CN114883163B - Transmission type semiconductor photocathode with high quantum efficiency and low intrinsic emittance and method - Google Patents
Transmission type semiconductor photocathode with high quantum efficiency and low intrinsic emittance and method Download PDFInfo
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Abstract
The invention discloses a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance and a method. The invention adopts the transmission type photocathode with the light trapping structure, the light trapping structure is a groove array with a periodic structure etched on the lower surface of the transparent conductive substrate, and the reflection of the substrate to incident laser is reduced, so that the absorption of the incident laser is improved; the surface of the transparent conductive substrate is provided with a light trapping structure which can change the transmission behavior of incident laser, reduce the reflectivity of the transparent conductive substrate and increase the absorption of the semiconductor photocathode film layer on incident light; by adopting laser back incidence, photoelectrons are fully thermalized after having longer transmission distance before emission, so that the transverse average energy and the intrinsic emittance of electron beams generated by a photocathode are reduced; the invention adopts a mode of driving the photocathode by the transmission laser, is beneficial to obtaining the electron beam with low emittance, and has important significance for hard X-ray free electron laser, ultrafast electron diffraction and other aspects.
Description
Technical Field
The invention relates to an electron beam emission technology, in particular to a light trapping structure transmission type semiconductor photocathode with high quantum efficiency and low intrinsic emittance and an implementation method thereof.
Background
The shortest wavelength that can be achieved by the Free Electron Laser (FEL) is limited by the emittance of the electron beam, and the decrease of the emittance is also beneficial to reducing the cost of the FEL, so the low emittance electron beam generated by the photocathode is very important for the Free Electron Laser (FEL). Continuous wave operation (CW) or high repetition rate is one of the current trends in X-ray free electron laser (XFEL), which puts new demands on the electron beam, both low emittance and high repetition rate. For example, LCLS-II from the national laboratory SLAC, USA, requires injectors capable of providing electron beams with a maximum beam charge of 300 pC, an emittance of less than 0.60 mm. The high-coherence and high-brightness light source is widely applied to various fields of material science, condensed state physics, chemistry, biology and the like and is used for analyzing the reaction process of 0.1-1 nm scale and fs magnitude. High brightness Free Electron Lasers (FEL) have created a need for photocathodes with high quantum efficiency, low emittance, and long lifetime. The alkali metal photocathode has the advantages that the high quantum efficiency (1-10%), the response wavelength as a visible light wave band and the long service life are the optimal choice. Various types of electron guns, such as dc electron guns, SRF electron guns, and ambient temperature radio frequency electron guns, are designed to use an alkali metal photocathode as the photocathode or as a backup cathode.
The main indicators of photocathodes include quantum efficiency, intrinsic emittance, and lifetime. Current metal photocathodes have low quantum efficiency, low intrinsic emittance, and long lifetime. In semiconductor photocathodes, GaAs has high quantum efficiency and low intrinsic emission, but has a very short lifetime and needs to be preserved
Under vacuum conditions of (1); alkali metal antimonide and telluride photocathodes have high quantum efficiency, low intrinsic emittance, long lifetime compared to GaAs, but low lifetime compared to metal photocathodes. Alkali metal photocathodes in contrast to other types of photocathodes, e.g. Cu, GaAs, Cs 2 Te, etc., has the following advantages:
1. low intrinsic emission, high quantum efficiency (around 10%);
2. at 10 -8 The service life of the product is long under the vacuum condition of Pa and can reach months;
3. the potential of applying the method to cathode surface engineering can provide an electron source design meeting the requirements of users.
With the development of electron gun technology and the continuous reduction of beam current emittance, the research on the increase of emittance caused by space charge effect and radio frequency field has become mature, the intrinsic emittance gradually becomes an important factor influencing the electron beam emittance, and therefore, the research on reducing the intrinsic emittance of photocathode becomes a very important direction in the field of low-emittance electron guns.
Degree of electron beam emission
The expression of (a) is:
wherein
Indicating photocathodeIntrinsic emittance, which is the main source of electron beam emittance, currently accounts for 70-90% of the final electron beam emittance;
the emittance increase caused by the space charge effect is divided into two parts of linear force and nonlinear force, wherein the emittance increase caused by the linear force can be compensated by a solenoid, and the emittance increase caused by the nonlinear force can be reduced only by shaping laser;
the emittance increase caused by the RF field can be reduced by calibration and precise mounting of the radio frequency structure and the microwave circuit.
From the above analysis, it can be seen that the intrinsic emittance and the contribution of the nonlinear space charge force contribute to the emittance of the electron beam in a large proportion, both of which are generated at the source of the electron gun. Therefore, this can be reduced only from the angle of the photocathode and the drive laser, thereby reducing the emittance of the final electron beam.
The international research on reducing the intrinsic emittance of the semiconductor photocathode is just started, and the related research is mostly the work within the recent years. Although some progress has been made in reducing the intrinsic emittance of photocathodes, there are also problems: 1) the intrinsic emittance is still high. For example, the intrinsic emittance of the current double-alkali photocathode is 0.5 mm. mrad/mm at room temperature, and the intrinsic emittance of the photocathode is expected to be further reduced in various applications (XFEL and the like); 2) although the method of driving the laser by adopting the specific wavelength can obtain lower intrinsic emittance, the repeatability is poor, and the method is difficult to be applied to beam experiments; 3) the intrinsic emittance reduction mechanism is not clear yet, and effective guidance for the research of reducing the emittance is lacked; 4) this results in a decrease in the quantum efficiency of the photocathode while reducing the intrinsic emittance.
