Harmonic generation at air-soil interface by femtosecond laser filament

We observe the third-harmonic generation and second-harmonic generation together with element fluorescence from the interaction of a femtosecond laser filament with a rough surface sample (sandy soil) in non-phase-matched directions. The harmonics prove to originate from the phase-matched surface harmonics and air filament, then scatter in non-phase-matched directions due to the rough surface. These harmonics occurr when the sample is in the region before and after the laser filament, where the laser intensity is not high enough to excite the element fluorescence. The observed harmonics are related to the element spectroscopy, which will benefit the understanding of the interaction of the laser filament with a solid and be helpful for the application on filament induced breakdown spectroscopy.