We study the ultra-strong coupling (USC) of Landau level transitions in strained Germanium quantum wells (sGe QW) to THz metasurfaces. The spin-splitting of the heavy-hole cyclotron resonance in sGe QWs due to the Rashba spin-orbit interaction in magnetic field offers an excellent platform to investigate ultra-strong coupling to a non-parabolic system. THz split ring resonators can be tuned to coincide with the single cyclotron transition (around 0.4 THz and a magnetic field of 1.5 T) or the spin-resolved transitions of the sGe QWs (at 1.3 THz and 4.5 T). Coupling to the single cyclotron yields a normalized USC rate of 25%, resulting from fitting with a Hopfield-like Hamiltonian model. Coupling to two or three cyclotron resonances in sGe QWs lead to the observation of multiple polaritons branches, one polariton branch for each oscillator involved in the system. An adaption of the theory allows to also describe this multiple-oscillator system and to determine the coupling strengths. The different Rabi-splittings for the multiple cyclotrons coupling to the same resonator mode relate to the underlying differences in the material. Furthermore, the visibility of an additional transition, possibly a light hole transition with very low carrier density, is strongly enhanced due to the coupling to the LC-resonance with a normalized strong coupling ratio of 4.7%. Future perspectives include controlling spin-flip transitions in USC and studying the impact of non-parabolicity on the ultra-strong coupling physics.
Janine Keller, Giacomo Scalari, Curdin Maissen, Gian Lorenzo Paravicini-Bagliani, Johannes Haase, Michele Failla, Maksym Myronov, David R. Leadley, James Lloyd-Hughes, and Jérôme Faist, "Ultra-strong coupling with spin-split heavyhole cyclotron resonances in sGe QWs (Conference Presentation)," Proc. SPIE 10102, Ultrafast Phenomena and Nanophotonics XXI, 101020X (Presented at SPIE OPTO: February 01, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2249856.5391659174001.
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