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The motions of electrons in solids may be highly correlated by strong, long-range Coulomb interactions. Correlated electron-hole pairs (excitons) are accessed spectroscopically through their allowed single-quantum transitions, but higher-order correlations that may strongly influence electronic and optical properties have been far more elusive to study. Here we report direct observation of bound exciton pairs (biexcitons) that provide incisive signatures of four-body correlations among electrons and holes in gallium arsenide (GaAs) quantum wells. Four distinct, mutually coherent, ultrashort optical purses were used to create coherent exciton states, transform these successively into coherent biexciton states and then new radiative exciton states, and finally to read out the radiated signals, yielding biexciton binding energies through a technique closely analogous to multiple-quantum two-dimensional Fourier transform (2D FT) nuclear magnetic resonance spectroscopy. A measured variation of the biexciton dephasing rate indicated still higher-order correlations.

The carrier recombination dynamics of InGaN quantum wells (QWs) grown on (0001)-oriented planar GaN and {1101}-oriented facets of GaN triangular prisms are demonstrated. The prisms are generated by lateral epitaxial overgrowth in a metal-organic chemical vapor deposition system.

The structural and photoluminescence (PL) characteristics of GaSbN/GaSb single quantum well (SQW) heterostructures grown by molecular beam epitaxy (MBE) is described. The temperature and excitation intensity dependences of PL as well as invariance of PL peaks elucidates N-related PL line features below the GaSb conduction band edge.

The negative capacitance (NC) effect is seen in a range of electronic devices. The NC effect in homogeneous semiconductors has been considered theoretically. The NC effect is now discussed, with a focus on the theoretical interpretation of the phenomenon. This can be based on the analysis of the time-domain transient current, in response to a small voltage step. The time-domain transient current approach is demonstrated using simulation results of quantum well infrared photodetectors.

An ensemble Monte Carlo particle method has been used to study transient response of n-type A1GaAs/GaAs in Inp/InGa As quantum well infrared photodectors (QWIPs). A sharp peak observed in the photocurrent initiated by an ultrashort pulse of infrared radiation, linked to the electron velocity overshoot effect. The plateaulike region in the QWIP frequency-dependent responsivity in the terahertz range could be used for heterodyne detection in QWIP-based mixers at terahertz frequencies.

The possibility of covering the short wavelength infrared region using intersubband transitions in GaAs-based quantum wells, is explored. InGaAs wells were investigated with A1As thin confining barriers, and the dark current was found to decrease with increasing detection wavelength. Photocurrents due to intersubband transition were see down to a wavelength of around 1 mum. Physical effects related to the indirect band minima at the X point were also revealed.

Molecular beam epitaxy has been used to grow ultrathin ZnSe/ZnS single quantum well structures, on GaP substrates with high Miller indices. Measurements of power and temperature dependence show that structural disorder is important in ZnSe/ZnS ultrathin single quantum wells. The luminescence emission is the response of local potentials with quantum dotlike characteristics, with luminescence peak a convolution of individual narrow lines.

Compressively strained, square potential profile GaInAsP quantum wells (QWs) with grown on InP substrates are useful to improve 1.55-mum lasers. A new approach is based on the creation of a linear variation of the bandgap within the well, through linear variations in the As concentration. Effective density of heavy hold band states is reduced in such V-shaped structures, and overlap between electron and hole wavefunction is thought to be higher than narrow abrupt-shaped QWs.

Design and experimental results for high-power, high-efficiency, low threshold current, 0.98-mum wavelength, broadened waveguide (BW) aluminium-free InGaAs-(In)GaAs(P)-InGaP lasers are presented. The observed fall in internal losses and rise in waveguide layer width for the separate-confinement heterostructure multiple-quantum-well structure is due to lower free-carrier absorption. BW lasers with InGaAsP and GaAs waveguides exhibit lower internal losses and threshold currents than those built for optimum optical confinement factors.

The study of semiconductor quantum-well (QW) lasers has usually concentrated on the use of rate equations coupling cold cariers to photons in the lasing cavity to determine static and dynamic properties. The carrier dynamics of QW lasers have been studied using a Monte Carlo method, and the results show that hot carrier effects in semiconductor lasers should be considered for analysis and design of semiconductor lasers.

A finite confining potential model is used to theoretically investigate the effects of an applied electric field perpendicular to well layers on the exciton linewidth due to free carrier scattering in semoconducting quantum-well structures. The free carrier-exciton linewidth broadening depends on carrier concentration, well width and temperature.

The use of three-level rate equations for a laser operating in single mode helps to develop a SPICE equivalent-circuit model for the design and analysis of quantum-well lasers. The model allows simulations of both small- and large-signals. It emphasizes the consideration of gateway states in long separate-confinement heterostructure (SCH) lasers for small-signal modulation response. Transport across the SCH determines the large-signal turn-on response for both short and long SCH lasers. The model is validated through a favorable comparison of simulated results with measured data.

