Infrared passbands from fractal slit patterns on a metal plate

We show that a fractal pattern of submicron wide slits, etched on a 0.1-μm-thick gold film, exhibit multiple pass and stop bands in the wavelength regime of 2–200 μm. In the midinfrared regime, the passbands show $\text{∼36\%}$ transmission (three orders of magnitude higher than the reference), and the stop bands exhibit $\text{∼80\%}$ reflection. In the far infrared regime, the passband transmittances are 25%, 29%, and 14%, respectively, at wavelengths of 34, 62, and 111 μm. These transmittances are rather high in view of the fact that supporting substrate is itself only about 40% transmitting, and a control pattern of holes with similar void-to-metal ratio is 3000 times less transmitting in all frequencies. We attribute the high transmittance from subwavelength slits to geometric resonances.