain angle according to a beam splitting ratio to present a light spot image;
and another beam of focused laser emitted by the beam splitter is used as a detection laser pulse and is emitted from the optical flange window to the tip of a photocathode filament of the electron gun of the ultrafast electron microscope, and the filament is excited to generate a detection photoelectron pulse.
Further, a collimating diaphragm is arranged between the reflecting mirror and the laser focusing lens.
Furthermore, when the optical flange window is positioned below the suppression electrode of the electron gun of the ultrafast electron microscope, the optical flange window further comprises a three-dimensional electric control reflector, and detection laser pulses incident through the optical flange window are reflected to the tip end of the filament of the photocathode through the three-dimensional electric control reflector.
Furthermore, the femtosecond detection laser pulse is obtained by triple frequency or quadruple frequency of the fundamental frequency near-infrared femtosecond laser to obtain the pulse laser of the ultraviolet band.
Furthermore, the laser focusing lens adopts a plano-convex lens made of ultraviolet-grade fused quartz with the front and rear surfaces both plated with ultraviolet antireflection films, and laser is vertically incident from the center of the curved surface of the plano-convex lens and is focused on the filament tip at the rear focus of the plano-convex lens for exciting and generating ultrafast detection photoelectron pulses with the same repetition frequency.
Further, the beam splitter adopts an ultraviolet fused quartz beam splitting piece.
Furthermore, the optical flange window is provided with leaded glass plated with an ultraviolet antireflection film.
Furthermore, the surface of the reflector is plated with a dielectric film or an aluminum film, and the reflector is arranged on the three-dimensional precise electric adjustable mirror frame.
Further, the laser focusing lens is arranged on the three-dimensional small-sized precision mechanical displacement table and used for adjusting the incident angle and position of the detection light path.
In a second aspect, the present invention provides a method for monitoring the position of a probe light spot of an ultrafast electron microscope, comprising:
the femtosecond detection laser pulse is shot near the tip of a photocathode filament of the ultrafast electron microscope through a rough adjusting reflector, whether a photoelectron image appears or not is observed, and if the femtosecond detection laser pulse is shot on the photocathode filament, the photoelectron image is generated;
the angle and the specific position of the femtosecond detection laser hitting the tip of the photocathode filament are regulated and controlled by finely regulating a laser focusing lens, if the resolution ratio of a photoelectron image is relatively highest, the brightness and the contrast are relatively maximum, and the image quality is kept stable, the femtosecond detection laser hitting the tip of the photocathode filament is considered to hit the tip of the photocathode filament, the light spot presented by a CCD camera is determined as the optimal light spot, and the position and the size of the optimal light spot are recorded;
the change condition of spot parameters (position, shape, size, intensity and the like) displayed on a CCD camera is monitored in real time, whether the femtosecond detection laser acts on the tip of the photocathode is judged, if the size and the position of the spot are inconsistent with the optimal spot, the light path is adjusted to ensure that the spot is basically consistent with the optimal spot, namely, the femtosecond laser pulse is always sent to the tip of a filament of the photocathode.
Due to the adoption of the technical scheme, the invention has the following advantages:
1. the invention utilizes the combination device of the ultraviolet focusing lens, the ultraviolet beam splitter and the CCD camera (charge coupled device) to realize the accurate positioning of the body position of the ultrafast photocathode of the electron gun of the detecting pulse laser incidence ultrafast electron microscope, thereby monitoring the position of the detecting pulse laser spot on the ultrafast photocathode of the electron gun in real time and ensuring the ultrafast electron microscope to be in the state of the optimal and stable photoelectron pulse excitation efficiency;
2. the invention can be used for accurately judging whether the femtosecond ultraviolet detection laser strikes the tip of the photocathode filament of the electron microscope, namely, by observing the quality and the position of a light spot presented on a CCD camera in real time, the detection laser is kept to act on the optimal site of the tip of the filament as far as possible by finely adjusting light path components such as a reflector, a focusing lens and the like in an incident light path, and finally the imaging effect of the ultrafast electron microscope is optimal;
in conclusion, the invention provides a novel convenient way for rapidly and accurately obtaining the real-time state of the photoelectron detection pulse generated by the probe laser spot acting on the filament tip, and provides guarantee for finally realizing ultrafast electron microscopic imaging based on a pumping-detection mechanism and analyzing the ultrafast dynamic process related to the structure and energy carriers in the material.
