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Applied Physics Letters

Issue 2 • Date Jul 2014

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Displaying Results 1 - 25 of 93
  • Pre-determining the location of electromigrated gaps by nonlinear optical imaging

    Page(s): 021101 - 021101-4
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    In this paper we describe a nonlinear imaging method employed to spatially map the occurrence of constrictions occurring on an electrically stressed gold nanowire. The approach consists at measuring the influence of a tightly focused ultrafast pulsed laser on the electronic transport in the nanowire. We found that structural defects distributed along the nanowire are efficient nonlinear optical sources of radiation and that the differential conductance is significantly decreased when the laser is incident on such electrically induced morphological changes. This imaging technique is applied to pre-determine the location of the electrical failure before it occurs. View full abstract»

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  • Ultra-broadband terahertz metamaterial absorber

    Page(s): 021102 - 021102-4
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    We demonstrated an ultra-broadband, polarization-insensitive, and wide-angle metamaterial absorber for terahertz (THz) frequencies using arrays of truncated pyramid unit structure made of metal-dielectric multilayer composite. In our design, each sub-layer behaving as an effective waveguide is gradually modified in their lateral width to realize a wideband response by effectively stitching together the resonance bands of different waveguide modes. Experimentally, our five layer sample with a total thickness 21 μm is capable of producing a large absorptivity above 80% from 0.7 to 2.3 THz up to the maximum measurement angle 40°. The full absorption width at half maximum of our device is around 127%, greater than those previously reported for THz frequencies. Our absorber design has high practical feasibility and can be easily integrated with the semiconductor technology to make high efficient THz-oriented devices. View full abstract»

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  • Utilization of chirped laser pulses to measure stimulated Raman scattering of organic liquids in the terahertz regime

    Page(s): 021103 - 021103-4
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    With the present paper, we demonstrate an experimental setup connecting two highly innovative fields of research: stimulated Raman scattering (SRS) spectroscopy and terahertz (THz) spectroscopy of liquids. For this purpose, we use the spectral focusing method allowing us to excite and measure molecular vibrations by chirped laser pulses. With our experimental setup, we apply this method to the THz regime in order to detect the frequency dependence of SRS processes in organic liquids. More specifically, we focus two chirped laser pulses onto the sample to drive molecular vibrations at THz frequencies. Thereby, the time delay between the laser pulses is directly related to the driving frequency. Due to SRS process, an energy transfer between the two laser pulses is established. By detecting the corresponding energy change of only one of the two laser pulses, we are able to observe inverse Raman scattering and stimulated Raman gain scattering as well as the transition between both processes at zero delay time. The peak positions and the line shapes of the measured spectra agree well with literature data obtained by spontaneous Raman scattering spectroscopy and optical heterodyne detection optical Kerr effect spectroscopy. With the present setup, we are able to excite and detect molecular vibrations ranging over two orders of magnitude from 10 THz to ultra-low frequencies at 100 GHz. View full abstract»

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  • Effect of graphene on photoluminescence properties of graphene/GeSi quantum dot hybrid structures

    Page(s): 021104 - 021104-4
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    Graphene has been discovered to have two effects on the photoluminescence (PL) properties of graphene/GeSi quantum dot (QD) hybrid structures, which were formed by covering monolayer graphene sheet on the multilayer ordered GeSi QDs sample surfaces. At the excitation of 488 nm laser line, the hybrid structure had a reduced PL intensity, while at the excitation of 325 nm, it had an enhanced PL intensity. The attenuation in PL intensity can be attributed to the transferring of electrons from the conducting band of GeSi QDs to the graphene sheet. The electron transfer mechanism was confirmed by the time resolved PL measurements. For the PL enhancement, a mechanism called surface-plasmon-polariton (SPP) enhanced absorption mechanism is proposed, in which the excitation of SPP in the graphene is suggested. Due to the resonant excitation of SPP by incident light, the absorption of incident light is much enhanced at the surface region, thus leading to more exciton generation and a PL enhancement in the region. The results may be helpful to provide us a way to improve optical properties of low dimensional surface structures. View full abstract»

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  • In-line single-mode fiber variable optical attenuator based on electrically addressable microdroplets

