Search

Focuses on a study which used ion implantation into the combined neodymium and yttrium aluminum garnet to produce waveguides capable of supporting laser action. Characterization of the refractive index profiles as a function of ion dose and energy, implant temperature, and thermal annealing; Analysis of results; Method by which the best mode confinement and attenuation was obtained; Discussion on several unusual features of the profiles.

Presents a study which focuses on a class of optical fibers based on lead germanate glass. Information on the maximum vibrational frequency of lead germanate glass; Advantages of fiber technology; Factors that provide the characterization of the optical properties of the rare-earth ion in the lead-germanate host; Details of the Levin-Block concept.

Describes a low threshold, room temperature 1.64 µm ytterbium:erbium:Y[sub3]Al[sub5]O[sub12] waveguide laser. Calculated mode spot sizes for the waveguides tested; Description of the experimental arrangement used to investigate the laser performance of the waveguides; Intensity profile of the ouput.

Hot-carrier solar cells offer the opportunity to harvest more energy than the limit set by the Shockley–Queisser model by reducing the losses due to the thermalization of photo-generated carriers. Previous reports have shown lower thermalization rates in thinner absorbers, but the origin of this phenomenon is not precisely understood. In this work, we investigate a series of ultrathin GaAs absorber layers sandwiched between AlGaAs barriers and transferred on host substrates with a gold back mirror. We perform power-dependent photoluminescence characterizations at different laser wavelengths from which we determine the carrier temperature in four absorber thicknesses between 20 and 200 nm. We observe a linear relationship between the absorbed power and the carrier temperature increase. By relating the absorbed and thermalized power, we extract a thermalization coefficient for all samples. It shows an affine dependence with the thickness, leading to the identification of distinct volume and surface contributions to thermalization. We confirm that volume thermalization is linked to LO phonon decay. We discuss the origin of the interface-related thermalization, showing that the effect of LO phonon transport is negligible. Overall, this work sheds new light on thermalization processes in ultrathin semiconductor layers and introduces a method to compare the performance of hot-carrier absorbers. [ABSTRACT FROM AUTHOR]

Observation of robust hot carrier effects in quantum-well structures has prompted hopes to increase the efficiency of solar cells beyond the Shockley–Queisser limit (33% for single junction solar cells at AM1.5G). One of the main studies in hot carrier effects is the determination of carrier temperature, which provides information on the thermalization mechanisms of hot carriers in semiconductor materials. Here, we investigate the spatial distribution of photo-generated hot carriers in a InGaAs multi-quantum-well structure via hyperspectral luminescence imaging. We discuss proper methods of extracting the temperature of carriers from a photoluminescence spectrum. Robust hot carrier effects are observed at the center of the laser spot at various lattice temperatures. In addition, it is seen that the local carrier temperature scales linearly with the local laser intensity as long as the illumination exceeds a threshold power; the carrier temperature at regions with local intensities below the threshold drops to the lattice temperature, i.e., experiences no hot carrier effects. Moreover, at large distances from the concentrated light, where the level of illumination is negligible, evidence of carrier radiative recombination is observed, which is attributed to carrier diffusion in the planar structure. The results of this study can be applied to investigate the influence of carrier diffusion and thermoelectric effects on the thermalization of hot carriers. [ABSTRACT FROM AUTHOR]

Current–voltage (I–V) curves and operational stability of hot-carrier solar cells are studied by a non-equilibrium theory considering three characteristic timescales of the hot-carrier dynamics (timescales for the extraction, equilibration, and thermalization). We find a hysteresis behavior in the I–V curves of high-efficiency hot-carrier solar cells, which could result in an operational instability. For practical application, we point out two types of instabilities that can degrade the device efficiency: one is intrinsic in a single cell and the other arises when plural cells are series-connected. It is also found that particle-number non-conserving processes, Auger recombination and impact ionization, increase the stability, showing an advantage of using a semiconductor material with a large Auger recombination coefficient for the light absorber. [ABSTRACT FROM AUTHOR]

