arXiv subjects

Markus B. Raschke

Publications and source records attributed to Markus B. Raschke.

At least 19 recordsLinked to original sources

arXiv2022

Observation of well-defined Kohn-anomaly in high-quality graphene devices at room temperature

Due to its ultra-thin nature, the study of graphene quantum optoelectronics, like gate-dependent graphene Raman properties, is obscured by interactions with substrates and surroundings. For instance, the use of doped silicon with a capping thermal oxide layer limited the observation to low temperatures of a well-defined Kohn-anomaly behavior, related to the breakdown of the adiabatic Born-Oppenheimer approximation. Here, we design an optoelectronic device consisting of single-layer graphene electrically contacted with thin graphite leads, seated on an atomically flat hexagonal boron nitride (hBN) substrate and gated with an ultra-thin gold (Au) layer. We show that this device is optically transparent, has no background optical peaks and photoluminescence from the device components, and no generation of laser-induced electrostatic doping (photodoping). This allows for room-temperature gate-dependent Raman spectroscopy effects that have only been observed at cryogenic temperatures so far, above all the Kohn-anomaly phonon energy normalization. The new device architecture by decoupling graphene optoelectronic properties from the substrate effects, allows for the observation of quantum phenomena at room temperature.

cond-mat.mes-hall↗

arXiv2021

Electron-phonon coupling in a magic-angle twisted-bilayer graphene device

The importance of phonons in the strong correlation phenomena observed in twisted bilayer graphene (TBG) at the so-called magic-angle is under debate. Here we apply gate-dependent micro-Raman spectroscopy to monitor the G band linewidth in TBG devices of twist angles $θ=0^{\circ}$, $\sim 1.1^{\circ}$ (magic-angle) and $\sim 7^{\circ}$ (large angle). The results show a broad and p/n-asymmetric doping behavior at the magic-angle, in clear contrast to the behavior observed in twist angles above and below. Atomistic modeling reproduces the experimental observations, revealing how the unique electronic structure of magic-angle TBGs influences the electron-phonon coupling and, consequently, the G band linewidth. Our findings indicate a connection between electron-phonon coupling and experimental observations of strongly correlated phenomena in magic-angle TBG.

cond-mat.mes-hall↗

arXiv2021

Semi-empirical Quantum Optics for Mid-Infrared Molecular Nanophotonics

Nanoscale infrared (IR) resonators with sub-diffraction limited mode volumes and open geometries have emerged as new platforms for implementing cavity quantum electrodynamics (QED) at room temperature. The use of infrared (IR) nano-antennas and tip nanoprobes to study strong light-matter coupling of molecular vibrations with the vacuum field can be exploited for IR quantum control with nanometer and femtosecond resolution. In order to accelerate the development of molecule-based quantum nano-photonic devices in the mid-IR, we develop a generally applicable semi-empirical quantum optics approach to describe light-matter interaction in systems driven by mid-IR femtosecond laser pulses. The theory is shown to reproduce recent experiments on the acceleration of the vibrational relaxation rate in infrared nanostructures, and also provide physical insights for the implementation of coherent phase rotations of the near-field using broadband nanotips. We then apply the quantum framework to develop general tip-design rules for the experimental manipulation of vibrational strong coupling and Fano interference effects in open infrared resonators. We finally propose the possibility of transferring the natural anharmonicity of molecular vibrational levels to the resonator near-field in the weak coupling regime, to implement intensity-dependent phase shifts of the coupled system response with strong pulses. Our semi-empirical quantum theory is equivalent to first-principles techniques based on Maxwell's equations, but its lower computational cost suggests its use a rapid design tool for the development of strongly-coupled infrared nanophotonic hardware for applications ranging from quantum control of materials to quantum information processing.

physics.optics↗

arXiv2021

Electric Field Control of Chirality

Polar textures have attracted significant attention in recent years as a promising analog to spin-based textures in ferromagnets. Here, using optical second harmonic generation based circular dichroism, we demonstrate deterministic and reversible control of chirality over mesoscale regions in ferroelectric vortices using an applied electric field. The microscopic origins of the chirality, the pathway during the switching, and the mechanism for electric-field control are described theoretically via phase-field modeling and second-principles simulations, and experimentally by examination of the microscopic response of the vortices under an applied field. The emergence of chirality from the combination of non-chiral materials and subsequent control of the handedness with an electric field has far-reaching implications for new electronics based on chirality as a field controllable order parameter.

