The NTU research team with their optical setup. From left: Dr. Jiaxin Zhao, Dr Antonio Fieramosca and Associate Professor Timothy Liew. Photo Credit: Dr Antonio Fieramosca.
Polaritons are hybrid particles formed by a combination of light and matter. These exotic particles typically exist only at very low temperatures close to absolute zero, or in complicated materials where they are difficult to control. Now, physicists at Nanyang Technological University (NTU Singapore), Tsinghua University in China, and CNR-Nanotec in Italy have achieved a breakthrough in creating polaritons and making them interact, entirely at room temperature.
In a paper published in the journal Nature Nanotechnology in March 2022, the researchers report on the observation of polaritons at room temperature in a 2D material (a one-atom-thick sheet) known as tungsten disulfide or WS2. They also performed the first experimental observation in a 2D material of inter-polariton parametric scattering, a key process in which two polaritons collide and change frequencies. This work, in combination with the recent demonstration of polariton lasing in the same material, may lead to a new class of “polaritonic” devices, such as ultrafast switches and transistors, that can operate at room temperature with low power consumption.
The team was led by Professor Qihua Xiong (Tsinghua University; a former NTU faculty member), Associate Professor Timothy C. H. Liew (NTU) and Professor Daniele Sanvitto (CNR-Nanotec). The experiments were performed in NTU’s School of Physical and Mathematical Sciences (SPMS).
Mixing light with matter
When a material is subjected to an extraordinarily intense light field, its properties can be strongly modified, which in turn alters the behavior of light waves within the material. Physicists refer to such a scenario using the term “nonlinear optics”, to distinguish it from the standard laws of “linear optics” that govern light in everyday settings. Nonlinear optical systems have many applications, from optical communications to optical computing, so a great deal of research has been devoted to optimizing their performance. One of their most common limitations is high power consumption, caused by the high-intensity light fields on which they rely.
Polaritons provide a way to generate nonlinear optical phenomena with significantly lower power consumption. Normally, there is a sharp distinction between particles of light, called photons, and particles of matter such as electrons, protons, or neutrons. But under specific conditions they can hybridize, leading to the formation of highly interacting particles called polaritons, which are neither purely light nor matter but a mix of the two. The polaritons in the present study are “exciton-polaritons”, formed by mixing photons with excitons – pairs of electrons and missing electrons, or “holes”, in semiconductor materials.
Exciton-polaritons were first discovered in the late 1990s, in experiments that needed to be performed at cryogenic temperatures close to absolute zero. If polaritons are to be utilized in practical devices, as many researchers hope, they need to exist at room temperature. But although researchers have identified a few materials capable of hosting exciton-polaritons at room temperature (e.g., organic semiconductors, perovskites, and zinc oxide), these have proven difficult to work with for a variety of reasons. For instance, the materials tend to have highly irregular atomic configurations, and are often chemically unstable when exposed to air.
Working at the nanoscale
The researchers from NTU, Tsinghua, and CNR-Nanotec overcame this difficulty by turning to 2D materials, sheets of atoms with a thickness of only one or two atoms (i.e., less than a nanometer). The first and most well-known 2D material is graphene, whose development was recognized by the 2010 Nobel Prize in Physics. Since then, scientists have developed many other 2D materials with a wide variety of physical properties that differ from standard 3D materials.
Dr Fieramosca (left) and Dr Zhao (right) setting up the experimental apparatus for creating and analyzing exciton-polaritons at room temperature. Photo Credit: Dr Antonio Fieramosca
Tungsten disulfide (WS2) belongs to a family of 2D materials known as transition metal dichalcogenides, which are semiconductors. (Graphene, by contrast, is a conductor.) Not only is WS2 itself chemically stable at room temperature, but the excitons within it are also extremely stable.
The researchers embedded a sheet of WS2 in a tiny optical resonator known as a microcavity, which traps light and magnifies it to high intensities. By directing different light beams into the microcavity, and carefully measuring the characteristics of the light escaping from it, they determined that exciton-polaritons were being formed within the device.
Nonlinear parametric scattering visualized in a plot of energy versus momentum produced from reflectivity measurements. A laser beam (Pump) injects polaritons, which interact with a weak laser beam called a “seed” and are then scattered into two new states (Idler and Amplified Seed). The latter forms the measured output beam.
Moreover, their analysis showed the exciton-polaritons colliding with each other, and altering each others’ energy and momentum in the process. This phenomenon, called parametric scattering, confirms that the polaritons truly behave as distinct particles. It had previously been observed in 3D materials at cryogenic temperatures, but had never before been seen in a 2D material.
The parametric scattering was observed at much lower power levels than most phenomena in nonlinear optics – about 100 times lower. This indicates that exciton-polaritons based on 2D materials can be used to create nonlinear optical devices that operate at low powers.
A bright future for polaritons
The observation of nonlinear parametric scattering between polaritons at room temperature is a significant milestone for the research field, as such interactions play a crucial role in plans to use polaritons for optical switches, optical transitors, and other useful devices. The results from the NTU/Tsinghua/CNR-Nanotec group are thus highly promising for the development of “polaritonic” devices that can operate at room temperature, and with low power consumption.
Another key advantage of WS2, and other transition metal dichalcogenides, is that it can be easily integrated into electrical devices. Looking ahead, the team envisions the development of electro-optical devices in which electrical signals are converted into exciton-polaritons, which can undergo transformations (e.g., amplification, switching, and frequency conversion) for ultra-fast information processing. They are also excited to undertake further investigations into the fundamental physics of exciton-polaritons, and the numerous fascinating behaviors they can manifest.
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About the Author
Dr Antonio Fieramosca is a Research Fellow at the School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore.