Fishing for bacteria might not sound like the most pleasant or relaxing pastime. But this is precisely what researchers from an international collaboration have been able to achieve, using a curved light-beam.
The scientists, from ITMO University, Tomsk State University, the University of Central Florida, the University of Ben-Gurion and the University of Bangor, published their results in Optics Letters and Scientific Reports. They refer to their curved light beam as a ‘photonic hook,’ and it can be used to manipulate the movement of small particles; including individual cells, bacteria, and nanoscale particles.
The radius of curvature of the photonic hook is half the size of its wavelength. This is the smallest radius of curvature that’s ever been recorded for electromagnetic waves. Although in classical optics it is considered that radiation propagates as straight beams of light, it is possible to produce ‘curved’ rays; exploiting the phase shifts that can occur when light passes through dielectric materials.
"[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 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."
The phenomenon of focus bending is caused by the interference of waves inside the dielectric particle as the phase velocity disperses. Almost as soon as the photonic hook was described in the literature, its potential applications for manipulating and moving particles around was noted by the scientific community.
"The reference article describing the photonic hook itself was followed by an article about its optomechanical application," commented Sergey Sukhov, a researcher at the University of Central Florida. "Even before the first paper was published, MIT included it in its weekly review of the most interesting preprints. Yet, it also raised a lot of questions from the reviewers. Soon after it was published, it hit the top downloads on the Optics Letters website. By that time, the second article on optomechanics was accepted for printing. We hope that the results of our experiments will cause even greater interest."
A certain degree of control over the shape and intensity of the photonic hook can be exerted by the researchers by changing the polarization and frequency of the incoming light wave. Different geometric and topological parameters of the dielectric particle used to “curve” the beam can also change the level of curvature in the hook. This ability to tune the photonic hook suggests a range of different applications, from redirecting optical signals, to overcoming the diffraction limit for signals, to moving particles or fishing for bacteria as discussed.
"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 Sukhov. "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 Nature article that first featured the use of the photonic hook to manipulate the direction of nanoparticles pointed out that, unlike traditional methods in optofluidics (using light beams to nudge nanoparticles around), the gold nanoparticle that they manipulated with the photonic hook followed a curved trajectory. The authors of the paper suggest that potentially this technique could be used to direct nanoparticles around obstacles.
The scientists in the collaboration hope to demonstrate a similar effect in bacteria.
"We are now going to make an experiment and attempt to move bacteria along a curved trajectory with a photonic hook," Alexander remarks. "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."
Given the number of bioengineered technologies that use bacteria as their chemical engines, it seems likely that this kind of fine-control nanoengineering technique will be highly useful in the future.