Volume 86, Issue 11, 14 March 2005
- 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:
We have developed fully integrated nanotubecomposite materials through the functionalization of multiwall carbon nanotubes(MWCNTs) by covalently attaching ferritin protein molecules onto the surface of MWCNTs. The investigation of the thermomechanical behavior was performed by dynamic mechanical thermal analysis. Results demonstrated dramatic enhancement in the mechanical properties of PVA, for example a 100%–110% increase in the modulus with the addition of 1.5 wt % of ferritin functionalized MWCNTs. Samples containing functionalized nanotubes showed a stronger influence on glass transition temperature in comparison to composites containing the same amount of nonfunctionalized nanotubes.
- LASERS, OPTICS, AND OPTOELECTRONICS
Polarization anisotropy in the electroluminescence of -plane multiple-quantum-well light-emitting diodes86(2005); http://dx.doi.org/10.1063/1.1875765View Description Hide Description
multiple-quantum-well light-emitting diodes were fabricated on plane films grown on plane substrates. The  axis of the epitaxial film is parallel to the  axis of the substrate. The surface is striated, with features running perpendicular to the axis and a maximum surface height difference of . Electroluminescence shows strong polarizationanisotropy, with more light emitted with polarization perpendicular to the axis compared to parallel to the axis. An Ahrrenius fit of the polarization ratio indicates that there is a difference in the energy gap between the two polarization states. This suggests that a high polarization ratio can be maintained at the high temperatures and drive current densities required for high-power light-emitting diode applications.
Experimental demonstration of the slow group velocity of light in two-dimensional coupled photonic crystal microcavity arrays86(2005); http://dx.doi.org/10.1063/1.1882755View Description Hide Description
We recently proposed two-dimensional coupled photonic crystalmicrocavity arrays as a route to achieve a slow-group velocity of light (flat band) in all crystal directions. In this letter we present the experimental demonstration of such structures with a measured group velocity below and discuss the feasibility of applications such as low-threshold photonic crystal lasers with increased output powers, optical delay components, and sensors.
86(2005); http://dx.doi.org/10.1063/1.1883332View Description Hide Description
Midinfrared single mode emission, up to , for a distributed feedback structure consisting of lateral index variation on both sides of a rib waveguide is achieved on a quantum cascade laser. Fabrication was made possible by a high density plasma process developed for high aspect ratio etching of structures. A lateral grating considerably enhances design freedom by decoupling the vertical waveguide structure from the grating strength. The grating coupling, now being mainly determined by waveguide width, can then be exploited for device applications.
Continuous-wave laser oscillation on the transition of atomic iodine pumped by produced in an electric discharge86(2005); http://dx.doi.org/10.1063/1.1883317View Description Hide Description
Laser action at on the transition of atomic iodine is conventionally obtained by a near-resonant energy transfer from which is produced using wet-solution chemistry. The difficulties in chemically producing has motivated investigations into purely gas phase methods to produce using low-pressure electric discharges. In this letter, we report on the demonstration of a continuous-wave laser on the transition of atomic iodine where the used to pump the iodine was produced by a radio-frequency-excited electric discharge. The electric discharge was sustained in a gas mixture upstream of a supersonic cavity which is employed to lower the temperature of the continuous gas flow and shift the equilibrium of atomic iodine in favor of the state. The laser output power was in a stable cavity composed of two 99.99% reflective mirrors.
86(2005); http://dx.doi.org/10.1063/1.1880446View Description Hide Description
Vertical-emitting microcavity pillars with a type of GaInAs quantum dots within a one cavity have been realized based on high reflectivitydistributed Bragg reflectors. High-quality factors were achieved due to an improved fabrication technology with a maximum quality factor of 27 700 for a micropillar with a diameter of . The dot dimensions could be enlarged by one order of magnitude using a low strain nucleation layer.
86(2005); http://dx.doi.org/10.1063/1.1875747View Description Hide Description
Finite difference time domain computations is used to study surface plasmon (SP) excitation around - and -shaped ridge nanoapertures made in silverfilm. The SP enhances optical transmission, in addition to the transmission mechanism of the waveguide propagation mode and Fabry-Pérot-like resonance. However, the near-field collimation of ridge aperture is found completely destroyed. On the other hand, using a bowtie-shaped aperture with sharp ridges made in silver, the loss of near-field collimation can be recovered. A super resolution optical spot with full width half magnitude as small as is achieved due to the resonant SP excitation localized at the tips of bowtie. Much higher field enhancement is also obtained compared to the bowtie aperture made in chromium.
