A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron beams. Electron beams generated with photocathodes are commonly used for free electron lasers and for ultrafast electron diffraction. Photocathodes are also commonly used as the negatively charged electrode in a light detection device such as a photomultiplier, phototube and image intensifier.
Quantum efficiency is a unitless number that measures the sensitivity of the photocathode to light. It is the ratio of the number of electrons emitted to the number of incident photons. [1] This property depends on the wavelength of light being used to illuminate the photocathode. For many applications, QE is the most important property as the photocathodes are used solely for converting photons into an electrical signal.
Quantum efficiency may be calculated from photocurrent (), laser power (), and either the photon energy () or laser wavelength () using the following equation. [1] [2]
For some applications, the initial momentum distribution of emitted electrons is important and the mean transverse energy (MTE) and thermal emittance are popular metrics for this. The MTE is the variance of the transverse momentum in a direction along the photocathode's surface and is most commonly reported in units of milli-electron volts. [3]
In high brightness photoinjectors, the MTE helps to determine the initial emittance of the beam which is the area in phase space occupied by the electrons. [4] The emittance () can be calculated from MTE and the laser spot size on the photocathode () using the following equation.
where is the rest mass of an electron. In commonly used units, this is as follows.
Because of the scaling of transverse emittance with MTE, it is sometimes useful to write the equation in terms of a new quantity called the thermal emittance. [5] The thermal emittance is derived from MTE using the following equation.
It is most often expressed in the ratio um/mm to express the growth of emittance in units of um as the laser spot grows (measured in units of mm).
An equivalent definition of MTE is the temperature of electrons emitted in vacuum. [6] The MTE of electrons emitted from commonly used photocathodes, such as polycrystalline metals, is limited by the excess energy (the difference between the energy of the incident photons and the photocathode's work function) provided to the electrons. To limit MTE, photocathodes are often operated near the photoemission threshold, where the excess energy tends to zero. In this limit, the majority of photoemission comes from the tail of the Fermi distribution. Therefore, MTE is thermally limited to , where is the Boltzmann constant and is the temperature of electrons in the solid. [7]
Due to conservation of transverse momentum and energy in the photoemission process, the MTE of a clean, atomically-ordered, single crystalline photocathode is determined by the material's band structure. An ideal band structure for low MTEs is one that does not allow photoemission from large transverse momentum states. [8]
Outside of accelerator physics, MTE and thermal emittance play a role in the resolution of proximity-focused imaging devices that use photocathodes. [9] This is important for applications such as image intensifiers, wavelength converters, and the now obsolete image tubes.
Many photocathodes require excellent vacuum conditions to function and will become "poisoned" when exposed to contaminates. Additionally, using the photocathodes in high current applications will slowly damage the compounds as they are exposed to ion back-bombardment. These effects are quantified by the lifetime of the photocathode. Cathode death is modeled as a decaying exponential as a function of either time or emitted charge. Lifetime is then the time constant of the exponential. [10] [11]
For many years the photocathode was the only practical method for converting light to an electron current. As such it tends to function as a form of 'electric film' and shared many characteristics of photography. It was therefore the key element in opto-electronic devices, such as TV camera tubes like the orthicon and vidicon, and in image tubes such as intensifiers, converters, and dissectors. Simple phototubes were used for motion detectors and counters.
Phototubes have been used for years in movie projectors to read the sound tracks on the edge of movie film. [12]
The more recent development of solid state optical devices such as photodiodes has reduced the use of photocathodes to cases where they still remain superior to semiconductor devices.
Photocathodes operate in a vacuum, so their design parallels vacuum tube technology. Since most cathodes are sensitive to air the construction of photocathodes typically occurs after the enclosure has been evacuated. In operation the photocathode requires an electric field with a nearby positive anode to assure electron emission. Molecular beam epitaxy is broadly applied in today's manufacturing of photocathode. By using a substrate with matched lattice parameters, crystalline photocathodes can be made and electron beams can come out from the same position in lattice's Brillouin zone to get high brightness electron beams.
