Magnetized Plasmas Twist Light

Converting a Gaussian laser beam into an optical vortex in magnetized plasma. An input Gaussian laser beam is sent through a plasma, which is mediated in an axial symmetric magnetic field generated by anti-Helmholtz coils. Traveling through the plasma twists the laser beam wavefront. (Source: K. Qu, Princeton Univ.)

To get the extremely high-reso­lution images vital to study new materials, microbes, and more, scientists often build micro­scopes based on optical vortices. Forming these tiny tornadoes of light is done using quartz or liquid crystals. However, using conven­tional materials for micro­scopes has its limits. As the power of the optical vortex increases, the material literally burns up and is destroyed. To produce the optical vortices, researchers needed a better approach. They devised a way to make optical vortices with 1000 times more power than previous methods. Their design uses strong, non­uniform magnetic fields to control plasmas, or ionized gases, to create the vortices.

The new approach, a plasma q-plate, will revolu­tionize sources for gene­rating optical vortices. The work will impact a broad range of appli­cations. For example, the new approach could lead to super-reso­lution micro­scopy. It could increase the bandwidth of optical fiber and milli­meter-wave wireless communi­cations. Also, the new approach could benefit quantum communi­cation with un­breakable encryp­tion.

For a laser beam of light moving uni­formly in one direction, the wave­fronts form parallel sheets with a centrally peaked inten­sity profile. There exists an optical vortex, whose wave­fronts twist and rotate as it passes through space. An optical vortex has rotating wave­fronts and a hollow inten­sity profile. This vortex can trap, rotate, and control micro­scopic particles or droplets, thereby func­tioning as an optical spanner that enhances the control flexi­bility of the optical tweezers that can trap particles. Super-reso­lution micro­scopes, with reso­lutions smaller even than the dif­fraction limit of light, can also be built using optical vortices.

Low inten­sity optical vortices can be formed using bire­fringent material media, such as quartz or liquid crystal, which split light into parallel and perpen­dicular polari­zations. However, using conven­tional material media for the micro­scopes has its limi­tations. As the inten­sity of the optical vortex increases, the material literally burns up. To produce high-power optical vortices, a team employed a plasma medium. The task of creating the required structure in plasma is chal­lenging because plasma is inherently unstruc­tured. The team’s approach circum­vents the diffi­culty of creating structure by intro­ducing anisotropy through a magnetic field. The team determined that a non-twisting laser beam, after propa­gating through magne­tized plasma, could be converted into an optical vortex. The magne­tized plasmas can mani­pulate the laser wave­front and directly convert a high-inten­sity Gaussian beam, say at a terahertz, into a twisted beam with high effi­ciency. (Source: Princeton Univ.)

Reference: K. Qu et al.: Plasma q-plate for generation and manipulation of intense optical vortices, Phys. Rev. E 96, 053207 (2018); DOI: 10.1103/PhysRevE.96.053207

Link: Dept. of Astrophysical Sciences, Princeton University, Princeton, USA

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