Van Hoi Pham

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.

TiO2/CdS nanocomposite films: fabrication, characterization, electronic and optical properties

In this work, TiO2 nanocrystalline thin films were obtained through evaporating Ti films by electron beam deposition (EBD) followed by thermal treatment. The results show that after annealing at 300, 400 and 450 C for 8h, the obtained TiO2 thin films have nanoparticle and nanorod structures of 15‐30nm diameter and 100‐300nm length. At 750 C for 8h, the rutile phase was formed. The incorporation of cadmium sulfide (CdS) into TiO2 nanoparticle thin films was investigated. A CdS thin film was vacuum deposited onto the pre-deposited TiO2 film by a thermal evaporation technique. The obtained TiO2 and TiO2/CdS nanocomposite films were characterized by x-ray diffraction (XRD) and a field emission scanning electron microscope (FE-SEM). The TiO2/CdS composite film was used in a photo-electrochemical (PEC) cell as a working electrode and in a platinum electrode as a counter electrode. The electrolyte solution contains 1M KCl and 0.1M Na2S. The results show that the cell with the TiO2/CdS composite film electrode has significantly improved photoelectric capability in comparison with that of pure TiO2 thin films, and the best thickness of the CdS thin film deposited on the ITO/TiO2 substrates is 70‐140nm.

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.

A microcavity based on a porous silicon multilayer

We present a new result for the wavelength-selective characteristics of a 1D photonic microcavity based on porous silicon. These properties are studied in both experimentation and simulation. The 1D Fabry–Perot cavity is fabricated by the electrochemical etching of a low-resistivity silicon wafer with modulation of applied current densities. The simulation relies on the transfer matrix method (TMM) to design and predict the optical properties of a 1D photonic microcavity as well as the relation between anodization parameters with reflection spectra. The experimental results show that the elaborated porous silicon photonic microcavities have the wavelength-selective property in a controllable range of 550–775 nm. We have grown cavity structures of 20 stacked layers and the line width at full-width half-maximum (FWHM) of the transmission band of cavity is 20 nm, centered at 643.27 nm. Measured spectral characteristics of photonic microcavity agree with the simulation results.

Control of whispering-gallery-mode spectrum from erbium-doped silica microsphere lasers

High-power whispering-gallery modes (WGMs) emitted from microcavity lasers have attracted attention for many applications, such as optical signal processing, spectroscopy, optical sensors, and large-bandwidth optical communications. In this paper, we present a simple approach for controlling the output WGMs of erbium-doped silica microsphere lasers. With the presented scheme, accurate adjustment of the coupling gap between the collection fiber taper and the spherical surface allows us to select different single modes of the microsphere laser or different multimode configurations (also functions of the waist diameter of taper and the Er-doped concentration). The nonlinear frequency shift of the microsphere cavity as a function of intracavity power has also been studied. The high intensity and high side mode suppression ratio of the obtained single WGM are suitable for spectroscopy, optical sensors, and communications.

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.

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%.

CdS sensitized ZnO electrodes in photoelectrochemical cells

In this work, ZnO nanocrystalline thin films were obtained by evaporating Zn films using a thermal evaporation technique and then thermal treatment. The results show that after annealing at 300, 350, 400 and 450 °C for 6 h, the obtained ZnO thin films have macro-structures. The results show that at an annealing temperature of 450 °C, the ZnO thin film has the best optical properties for photo-electrodes. The incorporation of cadmium sulfide (CdS) into ZnO macro-structure thin films was investigated. A CdS thin film was vacuum-deposited onto the pre-deposited ZnO film by the thermal evaporation technique. The obtained ZnO and ZnO/CdS bilayer films were characterized by x-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The ZnO/CdS bilayer film was used in a photoelectrochemical (PEC) cell as a working electrode and a platinum electrode as a counter electrode. The electrolyte solution contained 1 M KCl and 0.1 M Na2S. The results show that the cell with the ZnO/CdS bilayer electrode had significantly improved photoelectric properties in comparison with that of the pure ZnO thin films. The best thickness of the CdS thin film deposited on ITO/ZnO substrates was around 70 nm.

Characteristics of the Fiber Laser Sensor System Based on Etched-Bragg Grating Sensing Probe for Determination of the Low Nitrate Concentration in Water

The necessity of environmental protection has stimulated the development of many kinds of methods allowing the determination of different pollutants in the natural environment, including methods for determining nitrate in source water. In this paper, the characteristics of an etched fiber Bragg grating (e-FBG) sensing probe—which integrated in fiber laser structure—are studied by numerical simulation and experiment. The proposed sensor is demonstrated for determination of the low nitrate concentration in a water environment. Experimental results show that this sensor could determine nitrate in water samples at a low concentration range of 0–80 ppm with good repeatability, rapid response, and average sensitivity of 3.5 × 10−3 nm/ppm with the detection limit of 3 ppm. The e-FBG sensing probe integrated in fiber laser demonstrates many advantages, such as a high resolution for wavelength shift identification, high optical signal-to-noise ratio (OSNR of 40 dB), narrow bandwidth of 0.02 nm that enhanced accuracy and precision of wavelength peak measurement, and capability for optical remote sensing. The obtained results suggested that the proposed e-FBG sensor has a large potential for the determination of low nitrate concentrations in water in outdoor field work.

A Vapor Sensor Based on a Porous Silicon Microcavity for the Determination of Solvent Solutions

A porous silicon microcavity (PSMC) sensor has been made for vapors of solvent solutions, and a method has been developed in order to obtain simultaneous determination of two volatile substances with different concentrations. In our work, the temperature of the solution and the velocity of the air stream flowing through the solution have been used to control the response of the sensor for ethanol and acetone solutions. We study the dependence of the cavity-resonant wavelength shift on solvent concentration, velocity of the airflow and solution temperature. The wavelength shift depends linearly on concentration and increases with solution temperature and velocity of the airflow. The dependence of the wavelength shift on the solution temperature in the measurement contains properties of the temperature dependence of the solvent vapor pressure, which characterizes each solvent. As a result, the dependence of the wavelength shift on the solution temperature discriminates between solutions of ethanol and acetone with different concentrations. This suggests a possibility for the simultaneous determination of the volatile substances and their concentrations.