Investigation of Optical and Electro-optical Effects at Material Interfaces

Abstract

The key questions addressed in this thesis are related to light-matter interactions at the material interfaces and are related to both plasmonic as well as Pockels effects. Plasmonics enables the design of compact photonic circuits with sub-micron electric field confinements and authorizes optical signal processing at the nanoscale. At the same time, the Pockels effect is necessary for designing on-chip interferometers and ultra-high-speed modulators. These two are crucial for integrated optics and photonics engineering. Integrated optics deals with miniaturizing large-scale optical signal processing circuit functionalities on a small footprint. This leads to performance enhancement as well as low power consumption. An electro-optic modulator is one of the most integral parts of an integrated photonic circuit. LiNbO3 is a well-known material with asymmetric crystal symmetry with a high Pockels coefficient of around 40 pm/V. However, integrating asymmetric crystals on a chip leads to slow and expensive fabrication processes. Instead, considering amorphous and poly-crystalline materials, their fabrication procedure is cost-effective and can be deposited rapidly using solution-processed techniques. In the first part of this thesis, we present an observation of the Pockels effect at the interface of a sol-gel spin-coated amorphous titanium dioxide and a poly-crystalline metal. We have found nonlinear two-dimensional susceptibility χ2D(2)(omega; omega, 0) at the interface of these materials of the order of 10^7pm^2/V. The order of magnitude is similar to a recent report of χsurface(2)(2ω; ω, ω)~ 10^6pm^2/V for the silicon-air interface. This work presents the interface of centrosymmetric TiO2 and metal as a new electro-optic material. In the next part, we argue the possibility of free carrier absorption as the probable physics instead of the pure field Pockels in the observations regarding the electro-optic effect described in the beginning. We study theoretical models expressing the dependence of free carrier absorption and dispersion on the mobility of the medium. We note a quench in free carrier absorption for very low mobilities. Since our TiO2 dielectric is amorphous and has low measured mobility, we reject the free carrier absorption as a possibility. However, the plasma dispersion effect can dominate at higher mobilities. We then describe a mechanism to inject and modulate carriers up to 10^19 cm^-3 in high mobility. (μ≥ 1 cm^2V^-1s^-1) TiO2 with low intrinsic carrier concentration using ohmic contacts. Finally, we study these electro-modulated devices' optical properties using the transfer matrix method. It is possible to confine electromagnetic radiation at the interface of a metal and dielectric to nanoscale by converting it into surface plasmon polariton (SPPs). The SPPs are excellent tools for studying TiO2 and metal interface's electro-optical properties. In the last part of the thesis, we study the temporal coherence properties of these SPPs propagating on the surface of a metal and a dielectric. Despite the heavy loss, the coherence properties of the SPPs are preserved. We experimentally demonstrate this conservation of coherence up to 80 μm of propagation. Ultimately, we propose a miniaturized design of a plasmonic electro-optic modulator with TiO2 as dielectric.