Volume 90, Issue 16, 16 April 2007
- lasers, optics, and optoelectronics
- 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:
Fibonacci spirals are ubiquitous in nature, but the spontaneous assembly of such patterns has rarely been realized in laboratory. By manipulating the stress on Ag core/ shell microstructures, the authors obtained a series of Fibonacci spirals of definite chirality as a least elastic energy configuration. The Fibonacci spirals occur uniquely on conical supports-spherical receptacles result in triangular tessellations, and slanted receptacles introduce irregularities. These results demonstrate an effective path for the mass fabrication of patternedstructures on curved surfaces; they may also provide a complementary mechanism for the formation of phyllotactic patterns.
- LASERS, OPTICS, AND OPTOELECTRONICS
90(2007); http://dx.doi.org/10.1063/1.2722695View Description Hide Description
The authors propose the use of an optically trapped, dye doped polystyrene microsphere for spatial probing of the refractive index at any position in a fluid. Using the dye embedded in the microsphere as an internal broadband excitation source the authors eliminated the need for a tunable excitation source. They measured the full width at half maximum and frequency spacing of the transverse electric and transverse magnetic resonances as a function of the refractive index of the immersion fluid. The authors achieved a sensitivity of in refractive index, even when the exact size of the microsphere was not known.
90(2007); http://dx.doi.org/10.1063/1.2724922View Description Hide Description
An approach for wafer bonding based on boride-solution treatment was presented. The bonding energy is higher than the InP fracture energy by annealing at . An multiple-quantum-well(MQW) structure grown on InP was transferred onto Si substrate via the bonding process. X-ray diffraction and photoluminescence reveal that crystal quality of the bonded MQW was preserved. A thin oxide layer of about thick at the bonding interface was detected. X-ray photoelectron spectroscopy and Raman analyses indicate that the formation of oxygen bridging bonds by boridetreatment is responsible for the strong fusion obtained at such low temperature.
90(2007); http://dx.doi.org/10.1063/1.2723651View Description Hide Description
PbS quantum dots/multiwalled carbon nanotube (PbS/MWCNT) heterostructures were fabricated via a simple wet-chemical method. The structural and optical properties of as-prepared samples were characterized by transmission electron microscopy,x-ray diffraction, micro-Raman, and photoluminescence spectroscopies. It was found that the size, shape, and coverage density of the PbSquantum dots on the MWCNT can be controlled through varying synthesis conditions. Photoluminescence spectrum revealed that the emission of the PbSquantum dots was partially quenched when they were coupled to the MWCNT. This suggests that the photoinduced charge transfer may occur between the PbS and the MWCNT.
90(2007); http://dx.doi.org/10.1063/1.2723688View Description Hide Description
Pyramidal GaAs structures on top of distributed Bragg reflectors are investigated as candidates for true three-dimensional cavities with potentially low mode volume and high quality-factor. Different types of single and coupled resonators with base lengths of a few microns are realized using a combination of molecular-beam epitaxy, electron-beam lithography, and wet chemical etching. Embedded InGaAs quantum dots are utilized as light sources to verify the resonator modes. Furthermore, a spatially localized emission through the pyramid facets indicates the future possibility of coupling cavity modes to optical fibers. This could be interesting within the context of single photon emitters.
90(2007); http://dx.doi.org/10.1063/1.2722683View Description Hide Description
The degenerate fundamental mode of a microcavity pillar structure with circular cross section splits into two linearly polarized modes when the shape is changed to elliptical. The quality factor of these modes is very different. This letter demonstrates that the high mode provides better values of the figure of merit for strong coupling applications, , where is the modal volume, compared to values obtainable in circular structures. The difference in is shown to be a consequence of the polarization dependence of the losses through the microcavitymirrors.
90(2007); http://dx.doi.org/10.1063/1.2723078View Description Hide Description
We report unidirectional laser emission from -conjugated polymermicrocavities with broken symmetry geometries such as spiral and microdisk containing a “line defect,” in comparison with plain microdisk cavity having isotropic emission. We found that the laser emission directionality contrast ratio is 8–10 and far field lateral divergence angle is 12°–15° for both broken symmetry geometries, with no significant increase in the laser threshold intensity. Fourier transformanalysis of the laser emission spectra shows that unlike microdisks with line defect, the variation of light trajectories in the spiral microcavities leads to less defined laser modes.