Photocathodes divide into two broad groups; transmission and reflective. A transmission type is typically a coating upon a glass window in which the light strikes one surface and electrons exit from the opposite surface. A reflective type is typically formed on an opaque metal electrode base, where the light enters and the electrons exit from the same side. A variation is the double reflection type, where the metal base is mirror-like, causing light that passed through the photocathode without causing emission to be bounced back for a second try. This mimics the retina on many mammals.
The effectiveness of a photocathode is commonly expressed as quantum efficiency, that being the ratio of emitted electrons vs. impinging quanta (of light). The efficiency varies with construction as well, as it can be improved with a stronger electric field.
The surface of photocathodes can be characterized by various surface sensitive techniques like scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy.
Although a plain metallic cathode will exhibit photoelectric properties, the specialized coating greatly increases the effect. A photocathode usually consists of alkali metals with very low work functions.
The coating releases electrons much more readily than the underlying metal, allowing it to detect the low-energy photons in infrared radiation. The lens transmits the radiation from the object being viewed to a layer of coated glass. The photons strike the metal surface and transfer electrons to its rear side. The freed electrons are then collected to produce the final image.
The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, solid state, and quantum chemistry to draw inferences about the properties of atoms, molecules and solids. The effect has found use in electronic devices specialized for light detection and precisely timed electron emission.
X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique that measures the very topmost 200 atoms, 0.01 um, 10 nm of any surface. It belongs to the family of photoemission spectroscopies in which electron population spectra are obtained by irradiating a material with a beam of X-rays. XPS is based on the photoelectric effect that can identify the elements that exist within a material or are covering its surface, as well as their chemical state, and the overall electronic structure and density of the electronic states in the material. XPS is a powerful measurement technique because it not only shows what elements are present, but also what other elements they are bonded to. The technique can be used in line profiling of the elemental composition across the surface, or in depth profiling when paired with ion-beam etching. It is often applied to study chemical processes in the materials in their as-received state or after cleavage, scraping, exposure to heat, reactive gasses or solutions, ultraviolet light, or during ion implantation.
A laser diode is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction.
Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum tubes, more specifically vacuum phototubes. These detectors multiply the current produced by incident light by as much as 100 million times or 108 (i.e., 160 dB), in multiple dynode stages, enabling (for example) individual photons to be detected when the incident flux of light is low.
A synchrotron light source is a source of electromagnetic radiation (EM) usually produced by a storage ring, for scientific and technical purposes. First observed in synchrotrons, synchrotron light is now produced by storage rings and other specialized particle accelerators, typically accelerating electrons. Once the high-energy electron beam has been generated, it is directed into auxiliary components such as bending magnets and insertion devices in storage rings and free electron lasers. These supply the strong magnetic fields perpendicular to the beam that are needed to stimulate the high energy electrons to emit photons.
An electron gun is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy.
An image intensifier or image intensifier tube is a vacuum tube device for increasing the intensity of available light in an optical system to allow use under low-light conditions, such as at night, to facilitate visual imaging of low-light processes, such as fluorescence of materials in X-rays or gamma rays, or for conversion of non-visible light sources, such as near-infrared or short wave infrared to visible. They operate by converting photons of light into electrons, amplifying the electrons, and then converting the amplified electrons back into photons for viewing. They are used in devices such as night-vision goggles.
A quantum well is a potential well with only discrete energy values.
Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in the substance. The term refers to various techniques, depending on whether the ionization energy is provided by X-ray, XUV or UV photons. Regardless of the incident photon beam, however, all photoelectron spectroscopy revolves around the general theme of surface analysis by measuring the ejected electrons.
Photoemission electron microscopy is a type of electron microscopy that utilizes local variations in electron emission to generate image contrast. The excitation is usually produced by ultraviolet light, synchrotron radiation or X-ray sources. PEEM measures the coefficient indirectly by collecting the emitted secondary electrons generated in the electron cascade that follows the creation of the primary core hole in the absorption process. PEEM is a surface sensitive technique because the emitted electrons originate from a shallow layer. In physics, this technique is referred to as PEEM, which goes together naturally with low-energy electron diffraction (LEED), and low-energy electron microscopy (LEEM). In biology, it is called photoelectron microscopy (PEM), which fits with photoelectron spectroscopy (PES), transmission electron microscopy (TEM), and scanning electron microscopy (SEM).
