Indium antimonide (InSb) hot electron bolometer

Model QFI/X hot-electron bolometer

Technical specifications
Detector electrical responsivity >5 kV/W
Detector optical responsivity >3.5 kV/W
Detector electrical noise equivalent power (NEP) < 500 fW.Hz-1/2
Detector optical NEP < 750 fW.Hz-1/2
Useful frequency range 60 GHz (2 cm-1) to 500 GHz (30cm-1) at half-maximum
Frequency response ~1 MHz (-3dB)
Operating resistance 2 kOhm to 3 kOhm
Operating temperature 4.2 K

See below for notes relating to the above specification. 

Indium antimonide (InSb) bolometers offer much shorter time constants than composite germanium bolometers, at the expense of a narrower range of operating frequencies. Sensitivity at frequencies above 500 GHz can be improved with magnetically enhanced versions of this detector.

QMC Instruments Ltd. offers a high sensitivity InSb hot-electron bolometer that can be operated at or below 4.2 K. The detector is most commonly mounted in an optical integrating cavity behind Winston Cone coupling optics along with low-pass filters which ensure that unwanted higher frequencies are efficiently rejected.

An InSb (size 5 mm x 4.7 mm x 300 um) detector mounted on a quartz substrate is shown in the picture. Detectors can be purchased as a stand-alone unit or as part of a fully assembled, tested and calibrated detector system ready for use.


As with all our cryogenic detector systems, cooling can be provided either by a mechanical (pulse tube) cooler or by liquid helium. Mechanically cooled systems are complete and - apart from a vacuum pump for initial evacuation of the system - require only an electricity supply for operation.

Our liquid helium bath cryostats are designed and built to our specifications by our sister company, Thomas Keating Ltd. These cryostats offer the convenience of very long run-times. Cryostat operation is straightforward, and detector systems come with a comprehensive range of safety devices as standard. Superior cryogenic performance allows significant long-term saving on the cost of liquid helium. Larger cryostats are suitable for multi-channel detector systems or when very long liquid helium hold times are required.

Each detector system is evaluated and designed on an individual basis to provide optimum performance in all applications. Direction of view, throughput, field-of-view, filter selection, cryogenic performance and many more aspects of the detector system can be custom designed, often at no extra cost.

The ULN95 preamplifier is a high performance, wideband, low noise, rechargeable battery powered preamplifier designed specially for use with InSb hot electron bolometers. A low-noise JFET at the input stage ensures that the amplifier noise contribution does not dominate the system noise performance. The preamplifier is housed in a shielded housing and is designed to mount directly onto the side of the detector cryostat.

Notes on technical specifications

  • Detector optical responsivity is measured at 275 GHz
  • Detector optical NEP is measured at 275 GHz with  kHz modulation
  • Increased sensitivity is achieved by operating the detector at 1.5 K by reducing the vapour pressure above the helium-4 reservoir. Precise figures vary, but a typical detector will increase responsivity by 50% and will offer an NEP reduction by up to a factor of two

Example of System Optical Configuration

  • Side looking direction of view, approximately 80mm above base
  • f/2 quasi-parabolic Winston cone optic
  • 25 mm throughput (cone entrance aperture)
  • HDPE window
  • 1 THz low-pass multi-mesh filters at 77 K and 4.2 K

A two year guarantee is provided as standard on all QMC Instruments Ltd. products.

Operating principles

The speed of response of a conventional bolometer is limited by the thermal capacitance of the thermistor. Even in a composite structure, where a significantly smaller thermistor is thermally anchored to an absorbing metallic film, the device time constants are usually of order 1ms. This kind of detector is therefore clearly unsuitable for experiments where higher time resolution is desirable.

Indium Antimonide (InSb) is a III-V semiconductor used commonly as a detector in the near-infrared operating at liquid nitrogen temperature (77K.) 

At liquid helium temperature (4.2K) intra-band free-electron absorption of rather longer wavelengths results in changes to the electron mobility. The electron-electron interaction time is orders of magnitude shorter than that of the electron-phonon interaction by which energy is lost to the lattice. The electrons therefore come into thermal equilibrium at a temperature above the lattice. It is this temperature change and the resulting change in electron mobility, termed the hot-electron response, first predicted by E.H.Putley in 1960 that we make use of. The electron-phonon relaxation time constant is highly temperature dependent, and is measured to be 300ns at 4.2K.


Edited May11