As the laser peak power increases constantly, higher temporal contrast on target is demanded for physical experiments. Besides, a larger total gain, which is required for the amplification chain, will result in spectral bandwidth degeneration. Therefore, seed pulses of high contrast and ultra-broadband spectrum from the front end are crucial for the whole laser system.

The contrast can be decreased by the background noises which are introduced by many factors. Once the intensity of the focused noise reaches 10¹³W/cm², the interaction between main pulse and the target can be fiercely interfered. In dealing with this problem, the cross-polarized wave (XPW) generation is one of the most widely adopted method for contrast enhancement. However, in the state-of-art XPW schemes with BaF₂ adopted as the nonlinear crystal, the XPW conversion efficiency is usually less than 15% for single-BaF₂ schemes and ~25% for dual-crystal schemes. And the highest spectral broadening ratio (SBR) is reported merely ~2.3 in dual-BaF₂ scheme.

Recently, a research team from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has realized the XPW conversion efficiency with ~23.3%, contrast enhancement by ~9.7 orders of magnitude and spectrum of 626-1,167nm (FWHM~259nm) supporting Fourier-transfer-limit duration of 5.5 fs using the gadolinium gallium garnet (GGG) crystal by simulation. The results were published in Applied Physics Express.

In the experiment, researchers adopted the fourth-order Runge-Kutta algorithm to numerically simulate Eq. (1) for obtaining the output XPW conversion efficiency, contrast enhancement, SBR, temporal profile, etc.

The simulation also indicated that XPW injected by fundamental wave of higher-order super-Gaussian temporal profiles presented superior characterizations.

The comparison of XPW generation between GGG and BaF₂ has verified that the XPW efficiency for GGG is higher and the spectral broadening is much more remarkable, which implies a very promising application prospect of GGG crystal in XPW filters.

The work was supported by the National Natural Science Foundation of China.

XPW generation for GGG with injected FW of different temporal profiles. (Image by SIOM)