Carrier dynamics in InGaAs/GaAs quantum dots is analyzed with highly sensitive femtosecond transmission spectroscopy. In a first step, measurements on a large ensemble of nanoislands reveal the dynamical electronic filling of quantum dots from the surrounding wetting layer. Most interestingly, we find a spin-preserving phonon mediated scattering into fully localized states within a few picoseconds. Then, individual artificial atoms are isolated with metallic shadow masks. For the first time, a single self-assembled quantum dot is addressed in an ultrafast transmission experiment. We find bleaching signals in the order of 10^-5 that arise from individual interband transitions of one quantum dot. As a result, we have developed an ultrafast optical tool for both manipulation and read-out of a single self-assembled quantum dot.

We investigated near-field optical properties and images of single gold nanorods by using a near-field optical microscope. Observed transmission spectra show distinct transverse and longitudinal surface plasmon resonances. Transmission images observed near the surface plasmon resonances agree qualitatively with calculated maps of optical local density-of-states, and are assignable to plasmon wavefunctions. Ultrafast temporal responses in the single gold nanoparticles were observed by combining a near-field microscope with time-resolved techniques. Observed transient transmission images of the single nanorods show characteristic optical features, and are in good agreement with a calculated map of variation of local density of states arising from the elevation of electronic temperature in the nanorod induced by photoexcitation.

We demonstrate direct control over the level of lateral quantum coupling between two self-assembled InGaAs/GaAs quantum dots. This coupled system, which we also refer to as a lateral quantum dot molecule, was produced using a unique technique which combines molecular beam epitaxy and in-situ atomic layer etching. Atomic force microscopy measurements show that each molecule consists of two structurally distinct dots, which are aligned along the [1-10] direction. Each molecule exhibits a characteristic photoluminescence spectrum primarily consisting of two neutral excitonic and two biexcitonic transitions. The various transitions have been investigated using micro-photoluminescence measurements as a function of excitation power density, time, and applied electric field. Photon statistics experiments between the excitonic emission lines display strong antibunching in the second-order cross-correlation function which confirms that the two dots are quantum coupled. Cascaded emission between corresponding biexcitonic and excitonic emission has also been observed. Using a parallel electric field we can control the quantum coupling between the dots. This control manifests itself as an ability to reversibly switch the relative intensities of the two neutral excitonic transitions. Furthermore, detailed studies of the emission energies of the two neutral excitonic transitions as a function of parallel lateral electric field show a clear anomalous Stark shift which further demonstrates the presence of quantum coupling between the dots. In addition, this shift allows for a reasonable estimate of the coupling energy. Finally, a simple one-dimensional model, which assumes that the coupling is due to electron tunneling, is used to qualitatively describe the observed effects.

Open and close aperture Z-scans were performed on various CdS quantum dots embedded in either generation 4 (G4) and G5 poly(propyleneimine) dendrimer films using picosecond and femtosecond pulses between 350 nm and 1 &mgr;m. The films had an average thickness of 400nm. The measured values of the third order nonlinear coefficient were among the highest off-resonance nonlinearities reported for organic and/or hybrid composites materials. However, the nonlinear response with picosecond pulses were about an order of magnitude higher than the femtosecond counterpart. We show that the nonlinear response in these materials is also a function of the dynamics of the excited states involved and that measurements of the nonlinear optical coefficient with pulses of different duration is directly correlated to the dynamics of the excited states.