The use of InGaAs ternary substrates facilitates a performance enhancement of 1.3-micro-m vertical-cavity surface-emitting lasers (VCSEL's). Such substrates help to obtain a deep potential well, which leads to a larger optical gain. They also allow the development of a higher reflectivity distributed Bragg reflector. VSCEL's on such ternary substrates yield lower threshold current-density than those on the conventional InP substrates. The possibility of temperature-insensitive operation of the devices and the achievement of extremely low threshold current density are discussed.

Stress-strength studies of the catastrophic optical damage (COD) failure in Al-free uncoated 0.98-micrometer GaInAs-GaInP strained quantum-well lasers reveal the strong dependence of the aging-related critical power level (CPL) decrease on the injection current. A statistical investigation of the results shows the CPL data at each aging time to be distributed as per the Weibull statistics. The relationship between CPL data and injection current not only enables accurate prediction of COD failures for large/small current, but also shows them to be fatal for high-power operation.

Photoabsorption-induced disordering (PAID) provides an effective quantum-well intermixing mechanism for controlling the bandgap tuning in the InGaAs-InGaAsP system. The PAID approach exploits optical absorption in the active region of the multiple-quantum-well to generate optimum heat that induces interdiffusion between the wells and barriers. Photoconductivity analysis of the intermixed material not only reveals promising electrical/optical properties, but also demonstrates the suitability of PAID for device integration so as to produce photonic integrated circuits.

An extremely sensitive long wavelength infrared camera uses GaAs/AlGaAs quantum well focal plane arrays to provide excellent imagery that achieves a noise equivalent differential temperature of 26 mK. The GaAs/Al(sub x)Ga(sub 1-x)As material system not only serves as the basis for the development of a 9 micrometer cutoff 256 X 256 hand-held infrared photodetector, but also allows manipulation of the quantum well shape over a wide range. The camera may be fabricated entirely out of large bandgap materials to enable detection of light at wavelengths ranging from 6 to 25 micrometer.

A bound-to-quasi-bound intersubband transition technique for the development of GaAs/Al(sub x)Ga(sub 1-x)As quantum well infrared photodetectors provides extremely sensitive/long wavelength detection along with excellent imagery. The transition approach not only allows fabrication of random reflectors for efficient light coupling, but also achieves a noise equivalent differential temperature of 30 mK for a 15 micrometer cutoff 128 X 128 focal plane array imaging camera. A 1000 K blackbody source in conjunction with a grating monochromator uses back illumination to give the responsivity spectra.

A single-mode Fabry-Perot guided wave resonator, containing a single quantum well of InGaAs embedded in an InGaAsP-InP waveguide, facilitates the theoretical and experimental study of semiconductor all-optical nonlinear dichroic (SAND) devices. The dichroism generated by absorption edge selection rules for the quantum well governs the switching mechanism. It leads to two separate bandedges for TE and TM polarized light and real excitation refractive nonlinearities. The SAND device shows a signal gain of over 2 and a modulation depth of about 54%.

Hole saturated drift velocity in strained Si(sub 1-x)Ge(sub x) p-channel quantum well metal-oxide-semiconductor-field-effect-transistors (p-QWMOSFET's) increases with increasing Ge mole fractions. Holes in a strained SiGe well show average high-field velocities that are 50% higher than those in a Si-device. This enhanced hole transport is due to the high-energy Si-like optical phonon spectrum of the strained SiGe alloy. The p-QWMOSFET's are assessed for Ultra-Large-Scale-Integration applications.

A reverse step-tapered undulator field (RSTU) produces a smooth optical pulse throughout the saturated part of the machine pulse in a free-electron laser oscillator. An RSTU produces no decrease in the small-signal gain. Electrons are incapable of reabsorbing the optical field energy in the presence of an RSTU as the shifted ponderomotive potential well detraps electrons that reach the bottom of the well. The formation of subpulses thus reduces, resulting in one long optical pulse.

A quantum mechanical model based on the Wigner function explains carrier transport in separate confinement hetero-quantum well laser. The injected electrons and holes in the single quantum-well active layer have unequal concentrations. Heavy-hole density profiles are simulated for the two wells. Solutions are obtained for the quantum Liouville equations associated with the Wigner functions for electron, heavy-hole and light-hole.

Interband-resonant light modulation by intersubband-resonant light in undoped quantum wells is fast and is independent of the problems of carrier relaxation and heat dissipation. Calculation of the modulation incorporates the excitonic interband-transition and continuous level transition between the conduction and valence bands. A comparative study analyzes the modulation achieved using undoped and n-doped quantum wells. A real-time single-shot experiment at room temperature employs Ti-Al2O3 and CO2 lasers for interband- and intersubband-resonant lights in undoped quantum wells.

