Impact of polarization on quasi-two-dimensional exciton and barrier-width dependence of the exciton associated transition in wurtzite III-V nitride quantum wells

Excitonic states in AlxGa1-xN/GaN quantum wells (QWs) are studied within the framework of effective-mass theory. Spontaneous and piezoelectric polarizations are included and their impact on the excitonic states and optical properties are studied. We witnessed a significant blue shift in transition energy when the barrier width decreases and we attributed this to the redistribution of the built-in electric field between well layers and barrier layers. For the exciton the binding energies, we found in narrow QWs that there exists a critical value for barrier width, which demarcates the borderline for quantum confinement effect and the quantum confined Stark effect. Exciton and free carrier radiative lifetimes are estimated by simple argumentation. The calculated results suggest that there are efficient non-radiative mechanisms in narrow barrier QWs. (C) 2002 Elsevier Science Ltd. All rights reserved.

Effects of piezoelectricity and spontaneous polarization on electronic and optical properties of wurtzite III-V nitride quantum wells

A theoretical model accounting for the macropolarization effects in wurtzite III-V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction-valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times. (C) 2001 American Institute of Physics.

Fabrication of 4 x 4 tapered MMI coupler with large cross section

MMI coupler with large cross section has low coupling loss between the device and fiber. However, large chip area is required. Recently proposed N x N tapered MMI coupler shows a substantial reduction in device geometry. No such kind of devices with N > 2 has yet been realized up to now. The authors have demonstrated a 4 x 4 parabolically tapered MMI coupler with large cross section that can match the SM fiber in silicon-on-insulator (SOI) technology. The device exhibits a minimum uniformity of 0.36 dB and excess loss of 3.7 dB, It represents a key component for realization of MMI-based silicon integrated optical circuit technology.

Circular polarization of excitonic luminescence in CdTe quantum wells with excess electrons of different densities

The circular polarization of excitonic luminescence is studied in CdTe/Cd1-xMgxTe quantum wells with excess electrons of low density in an external magnetic field. It is observed that the circular polarization of X and X- emissions has opposite signs and is influenced by the excess electron density. If the electron density is relatively high so that the emission intensity of the negatively charged excitons X- is much stronger than that of the neutral excitons X, a stronger circular polarization degree of both X and X- emissions is observed. We find that the circular polarization of both X- and X emissions is caused by the spin polarization of the excess electrons due to the electron-spin-dependent nature of the formation of X-. If the electron density is relatively low and the emission intensity of X- is comparable to that of X, the circular polarization degree of X and X- emissions is considerably smaller. This fact is interpreted as due to a depolarization of the excess electron spins, which is induced by the spin relaxation of X-.

High-density InAs nanowires realized in situ on (100) InP

High-density InAs nanowires embedded in an In0.52Al0.48As matrix are fabricated in situ by molecular beam epitaxy on (100) InP. The average cross section of the nanowires is 4.5 x 10 nm(2). The linear density is as high as 70 wires/mu m. The spatial alignment of the multilayer arrays exhibit strong anticorrelation in the growth direction. Large polarization anisotropic effect is observed in polarized photoluminescence measurements. (C) 1999 American Institute of Physics. [S0003-6951(99)04134-0].

Polarization-Independent Micro-Ring Resonator on Silicon-on-Insulator

Polarization-independent laterally-coupled micro-ring resonator has been designed and demonstrated. The origin of the polarization-sensitivity of the photonic wire waveguide (PWW) was analyzed. A polarization-insensitive PWW structure was designed and a polarization-insensitive MRR based on this PWW structure was designed by finite difference time-domain method and was fabricated on an 8-inch silicon-on-insulator wafer. The offset between the resonant wavelengths of the quasi-TE mode and the quasi-TM mode is smaller than 0.15 nm. The FSR is about 17 nm, extinction ratio about 10 dB and Q about 620.

Mode analysis of photonic crystal polarization beam splitter and its application in integrated circuits design - art. no. 698437

In this work, the guided modes of a photonic crystal polarization beam splitter (PC-PBS) are studied. We demonstrate that the transmission of a low-loss photonic crystal 120 degrees waveguide bend integrated with the PBS will be influenced if the PBS is multi-moded. We propose a single-moded PC-PBS structure by introducing deformed structures, and it shows twice the enhancement of the transmission. This device with remarkable improvement of performance is promising in the use of photonic crystal integrated circuits design.

