arXiv subjects

Stephanie Reich

Publications and source records attributed to Stephanie Reich.

At least 19 recordsLinked to original sources

arXiv2025

Enhancement of the WS$_2$ A$_{1\text{g}}$ Raman Mode in MoS$_2$/WS$_2$ Heterostructures

When combined into van der Waals heterostructures, transition metal dichalcogenide monolayers enable the exploration of novel physics beyond their unique individual properties. However, for interesting phenomena such as interlayer charge transfer and interlayer excitons to occur, precise control of the interface and ensuring high-quality interlayer contact is crucial. Here, we investigate bilayer heterostructures fabricated by combining chemical-vapor-deposition-grown MoS$_2$ and exfoliated WS$_2$ monolayers, allowing us to form several heterostructures with various twist angles within one preparation step. In case of sufficiently good interfacial contact, evaluated by photoluminescence quenching, we observe a twist-angle-dependent enhancement of the WS$_2$ A$_{1g}$ Raman mode. In contrast, other WS$_2$ and MoS$_2$ Raman modes (in particular, the MoS$_2$ A$_{1g}$ mode) do not show a clear enhancement under the same experimental conditions. We present a systematic study of this mode-selective effect using nonresonant Raman measurements that are complemented with ab-initio calculations of Raman spectra. We find that the selective enhancement of the WS$_2$ A$_{1g}$ mode exhibits a strong dependence on interlayer distance. We show that this selectivity is related to the A$_{1g}$ eigenvectors in the heterolayer: the eigenvectors are predominantly localized on one of the two layers; yet, the intensity of the MoS$_2$ mode is attenuated because the WS$_2$ layer is vibrating (albeit with much lower amplitude) out of phase, while the WS$_2$ mode is amplified because the atoms on the MoS$_2$ layer are vibrating in phase. To separate this eigenmode effect from resonant Raman enhancement, our study is extended with near-resonant Raman measurements.

cond-mat.mes-hall↗

arXiv2025

Directed light emission from monolayers on 2D materials via optical interferences

Two-dimensional materials provide a rich platform to explore phenomena such as emerging electronic and excitonic states, strong light-matter coupling and new optoelectronic device concepts. The optical response of monolayers is entangled with the substrate on which they are grown or deposited on, often a two-dimensional material itself. Understanding how the properties of the two-dimensional monolayers can be tuned via the substrate is therefore essential. Here we employ angle-resolved reflectivity and photoluminescence spectroscopy on highly ordered molecular monolayers on hexagonal boron nitride (hBN) to systematically investigate the angle-dependent optical response as a function of the thickness of the hBN flake. We observe that light reflection and emission occur in a strongly directed fashion and that the direction of light reflection and emission is dictated by the hBN flake thickness. Transfer matrix simulations reproduce the experimental data and show that optical interference effects in hBN are at the origin of the angle-dependent optical properties. While our study focuses on molecular monolayers on hBN, our findings are general and relevant for any 2D material placed on top of a substrate. Our findings demonstrate the need to carefully choose substrate parameters for a given experimental geometry but also highlight opportunities in applications such as lighting technology where the direction of light emission can be controlled via substrate thickness.