    Page(s): 021105 - 021105-4
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    We report an in-line, fiber optic, broadband variable optical attenuator employing a side-polished, single-mode optical fiber integrated on a digital microfluidics platform. The system is designed to electrically translate a liquid droplet along the polished surface of an optical fiber using electrowetting forces. This fiber optic device has the advantage of no moving mechanical parts and lends itself to miniaturization. A maximum attenuation of 25 dB has been obtained in the wavelength range between 1520 nm and 1560 nm. View full abstract»

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  • Random lasing action in a polydimethylsiloxane wrinkle induced disordered structure

    Page(s): 021106 - 021106-4
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    This paper presents a chip-scale random lasing action utilizing polydimethylsiloxane (PDMS) wrinkles with random periods as disordered medium. Nanoscale wrinkles with long range disorder structures are formed on the oxidized surface of a PDMS slab and confirmed by atomic force microscopy. Light multiply scattered at each PDMS wrinkle-dye interfaces is optically amplified in the presence of pump gain. The shift of laser emission wavelength when pumping at different regions indicates the randomness of the winkle period. In addition, a relatively low threshold of about 27 μJ/mm2 is realized, which is comparable with traditional optofluidic dye laser. This is due to the unique sinusoidal Bragg-grating-like random structure. Contrast to conventional microfluidic dye laser that inevitably requires the accurate design and implementation of microcavity to provide optical feedback, the convenience in both fabrication and operation makes PDMS wrinkle based random laser a promising underlying element in lab-on-a-chip systems and integrated microfluidic networks. View full abstract»

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  • Picosecond dynamics of a silicon donor based terahertz detector device

    Page(s): 021107 - 021107-4
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    We report the characteristics of a simple complementary metal-oxide-semiconductor compatible terahertz detector device with low response time (nanoseconds) determined using a short-pulse, high intensity free-electron laser. The noise equivalent power was 1 × 10−11 W Hz−1/2. The detector has an enhanced response over narrow bands, most notably at 9.5 THz, with a continuum response at higher frequencies. Using such a device, the dynamics of donors in silicon can be explored, a system which has great potential for quantum information processing. View full abstract»

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  • A cesium bromide photocathode excited by 405 nm radiation

    Page(s): 021108 - 021108-2
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    In several applications, such as electron beam lithography and X-ray differential phase contrast imaging, there is a need for a free electron source with a current density at least 10 A/cm2 yet can be shaped with a resolution down to 20 nm and pulsed. Additional requirements are that the source must operate in a practical demountable vacuum (>1e-9 Torr) and be reasonably compact. In prior work, a photocathode comprising a film of CsBr on metal film on a sapphire substrate met the requirements except it was bulky because it required a beam (>10 W/cm2) of 257 nm radiation. Here, we describe an approach using a 405 nm laser which is far less bulky. The 405 nm laser, however, is not energetic enough to create color centers in CsBr films. The key to our approach is to bombard the CsBr film with a flood beam of about 1 keV electrons prior to operation. Photoelectron efficiencies in the range of 100–1000 nA/mW were demonstrated with lifetimes exceeding 50 h between electron bombardments. We suspect that the electron bombardment creates intraband color centers whence electrons can be excited by the 405 nm photons into the conduction band and thence into the vacuum. View full abstract»

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  • Terahertz intersubband absorption in non-polar m-plane AlGaN/GaN quantum wells

    Page(s): 021109 - 021109-4
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    We demonstrate THz intersubband absorption (15.6–26.1 meV) in m-plane AlGaN/GaN quantum wells. We find a trend of decreasing peak energy with increasing quantum well width, in agreement with theoretical expectations. However, a blue-shift of the transition energy of up to 14 meV was observed relative to the calculated values. This blue-shift is shown to decrease with decreasing charge density and is, therefore, attributed to many-body effects. Furthermore, a ∼40% reduction in the linewidth (from roughly 8 to 5 meV) was obtained by reducing the total sheet density and inserting undoped AlGaN layers that separate the wavefunctions from the ionized impurities in the barriers. View full abstract»

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  • A linear-to-circular polarization converter with half transmission and half reflection using a single-layered metamaterial

