arXiv subjects

D. L. Smith

Publications and source records attributed to D. L. Smith.

At least 19 recordsLinked to original sources

arXiv2026

Modeling Light Propagation and Amplification Efficiency in Highly Multimode, Yb-doped Fiber Amplifiers

Multimode fibers have been proposed for mitigating nonlinear effects in high-power fiber amplifiers, allowing for significant power scaling. Most previous studies on light propagation in continuous-wave fiber amplifiers focus on single mode or few mode fibers. Here we develop a tractable numerical model to simulate light propagation in narrowband, highly multimode fiber amplifiers, which takes into account gain saturation, pump depletion and mode-dependent gain. We consider a frequency domain, field based model, with modal gain being dependent on both intramodal gain and gain-induced mode coupling. We derive coupled equations for the evolution of signal modal amplitudes, pump power and population inversion, and numerically solve these equations using a finite-difference method. For highly multimode excitations, the optical intensity in the fiber is speckled and various modes grow at different rates, due to differential overlap with the gain medium and spatial hole burning. Our analysis is applied to Yb-doped fibers, with a quasi-quantitative analysis of the specific case of Yb, identifying different regimes in which either spontaneous emission (SE) or amplified spontaneous emission (ASE) limit amplifier efficiency, especially for larger core and multimode fibers. Finally, we incorporate ASE and spectrally resolved optical channels into our model and demonstrate the experimentally verifiable phenomenon of ASE suppression with sufficient input signal power. Our model can be combined with existing models for various nonlinear effects, providing a useful tool for quantitatively studying nonlinearity mitigation and power scaling in multimode fiber amplifiers.

physics.optics↗

arXiv2023

Harnessing the power of complex light propagation in multimode fibers for spatially resolved sensing

The propagation of coherent light in multimode optical fibers results in a speckled output that is both complex and sensitive to environmental effects. These properties can be a powerful tool for sensing, as small perturbations lead to significant changes in the output of the fiber. However, the mechanism to encode spatially resolved sensing information into the speckle pattern and the ability to extract this information is thus far unclear. In this paper, we demonstrate that spatially dependent mode coupling is crucial to achieving spatially resolved measurements. We leverage machine learning to quantitatively extract this spatially resolved sensing information from three fiber types with dramatically different characteristics and demonstrate that the fiber with the highest degree of spatially dependent mode coupling provides the greatest accuracy.

physics.optics↗

arXiv2019

Unrecognized Sources of Uncertainties (USU) in Experimental Nuclear Data

Evaluated nuclear data uncertainties are often perceived as unrealistic, most often because they are thought to be too small. The impact of this issue in applied nuclear science has been discussed widely in recent years. Commonly suggested causes are: poor estimates of specific error components, neglect of uncertainty correlations, and overlooked known error sources. However, instances have been reported where very careful, objective assessments of all known error sources have been made with realistic error magnitudes and correlations provided, yet the resulting evaluated uncertainties still appear to be inconsistent with observed scatter of predicted mean values. These discrepancies might be attributed to significant unrecognized sources of uncertainty (USU) that limit the accuracy to which these physical quantities can be determined. The objective of our work has been to develop procedures for revealing and including USU estimates in nuclear data evaluations involving experimental input data. We conclude that the presence of USU may be revealed, and estimates of magnitudes made, through quantitative analyses. This paper identifies several specific clues that can be explored by evaluators in identifying the existence of USU. It then describes numerical procedures to generate quantitative estimates of USU magnitudes. Key requirements for these procedures to be viable are that sufficient numbers of data points be available, for statistical reasons, and that additional supporting information about the measurements be provided by the experimenters. Realistic examples are described to illustrate these procedures and demonstrate their outcomes as well as limitations. Our work strongly supports the view that USU is an important issue in nuclear data evaluation, with significant consequences for applications, and that this topic warrants further investigation by the nuclear science community.

physics.data-an↗

arXiv2014

Spectrally-resolved hyperfine interactions between polaron and nuclear spins in organic light emitting diodes: Magneto-EL studies