cond-mat.mtrl-sci↗

arXiv2020

Adaptive tip-enhanced nano-spectroscopy

Tip-enhanced nano-spectroscopy and -imaging, such as tip-enhanced photoluminescence (TEPL), tip-enhanced Raman spectroscopy (TERS), and others, have become indispensable from materials science to single molecule studies. However, the techniques suffer from inconsistent performance due to lack of nanoscale control of tip apex structure, which often leads to irreproducible spectral, spatial, and polarization resolved imaging. Instead of refining tip-fabrication to resolve this problem, we pursue the inverse approach of optimizing the nano-optical vector-field at the tip apex via adaptive optics. Specifically, we demonstrate dynamic wavefront shaping of the excitation field to effectively couple light to the tip and adaptively control for enhanced sensitivity and polarization-controlled TEPL and TERS, with performance exceeding what can be achieved by conventional tip-fabrication and optimal excitation polarization. Employing a sequence feedback algorithm, we achieve 4.4$\times$10$^4$-fold TEPL enhancement of a WSe$_2$ monolayer which is >2$\times$ larger than the normal TEPL intensity without wavefront shaping, as well as the largest plasmon-enhanced PL intensity of a transition metal dichalcogenide (TMD) monolayer reported to date. In addition, with dynamical near-field polarization control in TERS, we demonstrate the investigation of conformational heterogeneity of brilliant cresyl blue (BCB) molecules as well as the controllable observation of IR-active modes due to a large gradient field effect. Adaptive tip-enhanced spectroscopy and imaging thus provides for a new systematic approach towards computational nanoscopy making optical nano-imaging more robust, versatile, and widely deployable.

physics.optics↗

arXiv2020

Radiative control of localized excitons at room temperature with an ultracompact tip-enhanced plasmonic nano-cavity

In atomically thin semiconductors, localized exciton (X$_L$) coupled to light shows single quantum emitting behaviors through radiative relaxation processes providing a new class of optical sources for potential applications in quantum communication. In most studies, however, X$_L$ photoluminescence (PL) from crystal defects has mainly been observed in cryogenic conditions because of their sub-wavelength emission region and low quantum yield at room temperature. Furthermore, engineering the radiative relaxation properties, e.g., emission region, intensity, and energy, remained challenging. Here, we present a plasmonic antenna with a triple-sharp-tips geometry to induce and control the X$_L$ emission of a WSe$_2$ monolayer (ML) at room temperature. By placing a ML crystal on the two sharp Au tips in a bowtie antenna fabricated through cascade domino lithography with a radius of curvature of <1 nm, we effectively induce tensile strain in the nanoscale region to create robust X$_L$ states. An Au tip with tip-enhanced photoluminescence (TEPL) spectroscopy is then added to the strained region to probe and control the X$_L$ emission. With TEPL enhancement of X$_L$ as high as ~10$^6$ in the triple-sharp-tips device, experimental results demonstrate the controllable X$_L$ emission in <30 nm area with a PL energy shift up to 40 meV, resolved by tip-enhanced PL and Raman imaging with <15 nm spatial resolution. Our approach provides a systematic way to control localized quantum light in 2D semiconductors offering new strategies for active quantum nano-optical devices.

physics.optics↗

arXiv2020

Ultrastrong plasmon-phonon coupling via epsilon-near-zero nanocavities

Vibrational ultrastrong coupling (USC), where the light-matter coupling strength is comparable to the vibrational frequency of molecules, presents new opportunities to probe the interactions of molecules with zero-point fluctuations, harness cavity-enhanced chemical reactions, and develop novel devices in the mid-infrared regime. Here we use epsilon-near-zero nanocavities filled with a model polar medium (SiO$_2$) to demonstrate USC between phonons and gap plasmons. We present classical and quantum mechanical models to quantitatively describe the observed plasmon-phonon USC phenomena and demonstrate a splitting of up to 50% of the resonant frequency. Our wafer-scale nanocavity platform will enable a broad range of vibrational transitions to be harnessed for USC applications.

physics.optics↗

arXiv2019

Tip-enhanced strong coupling spectroscopy, imaging, and control of a single quantum emitter

Optical cavities can enhance and control light-matter interactions. This has recently been extended to the nanoscale, and with single emitter strong coupling regime even at room temperature using plasmonic nano-cavities with deep sub-diffraction-limited mode volumes. However, with emitters in static nano-cavities, this limits the ability to tune coupling strength or to couple different emitters to the same cavity. Here, we present tip-enhanced strong coupling (TESC) spectroscopy, imaging, and control. Based on a nano-cavity formed between a scanning plasmonic antenna-tip and the substrate, by reversibly and dynamically addressing single quantum dots (QDs) we observe mode splitting > 160 meV and anticrossing over a detuning range of ~100 meV, and with sub-nm precision control over the mode volume in the ~1000 nm^3 regime. Our approach, as a new paradigm of nano-cavity quantum-electrodynamics near-field microscopy to induce, probe, and control single-emitter plasmon hybrid quantum states, opens new pathways from opto-electronics to quantum information science.

