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Resonantly excited high-density exciton gas studiedvia broadbandTHz spectroscopy

Description: We report the density-dependent crossover of a resonantly photoexcited exciton gas from insulating to conducting phases. Broadband terahertz spectroscopy gives direct access to the exciton binding energy via intra-excitonic 1s-2p transitions. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with decreasing inter-particle distance. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite free-carrier and exciton terahertz conductivity. Comparison with near-infrared absorption changes illustrates a significantly enhanced energy shift and broadening of the intra-excitonic resonance.
Date: June 25, 2005
Creator: Huber, Rupert; Kaindl, Robert A.; Schmid, Ben A. & Chemla, Daniel S.
Partner: UNT Libraries Government Documents Department

Ionization of Water Clusters is Mediated by Exciton Energy Transfer from Argon Clusters

Description: The exciton energy deposited in an argon cluster, (Arn ,< n=20>) using VUV radiation is transferred to softly ionize doped water clusters, ((H2O)n, n=1-9) leading to the formation of non-fragmented clusters. Following the initial excitation, electronic energy is channeled to ionize the doped water cluster while evaporating the Ar shell, allowing identification of fragmented and complete water cluster ions. Examination of the photoionization efficiency curve shows that cluster evaporation from excitons located above 12.6 eV are not enough to cool the energized water cluster ion, and leads to their dissociation to (H2O)n-2H+ (protonated) clusters.
Date: January 25, 2012
Creator: Golan, Amir & Ahmed, Musahid
Partner: UNT Libraries Government Documents Department

Femtosecond THz Studies of Intra-Excitonic Transitions

Description: Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu2O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10-14 cm2. Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.
Date: October 2, 2007
Creator: Huber, Rupert; Schmid, Ben A.; Kaindl, Robert A. & Chemla, Daniel S.
Partner: UNT Libraries Government Documents Department

Effects of chemical modifications on photophysics and exciton dynamics on {pi}-conjugation attenuated and metal-chelated photoconducting polymers

Description: Effects of two types of chemical modifications on photoconducting polymers consisting of polyphenylenevinylene (PPV) derivatives are studied by static and ultrafast transient optical spectroscopy as well as semi-empirical ZINDO calculations. The first type of modification inserts 2,2{prime}-bipyridyl-5-vinylene units (bpy V) in the PPV backbone, and the second type involves metal-chelation with the bpy sites. Photoluminescence and exciton dynamics of polymers 1 and 2 with PV:bpyV ratios of 1 and 3 were examined in solution, and compared to those of the homopolymer, poly(2,5-bis(2{prime}-ethylhexyloxy)-1,4-phenylenevinylene) (BEH-PPV). Similar studies were carried out for several metal-chelated polymers. These results can be explained by changes in {pi}-conjugation throughout the polymer backbone. The attenuation in {pi}-conjugation by the chemical modifications transforms a conducting polymer from one-dimensional semiconductor to molecular aggregates.
Date: March 11, 2000
Creator: Chen, L. X.; Jager, W. J. H.; Gosztola, D. J.; Niemczyk, M. P. & Wasielewski, M. R.
Partner: UNT Libraries Government Documents Department

Energy Transfer of Excitons Between Quantum Wells Separated by a Wide Barrier

Description: We present a microscopic theory of the excitonic Stokes and anti-Stokes energy transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch ({Delta}) at low temperatures (T). Exciton transfer through dipolar coupling, photon-exchange coupling and over-barrier ionization of the excitons through exciton-exciton Auger processes are examined. The energy transfer rate is calculated as a function of T and the center-to-center distance d between the two wells. The rates depend sensitively on T for plane-wave excitons. For located excitons, the rates depend on T only through the T-dependence of the localization radius.
Date: December 6, 1999
Partner: UNT Libraries Government Documents Department

