Ultrashort Pulse Second-Order Frequency Resolved Optical Gating with Thin Poled Nonlinear Polymers

Abstract

Frequency Resolved Optical Gating (FROG) [1] is a newly-developed technique for determining both the amplitude and phase of an ultrashort pulse.[2] Although there are several variations of the FROG technique which use different nonlinear processes, second harmonic generation (SHG) FROG is particularly well suited for measuring un-amplified nJ pulses directly from a modelocked Ti:sapphire oscillator, since SHG produces a much larger signal than the higher order nonlinear processes. In order for SHG FROG to accurately characterize ultrashort pulses, care must be taken to insure the second harmonic signal is not distorted by the nonlinear material, in phase-matched frequency doubling crystals for example, the group velocity mismatch (GVM) can cause severe distortion of the generated pulses over long interaction lengths. Since this finite phase matching bandwidth becomes less of a problem as the crystal thickness is reduced, the thinnest possible crystals (≈ 50 μm) are generally used for ultrashort-pulse measurement applications.[3] However, even using such short crystals, the crystal has to be optimally oriented to achieve proper phasematching for a given center frequency. Furthermore, fabricating such phasematched, polished, ultra-thin crystals is extremely difficult. Thus, an ideal SHG material for short pulse diagnostics is one that can generate an adequate signal intensity while being thin enough to avoid pulse broadening and precise angle tuning.