Quang Minh Ngo

Micro- and nanophotonic structures in the visible and near infrared spectral region for optical devices*

In this paper we present some research results on the micro and nano-photonic structures in the visible and near infrared spectral region for optical devices that have been done within the framework of Nanoscience and Nanotechnology Program of Institute of Materials Science. In the first part, we report the design and fabrication of 1D photonic structure based on porous silicon layers fabricated by electrochemical etching method and some of their potential applications such as optical filters, microcavity and optical sensors for distinguishing the content of bio-gasoline. In addition, we demonstrate some results on preparation of the 2D and 3D nanophotonic structures based on silica opal layers prepared by sol–gel and self-assembled methods. In the second part, we demonstrate the results of lasing emissions of erbium ions in the visible and near infrared zone from microcavity. The observation of emission of single-mode green light at the wavelength of 537 nm from erbium ions in the microcavity is interesting for the study of atom–photon interaction phenomenon. In the last part, we will show some new results of design and fabrication of nanocomposite based on nanoscale TiO2 and/or ZnO and nanoparticles of semiconductors and metals, which are oriented to the fabrication of energy conversion and photo-reactor devices.

Optical bistable devices based on guided-mode resonance in slab waveguide gratings

We investigate properties of nonlinear resonant gratings with emphasis on optical bistability. Slab waveguide gratings with various quality factors are designed and their characteristics analyzed with a finite-difference time-domain method. Considerable field enhancements are observed in the gratings and the performance compares favorably with metallic bistable devices. Bistability based on coupled gratings is also treated. Mechanically controllable switching intensity realized by varying a gap distance between two gratings is demonstrated. Resonant nonlinear elements in this work may find applications in all-optical information processing and optical switching, and our investigation on the dependence of the normalized switching intensity and the response time on quality factor will provide a general guide line for grating-based bistable device design.

Optical bistability based on guided-mode resonances in photonic crystal slabs

Optical bistability of nonlinear guided-mode resonances in photonic crystal slabs (PCSs) are numerically investigated. We perform finite-difference time-domain simulations to determine the linear and nonlinear characteristics of these guided-mode resonances in PCSs. The nonlinear characteristics such as switching intensity and switching time, which are suggested as the performance metric of all-optical switches, are similar to the all-optical switches in slab waveguide gratings. While the slab waveguide gratings are sensitive to the incoming light polarization, the PCSs can offer an advantage of avoiding reduction efficiency and a requirement of careful polarization stabilization of the light source. From the calculations, we introduce a dependency of the normalized switching intensity and the switching time of the all-optical switches on quality factor for our designed guided-mode resonances in PCSs.

Optical bistability based on Fano resonances in single- and double-layer nonlinear slab waveguide gratings

In this paper, we numerically investigate all-optical bistable switching at low input intensity based on Fano resonances available in nonlinear slab waveguide gratings with narrow slits. Fano resonances with various quality factors (Q-factors) in the single- and double-layer slab waveguide gratings are designed and their characteristics are studied by the finite-difference time-domain method. Dependencies on wavelengths of operation, various switching intensities, contrast, and bandwidth of all-optical bistabilities are observed. Comparing nonlinear characteristics of single- and double-layer grating configurations, the latter provides more bistable efficiency with the low input intensities needed and high contrast with high Q-factors at certain operating wavelengths. Both grating configurations in this work provide interesting venues for highly efficient Fano resonance-based all-optical bistable switching devices.

Metallic assisted guided-mode resonances in slab waveguide gratings for reduced optical switching intensity in bistable devices

A thin metal layer is introduced into a slab waveguide grating with guided-mode resonances to reduce the switching intensity in its bistable operational mode. A dielectric grating put on top of the metal layer plays the role of a coupling element between the normally incident light and the guided mode in the slab waveguide grating. The presence of the metal layer increases the reflectivity of the optical device that as a consequence exhibits high-reflection side bands and close-to-zero reflectivity drops. Several structures are designed by changing the grating depths and the metal layer thicknesses, and their influences on linear and nonlinear characteristics are analysed using the finite-difference time-domain method. We found that the metal layer increases the quality factor of guided-mode resonance filters. Numerical results show that the quality factor improves 5.6 times and the switching intensity is reduced 45 times when these devices are compared to typical slab waveguide gratings in the same working conditions, in terms of polarization and operating wavelength.

