Volume 92, Issue 13, 31 March 2008
- 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
- organic electronics and photonics
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
We report the experimental demonstration of a beam curvature in graded photonic crystals via a spectacular mirage effect. A two-dimensional structure of metallic rods is constructed to produce this effect in the microwave domain near . Experimental results are in excellent agreement with theoretical predictions, thus, proving the versatility of graded photonic crystals in view of their integration in future photonic circuits.
- LASERS, OPTICS, AND OPTOELECTRONICS
High efficiency diffractive grating couplers for interfacing a single mode optical fiber with a nanophotonic silicon-on-insulator waveguide circuit92(2008); http://dx.doi.org/10.1063/1.2905260View Description Hide Description
High efficiency diffractivegratingstructures to interface a single mode optical fiber and a nanophotonic integrated circuit fabricated on silicon-on-insulator are presented. The diffractivegratingstructures are designed to be inherently very directional by adding a silicon overlay before grating definition. 55% coupling efficiency at a wavelength of is experimentally demonstrated on devices fabricated using standard complementary metal-oxide semiconductor technology. By optimizing the grating parameters, we theoretically show that 80% grating coupling efficiency can be obtained for a uniform gratingstructure.
92(2008); http://dx.doi.org/10.1063/1.2904619View Description Hide Description
We report on a simple approach to obtain spectrally smoothed supercontinuum (SC) by using unstably mode-locked femtosecond laser pulses. The spectrum smoothing effect is due to the averaging of the supercontinua generated in a nonlinear photonic crystal fiber by modulated pulses. Theoretical analysis based upon solving the generalized nonlinear Schrödinger equation is presented, which qualitatively agrees well with the experimental observation. Finally, the application of the smoothed SC to broadband coherent anti-Stokes Raman scattering spectroscopy is demonstrated, which can result in a relatively smooth nonresonant background and suppress spurious peaks.
92(2008); http://dx.doi.org/10.1063/1.2904620View Description Hide Description
We present a fiber-based two-port terahertz electro-optic (EO) sampling system at wavelength, including an ultrafast photoconductive switch and a freely positionable prismatic EO probe. Frequency components are extended up to and the dynamic range is larger than , regardless of the direction of the electromagnetic wave propagating in the waveguide, thanks to the two-port system. The symmetrical two-port pigtailed EO probe allows to determine the direction of propagation of the guided picosecond electromagnetic pulses.
92(2008); http://dx.doi.org/10.1063/1.2904623View Description Hide Description
We have developed a method conceptually different from ellipsometric techniques which allows the determination of the complex refractive index by simultaneously measuring the unpolarized normal-incidence reflectivity relative to the vacuum and another reference media such as diamond, GaAs, CdTe, etc. From these two quantities, the complex optical response can be directly obtained without Kramers–Kronig transformation. Due to its transparency and large refractive index from the far-infrared to the soft ultraviolet regions, diamond can be ideally used as a second reference over the whole optical spectrum. The experimental arrangement is rather simple compared to ellipsometry.
Mechanisms of subdiffraction free-space imaging using a transmission-line metamaterial superlens: An experimental verification92(2008); http://dx.doi.org/10.1063/1.2904635View Description Hide Description
This work presents experimental verification of free-space subdiffraction imaging using a Veselago-Pendry superlens [Sov. Phys. Usp.10, 509 (1968);Phys. Rev. Lett.85, 3966 (2000)] based on the negative-refractive-index transmission-line approach. The superlens is able to resolve two sources apart not only at the design frequency of , where the metamaterial possesses and equal to and , respectively, but also at , where the metamaterial experiences a resonance in .
92(2008); http://dx.doi.org/10.1063/1.2904698View Description Hide Description
We describe optical and topographic imaging using a light emitting diode monolithically integrated on a silicon probe tip for near-field scanning optical microscopy (NSOM). The light emission resulted from a silicon dioxide layer buried between a phosphorus-doped silicon layer and a gallium-doped silicon region locally created at the tip by a focused ion beam. The tip was employed in a standard NSOM excitation setup. The probe successfully measured optical as well as topographic images of a chromium test pattern with imaging resolutions of 400 and , respectively. The directional resolution dependence of the acquired images directly corresponds to the shape, size, and polarity of the light source on the probe tip. To our knowledge, this report is the first successful near-field imaging result directly measured by such tip-embedded light sources.