physics.optics↗

arXiv2018

Mechanism of Electric Power Generation from Ionic Droplet Motion on Polymer Supported Graphene

Graphene-based electric power generation that converts mechanical energy of flow of ionic droplets over the device surface into electricity has emerged as promising candidate for a blue-energy network. Yet the lack of a microscopic understanding of the underlying mechanism has prevented ability to optimize and control the performance of such devices. This requires information on interfacial structure and charging behavior at the molecular level. Here, we use sum-frequency vibrational spectroscopy (SFVS) to probe the interfaces of devices composed of aqueous solution, graphene and supporting polymer substrate. We discover that the surface dipole layer of the polymer is responsible for ion attraction toward and adsorption at the graphene surface that leads to electricity generation in graphene. Graphene itself does not attract ions and only acts as a conducting sheet for the induced carrier transport. Replacing the polymer by an organic ferroelectric substrate could enhance the efficiency and allow switching of the electricity generation. Our microscopic understanding of the electricity generation process paves the way for the rational design of scalable and more efficient droplet-motion-based energy transducer devices.

physics.app-ph↗

arXiv2018

Polarization control with plasmonic antenna-tips: A universal approach for optical nano-crystallography and vector-field imaging

Controlling the propagation and polarization vectors in linear and nonlinear optical spectroscopy enables to probe the anisotropy of optical responses providing structural symmetry selective contrast in optical imaging. Here we present a novel tilted antenna-tip approach to control the optical vector-field by breaking the axial symmetry of the nano-probe in tip-enhanced near-field microscopy. This gives rise to a localized plasmonic antenna effect with significantly enhanced optical field vectors with control of both \textit{in-plane} and \textit{out-of-plane} components. We use the resulting vector-field specificity in the symmetry selective nonlinear optical response of second-harmonic generation (SHG) for a generalized approach to optical nano-crystallography and -imaging. In tip-enhanced SHG imaging of monolayer MoS$_2$ films and single-crystalline ferroelectric YMnO$_3$, we reveal nano-crystallographic details of domain boundaries and domain topology with enhanced sensitivity and nanoscale spatial resolution. The approach is applicable to any anisotropic linear and nonlinear optical response, and provides for optical nano-crystallographic imaging of molecular or quantum materials.

physics.optics↗

arXiv2017

Enhanced third-order optical nonlinearity driven by surface-plasmon field gradients

Achieving efficient nonlinear optical frequency conversion in small volumes is key for future on-chip photonic devices that would provide a higher-speed alternative to modern electronics. However, the already intrinsically low conversion efficiency severely limits miniaturization to nanoscale dimensions. Here we demonstrate that gradient-field effects can provide for an efficient, conventionally dipole-forbidden nonlinear response, offering a new approach for enhanced nonlinear optics in nanostructures. We show that a {\em longitudinal} nonlinear source current can dominate the third-order optical nonlinearity of the free electron response in gold in the technologically important near-IR frequency range where the nonlinearities due to other mechanisms are particularly small. Using adiabatic nanofocusing to spatially confine the excitation fields, from measurements of the $2ω_1 - ω_2$ four-wave mixing response as a function of detuning $ω_1 - ω_2$, we find up to $10^{-5}$ conversion efficiency with a gradient field contribution to $χ^{(3)}_{\mathrm{Au}}$ of up to $10^{-19}~\mathrm{m}^2 / \mathrm{V}^2$. The results are in good agreement with theory based on plasma hydrodynamics. Our results demonstrate an increase in nonlinear conversion efficiency with decreasing sample size that can offset and even overcompensate the volume decrease of conventional dipolar pathways. This will enable more efficient nonlinear optical devices and frequency converters and facilitate the extension of coherent multidimensional spectroscopies to the nanoscale.

cond-mat.mes-hall↗

arXiv2017

Observation of Diode Behavior and Gate Voltage Control of Hybrid Plasmon-Phonon Polaritons in Graphene-Hexagonal Boron Nitride Heterostructures

