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Applied Physics Letters

Issue 5 • Date Jul 2012

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Displaying Results 1 - 25 of 112
  • Silicon nanomembrane based photonic crystal waveguide array for wavelength-tunable true-time-delay lines

    Page(s): 051101 - 051101-4
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    We demonstrate a four-channel on-chip true-time-delay module based on a photonic crystal waveguide array. Using the photonic crystal taper to minimize the coupling loss, the delay lines with 1–3 mm long photonic crystal waveguides can operate up to a group index ng ∼ 23 without significant loss. The large group velocity dispersion enables continuous and wavelength-tunable time delays. Measurements show a highly linear phase-frequency relation, highest time delay up to 216.7 ps, and large tuning ranges of 58.28 ps, 115.74 ps, and 194.16 ps for 1–3 mm delay lines. The chip-scale true-time-delay module occupies only 0.18 mm2 area and can provide ±44.38° steering for an X-band phased-array-antenna. View full abstract»

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  • Extraordinary blueshift of a photonic crystal nanocavity by reducing its mode volume with an opaque microtip

    Page(s): 051102 - 051102-5
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    We demonstrate a method to reduce the mode volume of optical micro/nanocavities by positioning an opaque microtip in close proximity of the structures. This concept is used to blueshift the resonance of an active photonic crystal nanocavity by up to 16 nm. This tuning range is shown to be about 10 times larger than the redshift achieved with a bare dielectric microtip of the same size and shape. By imagining materials or multilayered devices with the ability to become transparent and opaque under external control, the blue and redshifts of the resonance would become possible with a single perturbing device. View full abstract»

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  • Demonstration of nonlinear magnetoelectric coupling in metamaterials

    Page(s): 051103 - 051103-4
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    We demonstrate nonlinear magnetoelectric coupling in a varactor-loaded metamaterial at microwave frequencies. The nonlinear magnetoelectric coupling takes the form of second-harmonic generation in which incident magnetic fields at frequency ω drive an electric polarization at frequency 2ω. The magnitudes and phases of the generated signals from two nonlinear metamaterials are measured, verifying the dominant nonlinear process in each sample. We expect the metamaterial design employed in this letter to form the prototype for future investigations into nonlinear magnetoelectric coupling. View full abstract»

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  • Tunable degree of localization in random lasers with controlled interaction

    Page(s): 051104 - 051104-3
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    We show that the degree of localization for the modes of a random laser (RL) is affected by the inter mode interaction that is controlled by shaping the spot of the pump laser. By experimentally investigating the spatial properties of the lasing emission we infer that strongly localized modes are activated in the low interacting regime while in the strongly interacting one extended modes are found lasing. Thus we demonstrate that the degree o localization may be finely tuned at the micrometer level. View full abstract»

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  • Absorption spectroscopy using interference between optical frequency comb and single-wavelength laser

    Page(s): 051105 - 051105-4
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    Optical frequency combs have the potential to be applied not only to frequency metrology but also to various spectroscopic measurements replacing incoherent wide-frequency band light sources and single-wavelength laser sources. In this study, we propose a system of absorption spectroscopy for single electronic transitions using the frequency-comb laser source. This spectroscopic method utilizes the interference between the probing frequency comb and an additional single-wavelength laser beam and detects power spectra of beat signals in the interfered laser beam. This method enables us to measure single-transition absorption spectra neither using a large-scale spectrometer nor scanning the laser-beam frequency. View full abstract»

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  • Polarization-sensitive photophoresis

    Page(s): 051106 - 051106-3
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    We photophoretically trap spherical airborne particles using a single radially or azimuthally polarized laser beam and show that the trapping efficiency is significantly higher for the radial polarization. The demonstrated polarization sensitivity of the photophoretic force, which is caused by polarization-dependent reflection from the particles, adds additional flexibility to the optical micromanipulation of light absorbing particles in gaseous media. View full abstract»

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  • Extended Jones matrix method for oblique incidence study of polarization gratings

    Page(s): 051107 - 051107-4
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    A fast and accurate extended Jones matrix method for characterizing the transmitted field of polarization gratings at oblique incidence is reported. The far field diffraction properties of the mth order of the grating are determined by vectorial Fourier coefficients of the transmitted field. With this method, polar plots of the various transmission orders can be obtained. The dependence of viewing angle performance on the grating pitch and grating thickness is studied. Liquid crystal polarization gratings with different pitches were fabricated with azo-dye SD-1 as photoalignment material, by polarization holography. The experimental results show very good agreement with the calculated data. View full abstract»

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  • A miniature ultrabright source of temporally long, narrowband biphotons

