(left) Perovskite metasurface capable of enhancing the circular dichroism, that is the difference in absorption between left and right circularly polarised light; (right) Perovskite metasurface able to generate the optical Rashba effect in light emission, manifested by the routing in different half-hemispheres of light with opposite chirality. Image credit – Dr Giorgio Adamo
Light beams containing an intrinsic twistedness, called chirality, have many important applications including the detection of toxins in drug manufacturing. But when chiral light interacts with molecules and other nanometer-scale objects, the resulting small shifts in chirality can be hard to detect. A team of scientists at Nanyang Technological University (NTU Singapore) have taken an important step to tackle this problem, by developing nano-structured films with an unprecedented ability to manipulate chiral light.
An object is “chiral” if it cannot be superimposed with its mirror image. Chirality is ubiquitous in nature phenomena, ranging from spiral galaxies to fundamental particles. A light beam is said to be chiral if its internal electric field twists clockwise or counter-clockwise as it travels; the chirality reaches a maximum value for light beams that are “circularly-polarized”.
One of the principal uses of chiral light is to detect objects that are also chiral. The detection and characterization of chiral molecules is particularly important for drug manufacturing. Often, one molecule is a beneficial drug, while its mirror-image counterpart is a dangerous toxin.
When a circularly-polarized light beam shines on chiral molecules, the amount that is absorbed depends on the relative chirality of the light and the molecules. By varying the chirality of the light beam, one can thus detect the presence of chiral molecules. But since molecules are much smaller than the wavelength of light (around one nanometer versus 500 nanometers), they interact rather weakly, so the variation with chirality tends to be small and quite difficult to measure.
A promising way past this problem has been found by a team of researchers at NTU’s School of Physical and Mathematical Sciences (SPMS), led by Associate Professor Cesare Soci. In a pair of papers recently published in Nature Communications and Advanced Materials, they showed that devices known as perovskite metasurfaces offer unprecedented capabilities for controlling chiral light.
Chiral hybrid organic-inorganic perovskites are a class of recently-discovered chiral materials with good chemical and electrical characteristics. Previous studies had revealed a modest amount of chiral activity, with an anisotropy factor – a common measure of how well a material detects chiral light – of around 0.04.
Associate Professor Soci, with research fellow Dr Guankui Long and co-workers, have found a way to achieve an anisotropy factor of 0.49, corresponding to a twelve-fold enhancement. To achieve this, they created a device known as a perovskite metasurface, consisting of a thin film with nanopatterns etched in it. By shaping the perovskite material into chiral patterns and making precise adjustments to their dimensions, they optimized the anisotropy factor measured by their optical apparatus.
Dr. Giorgio Adamo (left) and research fellow Dr. Guankai Long have found a way to achieve an anisotropy factor of 0.49, corresponding to a twelve-fold enhancement. Photo credit- Dr. Guankai Long
According to their numerical simulations, by further increasing the metasurface’s area, the anisotropy factor can be further increased to 1.11 (i.e., a 28-fold enhancement). This would be comparable to the anisotropy factor’s absolute theoretical upper limit of 2.
“Optimizing the shapes patterned onto the metasurface, a process that we call ‘superstructural design’, has a huge impact on how the device interacts with chiral light. This helps us overcome limitations in a material’s molecular ‘design’,” says Associate Professor Soci. The results of the study were reported in Nature Communications in March 2022.
Another paper from Associate Professor Soci’s group, authored by Dr Jingyi Tian and co-workers and published in Advanced Materials in January 2022, addressed a related problem: can the perovskite itself serve as an efficient source of chiral light?
Dr. Giorgio Adamo, Dr. Jingyi Tian (seated) and Assoc. Prof. Cesare Soci, co-authored a paper published by Advanced Materials.
Here, the team developed a perovskite metasurface containing a feature known as “broken in-plane inversion symmetry”. They then illuminated the device with a strong laser beam, and studied the re-emission of light from the metasurface – a process called photoluminescence. They found that the photoluminescence had a degree of circular polarization of around 60%, which is six times higher than what other researchers had previously achieved with chiral perovskites.
Moreover, the photoluminescence was strongly directional: the chiral light was almost entirely emitted in one specific direction, which is a desirable property for a light source.
Dr Giorgio Adamo, the co-principal author in both publications, said: “Our work shows the value of shifting from chemical structure engineering to structural engineering, when it comes to creating highly chiral materials. Based on the dramatic improvements that we found, these perovskite metamaterials might eventually be incorporated into practical devices for generating, manipulating, and detecting chiral light.”
References
- Jingyi Tian, Giorgio Adamo, Hailong Liu, Maciej Klein, Song Han, Hong Liu, Cesare Soci, Optical Rashba Effect in a Light-Emitting Perovskite Metasurface, Advanced Materials (2022).
- Guankui Long, Giorgio Adamo, Jingyi Tian, Maciej Klein, Harish N. S. Krishnamoorthy, Elena Feltri, Hebin Wang, and Cesare Soci, Perovskite metasurfaces with large superstructural chirality, Nature Communications (2022)