Whispering gallery mode-microlasers embedded into a dense medium

Lasing in fluorescent polystyrene microbeads embedded in gelatin of different density is reported up to a gelatin concentration of 30%, corresponding to a refractive index of 1.38. Above that value, the resonator modes become too broad for operation above threshold with a critical Q-factor of about 14,000. As a proof of applicability of these microlasers as remote microsensors inside of dense media, the resonator mode positions were used for quantification of the gelatin refractive indices, yielding the same linear dependency on gelatin concentration for both microlasers operated below and above threshold.