Nature of room-temperature photoluminescence in ZnO

Description: The temperature dependence of the photoluminescence (PL) transitions associated with various excitons and their phonon replicas in high-purity bulk ZnO has been studied at temperatures from 12 K to above room temperature (320 K). Several strong PL emission lines associated with LO phonon replicas of free and bound excitons are clearly observed. The room temperature PL spectrum is dominated by the phonon replicas of the free exciton transition with the maximum at the first LO phonon replica. The results explain the discrepancy between the transition energy of free exciton determined by reflection measurement and the peak position obtained by the PL measurement.
Date: November 11, 2004
Creator: Shan, W.; Walukiewicz, W.; Ager III, J.W.; Yu, K.M.; Yuan, H.B.; Xin, H.P. et al.
Partner: UNT Libraries Government Documents Department

Stimulated Terahertz Emission from Intra-Excitonic Transitions inCu2O

Description: We report the first observation of stimulated emission of terahertz radiation from internal transitions of excitons. The far-infrared electromagnetic response of Cu{sub 2}O is monitored via broadband terahertz pulses after ultrafast resonant excitation of three-dimensional 3p excitons. Stimulated emission from the 3p to the energetically lower 2s bound level occurs at a photon energy of 6.6 meV, with a cross section of {approx} 10{sup 14} cm{sup 2}. Simultaneous excitation of both exciton levels, in turn, drives quantum beats which lead to efficient terahertz emission sharply peaked at the difference frequency.
Date: June 16, 2005
Creator: Huber, Rupert; Schmid, Ben A.; Shen, Y. Ron; Chemla, Daniel S. & Kaindl, Robert A.
Partner: UNT Libraries Government Documents Department

Simple model relating recombination rates and non-proportional light yield in scintillators

Description: We present a phenomenological approach to derive an approximate expression for the local light yield along a track as a function of the rate constants of different kinetic orders of radiative and quenching processes for excitons and electron-hole pairs excited by an incident {gamma}-ray in a scintillating crystal. For excitons, the radiative and quenching processes considered are linear and binary, and for electron-hole pairs a ternary (Auger type) quenching process is also taken into account. The local light yield (Y{sub L}) in photons per MeV is plotted as a function of the deposited energy, -dE/dx (keV/cm) at any point x along the track length. This model formulation achieves a certain simplicity by using two coupled rate equations. We discuss the approximations that are involved. There are a sufficient number of parameters in this model to fit local light yield profiles needed for qualitative comparison with experiment.
Date: September 24, 2008
Creator: Moses, William W.; Bizarri, Gregory; Singh, Jai; Vasil'ev, Andrey N. & Williams, Richard T.
Partner: UNT Libraries Government Documents Department

Magneto-Excitons in (411)A and (100)-Oriented GaAs/AlGaAs Multiple Quantum Well Structures

Description: We report magneto-exciton spectroscopy studies of (411)A and (100)-oriented GaAs/Al{sub 0.3}Ga{sub 0.7}As multiquantum well structures. The samples consisted of seven GaAs quantum wells with widths varying between 0.6 and 12nm, were grown on (411)A and (100)-oriented GaAs substrates. The exciton diamagnetic energy shifts and linewidths were measured between 0 and 14T at 1.4K The dependence of the exciton diamagnetic shifts with magnetic field were calculated using a variational approach and good agreement with experiment for both substrate orientations was found.
Date: January 20, 1999
Creator: Bajaj, K.K.; Hiyamizu, S.; Jones, E.D.; Krivorotov, I.; Shimomura, S. & Shinohara, K.
Partner: UNT Libraries Government Documents Department

Microscopic Theory of Optical Nonlinearities and Spontaneous Emission Lifetime in Group-III Nitride Quantum Wells

Description: Microscopic calculations of the absorption/gain and luminescence spectra are presented for wide bandgap Ga{sub 1{minus}x}In{sub x}N/GaN quantum well systems. Whereas structures with narrow well widths exhibit the usual excitation dependent bleaching of the exciton resonance without shifting spectral position, a significant blue shift of the exciton peak is obtained for wider quantum wells. This blue shift, which is also present in the excitation dependent luminescence spectra, is attributed to the interplay between the screening of a strain induced piezoelectric field and the density dependence of many-body Coulomb effects. The calculations also show an over two orders of magnitude increase in the spontaneous electron-hole-pair lifetime with well width: due to the reduction of the electron-hole wavefunction overlap in the wider wells. The resulting decrease in spontaneous emission loss is predicted to lead to improved threshold properties in wide quantum well lasers.
Date: March 16, 1999
Creator: Chow, W.; Kira, M. & Koch, S.W.
Partner: UNT Libraries Government Documents Department

