Infrared thermal imaging detectorThe core function of is to convert invisible infrared radiation (the external manifestation of object temperature) into quantifiable electrical signals, ultimately generating thermal images. The performance directly depends on the efficiency of the photoelectric conversion mechanism, which determines whether the signal can be accurately captured. The following provides a detailed analysis from this core dimension:

The photon energy of infrared radiation is much lower than that of visible light (usually with a wavelength of 1-14 μ m), and cannot directly excite semiconductors to produce strong photoelectric effects like visible light detectors. Therefore, energy conversion needs to be achieved through "indirect or special direct" physical processes. According to the different conversion principles, mainstream infrared detectors can be divided into three types: pyroelectric, microbolometer (MEMS), and photon (quantum well/cadmium telluride mercury), and their conversion mechanisms have essential differences.

1. Pyroelectric detector: "temperature change → polarized charge → electrical signal"

Pyroelectric detectors utilize the physical properties of pyroelectric crystals (such as lithium tantalate and triglyceride sulfate TGS) to achieve conversion, belonging to the category of "thermal detection" (first converting infrared radiation into temperature changes, and then converting it into electrical signals), suitable for medium to long wave infrared (3-14 μ m).

Conversion process:

1. Infrared absorption: The metal black film (high absorption rate) on the surface of the detector absorbs infrared radiation, converts the radiated energy into thermal energy, and causes the temperature of the pyroelectric crystal to rise;

2. Polarization change: The "spontaneous polarization intensity" of a pyroelectric crystal changes with temperature (the polarization intensity decreases as the temperature increases);

3. Charge separation: Polarized charges are induced at the electrodes at both ends of the crystal. When the temperature change rate (dT/dt) is not zero, the charges on the electrodes will flow, forming a weak current signal;

4. Signal reading: The weak current is converted into a voltage signal through a preamplifier, and then processed into a quantifiable electrical signal through subsequent circuits.

Key features:

-Only sensitive to "temperature changes" (static temperature without signal), requires the cooperation of a mechanical chopper (periodically blocking infrared radiation to create temperature fluctuations) to work;

-Simple structure and low cost, but slow response speed (in milliseconds), suitable for scenarios with low frame rate requirements (such as security monitoring and temperature screening).

2. Micro bolometer type detector (MEMS technology): "Temperature change → Resistance change → Electrical signal"

Microbolometer is currently the mainstream technology for civilian infrared thermal imagers (such as security and consumer electronics), also belonging to "thermal detection". Its core is a thermistor based on MEMS microbridge structure, suitable for 8-14 μ m long wave infrared.

Conversion process:

1. Infrared absorption and temperature rise: The absorption layer (such as vanadium oxide, amorphous silicon) on the surface of MEMS microbridges (made of insulation materials such as silicon nitride, supporting thermistors) absorbs infrared radiation, and the temperature rises (the microbridge structure can reduce heat conduction to the substrate and improve thermal sensitivity);

2. Resistance change: The resistance of thermosensitive materials (such as vanadium oxide VOx) significantly changes with temperature (temperature coefficient TCR is usually -2%~-4%/K, with a negative sign indicating a decrease in resistance with increasing temperature);

3. Electrical signal conversion: Connect the thermistor to the Wheatstone bridge (through DC bias), and the change in resistance will cause the bridge to output a voltage difference signal (the voltage change is positively correlated with the temperature change);

4. Array based reading:Infrared thermal imaging detectorA focal plane array (FPA) is composed of hundreds of thousands or even millions of microbridge units, each corresponding to a pixel in the thermal image. The voltage signal is read point by point through a row column driving circuit and pieced together to form a complete thermal image.

Key features:

-No need for chopper (can directly detect static temperature), response speed faster than pyroelectric (microsecond level), frame rate can reach 30-60fps;

-MEMS technology can achieve miniaturization and low cost, but long-term use is susceptible to environmental temperature effects (requiring temperature compensation).

3. Photon detectors (quantum well/QWIP, mercury cadmium telluride MCT): "Infrared photons → charge carriers → electrical signals"

Photon type detectors directly utilize the quantum effects of semiconductors to convert infrared photons into electron hole pairs (carriers), belonging to the category of "quantum detection". They are suitable for mid to short wave infrared (1-5 μ m) and have high sensitivity and response speed. They are mainly used in scenarios such as military and aerospace.

Taking the widely used quantum well infrared photon detector (QWIP) as an example, its conversion process is as follows:

1. Quantum well structure: a multi-layer structure is formed by alternating growth of GaAs (gallium arsenide) and AlGaAs (aluminum gallium arsenide), with the energy band of the GaAs layer lower than that of the AlGaAs layer, forming a "quantum well" (electrons are confined within the GaAs layer and can only transition between specific energy levels);

2. Photon absorption and carrier excitation: When the energy of an infrared photon (E=h ν, where h is Planck's constant and ν is frequency) is equal to the energy level difference of a quantum well, electrons absorb the photon energy and transition from a low energy level to a high energy level (a freely moving conduction band), producing electron hole pairs;

3. Carrier separation and collection: Apply a bias voltage at both ends of the detector, and the electric field causes electrons and holes to move in opposite directions, forming a photocurrent signal (the magnitude of the photocurrent is positively correlated with the number of incident infrared photons);

4. Low temperature operation: To suppress the "dark charge carriers" (leakage current in the absence of light) generated by thermal excitation, photon detectors need to operate in a low-temperature environment (usually cooled to 77K/-196 ℃ by a Stirling cooler or liquid nitrogen), ensuring that only infrared photons can excite charge carriers.

Key features:

-High sensitivity (able to detect single photon level signals), fast response speed (nanosecond level), but complex structure, high cost, and dependence on refrigeration system, suitable for scenarios with strict requirements for detection accuracy.