Colloidal growth of plasmonic nanostructures may present some advantages such as shape control at the nm scale with atomic smoothness of the surfaces and possibly reduced damping. We show that the seed-mediated growth of gold nanostructures is strongly dependent on the gold seed nanocrystal structure. Starting with gold seed solutions prepared such that they are either single crystalline or multiply twinned, growth yields either nanorods with good control over the aspect ratio (~10%) or elongated bipyramidal nanoparticles. The nanorods are single crystalline while the gold bipyramids are penta-fold-twinned. The gold bipyramids are also strikingly monodisperse in shape with the sharpest ensemble surface plasmon resonance reported so far. Silver can be coated onto the gold nanostructures leading to a large blue-shift of the longitudinal plasmon resonance. Surprisingly, even a thin silver layer introduces much additional damping explained as scattering at the Au/Ag interface. Silver can be converted to silver sulphide yielding a large red-shift. The metal-semiconductor composite materials may present interesting nonlinear optical properties which are being currently investigated. Finally, the nonlinear optical scattering from individual Au nanorods was measured under excitation by ultrafast laser pulses on resonance with their longitudinal plasmon mode. Surprisingly, the ultrafast nonlinearity can be attributed entirely to the heating of conduction electrons and does not exhibit any response associated with coherent plasmon oscillation. This indicates an unanticipated damping of strongly driven plasmons.

Employing the quantum interference among one- and two-photon excitations induced by ultrashort two-color laser pulses it is possible to generate charge and spin currents in semiconductors and semiconductor nanostructures on femtosecond time scales. Here, it is reviewed how the excitation process and the dynamics of such photocurrents can be described on the basis of a microscopic many-body theory. Numerical solutions of the semiconductor Bloch equations (SBE) provide a detailed description of the time-dependent material excitations. Applied to the case of photocurrents, numerical solutions of the SBE for a two-band model including many-body correlations on the second-Born Markov level predict an enhanced damping of the spin current relative to that of the charge current. Interesting effects are obtained when the scattering processes are computed beyond the Markovian limit. Whereas the overall decay of the currents is basically correctly described already within the Markov approximation, quantum-kinetic calculations show that memory effects may lead to additional oscillatory signatures in the current transients. When transitions to coupled heavy- and light-hole valence bands are incorporated into the SBE, additional charge and spin currents, which are not described by the two-band model, appear.

The spontaneous and self-stimulated parametric emission from a semiconductor microcavity after resonant pulsed excitation is measured. The emission of the lower polariton branch is resolved in two-dimensional momentum space, using either time-resolved or time-integrated detection. Employing two pump directions, we experimentally probe polariton quantum correlations by exploiting quantum complementarity. Polaritons in two distinct idler-modes interfere if and only if they share the same signal-mode so that "which-way" information cannot be gathered. The experimental results prove the existence of polariton pair correlations that store the "which-way" information.

We demonstrate the first LEDs at 1.3-1.5um using GaAs deep-centers having higher (90%) efficiencies and larger Einstein B-coefficients than bulk InGaAs. An observed absence of deep-center self-absorption (from a Franck-Condon shift) could make possible near-zero threshold lasers. The fast capture (15- 40fs) of free holes by deep-centers, as well as the Einstein B-coefficient, are deduced from a combination of photoluminescence and electroluminescence measurements.

This work is devoted to correlation spectroscopy of individual II-VI CdTe/ZnTe QDs in view to determine non-resonant excitation mechanisms and provide information on spin relaxation of QD states. Second order photon autocorrelations and cross-correlations were measured in a Hanbury-Brown and Twiss setup for neutral and charged exciton and biexciton transitions, excited by pulses of a frequency-doubled femtosecond Ti:Sapphire laser. Some of the measurements were circular- or linear polarization resolved and performed in magnetic field. Besides, measurements of photoluminescence excited by pairs of laser pulses revealed fast excitation phenomena in the range of tens of ps. The results of measurements without polarization resolution were interpreted using a simple rate equation model and allowed us to establish the dominant role of single carrier capture in the non-resonant excitation of the QD. Polarization-dependent correlation measurements were used to study the magnetic field controlled transition between anisotropic QD exciton eigenstates active in linear polarization and those active in circular polarization. The same measurements provided information on spin relaxation of the carriers left in the dot after charged exciton recombination.