An optical hot-carrier solar cell (opt-HC-SC) is a variant of hot-carrier solar cells (HC-SCs). Hot luminescence in a specific narrow energy range is enhanced and extracted from the absorber through an optical energy-selective contact (opt-ESC) and illuminates a neighboring photovoltaic cell of a conventional type. Thus, this concept eliminates the requisites for carrier transport to achieve high conversion efficiency. In the present study, I clarified the issues for high efficiency using an improved detailed-balance model and determined the solutions. The serious issue is that the enhancement effect of the opt-ESC on the hot luminescence is considerably lower in practice than the optimal value, and this necessitates a high carrier density in the absorber for sufficient photon extraction and results in significant energy loss originating from carrier thermalization. When impact ionization occurs immediately, the average carrier energy lowers, the carrier density can decrease, and hence the thermalization energy loss significantly reduces. This, in turn, enables absorption of sub-bandgap photons arising from the intraband transition to improve the efficiency. Consequently, the synergy effect of both the aforementioned phenomena notably improves the efficiency to 53.6% under the practical upper limit of the enhancement effect, the carrier thermalization time of 1 ns, and 1000-times concentrated solar illumination, from 45.1% for no intraband transition or impact-Auger. By contrast, the individual effects are marginal. This is a unique feature of opt-HC-SCs and is not found in HC-SCs (carrier extraction). The present finding serves as a guide to material design to realize highly efficient opt-HC-SCs. [ABSTRACT FROM AUTHOR]

When ion irradiation introduces point-defects in semiconductors/insulators, discrete energy levels can be introduced in the bandgap, and then optical transitions whose energies are lower than the bandgap become possible. The electronic transitions between the discrete level and the continuous host band are observed as a continuous tail starting from the fundamental edge. This is the wellknown mechanism of the absorption tail close to the band-edge observed in many semiconductors/ insulators. In this paper, we propose another mechanism for the absorption tail, which is probably active in Nd-doped yttrium aluminum garnet (Nd:YAG) after ion irradiation and annealing. A Nd:YAG bulk crystal was irradiated with 15MeV Au5+ ions to a fluence of 8×1014 ions/cm². The irradiation generates an amorphous layer of ~3 μm thick with refractive index reduction of Δn=-0.03. Thermal annealing at 1000 ℃ induces recrystallization to randomly aligned small crystalline grains. Simultaneously, an extraordinarily long absorption tail appeared in the optical spectrum covering from 0.24 to ~2 μm without fringes. The origin of the tail is discussed based on two models: (i) conventional electronic transitions between defect levels and YAG host band and (ii) enhanced light scattering by randomly aligned small grains. [ABSTRACT FROM AUTHOR]

This paper suggests a method to convert heat to electricity by the use of devices called cold carrier emissive energy harvesters (cold carrier EEHs). The working principle of such converters is explained and theoretical power densities and efficiencies are calculated for ideal devices. Cold carrier EEHs are based on the same device structure as hot carrier solar cells, but works in an opposite way. Whereas a hot carrier solar cell receives net radiation from the sun and converts some of this radiative heat flow into electricity, a cold carrier EEH sustains a net outflux of radiation to the surroundings while converting some of the energy supplied to it into electricity. It is shown that the most basic type of cold carrier EEHs have the same theoretical efficiency as the ideal emissive energy harvesters described earlier by Byrnes et al. In the present work, it is also shown that if the emission from the cold carrier EEH originates from electron transitions across an energy gap where a difference in the chemical potential of the electrons above and below the energy gap is sustained, power densities slightly higher than those given by Byrnes et al. can be achieved. [ABSTRACT FROM AUTHOR]

Among the four features unique to hot-carrier solar cells (HC-SCs): (i) carrier thermalization time and (ii) carrier equilibration time in the absorber, (iii) energy-selection width and (iv) conductance of the energy-selective contacts (ESCs), requisites of (i)-(iii) for high conversion efficiency have been clarified. We have tackled the remaining issues related to (iv) in the present study. The detailed balance model of HC-SC operation has been improved to involve a finite value of the ESC conductance to find the required values, which in turn has been revealed to be feasible using resonant tunneling diodes (RTDs) consisting of semiconductor quantum dots (QDs) and quantum wells (QWs) by means of a formulation to calculate the conductance of the QD- and QW-RTDs derived using the rigorous solutions of the effective-mass Hamiltonians. Thus, all of the four requisites unique to HC-SCs to achieve high conversion efficiency have been elucidated, and the two requisites related to the ESCs can be fulfilled using the QD- and QW-RTDs. [ABSTRACT FROM AUTHOR]

