摘要:

Site‐selective spectroscopy reveals that Nd3+ions occupy more than 40 different crystal‐field environments in Sr5(VO4)3F. Preferential energy transfer to the site responsible for 1 &mgr;m lasing occurs but becomes less complete with increasing temperature. The4Iand4F3/2Stark levels of the lasing site have been determined and an analysis of the crystal field performed. From the crystal‐field fitting parametersBkq, a calculated energy‐level spectrum is determined up to 17 500 cm−1with a rms deviation from the available experimental levels of 6 cm−1. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359387

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Third‐order optical nonlinearities and recombination processes of photoexcited electron‐hole plasma in PbSe and PbTe are directly monitored by picosecond pump‐probe experiments in the wavelength range from 3 to 10 &mgr;m. After excitation, a strong blue shift of the absorption edge and large changes of the refractive index are found. Time‐resolved studies of the absorption changes at lattice temperatures between 70 and 160 K reveal a fast partial decrease of the carrier density within 100 ps at excitation densities exceeding 7×1017cm−3. Comparative emission spectra demonstrate that this behavior is due to recombination by stimulated emission. At room temperature the nonradiative Auger recombination dominates on a 0.5–2 ns time scale below the threshold of stimulated emission. The overall Auger coefficients for carrier densities of about 1018cm−3are derived from the data and have values ofcAPbSe=1.1±0.3×10−28cm6/s andcAPbTe=2.5±1.3×10−28cm6/s. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359388

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Experimental evidences of both weak and strong confinement regimes are reported on CdS nanocrystals embedded in a sodium borosilicate glass matrix. A method, based on the sol‐gel technique, is used for the preparation of CdS‐activated glass. This route is capable of providing nanocrystals covering a wide range of radii with small size dispersion. Low‐temperature linear‐absorption spectra have been analyzed in terms of excitons and electron‐hole confinements by fitting the results of a numerical calculation to experimental findings. The model used, in the envelope‐function formalism, involves both a Lorentzian broadening of the exciton energy states inside each nanocrystal and a Gaussian size distribution. The improvement of crystal quality and the sharpening of the size distribution by thermal annealing is also studied versus both time and temperature of treatment. It is shown that we can keep a tight control on the crystallinity, average size, and size distribution of the nanocrystals by rather simple adjustments and short treatments. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359389

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

High slope efficiency and low threshold laser performance have been achieved for both long pulsed and cw operation at 1.065 &mgr;m in Nd3+‐doped strontium fluorovanadate crystal, Nd3+:Sr5(VO4)3F, when pumped by narrow band pulsed Cr:LiSAF and cw Ti:sapphire lasers. However, there are inequivalent Nd3+sites in the crystal. The absorption of Nd3+ions in secondary sites, sites other than the site which contributes to lasing, may reduce the pumping efficiency and, consequently, the lasing efficiency. Strong concentration quenching of the Nd3+4F3/2state was also observed reducing the quantum efficiency of the laser transition from this state. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359391

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Measurements are reported on the infrared (IR) absorption, the optical band gap, and the dark conductivity of amorphous silicon‐phosphorus alloy thin films (a‐Si:P) with 20–44 at. % P prepared by coevaporation of Si and P. The results show that the optical band gap can be tailored in a range of 1.5–2.15 eV by varying the P concentration and the annealing temperature. The band gap for the sample with 20 at % P is the widest (1.70–1.82 eV) when annealed at temperatures ≤600 °C. From the IR‐absorption study, a stretching mode associated with Si—P bonds at 465–474 cm−1has been found for all alloy films. An evolution (shift or sharpening) of the Si—P absorption band with annealing temperature and P concentration have also been observed. Based on the IR‐absorption study the change of bonding structures in the alloy films is discussed. Conductivity measurements show that two electron conduction processes mainly exist in the investigated temperature range: extended‐state conduction in the conduction band at high temperatures and hopping conduction in the band tail at low temperatures. The transition temperature is around room temperature (14–55 °C), indicating that the carriers may mainly drift by hopping even at room temperature. The correlation between the optical band gap and the bonding structure is discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359392

