摘要:

We have systematically studied the optical properties of InGaN/GaN multiple quantum wells (MQWs) at 10 K under different excitation conditions using photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. We found that the PL emission consists of a strong main peak at 2.80 eV and a much weaker and broader secondary peak at∼2.25 eV.We observed that the peak position blueshifts and the spectral width narrows for the main peak when the excitation energies are varied from 3.81 eV (above the band gap of the AlGaN capping layer) to 2.99 eV (below the band gap of the GaN barrier layers). The intensity ratio of the main peak to the secondary peak also varied with excitation energy. The two observed emission peaks originate from different layers of the MQWs. Time-integrated and time-resolved PL revealed that the InGaN-related spontaneous emission processes are strongly affected by inhomogeneity and carrier localization in the MQWs. From these studies under varying excitation energies, we conclude that interface-related defects and roughness may play an important role in the InGaN-related emission mechanism during the carrier transfer between different layers of the MQWs. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122711

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Terahertz waveforms measured by free-space electro-optic sampling and a photoconductive dipole antenna were carefully compared. We show that the difference between the waveforms could be explained quantitatively in terms of carrier lifetime and frequency dependent response of the photoconductive receiver antenna. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122712

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A micro-optoelectromechanical switch that combines microactuator technology developed via the Lithographie Galvanformung Abformung process with lithographically defined polymeric alignment elements is described. The multimode optical switch achieves submillisecond switching times, low insertion loss (<1 dB), low cross talk (<70 dB), low voltage (3 V), and wavelength independence. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122713

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We demonstrate a composite system consisting of a GaAs quantum well structure placed in the evanescent field of a fused silica microsphere and show evanescent coupling between excitons in the quantum well and high-Qwhispering gallery modes of the composite system. The composite system allows separate engineering of photonic and electronic confinement and can take advantage of high-Qfactor and small mode volume of fused silica microspheres. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122714

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A saturable absorberCr4+:YAG,used for a passiveQswitching of Nd:YAG laser was studied in order to understand the physical mechanism of nonlinear absorption (photobleaching). An optical bleaching experiment was carried out using the two pulsed lasers with duration of pico- and nanoseconds at 1.064 &mgr;m wavelength. Experimental results were compared with numerical analysis of theoretical rate equations with all relevant energy levels. The inclusion of intersystem crossing between singlet- and triplet-excited states with reasonable rate of transitionkiscwas necessary to explain the difference in the saturation behaviors for nanosecond and picosecond laser pulses. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122715

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Photoluminescence and photoluminescence excitation (PLE) spectroscopy have been carried out on the ∼1550 nm4I13/2→4I15/2 Er3+emission from thin films of Er-dopedAs40Ge10Se25S25glass deposited on silicon substrates by radio frequency sputtering. The PLE spectroscopy shows that theEr3+emission is excited in the 1-&mgr;m-thick film by the Urbach absorption edge of the host glass rather than direct absorption by theEr3+intra-4fshell transitions. This enables the use of a broad range of pump wavelengths and novel pumping geometries. Comparison of the PLE spectra of the Er emission obtained before and after a rapid thermal anneal (RTA) clearly manifests a blue shift in the band gap induced by the RTA process. These results reveal that the broad band Er PLE mechanism discovered recently in Er-doped bulk chalcogenide glasses and attributed to host glass Urbach edge optical absorption and energy transfer mediated by native defects also occurs in sputtered films of Er-doped chalcogenide glass. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122716

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The concept of trapping vacuum-arc-generated macroparticles by a liquid is introduced. Experiments have been performed using a pulsed vacuum arc plasma source operating with a carbon cathode. Vacuum pump oil was used as the trapping liquid in vacuum. It was experimentally shown that trapping did not work as anticipated: a substantial number of particles experienced elastic reflection from the liquid surface. With simplified energy and momentum balances it is shown that the liquid behaves increasingly like a solid when the approaching particle is fast. A significant portion of its kinetic energy is transformed into deformation energy which can reappear as kinetic energy of the reflected particle. Particle reflection is likely to be additionally supported by a Leidenfrost-type effect: a thin oil vapor layer of relatively high pressure can be formed between the liquid surface and the macroparticle caused by the macroparticle’s high temperature. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122717

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We investigate the doping of vanadyl–phthalocyanine by a fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic dye films by cosublimation of matrix and dopant. The electrical parameters of the films derived from conductivity and Seebeck measurements show that the results largelyfollow standard models used to describe the doping of crystalline semiconductors; e.g., a smooth shift of the Fermi level towards the valence states with increasing doping is observed. Other effects, like the superlinear increase of conductivity with the molar doping ratio, need the inclusion of additional effects like percolation. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122718

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The surface structure of molecular beam epitaxially grown HgTe(001) has been studied underin situultrahigh vacuum conditions. The as-grown samples were investigated by means of x-ray photoelectron spectroscopy (XPS), spot profile analysis of low-energy electron diffraction, reflection of high-energy electron diffraction and scanning tunneling microscopy (STM). They exhibited ac(2×2)surface reconstruction with an additional weak(2×1)component in both diffraction experiments. The surface was shown by XPS to be terminated with Hg atoms. In addition, by means of STM experiments, we have been able to resolve the atomic structure of the reconstructed surface and to detect domain boundaries along the[11¯0]direction whose local symmetry is twofold and, therefore, the probable cause of the weak(2×1)reconstruction. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122719

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We have observed the formation of heteroepitaxial interfacial layers between silver nanoparticles and a single crystal copper surface by a phenomenon we term “contact epitaxy.” Upon depositing Ag nanoparticles (5–20 nm diameter) onto clean (001) Cu in an ultrahigh vacuumin situtransmission electron microscope, a thin (111)-oriented layer of Ag was detected at the interface between the substrate and particles. Molecular dynamics simulations reveal that the epitaxial layers form within picoseconds of impact, with rapid alignment arising from mechanical relaxation of the highly stressed interface formed upon initial contact. The simulations also show that multiple grains form in the nanoparticle as a consequence of this relaxation process. The unique structure of the nanoparticles, induced by contact epitaxy, is expected to significantly influence physical properties such as interfacial bonding, diffusion, chemical activity, and electrical transport, as well as forming a nucleus for grain growth and epitaxy which we also observe. Due to its simple origin, the phenomenon should also apply to materials systems beyond the field of nanoparticles with implications for cluster deposition, adhesion, rheology, and catalysis. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.122720

出版商:AIP    

年代:1998

数据来源: AIP