Transient Optical Absorption Research Articles

Photoluminescence and photoconductivity in CdTe crystals doped with Bi

Defect levels in CdTe doped with Bi are studied by low temperature photoluminescence, photoinduced current transient spectroscopy, photoconductivity measurements, and optical absorption. Two centers associated with the doping with Bi are reported. The first one, a deep level located at Ev+0.71eV, only present at low dopant concentrations, has donor character and hole-trap properties, and is mainly responsible for the high resistivity and very high photoconductivity of the samples. The second one, an acceptor center located at Ev+0.30eV, assigned to BiTe species, is only present at high dopant concentrations and is mainly responsible for the low resistivity and poor photoconductivity of these samples.

Transient optical absorption in pulsed-X-ray-irradiated pure-silica-core optical fibers: Influence of self-trapped holes

We investigated pulsed-X-ray-induced attenuation in two pure-silica-core (PSC) optical fibers with low-chloride (Cl) and different hydroxyl (OH) contents. We measured at room temperature the temporal (100ms–1000s) and spectral (0.73–3.1eV) variation of the induced optical absorption after a 1MeV X-ray pulse in both low- and high-OH PSC multi-mode fiber samples. A component of the transient loss in the low-OH sample was found to comprise absorption bands at 1.63 and 1.88eV (760 and 660nm) arising from self-trapped holes (STHs). Already known to be unstable at room temperature, STHs appear to play a key role in the transient responses of low-OH/low-Cl pure-silica-core fibers in the visible/near-infrared part of the spectrum.

Electronic excitation and optical responses of metal-nanoparticle composites under heavy-ion implantation

Measurements of optical transmission and ion-induced photon emission (IIPE) during implantation of copper ions into silica glass (a-SiO2) have been combined in order to study formation of Cu nanoparticles. It has been shown that in situ measurements are more advantageous than conventional experimental approaches of examining the properties of ion-implanted nanocomposites solely after implantation. Series of experiments have been done to prove that the band of IIPE at 545–550 nm originates from Cu+ solutes in a-SiO2. The combination of in situ optical techniques provides means of monitoring Cu nanoparticles and Cu solutes via the optical absorption in the range of the surface-plasmon resonance (SPR) of nanoparticles and the IIPE of Cu+ solutes. It has been shown that a simple linear approximation can be used to separate optical bands of defects, Cu solutes, and nanoparticles. Ion-induced transient optical absorption has been found in the ranges of SPR and defect bands. The transient optical response in the range of SPR has been tentatively attributed to a thermomodulation effect. We analyze relationships between electronic excitation, radiation-induced optical responses, and kinetics of nanoparticle formation. Several stages of nanoparticle formation have been distinguished: accumulation of Cu solutes, precipitation of Cu atoms, coalescence, growth of nanoparticles, and saturation of nanocomposites.

Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

Photosystem I has two branches of cofactors down which light-driven electron transfer (ET) could potentially proceed, each consisting of a pair of chlorophylls (Chls) and a phylloquinone (PhQ). Forward ET from PhQ to the next ET cofactor (FX) is described by two kinetic components with decay times of approximately 20 and approximately 200 ns, which have been proposed to represent ET from PhQB and PhQA, respectively. Immediately preceding each quinone is a Chl (ec3), which receives a H-bond from a nearby tyrosine. To decrease the reduction potential of each of these Chls, and thus modify the relative yield of ET within the targeted branch, this H-bond was removed by conversion of each Tyr to Phe in the green alga Chlamydomonas reinhardtii. Together, transient optical absorption spectroscopy performed in vivo and transient electron paramagnetic resonance data from thylakoid membranes showed that the mutations affect the relative amplitudes, but not the lifetimes, of the two kinetic components representing ET from PhQ to F(X). The mutation near ec3A increases the fraction of the faster component at the expense of the slower component, with the opposite effect seen in the ec3B mutant. We interpret this result as a decrease in the relative use of the targeted branch. This finding suggests that in Photosystem I, unlike type II reaction centers, the relative efficiency of the two branches is extremely sensitive to the energetics of the embedded redox cofactors.