    Page(s): 051108 - 051108-4
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    We demonstrate a miniature source of long biphotons utilizing the cluster effect and double-pass pumping in a monolithic doubly resonant parametric down-converter. We obtain a biphoton correlation time of 17.1 ns with a generation rate of 1.10×105 biphotons/(s mW) and an estimated linewidth of 8.3 MHz. View full abstract»

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  • Strong two-photon fluorescence enhanced jointly by dipolar and quadrupolar modes of a single plasmonic nanostructure

    Page(s): 051109 - 051109-5
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    A single gold nano-cylinder presenting multipolar plasmon resonances to enhance two-photon fluorescence is investigated employing three dimensional finite-difference time-domain method. Cylinders of large dimension usually display dipolar and quadrupolar plasmonic resonances. We demonstrate that the dipolar resonance can couple with the incident light resulting in a large localized field enhancement which increases the molecular excitation rate. At the same time, the radiative quadrupolar mode overlaps with the emission band of excited fluorophores to assist the fluorescence emission due to an enhancement in the quantum efficiency. Such dipole-quadrupole jointly enhanced two-photon fluorescence exhibits exceptionally promise in brighter label design. View full abstract»

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  • Band-edge transitions in hexagonal boron nitride epilayers

    Page(s): 051110 - 051110-4
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    Hexagonal boron nitride (hBN) epilayers have been synthesized on sapphire substrates by metal-organic chemical vapor deposition (MOCVD). These MOCVD grown epilayers exhibit highly efficient band-edge photoluminescence (PL) emission lines centered at around 5.5 eV. The results represent a remarkable improvement over the optical qualities of hBN films synthesized by different methods in the past. It was observed that the emission of hBN at 10 K is about 500 times stronger than that of high quality AlN epilayers. Polarization-resolved PL spectroscopy revealed that hBN epilayers are predominantly a surface emission material, in which the band-edge emission with electric field perpendicular to the c-axis (E⇀emic) is about 1.7 times stronger than the component along the c-axis (E⇀emi∥c⇀). This is in contrast to AlN, in which the band-edge emission is known to be polarized along the c-axis, (E⇀emi∥c⇀). Time-resolved PL measurements revealed a decay lifetime of around 4.3 ns at 10 K for the dominant band-edge transition line. The present result together with the ability of p-type doping of hBN represents a major step towards the realization of hBN based practical devices. View full abstract»

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  • Highly sensitive x-ray detectors in the low-energy range on n-type 4H-SiC epitaxial layers

    Page(s): 051111 - 051111-4
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    Schottky diodes on n-type 4H-SiC epitaxial layers have been fabricated for low-energy x-ray detection. The detectors were highly sensitive to soft x-rays and showed improved response compared to the commercial SiC UV photodiodes. Current-voltage characteristics at 475 K showed low leakage current revealing the possibility of high temperature operation. The high quality of the epi-layer was confirmed by x-ray diffraction and chemical etching. Thermally stimulated current measurements performed at 94–550 K revealed low density of deep levels which may cause charge trapping. No charge trapping on detectors’ responsivity in the low x-ray energy was found. View full abstract»

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  • Transmission properties of double-gap asymmetric split ring resonators in terahertz region

    Page(s): 051112 - 051112-5
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    We investigated the electromagnetic properties of the metamaterials that consist of double-gap split ring resonators (SRRs) in the terahertz region. We found that varying the position of one gap with respect to the other causes the resonant frequency of the SRRs to shift over a broad range. This frequency shift is attributed to the change in the combined capacitance that consists of two capacitances of gaps connected in series and an additional capacitance connected in parallel to the others. Our findings are also verified by obtaining good agreement between experiments and simulations. View full abstract»

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  • All-optical time-stretch digitizer

    Page(s): 051113 - 051113-5
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    We propose and demonstrate an all-optical time-stretch digitizer for real-time capture of ultrafast optical signals, beyond the bandwidths achievable by electronics. This approach uniquely combines four-wave mixing and photonic time-stretch technique to slow down and record high-speed optical signals. As a proof-of-concept, real-time recording of 40-Gb/s non-return-to-zero on-off-keying optical data stream is experimentally demonstrated using a stretch factor of 54 and 1.5-GHz back-end electronic bandwidth. We also report on the observation of dispersion penalty and its mitigation via single-sideband conversion enabled by an optical bandpass filter. Our technique may provide a path to real-time capture of ultrahigh-speed optical data streams. View full abstract»

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  • Formation energy of optically active Er3+ centers in Er doped GaN

