An in situ study of reconstructed gold electrode surfaces by second harmonic generation.

Second harmonic generation (SHG) was employed to monitor in situ the potential-induced reconstruction of Au(111) and Au(100) electrodes. Rotating the sample by 360° about the surface normal yields for the unreconstructed Au(111)-(1×1) surface the well-known threefold symmetry pattern in the SHG intensity, while Au(100)-(1×1) shows no rotational anisotropy, as expected for C4v symmetry. For the reconstructed Au(111)-(1×23) surface, however, an additional onefold symmetry pattern is observed, which allows in situ monitoring of the structural transition between (1×1) and (1×23). For the reconstructed Au(100)-(5×20) surface, a threefold symmetry pattern was found

Broadband THz absorption spectrometer based on excitonic nonlinear optical effects

A broadly tunable THz source is realized via difference frequency generation, in which an enhancement to χ (3) that is obtained via resonant excitation of III–V semiconductor quantum well excitons is utilized. The symmetry of the quantum wells (QWs) is broken by utilizing the built-in electric-field across a p–i–n junction to produce effective χ (2) processes, which are derived from the high χ (3) . This χ (2) media exhibits an onset of nonlinear processes at ~4 W cm −2 , thereby enabling area (and, hence, power) scaling of the THz emitter. Phase matching is realized laterally through normal incidence excitation. Using two collimated 130 mW continuous wave (CW) semiconductor lasers with ~1-mm beam diameters, we realize monochromatic THz emission that is tunable from 0.75 to 3 THz and demonstrate the possibility that this may span 0.2–6 THz with linewidths of ~20 GHz and efficiencies of ~1 × 10 –5 , thereby realizing ~800 nW of THz power. Then, transmission spectroscopy of atmospheric features is demonstrated, thereby opening the way for compact, low-cost, swept-wavelength THz spectroscopy

Phase-controlled nonlinear interferometry.

We have studied the influence of (relative) phases between fundamental and generated harmonic in a variety of nonlinear optical experiments. The concomitant large coherence length and the various methods of phase-control known from linear optics facilitate amplitude superposition and the generation of novel interference patterns, which may be used to determine phase and amplitude of the constituent processes