Moderate Excitation Research Articles

Properties of photoluminescence in type-II ZnMnSe∕ZnSeTe multiple quantum wells

The optical properties of type-II ZnMnSe∕ZnSeTe multiple quantum wells were investigated by photoluminescence (PL) measurement. It was found that the peak position of PL spectra shows a giant blueshift under a moderate optical excitation level. The giant blueshift can be interpreted in terms of the band-bending effect due to the spatially photoexcited carriers in a type-II alignment. We also found that the PL spectra exhibit a large in-plane polarization with respect to ⟨110⟩ axis with the polarization degree up to 12.4%. We showed that the polarization does not depend on the excitation intensity as well as temperature, which excludes the possibility of extrinsic mechanisms related to the in-plane anisotropy. The observed anisotropic effects of ZnMnSe∕ZnSeTe multiple quantum wells were attributed to the intrinsic property of the orientation of chemical bonds at the heterointerface of the type-II band alignment.

Dissociation Dynamics of Difluoroacetic Acid from the Ground and Excited Electronic States

The photodissociation dynamics of difluoroacetic acid (DFA) is investigated in both its ground and the first excited electronic states employing, respectively, pulsed CO2 and ArF (193 nm) lasers. DFA undergoes facile infrared multiphoton dissociation (IRMPD) on irradiation with a pulsed CO2 laser to open up various dissociation channels from the ground electronic state. Energetically the most preferred primary dissociation channel of DFA is 1,3-HF elimination, as demonstrated by ab initio molecular orbital (MO) calculations and the unimolecular rates for all molecular channels using Rice−Ramsperger−Kassel−Marcus theory. Contrary to an assumed dissociation mechanism for fluorinated carboxylic acids, the decarboxylation reaction is observed as a primary dissociation channel in the IRMPD of DFA. The vibrationally excited photoproducts CO, CO2, COFH, and HF have been detected by measurement of time-resolved infrared fluorescence. Photoexcitation of DFA to the electronically excited S1 state at 193 nm leads to the cleavage of the C−OH bonda higher energy channel absent in IRMPDproducing OH(v,J), which was detected state-selectively employing the laser-induced fluorescence technique. We measured partitioning of the available energy, and observed that the nascent OH(v,J) is generated vibrationally cold (i.e., in v = 0) with moderate rotational excitation (TR = 630 ± 80 K). But, a high fraction of energy goes to the relative translation (fT value of 0.37) of the photofragments. This observation is explained on the basis of the presence of an exit barrier for the C−OH bond cleavage reaction, and supported by modeling of the measured partitioning of energy and ab initio MO calculations. The transition-state structure for the C−OH bond cleavage producing OH could be calculated in the electronically excited state, T1.

Ultrafast Excited State Dynamics of Tri- and Hexaporphyrin Arrays

An investigation of the ultrafast excited state dynamics of triporphyrin and hexaporphyrin arrays consisting of covalently linked Zn tetraphenylporphine (ZnTPP) and free base tetraphenylporphine (FbTPP) units is reported. The interchromophoric distance in the hexamers is of the order of 13 Å, while it varies from 26 to 70 Å in the trimers. These arrays exhibit several features that differ substantially from those of the monomeric units: a broadening of the Soret band, a shortening of the S2 lifetime of the ZnTPP chromophores, and additional ultrafast decay components of the S1 fluorescence. In the hexaporphyrin arrays, most of these features are attributed to the presence of excitonic states that result from the strong coupling between the Bx,y transition dipoles. The time constants for S1 energy transfer between ZnTPP chromophores as well as between ZnTPP and FbTPP moieties, deduced from anisotropic and isotropic time-resolved fluorescence measurements, were found to be of the order of a few tens of picoseconds. Moreover, back energy transfer from the FbTPP to ZnTPP units is also observed. At high to moderate excitation intensity, S1−S1 annihilation becomes an important decay mechanism of the excited state population of the hexaporphyrins. In the triporphyrins, the differences relative to the monomer are ascribed to the interaction with the phenylacetylene linkers, which lifts the degeneracy of the S2 states. S2 and S1 energy transfer were found to take place in the triporphyrin with the shortest linker only. In the other triporphyrins, an efficient energy transfer from the linker to the porphyrin units was observed.

