Conventional scintillation detectors are typically single crystals of heavy-metal oxides or halides doped with rare-earth ions that record the recombination of electron-hole pairs by photon emission in the visible to ultraviolet. However, the light yields are typically low enough to require photomultiplier detection with the attendant instrumental complications. Here we report initial studies of gamma ray detection by zinc oxide (ZnO) nanowires, grown by vapor-solid deposition. The nanowires grow along the c-axis in a wurtzite structure; they are typically 80 nm in diameter and have lengths of 1- 2 μm. The nanowires are single crystals of high quality, with a photoluminescence (PL) yield from band-edge exciton emission in the ultraviolet that is typically one hundred times larger than the PL yield from defect centers in the visible. Nanowire ensembles were irradiated by 662 keV gamma rays from a Cs-137 source for periods of up to ten hours; gamma rays in this energy range interact by Compton scattering, which in ZnO creates F+ centers that relax to form singly-charged positive oxygen vacancies. Following irradiation, we fit the PL spectra of the visible emission with a sum of Gaussians at the energies of the known defects. We find highly efficient PL from the irradiated area, with a figure of merit approaching 106 photons/s/MeV of deposited energy. Over a period of days, the singly charged O+ vacancies relax to the more stable doubly charged O++ vacancies. However, the overall defect PL returns to pre-irradiation values after about a week, as the vacancies diffuse to the surface of these very thin nanowires, indicating that a self-healing process restores the nanowires to their original state.
Vertically oriented ZnO nanowires (NWs) are grown upon silicon substrates using a novel, modified vapor-solid
method. Electron-beam evaporation is then used to functionalize the sides of the ZnO NWs with MgO:Ag via glancing
angle deposition (GLAD). By varying the thickness of the deposited MgO insulating layer, it is possible to study the
underlying energetic mechanisms responsible for the quenching and enhancement of ZnO photoluminescence centers.
For the visible emission, strong quenching was observed to occur independently of the MgO thickness. In contrast, the
band-edge emission displayed an enhancement factor of 19 as the thickness of the MgO spacer was increased.
Second harmonic generation (SHG) laser spectroscopy has been demonstrated to be a powerful, sensitive,
and non-destructive analytical technique to study crystalline phases, domain structures, and molecular dipole
orientations of ferroelectric polymeric thin films such as PVDF. While other spectroscopic techniques, such as
WXRD and FTIR, do provide valuable information on the crystalline phases, they are not sensitive enough to
provide the detailed information at molecular levels and properties at interfaces. The current study of single layered
PVDF polymer has also shown that SHG can be further developed into an in-situ, sensitive and quantitative tool to
study ferroelectric polymeric thin film structures. In combination, SHG and electric field induced SHG (EFISH)
techniques will allow us to interrogate multilayered structures layer-by-layer, the effects of physical confinement
and interfacial physics.
Photoluminescence (PL) of Er-doped ZnO nanoparticle films was studied. The films were grown on silicon (100) or fused silica substrates using e-beam evaporation and subsequently annealed at 700 °C in air for an hour. PL was measured at two excitation wavelengths, 325 and 514.5 nm. The 325 nm was used for exciting the host semiconductor ZnO while 514.5 nm was used for directly exciting Er3+ ions in the ZnO host. Er3+ luminescence was observed from annealed films using either indirect (325 nm) or direct (514.5 nm) excitations. It has been found that the indirect excitation is significantly more efficient than the direct excitation in producing 1.54 μm photoluminescence. With indirect excitation, the Er3+ luminescence observed is attributed to energy transfer from ZnO host to the Er3+ ions doped. Energy transfer from e-h pairs resulting from ZnO host excitation may provide efficient routes for exciting Er3+ ions inside nano-crystalline particles of the films.
Zinc oxide is a wide bandgap insulator with significant promise for applications in optics, electro-optics and electronics.
However, there are challenges in growing high-quality material, and a prominent visible luminescence channel due to
defect recombination competes with the ultraviolet band-edge exciton decay. Here we demonstrate the possibility of
characterizing a specific defect by means of Purcell enhanced exciton-plasmon dynamics and photoluminescence.
Tb, Yb, and Ag co-doped glass nano-composites were synthesized in a lithium-lanthanum-aluminosilicate glass matrix
(LLAS) by a melt-quench technique. Ag nanoparticles (NPs) were formed in the glass matrix and confirmed by optical
absorption and transmission electron microscopy (TEM). Plasmon enhanced luminescence was observed. Cooperative
infrared to visible upconversion and visible to near-infrared quantum cutting were studied for samples with different
thermal annealing times. Because the Yb3+ emission at 940 - 1020 nm is matched well with the band gap of crystalline
Si, the quantum cutting effect may have its potential application in silicon-based solar cells.