Drawings
Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. Like reference numerals refer to like parts throughout the drawings. In the drawings:
FIG. 1 is a schematic view of a monitoring device for detecting a light spot by an ultrafast electron microscope with a photocathode of an electron gun excited laterally by an ultraviolet femtosecond laser according to an embodiment of the present invention;
FIG. 2 is a schematic view of a monitoring device for detecting light spots by an ultrafast electron microscope with an electron gun photocathode excited by UV femtosecond laser from bottom to top according to an embodiment of the present invention;
FIG. 3 is a flowchart of the method operation of an embodiment of the present invention;
FIG. 4 is a diagram of a probe spot with a diameter of about 50 μm focused on a photocathode according to an embodiment of the present invention.
Detailed Description
Exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be embodied in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
It is to be understood that the terminology used herein is for the purpose of describing particular example embodiments only, and is not intended to be limiting. As used herein, the singular forms "a", "an" and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "including," and "having" are inclusive and therefore specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order described or illustrated, unless specifically identified as an order of performance. It should also be understood that additional or alternative steps may be used.
Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as "first," "second," and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.
For convenience of description, spatially relative terms, such as "inner", "outer", "lower", "upper", and the like, may be used herein to describe one element or feature's relationship to another element or feature as illustrated in the figures. Such spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures.
The invention provides a device and a method for monitoring the detection spot position of an ultrafast electron microscope, wherein the device comprises a femtosecond detection laser pulse, a reflector, a laser focusing lens, a beam splitter, a CCD camera and an optical flange window; after being reflected by a reflector, the femtosecond detection laser pulse is focused by a laser focusing lens, and focused laser is incident to a beam splitter and is divided into two beams; a beam of focused laser emitted by the beam splitter is incident into the CCD camera at a certain angle according to the beam splitting ratio to present a light spot image; and another beam of focused laser emitted by the beam splitter is used as a detection laser pulse and is emitted from the optical flange window to the tip of the cathode filament of the ultrafast electron microscope electron gun, and the filament is excited to generate a detection photoelectron pulse. The invention provides a method and a device for monitoring the accurate action of ultraviolet femtosecond detection laser pulses on the optimal excitation site of the electron gun photocathode tip in an ultrafast electron microscope system in real time, and relates to an ultrafast electron microscope technology which combines a femtosecond laser light path system based on a pumping-detection principle and a field emission or thermal emission high-resolution electron microscope (a transmission electron microscope or a scanning electron microscope), is mainly used for detecting the ultrafast dynamic process of excited energy carriers in materials and can have ultrahigh space-time resolution at the same time.
Example 1
As shown in fig. 1, the device for monitoring the position of a detection spot of an ultrafast electron microscope provided by this embodiment is a detection light path of an ultrafast electron microscope in which an ultraviolet femtosecond laser excites an electron gun photocathode from a side surface, and includes a high repetition frequency femtosecond laser 1, an ultraviolet reflecting mirror 2, collimating diaphragms 3 and 4, an ultraviolet laser focusing lens 5, an ultraviolet beam splitter 6, a CCD camera 7, and an optical flange window 8.
The near-infrared femtosecond pulse laser generated by the high repetition frequency femtosecond laser 1 can be divided into two beams according to a power ratio of 1:1 (for example, but not limited thereto, the beam division ratio can be set as required): one beam of pumping laser obtained by frequency doubling or frequency tripling is generally used for exciting the dynamic process of a sample in an electron microscope, and the other beam of detection laser obtained by frequency doubling or frequency quadrupling and having a wavelength in an ultraviolet band is used for exciting a photocathode of an electron gun of an electron microscope to generate pulse photoelectrons for ultrafast dynamic imaging and detection, and the embodiment mainly focuses on the description of the process, and specifically comprises the following steps:
the femtosecond ultraviolet detection laser pulse is reflected by an ultraviolet reflector 2, then sequentially passes through two collimating diaphragms 3 and 4, and is focused in an ultraviolet band by an ultraviolet laser focusing lens 5, the focused laser is incident on an ultraviolet beam splitter 6 at a set angle such as 45 degrees and is divided into two beams according to a certain power ratio (the reflection and transmission ratio can be 1:9 or 2:8 and the like without limitation), and one beam of focused laser reflected by the ultraviolet beam splitter 6 is vertically incident into a CCD camera 7 to present a facula image; the transmitted laser through the ultraviolet beam splitter 6 is used as a detection laser pulse and is normally incident on a photocathode filament tip 9 of an electron gun of an electron microscope from an optical flange window 8, and the filament tip is excited by utilizing the Einstein photoelectric effect to generate a detection photoelectron pulse.
