arXiv subjects

Andrea Bergschneider

Publications and source records attributed to Andrea Bergschneider.

11 recordsLinked to original sources

arXiv2026

Compact, open, and tunable two-dimensional Fabry-Pérot cavity

In order to enhance light-matter interactions, optically active material, like a semiconductor, can be embedded into an optical cavity. For small cavity mode volumes, which are required for reaching the strong coupling regime, this is typically done by fabrication of microcavities. While guaranteeing a mechanically stable cavity, these systems lack tunability and are difficult to implement for small samples like van der Waals heterostructures (vdW). Here we present a design for a fully tunable two-dimensional (2D) Fabry-Pérot (FP) cavity which can host any material. The cavity can be freely positioned between two confocal high-NA aspheric lenses which allows for optical transmission microscopy and spectroscopy in real and momentum space. In order to maintain maximal mechanical stability, the cavity mirrors are held in a monolithic titanium frame. A pre-loaded flexure joint allows for manual coarse tuning of the cavity length and mirror parallelism, while three piezo actuators in a tripod layout can be used for fine tuning. We find cavity length fluctuations smaller than 100 pm, measure a finesse of 360 and reach the lowest accessible $m=3$ cavity mode (one electric field antinode within the air gap) at a minimal physical mirror separation of 380 nm. In order to demonstrate the 2D nature of the cavity mode we perform momentum space imaging in transmission as well as spectroscopy to measure the cavity dispersion.

physics.optics↗

arXiv2026

Aromatic molecular emitters in a hexagonal boron nitride stack

Single polycyclic aromatic hydrocarbon molecules embedded in organic matrices have proven to be an excellent family of narrow-linewidth quantum emitters. Extending this host-guest setting to van der Waals materials offers the opportunity to combine the preeminent properties of molecular emitters with the access to the versatility of two-dimensional hetero-structures and devices. In this work, we incorporate perylene molecules into multi-layered hexagonal boron nitride stacks and observe gigahertz-narrow zero-phonon-line transitions at cryogenic temperatures. We unambiguously verify the origins of photon emission through vibronic spectra analysis. By combining hyperspectral localization measurements with quantum chemistry calculations, we examine the insertion mechanisms of perylene molecules in the hexagonal boron nitride stacks, and conclude that pristine hBN layers tend to expel molecules from the sandwich, while extended morphological defects, hydroxyl groups and unpassivated boron and nitrogen atoms assist to stabilize molecular bindings to hBN. Our work provides valuable insight for future work to deterministically integrate narrow-linewidth molecular emitters into van der Waals devices.

physics.optics↗

arXiv2025

Numerical investigation of electrostatically confined excitons in monolayer $\text{MoSe}_2$

We investigate exciton confinement to a quantum wire in monolayer $\text{MoSe}_2$ where the confinement is achieved by a p-i-n junction. We employ an effective-mass exciton model and solve the problem numerically, reflecting device geometries found in experimental state-of-the-art set up. Our method allows us to investigate the entire spectrum of confined states. We show the emergence of quantum confinement and study the dependence of the confined states as a function of electrical gate voltages, which are experimentally tunable parameters. We find that the confined states can be divided into bright and dark states with the dark states having small but finite oscillator strengths. Their oscillator strengths are low enough that they have not yet been detected in experiments, whereas the spectrum of the bright exciton states reproduces recent experimental measurements. Our results provide insight into the theoretical background of confined exciton states beyond the ground state and pave the way for the development of new confinement schemes as well as avenues to access the previously not detected dark states.

cond-mat.mes-hall↗

arXiv2025

Coherent Interaction of 2s and 1s Exciton States in Transition-Metal Dichalcogenide Monolayers

We use femtosecond pump-probe spectroscopy to study the coherent interaction of excited exciton states in WSe2 and MoSe2 monolayers via the optical Stark effect. For co-circularly polarized pump and probe, we measure a blueshift which points to a repulsive interaction between the 2s and 1s exciton states. The determined 2s-1s interaction strength is on par with that of the 1s-1s, in agreement with the semiconductor Bloch equations. Furthermore, we demonstrate the existence of a 2s-1s biexciton bound state in the cross-circular configuration in both materials and determine their binding energy.

cond-mat.mes-hall↗

arXiv2024

Floquet-driven crossover from density-assisted tunneling to enhanced pair tunneling

We investigate the experimental control of pair tunneling in a double-well potential using Floquet engineering. We demonstrate a crossover from a regime with density-assisted tunneling to dominant pair tunneling by tuning the effective interactions. Furthermore, we show that the pair tunneling rate can be enhanced not only compared to the Floquet-reduced single-particle tunneling but even beyond the static superexchange rate, while keeping the effective interaction in a relevant range. This opens possibilities to realize models with explicit pair tunneling in ultracold atomic systems.

