Volume 90, Issue 11, 12 March 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
- interdisciplinary and general physics
Index of content:
Oxide and nitride nanotubes have gained attention for their large surface areas, wide energy band gaps, and hydrophilic natures for various innovative applications. These nanotubes were either grown by templates or multistep processes with uncontrollable crystallinity. Here the authors show that single crystal ZnOnanotubes can be directly grown on planar substrates without using catalysts and templates. These results are guided by the theory of nucleation and the vapor-solid crystal growth mechanism, which is applicable for transforming other nanowires or nanorods into nanotubular structures.
- LASERS, OPTICS, AND OPTOELECTRONICS
90(2007); http://dx.doi.org/10.1063/1.2713132View Description Hide Description
The authors investigate the potential of hollow core photonic crystal fibers (HCPCFs) for highly sensitive fluorescence detection by selectively coating the central hole of a HCPCF with fluorophores. After studying the influence of reabsorption and intermolecular effects on the emission, they show that dye concentrations down to can be detected using only nanoliter sample volumes. The results indicate that HCPCF are well suited for sensing applications involving internal emission or scattering processes, such as fluorescence based gas sensing or surface-enhanced Raman scattering, and may be integrated into compact optical devices.
Low threshold current density and negative characteristic temperature InAs self-assembled quantum dot lasers90(2007); http://dx.doi.org/10.1063/1.2713136View Description Hide Description
By combining optimized growth of the GaAs spacer layers and -type modulation doping of the quantum dots, a emitting self-assembledquantum dot laser exhibiting both a low threshold current density and negative-temperature behavior at room temperature is achieved. Spontaneous emission measurements provide no evidence for enhanced Auger recombination in doped devices. The negative exhibited by the doped device is consistent with a delayed thermalization of carriers within the quantum dot ensemble.
Observation of the splitting of degenerate surface plasmon polariton modes in a two-dimensional metallic nanohole array90(2007); http://dx.doi.org/10.1063/1.2713145View Description Hide Description
The authors report the experimental observation of the splitting of the degenerate surface plasmonpolariton modes excited in a metallic nanohole array integrated with microfluidic channel for delivery and precise control of the index of refraction of overlaying layer. They use high-resolution wavelength and angle scans for excitation of the surface modes and a polarizer-analyzer pair to suppress the nonresonant transmission. The two resultant modes include an intense, spectrally broad low-energy mode and a weaker, much narrower bandwidth high-energy mode.
90(2007); http://dx.doi.org/10.1063/1.2713868View Description Hide Description
The authors have considered the coexisting dressed four-wave mixing(FWM) and six-wave mixing (SWM) in an open four-level inverted Y configuration. The authors also report an experimental observation of optical pumping-assisted FWM and electromagnetically induced transparency (EIT)-assisted SWM. The efficient SWM can be selected by EIT window and controlled by the coupling as well as dressed field detuning and power. Due to EIT and optical pumping assistance, the enhanced SWM signal is more than ten times lager than the coexisting FWM signal.
High-efficiency switchable flat diffractive ophthalmic lens with three-layer electrode pattern and two-layer via structures90(2007); http://dx.doi.org/10.1063/1.2712773View Description Hide Description
A general design for switchable, flat, liquid crystal diffractive lens with three-layer electrode pattern and two-layer via structures is reported for near-, intermediate-, and distance-vision corrections of presbyopic eyes. The microfabricated transparent concentric ring electrodes are distributed in two layers and different voltages are applied to each electrode through bus lines in another layer. Connection between the electrodes and the bus lines is achieved by vias (conducting holes for vertical interconnections) in the third dimension. For demonstration, a lens is first tuned with a focal length of (1 diopter add power), eight-level phase modulation, and diffraction efficiency above 92% and then reconfigured to operate as a 2 diopter four-level lens with a diffraction efficiency of 78%. The lens operates with low voltages and a rise time of . This design methodology allows the aperture needed for practical use and a power-failure-safe configuration.