Many experiments at home and abroad can reduce the quantum efficiency while trying to reduce the intrinsic emittance of the photocathode, and because the quantum efficiency directly corresponds to the size of the beam current led out by the electron gun, the reduction of the quantum efficiency means the reduction of the led-out electron beam current, and the performance of the electron gun can be influenced. Therefore, how to maintain the high quantum efficiency of photocathodes while reducing the intrinsic emission degree, and finding a photocathode with both low intrinsic emission degree and high quantum efficiency is an important research direction in the field of electron sources.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a transmission type semiconductor photocathode with high quantum efficiency and low intrinsic emittance and an implementation method thereof, wherein the structure design is carried out on the substrate of the photocathode through FDTD, and the optical trap type transmission photocathode with a light trapping structure is prepared, so that the reflection of light can be reduced, and most incident light is absorbed by the substrate and thus by optical materials; meanwhile, the back incidence structure enables photoelectrons generated by the photocathode to be transmitted for a longer distance before being emitted, so that the transverse average energy of the electrons is reduced, and the intrinsic emittance is reduced.
One objective of the present invention is to provide a transmissive semiconductor photocathode with high quantum efficiency and low intrinsic emittance.
The invention provides a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, comprising: the light trapping structure comprises a semiconductor photocathode thin film layer, a transparent conductive substrate and a light trapping structure; forming a semiconductor photocathode film layer on the upper surface of the transparent conductive substrate; the lower surface of the transparent conductive substrate is used as the back of a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and a groove array with a periodic structure is etched on the lower surface of the transparent conductive substrate, so that a light trapping structure is formed on the lower surface of the transparent conductive substrate; the shape of the groove is a partial cylinder or a cuboid which is symmetrical about the central plane of the groove, the part of the groove which is in the shape of the partial cylinder and accounts for the cylinder is not more than a half cylinder, the section is an arc, the central angle of the arc is 2 theta, theta is more than 60 degrees, the period of the groove which is in the shape of the partial cylinder is more than or equal to the width of the groove, the period of the groove which is in the shape of the cuboid is more than the width of the groove, the section of the groove which is in the shape of the cuboid is rectangular, the short side of the rectangle is used as the bottom of the groove, and the long side of the rectangle is used as the side wall of the groove; the transmission type semiconductor photocathode with high quantum efficiency and low intrinsic emittance is positioned in a vacuum environment in the electron gun;
the laser is vertically incident from the back of a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and the laser is reflected and refracted at the interface of a vacuum-transparent conductive substrate; for the groove in the shape of a partial cylinder, a critical distance d determined by the radius R and the central angle of a circular arc exists according to the distance from the incident point of the laser to the central plane of the groove, and d = Rsin ((pi-theta)/3), when the distance from the incident point of the laser to the central plane of the groove is smaller than the critical distance d, the incident angle of the laser when reaching the interface of the vacuum-transparent conductive substrate is smaller than (pi-theta)/3, and the angle of reflected light is smaller than (pi-theta)/3, according to the theorem of the sag in the circle, the reflected light directly leaves the groove, and the laser only undergoes single reflection and refraction; when the distance from the incident point of the laser to the central plane of the groove is greater than the critical distance d, the incident angle when the laser reaches the interface of the vacuum-transparent conductive substrate is greater than (pi-theta)/3, and the angle of the reflected light is also greater than (pi-theta)/3 at this moment, according to the circle sag theorem, the reflected light reaches the interface of the vacuum-transparent conductive substrate again and is reflected and refracted, wherein the refracted light enters the transparent conductive substrate for transmission, so that the total laser power entering the transparent conductive substrate is increased; for the grooves in the shape of a cuboid, the groove array forms a grating structure, so that when laser is incident to the interface of the vacuum-transparent conductive substrate, high-order diffraction light including refraction diffraction and reflection diffraction is generated due to diffraction effect, and for the refraction light, the grating structure increases the efficiency of refraction diffraction, so that more laser enters the transparent conductive substrate at a higher diffraction angle for transmission; for reflected light, when the incident point of the laser is positioned between the grooves, the reflected diffracted light directly leaves the vacuum-transparent conductive substrate interface, and the laser only undergoes single reflection and refraction at the moment; when the incident point of laser is positioned in the groove, namely the short side of the rectangle, because the groove has a certain depth, the reflection angle of the reflected diffracted light of the high-grade diffracted light is not zero, so that the laser is obliquely incident to the side wall of the groove, namely the long side of the rectangle, and the vacuum-transparent conductive substrate interface at the side wall is reflected and refracted again, wherein the refracted light enters the transparent conductive substrate for transmission, the reflected light continues to be transmitted towards the vacuum, and the reflected light is reflected and refracted when reaching the side wall of the groove every time, so that the total laser power entering the transparent conductive substrate is increased, and the transmittance of the incident laser at the vacuum-transparent conductive substrate interface is increased;
for the laser transmitted in the transparent conductive substrate, when the laser reaches the interface of the transparent conductive substrate-the semiconductor photocathode thin film layer, the laser also generates reflection and refraction, wherein the reflected light is transmitted towards the interface of the vacuum-transparent conductive substrate, the reflection and refraction are generated again at the interface of the vacuum-transparent conductive substrate, and the refracted light enters the semiconductor photocathode thin film layer for transmission and is absorbed by the semiconductor photocathode thin film layer to generate photoelectrons; reflected light is transmitted inside the transparent conductive substrate, and due to the fact that the light trapping structure is arranged at the interface of the vacuum-transparent conductive substrate, laser transmitted inside the transparent conductive substrate reaches the interface of the transparent conductive substrate and the semiconductor photocathode thin film layer at different incidence angles, and when the reflection occurs at the interface of the transparent conductive substrate and the semiconductor photocathode thin film layer, the reflected laser is transmitted to the interface of the vacuum-transparent conductive substrate at different incidence angles; when the incident angle is larger than the critical angle of the laser in the transparent conductive substrate, the part of reflected light is totally reflected and is transmitted to the semiconductor photocathode film layer again; therefore, due to the existence of the light trapping structure, the reverse escape of the reflected laser reaching the interface of the vacuum-transparent conductive substrate is weakened, and the power of the incident laser entering the semiconductor photocathode film layer is increased;
