One of the next big steps in the fast-moving world of new technology is likely to be the replacement of silicon-based microchips with photonic integrated circuits or optical chips. Researchers are now looking into information processing using light in two dimensions, or optical surface waves, instead of electrons, which may lead to ultra-fast computing and exciting applications in biomedical science.
A surface wave is the light trapped at an interface, which can freely propagate in two-dimensional photonic systems. The manipulation and controlling of such surface waves at the nanoscale is becoming extremely important.
Within the large optical chip community are groups of scientists working on the development of nano-photonic devices using metamaterials. One of these is based in Birmingham: Dr Jensen Li and Shiyi Xiao, in collaboration with Professor Hui Liu from Nanjing University in China, are working towards a flexible and tunable holographic input interface for optical chips.
‘Our work is trying to give a flexible bridge between propagating light and surface waves,’ explains Jensen, a Senior Lecturer whose research focuses mainly on metamaterials and nano-photonics. ‘Previously, optical signals could be directly coupled from light in the free space by putting a series of grating or groove-like structures on the chip. However, they have very limited flexibility in controlling the profiles of the generated surface waves. In addition to bending and focusing waves into different patterns on an optical chip, we would need to tune and change the function in a dynamical way.’
One of the most promising ways to do this is the subject of their recent paper, ‘Flexible coherent control of plasmonic spin-Hall effect’, which was published in the journal Nature Communications and won the College’s Paper of the Month award late last year. In it, the researchers detail how, by shining a laser beam towards a micro-structured surface consisting of rings of nano-slots with tailor-made profile of orientation, it is possible to generate optical surface waves more flexibly than had previously been achieved.
‘We generate optical surface waves in the most flexible and tunable manner by borrowing a trick called spin-orbit coupling from electron to photon motion, in which the spin and the propagation directions are affecting each other,’ explains Shiyi, a Research Fellow in Physics and Astronomy. ‘It makes our design approach powerful while the design is still manageable for nano-fabrication. It also has the additional benefit of being dynamically tunable by changing the spin of the incident light, without using any nonlinear materials.’
Jensen continues: ‘What we are working on is spin-dependent optics by exploiting optical spin-Hall effect (SHE). We can generate different surface-standing wave patterns such as a triangle or the shape of a cross on the optical chip, depending on the spins of incident light. We can even get the two spins to cooperate to generate a movie. For example, you can watch while it writes the letter B.’
UoB 'written' using the effect
SHE is a spin-orbit coupling phenomenon, predicted by two Russian physicists in 1971, to induce electric currents with spin-dependent directions. The optical counterpart of SHE was been demonstrated only a decade ago, based on a semi-classical correction to geometrical optics. Up to now, only simple splitting of focal spots and a flip of propagation direction of light have been demonstrated. What Jensen and his colleagues’ paper shows is the consequence of a fully exploited version of the optical SHE.
‘We believe our demonstrations have huge potential in a wide range of applications requiring tunable surface wave control,’ says Shiyi. ‘By fully exploiting the spin-orbit coupling of light, we now have a flexible and tunable platform for generating inputs on an optical chip. All the required information is just stored in the orientation profile of the nano-slots – which collectively act like a hologram.
‘Such a holographic interface will be useful for applications of tunable optical devices, for example tunable sensors, tunable tweezers and holographic storage.’