Wei-Yi Tsai

Aluminum Plasmonic Multicolor Meta-Hologram

We report a phase-modulated multicolor meta-hologram (MCMH) that is polarization-dependent and capable of producing images in three primary colors. The MCMH structure is made of aluminum nanorods that are arranged in a two-dimensional array of pixels with surface plasmon resonances in red, green, and blue. The aluminum nanorod array is patterned on a 30 nm thick SiO2 spacer layer sputtered on top of a 130 nm thick aluminum mirror. With proper design of the structure, we obtain resonances of narrow bandwidths to allow for implementation of the multicolor scheme. Taking into account of the wavelength dependence of the diffraction angle, we can project images to specific locations with predetermined size and order. With tuning of aluminum nanorod size, we demonstrate that the image color can be continuously varied across the visible spectrum.

Versatile Polarization Generation with an Aluminum Plasmonic Metasurface

All forms of light manipulation rely on light-matter interaction, the primary mechanism of which is the modulation of its electromagnetic fields by the localized electromagnetic fields of atoms. One of the important factors that influence the strength of interaction is the polarization of the electromagnetic field. The generation and manipulation of light polarization have been traditionally accomplished with bulky optical components such as waveplates, polarizers, and polarization beam splitters that are optically thick. The miniaturization of these devices is highly desirable for the development of a new class of compact, flat, and broadband optical components that can be integrated together on a single photonics chip. Here we demonstrate, for the first time, a reflective metasurface polarization generator (MPG) capable of producing light beams of any polarizations all from a linearly polarized light source with a single optically thin chip. Six polarization light beams are achieved simultaneously including four linear polarizations along different directions and two circular polarizations, all conveniently separated into different reflection angles. With the Pancharatnam-Berry phase-modulation method, the MPG sample was fabricated with aluminum as the plasmonic metal instead of the conventional gold or silver, which allowed for its broadband operation covering the entire visible spectrum. The versatility and compactness of the MPG capable of transforming any incident wave into light beams of arbitrary polarizations over a broad spectral range are an important step forward in achieving a complete set of flat optics for integrated photonics with far-reaching applications.

Integrated plasmonic metasurfaces for spectropolarimetry

Plasmonic metasurfaces enable simultaneous control of the phase, momentum, amplitude and polarization of light and hence promise great utility in realization of compact photonic devices. In this paper, we demonstrate a novel chip-scale device suitable for simultaneous polarization and spectral measurements through use of six integrated plasmonic metasurfaces (IPMs), which diffract light with a given polarization state and spectral component into well-defined spatial domains. Full calibration and characterization of our device is presented, whereby good spectral resolution and polarization accuracy over a wavelength range of 500-700 nm is shown. Functionality of our device in a Müller matrix modality is demonstrated through determination of the polarization properties of a commercially available variable waveplate. Our proposed IPM is robust, compact and can be fabricated with a single photolithography step, promising many applications in polarization imaging, quantum communication and quantitative sensing.

Ultrafast Thermal Nonlinearity.

We show that when optical fields are plasmonically concentrated into the volumes on the scale of few tens of nanometers, the speed of the thermo-optical effects approaches picosecond scale, suitable for applications such as all-optical switching and routing.

Quasi-coherent thermal radiation with multiple resonant plasmonic cavities

This paper proposes a 1D plasmonic multilayer structure as a high-contrast mid-infrared thermal emitter with three distinct resonant wavelengths. The three resonance modes, based on the localized surface plasmon, provide an omnidirectional thermal emission. The emissivity spectrum reveals high polarization and strongly angle-independent properties. The resonance-assisted emissivity can be as high as 19.5 dB relative to off-resonant sideband emissivity. Such extremely low sideband emissivity makes the proposed plasmonic thermal emitter an efficient, high-contrast emitter, which will be useful for thermophotovoltaic and thermal sensing applications.

Material-assisted metamaterial: a new dimension to create functional metamaterial

A high Q-value reflective type metasurface consisting of 1D Au nanorods, a SiO2 spacer and a Au back reflector is demonstrated. It is shown that the sideband of the resonant mode can be suppressed as the resonant wavelength close to the phonon absorption of SiO2. By combining both designed structured resonance and inherent property of the based materials, a low angle-dependent metasurface with a Q-value of 40 has been demonstrated. The proposed structure will be useful for high sensitivity sensing and narrow band thermal emitter.

Optical toroidal response in three-dimensional plasmonic metamaterial

Toroidal dipole moments, the third kind of fundamental dipole moment, have unusual electromagnetic properties different from the electric and magnetic multipoles. We fabricate a new type of 3D plasmonic toroidal metamaterial by using mutual coupling between dumbbell-shaped gold apertures with vertical split-ring resonators (VSRRs) at optical frequency. The radiated power of multipole moments are calculated and analyzed to improve the meta-system is dominated by the toroidal dipole moment. This result paves a way for practical application on metamaterial based devices, such as biosensor and lasing spaser.

Investigation of degradation in beryllium chalcogenide II-VI semiconductors

The wurtzite-type Cd1−x−yZnxBeySe crystals grown by the high-pressure Bridgman method were investigated to obtain the suitable beryllium composition for improving the lifetime of II-VI based optoelectronic devices. A method based on the continuous exposure of the electron beam of cathodoluminescence measurement was introduced to probe the aging characteristics of optoelectronic materials. The results show that in order to improve the degradation problem, the incorporation of beryllium content in II-VI compounds should be less than 10%. In addition, the authors demonstrate that cathodoluminescence image is a very powerful tool to reveal the generation of defects of an optoelectronic solid under external perturbation.

Horizontal toroidal response in three-dimensional plasmonic(Conference Presentation)

The toroidal dipole moments of natural molecules are hard to be detected so the artificial toroidal materials made by metamaterial attract more attentions. Metamaterial, the sub-wavelength artificial structures, can modulate reflection or transmission of light. The toroidal metamaterial can not only amplify the toroidal moment but also repress the electric and magnetic dipole so it can be used to study the properties of toroidal dipole moment. However, there are many limitations for the experiments, such as the lateral light is necessary to excite the toroidal response. Most of the toroidal dipole moments oscillate perpendicularly to the substrate, therefore it is difficult to couple it with other dipole moments and could be only excited in the microwave region. In this paper, we design a toroidal metamaterial consisting of dumbbell-shaped aperture and vertical split ring resonator (VSRR) vertically. The toroidal dipole moment of our metamaterial is excited in the optical region. The arrangement of our nanostructures is vertical instead of planar annular arrangement to reduce the size of the unit cell and increase the density of the toroidal dipole moment. Moreover, the direction of toroidal dipole moment is parallel to the substrate which can be used for the study of the coupling effect with other kinds of dipolar moments.

Erratum: Current and Strain-Induced Spin Polarization inInGaN/GaNSuperlattices [Phys. Rev. Lett.98, 136403 (2007)]

The lateral current-induced spin polarization in InGaN/GaN superlattices (SLs) without an applied magnetic field is reported. The fact that the sign of the nonequilibrium spin changes as the current reverses and is opposite for the two edges provides a clear signature for the spin Hall effect. In addition, it is discovered that the spin Hall effect can be strongly manipulated by the internal strains. A theoretical work has also been developed to understand the observed strain-induced spin polarization. Our result paves an alternative way for the generation of spin polarized current.