Improving cavity spectroscopy in the fractional quantum Hall regime

Stefan Fält(1,2), Odysseas Williams(1), Christian Reichl(1) and Werner Wegscheider(1)
1 Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland
2 Institute of Quantum Electroncis, ETH Zürich, CH-8093 Zürich, Switzerland

The study of quantum Hall (QH) physics have historically benefitted from improvement in the epitaxial growth of 2-dimensional carrier gases. Recently, cavity spectroscopy has shown to be a sensitive probe of QH states in a device where the epitaxially grown optical cavity is integrated monolithically with the structure that confines the carrier gas. It has been used to study the optical properties of QH states [1] also in the nonlinear regime [2] and ferromagnetism in the valence band [3].

Improvements on this early generation of devices can be done in several ways. Here, both the design of the structure and material quality will be considered. For the latter, it is clear that the purity of the Al in the MBE is of importance as the cavity mirrors consist of distributed Bragg reflectors (DBRs) made of AlAs and AlGaAs. In-situ baking of the Al cells have already shown an increase in the mobility of GaAs-based 2-dimensional electron gases. For the structure design, optimizing the contrast in the refractive index of the DBRs will narrow the cavity linewidth and reduce the overall crystal strain relative to the GaAs substrate. Reducing the spacing between the cavity mirrors will increase the cavity coupling strength and reduce the total structure thickness.

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[2] P. Knüppel, S. Ravets, M. Kroner et al. Nonlinear optics in the fractional quantum Hall regime. Nature 572, 91–94 (2019).
[3] M. Lupatini, P. Knüppel, S. Faelt, R. Winkler, M. Shayegan, A. Imamoglu, and W. Wegscheider, Spin Reversal of a Quantum Hall Ferromagnet at a Landau Level Crossing. Phys. Rev. Lett. 125, 067404 (2020).