Compact polarization-insensitive arrayed waveguide grating based on SOI material

A compact polarization-insensitive 8x8 arrayed waveguide grating with 100GHz channel spacing at 1.55 mu m is presented on the material of silicon on insulator (SOI). Increasing the epitaxial layer thickness can reduce the birefringence of the waveguide, but the wvaeguide's bend radius also increases at the same time. We choose the SOI wafer with 3.0 mu m epitaxial layer to reduce the device's size and designed the appropriate structure of rib wave-guides to eliminate the polarization dependant wavelength shift. Compared to the other methods of eliminating the polarization dependant wavelength shift, the method is convenient and easy to control the polarization without additional etching process. The index differences between TE0 and TM0 of straight and bend waveguides are 1.4x10(-5) and 3.9x10(-5), respectively. The results showed that the polarization dependant wavelength shift is 0.1nm, and the device size is 1.5x1.0 cm(2).

Influence of band bending and polarization on the valence band offset measured by x-ray photoelectron spectroscopy

The influence of band bending and polarization on the valence band offset measured by x-ray photoelectron spectroscopy (XPS) is discussed, and a modification method based on a modified self-consistent calculation is proposed to eliminate the influence and thus increasing the precision of XPS. Considering the spontaneous polarization at the surfaces and interfaces and the different positions of Fermi levels at the surfaces, we compare the energy band structures of Al/Ga-polar AlN/GaN and N-polar GaN/AlN heterojunctions, and give corrections to the XPS-measured valence band offsets. Other AlN/GaN heterojunctions and the piezoelectric polarization are also introduced and discussed in this paper.

Polarization holographic high-density optical data storage in bacteriorhodopsin film

Optical films containing the genetic variant bacteriorhodopsin BR-D96N were experimentally studied in view of their properties as media for holographic storage. Different polarization recording schemes were tested and compared. The influence of the polarization states of the recording and readout waves on the retrieved diffractive image's intensity and its signal-to-noise ratio were analyzed. The experimental results showed that, compared with the other tested polarization relations during holographic recording, the discrimination between the polarization states of diffracted and scattered light is optimized with orthogonal circular polarization of the recording beams, and thus a high signal-to-noise ratio and a high diffraction efficiency are obtained. Using a He-Ne laser (633 nm, 3 mW) for recording and readout, a spatial light modulator as a data input element, and a 2D-CCD sensor for data capture in a Fourier-transform holographic setup, a storage density of 2 x 10(8) bits/cm(2) was obtained on a 60 x 42 mu m(2) area in the BR-D96N film. The readout of encoded binary data was possible with a zero-error rate at the tested storage density. (c) 2005 Optical Society of America.

Radiation-matter oscillations in attosecond polarization beats using twin color-locked noisy lights

Based on the phase-conjugate polarization interference between two one-photon processes. When the laser has broadband linewidth, the sum-frequency polarization beat (SFPB) signal shows the autocorrelation of SFPB exhibits hybrid radiation-matter detuning terahertz damping oscillation. As an attosecond ultrafast modulation process, it can be extended intrinsically to any sum-frequency of energy-levels. It hits been also found that the asymmetric behaviors of the polarization beat signals result from the unbalanced dispersion effects, (c) 2005 Elsevier B.V. All rights reserved.

Polarization multiplexed write-once-read-many optical data storage in bacteriorhodopsin films

In polymeric films of bacteriorhodopsin (BR) a photoconversion product, which was named the F620 state, was observed on excitation of the film with 532 nm nanosecond laser pulses. This photoproduct shows a strong nonlinear absorption. Such BR films can be used for write-once-read-many (WORM) optical data storage. We demonstrate that a photoproduct similar or even identical to that obtained with nanosecond pulses is generated on excitation with 532 mn femtosecond pulses. This photoproduct also shows strong anisotropic absorption, which facilitates polarization storage of data. The product is thermally stable and is irretrievable to the initial B state either by photochemical reaction or through a thermal pathway. The experimental results indicate that the product is formed by a two-photon absorption process. Optical WORM storage is demonstrated by use of two polarization states, but more polarization states may be used. The combination of polarization data multiplexing and extremely short recording time in the femtosecond range enables very high data volumes to be stored within a very short time. (c) 2005 Optical Society of America.

Photochromic diarylethene for polarization holographic optical recording

Photochromic diarylethene, 1,2-bis[2-methyl-5-(3-fluorophenyl)-3-thienyl] perfluorocyleopentene (1a), was synthesized. The compound showed good photochromic reactions both in solution and in PMMA matrix by photo-irradiation. Using the diarylethene lb/PMMA film as recording medium and a He-Ne laser for recording and readout, four types of polarization holographic optical recording were accomplished for the first time. The results show that the orthogonal circular polarization recording is the best method for holographic optical recording when the target photochromic diarylethene is used as recording material. (c) 2006 Published by Elsevier B.V.