arXiv subjects

T. Low

Publications and source records attributed to T. Low.

15 recordsLinked to original sources

arXiv2026

Twist-Controlled Symmetry Breaking in Surface Phonon Polariton Moir\'e Metasurfaces

Moire lattices provide a powerful route for engineering emergent symmetries and length scales through the relative rotation of periodic structures. However, their implementation in polaritonic systems remains relatively unexplored, and a general framework describing how twist modifies the interaction of optical modes in momentum space is still lacking. Here, we investigate how twist-induced moire periodicities can control symmetry and momentum-space coupling in surface phonon polariton (SPhP) metasurfaces. We fabricate twisted overlapping dual-grating metasurfaces on a polar dielectric substrate with dielectric overlayer and characterize their optical response using polarization-resolved Fourier-transform infrared microscopy. Experimental measurements are combined with full-wave simulations and momentum-space analysis to identify the resonant SPhP and SPhP-like waveguide (WG) modes arising from both individual grating periodicities and emergent moire periodicities. The results reveal twist-controlled symmetry breaking manifested as asymmetry between p to s and s to p polarization conversion, along with twist-dependent interactions between SPhP and SPhP-like WG modes. Our analysis reveals that the twist-engineered polarization-conversion asymmetry enables directional biasing of infrared radiative heat transfer. These findings establish twisted phonon-polaritonic metasurfaces as a versatile platform for geometry-controlled symmetry engineering in the mid-infrared. Future work may leverage such twist-programmable polaritonic interactions to enable directional thermal emission, polarization-selective detection, and reconfigurable infrared photonic devices.

physics.optics↗

arXiv2025

Optical Gain Through Metallic Electro-Optical Effects

Optical gain is a critical process in today's semiconductor technology and it is most often achieved via stimulated emission. In this theoretical study, we find a resonant TE mode in biased low-symmetry two-dimensional metallic systems which may lead to optical gain in the absence of stimulated emission. We do so by first modeling the optical conductivity using Boltzmann non-equilibrium transport theory and then simulating the scattering problem using a scattered-wave formalism. Assuming that the system may possess a Berry curvature dipole (BCD) and a non-zero Magnetoelectric tensor (MET), we find that the optical conductivity has a non-trivial dependence on the direction of the applied bias, which allows for probing the TE mode. After analyzing the system with one of each of the effects, we find that the resonant TE mode is only accessible when both effects are present. Further studies are necessary to find materials with a suitably large BCD and MET, in order to realize the predictions within this study.

cond-mat.mes-hall↗

arXiv2025

Roadmap for Photonics with 2D Materials

Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combination with layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary collection of those directions, where 2D materials contribute with polaritons of unique characteristics such as strong spatial confinement, large optical-field enhancement, long lifetimes, high sensitivity to external stimuli (e.g., electric and magnetic fields, heating, and strain), a broad spectral range from the far infrared to the ultraviolet, and hybridization with spin and momentum textures of electronic band structures. The explosion of photonics with 2D materials as a vibrant research area is producing breakthroughs, including the discovery and design of new materials and metasurfaces with unprecedented properties as well as applications in integrated photonics, light emission, optical sensing, and exciting prospects for applications in quantum information, and nanoscale thermal transport. This Roadmap summarizes the state of the art in the field, identifies challenges and opportunities, and discusses future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.

cond-mat.mtrl-sci↗

arXiv2025

Anisotropic Strain Relaxation-Induced Directional Ultrafast Carrier Dynamics in RuO2 Films

Ultrafast light-matter interactions inspire potential functionalities in picosecond optoelectronic applications. However, achieving directional carrier dynamics in metals remains challenging due to strong carrier scattering within a multiband environment, typically expected to isotropic carrier relaxation. In this study, we demonstrate epitaxial RuO2/TiO2 (110) heterostructures grown by hybrid molecular beam epitaxy to engineer polarization-selectivity of ultrafast light-matter interactions via anisotropic strain engineering. Combining spectroscopic ellipsometry, X-ray absorption spectroscopy, and optical pump-probe spectroscopy, we revealed the strong anisotropic transient optoelectronic response of strain-engineered RuO2/TiO2 (110) heterostructures along both in-plane [001] and [1-10] crystallographic directions. Theoretical analysis identifies strain-induced modifications in band nesting as the underlying mechanism for enhanced anisotropic carrier relaxation. These findings establish epitaxial strain engineering as a powerful tool for tuning anisotropic optoelectronic responses in metallic systems, paving the way for next-generation polarization-sensitive ultrafast optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Plasmon-enhanced near-field chirality in twisted van der Waals heterostructures

