Hot electron bolometer
The hot electron bolometer (HEB) operates at cryogenic temperatures, typically within a few degrees of absolute zero. At these very low temperatures, the electron system in a metal is weakly coupled to the phonon system. Power coupled to the electron system drives it out of thermal equilibrium with the phonon system, creating hot electrons.[6] Phonons in the metal are typically well-coupled to substrate phonons and act as a thermal reservoir. In describing the performance of the HEB, the relevant heat capacity is the electronic heat capacity and the relevant thermal conductance is the electron-phonon thermal conductance.
If the resistance of the absorbing element depends on the electron temperature, then the resistance can be used as a thermometer of the electron system. This is the case for bothsemiconducting and superconducting materials at low temperature. If the absorbing element does not have a temperature-dependent resistance, as is typical of normal (non-superconducting) metals at very low temperature, then an attached resistive thermometer can be used to measure the electron temperature.
Microwave measurement
A bolometer can be used to measure power at microwave frequencies. In this application, a resistive element is exposed to microwave power. A dc bias current is applied to the resistor to raise its temperature via Joule heating, such that the resistance is matched to the waveguide characteristic impedance. After applying microwave power, the bias current is reduced to return the bolometer to its resistance in the absence of microwave power. The change in the dc power is then equal to the absorbed microwave power. To reject the effect of ambient temperature changes, the active (measuring) element is in a bridge circuit with an identical element not exposed to microwaves; variations in temperature common to both elements do not affect the accuracy of the reading. The average response time of the bolometer allows convenient measurement of the power of a pulsed source.