The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device, or it may refer to the TMR effect of a magnetic tunnel junction.
Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There are a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or by various performance metrics, such as spectral response. Semiconductor-based photodetectors typically use a p–n junction that converts photons into charge. The absorbed photons make electron–hole pairs in the depletion region. Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.
A phototube or photoelectric cell is a type of gas-filled or vacuum tube that is sensitive to light. Such a tube is more correctly called a 'photoemissive cell' to distinguish it from photovoltaic or photoconductive cells. Phototubes were previously more widely used but are now replaced in many applications by solid state photodetectors. The photomultiplier tube is one of the most sensitive light detectors, and is still widely used in physics research.
Quantum-cascade lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jérôme Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.
Thermophotovoltaic (TPV) energy conversion is a direct conversion process from heat to electricity via photons. A basic thermophotovoltaic system consists of a hot object emitting thermal radiation and a photovoltaic cell similar to a solar cell but tuned to the spectrum being emitted from the hot object.
Angle-resolved photoemission spectroscopy (ARPES) is an experimental technique used in condensed matter physics to probe the allowed energies and momenta of the electrons in a material, usually a crystalline solid. It is based on the photoelectric effect, in which an incoming photon of sufficient energy ejects an electron from the surface of a material. By directly measuring the kinetic energy and emission angle distributions of the emitted photoelectrons, the technique can map the electronic band structure and Fermi surfaces. ARPES is best suited for the study of one- or two-dimensional materials. It has been used by physicists to investigate high-temperature superconductors, graphene, topological materials, quantum well states, and materials exhibiting charge density waves.
Multi-junction (MJ) solar cells are solar cells with multiple p–n junctions made of different semiconductor materials. Each material's p-n junction will produce electric current in response to different wavelengths of light. The use of multiple semiconducting materials allows the absorbance of a broader range of wavelengths, improving the cell's sunlight to electrical energy conversion efficiency.
Interband cascade lasers (ICLs) are a type of laser diode that can produce coherent radiation over a large part of the mid-infrared region of the electromagnetic spectrum. They are fabricated from epitaxially-grown semiconductor heterostructures composed of layers of indium arsenide (InAs), gallium antimonide (GaSb), aluminum antimonide (AlSb), and related alloys. These lasers are similar to quantum cascade lasers (QCLs) in several ways. Like QCLs, ICLs employ the concept of bandstructure engineering to achieve an optimized laser design and reuse injected electrons to emit multiple photons. However, in ICLs, photons are generated with interband transitions, rather than the intersubband transitions used in QCLs. Consequently, the rate at which the carriers injected into the upper laser subband thermally relax to the lower subband is determined by interband Auger, radiative, and Shockley-Read carrier recombination. These processes typically occur on a much slower time scale than the longitudinal optical phonon interactions that mediates the intersubband relaxation of injected electrons in mid-IR QCLs. The use of interband transitions allows laser action in ICLs to be achieved at lower electrical input powers than is possible with QCLs.
A photoinjector is a type of source for intense electron beams which relies on the photoelectric effect. A laser pulse incident onto the cathode of a photoinjector drives electrons out of it, and into the accelerating field of the electron gun. In comparison with the widespread thermionic electron gun, photoinjectors produce electron beams of higher brightness, which means more particles packed into smaller volume of phase space. Photoinjectors serve as the main electron source for single-pass synchrotron light sources, such as free-electron lasers and for ultrafast electron diffraction setups. The first RF photoinjector was developed in 1985 at Los Alamos National Laboratory and used as the source for a free-electron-laser experiment. High-brightness electron beams produced by photoinjectors are used directly or indirectly to probe the molecular, atomic and nuclear structure of matter for fundamental research, as well as material characterization.
In accelerator physics, the mean transverse energy (MTE) is a quantity that describes the variance of the transverse momentum of a beam. While the quantity has a defined value for any particle beam, it is generally used in the context of photoinjectors for electron beams.
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