arXiv subjects

Dimitris Sofikitis

Publications and source records attributed to Dimitris Sofikitis.

7 recordsLinked to original sources

arXiv2026

A plasma photocathode for spin-polarized electron beams via state-selected hydrogen halide photofragments

Spin-polarized electron beams are essential tools for probing fundamental symmetries and for the search beyond the Standard Model. While plasma-based accelerators are a promising pathway towards higher-energy frontiers, they have so far failed to deliver a competitive polarized source: existing proposals are challenging to realize and achievable polarizations remain far below conventional sources. Here, we introduce a photocathode-like scheme, applied to a gas of pre-polarized hydrogen and halogen atoms. A VUV and a visible laser pulse excite the halogen atoms to create a two-component ionization medium, consisting of low-threshold excited halogen atoms and high-threshold polarized hydrogen. Particle-in-cell simulations show witness beams with tens of pC charge retaining up to 97% of the initial polarization, rivaling state-of-the-art conventional sources.

physics.plasm-ph↗

arXiv2026

Pinching injection in wakefields for spin-polarized electron beams

Pinching of the driver beam in plasma wakefield acceleration is generally considered an unwanted effect that needs to be mitigated. Here, we propose that this effect can be utilized for the injection of spin-polarized electron beams from hydrogen halide targets into wakefields. Particle-in-cell simulations show that the electron spin is preserved on a level of 50% for a wide range of parameters due to the injection geometry. The presented injection scheme provides a possible pathway to alleviate some of the restrictions associated with pre-polarized hydrogen halide targets.

physics.plasm-ph↗

arXiv2024

High-energy polarized electron beams from the ionization of isolated spin polarized hydrogen atoms

We propose a laser-based method for the preparation of high-energy polarized electrons, from the ionization of isolated spin-polarized hydrogen (SPH) atoms. The SPH atoms are prepared from the photodissociation of HCl, using two consecutive UV pulses of ps duration. By appropriately timing and focusing the pulses, we can spatially separate the highly polarized SPH from other unwanted photoproducts, which then act as the target for the acceleration lasers. We show how elastic collisions define number density $n$ and polarization P regimes ($10^{16}\leq$ $n$ $\leq 10^{18}$ cm$^{-3}$, 0.99 $\geq$ P $\geq$ 0.40) for the pre-polarized targets, and use particle-in-cell simulations to demonstrate the method's feasibility.

physics.atom-ph↗

arXiv2018

Polarized Proton Beams from Laser-induced Plasmas

We report on the concept of an innovative source to produce polarized proton/deuteron beams of a kinetic energy up to several GeV from a laser-driven plasma accelerator. Spin effects have been implemented into the PIC simulation code VLPL to make theoretical predictions about the behavior of proton spins in laser-induced plasmas. Simulations of spin-polarized targets show that the polarization is conserved during the acceleration process. For the experimental realization, a polarized HCl gas-jet target is under construction using the fundamental wavelength of a Nd:YAG laser system to align the HCl bonds and simultaneously circular polarized light of the fifth harmonic to photo-dissociate, yielding nuclear polarized H atoms. Subsequently, their degree of polarization is measured with a Lamb-shift polarimeter. The final experiments, aiming at the first observation of a polarized particle beam from laser-generated plasmas, will be carried out at the 10 PW laser system SULF at SIOM/Shanghai.

physics.plasm-ph↗

arXiv2018

Ultrahigh-density spin-polarized H and D observed via magnetization quantum beats

We measure nuclear and electron spin-polarized H and D densities of at least 10$^{19}\, cm^{-3}$ with $\sim$10 ns lifetimes, from the photodissociation of HBr and DI with circularly-polarized UV light pulses. This density is $\sim$6 orders of magnitude higher than that produced by conventional continuous-production methods, and, surprisingly, at least 100 times higher than expected densities for this photodissociation method. We observe the hyperfine quantum beating of the H and D magnetization with a pick-up coil, i.e., the respective 0.7 and 3 ns periodic transfer of polarization from the electrons to the nuclei and back. The $\rm{10^{19}\,cm^{-3}}$ spin-polarized H and D density is sufficient for laser-driven ion acceleration of spin polarized electrons, protons, or deuterons, the preparation of nuclear-spin-polarized molecules, and for the demonstration of spin-polarized D-T or D-$\rm{{^3He}}$ laser fusion, for which a reactivity enhancement of $\rm{\sim50\%}$ is expected.

physics.chem-ph↗

arXiv2014

Evanescent-wave and open-air chiral sensing via signal-reversing cavity-enhanced polarimetry

Sensing chirality is of fundamental importance to many fields, including analytical and biological chemistry, pharmacology, and fundamental physics. Recent developments have extended optical chiral sensing using microwaves, fs pulses, superchiral light, and photoionization. The most widely used methods are the traditional methods of circular dichroism and optical rotation (OR). However, chiral signals are typically very weak, and their measurement is limited by larger time-dependent backgrounds and by imperfect and slow subtraction procedures. Here, we demonstrate a pulsed-laser bowtie-cavity-enhanced polarimeter with counter-propagating beams, which solves these background problems: the chiral signals are enhanced by the number of cavity passes; the effects of linear birefringence are suppressed by a large induced intracavity Faraday rotation; and rapid signal reversals are effected by reversing the Faraday rotation and subtracting signals from the counter-propagating beams. These advantages allow measurements of absolute chiral signals in environments where background subtractions are not feasible: we measure optical rotation from chiral vapour in open air, and from chiral liquids in the evanescent wave (EW) produced by total internal reflection at a prism surface. EW-OR of (+)-maltodextrin and (-)-fructose solutions confirm the Drude-Condon model for Maxwell's equations in isotropic optically active media. In particular, the effective optical rotation path length, near index matching, is equal to the Goos-H\"anchen shift of the EW. The limits of this polarimeter, when using a continuous-wave laser locked to a stable high-finesse cavity, should match sensitivity measurements for linear birefringence ($3\times 10^{-13}$ rad), which is several orders of magnitude more sensitive than current chiral detection limits, transforming the power of chiral sensing in many fields.

physics.chem-ph↗

arXiv2009

Molecular vibrational cooling by Optical Pumping with shaped femtosecond pulses

Some of us have recently reported in Science 321 232 (2008) vibrational cooling of translationally cold Cs_2 molecules into the lowest vibrational level v=0 of the singlet X 1Sigma_g ground electronic state. Starting from a sample of cold molecules produced in a collection of vibrational levels of the ground state, our method was based on repeated optical pumping by laser light with a spectrum broad enough to excite all populated vibrational levels but frequency-limited in such a way to eliminate transitions from v=0 level, in which molecules accumulate. In this paper this method is generalized to accumulate molecules into an arbitrary selected "target" vibrational level. It is implemented by using ultrashort pulse shaping techniques based on Liquid Crystal spatial light modulator. In particular a large fraction of the initially present molecule is transferred into a selected vibrational level such as v=1, 2 and 7. Limitations of the method as well as the possible extension to rotational cooling are also discussed.

physics.atom-ph↗