Yiting Chen

Broadband near-infrared metamaterial absorbers utilizing highly lossy metals

Radiation absorbers have increasingly been attracting attention as crucial components for controllable thermal emission, energy harvesting, modulators, etc. However, it is still challenging to realize thin absorbers which can operate over a wide spectrum range. Here, we propose and experimentally demonstrate thin, broadband, polarization-insensitive and omnidirectional absorbers working in the near-infrared range. We choose titanium (Ti) instead of the commonly used gold (Au) to construct nano-disk arrays on the top of a silicon dioxide (SiO2) coated Au substrate, with the quality (Q) factor of the localized surface plasmon (LSP) resonance being decreased due to the intrinsic high loss of Ti. The combination of this low-Q LSP resonance and the propagating surface plasmon (PSP) excitation resonance, which occur at different wavelengths, is the fundamental origin of the broadband absorption. The measured (at normal light incidence) absorption is over 90% in the wavelength range from 900 nm to 1825 nm, with high absorption persisting up to the incident angle of ~40°. The demonstrated thin-film absorber configuration is relatively easy to fabricate and can be realized with other properly selected materials.

Random-phase metasurfaces at optical wavelengths

Random-phase metasurfaces, in which the constituents scatter light with random phases, have the property that an incident plane wave will diffusely scatter, hereby leading to a complex far-field response that is most suitably described by statistical means. In this work, we present and exemplify the statistical description of the far-field response, particularly highlighting how the response for polarised and unpolarised light might be alike or different depending on the correlation of scattering phases for two orthogonal polarisations. By utilizing gap plasmon-based metasurfaces, consisting of an optically thick gold film overlaid by a subwavelength thin glass spacer and an array of gold nanobricks, we design and realize random-phase metasurfaces at a wavelength of 800 nm. Optical characterisation of the fabricated samples convincingly demonstrates the diffuse scattering of reflected light, with statistics obeying the theoretical predictions. We foresee the use of random-phase metasurfaces for camouflage applications and as high-quality reference structures in dark-field microscopy, while the control of the statistics for polarised and unpolarised light might find usage in security applications. Finally, by incorporating a certain correlation between scattering by neighbouring metasurface constituents new types of functionalities can be realised, such as a Lambertian reflector.

Plasmonic channel waveguides in random arrays of metallic nanoparticles.

We report detailed characterization of surface plasmon-polariton guiding along 1-, 1.5- and 2-μm-wide channels in high-density (~75 μm-2) random arrays of gold 70-nm-high and 50-nm-wide nanoparticles fabricated on a 70-nm-thin gold film supported by a 170-μm-thick silica substrate. The mode propagation losses, effective index dispersion, and scattering parameters are characterized using leakage-radiation microscopy, in direct and Fourier planes, in the wavelength range of 740-840 nm. It is found that the mode supported by 2-μm-wide channels propagates over > 10 μm in straight waveguides, with the corresponding S-bends and Y-splitters functioning reasonably well. The results show that the SPP waves can efficiently be guided by narrow scattering-free channels cut through randomly corrugated surface regions. The potential of this waveguiding mechanism is yet to be fully explored by tuning the scattering mean-free path and localization length via the density and size of random nanoparticles. Nevertheless, the results obtained are encouraging and promising diverse applications of these waveguide components in plasmonic circuitry.

Flexible long-range surface plasmon polariton single-mode waveguide for optical interconnects

We present the design, fabrication and characterization of long-range surface plasmon polariton waveguide arrays with materials, mainly silicones, carefully selected with the aim to be used as mechanically flexible single-mode optical interconnections, the so-called “plasmonic arc” working at 1.55µm. The fabricated plasmonic arcs show a TM/TE polarization ratio of ~25 dB. By using the cut-back method, the straight propagation loss at 1.55µm is estimated to 0.5-1 dB/mm and coupling loss to ~1-2 dB/facet after dicing. In the free-standing S-curved configuration, the bending loss of single cladding plasmonic arc is 2.2-2.8 dB/90° at bending radius 2.5 mm. For double cladding plasmonic arcs, it is decreased to 0.7-1.7 dB/90° for the same radius. The coupling loss with single-mode glass PCB waveguides is estimated to be 1.7 dB/interface in the best condition.

Efficient interfacing photonic and long-range dielectric-loaded plasmonic waveguides

Long-range dielectric-loaded surface plasmon-polariton waveguides (LR-DLSPPWs) operating at telecom wavelengths are efficiently (end-fire) interfaced with photonic waveguides by taking advantage of very similar lateral mode field profiles in these waveguide configurations. The LR-DLSPPWs are formed by 1-μm-high and 1-μm-wide polymer ridges fabricated atop 15-nm-thin and 500-nm-wide gold stripes supported by a 289-nm-thick ormoclear polymer deposited on a low-index (1.34) layer of cytop, whereas gold stripes are absent in the photonic waveguide configuration that is identical to the plasmonic one in all other respects. The coupling efficiency between LR-DLSPPWs and photonic waveguides is numerically estimated to be 97%, decreasing by only a few percents for non-centered gold stripes (as long as a gold stripe is kept inside the polymer ridge). The fabricated LR-DLSPPWs coupled to photonic waveguides are first characterized using amplitude- and phase-resolved near-field imaging of propagating radiation that reveals very similar mode field distributions in these waveguides as well as their efficient interfacing. The coupling efficiency is then experimentally characterized using the cutback approach resulting in an average level of 75% per interface, while the LR-DLSPPW mode propagation length is estimated to be on average 0.3 mm. Possible reasons for differences between experimental and simulation results are discussed, indicating that a 3-nm-thin titanium layer (used for improving adhesion between gold and ormoclear) introduces substantial mode absorption. The results obtained open new perspectives for realization of hybrid photonic-plasmonic components and circuits.

Chip-size plasmonic spectropolarimeters

Chip-size plasmonic spectropolarimeters for simultaneous polarization state and wavelength determination have been designed and experimentally demonstrated. The proof-of-concept 96-μm-diameter spectropolarimeter operating in the wavelength range of 750–950 nm exhibits expected polarization selectivity and high angular dispersion (0.0133 °/nm).