    Page(s): 051114 - 051114-4
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    Erbium doped GaN (GaN:Er) and low In-content InxGa1-xN (x∼0.05) epilayers were synthesized by metal organic chemical deposition. The 1.54 μm PL emission intensity was monitored for GaN:Er epilayers grown at different growth temperatures and utilized to establish a value of 1.8 ± 0.2 eV for the formation energy (EF) of the optically active Er3+ centers in GaN. The optically active Er+ centers are presumably Er and nitrogen vacancy (Er-VN) complexes. The experimentally measured value of the EF of the optically active Er3+ centers is about 0.98 eV larger than the calculated formation energy of Er ions at Ga sites; however, it is 1.1–2.2 eV lower than the formation energy of VN in GaN. Due to the large EF values, relatively high growth temperatures are required to improve the 1.54 μm emission efficiency in GaN:Er. View full abstract»

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  • Controlling the photoluminescence of acceptor and donor quantum dots embedded in a nonlinear photonic crystal

    Page(s): 051115 - 051115-4
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    We have studied energy transfer and photoluminescence in donor and acceptor quantum dots embedded in a nonlinear photonic crystal. The quantum dots are interacting with each other via the dipole-dipole interaction. The nonlinear photonic crystal modifies the dielectric constant of the hybrid system. Using the density matrix method, it is found that the energy transfer and photoluminescence in the donor quantum dot can be controlled by a pump field due to the nonlinearity of the photonic crystal. Additionally, our theoretical calculations agree with recent experiments. This hybrid system can be used to fabricate ultrafast switching and sensing nanodevices. View full abstract»

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  • Disorder-limited photon propagation and Anderson-localization in photonic crystal waveguides

    Page(s): 051116 - 051116-4
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    We investigate the disorder-limited propagation of photons in photonic crystal waveguides in the slow-light regime. We use analysis of Fabry-Perot resonances to map the mode dispersion and extract the photon localization length. Propagation lengths are deduced to be limited to less than 20 μm for group index >50. Anderson-localized modes are observed at high group indices, when the localization lengths are shorter than the waveguide lengths, consistent with the Fabry-Perot analysis. The results have consequences for integrated quantum-dot single-photon circuits which rely on slow-light effects to enhance emission rates. View full abstract»

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  • Electrical fine tuning of liquid crystal lasers

    Page(s): 051117 - 051117-4
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    We demonstrate high-precision, continuous, electrical tuning of a photonic band-edge laser based on a dye-doped cholesteric liquid crystal. A micro-patterned array of electrodes creates an electric field perpendicular to the cholesteric helix, which distorts the chiral order of the liquid crystal, thus shifting the resonant band edge modes. This configuration allows for smooth tuning of the laser emission in a wavelength range of about 3.5 nm, using low voltages (of the order of 10 V). View full abstract»

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  • Characterization of atomic step structures on CaF2(111) by their electric potential

    Page(s): 051601 - 051601-5
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    The structure and polarity of step edges on cleaved CaF2(111) are investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy. Ledges produced by cleaving the crystal appear with two distinctly different polarities denoted as type I and type II arising from the sectioning of ledges with steps having different polarities. With respect to the stoichiometric terrace, the surface potential is slightly reduced at ledges predominately composed of type I steps, while the potential of ledges predominantly composed of type II steps is significantly higher (typically 100 mV). We propose that the positive potential of type II steps stems from low coordinated Ca2+ ions inducing a dipole at step edges and confirm this by atomically resolved NC-AFM images revealing the Ca2+ ion sub-lattice with repulsive-mode imaging contrast. View full abstract»

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  • Atomic and electronic structure of graphene/Sn-Ni(111) and graphene/Sn-Cu(111) surface alloy interfaces

    Page(s): 051602 - 051602-4
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    First-principles density functional theory calculations were performed to investigate the effect of Sn surface alloying on the strength of interface interactions between graphene and Ni(111) or Cu(111) substrates. A substantial reduction of graphene-metal interactions was observed for the graphene/Sn-Ni(111) interface: binding energy was reduced from 0.055 eV/(C atom) to 0.015 eV/(C atom); interface distance was increased from 2.12 Å to 3.52 Å. The initially weak graphene/Cu(111) interface was hardly affected by Sn surface alloying. Electronic structure calculations, including local density of states and simulated scanning tunneling microscopy images, provide further details on the changing character of graphene-Ni(111) interactions upon Sn alloying. View full abstract»

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  • In situ x-ray studies of oxygen surface exchange behavior in thin film La0.6Sr0.4Co0.2Fe0.8O3-δ

