High-efficiency laser-like emission at low thresholds in dipolar organic
materials upon pulsed optical excitation, without using any external
mirrors. Unusually high conversion efficiencies and low thresholds in
laser-like emission have been observed in the solutions of organic
molecular salts having large dipole moments and specific dye molecules
having high photoluminescence efficiencies. Pumped with frequency-doubled
pulses from a Nd:YAG laser, conversion efficiencies in the range of 15-40%
were achieved without incorporation of external mirrors. The threshold
pump energies for such emission have been observed to be low (<8
.mu.J). The spectrally narrowed output beam was found to have low
divergence, high degree of polarization, and pulse-width less than that of
the excitation pulses (50 picoseconds). The exceptionally low threshold
(<1 .mu.J) and high energy conversion efficiencies observed in
molecular salts have been attributed to the large excited-state dipole
moment of these noncentrosymmetric molecules favoring strong cooperative
(laser-like) emission in spite of small photoluminescence quantum
efficiencies.
