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Sub-wavelength Modulation of Complex Refractive Index and X(2) Optical Nonlinearity in Organic Thin Films
Restricted (Penn State Only)
Doctor of Philosophy
Date of Defense:
December 05, 2016
Noel C. Giebink, Dissertation Advisor
Noel C. Giebink, Committee Chair
Thomas N. Jackson, Committee Member
Zhiwen Liu, Committee Member
Venkatraman Gopalan, Outside Member
photonic materials and devices
The spatial modulations of complex refractive index and optical nonlinearity are the essence to many functional components ranging from distributed feedback, photonic crystals, gain coupled lasers, long distance high efficiency second harmonic generation, etc. in today’s optical and optoelectronic technologies. In particular, the possibility to separately modulate the real and imaginary parts of the refractive indices have sparked intense theoretical interests because it enables the optical analogue of parity-time symmetry, a concept originated from quantum field theory. Such complex refractive index modulation (CIM) also leads to various intriguing optical phenomena such as unidirectional invisibility, coherent perfect absorption,and power oscillations. Beyond the regime of linear optics, modulating the second order nonlinear optical susceptibility X(2) of materials at the nanoscale represents an ongoing technological challenge that is important for a variety of nonlinear nanophotonic applications, such as mirrorless optical parametric generation. Here, we exploit vapor-deposited organic small molecules to realize arbitrary CIM and demonstrate passive PT-symmetry breaking in a composite organic thin film. The co-evaporation of three different organic materials a non-absorbing high index host, a non-absorbing low index dopant, and an absorbing dopant form the basis of individual layers, and the multiple CIM unit repeat units are designed for optical structures with n(r) = n*(−r) at = 532 nm. So that this condition of the real (n) and imaginary (k) components of the refractive index as even and odd functions of position, r, satisfies the PT symmetry requirements. Based on these composite small molecular organic thin films, we demonstrate unidirectional reflectivity at PT symmetry breaking point, open up opportunities for organic materials as a platform to explore PT symmetry in optics and more generally as a means to create optical alloys with arbitrary complex refractive index modulation. Going beyond the realm of linear optical systems, we exploit the large hyperpolarizability of intermolecular charge transfer (CT) states, naturally aligned at an organic semiconductor donor-acceptor (DA) interface, as a means to control the magnitude and sign of X(2) at the nanoscale. Focusing initially on a single pentacene-C60 DA interface, we confirm that the CT transition is strongly aligned orthogonal to the heterojunction and find that it is responsible for a large interfacial nonlinearity probed via second harmonic generation that is sufficient to achieve d33 > 10 pm/V when incorporated in a non-centrosymmetric DA multilayer stack. Using grating-shadowed oblique angle deposition to laterally structure the DA interface distribution in such multilayers subsequently enables the demonstration of a X(2) grating with 280 nm periodicity, which is the shortest reported to date.
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