We use spectrally-resolved magneto-electroluminescence (EL) measurements to study the energy dependence of hyperfine interactions between polaron and nuclear spins in organic LEDs. Using layered devices based on Bphen/MTDATA -- a well-known exciplex emitter -- we show that the increase in EL emission intensity $I$ due to small applied magnetic fields of order 100 mT is markedly larger at the high-energy blue end of the EL spectrum (dI/I ~11%) than at the low-energy red end (~4%). Concurrently, the widths of the magneto-EL curves increase monotonically from blue to red, revealing an increasing hyperfine coupling between polarons and nuclei and directly providing insight into the energy-dependent spatial extent and localization of polarons.

cond-mat.mtrl-sci↗

arXiv2014

Spin and Valley Noise in Two-Dimensional Dirac Materials

We develop a theory for optical Faraday rotation noise in two-dimensional Dirac materials. In contrast to spin noise in conventional semiconductors, we find that the Faraday rotation fluctuations are influenced not only by spins but also the valley degrees of freedom attributed to intervalley scattering processes. We illustrate our theory with two-dimensional transition metal dichalcogenides and discuss signatures of spin and valley noise in the Faraday noise power spectrum. We propose optical Faraday noise spectroscopy as a technique for probing both spin and valley relaxation dynamics in two-dimensional Dirac materials.

cond-mat.mes-hall↗

arXiv2012

Role of Nuclear Quadrupole Coupling on Decoherence and Relaxation of Central Spins in Quantum Dots

Strain-induced gradients of local electric fields in semiconductor quantum dots can couple to the quadrupole moments of nuclear spins. We develop a theory describing the influence of this quadrupolar coupling (QC) on the spin correlators of electron and hole "central" spins localized in such dots. We show that when the QC strength is comparable to or larger than the hyperfine coupling strength between nuclei and the central spin, the relaxation rate of the central spin is strongly enhanced and can be exponential. We demonstrate a good agreement with recent experiments on spin relaxation in hole-doped (In,Ga)As self-assembled quantum dots.

cond-mat.mes-hall↗

arXiv2012

Intrinsic spin fluctuations reveal the dynamical response function of holes coupled to nuclear spin baths in (In,Ga)As quantum dots

The problem of how single "central" spins interact with a nuclear spin bath is essential for understanding decoherence and relaxation in many quantum systems, yet is highly nontrivial owing to the many-body couplings involved. Different models yield widely varying timescales and dynamical responses (exponential, power-law, Gaussian, etc). Here we detect the small random fluctuations of central spins in thermal equilibrium (holes in singly-charged (In,Ga)As quantum dots) to reveal the timescales and functional form of bath-induced spin relaxation. This spin noise indicates long (400 ns) spin correlation times at zero magnetic field, that increase to $\sim$5 $μ$s as hole-nuclear coupling is suppressed with small (100 G) applied fields. Concomitantly, the noise lineshape evolves from Lorentzian to power-law, indicating a crossover from exponential to inverse-log dynamics.

cond-mat.mes-hall↗

arXiv2009

"Listening" to the spin noise of conduction electrons in bulk n:GaAs

We report a comprehensive study of stochastic electron spin fluctuations -- spin noise -- in lightly doped ($n$-type) bulk GaAs, which are measured using sensitive optical magnetometry based on off-resonant Faraday rotation. Frequency spectra of electron spin noise are studied as a function of electron density, magnetic field, temperature, probe laser wavelength and intensity, and interaction volume. Electron spin lifetimes $τ_s$ are inferred from the width of the spin noise spectra, and are compared with direct measurements of $τ_s$ using conventional Hanle effect methods. Both methods reveal a strong and similar dependence of $τ_s$ on the wavelength and intensity of the probe laser, highlighting the undesired influence of sub-bandgap absorption effects on the nominally `non-perturbative' spin noise measurements. As a function of temperature, the spin noise power increases approximately linearly from 1.5 K to 30 K, as expected for degenerate electrons obeying Fermi-Dirac statistics, but with an additional zero-temperature offset. Finally, as the cross-sectional area of the probe laser shrinks and fewer electrons are probed, the measured Faraday rotation fluctuations due to electron spin noise are shown to increase, as expected.

