Tailoring of Ultrafast Frequency Conversion with Quasi-Phase-Matching Gratings


Over the past decade, microstructured quasi-phase-matching (QPM) materials have been extensively used in nonlinear frequency conversion devices. QPM offers such advantages as large nonlinearities, noncritical propagation geometries and does not rely on natural birefringence of the material. More importantly, a single material can be tailored to allow interactions between almost any combination of wavelengths within the transparency range. Moreover, as it is particularly valuable for frequency conversion of ultrashort optical pulses, QPM allows tailoring of the amplitude and the phase response of the device.

In this dissertation we explore the new directions which have become possible due to realization that longitudinally nonuniform QPM gratings can bring much more to nonlinear frequency conversion than just tailoring a particular nonlinear material to phasematch a particular nonlinear interaction. We demonstrate several QPM devices which are not just improvements over the existing techniques but rather are conceptually novel devices.

In particular, we demonstrate QPM devices which combine second harmonic generation (SHG) with general shaping of light pulses on the femtosecond time scale. As a particular example of this QPM-SHG pulse shaping technique, we generate dual-wavelength synchronous second harmonic pulses from a single pump pulse. Careful accounting of the dispersion of the nonlinear material allowed design of a QPM-SHG pulse compressor for use with extremely short (less than 10 fs) pulses. Using such device we generate sub-6-fs pulses at 400 nm which, to the best of our knowledge, are the shortest pulses ever generated in the blue spectral region. We also demonstrate QPM pulse shaping devices which use the difference frequency generation, hence enabling shaped pulses to be obtained at any wavelength that can be phasematched by QPM.


December, 2000