arXiv subjects

Thomas Koschny

Publications and source records attributed to Thomas Koschny.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic Brightening and Nanoscale Imaging of Spin-Polarized Helical Edge Modes

Efficient sub-10 nm electric transport remains a major challenge for nanoelectronics due to high losses and impedance mismatches in conventional Drude metals. Despite their promise of dissipationless, reflection-free conduction, topologically protected chiral edge modes remain little explored in their nanoscale spin polarized transport-particularly regarding real-space visualization, magnetic field tunability, and high-frequency edge conductivity. Here, we report magnetic brightening and nanoscale visualization of highly spin-polarizable infrared helical edge states using cryogenic magneto-infrared scattering-type scanning near-field optical microscopy (cm-IR-sSNOM). Our measurements reveal magnetic field-induced near-field conductivity at step edges, uncovering quantum spin Hall spin-splitting modes with enhanced infrared polarizability and slightly narrowed near-field profiles. In addition, the infrared edge electrodynamic response scales nearly linearly with atomic layer number, providing compelling evidence that magnetic-field-induced gaps do not disrupt individual-layer edge states at energies of around 100 meV. These results sharply contrast with microwave and DC transport, where even small magnetically induced gaps decrease edge conduction. Magnetically tunable, topologically robust high-frequency edge modes open a pathway toward ultralow-loss nanoscale interconnects and quantum logic architectures for next-generation microelectronics, spintronics and quantum information science.

cond-mat.mes-hall↗

arXiv2026

Terahertz magneto-nanoscopy of encapsulated monolayer graphene

This study investigates the nanoscale conductivity of encapsulated monolayer graphene at temperatures down to 5 K and magnetic fields of up to 1 T. We use the scattering-type scanning near-field optical microscopy (s-SNOM) technique to probe magnetic-field-dependent responses from graphene close to charge neutrality in the terahertz spectral region. We observe the near-perfect high-$q$ reflector behavior of graphene but with subtle changes by the presence of magnetic fields. Measurements align with calculations of the magneto-optical conductivity and the near-field spectroscopic contrast that describes the field-tunable cyclotron resonance of Dirac fermions. Our result provides an initial step toward understanding temperature and magnetic-field effects on nanoscale terahertz transport in two-dimensional quantum materials.

cond-mat.mes-hall↗

arXiv2026

Cut-and-Project Density Functional Theory for Quasicrystals

Cut-and-project from a symmetric structure in a higher-dimensional space is a standard method for describing the structure of a large class of quasicrystals. By means of a novel localization procedure, we now show how local physical interactions within these quasicrystals are also accurately described by cut-and-project, from corresponding physical interactions in the higher-dimensional space. A density functional theory (DFT++) formulation allows the cut-and-project method to handle the Schroedinger equation for interactions in quasicrystals. The theory is both rigorous and computationally tractable. The resulting ab initio approach specifies quasicrystalline quantum states, in contrast to previous approaches which only worked with crystalline approximants of the quasi-periodic structures.

cond-mat.mtrl-sci↗

arXiv2026

Terahertz-nanoscale visualization of the microscopic spin-charge architecture of colossal magnetoresistive switching

Resolving sub-10 nm spin switching and the associated terahertz (THz) electrodynamics during the colossal magnetoresistance (CMR) transition is a definitive frontier in reaching the fundamental spatial, temporal, and energy-dissipation limits of spin-based microelectronics and quantum logic architectures. Yet, the requirement of simultaneous control of high magnetic field, cryogenic environment, and nanometer-scale resolution has remained an elusive benchmark for terahertz nanoscopy, leaving the obscured nano-scale high-frequency dynamics of these transitions largely unexplored. Here, we overcome these limitations by utilizing a custom-built cryogenic magneto-THz scattering-type scanning near-field optical microscopy (cm-THz-sSNOM) platform to resolve the nanoscale, THz spectroscopic evolution of the magnetic field-driven CMR transition in a manganite single crystal $\text{Pr}_{2/3}\text{Ca}_{1/3}\text{MnO}_{3}$. Our measurements provide a real-space visualization of the local THz conductivity, capturing the moment that magnetic-field-induced spin switching triggers the phase transition from an antiferromagnetic insulator to a ferromagnetic metal. THz nano-imaging, together with an ellipsoidal near-field model, reveals a multi-scale transition initiated by 1-2 nm isolated spin-flip sites at low magnetic fields, which coalesce into $\sim$15~nm conducting regions as the threshold field is approached. These results provide an in situ, previously inaccessible THz real-space view of CMR switching, establishing a general analysis framework for mapping spin-charge-lattice-orbit-coupled dynamics at spatial scales that transcend the nominal sSNOM resolution.

