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© 2019 Optical Society of America. Transient reflection was performed on large-area chemical vapor deposition (CVD) grown monolayer MoS2 flakes with silver nano-triangles coated on the surface to characterize its modifications to the optical properties of MoS2. Broadband transient reflection was carried out over a range of different pump wavelengths to tune in and out of the plasmonic resonance band of the nano-triangles. Large enhancement in transient reflectivity was observed, which approximately followed the plasmon resonance spectrum of the nano-triangles. Finite-difference time-domain simulationswere performed to verify that there is an increased absorption due to local surface plasmon-induced near-field enhancements. We also report observation of the dynamics of the MoS2 exciton bleaching bands, which showed subtle differences in recombination dynamics related to the exciton-plasmon coupling strength for excitation on-resonance and off-resonance at the silver nano-triangles plasmonic band.

Introduction Lymphocytic phlebitis is a benign condition characterised by inflammation of the veins and rarely affects the gastrointestinal tract. Reported cases present as acute abdomen and involve the colon or small intestine. We report the fourth case of gastric lymphocytic phlebitis in the literature. Presentation of case A 74-year-old female presented with eight weeks of abdominal pain. Findings at endoscopy were suggestive of a malignant ulcer on the greater curvature of antrum, while biopsies showed chronic gastritis without malignancy. Appearance at diagnostic laparoscopy was consistent with a malignant gastric ulcer with serosal changes. Due to persistent pain and the macroscopic appearance, she proceeded to have an open subtotal gastrectomy and D2 lymph node clearance. Despite macroscopic appearance, the microscopic examination demonstrated no malignancy, and was consistent with lymphocytic phlebitis with overlying ulceration. Discussion This case was a mimic for gastric malignancy, with the benign diagnosis only being made after surgical resection. Gastric lymphocytic phlebitis is a rare differential diagnosis for gastric ulcers when biopsies are negative, although preoperative diagnosis is difficult given the lesions do not involve the mucosa. Conclusion If clinical history and endoscopic findings are suspicious for malignancy, despite normal biopsies, an aggressive surgical resection remains reasonable given the rarity gastric lymphocytic phlebitis.

OBJECTIVE: To determine associations between cerebral microbleed (CMB) burden with recurrent ischemic stroke (IS) and intracerebral hemorrhage (ICH) risk after IS or TIA. METHODS: We identified prospective studies of patients with IS or TIA that investigated CMBs and stroke (ICH and IS) risk during ≥3 months follow-up. Authors provided aggregate summary-level data on stroke outcomes, with CMBs categorized according to burden (single, 2-4, and ≥5 CMBs) and distribution. We calculated absolute event rates and pooled risk ratios (RR) using random-effects meta-analysis. RESULTS: We included 5,068 patients from 15 studies. There were 115/1,284 (9.6%) recurrent IS events in patients with CMBs vs 212/3,781 (5.6%) in patients without CMBs (pooled RR 1.8 for CMBs vs no CMBs; 95% confidence interval [CI] 1.4-2.5). There were 49/1,142 (4.3%) ICH events in those with CMBs vs 17/2,912 (0.58%) in those without CMBs (pooled RR 6.3 for CMBs vs no CMBs; 95% CI 3.5-11.4). Increasing CMB burden increased the risk of IS (pooled RR [95% CI] 1.8 [1.0-3.1], 2.4 [1.3-4.4], and 2.7 [1.5-4.9] for 1 CMB, 2-4 CMBs, and ≥5 CMBs, respectively) and ICH (pooled RR [95% CI] 4.6 [1.9-10.7], 5.6 [2.4-13.3], and 14.1 [6.9-29.0] for 1 CMB, 2-4 CMBs, and ≥5 CMBs, respectively). CONCLUSIONS: CMBs are associated with increased stroke risk after IS or TIA. With increasing CMB burden (compared to no CMBs), the risk of ICH increases more steeply than that of IS. However, IS absolute event rates remain higher than ICH absolute event rates in all CMB burden categories.

