Ching-Wei Yu

Vapor-deposited thin films with negative real refractive index in the visible regime

A thin film comprising parallel tilted nanorods was deposited by directing silver vapor obliquely towards a plane substrate. The reflection and transmission coefficients of the thin film were measured at three wavelengths in the visible regime for normal-illumination conditions, using ellipsometry and walk-off interferometry. The thin film was found to display a negative real refractive index. Since vapor deposition is a well-established industrial technique to deposit thin films, this finding is promising for large-scale production of negatively refracting metamaterials.

Multilayered structures for p- and s-polarized long-range surface-plasmon-polariton propagation

A normalized admittance diagram assists in describing and designing multilayered structures to excite long-range surface-plasmon-polariton (LRSPP) waves of either the p- or the s-polarization state. These structures comprise symmetric periodic multilayers on one or both sides of a metal thin film in either the Kretschmann or the Sarid configuration. The normalized admittance diagram even assists in designing structures that can be used to excite LRSPP waves of both polarization states simultaneously.

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.

Deposited metamaterial thin film with negative refractive index and permeability in the visible regime.

Thin films are fabricated from arrays of silver nanorods with thicknesses of 160 nm and 200 nm, to function as a metamaterial. The negative refractive index and negative permeability are retrieved from measured reflection and transmission coefficients using walk-off interferometer in the visible regime. A negative-index-material thin film with negative permittivity or (and) permeability can be produced by glancing angle deposition.

Negative real parts of the equivalent permittivity, permeability, and refractive index of sculptured-nanorod arrays of silver

Thin films comprising parallel nanorods were deposited by directing silver vapor obliquely toward a plane substrate. The direction of the vapor flux was varied in two different ways to sculpture the nanorods in different shapes. The reflection and transmission coefficients of the thin films were measured at three wavelengths in the visible regime for normal-illumination conditions for two linear-polarization states, using walk-off interferometry and polarization interferometry. The authors found that sculpturing significantly affects the signs of the real parts of the equivalent permittivity, permeability, and refractive index of the silver thin films for the two polarization states at different wavelengths. Thus, vapor deposition combined with sculpturing can be useful for large-scale production of materials having different equivalent constitutive parameters with negative real parts.

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.

Modulation of the polarization state of light using a weak anisotropic thin film

The polarization state of light is modulated by an anisotropic thin film. An anisotropic MgF(2) film is deposited onto a plate that is put in contact with a BK7 prism to form a BK7 prism/film/air configuration. It is shown that the polarization state of reflected light can be easily modulated from a linear state to a circular state by rotating the thin-film plate.

Metal and dielectric duality for an aligned Al nanorod array

In this letter, aluminum (Al) is used to make a narrow rod array (NRA) by oblique angle deposition method. The Al NRA film exhibits both metallic and dielectric optical characteristics. Like metal films, the Al NRA film only weakly disperses visible wavelength of normally incident light. However, strong interference causes the obliquely incident s-polarized and p-polarized lights to resonate in the Al NRA film. Even though the optical constant of the Al NRA film is dielectriclike, the surface plasmon wave excited and propagating on the surface of Al NRA is demonstrated to be responsible for the p-polarized resonance.

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.

Shape effect on the real parts of equivalent permeability of chevron thin films of silver

One symmetric and two asymmetric chevron thin films of silver were fabricated by bidepositing oppositely tilted nanorods sequentially via oblique angle deposition. The equivalent electromagnetic parameters of the films for s and p polarizations were retrieved at a wavelength of 639 nm for normal incidence using walk-off and polarization interferometers. Experimental results indicate that the symmetric chevron thin film has the strongest magnetic field reversal among the three different shapes. The coupling of the transverse magnetic field between two separate rods makes the equivalent permeability negative real.