Automatic Phase Compensation for Extremely Short Optical-Pulse Generation Using Wavelet Transform

We introduce the wavelet transform (WT) method to automatic phase compensation for the generation of extremely short optical pulses with a duration of a few to several femtoseconds. This method is based on the immediate negative feedback of the spectral phase which is directly reconstructed by the wavelet-transform analysis with no uncertainty in phase retrieval without any manual decision, unlike the Fourier transform method. It is shown that the wavelet-transform method is useful for automatic compensations for three types of ultrabroadband pulses with different characteristics, such as: 1) a strong chirp; 2) a 600-rad spectral-phase variation over one octave bandwidth; and 3) a complicated spectral phase as well as a structured intensity spectrum

Exciton-exciton interaction and heterobiexcitons in GaN

The formation of not only A biexcitons (XXAA) but also heterobiexcitons that consist of A and B excitons (XXAB) in a free-standing bulk GaN is identified by polarization-sensitive spectrally resolved FWM measurements. The FWM spectra and delay-time dependence show that the interaction between A and B exciton gives rise to the energy shifts of the spectra and the phase shifts of the quantum beating, which is considered as the effect of the unbound state of XXAB (i.e., XX*AB) and XX is found to play an important role in the FWM signals for all polarizations. The unbound A biexciton (XX*AA) is also observed clearly in spectral and temporal domains

Valence-band-ordering of a strain-free bulk ZnO single crystal identified by four-wave-mixing spectroscopy technique

Spectroscopic and temporal four-wave-mixing (FWM) measurements are carried out on a strain-free bulk ZnO single crystal, in order to clarify the valence-band-ordering. Under the collinearly polarized lights with the electric-field component parallel to the c-axis, which can excite dipole-allowed Γ1-excitons, the FWM signal appears only in the energies corresponding to the B-exciton. Under the cross-linear polarization configuration exciting both Γ5- and Γ1-excitons, the FWM signal arising from the two-photon-coherence is absent in the energies corresponding to A-exciton. Both the results indicate that Γ1-exciton state belongs exclusively to B-exciton, meaning that the valence-band ordering is Γ9 - Γ7 - Γ7 in order of decreasing electron energy for the present strain-free ZnO single crystal