J.-Z. Zhang

Linewidth Enhancement Factor of Quantum-Dot Optical Amplifiers

The linewidth enhancement (alpha-) factor of quantum-dot (QD) semiconductor optical amplifiers in the small signal gain and nonlinear regimes is theoretically investigated. A microscopic polarization equation and a wave equation are used to model subpicosecond pulse propagation in the nonlinear regime. In addition, a population equation that takes into account spectral hole burning and carrier heating effects is used. A novel approach to obtain the alpha-factor from the output pulse amplitude and phase in the dynamic nonlinear regime is presented. An in-depth study reveals that the presence of excited states (ES) limits the alpha-factor to values greater than 1 except when the energy separation between the ground state and ES is large. The alpha-factor dependence on QD inhomogeneous broadening, carrier density, carrier temperature, energy level separation, and input pulse energy is analyzed. We find that these can change the alpha-factor considerably. In particular, the alpha-factor increases with increasing input pulse energy and can be greater than 10 for input pulse energies larger than the amplifiers input pulse saturation energy. In the light of our calculations, the optimum device engineering required to obtain a low alpha-factor is discussed

Intraband absorption for InAs/GaAs quantum dot infrared photodetectors

Using the envelope function theory, intraband absorption is calculated for InAs/GaAs pyramidal quantum dots. The effects of the quantum dot geometry, such as the dot shape and the wetting layer (WL) thickness, and the coupling between the WL and bound states on the intraband transitions are systematically studied. Strong in-plane polarized absorption from the first excited state occurs in the low mid-infrared region, while stronger broadband z-polarized absorption features are located at higher frequencies. This polarization dependence is in agreement with the experiment [Appl. Phys. Lett. 82, 630 (2003)] and is due to the dot geometry. The WL can induce both in-plane and z-polarized absorption. Absorption of in-plane polarized light from the ground state to the WL and continuum states is found to be negligible. Thus, for strong normal-incidence photodetection, absorption from the first excited state should be exploited.

Rodent motor and neuropsychological behaviour measured in home cages using the integrated modular platform SmartCage

To facilitate investigation of diverse rodent behaviours in rodents’ home cages, we have developed an integrated modular platform, the SmartCage™ system (AfaSci, Inc. Burlingame, CA, USA), which enables automated neurobehavioural phenotypic analysis and in vivo drug screening in a relatively higher‐throughput and more objective manner. The individual platform consists of an infrared array, a vibration floor sensor and a variety of modular devices. One computer can simultaneously operate up to 16 platforms via USB cables. The SmartCage™ detects drug‐induced increases and decreases in activity levels, as well as changes in movement patterns. Wake and sleep states of mice can be detected using the vibration floor sensor. The arousal state classification achieved up to 98% accuracy compared with results obtained by electroencephalography and electromyography. More complex behaviours, including motor coordination, anxiety‐related behaviours and social approach behaviour, can be assessed using appropriate modular devices and the results obtained are comparable with results obtained using conventional methods. In conclusion, the SmartCage™ system provides an automated and accurate tool to quantify various rodent behaviours in a ‘stress‐free’ environment. This system, combined with the validated testing protocols, offers powerful a tool kit for transgenic phenotyping and in vivo drug screening.

Enhancement and reduction of line broadening due to Auger scattering in modulation-doped InGaAs∕GaAs quantum dot devices

We calculate the line broadening of various Auger processes in modulation-doped InGaAs∕GaAs quantum dot (QD) semiconductor optical amplifiers (SOAs), involving scattering of carriers between wetting-layer states and confined QD states. We find that, as a result of p doping, the optical gain and the linewidth are significantly enhanced, while in shallow dots, n doping surprisingly leads to a reduction in the homogeneous linewidth. Our findings support the development of high-speed QD lasers and SOAs incorporating p doping and using optical amplifiers with n-doped shallow QDs for wavelength-division-multiplexing applications.

Rabi oscillations of ultrashort optical pulses in 1.55 μm InGaAs/InGaAsP quantum-well amplifiers

Using the Foreman effective mass Hamiltonian for InxGa1−xAs/InyGa1−yAszP1−z quantum wells, the propagation of a 150 fs pulse in an optical amplifier was calculated by solving the wave equation with the computed polarization as a source term. The multisubband carrier dynamics and heating as well as the polarization dynamics were taken into account. The intensity of the propagated pulse as well as its imposed frequency chirp, the carrier density, and temperature show strong Rabi oscillations. The Rabi oscillation imposes negative frequency chirp, redshifting the frequency components of a propagating pulse and thus its spectrum, as a result of the gain saturation-induced self-phase modulation.

Modelling of a 2D photonic crystal waveguide pulse reshaper integrated with a SOA

In this paper, an arbitrary chirped pulse (ACP)-finite difference time domain (FDTD) method is developed where pulse data can be read in from a data file. This has allowed the FDTD code to be used in combination with a semiconductor optical amplifier (SOA) model to study 2R regeneration using 2D photonic crystal waveguide for sub-picosecond pulses

Rapid hot-electron capture in self-assembled quantum dots via phonon processes

Electron capture induced by carrier heating in InAs∕GaAs quantum dots is studied theoretically. Room temperature capture rates due to single longitudinal-optical (LO) phonons, LO phonons plus acoustic phonons, and two LO phonons are compared. Due to energy broadening from carrier-carrier scattering, single LO-phonon processes are the fastest capture channel. Screening from wetting-layer (WL) carriers is studied comprehensively using a number of screening models. Due to the dispersion of the WL electron-hole plasma and dynamic effects of screening, antiscreening occurs expediting rather than slowing down electron capture, with capture times of several picoseconds when the carrier temperature is 100–200K above room temperature.

Electromagnetic Modelling of a Monolithic Pulse Reshaper based on a Photonic Crystal Waveguide Integrated with a SOA

Finite difference time domain (FDTD) and finite element (FE) frequency domain methods are used to study the propagation of arbitrary chirped pulses in photonic crystal (PhC) waveguide. An arbitrary chirped pulse is derived from a separate Semiconductor optical amplifier (SOA) model and is passed through a mini-stop band (MSB) in a photonic crystal waveguide. Good agreement is shown between the FDTD and FE models and pulse compression is observed

Modelling of a 2R regenerator based on a photonic crystal waveguide pulse reshaper integrated with a SOA

In this proceedings the Finite Difference Time Domain (FDTD) and frequency domain Finite Element (FE) methods are used to model both linear chirped pulse and arbitrary chirped pulse propagation in 2D Photonic Crystal (PhC) waveguides. An in-house FDTD code has been implemented which allows the study of pulse propagation in a very direct way. The carrier wavelength of the pulse is swept across the bandwidth of a mini-stopband feature and pulse compression behaviour is observed. In the case of linear chirped pulse, both round hole and square hole PhC waveguides are studied with the latter giving increased pulse compression. An input pulse is then derived from a SOA model which has arbitrary chirp. This is passed through a mini-stop band in a narrowed W3 PhC waveguide and pulse compression is observed.

Rabi Oscillations of Ultrashort Pulses in 1.55‐μm InGaAs/InGaAsP Quantum‐well Optical Amplifiers

Using the Foreman effective mass Hamiltonian for InxGa1−xAs/InyGa1−yAszP1−z quantum wells, the propagation of a 150 fs pulse in an optical amplifier was calculated taking into account the multi‐subband carrier dynamics and heating as well as the polarization dynamics. The intensity of the propagated pulse as well as its imposed frequency chirp, the carrier density and temperature show strong Rabi oscillations.