Yu-Ju Tsai

Infrared metamaterial phase holograms

As a result of advances in nanotechnology and the burgeoning capabilities for fabricating materials with controlled nanoscale geometries, the traditional notion of what constitutes an optical device continues to evolve. The fusion of maturing low-cost lithographic techniques with newer optical design strategies has enabled the introduction of artificially structured metamaterials in place of conventional materials for improving optical components as well as realizing new optical functionality. Here we demonstrate multilayer, lithographically patterned, subwavelength, metal elements, whose distribution forms a computer-generated phase hologram in the infrared region (10.6 μm). Metal inclusions exhibit extremely large scattering and can be implemented in metamaterials that exhibit a wide range of effective medium response, including anomalously large or negative refractive index; optical magnetism; and controlled anisotropy. This large palette of metamaterial responses can be leveraged to achieve greater control over the propagation of light, leading to more compact, efficient and versatile optical components.

Probing dynamically tunable localized surface plasmon resonances of film-coupled nanoparticles by evanescent wave excitation.

The localized surface plasmon resonance (LSPR) spectrum associated with a gold nanoparticle (NP) coupled to a gold film exhibits extreme sensitivity to the nanogap region where the fields are tightly localized. The LSPR of an ensemble of film-coupled NPs can be observed using an illumination scheme similar to that used to excite the surface plasmon resonance (SPR) of a thin metallic film; however, in the present system, the light is used to probe the highly sensitive distance-dependent LSPR of the gaps between NPs and film rather than the delocalized SPR of the film. We show that the SPR and LSPR spectral contributions can be readily distinguished, and we compare the sensitivities of both modes to displacements in the average gap between a collection of NPs and the gold film. The distance by which the NPs are suspended in solution above the gold film is fixed via a thin molecular spacer layer and can be further modulated by subjecting the NPs to a quasistatic electric field. The observed LSPR spectral shifts triggered by the applied voltage can be correlated with angstrom scale displacements of the NPs, suggesting the potential for chip-scale or flow-cell plasmonic nanoruler devices with extreme sensitivity.

Planar, flattened Luneburg lens at infrared wavelengths

Employing artificially structured metamaterials provides a means of circumventing the limits of conventional optical materials. Here, we use transformation optics (TO) combined with nanolithography to produce a planar Luneburg lens with a flat focal surface that operates at telecommunication wavelengths. Whereas previous infrared TO devices have been transformations of free-space, here we implement a transformation of an existing optical element to create a new device with the same optical characteristics but a user-defined geometry.

Plasmonic multi-mode interference couplers

Plasmonic multi-mode interference (MMI) couplers have been investigated both numerically and experimentally at the telecommunication wavelength of 1.55 mum. In this study, the couplers are implemented using thin Au stripes that support long-range surface plasmons. We first detail the operation principle of these devices with numerical simulations and show that useful effects can be obtained despite the high material losses inherent to metallic structures. A series of MMI couplers is subsequently fabricated and experimentally characterized, showing a quantitative agreement with our numerical predictions. We conclude by discussing some of the possible applications for these structures.

Analysis of a Gradient Index Metamaterial Blazed Diffraction Grating

The equivalent of a blazed diffraction grating can be formed from an array of metamaterial elements arranged so as to produce a linear gradient in the effective refractive index. By spreading the gradient over a multiwavelength distance, and repeating the pattern many times, a gradient index (GRIN) diffraction grating is formed. Using lithographically patterned, metallic metamaterial elements, dozens of distinguishable phase levels can be implemented by slightly modifying the design of each successive metamaterial element. We analyze here a multilayer metamaterial diffraction grating designed for operation at 10.6 μm, exploring the impact of material losses and impedance mismatch on the diffraction efficiency.

Arbitrary birefringent metamaterials for holographic optics at λ = 1.55 μm

This paper presents an optical element capable of multiplexing two diffraction patterns for two orthogonal linear polarizations, based on the use of non-resonant metamaterial cross elements. The metamaterial cross elements provide unique building blocks for engineering arbitrary birefringence. As a proof-of-concept demonstration, we present the design and experimental characterization of a polarization multiplexed blazed diffraction grating and a polarization multiplexed computer-generated hologram, for the telecommunication wavelength of λ = 1.55 μm. A quantitative study of the polarization multiplexed grating reveals that this approach yields a very large polarization contrast ratio. The results show that metamaterials can form the basis for a versatile and compact platform useful in the design of multi-functional photonic devices.

Metamaterial polarization multiplexed gratings

We demonstrate a metamaterial grating that has two diffraction periods for two orthogonal linear polarization states of illuminations. The proposed method will be useful in free space optical communications and novel optical imaging systems.

Infrared metamaterial hologram

We designed, fabricated, and characterized an infrared metamaterial hologram. The hologram correctly reproduces the design image. This work demonstrates that metamaterials can be used to fabricate devices with arbitrary 2D refractive index profiles.