In-line phase-contrast imaging of a biological specimen using a compact laser-Compton scattering-based x-ray source92(2008); http://dx.doi.org/10.1063/1.2903148View Description Hide Description
Laser-Compton scattering(LCS)x-raysources have recently attracted much attention for their potential use at local medical facilities because they can produce ultrashort pulsed, high-brilliance, and quasimonochromatic hard x rays with a small source size. The feasibility of in-line phase-contrast imaging for a “thick” biological specimens of rat lumbar vertebrae using the developed compact LCS-X in AIST was investigated for the promotion of clinical imaging. In the higher-quality images, anatomical details of the spinous processes of the vertebrae are more clearly observable than with conventional absorption radiography. The results demonstrate that phase-contrast radiography can be performed using LCS-X.
92(2008); http://dx.doi.org/10.1063/1.2903714View Description Hide Description
Propagation losses are paramount to the performance of microphotonic devices. In siliconphotonics, the expected contribution of known propagation-loss mechanisms is often insufficient to account for all the observed loss. Here, we identify a loss mechanism that we believe has not yet been reported in the literature. We observe loss reaching in silicon wire waveguides patterned in proximity of metals with low temperatures of silicide formation. The loss is attributed to formation of a dilute silicide at the waveguide sidewalls during reactive-ion etching.Sputteredmetal atoms originate from exposed metal on the wafer surface or from the reactive-ion etcher chamber and react with the bare silicon of the waveguide sidewall being formed.
92(2008); http://dx.doi.org/10.1063/1.2904696View Description Hide Description
Using a microscopic many-particle theory, we predict all-optical amplification of low-intensity light pulses in resonantphotonic bandgap structures realized by Bragg-spaced semiconductormultiple quantum wells. The discussed scheme is based on mirrorless parametric amplification.
92(2008); http://dx.doi.org/10.1063/1.2902174View Description Hide Description
We report a continuous wave laser output of a spiral-shaped InGaAsmicrocavity laser. When the boundary of the cavity is selectively pumped by current injection with a dc current, the laser generates a directional emission around the notch. We investigate the characteristics of the cw laser operation and observe whispering gallery type modes, whose factor is about 7000.
92(2008); http://dx.doi.org/10.1063/1.2905285View Description Hide Description
The propagation of microwaves through a chiralmetamaterial based on a magnetic dimer is experimentally studied. As proposed by our previous theoretical model, two resonance peaks are obtained in the transmission spectrum; these originate from the hybridization effect of magnetic resonance modes in this system. Optical activity is also observed in the transmission wave. The polarization state dramatically changes around the resonance frequency: the transmitted wave becomes elliptically polarized with its major polarization axis approximately perpendicular to that of the linear incident wave. This coupled magnetic dimer system provides a practical method to optically design tunable active medium and device.
92(2008); http://dx.doi.org/10.1063/1.2905295View Description Hide Description
We present an experimental demonstration of a polarization quasi-independent narrow-band (less than ) filter operating under high oblique incidence. The structure is a resonant grating with a hexagonal lattice which has been carefully designed to ensure polarization quasi-independence of the narrow-band resonance peak over a wide angular range around 60° of incidence. A good agreement between experimental results and theoretical calculations is shown.
Control of optical mode distribution through etched microstructures for improved broad area laser performance92(2008); http://dx.doi.org/10.1063/1.2906030View Description Hide Description
Etching microstructures into broad area diode lasers is found to lead to more uniform near field and increased power conversion efficiency, arising from increased slope. Self-consistent device simulation indicates that this improvement is due to an increase in the effective internal injection efficiency above threshold—the nonuniform near field leads to regions of inefficient clamping of the carrier density in the laser stripe. Measurements of spontaneous emission through the substrate confirm the predicted carrier profile. Both experiment and theory show that improved overlap between carrier and power distributions correlates with improved slope.
