arXiv subjects

Elsa Jöchl

Publications and source records attributed to Elsa Jöchl.

3 recordsLinked to original sources

arXiv2026

Absence of a Superradiant Phase Transition in Dirac Landau Polaritons

One of the most striking predictions in cavity quantum electrodynamics is the condensation of photons into a macroscopically populated ground state, the so-called superradiant phase transition (SRPT). SRPTs are theorized to occur in light-matter coupled systems above a critical coupling strength, yet have not been experimentally realized in equilibrium. On the contrary, the very existence of SRPTs has been largely disputed by No-Go theorems. In cavity-coupled electronic systems with Dirac dispersion, the diamagnetic $\vec{A}^2$-term crucial to No-go theorems is not present at leading order, making graphene Landau level transitions ultrastrongly coupled to terahertz cavities good candidates for SRPTs. In this work, we present the first terahertz spectroscopic measurements of an hBN-encapsulated monolayer graphene flake coupled to a highly sub-wavelength resonator mode. By tuning the graphene carrier density, we drive the resulting Landau polaritons into the ultrastrong coupling regime, with the normalized coupling reaching $\approx 40 \%$, approaching criticality. In this regime, the continuous SRPT would lead to a unique spectroscopic polariton softening, which we consistently rule out. The full polariton dispersion is instead quantitatively reproduced by a Hopfield Hamiltonian using a quasistatic near-field model that accounts for the sub-wavelength character of the cavity.

cond-mat.mes-hall↗

arXiv2025

Gate-tunable single terahertz meta-atom ultrastrong light-matter coupling

We study the electrical tunability of ultrastrong light-matter interactions between a single terahertz circuit-based complementary split ring resonator (cSRR) and a two-dimensional electron gas. For this purpose, transmission spectroscopy measurements are performed under the influence of a strong magnetic field at different set points for the electric gate bias. The resulting Landau polariton dispersion depends on the applied electric bias, as the gating technique confines the electrons in-plane down to extremely sub-wavelength dimensions as small as d = 410 nm. This confinement allows for the excitation of standing plasma waves at zero magnetic field and an effective tunability of the electron number coupled to the THz resonator. This allows the normalized coupling strength to be tuned in-situ from $η$ = 0.46 down to $η$ = 0.18. This is the first demonstration of terahertz far-field spectroscopy of an electrically tunable interaction between a single terahertz resonator and electrons in a GaAs quantum well heterostructure.

cond-mat.mes-hall↗

arXiv2021

An ultrastrongly coupled single THz meta-atom

Free-space coupling to strongly subwavelength individual optical elements is a central theme in quantum optics, as it allows to control and manipulate the properties of quantum systems. In this work, we show that by combining an asymmetric immersion lens setup and complementary design of metasurfaces we are able to perform THz time-domain spectroscopy of an individual, strongly subwavelength (d/λ0=1/20) meta-atom. We unravel the linewidth dependence of planar metamaterials as a function of the meta-atom number indicating quenching of the Dicke superradiance. On these grounds, we investigate ultrastrongly coupled Landau polaritons at the single resonator level, measuring a normalized coupling ratio of Ω/ω=0.60 resulting from coupling of the fundamental mode to a few thousand electrons. Similar measurements on a low loss, less doped two dimensional electron gas yield a coupling ratio Ω/ω=0.33 with a cooperativity C=4g^2/κγ= 94. Interestingly, the coupling strength of a coupled single resonator is the same as of a coupled array. Our findings pave the way towards the control of light-matter interaction in the ultrastrong coupling regime at the single electron/single resonator level. The proposed technique is way more general and can be useful to characterize the complex conductivity of micron-sized samples in the THz and sub-THz domain.

physics.optics↗