Spectral broadening and dispersion compensation of mid-infrared femtosecond pulses

Abstract

INTRODUCTIONSelf-phase modulation (SPM) is a nonlinear phenomenon widely used for broadening optical spectra in the visible to near-infrared (NIR) [1]. There have been, however, only a limited number of such reports in MIR. Corkum et al.[2] observed spectral broadening of picosecond pulses of 600-PJ energy at 9.3-Pm wavelength in semiconductor materials. Kapetanakos et al. [3] numerically studied the modulation instability inside GaAs with two closely-spaced radiation lines of a CO 2 laser. Recently, the supercontinuum extending from 0.8 Pm to 4.5 Pm has been demonstrated with ZBLAN fluoride fibers [4]. A drawback of such IR glass fibers is the difficulty in delivering high energy pulses because of the lower softening temperature. Kim et al. [5] demonstrated the supercontinuum generation at 2 ± 3.2 Pm by use of a single crystal sapphire fiber of high melting temperature. This material is useful in a range shorter than 5 Pm.Here we report that the efficient spectral broadening of MIR 100-fs pulses occurs via SPM in a GaAs single crystal. The second-order dispersion of the output pulses was compensated by the dispersive propagation in a CaF 2 plate to generate three-cycle pulses. The demonstrated scheme should work at any wavelength within the materials transparency range, and therefore be useful for spectral broadening of pulses generated from the well-established OPA-DFG system tunable around 3-18 Pm. Such MIR short pulses with broad spectra are of great importance in observing/controlling molecular vibrational dynamics.