Managing light in engineered nonlinear optical structures

Abstract

Optical frequency conversion process allows one to generate coherent light in wavelength ranges that are not readily available. It is well known that because of dispersion, the phase mismatch between interacting waves leads to low efficiency of frequency generation. Quasi-phase matching (QPM) that uses crystals with a spatial modulation of the second-order nonlinear coefficient χ(2), also known as nonlinear photonic crystals (NPC), is an important technique to solve the phase mismatch problem. With proper design of the quadratic nonlinearity modulation, one can not only obtain efficient frequency conversion, but also make diverse applications possible, including beam and pulse shaping, all-optical processing, entangled photon generation and manipulation. This thesis explores the fabrication, properties and application of various types of NPCs. In particular, we discuss NPC fabricated in single-domain ferroelectric crystals by using all-optical poling with near infrared femtosecond laser pulses and high voltage electric poling; as grown ferroelectric crystals with random sized ferroelectric domains; and orientation patterned semiconductors grown by hydride vapour phase epitaxy. Specifically, this research work involved the following topics: 1. Systematic investigation of a novel technique of direct writing of ferroelectric domains using near-infrared femtosecond laser pulses. Domain inversion in a LiNbO3 crystal was realized by its illumination with ultra-short infrared pulses only, without applying any external electric field. The quality of the inverted domains was characterized by Cerenkov second harmonic microscopy (CSHM) and selective chemical etching, respectively. The optically poled regions were not confined to the surface, but extended deep into the crystal. This is a significant result surpassing the capability of the traditional ultraviolet (UV) all-optical poling technique. As an experimental demonstration,Abstract x a QPM structure in a LiNbO3 channel waveguide was fabricated by this infrared laser poling technique allowing efficient frequency doubling of 815 nm light beam. 2. Experimental studies of the application of as-grown calcium barium niobate (CBN) crystal for a broadband frequency conversion. This frequency conversion process is similar to broadband harmonic generation in commonly used strontium barium niobate (SBN) crystal, but results in higher conversion efficiency reflecting a larger effective nonlinear coefficient of the CBN crystal. We also analyzed the spatial distribution of the intensity of the generated radiation as well as its polarization properties. This study contributes to a simpler and more efficient realization of broadband frequency conversion devices in a wide class of nonlinear optical media. 3. Experimental studies of multistep cascading frequency conversion processes in a custom-cut periodically poled lithium niobate crystal. By employing the total internal reflection inside the sample, we combined quasi-phase matched collinear and Cerenkov nonlinear sum frequency mixing to achieve enhanced fourth harmonic generation in a single periodically poled lithium niobate crystal. 4. Experimental investigation of nonlinear diffraction in an orientation-patterned semiconductor. By employing a new transverse geometry of interaction, nonlinear Cerenkov, nonlinear Raman-Nath and nonlinear Bragg diffractions were identified according to different configurations of quasi-phase matching conditions. The study extends the concept of transverse nonlinear parametric interaction toward infrared frequency conversion in semiconductors. It also offers an effective nondestructive method of visualization and diagnostic of spatial variations of second-order nonlinearity inside semiconductors. Show more