cond-mat.str-el↗

arXiv2025

Enhanced THz third-harmonic generation in graphene via hot-carriers in a topological cavity

Graphene is a promising material for nonlinear THz applications owing to its high third-order susceptibility and tunable optical properties. Its strong nonlinear response, driven by free-carrier thermodynamics, facilitates efficient third-harmonic generation (THG), which can be further enhanced using resonant metasurfaces and plasmonic structures. In this study, we use a platform that integrates graphene ribbons at the interface of topological photonic crystals designed to boost THG at low THz frequencies. To accurately capture the temperature dependence of the nonlinear optical response, we perform coupled simulations to model graphene's optical and thermodynamic responses under THz photoexcitation, incorporating photoinduced hot carrier effects within a multiphysics computational framework. This approach unveils the influence of carrier heating on THG, providing a more accurate description of nonlinear THz processes and revealing previously overlooked mechanisms of third-order conductivity modulation. We show that the elevated carrier temperature promotes high conversion efficiencies at moderate input intensities, within a design that operates in both reflection and transmission modes for advanced ultrafast optoelectronic devices in the THz regime.

physics.optics↗

arXiv2025

Correlating Superconducting Qubit Performance Losses to Sidewall Near-Field Scattering via Terahertz Nanophotonics

Elucidating dielectric losses, structural heterogeneity, and interface imperfections is critical for improving coherence in superconducting qubits. However, most diagnostics rely on destructive electron microscopy or low-throughput millikelvin quantum measurements. Here, we demonstrate noninvasive terahertz (THz) nano-imaging/-spectroscopy of encapsulated niobium transmon qubits, revealing sidewall near-field scattering that correlates with qubit coherence. We further employ a THz hyperspectral line scan to probe dielectric responses and field participation at Al junction interfaces. These findings highlight the promise of THz near-field methods as a high-throughput proxy characterization tool for guiding material selection and optimizing processing protocols to improve qubit and quantum circuit performance.

cond-mat.mes-hall↗

arXiv2024

Ultracompact, dynamically controllable circularly polarized laser enabled by chiral metasurfaces

We demonstrate a simple, low-cost and ultracompact chiral resonant metasurface design, which, by strong local coupling to a quantum gain medium (quantum emitters), allows to implement an ultra-thin metasurface laser, capable of generating tunable circularly polarized coherent lasing output. According to our detailed numerical investigations the lasing emission can be transformed from linear to circular and switch from right- to left-handed circularly polarized (CP) not only by changing the metasurface chiral response but also by changing the polarization of a linearly polarized pump wave, providing thus dynamic lasing-polarization control. Given the increasing interest for CP laser emission, our chiral metasurface laser design proves to be a versatile yet straightforward strategy to generate strong and tailored CP emission laser, promising great potential for future applications in both photonics and materials science.

physics.optics↗

arXiv2023

Multiresonant metasurfaces for arbitrarily-broadband pulse chirping and dispersion compensation

We show that ultrathin metasurfaces with a specific multiresonant response can enable simultaneously arbitrarily-strong and arbitrarily-broadband dispersion compensation, pulse (de-)chirping and compression or broadening. This breakthrough overcomes the fundamental limitations of both conventional non-resonant approaches (bulky) and modern singly-resonant metasurfaces (narrowband) for quadratic phase manipulations of electromagnetic signals. The required non-uniform trains of resonances in the electric and magnetic sheet conductivities that completely control phase delay, group delay, and chirp, are rigorously derived and the limitations imposed by fundamental physical constraints are thoroughly discussed. Subsequently, a practical, truncated approximation by finite sequences of physically-realizable linear resonances is constructed and the associated error is quantified. By appropriate spectral ordering of the resonances, operation can be achieved either in transmission or reflection mode, enabling full space coverage. The proposed concept is not limited to dispersion compensation, but introduces a generic and powerful ultrathin platform for the spatio-temporal control of broadband real-world signals with a myriad of applications in modern optics, microwave photonics, radar and communication systems.

physics.optics↗

arXiv2020

The local density of optical states in the 3D band gap of a finite photonic crystal

