Volume 91, Issue 18, 29 October 2007
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
Germaniumnanowires (GeNWs) were used to enhance the properties of organic photovoltaic devices. GeNWs were grown to a length of on by the vapor-liquid-solid method catalyzed by Au seeds. Once grown, the GeNWs were dispersed in solution with poly(3-hexylthiophene) and spin cast into films. The photoluminescence and external quantum efficiency of the films indicated a significant increase in exciton dissociation and photocurrent generation. The results imply that the GeNWs may act as an electron acceptor for bulk heterojunction hybrid-inorganic/organic photovoltaic devices. The impacts of GeNW on device characteristics are discussed.
- LASERS, OPTICS, AND OPTOELECTRONICS
Observations of interior whispering gallery modes in asymmetric optical resonators with rational caustics91(2007); http://dx.doi.org/10.1063/1.2800308View Description Hide Description
We propose asymmetric resonantcavities with rational caustics and experimentally demonstrate interior whispering gallery modes in monolithic silicon mesoscopic microcavities. These microcavities demonstrate unique robustness of cavity quality factor against roughness Rayleigh scattering. Distinct resonant families and directional radiation from interior whispering gallery modes are observed experimentally using angle-resolved tapered fiber measurements and near-field images, which can be used for microcavity laser and cavity quantum electrodynamics applications.
91(2007); http://dx.doi.org/10.1063/1.2794785View Description Hide Description
Refractive index changes induced by ion beamimplantation can be used to produce photonic devices such as waveguides. Here, we relate the measured three-dimensional changes in refractive index produced by ion beamimplantation to modeling of the implantation process. We use a quantitative phase microscopic method in conjunction with a tomographic reconstruction process to determine the change in the refractive index distribution within a silica optical fiber that has been selectively implanted with ions. The index profile is compared with numerical simulations of the ion vacancy and ionization using the stopping range of ions in matter program.
91(2007); http://dx.doi.org/10.1063/1.2801704View Description Hide Description
Defect related contributions to the reduction of the internal quantum efficiency of InGaN-based multiple quantum welllight emitting diodes under high forward bias conditions are discussed. Screening of localization potentials for electrons is an important process to reduce the localization at high injection. The possible role of threading dislocations in inducing a parasitic tunneling current in the device is discussed. Phonon-assisted transport of holes via tunneling at defect sites along dislocations is suggested to be involved, leading to a nonradiative parasitic process enhanced by a local temperature rise at high injection.
Electrically controllable and polarization-independent Fresnel zone plate in a circularly symmetric hybrid-aligned liquid crystal film with a photoconductive polymer layer91(2007); http://dx.doi.org/10.1063/1.2802568View Description Hide Description
This work reports a Fresnel zone plate in a circularly symmetric hybrid-aligned liquid crystal(LC)film with a photoconductive polymer layer. An ultraviolet-induced electrodelike pattern of polymer layer under a zone plate photomask results in alternating major and minor portions of external voltage dropping on LC layer in conductive and nonconductive regions, respectively. These effects cause the discrepancy in LC reorientation between adjacent zones, generating a Fresnel zone plate. The focusing of the zone plate is electrically controllable and polarization independent. Additionally, the zone plate has advantages of a zero focusing in the voltage-off state and a very small operating dc field range from .
91(2007); http://dx.doi.org/10.1063/1.2805224View Description Hide Description
We report a 20-layer quantum wire infrared photodetectorgrown on (001)-axis InP substrate by molecular beam epitaxy. High density InGaAs quantum wires were formed, utilizing the strained-induced lateral-layer ordering process by growing a strain-balanced short-period superlattice. This device shows a unique polarized photoresponse which favors the normal-incident infrared radiation polarizing perpendicular to the wire orientation. The photoresponse at exhibited a peak detectivity of at .
91(2007); http://dx.doi.org/10.1063/1.2800823View Description Hide Description
We report the application of full-field swept-source optical coherence tomography for fingerprint detection. This system consists of a superluminescent diode as broadband light source and an acousto-optic tunable filter as wavelength-tuning device. The conventional optical coherence tomographic system was modified by coating aluminum oxide on one side of the beam splitter which is used as reference mirror and fingerprints on the glass slide as object. Low-coherence interferometry, nonmechanical scanning, and compactness are the main advantages of the proposed system over conventional fingerprint detection techniques. The present technique is noninvasive in nature and does not require any physical or chemical processing.
