  Photonic crystals (PCs) offer unique ways to control light-matter
interactions. The measurement of dispersion relations is a fundamental
prerequisite if we are to create novel functionalities in PC devices.
Angle-resolved spectroscopic techniques are commonly used for characterizing
PCs that work in the visible and near-infrared regions. However, the techniques
cannot be applied to the mid- and long-wavelength infrared regions due to the
limited sensitivity of infrared detectors. Here, we propose an alternative
approach to measuring infrared dispersion relations. We construct a
high-precision angle-resolved setup compatible with a Fourier-transform
spectrometer with an angle resolution as high as 0.3 degrees. Hence, the
reflection spectra are mapped to the 2D photonic band structures of
In(Ga,Al)As/InP based PC slabs, which are designed as mid-infrared PC
surface-emitting lasers. We identify complex PC modes with the aid of
polarization selection rules derived by the group theory. Spectral analysis
makes it possible to evaluate the mode quality (Q) factors. Therefore,
angle-resolved reflection is a useful way of optimizing 2D PC parameters for
mid-infrared devices.

Optics;Applied Physics