Giant Rabi splitting of whispering gallery polaritons in GaN/InGaN core-shell wire
Researchers have first demonstrated a novel type of polariton system resulting from strong coupling between a two-dimensional exciton and whispering gallery mode photon using a core-shell GaN/InGaN hexagonal wire
Research involving exciton polaritons resulting from the strong coupling between photon and exciton has attracted great attention due to its exotic phenomena in solid-state systems. Rabi splitting energy is a standard quantity used for estimating how strong the coupling interaction is. Large Rabi splitting energy values provides a robust strong coupling regime that resists various dephasing processes (i.e., photon and exciton dephasing). Furthermore, a system having large Rabi splitting energy that exceeds the longitudinal optical phonon energy (~91 meV in GaN) could be thermodynamically decoupled from the phonon bath. This provides an important advantage for maintaining high coherence at high temperature. Many materials (e.g., wide-bandgap semiconductors, organic materials, two-dimensional atomic crystals) have been investigated with the aim of achieving large Rabi splitting energy.
In particular, Group III-nitride materials have a large coupling constant (between photon and exciton). They are very promising for use in both optically and electrically driven, room-temperature polariton systems due to their exceptional chemical and mechanical stability. A conventional exciton polariton system is a planar micro-cavity sandwiched between two distributed Bragg mirrors (DBR). Such systems contain multiple-quantum wells layer that confine excitons of high internal quantum efficiency. However, contrary to expectations, a system of this type has a smaller Rabi splitting energy (30–50 meV) than that of bulk GaN crystal (~100 meV). This results from the large built-in field of a nitride quantum well (i.e., quantum confined Stark effect), and from the substantial penetration of the Bragg reflectors by the electric field (i.e., small spatial overlap between electric field and quantum wells). This hinders the implementation of practical, room temperature, nitride-based polariton devices.
A novel exciton polariton system with a GaN/InGaN core-shell wire has been proposed. It represents a new type and has been experimentally demonstrated by Prof. Yong-Hoon Cho of the Department of Physics and coworkers. Whispering gallery modes are naturally formed inside the hexagonal wire without complicated DBR structures. Moreover, it contains high-quality non-polar multi-quantum wells that prevent the quantum-confined Stark effect. In this new system, the spatial overlap between whispering gallery modes and multiple-quantum wells is highly improved, compared to conventional planar microcavities. As a result, the researchers have achieved an extremely high Rabi splitting value (~180 meV, a much higher value in energy than an optical phonon), never before recorded using nitride materials.
Further improvement of their coupling is expected with high-period MQWs at well-defined positions in the wire. Polariton condensation could also be accomplished with longer photon lifetime, using larger radius and smoother surfaced wires. Semiconductor wires possessing MQWs layer opens up the possibility of high-efficiency electrically-driven whispering gallery polariton devices. Moreover, this structure provides robust polariton effect with a small footprint; thus, it has a great potential for a wide range of interesting applications.
This paper, authored by Su-Hyun Gong, Suk-Min Ko, Min-Ho Jang, and Yong-Hoon Cho, was published in the Nano Letters (June, 2015).
* lab webpage