    Page(s): 021110 - 021110-4
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    A linear-to-circular polarization converter with half transmission and half reflection using a single-layered metamaterial is theoretically and numerically demonstrated. The unit cell of the metamaterial consists of two coupled split-ring resonators with identical dimensions. A theoretical analysis based on an electrical circuit model of the coupled split-ring resonators indicates that the linear-to-circular polarization converter is achieved when the magnetic coupling between the split-ring resonators is set to a certain strength. A finite-difference time-domain simulation reveals that the single-layered metamaterial behaves as the linear-to-circular polarization converter and that the polarization converter has the combined characteristics of a half mirror and a quarter-wave plate. View full abstract»

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  • Micro-slotted whispering gallery mode resonators for optomechanical applications

    Page(s): 021111 - 021111-4
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    We present a study of augmented whispering gallery mode optical resonators that contain one or more narrow slots, through which the optical field exits the resonator's dielectric material and propagates in free space. We developed a theoretical model describing the micro-slotted resonator mode spectrum, and we find the theoretical results of the single-slot case are in close agreement with experimental observations. Furthermore, we examine a double-slot configuration that forms a cantilever-like partition within the circulating high-Q optical modes. The system exhibits high displacement sensitivity that could lead to optomechanical sensing applications. View full abstract»

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  • Microsphere-chain waveguides: Focusing and transport properties

    Page(s): 021112 - 021112-4
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    It is shown that the focusing properties of polystyrene microsphere-chain waveguides (MCWs) formed by sufficiently large spheres (D ≥ 20λ, where D is the sphere diameter and λ is the wavelength of light) scale with the sphere diameter as predicted by geometrical optics. However, this scaling behavior does not hold for mesoscale MCWs with D ≤ 10λ resulting in a periodical focusing with gradually reducing beam waists and in extremely small propagation losses. The observed effects are related to properties of nanojet-induced and periodically focused modes in such structures. The results can be used for developing focusing microprobes, laser scalpels, and polarization filters. View full abstract»

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  • Epitaxial polymorphism of La2O3 on Si(111) studied by in situ x-ray diffraction

    Page(s): 021601 - 021601-4
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    The phase formation of La2O3 epitaxial films during growth on Si(111) is investigated by synchrotron-based in situ grazing incidence x-ray diffraction and high resolution transmission electron microscopy. We find that first a 2–4 nm thick cubic bixbyite La2O3 layer grows at the interface to Si(111) substrate, followed by a hexagonal La2O3 film. Hence, to keep a cubic on cubic heteroepitaxy and to achieve high quality epitaxial nanostructures or multi-layers, the thickness of the interfacial La2O3 layer has to be restricted to 2 nm. The larger formation energy of the cubic phase can only partially be compensated by the biaxial strain in the epitaxial film based on density functional perturbation theory. Hence, the stabilization of the cubic phase is not due to bulk strain but could be related to a lower surface or interface free energy, or to kinetic effects. View full abstract»

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  • Dynamic grazing incidence fast atom diffraction during molecular beam epitaxial growth of GaAs

    Page(s): 021602 - 021602-4
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    A Grazing Incidence Fast Atom Diffraction (GIFAD) system has been mounted on a commercial molecular beam epitaxy chamber and used to monitor GaAs growth in real-time. In contrast to the conventionally used Reflection High Energy Electron Diffraction, all the GIFAD diffraction orders oscillate in phase, with the change in intensity related to diffuse scattering at step edges. We show that the scattered intensity integrated over the Laue circle is a robust method to monitor the periodic change in surface roughness during layer-by-layer growth, with oscillation phase and amplitude independent of incidence angle and crystal orientation. When there is a change in surface reconstruction at the start of growth, GIFAD intensity oscillations show that there is a corresponding delay in the onset of layer-by-layer growth. In addition, changes in the relative intensity of different diffraction orders have been observed during growth showing that GIFAD has the potential to provide insight into the preferential adatom attachment sites on the surface reconstruction during growth. View full abstract»

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  • Method for characterizing the contact resistance of metal-vanadium dioxide thin film interfaces

    Page(s): 021603 - 021603-4
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    The standard method for determining the contact resistance of planar metal-semiconductor interfaces can underestimate the true contact resistance under normal operating conditions, as it relies on the resistivity of the semiconductor material remaining constant during measurement. However, the strong temperature dependence of the resistivity of VO2 requires a modified approach that maintains a constant power density dissipated within the film to account for Joule heating. We develop a method for measuring contact resistance in semiconductors with a high thermal coefficient of resistivity, demonstrate this method with an example, and compare the results with the standard technique. View full abstract»

