Control of polarization for a broadband dichroic filter at 45 degrees incidence

A method for the control of polarization for a broadband dichroic filter was reported and some design examples were elaborated. This method could be applied over a wide range of wavelengths and a wide range of polarizations in the transmission region. A nonpolaiizing broadband dichroic filter and a broadband dichroic filter with certain polarization were designed and fabricated by electron beam evaporation with ion beam assisted deposition. The experimental spectral performances showed good agreement with their theoretical curves. In addition, the application of the method was discussed. (c) 2007 Optical Society of America

Proposal for the direct optical detection of pure spin currents in semiconductors

We suggest a different practical scheme for the direct detection of pure spin current by using the two-color Faraday rotation of optical quantum interference process (QUIP) in a semiconductor system. We demonstrate theoretically that the Faraday rotation of QUIP depends sensitively on the spin orientation and wave vector of the carriers, and can be tuned by the relative phase and the polarization direction of the omega and 2 omega laser beams. By adjusting these parameters, the magnitude and direction of the spin current can be detected.

Optical study of lateral carrier transfer in (In,Ga)As/GaAs quantum-dot chains

We have studied the lateral carrier transfer in a specially designed quantum dot chain structure by means of time-resolved photoluminescence (PL) and polarization PL. The PL decay time increases with temperature, following the T-1/2 law for the typical one-dimensional quantum system. The decay time depends strongly on the emission energy: it decreases as the photon energy increases. Moreover, a strong polarization anisotropy is observed. These results are attributed to the efficient lateral transfer of carriers along the chain direction. (c) 2008 American Institute of Physics.

Comparative study of optical properties between quantum dot chains band quantum dots

A comparative study of the steady-state and transient optical properties was made between InGaAs/GaAs quantum do chains (QDCs) and quantum dots (QDs). It was found that the photoluminescence (PL) decay time of QDCs exhibited a strong photon energy dependence, while it was less sensitive in QDs. The PL decay time increased much faster with the excitation power in the QDCs than that in QDs. When the excitation power was large enough, the PL decay time tended to be saturated. In addition, it was also found that the PL rise time was much shorter in QDCs than in QDs. All these experimental results show that there is a strong carrier coupling along the chain direction in the QD chain structure. The polarization PL measurements further confirm the carrier transfer process along the chain direction.

Numerical analysis of gain saturation, noise figure, and carrier distribution for quantum-dot semiconductor-optical amplifiers

The gain saturation behaviors and noise figure are numerically analyzed for quantum-dot semiconductor optical amplifiers (QD-SOAs). The carrier and photon distributions in the longitudinal direction as well as the photon energy dependent facet reflectivity are accounted in the rate equations, which are solved with output amplified spontaneous emission spectrum as iterative variables. The longitudinal distributions of the occupation probabilities and spectral-hole burning are presented for electrons in the excited and ground states of quantum dots. The saturation output power 19.7 dBm and device gain 20.6 dB are obtained for a QD-SOA with the cavity length of 6 rum at the bias current of 500 mA. The influences of them electron intradot relaxation time and the QD capture time on the gain spectrum are simulated with the relaxation time of 1, 30, and 60 ps and capture time of 1, 5, and 10 ps. The noise figure as low as 3.5 dB is expected due to the strong polarization sensitive spontaneous emission. The characteristics of gain saturation and noise figure versus input signal power for QD-SOAs are similar to that of semiconductor. linear optical amplifiers with gain clamping by vertical laser fields.

Design and fabrication of polarization-independent micro-ring resonators

Origin of polarization sensitivity of photonic wire waveguides (PWWs) is analysed and the effective refractive indices of two different polarization states are calculated by the three-dimensional full-vector beam propagation method. We find that PWWs are polarization insensitive if the distribution of its refractive index is uniform and the cross section is square. An MRR based on such a polarization-insensitive PWW is fabricated on an 8-inch silicon-on-insulator wafer using 248-nm deep ultraviolet lithography and reactive ion etching. The quasi-TE mode is resonant at 1542.25 nm and 1558.90 nm, and the quasi-TM mode is resonant at 1542.12 nm and 1558.94 nm. The corresponding polarization shift is 0.13 nm at the shorter wavelength and 0.04 nm at the longer wavelength. Thus the fabricated device is polarization independent. The extinction ratio is larger than 10 dB. The 3 dB bandwidth is about 2.5 nm and the Qvalue is about 620 at 1558.90 nm.