Photo-induced Metastable State of S8Clusters in Liquid Sulfur

AbstractIt is well known that liquid sulfur undergoes a polymerization transition from S8 rings to the mixture of S8 rings and polymeric chains at Tp=159°C. In addition to the transition, it has been known that there is a transformation from S8ring to a molecular species other than S8ring and polymeric chain below Tpsince 19thcentury. Such molecular species can also be obtained by the illumination of light. Several models of the molecular form have been presented. However, the conclusive model which explains all experimental results has not been settled yet. Recently we measured transient photo-induced optical absorption in liquid sulfur and Munejiri et al. performed first-principles molecular-dynamics simulations to clarify the microscopic origin of the experimental results. The results of the simulations presented a plausible molecular form as the molecular species, which is a tadpole-shaped S8 cluster. We show that the molecular form clearly explains the previous experimental results. A possibility of the formation of an intimate valence alternation pair in the small cluster is also discussed.

Optical studies of carrier and phonon dynamics in Ga1−xMnxAs

We present a time-resolved optical study of the dynamics of carriers and phonons in Ga1−xMnxAs layers for a series of Mn and hole concentrations. While band filling is the dominant effect in transient optical absorption in low-temperature-grown GaAs, induced absorption by trapped electrons become important with increasing Mn concentration in Ga1−xMnxAs, as inferred from the sign of the absorption change. We find that carrier trapping and trapped carrier recombination becomes faster as Mn concentration increases in GaMnAs. We also report direct observation on lattice vibrations in Ga1−xMnxAs layers via the reflective electro-optic sampling technique. The data show increasingly fast dephasing of longitudinal-optical phonon oscillations for samples with increasing Mn and hole concentration, which can be understood in term of phonon scattering by the holes.

Coherent and sequential contributions to femtosecond transient absorption spectra of a rhodamine dye in solution

A unified description is presented of sequential and coherent contributions to optical transient absorption measured by femtosecond pump-supercontinuum probe spectroscopy. All inherent transient terms are taken into account. The "coherence spike" seen during pump-probe overlap is thereby decomposed into its components. The method is demonstrated with rhodamine 110 in methanol. Pure homogeneous dephasing times are obtained from a simultaneous fit of all pertinent measurements. Vibronic structure in the coherence spectrum is assigned to stimulated Raman scattering between vibrational levels in the first excited electronic state. The time-zero spectrum for stimulated emission and the solvation relaxation function are also obtained.

Increase in the Mobility of Photogenerated Positive Charge Carriers in Polythiophene

We report the increase in the mobility of charge carriers in regioregular poly 3-hexyl thiophene (RR-P3HT) films by mixing them with tetracyanoethylene (TCNE), which is examined by in situ time-resolved microwave conductivity (TRMC) and transient optical spectroscopy (TOS). TCNE acts not only as an electron acceptor which increases the number of charge carriers on photoexposure but also as a functional additive which enhances the mobility of the charge carriers. This conclusion was deduced from the results of fluorescence quenching, transient optical absorption and photobleaching, and comparison of the TRMC signal with the TOS signal. The combination of the TRMC and TOS techniques represents a comprehensive and fully experimental approach to the determination of the intrinsic carrier mobility in conjugated polymers.

Solvent Adiabaticity Effects on Ultrafast Electron Transfer in Viologen Charge Transfer Complexes

Charge recombination (CR) kinetics following photoinduced charge transfer are measured by optical transient absorption for complexes of dimethyl viologen and diheptyl viologen with 4,4'-biphenol (MVBP and HVBP) in methanol. Exponential time constants for MVBP and HVBP are 480 and 790 fs, respectively. Kinetic differences cannot be rationalized with a standard equilibrium nonadiabatic rate formula using parameters obtained from linear absorption and resonance Raman measurements, which give nearly indistinguishable results for the two complexes. Solvent relaxation times and adiabaticities of MVBP are calculated using a full solvation correlation function approach. This analysis suggests that the smaller CR rate of HVBP is due to solvent reorganization differences, and is consistent with a greater adiabatic contribution for HVBP than MVBP. We conjecture that interactions between the diheptyl aliphatic groups of HVBP and the local solvent structure are responsible for the CR differences.