This study reports on time-of-flight (TOF) hole mobility measurements in aged 2,3,6,7,10,11-Hexakis(pentyloxy)triphenylene columnar liquid crystals. In contrast to the original samples reported in 2006, homeotropically aligned samples yielded TOF transients with an extended non-exponential rise. The experimental data were fit to a simple model that accurately reproduces the TOF transients assuming delayed charge release from traps near the optically excited electrode. While interfacial trapping appears only in the aged materials, the bulk mobility is similar to the pristine material. The model addresses dispersive transport in quasi-one-dimensional materials, determines the charge carrier mobility in systems with interfacial traps, and provides a method for characterizing the traps. [ABSTRACT FROM AUTHOR]

A range of ion beam techniques have been used to fabricate a variety of photonic guiding structures in the well-known lithium niobate (LiNbO3 or LN) crystals that are of great importance in integrated photonics/optics. This paper reviews the up-to-date research progress of ion-beam-processed LiNbO3 photonic structures and reports on their fabrication, characterization, and applications. Ion beams are being used with this material in a wide range of techniques, as exemplified by the following examples. Ion beam milling/etching can remove the selected surface regions of LiNbO3 crystals via the sputtering effects. Ion implantation and swift ion irradiation can form optical waveguide structures by modifying the surface refractive indices of the LiNbO3 wafers. Crystal ion slicing has been used to obtain bulk-quality LiNbO3 single-crystalline thin films or membranes by exfoliating the implanted layer from the original substrate. Focused ion beams can either generate small structures of micron or submicron dimensions, to realize photonic bandgap crystals in LiNbO3, or directly write surface waveguides or other guiding devices in the crystal. Ion beam-enhanced etching has been extensively applied for micro- or nanostructuring of LiNbO3 surfaces. Methods developed to fabricate a range of photonic guiding structures in LiNbO3 are introduced. Modifications of LiNbO3 through the use of various energetic ion beams, including changes in refractive index and properties related to the photonic guiding structures as well as to the materials (i.e., electro-optic, nonlinear optic, luminescent, and photorefractive features), are overviewed in detail. The application of these LiNbO3 photonic guiding structures in both micro- and nanophotonics are briefly summarized. [ABSTRACT FROM AUTHOR]

The equation describing the capacitance of capacitors is determined. It is shown that by optimizing the material of the conducting electrodes, the capacitance of capacitors reaching the quantum regime can be substantially enhanced or reduced. Dielectric capacitors with negative total capacitances are suggested and their properties analyzed. Resulting perspectives to enhance the performance of electronic devices are discussed. [ABSTRACT FROM AUTHOR]

We theoretically investigated the features of hot carrier solar cells, from which photogenerated carriers are extracted before they are completely thermalized. There are three channels of energy dissipation from photogenerated carriers that lowers the conversion efficiency: thermalization in the absorber, emission from the absorber, and thermodynamically unavoidable heat flux to the ambient. The emission increases with increasing carrier density in the absorber, whereas the heat flux decreases. Previous calculations of the conversion efficiency have been carried out under the supposition of no thermalization of carriers. In this case, the dominant process of energy dissipation is the emission, like conventional solar cells represented by the Shockley and Queisser formula. In practice, the carriers should be extracted to external circuits immediately after photogeneration because they are partially thermalized. This restriction leads to a much smaller carrier density and consequently more significant energy dissipation by heat flux, whereas the influence of the emission is negligible. As a result, the conversion efficiency is considerably lower than the values under the supposition of no thermalization. To suppress the heat flux to improve conversion efficiency, a smaller effective electron mass and a higher carrier temperature are required, as well as more intense irradiation. When the effective electron mass is much smaller than that of holes, the thermalization of holes has little influence on lowering the conversion efficiency. [ABSTRACT FROM AUTHOR]

Nd,Tm:YAG codoped single crystal waveguides were studied in order to discover if the presence of Nd3+ ions favors blue luminescence at 486 nm. Innovative implantation techniques were applied to locally change Δn and form varied H+ implanted channel structures in Nd,Tm:YAG buried epitaxial waveguiding layers. The guided blue luminescence due to the Tm3+ 1G4→3H6 transition was studied under infrared excitation at 785 nm (Tm3+ absorption) and 808 nm (Nd3+ absorption) for the epitaxial planar waveguides of different Tm3+ and Nd3+ concentrations for all the implanted channel waveguide structures. [ABSTRACT FROM AUTHOR]