We show ultrafast spin injection from a diluted magnetic semiconductor (DMS) into self-assembled quantum dots (QDs), where excitons or carriers are highly spin-polarized in the DMS under magnetic fields and are subsequently injected into the QDs resulting from the energy relaxation due to the potential difference. Two types of the sample structure have been studied for exploiting efficient spin injection by using time-resolved circularly polarized photoluminescence: one is the exciton-spin injection structure of CdSe QDs stacked with a Zn_0.80Mn_0.20Se layer and the other one is the electron-spin injection structure of QDs coupled with a Zn_0.68Cd_0.22Mn_0.10Se quantum well. In the former structure, exciton-spin injection takes place from the DMS layer into the QDs with a time constant of 10 ps after the pulse excitation for the DMS, followed by spin transfer among the QDs and spin relaxation in the QDs. In the latter case, we realize efficient electron-spin injection via quantum tunneling with a time constant of 20 ps, where the spin injection is resonantly assisted by LO-phonon scattering. These results imply importance of the spin-injection dynamics for the future applications of the QDs coupled with the DMS to ultrafast spintronic and spin-functional optical devices.

An ultrafast pump-probe method based on differing polarization properties of neutral and charged excitons in semiconductor quantum dots (QDs) is employed to study carrier dynamics in InGaAs QDs grown in nominally undoped, modulation doped and p-i-n structures. We find that at low temperature even in the nominally undoped samples there are large fractions of charged dots. It is also demonstrated that for bipolar electrical injection there is a high probability of the independent capture of electrons or holes into the dots, resulting in dot charging. Voltage-control of the charged exciton population, created via a combination of electrical and optical excitation, which exhibits a long lived spin-polarization (or spin-memory) is demonstrated.

We investigated the optical properties of epitaxial n- and p-type ZnO films grown on lattice-matched ScAlMgO₄ substrates. Chemical doping yielded a severe inhomogeneity in a statistical distribution of involved charged impurities. Two approaches are adopted to treat the inhomogeneity effects; Monte Carlo simulation technique for the n-doped films, and the fluctuation theory for the p-ZnO. The broadening of PL band of n-ZnO was significantly larger than predicted by theoretical results in which the linewidths of each individual emissions have been determined mainly from the concentration fluctuation of donor-type dopants by the simulation. Moreover, the rather asymmetrical line shape was observed. To explain these features, a vibronic model was developed accounting for contributions from a series of phonon replicas. In case of p-type ZnO:N, analysis of excitation-intensity dependence of the peak shift of donor-acceptor luminescence with a fluctuation model has also proven the importance of the inhomogeneity effect of charged impurity distribution, as in the case of ZnO:Ga. We extracted the inhomogeneity in the sample and acceptor activation energy prepared under the various growth conditions. It is shown that the theoretical results are in good agreement with the experimental 5-K time-resolved luminescence for the systems in a fluctuation field. Finally, localized-state distributions have been studied in N-doped ZnO thin films by means of transient photocurrent measurement.

Here we report on high field transport in GaN and GaN field effect devices, based on the rigid-ion model of the electron-phonon interaction within the Cellular Monte Carlo (CMC) approach. Using the rigid pseudo-ion method for the hexagonal wurzite structure, the anisotropic deformation potentials are derived from the electronic structure, the atomic pseudopotential, and the full phonon dispersion and eigenvectors for both acoustic and optical modes. Piezoelectric as well as anisotropic polar optical phonon scattering is accounted for as well. In terms of high field transport, the peak velocity is primarily determined by deformation potential scattering described through the rigid pseudo-ion model. The calculated velocity is compared with experimental data from pulsed I-V measurements. We simulate the effects of non-equilibrium hot phonons on the energy relaxation as well, using a detailed balance between emission and absorption during the simulation, and an anharmonic decay of LO phonons to acoustic phonons, as reported previously. Non-equilibrium phonons are shown to result in a significant degradation of the velocity field characteristics for high carrier densities, such as those encountered at the AlGaN/GaN interface due to polarization effects.

