Inferring periodic orbits from spectra of simply shaped microlasers

Dielectric microcavities are widely used as laser resonators and characterizations of their spectra are of interest for various applications. We experimentally investigate microlasers of simple shapes ͑Fabry-Perot, square, pentagon, and disk͒. Their lasing spectra consist mainly of almost equidistant peaks and the distance between peaks reveals the length of a quantized periodic orbit. To measure this length with a good precision, it is necessary to take into account different sources of refractive index dispersion. Our experimental and numerical results agree with the superscar model describing the formation of long-lived states in polygonal cavities. The limitations of the two-dimensional approximation are briefly discussed in connection with microdisks.