The Low Dimensional Structures and Devices group at Sheffield has recently made notable advances in the development of a highly versatile new technology, that of tunable, high finesse optical cavities which operate over a wide temperature range from 4 to 300K. Such cavities have application in a wide variety of subject areas including III-V polaritonics, 2D graphene-like structures and organics. The following text provides a short summary. Further details may be obtained from Mr Scott Dufferwiel, and Dr Feng Li. Details of this work may be found at Applied Physics Letters 104, 192107, 2014. Further publications are in preparation.
Semiconductor optical microcavities are significant structures for investigating light-matter interaction in solid state systems. The strong coupling between the cavity optical mode and the in-cavity emitter gives rise to cavity quantum electrodynamics (cavity QED), a promising approach towards the future solid-state quantum computing and communication. Conventional monolithic microcavities, though widely used in current quantum optics studies, have considerable limitations such as difficulties to achieve flexible spectral tuning, to incorporate foreign emitters at will and to implement ultra-strong three-dimensional optical confinement. In this sense, a new type of optical system is needed to meet these requirements.
Figure 1: (a) Schematic of the open cavity system. (b) Anticrossing between the cavity modes and the QW exciton revealing the formation of polaritons in the system. (c) Laguerre-Gaussian optical mode profiles in the open cavity.
In collaboration with the Photonic Nanomaterials Group, University of Oxford and Cavendish Laboratory, University of Cambridge, we have developed a novel optical system revealing the desired advantages. The tunable microcavity, operated at cryogenic temperatures, consists of a bottom semiconductor distributed Bragg reflector (DBR) with near-surface quantum wells (QWs) and a top dielectric concave DBR separated by a micrometre sized gap. Nanopositioners, manufactured by Attocube, allow positioning of the DBRs to form a hemispherical cavity and the spectral resonance can be tuned by controlling the separation (Figure 1a). While this setup has a large number of potential applications in areas such as optical properties of two-dimensional materials, improving the performance of single photon emitters and cavity optomechanics, it is especially valuable for exciton-polariton studies. The cavity polaritons, which are quasi-particles exhibiting dual nature of light and matter, can be confined to submicron size by the concave-planar microcavity, leading to increased polariton-polariton interactions – a key requirement to observe polariton quantum blockade, where the presence of a single polariton within the microcavity prevents the injection of a second polariton due to the strong inter-particle interactions. This has potential to result in single photon emission at significantly faster rates than current single photon sources. Furthermore, this study will also lead to the area of quantum polaritonics, enabling to manipulate exciton-polaritons at the single particle level.
At the current stage, the open cavity exhibits a maximum quality factor of ~35,000 with full tunability of the polariton energy. Strong exciton-photon coupling and the formation of polaritons have been demonstrated through a characteristic anticrossing between the cavity modes and the QW exciton (Figure 1b). The laterally-confined optical modes display Laguerre-Gaussian (LG) spatial distribution (Figure 1c), making it possible to create engineered phase and spin textures. Quasi-two-dimensional (2D) thin films, such as MoS2 and GaSe, have been incorporated into the cavity as active materials and exhibited strong enhancement of spontaneous emission (details in arXiv 1408.3612). Research work is currently in progress towards polariton quantum blockade as well as deep insight into the open cavity polariton condensates.
People involved in the project: S. Dufferwiel, F. Li, L. Giriunas, P. M. Walker, E. Clarke, E. Cancellieri, S. Schwarz, A. I. Tartakosvskii, M. S. Skolnick and D. N. Krizhanovskii (Sheffield); A. Trichet and J. M. Smith (Oxford); I. Farrer and D.A. Ritchie (Cambridge).