Quantum Hall Meets Metamaterial: Controlling and tuning light-matter interaction is crucial for fundamental studies of cavity quantum electrodynamics and for applications in classical and quantum devices. Scalari et al. (p. 1323 ) describe a system comprising an array of metamaterial split-ring resonators and a series of two-dimensional electronic gases (2DEG) formed in GaAs quantum wells. In a magnetic field, the electrons in the 2DEG performed cyclotron orbits and formed Landau levels. Strong coupling was observed between photon and magnetic cyclotron modes, producing a tunable semiconductor system for studying the light-matter interaction of two-level systems.

We review the most recent technological and application advances of quantum cascade lasers, underlining the present milestones and future directions from the Mid-infrared to the Terahertz spectral range. Challenges and developments, which are the subject of the contributions to this focus issue, are also introduced.

The prospect of controlling the electronic properties of materials via the vacuum fields of cavity electromagnetic resonators is emerging as one of the frontiers of condensed matter physics. We found that the enhancement of vacuum field fluctuations in subwavelength split-ring resonators strongly affects one of the most paradigmatic quantum protectorates, the quantum Hall electron transport in high-mobility two-dimensional electron gases. The observed breakdown of the topological protection of the integer quantum Hall effect is interpreted in terms of a long-range cavity-mediated electron hopping where the anti-resonant terms of the light-matter coupling Hamiltonian develop into a finite resistivity induced by the vacuum fluctuations. Our experimental platform can be used for any two-dimensional material and provides a route to manipulate electron phases in matter by means of vacuum-field engineering.

Two terahertz quantum cascade lasers based on $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{As}$ heterostructures are reported. Pulsed mode operation up to $84\mathrm{K}$ and continuous wave (cw) power of $0.36\mathrm{mW}$ at $10\mathrm{K}$ are demonstrated for the laser which emits from $1.34\text{to}1.58\mathrm{THz}$ . The other laser shows emission from $1.2\text{to}1.32\mathrm{THz}$ with pulsed mode operation up to $69\mathrm{K}$ and cw power of $0.12\mathrm{mW}$ at $10\mathrm{K}$ .

The ultrastrong light-matter interaction regime was investigated in metallic and superconducting complementary split-ring resonators coupled to the cyclotron transition of two-dimensional electron gases. The subwavelength light confinement and the large optical dipole moment of the cyclotron transition yield record high normalized coupling rates of up to $\frac{{\Omega }_R}{{\omega }_c}=0.87$. We observed a blue-shift of both polaritons due to the diamagnetic term of the interaction Hamiltonian.