Asymmetrical-cavity picosecond Raman laser at the water-air interface.

For the first time, to the best of our knowledge, we demonstrated a new type of Raman laser with asymmetrical cavity at the liquid-air interface. We observed an intriguing stimulated Raman scattering (SRS) threshold dependence when the pumping laser beam waist was transferred through the liquid-air interface, and we demonstrated a paradoxical 30-fold SRS threshold reduction in the vicinity of the water-air surface. The minimum SRS threshold was achieved when the pumping laser beam waist was located at the liquid surface. To explain the abnormal SRS threshold dependence, we suggested a new lasing mechanism. Finally, we demonstrated that threshold measurements at the liquid-air interface are a reliable method to quantitatively measure the interaction length in SRS experiments with a focused beam.