Phase-matched nonlinear wave-mixing processes in XUV region with multicolor lasers.

We report here experimental results of perturbative nonlinear optical wave-mixing processes in the extreme ultraviolet region by using two-color and three-color laser fields. Besides the usual odd-harmonic spectrum of high harmonic generation, new spectral components are observed when multiple incommensurate lasers (one driving plus one or two control field) interact with neutral krypton gas. To demonstrate the wave-mixing process underlying such an observation, we first couple the driving field with either the signal or the idler field of an optical parametric amplifier in the gaseous ensemble to generate certain mixing frequencies.

Cascaded four-wave mixing in the XUV region.

We present a detailed study of the wave-mixing process in the extreme ultraviolet (XUV) region (around 30 nm) by using two collinear multiple-cycle laser pulses with incommensurate frequencies (wavelengths 1400 and 800 nm). The experimental data provide evidence for the coherent accumulation of wave-mixing fields and a high third-order response of the medium in this spectral range. We show that the time evolution of the mixing fields can be used to study the coherence dynamics of the free-electron wave packet with a lifetime of 200-750 fs.

Coherent diffractive imaging microscope with a high-order harmonic source.

We report the generation of highly coherent extreme ultraviolet sources with wavelengths around 30 and 10 nm by phase-matched high-order harmonic generation (HHG) in a gas cell filled with argon and helium, respectively. We then perform coherent diffractive imaging (CDI) by using a focused narrow-bandwidth HHG source with wavelength around 30 nm as an illumination beam for two kinds of samples. The first is a transmission sample and the second is a absorption sample.

Ultrafast transient grating spectroscopy in silicon quantum dots.

Transient grating spectroscopy detects directly the relaxation of the excited carriers rather than time-resolved photoluminescence and thus it is particularly desired for the indirect semiconductors such as silicon quantum dots. We investigate ultrafast carrier dynamics in silicon quantum dots embedded in silicon oxide matrix using femtosecond transient grating spectroscopy. Two ultrafast decay components are observed with decay time of 800 fs and 4 ps at various detection wavelengths, which are attributed to the transverse optical and transverse acoustic phonon assisted relaxation.

Spectrally resolved femtosecond two-color three-pulse photon echoes: study of ground and excited state dynamics in molecules.

We report the use of spectrally resolved femtosecond two-color three-pulse photon echoes as a potentially powerful multidimensional technique for studying vibrational and electronic dynamics in complex molecules. The wavelengths of the pump and probe laser pulses are found to have a dramatic effect on the spectrum of the photon echo signal and can be chosen to select different sets of energy levels in the vibrational manifold, allowing a study of the dynamics and vibrational splitting in either the ground or the excited state. The technique is applied to studies of the dynamics of vibrational electronic states in the dye molecule Rhodamine 101 in methanol.