We propose and demonstrate an optical structure described as a guided mode slot resonator, which provides the strong resonant response of high contrast gratings, while at the same time concentrating the resonant field in nano-scale air voids. The resonant structures are fabricated from silicon-on-sapphire wafers and optically characterized by angle- and polarization-dependent infrared reflection and transmission spectroscopy. Experimental results show agreement with simulations, suggesting strong field enhancement of long wavelength light in nano-volume voids.