Volume 91, Issue 9, 27 August 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:
The self-organized formation of quantum dot chains was investigated using x-ray scattering. Two samples were compared grown on GaAs(100) by molecular beam epitaxy. The first sample with a single layer of dots shows weak quantum dot alignment and a corresponding elongated shape along , while the top layer of a multilayered sample exhibits extended and highly regular quantum dot chains oriented along . Numerical calculations of the three-dimensional strain fields are used to explain the initial stages of chain formation by anisotropicstrain relaxation induced by the elongated dot shape.
- LASERS, OPTICS, AND OPTOELECTRONICS
91(2007); http://dx.doi.org/10.1063/1.2775809View Description Hide Description
A room temperature detector of terahertz laser radiation ellipticity has been developed based on the simultaneous measurements of three different photoelectric phenomena: circular photogalvanic effect, linear photogalvanic effect, and photon drag effect. Each of these effects, which have subnanosecond time constants, is monitored by different detector units stacked together in one detector.
91(2007); http://dx.doi.org/10.1063/1.2775811View Description Hide Description
Photoluminescence spectroscopy of single InAs quantum dots at cryogenic temperatures is performed using a micron-scale optical fiber taper waveguide as a near-field optic. A lower bound on the measured collection efficiency of quantum dot spontaneous emission into the fundamental guided mode of the fiber taper is estimated at 0.1%, and spatially resolved measurements with resolution are obtained by varying the taper position with respect to the sample and using the fiber taper for both the pump and collection channels.
Time resolved microphotoluminescence studies of single InP nanowires grown by low pressure metal organic chemical vapor deposition91(2007); http://dx.doi.org/10.1063/1.2776358View Description Hide Description
The authors report optical studies of InP nanowires (NWs) grown by metal organic chemical vapor deposition. By means of low temperature microphotoluminescence experiments, the authors determined the optical properties of as-grown NWs. The emission of individual NWs is characterized by small linewidths as low as . Blueshifts of the NW emission energy between 25 and with respect to bulk InP are related to radial carrier confinement in nanowires with diameters between 15 and . Time resolved investigations reveal a low surface recombination velocity of and indicate thermally activated nonradiative surface recombination above approximately .
91(2007); http://dx.doi.org/10.1063/1.2776857View Description Hide Description
Photoluminescence properties of silicate and borosilicate glasses codoped with and ions have been characterized by excitation and emission spectroscopies. When excited by ultraviolet light the glasses emit a combination of green and orange-red wavelengths giving white light. The ratio of the intensities of orange-red to green emissions can be tuned by varying both the concentration of the ion and an the composition of the glass matrix. The excitation and emission spectra show a self-quenching effect for the ions and an energy transfer from to .
Phase-matched second-harmonic generation in cross-linking polyurethane films by thermal-assisted optical poling91(2007); http://dx.doi.org/10.1063/1.2776876View Description Hide Description
Phase-matched second-harmonic generation (SHG) in thermal-cross-linking polyurethane films was demonstrated using a thermal-assisted nonresonant optical poling technique. During the seeding process, samples heated in an oven were irradiated simultaneously by coherent superposition of the fundamental and second-harmonic lights of a femtosecond laser. The photoinduced second-order optical nonlinearity of the polymer films seeded at elevated temperatures kept stable at room temperature. The measurements for the dependence of SHG on the film thickness showed that a grating that satisfied the phase-matching condition for SHG was optically induced in the polymer films.
Fabrication of two-dimensional photonic crystal patterns on GaN-based light-emitting diodes using thermally curable monomer-based nanoimprint lithography91(2007); http://dx.doi.org/10.1063/1.2776980View Description Hide Description
The fabrication process of photonic crystals in a layer was established to improve the light extraction efficiency of light-emitting diodes(LEDs) by using nanoimprint lithography and inductively coupled plasma(ICP)etching process. Due to low etch selectivity of imprinted pattern, Cr mask patterns were lifted-off from the surface and ICPetch process was followed using -based plasma. As a result, two-dimensional pillar array patterns were uniformly fabricated on the layer and the photoluminescence intensity of the photonic crystalpatternedLED was increased by 2.6 fold compared to that of the same LED sample without photonic crystalpatterns.