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Decays of the diffraction efficiency of gratings written in Fe:LiNbO3have been studied in connection with photorefractive fixing. Measurements have been carried at five different temperatures within the range of interest in photorefractive fixing, 144–168 °C. The decay curves are nonexponential and can be fitted to a sum of three simple exponential components, whose effective activation energies and preexponential factors have been determined. The dependence of the last exponential component on the grating period &Lgr; has been measured and a 1/&Lgr;2law has been found. The decay constants have also been found to depend on the time the crystal is kept at the setting temperature before writing the grating. The difficulties for establishing a model taking into account the occurrence of several sites for protons in LiNbO3are discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359393

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Infrared attenuated total reflection spectra from thin SiO2films sandwiched between a Ge prism and a Si substrate were investigated. The measurements were performed in the range of Si‐O‐Si stretching vibrations and compared with calculated spectra using bulk values for the SiO2dielectric function. This comparison enabled confirmation of the experimentally observed peak broadening and peak shift of the longitudinal‐optical‐phonon mode at ∼1250 cm−1for films thicker than 30 A˚ by using the exact expression for calculatingp‐polarized spectra. It is also shown that the linear approximation for vibrational spectroscopy in this frequency range is only valid for thicknesses less than 15 A˚. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359394

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Room‐temperature yellow luminescence is a distinctive signature of boron‐related deep‐level defects in 6H‐SiC. This yellow luminescence is associated with the boron‐relatedDcenter, rather than the more shallow boron acceptor. However, the reported activation energy for photoluminescence (0.7–0.73 eV) is in disagreement with theDcenter’s reported thermal activation energy (0.58–0.63 eV) as determined by deep‐level transient spectroscopy (DLTS). We show that this discrepancy can be eliminated by correcting the DLTS results for the temperature dependence of hole capture at theDcenter. By use of independent capture and emission measurements, and a two‐stage deep‐level capture model, theDcenter’s ground state is resolved to beEv+0.74 eV±0.02 eV, in good agreement with photoluminescence data. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359395

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Time‐resolved luminescence of transiently excited porous silicon reveals the dynamics of the excitation‐luminescence cycle in which localized excitons efficiently generate orange‐red luminescence from this material at room temperature. Upon transient excitation with nanosecond pulses, both blue and orange luminescence bands appear coincidently. At long time delays, the orange luminescence dominates. We show here that the time‐delayed orange luminescence is thermally activated by hopping‐related processes. The activation energy for a 500 ns delay is 36 meV. Our observations are in principle capable of resolving outstanding contradictions in the porous silicon luminescence literature. Quantum confinement on a wire fluctuating width provides the strong energy disorder which facilitates exciton localization. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359396

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The modulation of an intense electromagnetic beam induced by the acousto‐optic (AO) effect has been analyzed in a strain‐dependent semiconductor crystal. The effect of the diffusion of charge carriers due to the doping of the medium has been investigated using the coupled‐mode theory. The origin of the AO interaction is assumed to lie in the induced nonlinear current density of the medium. The AO modulation process has been treated as a four‐wave parametric mixing process and the effective third‐order acousto‐optic susceptibility characterizing the instability process has been deduced. The AO modulation is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the acoustic mode. The threshold characteristics and the steady state growth rates are estimated from the acousto‐optic polarization of the medium. Analytical estimation reveals that the modulated beam can be amplified in a dispersionless acoustic wave interaction regime in the presence of enhanced diffusion due to excess charge carriers. The relative magnitude of the pump field at various doping levels exhibits entirely different steady‐state gain characteristics. A particular pump fieldErexhibits maximum gain in the lightly doped regime while the same field exhibits a minimum due to reverse energy flow from electromagnetic fields to the collision dominated space‐charge field in a heavily doped medium. At very high densities with the electron plasma frequency of the medium close to the pump frequency or under the influence of very high (drift) pump field the acoustoelectric domains are washed out due to domination of the drift process over diffusion instability process. This leads either to gain saturation in lightly doped regime or explosive increment of the gain constant in heavily doped regime. The magnitude of the third‐order nonlinear optical susceptibility for III‐V semiconductors obtained from our theoretical analyses is found to agree well with the previously reported values. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359325

出版商:AIP    

年代:1995

数据来源: AIP