Optical properties of InGaAs/GaAs quantum chains

We have investigated the steady-state and transient optical properties of InGaAs/GaAs quantum chains and found that the photoluminescence (PL) decay time exhibits a strong photon energy dependence. It increases with the decrease of the emission energy. It is also found that the PL decay time increases with the excitation power. When the excitation power is large enough the PL decay time tends to be saturated. All these experimental results show that there is a strong carrier coupling along the chain direction in the quantum dot chain structure. The polarization PL measurements further confirm the carrier transfer process along the chain direction.

Elasticity, band-gap bowing, and polarization of AlxGa1-xN alloys

Elastic constants, the bulk modulus, Young's modulus, band-gap bowing coefficients, spontaneous and piezoelectric polarizations, and piezoelectric coefficients of hexagonal AlxGa1-xN ternary alloys are calculated using first-principles methods. The fully relaxed structures and the structures subjected to homogeneous biaxial and uniaxial tension are investigated. We show that the biaxial tension in the plane perpendicular to the c axis and the uniaxial tension along the c axis all reduce the bulk modulus, whereas they reduce and enhance Young's modulus, respectively. We find that the biaxial and uniaxial tension can enhance the bowing coefficients. We also find that the biaxial tension can enhance the total polarization, while the uniaxial tension will suppress the total polarization. (C) 2008 American Institute of Physics.

Fabrication and Infrared Optical Properties of Nano Vanadium Dioxide Thin Films

Vanadium dioxide thin films were fabricated by ion beam sputtering on Si3N4/SiO2/Si after a post reductive annealing process in a nitrogen atmosphere. X-ray Diffraction (XRD), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS) were employed to analyze the effects of post annealing temperature on crystallinity, morphology, and composition of the vanadium oxide thin films. Transmission properties of vanadium dioxide thin films were measured by Fourier transform-infrared (FT-IR) spectroscopy. The results showed that the as-deposited vanadium oxide thin films were composed of non-crystalline V2O5 and a tetragonal rutile VO2. After annealing at 400 degrees C for 2 h, the mixed phase vanadium oxide (VOx) thin film changed its composition and structure to VO2 and had a (011) oriented monoclinic rutile structure. When increasing the temperature to 450 degrees C, nano VO2 thin films with smaller grains were obtained. FT-IR results showed that the transmission contrast factor of the nano VO2 thin film was more than 0.99 and the transmission of smaller grain nano VO2 thin film was near zero at its switched state. Nano VO2 thin film with smaller grains is an ideal material for application in optical switching devices.

Integration of a photonic crystal polarization beam splitter and waveguide bend

In this work, we present the design of an integrated photonic-crystal polarization beam splitter (PC-PBS) and a low-loss photonic-crystal 60 waveguide bend. Firstly, the modal properties of the PC-PBS and the mechanism of the low-loss waveguide bend are investigated by the two-dimensional finite-difference time-domain (FDTD) method, and then the integration of the two devices is studied. It shows that, although the individual devices perform well separately, the performance of the integrated circuit is poor due to the multi-mode property of the PC-PBS. By introducing deformed airhole structures, a single-mode PC-PBS is proposed, which significantly enhance the performance of the circuit with the extinction ratios remaining above 20dB for both transverse-electric (TE) and transverse-magnetic (TM) polarizations. Both the specific result and the general idea of integration design are promising in the photonic crystal integrated circuits in the future. (C) 2009 Optical Society of America

Four-channel reconfigurable optical add-drop multiplexer based on photonic wire waveguide

We designed and fabricated a four-channel reconfigurable optical add-drop multiplexer based on silicon photonic wire waveguide controlled through thermo-optic effect. The effective footprint of the device is about 1000 x 500 mu m(2). The minimum insertion loss is about 10.7 dB and the tuning bandwidth about 17 nm. The average tuning power efficiency is about 6.187 mW/nm and the tuning speed about 24.4 kHz. The thermo-optic polarization-rotation effect is firstly reported in this paper. (C) 2009 Optical Society of America

