Control of the lasing wavelength by a sampled grating in a complex-coupled DFB laser

The lasing wavelength of a complex-coupled DFB laser is controlled by a sampled grating. The key concepts of the approach are to utilize the -1st order (negative first order) reflection of a sampled grating for laser single mode operation, and use conventional holographic exposure combined with the usual photolithography to fabricate the sampled grating. The typical threshold current of the sampled grating based DFB laser is 32 mA, and the optical output is about 10 mW at an injected current of 100 mA. The lasing wavelength of the device is 1.5356 mu m, which is the -1st order wavelength of the sampled grating.

Efficient hole transport in asymmetric coupled InGaN multiple quantum wells

InGaN based light emitting devices (LEDs) with asymmetric coupled quantum wells (AS-QWs) and conventional symmetric coupled quantum wells (CS-QWs) active structures were grown by metal-organic chemical vapor deposition technique. The LEDs with AS-QWs active region show improved light emission intensity and reduced forward voltage compared with LEDs with CS-QWs active region. Based on the electroluminescence measurements and the devices structure analysis, it can be concluded that these improvements are mainly attributed to the efficient hole tunneling through barriers and consequently the uniform distribution of carriers in the AS-QWs. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3254232]

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.

Decoherence of two Josephson charge qubits coupled via sigma(x)sigma(x)-type coupling and exposed to sigma(x) noise

Decoherence properties of two Josephson charge qubits coupled via the sigma(x)sigma(x) type are investigated. Considering the special structure of this new design, the dissipative effects arising from the circuit impedance providing the fluxes for the qubits' superconducting quantum interference device loops coupled to the sigma(x) qubit variables are considered. The results show that the overall decoherence effects are significantly strong in this qubit design. It is found that the dissipative effects are stronger in the case of coupling to two uncorrelated baths than are found in the case of one common bath.

Laterally confined modes in wet-etched, metal-coated, quantum-dot-inserted pillar microcavities

We report the fabrication and the measurement of microcavities whose optical eigenmodes were discrete and were well predicted by using the model of the photonic dot with perfectly reflected sidewalls. These microcavities were consisted of the semiconductor pillar fabricated by the simple wet-etched process and successive metal coating. Angle-resolved photoluminescence spectra demonstrate the characteristic emission of the corresponding eigenmodes, as its pattern revealed by varying both polar (0) and azimuthal (45) angles. It is shown that the metal-coated sidewalls can provide an efficient way to suppress the emission due to the leaking modes in these pillar microcavities.

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.

Suppression of exciton recombination in symmetric GaAs0.7Sb0.3/GaAs/GaAs0.7P0.3 coupled quantum wells induced by an in-plane magnetic field

Suppression of the exciton recombination in GaAs0.7Sb0.3/GaAs/GaAs0.7P0.3 coupled quantum well (CQW) induced by an external magnetic field is investigated theoretically. Unlike the usual electro-Stark effect, the exciton energy dispersion of an exciton is modified by an external in-plane magnetic field, the ground state of the magnetoexciton shifts from a zero in-plane center of mass (CM) momentum to a finite CM momentum, and the Lorentz force induces the spatial separation of electron and hole. Consequently, this effect renders the ground state of magnetoexciton stable against radiative recombination due to momentum conservation. This effect depends sensitively on the thickness and height of GaAs0.7Sb0.3 layer, therefore it could provide us useful infometion about the band alignment of CQW. (C) 2004 American Institute of Physics.

Modes in square resonators

Modes in square resonators are analyzed and classified according to the irreducible representations of the point group C-4v. If the mode numbers p and q that denote the number of wave nodes in the directions of two orthogonal square sides are unequal and have the same even-odd characteristics, the corresponding double modes are accidentally degenerate and can be combined into two new distributions with definite parities relative to the square diagonal mirror planes. The distributions with odd parities belong to the whispering-gallery-like modes in square resonators. The mode frequencies and quality factors are also calculated by the finite-difference time-domain technique and Pade approximation method. The numerically calculated mode frequencies agree with the theoretical ones very well and the whispering-gallery-like modes have quality factors much higher than other modes, including their accidentally degenerate counterparts in square resonators.

Experimental observation of resonant modes in GaInAsP microsquare resonators

GaInAsP-InP microsquare resonators with InP pedestals are fabricated by two-step chemical etching, and obvious mode peaks are observed in the photoluminescence spectra of the resonators. The mode Q-factors about 500 are obtained for a microsquare resonator with the side length of 7 mu m. The experimental mode interval is in agreement with that predicted by the light ray method based on the cavity length, instead of that of the whispering-gallery (WG)-like modes, which has mode interval twice of that determined by the cavity length. The finite-difference time-domain simulation shows that a little asymmetry may greatly reduce the difference of the Q-factors between the WG-like modes and the other modes.