A three-dimensional (3D) photonic band gap crystal is an ideal tool to completely inhibit the local density of optical states (LDOS) at every position in the crystal throughout the band gap. This notion, however, pertains to ideal infinite crystals, whereas any real crystal device is necessarily finite. This raises the question as to how the LDOS in the gap depends on the position and orientation inside a finite-size crystal. Therefore, we employ rigorous numerical calculations using finite-difference time-domain (FDTD) simulations of 3D silicon inverse woodpile crystals filled with air or with toluene, as previously studied in experiments. We find that the LDOS versus position decreases exponentially into the bulk of the crystal. From the dependence on dipole orientation, we infer that the characteristic LDOS decay length $\ell_ρ$ is mostly related to far-field dipolar radiation effects, whereas the prefactor is mostly related to near-field dipolar effects. The LDOS decay length has a remarkably similar magnitude as the Bragg length for directional transport, which suggests that the LDOS in the crystal is dominated by vacuum states that tunnel from the closest interface towards the position of interest. Our work leads to design rules for applications of 3D photonic band gaps in emission control and lighting, quantum information processing, and in photovoltaics.

physics.optics↗

arXiv2019

Wavefront manipulation based of the excitation of bound states in dielectric photonic crystals and bilayer metasurfaces

We present the study of bound surface modes sustained at the termination of truncated bulk dielectric photonic crystals and isolated metasurfaces of dielectric meta-atoms. We discuss the origins of bound modes in the two systems and their relation. For both systems, we theoretically study and experimentally demonstrate wavefront manipulation, in particular directional emission, frequency splitting and beam collimation achieved by coupling of the bound states to radiation modes through leaky wave radiation mechanism using properly designed scattering gratings.

physics.app-ph↗

arXiv2019

Metasurfaces with Interleaved Electric and Magnetic Resonances for Broadband Arbitrary Group Delay in Reflection

Metasurfaces impart phase discontinuities on impinging electromagnetic waves that are typically limited to 0-2$π$. Here, we show that they can break free from this limitation and supply arbitrarily-large phase modulation over ultra-wide bandwidths. This is achieved by implementing multiple, properly-arranged resonances in the electric and magnetic sheet admittivities. We demonstrate metasurfaces that can perfectly reflect a broadband pulse imparting a prescribed group delay without distorting the pulse shape, opening new possibilities for temporal control and dispersion engineering across deeply subwavelength physical scales.

physics.app-ph↗

arXiv2019

Nonlinearity in the Dark: Broadband Terahertz Generation with Extremely High Efficiency

Plasmonic metamaterials and metasurfaces offer new opportunities in developing high performance terahertz emitters and detectors beyond the limitations of conventional nonlinear materials. However, simple meta-atoms for second-order nonlinear applications encounter fundamental trade-offs in the necessary symmetry breaking and local-field enhancement due to radiation damping that is inherent to the operating resonant mode and cannot be controlled separately. Here we present a novel concept that eliminates this restriction obstructing the improvement of terahertz generation efficiency in nonlinear metasurfaces based on metallic nanoresonators. This is achieved by combining a resonant dark-state metasurface, which locally drives nonlinear nanoresonators in the near field, with a specific spatial symmetry that enables destructive interference of the radiating linear moments of the nanoresonators, and perfect absorption via simultaneous electric and magnetic critical coupling of the pump radiation to the dark mode. Our proposal allows eliminating linear radiation damping, while maintaining constructive interference and effective radiation of the nonlinear components. We numerically demonstrate a giant second-order nonlinear susceptibility around Hundred-Billionth m/V, a one order improvement compared with the previously reported split-ring-resonator metasurface, and correspondingly, a 2 orders of magnitude enhanced terahertz energy extraction should be expected with our configuration under the same conditions. Our study offers a paradigm of high efficiency tunable nonlinear metadevices and paves the way to revolutionary terahertz technologies and optoelectronic nanocircuitry.

physics.optics↗

arXiv2018

Investigation of broadband terahertz generation from metasurface

The nonlinear metamaterials have been shown to provide nonlinear properties with high nonlinear conversion efficiency and in a myriad of light manipulation. Here we study terahertz generation from nonlinear metasurface consisting of single layer nanoscale split-ring resonator array. The terahertz generation due to optical rectification by the second-order nonlinearity of the split-ring resonator is investigated by a time-domain implementation of the hydrodynamic model for electron dynamics in metal. The results show that the nonlinear metasurface enables us to generate broadband terahertz radiation and free from quasi-phase-matching conditions. The proposed scheme provides a new concept of broadband THz source and designing nonlinear plasmonic metamaterials.

