Ultrafast dynamics of nonequilibrium

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From the observed single-picosecond electrical transients, we were able to extract the characteristic electron thermalization and electron-phonon relaxation time.

Superconducting and Related Oxides: Physics and Nanoengineering III, ed. by I. Bozovic and D. Pavuna Proc. SPIE. 3481, 480-491 (1998).

Ultrafast dynamics of nonequilibrium quasiparticles in high-temperature superconductors Roman Sobolewski* Department of Electrical and Computer Engineering and Laboratory of Laser Energetics University of Rochester, Rochester, NY 14627-0231, USA ABSTRACT The discovery of high-temperature superconductors (HTS) and associated expectations of application of these materials in ultrafast electronics and optoelectronics has created an urgent need for a better understanding of carrier dynamics in HTS, including the HTS electrical and optical properties and their response to pulsed, external electromagnetic perturbations. The above goals were accomplished via comprehensive transient photoexcitation measurements of light-induced nonequilibrium phenomena in high-quality, epitaxial YBa2 Cu 3 O7-x (YBCO) thin-film microbridge samples. The photoresponse from 2 × 10 6 A/cm2 at 77 K. Excitation beam Sampling beam YBCO coplanar waveguide LiTaO3 crystal

Fig. 4. Experimental sample configuration, including the excitation and probe optical beams, sample biasing, and the LiTaO3 EO crystal. Z2200

The sample was mounted on a gold-plated alumina substrate, attached to a copper block inside an exchange-gas, liquid-helium dewar with optical access through a pair of fused-silica windows. During measurements, the sample was in He exchange gas, and the temperature was stabilized to ±0.2 K by a temperature controller. One end of the CPW was wirebonded directly to a semirigid, 50-Ω coaxial cable, while the other end was wirebonded to ground on the alumina plate. The 1.2-mlong cable brought the signal out of the dewar, and, together with an 18-GHz-bandwidth bias-tee and a 20-GHz-bandwidth amplifier (30-dB gain), allowed us to observe the bolometric part of the bridge response on a 14-GHz-bandwidth sampling oscilloscope. The oscilloscope signal was also used to optimize the alignment of the excitation beam for maximum response. As shown in Fig. 4, the entire CPW structure was overlaid with an EO LiTaO 3 crystal to facilitate the EO sampling measurements. The complete experimental EO setup is shown in Fig. 5. A commercial Ti:sapphire laser, pumped by an Ar-ion laser, was used to generate the microbridge photoresponse and electro-optically measure the propagating transient. The laser provided ~100-fs-wide optical pulses with 780-nm wavelength and 76-MHz repetition rate, at an average power of 1 W. The beam was split into two paths by a 70/30 beamsplitter. The first (excitation) beam (700 mW) was frequency doubled in a nonlinear ß-bariumborate (BBO) crystal, and a reflective filter was used to eliminate the remaining 780-nm light. The excitation beam was intensity modulated by an acousto-optic modulator and focused by a microscope objective to a 10-µmdiam spot on the microbridge. The microscope objective was also a part of the viewing arrangement, which allowed us to observe the sample during positioning of the beams. The average optical power of the 390-nm light, measured at a position just outside the dewar, was ~2 mW. By measuring the amount of light absorption/reflection in the two dewar windows and the LiTaO3 crystal, we found that the incident power was further reduced to ~1 mW at the YBCO detector surface, corresponding to a fluence of 17 µJ/cm 2 . Taking the geometry as well as the reflectance and transmittance of YBCO into account, we estimate the power actually absorbed by the microbridge to be only ~60 µW (fluence 1 µJ/cm 2 ), what corresponds to approximately 2 × 1017 3-eV photons per cm3 . According to previous experiments,8 the permanent temperature increase due to laser illumination was ~3 K/mW; thus, the temperature of the light-illuminated bridge increased only ~0.2 K in our case.

Beam splitter

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