A team of scientists led by Prof. Qihua Xiong at Tsinghua University was invited by Co-Founding-Editors-in-Chief, Prof. Lei Zhou and Prof. Din Ping Tsai, to contribute a comprehensive review paper entitled "Microcavity exciton polaritons at room temperature", which was published on the first issue of Photonics Insights as " On the Cover". (Sanjib Ghosh, Rui Su, Jiaxin Zhao, Antonio Fieramosca, Jinqi Wu, Tengfei Li, Qing Zhang, Feng Li, Zhanghai Chen, Timothy Liew, Daniele Sanvitto, Qihua Xiong. Microcavity exciton polaritons at room temperature[J]. Photonics Insights, 2022, 1(1): R04)

Microcavity exciton polaritons are half-light half-matter bosonic quasiparticles resulting from the strong coupling between excitons and photons in optical microcavities. The formation process and physical properties of exciton polaritons are shown in Figure 1. While possessing both the properties of light and matter, exciton polaritons can be essentially viewed as exciton-dressed photons. The core advantage lies in inheritance of the strong nonlinear interaction and sensitivity to external stimuli, such as electric and magnetic fields, from the excitonic part, thus, compensating the shortcomings of purely photonic systems.

Figure 1 Formation process and physical properties of exciton polaritons in a microcavity.

Meanwhile, the photonic component offers a low effective mass (one billionth of atomic effective mass), fast propagation speed, and easy manipulation, among other superior properties. These unique physical properties make it promising for applications including low-threshold lasers, optical modulators, switching devices, neural network computing, quantum simulation and computation.

Early research on exciton polaritons was mostly based on traditional III-V quantum well semiconductor systems at very low temperatures (liquid helium or nitrogen temperatures), for which cryogenic cooling devices were required. Thus, working temperature was one of the biggest challenges for exciton polaritons to move towards practical applications.

To this end, researchers have made a lot of efforts to move towards the room temperature operation. Due to the small effective mass of exciton polaritons in microcavities, their quantum states can theoretically exist at room temperature. In the early 21st century, a series of semiconductors with large exciton binding energies and high oscillator strengths gradually appeared, allowing to successfully observe exciton polaritons and Bose-Einstein condensates at room temperature, marking a new stage in the field. Realizing novel quantum effects of exciton polaritons no longer needs to rely on complex low temperature systems but can be easily realized at room temperature. At the same time, polaritonic devices based on room temperature microcavities have also entered a period of rapid development.

Based on this significant historical process, researchers have further made outstanding contributions in the basic quantum physics phenomena and practical applications such as lasing, nonlinear optical devices, and artificial lattice simulation using different microcavities such as organic semiconductor systems, perovskite systems, transition metal dichalcogenide systems, and carbon nanotube systems.

Wide-bandgap inorganic semiconductor systems

In the early days, wide bandgap inorganic semiconductor systems (such as gallium nitride and zinc oxide systems) with stable excitons at room temperature and mature fabrication processes were considered suitable platforms for studying exciton polaritons. For example, early gallium nitride microcavities fabricated using MOCVD successfully achieved exciton polaritons and their low-threshold lasing phenomena at room temperature under optical pumping (Figure 2a). In the development of on-chip integrated optoelectronic devices, researchers further combined complex PN junction structures with microcavities and successfully achieved electrically pumped exciton polariton lasers at room temperature (Figure 2b).

Figure 2 Development of wide-bandgap inorganic semiconductor systems.

Zinc oxide is also an excellent wide-bandgap inorganic semiconductor which has shown enormous potential. For example, Prof. Zhanghai Chen's group synthesized one-dimensional Zinc oxide microrods with hexagonal cross-sections via a carbon reduction method. The microrods themselves can simultaneously serve as microcavities and gain layers to achieve polariton lasing and nonlinear parametric scattering at room temperature (Figure 2c). The team further combined the Zinc oxide microrods with a grating structure and used the periodic structure to control the properties of the Zinc oxide polaritons, achieving novel phenomena such as the opening of the bandgap and weak lasing, providing a pathway for the effective control of room temperature exciton polaritons.

Organic semiconductor system

Organic semiconductors, as semiconductors with Franz-Keldysh exciton characteristics, have advantages such as large exciton binding energy, high oscillator strength, ease of synthesis, a wide variety, and strong controllability, providing an excellent platform for the study of room temperature exciton polariton.

Research on exciton polaritons in organic semiconductors can be traced back to the strong coupling effect achieved by Lidzey and colleagues in a metal Bragg reflector mixed microcavity in 1998. Since then, a series of novel phenomena have been realized in the organic semiconductor system at room temperature, such as exciton polariton condensation and lasing in different types of organic semiconductors (Figure 3a). Furthermore, Bose-Einstein condensation of exciton polaritons, frictionless superfluidity have also been achieved at room temperature (Figure 3b).

Figure 3 Development of organic semiconductor exciton polariton.

In addition, due to the strong nonlinear properties of exciton polariton, researchers have also achieved room temperature all-optical transistors in organic systems (Figure 3c). The recent successful implementation of periodic structures in organic semiconductor microcavities has also promoted the realization of topological optical devices, such as topological lasers (Figure 3d), opening up new avenues for on-chip integrated optical devices.

