arXiv subjects

Ulrich Kuhl

Publications and source records attributed to Ulrich Kuhl.

At least 37 records · Page 2Linked to original sources

arXiv2020

Encircling exceptional points as a non-Hermitian extension of rapid adiabatic passage

The efficient transfer of excitations between different levels of a quantum system is a task with many applications. Among the various protocols to carry out such a state transfer in driven systems, rapid adiabatic passage (RAP) is one of the most widely used. Here we show both theoretically and experimentally that adding a suitable amount of loss to the driven Hamiltonian turns a RAP protocol into a scheme for encircling an exceptional point including the chiral state transfer associated with it. Our work thus discloses an intimate connection between a whole body of literature on RAP and recent studies on the dynamics in the vicinity of an exceptional point, which we expect to serve as a bridge between the disjoint communities working on these two scenarios.

physics.optics↗

arXiv2020

Self-Shielded Topological Receiver Protectors

Receiver protectors (RPs) shield sensitive electronics from high-power incoming signals that might damage them. Typical RP schemes range from simple fusing and PIN diodes, to superconducting circuits and plasma cells - each having a variety of drawbacks ranging from unacceptable system downtime and self-destruction to significant insertion losses and power consumption. Here, we theoretically propose and experimentally demonstrate a unique self-shielding RP based on a coupled-resonator-microwave-waveguide (CRMW) with a topological defect being inductively coupled to a diode. This RP utilizes a charge-conjugation (C) symmetric resonant defect mode that is robust against disorder and demonstrates high transmittance at low incident powers. When incident power exceeds a critical value, a self-induced resonant trapping effect occurs leading to a dramatic suppression of transmittance and a simultaneous increase of the reflectance close to unity. The proposed RP device is self-protected from overheating and electrical breakdown and can be utilized in radars, reflection altimeters, and a broad range of communication systems.

physics.app-ph↗

arXiv2020

Non-Hermitian CT-Symmetric Spectral Protection of Nonlinear Defect Modes

We investigate, using a microwave platform consisting of a non-Hermitian Su-Schrieffer-Heeger array of coupled dielectric resonators, the interplay of a lossy nonlinearity and CT-symmetry in the formation of defect modes. The measurements agree with the theory which predicts that, up to moderate pumping, the defect mode is an eigenstate of the CT-symmetric operator and retains its frequency at the center of the gap. At higher pumping values, the system undergoes a self-induced explicit \CT-symmetry violation which removes the spectral topological protection and alters the shape of the defect mode.

physics.app-ph↗

arXiv2020

Observation of supersymmetric pseudo-Landau levels in strained microwave graphene

Using an array of coupled microwave resonators arranged in a deformed honeycomb lattice, we experimentally observe the formation of pseudo-Landau levels in the whole crossover from vanishing to large pseudomagnetic field strength. This is achieved by utilizing an adaptable set-up in a geometry that is compatible with the pseudo-Landau levels at all field strengths. The adopted approach enables to observe fully formed flat-band pseudo-Landau levels spectrally as sharp peaks in the photonic density of states, and image the associated wavefunctions spatially, where we provide clear evidence for a characteristic nodal structure reflecting the previously elusive supersymmetry in the underlying low-energy theory. In particular, we resolve the full sublattice polarization of the anomalous 0th pseudo-Landau level, which reveals a deep connection to zigzag edge states in the unstrained case.

cond-mat.mes-hall↗

arXiv2020

Current vortices in aromatic carbon molecules

The local current flow through three small aromatic carbon molecules, namely benzene, naphthalene and anthracene, is studied. Applying density functional theory and the non-equilibrium Green's function method for transport, we demonstrate that pronounced current vortices exist at certain electron energies for these molecules. The intensity of these circular currents, which appear not only at the anti-resonances of the transmission but also in vicinity of its maxima, can exceed the total current flowing through the molecular junction and generate considerable magnetic fields. The $π$ electron system of the molecular junctions is emulated experimentally by a network of macroscopic microwave resonators. The local current flows in these experiments confirm the existence of current vortices as a robust property of ring structures. The circular currents can be understood in terms of a simple nearest-neighbor tight-binding Hückel model. Current vortices are caused by the interplay of the complex eigenstates of the open system which have energies close-by the considered electron energy. Degeneracies, as observed in benzene and anthracene, can thus generate strong circular currents, but also non-degenerate systems like naphthalene exhibit current vortices. Small imperfections and perturbations can couple otherwise uncoupled states and induce circular currents.

cond-mat.mes-hall↗

arXiv2020

Experimental realization of optimal energy storage in resonators embedded in scattering media

The ability to enhance light-matter interactions by increasing the energy stored in optical resonators is inherently dependent on their coupling to the incident wavefront. In practice, weak coupling may result from resonators' irregular shapes and/or the scrambling of waves in the surrounding scattering environment. Here, we present a non-invasive wavefront shaping technique providing optimal coupling to resonators. The coherent control of the incident wavefront relies on the lengthening of delay times of waves efficiently exciting the resonator. We demonstrate our concept in microwave experiments by injecting in-situ optimal wavefronts that maximize the energy stored in multiple high-permittivity dielectric scatterers and extended leaky cavities embedded in a complex environment. We expect our framework to find important applications in the enhancement of light-matter interactions in photonic materials as well as to enhance energy harvesting.