92(2008); http://dx.doi.org/10.1063/1.2906904View Description Hide Description
We report on large three-photon absorption (3PA) in thioglycol-capped ZnSsemiconductorquantum dots(QDs) at different wavelengths with femtosecond Z-scan technique. Both the intrinsic 3PA coefficients and cross sections of the sized ZnSQDs in aqueous solution are nearly one order of magnitude greater than that of ZnSbulk crystal. The 3PA resonance in ZnSQDs is observed at the lowest excitonic transition of .
A route to increase the enhancement factor of surface enhanced Raman scattering (SERS) via a high density Ag flower-like pattern92(2008); http://dx.doi.org/10.1063/1.2895639View Description Hide Description
We explored a route to prepare a high enhancement factor of SERS substrate via a high density of Ag flowerlike pattern. The finite difference time domain(FDTD) calculations indicate that the Ag flowerlike pattern may demonstrate a high quality SERS property owing to the high density and abundant hot spot characteristic. Using an unusually high overpotential with electrodeposition system, the fractal flowerlike patterns and the high density nanoparticle arrays were experimental synthesized. The SERS measurement of above different Agnanostructures verified the predications from the FDTD calculation.
92(2008); http://dx.doi.org/10.1063/1.2898138View Description Hide Description
An uncooled microbolometer focal plane array (FPA) has been developed and used for imaging of objects illuminated by monochromatic coherent radiation of a free electron laser tunable in the range of . A sensitivity threshold of was obtained for the FPA with a homemade absolute interferometric power meter. Videos up to were recorded in both transmission and reflection/scattering modes. When objects were illuminated by laser radiation scattered by a rough metal surface, speckled images were observed. Good quality terahertz images were achieved through the fast rotation of the scatterer.
92(2008); http://dx.doi.org/10.1063/1.2907335View Description Hide Description
The electron capture time in superlattice structures consisting of periodically spaced layers of self-assembled ErAs nanoislands and is investigated on photoconductive switches as a function of the superlattice period using photocurrent autocorrelation and pulsed laser excitation at . The capture time can be tuned from picoseconds all the way down to by changing the periodicity. Two different Be doping schemes are explored to reduce the dark current. The resulting characteristics indicate that ErAs:InGaAs may serve as a high performance photoconductive material at this wavelength for pulsed terahertz emission and detection.
The physical picture and the essential elements of the dynamical process for dispersive cloaking structures92(2008); http://dx.doi.org/10.1063/1.2840703View Description Hide Description
We investigate the dynamical process of dispersive cloak by finite-difference time-domain numerical experiments. We find that there is a strong scattering process before achieving the stable state and its time length can be tuned by the dispersive strength. Poynting-vector directions show that the stable cloaking state is constructed locally while an intensity front sweeps through the cloak. Deeper studies demonstrate that the group velocity tangent component is the dominant element in the process. This study is helpful not only for clear physical pictures but also for designing better cloaks to defend pulsive radars.
92(2008); http://dx.doi.org/10.1063/1.2903134View Description Hide Description
A straightforward and inexpensive solution-based method to coat cylindrical microcavities with silicon nanocrystals is described. By using this method, high-quality films of oxide-embedded silicon nanocrystals (Si-ncs) were formed on the inner wall of hollow glass fibers. The resulting films were uniform and crack-free over lengths of and were strongly luminescent due to the presence of well-passivated Si-ncs. The optical confinement provided by the film gave rise to resonant modes in the photoluminescence spectrum, with high-quality factors compared to Si-ncs in planar microcavities or microdisks or coated on glass spheres.
- PLASMAS AND ELECTRICAL DISCHARGES
92(2008); http://dx.doi.org/10.1063/1.2897308View Description Hide Description
We have determined -band electric field strengths required to obtain air breakdown at atmospheric pressure in the presence of metallic initiators, which are irradiated with repetitive microwave pulses of duration and peak power. Using a half-wavelength initiator, a factor of 40 reduction (compared to no initiator) was observed in the electric field required to achieve breakdown. The present measurements are compared to a previously published model for air breakdown, which was originally validated with -band frequencies and single pulses. We find good agreement between this previous model and our present measurements of breakdown with -band frequencies and repetitive pulses.