Sometimes ideas for research projects can come from unusual places. For me, this project started when I signed up to the ACT Science Mentors program to mentor a high school student through a research project. The student I was partnered with was very interested in quantum mechanics, and had already measured the Boltzmann constant in his project the previous year. So I was looking around trying to think of a quantum mechanics experiment that would be interesting and challenging for my student, but could be done with the resources available to the Science Mentors program. One project that my student initially took an interest in was creating an optical vortex. Although my student went on the do a project on quantum key distribution instead, I realized I could use my optical phased array to make an optical vortex.
What the heck is an optical vortex?
An optical vortex, also called a photonic quantum vortex or phase singularity, is a beam of light where the light twists in a helical path around its direction of travel. When this light is shone onto a wall it simply looks like a ring of light with a dark spot in the middle. However, the light is carrying orbital angular momentum which makes it useful for a range of science experiments and applications.
Orbital angular momentum can be harnessed to give light some very useful properties. Beams of light with orbital angular momentum can be used to enhance the resolution of some types of microscopes. Beams with different amounts of orbital angular momentum can be used to increase the amount of data transmitted through an optical fiber, or through the air, because each individual beam can be used to transmit a separate stream of data without interfering with the other beams. The properties of optical vortices mean they have uses in quantum communications and quantum memory storage experiments. Because the light has rotational momentum, that momentum can be transferred to physical objects, so optical vortex beams can be used as ‘optical tweezers‘ to precisely rotate tiny objects such as bacteria or cells for microbiology experiments.
Making an optical vortex
Optical vortex beams can be created in many different ways. including special lenses, holograms, spatial light modulators, and carefully designed microscopic antennas. However, my optical phased array can also be made to produce optical vortex beams, with the advantage that the amount of twist of the vortex can be adjusted billions of times per second, and the direction of emission of the beam can be controlled very rapidly and precisely.
Basically, an optical phase array (OPA) is an array of multiple lasers where the beams combine together to make a single, more powerful laser beam that can be controlled with exquisite precision. By using the outer ring of 6 laser emitters of my OPA and making one laser emitter in the ring emit light slightly ahead of the one next to it, the individual laser beams combine into the helix required to make an optical vortex.
In my paper, I simulated what the vortex beam from my OPA should look like when it is shone onto an infra-red camera. The simulation showed the classic ring of light with a dark spot in the centre that indicates an optical vortex. The multiple rings and hexagonal shape seen in the images are due to the fact there were only 6 laser emitters. More emitters, and a tighter grouping of the emitters, would result in fewer, cleaner rings.
I then programmed my OPA to adjust the laser beams to the correct offsets. With 6 laser emitters, I could achieve 5 different amounts of helical rotation (5 different amounts of orbital angular momentum). They looked exactly like the simulations predicted, and my other measurements showed that the OPA was producing the lasers offsets needed to make the helix with extremely high precision.
The range of different amounts of beam helicity can be increased simply by increasing the number of laser emitters in the OPA. More emitters also means the beam will have greater fidelity. Vortex beams generated by OPAs have exciting possibilities in laser communications, fundamental research, and quantum communications experiments.