Recently a melt-processed blend of
1,4-bis(α-cyano-4-octadecyloxystyryl)-2,5-dimethoxybenzene (C18-RG) dye and
polyethylene terephthalate glycol (PETG) has been demonstrated as a promising 3-dimentional optical data storage
(ODS) medium 1. ODS in this novel system relies on the laser-induced switching of the aggregation state of the excimerforming
fluorescent dye in the inert host polymer. Here we investigate the mechanism and the time scales involved in the
writing process. The optical writing was realized by the laser-induced localized excimer to monomer conversion and was
characterized by the emergence of the monomer fluorescence. We obtained the dependence of the excimer to monomer
conversion on the writing time. Our result indicates that the effective optical writing time is controlled by heating and
cooling time of the host polymer and the excimer-to-monomer conversion time. The effective laser writing time, under
the specific writing conditions employed in our experiments, is on the order of 10 ms.
Tb3+ and Ag co-doped glass nano-composites were synthesized in a glass matrix Li2O-LaF3-Al2O3-SiO2 (LLAS) by a melt-quench technique. The growth of Ag nanoparticles (NPs) was controlled by a thermal annealing process. A broad absorption band peaking at about 420 nm was observed due to surface plasmon resonance (SPR) of Ag NPs. The intensity of this band grows with increasing annealing time. The transmission electron microscopic image (TEM) reveals the formation of Ag NPs in glass matrix. Photoluminescence (PL) emission and excitation spectra were measured for glass samples with different Ag concentrations and different annealing times. Plasmon enhanced Tb3+ luminescence was observed at certain excitation wavelength regions. Luminescence quenching was also observed for samples with high Ag concentration and longer annealing time. Our luminescence results suggest that there are two competitive effects, enhancement and quenching, acting on Tb3+ luminescence in the presence of Ag NPs. The enhancement of Tb3+ luminescence is mainly attributed to local field effects due to SPR. The quenching of luminescence suggests an energy transfer from Tb3+ ions to Ag NPs.
Tb3+ and Ag co-doped glass nano-composites are synthesized in a glass matrix Li2O-LaF3-Al2O3-SiO2 (LLAS) by a melt-quench
technique. The nucleation and growth of Ag nanoparticles (NPs) were controlled by a thermal annealing process.
A broad absorption band peaking at about 420 nm was observed due to surface plasmon resonance (SPR) of Ag NPs.
Annealing of glass samples results in the growth of Ag NPs. Photoluminescence (PL) emission and excitation spectra
were measured from glass samples with different Ag concentrations and different annealing times. Plasmon enhanced
Tb3+ luminescence was observed at certain excitation wavelength regions. Luminescence quenching was observed for
samples with high Ag concentration and longer annealing time. Our luminescence results suggest that there are two
competitive effects, enhancement and quenching, acting on Tb3+ luminescence in the presence of Ag NPs. The
enhancement of Tb3+ luminescence is mainly attributed to local field effects: the SPR of Ag NPs causes an intensified
electromagnetic field around the NPs, resulting in enhanced optical transitions of Tb3+ ions in the vicinity. The
quenching effect in the presence of Ag NPs suggests an energy transfer from Tb3+ ions to Ag NPs. The competition
between the plasmonic enhancement and the quenching effect is discussed for samples with different Ag concentrations
and annealing times.
Nanostructured metal-ZnO systems provide an ideal workbench for studying the dynamics of exciton-plasmon coupling.
In order to characterize the interactions, we grew tri-layer structures comprising thin films of ZnO, variable-thickness
spacer layers of MgO, and thin films of Ag or Au. Analysis of the photoluminescence of these structures as a function
of increasing MgO thickness confirms the existence of surface plasmon polariton-exciton coupling through Purcell
enhancement of the excitonic emission for MgO films thinner than 30 nm, and through emission at the SPP resonance
for MgO films thicker than 30 nm. Further, we demonstrate the enhancement of the ZnO impurity photoluminescence
through dipole-dipole scattering with Ag and Au LSPs. Preliminary degenerate band-edge pump-probe measurements
confirm the conclusions developed from photoluminescence measurements. In order to disentangle and further quantify
the interactions seen in these systems, we are lithographically patterning metal nanoparticle arrays and metal hole arrays
on ZnO quantum wells and beginning to perform white-light pump-probe spectroscopy to fully characterize the
dynamics of energy transfer within these systems.
