Near-field coupling of absorbing material to subwavelength cavities

Abstract

In conjunction with our previous work on spectral transmission dependence on geometric parameters for subwavelength slits, we have investigated the optical behavior with the inclusion of an absorber on the extraordinary optical transmission of sliver slits resonant in the mid-wave infrared (MWIR). The placement of an absorbing layer causes a dramatic change to the dielectric environment of the subwavelength slit causing the cavity to become energetically leaky. We have found this broadens the spectral response of the cavity by increasing the imaginary component of the effective cavity index, reducing the cavity quality by a factor of more than seven. To mitigate this undesired effect, we have found that partially decoupling the absorber with a thin isolating layer helps restore the slit’s narrow spectral response and we explore the dependence of optical properties on the isolating layer’s depth. The optimum thickness of a silicon dioxide isolation layer for best quantum efficiency (Q.E.) was found to be 100 nm with a maximum Q.E. of 37.5%. This is more than double the Q.E. of the directly coupled absorber and the cavity Q increase is of the same order, with a corresponding narrowing of the resonance bandwidth. In addition, we explore the effect on the cavity of changing the dielectric environment at the input of the slit to improve the resonance properties in the propagation direction improving the Q by 20% and improving the Q.E. to 40.2%.