摘要:

We have investigated the photoexcitation dynamics of poly(1,4-phenylene vinylene) (PPV) and poly(2-fluoro-1,4-phenylene vinylene) (PFPV) by using the femtosecond transient absorption spectroscopic technique. It was demonstrated that photoinduced absorption originates from nonemissive indirect polaron pairs which are more efficiently produced in PFPV than in PPV. The decay processes of polaron pairs are mainly contributed by collisional annihilation and internal conversion by phonon emission in PFPV and PPV, respectively. The differences in photoexcitation dynamics between PPV and PFPV are believed to be caused by the strong electronegativity of substituted fluorine atom. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120285

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Luminescence measurements of chromate ions(CrO42−)adsorbed onto disperse silica(SiO2)surfaces under intense pulsed ultraviolet excitation (&lgr;exc=354.7or 266 nm,&tgr;p=20 ns) were performed at room temperature. The luminescence spectra were collected for different values of the excitation intensity (I). The luminescence yield in the red band corresponding to emission fromCrO42−ions varies linearly withIunder 354.7 nm excitation. However, the intensity dependence of the luminescence yield under 266 nm excitation is characterized by a threshold(I∼0.6×106 W cm−2),after which the intensity dependence of the luminescence yield is described by a power law with indexn=1.7and then saturated atI∼1.0×106 W cm−2.This feature of the excitation dynamics has been attributed to stimulated emission and considered by means of a four-level scheme. The cross section of stimulated emission was estimated as∼3.3×10−21 cm2.©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119796

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Electro- and photoluminescence spectra of high-brightness light-emitting AlGaN/InGaN/GaN single-quantum-well structures are studied over a broad range of temperatures and pumping levels. Blue shift of the spectral peak position was observed along with an increase of temperature and current. An involvement of band-tail states in the radiative recombination was considered, and a quantitative description of the blue temperature-induced shift was proposed assuming a Gaussian shape of the band tail. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119797

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In a self-consistent multicomponent envelope function approach, we showed that&khgr;xxx(2)component of the second-harmonic susceptibility tensor of a symmetricp-type quantum well (QW) exists, because of the valence-band mixing at nonzero values of the in-plane wave vector. For a highly doped narrowGaAs/Al0.3Ga0.7AsQW, our calculation indicates that at low temperatures a large value(∼10−7m/V) of&khgr;xxx(2)can be achieved in the far-infrared frequency range without satisfying double resonance conditions. This value is comparable to the experimentally observed value of&khgr;zzz(2)inn-type asymmetric QW structures. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120432

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this letter, we demonstrate the ability of our reflection mode scanning near-field optical microscope functioning in the mid-infrared to reveal infrared dielectric contrast in absence of any topographical contrast. This contrast is induced by local structures prepared by low energy boron implantation in silicon. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119798

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A systematic study of the noise performances of polycrystalline silicon (polysilicon) thin-film transistors (TFTs) made by excimer laser crystallization is presented. The drain current spectral density of these devices shows an evident1/fbehavior and the origin of the noise was attributed to carrier number fluctuations. The flat-band voltage spectral density was found to be strongly correlated with the field-effect mobility, suggesting that the microscopic mechanism causing the carrier number fluctuations involves the localized states present at the grain boundaries. The noise level in the devices with the best electrical characteristics is comparable with that observed inc-Si metal–oxide–semiconductor field effect transistors, a major improvement if compared to polysilicon TFTs made by solid-phase crystallization. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119799

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have investigated the regrowth of GaAs/AlAs quarter-wave Bragg reflectors on patterned mesa InP-based quantum well heterostructures that can be fabricated into 1.55 &mgr;m vertical cavity surface emitting lasers. It is seen from transmission electron and scanning electron microscopy that the multiple layer GaAs-based mirrors can be grown on InP-based heterostructure mesas of diameters 10–40 &mgr;m without noticeable propagation of defects into the reflector layers or the quantum well region below. At the same time the photoluminescence from the quantum wells after regrowth indicates that lasers can be fabricated. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119800

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

An angle tuned silver gallium selenide(AgGaSe2)optical parametric oscillator (OPO), pumped by the fixed wavelength 1.57 &mgr;m output of a noncritically phase-matchedKTiOPO4OPO, yielded radiation continuously tunable from 6 to 14 &mgr;m. Energies of up to 1.2 mJ/pulse with bandwidths of∼5 cm−1   (full width at half-maximum) were obtained using a6.5×6.5×35.3  mmlong, type IAgGaSe2crystal. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119920

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Erbium-doped silica glasses prepared by the sol–gel process are desirable for applications in high-power lasers and integrated optical devices. However, its fluorescence could be quenched due to the high hydroxyl(OH−)content. Although thisOH−content can be greatly reduced through special gas treatment [Y. Zhou, S. S. Wang, H. L. Liu, Y. L. Lam, Y. C. Chan, and C. H. Kam, EEE J.8, 109 (1996).], the fluorescence is still considered weak for practical applications. We have used sol–gel aluminum codoping to increase the solubility of theEr3+ions in a silica glass host and, consequently, significantly enhanced the fluorescence intensity ofEr3+-doped sol–gel glass to the extent suitable for practical usage. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119801

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Polarization control is achieved in vertical-cavity surface-emitting lasers (VCSELs) by using structurally anisotropic self-assembled quantum dots (QDs) as active layers. The dot shape is long in the [01¯1] direction on the (100) surface. The photoluminescence from the dots has a polarization dependence whose intensity along the [01¯1] direction is 1.37 times stronger than that along the orthogonal [011] direction. The QD VCSEL operates at the wavelength of the QD ground state transition. Lasing emission shows polarization along the [01¯1] direction and an orthogonal polarization suppression ratio of 18 dB. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119802

出版商:AIP    

年代:1997

数据来源: AIP