A detailed calculation of the amplitude and phase response of ultrathin ZnTe and GaP electro-optic sensors is presented. We demonstrate that the inclusion of the dispersion of the second-order nonlinearity is essential. Significant structures in experimental data can be explained by the theoretical response function. Correcting for the detector characteristics, we determine the precise shape of electromagnetic transients with a time resolution of 20 fs. In addition, we show that ultrafast transport of photocarriers in semiconductors can act as an efficient source for coherent electromagnetic radiation covering the entire far-to-mid-infrared regime.

We demonstrate tomographic T-ray imaging, using the timing information present in terahertz (THz) pulses in a reflection geometry. THz pulses are reflected from refractive-index discontinuities inside an object, and the time delays of these pulses are used to determine the positions of the discontinuities along the propagation direction. In this fashion a tomographic image can be constructed.

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We investigate the dynamics of coherent optical phonons in tellurium after high-density excitation with femtosecond laser pulses. The data show a continuous redshift of the phonon frequency with increasing excitation density. Experiments with double-pulse excitation prove that the observed frequency shift is of purely electronic origin. We demonstrate that coherent phonons allow monitoring the pathway to nonthermal laser-induced melting of crystalline materials. 64.70.Dv, 78.47.+p Since the optical excitation of semiconductor crystals corresponds to a promotion of electrons from bonding into antibonding states, high-density excitation may lead to a nonthermal melting, without heating of the lattice to the melting temperature [1]. The occurrence of laser-induced phase transitions has been experimentally demonstrated in time-resolved studies of second-harmonic generation on Si and GaAs [2,3]. In these experiments, a loss of the crystal structure was observed on a femtosecond time scale, i.e., before significant energy transfer between the excited electrons and the lattice can occur. However, these experiments are sensitive to the crystal symmetry and can only indicate whether the phase transition has taken place or not. In this Letter, we present a study of coherent phonons in highly excited tellurium, where we monitor the weakening of the crystal via the reduction of the phonon frequency. We show that this shift is of almost purely electronic origin and directly related to the electron density. Therefore our experiments provide direct information about the pathway towards a laserinduced phase transition.Optical excitation and time-domain detection of coherent lattice vibrations require femtosecond laser pulses with a duration much shorter than the vibration period and has been demonstrated in various materials [4][5][6][7]. One of the most efficient generation mechanisms is the "displacive excitation of coherent phonons" (DECP) that has been identified in experiments with several materials, including tellurium [5]. This mechanism results from the fact that the lattice equilibrium position of an electronically excited state differs from that of the ground state. Therefore above-band-gap optical excitation with an ultrashort laser pulse prepares the lattice in astate displaced from its new equilibrium position, leading to coherent lattice vibrations with a characteristic cosinelike time dependence [8].Up to now, most studies of coherent phonons have been performed in a low-excitation regime, so that the coherent phonon frequencies match exactly the frequencies observed in nonresonant Raman measurements. Only in two recent studies has a transient shift of the phonon frequency been reported, which has been assigned to the large ionic displacement [9], i.e., anharmonicity of the phonon, and to "ionic screening" by the photoexcited carriers [10], where the time dependence has been interpreted in terms of screening efficiency. However, recent theoretical investigations predict a continuous weakening of the lattice w...

Diffusion-tensor MR imaging was compared at 1.5 and 3.0 T. With sufficient signal-to-noise ratio, we found no differences in fractional anisotropy. With a 40% higher signal-to-noise ratio at 3.0 T, higher resolution could be obtained without introduction of noise-related errors, albeit at the cost of increased geometric distortions caused by 3.0-T magnetic field inhomogeneities.

The transient current response of bulk GaAs and InP is investigated at ultrahigh electric fields. On ultrashort time scales, the electronic system is far from equilibrium and overshoot velocities as high as 8 3 10 7 cm͞s are observed. Our studies also lead to a detailed understanding of the ionic response of polar semiconductors. For the first time, carrier motion is determined with a resolution of 20 fs at fields up to 130 kV͞cm. The dependence of the ultrafast dynamics on material and electric field provides new insights into the microscopic mechanisms governing nonequilibrium transport.[S0031-9007(99)09427-2]