Richard Mu, Don Henderson, Akira Ueda, Marvin Wu, Johnathan Bennett, M. Paliza, M. Huang, J. Keay, Leonard Feldman, K. Kwiatkowski, C. Lukehart, Jennifer Hollingsworth, W. Buhro, J. Harris, E. Gordon, A. Hepp
Structural, optical and electrical characterization has been conducted on CuInS2 (CIS) thin film fabricated via spray CVD technique. Both RBS and Raman scattering analyses suggest that the film is slightly Cu rich by approximately 2%. XRD measurement indicates the film is polycrystalline CuInS2 with [220] orientation on a quartz substrate with a possible Cu1.7In0.05S secondary phase (< 3%) co-existing with CIS. The measured optical band gap of the film is about 1.44 eV. Hall effect measurements suggest that the film is p-type. Both measured mobility and resistivity are consistent with those of the bulk. The combination of AFM, STM and electrical measurements indicate that grain boundaries may be the charge carrier transport limiting factors. However, the presence of a secondary non- chalcopyrite phase is complicated the current study. It is still not clear the nature of the hole transport is due to so called `intrinsic doping' and `native defects' in the micron sized CIS crystals, or to the presence of a secondary phase or to the grain boundaries. It is shown that Raman and IR spectroscopy can be powerful tools to study the film stoichiometry, structural composition and molecular species present in the film.
Silicon ions were implanted into fused silica substrates at doses of 1 by 1021, 2 by 1021, 5 by 1021, and 1 by 1022 ions/cm3. The implanted substrates were subsequently annealed at 1100 degrees C for one hour in a reducing atmosphere. Optical absorption spectra recorded after the annealing treatment showed absorption onsets at 316, 373, 434 and 493 nm for substrates implanted with 1 by 1021, 2 by 1021, 5 by 1021, and 1 by 1022 ions/cm3 respectively. Static photoluminescence (PL) measurements indicated red emission between 720 and 770 nm with a slightly increasing red shift with ion dose. Time resolved PL at room temperature revealed slow and fast lifetimes which increased with decreasing temperature. TEM studies showed that the particles size increased with increasing ion dose with typical particle sizes ranging between 2 and 5 nm indicating quantum confinement of the exciton which can account for the blue shift in the absorption edge with decreasing ion dose. However, the maxima in the PL spectra for all ion doses are relatively independent of the ion dose and are strongly Stokes shifted from the absorption spectra suggesting that radiative recombination occurs from a common luminescent center, possibly a surface or interfacial state in the SiOx layer surrounding the nanocrystal.
Theoretical considerations of thermal lens effect due to linear and nonlinear optical absorption is presented. Based on this model, Z-scan technique, especially two-color Z-scan due can be used to detect very low level of impurities or defects in optical materials. Depending upon the optical cross section of the particular species being probed, two-color Z-scan can detect impurities, for example, the OH groups in fused silica at sub-ppm level by weight or better.
Infrared spectral hole burning studies have provided a wealth of information concerning site reorientation of defects in solids and vibrational relaxation dynamics. The most investigated systems appear to be impurities trapped in alkali halides. Limited studies on molecules trapped in noble gas matrices have demonstrated that these systems are good candidates for investigating persistent spectral holes. However, most infrared spectral hole burning studies have been limited by the tunability of commercially available infrared lasers which in turn restricts the spectral feature which can be burned. On the other hand, the tunability of Infrared Free Electron Lasers (IR-FELs) allows for targeting radiation into vibrational of the molecular system under study. We have used the Free Electron Laser-Fourier Transform Infrared Spectroscopy to investigate infrared hole burning of formic acid (HCOOD) isolated in an Ar matrix at a matrix/sample ratio of 4000/1. The results of the FEL radiation tuned to v2 mode of HCOOD are discussed together with matrix induced frequency shifts and matrix induced band splittings.
The vibrational dynamics of O-H groups in fused silica have been examined by a time- resolved pump-probe technique using the Vanderbilt Free Electron Laser (FEL). We consider two effects, local heating and transient thermal lensing, which can influence measured T1 values in one color pump-probe measurements. The dependence of these two effects on both the micropulse spacing and the total number of micropulses delivered to the sample are analyzed in detail for the O-H/SiO2 system. The results indicate that transient thermal lensing can significantly influence the measured probe signal. The local heating may cause thermally induced changes in the ground state population of the absorber, thereby complicating the analysis of the relaxation dynamics.
Infrared reflectance between 4000 and 100 cm-1 and optical spectra between 1.8 and 6.2 eV of high purity silica implanted with nominal doses of 1, 3, and 6 X 1016 Pb ions/cm2 were recorded before and after annealing at 400, 600, and 800 degree(s) C for 1 hour. Curve resolution analysis of the Si-O stretching region resulted in six peaks which were characterized by their lineshape parameters. The oscillator strength of the ion induced defect peak at 1035 cm-1 was found to depend on ion dose. The defect band at 1035 cm-1 decreased to an intensity comparable to that of the unimplanted glass after thermal annealing for 1 hour at 800 degree(s) C. Far infrared spectra indicated the formation of lead silicate particles after annealing.
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