Recombination mechanism of luminescence excitation of self‐trapped excitons in BeO

Time-resolved spectra, polarization of the luminescence and transient optical absorption (TOA) induced by irradiation of beryllium oxide crystals with electron pulses have been studied. Exponential stages with decay time τ = 6.5 ms in luminescence bands at 4.0, 5.0 and 6.7 eV coinciding in spectral and polarized properties with the luminescence of two different types of self-trapped excitons (STE) have been found. Efficiency of formation of centers with decay time 6.5 ms appeared to be comparable with that of formation of triplet STE. The electron fluence increase does not change the exponential character and relaxation of TOA decaying with time 6.5 ms and results in a linear growth of optical density. These data are typical of monomolecular recombination processes. Similarity of TOA spectra of centers with decay time 6.5 ms with those of V-type hole centers and hole component of STE shows that the stage 6.5 ms is due to the recombination of cation Frenkel defects. The recombination of close spatially correlated Frenkel pairs Be+ V– manifests itself in a form of exponential components with decay time 6.5 ms in the luminescence of two types of STE in BeO. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitation of Self-Trapped-Exciton Luminescence in the Recombination of Frenkel Defects in BeO

Polarized luminescence and transient optical absorption (TOA) induced by pulsed electron irradiation in beryllium oxide crystals were studied. Exponential stages with decay times τ = 6.5 ms were observed to exist in luminescence bands at 4.0, 5.0, and 6.7 eV, which coincide in spectral composition and polarization characteristics with the luminescence of self-trapped excitons (STEs) of two types. The formation efficiency of centers with a 6.5-ms decay time is comparable to that of triplet STEs. The general characteristics of the kinetics and the decay times of the TOA of these centers do not depend on electron fluence and are governed by the monomolecular recombination process. The spectra of TOA centers with a decay time of 6.5 ms were found to be similar to those of V-type hole centers and STE hole components. The mechanism by which recombination of closely spaced, spatially correlated Frenkel pairs, Be+ and V− centers, brings about an exponential component with a 6.5-ms decay time in the luminescence of STEs of two types in BeO is discussed.

Characterization of Nonlinear Optical Properties of Silver Oxide Super Resolution Near-Field Structures by Z-Scan Measurements

Femtosecond laser Z-Scan measurements are used to characterize the nonlinear optical properties of AgOx super resolution near-field structures (super-RENS). Open aperture Z-Scan measurements reveal a complex transient nonlinear optical absorption response of AgOx super-RENS under intensive laser irradiation. This is attributed to the thermal decomposition of AgOx into Ag nanoparticles in the near-field optical mask layer. A bi-compositional model is proposed to explain this complex nonlinear optical response. The threshold laser peak intensity for the phase transition and the nonlinear optical absorption coefficient of an Ag nanoparticle are determined to be Ith∼0.3 GW/cm2 and β=(-5.6±0.5)×103 cm/GW at λ=800 nm, respectively.

Oxygen lone-pair states near the valence band edge of aluminum oxide thin films

Deep level transient spectroscopy and optical absorption spectroscopy measurement revealed three outstanding features of density-of-states (DOS) appeared above the valence band edge (Ev) of Al oxide thin films. The broad peak located at 0.39 eV above Ev disappears while the other two located at 1.0 and 1.3 eV shift in position and attenuate in intensity upon annealing at 200 °C. The latter two peaks are removed by annealing at temperature up to 400 °C. The observed midgap DOS feature dynamics is in accordance with the oxygen lone pair features as confirmed earlier with Raman spectroscopy in the low-frequency ranges [C. Q. Sun et al., J. Appl. Phys. 90, 2615 (2001)] and thermal desorption measurements of other oxide surfaces.

