Optimizing phonon-cooled Nb hot-electron bolometers

A Nb hot-electron bolometer designed for operation as a sensor of millimete r waves and far-infrared radiation has been analyzed. This optically couple d detector has a very wide spectral bandwidth, a response speed of similar to 1 ns determined by electron-phonon scattering, and an impedance of 50 Om ega. The geometry employed here has traditionally been affected by stabilit y problems manifesting as random voltage fluctuations, which have hindered the usefulness of these bolometers. This issue has been addressed by explor ing the underlying physical causes and adapting the fabrication technique. Analytical results on substrate-induced bolometric effects are summarized a nd other factors concerning detector performance addressed, and the reality of an isothermal regime of operation is questioned. A noise equivalent pow er of 4 pW/Hz(1/2) was measured at 100 mum, with a coupling efficiency clos e to unity that was achieved by reducing the optical reflection losses. Ope ration of a similar device is shown able to resolve the macropulses of FELI X, the Free-Electron Laser for Infrared eXperiments, into its 40 ns spaced micropulses. (C) 2000 American Institute of Physics. [S0021-8979(00)03423-X ].