Meng-Jie Lin

Biologically inspired achromatic waveplates for visible light

Waveplates are planar devices used in optics and optoelectronics to change the polarization state of light. Made of anisotropic dielectric materials such as crystals and thin films, waveplates are not known to exhibit achromatic performance over the visible regime. Inspired by the microvillar structure of R8 cells functioning as polarization converters in the eyes of stomatopod crustaceans, we conceived, designed, fabricated and tested periodically multilayered structures comprising two different types of arrays of nanorods. Morphologically analogous to the ocular cells, here we show that the periodically multilayered structures can function as achromatic waveplates over the visible regime.

Metal/dielectric/metal sandwich film for broadband reflection reduction.

A film comprising randomly distributed metal/dielectric/metal sandwich nanopillars with a distribution of cross-sectional diameters, displayed extremely low reflectance over the blue-to-red regime, when coated on glass and illuminated normally. When it is illuminated by normally incident light, this sandwich film (SWF) has a low extinction coefficient, its phase thickness is close to a negative wavelength in the blue-to-red spectral regime, and it provides weakly dispersive forward and backward impedances, so that reflected waves from the two faces of the SWF interfere destructively. Broadband reflection-reduction, over a wide range of incidence angles and regardless of the polarization state of the incident light, was observed when the SWF was deposited on polished silicon.

Silver/silicon dioxide/silver sandwich films in the blue-to-red spectral regime with negative-real refractive index

Sandwich films comprising arrays of silver/silicon-dioxide/silver nanosandwiches were grown by the oblique angle deposition technique. For normally incident light, these films present a negative-real refractive index (NRRI) over almost the entire visible regime, with quite high transparency and figures of merit, regardless of the orientation of the incident electric field. A broad distribution of nanosandwich sizes is responsible for the breadth of the NRRI spectral regime.

An interference coating of metamaterial as an ultrathin light absorber in the violet-to-infrared regime

A metamaterial with brief and ultrathin structure performs high efficiency in light absorption. An upright aluminum nanorod array (Al NRA) is obliquely deposited, measured, and analyzed its optical property. The Al NRA performs high efficiency of light absorption and low reflectance simultaneously. Based on the measured refractive index and impedances, the wave propagation through the Al NRA is traced to demonstrate the destructive interference that leads to antireflection. According to the analysis of wave tracing, an Al semicontinuous film with thickness of 15nm is introduced under an Al NRA with thickness of only 245nm as a brief and thin two-layered structure. The broadband and polarization-independent light absorption is measured over the violet-to-infrared regime.

Slanted S-shaped nano-columnar thin films for broadband and wide-angle polarization conversion

A slanted S-shaped nano-columnar (SSNC) thin film sculptured by oblique angle deposition is applied as an enhanced polarization converter. A growth model of a SSNC film is developed using available empirical equations for columnar thin films. The optimum sculpture parameters for a SSNC film to have high efficient polarization conversion are theoretically estimated and applied in fabrication. The designated SSNC films are sculptured and demonstrated to have high polarization conversion reflectance over a broadband and wide-angle range.

Optical coating on nano-optical antennas to enhance directional radiation

Abstract. A traditional high-reflection optical coating was applied to enhance the directional radiation of nanoantennas. A highly reflective multilayer upon the top lateral side of a horizontally lying silver nanorod enhances the forward scattering when an optical wave is incident on the bare bottom side of the nanorod. Enhanced forward scattering can thus be observed from a glancing deposited silver nanorod array (NRA). An effective method for coupling the energy of incident light into a NRA involves arranging the NRA in a prism coupling. The highly efficient light coupling effect over a broadband and a wide range of angles results in extra-strong forward light scattering.

Design of an achromatic optical coating waveplate

We propose a design method of an achromatic waveplate that comprises anisotropic thin films as a multi-cell layered system. Each cell is a symmetrical three-layered ABA structure with equivalent refractive indices and phase thicknesses. A merit function defined as the rate of change of the phase retardation with respect to wavelength is introduced to control the variation of phase retardation by selecting proper birefringence and thickness for each film. The allowed thickness of each birefringent film is investigated for different requirement about the uniformity of phase retardation. An achromatic waveplate with a smaller number of cells is demonstrated by arranging the two composed films made of different materials with high contrast of refractive index between them. The arrangement of a matching layer, which is associated with the performance of the achromatic waveplate, is also discussed.

Local field enhancement effects for dielectric coatings on silver nanorod arrays

A slanted silver nanorod array (NRA) deposited with glancing angle of deposition around 89°. By controlling the deposition angle, SiO2 and Ta2O5 grow on sliver rods in different morphologies. The multilayer designed as high reflective multilayer by arranging SiO2 and Ta2O5 alternatively on a cylindrical silver rod with diameter of 80 nm and a length of 200 nm would enhance the local field intensity and scattering when the rod is illuminated by s-polarized and ppolarized light waves. In this work, the reflective multilayer is designed at wavelengths of 450nm and 750nm that are associated with transverse plasmonic mode and longitudinal plasmonic mode, respectively. It is demonstrated experimentally that the intensity of light scattering from the capped NRA is enhanced due to the local field confinement around silver rod.

Effect of size of aluminum/silicon dioxide/aluminum nanosandwich films on their optical properties

Abstract. Three aluminum (Al)/silicon dioxide (SiO2)/aluminum (Al) nanosandwich films (SWFs) of various heights were fabricated using glancing angle deposition. An SWF comprises a 45-nm thick SiO2 layer sandwiched between two Al nanopillars. The thicknesses of both top and bottom nanopillars were varied from 187.5 to 217.5 nm. The equivalent constitutive and related parameters of each SWF were obtained from the reflection coefficients and transmission coefficients that were measured using a walk-off interferometer. Both the equivalent permittivity and the equivalent permeability of each SWF turned out to be negative real. Exactly how the height of the Al nanopillars of the double negative SWF affects its low reflectance through destructive interference is also examined using the wave tracing method. Moreover, the localized reversed magnetic field in the SiO2 layer of each SWF was simulated by finite-difference time-domain method to qualitatively interpret the negative real permeability.

Optical property of obliquely deposited dielectric films upon a metamaterial thin film

The equivalent electromagnetic parameters of a three-layered system Air/ Ta2O5 NRA (165nm)/ SiO2 NRA (133nm)/ Ag NRA(130nm) /BK7, fabricated by glancing angle deposition, are derived. The equivalent optical constants are used to estimate the optical performance that is achieved when additional films are deposited upon the NRA. First, the Ag NRA is deposited at a deposition angle of 89deg. An SiO2 NRA is then deposited at a deposition angle of -89deg. Finally a Ta2O5 NRA is deposited upon the SiO2 NRA at a deposition angle of 80deg. The reflectance and transmittance of the three-layered system Air/ Ta2O5 NRA (165nm)/ SiO2 NRA (133nm)/ Ag NRA(130nm) /BK7 glass are measured and compared with the theoretically estimated reflectance. At a wavelength of 568nm, the theoretically estimated p-polarization reflectance is 21.75%, which is close to the measured reflectance of 20.3%. A suitable arrangement of dielectric thin films upon a metamaterial thin film is demonstrated to induce p-polarized transmission.