90(2007); http://dx.doi.org/10.1063/1.2721134View Description Hide Description
To produce surface plasmons,silvernanostructures were fabricated using a thermal evaporation method and the surface plasmon wavelength was tuned via an annealing. These nanostructures were located between the indium tin oxide (ITO) and poly(3,4-ethylenedioxy thiophene)/poly(styrenesulfonate) for the coupling of the surface plasmon resonance with organic fluorophores. To prevent the quenching of emission, spacer was placed between the light emitting polymers and the ITO substrate. As a result, the authors were able to observe an increase in photoluminescence of conjugated polymers using the silvernanostructures.
90(2007); http://dx.doi.org/10.1063/1.2723677View Description Hide Description
The authors studied the laser emission from an anisotropic one-dimensional (1D) photonic crystal(PC) laser. An active medium layer, consisted of an epoxy resin doped with fluorescent dye, was sandwiched between two anisotropic 1D PCfilms. Efficient laser emissions were generated by optical pumping at relatively low lasing thresholds. The wavelengths of the emitted lasers were 611 and , which correspond to the two split eigenmodes at the low-energy band edges due to the anisotropy of the PCs. The authors also demonstrated that the polarization of the lasing emission can be controlled by adjusting the birefringence of the PCs.
Hole escape processes detrimental to photoluminescence efficiency in a blue InGaN multiple-quantum-well diode under reverse bias conditions90(2007); http://dx.doi.org/10.1063/1.2723683View Description Hide Description
Photoluminescence (PL) properties of a blue multiple-quantum-well (MQW) diode with an additional -doped electron reservoir layer (ERL) have been investigated at as a function of reverse bias under indirect barrier excitation. A PL intensity ratio of MQW/ERL is observed to be significantly quenched by increasing the reverse field due to electron-hole separation and carrier escape, in spite of observed blueshifts, when the excitation power is decreased by two orders of magnitude. The PL intensity reduction suggests that the hole escape process plays an important role for determination of the PL efficiency under the reverse bias.
90(2007); http://dx.doi.org/10.1063/1.2724903View Description Hide Description
The authors report on the enhancement of the light extraction efficiency of GaN-based light-emitting diodes(LEDs) via the texturing of -type layers. Compared with standard LEDs,LED fabricated with the textured -type layers produced a significant improvement in the output power, depending on the reflectivity of the electrode, the etch-pit size, and the chip dimension. The textured LEDs were found to yield the output power enhancement as high as 54%. However, it was also found that the electrical property of the textured LEDs can be degraded when the size of the etch pits is too large, indicating that a well-controlled texturing process is required for the realization of high-efficiency LEDs.
90(2007); http://dx.doi.org/10.1063/1.2724908View Description Hide Description
The authors report complementary investigations on the coherence properties of spontaneous and stimulated emission from quantum-dot-based high-quality semiconductor micropillar cavities. Low temperature microphotoluminescencemeasurements on an elliptically shaped micropillar revealed a clear polarization splitting of its fundamental mode. Full conformity is found with an oscillatory behavior observed in corresponding first-order field correlation measurements. In addition, power-dependent series on a single polarization component of the lasing mode have systematically revealed a strong coherence time increase from , which traces the change of emission characteristics from thermal to coherent light.
Influence of unsymmetrical periodicity on extraordinary transmission through periodic arrays of subwavelength holes90(2007); http://dx.doi.org/10.1063/1.2724914View Description Hide Description
Quadrate hole array is explored to study the influence of unsymmetrical periodicity on extraordinary optical transmission through periodic arrays of subwavelength holes. It is found that the transmission efficiency of light and the ratio between transmission efficiencies of horizontal and vertical polarized light can be continuously tuned by rotating the quadrate hole array. The authors can calculate out the transmission spectra (including the heights and locations of peaks) for any rotation angle with a simple theoretical model.
Experimental determination of the band gap dependence of Auger recombination in multiple quantum well lasers at room temperature90(2007); http://dx.doi.org/10.1063/1.2722041View Description Hide Description
The band gap dependencies of the threshold current and its radiative component are measured using high pressure techniques. Detailed theoretical calculations show that the band gap dependence of the internal losses plays a significant role in the band gap dependence of the radiative current. Temperature dependence measurements show that the radiative current accounts for 20% of the total threshold current at room temperature. This allows us to determine the pressure dependence of the non-radiative Auger recombination current, and hence to experimentally obtain the variation of the Auger coefficient with band gap.
