Light craft: creating unusual properties in glass with light

Glass dominates modern optical technologies. Nonlinear optical processes, such as second-harmonic generation and parametric frequency conversion, are technological very attractive and require a second-order optical nonlinearity - a χ(2), which is normally absent in glass owing to its inversion symmetry. Thus, when light-induced frequency doubling was first discovered wide-ranging studies ensued into the mechanism and properties of this unexpected phenomenon. The mystery of self-organized χ(2) gratings was finally solved on the basis of a new physical phenomenon - the coherent photogalvanic effect, consisting in quantum interference between light fields at two different frequencies, ω and 2ω, which excites a phase dependent current (coherent photocurrent). Coherent photocurrent creates quasi-phase matching χ(2) gratings. Moreover in the experiments on electric-field second harmonic generation in optical fibres 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