  The interaction between intersubband resonances (ISRs) and metamaterial
microcavities can form a strongly coupled system where new resonances form that
depend on the coupling strength. Here we present experimental evidence of
strong coupling between the cavity resonance of a THz metamaterial and the ISR
in a high electron mobility transistor structure with a triangular confinement.
The device is electrically switched from an uncoupled to a strongly coupled
regime by tuning the ISR with epitaxially grown transparent gates. An
asymmetric triangular potential in the heterostructure enables ultrawide
electrical tuning of ISR which is an order of magnitude higher as compared to
the equivalent square well. For a single triangular well, we achieve a coupling
strength of 0.52 THz, with a normalized coupling ratio of 0.26.