86(2005); http://dx.doi.org/10.1063/1.1875760View Description Hide Description
Dynamic behaviors of two coupled bistable elements optically connected in a series are theoretically predicted and discussed based on a phenomenological model with respect to photocarrier densities, where each bistable element is a self-electro-optic effect device (SEED). Feedback from one of the two SEEDs to the power of an incident light beam was introduced, which resulted in various types of oscillatory solutions and bifurcation structures depending on incident light power and choices of parameter values included in our model.
Enhanced performance of bipolar cascade light-emitting diodes by doping the aluminum oxide apertures86(2005); http://dx.doi.org/10.1063/1.1885168View Description Hide Description
Performance improvements in multiple-stage, single-cavity bipolar cascade light-emitting diodes including reduced operating voltages, enhanced light generation, and reduced device heating are obtained by doping intracavity aluminum oxide apertures with silicon. This doping results in a reduced electron energy barrier and, therefore, a reduced series resistance which leads to better power and heating characteristics. Nearly 50% reductions in operating voltages, 200% increases in light power, and increased operating range are demonstrated. We discuss the direct implications of these results for the design of bipolar cascade vertical-cavity surface-emitting lasers.
Evidence for charge-carrier mediated magnetic-field modulation of electroluminescence in organic light-emitting diodes86(2005); http://dx.doi.org/10.1063/1.1883322View Description Hide Description
Electroluminescence(EL) from organic light-emitting diodes can be surprisingly sensitive to modest magnetic fields . The origin of this magnetic-field effect has not been clearly identified, although the magnetic-field effect in some devices resembles that of delayed fluorescence in anthracene, which originates from magnetic-field-dependent singlet-exciton production via triplet-triplet annihilation (TTA). Here, we test the role of TTA at low magnetic fields by measuring transient EL and by employing dc drive levels so low that the bimolecular rate of TTA is unimportant. Under these conditions, we find enhancements of EL exceeding 14% at the lowest drives, which excludes TTA-mediated magnetic-field effects and indicates a role for charge-carrier pair states.
86(2005); http://dx.doi.org/10.1063/1.1883334View Description Hide Description
We describe the simultaneous excitation and direct far-field imaging of the scattering from surface plasmonpolariton modes in a two-dimensional metallic hole array grating. Conditions for the coupling and imaging are discussed, where the coupling is shown to be consistent with both measured and calculated dispersion relations. Excitation is accomplished at several different wavelengths (from 1.31 to ), incidence angles, and grating periods, enabling the observation of a number of distinct modes with various in-plane wave vectors.
Propagation loss measurements and Fabry–Pérot mode analysis using out-of-plane light scattering in photonic crystal waveguides86(2005); http://dx.doi.org/10.1063/1.1880447View Description Hide Description
An experimental technique is presented for the measurements of propagation losses in planar photonic crystal waveguides (PhC-WGs). When the guided light interacts with the air holes, it is partially scattered out of the propagation plane. The intensity of this diffracted light is measured along the PhC-WG and the propagation losses are deduced from its attenuation. To illustrate this, the Fabry–Pérot-type mode of a single-line defect PhC-WG is investigated. The propagation loss values obtained experimentally agree well with theoretical predictions. Finally, a study of single-line defect 60° bends shows that this technique also provides an efficient method to obtain modal information along PhC structures.
86(2005); http://dx.doi.org/10.1063/1.1885188View Description Hide Description
An external-cavity laser using an integrated two-segment design allows broad and continuous wavelength tuning with independent control of power and wavelength. The laser has a gain segment and a surface-emitting Bragg grating segment that was designed to be off-band for zero reflection inside the cavity, and with a strong free-space coupling to an external mirror to form a linear cavity. Wavelength control was achieved with a combination of coarse and broad wavelength tuning with the external mirror, and continuous, fine wavelength tuning via current-induced phase shift in the Bragg grating. Separate controls of the two segments allowed wavelength fine tuning without power variation and vice versa. The concept was applied to a laser, and is applicable to other wavelength.