cond-mat.mes-hall↗

arXiv2025

Ultrastrong Light-Matter Coupling in Materials

Ultrastrong light-matter coupling has traditionally been studied in optical cavities, where it occurs when the light-matter coupling strength reaches a significant fraction of the transition frequency. This regime fundamentally alters the ground and excited states of the particle-cavity system, unlocking new ways to control its physics and chemistry. However, achieving ultrastrong coupling in engineered cavities remains a major challenge. Here, we show that ultra- and deep-strong coupling naturally occur in bulk materials without the need for external cavities. By analyzing experimental data from over 70 materials, we demonstrate that phonon-, exciton-, and plasmon-polaritons in many solids exhibit ultrastrong coupling, systematically surpassing the coupling strengths achieved in cavity-based systems. To explain this phenomenon, we introduce a dipole lattice model based on a generalized Hopfield Hamiltonian, which unifies photon-matter, matter-matter, and photon-photon interactions. The complete overlap between the photonic and collective dipole modes in the lattice enables ultrastrong coupling, leading to excited-state mixing, radiative decay suppression, and potential phase transitions into collective ground states. Applying our model to real materials, we show that it reproduces light-matter coupling across broad material classes and may underlie structural phase transitions that give rise to emergent phenomena such as ferroelectricity, insulator-to-metal transitions, and exciton condensation. Recognizing ultrastrong coupling as an intrinsic property of solids reshapes our understanding of light-matter interactions and opens new avenues for exploring quantum materials and exotic phases of matter.

physics.optics↗

arXiv2025

THz-Driven Coherent Phonon Fingerprints of Hidden Symmetry Breaking in 2D Layered Hybrid Perovskites

Metal-halide perovskites (MHPs) emerged as a family of novel semiconductors with outstanding optoelectronic properties for applications in photovoltaics and light emission. Recently, they also attract interest as promising candidates for spintronics. In materials lacking inversion symmetry, spin-orbit coupling (SOC) leads to the Rashba-Dresselhaus effect, offering a pathway for spin current control. Therefore, inversion symmetry breaking in MHPs, which are characterized by strong SOC, has crucial implications. Yet, in complex low-dimensional hybrid organic-inorganic perovskites (HOIPs), the presence of and structural contributions to inversion symmetry breaking remain elusive. Here, employing intense THz fields, we coherently drive lattice dynamics carrying spectroscopic fingerprints of inversion symmetry breaking in Ruddlesden-Popper (PEA)$_2$(MA)$_{n-1}$PbnI${3n+1}$ perovskites, which are globally assigned to a centrosymmetric space group. We demonstrate coherent control by THz pulses over specific phonons, which we assign to either purely inorganic or highly anharmonic hybrid cage-ligand vibrations. By developing a general polarization analysis for THz-driven phonons, we pinpoint linear and nonlinear driving mechanisms. From this, we identify simultaneous IR- and Raman-activity of inorganic cage modes below 1.5 THz, indicating mode-selective inversion symmetry breaking. By exploring the driving pathways of these coherent phonons, we lay the groundwork for simultaneous ultrafast control of optoelectronic and spintronic properties in 2D HOIPs.

cond-mat.mtrl-sci↗

arXiv2024

Nanocavities for Molecular Optomechanics: their fundamental description and applications

Vibrational Raman scattering -- a process where light exchanges energy with a molecular vibration through inelastic scattering -- is most fundamentally described in a quantum framework where both light and vibration are quantized. When the Raman scatterer is embedded inside a plasmonic nanocavity, as in some sufficiently controlled implementations of surface-enhanced Raman scattering (SERS), the coupled system realizes an optomechanical cavity, where coherent and parametrically amplified light-vibration interaction becomes a resource for vibrational state engineering and nanoscale nonlinear optics. The purpose of this Perspective is to clarify the connection between the languages and parameters used in the fields of molecular cavity optomechanics (McOM) vs. its conventional, `macroscopic' counterpart, and to summarize the main results achieved so far in McOM and the most pressing experimental and theoretical challenges. We aim to make the theoretical framework of molecular cavity optomechanics practically usable for the SERS and nanoplasmonics community at large. While quality factors ($Q$'s) and mode volumes ($V$'s) essentially describe the performance of a nanocavity in enhancing light-matter interaction, we point to the light-cavity coupling efficiencies ($\eta$'s) and optomechanical cooperativities ($\mathcal{C}$'s) as the key parameters for molecular optomechanics. As an illustration of the significance of these quantities, we investigate the feasibility of observing optomechanically induced transparency with a molecular vibration -- a measurement that would allow for a direct estimate of the optomechanical cooperativity.