Crossing behavior of the singlet and triplet State of the negatively charged magneto-exciton in a GaAs/AlGaAs quantum well

Description: Polarized magneto-photoluminescence (MPL) measurements on a high mobility {delta}-doped GaAs/AlGaAs single quantum well from 0--60 T at temperatures between 0.37--2.1 K are reported. In addition to the neutral heavy hole magneto-exciton (X{sup 0}), the singlet (X {sub s}{sup {minus}}) and triplet (X {sub t}{sup {minus}}) states of the negatively charged magneto-exciton are observed in both polarizations. The energy dispersive and time-resolved MPL data suggest that their development is fundamentally related to the formation of the neutral magneto-exciton. At a magnetic field of 40 T the singlet and the triplet states cross as a result of the role played by the higher Landau levels and higher energy subbands in their energetic evolution, confirming theoretical predictions. The authors also observed the formation of two higher energy peaks. One of them is completely right circularly polarized and its appearance can be considered a result of the electron-hole exchange interaction enhancement with an associated electron g-factor of 3.7 times the bulk value. The other peak completely dominates the MPL spectrum at fields around 30 T. Its behavior with magnetic field and temperature indicates that it may be related to previous anomalies observed in the integer and fractional quantum Hall regimes.
Date: January 27, 2000
Partner: UNT Libraries Government Documents Department

Magnetic Field Induced Charged Exciton Studies in a GaAs/Al(0.3)Ga(0.7)As Single Heterojunction

Description: The magnetophotoluminescence (MPL) behavior of a GaAs/Al<sub>0.3</sub>Ga<sub>0.7</sub>As single heterojunction has been investigated to 60T. We observed negatively charged singlet and triplet exciton states that are formed at high magnetic fields beyond the {nu}=l quantum Hall state. The variation of the charged exciton binding energies are in good agreement with theoretical predictions. The MPL transition intensities for these states showed intensity variations (maxima and minima) at the {nu}=l/3 and 1/5 fractional quantum Hall (FQH) state as a consequence of a large reduction of electron-hole screening at these filling factors.
Date: May 25, 1999
Creator: Kim, Yongmin; Munteanu, F.M.; Perry, C.H.; Reno, J.L.; Rickel, D.G. & Simmons, J.A.
Partner: UNT Libraries Government Documents Department

Spectral ellipsometry of GaSb: Experiment and modelling

Description: The optical constants {epsilon}(E)[{equals}{epsilon}{sub 1}(E) + i{epsilon}{sub 2}(E)] of single crystal GaSb at 300K have been measured using spectral ellipsometry in the range of 0.3--5.3 eV. The {epsilon}(E) spectra displayed distinct structures associated with critical points (CPs) at E{sub 0}(direct gap), spin-orbit split E{sub 0} + {Delta}{sub 0} component, spin-orbit split (E{sub 1}), E{sub 1} + {Delta}{sub 1} and (E{sub 0}{prime}), E{sub 0}{prime} + {Delta}{sub 0}{prime} doublets, as well as E{sub 2}. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys.Rev.B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs plus excitonic and band-to-band Coulomb enhancement effects at E{sub 0}, E{sub 0} + {Delta}{sub 0}and the E{sub 1}, E{sub 1} + {Delta}{sub 1} doublet. In addition to evaluating the energies of these various band-to-band CPs, information about the binding energy (R{sub 1}) of the two-dimensional exciton related to the E{sub 1}, E{sub 1} + {Delta}{sub 1} CPS was obtained. The value of R{sub 1} was in good agreement with effective mass/{rvec k} {center_dot} {rvec p} theory. The ability to evaluate R{sub 1} has important ramifications for recent first-principles band structure calculations which include exciton effects at E{sub 0}, E{sub 1}, and E{sub 2}.
Date: May 1, 1999
Creator: Charache, G.W.; Mu {tilde n}oz, M.; Wei, K.; Pollak, F.H. & Freeouf, J.L.
Partner: UNT Libraries Government Documents Department

