We suggest a novel scheme for multistep cascaded parametric processes based on a periodically poled nonlinear directional coupler and derive the conditions for simultaneous phase matching of the second-harmonic generation and sum-frequency mixing processes. We predict an efficient wavelength conversion by the cascaded third-harmonic generation from the third optical communication window (1.53-1...

We review recent experimental and theoretical work on the use of counterpropagating light to enhance high-order harmonic generation through all-optical quasi-phase matching. Also presented is a new technique for measuring the coherence of high harmonics in the nonlinear medium. This information is crucial for understanding the process of harmonic generation over extended distances, as well as f...

We have executed a series of Young’s two-slit experiments to measure the spatial coherence of soft x rays produced by high order harmonic generation in helium within the 270 to 480 Å wavelength range. We find that the harmonics exhibit good fringe visibility and high spatial coherence, though the coherence is somewhat degraded at high intensity because of the production of free electrons throug...

Techniques of coherent nonlinear spectroscopy (second harmonic generation and third harmonic generation) are combined with near-field scanning optical microscopy for imaging selected chemical and physical environments in biological matter on a nanoscopic scale. Resonant enhancement of nonlinear signals is utilized as a method of producing chemically selective contrast while the order of the pro...

We theoretically propose a new procedure of designing quasi-phase-matching (QPM) gratings for compressing optical ultra-short pulses during second-harmonic generation. The grating consists of blocks of crystal with same block length and the direction of spontaneous polarization of each block is determined by optimal algorithm, by which the sign of the nonlinear coefficient of each block is opti...

A biophotonics approach based on the nonlinear optical process of second harmonic generation is presented and demonstrated on malignant human cell lines labelled by harmonic nanoparticles. The method enables independent imaging and therapeutic action, selecting each modality by simply tuning the excitation laser wavelength from infrared to visible. In particular, the generation of deep ultravio...