    Page(s): 051603 - 051603-5
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    In situ synchrotron x-ray techniques were used to investigate oxygen surface exchange behavior in thin film La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)/Gd2O3-doped CeO2/Y2O3-stabilized ZrO2 heterostructures. Applying electrical potentials across the heterostructures results in significant expansion or contraction of the out-of-plane LSCF lattice parameter, indicating changes in the LSCF oxygen vacancy concentration. Oxygen transport across the LSCF/atmosphere interface is found to be rate limiting under both cathodic and anodic conditions. View full abstract»

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  • Inhomogeneous ohmic contacts: Barrier height and contact area determination

    Page(s): 051604 - 051604-5
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    The current-voltage characteristics at a uniform ohmic contact are, by definition, dominated by its series resistance and ordinarily offer little information, beyond an upper-bound, on the Schottky barrier height (SBH) of the metal-semiconductor junction. We demonstrate through temperature-dependent measurements of Au “ohmic” contacts on n-type Si(100)1 × 1-S, however, that quantitative information on not only the magnitude of the SBH, to within ± 0.03 eV, but also the effective conduction area can still be deduced. Details of these analyses, which require the use of a thin Si epi-layer, and the “partisan interlayer” mechanism, which lead to the formation of ohmic contacts, are discussed. View full abstract»

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  • Probing the intrinsic electrical properties of thin organic layers/semiconductor interfaces using an atomic-layer-deposited Al2O3 protective layer

    Page(s): 051605 - 051605-5
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    The intrinsic electrical properties of thin organic layers/semiconductor interfaces are challenging to probe because of large tunneling currents through the molecular layer and damage during device fabrication. We present here a method to protect the interface with an Al2O3 layer gently deposited on top using atomic layer deposition. The resulting two-layered gate stack can be characterized with capacitance and conductance measurements. The protected interface shows an inherent high quality with low Dit, less than 2 × 1011 cm-2 eV-1 for n-type Si(111) surface, best measured using mercury probe. The additional procedures required to fabricate MOS capacitors lead to an increase in Dit. View full abstract»

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  • Substrate-induced disorder in V2O3 thin films grown on annealed c-plane sapphire substrates

    Page(s): 051606 - 051606-5
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    We investigate the structural and electronic properties of V2O3 thin films deposited by oxygen plasma-assisted molecular beam epitaxy onto annealed and unannealed c-plane sapphire substrates. Annealing the substrates before growth to produce ultra-smooth surfaces improved initial epitaxy, according to in situ reflection high-energy electron diffraction. Surprisingly, films deposited on annealed substrates had a more island-like surface, broader x-ray diffraction peaks, and an increased resistivity of V2O3’s normally metallic high-temperature phase. We attribute these results to enhanced strain coupling at the interface between the substrate and film, highlighting the vulnerability of V2O3’s strongly correlated metallic phase to crystalline defects and structural disorder. View full abstract»

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  • Strain rate sensitivity and activation volume of Cu/Ni metallic multilayer thin films measured via micropillar compression

    Page(s): 051901 - 051901-4
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    Micropillar compression testing with repeated jumps in strain rate is used to circumvent inherent difficulties associated with nanoindentation and tensile testing of free-standing films. Application to sputtered 21 nm/21 nm Cu/Ni multilayer thin films with a cube-on-cube texture reveals an average strain rate sensitivity (m = 0.014) and activation volume (V = 17 b3), comparable to nanocrystalline face-centered cubic metals. Yet, m increases by ∼50% and V decreases by 70% with increasing strain, opposite to trends reported for nanotwinned Cu. The large, strain-dependent shifts in m and V are dependent on the underlying misfit dislocation structure of Cu/Ni interfaces. View full abstract»

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  • Diamond/aluminium nitride composites for efficient thermal management applications

    Page(s): 051902 - 051902-4
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    Synthetic diamond/AlN composite materials have been fabricated by a combination of microwave plasma-assisted chemical vapor deposition and molecular beam epitaxy. These wide band gap semiconductor heterojunctions show promises for many applications, including thermal management, deep ultraviolet light emitting devices, and high power and high temperature electronics. Here, we report results of an interface study of polycrystalline diamond layers grown on single crystal AlN(0001). High resolution transmission microscopy revealed atomically sharp interfaces between diamond and AlN. Temperature dependent Raman spectroscopy measurements showed reduced thermal resistance on diamond-coated AlN substrates compared to uncoated AlN at temperatures above 330 K. View full abstract»

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Applied Physics Letters, published by the American Institute of Physics, features concise, up-to-date reports on significant new findings in applied physics.

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Nghi Q. Lam
Argonne National Laboratory