Within the intracranial vasculature, atherosclerosis occurs in two distinctive patterns: (1) in Western populations who have severe extracranial and systemic atherosclerosis, the severity of intracranial involvement is consistently less than that within extracranial arteries; and (2) in Asians, Africans, and Hispanics, who often have isolated intracranial arterial disease that is found to be more often accompanied by brain infarction than comparable extracranial atherosclerotic disease. Compared to coronary and extracranial carotid atherosclerosis, intracranial atherosclerosis has distinct pathological characteristics compared to that of extracranial arteries. Intracranial atherosclerosis (ICAS) had been understudied due to the relative inaccessibility of cerebral artery specimens under current treatment strategies. Acquiring post-mortem cerebral vessel specimens for histology processing is the most direct method to analyze the pathological characteristics of ICAS, in order to analyze both lumen stenosis and plaque components contributing to brain infarctions. The developments in high resolution magnetic resonance imaging (HRMRI) make it feasible to assess human ICAS in vivo. It is nevertheless challenging to understand vessel wall changes within brain vasculature demonstrated on HRMRI, as well as to identify biomarkers for stroke risk stratification and treatment strategy modification. Knowledge about intracranial atherosclerosis remains limited due to lack of human arterial specimens, and the development of proper animal models of human cerebral atherosclerosis is necessary to explore the pathogenesis of intracranial atherosclerosis and to assess various strategies preventing or treating ICAS-related stroke.

INTRODUCTION: We aim to investigate the correlation between presence of anterior temporal artery (ATA), the first major branch of middle cerebral artery (MCA), on conventional angiography and clinical outcome in patients with acute atherosclerotic M1-MCA occlusion. METHODS: Consecutive patients with acute atherosclerotic M1-MCA occlusion from Nanjing Stroke Registry Program (NSRP) between January 2007 and December 2012 were included in this study. All patients underwent MRI followed by conventional angiography. From their data, we analyzed baseline characteristics, infarction patterns, DWI-ASPECTS, and collateral circulation. The correlation of ATA presence and good clinical outcome, modified Rankin Scale (mRS) score ≤ 2, at 3 months was also calculated. RESULTS: In 98 patients meeting entry criteria, the presence of ATA was found in 44 patients. Patients with ATA present were found to have less hypertension (p = 0.042), lower baseline National Institutes of Health Stroke Scale (NIHSS) (p = 0.043), more small infarcts in perforating artery territory (p = 0.013), and a higher number of DWI-ASPECTS ≥ 7 (p = 0.034). Binary logistic regression analysis showed an adjusted odds ratio of 4.45 for a good outcome in patients with ATA presence (95% CI 1.52 to 13.03, p = 0.007). CONCLUSION: The presence of ATA can be used as a predictor of good outcome in patients with acute atherosclerotic M1-MCA occlusion.

We report on observation of simultaneous strong enhancements of second-harmonic generation (SHG) and third-harmonic generation (THG) in visible spectra region in a one-dimensional ZnSe-ZnMgS semiconductor photonic crystal. These enhancements come from phase matching and a high density of modes near the photonic band edge. Our result shows that the THG is due to direct X-(3) process instead of the cascaded two-step X-(2) process. Measured forward SH conversion efficiency close to 1\% for only a few microns thick film demonstrates potential device applications. Theoretical calculations are in good agreement with the experimental measurements.

We report the simulation and fabrication of two-dimensional hetero-binary and honeycomb photonic crystals by one-step holographic lithography. These structures are realized by introducing one or three auxiliary beams into the three basic beams forming a regular hexagonal structure. The size contrast between the center rod and its six neighbors in a hetero-binary structure can be tuned by adjusting the intensity contrast between the auxiliary beam and basic beams. The idea of creating heterogeneity into the interference pattern by auxiliary beams could be easily extended to three-dimensional case by adding more beams in the vertical direction. (c) 2006 American Institute of Physics.

Picosecond carrier dynamics of deep bandtail states (3.1 eV) in an unintentionally n-doped GaN epilayer at room temperature under high excitation densities (i.e., N-0 = 1.0 x 10(19) - 1.1 x 10(20) cm(-3)) have been investigated with nondegenerate femtosecond pump - probe (267/400 nm) reflectance (Delta R/R-0). All Delta R/R-0 traces possess a similar to 2 ps buildup time that represents an overall time for the initial non-thermal carrier population to relax towards the continuum extremes and then into the probed tail states. We observe a saturation of Delta R/R-0 initial ( first 10 ps) recovery rate gamma(i) at a density of 5 - 6 x 10(19) cm(-3) close to the Mott transition threshold obtained from time-integrated PL measurements. Such a saturation phenomenon has been identified as the trap-bottleneck due to the bandtail states and deep traps. As N-0 is further increased, gamma(i) accelerates due to the onset of Auger recombination as the trap-bottleneck becomes effective. The best fit by the Auger model for N0 in the range of the mid-10(19) - 10(20) cm(-3) yields an Auger coefficient of C-a similar to 5.0 x 10(-30) cm(6) s(-1).

