Ultrastrong Coupling of the Cyclotron Transition of a 2D Electron Gas to a THz Metamaterial

Abstract

Artificial cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems, due to the possibility of manipulating the cavity quantum electrodynamic ground state with controllable physical properties. We report here experiments showing ultrastrong light-matter coupling in a terahertz metamaterial where the cyclotron transition of a high mobility two-dimensional electron gas is coupled to the photonic modes of an array of electronic split-ring resonators. We observe a normalized coupling ratio Ω ωc = 0.58 between the vacuum Rabi frequency Ω and the cyclotron frequency ω c . Our system appears to be scalable in frequency and could be brought to the microwave spectral range with the potential of strongly controlling the magnetotransport properties of a highmobility 2DEG.

Collective non-perturbative coupling of 2D electrons with high-quality-factor terahertz cavity photons

“…1, g, h, and i, the full-width-at-halfmaximum (FWHM) values, or κ, of these cavity modes were 2.7 GHz, 5.0 GHz, and 3.8 GHz, corresponding to Q factors of 150, 243, and 532, respectively; note that these numbers are slightly lower than those for an empty cavity without including the 2DEG, which were 183, 450, and 810. These Q-factors are one to two orders of magnitude higher than those reported for the THz metamaterial resonators employed in previous untrastrong-coupling studies using 2DEG CR [12,13]. In the following, experimental data recorded with Cavity 1 are shown.…”

“…Intraband transitions, such as intersubband transitions (ISBTs) [1] and cyclotron resonance (CR) [22], are much better candidates for accomplishing ultrastrong coupling because of their small ω 0 , typically in the midinfared and terahertz (THz) range, and their enormous dipole moments (10s of e-Å). Experimentally, ultrastrong coupling has indeed been achieved in GaAs QWs using ISBTs [10,11] and CR [12,13]. In the latter case, a record high value of g/ω 0 = 0.87 has been reported [13].…”

“…In addition, strong coherent light-matter interaction in a solid-state environment is of great interest to emerging quantum-based technologies [8,9]. However, creating a system that combines a long electronic coherence time, a large dipole moment, and a high cavity quality (Q) factor has been a challenging goal [10][11][12][13]. Here, we report collective ultrastrong light-matter coupling in an ultrahigh-mobility two-dimensional electron gas in a high-Q terahertz photonic-crystal cavity in a quantizing magnetic field, demonstrating a cooperativity of ∼360.…”

Topologically nontrivial Floquet band structure in a system undergoing photonic transitions in the ultrastrong-coupling regime

“…This phase transition has no counter part in weak-coupling systems, and its nature is directly related to the non-trivial topology of the quasi-energy space. In the context of recent significant fundamental interests in exploring the ultra-strong coupling physics [24][25][26][27][28][29][30][31][40][41][42][43], our work points to the exciting prospect of exploring non-trivial topological effects in ultra-strong coupling regime.…”

“…[14] for achieving a photonic gauge potential. Unlike the electronic transition, where reaching the ultra-strong coupling regime is a significant challenge [40][41][42][43], for photonic transition [33] it is in fact rather natural that the system operates in the ultrastrong coupling regime. Thus, systems exhibiting photonic transition can be readily used to explore the physics of ultra-strong coupling.…”