摘要:

We present data concerning the dynamics of photoexcitation of defect centers in germanium‐doped silica glass optical fibers and fiber preforms. It is shown that two‐photon absorption of mode‐locked andQ‐switched light at 527 nm results in partial ionization of the germanium oxygen deficiency center (GODC) 400‐nm luminescence band. This bleaching induces loss in the 320–600‐nm wavelength range, which we argue is due to the introduction of Ge(1) centers and charges trapped at structural defects in the glass. The excitation and relaxation of dynamics of the GODC are analyzed by examining the photoluminescence and absorption spectra, and two models are proposed to explain the observed behavior.

ISSN:0021-8979    

DOI:10.1063/1.354625

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Thermal stresses in selectively etched and selectively grown GaAs/Si (100) stripes of different width‐to‐thickness ratios are investigated by low‐temperature cathodoluminescence (CL) spectroscopy and elastic finite element (FE) analysis. A qualitative description of the expected stress state based on FE results is followed by experimental stress measurement by low‐temperature (10 K) CL spectroscopy. Precise determination of the heavy‐hole (HH;mj=± 3/2) and light‐hole (LH;mj=± 1/2) excitonic peak energies through deconvolution of the CL spectra allows for measurement of the longitudinal and transverse stress components (parallel and perpendicular to the long stripe dimension, respectively). The CL results show that the longitudinal stress remains fairly constant (tensile) as a function of position, whereas the transverse stress decreases monotonically with decreasing distance from stripe edges, assuming small negative (compressive) values in the immediate vicinity of the latter. No significant differences were found between selectively etched and selectively grown stripes. The experimental results are compared with finite element elastic calculations and discussed in view of the experimental and computational parameters.

ISSN:0021-8979    

DOI:10.1063/1.354626

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The photoconduction of synthetic pyrite FeS2single crystals with low impurity concentration prepared by chemical vapor transport method is reported over a temperature range from 15 K to 300 K. The energy gap is determined from the photoconductive spectral response by the Moss rule. At room temperature the indirect energy gap is found to be 0.83±0.02 eV. According to the experimental evidence for a sharp antibondingp‐like state above the Fermi level of FeS2reported by Folkertsetal. [J. Phys. C20, 4135 (1987)], the indirect gap is assigned as the transition between Fe 3dt2gand S 3p&sgr;* states. The temperature variation of the indirect band gap shows a linear dependence between 100 K and 300 K with a negative temperature coefficience equal to 1.0±0.1 meV/K. A distinct feature at higher energy side of the spectrum is observed and attributed to the direct band‐gap transition. At temperature lower than 60 K, a very sharp peak around 0.9 eV emerged below the absorption edge. This feature might be associated with the excess carriers induced by the photothermal ionization between the impurity levels and conduction band.

ISSN:0021-8979    

DOI:10.1063/1.354627

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Coherent Si1−xGexalloys and multilayers synthesized by molecular beam epitaxy (MBE) on Si(100) substrates have been characterized by low‐temperature photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). Phonon‐resolved transitions originating from excitons bound to shallow impurities were observed in addition to a broad band of intense luminescence. The broad PL band was predominant when the alloy layer thickness was greater than 40–100 A˚, depending onxand the strain energy density. The strength of the broad PL band was correlated with the areal density (up to ∼109cm−2) of strain perturbations (local lattice dilation ∼15 A˚ in diameter) observed in plan‐view TEM. Thinner alloy layers exhibited phonon‐resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. Photoluminescence excitation spectroscopy, external quantum efficiency, time‐resolved PL decay, together with the power and temperature dependence of luminescence intensity, have been used to characterize Si1−xGex/Si heterostructures exhibiting both types of PL spectra. The role of MBE growth parameters in determining optical properties was investigated by changing the quantum well thickness and growth temperature. The transition from phonon‐resolved, near‐band‐gap luminescence in thin layers to the broad PL band typical of thick layers is discussed in terms of a strain energy balance model which predicts a ‘‘transition thickness’’ which decreases with increase inx.

