Gray Scale Lithography Technology for Asymmetric Resonant Cavities of Micro - and Meso-Scale Lasers, Amplifiers and Filters

University Of California-San Diego, La Jolla CA

Investigators

Abstract

0000170 Lee The PI and his group will study novel photonic devices fabricated by gray-scale lithography which exploit a new type of optical resonant micro-cavity known as an Asymmetric Resonant Cavity (ARC). The ARC is a compact dialectic micro-cavity based on total (or partial) internal reflection which has been shown to support high-Q modes, emit with high directionality and (in the case of semiconductor mid-infrared lasers) with high output power. The ARC concept is based on smooth oval deformations of cylindrical or spherical whispering gallery resonators. Prior to the work proposed below, only limited use has been made of sophisticated modem lithographic techniques in developing the ARC-based devices, which require sub-micron accuracy and well-controlled height profiles. They will investigate ARCs based on glass, Er-doped glass, GaAs, InGaAs/GaAlAs and silicon/germanium. The proposed project will demonstrate further useful properties of ARCs for meso-optical devices, by employing the newly developed gray scale lithography. Specifically they hope to test and demonstrate the following important features: Improved lasing characteristics of ARC micro-lasers over a wide range of wavelength bands and gain materials. The possibility of lasing emission from ARC micro-lasers from two qualitatively different modes, i.e. different in both emission directionality and wavelength. Demonstrate similar properties for ARC-based optical amplifiers. Improved input-output coupling between waveguides and ARC micro-resonators due to nonevanescent coupling to ARCS, resulting in much improved design tolerances. Demonstrate how to exploit unique directional properties of ARC resonances for filtering operations useful for wavelength division multiplexing (WDM) applications in planar integrated optics. Demonstrate device configurations which convert planar propagation or emission to propagation or emission perpendicular to the plane. Configurations proposed are ARCs with auxiliary mesoreflectors, or axicon-domed ARCS. These configurations rely essentially on unique features of gray-scale lithography. Lithography can be applied to define lateral shapes, but exposure control will define the surface profiles. UCSD will develop two complimentary fabrication methods. One is based on dry etching for ARCs of height profiles < 5 microns and another on Sandia's LIGA foundry process for profiles > 5 microns. Both methods will employ gray scale lithography, which UCSD pioneered, and which is superior to binary lithography for producing higher quality and lower cost optical components. However, the gray scale technology must be modified by employing thicker diamond-like-coatings on the gray scale masks for wider dynamic ranges of exposure, and thicker analog photoresist layers. The LIGA process at Sandia now works only with binary lithography; but they have agreed to work with us to introduce gray scale lithography into their LIGA process. Moreover, Sandia has produced LIGA parts only in PMMA, metals and ceramics to date, whereas the preferred materials for ARCs are glass, silicon and germanium. The exciting possibility of designing, producing and testing meso-optics of arbitrary 3D shapes, is within reach with gray-scale technology. Application of gray scale technology to Sandia's LIGA foundry process as well as meso-optics for redirecting ARC emission is totally new. This is a collaborative project between UCSD and Yale. Yale (ADS and RKC) will be responsible in defining the shapes, sizes and materials of ARCs to be fabricated. UCSD (SHL) will be responsible for the fabrication of ARCs and sample examination. Yale (RKC) will measure the fabricated ARCs in terms of their performance and provide feedback to the Yale design group and to the fabrication group at UCSD, while focusing on possible application to the integrated optics field and WDM part of the telecommunications field. ***