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

Ngo, Quang Minh,Kim, Sangin,Song, Seok Ho,Magnusson, Robert The Optical Society 2009 Optics express Vol.17 No.26

<P>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.</P>

Enhanced UV/blue fluorescent sensing using metal-dielectric-metal aperture nanoantenna arrays

Quang Minh Ngo,Ying-Lung D. Ho,Jon R. Pugh,Andrei Sarua,Martin J. Cryan 한국물리학회 2018 Current Applied Physics Vol.18 No.7

Subwavelength aperture antenna arrays are designed and fabricated for potential applications in fluorescence sensing in the near UV/blue range. They are designed using finite-difference time-domain (FDTD) simulation, fabricated using focused ion beam etching and characterised using angular Fourier spectroscopy. The aperture arrays are formed in the top layer of an aluminum-silica-aluminum trilayer and produce a maximum simulated field intensity enhancement of 5.8 times at 406 nm and highly directive emission with a beamwidth of 8.3 deg. The normal incidence reflection response has been measured and shows reasonable agreement with modelled results. In addition, to investigate higher field intensity enhancements, bowtie aperture arrays are simulated and the influence of parameters such as dielectric gap, position of dipole source, and aperture shape and size are discussed and show enhancements up to 67 times are possible.

An Energy Harvester Based on a Bistable Origami Mechanism

Tien-Hoang Ngo,I-Ting Chi,Minh-Quang Chau,Dung-An Wang 한국정밀공학회 2022 International Journal of Precision Engineering and Vol.23 No.2

This paper describes a vibration energy harvester based on an origami mechanism through piezoelectric energy conversion. The device is capable of broadband energy harvesting from low frequency ambient vibrations. This design utilizes bistability inherent in the waterbomb base origami structure to increase the bandwidth of its frequency response. The folding along the crease pattern of the origami mechanism can facilitate large bending deformations of piezoelectric films to generate high electric power output. The compact size and light weight of the origami mechanism render it convenient for integration into various hosts subjected to vibrations. Performance of the origami mechanism is investigated under external excitations with various vibration magnitudes and acceleration levels. This work demonstrates a scheme of broadband vibration energy harvesting by integration of an origami structure with piezoelectric materials. A high fractional bandwidth of 40% is attained under a sinusoidal excitation with a peak acceleration of 0.1 g.

Transient Super-ballistic Spreading of Wave Packets with Large Spreading Exponents in Some Hybrid Ordered-quasiperiodic Lattices

Ba Phi Nguyen,Quang Minh Ngo,Kihong Kim 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.68 No.3

We consider the spreading of an initially localized wave packet in one-dimensional hybrid orderedquasiperiodic lattices. We consider two diffrent kinds of quasiperiodic sequences, which are the Cantor and the period-doubling sequences. From numerical calculations based on the discrete Schr¨odinger equation, we demonstrate that hybrid ordered-quasiperiodic lattices can support the super-ballistic spreading of a wave packet with very large spreading exponents for certain transient time windows. Remarkably, in the case of the sublattice with the on-site potential obeying the period-doubling quasiperiodic sequence, we find that the super-ballistic exponent can be larger than six. We also point out that previous explanations of this phenomenon based on a generalized version of the point source model are incorrect.

English as a Medium of Instruction: A Case Study at a Gifted High School in Vietnam

Long V Nguyen,Ngo Quang Minh Hai 아시아테플 2018 The Journal of Asia TEFL Vol.15 No.4

This paper aims to examine the perceptions of teachers and students at a gifted high school in Central Vietnam about teaching science subjects such as Maths, Chemistry, Physics, and Biology through English. Using questionnaires, in-depth interviews, and class observations as research instruments, this study also explores the challenges the administrators, teachers, and students are facing in adopting English as a medium of instruction (EMI). The findings revealed that leaders, teachers, and students appreciate the project and show interest in EMI, but that its implementation experiences a lot of dilemmas and tensions partly owing to English competence, pedagogical methods, materials, and management. From these findings, the study suggests some implications to help make EMI implementation more applicable and effective at the high school level.

