There is great current interest in the infrared radiative properties of high-Tc superconducting films and crystals. This interest stems from both fundamental considerations and promising applications. One promising application of high-Tc superconducting films is as infrared radiation detectors. It has also been proposed to cool superconducting electronics for space applications passively by thermal radiation. If the high-Tc superconductors are perfect reflectors at wavelengths greater than that corresponding to the energy gap, then a high-Tc superconductor radiation shield would show excellent performance if kept at a temperature of 20 K. Precise knowledge of optical properties, and particularly the complex refractive indices of high-Tc superconducting films, are required to evaluate these applications. In this study the monochromatic normal reflectance of high- quality films of YBaCuO and BiSrCaCuO is being measured with sets of four identical samples for temperatures between 6 K and room temperature, at wavelengths from 0.5 um to 30 um. The measurements are performed with a high-precision grating spectrometer which was especially designed for the measurement of the low-temperature radiative properties of noble-metal films. This apparatus does not employ a reference mirror. For the background measurement, the infrared beam travels through the sample compartment without optical interaction. The reflectance data are obtained by passing the beam through eight reflections from four identical samples. This technique avoids both shortcomings associated with the reference-mirror single-reflection method. Due to the absence of a mirror, absolute reflectance data are obtained, not values relative to a mirror reflectance less than unity. The measurements are carried out with films of the highest quality available, fabricated by Siemens, Erlangen, West Germany, and the IBM T.J. Watson Research Center, Yorktown Heights. These films are of pseudocrystalline nature, with strong c axis upward orientation, such that the radiative properties of the ab plane of the crystal unit cell are measured. These measurements are expected to set a new standard for the high- Tc materials. This standard will help to clarify the applicability of BCS-based theories to the optical properties of superconducting high-Tc films. The new precision measurements are also expected to shed light on the problem of the energy gap as observed by infrared spectra.
