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J. O'Hara

Publications and source records attributed to J. O'Hara.

7 recordsLinked to original sources
arXiv2023

LANSCE 805 MHz Low Level RF Resonance Control Water System Upgrade

The Los Alamos Neutron Science Center (LANSCE) accelerator at Los Alamos National Laboratory (LANL) has been in service for over 50 years. Efforts to update and modernize crucial systems, many of which are origi-nal, are ongoing. This paper reports on the refurbishment of the Low-Level Radio Frequency (LLRF) Resonance Control Water System (RCWS) for the half-mile long Cavity-Coupled LINAC (CCL). The RCWS controls the resonance frequency of the cavities by controlling the temperature of the cooling water delivered to each of 44 accelerator modules. Of the 44 modules making up the CCL, 20 now have an upgraded RCWS. Upgrading in-cludes removing the old hardware and installing new components including: water pumps, mix tanks, valves, temperature switches, flow switches and plumbing. This paper describes the design of the new system, material and component selection, installation, and technical challenges.

physics.acc-ph↗
arXiv2018

High Purcell factor generation of coherent on-chip single photons

On-chip single-photon sources are key components for integrated photonic quantum technologies. Semiconductor quantum dots can exhibit near-ideal single-photon emission but this can be significantly degraded in on-chip geometries owing to nearby etched surfaces. A long-proposed solution to improve the indistinguishablility is by using the Purcell effect to reduce the radiative lifetime. However, until now only modest Purcell enhancements have been observed. Here we use pulsed resonant excitation to eliminate slow relaxation paths, revealing a highly Purcell-shortened radiative lifetime (22.7 ps) in a waveguide-coupled quantum dot-photonic crystal cavity system. This leads to near-lifetime-limited single-photon emission which retains high indistinguishablility (93.9%) on a timescale in which 20 photons may be emitted. Nearly background-free pulsed resonance fluorescence is achieved under $π$-pulse excitation, enabling demonstration of an on-chip, on-demand single-photon source with very high potential repetition rates.

quant-ph↗
arXiv2012

A modal analysis method to describe weak nonlinear effects in metamaterials

We apply a rigorous eigenmode analysis to study the electromagnetic properties of linear and weakly nonlinear metamaterials. The nonlinear response can be totally described by the linear eigenmodes when weak nonlinearities are attributed to metamaterials. We use this theory to interpret intrinsic second-harmonic spectroscopy on metallic metamaterials. Our study indicates that metamaterial eigenmodes play a critical role in optimizing a nonlinear metamaterial response to the extent that a poorly optimized modal pattern overwhelms the widely recognized benefits of plasmonic resonant field enhancements.

physics.optics↗
arXiv2011

Space of subspheres and conformal invariants of curves

A space curve is determined by conformal arc-length, conformal curvature, and conformal torsion, up to Möbius transformations. We use the spaces of osculating circles and spheres to give a conformally defined moving frame of a curve in the Minkowski space, which can naturally produce the conformal invariants and the normal form of the curve. We also give characterization of canal surfaces in terms of curves in the set of circles.

math.DG↗
arXiv2004

Conformally invariant energies of knots

Conformally invariant functionals on the space of knots are introduced via extrinsic conformal geometry of the knot and integral geometry on the space of spheres. Our functionals are expressed in terms of a complex-valued 2-form which can be considered as the cross-ratio of a pair of infinitesimal segments of the knot. We show that our functionals detect the unknot as the total curvature does, and that their values explode if a knot degenerates to a singular knot with double points.

math.GT↗
arXiv2004

Conformally invariant energies of knots II

This paper has been withdrawn by the authors, as it was combined with "Conformally invariant energies of knots I" (math/0409396) to be "Conformally invariant energies of knots" which has replaced the former.

math.GT↗