Establish Cultivation by Mixing Crops of Different Strains of Eucheuma and Kappaphycus Species

Dang, Diem Hong,Hoang, Minh Hien,Ngo, Thi Hoai Thu,Dinh, Thi Thu Hang,Huynh, Quang Nang The Korean Society for Marine Biotechnology 2010 한국해양바이오학회지 Vol.4 No.1

Species Kappaphycus alvarezii (Doty) Doty, Kappaphycus striatum (Schmitz.) Doty and Eucheuma denticulatum (N. L. Burman) Collins et Harvey, which was brought to Vietnam from Japan in 1993 and Coco island, Martan Sea, Cebu, Philippines in 2005 have been cultivated in the different coasts of South Central Vietnam. Their growth rates and physical properties of carrageenan, then, were analyzed. The obtained results showed that the growth rate of E. denticulatum and K. striatum strains is higher than those of K. alvarezii. Species of K. striatum could grow over wide range of temperature and tolerate more strongly to high temperature compared with K. alvarezii, but their content and gel strength of kappa-carrageenan were almost the same and high. For purpose of the Kappahycus cultivation farms with stable and high production all year round (especially in the seawaters of shallow, semi-closed Lagoons where the water movement is not good and with high temperature in the hot season), mixed cropping of K. alvarezii and K. striatum as seeds stock during different cropping seasons was established. Our results suggested that K. alvarezii and K. striatum could be grown in the cool season (from Oct. to next March) with the same and high content and gel strength of kappa - carrageenan, but in the hot season need to chose K. striatum for cultivation only (from Apr. to Sept.).

Carbon-Nanotube Growth over Iron Nanoparticles Formed on CaCO3 Support by Using Hydrogen Reduction

Nguyen Duc Dung,Nguyen Van Chuc,Ngo Thi Thanh Tam,Nguyen Hong Quang,Phan Hong Khoi,Phan Ngoc Minh 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.5

Carbon nanotubes (CNTs) were grown by chemical vapor deposition on a mixture of iron salt and CaCO3. Salt mixtures of Fe(NO₃)₃9H₂O/CaCO₃ and FeCl₃6H₂O/CaCO₃ with various Fe weight contents were used as catalysts for growing the CNTs. A scanning electron microscope study revealed that the CNTs were densely grown on the Fe(NO₃)₃9H₂O/CaCO₃ catalyst containing 5 wt.% Fe. The effect of growth temperature on the segregation of Fe nanoparticles formed by hydrogen reduction is discussed. The result shows that 800 ℃ is the optimal temperature for the formation of Fe nanoparticles over which CNTs grow with the highest yield of 78.61 %. A raman spectroscope and a scanning transmission electron microscope were utilized to characterize the multiwall structure of the CNTs. The 92.16 % purity of the CNTs was determined by using thermal gravimetric analysis. Carbon nanotubes (CNTs) were grown by chemical vapor deposition on a mixture of iron salt and CaCO3. Salt mixtures of Fe(NO₃)₃9H₂O/CaCO₃ and FeCl₃6H₂O/CaCO₃ with various Fe weight contents were used as catalysts for growing the CNTs. A scanning electron microscope study revealed that the CNTs were densely grown on the Fe(NO₃)₃9H₂O/CaCO₃ catalyst containing 5 wt.% Fe. The effect of growth temperature on the segregation of Fe nanoparticles formed by hydrogen reduction is discussed. The result shows that 800 ℃ is the optimal temperature for the formation of Fe nanoparticles over which CNTs grow with the highest yield of 78.61 %. A raman spectroscope and a scanning transmission electron microscope were utilized to characterize the multiwall structure of the CNTs. The 92.16 % purity of the CNTs was determined by using thermal gravimetric analysis.

Surface plasmon-assisted optical switching/bistability at telecommunication wavelengths in nonlinear dielectric gratings

Thu Trang Hoang,Khai Q. Le,Quang Minh Ngo 한국물리학회 2015 Current Applied Physics Vol.15 No.9

In this paper, we numerically investigate nonlinear optical responses in dielectric gratings coated by flat metallic surface, where the structural parameters are chosen for wavelengths of operation at the telecom regime. The presented designs show the output light reflection with respect to the incident intensity of increasing, decreasing, and non-monotonic belt-shape functions. Nonlinear optical responses including switching and bistability appeared in the telecom region owing to assistance of surface plasmon resonance excited at the metal/dielectric interface, which enhances the diffraction efficiency and induces the momentum matching for the leaky modes of the dielectric grating. This results in electric intensityeffective refractive index dependence enhancement in the Kerr nonlinear media. Our design and investigation of the linear and nonlinear optical responses will provide a general guideline for a hybrid metallic-dielectric structure-based optical switching/bistability device design.

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

Van Hoi Pham,Huy Bui,Le Ha Hoang,Thuy Van Nguyen,The Anh Nguyen,Thanh Son Pham,Quang Minh Ngo 한국광학회 2013 Current Optics and Photonics Vol.17 No.5

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%.

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

Huy Bui,Thuy Van Nguyen,The Anh Nguyen,Thanh Binh Pham,Quoc Trung Dang,Thuy Chi Do,Quang Minh Ngo,Roberto Coisson,Van Hoi Pham 한국광학회 2014 Current Optics and Photonics Vol.18 No.4

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.