when laser enters the semiconductor photocathode thin film layer, one part of the laser is absorbed by the semiconductor photocathode thin film layer to generate photoelectrons, and the other part of the laser penetrates through the semiconductor photocathode thin film layer to enter vacuum; because the light trapping structure is arranged at the interface of the vacuum-transparent conductive substrate, laser vertically incident to the transparent conductive substrate has different incident angles corresponding to different incident points for the groove in the shape of a partial cylinder, so that the refraction angle is increased, and for the groove in the shape of a cuboid, the diffraction effect of the grating structure formed by the groove in the cuboid generates high-grade diffraction light, so that the refraction angle of the refraction light entering the transparent conductive substrate for transmission is increased by the light trapping structure; for the refracted light entering the transparent conductive substrate for transmission, the incident angle is increased when the refracted light is transmitted to the interface of the transparent conductive substrate and the semiconductor photocathode thin film layer, and the refraction angle when the refraction occurs is also increased due to the increase of the incident angle, so that the refracted light at the interface of the transparent conductive substrate and the semiconductor photocathode thin film layer has a larger refraction angle, and the refracted light at the interface of the transparent conductive substrate and the semiconductor photocathode thin film layer enters the semiconductor photocathode thin film layer at a larger angle due to the existence of the light trapping structure; because the emergent power of the incident laser exponentially attenuates along with the increase of the optical path in the substance, the laser transmitted in the semiconductor photocathode thin film layer has a longer optical path under the condition that the thickness of the semiconductor photocathode thin film layer is not changed due to a larger refraction angle, so that the absorption rate of the semiconductor photocathode thin film layer on the incident laser is increased; due to the existence of the light trapping structure, the refracted light experiences longer transmission distance before reaching the semiconductor photocathode film layer-vacuum interface, so that the absorbed power is increased, more photoelectrons are generated in the semiconductor photocathode film layer, and the quantum efficiency is increased; meanwhile, for photoelectrons generated in the semiconductor photocathode thin film layer, before reaching a semiconductor photocathode thin film layer-vacuum interface, collision loss energy is experienced in the transmission process, namely thermalization is generated, a longer transmission distance is experienced, so that the loss energy is more, the thermalization is more sufficient, the lateral average energy of the emitted free electrons is also smaller, and the corresponding intrinsic emittance is lower, so that the lateral average energy and the intrinsic emittance of electron beams generated by the transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance are reduced.
The period of the groove in the shape of a partial cylinder is 5-500 nm; the width of the groove is 5-500 nm; the recess in the shape of a partial cylinder occupies a portion of the cylinder greater than 1/6 semicylinders. The period of the groove in the shape of a cuboid is 20-600 nm; the width of the groove, namely the short side of the rectangle, is 5-400 nm, and the depth, namely the long side of the rectangle, is 5-500 nm. The distance between the bottom end of the groove and the photocathode film layer is 0.2-2 mm.
The semiconductor photocathode thin film layer adopts an alkali metal photocathode thin film layer, and one of tellurium cesium, antimony potassium cesium, antimony sodium cesium, antimony rubidium cesium, antimony sodium potassium cesium and antimony sodium potassium rubidium cesium; the thickness is 40 to 200 nm.
The transparent conductive substrate is made of one of strontium titanate, lithium titanate, quartz glass plated with indium tin oxide, quartz glass doped with zinc oxide and graphene; the diameter is 10-50 mm circular, or the side length is 10-50 mm square.
Another objective of the present invention is to provide a method for implementing a transmissive semiconductor photocathode with both high quantum efficiency and low intrinsic emittance.
The invention discloses a method for realizing a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, which comprises the following steps:
1) preparing a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance:
a) providing a transparent conductive substrate;
b) forming a semiconductor photocathode film layer on the upper surface of the transparent conductive substrate;
c) the lower surface of the transparent conductive substrate is used as the back of a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and a groove array with a periodic structure is etched on the lower surface of the transparent conductive substrate, so that a light trapping structure is formed on the lower surface of the transparent conductive substrate;
d) the shape of the groove is a partial cylinder or a cuboid which is symmetrical about the central plane of the groove, the part of the groove which is in the shape of the partial cylinder and accounts for the cylinder is not more than a half cylinder, the section is an arc, the central angle of the arc is 2 theta, theta is more than 60 degrees, the period of the groove which is in the shape of the partial cylinder is more than or equal to the width of the groove, the period of the groove which is in the shape of the cuboid is more than the width of the groove, the section of the groove which is in the shape of the cuboid is rectangular, the short side of the rectangle is used as the bottom of the groove, and the long side of the rectangle is used as the side wall of the groove;
e) the transmission type semiconductor photocathode with high quantum efficiency and low intrinsic emittance is positioned in a vacuum environment in the electron gun;
2) the laser is vertically incident from the back of a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and the laser is reflected and refracted at the interface of a vacuum-transparent conductive substrate; for a groove in the shape of a partial cylinder, a critical distance d determined by the radius R and the central angle of a circular arc exists according to the distance from the incident point of laser to the central plane of the groove, and d = Rsin ((pi-theta)/3), when the distance from the incident point of the laser to the central plane of the groove is smaller than the critical distance d, the incident angle of the laser when reaching the interface of the vacuum-transparent conductive substrate is smaller than (pi-theta)/3, and the angle of reflected light is smaller than (pi-theta)/3, according to the principle of the sag in the circle, the reflected light directly leaves the groove, and the laser only has single reflection and refraction; when the distance from the incident point of the laser to the central plane of the groove is greater than the critical distance d, the incident angle when the laser reaches the interface of the vacuum-transparent conductive substrate is greater than (pi-theta)/3, and the angle of the reflected light is also greater than (pi-theta)/3 at this moment, according to the circle sag theorem, the reflected light reaches the interface of the vacuum-transparent conductive substrate again and is reflected and refracted, wherein the refracted light enters the transparent conductive substrate for transmission, so that the total laser power entering the transparent conductive substrate is increased; for the grooves in the shape of a cuboid, the groove array forms a grating structure, so that when laser is incident to the interface of the vacuum-transparent conductive substrate, high-order diffraction light including refraction diffraction and reflection diffraction is generated due to diffraction effect, and for the refraction light, the grating structure increases the efficiency of refraction