Quantum interference and poling in optical glass waveguides

Optical glass-fibres and waveguides dominate optical communications. The development of linear electrooptic modulators/switches and parametric frequency converters directly integrated into optical glass waveguide structures technologically is very attractive. However such components require a second-order nonlinearity - a χ(2), which is normally absent in glass owing to its inversion symmetry. Thus, when self-organised frequency doubling was first discovered wide-ranging studies ensued into the mechanism and properties of this unexpected phenomenon. The mystery of photoinduced χ(2) gratings was finally solved on the basis of a new physical phenomenon - the coherent photogalvanic effect, consisting in quantum interference, which excites a phase dependent current (coherent photocurrent). Coherent photocurrent induces quasi-phase matching χ(2) gratings. More recently the value of nonlinearity has been increased to a level, comparable to the best nonlinear crystals by new poling techniques. Moreover in the experiments on fibre electric field poling the first evidence of phase dependent modulation of a total cross-section of ionization due to quantum interference (coherent photoconductivity) in solid state materials has been obtained