Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon's quark constituents; indeed, recent proton data have attracted intense theoretical interest. In this Letter, we report new measurements of the proton electromagnetic form factor ratio using the recoil polarization method, at momentum transfers Q2=5.2, 6.7, and 8.5  GeV2. By extending the range of Q2 for which G(E)(p) is accurately determined by more than 50%, these measurements will provide significant constraints on models of nucleon structure in the nonperturbative regime.

Here we introduce a new paradigm of far-field optical lithography, optical force stamping lithography. The approach employs optical forces exerted by a spatially modulated light field on colloidal nanoparticles to rapidly stamp large arbitrary patterns comprised of single nanoparticles onto a substrate with a single-nanoparticle positioning accuracy well beyond the diffraction limit. Because the process is all-optical, the stamping pattern can be changed almost instantly and there is no constraint on the type of nanoparticle or substrates used. Keywords optical lithography; optical forces; spatial light modulator; colloidal nanoparticlesOptical lithography techniques are widely used to fabricate nanoscale devices for optoelectronics, biological and medical applications 1,2,3 . The lateral feature size achievable by conventional far-field optical lithography is diffraction-limited 4 . A number of far-field approaches have been proposed to overcome the diffraction limit 5,6,7,8 , but these are not suitable for arbitrary pattern formation. Scanning near-field optical microscopy techniques can also go beyond the diffraction limit 9,10,11,12 , but due to their serial essence the scanning methods suffer from low throughput and limited patterning areas.An elegant way of manipulating small particles is to employ the optical forces exerted by light on micro-and nanoobjects upon interaction with them. These forces can be used to optically trap individual particles by a tightly focused laser beam 13 , move them to target positions and, finally, fix them onto a substrate by various mechanisms 14,15,16 . Although this approach offers wide flexibility in assembling arbitrary patterns composed of particles, it is of limited use for mass production due to its extremely low throughput and inherent complexity of manipulation of several simultaneously trapped particles. In Optical Force Stamping Lithography (OFSL) the optical forces are not used to optically trap nanoparticles. Instead by using the repulsive force exerted by a laser beam resonant to nanoparticles' extinction maxima 17,18 , the nanoparticles are accelerated along the directions of the light energy flux and fixed at desired positions on a substrate 19,20 .OFSL is an all-optical, far-field and maskless technique offering high flexibility for surface processing over large areas using tailored colloidal micro-and nanoparticles with desired optical, electric and magnetic properties. The method merges advantages of optical far-field lithography and colloidal chemistry by applying a two step fabrication of nanoscale devices. First, individual components of the future device with required properties and functionalities are prepared using well established colloidal synthesis protocols 21,22 . These components are then arranged on a substrate by light which enables an easy parallelization of this process. The concept of OFSL can be defined as follows: a spatial light modulator (SLM) is used to split a laser beam into several beams, creating an optical pattern, which ...

Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton electric to magnetic form factor ratio, G(E)/G(M), obtained separately from cross section and polarization transfer measurements. One possible explanation for this difference is a two-photon-exchange contribution. In an effort to search for effects beyond the one-photon-exchange or Born approximation, we report measurements of polarization transfer observables in the elastic H(e[over →],e(')p[over →]) reaction for three different beam energies at a Q(2)=2.5  GeV(2), spanning a wide range of the kinematic parameter ε. The ratio R, which equals μ(p)G(E)/G(M) in the Born approximation, is found to be independent of ε at the 1.5% level. The ε dependence of the longitudinal polarization transfer component P(ℓ) shows an enhancement of (2.3±0.6)% relative to the Born approximation at large ε.

In this article, we report how Janus particles, composed of a silica sphere with a gold half-shell, can be not only stably trapped by optical tweezers but also displaced controllably along the axis of the laser beam through a complex interplay between optical and thermal forces. Scattering forces orient the asymmetric particle, while strong absorption on the metal side induces a thermal gradient, resulting in particle motion. An increase in the laser power leads to an upward motion of the particle, while a decrease leads to a downward motion. We study this reversible axial displacement, including a hysteretic jump in the particle position that is a result of the complex pattern of a tightly focused laser beam structure above the focal plane. As a first application we simultaneously trap a spherical gold nanoparticle and show that we can control the distance between the two particles inside the trap. This photonic micron-scale “elevator” is a promising tool for thermal force studies, remote sensing, and optical and thermal micromanipulation experiments.

We demonstrate that optical trapping of multiple silver nanoparticles is strongly influenced by plasmonic coupling of the nanoparticles. Employing dark-field Rayleigh scattering imaging and spectroscopy on multiple silver nanoparticles optically trapped in three dimensions, we experimentally investigate the time-evolution of the coupled plasmon resonance and its influence on the trapping stability. With time the coupling strengthens, which is observed as a gradual red shift of the coupled plasmon scattering. When the coupled plasmon becomes resonant with the trapping laser wavelength, the trap is destabilized and nanoparticles are released from the trap. Modeling of the trapping potential and its comparison to the plasmonic heating efficiency at various nanoparticle separation distances suggests a thermal mechanism of the trap destabilization. Our findings provide insight into the specificity of three-dimensional optical manipulation of plasmonic nanostructures suitable for field enhancement, for example for surface-enhanced Raman scattering.

We have studied the reaction p + 27 Al → 3 He + p + π − + X at recoil-free kinematics. An η meson possibly produced in this reaction would be thus almost at rest in the laboratory system and could therefore be bound with high probability, if nuclear η states exist. The decay of such a state through the N * (1535) resonance would lead to a proton-π − pair emitted in opposite directions.For these conditions we find some indication of such a bound state. An upper limit of ≈ 0.5 nb is found.

We investigate the optical and morphological properties of gold nanoparticles grown by reducing a gold salt with Na2S. Lasers are tuned to the observed plasmon resonances, and the optical forces exerted on the nanoparticles are used to selectively print individual nanoparticles onto a substrate. This enables us to combine dark-field spectroscopy and scanning electron microscopy to compare the optical properties of single nanoparticles with their morphology. By arresting the synthesis at different times, we are able to investigate which type of nanoparticle is responsible for the respective resonances. We find that thin Au nanotriangles are the source of the observed near infrared (NIR) resonance. The initial lateral growth of these triangles causes the plasmon resonance to redshift into the NIR, whereas a subsequent thickening of the triangles and a concomitant truncation lead to a blueshift of the resonance. Furthermore, we find that the nanotriangles produced have extremely narrow line widths (187 ± 23 meV), show nearly isotropic scattering, and are stable for long periods of time. This shows their vast potential for applications such as in vivo imaging and bio(chemical) sensing. The method used here is generally applicable to other syntheses, and shows how complex nanostructures can be built up on substrates by selectively printing NPs of varying plasmonic resonances.

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