diffraction, so that more laser enters the transparent conductive substrate at a higher diffraction angle for transmission; for reflected light, when the incident point of the laser is positioned between the grooves, the reflected diffracted light directly leaves the vacuum-transparent conductive substrate interface, and the laser only undergoes single reflection and refraction at the moment; when the incident point of the laser is positioned in the groove, namely the short side of the rectangle, because the groove has a certain depth, the reflection angle of the reflected diffraction light of the high-level diffraction light is not zero, so that the laser is obliquely incident to the side wall of the groove, namely the long side of the rectangle, and the reflection and refraction occur again at the vacuum-transparent conductive substrate interface at the side wall, wherein the refracted light enters the transparent conductive substrate for transmission, the reflected light continues to be transmitted towards the vacuum, and the reflection and refraction occur when the reflected light reaches the side wall of the groove every time, so that the total laser power entering the transparent conductive substrate is increased, and the transmittance of the incident laser at the vacuum-transparent conductive substrate interface is increased;
3) for the laser transmitted in the transparent conductive substrate, when the laser reaches the interface of the transparent conductive substrate-the semiconductor photocathode thin film layer, the laser also generates reflection and refraction, wherein the reflected light is transmitted towards the interface of the vacuum-transparent conductive substrate, the reflection and refraction are generated again at the interface of the vacuum-transparent conductive substrate, and the refracted light enters the semiconductor photocathode thin film layer for transmission and is absorbed by the semiconductor photocathode thin film layer to generate photoelectrons; reflected light is transmitted inside the transparent conductive substrate, and due to the fact that the light trapping structure is arranged at the interface of the vacuum-transparent conductive substrate, laser transmitted inside the transparent conductive substrate reaches the interface of the transparent conductive substrate and the semiconductor photocathode thin film layer at different incidence angles, and when the reflection occurs at the interface of the transparent conductive substrate and the semiconductor photocathode thin film layer, the reflected laser is transmitted to the interface of the vacuum-transparent conductive substrate at different incidence angles; when the incident angle is larger than the critical angle of the laser in the transparent conductive substrate, the part of reflected light is totally reflected and is transmitted to the semiconductor photocathode film layer again; therefore, due to the existence of the light trapping structure, the reverse escape of the reflected laser reaching the interface of the vacuum-transparent conductive substrate is weakened, and the power of the incident laser entering the semiconductor photocathode film layer is increased;
4) when laser enters the semiconductor photocathode thin film layer, one part of the laser is absorbed by the semiconductor photocathode thin film layer to generate photoelectrons, and the other part of the laser penetrates through the semiconductor photocathode thin film layer to enter vacuum; because the light trapping structure is arranged at the interface of the vacuum-transparent conductive substrate, laser vertically incident to the transparent conductive substrate has different incident angles corresponding to different incident points for the groove in the shape of a partial cylinder, so that the refraction angle is increased, and for the groove in the shape of a cuboid, the diffraction effect of the grating structure formed by the groove in the cuboid generates high-grade diffraction light, so that the refraction angle of the refraction light entering the transparent conductive substrate for transmission is increased by the light trapping structure; for the refracted light entering the transparent conductive substrate for transmission, the incident angle is increased when the refracted light is transmitted to the interface of the transparent conductive substrate and the semiconductor light cathode thin film layer, and the refraction angle when the refraction occurs is also increased due to the increase of the incident angle, so that the refracted light at the interface of the transparent conductive substrate and the semiconductor light cathode thin film layer has a larger refraction angle, and the refracted light at the interface of the transparent conductive substrate and the semiconductor light cathode thin film layer enters the semiconductor light cathode thin film layer at a larger angle due to the existence of the light trapping structure; because the emergent power of the incident laser exponentially attenuates along with the increase of the optical path in the substance, the laser transmitted in the semiconductor photocathode thin film layer has a longer optical path under the condition that the thickness of the semiconductor photocathode thin film layer is not changed due to a larger refraction angle, so that the absorption rate of the semiconductor photocathode thin film layer on the incident laser is increased; due to the existence of the light trapping structure, the refracted light experiences longer transmission distance before reaching the semiconductor photocathode film layer-vacuum interface, so that the absorbed power is increased, more photoelectrons are generated in the semiconductor photocathode film layer, and the quantum efficiency is increased; meanwhile, for photoelectrons generated in the semiconductor photocathode thin film layer, before reaching a semiconductor photocathode thin film layer-vacuum interface, collision loss energy is experienced in the transmission process, namely thermalization is generated, a longer transmission distance is experienced, so that the loss energy is more, the thermalization is more sufficient, the lateral average energy of the emitted free electrons is also smaller, and the corresponding intrinsic emittance is lower, so that the lateral average energy and the intrinsic emittance of electron beams generated by the transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance are reduced.
The invention has the advantages that:
the invention provides a transmission type photocathode with a light trapping structure for the first time, which can greatly reduce the reflection of incident laser; the reflectivity of the existing reflective photocathode to incident light is 30-50%, the method can greatly reduce the reflection of the transparent conductive substrate to incident laser, thereby greatly improving the absorption of the photocathode to the incident laser; for a traditional transparent substrate without a light trapping structure, reflected light from a semiconductor photocathode thin film layer is incident to a substrate-vacuum interface at a reflection angle, and the condition of total reflection does not exist; the surface of the transparent conductive substrate is provided with the light trapping structure, so that the transmission behavior of incident laser can be changed, the reflectivity of the transparent conductive substrate is reduced, and the absorption of a semiconductor photocathode film layer on incident light is increased; compared with the traditional reflection type photocathode structure, the transmission type semiconductor photocathode with high quantum efficiency and low intrinsic emittance of the invention adopts the scheme of laser back incidence, photoelectrons are generated more nearby a substrate-photocathode interface, and the transmission distance is longer and the photoelectrons are sufficiently thermalized before emission, so that the transverse average energy and the intrinsic emittance of electron beams generated by the photocathode are reduced; the invention adopts a mode of driving the photocathode by the transmission laser, is beneficial to obtaining the electron beam with low emittance, and has important significance for hard X-ray free electron laser (XFLE), Ultrafast Electron Diffraction (UED) and other aspects of application.
