Cascaded Nonlinear Optical Mixing in a Noncollinear Optical Parametric Amplifier

Ultrafast optical science is a rapidly evolving multidisciplinary field: the ability to excite matter with femtosecond light pulses and probe its subsequent evolution on ultrashort time scales opens up completely new fields of research in physics, chemistry, and biology.[1] Furthermore, the high intensities that can be generated using femtosecond light pulses allow us to explore new regimes of light-matter interaction.[2] The implementation of more sophisticated spectroscopic techniques has been accompanied by improvements in laser sources. Considerable effort has been dedicated to the achievement of shorter light pulses,[3-5] to improve temporal resolution; other efforts have worked to expand the frequency tunability of the pulses, since this would make it possible to excite in resonance different materials, and to probe optical transitions occurring at different frequencies. Early sources of femtosecond optical pulses were based on dye laser technology; in that case, some frequency tunability could be achieved by simply changing the laser dye.[6] This flexibility, however, came at the expense of a complicated and time consuming reoptimization. In recent years, femtosecond laser systems have become readily available. Rapid developments of widely tunable femtosecond laser radiation has also become power tools in the field of ultrafast spectroscopy, especially the transient ultrafast pump-probe spectroscopy on various from small organic compounds to complex enzymes, has opened up new research interests in femto-chemistry and femto-biology in the recent years.[7-9] The combination of high-intensity, ultrashort pulses from amplified solid-state lasers, for example, the regeneratively amplified Ti: sapphire laser systems running at 800-nm, with nonlinear optical techniques, such as optical parametric generator /amplifier (OPG/OPA), has made widely tunable femtosecond laser pulses routinely available in many research laboratories. Generally, the OPG process transfers energy from a high-power, fixed frequency pump beam to a low-power, variable frequency signal beam, thereby generating also a third idler beam. To be efficient, this process requires very high intensities of the order of tens of GW/cm2; it is therefore eminently suited to femtosecond laser systems, which can easily achieve such intensities even with modest energies, of the order of a few microjoules. They have proved versatile as widely tunable coherent sources, especially for short pulses since they can offer both high gain and high-gain bandwidth. Gain bandwidths of several thousand wave numbers are possible,[10-15] corresponding to transform-limited pulse durations down to a few femtoseconds, and amplification of such short pulses has