Structured light and lasers with orbital angular momentum
A new twist for plasma physics?
Structured lasers have non-trivial phase structures and polarisations. A well known example of structured light is the orbital angular momentum. Lasers with orbital angular momentum are characterized by helical wavefronts, with a phase singularity at the center of the laser, and have doughnut shaped intensity profiles. They are interesting from a fundamental point of view because they possess quantized amounts of angular momentum. Unlike circularly polarized light, which is featured by spin-angular momentum, twisted lasers contain angular momentum even if they are linearly polarized.
These lasers are also important for applications, and they have been providing a transformative set of tools for ultra-fast optical communications (the orbital angular momentum is a degree of freedom than can be used to encode information), for super-resolution microscopy, nano-particle manipulation and astrophysics.
What happens when these lasers propagate in plasmas has been nearly unexplored, but it promises exciting and unexpected physics. My work has focused on the possibilities that these lasers bring to plasma accelerators and how could plasmas be used to generate and amplify ultra-intense twisted lasers. Follow the links below to find out more.
(left) Electric field iso-surfaces of a twisted laser, which illustrate the helical wavefront structures. (right) Transverse intensity of profile. Brighter (darker) colors mean higher (lower) laser intensities.
All optical control of the topology of laser-plasma accelerators
J. Vieira et al., Accepted for publication in Physical Review Letters (2018)
Plasma accelerators may lead to more compact, yet powerful particle accelerators. In addition to their compactness, plasma accelerators may also provide unprecedented flexibility on the topology of the accelerating structures. Using theory and computer simulations, we propose a path to explore the topological freedom of plasma accelerators. Our work opens a new direction to explore plasma accelerators, leading to the generation relativistic beams with unprecedented spatiotemporal shapes, which cannot be commonly produced by conventional devices. The topological flexibility of the plasma is an emergent feature that becomes possible because plasma electrons, which sustain the accelerating structure, can move along complicated paths and thus be re-arranged to form non-trivial topologies.
Considering a “light spring” (laser with helical intensity profile) in a plasma, our work reveals a new type of acceleration that creates helical electron beams that rotate about their axis as they reach velocities close to the speed of light. They can be seen as small electron tornadoes. Surprisingly, the velocity at which they rotate is not arbitrary but quantised. Although the effect is purely classical, it mimics the quantum behaviour and reproduces the quantisation properties of twisted electron quantum vortices. Our work can have broad implications in different fields, from compact x-ray sources with novel properties and intense magnetic field generation, to novel pathways to control laser-matter interactions.
Example of a plasma wave with orbital angular momentum (red and blue) driven by a light spring (yellow) leading to the generation of a vortex electron beam (spheres).
High Orbital Angular Momentum Harmonics Generation
J. Vieira et al., Physical Review Letters 117 265001 (2016)
High harmonic generation (HHG) results from non-linear optical processes by which n photons, each with a given frequency ω, combine into a single photon with frequency nω. Because it can lead to a strong laser frequency upshift, HHG is routinely used in advanced microscopy techniques. In recent HHG experiments, where the incident photon beam contained an initial level of orbital angular momentum(OAM) ℓ, the final photon contained, in addition, an OAM level given by nℓ. In these experiments, which demonstrated the conservation of angular momentum (ℓ) and energy (ω), the OAM harmonics were coupled to the frequency harmonics.
We have recently investigated an unexplored mechanism that opens the possibility to create high OAM harmonics while leaving the laser frequency unchanged, thereby treating the orbital angular momentum as a true independent degree of freedom. Our theoretical calculations and numerical simulations show that the order of the high harmonics obeys a simple algebraic selection rule, which depends only on the initial OAM of the incident laser beams. Using our scheme, we have proposed an all-optical realization of the Green-Tao prime number theorem.
Our configuration employs stimulated Raman scattering in plasmas, using a pump beam in a fractional OAM state, i.e. containing more than a single OAM level. Although the plasma the advantage of supporting very intense fields, the physics also occurs in media with Kerr nonlinearity.
Initial laser set-up for the realization of the high OAM harmonics based on stimulated Raman scattering in the plasma. Similar results hold for any three-wave parametric process in nonlinear optics.
Amplification of exotic light in plasmas
J. Vieira et al, Nature Communications, 7 10371 (2016).
Raman backscattering amplification is a three-wave coupling mechanism where the energy stored in a long pump laser transfers its energy to a counter-propagating probe laser through a daughter plasma wave.
Raman amplification has been demonstrated experimentally and recent work has identified the ideal parameter region to avoid parasitic instabilities.
Together with Dr. Raoul Trines from Rutherford Appleton Laboratory, I have recently shown that Raman backscattering can efficiently generate and amplify twisted lasers with orbital angular momentum.
A laser with orbital angular momentum in a special configuration of stimulated Raman backscattering in plasmas.
High gradient positron acceleration
J. Vieira et al Phys.Rev. Lett. 112 215001 (2014).
Like boats and strong winds create waves in the sea, plasma accelerators use intense lasers or particle beam drivers to create ultra-fast plasma waves. Charged surfing particles can catch these waves, gaining very high energies.
Large amplitude plasma waves are ideally suited for electron acceleration. They are characterized by linear focusing fields, which preserve beam emittance, and by linear accelerating fields, which preserve the energy spread of beams with tailored current profiles.
Positron acceleration has been a long-standing challenge, because large amplitude plasma waves quicly defocus positrons. The total accelerating distance for positrons would then be limited to the distance they would take before leaving the plasma wave. Energy gains would be very small, and overall bunch quality would be poor.
In collaboration with Prof. Tito Mendonça (GoLP/IST) I have recently found that intense lasers with orbital angular momentum could provide ideal structures for positron acceleration in large amplitude plasma waves (see Figure). This could allow for future designs of plasma based linear colliders.
These lasers can also be used to accelerate ring electron bunches. These could be useful as lenses to collimate proton bunches (in connection with accelerator physics) and for electron diffraction (in connection with advanced imaging devices).
A doughtnut plasma wave driven by a twisted laser. The laser propagates towards the right. Laser projections in red-black. Plasma wave projections in gray (white – low plasma electron density; black – high plasma electron density). Colors are plasma electron density isosurfaces. Blue spheres are plasma electrons that spontaneously enter into the plasma wave and gain energy. Positrons accelerate at the front.