Drawings
FIG. 1 is a schematic diagram of a transmissive semiconductor photocathode with high quantum efficiency and low intrinsic emission according to a first embodiment of the present invention;
FIG. 2 is a diagram of a second embodiment of a transmissive semiconductor photocathode with high quantum efficiency and low intrinsic emission according to the present invention;
FIG. 3 is a schematic diagram of a transmissive semiconductor photocathode of the present invention having both high quantum efficiency and low intrinsic emittance, with respect to a groove having a partial cylindrical shape, with laser light incident perpendicularly to the vacuum-transparent conductive substrate interface.
Detailed Description
The invention will be further elucidated by means of specific embodiments in the following with reference to the drawing.
Example one
As shown in fig. 1, the transmissive semiconductor photocathode with both high quantum efficiency and low intrinsic emittance of the present embodiment includes: a semiconductor photocathode thin film layer 11, a transparent conductive substrate 12 and a light trapping structure 13; forming a semiconductor photocathode film layer on the upper surface of the transparent conductive substrate; the lower surface of the transparent conductive substrate is used as the back of a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and a groove array with a periodic structure is etched on the lower surface of the transparent conductive substrate, so that a light trapping structure is formed on the lower surface of the transparent conductive substrate; the shape of the groove is a semi-cylinder, the period of the groove is equal to the width of the groove, the groove is a symmetrical figure about the central plane of the groove, and the section of the groove is a semi-arc; a transmissive semiconductor photocathode having both high quantum efficiency and low intrinsic emittance is located in a vacuum environment within an electron gun.
In this embodiment, the distance from the bottom end of the groove to the photocathode thin film layer is 1 mm; the period of the grooves was 200 nm, the width of the grooves was 200 nm, and the radius of the corresponding cylinder was 100 nm.
The method for implementing a transmissive semiconductor photocathode with both high quantum efficiency and low intrinsic emission of the present embodiment includes the following steps:
1) preparing a transmissive semiconductor photocathode with both high quantum efficiency and low intrinsic emittance, as shown in fig. 1;
2) laser is vertically incident from the back of a transmission type semiconductor photocathode with high quantum efficiency and low intrinsic emittance; the vertically incident laser 311 is reflected and refracted at the vacuum-transparent conductive substrate interface 21, the critical distance is half of the radius of the circular arc according to the distance from the incident point of the laser to the central plane of the groove, when the distance from the incident point of the laser to the central plane of the groove is less than half of the radius of the circular arc, the incident angle of the laser when reaching the vacuum-transparent conductive substrate interface 21 is less than 30 degrees, the reflection angle of the reflected light is also less than 30 degrees, according to the circle sag theorem, the reflected light directly leaves the groove, and the laser only undergoes single reflection and refraction at the moment; when the distance from the incident point of the laser to the central plane of the groove is greater than half of the radius of the arc, the incident angle when the laser reaches the vacuum-transparent conductive substrate interface 21 is greater than 30 °, at this time, the reflection angle of the reflected light is also greater than 30 °, according to the circle sag theorem, the reflected light 312 with the reflection angle greater than 30 ° reaches the vacuum-transparent conductive substrate interface 21 again and undergoes reflection and refraction, wherein the refracted light enters the transparent conductive substrate 12 to be transmitted to form the laser 313 transmitted inside the transparent conductive substrate 12, so that the total laser power entering the transparent conductive substrate 12 is increased, and the transmittance of the incident laser at the vacuum-transparent conductive substrate interface 21 is increased;
3) for the laser light 313 transmitted inside the transparent conductive substrate 12, when reaching the transparent conductive substrate-semiconductor photocathode thin film layer interface 22, reflection and refraction also occur, wherein the reflected light is transmitted towards the vacuum-transparent conductive substrate interface 21, reflection and refraction occur again at the vacuum-transparent conductive substrate interface 21, and the refracted light enters the semiconductor photocathode thin film layer 11 to transmit the laser light 323 transmitted in the semiconductor photocathode thin film layer 11 and is absorbed by the semiconductor photocathode thin film layer 11 to generate photoelectrons; because the light trapping structure 13 is arranged at the vacuum-transparent conductive substrate interface 21, the laser light refracted from the vacuum-transparent conductive substrate interface 21 enters the transparent conductive substrate 12, and the laser light transmitted to the transparent conductive substrate-semiconductor photocathode thin film layer interface 22 has different angles, when the laser light is reflected at the transparent conductive substrate-semiconductor photocathode thin film layer interface 22, the reflected laser light is also transmitted to the vacuum-transparent conductive substrate interface 21 at different incident angles; when the incident angle is larger than the critical angle of the laser in the transparent conductive substrate 12, the part of the reflected light 322 with the incident angle larger than the critical angle is totally reflected and is transmitted again towards the semiconductor photocathode thin film layer 11; therefore, due to the existence of the light trapping structure 13, the reverse escape of the reflected laser reaching the vacuum-transparent conductive substrate interface 21 is weakened, and the power of the incident laser entering the semiconductor photocathode thin film layer 11 is increased;
4) after laser enters the semiconductor photocathode thin film layer 11, one part of the laser is absorbed by the semiconductor photocathode thin film layer 11 to generate photoelectrons, and the other part of the laser penetrates through the semiconductor photocathode thin film layer 11 to enter vacuum; because the light trapping structure 13 is arranged at the vacuum-transparent conductive substrate interface 21, for the laser vertically incident to the transparent conductive substrate, the refraction angle is increased due to different incident angles corresponding to different incident points; for the refracted laser light entering the transparent conductive substrate 12 for transmission, the incident angle when transmitted to the transparent conductive substrate-semiconductor light cathode thin film layer interface 22 is increased, and the refraction angle when refraction occurs is also increased due to the increase of the incident angle, so that the refracted light at the transparent conductive substrate-semiconductor light cathode thin film layer interface 22 has a larger refraction angle, and thus the refracted light at the transparent conductive substrate-semiconductor light cathode thin film layer interface 22 enters the semiconductor light cathode thin film layer 11 at a larger angle due to the light trapping structure 13; since the output power of the incident laser exponentially decays with the increase of the optical path length in the substance, the larger refraction angle allows the laser 323 transmitted in the semiconductor photocathode thin film layer 11 to have a longer optical path length without changing the thickness of the semiconductor photocathode thin film layer 11, thereby increasing the absorption of the semiconductor photocathode thin film layer 11 to the incident laser; due to the presence of the light trapping structure, the refracted light experiences a longer transmission distance before reaching the semiconductor photocathode thin film layer-vacuum interface 23, so that the absorbed power is increased, more photoelectrons are generated in the semiconductor photocathode thin film layer 11, and the quantum efficiency is increased; meanwhile, for photoelectrons generated in the semiconductor photocathode thin film layer, before reaching a semiconductor photocathode thin film layer-vacuum interface, collision loss energy is experienced in the transmission process, namely thermalization is generated, a longer transmission distance is experienced, so that the loss energy is more, the thermalization is more sufficient, the transverse average energy of the emitted free electrons is also smaller, and the corresponding intrinsic emittance is lower, so that the transverse average energy and the intrinsic emittance of electron beams generated by the transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance are reduced.