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  • Kinetic model for electric-field induced point defect redistribution near semiconductor surfaces

    Page(s): 021604 - 021604-5
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    The spatial distribution of point defects near semiconductor surfaces affects the efficiency of devices. Near-surface band bending generates electric fields that influence the spatial redistribution of charged mobile defects that exchange infrequently with the lattice, as recently demonstrated for pile-up of isotopic oxygen near rutile TiO2 (110). The present work derives a mathematical model to describe such redistribution and establishes its temporal dependence on defect injection rate and band bending. The model shows that band bending of only a few meV induces significant redistribution, and that the direction of the electric field governs formation of either a valley or a pile-up. View full abstract»

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  • Non contact probing of interfacial stiffnesses between two plates by zero-group velocity Lamb modes

    Page(s): 021605 - 021605-4
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    A non contact technique using zero-group velocity (ZGV) Lamb modes is developed to probe the bonding between two solid plates coupled by a thin layer. The layer thickness is assumed to be negligible compared with the plate thickness and the acoustic wavelength. The coupling layer is modeled by a normal and a tangential spring to take into account the normal and shear interfacial stresses. Theoretical ZGV frequencies are determined for a symmetrical bi-layer structure and the effect of the interfacial stiffnesses on the cut-off and ZGV frequencies are evaluated. Experiments are conducted with two glass plates bonded by a drop of water, oil, or salol, leading to a few micrometer thick layer. An evaluation of normal and shear stiffnesses is obtained using ZGV resonances locally excited and detected with laser ultrasonic techniques. View full abstract»

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  • Improving the oxidation potential of Sb-doped SnO2 electrode by Zn/Sb co-doping

    Page(s): 021606 - 021606-3
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    Inorganic oxides are recognized as attractive materials for developing anodes for wastewater treatment, potentially offering a cost effective solution for electro-oxidation. A key parameter in measuring the effectiveness of different anode materials is the oxygen over potential. In this paper, we study the role of Zn and Sb co-doping of SnO2 thin films to achieve enhanced oxidation potentials, suitable for use in wastewater treatment. The morphology, chemical, and electrochemical properties of the films were characterized, and as a result of an optimization study, suitable anode materials for wastewater treatment are identified. View full abstract»

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  • Intrinsic inhomogeneity in barrier height at monolayer graphene/SiC Schottky junction

    Page(s): 021607 - 021607-5
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    Graphene interfaced with a semiconductor forms a Schottky junction with rectifying properties. The inherent spatial inhomogeneity due to the formation of ripples and ridges in graphene can lead to fluctuations in the Schottky barrier height (SBH). The non-ideal behavior of the temperature dependent barrier height and ideality factor greater than 4 can be attributed to these spatial inhomogeneities. Assuming a Gaussian distribution of the barrier, mean SBHs of 1.30 ± 0.18 eV and 1.16 ± 0.16 eV are found for graphene/SiC junctions on the C- and Si-face, respectively. These findings reveal intrinsic spatial inhomogeneities in the SBHs in graphene based Schottky junctions. View full abstract»

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  • Controlling solid elastic waves with spherical cloaks

    Page(s): 021901 - 021901-4
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    We propose a cloak for coupled shear and pressure waves in solids. Its elastic properties are deduced from a geometric transform that retains the form of Navier equations. The spherical shell is made of an anisotropic and heterogeneous medium described by an elasticity tensor ℂ′ (without the minor symmetries), which has 21 non-zero spatially varying coefficients in spherical coordinates. Although some entries of ℂ′, e.g., some with a radial subscript, and the density (a scalar radial function) vanish on the inner boundary of the cloak, this metamaterial exhibits less singularities than its cylindrical counterpart studied in [M. Brun, S. Guenneau, and A. B. Movchan, Appl. Phys. Lett. 94, 061903 (2009).] In the latter work, ℂ′ suffered some infinite entries, unlike in our case. Finite element computations confirm that elastic waves are smoothly bent around a spherical void. View full abstract»

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  • Multiple-shocks induced nanocrystallization in iron