Femtosecond laser pulses with suitable pulse areas are shown to decelerate and stop inside a resonant photonic crystal. The release of the stored fields can be achieved through pulse collision by use of an external optical pulse as a control field. The duty cycle efficiency of the storage and release of light is shown to be as much as 96\%. The proposed technique is applied to a real photonic system with finite thickness of atomic layers. (c) 2005 Optical Society of America.

Dramatic enhancement of second harmonic generation (SHG) near the photonic band edge was observed in a one-dimensional ZnSe/ZnMgS semiconductor photonic crystal (PC) structure. Over two orders of magnitude increase in SHG intensity was observed at the photonic band edge at similar to 1400 nm compared to the nonphase matching region. The maximum SHG conversion efficiency of 0.8\% is observed in only seven micrometers length of crystal. This enhancement came from a combination of large ZnSe second order susceptibility coefficient (chi((2))), high density of optical modes and phase matching of the fundamental and second harmonic waves near the photonic band edge due to modification of the dispersion curve by the PC structure. (C) 2005 American Institute of Physics.

A ternary materials system, which comprises two-photon initiator, oligomer and poly(methyl methacrylate), was prepared. A polymeric waveguide film was manufactured by spinning the materials on ITO, and the two-photon initiated photopolymerization (TPIP) was carried out successfully in the waveguide film. Consequently, an approach using TPIP is demonstrated for fabrication of coupling grating in a thin polymeric film possessing waveguide properties. The results show that the incident laser can be coupled into the polymeric film by this coupling grating obtained from TPIP. The microstructure feature of coupling grating is illustrated, and waveguide mode and transmission loss of the polymeric film are investigated. (C) 2004 Elsevier B.V. All rights reserved.

The highly aligned ZnO whiskers, which are grown by hydrothermal oxidation of metallic zinc plate in the presence of ethylenediamine molecules, exhibit single and/or a few modes of supernarrow spectral emissions (similar to0.7 nm) at near 378 nm and negligible deep level defects emissions. Time-integrated and time-resolved photoluminescence show that the supernarrow spectral peaks are due to the laser action with an excitation threshold about 70 muJ/cm(2) and emission lifetime of <30 ps. The lasing emission is highly polarized along the excitation laser polarization direction. The low lasing threshold, stable and regular supernarrow longitudinal modes, and strong lasing polarization effects can be well explained by the model of microcavity laser where the two end facets of whisker form the microcavity. (C) 2004 American Institute of Physics.

The excitation spectra for linear fluorescence and for frequency upconversion are recorded in Er3+ doped crystal Er:YAG. An excitation induced spectral broadening and hole-burning are shown to appear at the resonant frequency of the linear fluorescence excitation spectrum, where the upconversion is simultaneously enhanced. Rate equation analysis is applied to model the optical pumping processes in the crystal. We present evidence that the induced resonance broadening and hole-burning are the result of excitation competition between the linear absorption and the excited state absorption. Femtosecond pulse pairs are applied to excite the linear and nonlinear frequency conversion, resulting in coherent controlled branching ratio between the linear and nonlinear frequency conversion. The experimental reasults are in good agreement with the numerical evaluation based on optical Bloch equation.

A noise study on a set of ZnS0.58Se0.42 UV photodiodes with 1 mum thick Al bonding pads selected from a 2'' wafer was carried out. Based on their 1/f noise behaviors, they were divided into three groups. The noise spectral density, impulse response time, bandwidth, responsivity, dark current at reverse bias up to 12 V and zero bias resistance were determined for three typical detectors, i.e. one from each of the three groups. The uneven performance of these detectors is mostly likely due to the different damage level induced in the wire-bonding process. With thicker Al bonding pads of 1.5 mum, a detectivity as high as 7.1 x 10(14) cmHz(1/2) W-1 has been achieved. (C) 2003 Elsevier Science B.V. All rights reserved.