Image enhanced polymer-based multimode interference coupler covering and bands using deeply etched air trenches90(2007); http://dx.doi.org/10.1063/1.2713626View Description Hide Description
A design of multimode interference coupler with deeply etched air trenches at the boundary of the multimode section is proposed for photonic integrated circuitry on low-index-contrast materials. The device length decreases from for a polymer multimode interference coupler. Due to the enhanced optical confinement, the optimized coupler with air trenches experimentally achieved reduced insertion loss, improved contrast ratio for transverse-electric and transverse-magnetic polarization, compared with conventional multimode interference coupler. The device entirely covers the wavelength range of both and bands, which is sufficient for broadband communications.
90(2007); http://dx.doi.org/10.1063/1.2539745View Description Hide Description
The authors develop a rigorous theory of the enhancement of spontaneous emission from a light emitting device via coupling the radiant energy in and out of surface plasmonpolaritons (SPPs) on the metal-dielectric interface. Using the system as an example, the authors show that using SPP pays off only for emitters that have a low luminescence efficiency.
90(2007); http://dx.doi.org/10.1063/1.2713356View Description Hide Description
The authors fabricated and tested a compact optical sensor with an integrated waveguide and a microchannel in spiral geometry. The multimode waveguide, consisting of an SU-8 core of thick and wide and a fluid cladding layer of thick, realizes a light-fluid interaction length of within a device area of . The waveguidesensor measures changes in liquid sample concentration and is sensitive to changes in liquid refractive index. Absorbance measurements using the spiral waveguidesensor demonstrate increased sensitivity compared with a linear geometry sensor.
90(2007); http://dx.doi.org/10.1063/1.2713760View Description Hide Description
A GaSb based homojunction interfacial work function internal photoemission far-infrared detector is presented. Metal-organic vapor phase epitaxygrown samples show peak responsivity and a peak detectivity of Jones with effective quantum efficiency of 33% at and . The detector exhibits a free carrier response threshold wavelength. Results indicate that homojunction internal work function internal photoemissiondetectors are promising candidates to be a competitor for terahertz applications.
90(2007); http://dx.doi.org/10.1063/1.2713762View Description Hide Description
Optically pumped organic distributed feedback lasers, based on the same cavity geometry and polymeric active material, but patterned with different methods, are compared. In one case, the pattern is transferred into the polymer by spin coating the latter on top of a silica master. In the other case, the master is used as a mold to imprint a polymer film. The comparison shows that the imprinted structures, even though a lower quality copy of the master at morphological level, exhibit better performance. This result is checked for different laser cavities, and a qualitative explanation in terms of optical contrast is provided.
90(2007); http://dx.doi.org/10.1063/1.2713774View Description Hide Description
Nanowires can potentially be used with low-cost flexible plastic substrates for applications such as large-area displays and sensor arrays. However, high temperature processing steps such as thermal annealing that are incompatible with plastic substrates are still a major hindrance. Laser annealing permits localized energy input without affecting the underlying substrate and can help overcome this problem. In this study, the excimer laserannealing of siliconnanowires is demonstrated to be an efficient means of activating implanted dopants. The optical absorption of the nanowires is discussed and the effect of parameters such as fluence and number of pulses is investigated.
microcavities with double dielectric mirrors fabricated by selective removal of an (Al,In)N sacrificial layer90(2007); http://dx.doi.org/10.1063/1.2712786View Description Hide Description
Comparable microcavities with active regions containing distributed (In,Ga)N quantum wells,grown on GaN substrates and bounded by two dielectricmirrors, have been fabricated by two different routes: one using laser lift-off to process structures grown on GaN-on-sapphire templates and the second using freestanding GaN substrates, which are initially processed by mechanical thinning. Both exploit the properties of an layer, lattice matched to the GaN substrate and spacer layers. In both cases cavity quality factors are demonstrated by measurements of the cavity-filtered room-temperature excitonic emission near .
90(2007); http://dx.doi.org/10.1063/1.2713168View Description Hide Description
Using numerical simulations, the authors find that the electrons can be captured and accelerated to high energies (GeV) in a slit laser beam with an intensity of , where is the laser wavelength in units of μm. The range of the optimum incident energy is very wide, even up to GeV. These results are of interest for experiments because the relatively low intensity can be achieved with present chirped pulse amplification technique and a wide range of incident energies means that a multistage acceleration is possible.
