Pham Van Hoi

Long-band emission of microsphere lasers based on erbium-doped sol-gel silica-alumina glasses

This letter reports on long-band (L band) lasing whispering-gallery modes (WGMs) in microspheres based on sol-gel silica-alumina glasses doped with high concentration of erbium ions (Er+3 from 1250 to 15 000 ppm). The sol-gel method for making these Er-doped silica-alumina glasses is described. The microspheres with diameters from 40 to 200μm were built by electrical arc. The experimental results show that laser oscillations with high gain at L band were obtained with different coupling schemes, especially with forward pumping and/or backward pumping coupling. The collected laser output power of one WGM could reach to −5dBm in the L band of 1570–1610 nm.

Silicon–Rich Silicon Oxide Thin Films Fabricated by Electro-Chemical Method

Porous silicon (PS) has attracted increasing research interest in basic physics as well as applications since 1990 when Canham reported on the efficient visible photoluminescence (PL) of porous silicon (Canham, 1990). Structurally, PS consists of many pores and silicon residuals and usually can be described as a homogeneous mixture of silicon, air and, even silicon dioxide. Based on porosity, PS can be classified into three types: nano, mesoand macro-pores. In the case of PS nano-pores, the size of both the silicon residuals and the air voids (pores) can be in the range of few nanometers. The exciton Bohr radius in Si is around 4.3 nm, so that quantum confinement can occur and change the electronic structure of those silicon nanocrystals. On the other hand, because the value of porosity is directly linked to the effective index of refraction of the PS layer, this layer appears as an effective medium, where the refractive index has a tunable value between the index of refraction of bulk Si and that of the air (pores). Those changes in the electronic structure and refractive index of PS when compared with bulk Si make it fascinating as both a low-dimensional material and an optical one. The considerable and controllable changes in the electronic structure and refractive index of PS fabricated by electrochemical anodization make it a promising material for photonics in comparison with bulk silicon and/ or pure silica. Using the oxidation process in O2 environment at high temperature, the PS samples become siliconrich silicon oxides (SRSO), which has high chemical instability and avoids the aging of the PS that is important condition for optical devices such as planar optical waveguides, optical interference filters, micro-cavities, etc (Bettotti et al., 2002). During the last decade, Erbium (Er)-doped silicon-rich silicon oxide has attracted much interest due to its big potential application in Si-based optoelectronic devices for telecom and optical sensors. The Er-ions implanted in SRSO materials produce light emission at around wavelength range of 1540 nm, which corresponds to minimum light absorption in silica-based glass fibers. In this regard, a lot of studies have been carried out to improve the luminescence efficiency of this material. Such studies have revealed that co-implantation of Er and O2 induce a strong enhancement in the Er-ions related emission at range of 1540 nm. In first case, samples were prepared by co-implanting Si and Er into silica thin films or co-sputtering Si, Er2O3 and SiO2 on the silicon substrate (Shin et al., 1995). In second case, samples were prepared by implanting Er-ions into SiO2 films containing Si-nanocrystals (nc-Si) and/or by Er-ion electrochemical deposition on silicon-rich oxide (SRSO) layers. The room temperature luminescence emission at the range of 1540 nm from Er-electrochemically doped porous

Porous silicon as a promising material for photonics

Electrochemical etching – a usual technique in nanotechnology – creates porous silicon with novel and useful properties. The considerable and controllable changes in the electronic structure and refractive index of porous silicon make it a promising material for photonics in comparison with bulk silicon. In this paper, we review as well as report on some interesting and unique properties of porous silicon material. In studying porous silicon as a low-dimensional material, we focus on the effect of the surface passivation of silicon nanocrystals on photoluminescence characteristics of such zero-dimensional crystals. As an optical material, we demonstrate the fabrication method and optical properties of the planar waveguide as well as the active waveguide and optical interference filters operated in infrared wavelengths. In addition, we investigated the effect of energy transfer from silicon nanocrystals to erbium ions in the erbium-doped porous silicon waveguide and also elaborate on the origins of the difference between the reflectivity spectra from fabricated filters and that of the simulation program.

Porous silicon as a low dimensional and optical material

The considerable and controllable changes in electronic structure and refractive index of porous silicon fabricated by electrochemical anodization make it become a promising material for photonics in comparison with bulk silicon. As the study of PS in terms of a low-dimensional material, we reviewed the effect of the surface passivation of silicon nanocrystals on photoluminescence spectra of such zero-dimensional crystals. In terms of an optical material, we show the fabrication method and optical properties of planar waveguide as well as active waveguide operated in the range of 1.54 μm wavelengths. We have also investigated the effect of energy transfer from silicon nanocrystals to erbium ions based on erbium-doped porous silicon waveguide.

