  We investigate the use of guided modes bound to defects in photonic crystals
for achieving double resonances. Photoluminescence enhancement by more than
three orders of magnitude has been observed when the excitation and emission
wavelengths are simultaneously in resonance with the localized guided mode and
cavity mode, respectively. We find that the localized guided modes are
relatively insensitive to the size of the defect for one of the polarizations,
allowing for flexible control over the wavelength combinations. This double
resonance technique is expected to enable enhancement of photoluminescence and
nonlinear wavelength conversion efficiencies in a wide variety of systems.