Observation of electroluminescence from low-temperature sputtered thin films on top of the heterostructure91(2007); http://dx.doi.org/10.1063/1.2777175View Description Hide Description
The authors report on the UV light emission from low-temperature sputtered thin films on top of the heterostructure. They compare samples with and without a current blocking layer. With a layer, electroluminescencespectrum shows a sharp emission peak at , which is attributed to the recombination of accumulated carriers between and junctions. As for the sample without a layer, a broadband ranging from is observed, which is due to deep-level transition in the GaN along with defects in the ZnO layers.
91(2007); http://dx.doi.org/10.1063/1.2775095View Description Hide Description
Coherent terahertz pulses have been generated at by frequency-mixing two laser lines near based on collinearly phase-matched difference-frequency generation in GaSe crystals. The highest average output power was measured to be , corresponding to a peak output power of . Both the average powers and normalized conversion efficiencies have been significantly improved over the previous results. The generated terahertz pulses are expected to have the narrowest linewidth dictated by the pulse width through Fourier transform based on the characteristics of the two laser beams.
91(2007); http://dx.doi.org/10.1063/1.2775326View Description Hide Description
The authors report a highly sensitive wavelength-encoded strain sensor made from a piece of photonic crystal fiber(PCF) spliced to standard fibers. The authors intentionally collapse the PCF air holes over a short region to enlarge the propagating mode of the lead-in fiber which allows the coupling of only two modes in the PCF. The transmission spectrum of the interferometer is stable and sinusoidal over a broad wavelength range. The sensor exhibits linear response to strain over a large measurement range, its temperature sensitivity is very low, and for its interrogation a battery-operated light emitting diode and a miniature spectrometer are sufficient.
Polarization engineering via staggered InGaN quantum wells for radiative efficiency enhancement of light emitting diodes91(2007); http://dx.doi.org/10.1063/1.2775334View Description Hide Description
Staggered InGaN quantum wells(QWs) grown by metal-organic chemical vapor deposition are demonstrated as improved active region for visible light emitters. Theoretical studies indicate that InGaN QW with step-function-like In content in the quantum well offers significantly improved radiative recombination rate and optical gain in comparison to the conventional type-I InGaN QW. Experimental results of light emitting diode(LED) structure utilizing staggered InGaN QW show good agreement with theory. Polarization band engineering via staggered InGaN quantum well allows enhancement of radiative recombination rate, leading to the improvement of photoluminescence intensity and LED output power.
91(2007); http://dx.doi.org/10.1063/1.2768640View Description Hide Description
The authors experimentally demonstrate a slot photonic crystalstructure for guiding light in a sub--wide low-index region. A multimode interference-based coupling structure is introduced to couple light into such a narrow slot photonic crystal waveguide. A coupler of long enhances the coupling efficiency by for the quasi-transverse-electric mode over optical bandwidth centered at . The measured transmission spectra are in good agreement with the simulated band diagram.
91(2007); http://dx.doi.org/10.1063/1.2776353View Description Hide Description
Focal plane array fabrication requires a well passivated material that is resistant to aggressive processes. The authors report on the ability of type-II superlattice heterodiodes to be more resilient than homojunctions diodes in improving sidewall resistivity through the use of various passivation techniques. The heterostructure consisting of two wide band gapsuperlattice contacts and a low band gap active region exhibits an averaging of . The devices passivated with , and or polyimide did not degrade compared to the unpassivated sample and the resistivity of the sidewalls increased to .
Resonant optical in-well pumping of an (AlGaIn)(AsSb)-based vertical-external-cavity surface-emitting laser emitting at91(2007); http://dx.doi.org/10.1063/1.2773970View Description Hide Description
The semiconductor heterostructure design and lasing characteristics of an optically in-well pumped (AlGaIn)(AsSb)-based vertical-external-cavity surface-emitting laser (VECSEL) emitting at are presented. The pump absorption in the active quantum wells at has been enhanced by a higher-order microcavity resonance. VECSEL operation in-well pumped by a thulium-doped fiber laser has been demonstrated. Compared to a VECSEL barrier pumped at , the in-well pumped device reaches a significantly higher power efficiency, and thus a higher output power at a given pump power, due to the smaller quantum deficit and hence reduced internal heat generation. Using an intracavity SiC heat spreader, a cw output power in excess of has been achieved at a heat sink temperature of , and still more than at .