cond-mat.mes-hall↗

arXiv2009

Spin polarization control by electric stirring: proposal for a spintronic device

We propose a spintronic device to generate spin polarization in a mesoscopic region by purely electric means. We show that the spin Hall effect in combination with the stirring effect are sufficient to induce measurable spin polarization in a closed geometry. Our device structure does not require the application of magnetic fields, external radiation or ferromagnetic leads, and can be implemented in standard semiconducting materials.

cond-mat.mes-hall↗

arXiv2009

Bias-controlled sensitivity of ferromagnet/semiconductor electrical spin detectors

Using Fe/GaAs Schottky tunnel barriers as electrical spin detectors, we show that the magnitude and sign of their spin-detection sensitivities can be widely tuned with the voltage bias applied across the Fe/GaAs interface. Experiments and theory establish that this tunability derives not just simply from the bias dependence of the tunneling conductances $G_{\uparrow,\downarrow}$ (a property of the interface), but also from the bias dependence of electric fields in the semiconductor which can dramatically enhance or suppress spin-detection sensitivities. Electrons in GaAs with fixed polarization can therefore be made to induce either positive or negative voltage changes at spin detectors, and some detector sensitivities can be enhanced over ten-fold compared to the usual case of zero-bias spin detection.

cond-mat.mtrl-sci↗

arXiv2009

Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

We present a comparative study of ultrafast photo-conversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals is activated and temperature-independent respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C$_{60}$-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.

cond-mat.other↗

arXiv2007

Effect of Intra-molecular Disorder and Inter-molecular Electronic Interactions on the Electronic Structure of Poly-p-Phenylene Vinylene (PPV)

We investigate the role of intra-molecular conformational disorder and inter-molecular electronic interactions on the electronic structure of disorder clusters of poly-p-phenylene vinylene (PPV) oligomers. Classical molecular dynamics is used to determine probable molecular geometries, and first-principle density functional theory (DFT) calculations are used to determine electronic structure. Intra-molecular and inter-molecular effects are disentangled by contrasting results for densely packed oligomer clusters with those for ensembles of isolated oligomers with the same intra-molecular geometries. We find that electron trap states are induced primarily by intra-molecular configuration disorder, while the hole trap states are generated primarily from inter-molecular electronic interactions.

cond-mat.soft↗

arXiv2006

Optical and electrical spin injection and spin transport in hybrid Fe/GaAs devices

We discuss methods for imaging the nonequilibrium spin polarization of electrons in Fe/GaAs spin transport devices. Both optically- and electrically-injected spin distributions are studied by scanning magneto-optical Kerr rotation microscopy. Related methods are used to demonstrate electrical spin detection of optically-injected spin polarized currents. Dynamical properties of spin transport are inferred from studies based on the Hanle effect, and the influence of strain on spin transport data in these devices is discussed.

cond-mat.mtrl-sci↗

arXiv2006

Bias-dependent electron spin lifetimes in n-GaAs and the role of donor impact ionization

In bulk n-GaAs epilayers doped near the metal-insulator transition, we study the evolution of electron spin lifetime $τ_s$ as a function of applied lateral electrical bias $E_x$. $τ_s$ is measured via the Hanle effect using magneto-optical Kerr rotation. At low temperatures (T<10 K, where electrons are partially localized and $τ_s > 100$ ns at zero bias), a marked collapse of $τ_s$ is observed when $E_x$ exceeds the donor impact ionization threshold at $\sim$10 V/cm. A steep increase in the concentration of warm delocalized electrons -- subject to Dyakonov-Perel spin relaxation -- accounts for the rapid collapse of $τ_s$, and strongly influences electron spin transport in this regime.

cond-mat.mes-hall↗

arXiv2005

Effects of strain, electric, and magnetic fields on lateral electron spin transport in semiconductor epilayers

