Flavin mononucleotide biomolecular laser: longitudinal mode structure, polarization, and temporal characteristics as probes of local chemical environment.

A detailed characterization of the flavin mononucleotide (FMN) biomolecular laser, optically pumped in a stable resonator, is reported here. Photoexcitation of the molecule at 355 nm results in lasing over the ~566.5-573.5 nm spectral region, and the threshold pump energy density is measured to be 110 ± 10 µJ/mm² for a 10 mM FMN/water solution. Over twenty longitudinal modes are observed when the cavity length L and the energy pump fluence E_p are 375 µm and 300 µJ/mm², respectively. Partial substitution of glycerol for water as the solvent results in a factor of four reduction in the threshold pump energy fluence (to < 30 µJ/mm²) and a quadrupling of the slope efficiency. This effect is attributed to the O₂ - mediated photoconversion of FMN molecules in the triplet state to the singlet species. For pump intensities a factor of 2.5 above threshold, the laser pulse width is ~2 ns FWHM, and the output intensity decays exponentially with a photon lifetime of 1.7 ns. The addition of glycerol to a FMN/water solution also suppresses s-polarized emission (yielding P = 0.78 ± 0.08), presumably as a result of the inhibition of FMN rotational diffusion. The sensitivity of the spectral and optical properties of this and other biomolecular lasers to the chemical environment underscores the value of coherent emission as a biochemical or biomedical diagnostic tool, particularly insofar as molecule-molecule interactions are concerned.