Ferromagnetic particle collections typically possess anisotropic terahertz (THz) transmission properties that are sensitive to the orientation of an applied external magnetic field. Here, we show that the particle surface morphology can have a large and dominant effect on the magnetic field orientation dependence of the THz transmission. In particular, the THz transmission through highly porous ferromagnetic Ni particles shows isotropic dependence on the external magnetic field orientation. This isotropic magnetic phenomenon suggests the possibility of innovative photonic materials with tailored magnetic properties.

In this work, we explore the interaction of terahertz electromagnetic pulses with chiral metallic mesostructures. In contrast to conventional continuous wave experiments conducted in the visible and microwave regimes, time-domain THz spectroscopy enables direct measurement of the electric field and polarization dynamics of electromagnetic waves propagated through the chiral structure. With this experimental methodology, we discover significant polarization circularization of the radiation scattered from a sub-wavelength sized helix in the axial configuration. Numerical simulations are in excellent quantitative agreement with the experimental results. Understanding light behaviour in a helical structure is not only of fundamental significance, but could potentially lead to the development of entirely new materials to improve communication and information technology.

The two-photon QWIP approach involves three equidistant subbands, two of which are bound in the quantum well, and the third state is located in the continuum. The intermediate subband induces a resonantly enhanced optical nonlinearity, which is about six orders of magnitude stronger than in usual semiconductors. Temporal resolution is only limited by the sub-ps intrinsic time constants of the quantum wells, namely the intersubband relaxation time and the dephasing time of the intersubband polarization. Both properties make this device very promising for pulse diagnostics of pulsed midinfrared lasers. We have performed autocorrelation measurements of ps optical pulses from the free-electron laser (FEL) facility FELBE at the Forschungszentrum Dresden Rossendorf. Using a rapid-scan autocorrelation scheme at a scan frequency of 20 Hz, high-quality quadratic autocorrelation traces are obtained, yielding ratios close to the theoretically expected value of 8:1 between zero delay and large delay for interferometric autocorrelation, and 3:1 for intensity autocorrelation. Thus, two-photon QWIPs provide an excellent new technique for online pulse monitoring of the FEL. In addition, we have investigated the saturation mechanism of the photocurrent signal, which is due to internal space charges generated in the detector.

The concept and implementation of a near-field microwave measurement system that relies on the Pockels effect in fiber-coupled, semi-insulating GaAs probes to acquire polarization-sensitive maps of electric-field patterns in close proximity to antenna arrays, integrated circuits, and packaged components, is presented. The evolution of the electro-optic field-mapping technique, which has subsequently addressed magnetic-field characterization via magneto-optic sensing and temperature measurement through semiconductor band-gap modulation, will also be discussed. The use of emerging materials, such as diluted magnetic semiconductors, is also considered.

We present theoretical investigations of the intrinsic dynamics of long-wavelength non-equilibrium optical phonons in bulk and low-dimensional semiconductors. The theory is based on the application of Fermi's golden rule formula, with phonon dispersion relations as well as crystal anharmonicity considered in the framework of isotropic continuum model. Contributions to the decay rates of the phonon modes are discussed in terms of four possible channels: Klemens channel (into two acoustic daughter modes), generalised Ridley channel (into one acoustic and one optical mode), generalised Vallee-Bogani channel (into a lower mode of the same branch and an acoustic mode), and Barman-Srivastava channel (into two lower-branch optical modes). The role of crystal structure and cation/anion mass ratio in determining the lifetime of such modes in bulk semiconductors is highlighted. Estimates of lifetimes of such modes in silicon nanowires and carbon nanotubes will also be presented. The results support and explain available experimental data, and make predictions in some cases.

The lifetime of longitudinal optical phonon mode in GaN has been measured by subpicosecond time-resolved Raman spectroscopy for photoexcited electron-hole pair density ranging from 10¹⁶ cm^-3 to 2x10¹⁹cm^-3 and at T = 300K. The lifetime has been found to decrease from 2.5 ps, at the lowest density to 0.35 ps, at the highest density. Possible mechanism for this observation has been discussed. Our experimental findings help resolve the recent controversy over the lifetime of LO phonon mode in GaN.

