Optically and thermally tunable superconducting transmission lines

Using the method of time-domain optoelectronic sampling spectroscopy, we have tuned the ultrafast electrical pulse propagating through YBa₂Cu₃O_7-x (YBCO) transmission lines as a function of temperature and optical pulse energy. Electrical pulses are generated by photoconductively shorting charged semiconducting optical switches with 70 psec optical pulses at 532 nm. The optically generated and detected electrical pulses have frequencies extending up to 100 GHz. The superconducting transmission lines are microstrip spiral lines on the LaAlO₃ substrates. By varying the optical pulse energy from 0 to 23 pJ, the delay time through the 66-mm-long superconducting line is tuned by 54 psec at 55 K and shows a quadratic dependence on the optical pulse energy. As for the 250 mm-long transmission line, the delay of 69 psec is observed as the temperature increases from 26 K to 63 K.