We construct a spin-drift-diffusion model to describe spin-polarized electron transport in zincblende semiconductors in the presence of magnetic fields, electric fields, and off-diagonal strain. We present predictions of the model for geometries that correspond to optical spin injection from the absorption of circularly polarized light, and for geometries that correspond to electrical spin injection from ferromagnetic contacts. Starting with the Keldysh Green's function description for a system driven out of equilibrium, we construct a semiclassical kinetic theory of electron spin transport in strained semiconductors in the presence of electric and magnetic fields. From this kinetic theory we derive spin-drift-diffusion equations for the components of the spin density matrix for the specific case of spatially uniform fields and uniform electron density. We solve the spin-drift-diffusion equations numerically and compare the resulting images with scanning Kerr microscopy data of spin-polarized conduction electrons flowing laterally in bulk epilayers of n-type GaAs. The spin-drift-diffusion model accurately describes the experimental observations. We contrast the properties of electron spin precession resulting from magnetic and strain fields. Spin-strain coupling depends linearly on electron wave vector and spin-magnetic field coupling is independent of electron wave vector. As a result, spatial coherence of precessing spin flows is better maintained with strain than with magnetic fields, and the spatial period of spin precession is independent of the applied electrical bias in strained structures whereas it is strongly bias dependent for the case of applied magnetic fields.

cond-mat.mtrl-sci↗

arXiv2005

Different regimes of Forster energy transfer between an epitaxial quantum well and a proximal monolayer of semiconductor nanocrystals

We calculate the rate of non-radiative, Forster-type energy transfer (ET) from an excited epitaxial quantum well (QW) to a proximal monolayer of semiconductor nanocrystal quantum dots (QDs). Different electron-hole configurations in the QW are considered as a function of temperature and excited electron-hole density. A comparison of the theoretically determined ET rate and QW radiative recombination rate shows that, depending on the specific conditions, the ET rate is comparable to or even greater than the radiative recombination rate. Such efficient Forster ET is promising for the implementation of ET-pumped, nanocrystal QD-based light emitting devices.

cond-mat.mes-hall↗

arXiv2004

Imaging spin flows in semiconductors subject to electric, magnetic, and strain fields

Using scanning Kerr microscopy, we directly acquire two-dimensional images of spin-polarized electrons flowing laterally in bulk epilayers of n:GaAs. Optical injection provides a local dc source of polarized electrons, whose subsequent drift and/or diffusion is controlled with electric, magnetic, and - in particular - strain fields. Spin precession induced by controlled uniaxial stress along the <110> axes demonstrates the direct k-linear spin-orbit coupling of electron spin to the shear (off-diagonal) components of the strain tensor.

cond-mat.mtrl-sci↗

arXiv2004

Electron transmission through a short interacting wire: 0.7 conductance anomaly

We investigate tunneling through a short interacting wire. We identify two temperature regimes (a) $T_{Kondo}<T\le T^{wire}=\hbar v_F/k_Bd$ ($d$ is the length of the short wire) and (b) $T<T_{Kondo}\ll T^{wire}$. In the first regime the effective (renormalized) electron-electron interaction is smaller than the tunneling matrix element. This is the situation at finite temperature $T$ where the single particle spectrum of the wire is characterized by a multilevel "quantum dot" system with magnetic quantum number S=0 which is higher in energy than the SU(2) spin doublet $S=\pm1/2$. Due to the single particle energy we find that the tunneling electron into the wire must have an opposite spin to the one in the short wire giving rise to a conductance, $G= G_\uparrow+G_\downarrow$, $e^2/h\le G\ll 2e^2/h$. In the second regime, when $T \to0$ we have a situation that the effective (renormalized) electron-electron interaction is larger than the tunneling matrix element. This problem is equivalent to a Kondo problem. We find for $T<T_{Kondo}$ that the conductance is given by $G=2e^2/h$. These results are in agreement with recent experiments where for $T_{Kondo}<T<T^{wire}$ the conductance $G$ obeys $e^2/h\le G\ll 2e^2/h$, and for $T< T_{Kondo}$, $G=2e^2/h$. In both regimes the current is not polarized and the SU(2) symmetry is not broken.

cond-mat.mes-hall↗