We report room-temperature time-integrated and time-resolved photoluminescence (PL) measurements on a nominally undoped wurtzite ZnO thin film grown on (001) silicon. A linear and sublinear excitation intensity I-ex dependence of the PL intensity were observed for the 379.48-nm exciton line and the weak broad green band (similar to510 nm), respectively. The green luminescence was found to decay as hyperbolic t(-1), and its peak energy was observed to increase nearly logarithmically with increased I-ex. These results are in an excellent agreement with the tunnel-assisted donor-deep-acceptor pair (DAP) model so that its large blueshifts of about 25 meV per decade increase in I-ex can be accounted for by the screening of the fluctuating impurity potential. Also, the 30-ps fast decay of the exciton emission was attributed to the rapid trapping of carriers at luminescent impurities, while the short lifetime of tau(1/e)=200 ps for the green luminescence may be due to an alternative trapping by deeper centers in the ZnO. Finally, singly ionized oxygen and zinc vacancies have been tentatively invoked to act as donor-deep-acceptor candidates for the DAP luminescence, respectively. (C) 2003 American Institute of Physics.

The energy transfer processes between polyfluorene and tetraphenyl porphyrin have been investigated through steady state and time resolved measurements. Radiative transfer is found to be negligible and so the results are analyzed in terms of Forster resonance energy transfer. Rate equations are fitted to the experimental data to obtain values for the concentration dependent energy transfer rate. From this, we calculate a value of 38 Angstrom for the energy transfer distance. This compares well with the value for the Forster radius we calculate from the spectral overlap, even though Forster theory is not strictly valid for this system. (C) 2003 American Institute of Physics.

By employing picosecond time-resolved photoluminescence, we investigated the temporal behavior of the near band-gap exciton emission located at similar to3.264 eV of a ZnO epilayer deposited onto a (10 0) silicon substrate by plasma-assisted MOCVD. The emission exhibits a biexponential decay behavior composed of an initial fast component (3050 ps) followed by a second slower component (100-400 ps). The extracted time constant for the capture of free-excitons at the band-tail states is in the order of 30ps. The results strongly suggest that the decay of the exciton population is governed by the initial fast decay due to the capture of excitons and trapping of carriers by deep centers at defects and/or impurities, and the measured slower decay component is due to the radiative recombination of free- or localized-excitons. (C) 2003 Elsevier Science B.V. All rights reserved.

Femtosecond time-resolved reflectivity was used to investigate below-band-gap (3.1 eV) carrier dynamics in a nominally undoped GaN epilayer under high excitation. A 2.5-ps rising process can be observed in the transient trace. This shot rising time results from the hot phonon effects which can cause a delayed energy relaxation of the initial photocarriers toward the band edge. From the density dependence of the carrier dynamics, the Mott density was estimated to be 1.51-1.56 x 10(19) cm(-3). Below the Mott density, the initial probed carrier dynamics was explained to the effect of acoustic phonon-assisted tunnelling for localized states, where a significant excitation density dependence of the tunnelling probability was observed due to the optically induced bandtail extension to lower energies. Above the Mott density, the measured carrier dynamics reflected the relaxation of an electron-hole plasma, in which a distinct fast decay component of 2.3ps was observed due to the onset of nonlinear relaxation processes such Auger recombination.

We demonstrate a noncollinear optical parametric amplifier(OPA) which generates 13.5fs pulses tunable in the visible region from 510 to 650nm. The ultrashort pulse is generated via pulse compression using chirp mirrors. The OPA output pulse energy is over 1.5muJ and the spectral bandwidth can be controlled by the chirp in white light seed.

We investigated four conjugated polymers of the same backbone with different lengths of side chains attached with europium (Eu3+) complex (herein labeled P2, P4, P6 and P8). All samples exhibited strong luminescence belonging to f-f{*} electronic transitions of Eu3+. Sharp emission peaks of D-5(0)-F-7(J) (J=0, 1, 2, 3) transitions were observed with the strongest peak at similar to612 nm (i.e. D-5(0))-F-7(2) transition). Ligand field splitting was seen for the D-5(0)-F-7(J) (J=1, 2, 3) transitions. Weaker and very broad emission due to singlet S-0 --> S-1 transition of the polymer was also observed. The temperature dependent fluorescence measurement indicating the energy transfer from polymer S-1 state to the Eu excited D levels is not a barrierless process; some kind of activation energy may be involved. The fluorescence rise-time indicates that the energy transfer process may take several hundred nanoseconds to be fully completed. Emission lifetimes at 612 nm were measured at 0.72, 0.37, 0.39 and 0.61 ms for P2, P4, P6 and P8, respectively. However, there was a fast decay component for the 612 nm emission, indicating some initial rapid relaxation process. (C) 2002 Elsevier Science B.V. All rights reserved.

