High-speed asynchronous optical sampling ͑ASOPS͒ is a novel technique for ultrafast time-domain spectroscopy ͑TDS͒. It employs two mode-locked femtosecond oscillators operating at a fixed repetition frequency difference as sources of pump and probe pulses. We present a system where the 1 GHz pulse repetition frequencies of two Ti:sapphire oscillators are linked at an offset of ⌬f R = 10 kHz. As a result, their relative time delay is repetitively ramped from zero to 1 ns within a scan time of 100 s. Mechanical delay scanners common to conventional TDS systems are eliminated, thus systematic errors due to beam pointing instabilities and spot size variations are avoided when long time delays are scanned. Owing to the multikilohertz scan-rate, high-speed ASOPS permits data acquisition speeds impossible with conventional schemes. Within only 1 s of data acquisition time, a signal resolution of 6 ϫ 10 −7 is achieved for optical pump-probe spectroscopy over a time-delay window of 1 ns. When applied to terahertz TDS, the same acquisition time yields high-resolution terahertz spectra with 37 dB signal-to-noise ratio under nitrogen purging of the spectrometer. Spectra with 57 dB are obtained within 2 min. A new approach to perform the offset lock between the two femtosecond oscillators in a master-slave configuration using a frequency shifter at the third harmonic of the pulse repetition frequency is employed. This approach permits an unprecedented time-delay resolution of better than 160 fs. High-speed ASOPS provides the functionality of an all-optical oscilloscope with a bandwidth in excess of 3000 GHz and with 1 GHz frequency resolution.

Coherent zone-folded acoustic phonons are excited in GaAs͞AlAs superlattices by femtosecond laser pulses via resonant impulsive stimulated Raman scattering in both forward and backward scattering directions. The relative amplitudes of three distinct modes of first and second backfolded order match well with scattering intensities calculated within an elastic continuum model. The detection of the coherent acoustic modes is based on the modulation of the interband transitions via the acoustic deformation potential and exhibits a strong enhancement at interband transitions. [S0031-9007(98)08288-X] PACS numbers: 78.66.Fd, 63.22. + m, 78.47. + p The investigation of low-energy elementary excitations in semiconductor heterostructures is driven by their relevance as the final state in the energy relaxation process. In particular, acoustic phonons play a dominant role for dephasing processes at low lattice temperature and for heat transport in general. Acoustic phonons are commonly investigated by continuous wave (cw) Brillouin scattering. Time resolved experiments based on ultrashort pulse lasers have largely contributed to the understanding of acoustic phonon dynamics. Recently, the generation and propagation of ballistic acoustic phonons in a single semiconductor quantum well was observed by surface deflection spectroscopy [1]. Semiconductor superlattices exhibit zone folding of the acoustic branches within the mini-Brillouinzone (mini-BZ) due to the artificial periodicity of the elastic properties along the growth direction. Here, light can couple to zone-folded acoustic modes of the superlattice at higher frequencies in the 100 GHz to THz range. These modes have been extensively studied in cw Raman spectroscopy [2][3][4]. The folded bulk acoustic branches are optical branches within the superlattice zone scheme; thus light scattering from those modes is referred to as Raman scattering. The coherent excitation of a single first-order zone-folded mode was observed in GaAs͞AlAs superlattices using a time-derivative detection scheme [5]. However, the excitation and detection mechanisms are not yet fully clarified. In this paper we report on the nature of the excitation and detection mechanisms relevant for coherent zone-folded acoustic vibrations of first and second order in GaAs͞AlAs superlattices.Most time resolved experiments on coherent lattice excitations dealt with longitudinal optical phonons [6] or phonon polaritons [7]. While the excitation of phonon polaritons relies on impulsive stimulated Raman scattering (ISRS) [8], in the case of longitudinal optical phonons, a variety of excitation processes have been identified which cannot be explained within the context of stimulated Raman scattering. The most prominent non-Raman type mechanisms are the displacive excitation of coherent phonons relevant for symmetry maintaining optical modes [9] and the generation of coherent LO phonons via rapid surface field screening in polar semiconductors [10]. One important hint towards the determination of the excitation pro...

A femtosecond laser-based optical frequency synthesizer is referenced to an optical standard, and we use it to demonstrate the generation and control of the frequency of electromagnetic fields over 100 terahertz of bandwidth with fractional uncertainties approaching 1 part in 10(19). The reproducibility of this performance is verified by comparison of different types of femtosecond laser-based frequency synthesizers from three laboratories.