The use of multistack multiquantum barrier (MS-MQB) structure produces a higher potential barrier for graded-index separate confinement heterostructure lasers. The results of simulation by transfer matrix method and the experimental results using GaAs-AlAs MS-MQBs are in agreement with each other. The devices employing MS-MQBs have higher characteristic temperature and modulation bandwidth, and lower threshold current resulting from the enhancement in the carrier and optical confinements. Design methodology of 0.78 and 1.3 micrometers graded-index separate confinement heterostructure lasers is presented.

A model describes the effects of tensile strain on threshold current densities in broad-area stripe GaAsP-AlGaAs single-quantum-well lasers. The model is based on a six band Luttinger-Kohn valence dispersion model. Constant-gain contours generated from the theoretical model describe the behaviour of tensile-strained lasers in different regimes of operation. The results show that reduction in threshold currents due to application of tensile strain in GaAsP-AlGaAs lasers is only present in the high-gain or short cavity regime.

The design and construction of a ten-channel tunable multisection DFB laser array was carried out. Low threshold current of 5-7m A was achieved by using strained-In GaAsP MQW's with a 0.9% compressive strain. The spectral linewidth was lower than derived for the modification of the side electrode in a multisection DFB laser. The ten channels were tuned at the lasing frequency of 10GHz with each channel sustaining a narrow linewidth of lower than 2.3 MHz and an output power of over l2mW.

The development and detailed analysis of 1.55 micrometer, partly gain-coupled InGaAsP-InP DFB lasers, combined with a strained layer MQW active grating is discussed. Low threshold CW functioning was derived at room temperature. The influence of cavity length on the longitudinal-mode behavior pattern of the laser was characterized experimentally and theoretically. The novel longitudinal-mode behavior analyzed was simulated by employing a transfer-matrix analysis method. It was concluded that this experiment produced accurate data useful for interpreting structures in the laser.

Efficiency of the injection of quantum well lasers and other static qualities are affected by the parameters of carrier transport. Transport time across the current and optical confinement zones must be reduced. Reduced efficiencies of lasers result from too many changes in the separate confinement heterostructure zone, which in turn is due to inadequate carrier confinement and enhanced carrier transport periods.

Arrhenius behavior was detected in dc photocurrent variations with temperature and bias from GaAs-AlxGa1-xAs single quantum wells present in p-i-n diodes. An activation energy determined by the field was observed close to the hole well depth. The assumption used here was that the rapidly escaping carriers determine the recombination rate, and that their carriers are light-hole driven processes.

The band edge transition types in quantum-well heterostructures are identified by a new optical technique which is based on photoluminescence excitation spectroscopy (PLE). Direct identification of band-edge transition types in quantum wells is possible with the PLE. The orthogonal polarizations of the excitation beam yields two PLE spectra which are used in the determination of the transition types near the band-edge.

Vanishing conductivities in materials with a band gap, result in diverging resistivities, although other insulators exhibit more complex behaviour, especially in the presence of a high magnetic field. Magnetoresistance diverges as temperatures approach absolute zero, although the transverse resistance remains finite. Experimental evidence for such a Hall insulator is reported in a two-dimensional electron system.

Scanning tunneling spectroscopy is used to observe the quantum well (QW) states in Au/Fe(100) and their energy dependence on tip-sample distance. The shifts in QW states might be caused by the phase shift of the QW wave functions in the Au film as well as at the vacuum/Au interface.

A study investigates the optical and structural properties of AlInGaN quaternary single and multiple quantum-well structures by means of photoluminescence and x-ray diffraction. Details of the comparative study of single quantum-well (SQW) and multiple quantum-well (MQW) structures that was carried out in terms of the exciton localization effect and the strain relaxation are discussed.

How many pairs of electrons and 'missing electrons' can sustain collective motion in a semiconductor? The limits of this electron-hole dance are found by probing the dance floor using ultrashort laser pulses. Three's the limit for multiple excitons Long-range correlations between charge particles (electrons and 'holes') in semiconductors lead to many-body effects, which are of fundamental interest and also of importance in optoelectronic applications. The exciton state, in which an electron and hole are paired, has been extensively studied, but the properties of multiple exciton states involving three or more charge particles are largely unknown as they are challenging to observe experimentally. Daniel Turner and Keith Nelson have extended a spectroscopy technique called multidimensional Fourier transform optical spectroscopy, and demonstrate its ability to generate and characterize bi-excitons, tri-excitons and other unbound correlations in a gallium arsenide nanostructure. This experiment involves controlling the geometry, temporal delays and optical phases of up to seven light fields simultaneously. The findings are of particular interest as it was previously not known whether tri-excitons -- involving correlations between six particles -- could exist at all. The authors also present clear evidence that four-exciton states do not exist, indicating an upper limit for many-body correlations in this type of semiconductor system., Author(s): Gregory D. Scholes [sup.1] Author Affiliations: (1) Gregory D. Scholes is in the Department of Chemistry and the Center for Quantum Information and Quantum Control, University of Toronto, Toronto, [...]