Organic light-emitting diodes with photonic crystals on glass substrate fabricated by nanoimprint lithography90(2007); http://dx.doi.org/10.1063/1.2713237View Description Hide Description
The authors have fabricatedorganic light-emitting diodes(OLEDs) having two-dimensional photonic crystals(PC) as light extraction elements by employing nanoimprint lithography technique. PCs were imprinted on the glass substrate and OLED layers were formed on the imprinted side of the glass substrate. The device having PC showed the improvement of luminance by a factor of 1.5 compared to normal devices. The authors conclude that the nanoimprint lithography is very useful for the fabrication of the OLEDs with PC.
90(2007); http://dx.doi.org/10.1063/1.2713787View Description Hide Description
Three dimensional microstructure fabrication by two-photon polymerization is an established technique that normally uses single beam serial writing. Recently the use of a micro-optical element, to give multipoint beam delivery, was reported to give a degree of parallel processing. The authors describe an alternative approach to parallel processing using an axicon lens. This is a refractive element that, in combination with a high power microscope objective, efficiently transforms the laser beam from a Gaussian spot to an annulus. The authors demonstrate that the beam can polymerize a three dimensional shape, with nanoscale resolution. The use of more sophisticated refractive beam shaping is also discussed.
90(2007); http://dx.doi.org/10.1063/1.2712804View Description Hide Description
A photoluminescence study showed that the self-assembled InGaNquantum dots(QDs) provide strongly localized recombination sites for carriers and that the piezoelectric field-induced quantum-confined Stark effect(QCSE) is small because the height of QDs is too small to separate the wave functions of electrons and holes. The InGaNQDlight-emitting diode(LED) showed an emission peak at , and the peak was redshifted with increasing injection current, indicating a small QCSE. The light output power of an InGaNQDLED increased linearly with increasing injection current due to the strongly localized recombination sites of the InGaNQDs.
Weak coupling interactions of colloidal lead sulphide nanocrystals with silicon photonic crystal nanocavities near at room temperature90(2007); http://dx.doi.org/10.1063/1.2714097View Description Hide Description
The authors observe the weak coupling of lead sulphide nanocrystals to localized defect modes of two-dimensional silicon nanocavities. Cavity resonances characterized with ensemble nanocrystals are verified with cold-cavity measurements using integrated waveguides. Polarization dependence of the cavity field modes is observed. The linewidths measured in coupling experiments are broadened in comparison to the cold-cavity characterization, partly due to large homogeneous linewidths of the nanocrystals. The calculated Purcell factor [Phys. Rev.69, 681 (1946)] for a single exciton is 100, showing promise toward applications in single photon systems. These novel light sources operate near wavelengths at room temperature, permitting integration with current fiber communication networks.
90(2007); http://dx.doi.org/10.1063/1.2714098View Description Hide Description
Quantum cascade lasers emitting below are demonstrated. The lasers based on the material system emit at in pulsed mode with threshold current densities near at and operate up to room temperature. No degradation has been observed in laser performances compared with quantum cascade lasers emitting at . The obtained results show no influence of the valley in InAs on operation of these short wavelength quantum cascade lasers.
90(2007); http://dx.doi.org/10.1063/1.2714312View Description Hide Description
The authors describe the performance of submicron microdisk lasersfabricated within quantum well material working at room temperature. The smallest lasers, with diameters of approximately , feature ultrasmall mode volumes and exhibit single mode operation at low threshold powers. Their small cavity volumes of approximately enable microdisk lasers to be used as spectroscopic sources. Here the authors demonstrate the fabrication and characterization of visible, monolithically fabricated, submicron microdisk lasers.
- PLASMAS AND ELECTRICAL DISCHARGES
90(2007); http://dx.doi.org/10.1063/1.2714095View Description Hide Description
A laser-produced plasma was employed as an ultrafast shutter for shielding high-energy ions. Fast ions with energy on the order of keV were prevented from reaching the detector by a shutter plasma. The ion signal was suppressed to 1% of the unshielded peak ion signal. Suppression of the ion signal was observed up to from the target surface of the shutter plasma. The shutter closing time was at the distance of . The plasma shutter could operate even at a repetition rate. The high-density plasma acts as a shutter by capturing a low-density plasma.