In this letter, we demonstrated an autocorrelator based on two-photon absorption (TPA) of ZnSSe photodetectors, which is capable of measuring femtosecond laser pulses from 800 to 400 nm. This broad bandwidth autocorrelator has high sensitivity and is capable of measuring laser pulses as short as 27 fs and pulse energy of a few picojoules. These results could lead to cost-effective ultrafast autocorrelator with a single detector head to cover the whole visible to near ultraviolet spectral region. Our TPA measurement shows that the defect and impurity states below the bandgap (E-g) have detrimental effect for excitation wavelengths near the bandedge, however, these states are also beneficial by extending the operation wavelengths of the detector beyond E-g/2. Using a ZnMgS photodetector with a larger Eg enables the autocorrelator to be more sensitive at 400 nm, and extends the responsivity further into the ultraviolet wavelengths.

Zn1-xMgxS alloy thin films covering Mg composition, x, from 0 to 1 were grown on GaP (100) substrates using molecular beam epitaxy. As observed by reflection high energy electron diffraction patterns, the alloys can be grown with stable zinc-blende structures up to x around 30\%. For x > 30\%, a structural transition will occur at a critical thickness which is sensitively dependent on the composition x. Several Schottky barrier photodetectors using a Zn1-xMgxS layer with a thickness less than the critical thickness as the active layer: were fabricated. The shortest cutoff wavelength achieved so far is 270 nm. Temporal response of the device shows that the photocurrent decays with a time constant as fast as 13 ns for a mesa area of 1.5 mm(2).

The time-resolved differential absorption of the ZnSe/ZnSTe superlattice is studied using femtosecond pump-probe measurements. Transient spectral hole burning due to the initial nonthermal carrier distribution is observed at zero time delay and the carriers an thermalized within 0.5 ps. The high-energy tail of the differential absorption spectra was used to deduce the effective temperature of the thermalized carriers. Rapid hot-carrier cooling from a temperature of 763 to 450 K within the first 4 ps is observed, with carrier cooling slowing dawn hence. This initial fast hot-carrier cooling is consistent with the strong carrier-phonon interaction in large gap II-VT semiconductors. (C) 2001 Academic Press.

We demonstrated for the first time an autocorrelator based on two-photon absorption of ZnSSe photodetectors which is capable of measuring femtosecond laser pulses from 800 to 400 nm. This antocorrelator has a very broad bandwidth, high sensitivity and is capable of measuring laser pulses with high repetition rate. These results could lead to cost effective ultrafast autocorrelators with a single detector head to cover the whole of visible to near UV spectral region. Using a ZnMgS photodetector with a larger bandgap enables the autocorrelator to be more sensitive at 400 nm and extends the responsivity further into the ultraviolet wavelength of 350 nm. (C) 2001 Elsevier Science B.V. All rights reserved.

Epitaxial growth of Zn1-xMgxS alloy thin films on GaP(100) substrates was carried out using the molecular-beam-epitaxy technique. In situ reflection high-energy electron diffraction studies show that the alloys can be grown with a stable zinc-blende structure up to x around 30\%. For x > 30\%, a structural transition will occur at a critical thickness which is sensitively dependent on the x composition. A near-band-edge peak with a full width at half maximum of about 10 nm was observed in room-temperature photoluminescence measurements made on as-grown alloy thin films. Several Zn1-xMgxS-based Schottky barrier photodetectors were fabricated. Room-temperature photoresponse measurements were performed on these detectors and abrupt long-wavelength cutoffs covering 325, 305, 295, and 270 nm were achieved for devices with Mg composition of 16\%, 44\%, 57\%, and 75\%, respectively. The response curve of the Zn0.43Mg0.57S device offers a close match to the erythemal action spectrum that describes human skin sensitivity to UV radiation. (C) 2001 American Institute of Physics.