Nanostructured Metal–Insulator–Metal Metamaterials for Refractive Index Biosensing Applications: Design, Fabrication, and Characterization

Nanofabrication of nanostructured metal-insulator-metal metamaterial (NMIM2) by the electron beam lithography and the liftoff technique is reported in this paper. The NMIM2 consists of periodic arrays of gold (Au) nanotriangles deposited on a gold layer (acting as a mirror) separated by a thin dielectric insulator. Such an NMIM2 stack was fabricated on a silicon wafer. Reflection of the NMIM2 was measured by the Fourier transform infrared spectroscope. The experimental measurement for reflection coefficients are in good agreement with the full-wave finite-different time-domain simulation results. At resonance, we numerically observed a strong field localized inside the gap between the Au nanostructures and the Au mirror, resulting in a strong confinement of incoming light within the insulator spacer and, thus, a pronounced absorption at telecommunication wavelengths. This resonant spectral response is investigated for sensitive label-free refractive index biosensing applications.

Nano-porous Silicon Microcavity Sensors for Determination of Organic Fuel Mixtures

We present the preparation and characteristics of liquid-phase sensors based on nano-porous silicon multilayer structures for determination of organic content in gasoline. The principle of the sensor is a determination of the cavity-resonant wavelength shift caused by refractive index change of the nano-porous silicon multilayer cavity due to the interaction with liquids. We use the transfer matrix method (TMM) for the design and prediction of characteristics of microcavity sensors based on nano-porous silicon multilayer structures. The preparation process of the nano-porous silicon microcavity is based on electrochemical etching of single-crystal silicon substrates, which can exactly control the porosity and thickness of the porous silicon layers. The basic characteristics of sensors obtained by experimental measurements of the different liquids with known refractive indices are in good agreement with simulation calculations. The reversibility of liquid-phase sensors is confirmed by fast complete evaporation of organic solvents using a low vacuum pump. The nano-porous silicon microcavity sensors can be used to determine different kinds of organic fuel mixtures such as bio-fuel (E5), A92 added ethanol and methanol of different concentrations up to 15%.

Fabrication and Numerical Characterization of Infrared Metamaterial Absorbers for Refractometric Biosensors

We report the successful fabrication of infrared plasmonic metamaterial absorbers by electron beam lithography and the lift-off technique. The absorber consists of periodic arrays of gold (Au) nanostructures deposited on a stack of thin silica spacer and gold film (acting as a mirror) on a silicon wafer. At resonance, we numerically observed a strong field enhancement between the metallic nanostructures and the Au film, resulting in a strong confinement of incident light within the silica spacer and thus a high absorption of up to 80% at infrared wavelengths. Our experimental measurement for reflection coefficients are in excellent agreement with the full-wave simulation results. We show that the resonant absorption spectral response can be used for highly-sensitive, label-free refractive-index biosensors. By tailoring various forms of nanostructures, we investigate their refractive index sensitivities to identity the most sensitive sensor at specific infrared wavelengths.

All-Optical Switches Based on Multiple Cascaded Resonators With Reduced Switching Intensity-Response Time Products

The general nonlinear behavior of multiple cascaded resonators is analyzed by using the coupled-mode theory in time and then compared to a single resonator from the viewpoint of optical bistable operation. Our analysis reveals that from the perspective of switching intensity, the multiple resonator structure is more advantageous than the single resonator with an equivalently increasing quality factor. It is also shown that a switching intensity-response time product, suggested as a performance metric for optical bistable devices, decreases linearly as the number of cascaded resonators increases. The effect of coupling n resonators is equivalent to a single resonator device with n-time increased quality factor in terms of the performance metric. The coupled-mode theory analysis is confirmed by a numerical study of the bistable operation of one-dimensional photonic crystal resonator structures using the finite-difference time-domain method.

Cavity induced perfect absorption in metamaterials

We present novel resonant modes at the THz regime in a structure combining conventional metamaterial absorber (MA) with a cavity (MAC). The well-known structure consisting of three individual layers of periodic metallic dishes on the top, a dielectric layer in the middle, and a metallic film in the bottom is used, and the cavity is formed on the top layer by changing the geometry of the metallic dishes. MACs with various cavity parameters are designed and their absorption characteristics, such as magnetic field distribution, surface current, and power loss density at resonant frequencies of the designed structure, are numerically investigated. Resonant effects in this work may find applications in THz tunable and broadband MA, and our investigation on the dependence of the absorption frequency and absorption intensity on the geometric cavity of the designed structure will provide a general guideline for MAC design.