Fourfold increase of the ultraviolet electroluminescence from layers by fluorine coimplantation and flash lamp annealing91(2007); http://dx.doi.org/10.1063/1.2803855View Description Hide Description
Efficient ultraviolet electroluminescence (UVEL) is obtained from metal-oxide-silicon (MOS) structures with the active layers prepared by flash lamp annealing and Gd and F coimplantation. We observed a doubling of both the UVEL intensity and the defect related luminescence by increasing the fluorine concentration. This is due to suppression of the hot electron scattering on the donor-type level of the center, the number of which is reduced by fluorine, and to increase of the optical active centers by molecule formation. Also, fluorine coimplantation has no influence on the operating time of the MOS diode. Additionally, the flash lamp annealing doubled the ultraviolet electroluminescence from layers implanted by gadolinium alone or in combination with fluorine. This is related to the suppression of cluster formation of rare earth atoms occurring during conventional annealing methods.
91(2007); http://dx.doi.org/10.1063/1.2804001View Description Hide Description
An ultrathin ( thick) flexible light plate was designed and fabricated on a parylene template using a combination of self-alignment and lift-off techniques. The solid-state InGaNlight-emitting diodes(LEDs) was used as the light source to overcome the problem of conventional organic light-emitting devices which require perfect encapsulation against the permeation of water and oxygen. After the sidewalls of LEDs were passivated by the photodefinable polymer, the LED chip array can be further sandwiched by the indium-tin oxide (ITO) and Al electrodes to form a thin-film package with all the processing temperatures below . The ITO-coated transparent parylene template can be peeled off from the glass carrier after forming the ultrathin LED light plate. The flexible light plates present no damage even after they were flexed 1000 times around a -diameter cylinder. The present self-alignment or mask-less process is a very promising approach to flexible backlight applications.
91(2007); http://dx.doi.org/10.1063/1.2804005View Description Hide Description
This letter reports the properties of GaN-based green light-emitting diodes(LEDs) having a photonic crystal layer with a photonic bandgap (PCWG) and without a photonic bandgap (PCOG). With decreasing the photoluminescence(PL) detection angle from 140° to 60°, the enhancement of PL intensity of LED with PCWG was largely increased from 9 to 25 times, compared to that of LEDs without a patterned structure, while the PL intensity of LED with PCOG was increased from 4.6 to 5.6 times. The electroluminescence output power of green LEDs with a PCWG was enhanced about two times compared to LEDs with a PCOG. These results suggest that the light extraction of green LEDs can be greatly increased by using PCWG instead of PCOG.
91(2007); http://dx.doi.org/10.1063/1.2804572View Description Hide Description
We report a semiconductor based mechanism for electrically controlling the frequency of light transmitted through extraordinary optical transmission gratings. In doing so, we demonstrate active control over the surface plasmon (SP) resonance at the metal/dielectric interface. The gratings, designed to operate in the midinfrared spectral range, are fabricated upon a dopedGaAs epilayer. Tuning of over is achieved, and the devices are modeled to investigate the physical origin of the tuning mechanism. Though our structures are designed for the midinfrared, the tuning mechanism demonstrated could be applied to other wavelength ranges, especially the visible and near infrared.
91(2007); http://dx.doi.org/10.1063/1.2805018View Description Hide Description
The built-in electric field may impose a drift on charged point defects and may thus enhance or retard the intermixing during annealing.Electric field is built-in near the surface due to the pinning of surface Fermi level after argon plasma treatment on InPsurfaces of quantum well samples. Subsequent annealing leads to different intermixing results due to the different field directions on InP cap layers in different doping types. Experiments also showed different influences of the built-in field on the two sublattices largely due to different charge numbers of point defects on the respective sublattices.
91(2007); http://dx.doi.org/10.1063/1.2805073View Description Hide Description
Ordered ZnOnanorod arrays with almost uniform rod size have been grown perpendicularly on substrates by a controlled vapor phase transport growth method. The ZnOnanorods are  oriented single crystals with diameter of and length of , with a rod-to-rod spacing of . Photoluminescence spectra of the rod arrays indicate that the rods are of high crystal quality: very strong, well-separated bound and free exciton emission in the ultraviolet (UV) region are resolved at low temperature. Time resolved microphotoluminescence measurements are performed on single nanorods standing on the substrate which demonstrates lasing behavior with multiple UV lasing modes. Under quasistationary excitation lasing is observed up to room temperature. The lasing emission peaks are sharp, with a linewidth about , and have a fast decay time of . These high crystal quality nanorod arrays may be promising candidates for UV nanolaser devices.
