  We investigate the lasing modes in fully chaotic polymer microstadiums under
optical pumping. The lasing modes are regularly spaced in frequency, and their
amplitudes oscillate with frequency. Our numerical simulations reveal that the
lasing modes are multi-orbit scar modes. The interference of partial waves
propagating along the constituent orbits results in local maxima of quality
factor at certain frequencies. The observed modulation of lasing mode amplitude
with frequency results from the variation of quality factor, which provides the
direct evidence of wave interference effect in open chaotic microcavities.