It is shown that chiral plasmons, characterized by a longitudinal magnetic moment accompanying the longitudinal charge plasmon, lead to electromagnetic near-fields that are also chiral. For twisted bilayer graphene, we estimate that the near field chirality of screened plasmons can be several orders of magnitude larger than that of the related circularly polarized light. The chirality also manifests itself in a deflection angle that is formed between the direction of the plasmon propagation and its Poynting vector. Twisted van der Waals heterostructures might thus provide a novel platform to promote enantiomer-selective physio-chemical processes in chiral molecules without the application of a magnetic field or external nano-patterning that break time-reversal, mirror plane or inversion symmetry, respectively.

cond-mat.mes-hall↗

arXiv2019

Strain-Engineered High Responsivity MoTe2 Photodetector for Silicon Photonic Integrated Circuits

In integrated photonics, specific wavelengths are preferred such as 1550 nm due to low-loss transmission and the availability of optical gain in this spectral region. For chip-based photodetectors, layered two-dimensional (2D) materials bear scientific and technologically-relevant properties leading to strong light-matter-interaction devices due to effects such as reduced coulomb screening or excitonic states. However, no efficient photodetector in the telecommunication C-band using 2D materials has been realized yet. Here, we demonstrate a MoTe2-based photodetector featuring strong photoresponse (responsivity = 0.5 A/W) operating at 1550nm on silicon photonic waveguide enabled by engineering the strain (4%) inside the photo-absorbing transition-metal-dichalcogenide film. We show that an induced tensile strain of ~4% reduces the bandgap of MoTe2 by about 0.2 eV by microscopically measuring the work-function across the device. Unlike Graphene-based photodetectors relying on a gapless band structure, this semiconductor-2D material detector shows a ~100X improved dark current enabling an efficient noise-equivalent power of just 90 pW/Hz^0.5. Such strain-engineered integrated photodetector provides new opportunities for integrated optoelectronic systems.

physics.app-ph↗

arXiv2019

Unidirectional plasmonic edge modes on general two-dimensional materials

We investigate the field and spin-momentum coupling of edge plasmons hosted by general two-dimensional materials and identify sweet spots depending on the polarisation plane, ellipticity and the position of an electric dipole relative to the plane and edge. Exciting the dipole at these sweet spots by propagating light leads to uni-directional propagating edge plasmons or edge modes are totally suppressed. We also extent previous approximate treatments [A. Fetter Phys. Rev. B 32, 7676 (1985)] to include anisotropy and hyperbolic systems, elucidating its predictions for the existence of edge modes. A thorough assessment of the approximate description is carried out, comparing its spin-momentum coupling features in the near field with exact results from Wiener-Hopf techniques. Simulations are also performed confirming the overall picture. Our results shed new light on the quest of chiral plasmonics in 2D materials and should be relevant for future experiments.

cond-mat.mes-hall↗

arXiv2018

Linear response of twisted bilayer graphene: continuum vs. tight-binding models

We present a linear response calculation for twisted bilayer graphene. The calculation is performed for both the continuum and tight-binding models, with the aim of assessing the validity of the former. All qualitatively important features previously reported by us [T. Stauber et al. Phys. Rev. Lett. 120, 046801 (2018)] for the Drude matrix in the continuum model are also present in the tight-binding calculation, with increasing quantitative agreement for decreasing twist angle. These features include the chiral longitudinal magnetic moment associated with plasmonic modes, and the anomalous counterflow around the neutrality point, better interpreted as a paramagnetic response. We have addressed the differences between Drude and equilibrium response, and shown that orbital paramagnetism is the equilibrium response to a parallel magnetic field over a substantial doping region around the neutrality point. Chirality also makes the equilibrium response to exhibit a non trivial current structure associated with the non-vertical character of interlayer bonds in the tight-binding calculation.