Confined excitons, phonons and their interactions in Ge nanocrystals embedded in SiO2

Description: The authors report the resonant Raman scattering of the optical phonon in Ge nanocrystals with radius ranging from 2 to 5 nm. They have observed the effect of quantum confinement on both the optical phonon and the E{sub 1} exciton. The confinement energy of the E{sub 1} exciton has been explained within the effective mass approximation.
Date: December 31, 2000
Creator: Teo, K.L.; Kwok, S.H. & Yu, P.Y.
Partner: UNT Libraries Government Documents Department

Exciton and biexciton signatures in the femtosecond transient absorption of phenylene-based polymers and oligomers

Description: The femtosecond transient absorption of phenylene-based polymers at low intensities is dominated by singlet excitons. In films, inter-chain excitons are created at high intensities via two-exciton states. In solution, the authors observe signatures of stable biexcitons.
Date: March 1, 1998
Creator: McBranch, D.; Klimov, V. & Kraabel, B.
Partner: UNT Libraries Government Documents Department

Effect of magnetic confinement on manybody nonlinearities of Lorentzian excitons

Description: 100-fs time- and frequency-resolved four-wave mixing in GaAs at T=1.6K under magnetic field, 0{<=}|{rvec B}|{<=}12T, is used for investigating how magnetic confinement modifies the processes that govern the manybody nonlinearities of the Lorentzian excitons.
Date: November 1, 1995
Creator: Kner, P.; Bar-Ad, S.; Marquezini, M.V. & Chemla, D.S.
Partner: UNT Libraries Government Documents Department

Tailorable, Visible Light Emission From Silicon Nanocrystals

Description: J. P. Wilcoxon and G. A. Samara Crystalline, size-selected Si nanocrystals in the size range 1.8-10 nm grown in inverse micellar cages exhibit highly structured optical absorption and photoluminescence (PL) across the visible range of the spectrum. The most intense PL for the smallest nanocrystals produced This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. to induce a useful level of visible photoluminescence (PL) from silicon (Si). The approaches understood. Visible PL has been observed from Si nanocrystals, or quantum dots, produced by a variety of techniques including aerosols,2 colloids,3 and ion implantation.4 However, all of The optical absorption spectra of our nanocrystals are much richer in spectral features spectrum of bulk Si where the spectral features reflect the details of the band structure shown in nanocrystals estimated to have a Si core diameter of 1-2 nm. These measured quantum those in the spectrum of bulk Si in Fig. 1 are striking indicating that nanocrystals of this size 8-Room temperature PL results on an HPLC size-selected, purified 2 nm nanocrystals but blue shifted by -0.4 eV due to ...
Date: July 20, 1999
Creator: Samara, G.A. & Wilcoxon, J.P.
Partner: UNT Libraries Government Documents Department

Defects and morphological concerns in electroluminescent polymers

Description: The degradation of luminescence in phenylenevinylene polymers is due to exciton diffusion to quenching defects. The microscopic structure of these defects is identified by in-situ vibrational spectroscopy. The authors present evidence that the defect quenching is due to charge transfer by studies on model phenylenevinylene oligomer. In the absence of defect quenchers, the authors have achieved nearly exponential photoluminescence decay with observed lifetimes > 1 ns and a fourfold increase in electroluminescence. They have also utilized picosecond laser spectroscopy to study the formation yield of emissive excitons in the polymer PPVs with different morphology. They have found that increasing polymer chain separation would greatly increases the luminescent efficiency due to avoiding the interchain excitons (exciplexes). Clarification of the nature of photophysics of conjugated polymers suggests avenues for improvement in fabrication of emissive polymers and electroluminescent polymers devices.
Date: July 24, 1995
Creator: Yan, M.; Rothberg, L. & Galvin, M.E.
Partner: UNT Libraries Government Documents Department

UV Crosslinkable Polythiophene for Nano-imprinting and Photolithography toward Ordered Bulk Heterojunction in Organic Photovoltaics

