| The central theme of this thesis is the design and use of bolometers for detection of far infrared and submillimeter wavelength radiation. A new material, micrometer thick membranes of silicon nitride, is used in modern bolometer designs. An understanding of thermal transport in silicon nitride is critical to evaluate and optimize detector performance. We have measured the thermal conductance, G, of {dollar}{lcub}approx{rcub}1mu m{dollar} thick low-stress silicon nitride membranes over the temperature range, {dollar}0.06 < T < 6K{dollar}, as a function of surface morphology. For {dollar}T > 4K, G{dollar} is independent of surface morphology indicating that the thermal transport is determined by bulk scattering. For {dollar}T < 4K,{dollar} scattering from membrane surfaces becomes significant. We find that G is reduced by a factor as large as 5 for membranes which have sub-micron sized Ag particles glued to the surface or are micromachined into narrow strips as are required in many applications when compared with that of clean, solid membranes with the same ratio of cross section to length.; We have used optimized bolometers for the study of two novel materials, single walled carbon nanotubes (SWNT) and single crystals of high temperature superconductors. We have measured the transmittance of several samples of bundles of SWNT over the frequency range {dollar}10 < nu < 300cmsp{lcub}-1{rcub}{dollar} at temperatures {dollar}1.2 < T < 300K{dollar}. The broadband shape of the transmittance has a temperature dependence similar to the DC transport measurements. We find a temperature dependent feature near {dollar}nuapprox 30cmsp{lcub}-1{rcub}{dollar} that is consistent with the prediction of a small energy gap {dollar}Esb{lcub}g{rcub}approx 4meV{dollar} and also with a soft librational mode in SWNT bundles.; We have directly measured the absorptivity of high quality single crystals of {dollar}rm YBasb2Cusb3Osb{lcub}6.5{rcub}{dollar} and {dollar}rm Tlsb2Basb2Casb2Cusb3Osb{lcub}10-delta{rcub}{dollar} over the frequency range {dollar}50 < nu < 800cmsp{lcub}-1{rcub}{dollar} at a temperature of 1.2K. Direct absorptivity measurements are powerful for studying materials in the superconducting state since in conventional superconductors the loss at frequencies below the energy gap is zero. The BCS theory, which describes the superconducting state of simple metals, fails to describe the electronic properties of the high {dollar}Tsb{lcub}c{rcub}{dollar} superconductors in the superconducting state. In particular, experimental studies have shown that the energy gap has a d-wave anisotropy. We measure non-zero absorption at frequencies below {dollar}3.5kTsb{lcub}c{rcub}/h{dollar} which is consistent with the d-wave model. We measure an increase in absorptivity below 55{dollar}cmsp{lcub}-1{rcub}{dollar}. Our data suggest a quasiparticle scattering rate much lower than reported for other high quality samples at these frequencies. We discuss our tests for systematic effects. |