This work discusses critical role played by hot phonons in limiting high speed performance of electronic and optical devices based on wide gap nitride semiconductors A simple model is introduced that explains velocity saturation in the wide bandgap semiconductors and, based, on the experimental data, show that hot phonons in nitrides present unique challenge. Various methods of mitigating the effects of hot phonons - ranging from creating conditions for stimulated phonon emission to use of disorder are discussed.

The photosensitivity linearity of a back-illuminated, pin photodiode arrays built on 75-&mgr;m thick single silicon dies is discussed. Photosensitivity linearity measurements were performed in the range of input light fluxes above ~1nW/pixel and the linearity was found to be better than 0.01% within the spectral range from 450 to 1000 nm. For lower light fluxes, the non-linearity of the photo-sensitivity was smaller than the noise current of the array pixels and different methods should be applied to measure the photosensitivity linearity with an accuracy of better than 0.1%. The theoretical limits for the sensitivity linearity measurements are discussed. This work describes also the automatic probe system for opto-electrical testing of the front- and backside illuminated photodiode arrays. The system allows 100% testing of wafers and dies before die attach. The system is configured to work on wafers up to 150 mm in size or single multi-pixel dies.

Large area optical detection systems are required for applications including cell imaging, spectroscopy, nuclear medicine, bio diagnostics, radiation detection and high energy physics. Each of these applications requires that a detector or detector arrays be closely coupled with light sources or optical couplers such as fibres or light couplers. In this paper, the scaling of novel Silicon Photomultiplier detectors to tile across a large area is presented. In particular, a novel method is discussed for compact packaging of SPM detectors into a tiled 2D detector array for large area imaging and 2D spatial detection. The SPM detector has performance characteristics comparable to vacuum photon multiplier tubes used in these applications today but offers several performance and system design advantages including spatial resolution, optical over exposure, small form factor, weight, magnetic insensitivity and low bias operation.

A UK consortium (MI3) has been founded to develop advanced CMOS pixel designs for scientific applications. Vanilla, a 520x520 array of 25&mgr;m pixels benefits from flushed reset circuitry for low noise and random pixel access for region of interest (ROI) readout. OPIC, a 64x72 test structure array of 30&mgr;m digital pixels has thresholding capabilities for sparse readout at 3,700fps. Characterization is performed with both optical illumination and x-ray exposure via a scintillator. Vanilla exhibits 34±3e^- read noise, interactive quantum efficiency of 54% at 500nm and can read a 6x6 ROI at 24,395fps. OPIC has 46±3e^- read noise and a wide dynamic range of 65dB due to high full well capacity. Based on these characterization studies, Vanilla could be utilized in applications where demands include high spectral response and high speed region of interest readout while OPIC could be used for high speed, high dynamic range imaging.

Crosstalk of CMOS Image Sensor (CIS) causes degradation of spatial resolution, color mixing and leads to image noise. Crosstalk consists of spectral, optical and electrical components, but definition of each component is obscure and difficult to quantify. For the first time, quantifiable definition of each component is proposed to perform crosstalk analysis in this paper. Contribution of each component to the total crosstalk is analyzed using opto-electrical simulation. Simulation is performed with an internally developed 2D finite difference time domain (FDTD) simulator coupled to a commercial device simulator. Simulation domain consists of set of four pixels. Plane wave propagation from micro-lens to the photodiode is analyzed with FDTD and the optical simulation result is transformed into the photo-current in the photodiode using electrical simulation. The total crosstalk consists of 43% of spectral crosstalk, 14% of optical cross talk, and 43% of electrical crosstalk at the normal incident light. Spectral crosstalk can be suppressed through careful selection of color filter materials with good selectivity of color spectrum. Characteristics of crosstalk and photosensitivity show contrary trend to one another as a function of color filter thickness. Therefore, the crosstalk target is fixed and simulation is performed to determine the minimum color filter thickness that satisfies the crosstalk target. By color filter material and thickness optimization, 10% increase in photosensitivity and 7% decrease spectral crosstalk were obtained. Electrical crosstalk showed 11% and 9% improvement through applying to new implantation process and stacking multi-epi layer on the p-type substrate, respectively.