Radiation-induced processes and defects in alkali and alkaline-earth borate crystals

The paper presents the results of a study of the radiation-induced processes and defects in nonlinear optical crystals Li2B4O7 (LTB), LiB3O5 (LBO), CsLiB6O10, KB5O8·4H2O, β-BaB2O4. It was revealed that a pulsed electron beam irradiation at 290K forms the radiation-induced pairs of the ‘vacancy—interstitial atom’ defects in the cation sublattice of these crystals. This gives rise to a creation of metastable electronic (interstitial atom) and hole (small-radius polaron near the cation vacancy) centers in high concentrations. Optical hole-transitions from the local level of the trapped hole centers to the valence band states are responsible for the transient optical absorptions (TOA) of borates in the visible and UV spectral ranges. A sublattice of the weakly bound mobile lithium cations in LTB and LBO favors a spatial separation of the radiation-induced pair defects ‘hole polaron near Li-vacancy—mobile interstitial Li0 atom’. Their decay rated by the electron–hole nonradiative tunnel recombination determines a peculiar feature of the TOA decay kinetics in LTB and LBO.

Exciplex formation of copper(II) octaethylporphyrin revealed by pulsed X-rays

Abstract The triplet excited structures of Cu(II) octaethylporphyrin (CuOEP) in toluene and in 1:1 mixture of toluene and tetrahydrofuran (THF) were investigated by time-domain laser pulse pump, X-ray pulse probe X-ray absorption spectroscopy (pump–probe XAS) at room temperature using X-rays from a third generation synchrotron source with 100-ps time resolution. The transient optical absorption measurements indicate a strong solvent dependency of the triplet excited state lifetime due to the presence of oxygen-containing solvent molecules. While the ground state CuOEP molecules remain square-planar in both solvents, the attenuation of a peak attributed to the 1s → 4p z transition at the Cu K-edge for the laser excited CuOEP in the THF/toluene mixture revealed the penta-coordinated exciplex formation which is responsible for the shortening of the triplet excited state lifetime. Meanwhile, the average Cu–N distance in the triplet excited state is lengthened by 0.03 A due to ligation with a THF solvent molecule, which agrees with a domed coordination structure for copper in the penta-coordinated exciplex.

Modifications and Color Markings in Glasses by UV Laser Radiation

ABSTRACTRare earth-, transition metal- and metal doped soda lime-, BK7- and photosensitive lithium aluminosilicate glass are irradiated by pulsed UV laser radiation (wavelength λL = 355nm, pulse length tp = 10 to 80 ns) as a function of repetition rate, pulse energy and overlap between successive pulses. The solid state laser radiation is focused into the volume to fluences below the removal-threshold. Depending on the chemical composition, the laser parameters, and the processing variables the laser radiation is absorbed by optically- and photochemically active polyvalent ions with conversion of the optical energy into fluorescence stimulation and ionization as well as color center formation resulting in modifications and/or color markings in the VIS. The absorption and concentration changes of the polyvalent ions are probed by optical spectroscopy and transient absorption spectroscopy. The photochemical and -physical processes such as oxidation and reduction of polyvalent ions and/or color center generation underlying the modifications and color markings are discussed.

Electronic structure calculations of intrinsic and extrinsic hydrogen point defects inKH2PO4

We report first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of H intrinsic (Frenkel pair) and extrinsic (H vacancy or interstitial) point defects in ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}.$ The relaxed atomic structures, the formation energy, the ionization energy, and electron and hole affinities for the various defects have been calculated. For the Frenkel pair, the additional hole leads to a decrease of the $\mathrm{O}---\mathrm{O}$ bond length between the two O atoms next to the H vacancy, while the effect of the additional electron is small. For the H vacancy, the added hole is trapped and shared by the two O atoms adjacent to the vacancy, reducing dramatically the $\mathrm{O}---\mathrm{O}$ bond length, thus forming a molecular-type polaron. We find that the positively charged H vacancy introduces states in the gap, in contrast with its neutral state, confirming the experimental suggestion that it is a relevant absorbing center. The negatively charged H vacancy leads to an increase of the two O atoms close to the H vacancy, and does not induce states in the gap. The H interstitial does not interact with the host atoms in the neutral state. However, the addition of an electron leads to the ejection of a H host atom and the subsequent formation of a ${\mathrm{H}}_{2}$ interstitial molecule and a H vacancy, in agreement with experimental suggestions. In the positively charged state the H interstitial binds to its nearest-neighbor O atom forming a hydroxyl bond. The H interstitial in both positive and negative charge states induces no defect states in the band gap, in contrast with its neutral state. The calculations provide insights into the role of the charged and neutral defects on the transient optical absorption under irradiation by high-intensity laser beam.

