Whispering Gallery Mode Emission of a Cylindrical Droplet Laser

A cylindrical droplet laser was fabricated in a silicone rubber by using a polyethylene-glycol solution of rhodamine 6G. The silicone rubber provided a simple molding process for enclosing the droplet, since silicone oil solidified at room temperature by only adding a curing agent. Polyethylene glycol dissolved a large amount of dye molecules, yielding a fluorescent solution whose refractive index (1.46) was higher than that of the silicone rubber (1.40). Consequently, some fluorescence rays circulated in the cylindrical droplet owing to the total internal reflection on the side surface (the whispering gallery mode). Other fluorescence rays made round trips in the radial or axial directions of the cylindrical droplet (the radial and axial modes) being reflected at the side or bottom surfaces. When the droplet was excited by a green laser pulse (wavelength: 527 nm, pulse duration: 10 ns), these emission modes competed with one another to induce a stimulated emission. In a droplet with 2.0 mm diameter and 1.4 mm height, the whispering gallery mode conquered the other emission modes, exhibiting a non-linear peak growth and a peak-width narrowing when the excitation energy exceeded 20 mu J (the threshold energy of the stimulated emission).