cond-mat.mes-hall↗

arXiv2017

Theoretical overview of black phosphorus

We review the basic optical, electronic, optoelectronic, thermoelectric and mechanical properties of few-layer black phosphorus (BP), a layered semiconductor that can be exfoliated from bulk BP, the most stable allotrope of phosphorus. The distinguishing trait of BP is its highly anisotropic crystal structure, which leads to strong optical linear dichroism, elliptical plasmonics energy surfaces, anisotropic electronic and thermal conductivities and elasticity. We provide tutorial-like discussion of these phenomena and their theoretical models.

cond-mat.mes-hall↗

arXiv2017

Electrical control of inter-layer excitons in van der Waals heterostructures

We investigate excitons in stacked transition metal dichalcogenide (TMDC) layers under perpendicularly applied electric field, herein MoSe$_2$/WSe$_2$ van der Waals heterostructures. Band structures are obtained with density functional theory calculations, along with the electron and hole wave functions in conduction and valence bands, respectively. Although the type-II nature of the heterostructure leads to fully charge separated inter-layer excitons, charge carriers distribution among the layers is shown to be easily tunable by external field. Our results show that moderate values of electric field produce more evenly distributed wave functions along the heterostructure, thus enhancing both the inter-layer exciton binding energy and, most notably, its oscillator strength.

cond-mat.mes-hall↗

arXiv2017

Chiral response of twisted bilayer graphene

We present an effective (minimal) theory for chiral two-dimensional materials. These materials possess an electro-magnetic coupling without exhibiting a topological gap. As an example, we study the response of doped twisted bilayers, unveiling unusual phenomena in the zero frequency limit. An in-plane magnetic field induces a huge paramagnetic response at the neutrality point and, upon doping, also gives rise to a substantial longitudinal Hall response. The system also accommodates nontrivial longitudinal plasmonic modes which are associated with a longitudinal magnetic moment, thus endowing them with a chiral character. Finally, we note that the optical activity can be considerably enhanced upon doping.

cond-mat.mes-hall↗

arXiv2015

Anisotropic exciton Stark shift in black phosphorus

We calculate the excitonic spectrum of few-layer black phosphorus by direct diagonalization of the effective mass Hamiltonian in the presence of an applied in-plane electric field. The strong attractive interaction between electrons and holes in this system allows one to investigate the Stark effect up to very high ionizing fields, including also the excited states. Our results show that the band anisotropy in black phosphorus becomes evident in the direction dependent field induced polarizability of the exciton.

cond-mat.mes-hall↗

arXiv2014

Anomalous Reflection Phase of Graphene Plasmons and its Influence on Resonators

The phase picked up by a graphene plasmon upon scattering by an abrupt edge is commonly assumed to be $-\pi$. Here, it is demonstrated that for high plasmon momenta this reflection phase is $\approx -3\pi/4$, virtually independent on either chemical potential, wavelength or dielectric substrate. This non-trivial phase arises from a complex excitation of highly evanescent modes close to the edge, which are required to satisfy the continuity of electric and magnetic fields. A similar result for the reflection phase is expected for other two-dimensional systems supporting highly confined plasmons (very thin metal films, topological insulators, transition polaritonic layers, etc.). The knowledge of the reflection phase, combined with the phase picked up by the plasmon upon propagation, allows the estimation of resonator properties from the dispersion relation of plasmons in the infinite monolayer.

cond-mat.mes-hall↗

arXiv2010

Gaps tunable by electrostatic gates in strained graphene

We show that when the pseudomagnetic fields created by long wavelength deformations are appropriately coupled with a scalar electric potential, a significant energy gap can emerge due to the formation of a Haldane state. Ramifications of this physical effect are examined through the study of various strain geometries commonly seen in experiments, such as strain superlattices and wrinkled suspended graphene. Of particular technological importance, we consider setup where this gap can be tunable through electrostatic gates, allowing for the design of electronic devices not realizable with other materials.

cond-mat.mes-hall↗

arXiv2010

Band structure and gaps of triangular graphene superlattices

General properties of long wavelength triangular graphene superlattice are studied. It is shown that Dirac points with and without gaps can arise at a number of high symmetry points of the Brillouin Zone. The existence of gaps can lead to insulating behavior at commensurate fillings. Strain and magnetic superlattices are also discussed.

cond-mat.mes-hall↗