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We report the development of a waveguide-coupled monolithic Si bolometer for applications in low-background millimeter-wave astrophysical observations. In this device, the absorber of the bolometer is a narrow Bi-coated Si substrate oriented along the E-plane of the waveguide. This design allows efficient coupling of bolometers to waveguide. We measured these devices to have a high coupling efficiency, an electrical responsivity /spl ap/2/spl times/10/sup 9/ V/W, and electrical noise equivalent power (NEP) /spl ap/10/sup -17/ W//spl radic/(Hz) at /spl sim/100 mK. >

The hot-electron microbolometer is a novel low-temperature thermal detector which has potential for very low-noise power measurements at millimeter wavelengths. We are investigating issues in the practical implementation of this detector for low-background applications. We present results from tests of an efficient scheme to couple the detector to waveguide, as well as the expected noise performance of a microbolometer optimized for balloon-borne measurements. The expected NEP of microbolometer in such a measurement is 7/spl times/10/sup 18/ W//spl radic/(Hz) which is nearly a factor of two better than bolometers currently in use. >