Stimulated emission and time-resolved photoluminescence in rf-sputtered ZnO thin films

Stimulated emission (SE) was measured from ZnO thin films grown on c-plane sapphire by rf sputtering. Free exciton transitions were clearly observed at 10 K in the photoluminescence (PL), transmission, and reflection spectra of the sample annealed at 950 °C. SE resulting from both exciton-exciton scattering and electron hole plasma formation was observed in the annealed samples at moderate excitation energy densities. The SE threshold energy density decreased with increasing annealing temperature up to ∼950 °C. The observation of low threshold exciton-exciton scattering-induced SE showed that excitonic laser action could be obtained in rf-sputtered ZnO thin films. At excitation densities below the SE threshold, time-resolved PL revealed very fast recombination times of ∼74 ps at room temperature, and no significant change at 85 K. The decay time for the SE-induced PL was below the system resolution of <45 ps.

Nonlinear absorption of PbS nanocrystals in silicate glass

Nonlinear transient absorption of glasses doped with PbS nanocrystals was investigated by means of picosecond pump–probe absorption spectroscopy. The entire lowest energy absorption band was bleached on glass excitation at any spectral position inside this band, indicating that homogeneous broadening determines the bandwidth. Three different induced absorption mechanisms taking place at different incident light intensities were distinguished: absorption to the two-electron–hole pair states limits absorption bleaching to about 50% of the steady-state value at moderate excitation intensity; short-lived absorption from the two-electron–hole pair states appears at higher intensity when these states are populated; and two-photon absorption occurs at very high light intensities. The two-photon absorption populates high-energy exciton states which relax bypassing the lowest energy exciton state. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Conversion of an electron–hole plasma into a high density excitonic state in ZnO epitaxial thin films

Time-resolved luminescence spectra of an exciton-exciton scattering process and an electron–hole plasma (EHP) were measured in ZnO epitaxial thin films by the optical Kerr gate method. At moderate excitation density below critical Mott density, stimulated emission of luminescence due to the exciton–exciton scattering process (P band) was observed. The P band shows a red shift and spectral narrowing in the first 1–2 ps after the excitation which reflects a thermal redistribution of excitons in the n = 1 excitonic state. On the other hand, at high excitation density above the critical Mott density, stimulated emission of the luminescence due to the EHP was observed. After thermalization of the EHP, the luminescence intensity of the EHP band decreases with time, and the EHP band converts to the P band after ∼10 ps through a reverse Mott transition. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Coherence properties of polaritons in semiconductor microcavities

In this work we present the coherent control of polariton parametric scattering with weak phase locked probe pulses. It is demonstrated that the signal emission arising from the parametric scattering is completely coherent for moderate excitation power, whereas at high pump density the dynamics appears less coherent due to the depletion of the pump polariton reservoir. It is furthermore demonstrated that the phase of the probe pulses controls the emission from signal, pump, and idler at the same time. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical gain in self-assembled ZnO microcrysallite thin films

The lasing properties in ZnO thin films grown by laser molecular beam epitaxy on sapphire substrates have been studies. The excitation dependence of the optical gain spectra has been obtained at room temperature using a variable stripe length method. At moderate excitation intensities, the optical gain responsible for lasing is shown as a result of an exciton–exciton scattering process. At higher excitation intensities, the exciton–hole plasma processes become more dominant and exhibit the highest gain reaching value 571 cm −1 at an excitation density of 280 kW/cm 2. An optical loss in ZnO thin film at room temperature was found to be a low value of 68 cm −1. A lasing threshold of 85 kW/cm 2 has been observed at room temperature.

Optical readouts of cantilever bending designed for high magnetic field application

A single-fiber optical readout of cantilever deflection caused by Lorentz force was investigated. A good agreement of the optical near field model with the measured nonlinear characteristic was found. The ac-excitation mode of the cantilever is preferable, due to the uncontrollable static light reflections in a single-fiber setup. The presented sensor design seems suitable for a measurement range of the magnetic flux density of at least 10 mT to 50T with moderate excitation current amplitudes. Two-fiber setups may reduce the background light and could enable dc excitation but require smaller offset between cantilever and fiber axis.

