Electro-Optic Devices with Nanometer Scale Structures Fabricated by Electron Beam

Decomposition of chromophore molecules under direct electron beam irradiation reduces the refractive index of chromophore containing polymers. The induced refractive index contrast between the exposed and unexposed regions is high enough for waveguide bends of small radius and thus micro-ring resonator devices. This electron beam bleaching of chromophore-containing polymers provides a fabrication approach for nonlinear polymer optical waveguide devices. Fabrication of high quality micro-ring resonators with critical feature size on the order of 100 nm was demonstrated with this technique in a PMMA electron-beam resist that contains nonlinear optic chromophores. Summary of Research: Photonic integration can provide size, weight, and power (SWAP) reductions together with better performance analogous to electronic integration. Micro-ring resonators are regarded as the promising building blocks because of their compactness and multifunctionality. For example, versatile functions of electrooptic modulation and switching, optical rectification, wavelength conversion and all-optical switching can be realized by micro-ring resonators made of organic nonlinear optical materials [1]. Exponential increasing of the electro-optic coefficients in a rate consistent with Moore’s Law and the intrinsic ultrafast response time of these materials could allow for ‘lossless’ conversion or even gain in the electrical-optical-electrical signal transduction process and terahertz bandwidths. Typical organic nonlinear optical materials are guesthost systems with optical polymers doped with organic chromophores. Systematic analysis and simulation of polymer micro-ring resonators shows that fabrication of the submicron coupling gap with a precision < 100 nm is critical to achieve high quality factor (Q-factor) and sharp resonance. This high resolution can be achieved through electron beam lithography combined with reactive ion etching (RIE) or nanoimprinting. Traditional photolithography plus RIE have also been used to fabricate micro-ring resonators with lateral coupling gaps greater than 1 μm or vertical coupling gaps. However, many device designs require lateral coupling gaps smaller than 2 μm. For the chromophorecontaining polymers, we have found that high energy electron beam can break bonds and decompose the organic chromophores. This decomposition bleaches out the color and reduces the index of refraction of the polymers. Electron beam irradiation induced a refractive index decrease of about 0.06 in chromophore-containing polymethyl methacrylate (PMMA) polymers, which is large enough for ring waveguides of small radius. Electron beam provides nanometer scale resolution, which couldn’t be achieved with photobeaching. Without using expensive high resolution photomasks, different designs of micro-ring resonator devices can be generated easily with computer assisted design and control systems. This is especially cost effective for device prototyping and design optimization. Electron beam bleaching is a single step process, which simplifies the device fabrication and reduces the sources of error from multiple fabrication steps. It also eliminates the use of wet chemicals which can dissolve the chromophorecontaining polymer waveguides and greatly increase the propagation loss.