    Page(s): 021902 - 021902-4
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    We found that multiple shots of femtosecond laser-driven shock pulses changed coarse crystalline iron grains with a size of 140 μm into nanocrystals with a high density of dislocations, which had never been observed in conventional shock processes. We performed metallurgical microstructure observations using transmission electron microscopy (TEM) and hardness measurements using nanoindentation on cross-sections of shocked iron. TEM images showed that grains with sizes from 10 nm through 1 μm exist within 2 μm of the surface, where the dislocation density reached 2 × 1015 m−2. Results of the hardness measurements showed a significant increase in hardness in the nanocrystallized region. We suggest that the formation of a high density of dislocations, which is produced by a single shock, induces local three-dimensional pile-up by the multiple-shocks, which causes grain refinement at the nanoscale. View full abstract»

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  • Photoluminescence characteristics of polariton condensation in a CuBr microcavity

    Page(s): 021903 - 021903-4
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    We have investigated the photoluminescence (PL) properties of a CuBr microcavity at 10 K, including the temporal profiles, from the viewpoint of cavity-polariton condensation. The excitation energy density dependence of the PL intensity (band width) of the lower polariton branch at an in-plane wave vector of k// = 0 exhibits a threshold-like increase (decrease). A large blueshift in the PL energy of ∼10 meV caused by the cavity-polariton renormalization is correlated with the excitation energy density dependence of the PL intensity. The estimated density of photogenerated electron-hole pairs at the threshold is two orders lower than the Mott transition density. These results consistently demonstrate the occurrence of cavity-polariton condensation. In addition, we found that the PL rise and decay times are shortened dramatically by the cavity-polariton condensation, which reflects the bosonic final state stimulation in the relaxation process and the intrinsic cavity-polariton lifetime in the decay process. View full abstract»

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  • Sub-100 ps laser-driven dynamic compression of solid deuterium with a ∼40 μJ laser pulse

    Page(s): 021904 - 021904-5
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    We dynamically compress solid deuterium over <100 ps from initial pressures of 22 GPa to 55 GPa, to final pressures as high as 71 GPa, with <40 μJ of pulse energy. At 25 GPa initial pressure, we measure compression wave speeds consistent with quasi-isentropic compression and a 24% increase in density. The laser drive energy per unit density change is 109 times smaller than it is for recent longer (∼30 ns) time scale compression experiments. This suggests that, for a given final density, dynamic compression of hydrogen might be achieved using orders of magnitude lower laser energy than currently used. View full abstract»

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  • Raman scattering analysis of electrodeposited Cu(In,Ga)Se2 solar cells: Impact of ordered vacancy compounds on cell efficiency

    Page(s): 021905 - 021905-4
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    This work reports the detailed Raman scattering analysis of Cu-poor Cu(In,Ga)Se2 (CIGS) electrodeposited solar cells using different excitation wavelengths. The systematic assessment of cells fabricated with Cu-poor absorbers that were synthesized with different Cu contents has allowed identifying the existence of a quasi-resonant excitation of a Raman peak characteristic of an Ordered Vacancy Compound (OVC) secondary phase when using a 785 nm excitation wavelength. The enhanced sensitivity of the spectra measured with these conditions to the presence of the OVC phase provides with a suitable tool for the non destructive assessment on the occurrence of this Cu-poor secondary phase in the surface region of the CIGS absorbers from measurements performed on finished cells. The correlation between the Raman scattering data and the optoelectronic parameters of the devices shows the existence of an optimum OVC content range leading to devices with highest open circuit voltage and efficiency. These data provide with a clear experimental evidence on the impact of the OVC phases on the device efficiency. View full abstract»

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  • All-optical characterization of cryogenically cooled argon clusters in continuous gas jets

    Page(s): 021906 - 021906-4
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    We present an all-optical method for measuring the average size, density, and mass fraction of large (>50 nm) clusters produced in cryogenically cooled, continuous gas jets. The technique combines three optical diagnostics—forward/backward Mie scattering detection, 90° scattering imaging, and neutral gas interferometry. Our measurement shows that cryogenic cooling and high backing gas pressure greatly enhance both cluster size and clustering ratio in continuous gas flow. Such a cluster source can be used as a potential target for intense, high-repetition-rate (>kHz) laser pulses. View full abstract»

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Aims & Scope

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