Efficiency droop behaviors of multiple-quantum-well light-emitting diodes with varying quantum well thickness91(2007); http://dx.doi.org/10.1063/1.2805197View Description Hide Description
multiple-quantum-well(MQW)light-emitting diodes with varied InGaN quantum well thicknesses are fabricated and characterized. The investigation of luminous efficiency versus current density reveals a variety of efficiency droop behaviors. It is found that the efficiency droop can be drastically reduced by increasing the quantum well thickness of the MQW structures. On the other hand, relative internal quantum efficiency(IQE) measurements indicate that a thinner well results to higher IQEs owing to the greater spatial overlap of electron and hole distribution functions.
91(2007); http://dx.doi.org/10.1063/1.2805228View Description Hide Description
The use of metal stripe step-in-width Bragg gratings operating in the long-range surface plasmon-polariton mode as distributed Bragg reflectors in external cavity lasers is explored. Lasing using a gain element coupled via an optical fiber to such a grating is demonstrated at , and a high output polarizationextinction ratio of about is measured. Single mode lasing is feasible using direct end-fire coupled gain and grating chips, and wavelength tuning over a broad range is possible using thermally tuned Augratings embedded in polymer.
Efficiency enhancement using electron energy detuning in a laser seeded free electron laser amplifier91(2007); http://dx.doi.org/10.1063/1.2803772View Description Hide Description
We report the experimental characterization of efficiency enhancement in a single-pass seeded free-electron laser(FEL) where the electron energy is detuned from resonance. Experiments show a doubling of the efficiency for beam energies above the resonant energy. Measurements of the FEL spectra versus energy detuning shows that the wavelength is governed by the seed laser. The variation in the gain length with beam energy was also observed. Good agreement is found between the experiment and numerical simulations using the MEDUSA simulation code.
91(2007); http://dx.doi.org/10.1063/1.2803851View Description Hide Description
By using a semiconductor/metal grating formed on the episide of a quantum-cascade structure, distributed feedback lasing has been achieved with beam emission through the substrate. Using short-pulse excitation ( , ), single-longitudinal-mode operation near is demonstrated over wide ranges in heatsink temperature and drive current. The beam divergence in the longitudinal direction at a distance away from the wide aperture is .
91(2007); http://dx.doi.org/10.1063/1.2805019View Description Hide Description
In this paper, an electrical measurement method to identify shallow dopants in lowly dopedsemiconductornanowires was suggested. Room temperature electrical measurement indicates that electron concentrations of the nanowires are about . Temperature-dependentmeasurement of conductivities of single nanowires in low temperature region gives activation energy of , which is consistent with the reported activation energy of for Si donor in films with donor concentration of . Our results confirm that the shallow donors in the as-synthesized GaNnanowires are silicon. We consider such a method may be applicable to other semiconductornanowires.
Enlarging the bandwidth of nanoscale propagating plasmonic modes in deep-subwavelength cylindrical holes91(2007); http://dx.doi.org/10.1063/1.2803849View Description Hide Description
Subwavelength cylindrical holes in optically thick metallic films always support a propagating mode near the surface plasmon frequency, regardless of how small the holes are. For holes filled with a uniform dielectric material, the bandwidth of the mode asymptotically approaches zero as the hole size is reduced to deep-subwavelength scales. We show that it is possible to create nanoscale propagating plasmonic modes with a very large bandwidth in holes that are concentrically filled with two different dielectric materials, even when the hole radius goes to zero.
91(2007); http://dx.doi.org/10.1063/1.2804565View Description Hide Description
We report on the optical mode structure of laterally confined organic microcavities. For preparation, an organic semiconductor is evaporated through a mask with square sized holes, resulting in photonic dots with approximately diameter. Using a microscope setup, we observe a complex mode structure in transmission and photoluminescence. From the mode mapping, we conclude a strong three-dimensional optical confinement. The near and far field spectra are modeled by transfer matrix calculations and a Fourier transform of the internal electric field distribution, respectively.
Study of nonpolar -plane multiquantum well light emitting diodes grown by homoepitaxial metal-organic chemical vapor deposition91(2007); http://dx.doi.org/10.1063/1.2805029View Description Hide Description
Nonpolar -plane InGaN-based light emitting diodes(LEDs)grown on low-extended defect density bulk -plane GaN substrates offer great potential for high performance devices due to the absence of polarization-related internal electric fields. To optimize the quantum well(QW) structure, systematic sets of near blue-ultraviolet LEDs using different well widths, barrier widths, and QW periods were packaged and tested. With increasing current, high power LEDs were realized with fairly flat external quantum efficiency and blueshift-free peak wavelength for QWs with thicknesses from , barrier widths from , and QW numbers from 4 to 10.