The nondissipative quasistatic longitudinal optical response of insulators is characterized by an intrinsic geometric capacitance, determined by the ratio of the quantum metric to the energy gap, as recently established. We study the photonic spin Hall effect (PSHE) in this low-frequency regime, induced by reflection from an atomically thin material serving as the interface between two dielectrics. As a function of the magnitude of this geometric capacitance, covering values typical of graphene-family systems and for incidence close to the Brewster angle, the calculated in-plane and transverse beam shifts become comparable to the wavelength and thus detectable. At sufficiently low frequencies, we derive linear and quadratic approximations that allow the geometric capacitance to be extracted from measurements of the beam displacements. We provide upper bounds for the beam shifts. The results suggest that THz PSHE might be a useful probe of the quantum geometric capacitance of two-dimensional insulators.

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• Capacitance
• Optical conductivity
• Spin Hall effect
• Dirac semimetal
• Graphene
• Two-dimensional electron system
• Linear response theory
• Optical spectroscopy
• Reflectivity