ISSN:0021-8979    

DOI:10.1063/1.354628

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Single crystal epitaxial films of CdTe were grown on (0001) sapphire and (100) GaAs substrates by the ionized cluster beam deposition method. The low‐temperature photoluminescence spectra depend strongly on the ionization current and the acceleration voltage of the CdTe cluster. The change of the excitonic edge emission can be explained by the improvement of the stoichiometry due to the ionization. The crystalline quality of the epitaxial film was also improved by the small acceleration (0.5 kV).

ISSN:0021-8979    

DOI:10.1063/1.354629

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Theoretical investigations are presented of the electric‐field dependence of normal‐incidence interconduction subband absorption in Ga1−xAlxSb/AlSbL‐valley quantum wells. Under an applied electric field of 50 kV/cm, a blue shift of the absorption peak from 4.94 to 4.82 &mgr;m was found in a Ga0.7Al0.3Sb/AlSb structure with well width of 25 A˚. The ability to absorb normally incident light and to achieve significant Stark shifts with bias makes the Ga1−xAlxSb/AlSbL‐valley system an attractive choice for the 3–5 &mgr;m vertical optical modulators.

ISSN:0021-8979    

DOI:10.1063/1.354630

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An original configuration of propagation of acoustic shear wave in Paratellurite slightly off the [110] axis in the (001) plane is described. It allows greatly reducing the spreading of the acoustic energy. The main characteristics of an acousto‐optic interaction with this acoustic shear wave are then discussed. An experimental validation based on the schlieren method is finally proposed. It shows the effective reduction of the acoustic beam diffraction of the shear wave propagating with a 5° angular tilt. It is compared with the acoustic diffraction obtained for a shear wave propagating in TeO2along the [110] axis without angular tilt.

ISSN:0021-8979    

DOI:10.1063/1.354631

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The photoluminescence and optical absorption properties of Zn‐doped GaS crystals prepared by the iodine vapor transport method are reported. Undoped GaS crystals are also used for comparison. Zn substituted for a Ga site acts as an acceptor having a deep energy level and forms a complex with the iodine coactivator, which can be the luminescence center. When increasing the charged amount of Zn, the near‐blue emission band of 2.47 eV at 97 K becomes dominant, while the 2.17‐eV emission band due to the Ga vacancies, which is dominant in the undoped crystals, gradually vanishes because of the reduction of the Ga vacancies. It is thus described that Zn is a promising dopant in GaS for near‐blue‐light‐emitting devices.

ISSN:0021-8979    

DOI:10.1063/1.354632

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Intrinsic structural disorder in scandium‐substituted garnets, attributed to mixed occupancy of certain sites in the crystal lattice by different cations, has direct consequences for the optical spectra of rare‐earth activator ions dispersed over multiple sites. In trivalent thulium‐doped yttrium scandium gallium garnet (Tm3+:YSGG), site‐selective laser excitation spectra reveal the presence of Tm3+ions in regularD2sites, disturbed regular sites, and in octahedralC3isites. Absorption spectra obtained at 4 K between 0.26 and 1.85 &mgr;m are broader than those observed in more‐ordered crystal hosts and include structure attributed to Tm3+ions in sites of other thanD2symmetry. A crystal‐field splitting calculation was carried out in which a parametrized Hamiltonian (including Coulombic, spin‐orbit, and crystal‐field terms for Tm3+ions inD2symmetry) was diagonalized for all manifolds of the Tm3+(4f12) configuration. The rms deviation between 52 experimental and calculated Stark levels of Tm3+in regularD2sites was 5 cm−1.

ISSN:0021-8979    

DOI:10.1063/1.354633

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The ablation of polyimide by intense UV laser beams is modeled with a photothermal process that considers the shielding of the laser beam by the decomposed polymer. The heating of the decomposed material may drive the polymer degradation process for fluences above several J/cm2.

ISSN:0021-8979    

DOI:10.1063/1.354634

出版商:AIP    

年代:1993

数据来源: AIP