M. Schall

Far-infrared vibrational spectra of all-trans, 9-cis and 13-cis retinal measured by THz time-domain spectroscopy

Low-frequency torsional vibration modes of the conjugated polyene chain of the biologically important chromophore retinal are of major relevance when studying its photoisomerization dynamics. We present absorptivity and dispersion spectra of three diAerent retinal isomers in the far-infrared region between 10 and 100 cm ˇ1 (0.3‐3.0 THz) measured by terahertz time-domain spectroscopy at 298 and 10 K. At low temperatures the observed broad absorption bands resolve into narrow peaks correlating to torsional modes of the molecule. Comparison of the absorption spectra of diAerent retinal isomers enables the approximate localization of the modes within the molecule. Experimental linewidths and transition strengths are reported. ” 2000 Elsevier Science B.V. All rights reserved.

Far infrared properties of electro-optic crystals measured by THz time-domain spectroscopy

In this article we report an experimental investigation of the far-infrared properties of several nonlinear crystals, LiNbO3, LiTaO3, ZnTe and CdTe. Using Terahertz Time-Domain Spectroscopy (THz-TDS) we have measured the complex frequency response, i.e. both index of refraction and absorption up to 3 THz (100 cm−1) for the electro-optic crystals at room temperature. The single Lorentzian oscillator model is used to describe the aquired data. Additional resonance features are observed, especially in the II-VI compounds.

Electro-optic detection of THz radiation in LiTaO3, LiNbO3 and ZnTe

Freely propagating THz pulses are detected in electro-optic (eo) crystals by monitoring the phase retardation (PR) of an infrared femtosecond probe pulse. This technique permits the determination of the temporal shape of the THz pulse in the subpicosecond time domain. We present measurements in LiTaO3, LiNbO3, and ZnTe and compare their signal performance as eo crystals with theoretical calculations for the PR signal. ZnTe shows the best performance for eo detection.

Photoexcited GaAs surfaces studied by transient terahertz time-domain spectroscopy

The transmission characteristics of an air-GaAs interface and the transient absorption and index spectra of the thin, photoexcited surface layer are investigated subsequent to excitation by a femtosecond laser pulse. We find that the total phase change and transmission of a terahertz (THz) probe pulse are dominated by interface effects. This observation has important implications in the interpretation of THz time-domain spectroscopy data of absorbing media. We also observe that the THz pulse apparently arrives at the detector as much as 60 fs earlier when it is transmitted through an optically excited GaAs wafer. This effect is fully explained in terms of a frequency-dependent transmission and phase shift at the air-GaAs interface and is not associated with superluminal propagation.

Ultrafast carrier trapping in microcrystalline silicon observed in optical pump–terahertz probe measurements

We report on direct evidence of ultrafast carrier dynamics displaying features on the picosecond time scale in microcrystalline silicon (μc-Si:H). The dynamics of photogenerated carriers is studied by using above-band-gap optical excitation and probing the instantaneous carrier mobility and density with a THz pulse. Within the first picoseconds after excitation, the THz transmission transients show a fast initial decay of the photoinduced absorption followed by a slower decrease due to carrier recombination. We propose that the initial fast decay in the THz transients is due to carrier capture in the trapping states.

Characterization and application of dichroic filters in the 0.1-3-THz region

Low-loss dichroic filters, a subgroup of frequency-selective components, have been characterized by terahertz time-domain spectroscopy in the region from 0.1 to 3 THz and with Fourier transform spectroscopy. The two data sets are fully consistent. The time-domain spectrometer is used to investigate the phase velocity behavior of dichroic filters. The dichroic filters have various applications in frequency mixing, multiplying, and diplexing experiments. In a novel application, cascaded filters were used to limit the terahertz pulse bandwidth and to monitor molecular transitions of atmospheric water vapor in a selected frequency band.

Above-band gap two-photon absorption and its influence on ultrafast carrier dynamics in ZnTe and CdTe

We present measurements of transient carrier dynamics subsequent to intense above-band gap femtosecond excitation of the II–VI compounds ZnTe and CdTe, probed in the far infrared by transient terahertz time-domain spectroscopy. The intensity and temperature dependence of the carrier dynamics illuminates the role of two-photon absorption (TPA) of pump photons. At cold temperatures and high excitation intensities TPA results in a photoexcited carrier distribution which requires several picoseconds to thermalize. As a consequence, the dielectric function of the carrier distribution deviates strongly from the Drude model for as long as 20 ps after excitation.

Freeze-out of difference-phonon modes in ZnTe and its application in detection of THz pulses

Generation by optical rectification and detection by electro-optic sampling of freely propagating subpicosecond electromagnetic pulses (terahertz pulses) in ZnTe is limited by two broad difference-phonon absorption bands in the spectral region below the transverse optical resonance at 5.32 THz. In this letter, we show that, at temperatures below 80 K, these difference modes are frozen out and consequently ZnTe becomes completely transparent within our detection bandwidth. We utilize this effect to obtain a three-fold increase in detection efficiency at frequencies above 3 THz in a thin ZnTe detector operated at 80 K, compared to the performance at room temperature.

Detection of High Power THz Pulses by Phase Retardation in an Electro-Optic Crystal

In the last decade much research work has been done on the generation, detection and application of short-pulse THz radiation, see for an overview.1

Subpicosecond time-resolved terahertz time-domain spectroscopy of transient carrier dynamics in semiconductors

We have applied a newly developed transient terahertz time- domain spectrometer to study the temporal development of the dynamics of photogenerated carriers in semiconductor materials. The study presented here include semi-insulating (SI) and low-temperature-grown (LT) GaAs. By measuring the detailed shape of a subpicosecond electrical field pulse (THz pulse) transmitted through the sample at a time T after excitation with a femtosecond laser pulse, the absorption coefficient and refractive index in the region between 0.1 THz and 3 THz can be measured with high accuracy. By varying the time T, the transient absorption and index spectra can be measured with subpicosecond time resolution. Temporal and spectral behavior of the carrier dynamics in SI and LT GaAs, in dependence of intensity and wavelength of the excitation pulse, is measured. We directly observe carrier scattering to the sidevalleys and the subsequent return of the carriers to the central valley. The experimental data strongly suggest that the transmission of the THz pulse through the photoconducting surface layer of the semiconductor can be described as instantaneous tunneling of the electric field through a metal-like barrier.