physics.optics↗

arXiv2024

Collective states of {\alpha}-sexithiophene chains inside boron nitride nanotubes

Nanotubes align molecules into one dimensional chains creating collective states through the coupling of the molecular transition dipole moments. These collective excitations have strong fluorescence, narrow bandwidth, and shifted emission/absorption energies. We study the optical properties of {\alpha}-sexithiophene chains in boron nitride nanotubes by combining fluorescence with far- and near-field absorption spectroscopy. The inner nanotube diameter determines the number of encapsulated molecular chains. A single chain of {\alpha}-sexithiophene molecules has an optical absorption and emission spectrum that is red-shifted by almost 300 meV compared to the monomer emission, which is much larger than expected from dipole-dipole coupling. The collective state splits into excitation and emission channels with a Stokes shift of 200 meV for chains with two or more files. Our study emphasises the formation of a delocalized collective state through Coulomb coupling of the transition moments that shows a remarkable tuneability in transition energy.

physics.atm-clus↗

arXiv2024

Double tips for in-plane polarized near-field microscopy and spectroscopy

Near-field optical microscopy and spectroscopy provide high-resolution imaging below the diffraction limit, crucial in physics, chemistry, and biology for studying molecules, nanoparticles, and viruses. These techniques use a sharp metallic tip of an atomic force microscope (AFM) to enhance incoming and scattered light by excited near-fields at the tip apex leading to high sensitivity and a spatial resolution of a few nanometers. However, this restricts the near-field orientation to out-of-plane polarization, limiting optical polarization choices. We introduce double tips that offer in-plane polarization for enhanced imaging and spectroscopy. These double tips provide superior enhancement over single tips, although with a slightly lower spatial resolution (~30nm). They enable advanced studies of nanotubes, graphene defects, and transition metal dichalcogenides, benefiting from polarization control. The double tips allow varied polarization in tip-enhanced Raman scattering and selective excitation of transverse-electric and -magnetic polaritons, expanding the range of nanoscale samples that can be studied.

physics.optics↗

arXiv2024

Cross-Dimensional Exciton Coupling in Quantum Dot - Carbon Nanotube Hybrid Thin Films

Dipole-dipole interactions provide a route to couple excitons across materials of different dimensionality. Here, we introduce hybrid films as a cross-dimensional excitonic platform and develop a theory of resonant Raman scattering in the presence of dipolar exciton coupling. Combining a coupled-oscillator model with perturbation theory, we show that coupling renormalizes the exciton-photon matrix elements and modifies nanotube Raman excitation profiles. We test this framework in vacuum-filtered films containing InP/ZnS quantum dots and mixed chirality metallic single-walled carbon nanotubes. The quantum-dot exciton remains near 1.83 eV, while chirality-dependent nanotube excitons span 1.7-1.93 eV, enabling systematic control of excitonic detuning. Relative to pristine nanotube films, the hybrids exhibit detuning-dependent redshifts and blueshifts, Raman intensity enhancement, reduced effective linewidths, and near-resonant two-branch profiles. The model yields an effective splitting of approximately 110 meV. Resonant Raman scattering thus provides component-selective access to dipolar interactions that are only weakly apparent in ensemble absorption spectra.

cond-mat.mes-hall↗

arXiv2024

Raman resonances mediated by excitonic polarons in BiVO$_4$

Excitonic polarons are quasiparticles formed by a Coulomb-bound electron-hole pair with strong coupling to lattice vibrations. Despite high fundamental interest in excitonic polarons, the experimental investigation of these particles remains challenging. In this work, we exploit the resonant Raman effect to probe the excitonic polarons in bismuth vanadate. We track enhancement of Raman modes as a function of excitation energy and reveal two optical resonances: one inside the band gap at 1.94 eV and another one near the optical absorption edge at 2.45 eV. The high-energy resonance originates from free excitons, which exhibit a characteristic 40 meV anisotropy between polarizations parallel and perpendicular to the c axis. Remarkably, the low-energy resonance shows no contrast in the optical absorption spectra. We attribute this resonance to an excitonic polaron formed through strong exciton-phonon coupling, making excitonic and excitonic polaron Raman resonances similar in strength. We probe the energy level of the excitonic polaron and compare its coupling strength to the different vibrational modes. Our results establish resonant Raman spectroscopy as a unique and powerful tool for probing quasiparticles of polaronic and excitonic nature in oxide materials.

