Jennifer L. W. Siders

Conductivity artifacts in optical-pump THz-probe measurements of YBa 2 Cu 3 O 7

Transient changes in the complex conductivity of the high-temperature superconductor YBa2Cu3O7 have been measured with optical-pump THz-probe spectroscopy. Artifacts in the conductivity arise when optical excitation induces changes in the material response that occur on a time scale comparable with or faster than the THz pulse width. A model is presented that agrees with experiments and provides insight in distinguishing between artifacts and real features in transient conductivity measurements.

Generation and characterization of terahertz pulse trains from biased, large-aperture photoconductors.

The saturation properties of terahertz emission from biased, large-aperture photoconductors excited by trains of amplified femtosecond optical pulses are presented. A direct comparison is made of the multiple-pulse saturation properties of terahertz emission from semi-insulating GaAs and low-temperature-grown GaAs emitters with different carrier lifetimes. When the carrier lifetime is less than or comparable with the interpulse spacing, a significant enhancement of the narrow-band terahertz output is observed. The enhancement is not observed for emitters with long carrier lifetimes, consistent with the results of a previously derived saturation theory [Opt. Lett. 18, 1340 (1993)].

Nonequilibrium superconductivity and quasiparticle dynamics in YBa 2 Cu 3 O 7-δ

We use optical pump, coherent terahertz probe spectroscopy to transiently excite nonequilibrium populations of quasiparticles in YBa2Cu3O7−δ and monitor, with picosecond resolution, the superfluid and quasiparticle dynamics.

Femtosecond Coherent Spectroscopy at 800nm: MI-FROG Measures High-Field Ionization Rates in Gases

The authors report the first quantitative phase-sensitive measurement of ultrafast ionization rates in gases using Multi-phase Interferometric Frequency-Resolved Optical Gating. Ultrafast probe depletion via frequency mixing in the ionization front is observed.

Nonequilibrium superconductivity and quasiparticle dynamics in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//

Summary form only given. We create a nonequilibrium distribution of quasiparticles in superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films using 150-fs, 1.5-eV optical pulses. The subsequent interplay between the superfluid and quasiparticle dynamics is investigated with coherent terahertz (THz) spectroscopy with the ultimate goal of understanding the character of the excited quasiparticles, their decay channels and relaxation times.

Enhancement of narrow-band terahertz radiation from large aperture photoconductors through multiple pulse excitation

Summary form only given. We present the saturation properties of the THz emission from biased, large aperture photoconductors excited by trains of amplified fs optical pulses. Our studies demonstrate avoidance of saturation effects through multiple pulse excitation, and hence a strong enhancement of the peak power spectral density, for emitters with carrier lifetimes shorter than the interpulse spacing. Such an enhancement was previously observed for photoconductive dipole antennas excited by pulses from a mode-locked oscillator; the current experiments extend this to the much higher THz field regime available using large aperture emitters and amplified laser pulses. Our results also confirm that the saturation recovery time depends on the photocurrent lifetime, consistent with previous theoretical results.

High-power narrow-band terahertz generation using multiple-pulse excitation of photoconductive antennas

Large-aperture biased photoconductive emitters which can generate high-power narrow-band terahertz (THz) radiation are developed. These emitters avoid saturation at high fluence excitation and achieve enhanced peak power spectral density by employing a thick layer of short lifetime low- temperature-grown GaAs (LT-GaAs) photoconductor and multiple-pulse excitation. THz waveforms are calculated from the saturation theory of large-aperture photoconductors, and a comparison is made between the theory and the measurement. A direct comparison of the multiple-phase saturation properties of terahertz emission from semi-insulating GaAs and LT-GaAs emitters with different carrier lifetimes reveals a strong dependence of the multiple pulse saturation properties of terahertz emission on the carrier lifetime. In particular, the data demonstrate that saturation is avoided only when the interpulse spacing is longer than the carrier lifetime.

Terahertz emission from YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films via bulk electric quadrupole optical rectification

The dependence of the terahertz emission on polarization and crystal orientation for YBa/sub 2/Cu/sub 3/O/sub 6.5/ at T = 300 K and excited at 1.5 eV is shown. This data exhibits the same symmetry for consistence with a bulk electric quadrupole XZ process. The peak emission from this film is about three times smaller than normal, consistent with the lower number of free carriers in this partially quenched material.