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Over the last two decades, silicon photonics has rapidly matured, leading to a growing interest in building large complex systems consisting of thousands of integrated optical components. A direct consequence of this push towards large scale integration is the need for high efficiency silicon photonic building blocks.In this work, we present a concrete path towards realizing these essential photonic building blocks. The foundation of our approach to designing photonic components is gradient-based shape optimization, which employs boundary smoothing based on high numerical precision polygons. In addition to helping us calculate accurate device sensitivities, this method affords us a great amount of flexibility when representing device geometries and enables us to incorporate design constraints directly into optimizations in a simple and intuitive way.Our approach to gradient-based optimization shares an important similarity to other forms of shape and topology optimization employed in the nanophotonics community: on its own, it is not a complete solution to designing high performance and robust devices. Due to the inherently non-convex nature of electromagnetic optimization problems, we cannot expect convex optimization to universally yield good devices without outside input. In order to overcome this obstacle, we have systematized the process of providing "outside input" through our hierarchical approach to design and optimization. Using a strategic combination of simple physical analysis to find good starting geometries and optimization with both coarse and fine parameterizations, we show that efficient and robust devices can be designed with minimal guesswork.Using this hierarchical approach, we demonstrate how a variety of silicon photonic components can be designed with superior performance. In particular, we design three port splitters, broadband four port splitters, fabrication-insensitive waveguide crossings, and a variety of efficient grating couplers which set a new standard for device performance. These components form the foundation for an optimized silicon photonic component library which will be important for demanding applications of the future.

The theory of mode-sorting in bimodal asymmetric Y-junctions is extended to multimode asymmetric Y-junctions with multiple output arms. This theory allows for the optimization of these mode-sorting planar structures. Asymmetric Y-junctions provide unique opportunities for spatial mode division multiplexing (MDM) of optical fiber. Spatial MDM is considered paramount to overcoming the bandwidth limitations of single-mode fiber. The design criteria presented in this paper facilitate their design. Refereed/Peer-reviewed

We present a detailed study of the localized coupled-cavity modes in 2-D dielectric photonic crystals. The transmission, phase, and delay time characteristics of the various coupled-cavity structures are measured and calculated. We observed the eigenmode splitting, waveguiding through the coupled cavities, splitting of electromagnetic waves in waveguide ports, and switching effect in such structures. The corresponding field patterns and the transmission spectra are obtained from the finite-difference-time-domain (FDTD) simulations. We also develop a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.

Polarization selectors, especially, those involving quantum dots (QDs) in photonic crystals (PCs) are interesting as they offer scalability for planar architectures. Three-dimensional finite difference time domain simulations were used to design PC structures suitable for polarization-selective transmission of QD emission into different regions as well as polarization beam splitters with different splitting ratios. Different waveguide out couplers were also designed for these coupled waveguide PC structures. As the transmitted intensity is the net result of superposition of all scattered beams, QD position defines the coupling efficiency to different paths. A detailed study showed that the transmission efficiency has intermediate maxima when the QD is shifted in either x- or z-directions. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

We review our work done for topology optimization of passive photonic crystal component parts for broadband and wavelength dependent operations. We show examples of low-loss topology-optimized bends and splitters optimized for broadband transmission and demonstrate the applicability of topology optimization for designing slow-light and/or wavelength selective component parts. We also present how the dispersion of light in the slow-light regime of photonic crystal waveguides can be tailored to obtain filter functionalities in passive devices and/or to obtain semi-slow light having a group velocity in the range ∼(c0/15 - c0/100); vanishing, positive, or negative group velocity dispersion (GVD); and low-loss propagation in a practical ∼5-15 nm bandwidth.