Perovskite system

Halide perovskites, as unique combinations of organic and inorganic semiconductors, have shown great success in the study of room temperature exciton polariton. Perovskites have broad prospects in photovoltaic and optoelectronic devices due to their excellent properties such as easy growth, direct bandgap, tunable band edges, high optical gain, and large exciton binding energy. Besides, their outstanding optical properties have recently enabled significant progress in the study of room temperature exciton polariton.

The research on perovskite exciton polariton can be traced back to the strong coupling effect based on two-dimensional hybrid perovskite semiconductors in a distributed feedback microcavity in 1998, This has led to a series of research works, such as the achievement of room temperature condensation (Figure 4a) and long-range high-speed propagation based on all-inorganic perovskite by Prof. Qihua Xiong's group.

Furthermore, researchers have also found that the perovskite microcavities has strong spin-dependent and highly nonlinear properties, providing a new platform for the fundamental physics research of exciton polariton, nonlinear optical devices, and on-chip integrated optical circuits. In 2020, one-dimensional periodic structures were successfully realized in the perovskite system (Figure 4b), which provided an extremely controllable and scalable approach to effectively manipulate perovskite polaritons and promoted the effective implementation of quantum simulations for novel topological phenomena in both the Hermitian (Figure 4c) and non-Hermitian (Figure 4d) regimes.

Figure 4: Development of Exciton polariton in Perovskite Semiconductors

Recently, Prof. Qihua Xiong's group has made numerous outstanding achievements in the optical manipulation of Bose-Einstein condensate, lasing, nonlinear interactions, and artificial lattice potentials in the context of exciton polariton in perovskite and 2D layered semiconductor microcavities.

Transition metal dichalcogenides (TMD) system

The emergence of transition metal dichalcogenides system has also provided an excellent platform for the study of room temperature exciton polaritons. Due to the two-dimensional confinement effect and weak dielectric screening, single-layer transition metal dichalcogenides such as MoS₂, MoSe₂, WS₂ and WSe₂ exhibit strong exciton effects at room temperature, laying the foundation for the realization of room temperature exciton polariton (Figure 5a).

At the same time, their strong spin-orbit coupling and symmetry breaking also bring novel physical phenomena to the study of exciton polariton, such as room temperature valley-dependent circular polarization (Figure 5b) and nonlinear parametric scattering (Figure 5c). Their unique quantum well structure and controllability also bring new opportunities for the control and development of exciton polaritons, such as using Lego structures to achieve Moore's law periodic control, tunneling structure light emitting diodes (Figure 5d), and so on.

Figure 5. Development of exciton polariton in transition metal dichalcogenide semiconductors

Carbon nanotube system

The carbon nanotube system has also shown promising potential for the room temperature exciton polariton research. It is a special one-dimensional tubular carbon material with novel characteristics such as the ability to adjust band size and conductivity through structural modification. The carbon nanotube system has been found to exhibit near-infrared luminescence and strong exciton binding energy of up to 300~500 meV, laying the foundation for its research on room temperature exciton polaritons. For example, in a metal microcavity, strong coupling between microcavity photons and carbon nanotubes has been successfully achieved (Figure 6a).

Figure 6. Carbon nanotube system

Based on this structure, researchers further introduced an electrode structure to successfully achieve electrically injected exciton polariton and gate-tuned behavior (Figure 6b). Due to the unique one-dimensional structure of carbon nanotubes, the polarization dependence of the coupling strength in their microcavities was discovered, leading to the confirmation of the existence of exceptional points in momentum space. As an emerging system, exciton polariton in carbon nanotubes is relatively limited, and more novel phenomena and devices are still being explored.

Although strong light-matter coupling can be achieved in different systems mentioned above, each system has its own advantages and disadvantages. This article systematically summarizes and compares the physical properties of exciton polaritons in different systems, providing useful references for applications based on exciton polaritons.

Summary

Prof. Qihua Xiong's team and their collaborators have provided a detailed summary and discussion of the progress in both theory and experiment in the field of exciton polaritons. The article first introduces the relevant background of the Bose quantum field theory, and then provides a detailed description of the mean field based on the driven-dissipative Gross-Pitaevskii equation. The theoretical progress indicates that exciton polariton has enormous potential for applications in quantum computing and quantum simulation.

They also focus on the development and highlights of room temperature exciton polaritons in various semiconductor systems, such as organic semiconductor systems, perovskite systems, transition metal dichalcogenide systems, and carbon nanotube systems. Finally, they provide a prospect for future developments, such as the enhancement of nonlinear effects towards high energy efficiency nonlinear optical devices, electrically injected devices, optical simulators, quantum devices, and emerging systems.

This review article has received high praise from the renowned scientist Prof. Alexey Kavokin in the field of exciton polariton, who wrote a commentary article entitled as "Liquid light at room temperature" and pointed out that this is an excellent review of exciton polaritons in classical and quantum computing directions, which complements his book 'Microcavities' published by Oxford University Press.