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Fourier transform (FT) spectroscopy is a versatile technique for studying theinfrared (IR) optical response of solid-, liquid-, and gas-phase samples. Instandard FT-IR spectrometers, a light beam passing through a Michelsoninterferometer is focused onto a sample with condenser optics. This designenables us to examine relatively small samples, but the large solid angle ofthe focused infrared beam makes it difficult to analyze angle-dependentcharacteristics. Here we design and construct a high-precision angle-resolvedreflection setup compatible with a commercial FT-IR spectrometer. Our setupconverts the focused beam into an achromatically collimated beam with an angledispersion as high as 0.25$^\circ$. The setup also permits us to scan theincident angle over ~8$^\circ$ across zero (normal incidence). The beamdiameter can be reduced to ~1 mm, which is limited by the sensitivity of anHgCdTe detector. The small-footprint apparatus is easily installed in an FT-IRsample chamber. As a demonstration of the capability of our reflection setup wemeasure the angle-dependent mid-infrared reflectance of two-dimensionalphotonic crystal slabs and determine the in-plane dispersion relation in thevicinity of the $\Gamma$ point in momentum space. We observe the formation ofphotonic Dirac cones, i.e., linear dispersions with an accidental degeneracy at$\Gamma$, in an ideally designed sample. Our apparatus is useful forcharacterizing various systems that have a strong in-plane anisotropy,including photonic crystal waveguides, plasmonic metasurfaces, and molecularcrystalline films.

Apparatus for high-precision angle-resolved reflection spectroscopy in the mid-infrared region