Pulse radiolysis was used to determine the triplet state energy (2.3 eV) of Poly(2,5-pyridine diyl) (PPY) film which was found to be coincident with the emission energy. Derailed time-resolved photoluminescence (PL) and pump-probe measurements have been applied to study the photoexcited state relaxation dynamics. In films, a very large spectral red-shift (similar to0.35 eV) for the PL occurred within the first 100 ps whereas no spectral red-shift was observed for the PPY in solution. This result shows clear evidence for the evolution of short-lived singlet emission (S-1 --> S-0) at similar to 450 nm to long-lived tripler emission (T-1 --> S-0) at similar to 520 nm for the PPY thin film. Streak camera measurement indicates the long live component has a decay time constant of several ns. The picosecond photo-induced triplet state absorption (T-1 --> T-2 transition) peaks at similar to 600 nm as measured by pump-probe which is consistent with both the radiolysis and cw photoinduced absorption measurements. A triplet lifetime of similar to6 ns is measured which is again consistent with the streak camera measurement. These results lead us to believe that the long live component of the emission from PPY film is in fact phosphorescence. Furthermore, there is evidence that oxygen plays a very important role in the fast tripler radiative lifetime in PPY films. (C) 2001 Elsevier Science B.V. All rights reserved.

Si phototransistors with a punchthrough base were fabricated with regular planar technology. Optical conversion gains larger than 15 000 were observed. In addition to very high gain, the unique structure of the device also resulted in a fast transient response as well as low output noise. The measured full width at half maximum of the device transient response is 1.6 ns and a -3 dB bandwidth of 300 MHz. The measured output noises at different currents can be well fitted by the relation i(n)((2) over bar)=2qI(C). (C) 2001 American Institute of Physics.

We experimentally and theoretically investigated the stimulated emission band in neatly prepared 2,5-dioctyloxy poly(p-phenylene vinylene) (DOO-PPV) films. We found that at moderately high excitation intensity spectral narrowing (SN) occurs due to the amplified spontaneous emission (ASE). A complex ASE build-up and decay dynamics as a function of excitation intensity and stripe length is observed. We also found that the exciton energy relaxation process within the inhomogenously broadened energy distribution becomes faster when ASE occurs. A theoretical model taking into account the polymer film gain and loss, the propagation effect and the time dependent exciton generation process is developed and used to explain the experimental data. (C) 2001 Published by Elsevier Science B.V.

Transmittance difference spectroscopy has been applied to study the optical properties of spin-cast rigid-rod polymer films. Significant in-plane optical anisotropy is observed for both the real and the imaginary parts of the transmittance of the poly(2,5-pyridine diyl) films. This in-plane optical anisotropy is a result of the partial alignment of the polymer chains, oriented radially outward from the center of the film, during the spin casting process. This observation has important implications for emissive polymer devices, particularly for devices sensitive to polarization direction and those using waveguide confinement where relatively long propagation distance within the films are required. (C) 2000 American Institute of Physics. [S0003-6951(00)04411-9].

Femtosecond time-resolved studies of photoluminescence (PL) and stimulated emission (SE) have been conducted, for the first time, in neat thin films of poly(p-phenylenevinylene) derivatives. We found that the SE spectral narrowing is accompanied by a dramatic lifetime shortening from similar to 250 to similar to 3 ps, which enhances energy migration within the inhomogeneously broadened exciton distribution. In addition, the SE is delayed with respect to the PL onset by a few picoseconds depending on excitation intensity and the illumination stripe length. In some cases, a second, delayed SE is formed in the film due to the continuing energy migration into exciton states with superior radiative coupling. A set of coupled rate equations, which take into account the time-dependent exciton generation, propagation effect, optical gain and absorption loss, can simulate correctly all the observed SE dynamics. (C) 2000 Elsevier Science S.A. All rights reserved.

Femtosecond time-resolved studies of photoluminescence (PL) and stimulated emission (SE) have been conducted in neat thin films of poly(p-phenylene vinylene) derivatives. For high excitation intensity I, we found that the SE spectral narrowing is accompanied by a dramatic lifetime shortening from similar to 250 to similar to 3 ps, which enhances energy migration within the inhomogeneously broadened exciton distribution. In addition, the SE is delayed with respect to the PL onset by a few picoseconds, depending on I and the illumination stripe length. In some cases, a second delayed SE is formed in the film due to the continuing energy migration into exciton states with superior radiative coupling. (C) 1999 Elsevier Science B.V. All rights reserved.