Influence of higher Coulomb correlations on optical coherent-control signals from a ZnSe quantum well

The manipulation of the coherent optical polarization in a quantum well with a pair of phase-locked laser pulses is investigated by using wave-mixing signals. The measurements are performed in four- and six-wave-mixing geometries with different polarization states of the excitation pulses. Even at moderate excitation densities, the signals are modulated by higher harmonics that correspond to high-order optical nonlinearities. A significant effect of higher Coulomb correlations on the coherent-control signals is demonstrated by comparing the experiments with calculations based on a microscopic density-matrix theory. The theoretical approach makes use of the dynamics-controlled truncation scheme. The corresponding numerical results are in good agreement with the experiment.

Spectral response of the pulsationally induced shocks in the atmosphere of BW Vulpeculae

BW Vulpeculae (BW Vul) is remarkable for exciting an extremely strong radial pulsation mode which grows through its outer envelope and forms visible shock features in the atmosphere. Material propelled upwards by the shock returns violently to the lower photosphere where it creates a second shock just before the start of the next cycle. We have obtained three nights of echelle data for this star over about five pulsation cycles (P= 0.201 d) in 2000 September in order to investigate the effects of atmospheric shocks on important lines in the optical red spectrum. These lines include He iλ5875 and λ6678, C iiλλ6578–83 doublet, and other moderate and high excitation lines. To these data we have added 37 archival IUE/SWP echelle spectra obtained in 1994. We have investigated the equivalent widths and shapes of the optical lines for evidence of inter alia lags and have compared our results to the IUE fluxes extracted from the far-ultraviolet continuum, He iiλ1640, and several resonance lines. A comparison of He iλ5875 and λ6678 line profiles during the peak of the infall activity suggests that differences in the development of a second blue lobe in the profile at this time are due to heating and a short-lived formation of an optically thin layer above the region compressed by the infall. This discovery and the well-known decreases in equivalent widths of the C ii doublet at the two shock phases further suggest that shock heating flattens the atmospheric temperature gradient. Except for evidence of wind absorption in the far blue wings of the ultraviolet resonance lines, we find no evidence for a shock delay arriving at different photospheric strata (i.e., a ‘Van Hoof effect’). Line-to-line differences in the relative strengths of double lobes can be false indicators arising from varying degrees of desaturation of multiple lines, such as for the red He i lines.

Ultrashort, nonlinear, optical time response of Fe-doped InGaAs/InP multiple quantum wells in 1.55-μm range

Pump-probe experiments are used to characterize Fe-doped InGaAs/InP multiple quantum wells for ultrafast saturable absorption applications. Sample photoresponse time is well controlled by iron doping from the nanosecond range to a value as short as 0.45 ps for an iron concentration of 6×1018 cm−3. Furthermore, the switching amplitude, that is, the differential transmission ratio (DTR), of samples having increasing Fe doping (null to 6×1018 cm−3) presents two decreasing regimes at moderate optical excitation levels. The first one is explained well by the degree of quality of the material, whereas the second one is interpreted as an inhibition of exciton formation at high doping levels. This interpretation is highlighted by an analysis of the temporal evolution of the DTR. The influence of the doping localization (in the well, in the barrier, or in the whole structure) is studied as well. Finally, an attractive way to maintain a constant value of the DTR with an ultrafast response time (2.6 ps) is proposed and demonstrated.

The initiation of bursts in thalamic neurons and the cortical control of thalamic sensitivity.

Thalamic neurons generate high-frequency bursts of action potentials when a low-threshold (T-type) calcium current, located in soma and dendrites, becomes activated. Computational models were used to investigate the bursting properties of thalamic relay and reticular neurons. These two types of thalamic cells differ fundamentally in their ability to generate bursts following either excitatory or inhibitory events. Bursts generated with excitatory inputs in relay cells required a high degree of convergence from excitatory inputs, whereas moderate excitation drove burst discharges in reticular neurons from hyperpolarized levels. The opposite holds for inhibitory rebound bursts, which are more difficult to evoke in reticular neurons than in relay cells. The differences between the reticular neurons and thalamocortical neurons were due to different kinetics of the T-current, different electrotonic properties and different distribution patterns of the T-current in the two cell types. These properties enable the cortex to control the sensitivity of the thalamus to inputs and are also important for understanding states such as absence seizures.

