Detailed model for the In0.18Ga0.82N/GaN self-assembled quantum dot active material for λ = 420 nm emission.

We present a comprehensive model for In(0.18)Ga(0.82)N/GaN self-assembled quantum dot (QD) active material. The strain distribution in the QD structure is studied using linear elastic theory with the application of the shrink-fit boundary condition at the material interface. Subsequent calculations also predict the strain-induced quantum-confined Stark effect (QCSE). Under carrier injection, the overall effect of band bending and charge screening is studied by solving the Schrödinger and Poisson equations self-consistently. The optical gain spectrum of the InGaN/GaN QD active material is calculated based on the electronic states solved from the Schrödinger-Poisson equation, and both the calculated material gain peak and emission wavelength agree well with the measured experimental data.

Comprehensive model of 1550 nm MEMS-tunable high-contrast-grating VCSELs.

A comprehensive theoretical model for the long-wavelength micro-electro-mechanical-tunable high-contrast-grating vertical-cavity surface-emitting lasers is presented. Our band structure model calculates the optical gain and spontaneous emission of the InGaAlAs quantum well active region. The grating reflectivity and the cavity resonance condition are investigated through optical modeling. Correlating the results with the electrostatic model for the micro-electro-mechanical system, we accurately predict the measurements on the voltage-contolled lasing wavelength. Furthermore, our calculated temperature-dependent wavelength-tunable light output vs. current (L-I) curves show excellent agreement with experiment.

Theory and experiment of submonolayer quantum-dot metal-cavity surface-emitting microlasers.

We present a theoretical model for metal-cavity submonolayer quantum-dot surface-emitting microlasers, which operate at room temperature under electrical injection. Size-dependent lasing characteristics are investigated experimentally and theoretically with device radius ranging from 5 µm to 0.5 µm. The quantum dot emission and cavity optical properties are used in a rate-equation model to study the laser light output power vs. current behavior. Our theory explains the observed size-dependent physics and provides a guide for future device size reduction.

Electronic band structures and optical properties of type-II superlattice photodetectors with interfacial effect.

The electronic band structures and optical properties of type-II superlattice (T2SL) photodetectors in the mid-infrared (IR) range are investigated. We formulate a rigorous band structure model using the 8-band k · p method to include the conduction and valence band mixing. After solving the 8 × 8 Hamiltonian and deriving explicitly the new momentum matrix elements in terms of envelope functions, optical transition rates are obtained through the Fermi's golden rule under various doping and injection conditions. Optical measurements on T2SL photodetectors are compared with our model and show good agreement. Our modeling results of quantum structures connect directly to the device-level design and simulation. The predicted doping effect is readily applicable to the optimization of photodetectors. We further include interfacial (IF) layers to study the significance of their effect. Optical properties of T2SLs are expected to have a large tunable range by controlling the thickness and material composition of the IF layers. Our model provides an efficient tool for the designs of novel photodetectors.

Metal-cavity surface-emitting microlaser with hybrid metal-DBR reflectors.

We demonstrate a metal-cavity surface-emitting microlaser at room temperature using hybrid metal/distributed Bragg reflectors as well as substrate removal. Our devices operate under continuous-wave current injection at room temperature. The smallest laser is 1.0 µm in radius and ~4.0 µm in height with a circular beam shape and an output over 8 µW. The device lases at 995 nm wavelength with a threshold current of about 2.6 mA.

A surface-emitting 3D metal-nanocavity laser: proposal and theory.

A novel three-dimensional (3D) metal-nanocavity (or nano-coin) semiconductor laser suitable for electrical injection is proposed and analyzed. Our design uses metals as both the cavity sidewall and the top/bottom reflectors (i. e., a fully metal encapsulated nanolaser) and maintains the surface-emitting nature. As a result of the large permittivity contrast between the dielectric and metal, the optical energy can be well-confined inside the metal nanocavity. With a proper design and the choice of the HE111 mode, which has the best top surface radiation pattern, a laser with a physical size smaller than 0.01λ(0)(3) is achievable at 1.55 µm wavelength with a reasonable semiconductor gain at room temperature. We provide a detailed theoretical model starting from the waveguide analysis to full 3D structure simulations by taking into account both geometry and metal dispersion. We show a systematic procedure for analyzing this class of 3D metal-cavity (or nano-coin) lasers with discussions on the optimization of the performance such as light output power, threshold reduction, and output beam shaping.

Theory of plasmonic fabry-perot nanolasers.

Semiconductor plasmonic Fabry-Perot lasers at submicron and nanometer scales exhibit many characteristics distinct from those of their conventional counterparts at micron scale. The differences originate from their small sizes and the presence of plasma metal in the cavity. To design a laser of this type, these features have to be taken into account properly. In this paper, we provide a comprehensive approach to the design and performance evaluation of the plasmonic Fabry-Perot nanolasers. In particular, we show the proper procedure to obtain the key parameters for lasing action, which are usually neglected in the conventional semiconductor Fabry-Perot lasers but become important for nanolasers.

Optical characteristics of a quantum-dot laser with a metallic waveguide.