Two-dimensional photonic band-gap defect modes with deformed lattice

A numerical study of the defect modes in two-dimensional photonic crystals with deformed triangular lattice is presented by using the supercell method and the finite-difference time-domain method. We find the stretch or shrink of the lattice can bring the change not only on the frequencies of the defect modes but also on their magnetic field distributions. We obtain the separation of the doubly degenerate dipole modes with the change of the lattice and find that both the stretch and the shrink of the lattice can make the dipole modes separate large enough to realize the single-mode emission. These results may be advantageous to the manufacture of photonic crystal lasers and provide a new way to realize the single-mode operation in photonic crystal lasers.

Analysis of modes in a freestanding microsquare resonator by 3-D finite-difference time-domain

Modes in a microsquare resonator slab with strong vertical waveguide consisting of air/semiconductor/air are analyzed by three-dimensional (3-D) finite-difference time-domain simulation, and compared with that of two-dimensional (2-D) simulation under effective index approximation. Mode frequencies and field distributions inside the resonator obtained by the 3-D simulation are in good agreement with those of the 2-D approximation. However, field distributions at the boundary of the resonator obtained by 3-D simulation are different from that of the 2-D simulation, especially the vertical field distribution near the boundary is great different from that of the slab waveguide, which is used in the effective index approximation. Furthermore the quality factors obtained by 3-D simulation are much larger. than that by 2-D simulation for the square resonator slab with the strong vertical waveguide.

Butt-coupled MOVPE growth for high-performance electro-absorption modulator integrated with a DFB laser

A novel butt-joint coupling scheme is proposed to improve the coupling efficiency for the integration of a GalnAsP MQW distributed feedback (DFB) laser with an MQW electro-absorption modulator (EAM). The proposed method gives more than 90% coupling efficiency, being much higher than the 26% coupling efficiency of the common MQW-MQW coupling technique. The differential quantum efficiency of the MQW-bulk-MQW coupled device is also much higher than that of the MQW-MQW device, 0.106 mW/mA versus 0.02 mW/mA. The EAM-DFB devices fabricated by the proposed method exhibit a very high modulation efficiency (12 dB/V) from 0 to I V. By adopting a high-mesa ridge waveguide and buried polyimide, the capacitance of the modulator is reduced to about 0.28 pF. The experimental results demonstrate that the method can replace the conventional MQW-MQW coupling technique to fabricate high-quality integrated photonic devices. (C) 2007 Elsevier B.V. All rights reserved.

Plasmons in vertically coupled InAs/GaAs quantum dots

We investigate plasmon excitations in a quantum wire that consists of an infinite one-dimensional array of vertically coupled InAs/GaAs strained quantum dots (QDs). The research is carried out in the framework of random-phase approximation using effective-mass theory. Our formalism is capable of studying plasmons with strong tunneling among QDs, which frustrate the conventionally adopted tight-binding approximation. Based on this formalism, a systematic study on the intraminiband or intrasubband plasmon in vertically coupled InAs/GaAs strained QDs is presented. It is found that an increase of the dot spacing will inevitably reduce the plasmon energy. In contrast, the role of dot height is relatively complex and depends on the dot spacing. The results demonstrate the possibility to engineer collective excitations in low dimensional systems by simply changing their geometric configuration.

Coherent tunnelling in coupled quantum wells under a uniform magnetic fields

Coherent tunnelling is studied in framework of the effective mass approximation for an asymmetric coupled quantum well. The Hartree potential due to the electron-electron interaction is considered in our calculation. The effects of the longitudinal and transverse magnetic field on coherent tunnelling characteristics are discussed. It has been found that the external field plays an important role in modulating the electron states.

1.3 mu m gain coupled DFB laser with InGaAlAs MQW grown on absorptive InGaAsP corrugation

An InGaA1As multiquantum well (MQW) has been successfully overgrown on the absorptive InGaAsP corrugation for fabricating the 1.3 mu m gain coupled distributed feedback (DFB) lasers. The absorptive InGaAsP corrugation was efficaciously preserved during the overgrowth of the InGaA1As MQW active region. The absorptive InGaAsP corrugation has a relatively high intensity around the PL peak wavelength in comparison with that of the InGaA1As MQW. The fabricated DFB laser exhibited a side mode suppression ratio of 40 dB together with a high single-mode yield of 90%.