Informal Condensed Matter Physics Seminar: Ab-initio theory of emission spectra from gain media

Absract:

We present an ab-initio theory which yields quantitatively accurate formulas for the emission spectra from gain media, accounting for inhomogeneity and nonlinearity of the gain and including systems with exceptional points (EPs). Nonorthogonality of the modes in open resonators leads to enhancement of the local density of states and, consequently, to enhanced spontaneous emission rates. Traditional expressions for the enhancement (Petermann) formally diverge at EPs. However, by using an appropriate choice of basis vectors (including Jordan vectors), we show that the enhancement is actually finite. Moreover, the spectral lineshape near the resonance peaks changes from a simple Lorentzian for non-degenerate eigenvalues to a squared Lorentzian near EPs. Above the lasing thresholds, our analysis produces a generalized formula for the multimode laser linewidths that contains nearly all previously known effects and also finds new nonlinear and multimode corrections which become significant in microcavity lasers (e.g., random lasers and photonic-crystal lasers).