Electromagnetic Transmission Through Resonant Structures.

Abstract

Electromagnetic resonators store energy in the form of oscillatory electric and magnetic fields and gradually exchange that energy by coupling with their environment. This coupling process can have profound effects on the transmission and reflection properties of nearby interfaces, with rapid transitions from high transmittance to high reflectance over narrow frequency ranges, and has been exploited to design useful optical components such as spectral filters and dielectric mirrors. This dissertation includes analytic, numeric, and experimental investigations of three different electromagnetic resonators, each based on a different method of confining electromagnetic fields near the region of interest.

First, we show that a structure with two parallel conducting plates, each containing a subwavelength slit, supports a localized resonant mode bound to the slits and therefore exhibits (in the absence of nonradiative losses), perfect resonant transmission over a narrow frequency range. In practice, the transmission is limited by conduction losses in the sidewalls; nevertheless, experimental results at 10 GHz show a narrowband transmission enhancement by a factor of 10^4 compared to the non-resonant transmission, with quality factor (ratio of frequency to peak width) Q~3000.

Second, we describe a narrowband transmission filter based on a single-layer dielectric grating. We use a group theory analysis to show that, due to their symmetry, several of the grating modes cannot couple to light at normal incidence, while several others have extremely large coupling. We then show how selectively breaking the system symmetry using off-normal light incidence can produce transmission peaks by enabling weak coupling to some of the previously protected modes. The narrowband filtering capabilities are validated by an experimental demonstration in the long wavelength infrared, showing transmission peaks of quality factor Q~100 within a free-spectral range of 8-15 um.

Third, we demonstrate that defect-free periodic structures of finite extent can support extended, surface-avoiding, high-quality factor resonant modes, even without mirror-like structures at the boundaries to confine electromagnetic energy. After discussing the necessary conditions for mode confinement to occur, several numerical examples are given. Finally, an experiment at microwave frequencies (2-9 GHz) demonstrates mode confinement, with quality factors Q~150, in a 12-period array of short dielectric rods.