Guangyuan Si

Enhanced Surface Plasmon Resonance on a Smooth Silver Film with a Seed Growth Layer

This paper reports an effective method to enhance the surface plasmon resonance (SPR) on Ag films by using a thin Ni seed layer assisted deposition. Ag films with a thickness of about 50 nm were deposited by electron beam evaporation above an ultrathin Ni seed layer of approximately 2 nm on both silicon and quartz substrates. The root-mean-square (rms) surface roughness and the correlation length have been reduced from >4 nm and 28 nm for a pure Ag film to approximately 1.3 and 19 nm for Ag/Ni films, respectively. Both experimental and simulation results show that the Ag/Ni films exhibit an enhanced SPR over the pure Ag film with a narrower full width at half-maximum. Ag films with a Ge seed layer have also been prepared under the same conditions. The surface roughness can be reduced to less than 0.7 nm, but narrowing of the SPR curve is not observed due to increased absorptive damping in the Ge seed layer. Our results show that Ni acts as a roughness-diminishing growth layer for the Ag film while at the same time maintaining and enhancing the plasmonic properties of the combined structures. This points toward its use for low-loss plasmonic devices and optical metamaterials applications.

Liquid-Crystal-Enabled Active Plasmonics: A Review

Liquid crystals are a promising candidate for development of active plasmonics due to their large birefringence, low driving threshold, and versatile driving methods. We review recent progress on the interdisciplinary research field of liquid crystal based plasmonics. The research scope of this field is to build the next generation of reconfigurable plasmonic devices by combining liquid crystals with plasmonic nanostructures. Various active plasmonic devices, such as switches, modulators, color filters, absorbers, have been demonstrated. This review is structured to cover active plasmonic devices from two aspects: functionalities and driven methods. We hope this review would provide basic knowledge for a new researcher to get familiar with the field, and serve as a reference for experienced researchers to keep up the current research trends.

Annular aperture array based color filter

In this letter, we propose and experimentally demonstrate a color filter based on an annular aperture geometry working in the visible range. The device is built by configuring an array of annular apertures in a gold film suitable for transmission measurement. We show effective fine tuning of resonance peaks through precise geometric control of the aperture dimensions. Selective transmission through annular apertures of various sizes leads to continuous color tuning of transmitted electromagnetic waves. This may find potential for application in high-definition displays, optical filters, ultrafast switching, and bio-sensing.

Nano-antenna in a photoconductive photomixer for highly efficient continuous wave terahertz emission

We report highly efficient continuous-wave terahertz (THz) photoconductive antenna based photomixer employing nano-gap electrodes in the active region. The tip-to-tip nano-gap electrode structure provides strong THz field enhancement and acts as a nano-antenna to radiate the THz wave generated in the active region of the photomixer. In addition, it provides good impedance matching to the THz planar antenna and exhibits a lower RC time constant, allowing more efficient radiation especially at the higher part of the THz spectrum. As a result, the output intensity of the photomixer with the new nano-gap electrode structure in the active region is two orders of magnitude higher than that of a photomixer with typical interdigitated electrodes. Significant improvement in the THz emission bandwidth was also observed. An efficient continuous wave THz source will greatly benefit compact THz system development for high resolution THz spectroscopy and imaging applications.

Suspended slab and photonic crystal waveguides in lithium niobate

Suspended waveguides have been widely applied to silicon-on-insulator structures because they are easily fabricated with processing techniques similar to those of integrated circuit design. However, it is difficult to fabricate such structures in lithium niobate, which is also a very important material for optoelectronics. One main challenge is the difficulty of etching lithium niobate. In this work, the authors show a method to fabricate suspended slab waveguides in lithium niobate by combining ion implantation, focused ion beam milling, and selective wet etching techniques. The method does not involve wafer bonding or crystal ion slicing and is entirely monolithic. Lattice damage can be introduced to a buried thin layer of a certain depth beneath the sample surface by ion implantation, resulting in a considerable wet etching selectivity to bulk material. The etching rate has been investigated to control the size of the suspended membrane. Fabrication of suspended photonic crystal waveguides has also bee...

Photonic crystal structures with ultrahigh aspect ratio in lithium niobate fabricated by focused ion beam milling

Lithium niobate (LiNbO3, LN) is an important material which is widely applied in fabricating photonic and acoustic devices. However, it is difficult to either wet etch or dry etch LN due to the material’s properties. Here, the authors report novel pattern fabrication based on LN using focused ion beam (FIB) milling. When an array of small holes is etched, a severe tapering problem is observed as is common, but by replacing the nanocylindrical hole array with a nanoring structure, the authors obtain photonic crystals with an aspect ratio of up to 50:1 (2 μm total etching depth and 40 nm gap aperture). Dense nanorod arrays with sub-30-nm ultrasmall gaps and more than 2.5 μm etching depth are also achieved with FIB milling.

Direct and accurate patterning of plasmonic nanostructures with ultrasmall gaps

We report an improved method to directly and accurately fabricate plasmonic nanostructures with ultrasmall gaps. The fabrication is based on high-resolution focused ion beam milling with closely packed nanoring patterns. With fine and precise adjustment of the ion beam, elegant plasmonic nanostructures with ultrasmall dimensions down to 10 nm are achieved. We also show that the gap dimensions have a strong effect on the optical reflectance and transmittance of the plasmonic nanostructures. Measured results show reasonable agreement with finite-difference time-domain calculations. Our approach could find promising applications in plasmon-assisted sensing and surface-enhanced spectroscopy.

Fabrication and characterization of well-aligned plasmonic nanopillars with ultrasmall separations

We show the fabrication of well-aligned gold and silver nanopillars with various array parameters via interference lithography followed by ion beam milling and compare the etching rates of these two metallic materials. Silver is suitable for fabricating ultrafine arrays with ultrasmall separations due to high milling rates. The optical properties of the fabricated nanopillars are specifically characterized from both normal incidence and oblique incident angles. Tunable surface plasmon resonances are achieved with varying structural parameters. Strong coupling effects are enabled when the separation between adjacent nanopillars is dramatically reduced, leading to useful applications in sensing and waveguiding.

Plasmon-enhanced sensing: current status and prospects

By combining different plasmonic nanostructures with conventional sensing configurations, chemical/biosensors with significantly enhanced device performance can be achieved. The fast development of plasmon-assisted devices benefits from the advance of nanofabrication technology. In this review, we first briefly show the experimental configurations for testing plasmon enhanced sensing signals and then summarize the classic nanogeometries which are extensively used in sensing applications. By design, dramatic increment of optical signals can be obtained and further applied to gas, refractive index and liquid sensing.

Subwavelength lithography by waveguide mode interference

A subwavelength lithography method is demonstrated theoretically and experimentally through the interference of transverse electric (TE) modes in a metal-dielectric waveguide (MDW). Like surface plasmon polaritons (SPPs) on metal surfaces, the TE modes have evanescent waves leaking out of the MDW and are used to do subwavelength patterning but with larger pattern area than SPPs for their low propagation loss. The patterning resolution and depth could be optimized by modifying the thickness of the dielectric layer in the MDW. Two-dimensional subwavelength patterning using TE modes is also proposed with azimuthally polarized light exposure.