In some preferred embodiments of the present invention, the femtosecond detection laser pulse for exciting the photocathode of the electron microscope electron gun is a pulse laser of an ultraviolet band (which can meet the work function requirement of most photocathode filament materials) obtained by triple or quadruple frequency of the fundamental frequency near-infrared femtosecond laser (1030nm), and meets the basic conditions of a certain pulse width, repetition frequency, single pulse energy, polarization direction, etc., for example, the pulse width range may be 250-300fs, the repetition frequency range may be 200KHz-25MHz, the single pulse energy may be 1-10nJ, and the polarization direction is parallel to the filament, i.e., the vertical polarization direction.
In some preferred embodiments of the present invention, the ultraviolet laser focusing lens 5 is a plano-convex lens made of ultraviolet-level fused quartz with both front and back surfaces coated with ultraviolet antireflection films, which allows the deep ultraviolet band laser to transmit completely and effectively avoids the generation of laser-induced fluorescence. The detection laser is vertically incident from the center of the curved surface of the plano-convex lens and is focused on the filament tip at the rear focus of the plano-convex lens, and is used for exciting and generating ultrafast detection photoelectron pulses with the same repetition frequency. Because the area of the focusing light spot is small and the average power density is high, the device can effectively excite and generate pulse photoelectrons for imaging and detecting dynamic information.
In some preferred embodiments of the present invention, the electron gun photocathode is primarily directed to a schottky thermal field emission photocathode (which may also be a conventional thermal emission tungsten cathode, yttria iridium (Y)2O3-Ir) cathode, lanthanum hexaboride (LaB)6) Cathode, schottky cold field emission cathode, etc., by way of example and not limitation), with a hot field cathode filament mounted at the center of the gate hole and at a suitable distance, the filament being of single-crystal tungsten W and having a tip (radius of curvature of about 300nm to 500nm) resembling a "Y" shaped needle tip and coated with a small cluster of metal oxide (e.g., zirconia ZrO, for the purpose of effectively reducing the electron work function in single-crystal tungsten to about 2.5eV) near the tip. Since the work function of an electron after photoexcitation is generally affected by the crystal orientation of the surface of the material from which it has exited, a schottky thermal field emission cathode is generally used<100>Crystal orientation, thereby obtaining the lowest possible work function and high photoelectron emissivity.
In some preferred embodiments of the present invention, the uv beam splitter 6 is an uv fused quartz beam splitter, which is installed between the uv laser focusing lens 5 and the optical flange window 8, and has a certain splitting ratio when the detection femtosecond laser is incident on the surface thereof at a certain angle (for example, usually at an angle of 45 °), and the electromagnetic wave response band of the uv fused quartz beam splitter mainly covers about 200nm to 450nm, and has low light absorption rate in this wavelength range and good thermal stability. Generally, one side of the ultraviolet fused quartz beam splitting piece is flat, the other side of the ultraviolet fused quartz beam splitting piece is provided with a bulge with a certain radian, and the whole ultraviolet fused quartz beam splitting piece is wedge-shaped, so that unnecessary interference caused by reflection of the front surface and the rear surface can be effectively reduced.
In some preferred embodiments of the invention, a CCD camera 7 is mounted on an additional bread board outside the electron gun near the optical flange window 8 for collecting another small fraction of the reflected split laser light that is split off from the probe laser beam at the excitation light cathode by the uv beam splitter 6. Because the small part of the beam-splitting laser and the laser acting on the tip of the photocathode come from the same main light path, the beam-splitting laser collected by the CCD camera 7 can completely reflect the real state of the tip of the photocathode under the action of the detection laser in real time.
In some preferred embodiments of the present invention, leaded glass coated with an ultraviolet antireflection film is mounted on the optical flange window 8.
