Gradient-index (GRIN) materials feature spatially-varying indices of refraction (i.e., permittivity and/or permeability) which enables them to bend light throughout their volume unlike conventional homogeneous materials which can only bend light at their surfaces. This power affords optical designers with additional degrees of design freedom to realize performance not achievable by conventional materials and designs. CEARL is at the forefront of freeform GRIN lens design both in the optical and RF regimes and has developed custom numerical solvers and optimization techniques to realize bespoke GRIN lenses that meet user-specified performance objectives. 

Metalenses exploit the generalized form of Snell's law which introduced a phase-gradient term to the standard formula. This inclusion enables a host of new behaviors not previously achievable with conventional optics. Among these behaviors include anamlous reflection and refraction, dispersion-engineered focusing behaviors, and optically-reconfigurable imaging performances. CEARL researchers partner with leading fabrication experts at Harvard, MIT, and Sandia National Labs to realize new metasurface and metalens designs that incorporate our state of the art design and optimization tools with cutting-edge nanofabrication techniques. 

Topology optimization (TO) is a core local optimization technique of high-performance nanophotonic device design which has become very popular due to its geometric flexibility. In a commonly used version of TO, structures are formulated as a “density field” of variable permittivity blocks (hundreds or even thousands), each of which are adjusted individually. In almost every area of engineering design, more degrees of freedom entails more possibility for improved device performance. This is certainly true in the area of nanophotonics, where geometric parameterizations which are highly flexible have raised the bar on all kinds of performance metrics (conversion and diffraction efficiencies, confinement, etc.). However, as the number of design variables increases, so does the difficulty in optimizing. The secret sauce of TO is it’s use of adjoint analysis, allowing computation of an update gradient for thousands of variables using just two full-wave simulations.

As part of multiple DARPA programs, CEARL researchers have developed custom optical design tools to meet the needs of future optical engineers and enable them to design with disruptive components based on freeform GRIN and metasurface components. These tools enable us to design systems not possible with standard commercial solvers and investigate the performances and SWaP (size, weight, and power) reduction potential that these new components offer.

Bound States in Continuum (BIC), topologically protected non-radiative modes localized at highly symmetrical point in reciprocal space, have attracted tremendous research interest due to their unique properties including a diverging Q-factor and extreme field localization effect. Numerous methods have been proposed to realize a quasi-BIC scattering system with slightly broken symmetry, such as k-detuning and spatial symmetry breaking. By exploiting the winding nature of quasi-BIC modes in reciprocal space, an active OAM generation photonic system is demonstrated through Temporal Coupling Mode Theory and full wave simulations. Meanwhile, a polarization abundance domain adjacent to the topological charges of the vectorized scattering field is observed in the k-space. By leveraging the nonlinear response of the active material (e.g., α-Si or ZnO), CEARL researchers have demonstrated hybrid metasurfaces capable of ultrafast all-optical modulation.