Experimental investigation of second-harmonic emission pattern from dual-laser-induced air plasma

We measured the emission pattern of a counterintuitive second-harmonic radiation from an air plasma co-excited by two ultrafast laser pulses. Contrary to a typical dual-beam second-order nonlinear process, which demands phase-matched nonlinear radiation to propagate between the two fundamental components, the emission of interest was found to be generated along two dominant directions that tailed closely after the two fundamental field beam paths. The measured divergence angle of the second-harmonic beam was less than half of that of the fundamental counterpart. We further compared the emission strength and its dependences on both fundamental beam optical polarizations along these two streams of second-harmonic signal. The comparison verified the symmetry between the two output beams. These observations provide a new, to the best of our knowledge, experimental basis that supports future theoretical understanding of this nonlinear effect in dual-laser-induced air plasma.