In some preferred embodiments of the present invention, the surface of the ultraviolet reflecting mirror 2 is plated with a dielectric film, for example, a multilayer dielectric film (which is totally reflective to ultraviolet laser light and has high resistance to laser light) or an aluminum film with different refractive indexes is plated on a BK7 substrate with a surface precision of λ/10. Further, the ultraviolet reflector 2 can be arranged on the three-dimensional precise electric adjustable lens frame and used for initially adjusting the incident angle and position of detection laser when a detection light path is built, so that the detection laser basically reaches the filament after passing through the centers of the collimation diaphragm 3, the collimation diaphragm 4 and the ultraviolet laser focusing lens 5 respectively, and the ultraviolet reflector is used for calibrating and optimizing the optimal position point where the detection laser hits the tip of the filament at any time during the use period after the light path is built, and the fine regulation and control can be realized through the electric lens frame due to the fact that the tip of the filament is very small and the manual error is large.
In some preferred embodiments of the present invention, the size of the opening of the collimating diaphragms 3 and 4 can be adjusted to determine the incident direction of the detection laser and fix the detection light path to facilitate the subsequent collimation of the light path at any time.
In some preferred embodiments of the present invention, the ultraviolet laser focusing lens 5 is placed on the three-dimensional small-sized precise electric displacement stage 10, the incident angle and position of the detection light path are finely adjusted and controlled by adjusting the X axis and the Y axis of the electric displacement stage 10, so that the ultraviolet laser focusing lens can vertically pass through the center of the ultraviolet laser focusing lens 5 and converge to the tip of the filament, and the detection laser focusing position is changed by adjusting the Z axis of the electric displacement stage 10 so that the tip of the filament is located near the focus as much as possible, thereby improving the excitation efficiency of photoelectrons.
Example 2
The device for monitoring the detection light spot position of the ultrafast electron microscope provided by the embodiment is a detection light path of the ultrafast electron microscope, wherein a detection laser light path excites an electron gun photocathode from bottom to top, the difference from embodiment 1 is that the photoelectron excitation mode is such that the focused detection laser is reflected upward to the tip of the photocathode through the fixed mirror 11 placed in the lens barrel as shown in fig. 2 to excite the pulsed photoelectron, therefore, the detection laser spot monitoring device constructed for the excitation mode in the embodiment is that detection laser enters the reflecting mirror 11 after being focused by the ultraviolet laser focusing lens 5 and is firstly divided into two beams by the ultraviolet beam splitter 6, the reflected beam enters the CCD camera 7, and the transmitted beam passes through the optical flange window 8 on the side of the lens barrel (at this time, the optical flange window 8 is located below the suppression electrode 12), and then enters the reflector 11 to be reflected upward as the detection laser pulse excitation light cathode to generate the detection pulse electron beam. Similarly, the laser spot of the reflected beam on the screen of the CCD camera 7 can reflect the actual situation of the transmitted beam acting on the tip of the filament in real time, and the excitation of the optimal pulsed photon beam can be maintained by adjusting the ultraviolet mirror 2 and the ultraviolet laser focusing lens 5 on the mechanical displacement stage.
Example 3
The invention can completely copy the situation of the transmission laser spot in the electron gun by using the reflected laser spot which is displayed by the CCD camera 7 in real time, realizes the complete visualization of the situation of the detection photoelectrons generated by the detection laser acting on the photocathode, replaces the original low-efficiency mode of blindly finding the tip of the filament by scanning the detection laser in a large range, and can be used as an accurate basis for efficiently and directly monitoring the real situation of the detection laser spot acting on the tip of the photocathode.
As shown in fig. 3, the method for monitoring the position of the probe spot of the ultrafast electron microscope according to the embodiment includes the steps of:
s1, by coarsely adjusting the motor-driven mirror 2, the uv femtosecond laser is applied near the tip of the photocathode filament, where a photoelectron image may appear, which would be generated if applied to the photocathode filament.