cond-mat.mtrl-sci↗

arXiv2024

Dielectric Screening Inside Carbon Nanotubes

Dielectric screening plays a vital role for the physical properties in the nanoscale and also alters our ability to detect and characterize nanomaterials by optical techniques. We study the dielectric screening inside of carbon nanotubes and how it changes electromagnetic fields and many-body effects for encapsulated nanostructures. First, we show that the local electric field inside a nanotube is altered by one-dimensional screening with dramatic effects on the effective Raman scattering efficiency of the encapsulated species for metallic walls. The scattering intensity of the inner tube is two orders of magnitude weaker than for the tube in air, which is nicely reproduced by local field calculations. Secondly, we find that the optical transition energies of the inner nanotubes shift to lower energies compared to a single-walled carbon nanotubes of the same chirality. The shift is higher if the outer tube is metallic than when it is semiconducting. The magnitude of the shift suggests that the excitons of small diameter inner metallic tubes are thermally dissociated at room temperate if the outer tube is also metallic and in essence we observe band-to-band transitions.

cond-mat.mes-hall↗

arXiv2023

Longitudinal Polaritons in Crystals

The collective excitations of solids are classified as longitudinal and transverse depending on their relative polarization and propagation direction. This seemingly formal classification results in surprisingly distinct types of excitations if calculated within the Coulomb gauge. Transverse modes couple to free-space photons and hybridize into polaritons for strong light-matter coupling. Longitudinal modes, in contrast, are seen as pure matter excitations that produce a dynamic polarization inside the material without photon coupling. Here we show that both longitudinal and transverse modes become polaritons in the explicitly covariant Lorenz gauge. Longitudinal excitations couple to longitudinal and scalar photons, which have been considered elusive so far. We show that the dipolar excitations become three-fold degenerate in the long-wavelength limit when including all photonic degrees of freedom, as expected from symmetry. Our findings demonstrate how choosing a gauge determines our thinking about materials excitations and how gauge fixing reveals new pathways for tailoring polaritons in crystals, metamaterials, and surfaces. Longitudinal polaritons will interact with longitudinal near fields located at surfaces, which provides additional excitation channels to engineer scanning near-field microscopy and surface-enhanced spectroscopy.

physics.optics↗

arXiv2023

Strong Coupling of Two-Dimensional Excitons and Plasmonic Photonic Crystals: Microscopic Theory Reveals Triplet Spectra

Monolayers of transition metal dichalcogenides (TMDC) are direct-gap semiconductors with strong light-matter interactions featuring tightly bound excitons, while plasmonic crystals (PCs), consisting of metal nanoparticles that act as meta-atoms, exhibit collective plasmon modes and allow one to tailor electric fields on the nanoscale. Recent experiments show that TMDC-PC hybrids can reach the strong-coupling limit between excitons and plasmons forming new quasiparticles, so-called plexcitons. To describe this coupling theoretically, we develop a self-consistent Maxwell-Bloch theory for TMDC-PC hybrid structures, which allows us to compute the scattered light in the near- and far-field explicitly and provide guidance for experimental studies. Our calculations reveal a spectral splitting signature of strong coupling of more than $100\,$meV in gold-MoSe$_2$ structures with $30\,$nm nanoparticles, manifesting in a hybridization of exciton and plasmon into two effective plexcitonic bands. In addition to the hybridized states, we find a remaining excitonic mode with significantly smaller coupling to the plasmonic near-field, emitting directly into the far-field. Thus, hybrid spectra in the strong coupling regime can contain three emission peaks.

