Arvinder Singh Chadha

Approaching total absorption at near infrared in a large area monolayer graphene by critical coupling

We demonstrate experimentally close to total absorption in monolayer graphene based on critical coupling with guided resonances in transfer printed photonic crystal Fano resonance filters at near infrared. Measured peak absorptions of 35% and 85% were obtained from cavity coupled monolayer graphene for the structures without and with back reflectors, respectively. These measured values agree very well with the theoretical values predicted with the coupled mode theory based critical coupling design. Such strong light-matter interactions can lead to extremely compact and high performance photonic devices based on large area monolayer graphene and other two–dimensional materials.

Coupled double-layer Fano resonance photonic crystal filters with lattice-displacement

We present here ultra-compact high-Q Fano resonance filters with displaced lattices between two coupled photonic crystal slabs, fabricated with crystalline silicon nanomembrane transfer printing and aligned e-beam lithography techniques. Theoretically, with the control of lattice displacement between two coupled photonic crystal slabs layers, optical filter Q factors can approach 211 000 000 for the design considered here. Experimentally, Q factors up to 80 000 have been demonstrated for a filter design with target Q factor of 130 000.

Comparative study of metallic and dielectric helix photonic metamaterial

We compare the performance of metallic and dielectric thin film helix photonic metamaterial numerically. The simulated metal helix provides a 4 μm broad band circular dichroism with a polarization suppression ratio of 20:1. The transmission efficiency of the metal helix drops significantly with increasing number of helix turns and angle of incidence. Contraire, the simulated silicon dielectric helix structure provides an extremely high polarization suppression ratio above 2,300:1 with almost 100% transmission over a wide range of incident angles. The results provided highlights the trade-offs between the transmission efficiency and the specular and angular bandwidth for designing practical thin-film circular polarizers.

Polarization- and angle-dependent characteristics in two dimensional photonic crystal membrane reflectors

We report here measured angle- and polarization-dependent reflection properties in two-dimensional photonic crystal broadband membrane reflectors. Polarization independent reflection was obtained at surface normal direction. High reflection can also be obtained for one transverse electric (TE) polarization state over a wide range of incident angles. With the target of 1550 nm spectral band, the measured spectral bandwidths for TE polarization with reflectivity greater than 90% are 180 nm and 56 nm at surface normal and oblique incidence of 45°, respectively. The experimental spectrum approximates the theoretical spectral response of this membrane reflector.

High-order grating coupler for high-efficiency vertical to in-plane coupling

We present here the design of a robust broadband high efficiency surface-normal vertical to in-plane optical coupler using fourth-order gratings. The fourth-order gratings can be designed such that the zero-order diffraction is suppressed while the diffraction efficiencies of the higher orders are enhanced for the in-plane coupling. We numerically demonstrated the surface normal incidence light coupling efficiency of 88.5% at 1,535 nm with a 3 dB bandwidth of 42 nm and 1dB bandwidth of 28 nm. Large fabrication tolerance of the fourth-order grating is also assessed.

Tuning the refractive index of blended polymer films by RIR-MAPLE deposition

Graded index polymer films enable novel optics using rigid or flexible substrates, such as waveguides or anti-reflection coatings. Previously, such films have been fabricated by nanoimprint lithography or the decomposition of a single component in polymer blends. Yet, it is desirable to have precise control over the polymer film composition in order to have the most flexibility in designing refractive index profiles. Resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is a polymer thin film deposition technique that enables multi-layer structures on a wide variety of substrate materials, regardless of the solubilities of constituent polymers. In this work, the feasibility of tuning the refractive index of blended polymer films of polystyrene and poly(methyl methacrylate) deposited by RIR-MAPLE is demonstrated. Different polymer blend film compositions are deposited using RIR-MAPLE by varying the polymer target ratio. Transmission electron microscopy and atomic force microscopy are used to characterize the film morphology.

An all-dielectric broadband high-transmission efficiency circular polarizer

We investigated numerically an all-dielectric semiconductor-based circular polarizer consisting of silicon helix on glass substrate. Only three turns of the dielectric helix structure provides extremely high polarization suppression ratio above 2,300:1 with almost 100% transmission. Transmission efficiency greater than 85% for one circularly polarized light is maintained for incident angles as high as 300. The influence of the various helix lattice parameters on the polarization gap and the circular dichroism is also presented.

Design criteria to optimize the near perfect anti-reflection coating

We present design criteria to demonstrate nanostructured high performance anti-reflection coatings that allows over 98.5% transmission over a broad visible spectral range with cone angles up to 120°. We propose a methodology to relate the nanostructures with different shapes and sizes as an equivalent of the graded index (GRIN) film. Starting from GRIN film we find the rate of change of refractive index with reference to the film thickness, we then design nanostructures such that their effective refractive index (RI) satisfies the above equality to achieve broadband wide-angle low reflection coating. With the theory presented we can accurately predict the reflectance performance of the nanostructure. We will present the simulation results of the angular and spectral dependences of nanocone anti-reflection structures, with the principle applicable to all other types of nano-structures anti-reflection coatings.

Large area imprinted surface textures for omnidirectional conformal AR coatings on flexible amorphous silicon solar cells

Inverted pyramid surface textures are demonstrated on flexible amorphous Si solar cells based on a resin imprint process. The surface texture reduces the reflectivity of the cells over a broad spectrum. The texture leads to relative cell efficiency increases from 7% at 0° (surface-normal incident) to as high as 22% at 60°. Larger efficiency improvement at higher incident angles is a result of the broad spectrum omni-directional suppression in reflection. The current-voltage characteristics show that the open circuit voltage remains almost constant but the surface texture increases the short circuit current in the amorphous Si solar cell. This suggests that the efficiency improvement is primarily due to improved coupling of light into the cell. This omnidirectional surface texture is a promising method for anti-reflection in all kinds of cells-crystalline Si, amorphous Si, thin films, and organic solar cells.

Spectral selective absorption enhancement from stacked ultra-thin InGaAs/Si Fano resonance membranes

We report here modified absorption property of InGaAs nano-membrane on a Fano filter made of patterned single crystalline silicon nano-membrane transferred onto glass substrate. Placement of an ultra-thin InGaAs film on Si Fano resonant membrane enhances the absorption with simulated enhancement factor ~35 and measured enhancement factor of ~26. Leaky modes in the photonic crystal (PC) consist of high field standing waves that can be coupled to the out of plane radiation mode provided by lattice matching of the PC. We will present simulation, device fabrication and experimental characterization of stacked ultra-thin InGaAs/Si Fano resonance membrane in the IR regime.