86(2005); http://dx.doi.org/10.1063/1.1885182View Description Hide Description
Theoretical analysis was performed by means of the finite-difference time-domain method to examine the mode properties of two-dimensional surface-emitting photonic-crystal lasers with elliptical air holes. It was found that a single-lobed beam pattern could be obtained from the device surface by introducing a phase shift to the center of the crystal. The effects of the shift on the polarization mode and on two-dimensional optical coupling (which is needed for single-mode lasing over a large area) were also investigated. It was shown that the phase shift emphasized the linear polarization of the emitted beam due to symmetry reversal. On the other hand, it was also found that the shift did not spoil the two-dimensional optical coupling.
86(2005); http://dx.doi.org/10.1063/1.1885170View Description Hide Description
Topologyoptimization based on nonlinear programming techniques and design sensitivity analysis are used to maximize directional emission from a waveguide termination in a two-dimensional square lattice rods-in-air photonic crystal. Maximizing directional emission from a photonic crystal waveguide termination is desirable since a horn antenna style termination of comparable functionality is very large, while small simple terminations show strong angular spread. The optimized waveguide termination designed here is compact and exhibits a fivefold increase in power incident upon the target area over the simple termination. This improvement is achieved by creating surface modes that interfere to produce highly directional emission.
Measurement of subband electronic temperatures and population inversion in THz quantum-cascade lasers86(2005); http://dx.doi.org/10.1063/1.1886266View Description Hide Description
We compare the electronic temperatures and the population inversion both below and above the lasing threshold in three quantum-cascade lasers (QCLs) operating at , , and using microprobe band-to-band photoluminescence. In the lasing range, while the ground-statetemperature remains close to the lattice one , the upper radiative state heats up to . From the measured thermal resistance and the power dependence of the ground-state electronic temperature, we get a value of the electron-lattice energy relaxation rate comparable with that typical of midinfrared QCLs.
86(2005); http://dx.doi.org/10.1063/1.1883706View Description Hide Description
We propose and demonstrate a pulse compression technique using a hollow fiber with a pressure gradient. This technique improves the spatial and spectral qualities of femtosecond laser pulses spectrally broadened by self-phase modulation, and allows an increase of the pulse energy for pulse compression. Using chirped mirrors for dispersion compensation, we have successfully compressed the pulse to less than 10 fs with an energy as high as 5 mJ.
86(2005); http://dx.doi.org/10.1063/1.1883331View Description Hide Description
We report room-temperature light emission from erbium-doped silicon-rich oxide integrated in a silica microdisk. Silica disks are fabricated by standard optical lithography and etching techniques. Erbium-doped silicon-rich oxide is then deposited by coevaporation of silicon and erbium under oxygen flux. A spatially resolved photoluminescence experiment highlights the efficient coupling of the signal to whispering gallery modes when the excitation beam is focused near the edge of the disk. Quality factors as high as 3000 are measured, limited by the setup’s spectral resolution.
86(2005); http://dx.doi.org/10.1063/1.1886897View Description Hide Description
Highly transparent ceramics with uniform doping up to Nd and with (Nd, Yb) codoping are reported. The spectroscopic and emission decay properties of ceramics are similar to those of single crystals. Highly efficient Nd-to-Yb energy transfer is observed in the codoped ceramics. It is inferred that these materials have potential for construction of Nd or Nd-sensitised-Yb lasers.
86(2005); http://dx.doi.org/10.1063/1.1886903View Description Hide Description
We present a charge integration photondetector (CIPD) that enables the efficient measurement of photon number states at the telecom-fiber wavelengths with a quantum efficiency of 80% and a resolution less than 0.5 electrons at sampling. The CIPD consists of an InGaAs pin photodiode and a GaAs JFET in a charge integration amplifier, which is cooled to to reduce thermal noise and leakage current. The charge integration amplifier exhibits a low noise level of . The dark count is as low as .
86(2005); http://dx.doi.org/10.1063/1.1884262View Description Hide Description
A large-aperture design for terahertz traveling-wave photomixers, continuously pumped free space by two detuned diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs (LT-GaAs). It combines the advantages of conventional interdigitated small-area structures and traveling-wave devices. An output power of 1 μW at the mixing frequency of 1 THz was measured in initial testing, which meets local oscillator power requirements for superconducting heterodyne mixer devices.