cond-mat.mes-hall↗

arXiv2023

Collective States in Molecular Monolayers on 2D Materials

Collective excited states form in organic two-dimensional layers through the Coulomb coupling of the molecular transition dipole moments. They manifest as characteristic strong and narrow peaks in the excitation and emission spectra that are shifted to lower energies compared to the monomer transition. We study experimentally and theoretically how robust the collective states are against homogeneous and inhomogeneous broadening as well as spatial disorder that occur in real molecular monolayers. Using a microscopic model for a two-dimensional dipole lattice in real space we calculate the properties of collective states and their extinction spectra. We find that the collective states persist even for 1-10% random variation in the molecular position and in the transition frequency, with similar peak position and integrated intensity as for the perfectly ordered system. We measure the optical response of a monolayer of the perylene-derivative MePTCDI on two-dimensional materials. On the wide band-gap insulator hexagonal boron nitride it shows strong emission from the collective state with a line width that is dominated by the inhomogeneous broadening of the molecular state. When using the semimetal graphene as a substrate, however, the luminescence is completely quenched. By combining optical absorption, luminescence, and multi-wavelength Raman scattering we verify that the MePTCDI molecules form very similar collective monolayer states on hexagonal boron nitride and graphene substrates, but on graphene the line width is dominated by non-radiative excitation transfer from the molecules to the substrate. Our study highlights the transition from the localized molecular state of the monomer to a delocalized collective state in the two-dimensional molecular lattice that is entirely based on Coulomb coupling between optically active excitations of the electrons or molecular vibrations.

physics.atm-clus↗

arXiv2023

Nanomechanical absorption spectroscopy of 2D materials with femtowatt sensitivity

Nanomechanical spectroscopy (NMS) is a recently developed approach to determine optical absorption spectra of nanoscale materials via mechanical measurements. It is based on measuring changes in the resonance frequency of a membrane resonator vs. the photon energy of incoming light. This method is a direct measurement of absorption, which has practical advantages compared to common optical spectroscopy approaches. In the case of two-dimensional (2D) materials, NMS overcomes limitations inherent to conventional optical methods, such as the complications associated with measurements at high magnetic fields and low temperatures. In this work, we develop a protocol for NMS of 2D materials that yields two orders of magnitude improved sensitivity compared to previous approaches, while being simpler to use. To this end, we use electrical sample actuation, which simplifies the experiment and provides a reliable calibration for greater accuracy. Additionally, the use of low-stress silicon nitride membranes as our substrate reduces the noise-equivalent power to $NEP = 890 fW/\sqrt{Hz}$, comparable to commercial semiconductor photodetectors. We use our approach to spectroscopically characterize a two-dimensional transition metal dichalcogenide (WS$_2$), a layered magnetic semiconductor (CrPS$_4$), and a plasmonic supercrystal consisting of gold nanoparticles.

cond-mat.mes-hall↗

arXiv2022

Observation of multi-directional energy transfer in a hybrid plasmonic-excitonic nanostructure

Hybrid plasmonic devices involve a nanostructured metal supporting localized surface plasmons to amplify light-matter interaction, and a non-plasmonic material to functionalize charge excitations. Application-relevant epitaxial heterostructures, however, give rise to ballistic ultrafast dynamics that challenge the conventional semiclassical understanding of unidirectional nanometal-to-substrate energy transfer. We study epitaxial Au nanoislands on WSe$_2$ with time- and angle-resolved photoemission spectroscopy and femtosecond electron diffraction: this combination of techniques resolves material, energy and momentum of charge-carriers and phonons excited in the heterostructure. We observe a strong non-linear plasmon-exciton interaction that transfers the energy of sub-bandgap photons very efficiently to the semiconductor, leaving the metal cold until non-radiative exciton recombination heats the nanoparticles on hundreds of femtoseconds timescales. Our results resolve a multi-directional energy exchange on timescales shorter than the electronic thermalization of the nanometal. Electron-phonon coupling and diffusive charge-transfer determine the subsequent energy flow. This complex dynamics opens perspectives for optoelectronic and photocatalytic applications, while providing a constraining experimental testbed for state-of-the-art modelling.

