Design of quantum cascade microcavity lasers based on Q factor versus etching depth

The choice of the etching depth for semiconductor microcavities is a compromise between a high Q factor and a difficult technique in a practical fabricating process. In this paper, the influences of the etching depth on mode Q factors for mid-infrared quantum cascade microcylinder and microsquare lasers around 4.8 and 7.8 mu m are simulated by three-dimensional (3D) finite-difference time-domain (FDTD) techniques. For the microcylinder and the microsquare resonators, the mode Q factors of the whispering-gallery modes (WGMs) increase exponentially and linearly with the increase in the etching depth, respectively Furthermore, the mode Q factors of some higher order transverse WGMs may be larger than that of the fundamental transverse WGM in 3D microsquares. Based on the field distribution of the vertical multilayer slab waveguide and the mode Q factors versus the etching depth, the necessary etching depth is chosen at the position where the field amplitude is 1% of the peak value of the slab waveguide. In addition, the influences of sidewall roughness on the mode Q factors are simulated for microsquare resonators by 2D FDTD simulation. (C) 2009 Optical Society of America

Electron spin quantum beats and room temperature g factor in GaAsN

We report on the investigation of electron spin quantum beats at room temperature in GaAsN thin films by time-resolved Kerr rotation technique. The measurement of the quantum beats, which originate from the Larmor precession of electron spins in external transverse magnetic field, yields an accurate determination of the conduction electron g factor. We show that the g factor of GaAs1-xNx thin films is significantly changed by the introduction of a small nitrogen fraction.

Optical bistability in InP/GaInAsP equilateral-triangle-resonator microlasers

Optical bistability is reported in InP/GaInAsP equilateral-triangle-resonator (ETR) microlasers, which are fabricated by planar technology. For a 30 mu m side ETR microlaser with a 2-mu m-wide output waveguide connected to one of the vertices of the ETR, hysteresis loops are observed for the output power versus the injection current from 215 to 235 K. The laser output spectra are measured in the upper and lower states of the hysteresis loop, which show strong mode competition among transverse modes. The hysteresis loops are demonstrated by two-mode rate equations with asymmetric cross gain saturation and different output efficiencies. (C) 2009 Optical Society of America

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

Effect of Ka-band microwave on the spin dynamics of electrons in a GaAs/Al0.35Ga0.65As heterostructure

We report experimental results of the effect of Ka-band microwave on the spin dynamics of electrons in a two-dimensional electron system (2DES) in a GaAs/Al0.35Ga0.65As heterostructure via time-resolved Kerr rotation measurements. While the microwave reduces the transverse spin lifetime of electrons in the bulk GaAs, it significantly increases that in the 2DES, from 745 to 1213 ps, when its frequency is close to the Zeeman splitting of the electrons in the magnetic field. Such a microwave-enhanced spin lifetime is ascribed to the microwave-induced electron scattering which leads to a "motional narrowing" of spins via D'yakonov-Perel' mechanism.

Design and realization of a microracetrack resonator based polarization splitter in silicon-on-insulator

An efficient polarization splitter based on a microracetrack resonator in silicon-on-insulator has been designed and realized using electron beam lithography and inductively coupled plasma etching. Polarization-dependent waveguides and the microracetrack resonator are combined and exploited to split two orthogonal polarizations. Rib waveguides are employed to enhance the coupling efficiency for the transverse-electric mode and endow the resonator with high performance for both polarizations. In experiments, a splitting ratio has been achieved of about 20 dB at the drop port around 1550 nm for each extracted polarization, in good agreement with the prediction.

Spin dynamics of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system

We report on time-resolved Kerr rotation measurements of spin coherence of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system in a GaAs/Al0.35Ga0.65As heterostructure. While the transverse spin lifetime (T-2(*)) of electrons decreases monotonically with increasing magnetic field, it has a nonmonotonic dependence on the temperature and reaches a peak value of 596 ps at 36 K, indicating the effect of intersubband electron-electron scattering on the electron-spin relaxation.

Symmetry and optical transition rule for low-dimensional semiconductor system with spin-orbit interaction and magnetic field

Starting from effective mass Hamiltonian, we systematically investigate the symmetry of low-dimensional structures with spin-orbit interaction and transverse magnetic field. The position-dependent potentials are assumed to be space symmetric, which is ever-present in theory and experiment research. By group theory, we analyze degeneracy in different cases. Spin-orbit interaction makes the transition between Zeeman sub-levels possible, which is originally forbidden within dipole approximation. However, a transition rule given in this paper for the first time shows that the transition between some levels is forbidden for space symmetric potentials. (C) 2009 Elsevier Ltd. All rights reserved.