Current state of the art high resolution counting modules, specifically designed for high timing resolution applications, are largely based on a computer card format. This has tended to result in a costly solution that is restricted to the computer it resides in. We describe a four channel timing module that interfaces to a computer via a USB port and operates with a resolution of less than 100 picoseconds. The core design of the system is an advanced field programmable gate array (FPGA) interfacing to a precision time interval measurement module, mass memory block and a high speed USB 2.0 serial data port. The FPGA design allows the module to operate in a number of modes allowing both continuous recording of photon events (time-tagging) and repetitive time binning. In time-tag mode the system reports, for each photon event, the high resolution time along with the chronological time (macro time) and the channel ID. The time-tags are uploaded in real time to a host computer via a high speed USB port allowing continuous storage to computer memory of up to 4 millions photons per second. In time-bin mode, binning is carried out with count rates up to 10 million photons per second. Each curve resides in a block of 128,000 time-bins each with a resolution programmable down to less than 100 picoseconds. Each bin has a limit of 65535 hits allowing autonomous curve recording until a bin reaches the maximum count or the system is commanded to halt. Due to the large memory storage, several curves/experiments can be stored in the system prior to uploading to the host computer for analysis. This makes this module ideal for integration into high timing resolution specific applications such as laser ranging and fluorescence lifetime imaging using techniques such as time correlated single photon counting (TCSPC).

With the rapid growth of optoelectronics technologies, photodiodes (PDs) has been widely used in optical measurement systems, color measurement and analysis systems, etc. To meet most of the measurement requirements, the determination of PD spectral responses is very important. The goal of this paper is to develop a high-accuracy and cost-effective spectral response measurement system for PDs. In this paper, the proposed system contains a grating-based spectral filtering module, an amplifier module, and a digital-signal-processing (DSP) based platform. In the spectral filtering module, a single-grating monochromator based on a Czerny-Turner configuration is first analyzed and simulated, and then the experiments are conducted to check if the measurement accuracy is satisfactory. In the measurement system, optoelectronic signals from the PD under test are acquired from the amplifier module and the DSP-based platform is developed to communicate and manipulate the measured data. Through comparison with the measurement data from a commercially available system, it is found that our approach gives quite satisfactory results.

We designed and fabricated traveling-wave photodetector with enhanced bandwidth. Because the saturation velocity of hole is smaller than that of electron, bandwidth limitation in conventional symmetric TWPD results from the difference in electron and hole transit times. For solving this problem, we designed a new structure with asymmetric intrinsic region to equalize the carrier transit times. The intrinsic region on the epitaxial layer consists of InGaAs core and 1.3Q InGaAsP cladding regions. In the whole i-layer thickness with 1 &mgr;m, the core region is 0.2 &mgr;m thick and the thickness of cladding region is asymmetrically made up. As the thickness of upper cladding region to p-side is decreased, the transit lengths of electron and hole are matched and the bandwidth of TWPD is enhanced. By fabricating TWPD's from three kinds of epitaxial wafers, we prove the enhanced bandwidth of TWPD with asymmetric intrinsic region.

The contact of p-GaN was formed under different annealing condition, and its effect on p-i-n GaN-based detectors was studied by current-voltage (I-V) measurements and the response spectra. The parameters of metal/p-GaN interface were obtained by fitting the forward I-V curves. The results show that ideal factor of metal-semiconductor ( M-S) contacts annealed at 550°C for 3min is about 1.19, which means the formation of good ohmic contacts at the M-S interface and leads a lower turn-on voltage. But metal/p-GaN contacts have no obvious effect on response spectra of detectors.