Photonics is the study of the generation, detection, and manipulation of light waves in the form of photons. One interesting property of light is polarisation, defined by its electric and magnetic fields oscillating in any direction perpendicular to the direction of propagation. This oscillation is not restricted to one plane either. Circular polarisation occurs when light waves have electric fields that follow a spiral trajectory along the direction of propagation.
These circularly polarised waves have applications in biological and chemical sensing, optical communications, and quantum computing, but controlling them is challenging. "When we generate circularly polarised light, we want to make sure that it is directed at a specific angle for us to collect and efficiently use it," explained Wu Lin, Associate Professor at the Singapore University of Technology and Design (SUTD).
The best compact emitters of light are quantum dots—semiconductor nanocrystals with quantum mechanical behaviours thanks to their small size (2–10 nanometres). The emitted light goes in all directions and has poor polarisation, but placing it next to nanostructures enables directional emission or circular polarisation. Simultaneous control of both direction and polarisation, however, has never been achieved. In their paper " Unidirectional chiral emission via twisted bi-layer metasurfaces ", Associate Prof Wu and her team set out to bridge this gap.
Essentially, the team was faced with a multidimensional issue. They had to control the direction of the emitted beam and the polarisation of the light, while using a precisely engineered resonance of the structure. Moreover, a circularly polarised wave is chiral, which means that its mirror image cannot be superimposed on top of it. To create a structure that emits chiral waves, all mirror symmetries of the structure must be broken, causing them to have unusual designs.
The solution was proposed by SUTD-NUS PhD student and first co-author of the paper, Dmitrii Gromyko, working under the supervision of Associate Prof Wu. Inspired by a spiral ladder and a double-headed drum, Dmitrii first developed the design of twisted bilayer metasurfaces, consisting of two layers of periodically arranged discs with notches carved at specific angles. This innovative approach was further refined by Associate Prof Wu and the research team. The key element that makes this structure successful is its bilayer design. Since both layers can be individually controlled prior to their coupling, the metasurface provides both versatility and synergy. This design was
"While the electric fields of our waves are smooth and continuous in space, they can be generated by just two layers of discs which resemble a two-step spiral ladder. Just climbing two steps of this ladder is enough for you to know if you are taking a clockwise or counterclockwise path. In the case of photonic nanostructures, you just have to choose the right size for the steps of your ladder," said Dmitrii.
For the drum analogy, sound from hitting the top drumhead causes oscillations in the bottom drumhead, coupling their vibrations. The same goes for the nanostructure, except that the notches in its discs cause asymmetry. This asymmetry allows the polarisation of emitted waves to be controlled by rotating the notched discs in each layer.
With this design, the team could control three parameters essential for the precise control of the emission: the distance between the two layers, the angle between the notches in the top and bottom discs, and the lateral shift of the centres of the top discs with respect to the bottom discs. However, creating such a nanostructure was a huge feat.
"This is a real challenge because you have to vertically align the two layers with a precision of 10 nanometres. I'm proud that we have such capabilities in Singapore, as our colleagues from the Agency for Science, Technology and Research and the National University of Singapore performed the fabrication and measurement steps," stated Associate Prof Wu. The primary experimental work was conducted by the team led by Associate Prof Zhaogang Dong, who recently joined SUTD's Science, Mathematics, and Technology cluster.
Developing these bilayer metasurfaces has several benefits. Theoretically, it advances the field's understanding of resonances in multi-layer systems, design approaches, and fabrication technology. Practically, it enables the asymmetrical directional emission of waves with tailored properties. The nanostructure can then function as efficient emitters, routers, or grating couplers of circularly polarised waves, among other things.
As a next step, the team aims to integrate their bilayer design with nano-electro-mechanical systems to achieve reconfigurable chiral metasurface systems that can actively manipulate light emission angle, wavelength, and polarisation.
Embodying SUTD's principle of intersecting design and technology, this study paves the way for making ultra-compact devices with specific properties that meet the needs of modern science and technology. "There is a myriad of challenges and practical problems waiting to be resolved with a smart design," said Associate Prof Wu. "It is all about designing new solutions and advancing the current technology."
