Nanyang Assistant Professor Rui Su and his research group. From left: Prof. Rui Su, Dr Jie Liang and Mr Feng Jin. Photo credit: M. Fadly.
Imagine a bowling ball zooming down a bowling alley, spinning as it moves. Because of how its spin interacts with the forward motion, the ball does not travel in a straight line, but curls to one side and lands in the gutter. Had we thrown the ball with the opposite spin, it would have curled the other way, landing in the opposite gutter.
The electrons in certain electrically conducting materials behave similarly. As they flow through the material, some veer to the left and others to the right, depending on their internal spin. Thus, different amounts of spin angular momentum are deposited on opposite sides perpendicular to the current, a phenomenon called the “spin Hall effect” (named after a related effect discovered by the 19th century physicist Edwin Hall).
However, electrons are not the only microscopic particles that have spin. Using other kinds of particles, it should be possible to observe new versions of the spin Hall effect.
A team of physicists from Nanyang Technological University (NTU), Singapore, has recently observed the spin Hall effect with exciton polaritons – hybrid particles of light and matter. The team, led by Nanyang Assistant Professor Rui Su and Associate Professor Timothy C. H. Liew from NTU’s School of Physical and Mathematical Sciences, in collaboration with Professor Yuri G. Rubo at the Universidad Nacional Autónoma de México, reported their advance in the journal Nature Photonics in February 2024.
Using the polaritonic spin Hall effect, the researchers were able to generate polaritons with a record-breaking level of spin purity. In the future, this technique may allow light fields to be controlled more efficiently and effectively than with existing devices like liquid crystal displays.
Nanyang Assistant Professor Rui Su (left) aligning the optical setup for the polariton experiment with research fellow Dr. Jie Liang (middle) and PhD student Mr Feng Jin (right). Photo credit: M. Fadly.
Spinning Particles of Light and Matter
Before we can understand the polaritonic spin Hall effect, think about an ordinary beam of light, which is an electromagnetic wave. A property of the wave called polarization describes how its electric field is oriented. In “linearly-polarized” light (Fig. 1, left panel), the polarization points in a fixed direction perpendicular to the beam. In “circularly-polarized” light (Fig. 1, middle and right panels), the polarization spirals like a corkscrew, and the clockwise or counter-clockwise direction of the spiral behaves as a spin.
Figure 1. Schematic illustration of “linearly-polarized” light (left), “right-handed circularly-polarized” light (middle), and “left-handed circularly-polarized” light (right). Figure credit: J. Liang.
While this polarization-based spin is usually regarded as a property of a wave, it is also possessed by photons, the individual quantum particles of light. Polaritons, on the other hand, are quantum particles that are a mix of light and matter. These hybrid particles appear only in special settings where light interacts strongly with a material. In particular, polaritons can carry a spin based on the photon spin.
In their study, the NTU team used exciton-polaritons, a variety of polariton whose matter component consists of positive and negative electric charges in a semiconductor. These exciton-polaritons are generated in a tiny optical cavity (or “microcavity”) containing a so-called lead halide perovskite material, CsPbBr3, a semiconductor with strong light-matter interactions (Fig. 2, left panel).
Figure 2. Left: photograph of a sample of CsPbBr3, a perovskite material that can host hybrid particles of light and matter called exciton-polaritons. Right: experimental setup for the polariton spin-Hall effect: a microcavity formed by two mirrors (dark cyan strips), containing a layer of CsPbBr3 and a layer of liquid crystal. Figure excerpted from Liang et al., Nature Photonics (2024).
Synthetic Spin-Orbit Coupling
For the spin Hall effect to occur, not only must a particle have spin, but its motion must be affected by spin, like the spinning bowling ball described at the start of this article.
“The simplest way for spin to affect motion is to have a magnetic field, but that is impractical. We want these polaritons to have technological applications down the line, and strong magnets are a no-go for most devices,” notes Nanyang Assistant Professor Rui Su.
To resolve this problem, Su teamed up with Associate Professor Timothy Liew, an expert in the theory of polaritons. They designed an exciton-polariton system with strong “spin-orbit coupling,” a phenomenon whereby spin and motion influence each other without a magnetic field.
Their solution was to add a layer of liquid crystal to the microcavity (Fig. 2, right panel). Liquid crystals are made of cylindrically-shaped molecules and possess a property called birefringence, causing light traveling parallel or perpendicular to the cylinders have different speeds. The orientation of cylindrically-shaped molecules can be actively tuned by electrical voltages and the effective speeds are thus controllable inside the microcavity. Using theoretical calculations, the researchers predicted that when exciton-polaritons move through the perovskite, the influence of the adjacent liquid crystal produces synthetic spin-orbit coupling controlled by external electrical voltages.
A close-up view of the liquid crystal-filled perovskite microcavity. Photo credit: M. Fadly.
A Spin Filter For Polaritons
The NTU team’s analysis also indicated that these exciton-polaritons would exhibit the spin-Hall effect. When polaritons are shot across the CsPbBr3 layer, the spin-orbit coupling makes them veer left or right, depending on their spin (Fig. 3, left panel).
Figure 3. Left: schematic showing exciton-polaritons with different spins (indicated by blue and red) veering in different directions as they move across the CsPbBr3. Right: experimental results, based on angle-resolved optical spectroscopy, revealing two jets of exciton-polaritons with high-purity spins. Figure excerpted from Liang et al., Nature Photonics (2024).
To observe this effect experimentally, he team employed a technique known as angle-resolved optical spectroscopy. Some polaritons escape the microcavity in the form of photons, which remember the spin orientation of the original exciton-polaritons. By carefully measuring the polarization of this escaping light, the researchers can reconstruct the spins of the exciton-polaritons at different positions in the microcavity.
Their experimental results (Fig. 3, right panel) precisely match the spin-Hall effect. “We see two jets of polaritons, carrying opposite spins,” explains Dr Jie Liang, a postdoctoral researcher at NTU and the first author of the Nature Photonics paper. “The jets are clearly separated, and each jet’s spin has about 90 percent purity, much better than any other polariton experiment has seen before.”
“Our findings provide a feasible way to generate and manipulate near pure polariton spins at room temperature,” adds Nanyang Assistant Professor Su. “This discovery offers an exciting opportunity to develop so-called spin-optoelectronic devices, which use electric currents to manipulate the polarization of light or vice versa.”
Moving ahead, the team is exploring how to use this exciton-polariton system to create elementary spin-optoelectronic devices such as optical spin filters. They are also exploring how to fine-tune the properties of the liquid crystals, in order to give the exciton-polaritons new properties and behaviors that have not previously been found.