Properties of photoluminescence in type-II GaAsSb/GaAs multiple quantum wells

The optical properties of type-II GaAsSb/GaAs multiple quantum wells were investigated by photoluminescence. It was found that the peak position of photoluminescence (PL) spectra shows a giant blueshift under a moderate optical excitation level. As the pumping intensity increases further, the saturation of the peak position was observed. The giant blueshift can be interpreted in terms of the band-bending effect due to the spatially photoexcited carriers in a type-II alignment. The saturation effect was attributed to the creation of an energy barrier induced by photocarriers, which prevents the charge transfer. The change of integrated photoluminescence intensity with increasing excitation intensity is also consistent with the band-bending model. In addition, the temperature dependence of the PL spectra reveals that the thermal escape of electrons from the GaAs well into the GaAsSb barrier is responsible for the PL quenching.

Ballistic phonon production in photoexcited Ge, GaAs, and Si

Phonon imaging and photoluminescence measurements are used to determine the frequency and spatial distribution of optically generated nonequilibrium phonons in Si, Ge, and GaAs at 1.7 K. At low excitation levels the thermalization of photoexcited carriers and the subsequent phonon down-conversion produce a broad frequency distribution of acoustic phonons that ``quasidiffuse'' in the crystal. These phonons produce a temporally broad heat pulse when detected at a distance from the excitation point. At moderate excitation levels (typically a 10-nS pulse with a power density of \ensuremath{\sim}20 ${\mathrm{W}/\mathrm{m}\mathrm{m}}^{2}$), the laser pulse produces a dense electron-hole plasma that can radically change the frequency distribution of nonequilibrium phonons. The plasma is a potentially rich source of low-frequency acoustic phonons, characterized by a temporally sharp heat pulse at a remote detector. The fraction of low-frequency phonons in the heat pulses is smallest in the direct-gap semiconductor GaAs, where rapid recombination depletes the populations of electrons and holes in just a few nanoseconds. More noticeable low frequency phonon components are seen in heat pulses in the indirect-gap semiconductors Ge and Si. At sufficiently high excitation densities (\ensuremath{\sim}60 ${\mathrm{W}/\mathrm{m}\mathrm{m}}^{2}$) in Ge, there is a suppression of the low-frequency phonon signal, which may result from phonon absorption within a cloud of electron hole droplets. An interesting alternative hypothesis is that the acoustic phonons created in the plasma are sufficiently dense to initiate phonon coalescence, whereby phonons are localized by phonon-phonon scattering over a relatively long period (500 ns). This localized ``hot spot'' could provide the phonon wind that drives the initial rapid expansion of the electron-hole plasma into the crystal.

Diffuse Reflectance IR Spectra of Molecular Hydrogen and Deuterium Adsorbed on Zinc Oxide

Diffuse reflectance IR spectroscopy is used to study hydrogen and deuterium adsorption on zinc oxide at room temperature and 77 K. At room temperature, H2 and D2 molecules are dissociatively adsorbed with the formation of hydrides and hydroxy groups of three types. At 77 K, diffuse reflectance spectra reveal the bands from molecular hydrogen and deuterium in addition to the dissociatively adsorbed forms. The presence of several bands of stretching H–H and D–D vibrations points to the nonuniformity of adsorption sites. This nonuniformity is also confirmed by the fact that, after heating zinc oxide from 77 K to room temperature in an atmosphere of hydrogen, only an insignificant portion of adsorbed molecular hydrogen dissociates. Most of dissociatively adsorbed hydrogen is formed without a molecular precursor. The dissociation of H2 and D2 most likely occurs on very active adsorption species so rapidly that the molecular precursor is not observed. The bond energy in molecular deuterium precursors of dissociation estimated from the fundamental vibration frequency and the overtone of D–D vibrations suggests moderate excitation of the bond. This agrees well with the conclusion that the dissociative adsorption of hydrogen and deuterium occurs without a molecular precursor.

ULTRAFAST CARRIER DYNAMICS IN ZnO EPITAXIAL THIN FILMS STUDIED BY OPTICAL KERR GATE LUMINESCENCE SPECTROSCOPY

Time-resolved luminescence spectra of ZnO epitaxial thin films have been measured by the optical Kerr gate luminescence spectroscopy under resonant excitation of the excitonic state. At moderate excitation density, a luminescence band (P band) which is attributed to an exciton-exciton collision process was observed. With increasing the excitation density, the P band disappears while a new luminescence band (EHP band) due to radiative recombination of electrons with holes in an electron-hole plasma appears. The EHP band has a rise time of ~1 ps, and shifts to lower energy with time and then finally shifts to higher energy. The rise time of the EHP band shows a transition time from a high density excitonic state to an unbound electron-hole system due to the screening of Coulomb interaction. The red shift of the EHP band comes from the reduced band-gap effect, while the blue shift to the EHP band is mainly attributed to recovery of the renormalized band-gap, which occurs with decreasing the carrier density due to the radiative recombination of electrons with holes.