The observed Dy3+ ion upconversion luminescence in LiNbO3 crystal heavily codoped with ZnO was spectrally and temporally analyzed by microsecond time-resolved spectrum under 806 nm intense femtosecond laser excitation at room temperature. Absorption spectrum and modified Judd-Ofelt approach were used to investigate its spectroscopic properties. The bright blue and intense yellow emissions are assigned to transitions 4F9/2→6H15/2 and 4F9/2→6H13/2, respectively. It is concluded that the main upconversion mechanism is excited state absorption by pump power dependence in combination with luminescence intensity temporal evolvements. [ABSTRACT FROM AUTHOR]

In n-type diamond doped with phosphorus, exciton properties have been investigated by cathodoluminescence as a function of the phosphorus concentration and the temperature. The homoepitaxial diamond layers were grown by microwave plasma-assisted chemical vapor deposition and doped using a liquid organic precursor of phosphorus (tertiarybutylphosphine). The phosphorus concentration ranges from 5.2×1016 to 3.3×1018 cm-3 as measured by secondary ion mass spectrometry. It is shown that the ratio between the luminescence intensities of the neutral phosphorus-bound exciton and the free exciton follows the dopant concentration. Calibration graphs are presented to determine the phosphorus contents in diamond using cathodoluminescence spectroscopy. [ABSTRACT FROM AUTHOR]

A simple theoretical model is proposed for a description of light propagation in ion-implanted leaky waveguides. It permits interpretation of existing published experimental results. A single parameter, the imaginary component of the refractive index, is sufficient to describe losses in the barrier. Simple analytical expressions are obtained for both the lateral resonance condition and the confinement coefficient. For each quasiguided mode, we calculate an optimal barrier thickness which gives the lowest attenuation coefficient. Interestingly, for thick barriers, high-order modes give the best confinement coefficient. Our approach can also describe prism-loaded slab type waveguides. [ABSTRACT FROM AUTHOR]

Describes the synthesis, characterization, and application of several different Er-doped thin film photonic materials. Focus on oxide glasses, ceramic thin films, and amorphous and crystalline silicon; MeV ion implantation as a technique suited for doping of the materials; Role of implantation defects, effect of annealing, concentration dependent effects, and optical activation.

Focuses on a study which described the photoluminescence spectra of Al[subx]Ga[sub1-x]As alloys. Discussion on photoluminescence as a characterization technique; Interband transitions and excitonic recombinations; Lineshape broadening effects.

Presents a study that examined the liquid phase epitaxy of gallium-arsenic from bismuth (Bi) melt. Use of photoluminescence; Dependence of carrier concentration determined by Hall effect on tin concentration in Bi melt; Information on the low temperature photoluminescence spectrum of a high quality epitaxial layer.

Studies epitaxial waveguides of high refractive index aluminum garnets. Overview of previous studies on garnet waveguides; Description of procedures specific to the epitaxy of aluminum garnets; Characterization of garnet waveguides.

Presents a study that observed an extra strange mode in the extraordinary index mode spectrum of certain optical waveguides produced by helium ion implantation in lithium niobate waveguides. Information on the extra strange mode; Discussion; Conclusion.

Focuses on a study which analyzed the effect of electric fields on time-resolved photoluminescence (TRPL) in semiconductors following excitation by a short laser pulse. Description of the semiconductor structure used in the calculations; Illustration of normalized photoluminescence decay curves calculated for three different initial injection levels; Discussion on the behavior of TRPL spectra in the presence of electric fields.

We report measurements of chemical sputtering yields of graphite exposed to low temperature nitrogen plasmas. The influence of surface temperature and incoming ion energy on the sputtering yields has been investigated in two distinct ion flux density regimes. Sputtering yields grow consistently with increasing temperatures in experiments with low flux density (Γi≈1020 m-2s-1-1021 m-2s-1) and high flux density (Γi≈1023 m-2s-1). Moreover, empirical fitting of the data suggests that the temperature of 670 °C is optimal for chemical sputtering at high flux density. Negative biasing of the samples was used to vary the ion energy in the low flux density regime. The sputtering yield in this case increases from 0.07 atoms/ion for Ei = 1.5 eV to 0.19 atoms/ion for Ei = 35 eV. After taking into account the dependence of the yields on temperature and ion energy, we evidenced a flux dependence of sputtering, similar to that found for chemical sputtering of carbon by hydrogen. [ABSTRACT FROM AUTHOR]