Single mode vertical-cavity surface emitting laser with surface plasmon nanostructure

The vertical-cavity surface-emitting laser(VCSEL) has proved to be a low cost light source with attractive properties such as surface emission, circular and low divergence output beam, and simple integration in two-dimensional array. Many new applications such as in spectroscopy, optical storage, short distance fiber optic interconnects, and in longer distance communication, are continuously arising. Many of these applications require stable and single-mode high output power. Several methods that affect the transverse guiding and/or introduce mode selective loss or gain have been developed. In this study, a method for improving the single mode output power by using metal surface plasmons nanostructure is proposed. Theoretical calculation shows that the outpout power is improved about 50% compared to the result of standard VCSELs.

Two-dimensional simulation of high-order laterally-coupled GaAs-AlGaAs DFB laser diodes

Laterally-coupled distributed feedback (LC-DFB) laser diodes made without an epitaxial re-growth process have the advantage of a simple fabrication process. In this paper, two-dimensional optical field distribution of the fundamental quasi TE (transverse electric) mode is calculated by means of a semivectorial finite-difference method (SV-FDM). The dependence of the effective coupling coefficient (kappa(eff)) on the dutycycle of first-, second- and third-order LC-DFB LDs is investigated using modified coupled wave equations.

Mode characteristics of semiconductor equilateral triangle microcavities with side length of 5-20 mu m

Semiconductor equilateral triangle microresonators (ETRs) with side length of 5, 10, and 20 mum are fabricated by the two-step inductively coupled plasma (ICP) etching technique. The mode properties of fabricated InGaAsP ETRs are investigated experimentally by photoluminescence (PL) with the pumping source of a 980-nm semiconductor laser and distinct peaks are observed in the measured PL spectra. The wavelength spacings of the distinct peaks agree very well with the theoretical longitudinal mode intervals of the fundamental transverse modes in the ETRs, which verifies that the distinct peaks are corresponding to the enhancement of resonant modes. The mode quality factors are calculated from the width of the resonant peaks of the PL spectra, which are about 100 for the ETR with side length of 20 mum.

Single-mode condition for silicon rib waveguides with trapezoidal cross-section

Single-mode condition for silicon rib waveguides with trapezoidal cross-section was obtained using a numerical method based on imaginary-distance beam propagation method with non-uniform discretization. Both quasi-transverse-electric and quasi-transverse-magnetic modes were investigated. Simulated single-mode condition is given by a modified equation. Comparison with reported results shows that the Marcatili's method is in a better agreement with our results. (C) 2003 Elsevier B.V. All rights reserved.

Electron transport through hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots

The transmission of electrons through a hierarchical self-assembly of GaAs/AlxGa(1-)xAs quantum dots (QDs) is calculated using the coupled-channel recursion method. Our results reveal that the number of conductance peaks does not change when the barrier widths change, but the intensities decrease as the barrier widths increase. The conductance peaks will shift towards low Fermi energies as the transverse width of GaAs QD increases, as the thickness of GaAs quantum well increases, or as the height of GaAs QDs decreases. Our calculated results may be useful in the application of QDs to photoelectric devices. (c) 2005 American Institute of Physics.

1.55-mu m ridge DFB laser integrated with a buried-ridge-stripe dual-core spot-size converter by quantum-well intermixing

A ridge distributed feedback laser monolithically integrated with a buried-ridge-stripe spot-size converter operating at 1.55 mu m was successfully fabricated by means of low-energy ion implantation quantum-well intermixing and dual-core technologies. The passive waveguide was optically combined with a laterally exponentially tapered active core to control the mode size. The devices emit in a single transverse and single longitudinal mode with a sidemode suppression ratio of 38.0 dB. The threshold current was 25 mA. The beam divergence angles in the horizontal and vertical directions were as small as 8.0 degrees x 12.6 degrees, respectively, resulting in 3.0-dB coupling loss with a cleaved single-mode optical fiber.

Laser diode monolithically integrated with an electroabsorption modulator and dual-waveguide spot-size converter

A 1.60-mu m laser diode and electroabsorption modulator monolithically integrated with a dual-waveguide spot-size converter output for low-loss coupling to cleaved single-mode optical fiber is demonstrated. The devices emit in a single transverse and quasi-single longitudinal mode with a side mode suppression ratio of 25.6 dB. These devices exhibit a 3-dB modulation bandwidth of 16.0 GHz, and modulator extinction ratios of 16.2 dB dc. The beam divergence angle is about 7.3x10.6 deg, resulting in 3.0-dB coupling loss with cleaved single-mode optical fiber. (c) 2005 Society of Photo-optical Instrumentation Engineers.