K. Reimann

Band gaps, crystal-field splitting, spin-orbit coupling, and exciton binding energies in ZnO under hydrostatic pressure

Abstract Using two-photon absorption under hydrostatic pressures up to 7.3 GPa at a temperature of 6 K, we have determined the pressure dependences of the three lowest-energy band gaps in the wurtzite semiconductor ZnO. Additionally, we have measured exciton binding energies, crystal-field splitting, and spin-orbit coupling as a function of pressure.

Generation of single-cycle THz transients with high electric-field amplitudes.

Single-cycle terahertz (THz) transients in the frequency range 0.3-7 THz with electric-field amplitudes of more than 400 kV/cm are generated by four-wave mixing of the fundamental and the second harmonic of 25 fs pulses from a Ti:sapphire amplifier in ionized air. These transients are fully characterized by electro-optic sampling with ZnTe and GaP crystals. One can tune the center frequency of the THz transients by varying the length of the incident pulse. The electric-field amplitude increases linearly with the incident pulse energy.

Generation, shaping, and characterization of intense femtosecond pulses tunable from 3 to 20 µm

We report on an intense mid-infrared light source that provides femtosecond pulses on a microjoule energy level, broadly tunable in the 3–20-µm wavelength range with pulse durations as short as 50 fs at 5 µm. The pulses are generated by phase-matched difference-frequency mixing in GaSe of near-infrared signal and idler pulses of a parametric device based on a 1-kHz Ti:sapphire amplifier system. Pulse durations are characterized with different techniques including autocorrelation measurements in AgGaS2, two-photon absorption in InSb, and cross-correlation measurements with near-infrared pulses in a thin GaSe crystal. A subsequent zero-dispersion stretcher of high transmission allows for optimum pulse compression, a more detailed amplitude and phase characterization and, ultimately, amplitude shaping of the mid-infrared pulses.

Table-top sources of ultrashort THz pulses

In this paper techniques for the generation and measurement of ultrashort pulses in the frequency range from about 0.1 to 10 THz are reviewed. The methods for generation are restricted to table-top systems based on short-pulse lasers in the visible or in the near-infrared. Three techniques are dealt with in detail: photoconductive switches, difference frequency generation and plasma sources. Definitions and methods to measure the pulse width are given, among them cross-correlation and measurements of the electric field of these pulses as a function of time by photoconductive switches and electro-optic sampling.

Direct field-resolved detection of terahertz transients with amplitudes of megavolts per centimeter

Phase-matched difference-frequency mixing in a thin GaSe crystal within the broad spectrum of 25-fs pulses from a Ti:sapphire oscillator multipass amplifier system permits the generation of few-cycle electric field transients, frequencies up to 30 THz, and amplitudes of more than 1 MV/cm. The field transients generated at a 1-kHz repetition rate are directly measured by electro-optic sampling by 12-f probe pulses from the 75-MHz repetition-rate Ti:sapphire oscillator in combination with a novel electronic gating technique.

Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO2⋅Y2O3

First measurements of oxygen grain boundary diffusion coefficients in nanocrystalline yttria-doped ZrO2 (n-ZrO2⋅6.9 mol % Y2O3) are presented. The 18O diffusion profiles measured by secondary ion mass spectroscopy are much deeper in the nanocrystalline specimens than in single crystals. An oxygen diffusivity, DB, in the grain boundaries can be deduced, which is ≈3 orders of magnitude higher than in single crystals. From the present data the temperature variation of the oxygen grain boundary diffusivity, DB = 2.0 × 10−5 exp (−0.91 eV/kBT) m2/s, and the oxygen surface exchange coefficient, k = 1.4 × 10−2 exp (−1.13 eV/kBT) m/s, are derived.

Internal motions of a quasiparticle governing its ultrafast nonlinear response.

A charged particle modifies the structure of the surrounding medium: examples include a proton in ice, an ion in a DNA molecule, an electron at an interface, or an electron in an organic or inorganic crystal. In turn, the medium acts back on the particle. In a polar or ionic solid, a free electron distorts the crystal lattice, displacing the atoms from their equilibrium positions. The electron, when considered together with its surrounding lattice distortion, is a single quasiparticle, known as the Fröhlich polaron. The basic properties of polarons and their drift motion in a weak electric field are well known. However, their nonlinear high-field properties—relevant for transport on nanometre length and ultrashort timescales—are not understood. Here we show that a high electric field in the terahertz range drives the polaron in a GaAs crystal into a highly nonlinear regime where, in addition to the drift motion, the electron is impulsively moved away from the centre of the surrounding lattice distortion. In this way, coherent lattice vibrations (phonons) and concomitant drift velocity oscillations are induced that persist for several hundred femtoseconds. They modulate the optical response at infrared frequencies between absorption and stimulated emission. Such quantum coherent processes directly affect high-frequency transport in nanostructures and may be exploited in novel terahertz-driven optical modulators and switches.

Phase-resolved two-dimensional spectroscopy based on collinear n-wave mixing in the ultrafast time domain

We present a novel approach for femtosecond two-dimensional (2D) spectroscopy in the midinfrared combining a collinear beam geometry and phase-resolved detection. Two phase-locked pulses of variable time delay tau interact with the sample. The transmitted electric fields are measured in real time t by electro-optic sampling. 2D spectra are generated by Fourier transforming the signal along the two time axes tau and t. In the 2D spectra, nonlinear signals originating from different orders n in the electric field are separated. Such decomposition of the overall response is demonstrated by mapping the nonlinear response of intersubband transitions in GaAs/AlGaAs multiple quantum wells.

Experimental determination of the electronic band structure of SnO2

Abstract The S exciton series belonging to the fundamental gap and two hitherto unknown valence bands with even parity have been discovered by two-photon spectroscopy in the rutile-type semiconductor tin oxide (SnO2). Since at least three excitons were observed for each band, it was possible to determine accurately exciton binding energies and band gaps.

Above-room-temperature mid-infrared lasing from vertical-cavity surface-emitting PbTe quantum-well lasers

Above-room-temperature operation of vertical-cavity surface-emitting lasers emitting in the mid-infrared is reported. The stimulated emission is generated in PbTe quantum wells embedded in two-wavelength microcavities by optically pumping with fs laser pulses. The spectrum of the laser modes is broadened and blue-shifted due to dynamic band filling. The intensity of the mid-infrared emission and the laser threshold depends on the energy of the microcavity resonance. At a wavelength of 3.1 μm, laser operation is obtained up to a temperature of 65 °C, limited by nonradiative recombination processes.