Catching Bacteria With a Photonic Hook

Scheme of a photonic hook to move particles along a curved path around transparent obstacles. (Source: ITMO)

An inter­national team of physicists has discovered a new type of curved light beams, dubbed a “photonic hook”. Photonic hooks are unique, as their radius of curvature is two times smaller than their wavelength. This is the first time that such a small curvature radius of electro­magnetic waves has been recorded. A photonic hook can be used to improve the resolution of optical scanning systems, as well as to control the movement of nano­particles, individual cells, viruses or bacteria.

For the longest time, physicists claimed that electro­magnetic radiation propagates along a straight line; however, in 2007 the existence of a curved electro­magnetic ray was experi­mentally confirmed. Dubbed the airy beam was considered a singular example of a curved ray. Recently, scientists from ITMO Univer­sity, along with their colleagues from Tomsk State Univer­sity, the Uni­versity of Central Florida, the University of Ben-Gurion and the Univer­sity of Bangor, have dis­covered a new type of curved light beam – the photonic hook.

“The photonic hook is formed when we direct a plane light wave to a dielectric particle of an asym­metric shape,” says Alexander Shalin, head of the Inter­national Laboratory of Nano-opto-mechanics at ITMO Univer­sity. “We studied a cuboid-particle. It has the appearance of a cube with a prism located on one side. Due to this shape, the time of the complete phase of the wave oscil­lations varies irregularly in the particle. As a result, the emitted light beam bends.” Scientists have shown that the photonic hook’s curva­ture radius can be much smaller than its wave­length. The curvature can also be adjusted by varying wave­length, incident light polarization as well as geometric para­meters of the emitting particle. This property can be used to redirect an optical signal, to overcome the diffrac­tion limit in optical systems or to move indi­vidual particles on a nanoscale.

“This idea was ini­tially suggested by our colleagues from Tomsk State Univer­sity. As soon as we made the necessary calcu­lations and described this phenomenon, we decided to check whether a photon hook can be used in opto­mechanics,” says Sergey Sukhov, researcher at the Univer­sity of Central Florida. “It turned out that, using a photonic hook, we can make a mani­pulator to move particles along a curved path around transparent obstacles. This is possible due to radiation pressure and gradient optical force. When some particle hits the region of the highest inten­sity of the beam, the gradient force keeps it inside the beam while radiation pressure pushes it along the curved path of energy flow propa­gation.”

Such a method of control over particles movement is promising for opto­fluidics. This techno­logy uses light beams to direct micro-streams of dissolved nano- and micro­particles. This allows scientists to make micro-reactors on chips and to inves­tigate, for example, bacteria, viruses or individual cells. “We are now going to make an experi­ment and attempt to move bacteria along a curved tra­jectory with a photonic hook,” Alexander continues. “First of all, we need to get the hook itself in experi­mental conditions. We need to check, for instance, if a substrate under our cuboid would affect the hook emission. Next we will make a proto­type of the micro-reactor and study how particles move.” (Source: ITMO)

References: L. Yue et al.: Photonic hook: a new curved light beam, Opt. Lett. 43, 771 (2018); DOI: 10.1364/OL.43.000771 • A. S. Ang et al.: ‘Photonic Hook’ based optomechanical nanoparticle manipulator, Sci. Rep. 8, 2029 (2018); DOI: 10.1038/s41598-018-20224-4

Link: Nanooptomechanics Laboratory, ITMO University, St Petersburg, Russia

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