China's first international development of quantum cascade detector infrared focal plane array
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[Chinese instrument network instrument research and development] The essence of light is electromagnetic waves. The electromagnetic waves that human eyes can perceive are called visible light, which is known as the seven colors of red, orange, yellow, green, blue, and purple. Visible light is only a small part of the entire electromagnetic spectrum. To perceive electromagnetic waves other than visible light, it must be achieved by external means such as photodetectors.

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Infrared light, also known as infrared light, was discovered in the laboratory by British scientist Herschel in 1800. It is an electromagnetic wave with a longer wavelength than red light. It has obvious thermal effects and makes people feel it and cannot see it. The terminology says that infrared radiation is present in all objects whose absolute temperature is above zero. In layman's terms, the objects we are currently exposed to are constantly emitting infrared light. Therefore, we can observe objects through infrared detection. Infrared detection technology can be used for night vision, medical treatment, gas detection, astronomical detection and so on.

Electromagnetic spectrum
An infrared detector is a photoelectric conversion device that senses infrared radiation with high sensitivity. The early infrared detection was based on the thermal effect of infrared radiation, that is, the irradiation of infrared light caused the temperature of the detector to rise. The temperature change caused the physical parameters of the infrared detector to change, and the strength of the infrared light was judged accordingly. Since this method is based on a temperature change and the temperature change is a slow process, this thermal effect-based infrared detector has a slower sensing speed.
Most modern infrared detectors are designed based on the photoelectric effect, and are very similar to the visible light band CCD or CMOS detectors, which are widely used in the photosensitive components of the camera. The only difference is that the photoelectric conversion pixels in the infrared detector are Made of photoelectric materials that can sense infrared light waves. Because light has wave-particle duality, light waves can often be called photons. Photons can directly act on the electrons in the infrared detector, making direct changes in the current or voltage output by the infrared detector. By testing this change, the intensity of the incident light can be directly estimated based on its conversion efficiency. This method is based on the photoelectric effect, avoiding the temperature change process, so the photodetector response speed is faster.
Quantum cascade detector (QCD) is a new type of photodetector, which was proposed at the beginning of the 21st century. It is an artificial structural crystal material. Quantum cascade detectors are usually formed by alternately growing two semiconductor materials with different bandgap widths. The band of the material is designed to be a quantum well structure through energy band engineering. The detection wavelength is mainly limited by the height of the barrier and can cover the infrared. With terahertz band. For example, a barrier is like a wall. A quantum well is like a flat wall between a wall and a wall. By adjusting the thickness of the wall, the height of the wall, and the distance between the wall and the wall, various energy levels can be distributed between the walls. According to the principle of quantum mechanics, energy levels will be bound between walls and walls, not higher than the wall.

Energy band structure and working method of quantum cascade detector
The energy level distribution of the quantum cascade detector is shown in the above figure. Its structure can be roughly divided into two parts, the absorption zone and the transport zone. The absorption zone is responsible for photon absorption, absorbing one incident photon while exciting one electron; the transport zone is responsible for the directional movement of this electron. In the absorption region of the figure above, an incident photon can increase the electron at the E1 level to the E6 energy level, and then the energy level in the transport region is designed to be a down-step pattern, enabling the electrons to move directionally. Is this photoelectric process that climbs up and slides down a bit familiar? That's right, the slides we've played with all have the same purpose! This system of multiple quantum levels is called a "quantum cascade." At this point, one might ask, is the energy level not limited to two "walls"? So how can electrons "pass through the wall"? Here is another interesting concept in quantum mechanics: the quantum tunneling effect. From the point of view of quantum mechanics, electrons are fluctuating. Therefore, electrons have a certain probability of “passing through the wall” directly. This is incredible in classical physics, but it actually occurs in quantum mechanics. This phenomenon is called the quantum tunneling effect. And under certain conditions, the probability of "wall crossing" of electrons can approach 100%.
The asymmetrical structure of quantum cascade detectors allows them to exhibit photovoltaic characteristics, allowing photo-excited electrons to spontaneously transport in one direction without the use of other external forces such as an applied electric field. This photovoltaic property makes the output and acquisition of photoelectric signals more convenient. When no electric field is applied, the quantum cascade detector will not generate current (no dark current) in the absence of light, and pure photocurrent will only be output if photons are incident. Therefore, quantum cascade detectors have low power consumption, low heat generation, and low thermal load and can be used to prepare low energy imaging focal plane arrays for imaging chips.
Based on various advantages, quantum cascade detectors have become very promising infrared detectors in applications such as low-light detection, satellite remote sensing, high-speed laser communications from Starfield, and high-contrast infrared imaging.

Quantum Cascade Detector IR Focal Plane Array Infrared Imaging of Electric Iron
At present, the Lu Wei research team of the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences has developed the infrared focal plane array of quantum cascade detectors for the first time in the world. The detector is based on GaAs/AlGaAs materials and has a peak detection wavelength of 8.5 microns and is located in the long-wave infrared region. The array size reached 320 × 256 (81920 pixels) and preliminary infrared imaging experiments were conducted.
(Original Title: New Technology for Infrared Detection - Quantum Cascade Detectors)

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