physics.optics↗

arXiv2019

Optimized Analog Information Multiplexing Across Custom-Tailored MetaCavity Configurations

Combining tunable metasurfaces with chaotic cavities opens new avenues for finely tailored dynamic control of microwaves with programmable coding metacavities (PCMs). There is currently a strong interest in utilizing PCMs to overcome the notorious difficulty of coherent measurements across large apertures at radiofrequencies, with important applications in imaging and sensing for security screening, medical diagnosis and human-computer interaction. Such approaches rely on multiplexing spatially encoded information across a random sequence of PCM coding patterns for single-port single-frequency acquisition. Here, it is shown that a judiciously tailored rather than random coding sequence is necessary to unlock the full potential of PCMs for analog multiplexing. Specifically, the singular value spectrum of the multiplexing channel matrix is tailored to be perfectly flat - as opposed to downward sloping. In-situ experiments show that thereby the number of necessary measurements to achieve a given reconstruction quality is lowered by a factor of 2.5. Computational imaging and other microwave metrology applications are expected to benefit from the resulting reduction in acquisition time, processing burden and latency. The proposed approach and platform also set the stage for future studies in the emerging field of wave control through engineered wave chaos.

physics.app-ph↗

arXiv2019

Microwave limiters implemented by coupled dielectric resonators based on a topological defect mode and CT-symmetry breaking

We present a microwave realization of a reflective topological limiter based on an explicit self-induced violation of a charge-conjugation (CT) symmetry. The starting point is a bipartite structure created by coupled dielectric resonators with a topological defect placed at the center and two lossy resonators placed on the neighboring sites of the defect. This defect supports a resonant mode if the CT-symmetry is present, while it is suppressed once the symmetry is violated due to permittivity changes of the defect resonator associated with high irradiances of the incident radiation. This destruction leads to a suppression of transmittance and a subsequent increase of the reflectance while the absorption is also suppressed.

physics.app-ph↗

arXiv2019

Optimal Wave Fields for Micro-manipulation in Complex Scattering Environments

The manipulation of small objects with light has become an indispensable tool in many areas of research ranging from physics to biology and medicine. Here we demonstrate how to implement micro-manipulation at the optimal level of efficiency for targets of arbitrary shape and inside complex environments such as disordered media. Our approach is to design wave-fronts in the far-field that have optimal properties in the near-field of the target such as to apply to it the strongest possible force, pressure or torque as well as to achieve the most efficient focus at the target position. Free of any iterative optimization, our approach only relies on a simple eigenvalue problem established from the scattering matrix of the system and its dependence on the target parameters. To illustrate this theoretical concept, we perform a proof-of-principle experiment in the microwave regime, which fully confirms our predictions.

physics.optics↗

arXiv2019

Localisation and transport in bidimensional random models with separable Hamiltonians

We consider two bidimensional random models characterised by the following features: a) their Hamiltonians are separable in polar coordinates and b) the random part of the potential depends either on the angular coordinate or on the radial one, but not on both. The disorder correspondingly localises the angular or the radial part of the eigenfunctions. We analyse the analogies and the differences which exist between the selected 2D models and their 1D counterparts. We show how the analogies allow one to use correlated disorder to design a localisation length with pre-defined energy dependence and to produce directional localisation of the wavefunctions in models with angular disorder. We also discuss the importance of finite-size and resonance effects in shaping the eigenfunctions of the model with angular disorder; for the model with disorder associated to the radial variable we show under what conditions the localisation length coincides with the expression valid in the 1D case.

cond-mat.dis-nn↗

arXiv2019

Uncorrelated Configurations and Field Uniformity in Reverberation Chambers Stirred by Tunable Metasurfaces

Reverberation chambers are currently used to test electromagnetic compatibility as well as to characterize antenna efficiency, wireless devices, and MIMO systems. The related measurements are based on statistical averages and their fluctuations. We introduce a very efficient mode stirring process based on electronically reconfigurable metasurfaces (ERMs). By locally changing the field boundary conditions, the ERMs allow to generate a humongous number of uncorrelated field realizations even within small reverberation chambers. We fully experimentally characterize this stirring process by determining these uncorrelated realizations via the autocorrelation function of the transmissions. The IEC-standard uniformity criterion parameter $σ_{dB}$ is also investigated and reveals the performance of this stirring. The effect of short paths on the two presented quantities is identified. We compare the experimental results on the uniformity criterion parameter with a corresponding model based on random matrix theory and find a good agreement, where the only parameter, the modal overlap, is extracted by the quality factor.

physics.app-ph↗

arXiv2018

Refining the Experimental Extraction of the Number of Independent Samples in a Mode-Stirred Reverberation Chamber

