Terahertz Hilbert Spectroscopy by High-T c Josephson Junctions

Abstract

Hilbert spectroscopy is based on the frequency-selective detection of weak electromagnetic radiation by Josephson junctions described by the resis- tively shunted junction (RSJ) model. The YBa2Cu3O7−x grain-boundary Joseph- son junctions fabricated on twin-free NdGaO3 bicrystals were found to be close to the RSJ model. Laboratory prototypes of general-purpose Hilbert spectrometers were developed and characterized. The spectral bandwidth of Hilbert spectroscopy for any junction temperature between 30 and 85K is as large as one frequency decade. The middle frequency of this bandwidth scaled with the characteristic fre- quency fc =(2e/h)IcRn of the Josephson junction and a total bandwidth of almost two orders, from 50GHz to 4 THz, was covered by one Josephson junction at two temperatures. A spectral resolution δf/f of the order of 10−3 was realized in the terahertz range. The dynamic range of intensities of radiation which can be mea- sured by Hilbert spectroscopy is close to five orders. With the help of Hilbert spec- troscopy we measured the following emission spectra: Lorentz spectra of Josephson oscillations, spectra of high-harmonic content in commercial millimeter-wave oscil- lators, spectra of terahertz radiation from optically-pumped gas lasers and spectra of transition radiation from relativistic electron bunches at DESY (Hamburg).