브래그 및 폴리머 회절격자에서의 공간-시간 대칭적 광학 포텐셜 형성

Abstract

Parity-time (PT) symmetric Hamiltonians have opened a new venue for interesting physical systems respecting non-Hermitian quantum mechanics and associated optical phenomena. In general, such systems require complex-valued potentials with particular symmetry properties. In optics, it is possible to form a complex-valued potential by employing appropriate combinations of optical gain and losses. In this thesis, we perform comprehensive theoretical and experimental study on periodic optical systems possessing PT symmetric coupling properties. We first establish formal analogy between PT symmetric Hamiltonians and optical coupling in diffraction gratings. To realize PT symmetric grating structures and related unusual effects, Rhodamine 6G and IR140 dyes are employed as an organic gain material. Integrated plasmonics architectures are considered in depth as a promising system since they naturally have losses that is necessary for forming PT symmetric optical potentials. We experimentally obtain modal gain values of 1000 cm–1 at 594 nm for Rhodamine 6G and independently 16.7 cm–1 at 865 nm for IR140. Standard Kretchmann configurations and variable stripe-length methods are used for the gain measurements. These gain levels are enough to create PT symmetric waveguide systems operating with long-range SPP modes at corresponding wavelengths. Dielectric-loaded long-range SPP waveguide structures are characterized using the finite element method. Assuming IR140-doped PMMA as gain material, cutoff top-ridge dimensions (thickness and width) are numerically found for single-mode operation. A step-in-width waveguide Bragg grating structure is designed for exploiting spontaneous PT symmetry breaking and consequent unidirectionality. We employ two pairs of waveguides with different widths, which have identical imaginary parts but different real parts of effective propagation constants. These plausible combinations are carefully arranged within one grating period. In the last part of this thesis, we propose and experimentally establish PT symmetric diffraction gratings permitting active optical controls. We consider complex-valued refractive index formation in azo polymer film under two interfering coherent pump beams. The complex-valued refractive index is characterized by measuring the diffraction behaviors. We show that the phase difference and balance of the real and imaginary index modulations can be controlled by the polarization states of two pump beams. Therefore, the polarization-sensitive complex gratings in this proposal are applicable for investigating non-Hermitian physics that potentially provide novel mechanisms for advanced optical components and systems.