Strong double space-time wave packets using optical parametric amplification

Space-time wave packets are optical pulses synthesized from light waves carrying strong correlations between their spatial and temporal degrees of freedom and show many anomalous behaviors such as superluminal propagation. In this study, the authors proposed a way of amplifying space-time wave packets by nonlinear optical parametric processes, which will widen the applications of the new optical wave packets. The space-time (ST) wave packet, a type of light source with many anomalous behaviors, has already found application in weak-field optics, however its generation method limits its energy and therefore its application in strong-field optics. Here we show that the type-I collinear optical parametric amplification (OPA) is a natural amplifier for a ST wave packet, because in it, the ST spectrum (i.e., angle-dependent phase-matching spectrum) of the signal and idler has the same analytical expression as that of a ST wave packet under the narrowband approximation. The high gain in a thin-crystal OPA allows for large energy amplification while ensuring unchanged ST correlation. Meanwhile, a double ST wave packet containing two collinear-propagating ones of the amplified signal and the generated idler via the nondegenerate OPA is reported, which with short- and long-wavelengths (relative to the degenerate-wavelength) have superluminal and subluminal velocities, respectively. This study realizes energy amplification, wavelength conversion, and velocity switching of a ST wave packet and will expand its application in many fields.