We report the femtosecond time-resolved studies of room temperature exciton recombination and ultrafast stimulated emission dynamics in ZnO microcrystallite thin films. A free exciton photoluminescence lifetime of a few tens of picoseconds and a decay time of a few picoseconds for the stimulated emission were observed. The relatively slow rise time (3ps) for the P band as the result of exciton-exciton scattering compared with the 0.8ps rise time for the N band attributed to electron-hole plasma recombination clearly distinguished the two stimulated emission processes.

Transient third-order optical nonlinearity chi((3)) Of ZnO microcrystallite thin films is measured at various temperatures by using femtosecond degenerate four-wave-mixing. Room-temperature excitonic enhancement of chi((3)) is observed. The magnitude of chi((3)) ranges between 10(-4) to 10(-6) esu from 4.2 K to room temperature, The measured chi(3) response time ranging from 200 to 300 fs is ultrafast for temperature down to 4.2 K.

We have investigated emission properties of a series of poly(phenylacetylene) samples with controlled amount of cis/trans ratio. We observed that both the emission efficiency and PL lifetime decrease with increasing cis content. These results seem to rule out the soliton model as the origin for the strong photoluminescence (PL) in PPAs. However, the PL results can be explained by the lower degree of stereoregularity with decreasing cis content resulting in short segments of cia PPA which give rise to strong PL due to the confinement effect.

ZnSe self-organized dot structures on ZnS thin films were fabricated by the molecular beam epitaxy technique. Atomic force microscopy (AFM) images of exposed dots taken within the same day of growth reveal that the dot density increases with increasing ZnSe coverage. AFM images taken at later limes (up to six months) shows ripening effect. Room temperature photoluminescence studies on buried-dot structures show promising optical characteristics that are sensitive to the growth temperature used for dot formation, (C) 1999 Published by Elsevier Science B.V. All rights reserved.

Nd:Ca4YO(BO3)(3) (Nd:YCOB) crystal was grown by the Czochralski method, and its structure was measured by using a four circle X-ray diffractometer. The transparent spectrum from 200 to 2600 nm was measured at room temperature. The fluorescence spectrum near 1.06 mu m showed that the main emission wavelength of Nd:YCOB crystal was centered at 1060.8 nm. Laser output at 1.06 mu m has been demonstrated when it was pumped by a Ti:sapphire laser at the wavelength of 794 nm, the highest output power was 68 mW under pumping power of 311 mW, the pumping threshold was 163 mW and slope efficiency was 46.9\%. The self-frequency doubled green light has been observed when it was pumped by a Ti:sapphire or a laser diode (LD). A 14.5 mm Nd:YCOB crystal sample cut at (theta, phi)=(90 degrees, 33 degrees) was used for type I second-frequency generation (SHG) of the 1.06 mu m laser pulse. The SHG conversion efficiency was 22\%. (C) 1999 Elsevier Science B.V. All rights reserved.

The emission properties of tetra and hexa methyl sulphanyl (SMe) substituted sexithiophenes have been studied in solution form using time-resolved up-conversion photoluminescence technique. The SMe substituents have an electron donating effect which compensates for the loss of pi-electron conjugation caused by the twisted conformation of the molecular geometry. Enhanced conjugation however does not lead to long recombination lime due to the heavy atom effect, which increases the nonradiative decay rate.

We report the measurement of third-order optical nonlinearity (chi((3))) of ZnO microcrystallite thin films near the excitonic resonance at various temperatures using the femtosecond degenerate four-wave-mixing technique. The measured chi((3)) response times are 270, 240, and 160 fs at 4.2 K, 77 K, and room temperature, respectively. The values of chi((3)) range from 10(-7) to 10(-4) esu. The maximum values of chi((3)) always correspond to the absorption peaks at different temperatures. Room-temperature excitonic enhancement of chi((3)) is also observed. (C) 1999 American Institute of Physics. [S0003-6951(99)05047-0].

A two-step molecular beam epitaxy growth approach was developed to fabricate a ZnSTe-based Schottky barrier photovoltaic ultra-violet (UV) detector array on GaP(1 0 0) substrate. The detectors show visible blind and UV sensitive response with peak UV responsivity of 0.13 A/W and 1.2 x 10(6) V/W at 320 nm. The built-in potential of the detector was determined to be 1.7 V. Temporal response of the device shows that the photocurrent decays with a time constant as fast as 1.2 ns, limited apparently by the RC constant of the detector structure. (C) 1999 Elsevier Science B.V. All rights reserved.