Electron paramagnetic resonance and electron-nuclear double resonance study of trapped-hole centers inLiB3O5crystals

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) have been used to characterize two distinct hole centers in single crystals of ${\mathrm{LiB}}_{3}{\mathrm{O}}_{5}$ (commonly referred to as LBO). Irradiating a crystal at 77 K with x rays produces an intense four-line holelike EPR signal, with the structure arising from the hyperfine interaction with one ${}^{11}\mathrm{B}$ nucleus. Warming the crystal to approximately 130 K destroys the first hole center and allows a second less intense four-line holelike EPR signal to be observed (also interacting with one ${}^{11}\mathrm{B}$ nucleus). The second hole center decays between 150 and 200 K. EPR and ENDOR angular dependence data were used to determine the g matrix and the ${}^{11}\mathrm{B}$ hyperfine and nuclear quadrupole matrices for each hole center. We suggest that the first (less thermally stable) center is a self-trapped hole. In this defect, the hole is localized primarily on an oxygen ion between a threefold bonded boron and a fourfold bonded boron, and the self-trapping occurs because of a significant relaxation of the neighboring fourfold boron away from the hole. GAUSSIAN 98 calculations, using a $({\mathrm{B}}_{3}{\mathrm{O}}_{7}{\mathrm{H}}_{4}{)}^{0}$ cluster to represent the defect and the nearby lattice, support this self-trapping mechanism. A similar model is suggested for the second hole center, except in this case a neighboring lithium vacancy is included to provide the increased thermal stability. These trapped-hole centers are of interest because of their possible role in the unwanted transient optical absorption produced in ${\mathrm{LiB}}_{3}{\mathrm{O}}_{5}$ crystals at room temperature by high-power pulsed ultraviolet lasers.

Transient optical absorption and luminescence of lithium triborate LiB3O5

The transient optical absorption and luminescence of LiB3O5 (LBO) nonlinear crystals in the visible and UV spectral ranges were studied. Measurements made using absorption optical spectroscopy with nsscale time resolution revealed that the transient optical absorption (TOA) in LBO originates from optical transitions in hole centers and that the kinetics of optical density relaxation are rate-limited by interdefect nonradiative tunneling recombination involving these hole centers and the Li0 electronic centers, which represent neutral lithium atoms. At 290 K, the Li0 centers can migrate in a thermally stimulated, one-dimensional manner, a process which is not accompanied by carrier delocalization into the conduction or valence band. It is shown that the pulsed LBO cathodoluminescence kinetics is rate-limited by a recombination process involving two competing valence-band-mediated hole centers and shallow B2+ electronic centers. The radiative recombination accounts for the characteristic σ-polarized LBO luminescence in the 4.0-eV region.

The role of Fe and Cu dopants in electron–hole trapping and relaxation process in congruent LiNbO3

Abstract The transient optical absorption and kinetics of absorption decay is studied in undoped, Fe doped, and Cu doped LiNbO 3 crystals irradiated by pulsed electron beam. The 1.6 eV band of electron polaron trapped at antisite niobium was observed in all crystal samples. The nature of centers responsible for transient absorption is discussed. Electron polarons are shown to be less stable in LiNbO 3 :Fe compared with LiNbO 3 :Cu and undoped LiNbO 3 . It is suggested that a major part of electrons and holes created by irradiation are trapped in the vicinity of dopants.