Recent research revealed that in resonators with deep subwavelength gaps coupled to two-dimensional electron gases, propagating plasmons lead to energy leakage, hindering polaritonic resonance. This study introduces plasmonic reflectors to create an artificial energy stopband, confining terahertz-range plasmons and recovering polaritonic resonances. Using this approach demonstrates a normalized coupling ratio of 0.36, enabling the observation of polaritonic resonances not seen without plasmonic reflectors.

We will present experiments on ultrastrongly coupled, strongly subwavelength resonators in arrays down to single element. We will discuss the limitations of the planar resonators approach when employing extremely subwavelength gaps and we will present the new developments towards single-object, few electrons ultrastrongly coupled systems.

It was recently demonstrated that, in deep subwavelength gap resonators coupled to two-dimensional electron gases, propagating plasmons can lead to energy leakage and prevent the formation of polaritonic resonances. This process, akin to Landau damping, limits the achievable field confinement and thus the value of light-matter coupling strength. In this work, we show how plasmonic reflectors can be used to create an artificial energy stopband in the plasmon dispersion, confining them and enabling the recovery of the polaritonic resonances. Using this approach we demonstrate a normalized light-matter coupling ratio of ^Ω_ω^R₀ = 0.36 employing a single doped quantum well with a resonator’s gap size of 250 nm equivalent to λ/3000 in vacuum, a geometry in which the polaritonic resonances would not be observable in the absence of the plasmonic reflectors.

In this work, by combining an asymmetric Silicon immersion lens configuration and a complementary resonator design, far-field transmission measurement of a single subwavelength split-ring resonator, ultra-strongly coupled to inter Landau level transitions in a single quantum well, is resolved. The highest coupling is 60%, achieved for only 2000 coupled electrons for a single resonator on an InSb quantum well. This technique can be useful to characterize the complex conductivity of micron-sized samples, such as exfoliated graphene or transition metal dichalcogenide flakes, in the THz domain.

In this work, we theoretically and experimentally show that the confinement of an electromagnetic field below critical length-scales can excite high momentum matter resonances and can ultimately limit the light-matter coupling enhancement in an ultrastrong coupling regime.

Subwavelength electromagnetic field localization has been central to photonic research in the last decade, allowing us to enhance sensing capabilities as well as increase the coupling between photons and material excitations. The strong and ultrastrong light–matter coupling regime in the terahertz range using split-ring resonators coupled to magnetoplasmons has been widely investigated, achieving successive world records for the largest light–matter coupling ever achieved. Ever shrinking resonators have allowed us to approach the regime of few-electron strong coupling, in which single-dipole properties can be modified by the vacuum field. Here, we demonstrate, theoretically and experimentally, the existence of a limit to the possibility of arbitrarily increasing electromagnetic confinement in polaritonic systems. Strongly subwavelength fields can excite a continuum of high-momenta propagative magnetoplasmons. This leads to peculiar nonlocal polaritonic effects, as certain polaritonic features disappear and the system enters the regime of discrete-to-continuum strong coupling.

We report a physical limit for reducing the modal volume of a cavity and ultimately increasing the light-matter coupling strength. Extremely confined photonic mode below a critical length-scale can introduce a large in-plane wave vector and excite a continuum of high momentum matter resonances. This excitations act as loss channels and reduce the field confinement by smearing the distribution of surface charges, thus consequently limiting the achievable field enhancement.

In this work, we theoretically and experimentally show that the confinement of an electromagnetic field below critical length-scales can excite high momentum matter resonances and can ultimately limit the light-matter coupling enhancement in an ultrastrong coupling regime.

We present our investigation on ultrastrong coupling of the Landau level transitions in a two dimensional electron gas to complementary split ring resonator arrays, as a terahertz metasurface. To achieve a higher coupling rate, the capacitive gap of the resonator is reduced systematically. This increment in the coupling rate is expected due to the enhanced vacuum field fluctuations in a smaller volume mode. However, our results indicate that the gap size reduction introduces a broadening in the upper polariton branch below a certain gap width.

Ultrastrong coupling (USC) regime has been an intriguing research topic in cavity quantum electrodynamics (QED) experiments due to its capability to modify the ground and the excited states of a coupled system[1-3]. We investigate the USC of Landau level (LL) transitions in a two dimensional electron gas to teraherz (THz) metasurfaces using THz time domain spectroscopy. Complementary THz split ring resonator (cSRR) arrays are used as metasurfaces[4]. A higher coupling rate can be achieved by reducing the capacitive gap of the resonator due to the increased vacuum field fluctuations. However, our results indicate that this reduction introduces an inhomogenous broadening of the upper polariton branch below a threshold gap width.

We fabricate high temperature superconducting metasurfaces from yttrium-barium-copper oxide (YBCO) films via focused ion beam milling. The design of the complementary split ring resonators yields a high quality factor (up to Q = 31) resonance, which is fully switchable when exploiting the transition from the superconducting to normal conducting state, which is achieved by heating the sample above the critical temperature. Meanwhile, the resonance is continuously visible and frequency stable under the action of a magnetic field, up to B = 9 T. This makes it an ideal candidate for light-matter coupling experiments with magnetic field tunable Landau level transitions. We present two techniques to bring the metasurface in close vicinity of a 2D electron gas in an AlGaAs/GaAs QW without direct deposition, and we achieve a normalized vacuum Rabi frequency of ω_R/λ_cyc = 24%.

Complementary double split ring THz resonators fabricated on a 10 µm thin Si-membrane are studied. The linewidths of the fundamental LC mode and dipolar mode are drastically narrowing with increased resonator spacing. The extracted decay rate of the LC mode as a function of the resonator density shows a linear dependence, evidencing a collective superradiant effect of the resonator array. Furthermore, it is shown that a metamaterial can be designed for a low superradiant broadening of the resonance at high resonator densities, that is, in the metamaterial condition. The use of a thin membrane as a substrate is crucial, since it shifts the THz surface plasmon polaritons modes to much higher frequencies, preventing them to couple to the LC mode and unveiling the superradiant broadening mechanism for a large range of lattice spacings. At higher frequencies, not interfering with the high Q LC mode, other additional modes, which are ascribed to photonic crystal modes, form in the Si-membrane. The angle-dependent band structure is mapped and corresponding simulated electric field distributions are shown.

We study complementary THz metasurfaces with changing lattice constant. On thick substrates the LC-resonance anti-crosses with a surface plasmon polaritons mode. On 10 μm Simembranes, we reveal a superradiantly limited linewidth of the LC-resonance.