Light control of lasing from liquid-crystal micro-droplet light switch /

The all-optical control of light has been explored in hard matter, but to control light by light in soft matter is a major experimental and conceptual challenge that remains largely unexplored. We propose and verify experimentally and numerically that nanosecond light pulses can be controlled by light pulses using resonant stimulated-emission depletion (STED) in a liquid-crystal optical cavity that acts as a nanosecond optical switch. This light-by-light control is realized in a micrometer-diameter droplet of fluorescent-dye-doped nematic liquid crystal that is submersed in an aqueous solution. A set of laser-printed optical waveguides touches the surface of the droplet and provides flow of light in and out of the droplet. We demonstrate that nanosecond light pulses launched through the waveguides initiate the lasing of whispering gallery modes in the droplet. However, the lasing can be suppressed very efficiently on a nanosecond scale by another, red-shifted light pulse before the lasing takes place. The proposed concept of light control in soft matter using STED in an optical cavity requires fewer production steps because of self-assembly. It could be massively replicated using soft imprint lithography, uses less production energy due to low production temperatures, requires less toxic materials, and could be made entirely biocompatible and flexible.