Keck spectroscopy and imaging of faint galaxies identified as microjansky radio sources

We investigate the nature of the faintest radio sources detected in three Very Large Array surveys, to F(8.5 GHz)∼8 μJy. Using the Keck low-resolution imaging spectrograph in BRI and the near-infrared camera in K′ (2.1 μm), we image 51 radio sources, and identify probable optical counterparts for 50. With low-resolution imaging spectroscopy, we successfully acquire new redshifts for 17 sources. Combining these with nine prior redshifts, we can then analyse a sample of 26 sources with spectroscopic redshifts. Based on this sample of 26, we find the largest contribution, about 60 per cent (15), to be from disc galaxies with high radio-to-optical ratios, indicating ongoing or very recent starbursts, at redshifts of z=0–1. Most of these have large optical luminosities (MB≃−22±1), although two are sub-L*(MB>−20). About 20 per cent (five) of the sample have relatively low ratios of radio-to-optical flux, consistent with normal, non-interacting galaxies, all at z<0.4. Four are quasi-stellar objects (QSOs), of which one appears to be interacting and shows strong Mg ii absorption. Two others are very radio-luminous giant ellipticals and presumably contain active galactic nuclei. All of the 14 non-QSO galaxies with spectra show Hβ and [O ii] λ3727 emission lines, with luminosities corresponding to star formation rates (SFRs) of the order of ∼10 M⊙ yr−1. These SFRs are about an order of magnitude lower than estimated from the radio luminosities. This result may be explained if the galaxies are observed ∼107–108 yr after a starburst and/or their regions of current star formation are heavily obscured by dust. Three of the galaxies show strong Hδ absorption and can be classed as post-starburst or ‘e(a)’ galaxies. The Keck spectra are of high enough resolution to provide approximate emission linewidths and rotation curves, as well as [O iii]/Hβ ratios (‘excitation’). Most of the galaxies (12/14) have high-amplitude kinematics (100–400 km s−1), implying large dynamical masses, and moderate excitation (∼0.51±0.12), consistent with their morphological appearance as non-AGN, L⩾L* spirals, of probable near-solar metallicity. The two with low optical luminosities appear to be low-mass galaxies with much higher excitations (∼2.8), and may be more similar to local H ii galaxies than to normal spirals.

Identification of soil–structure interaction effect in base-isolated bridges from earthquake records

Identification of system parameters with the help of records made on base-isolated bridge during earthquakes provides an excellent opportunity to study the performance of the various components of such bridge systems. Using a two-stage system identification methodology for non-classically damped systems, modal and structural parameters of four base-isolated bridges are reliably identified using acceleration data recorded during 18 earthquakes. Physical stiffness of reinforced concrete columns, dynamic properties of soil and foundation impedance are found by available theoretical models in conjunction with pertinent information from the recorded accelerographs. Soil–structure interaction (SSI) effect in these bridges is examined by comparing the identified and physical stiffness of the sub-structure components. It is found that SSI is relatively pronounced in bridges founded in weaker soils and is more strongly related to the ratio of pier flexural stiffness and horizontal foundation stiffness than soil shear modulus, G s, alone. However, substantial reduction in G s is observed for moderate seismic excitation and this effect should be taken into account while computing foundation impedance.

Covalently bound molecular structures in the α + 16O system

Using the concept of covalent molecular orbitals for neutrons and the known properties of the local α + 16O potential the formation of asymmetric molecular structures in neon isotopes is discussed. Experimental evidence for parity doublets in 21Ne is reviewed and a corresponding band structure for the states in 21Ne at moderate excitation energy of E x = 0-8 MeV is proposed. The structure of some bands can be interpreted as consisting of an instrinsic asymmetric ( 4He + 16O) structure bound by a covalent neutron in σ and π orbitals. An extension of the observed structures to symmetric molecular structures in isotopes of Mg and heavier nuclei is suggested. In particular shape isomers in isotopes of magnesium, namely (He)2O molecules, can be predicted and an extended Ikeda diagram is proposed.