Optical tweezers have been widely used to manipulate biological and colloidal material, but the diffraction limit of far-field optics makes focused beams unsuitable for manipulating nanoscale objects with dimensions much smaller than the wavelength of light. While plasmonic structures have recently been successful in trapping nanoscale objects with high positioning accuracy, using such structures for manipulation over longer range has remained a significant challenge. In this work, we introduce a conveyor belt design based on a novel plasmonic structure, the resonant C-shaped engraving (CSE). We show how long-range manipulation is made possible by means of handoff between neighboring CSEs, and we present a simple technique for controlling handoff by rotating the polarization of laser illumination. We experimentally demonstrate handoff between a pair of CSEs for polystyrene spheres 200, 390, and 500 nm in diameter. We then extend this technique and demonstrate controlled particle transport down a 4.5 μm long "nano-optical conveyor belt."
We demonstrate the all-optical recording of deeply subwavelength data bits in Ge2Sb2Te5 using a near-field scanning optical microscope (NSOM) probe that utilizes a C-aperture fabricated using through membrane focused ion beam milling. Data bits recorded with various optical powers were read out optically by C-aperture NSOM and the physical bit size was measured by atomic force microscopy (AFM). Both optical and AFM measurements were found to be in excellent agreement with simulation. We achieved a minimum physical bit size of 53.5×50.2 nm2 at a wavelength of 980 nm (λ/20) indicating a data density of 223 Gbit/in.2.
We report an improved fabrication method for C-shaped near-field apertures resonant in the near-IR regime. The apertures are created in a metal layer on a silicon nitride membrane using a focused ion beam and a through membrane milling technique that avoids two problems with fabricating very small apertures: gallium contamination and edge rounding. Finite-difference time-domain simulations predict a 63x more intense near field with a 2.2x smaller spot versus conventionally milled apertures. We verify the position of the simulated resonance peaks with experimental far-field transmission measurements where we also find an increase of 8.8x in intensity. Our method has applications to many other plasmonic devices including bow-tie and fractal apertures, periodic arrays, and gratings.
An optoelectronic tweezers (OETs) system employing a non-uniform background electric field is presented. In addition to optically induced electrodes, physical electrodes are incorporated into the design. The geometries of the physical electrodes are selected to create a background field with gradients along a specific axis. Due to the resulting background force, the proposed scheme traps particles along an axis around the rim of the optical spot. This is a resolution improvement over conventional OETs where particle trapping occurs uniformly around the spot. Numerical simulations of the device including conductivity, electric fields, and force profiles are presented. The trapping and manipulation of micro-particles using the device are experimentally demonstrated. The experiment verifies that trapping occurs along a specific axis of the optical beam.
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.
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