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The effect of pumping methods on the gain and phase dynamics of quantum-dot vertical cavity semiconductor optical amplifier is investigated. The gain and phase recovery time is considered at the ground state transition wavelength. By including the effect of the excited state and wetting layer on the refractive index change, it is shown that under electrical pumping, the better gain dynamics is achieved compared to the optical pumping scheme. However, the phase recovery time can be accelerated by optical pumping. These behaviors result from the different mechanisms of population inversion in these two pumping methods. [ABSTRACT FROM PUBLISHER]

The acceleration of gain recovery in quantum dot vertical cavity semiconductor optical amplifiers (QD-VCSOA) is investigated. The 1-Tb/s pattern effect free cross gain modulation in QD-VCSOA can be achieved by using an additional light beam. We can overcome the limitation of the slow transition of carriers from the wetting layer to the excited state by applying a light beam which increases the carrier density in the excited state and thus strongly improves the gain recovery time. [ABSTRACT FROM AUTHOR]

Two heavily doped n-type Zn0.46Cd0.54Se/Zn0.24 Cd0.25Mg0.51Se multiple quantum well (MQW) structures have been grown on InP (0 0 1) substrates by molecular beam epitaxy. Photoluminescence (PL), time-resolved PL, and Fourier transform infrared (FTIR) spectroscopy were performed to characterize their interband and intersubband (ISB) properties. These two MQW samples have similar structures except for different well widths and a different number of periods. Excitation-intensity-dependent PL shows no electronic coupling between the multiquantum wells. The integrated PL intensities and the PL decay times of the MQWs were measured as functions of temperature in the range from 77 to 290 K. Theoretical fittings of temperature dependences of integrated PL intensities and PL decay times indicate that the nonradiative recombination processes observed in our samples can be well described by hole capture by acceptor-like defect centers through multiphonon emissions. ISB absorption spectra of the samples were measured by FTIR and show peak absorption at wavelengths of 3.99 and 5.35 mum for the MQWs with well widths of 28 and 42 A, respectively. Theoretical calculations based on the envelope function approximation confirm that these peaks are due to the transitions from the ground state E1 to the first excited state E2. [ABSTRACT FROM PUBLISHER]

Electrons are bound to the surface of liquid helium by the image potential due to the induced polarization of the helium. The potential varies as the reciprocal of the distance of the electron from the surface so that the energy levels for the electron motion perpendicular to the surface are those of a one-dimensional hydrogen atom with very small charge. In order to make quantum logic elements (qubits) using the ground state and first excited state in this potential, individual electrons must be confined over microelectrodes which are individually connected to variable voltage sources so that the electron energy levels can be shifted in and out of resonance with applied microwave radiation. The electrodes consist of an array of gold columns, about 1.2 mum tall and 200 nm in diameter, separated by 500 nm. These are grown by electroplating on the ends of leads deposited on a silicon substrate by e-beam lithography. The leads are covered by a dielectric layer and then a metal ground plane, so that the electric field from the leads is screened. We describe, here, our technique for fabricating this system and present the numerical computations of the electric fields from the electrodes [ABSTRACT FROM PUBLISHER]

We have performed simulations on a coplanar structured alternating current plasma display panel cell sustained in the ternary He-Ne-Xe gas mixture and its discharge characteristics were compared with those of the conventional binary Ne-Xe mixture, by using the two-dimensional fluid model simulator. Since plasma display panels use micro-discharges in the Xe mixed gas to generate vacuum ultraviolet light to excite phosphor for visible light emission, the condition of discharge gas mixtures is one of the main factors that determine luminous efficacy and thus, intensive studies on discharge gas mixture are essential to optimize it. As the He concentration increases in the He-Ne-Xe mixture, luminance and luminous efficacy increase but power consumption decreases, which agrees well with the recent experimental findings. We have found that the enhanced ion mobilities in the gas mixture by adding He gas to the conventional Ne-Xe mixture play an important role in increasing luminous efficacy of a plasma display panel cell, by reducing the cathode fall potential. In addition, He atoms increase the number of Xe*(3P2) in the excited state and thus, the dimer emission is increased while the resonant line is reduced with the He content. As a result, the total vacuum ultraviolet emission is increased and higher luminance and higher luminous efficacy can be obtained. [ABSTRACT FROM PUBLISHER]

We report the device characteristics of stacked InAs-GaAs quantum dot (QD) lasers cladded by an Al0.4Ga0.6As layer grown at low temperature by metal-organic chemical vapor deposition. In the growth of quantum dot lasers, an emission wavelength shifts toward a shorter value due to the effect of postgrowth annealing on quantum dots. This blueshift can be suppressed when the annealing temperature is below 570°C. We achieved 1.28-μm continuous-wave lasing at room temperature of five layers stacked InAs-GaAs quantum dots embedded in an In0.13Ga0.87As strain-reducing layer whose p-cladding layer was grown at 560°C. From the experiments and calculations of the gain spectra of fabricated quantum dot lasers, the observed lasing originates from the first excited state of stacked InAs quantum dots. We also discuss the device characteristics of fabricated quantum dot lasers at various growth temperatures of the p-cladding layer. [ABSTRACT FROM PUBLISHER]

