Filtering of unpolarized light by gratings

A Spectrally Tunable Dielectric Subwavelength Grating based Broadband Planar Light Concentrator

Energy consumption of buildings is increasing at a rapid pace due to urbanization, while net-zero energy buildings offer a green and sustainable solution. However, limited rooftop availability on multi-story buildings poses a challenge for large-scale integration of photovoltaics. Conventional silicon solar panels block visible light making them unfeasible to cover all the surfaces of a building. Here, we demonstrate a novel dielectric grating based planar light concentrator. We integrate this functional device onto a window glass transmitting visible light while simultaneously guiding near infrared (NIR) portion of sunlight to edges of the glass window where it is converted to electricity by a photovoltaic cell. Gratings are designed to guide NIR region and realize polarization independent performance. Experimentally, we observe 0.72% optical guiding efficiency in the NIR region (700–1000 nm), transmitting majority of the visible portion for natural room lighting. Integrating solar cell at the window edge, we find an electrical conversion efficiency of about 0.65% of NIR light with a 25 mm² prototype. Major losses are coupling and guiding losses arising from non-uniformity in fabrication over a large area. Such a functional window combining energy generation, natural room lighting and heat load reduction could mitigate urban heat island effect in modern cities.

Properties of two-dimensional resonant reflectors with zero-contrast gratings

The spectral properties of high-reflection mirrors imbued with two-dimensional (2D) subwavelength periodicity are investigated. The reflectors are designed in a silicon-on-glass film that is partially etched to implement a zero-contrast interface between the grating pillars and the sublayer, thereby annulling the local reflections and phase changes associated with hard interfaces. This approach has shown impressive results for 1D polarized reflectors; here we strive to discover analogous wideband unpolarized reflectors. Using particle swarm optimization, we report wideband unpolarized reflectors in the 1.4-2.0 μm wavelength band. A 2D reflector with square pillars exhibits 99% reflectance across a bandwidth exceeding 350 nm and possesses tolerance against angular deviations. The complementary structure with rectangular periodic voids achieves a bandwidth of 370 nm. A comparable, optimized 2D high-contrast grating reflector with grating pillars residing directly on the substrate yields a 99% bandwidth of 240 nm.

Computational design for guided-mode grating resonances

We are concerned with optimal design of guided-mode grating resonant structures (GMGRs). A typical structure is the integration of a zeroth-order grating and a planar waveguide. Our approach has two main steps. The first is to find the resonant wavelength. For any fixed grating structure the resonant wavelength is found by solving a nonlinear eigenvalue problem. The second step is to develop a Newton-type local optimization method. A crucial step is to determine an appropriate initial guess of the design parameters. Numerical design examples for both TE and TM polarization are presented. The design algorithm is expected to provide systematic guidance in engineering design of GMGRs.

Polarization independence of a one-dimensional grating in conical mounting

We demonstrate theoretically a polarization-independent guided-mode resonant filter with only a one dimensional grating. A rigorous method, the modal method by Fourier expansion, is used to compute the diffracted efficiencies of the grating. Wave-vector analysis fails to correctly design a polarization-independent structure. We show that a rigorous analysis of the resonances must be employed to obtain such a device; using a pole approach, we study the effects of grating parameters on the resonances of both polarizations.

Study of waveguide grating eigenmodes for unpolarized filtering applications

The dispersion relation of eigenmodes of two-dimensional waveguide gratings is studied with a perturbative model. The analytic expression of the complex wavelength of the modes permits us to predict the shape of the anomalies in the grating reflectivity with respect to the wavelength and the polarization of the incident plane wave. The simultaneous excitation of two independent modes is necessary for obtaining high-efficiency filtering of unpolarized light. We show how this requirement can be met.