Eunice Sok Ping Leong

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.

Polarization-Independent Multiple Fano Resonances in Plasmonic Nonamers for Multimode-Matching Enhanced Multiband Second-Harmonic Generation

Plasmonic oligomers composed of metallic nanoparticles are one class of the most promising platforms for generating Fano resonances with unprecedented optical properties for enhancing various linear and nonlinear optical processes. For efficient generation of second-harmonic emissions at multiple wavelength bands, it is critical to design a plasmonic oligomer concurrently having multiple Fano resonances spectrally matching the fundamental excitation wavelengths and multiple plasmon resonance modes coinciding with the harmonic wavelengths. Thus far, the realization of such a plasmonic oligomer remains a challenge. This study demonstrates both theoretically and experimentally that a plasmonic nonamer consisting of a gold nanocross surrounded by eight nanorods simultaneously sustains multiple polarization-independent Fano resonances in the near-infrared region and several higher-order plasmon resonances in the visible spectrum. Due to coherent amplification of the nonlinear excitation sources by the Fano resonances and efficient scattering-enhanced outcoupling by the higher-order modes, the second-harmonic emission of the nonamer is significantly increased at multiple spectral bands, and their spectral positions and radiation patterns can be flexibly manipulated by easily tuning the length of the surrounding nanorods in the nonamer. These results provide us with important implications for realizing ultrafast multichannel nonlinear optoelectronic devices.

Active molecular plasmonics: Tuning surface plasmon resonances by exploiting molecular dimensions

Abstract: Molecular plasmonics explores and exploits the molecule–plasmon interactions on metal nanostructures to harness light at the nanoscale for nanophotonic spectroscopy and devices. With the functional molecules and polymers that change their structural, electrical, and/or optical properties in response to external stimuli such as electric fields and light, one can dynamically tune the plasmonic properties for enhanced or new applications, leading to a new research area known as active molecular plasmonics (AMP). Recent progress in molecular design, tailored synthesis, and self-assembly has enabled a variety of scenarios of plasmonic tuning for a broad range of AMP applications. Dimension (i.e., zero-, two-, and threedimensional) of the molecules on metal nanostructures has proved to be an effective indicator for defining the specific scenarios. In this review article, we focus on structuring the field of AMP based on the dimension of molecules and discussing the state of the art of AMP. Our perspective on the upcoming challenges and opportunities in the emerging field of AMP is also included.

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.

Plasmon-induced transparency in coupled triangle-rod arrays

We demonstrate polarization-dependent plasmon-induced transparency in coupled triangle-rod arrays. The observed phenomenon is the result of the destructive interference between the bright and dark resonators in this coupled system, which is verified through the numerical simulations using the finite-difference time-domain (FDTD) method. By precisely controlling the structural parameters of the coupled triangle-rod system, the plasmon-induced transparency can be effectively manipulated. This plasmonically coupled nanostructure could be potentially useful for designing and developing artificial plasmonic molecules and metamaterials with desired functions, which may further find promising applications in biosensing, nanoparticle trapping and optical filters.

Incident-angle dependent color tuning from a single plasmonic chip

We report on a broad color tuning effect covering the visible range from a single plasmonic chip. By simply tilting the orientation of the designed plasmonic chip within a certain range, the photon-plasmon coupling interactions between the incident light and the plasmonic nanostructures on the chip can be finely tuned, resulting in an angle-dependent continuous color filtering effect. The physical mechanism of the device is investigated through the full-wave calculations, which provide important guidance for the design and optimization of the proposed devices. The broad color tuning from the demonstrated single chip will potentially benefit visualization and display technologies, and is particularly useful for the construction of reflection-based spatial light modulators.

Azo-dye-doped absorbing photonic crystals with purely imaginary refractive index contrast and all-optically switchable diffraction properties

We demonstrate a two-dimensional absorbing photonic crystal with a uniform real part of the refractive index, but a periodically modulated imaginary part. It was realized through back-filling the voids of a periodic array of azo-dye-doped polymeric disks with the same undoped polymers. The photonic crystals were characterized using the diffraction method. The experimental results showed that only the light in the spectral range where the azo-dye absorbed was diffracted, indicating that a purely absorbing photonic crystal was formed. This absorbing photonic crystal also showed switchable diffraction properties due to the trans-cis isomerization of the azo-dye under the light pump.

Enhanced photoelectrochemical performance of bridged ZnO nanorod arrays grown on V-grooved structure.

Bridged ZnO nanorod arrays on a V-grooved Si(100) substrate were used as the photoanode of a photoelectrochemical (PEC) cell for water splitting. Photolithography followed by reactive ion etching was employed to create a V-grooved structure on a Si substrate. ZnO nanorod arrays were grown via a hydrothermal method. The light trapping and PEC properties are greatly enhanced using the bridged ZnO nanorod arrays on a V-grooved Si substrate compared with those on a flat one. Increased short circuit photocurrent density (J(SC), 0.73 mA cm(-2)) and half-life time (1500 s) are achieved. This improved J(SC) and half-life time are 4 times and 10 times, respectively, higher than those of the ZnO nanorod arrays grown on a flat substrate. The overall PEC cell performance improvement for the V-groove grown ZnO array is attributed to the reduced light reflection and enhanced light trapping effect. Moreover, V-groove ZnO showed stronger adhesion between ZnO nanorod arrays and the substrate.

Fabrication of suspended, three-dimensional chiral plasmonic nanostructures with single-step electron-beam lithography

Recent years have witnessed explosive development of chiral plasmonics due to the fact that chiral plasmonic nanostructures give rise to broadband and scalable chiroptical effects orders of magnitude larger than naturally occurring materials. Various chiral plasmonic nanostructures have been demonstrated based on top-down and bottom-up fabrication techniques. However, three-dimensional (3D) chiral plasmonic nanostructure fabrication still remains challenging in many aspects. Here, we demonstrate suspended 3D chiral plasmonic nanostructures fabricated with only one-step electron-beam lithography. Our approach is unique since no alignment is required in the fabrication processes and the top and the bottom structures are self-aligned. Our 3D chiral plasmonic nanostructure consists of a suspended ultrathin silicon nitride membrane with perfectly-aligned L-shape and disk-shape gold nanostructures on its two respective sides. Such suspended chiral plasmonic nanostructures possess strong chiroptical properties at optical frequencies, which can be engineered by simply changing the disk size on one side of the membrane. The origin of the chiroptical properties is also analyzed using the plasmon hybridization model. Experimental results are in good agreement with the finite-difference time-domain simulations. Such suspended chiral plasmonic nanostructures could be highly applicable for chirality analysis of biomolecules, drugs, and chemicals.

Developing novel liquid crystal technologies for display and photonic applications

Abstract Modern liquid crystal displays (LCDs) require novel technologies, such as new alignment methods to eliminate alignment layers, fast response and long operation time. To this end, we report an overview of recent efforts in LCD technologies devoted to realize more display modes having no alignment layer, faster switching time and low battery consumption. In particular, we overview recent advances on the liquid crystals (LCs) alignment for display applications, which includes superfine nanostructures, polymeric microchannels and polymer stabilized LCs. Furthermore, we analyze the main optical and electro-optical properties of new generation LCDs displays addressing a particular attention to LCs blue phase hosting gold nanoparticles. Moreover, we focus on the progress of electrofluidic displays, which demonstrates characteristics that are similar to LCDs, with attention on various pixel designs, operation principles and possible future trends of the technology.