physics.optics↗

arXiv2016

Ultrathin metasurface with topological transition for manipulating spoof surface plasmon polaritons

Metasurfaces, with intrinsically planar nature and subwavelength thickness, provide us unconventional methodologies to not only mold the flow of propagating waves but also manipulate near-field waves. Plasmonic metasurfaces with topological transition for controlling surface plasmon polaritons (SPPs) recently have been experimentally demonstrated, which, however, are limited to optical frequencies. In this work, we proposed and experimentally characterized an ultrathin metasurface with the topological transition for manipulating spoof SPPs at low frequency. We demonstrated rich interesting phenomena based on this metasurface, including frequency-dependent spatial localization, non-diffraction propagation, negative refraction, and dispersion-dependent spin-momentum locking of spoof SPPs. Comparing with traditional three-dimensional metamaterials, our metasurface exhibits low propagation loss and compatibility with the photonic integrated circuit, which may find plenty of applications in spatial multiplexers, focusing and imaging devices, planar hyperlens, and dispersion-dependent directional couplers, in microwave and terahertz frequencies.

physics.optics↗

arXiv2014

Comparison of gold- and graphene-based resonant nano-structures for terahertz metamaterials and an ultra-thin graphene-based modulator

Graphene exhibits unique material properties and in electromagnetic wave technology, it raises the prospect of devices miniaturized down to the atomic length scale. Here we study split-ring resonator metamaterials made from graphene and we compare them to gold-based metamaterials. We find that graphene's huge reactive response derived from its large kinetic inductance allows for deeply subwavelength resonances, although its resonance strength is reduced due to higher dissipative loss damping and smaller dipole coupling. Nevertheless, tightly stacked graphene rings may provide for negative permeability and the electric dipole resonance of graphene meta-atoms turns out to be surprisingly strong. Based on these findings, we present a terahertz modulator based on a metamaterial with a multi-layer stack of alternating patterned graphene sheets separated by dielectric spacers. Neighbouring graphene flakes are biased against each other, resulting in modulation depths of over 75% at a transmission level of around 90%.

physics.optics↗

arXiv2014

Graphene for terahertz applications

Graphene is a one-atom-thick sheet of carbon atoms arranged in a honeycomb lattice. It was first obtained by exfoliation of graphite in 2004 and has since evolved into a thriving research topic because of its attractive mechanical, thermal, and electrical properties. Graphene's unique electrical properties derive from the relativistic nature of its quasiparticles, resulting in exceptionally high electron mobility. Graphene promises to revolutionize many applications, ranging from solar cells and light-emitting devices to touch screens, photodetectors, microwave transistors, and ultrafast lasers.

physics.optics↗

arXiv2014

Robust wedge demonstration to optical negative index metamaterials

A robust wedge setup is proposed to unambiguously demonstrate negative refraction for negative index metamaterials. We applied our setup to several optical metamaterials from the literature and distinctly observed the phenomena of negative refraction. This further consolidates the reported negative-index property. It is found there generally exists a lateral shift for the outgoing beam through the wedge. We derived a simple expression for calculating this beam shift and interestingly, it provides us a strategy to quantitatively estimate the loss of the wedge material (Im[n]). Addition- ally, we offered a design of metamaterials, compatible with nano-imprinting-lithography, showing negative refractive index in the visible regime (around yellow-light wavelengths). The multi-layer- system retrieval was utilized to extract the effective refractive index of the metamaterial. It was also intuitively characterized through our wedge setup to demonstrate corresponding phenomena of refraction.

physics.optics↗

arXiv2014

One- and two-dimensional photo-imprinted diffraction gratings for manipulating terahertz waves

Emerging technology based on artificial materials containing metallic structures has raised the prospect for unprecedented control of terahertz waves through components like filters, absorbers and polarizers. The functionality of these devices is static by the very nature of their metallic or polaritonic composition, although some degree of tunability can be achieved by incorporating electrically biased semiconductors. Here, we demonstrate a photonic structure by projecting the optical image of a metal mask onto a thin GaAs substrate using a femtosecond pulsed laser source. We show that the resulting high-contrast pattern of photo- excited carriers can create diffractive elements operating in transmission. With the metal mask replaced by a digital micromirror device, our photo-imprinted photonic structures provide a route to terahertz components with reconfigurable functionality.

physics.optics↗