cond-mat.mes-hall↗

arXiv2022

Light Control over Chirality Selective Functionalization of Substrate Supported Carbon Nanotubes

Diazonium reactions with carbon nanotubes form optical $sp^3$ defects that can be used in optical and electrical circuits. We investigate a direct on-device reaction supported by confined laser irradiation and present a technique where an arbitrary carbon nanotube can be preferentially functionalized within a device by matching the light frequency with its transition energy. An exemplary reaction was carried out between (9,7) nanotube and 4-bromobenzenediazonium tetrafluoroborate. The substrate supported nanotubes of multiple semiconducting chiralities were locally exposed to laser light while monitoring the reaction kinetics in-situ via Raman spectroscopy. The chiral selectivity of the reaction was confirmed by resonant Raman spectroscopy, reporting a 10 meV $E_{22}$ transition energy red-shift only of the targeted species. We further demonstrated this method on a single tube (9,7) electroluminescent device and show a 25 meV red-shifted emission of the ground state $E_{11}$ compared to the emission from the pristine tubes.

cond-mat.mes-hall↗

arXiv2022

Nanomechanical spectroscopy of 2D materials

We introduce a nanomechanical platform for fast and sensitive measurements of the spectrally-resolved optical dielectric function of 2D materials. At the heart of our approach is a suspended 2D material integrated into a nanomechanical resonator illuminated by a wavelength-tunable laser source. From the heating-related frequency shift of the resonator as well as its optical reflection measured as a function of photon energy, we obtain the real and imaginary parts of the dielectric function. Our measurements are unaffected by substrate-related screening and do not require any assumptions on the underling optical constants. This fast ($\tau_{rise}$ $\sim$ 135 ns), sensitive (noise-equivalent power = 90 $\frac{pW}{\sqrt{Hz}}$ ), and broadband (1.2 $-$ 3.1 eV, extendable to UV-THz) method provides an attractive alternative to spectroscopic or ellipsometric characterisation techniques.

cond-mat.mes-hall↗

arXiv2021

Synthesis of Multifunctional Charge Transfer Agents: Towards Single Walled Carbon Nanotubes with Defined Covalent Functionality and Preserved {\pi} System

The attachment of well-defined charge transfer agents to the surface of nanomaterials is an efficient strategy to control their charge density and also to tune their optical, electrical, and physicochemical properties. Particularly interesting are charge transfer agents that either donate or withdraw electrons depending on the arrangements of their building units and that promise a non-destructive attachment to delicate nanomaterials like sp$^2$ compounds. In this work, we rationally synthesize molecular moieties with versatile functionalities. A reactive anchor group allows to attach them to carbon nanotubes as defined charge transfer agents while preserving the tube $\pi$-conjugation. The charge transfer agents were synthesized through the stepwise nucleophilic substitution of either one (monosubstituted series) or two chlorine (disubstituted series) atoms of cyanuric chloride by aniline derivatives containing one, two or three methoxy groups in the para and meta positions. Variation in the number and position of methoxy as an electron transferring group help us to manipulate the electronic and optical properties of the molecular probes and their charge transfer to the SWNTs systematically. The correlation between the optical properties of these molecular probes and their functionality was investigated by experiments and quantum chemical calculations. While the optoelectronic properties of the conjugated charge transfer agents were dominated by the aniline segments, the triazine warrants the ability to nondestructively attach to the surface of SWNTs. This study is one step ahead towards the production of SWNTs with desired optical and electrical properties by covalent $\pi$-preserving functionalization.

cond-mat.mes-hall↗