We investigate the number of independent samples in a chaotic reverberation chamber. Its evaluation as defined by the IEC standard can be made more precise when using not the index of the first value larger then the correlation length but using the value obtained by a linear interpolation instead. The results are validated by a juxtaposition with values from a measurement using a high stirrer-angle resolution. A comparison with estimates known from the literature validates our findings. An alternative approach using the local maxima of the parametric dependence of the transmission is presented in order to show the applicability of the extracted correlation length over a large range of frequencies.

physics.data-an↗

arXiv2017

Particlelike scattering states in a microwave cavity

We realize scattering states in a lossy and chaotic two-dimensional microwave cavity which follow bundles of classical particle trajectories. To generate such particlelike scattering states we measure the system's transmission matrix and apply an adapted Wigner-Smith time-delay formalism to it. The necessary shaping of the incident wave is achieved in situ using phase and amplitude regulated microwave antennas. Our experimental findings pave the way for establishing spatially confined communication channels that avoid possible intruders or obstacles in wave-based communication systems.

physics.class-ph↗

arXiv2017

Statistics of Reflection and Transmission in the Strong Overlap Regime of Fully Chaotic Reverberation Chambers

We have experimentally investigated a chaotic reverberation chamber in the regime of strong modal overlap ($1{<}d{<}150$) varying the opening as well as the coupling strength $κ$ of the two attached antennas. We find a good agreement with numerical distribution of the reflection $R=|S_{ii}|^2$ and transmission $T=|S_{21}|^2$ obtained via the effective Hamiltonian formalism using random matrix theory. The two parameters entering the numerics, $κ$ and the decay rate $τ$ were determined beforehand experimentally. Additionally we verified the relation predicted by Schroeder and Kuttruff for acoustic rooms between the averaged frequency spacing of maximal transmission $\langleδf_{max}\rangle$ and the decay rate.

cond-mat.dis-nn↗

arXiv2017

Introducing Enhanced Transport to the Effective Hamiltonian Approach via Random Matrices with a Pair of Connecting States

Direct transport processes play an important role in wireless communications where an ideal setup uses microwave fields to establish reliable communication channels between transmitter and receiver. But it is inherent to the problem that one cannot fully control the environment. While the influence of a complex scattering surrounding can be very well described using Random Matrix Theory it is not always obvious how to combine this universal approach with concrete communication channels. In this work we present an approach introducing an enhanced path between two antennas to the Hamilton operator to account for a prototypical problem. In order to be able to describe the stability of wireless chip-to-chip communication, we analyze the transport properties and predict the stability of the transmission under increasing importance of the environment.

cond-mat.dis-nn↗

arXiv2017

Fluctuations in an established transmission in the presence of a complex environment

In various situations where wave transport is preeminent, like in wireless communication, a strong established transmission is present in a complex scattering environment. We develop a novel approach to describe emerging fluctuations, which combines a transmitting channel and a chaotic background in a unified effective Hamiltonian. Modeling such a background by random matrix theory, we derive exact non-perturbative results for both transmission and reflection distributions at arbitrary absorption that is typically present in real systems. Remarkably, in such a complex scattering situation, the transport is governed by only two parameters: an absorption rate and the ratio of the so-called spreading width to the natural width of the transmission line. In particular, we find that the established transmission disappears sharply when this ratio exceeds unity. The approach exemplifies the role of the chaotic background in dephasing the deterministic scattering.

cond-mat.mes-hall↗

arXiv2017

Focusing inside Disordered Media with the Generalized Wigner-Smith Operator

We introduce a wavefront shaping protocol for focusing inside disordered media based on a generalization of the established Wigner-Smith time-delay operator. The key ingredient for our approach is the scattering (or transmission) matrix of the medium and its derivative with respect to the position of the target one aims to focus on. A specifc experimental realization in the microwave regime is presented showing that the eigenstates of a corresponding operator are sorted by their focusing strength - ranging from strongly focusing on the designated target to completely bypassing it. Our protocol works without optimization or phase-conjugation and we expect it to be particularly attractive for optical imaging in disordered media.

physics.optics↗

arXiv2017

Transport gap engineering by contact geometry in graphene nanoribbons: Experimental and theoretical studies on artificial materials

Electron transport in small graphene nanoribbons is studied by microwave emulation experiments and tight-binding calculations. In particular, it is investigated under which conditions a transport gap can be observed. Our experiments provide evidence that armchair ribbons of width $3m+2$ with integer $m$ are metallic and otherwise semiconducting, whereas zigzag ribbons are metallic independent of their width. The contact geometry, defining to which atoms at the ribbon edges the source and drain leads are attached, has strong effects on the transport. If leads are attached only to the inner atoms of zigzag edges, broad transport gaps can be observed in all armchair ribbons as well as in rhomboid-shaped zigzag ribbons. All experimental results agree qualitatively with tight-binding calculations using the nonequilibrium Green's function method.

cond-mat.mes-hall↗