We study the ultrastrong driving of Rydberg excitons in Cu
2O by a microwave field. The effect of the field is studied using optical absorption spectroscopy, and through the observation of sidebands on the transmitted laser light. A model based on Floquet theory is constructed to study the system beyond the rotating wave approximation. We obtain near quantitative agreement between theory and experiment across a 16-fold range of microwave field strengths spanning from the perturbative to the ultrastrong driving regime. Compared to Rydberg atoms, the large non-radiative widths of Rydberg excitons leads to new behaviour such as the emergence of an absorption continuum without ionization.
The coherent interaction of Rydberg atoms with microwave fields in the ultrastrong-driving regime, in which the Rabi frequency is of the same order of magnitude as the transition frequency, has been studied for states with principal quantum number
n=105 in helium. Experiments were performed in pulsed supersonic beams, and the effects of the ultrastrong 1.280 GHz microwave driving field, tuned to near resonance with the $1s105s\phantom{\rule{0.16em}{0ex}}^{3}S_{1}\ensuremath{\rightarrow}1s105p\phantom{\rule{0.16em}{0ex}}^{3}P$ transition, were identified from Autler-Townes splittings of the $1s3p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}\ensuremath{\rightarrow}1s105s\phantom{\rule{0.16em}{0ex}}^{3}S_{1}$ transition by cw laser spectroscopy. The microwave field strength was calibrated in situ in the apparatus from Autler-Townes splittings measured in the weak-driving regime in which the rotating-wave approximation holds. The results of the experiments were compared to the energy-level structure of the atoms in the presence of the microwave field calculated using Floquet methods. From this comparison, the microwave-field strengths for which the rotating-wave approximation and the two-level approximation break down have been identified. The feasibility of implementing microwave traps for Rydberg atoms and molecules, which operate in the ultrastrong-driving regime, has been evaluated.
Microwave–optical conversion is key to future networks of quantum devices, such as those based on superconducting technology. Conversion at the single quantum level requires strong nonlinearity, high bandwidth, and compatibility with a millikelvin environment. A large nonlinearity is observed in Rydberg atoms, but combining atomic gases with dilution refrigerators is technically challenging. Here, we demonstrate a strong microwave–optical nonlinearity in a cryogenic, solid-state system by exploiting Rydberg states of excitons in Cu2O. We measure a microwave–optical cross-Kerr coefficient of B0 = 0.022 ± 0.008 m V−2 at 4 K, which is several orders of magnitude larger than other solid-state systems. The results are in quantitative agreement with a nonlinear susceptibility model based on the giant microwave dipole moment between nearby excitonic states. Our results highlight the potential of Rydberg excitons for nonlinear optics and form the basis for a microwave–optical frequency converter based on Cu2O.
A theoretical study of the dynamics of Rydberg atoms in a microwave cavity driven by a strong bichromatic field is presented. The resonator is assumed to operate in the low-Q regime. As a consequence, photons emitted by the atoms are dissipated in the cavity walls during the interaction time of the atoms inside the resonator. In this situation the cavity field follows the atomic dynamics adiabatically. The transient behavior of the system is analyzed in terms of Floquet states, and cavity-induced transition rates between these states are calculated for a large range of parameters of the bichromatic field. Narrow resonances are found in the transition rates, in agreement with recent experimental investigations of cavity Rydberg atoms subjected to strong bichromatic driving. We explain in detail the structure of the resonances, which is determined by the frequency-dependent cavity-mode density as well as the Rabi frequencies of the applied fields. The intensity-dependent shifts of the resonance frequencies are also calculated and found to be largely insensitive to inhomogeneous broadening. Finally, the numerical results are compared with experimental observations.
Over the past decade, Rydberg excitons in bulk Cu 2 O have emerged as a solid-state analog to atomic Rydberg systems, exhibiting exaggerated quantum optical properties including large dipole moments, strong dipole–dipole interactions, and enhanced nonlinear susceptibilities. High-resolution absorption spectroscopy has revealed elegant scaling laws with principal quantum number across temperatures from tens of mK to above 100 K. While many features follow a hydrogen-like model, non-hydrogenic effects arise from complex bandstructures and many-body interactions. This review summarizes experimental advances in accessing high- n Rydberg states and quantum phenomena arising from diverse exciton interaction mechanisms. It also discusses prospective quantum and nonlinear optical applications.
The primary objective of this thesis is to investigate the feasibility of cuprous oxide (cuprite) as a competitive platform for microwave-to-optical conversion utilising Rydberg excitons. While the ultimate goal is the simultaneous enhancement of both microwave and optical domains to reach the strong-coupling regime, this work focuses on characterising the two domains independently to establish the necessary foundational parameters. The approach follows a two-pronged strategy: increasing the optical field interaction by embedding cuprite within a Fabry-Pérot microcavity and, in parallel, enhancing the microwave field through integration of cuprite with superconducting microwave resonators. Within the microcavity, high-resolution spectroscopy enabled the resolution of Rydberg resonances as high as 17P. A coupled-oscillator model fitting confirmed the achievement of the strong-coupling regime for states
n≤4. In the second-harmonic generation regime, the microcavity facilitated the identification of doubly resonant locations satisfying the quasi-phase matching conditions required to simultaneously enhance the pump field and the SHG output. This architecture permitted the observation of Rydberg states up to 10D, the highest reported to date in a microcavity sample and revealed significant cavity-induced splittings of the 3S and 3D excitonic resonances. Microwave field confinement was addressed through the development of superconducting lumped-element resonators. A nested split-ring resonator design achieved peak electric field intensities exceeding
1.2×106 V/m and a sub-wavelength mode volume of
Vm≈2156 $\upmu\mathrm{m}^3$, satisfying the high-confinement requirements for efficient microwave-to-optical coupling. Leveraging the multi-mode response of this platform, we provide the first reported measurement of cuprite's relative permittivity at microwave frequencies and sub-Kelvin temperatures of
εr=6.66±0.08. Furthermore, the loss tangent of cuprite was constrained to the
10−5-
10−4 range using a lumped-element interdigitated capacitor resonator architecture. The convergence of these optical and microwave architectures provides a definitive roadmap for a hybrid quantum system where Rydberg excitons couple concurrently to both domains. By accessing high-n states within a co-localized field geometry, this work establishes the engineering strategies necessary to realise scalable, cuprite-based quantum transduction devices for next-generation communication systems.