Jorge Vieira

 

 

 

Structured Light

 

 

 

Most of my research with structured light focuses on ultra-intense lasers with orbital angular momentum. Light, just as ordinary matter, carries momentum. Light's momentum is proportional to the Poynting vector and can be exploited to provide mechanical control over material bodies. An example that you might have seen already is a light-mill (Crookes radiometer [crookes]) that starts working when light is incident upon it.

In a plane wave, the Poynting vector points towards light's propagation direction. This observation was at the core of the invention of optical tweezers [ashkin'70], which gave the Nobel prize to Arthur Ashkin in 2018. There are other examples of waves, however, where the Poynting vector does not point exactly towards the wave propagation direction. A fascinating class of such exotic waves are those given by the Laguerre-Gaussian modes. Laguerre-Gaussian modes form a complete set of solutions of Maxwell's equations in cylindrical geometry. These modes are interesting because they carry quantized orbital angular momentum.

It had been known, since the early days of quantum mechanics, that quantum wave-packets could carry orbital angular momentum. It therefore seems natural that light, which, under the paraxial approximation, obeys a Schroedinger-like equation, could also carry orbital angular momentum. This was a purely mathematical analogy until 1992 when Les Allen and co-workers identified a simple laboratory setup that could be used to produce Laguerre-Gaussian laser modes from readily available optical components [allen'92].

This achievement triggered an explosive growth of exciting discoveries enabled by the orbital angular momentum, which culminated with twisted optical tweezers, advanced techniques for ultra-fast optical communications, and advanced microscopy. For example, one of the pioneering super-solution microscopy techniques, STimulated Emission Depletion (STED) [hell'94], which have the Nobel prize to Stefan Hell in Chemistry in 2014, makes direct use of the doughnut shaped intensity profile of twisted lasers.

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Light's orbital angular momentum has long been recognized as an intrinsic degree of freedom that stands in equal footage to the intensity and frequency of a laser pulse. This aspect has been, and continues to be, actively investigated at low intensities, below ionisation thresholds, in the fields of optics and photonics [padgett'94].

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Thanks to the invention of the Chirped-Pulse Amplification (CPA) technique that gave the other half of 2018's Nobel prize in Physics to Donna Strickland and Gerard Mourou [stickland'85] it is currently possible to create ultra-intense laser pulses with powers exceeding one Peta-Watt and intensities above 10^19 W/cm^2. When interacting with any known material these lasers instantly ionize it and create a  plasma.

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These lasers, which re both ultra-intense and ultra-short, with few fs durations, unlocked relativistic nonlinear optics, which lead to the discovery many flavors of plasma-based x-ray light sources, such as relativistic plasma mirrors, and compact plasma accelerators, among many other concepts.

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Most experiments at ultra-high intensities still work with lasers that can be well approximated by plane-waves. However, the interaction of ultra-intense lasers with advanced spatiotemporal couplings, such as the orbital angular momentum, with matter can lead to a whole new range of physical phenomena, which cannot be accessed by pure plane-waves. It allows, for example, to control the topology of the accelerating structures in plasma accelerators and to make plasma accelerators suitable for positron acceleration.

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The movie below shows simulation results high orbital angular momentum harmonics emerging in an initially Gaussian laser in a Raman backscatter geometry [vieira'16]. The changes on the transverse intensity profile of the laser indicate OAM contents. The laser pulse moves towards positive x1 and the simulation is in a moving frame that moves at c. The simulation was performed using Osiris and the visualization uses VisXD.