90(2007); http://dx.doi.org/10.1063/1.2724919View Description Hide Description
The authors report the generation of pulses with a duration of from a neon filled hollow-core fiber seeded with carrier-envelope phase stabilized , pulses. The carrier-envelope phase after the fiber was measured by a second, out-loop -to-interferometer. With seed pulse power locked, the carrier-envelope phase of the two-cycle pulses is controlled to a standard deviation of . The peak power of the carrier-envelope phase stabilized pulses, , is twice that previously generated. The significance of seed pulse energy stability for carrier-envelope phase stabilization of few-cycle laser pulses is demonstrated.
Dichromatic InGaN-based white light emitting diodes by using laser lift-off and wafer-bonding schemes90(2007); http://dx.doi.org/10.1063/1.2722672View Description Hide Description
An InGaN-based dual-wavelength blue/green light emitting diode(LED) with three terminal operations has been designed and fabricated by using sapphire laser lift-off and wafer-bonding schemes. The device is equivalent to a parallel connection of blue and green LEDs; thus the effective electrical resistance of the device could be reduced. The luminous efficiency is at , accompanied by a broad electroluminescence emission with a combination of blue and green colors. This monolithically integrated dichromatic lighting structure has great potential in the application of the solid-state lighting.
90(2007); http://dx.doi.org/10.1063/1.2724902View Description Hide Description
The authors demonstrate three-dimensional erasable bit optical data storage in a quantum-dot doped photopolymer under two-photon excitation by a near-infrared femtosecond pulsed laser beam. It is shown that the photorefractive polymer consisting of poly(vinyl carbazole), ethyl carbazole, 4-(diethylaminobenzylidene)-malononitrile, and CdSquantum dots exhibits the changes not only in refractive index but also in fluorescence. Such a photosensitivity provides a multimode readout mechanism. In particular, the use of S rich surface quantum dots not only allows the two-photon-induced bit optical data storage with greater contrast but also expands the margin between permanent and erasable recording thresholds.
90(2007); http://dx.doi.org/10.1063/1.2724913View Description Hide Description
Using spatially resolved cathodoluminescence spectroscopy, the authors have measured the spectral and spatial distribution of the luminescence intensity of nanocolumns containing quantum disks. The optical emission of the quantum disk and of the thick (Al,Ga)N layer in the columns were clearly identified. The disk spectra of single columns are as broad as . A significant contribution to this broadening is probably due to the laterally inhomogeneous strain distribution within the disks. The optical emission of the thick (Al,Ga)N layer in the columns is spread out over a wide spectral range of several , which is caused by an inhomogeneous incorporation of Al along the growth direction of the columns.
90(2007); http://dx.doi.org/10.1063/1.2724921View Description Hide Description
The authors report low-power optical bistability under continuous wave pumping conditions in five-cell photonic crystal linear resonators containing InGaAsP quantum wells, by employing the fiber-coupling technique. The threshold bistable power is measured to be at the normalized detuning of . Owing to the high band-edge nonlinearities of quantum wells and the efficient fiber coupling, minimal instability is observed. In addition, all-optical switching is demonstrated with switching energy less than .
90(2007); http://dx.doi.org/10.1063/1.2724924View Description Hide Description
A method to enhance the photomixing efficiency by reducing the transit time of majority of carriers in photomixers and photodetectors to is proposed. Enhanced optical fields associated with surface plasmonpolaritons coupled with velocity overshoot phenomenon results in net decrease of transit time of carriers. As an example, model calculations demonstrating (or and at 1 and , respectively) improvement in terahertz power generation efficiency of a photomixer based on low temperature grown GaAs are presented.
Tunable ferroelectric photonic crystals based on porous silicon templates infiltrated by sodium nitrite90(2007); http://dx.doi.org/10.1063/1.2724928View Description Hide Description
Ferroelectric photonic band-gap(PBG)structures are fabricated by the infiltration of sodium nitrite into the nanoporous silicon templates composed by the electrochemical etching of -type silicon. Reversible spectral tuning of the PBG and of the microcavity mode up to is attained in the temperature interval from due to the temperature changes of the refractive index of sodium nitrite in nanoporous silicon matrix. Optical second-harmonic generation measurements prove the ferroelectric state of the composed nanocomposite PBGstructures.