Laser-induced echoes: from molecular alignment to harmonics generation in free electron lasers
ORAL
Abstract
Recently we predicted a novel phenomenon of molecular alignment echoes, and demonstrated it by measuring laser-induced birefringence in a thermal CO$_{\mathrm{2}}$ gas excited by a pair of femtosecond laser pulses [PRL 114, 153601 (2015)]. Here we report a new effect of fractional echoes observed in the same system via the third harmonic generation from a probe pulse [PRA 94, 033404 (2016)]. Moreover, using the technique of coincidence Coulomb explosion imaging (COLTRIMS) for the direct spatiotemporal analysis of molecular angular distribution, we observed a gamut of novel types of echoes, including fractional echoes of high order, spatially rotated echoes, and the counter-intuitive imaginary echoes at negative times [PRX 6, 041056 (2016)]. Remarkably, a phenomenon similar to fractional echoes of high order lies behind the recent demonstration of the efficient generation of high harmonics in free-electron lasers [Nat. Photonics 10, 512 (2016)].
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