Specifically, the filament does not emit thermal electron for imaging by setting the filament parameters on the existing electron microscope operating software, and can only image by detecting photoelectrons generated by laser excitation, namely, whether the detection laser is applied to the filament or not is judged by switching the detection laser, and whether photoelectron images appear or not is judged: when the detection laser is turned off, the window of the electron microscope is completely black, and no secondary electron image exists; when the detection laser is turned on, a secondary electron image appears in an electron microscope window at once, at this time, the detection laser is irradiated on a photocathode filament to excite pulsed photoelectrons to obtain a photoelectron image, and at this time, the situation of poor image quality may occur, wherein the poor image quality means that the resolution of the image is not high, the overall brightness and contrast of the image are low and the like because photoelectron signals are weak.
And S2, finely adjusting the ultraviolet laser focusing lens 5, and regulating and controlling the angle and the specific position of the ultraviolet femtosecond laser on the tip 12 of the photocathode filament.
Specifically, the incident angle and position of a detection light path are finely regulated and controlled by adjusting the X axis and the Y axis of the electric displacement table 10, so that the detection light path can vertically penetrate through the center of the ultraviolet laser focusing lens 5 and converge to the tip of the filament, the detection laser focusing position is changed by adjusting the Z axis of the electric displacement table 10, the tip of the filament is located near the focus of the lens as far as possible, and the photoelectron excitation efficiency is improved.
If the resolution of the photoelectron image is relatively highest, the brightness and the contrast are relatively maximum, and the image quality is kept stable, the photoelectron excitation efficiency is highest when the filament tip is hit, and if the image is not hit at the filament tip, the photoelectron signal is weak and the image quality is poor. Because the incident light has different refractive indexes and different focusing degrees in different thicknesses of the focusing lens, the method can finally realize excitation and generation of a large number of pulse photoelectrons by the fine adjustment method, namely, the excitation is optimized to the state of the photoelectron image with the maximum contrast, the strongest brightness and the highest resolution, and the clearer ultrafast carrier dynamics information can be detected.
And S3, when the image quality is kept to be optimal and stable for a long time, determining the laser beam to hit the tip of the photocathode filament, determining the light spot presented by the CCD camera 7 as the optimal light spot, recording the position and the size of the optimal light spot, monitoring the change of the light spot parameters of the CCD camera 7 in real time during use, judging whether the femtosecond detection laser beam acts on the tip of the photocathode filament, and if the size and the position of the light spot are not consistent with the optimal light spot, adjusting the light path to enable the light path to be consistent with the optimal light spot, namely ensuring that the femtosecond laser pulse always hits the tip of the photocathode filament.
Specifically, the photoelectron image is calibrated on a computer screen synchronously connected with the CCD camera 7 by using a small circle to show the position and the size of the optimal light spot (the optimal light spot is relatively most circular in shape and has Gaussian distribution and relatively strong intensity), the two laser beams obtained by beam splitting are from the same laser beam, so the behaviors and the properties of the two laser beams are consistent, that is, although the two light spots finally focused are not the same, the change trends including the shape, the size, the position, the intensity distribution and the like of the light spot are the same, the change trends can be used as a monitoring standard, the real-time monitoring and detection of the position, the shape, the intensity and the like of the light spot can be realized through the circle, and most importantly, the laser beam is accurately shot at the tip of a photocathode filament as long as the light spot is adjusted to the position calibrated by the circle in the following process. The reason is that the transmitted laser beam incident on the photocathode filament and the reflected laser beam simultaneously entering the CCD camera 7 obtained by splitting the detection laser beam are both from the same pulse laser beam, and when the main optical path is changed (by adjusting the ultraviolet reflecting mirror 2 or the ultraviolet laser focusing lens 5), the change is completely consistent, including the basic parameters such as the position, shape, size and intensity of the light spot, and the main laser beam does not pass through other optical elements after being split once, so that the distortion of any information can be effectively avoided. As shown in fig. 4, the light spots of the uv femtosecond probe laser with a diameter of about 50 μm focused on the photocathode, which are displayed on the screen of the CCD camera 7 in real time, include information such as the position, shape, size, and intensity distribution of the light spots, so that the position and size of the uv femtosecond laser acting on the tip of the photocathode can be directly judged, so as to adjust the incidence direction and position point of the probe laser in time, and finally, the imaging quality of the probe photoelectron pulse is optimized, and further, clear ultrafast dynamics information is obtained.
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: it is to be understood that modifications may be made to the above-described arrangements in the embodiments or equivalents may be substituted for some of the features of the embodiments without departing from the spirit or scope of the present invention.