ZnO/CdS Bilayer used for Electrode in Photovoltaic Device

In this article we present the fabrication and characterization of the nanoporous ZnO and/or ZnO/CdS thin films onto indium doped-tin oxide (ITO) substrates, based on the thermal evaporation technique followed by thermal treatment. The preparation method was relatively simple and low-cost for large scale uniform coating to produce clean, dense and strong adhesion to substrate thin films. The nanostructured ZnO and ZnO/CdS thin films were characterized by X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM). The nanostructured ZnO/CdS bilayer film was used in a photo-electrochemical (PEC) cell as a working electrode and a Pt net as a counter electrode. The results show that the photovoltaic cell with nanostructured ZnO/CdS bilayer film electrode has significantly improved photoelectric capability in comparison with that of ZnO electrode.

Precision Frequency Measurement for He-Ne/ I 2 Laser Emission with kHz- Accuracy

An accurate and precise frequency measurement of S.I. unit traceability is essential not only in the field of frequency metrology but also in many fundamental scientific researches, because an accurate and precise frequency will be able to provide stable reference frequency related quantities like length and time – the two important elements of physics. In this paper we present our frequency measurement for laser emission of the He-Ne/I2 laser system at the kHz accuracy and precision level. A frequency beat measurement of the laser under test with the standard frequency from a frequency optical comb is conducted. The second measurement method based on the heterodyne technique, where the beats between the hyperfine frequencies of 127I2 isotope at frequency of 474 THz and the testing He-Ne laser frequency recorded. The measurement data from different measurement techniques show a good consistency and therefore the given data are reliable.

Investigation of 1d Photonic Crystal Based on Nano-porous Silicon Multilayer for Optical Filtering

We present the fabrication, simulation, and measurements of 1D photonic crystal based on nano-porous silicon multilayer designed as an optical interference filter. Using electro-chemical etching with timely repeat steps of applied current densities, we fabricated a multilayer structure composed of alternating high- and low-index layer which achieved 90% power reflectivity at wavelength range of 1400-3000 nm. The simulation is relying on the Transfer Matrix Method (TMM) to design and predict the optical properties of nano-porous silicon multilayer as well as the relation between anodization parameters with reflection spectra. The measured reflection and transmission spectra of the nano-porous silicon multilayer show good agreement with simulation. This technique could provide a convenient and economical method to produce filters, cavities, and graded-index dielectric waveguides in the future.

Nanocomposite Thin Film TiO 2 /CdS Electrodes Prepared by Thermal Evaporation Process for Photovoltaic Applications

The incorporation of cadmium sulfide (CdS) into TiO2 nanoparticle thin films was investigated. The nanoparticle TiO2 thin film onto an indium dopedtin oxide (ITO) substrate was deposited by Electron Beam Deposition (EBD) combined with thermal process. Then a CdS thin film was vacuum-deposited onto the pre-deposited TiO2 film by a thermal evaporation technique. The obtained TiO2/CdS was characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM). The TiO2/CdS nanocomposite film was used in a photoelectrochemical (PEC) cell as a working electrode and a platinum electrode as a counter electrode. The electrolyte solution contains 1 M KCl and 0.1M Na2S. The results show that the cell with TiO2/CdS composite film electrode has significantly improved photoelectric capability in comparison with that of the pure TiO2 thin films.

Controlling the Bragg Wavelength Spectral Profile Expansion of FBG Sensors by Nano-Particle Coated Layers

This paper presents the results of a detailed study of spectra profile expansion of Bragg wavelength on the embedded fiber-Bragg-grating (EFBG) and the method for controlling it for optical sensors. The fiber-Bragg-grating (FBG) was coated by nano-particle layers with various thicknesses (600–2000 nm) and bonded to substrates of various materials with a large thermal expansion coefficient. With this embedding method, the variation of the line-width expansion of Bragg wavelength with cooling down FBG has been controlled. The nano-EFBG morphology was investigated by FE-SEM and the nano-EFBG sensors are studied in ambient from 77K (liquid nitrogen) to 393K. The expansion of spectral profile caused by transverse loading from nano-particle/embedded layers, can be changed in the range of 0.1–1.3 nm between before and after cooling down. This result may be used for the strain-temperature sensors, but has potential application in FBG dispersion compensation devices.

Toroidal microcavity lasers based on Er-doped silica-alumina glasses

The toroidal microcavity laser based on high-concentration erbium-doped silica-alumina glasses, which was developed by molten method using CO2-laser beam, is presented in detail. Average erbium ion concentrations were in the range from 2500 to 3000 ppm and an output lasing power as high as -5.0 dBm is obtained from toroidal microcavities with equatorial diameters in the range 90-140 mum. The advantages of toroidal cavity are followings: Q-factor of the cavity is increasing in comparison with spherical cavity and number of whispering-gallery modes (WGM) is decreased. Controlling lasing wavelength and number of lasing mode was possible by changing the distance between the tapered optical fiber and the toroidal cavity. The very narrow line-width of laser spectra may be useful for photonic sensor.