As shown in fig. 3, for a groove having a shape of a partial cylinder, the cross section is a circular arc, the central angle of the circular arc is 2 θ, there is a critical distance d determined by the radius R of the circular arc and the central angle according to the distance from the incident point of the laser to the central plane of the groove, d = Rsin ((pi- θ)/3), when the distance from the incident point of the laser to the central plane of the groove is less than the critical distance d, the incident angle when the laser reaches the interface of the vacuum-transparent conductive substrate is less than the critical angle α, α = (pi- θ)/3, and the angle of the reflected light is also less than (pi- θ)/3, according to the principle of the sag within the circle, the reflected light will directly leave the groove, and the laser will only undergo single reflection and refraction; when the distance between the incident point of the laser and the central plane of the groove is greater than the critical distance d, the incident angle when the laser reaches the interface of the vacuum-transparent conductive substrate is greater than (pi-theta)/3, and the angle of the reflected light is also greater than (pi-theta)/3 at this moment, according to the circle sag theorem, the reflected light reaches the interface of the vacuum-transparent conductive substrate again and is reflected and refracted, wherein the refracted light enters the transparent conductive substrate for transmission, so that the total laser power entering the transparent conductive substrate is increased.
Example two
As shown in fig. 2, the transmissive semiconductor photocathode with both high quantum efficiency and low intrinsic emittance of the present embodiment includes: a semiconductor photocathode thin film layer 11, a transparent conductive substrate 12 and a light trapping structure 13; forming a semiconductor photocathode film layer on the upper surface of the transparent conductive substrate; the lower surface of the transparent conductive substrate is used as the back of a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and a groove array with a periodic structure is etched on the lower surface of the transparent conductive substrate, so that a light trapping structure is formed on the lower surface of the transparent conductive substrate; the shape of the groove is cuboid, and the period of the groove is greater than the width of the groove; a transmissive semiconductor photocathode having both high quantum efficiency and low intrinsic emittance is located in a vacuum environment within an electron gun.
In this embodiment, the distance from the bottom end of the groove to the photocathode thin film layer is 1mm, and the period of the groove is 60 nm; the width of the groove was 50 nm and the depth was 55 nm.
The method for implementing a transmissive semiconductor photocathode with both high quantum efficiency and low intrinsic emission of the present embodiment includes the following steps:
1) preparing a transmissive semiconductor photocathode with both high quantum efficiency and low intrinsic emittance, as shown in fig. 2;
2) the laser is vertically incident from the back of the transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and the vertically incident laser 311 is reflected and refracted at the vacuum-transparent conductive substrate interface 21; the groove array forms a grating structure, so that when laser is incident to the vacuum-transparent conductive substrate interface 21, high-order diffraction light is generated due to diffraction effect, including refraction diffraction and reflection diffraction, for the refraction light, the grating structure increases the efficiency of refraction diffraction, and more laser enters the transparent conductive substrate 12 at a higher diffraction angle for transmission; for the reflected light, when the incident point of the laser is located in the groove, i.e. the short side of the rectangle, because the groove has a certain depth, the reflection angle of the reflected diffracted light 314 of the high-order diffracted light is not zero, so that the reflected light obliquely enters to the side wall of the groove, i.e. the long side of the rectangle, and the vacuum-transparent conductive substrate interface 21 at the side wall is reflected and refracted again, wherein the refracted light enters into the transparent conductive substrate 12 to be transmitted to form the laser 323 transmitted in the semiconductor photocathode thin film layer 11, the reflected light will continue to be transmitted towards the vacuum, and the reflection and refraction occur each time the reflected light reaches the side wall of the groove, so that the total laser power entering into the transparent conductive substrate 12 is increased; thereby increasing the transmittance of the incident laser light at the vacuum-transparent conductive substrate interface 21;
3) for the laser beam 313 transmitted inside the transparent conductive substrate 12, when reaching the transparent conductive substrate-semiconductor photocathode thin film layer interface 22, the reflection and refraction also occur, wherein the reflected light is transmitted towards the vacuum-transparent conductive substrate interface 21, the reflection and refraction occur again at the vacuum-transparent conductive substrate interface 21, the refracted light enters the semiconductor photocathode thin film layer 11 for transmission, and is absorbed by the semiconductor photocathode thin film layer 11 to generate photoelectrons; due to the light trapping structure 13 at the vacuum-transparent conductive substrate interface 21, the laser light transmitted inside the transparent conductive substrate 12 will reach the transparent conductive substrate-semiconductor photocathode thin film layer interface 22 at different incident angles, and when reflection occurs at the transparent conductive substrate-semiconductor photocathode thin film layer interface 22, the reflected laser light is also transmitted to the vacuum-transparent conductive substrate interface 21 at different incident angles; when the incident angle is larger than the critical angle of the laser in the transparent conductive substrate 12, the part of the reflected light 322 with the incident angle larger than the critical angle is totally reflected and is transmitted again to the semiconductor photocathode thin film layer 11; therefore, due to the existence of the light trapping structure 13, the reverse escape of the reflected laser reaching the vacuum-transparent conductive substrate interface 21 is weakened, and the power of the incident laser entering the semiconductor photocathode thin film layer 11 is increased;
