Sculpting ultrastrong light-matter coupling through spatial matter structuring

The central theme of cavity quantum electrodynamics is the coupling of a single optical mode with a single matter excitation, leading to a doublet of cavity polaritons which govern the optical properties of the coupled structure. Especially in the ultrastrong coupling regime, where the ratio of the vacuum Rabi frequency and the quasi-resonant carrier frequency of light, $\Omega_{\mathrm R}/\omega_{\mathrm c}$, approaches unity, the polariton doublet bridges a large spectral bandwidth $2\Omega_{\mathrm R}$, and further interactions with off-resonant light and matter modes may occur. The resulting multi-mode coupling has recently attracted attention owing to the additional degrees of freedom for designing light-matter coupled resonances, despite added complexity. Here, we experimentally implement a novel strategy to sculpt ultrastrong multi-mode coupling by tailoring the spatial overlap of multiple modes of planar metallic THz resonators and the cyclotron resonances of Landau-quantized two-dimensional electrons, on subwavelength scales. We show that similarly to the selection rules of classical optics, this allows us to suppress or enhance certain coupling pathways and to control the number of light-matter coupled modes, their octave-spanning frequency spectra, and their response to magnetic tuning. This offers novel pathways for controlling dissipation, tailoring quantum light sources, nonlinearities, correlations as well as entanglement in quantum information processing

Subcycle dynamics of deep-strong light-matter coupling

In an optical microcavity, fundamentally new quantum states of matter can be created by dressing electronic excitations with virtual photons of the cavity modes. In deep- and ultrastrongly coupled systems, the rate of exchange between cavity modes and electronic excitations – the vacuum Rabi frequency – approaches or even exceeds the carrier frequency of light 0, giving rise to interesting phenomena. In this thesis, two different material systems for extremely strong light-matter coupling are presented: GaAs and InAs based semiconductor heterostructures. The combination of such heterostructures and subwavelength THz resonators enabled the observation of extremely high light-matter coupling strengths. The presented structures couple cyclotron resonances of two-dimensional electron gases in semiconductor heterostructures to custom-tailored THz nanoresonators, leading to the formation of cavity polaritons. Furthermore, this thesis describes a novel architecture for deep-strongly coupled structures in which the coupling strength, can be modulated more than an order of magnitude faster than the oscillation cycle of light. Here, cavity polaritons characterised by normalized coupling strengths of up to 1.3 were non-adiabatically modulated. Guided by parameter-free electrodynamical simulations, the resonators are equipped with InGaAs structures located in the area of maximum field enhancement. Femtosecond near-infrared photoexcitation of these switch elements rapidly reshapes the fundamental optical mode, decoupling it from the cyclotron resonance and completely collapsing ΩR, as verified by steady-state THz transmission experiments. Moreover, the intriguing subcycle dynamics that arises when light-matter coupling is switched off by excitation with near-infrared pulses of a duration of 70 fs are explored. In this setting, the response function exhibits sub-polariton-cycle oscillations of the transmission with frequency components exceeding the polariton frequency by more than an order of magnitude. A dynamical quantum model quantitatively links these oscillations to a strongly non-adiabatic collapse of the coupling strength 20 times faster than the cycle duration of the lower polariton

Sculpting ultrastrong light-matter coupling through spatial matter structuring

The central theme of cavity quantum electrodynamics is the coupling of a single optical mode with a single matter excitation, leading to a doublet of cavity polaritons which govern the optical properties of the coupled structure. Especially in the ultrastrong coupling regime, where the ratio of the vacuum Rabi frequency and the quasi-resonant carrier frequency of light, 𝛀𝐑/𝝎𝐜, approaches unity, the polariton doublet bridges a large spectral bandwidth 𝟐𝛀𝐑, and further interactions with off-resonant light and matter modes may occur. The resulting multi-mode coupling has recently attracted attention owing to the additional degrees of freedom for designing light-matter coupled resonances, despite added complexity. Here, we experimentally implement a novel strategy to sculpt ultrastrong multi-mode coupling by tailoring the spatial overlap of multiple modes of planar metallic THz resonators and the cyclotron resonances of Landau-quantized two-dimensional electrons, on subwavelength scales. We show that similarly to the selection rules of classical optics, this allows us to suppress or enhance certain coupling pathways and to control the number of light-matter coupled modes, their octave-spanning frequency spectra, and their response to magnetic tuning. This offers novel pathways for controlling dissipation, tailoring quantum light sources, nonlinearities, correlations as well as entanglement in quantum information processing