In this study, the phonon dispersion of a wurtzite InN-GaN multiple quantum well superlattice was produced using a modified adiabatic bond charge model. Bulk binary compound parameters obtained from ab initio calculations were used as input parameters for the modified bond charge model. The phononic bandgap, visualization of the vibrational modes, specific heat, and thermal conductivity in the quantum confined direction for the superlattice material are reported. The size of the phononic bandgap is predicted to be between that of the two bulk materials in all symmetry directions and insufficient to prevent first order phonon decay. Surprisingly, the A1-like mode has a greater vibration frequency than the E1-like mode, which is opposite to what was believed in the bulk counterparts. The specific heat of the superlattice is similar to that of GaN, despite similar volumetric fractions of the materials. The low thermal conductivity of InN limits the thermal conduction in the confined direction. [ABSTRACT FROM AUTHOR]

An electro-static model has been formalized to analyze the effect of carrier screening on the energy relaxation of electrons in polar-semiconductor multiple-quantum-well superlattices. The screened polar potential in this structure has been obtained in terms of its 0th order and 1st order Fourier components. The 1st order correction gives a further reduction of the coupling strength due to the uneven spatial distribution of carriers, and it heavily depends on the relative bi-layer thickness compared to the Debye screening length. The reductions of electron energy relaxation in In0.25GaAs/GaAsP0.33 multiple-quantum-well superlattices have been numerically analyzed yielding conclusions which agree with the recent experiments. The model provides a quantified approach to optimize the structure configuration in terms of the carrier cooling rate, allowing the multiple-quantum-well superlattice to be an ideal absorber candidate for realizing the hot carrier solar cell. [ABSTRACT FROM AUTHOR]

The effects of crystallite size, surface structure, and dopants on the magnetic properties of semiconducting oxides are highly controversial. In this work, Fe:SnO2 nanoparticles were prepared by four wet-chemical methods, with Fe concentration varying from 0% to 20%. Analysis confirmed pure single-phase cassiterite with a crystallite size of 2.6 ± 0.1 nm that decreased with increasing. Fe% doped substitutionally as Fe3+. Pure SnO2 showed highly reproducible weak magnetization that varied significantly with synthesis method. Interestingly, doping SnO2 with Fe < 2.5% produced enhanced magnetic moments in all syntheses; the maximum of 1.6 × 10-4 µB/Fe ion at 0.1% Fe doping was much larger than the 2.6 × 10-6 µB/Fe ion of pure Fe oxide nanoparticles synthesized under similar conditions. At Fe ≥ 2.5%, the magnetic moment was significantly reduced. This work shows that (1) pure SnO2 can produce an intrinsic ferromagnetic behavior that varies with differences in surface structure, (2) very low Fe doping results in high magnetic moments, (3) higher Fe doping reduces magnetic moment and destroys ferromagnetism, and (4) there is an interesting correlation between changes in magnetic moment, bandgap, and lattice parameters. These results support the possibility that the observed ferromagnetism in SnO2 might be influenced by modification of the electronic structure by dopant, size, and surface structure. [ABSTRACT FROM AUTHOR]

We report the unusual growth of pure and Fe-doped ZnO nanoparticles prepared by forced hydrolysis and the weakening of ferromagnetism due to aging in ambient conditions. More than four dozen nanoparticle samples in the size range of 4-20 nm were studied over 1 to 4 years. The as-prepared samples had significant changes in their crystallite sizes and magnetization as they aged in ambient conditions. Detailed studies using x ray diffraction and transmission electron microscopy (TEM) demonstrated that the crystallite size increased by as much as 1.4 times. Lattice parameters and strain also showed interesting changes. Magnetometry studies of Zn1-xFexO with x = 0-0.2 showed ferromagnetism at room temperature; however, keeping the samples in ambient conditions for one year resulted in modifications in the crystallite size and magnetization. For the Zn0.95Fe0.05O sample, the size changed from 7.9 nm to 9.0 nm, while the magnetization decreased from 1×10-3emu/g (memu/g) to 0.2 memu/g. Both magnetic and structural changes due to aging varied with the environment in which they were stored, indicating that these changes are related to the aging conditions. [ABSTRACT FROM AUTHOR]