4) after laser enters the semiconductor photocathode thin film layer 11, one part of the laser is absorbed by the semiconductor photocathode thin film layer 11 to generate photoelectrons, and the other part of the laser penetrates through the semiconductor photocathode thin film layer 11 to enter vacuum; since the light trapping structure 13 is provided at the vacuum-transparent conductive substrate interface 21, for laser light perpendicularly incident to the transparent conductive substrate, the diffraction effect of the grating structure formed by the grooves of the rectangular parallelepiped generates high-order diffracted light, so that the refraction angle is increased by the light trapping structure 13; for the refracted laser light entering the transparent conductive substrate 12 for transmission, the incident angle when transmitted to the transparent conductive substrate-semiconductor light cathode thin film layer interface 22 is increased, and the refraction angle when refraction occurs is also increased due to the increase of the incident angle, so that the refracted light at the transparent conductive substrate-semiconductor light cathode thin film layer interface 22 has a larger refraction angle, and thus the refracted light at the transparent conductive substrate-semiconductor light cathode thin film layer interface 22 enters the semiconductor light cathode thin film layer 11 at a larger angle due to the light trapping structure 13; since the output power of the incident laser exponentially decays with the increase of the optical path length in the substance, the larger refraction angle allows the laser 323 transmitted in the semiconductor photocathode thin film layer 11 to have a longer optical path length without changing the thickness of the semiconductor photocathode thin film layer 11, thereby increasing the absorption rate of the semiconductor photocathode thin film layer 11 to the incident laser; due to the presence of the light trapping structure 13, the refracted light experiences a longer transmission distance before reaching the semiconductor photocathode thin film layer-vacuum interface 23, so that the absorbed power is increased, more photoelectrons are generated in the semiconductor photocathode thin film layer 11, and the quantum efficiency is increased; meanwhile, for photoelectrons generated in the semiconductor photocathode thin film layer, before reaching a semiconductor photocathode thin film layer-vacuum interface, collision loss energy is experienced in the transmission process, namely thermalization is generated, a longer transmission distance is experienced, so that the loss energy is more, the thermalization is more sufficient, the transverse average energy of the emitted free electrons is also smaller, and the corresponding intrinsic emittance is lower, so that the transverse average energy and the intrinsic emittance of electron beams generated by the transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance are reduced.
It is finally noted that the disclosed embodiments are intended to aid in the further understanding of the invention, but that those skilled in the art will appreciate that: various substitutions and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the embodiments disclosed, but the scope of the invention is defined by the appended claims.
Claims (3)
1. A transmissive semiconductor photocathode with both high quantum efficiency and low intrinsic emittance, comprising: the light-trapping structure comprises a semiconductor photocathode thin film layer, a transparent conductive substrate and a light-trapping structure; forming a semiconductor photocathode film layer on the upper surface of the transparent conductive substrate; the lower surface of the transparent conductive substrate is used as the back of a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and a groove array with a periodic structure is etched on the lower surface of the transparent conductive substrate, so that a light trapping structure is formed on the lower surface of the transparent conductive substrate; the shape of the groove is a partial cylinder or a cuboid which is symmetrical about the central plane of the groove, the part of the groove which is in the shape of the partial cylinder and accounts for the cylinder is not more than a half cylinder, the section is an arc, the central angle of the arc is 2 theta, theta is more than 60 degrees, the period of the groove which is in the shape of the partial cylinder is more than or equal to the width of the groove, the period of the groove which is in the shape of the cuboid is more than the width of the groove, the section of the groove which is in the shape of the cuboid is rectangular, the short side of the rectangle is used as the bottom of the groove, and the long side of the rectangle is used as the side wall of the groove; the transmission type semiconductor photocathode with high quantum efficiency and low intrinsic emittance is positioned in a vacuum environment in the electron gun; the period of the groove in the shape of a partial cylinder is 5-500 nm; the period of the groove in the shape of the cuboid is 20-600 nm; the width of the groove, namely the short side of the rectangle is 5-400 nm, and the depth, namely the long side of the rectangle is 5-500 nm; the distance between the bottom end of the groove and the photocathode film layer is 0.2-2 mm; the semiconductor photocathode film layer adopts an alkali metal photocathode film layer; the semiconductor photocathode thin film layer adopts one of tellurium cesium, antimony potassium cesium, antimony sodium cesium, antimony rubidium cesium, antimony sodium potassium cesium and antimony sodium potassium rubidium cesium;
the laser is vertically incident from the back of a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and the laser is reflected and refracted at the interface of a vacuum-transparent conductive substrate; for a groove in the shape of a partial cylinder, a critical distance d determined by the radius R and the central angle of a circular arc exists according to the distance from the incident point of laser to the central plane of the groove, and d = Rsin ((pi-theta)/3), when the distance from the incident point of the laser to the central plane of the groove is smaller than the critical distance d, the incident angle of the laser when reaching the interface of the vacuum-transparent conductive substrate is smaller than (pi-theta)/3, and the angle of reflected light is smaller than (pi-theta)/3, according to the principle of the sag in the circle, the reflected light directly leaves the groove, and the laser only has single reflection and refraction; when the distance from the incident point of the laser to the central plane of the groove is greater than the critical distance d, the incident angle when the laser reaches the interface of the vacuum-transparent conductive substrate is greater than (pi-theta)/3, and the angle of the reflected light is also greater than (pi-theta)/3 at this moment, according to the circle sag theorem, the reflected light reaches the interface of the vacuum-transparent conductive substrate again and is reflected and refracted, wherein the refracted light enters the transparent conductive substrate for transmission, so that the total laser power entering the transparent conductive substrate is increased; for the grooves in the shape of a cuboid, the groove array forms a grating structure, so that when laser is incident to the interface of the vacuum-transparent conductive substrate, high-order diffraction light including refraction diffraction and reflection diffraction is generated due to diffraction effect, and for the refraction light, the grating structure increases the efficiency of refraction diffraction, so that more laser enters the transparent conductive substrate at a higher diffraction angle for transmission; for reflected light, when the incident point of the laser is positioned between the grooves, the reflected diffracted light directly leaves the vacuum-transparent conductive substrate interface, and the laser only undergoes single reflection and refraction at the moment; when the incident point of laser is positioned in the groove, namely the short side of the rectangle, because the groove has a certain depth, the reflection angle of the reflected diffracted light of the high-grade diffracted light is not zero, so that the laser is obliquely incident to the side wall of the groove, namely the long side of the rectangle, and the vacuum-transparent conductive substrate interface at the side wall is reflected and refracted again, wherein the refracted light enters the transparent conductive substrate for transmission, the reflected light continues to be transmitted towards the vacuum, and the reflected light is reflected and refracted when reaching the side wall of the groove every time, so that the total laser power entering the transparent conductive substrate is increased, and the transmittance of the incident laser at the vacuum-transparent conductive substrate interface is increased;
