Photonic Hook Could Enable Optical Manipulation of Nanoparticles

A curved photonic nanojet, called a photonic hook, was created using an asymmetric cuboid, formed by appending a triangular prism to one side of a cube. Using numerical methods, scientists showed that the trajectory of a probe nanoparticle in the shadow side of the cuboid was curved. When a glass obstacle was added to the system, the trajectory shifted toward another path, losing some of its curvature in the long range. When a gold obstacle was added, the jet was reduced in size, and the forces in the long range decreased as well.

The scientific team comprised researchers from ITMO University, Tomsk State University, the University of Central Florida (UCF), the University of Ben-Gurion and the University of Bangor.

This is a model of a particle, hit by a laser pulse, that emits a new type of curved beam. Courtesy of ITMO University.

“The photonic hook is formed when we direct a plane light wave to a dielectric particle of an asymmetric shape. We studied a particle called cuboid. 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,” said ITMO professor Alexander Shalin.

The photonic hook’s curvature radius can be much smaller than its wavelength and can be adjusted by varying wavelength, incident light polarization and geometric parameters of the emitting particle. This property could 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,” said Sergey Sukhov, researcher at UCF. “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. 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.”

The optomechanical effect that allows particles to be moved around a specific path could pave the way for more flexible optical manipulation of nanoparticles and their transport along nonstraight trajectories, without the use of Airy beams. Moreover, the team says that it has shown that its method works on a scale much smaller than Airy beams.

In contrast to traditional self-accelerating beams, the photonic hook can be created using a compact microscopic optical element. The microscopic dimensions and simplicity of the photonic hook could enable it to be integrated into microfluidic devices and lab-on-a-chip platforms.

Alexander said that the team plans to experiment with moving bacteria along a curved trajectory using a photonic hook. Embedding the cuboid on a substrate could allow a more realistic representation of how the system could be used in an integrated device.

“First of all, we need to get the hook itself in experimental conditions,” Alexander said. “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 microreactor and study how particles move.”

The research was published in Scientific Reports (doi:10.1038/s41598-018-20224-4).