Second harmonic generation in centrosymmetric gas with spatiotemporally focused intense femtosecond laser pulses

Abstract

We demonstrate unexpectedly strong second harmonic generation (SHG) in argon gas by use of spatio-temporally focused (SF) femtosecond laser pulses. The resulting SHG by the SF scheme at a 75 cm distance shows a significantly enhanced efficiency than that achieved with conventional focusing schemes, which offers a new promising possibility for standoff applications. Our theoretical calculations reasonably reproduce the experimental observations, which indicate that the observed SHG mainly originates from the gradient of non-uniform plasma dynamically controlled by the SF laser field.

Characteristics and Applications of Spatiotemporally Focused Femtosecond Laser Pulses

“…The incident pulse energy is~1 mJ and the focal length of the focusing lens is 75 cm. Reproduced with permission from [25], Copyright The Optical Society, 2014.…”

“…Shortly after the establishment of the technique, it was discovered that the SSTF spot has some unconventional characteristics, including a pulse front tilt (PFT) and an intensity plane tilt [23,24]. Moreover, the interaction between the SSTF pulses and transparent media can induce some intriguing phenomena, such as a nonreciprocal writing effect, second harmonic generation in isotropic media, and curved plasma formation in fused silica glass [23,25,26]. Until now, the physics mechanism of the nonlinear interaction of SSTF pulses with transparent materials has not been fully understood.…”

“…Furthermore, in 2014, Li et al reported an unexpected intense second harmonic generation in the gas chamber filled with 1 atm argon using the SSTF scheme, which seems counterintuitive in centrosymmetric media [25]. As it can be seen in Figure 11, when the polarization direction of the pump pulses is along x axis (i.e., linearly polarized along the direction of the spatial chirp), the SSTF pulses can generate a second harmonic efficiently with an extremely high signal-to-noise ratio.…”

“…By changing the polarization of input laser pulses to the y axis (i.e., linearly polarized perpendicular to the direction of the spatial chirp), the second harmonic beam shows two lobes separated from each other and the signal-to-noise ratio is dramatically decreased. The underlying mechanism for such a second harmonic generation by the spatiotemporal focusing scheme is ascribed to the gradient of non-uniform plasma induced by the PFT of the SSTF pulses [25]. …”

Femtosecond laser three-dimensional micro- and nanofabrication

“…130,135,136 In particular, a latest investigation reveals that a spatiotemporally focused pulse possesses not only a pulse front tilt, but also an intensity plane tilt around the focus. 137…”