for the laser transmitted in the transparent conductive substrate, when the laser reaches the interface of the transparent conductive substrate-the semiconductor photocathode thin film layer, the laser also generates reflection and refraction, wherein the reflected light is transmitted towards the interface of the vacuum-transparent conductive substrate, the reflection and refraction are generated again at the interface of the vacuum-transparent conductive substrate, and the refracted light enters the semiconductor photocathode thin film layer for transmission and is absorbed by the semiconductor photocathode thin film layer to generate photoelectrons; reflected light is transmitted inside the transparent conductive substrate, and due to the fact that the light trapping structure is arranged at the interface of the vacuum-transparent conductive substrate, laser transmitted inside the transparent conductive substrate reaches the interface of the transparent conductive substrate and the semiconductor photocathode thin film layer at different incidence angles, and when the reflection occurs at the interface of the transparent conductive substrate and the semiconductor photocathode thin film layer, the reflected laser is transmitted to the interface of the vacuum-transparent conductive substrate at different incidence angles; when the incident angle is larger than the critical angle of the laser in the transparent conductive substrate, the part of reflected light is totally reflected and is transmitted to the semiconductor photocathode film layer again; therefore, due to the existence of the light trapping structure, the reverse escape of the reflected laser reaching the interface of the vacuum-transparent conductive substrate is weakened, and the power of the incident laser entering the semiconductor photocathode thin film layer is increased;
when laser enters the semiconductor photocathode thin film layer, one part of the laser is absorbed by the semiconductor photocathode thin film layer to generate photoelectrons, and the other part of the laser penetrates through the semiconductor photocathode thin film layer to enter vacuum; because the light trapping structure is arranged at the interface of the vacuum-transparent conductive substrate, laser vertically incident to the transparent conductive substrate has different incident angles corresponding to different incident points for the groove in the shape of a partial cylinder, so that the refraction angle is increased, and for the groove in the shape of a cuboid, the diffraction effect of the grating structure formed by the groove in the cuboid generates high-grade diffraction light, so that the refraction angle of the refraction light entering the transparent conductive substrate for transmission is increased by the light trapping structure; for the refracted light entering the transparent conductive substrate for transmission, the incident angle is increased when the refracted light is transmitted to the interface of the transparent conductive substrate and the semiconductor light cathode thin film layer, and the refraction angle when the refraction occurs is also increased due to the increase of the incident angle, so that the refracted light at the interface of the transparent conductive substrate and the semiconductor light cathode thin film layer has a larger refraction angle, and the refracted light at the interface of the transparent conductive substrate and the semiconductor light cathode thin film layer enters the semiconductor light cathode thin film layer at a larger angle due to the existence of the light trapping structure; because the emergent power of the incident laser exponentially attenuates along with the increase of the optical path in the substance, the laser transmitted in the semiconductor photocathode thin film layer has a longer optical path under the condition that the thickness of the semiconductor photocathode thin film layer is not changed due to a larger refraction angle, so that the absorption rate of the semiconductor photocathode thin film layer on the incident laser is increased; due to the existence of the light trapping structure, the refracted light experiences longer transmission distance before reaching the semiconductor photocathode film layer-vacuum interface, so that the absorbed power is increased, more photoelectrons are generated in the semiconductor photocathode film layer, and the quantum efficiency is increased; meanwhile, for photoelectrons generated in the semiconductor photocathode thin film layer, before reaching a semiconductor photocathode thin film layer-vacuum interface, collision loss energy is experienced in the transmission process, namely thermalization is generated, a longer transmission distance is experienced, so that the loss energy is more, the thermalization is more sufficient, the lateral average energy of the emitted free electrons is also smaller, and the corresponding intrinsic emittance is lower, so that the lateral average energy and the intrinsic emittance of electron beams generated by the transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance are reduced.
2. The transmissive semiconductor photocathode of claim 1 having both high quantum efficiency and low intrinsic emittance, wherein the transparent conductive substrate is a circle having a diameter of 10 to 50 mm or a square having a side of 10 to 50 mm.
3. A method of preparing a transmissive semiconductor photocathode with both high quantum efficiency and low intrinsic emittance according to claim 1, comprising the steps of:
a) providing a transparent conductive substrate;
b) forming a semiconductor photocathode film layer on the upper surface of the transparent conductive substrate;
c) the lower surface of the transparent conductive substrate is used as the back of a transmission-type semiconductor photocathode with high quantum efficiency and low intrinsic emittance, and a groove array with a periodic structure is etched on the lower surface of the transparent conductive substrate, so that a light trapping structure is formed on the lower surface of the transparent conductive substrate;
d) a transmissive semiconductor photocathode having both high quantum efficiency and low intrinsic emittance is located in a vacuum environment within an electron gun.
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