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.