I. Galbraith

Diffusion and diffraction in dispersive optical bistability

We present some numerical calculations on the effect of diffusion and diffraction on the switching characteristics of bistable optical etalons, together with an approximate analytic model demonstrating the major features and a discussion of switching waves in such systems. A diffusion length of one third of the input-beam spot size gives large hysteresis, whereas diffraction alone gives no hysteresis in first order.

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.

SUPPRESSION OF AUGER RECOMBINATION IN ARSENIC-RICH INAS1-XSBX STRAINED LAYER SUPERLATTICES

Room-temperature pump-probe transmission experiments have been performed on an arsenic-rich InAs/InAs1-xSbx strained layer superlattice (SLS) above the fundamental absorption edge near 10 mu m, using a ps far-infrared free-electron laser. Measurements show complete bleaching at the excitation frequency, with recovery times which are found to be strongly dependent on the pump photon energy. At high excited carrier densities, corresponding to high photon energy and interband absorption coefficient, the recombination is dominated by Auger processes, A direct comparison with identical measurements on epilayers of InSb, of comparable room-temperature band gap, shows that the Auger processes have been substantially suppressed in the superlattice case as a result of both the quantum confinement and strain splittings in the SLS structure, In the nondegenerate regime, where the Auger lifetime scales as tau(aug)(-1)=C1Ne2, a value of C-1 some 100 times smaller is obtained for the SLS structure. The results have been interpreted in terms of an 8x8 k . p SLS energy band calculation, including the full dispersion for both k in plane and k parallel to the growth direction. This is the strongest example of room-temperature Auger suppression observed to date for these long-wavelength SLS alloy compositions and implies that these SLS materials may be attractive for applications as room-temperature mid-IR diode lasers

Diffusive transverse coupling of bistable elements - Switching waves and crosstalk

Diffusion is the dominant transverse coupling mechanism in some bistable optical systems, including InSb. We report calculations on switching waves, radial variations of hysteresis loops, and crosstalk between arrays of devices with diffusive coupling.

Optical Excitations in Star-Shaped Fluorene Molecules

A detailed study of the low-energy optical transitions in two families of star-shaped molecules is presented. Both families have 3-fold rotational symmetry with oligofluorene arms attached to a central core. In one family, the core of the molecule is a rigid meta-linked truxene, while the other is a meta-linked benzene moiety. The low-energy transitions were studied both experimentally and using time-dependent density functional theory (TD-DFT). The optical transitions of these new star-shaped molecules were compared with corresponding linear oligofluorenes. Both families of star-shaped molecules showed higher absorption and fluorescence dipoles and photoluminescence quantum yields than straight chain oligofluorenes. TD-DFT calculations show that absorption takes place across the entire molecule, and after excited state relaxation, the emission results from a single arm. In both theory and experiment the transition dipole moments show an approximate n(0.5) dependence on the number of fluorene units in each arm.

Si:P as a laboratory analogue for hydrogen on high magnetic field white dwarf stars.

Laboratory spectroscopy of atomic hydrogen in a magnetic flux density of 10(5) T (1 gigagauss), the maximum observed on high-field magnetic white dwarfs, is impossible because practically available fields are about a thousand times less. In this regime, the cyclotron and binding energies become equal. Here we demonstrate Lyman series spectra for phosphorus impurities in silicon up to the equivalent field, which is scaled to 32.8 T by the effective mass and dielectric constant. The spectra reproduce the high-field theory for free hydrogen, with quadratic Zeeman splitting and strong mixing of spherical harmonics. They show the way for experiments on He and H(2) analogues, and for investigation of He(2), a bound molecule predicted under extreme field conditions.

Effect of exciton self-trapping and molecular conformation on photophysical properties of oligofluorenes

Electronic absorption and fluorescence transitions in fluorene oligomers of differing lengths are studied experimentally and using density functional theory (DFT) and time-dependent DFT. Experimental values are determined in two ways: from the measured molar absorption coefficient and from the radiative rate deduced from a combination of fluorescence quantum yield and lifetime measurements. Good agreement between the calculated and measured transition dipoles is achieved. In both theory and experiment a gradual increase in transition dipoles with increasing oligomer length is found. In absorption the transition dipole follows an approximately n(0.5) dependence on the number of fluorene units n for the range of 2 < or = n < or = 12, whereas a clear saturation of the transition dipole with oligomer length is found in fluorescence. This behavior is attributed to structural relaxation of the molecules in the excited state leading to localization of the excitation (exciton self-trapping) in the middle of the oligomer for both twisted and planar backbone conformations. Twisted oligofluorene chains were found to adopt straight or bent geometries depending on alternation of the dihedral angle between adjacent fluorene units. These different molecular conformations show the same values for the transition energies and the magnitude of the transition dipole.

Ionization degree of the electron-hole plasma in semiconductor quantum wells

The degree of ionization of a nondegenerate two-dimensional electron-hole plasma is calculated using the modified law of mass action, which takes into account all bound and unbound states in a screened Coulomb potential. Application of the variable phase method to this potential allows us to treat scattering and bound states on the same footing. Inclusion of the scattering states leads to a strong deviation from the standard law of mass action. A qualitative difference between mid- and wide-gap semiconductors is demonstrated. For wide-gap semiconductors at room temperature, when the bare exciton binding energy is of the order of T, the equilibrium consists of an almost equal mixture of correlated electron-hole pairs and uncorrelated free carriers.

Dynamics of fluorescence depolarisation in star-shaped oligofluorene-truxene molecules.

Star-shaped molecules are of growing interest as organic optoelectronic materials. Here a detailed study of their photophysics using fluorescence depolarisation is reported. Fluorescence depolarisation dynamics are studied in branched oligofluorene-truxene molecules with a truxene core and well-defined three-fold symmetry, and are compared with linear fluorene oligomers. An initial anisotropy value of 0.4 is observed which shows a two-exponential decay with time constants of 500 fs and 3-8 ps in addition to a long-lived component. The femtosecond component is attributed to exciton localisation on one branch of the molecule and its amplitude reduces when the excitation is tuned to the low energy tail of the absorption spectrum. The picosecond component shows a weak dependence on the excitation wavelength and is similar to the calculated rate of the resonant energy transfer of the localised exciton between the branches. These assignments are supported by density-functional theory calculations which show a disorder-induced splitting of the two degenerate excited states. Exciton localisation is much slower than previously reported in other branched molecules which suggests that efficient light-harvesting systems can be designed using oligofluorenes and truxenes as building blocks.