A Quantum Well Infrared Photodetector (QWIP) is an infrared photodetector, which uses electronic intersubband transitions in quantum wells to absorb photons. In order to be used for infrared detection, the parameters of the quantum wells in the quantum well infrared photodetector are adjusted so that the energy difference between its first and second quantized states match the incoming infrared photon energy. QWIPs are typically made of gallium arsenide, a material commonly found in smartphones and high-speed communications equipment. [1] Depending on the material and the design of the quantum wells, the energy levels of the QWIP can be tailored to absorb radiation in the infrared region from 3 to 20 µm. [2]
QWIPs are one of the simplest quantum mechanical device structures that can detect mid-wavelength and long-wavelength infrared radiation. They are known for their stability, high pixel-to-pixel uniformity, and high-pixel operability. [3]
In 1985, Stephen Eglash and Lawrence West observed strong intersubband transition in multiple quantum wells (MQW) that prompted more serious consideration into using quantum wells for infrared detectors. [4] Previously, attempts to use quantum wells for infrared detection were based on free absorption in quantum wells that bring the electrons over the top of the barriers. However, resulting detectors displayed low sensitivity. [5]
By 1987, the basic operating principles for a quantum well infrared photodetector that demonstrated sensitive infrared detection were formulated. In 1990, the low-temperature sensitivity of the technology was further improved by increasing the barrier thickness, which suppressed the tunneling current. [5] At this point, these devices were formally known as quantum well infrared photodetectors. [5] [6] In 1991, the first infrared image was obtained using this approach. [5]
In 2002, researchers at the U.S. Army Research Laboratory (ARL) developed a voltage-tunable, two-color QWIP with effective wavelength switching for remote temperature sensing. The instrument exhibited a peak detection wavelength of 7.5 micrometers for positive bias at 10 K when the electrons resided in one of the quantum wells and switched to 8.8 micrometers at a large negative bias when the electrons were transferred to the other well. [7] [8]
Yet despite its use in civilian applications, QWIP technology was considered insufficient by the U.S. military for military use. At the time, the photodetectors could only sense the one-dimensional quantization when the light traveled in parallel to the material layers, which typically occurred when light was shined at the edge of the detector. As a result, the QWIP technology had a quantum efficiency of only 5 percent. In addition, the reflection gratings commonly used in the industry to alleviate this problem were made of very fine periodic posts and were difficult to produce in large formats. [1]
To address this problem, researchers at the Army Research Laboratory developed the corrugated quantum infrared photodetector (C-QWIP) in 2008, which used micromirrors on the photodetector to increase the effectiveness of redirecting the light onto the quantum well region at any wavelength. [9] In essence, the 45-degree inclined detector sidewalls allowed light to be reflected parallel to the material layers to produce an electrical signal. [10] Tests conducted by researchers at ARL and L-3 Communications Cincinnati Electronics determined that the C-QWIP demonstrated bandwidths exceeding 3 micrometers, which was 5 times wider than the commercial QWIP at the time. [9] Since C-QWIPs can be manufactured using gallium arsenide, they served as a more affordable alternative to conventional infrared detectors for Army helicopters without sacrificing resolution and requiring less calibration and maintenance. [11]
In February 2013, NASA launched a satellite that featured the Thermal Infrared Sensor (TIRS) instrument as part of its Landsat Data Continuity Mission. The TIRS utilized three C-QWIPs designed by the Army Research Laboratory to detect long wavelengths of light emitted by the Earth and track how the planet's water and land are being used. This application marked the first time a QWIP was used in space. [1] [11] [12]
Infrared detectors generally work by detecting the radiation emitted by an object, and the intensity of the radiation is determined by factors such as the object's temperature, distance, and size. Unlike most infrared photodetectors, QWIPs are independent of the band gap of the detecting material, because they are based on the optical transition within a single energy band. As a result, it can be used to detect objects with much lower energy radiation than what was previously possible. [5]
The basic elements of a QWIP are quantum wells, which are separated by barriers. The quantum wells are designed to have one confined state inside the well and a first excited state which aligns with the top of the barrier. The wells are n-doped such that the ground state is filled with electrons. The barriers are wide enough to prevent quantum tunneling between the quantum wells. Typical QWIPs consists of 20 to 50 quantum wells. When a bias voltage is applied to the QWIP, the entire conduction band is tilted. Without light the electrons in the quantum wells just sit in the ground state. When the QWIP is illuminated with light of the same or higher energy as the intersubband transition energy, an electron is excited.
Once the electron is in an excited state, it can escape into the continuum and be measured as photocurrent. To externally measure a photocurrent the electrons need to be extracted by applying an electric field to the quantum wells. The efficiency of this absorption and extraction process depends on several parameters.
Photocurrent
Assuming that the detector is illuminated with a photon flux (number of photons per unit time), the photocurrent is
where is the elementary charge, is the absorption efficiency and is the photoconductive gain. [13] and are the probabilities for a photon to add an electron to the photocurrent, also called quantum efficiency. is the probability of a photon exciting an electron, and depends on the electronic transport properties.
Photoconductive gain
The photoconductive gain is the probability that an excited electron contributes to the photocurrent—or more generally, the number of electrons in the external circuit, divided by the number of quantum well electrons that absorb a photon. Although it might be counterintuitive at first, it is possible for to be larger than one. Whenever an electron is excited and extracted as photocurrent, an extra electron is injected from the opposite (emitter) contact to balance the loss of electrons from the quantum well. In general the capture probability , so an injected electron might sometimes pass over the quantum well and into the opposite contact. In that case, yet another electron is injected from the emitter contact to balance the charge, and again heads towards the well where it might or might not get captured, and so on, until eventually an electron is captured in the well. In this way, can become larger than one.