We report the optical characteristics of a quantum-dot laser with a metal-coated waveguide, which shows a large group index of 4.2 compared to 3.2 for an uncoated laser. Temperature-dependent measurements reveal a high characteristic temperature in the temperature range of 7 degrees C-25 degrees C. The optical gain, refractive index change, and linewidth enhancement factor are extracted from the measured Fabry-Perot amplified spontaneous emission spectra. We use the pulse measurements to eliminate the thermal effect and obtain a low linewidth enhancement factor of 0.35.

Whispering gallery mode lasing from zinc oxide hexagonal nanodisks.

Disk-shaped semiconductor nanostructures provide enhanced architectures for low-threshold whispering gallery mode (WGM) lasing with the potential for on-chip nanophotonic integration. Unlike cavities that lase via Fabry-Perot modes, WGM structures utilize low-loss, total internal reflection of the optical mode along the circumference of the structure, which effectively reduces the volume of gain material required for lasing. As a result, circularly resonant cavities provide much higher quality (Q) factors than lower reflection linear cavities, which makes nanodisks an ideal platform to investigate lasing nanostructures smaller than the free-space wavelength of light (i.e., subwavelength laser). Here we report the bottom-up synthesis and single-mode lasing properties of individual ZnO disks with diameters from 280 to 900 nm and show finite difference time domain (FDTD) simulations of the whispering gallery mode inside subwavelength diameter disks. These results demonstrate ultraviolet WGM lasing in chemically synthesized, isolated nanostructures with subwavelength diameters.

Theory for n-type doped, tensile-strained Ge-Si(x)Ge(y)Sn1-x-y quantum-well lasers at telecom wavelength.

We propose and develop a theoretical gain model for an n-doped, tensile-strained Ge-Si(x)Ge(y)Sn(1-x-y) quantum-well laser. Tensile strain and n doping in Ge active layers can help achieve population inversion in the direct conduction band and provide optical gain. We show our theoretical model for the bandgap structure, the polarization-dependent optical gain spectrum, and the free-carrier absorption of the n-type doped, tensile-strained Ge quantum-well laser. Despite the free-carrier absorption due to the n-type doping, a significant net gain can be obtained from the direct transition. We also present our waveguide design and calculate the optical confinement factors to estimate the modal gain and predict the threshold carrier density.

Electron spin polarization induced by linearly polarized light in a (110) GaAs quantum-well waveguide.

We report an experimental demonstration of generating electron spin polarization with linearly polarized light in a (110) GaAs quantum well. A detailed frequency-domain pump-probe study shows that the dynamic nuclear spin polarization arising from the oriented electron spins results in a strong dependence of the electron spin splitting on the photon energy and intensity of the linearly polarized excitation laser.

Normal modes for plasmonic nanolasers with dispersive and inhomogeneous media.

We derive the quasi-orthogonality condition of optical modes in a dispersive and inhomogeneous medium. This condition leads to the proper normalization rule of the optical field in a general linear medium with inhomogeneity. The derived rule also provides a physically meaningful method in estimating the magnitude of the field-enhanced spontaneous and stimulated emission rates, for example, in a microcavity or nanocavity.

Theory for bowtie plasmonic nanolasers.

We develop a fundamental formulation for electrically-pumped plasmonic semiconductor nanolasers based on a metallic bowtie structure. Because of the negative dielectric constant of the metal at optical frequencies, the effective modal volume of the plasmonic mode can be compressed to the nanometer scale. In addition, the curvature effect of the bowtie tips provides additional field enhancement in the bowtie gap and further reduces the modal volume. With this small modal volume, the required volume of the active region is reduced correspondingly, which significantly decreases the threshold current. The huge field enhancement due to the small modal volume at the gap of the bowtie may overcome the material and radiation losses by increasing both the spontaneous and stimulated emission rates, and it makes the lasing action possible.

Slow-to-fast light using absorption to gain switching in quantum-well semiconductor optical amplifier.

Room temperature quantum-well semiconductor optical amplifier with large input power is utilized in both the absorption and gain regime as an optical group delay and advance (slow and fast light), respectively. Material resonance created by coherent population oscillation and four wave mixing is tuned by electrical injection current, which in turn controls the speed of light. The four-wave mixing and population oscillation model explains the slow-to-fast light switching. Experimentally, the scheme achieves 200 degrees phase shift at 1 GHz, which corresponds to 0.56 delay-bandwidth product. The device presents a feasible building block of a multi-bit optical buffer system.

Room-temperature slow light with semiconductor quantum-dot devices.

We demonstrate room-temperature slow light that is electrically and optically controllable by using a quantum-dot (QD) semiconductor optical amplifier (SOA) at zero and low bias below the transparency current. The absorption spectrum of the QD SOA exhibits a spectral dip with a corresponding group-index dispersion and group delay owing to coherent population oscillation caused by the interaction of pump and probe laser light near resonance of the first heavy-hole-conduction-state transition. At an optical pump power of approximately 0.3 mW inside the single-mode waveguide without current injection, a group-index change of 3.0 with a bandwidth of 2 GHz was measured. This group-index change can be controlled by injection of electrical current and by changing the optical pump power.

Slow light in semiconductor quantum wells.

We demonstrate slow light via population oscillation in semiconductor quantum-well structures for the first time. A group velocity as low as 9600 m/s is inferred from the experimentally measured dispersive characteristics. The transparency window exhibits a bandwidth as large as 2 GHz.