The electronic structure of strained ZnO/MgxZn1-xO superlattices and the influence of polarization

Based on the effective-mass model, the lower energies of the electron and the hole of ZnO/MgxZn1-xO superlattices are calculated. Because of the mismatch of the lattice constant between the ZnO well and the MgxZn1-xO barrier, piezoelectric and spontaneous polarization exist in ZnO/MgxZn1-xO superlattices and a macroscopical internal electric held is found when well width L-w >4 nm and Mg concentration x > 0.2. The parameters of ZnO/MgxZn1-xO superlattices such as lattice constant, band offset, etc. are also proposed. Through calculations, we found the internal electric field can change the lowest energies of the electron and hole to 105.4 and 85.1 meV when well width L-w up to 70 angstrom, which will influence the electronic and optical properties of ZnO/MgxZn1-xO superlattices greatly, while the Rashba effect from the internal electric field is so small that it can be neglected. The ground state exciton energies with different Mg concentration x are also calculated by variational method, our results are very close to the experimental results when Mg concentration x <= 0.3. (C) 2008 Elsevier B.V. All rights reserved.

Ballistic electron transport in hybrid ferromagnet/two-dimensional electron gas sandwich nanostructure: Spin polarization and magnetoresistance effect

We have theoretically investigated ballistic electron transport through a combination of magnetic-electric barrier based on a vertical ferromagnet/two-dimensional electron gas/ferromagnet sandwich structure, which can be experimentally realized by depositing asymmetric metallic magnetic stripes both on top and bottom of modulation-doped semiconductor heterostructures. Our numerical results have confirmed the existence of finite spin polarization even though only antisymmetric stray field B-z is considered. By switching the relative magnetization of ferromagnetic layers, the device in discussion shows evident magnetoconductance. In particular, both spin polarization and magnetoconductance can be efficiently enhanced by proper electrostatic barrier up to the optimal value relying on the specific magnetic-electric modulation. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3041477]

Optical matching layer structures in evanescent coupling photodiodes at a wavelength of 1.55 mu m: physics, design and simulation

We have studied the optical matching layers (OMLs) and external quantum efficiency in the evanescent coupling photodiodes (ECPDs) integrating a diluted waveguide as a fibre-to-waveguide coupler, by using the semi-vectorial beam propagation method (BPM). The physical basis of OML has been identified, thereby a general designing rule of OML is developed in such a kind of photodiode. In addition, the external quantum efficiency and the polarization sensitivity versus the absorption and coupling length are analysed. With an optical matching layer, the absorption medium with a length of 30 mu m could absorb 90% of the incident light at 1.55 mu m wavelength, thus the total absorption increases more than 7 times over that of the photodiode without any optical matching layer.

Structural and optical properties of ZnO films on SrTiO3 substrates by MOCVD

ZnO films have been fabricated on (0 0 1), (0 1 1) and (1 1 1) SrTiO3 (STO) substrates by metal-organic chemical vapour deposition (MOCVD). It is interesting that the ZnO films on (0 0 1) and (0 1 1) STO substrates show polar and semipolar orientations, which are different from previous reports, while the same growing direction of polar ZnO with previous results is found on (1 1 1) STO. For the atomic arrangements, two orthogonal domains and a single domain are observed on (0 0 1) and (1 1 1) STO, respectively. Photoluminescence spectra show that every sample has a sharp near-band-edge emission peak at about 3.28 eV without any deep-level emission band between 1.5 and 2.8 eV, implying a high optical quality. A violet emission around 3.0 eV is observed only in ZnO films on (0 0 1) and (0 1 1) STO substrates grown at 600 degrees C, which is discussed briefly. Additionally, the semipolar ZnO does not weaken the emission efficiency along with the reduction in the polarization effect compared with polar ZnO. These results show that high-quality polar and semipolar ZnO films can be grown on STO substrates by MOCVD.