The exact value of is determined by the ratio of capture probability and escape probability .
where is the number of quantum wells. The number of quantum wells appears only in the denominator, as it increases the capture probability , but not the escape probability .
Spontaneous emission is the process in which a quantum mechanical system transits from an excited energy state to a lower energy state and emits a quantized amount of energy in the form of a photon. Spontaneous emission is ultimately responsible for most of the light we see all around us; it is so ubiquitous that there are many names given to what is essentially the same process. If atoms are excited by some means other than heating, the spontaneous emission is called luminescence. For example, fireflies are luminescent. And there are different forms of luminescence depending on how excited atoms are produced. If the excitation is effected by the absorption of radiation the spontaneous emission is called fluorescence. Sometimes molecules have a metastable level and continue to fluoresce long after the exciting radiation is turned off; this is called phosphorescence. Figurines that glow in the dark are phosphorescent. Lasers start via spontaneous emission, then during continuous operation work by stimulated emission.
Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron, causing it to drop to a lower energy level. The liberated energy transfers to the electromagnetic field, creating a new photon with a frequency, polarization, and direction of travel that are all identical to the photons of the incident wave. This is in contrast to spontaneous emission, which occurs at a characteristic rate for each of the atoms/oscillators in the upper energy state regardless of the external electromagnetic field.
Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, it measures the electrical output per optical input.
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation.
Cathodoluminescence is an optical and electromagnetic phenomenon in which electrons impacting on a luminescent material such as a phosphor, cause the emission of photons which may have wavelengths in the visible spectrum. A familiar example is the generation of light by an electron beam scanning the phosphor-coated inner surface of the screen of a television that uses a cathode ray tube. Cathodoluminescence is the inverse of the photoelectric effect, in which electron emission is induced by irradiation with photons.
An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of photomultiplier tubes. The avalanche photodiode was invented by Japanese engineer Jun-ichi Nishizawa in 1952. However, study of avalanche breakdown, microplasma defects in silicon and germanium and the investigation of optical detection using p-n junctions predate this patent. Typical applications for APDs are laser rangefinders, long-range fiber-optic telecommunication, and quantum sensing for control algorithms. New applications include positron emission tomography and particle physics.
A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate. Sometimes, the excited state is metastable, so the relaxation back down from the excited state to lower states is delayed. The process then corresponds to one of two phenomena: delayed fluorescence or phosphorescence. The correspondence depends on the type of transition and hence the wavelength of the emitted optical photon.
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.
A quantum well is a potential well with only discrete energy values.
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.
Hg1−xCdxTe or mercury cadmium telluride is a chemical compound of cadmium telluride (CdTe) and mercury telluride (HgTe) with a tunable bandgap spanning the shortwave infrared to the very long wave infrared regions. The amount of cadmium (Cd) in the alloy can be chosen so as to tune the optical absorption of the material to the desired infrared wavelength. CdTe is a semiconductor with a bandgap of approximately 1.5 electronvolts (eV) at room temperature. HgTe is a semimetal, which means that its bandgap energy is zero. Mixing these two substances allows one to obtain any bandgap between 0 and 1.5 eV.
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.
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule.
The Mainz Microtron, abbreviated MAMI, is a microtron which provides a continuous wave, high intensity, polarized electron beam with an energy up to 1.6 GeV. MAMI is the core of an experimental facility for particle, nuclear and X-ray radiation physics at the Johannes Gutenberg University in Mainz (Germany). It is one of the largest campus-based accelerator facilities for basic research in Europe. The experiments at MAMI are performed by about 200 physicists of many countries organized in international collaborations.
The theoretical and experimental justification for the Schrödinger equation motivates the discovery of the Schrödinger equation, the equation that describes the dynamics of nonrelativistic particles. The motivation uses photons, which are relativistic particles with dynamics described by Maxwell's equations, as an analogue for all types of particles.
Photoelectrochemical processes are processes in photoelectrochemistry; they usually involve transforming light into other forms of energy. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, and photochromism.
Photoconductive polymers absorb electromagnetic radiation and produce an increase of electrical conductivity. Photoconductive polymers have been used in a wide variety of technical applications such as Xerography (electrophotography) and laser printing. Electrical conductivity is usually very small in organic compounds. Conductive polymers usually have large electrical conductivity. Photoconductive polymer is a smart material based on conductive polymer, and the electrical conductivity can be controlled by the amount of radiation.
A. G. Unil Perera is a Sri Lankan-born American physicist with an assortment of research interests in experimental condensed matter physics, especially semiconductor infrared detectors and applications. He has authored over 200 publications covering a variety of disciplines inside. He is a Regents’ Professor of Physics at Georgia State University, in Atlanta, Georgia. After his basic Education in Sri Lanka, he obtained his doctoral degree in (applied) physics from the University of Pittsburgh under the supervision of Darry D. Coon. During his graduate research, he developed a detector, which can detect infrared (IR) radiation without the use of any amplifiers. (Solid State Electronics, 29, 929,. Then he introduced the concept of a two-terminal artificial neuron (International Journal of Electronics, 63, 61, , a parallel asynchronous processing based on artificial neurons , Neural Networks 2, 143, .( Phys. Rev. Lett., 58, 1139, .