Kinetic inductance bolometers and calorimeters, each consisting of a kinetic inductance device (KID) suspended on a membrane and embedded in a resonant circuit, are being developed for applications such as planetary science, climate science, and X-ray spectroscopy. Arrays of these resonator-bolometers, each with a unique resonant frequency, are coupled to a single feedline, allowing many bolometers or calorimeters to be multiplexed using microwave readout. These devices potentially offer a combination of high sensitivity, large arrays, and broadband response not available from competing technologies.
In a kinetic inductance bolometer (KIB), the KID is suspended on a membrane to decouple its temperature from the bath temperature of the silicon substrate upon which the device is fabricated. The KID's inductance depends on the temperature of the membrane, making it a sensitive thermometer. The KID is also embedded in a resonant circuit so that the bolometer response may be read out by biasing the bolometer at a frequency near resonance (typically near a GHz). Power radiated on the bolometer heats the membrane and KID, increases inductance, and decreases the resonance frequency, resulting in a change in the response at the bias frequency. Kinetic inductance calorimeters (KICs) are similar except they measure the energy of absorbed particles rather than power.
The thermal bandwidth of bolometers and calorimeters is set by the combination of the bolometer heat capacity (which is determined by the size and material composition of the bolometer) and thermal conductance. The thermal conductance must be small to minimize thermal noise in the bolometer, resulting in slow devices. If bolometers are deployed in an optical instrument such as an imaging Fourier transform infrared spectrometer, the response time limit may restrain the spatial resolution or wavelength resolution of the instrument. But increasing the thermal conductance to speed up the bolometers results in an increase in noise and loss of sensitivity. Achieving faster bolometers without losing sensitivity could greatly improve the capabilities of these instruments.
A novel form of electrothermal feedback can be employed to speed up the response time of KIBs and KICs. The electrothermal feedback is associated with varying reactance in the kinetic inductor, which shifts the resonance frequency as the temperature changes. For positive detuning, a positive temperature fluctuation causes increased inductance and increased detuning, resulting in decreased power in the bolometer. If the applied bias power in the bolometers is sufficient to significantly elevate the membrane temperature above the bath temperature, then the detuning may significantly decrease the power into the device. The decrease in bias power cools the bolometer back to equilibrium faster than through thermal cooling alone, resulting in feedback that speeds up the bolometer. With large bias powers, the speedup may be by a factor of 10 to 100 times for KIBs.