cond-mat.quant-gas↗

arXiv2023

Coupled high-finesse optical Fabry-Perot microcavities

Optical fiber Fabry-Perot cavities have been a development facilitating the efficient integration of high-finesse cavities into fiber-optic assemblies. In this work, we demonstrate coupling of two high-finesse fiber cavities by direct photon tunneling between them. We detect the coupled mode spectra and demonstrated the variability of coupling strength and dissipation rates for different transverse modes. Moreover, we observe very narrow spectral features resulting from a dynamical generalizaton of electromagnetically-induced transparency showing that even dissipative systems without metastable states can feature long-lived coherences.

physics.optics↗

arXiv2019

Experimental Characterization of Two-Particle Entanglement through Position and Momentum Correlations

Quantum simulation is a rapidly advancing tool to gain insight into complex quantum states and their dynamics. Trapped ion systems have pioneered deterministic state preparation and comprehensive state characterization, operating on localized and thus distinguishable particles. With ultracold atom experiments, one can prepare large samples of delocalized particles, but the same level of characterization has not yet been achieved. Here, we present a method to measure the positions and momenta of individual particles to obtain correlations and coherences. We demonstrate this with deterministically prepared samples of two interacting ultracold fermions in a coupled double well. As a first application, we use our technique to certify and quantify different types of entanglement.

cond-mat.quant-gas↗

arXiv2019

Interacting Polaron-Polaritons

Two dimensional semiconductors provide an ideal platform for exploration of linear exciton and polariton physics, primarily due to large exciton binding energy and strong light-matter coupling. These features, however, generically imply reduced exciton-exciton interactions, hindering the realisation of active optical devices such as lasers or parametric oscillators. Here, we show that electrical injection of itinerant electrons into monolayer molybdenum diselenide allows us to overcome this limitation: dynamical screening of exciton-polaritons by electrons leads to the formation of new quasi-particles termed polaron-polaritons that exhibit unexpectedly strong interactions as well as optical amplification by Bose-enhanced polaron-electron scattering. To measure the nonlinear optical response, we carry out time-resolved pump-probe measurements and observe polaron-polariton interaction enhancement by a factor of 50 ($0.5 μ$eV $μ$m$^2$) as compared to exciton-polaritons. Concurrently, we measure a spectrally integrated transmission gain of the probe field of $\gtrsim 2$ stemming from stimulated scattering of polaron-polaritons. We show theoretically that the non-equilibrium nature of optically excited quasiparticles favours a previously unexplored interaction mechanism stemming from a phase-space filling in the screening cloud, which provides an accurate explanation of the strong repulsive interactions observed experimentally. Our findings show that itinerant electron-exciton interactions provide an invaluable tool for electronic manipulation of optical properties, demonstrate a new mechanism for dramatically enhancing polariton-polariton interactions, and pave the way for realisation of nonequilibrium polariton condensates.

cond-mat.mes-hall↗

arXiv2019

High-Contrast Interference of Ultracold Fermions

Many-body interference between indistinguishable particles can give rise to strong correlations rooted in quantum statistics. We study such Hanbury Brown-Twiss-type correlations for number states of ultracold massive fermions. Using deterministically prepared $^6$Li atoms in optical tweezers, we measure momentum correlations using a single-atom sensitive time-of-flight imaging scheme. The experiment combines on-demand state preparation of highly indistinguishable particles with high-fidelity detection, giving access to two- and three-body correlations in fields of fixed fermionic particle number. We find that pairs of atoms interfere with a contrast close to 80%. We show that second-order density correlations arise from contributions from all two-particle pairs and detect intrinsic third-order correlations.

cond-mat.quant-gas↗

arXiv2018

Spin-resolved single-atom imaging of $^6$Li in free space

We present a versatile imaging scheme for fermionic $^6$Li atoms with single-particle sensitivity. Our method works for freely propagating particles and completely eliminates the need for confining potentials during the imaging process. We illuminate individual atoms in free space with resonant light and collect their fluorescence on an electron-multiplying CCD camera using a high-numerical-aperture imaging system. We detect approximately \num{20} photons per atom during an exposure of 20 $μ$s and identify individual atoms with a fidelity of $99.4\pm0.3$ % . By addressing different optical transitions during two exposures in rapid succession, we additionally resolve the hyperfine spin state of each particle. The position uncertainty of the imaging scheme is 4.0 $μ$m, given by the diffusive motion of the particles during the imaging pulse. The absence of confining potentials enables readout procedures, such as the measurement of single-particle momenta in time of flight, which we demonstrate here. Our imaging scheme is technically simple and easily adapted to other atomic species.

cond-mat.quant-gas↗

arXiv2015

Two Fermions in a double well: Exploring a fundamental building block of the Hubbard model

We have prepared two ultracold fermionic atoms in an isolated double-well potential and obtained full control over the quantum state of this system. In particular, we can independently control the interaction strength between the particles, their tunneling rate between the wells and the tilt of the potential. By introducing repulsive (attractive) interparticle interactions we have realized the two-particle analog of a Mott-insulating (charge-density-wave) state. We have also spectroscopically observed how second-order tunneling affects the energy of the system. This work realizes the first step of a bottom-up approach to deterministically create a single-site addressable realization of a ground-state Fermi-Hubbard system.

cond-mat.quant-gas↗