91(2007); http://dx.doi.org/10.1063/1.2775043View Description Hide Description
White light was produced exploiting an additive synthesis of red, green, and blue fluorescence through frequency upconversion in fluorolead-germanate glass codoped with , , and . The 475, 540, and signals were, respectively, assigned to thulium and holmium and , excited via energy tranfer from ytterbium. The dopant concentrations were adjusted, yielding the emission of a wide color gamut in the visible spectrum and the production of white light using excitation at . The spectral positions and purity (blue, 97%; green, 100%; red, 95%) of the three colors produced and coordinates.
Random lasing emission from a red fluorescent dye doped polystyrene film containing dispersed polystyrene nanoparticles91(2007); http://dx.doi.org/10.1063/1.2778550View Description Hide Description
The authors report a random lasing emission from 4-(dicy-anomethylene)-2--butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)--pyran doped polystyrene thin films by introducing polystyrene nanoparticles. The aspects of concentration and diameter of polystyrene nanoparticles have been intensively investigated and found that the lasing occurs due to the scattering role of polystyrene nanoparticles. The devices emit a resonance multimode peak centered at a wavelength of with a mode linewidth of less than and exhibit threshold excitation intensity of as low as . The microscopic laser cavities formed by multiple scattering have been captured. The demonstration of random laser opens up the possibility of using organic scattering as alternative sources of coherent light emission.
Structural and lasing characteristics of ultrathin hexagonal ZnO nanodisks grown vertically on silicon-on-insulator substrates91(2007); http://dx.doi.org/10.1063/1.2779094View Description Hide Description
Hexagonal ZnO nanodisks were grown on silicon-on-insulator substrates by vapor-transport method without catalysts. The resulting nanodisks possess ultrathin thickness and most of them present vertical orientation on the substrate surface. Under optical excitation, these ultrathin nanodisks exhibit ultraviolet lasing emission with the excitation threshold of and emission lifetime of at room temperature. The detailed lasing analysis indicates that whispering-gallery-mode lasing may occur in the ZnO nanodisks.
Experimental demonstration of a wavelength demultiplexer based on negative-refractive photonic-crystal components91(2007); http://dx.doi.org/10.1063/1.2779927View Description Hide Description
Negative refraction of light was observed at near-infrared wavelengths in a silicon-on-insulator photonic-crystal-slab superprism having low-loss interface structures. It was used for wavelength demultiplexing as a diffraction grating, in combination with a photonic-crystal superlens used for focusing light beams. Coarse wavelength demultiplexing action with a channel spacing of was demonstrated in a device whose size was only (excluding input and output waveguides). These results agree well with those obtained by a finite-difference time-domain simulation.
91(2007); http://dx.doi.org/10.1063/1.2778759View Description Hide Description
The authors report a study of the linear electro-optic coefficient in waveguides containing quantum dots with a band gap at . The Pockels effect is investigated in the telecommunication window. The measured linear electro-optic coefficient for is , much higher than that of the bulk or quantum well material. An enhancement of the phase variation is achieved compared to that obtained in bulk GaAs waveguides. Finally, a spectral bandwidth of is demonstrated in the window.
91(2007); http://dx.doi.org/10.1063/1.2775321View Description Hide Description
We report an optical fiber tweezer based on high-index material for trapping and optical manipulation of microscale particles in water. The use of a high-index material increases the trapping force with respect to the more common silica, through tighter focusing of light. We demonstrate the potential of this simple and versatile device by trapping and rotating nematic liquid crystal drops. We monitor the rotation of the drop by detecting light modulation observed with the same fiber using backscattered light, which exhibits modulation in intensity due to the rotation of the drop; this further extends the capabilities of the fiber tweezers.
- PLASMAS AND ELECTRICAL DISCHARGES
91(2007); http://dx.doi.org/10.1063/1.2776021View Description Hide Description
The established contact charge relaxation scheme of Matsuyama and Yamamoto [J. Phys. D28, 2418 (1995);30, 2170 (1997)] is reassessed in light of the observations of Horn and Smith [Science256, 362 (1992)] and Horn et al. [Nature (London)366, 442 (1993)] of charge relaxation between separated charged surfaces. The multiple partial discharges observed in these studies are inconsistent with a scheme where onset and extinction are both described by a single Paschen curve. A modified version of the established scheme is therefore proposed.