Ultralow threshold on-chip microcavity nanocrystal quantum dot lasers

Chemically synthesized nanocrystal, $CdSe∕ZnS$ (core/shell), quantum dots are coated on the surface of an ultrahigh-$Q$ toroidal microcavity and the lasing is observed at room and liquid nitrogen temperature by pulsed excitation of quantum dots, either through tapered fiber or free space. Use of a tapered fiber coupling substantially lowered the threshold energy when compared with the case of free space excitation. The reason for the threshold reduction is attributed to the efficient delivery of pump pulses to the active gain region of the toroidal microcavity. Further threshold reduction was possible by quantum dot surface-coverage control. By decreasing the quantum dot numbers on the surface of the cavity, the threshold energy is further decreased down to $9.9fJ$⁠.