The method can also be popularized to the information of monitoring the position, the shape, the size, the intensity distribution and the like of the pumping light spot of the ultrafast electron microscope.
Claims (10)
1. A device for monitoring the detection spot position of an ultrafast electron microscope is characterized by comprising a femtosecond detection laser pulse, a reflector, a laser focusing lens, a beam splitter, a CCD camera and an optical flange window;
after being reflected by the reflector, the femtosecond detection laser pulse is focused by the laser focusing lens, and focused laser is incident to the beam splitter and is divided into two beams;
a beam of focused laser emitted by the beam splitter is vertically incident into the CCD camera to present a light spot image;
and another beam of focused laser emitted by the beam splitter is used as a detection laser pulse and is emitted from the optical flange window to the tip of a photocathode filament of the electron gun of the ultrafast electron microscope, and the filament is excited to generate a detection photoelectron pulse.
2. The device for monitoring the position of the probe light spot by the ultrafast electron microscope according to claim 1, wherein a collimating diaphragm is disposed between the reflector and the laser focusing lens.
3. The apparatus according to claim 1, further comprising a three-dimensional electrically controlled mirror, when the optical flange window is located below the suppressor of the ultrafast electron microscope electron gun, wherein the detection laser pulses incident through the optical flange window are reflected by the three-dimensional electrically controlled mirror to the tip of the filament of the photocathode.
4. The apparatus according to claim 1, wherein the femtosecond probe laser pulse is obtained by tripling or quadrupleing a fundamental frequency near-infrared femtosecond laser to obtain a pulse laser in an ultraviolet band.
5. The device for monitoring the detection spot position of the ultrafast electron microscope according to any one of claims 1 to 4, wherein the laser focusing lens is a plano-convex lens made of ultraviolet-grade fused silica with ultraviolet antireflection films coated on the front and back surfaces, and the laser is vertically incident from the center of the curved surface of the plano-convex lens and focused on the filament tip at the back focal point of the plano-convex lens for exciting and generating ultrafast detection photoelectron pulses with the same repetition frequency.
6. The device for monitoring the position of the detection light spot of the ultrafast electron microscope according to any one of claims 1 to 4, wherein the beam splitter adopts an ultraviolet fused quartz beam splitter.
7. The device for monitoring the position of a detection light spot of an ultrafast electron microscope according to any one of claims 1 to 4, wherein a leaded glass coated with an ultraviolet antireflection film is mounted on the optical flange window.
8. The device for monitoring the position of a detection light spot of an ultrafast electron microscope according to any one of claims 1 to 4, wherein the surface of the reflector is plated with a dielectric film or an aluminum film, and the reflector is arranged on the three-dimensional precise electrically adjustable frame.
9. The device for monitoring the position of the detection light spot of the ultrafast electron microscope according to the claim 1 to 4, wherein the laser focusing lens is arranged on the three-dimensional small-sized precision mechanical displacement platform for adjusting the incident angle and position of the detection light path.
10. A method for monitoring the position of a detection light spot of an ultrafast electron microscope is characterized by comprising the following steps:
the femtosecond detection laser pulse is shot near the tip of a photocathode filament of the ultrafast electron microscope through a rough adjusting reflector, whether a photoelectron image appears or not is observed, and if the femtosecond detection laser pulse is shot on the photocathode filament, the photoelectron image is generated;
the angle and the specific position of the femtosecond detection laser hitting the tip of the photocathode filament are regulated and controlled by finely regulating a laser focusing lens, if the resolution ratio of a photoelectron image is relatively highest, the brightness and the contrast are relatively maximum, and the image quality is kept stable, the femtosecond detection laser hitting the tip of the photocathode filament is considered to hit the tip of the photocathode filament, the light spot presented by a CCD camera is determined as the optimal light spot, and the position and the size of the optimal light spot are recorded;
the change condition of the spot parameters displayed on the CCD camera is monitored in real time, whether the femtosecond detection laser acts on the tip of the photocathode is judged, if the size and the position of the spot are inconsistent with the optimal spot, the light path is adjusted to keep consistent with the optimal spot, and the femtosecond laser pulse is ensured to always hit the tip of the filament of the photocathode.
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