Description: Exciton travel distance in organic material is on the order of 10-20 nm, thus the morphology of the organic active layer is critical to achieve high performance in OPVs. An ordered bulk heterojunction (BHJ) morphology with phase separation on the order of 10-20 nm will collect all excitons at the interface and give uninterrupted paths to all separated charges to reach the corresponding electrodes.
Date: July 25, 2010
Creator: Yang, Q.; Hlaing, H.; Ocko, B.; Black, C. & Grubbs, R.B.
Partner: UNT Libraries Government Documents Department

Excitations, optical absorption spectra, and optical excitonic gaps of heterofullerenes: I. C60, C59N+ and C48N12

Description: Low-energy excitations and optical absorption spectrum of C{sub 60} are computed by using time-dependent (TD) Hartree-Fock (HF), TD-density functional theory (TD-DFT), TD-DFT-based tight-binding (TD-DFT-TB) and a semiempirical ZINDO method. A detailed comparison of experiment and theory for the excitation energies, optical gap and absorption spectrum of C{sub 60} is presented. It is found that electron correlations and collective effects of exciton pairs play important roles in assigning accurately the spectral features of C{sub 60} and the TD-DFT method with non-hybrid functionals or a local spin density approximation leads to more accurate excitation energies than with hybrid functionals. The level of agreement between theory and experiment for C{sub 60} justifies similar calculations of the excitations and optical absorption spectrum of a monomeric azafullerene cation C{sub 59}N{sup +} exhibits distinguishing spectral features different from C{sub 60}: (1) the first singlet is dipole-allowed and the optical gap is redshifted by 1.44 eV; (2) several weaker absorption maxima occur in the visible region; (3) the transient triplet-triplet absorption at 1.60 eV (775 nm) is much broader and the decay of the triplet state is much faster. The calculated spectra of C{sub 59}N{sup +} characterize and explain well our measured ultraviolet-visible (UV-vis) and transient absorption spectra of the carborane anion salt [C{sub 59}N][Ag(CB{sub 11}H{sub 6}Cl{sub 6}){sub 2}]. For the most stable isomer of C{sub 48}N{sub 12}, we predict that the first singlet is dipole-allowed, the optical gap is redshifted by 1.22 eV relative to that of C{sub 60}, and optical absorption maxima occur at 585, 528, 443, 363, 340, 314 and 303 nm. We point out that the characterization of the UV-vis and transient absorption spectra of C{sub 48}N{sub 12} isomers is helpful in distinguishing the isomer structures required for applications in molecular electronics. For C{sub 59}N{sup +} and C{sub 48}N{sub 12} as well as ...
Date: October 2, 2003
Creator: Xie, R; Bryant, G W; Sun, G; C. Nicklaus, M; Heringer, D; Frauenheim, T et al.
Partner: UNT Libraries Government Documents Department

Active infrared materials for beam steering.

Description: The mid-infrared (mid-IR, 3 {micro}m -12 {micro}m) is a highly desirable spectral range for imaging and environmental sensing. We propose to develop a new class of mid-IR devices, based on plasmonic and metamaterial concepts, that are dynamically controlled by tunable semiconductor plasma resonances. It is well known that any material resonance (phonons, excitons, electron plasma) impacts dielectric properties; our primary challenge is to implement the tuning of a semiconductor plasma resonance with a voltage bias. We have demonstrated passive tuning of both plasmonic and metamaterial structures in the mid-IR using semiconductors plasmas. In the mid-IR, semiconductor carrier densities on the order of 5E17cm{sup -3} to 2E18cm{sup -3} are desirable for tuning effects. Gate control of carrier densities at the high end of this range is at or near the limit of what has been demonstrated in literature for transistor style devices. Combined with the fact that we are exploiting the optical properties of the device layers, rather than electrical, we are entering into interesting territory that has not been significantly explored to date.
Date: October 1, 2010
Creator: Brener, Igal; Reno, John Louis; Passmore, Brandon Scott; Gin, Aaron V.; Shaner, Eric Arthur; Miao, Xiaoyu et al.
Partner: UNT Libraries Government Documents Department