In a plasma with ions streaming at a uniform velocity ∼cs, dust grains can be accurately modeled as particles interacting via the dynamically-screened Coulomb interaction, calculated from linear response theory for the plasma. This force is nonreciprocal, i.e., action does not equal reaction, which has remarkable dynamical consequences. We show that up to four grains can form a stable self-bound molecule, which propels itself upstream against the ion flow. Stable equilibria are also found for pairs of grains confined in harmonic or quartic external potentials. For two grains in an anharmonic potential, or for three or more grains in any potential, there is no conserved quantity and self-excited oscillations can occur. In general, there are multiple equilibria, hysteresis occurs as parameters are varied, and it is not possible to distinguish ground and excited states. We show how the organizational and dynamical principles that govern the behavior of few-grain and low-dimensional systems also elucidate the more complex dynamics of crystals. [ABSTRACT FROM PUBLISHER]

The dynamic characteristics, and in particular the modulation bandwidth, of high-speed semiconductor lasers are determined by intrinsic factors and extrinsic parameters. In particular, carrier transport through the heterostructure and thermalization, or quantum capture in the gain region, tend to play an important role. We have made a detailed study of carrier relaxation and quantum capture phenomena in In(Ga)As-Al(Ga)As self-organized quantum dots (QD's) and single-mode lasers incorporating such dots in the gain region through a variety of measurements. The modulation bandwidth of QD lasers is limited to 5-6 GHz at room temperature and increases to ~30 GHz only upon lowering the temperature to 100 K. This behavior is explained by considering electron-hole scattering as the dominant mechanisms of electron relaxation in QD's and the scattering rate seems to decrease with increase of temperature. The switching of the emission wavelength, from the ground state to an excited state, has been studied in coupled cavity devices. It is found that the switching speed is determined intrinsically by the relaxation time of carriers into the QD states. Fast switching from the ground to the excited state transition is observed. The electrooptic coefficients in the dots have been measured and linear coefficient τ=2.58×10-11 m/V. The characteristics of electrooptic modulators (EOM's) are also described [ABSTRACT FROM PUBLISHER]

Junction temperature of InAs quantum-dot laser diodes (LDs) is measured by utilizing a forward voltage- temperature method. Although the forward voltage decrease with junction temperature increment is low, the linear relation between forward voltage and temperature clearly occurs. It is found that SiNx is more useful than SiO2 to keep junction temperature low. Injection current to shift the lasing wavelength from ground to excited state is increased over 200 mA by using SiNx instead of SiO2 as the insulating layer. As a result, ground state optical power is doubled. [ABSTRACT FROM PUBLISHER]

Excited-state-mediated capture of carriers from the waveguide into the lasing ground-state in quantum dots (QDs) is studied. Such a two-step capture places a fundamental limitation on ground-state lasing-the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD. The longest, cut-off transition time exists, beyond which no ground-state lasing is possible [ABSTRACT FROM PUBLISHER]

The small-signal cross-gain modulation (XGM) characteristics of a p-doped quantum-dot (QD) semiconductor optical amplifier (SOA) are measured and compared with a theoretical model. The measured small-signal XGM responses show an average 3-dB bandwidth of 3.5 GHz and have no significant dependence on the pump-probe detuning wavelength. This indicates that the main XGM mechanism of the p-doped QD SOA is the total carrier density change rather than spectral hole burning. The measured small-signal conversion efficiency is compared with a theoretical model, which is derived from rate equations for QD SOAs. Based on our theoretical results, we find that the reduced intersubband (from the QD excited to ground state) carrier relaxation time due to p-type doping will decrease the small-signal 3-dB XGM bandwidth of QD SOAs [ABSTRACT FROM PUBLISHER]

The optical linewidth enhancement factor (LEF) of a p-doped quantum-dot (QD) laser is measured below threshold and compared with a theoretical calculation. The optical gain, refractive index, and LEF are well matched with our theoretical model when the thermal effect is isolated by an additional pulse current measurement of the LEF. We also theoretically calculate the LEF of an undoped QD Fabry-Pe´rot (FP) laser assuming that the structure of the undoped FP QD laser is the same as that of the p-doped QD FP laser except the p-type doping. The changes in modal gain and refractive index due to the respective QD ground and excited states are calculated. Based on the theoretical results, we show that the LEF of the p-doped QD laser is smaller than that of the undoped QD laser due to the reduced transparency carrier density. [ABSTRACT FROM PUBLISHER]

We report ridge-waveguide superluminescent diodes based on five stacks of self-assembled InAs-GaAs quantum dots. Devices with output powers up to 10 mW emitting around 1.3 μm are demonstrated. Spectral analysis shows a broad emission peak (26-nm full-width at half-maximum) from the dot ground state at low injection, and an additional peak from the excited state at higher bias. Temperature characteristics in the range 10°C-80°C are also reported. The experimental curves are in good agreement with simulations performed using a traveling-wave rate equation model. [ABSTRACT FROM PUBLISHER]

Measurements of ultrafast gain recovery in self-assembled InAs quantum-dot (QD) amplifiers are explained by a comprehensive numerical model. The QD excited state carriers are found to act as a reservoir for the optically active ground state carriers resulting in an ultrafast gain recovery as long as the excited state is well populated. However, when pulses are injected into the device at high-repetition frequencies, the response of a QD amplifier is found to be limited by the wetting-layer dynamics [ABSTRACT FROM PUBLISHER]