Light-matter interaction in two-dimension photonic materials allows for confinement and control of free-space radiation on sub-wavelength scales. Most notably, the van der Waals heterostructure obtained by stacking graphene (G) and hexagonal Boron Nitride (hBN) can provide for hybrid hyperbolic plasmon phonon-polaritons (HP3). Here, we present a polariton diode effect and low-bias control of HP3 modes confined in G-hBN. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP3 waves over both hBN Reststrahlen bands and observe the unidirectional propagation of HP3 modes at in-plane heterointerfaces associated with the transition between different substrate dielectrics. By electric gating we further control the HP3 hybridization modifying the coupling between the continuum graphene plasmons and the discrete hyperbolic phonon polaritons of hBN as described by an extended Fano model. This is the first demonstration of unidirectional control of polariton propagation, with break in reflection/transmission symmetry for HP3 modes. G-hBN and related hyperbolic metamaterial nanostructures can therefore provide the basis for novel logic devices of on-chip nano-optics communication and computing.

cond-mat.mes-hall↗

arXiv2017

Radiative control of dark excitons at room temperature by nano-optical antenna-tip induced Purcell effect

Excitons, Coulomb-bound electron and hole pairs, are elementary photo-excitations in semiconductors, that can couple directly to light through radiative relaxation. In contrast to these bright excitons, dark excitons X$\rm{_D}$ with anti-parallel electron spin polarization exist, with generally forbidden radiative emission. Because of their associated long lifetimes, these dark excitons are appealing candidates for quantum computing and opto-electronic devices. However, optical read-out and control of X$\rm{_D}$ states have remained a major challenge due to their decoupling from light. Here, we present a novel tip-enhanced nano-optical approach to precisely switch and programmably modulate the X$\rm{_D}$ emission even at room temperature. Using monolayer two-dimensional transition metal dichalcogenide (TMD) WSe$\rm{_2}$ on a gold film as model system, we demonstrate ${\sim} 6 \times 10^5$-fold enhancement in dark exciton photoluminescence quantum yield. This is achieved by the unique coupling of the nano-optical antenna-tip to the dark exciton \textit{out-of-plane} optical dipole moment, with an extraordinary Purcell factor of $\ge 2 \times 10^3$ of the tip-sample nano-cavity. Compared to the necessity of cryogenic temperatures and high magnetic fields in conventional approaches, our work provides a new way to harness excitonic properties in low-dimensional semiconductors and new strategies for quantum opto-electronic devices.

cond-mat.mtrl-sci↗

arXiv2016

A nanofocused plasmon-driven sub-10 femtosecond electron point source

Progress in ultrafast electron microscopy relies on the development of efficient laser-driven electron sources delivering femtosecond electron pulses to the sample. In particular, recent advances employ photoemission from metal nanotips as coherent point-like femtosecond low-energy electron sources. We report the nonlinear emission of ultrashort electron wave packets from a gold nanotip generated by nonlocal excitation and nanofocusing of surface plasmon polaritons. We verify the nanoscale localization of plasmon-induced electron emission by its electrostatic collimation characteristics. With a plasmon polariton pulse duration below 8 fs at the apex, we identify multiphoton photoemission as the underlying emission process. The quantum efficiency of the plasmon-induced emission exceeds that of photoemission from direct apex illumination. We demonstrate the application for plasmon-triggered point-projection imaging of an individual semiconductor nanowire at 3 $μ$m tip-sample distance. Based on numerical simulations we estimate an electron pulse duration at the sample below 10 fs for tip-sample distances up to few micrometers. Plasmon-driven nanolocalized electron emission thus enables femtosecond point-projection microscopy with unprecedented temporal and spatial resolution, femtosecond low-energy electron in-line holography, and opens a new route towards femtosecond scanning tunneling microscopy and spectroscopy.

cond-mat.mes-hall↗

arXiv2016

Hybrid tip-enhanced nano-spectroscopy and -imaging of monolayer WSe2 with local strain control

Many classes of two-dimensional (2D) materials have emerged as potential platforms for novel electronic and optical devices. However, the physical properties are strongly influenced by nanoscale heterogeneities in the form of edges, grain boundaries, and nucleation sites. Using combined tip-enhanced Raman scattering (TERS) and photoluminescence (TEPL) nano-spectroscopy and -imaging, we study the associated effects on the excitonic properties in monolayer WSe2 grown by physical vapor deposition (PVD). With <15 nm spatial resolution we resolve nonlocal nanoscale correlations of PL spectral intensity and shifts with crystal edges and internal twin boundaries associated with the expected exciton diffusion length. Through an active atomic force tip interaction we can control the crystal strain on the nanoscale, and tune the local bandgap in reversible (up to 24 meV shift) and irreversible (up to 48 meV shift) fashion. This allows us to distinguish the effect of strain from the dominant influence of defects on the PL modification at the different structural heterogeneities. Hybrid nano-optical and nano-mechanical imaging and spectroscopy thus enables the systematic study of the coupling of structural and mechanical degrees of freedom to the nanoscale electronic and optical properties in layered 2D materials.

physics.optics↗

arXiv2015

Optical gradient force nano-imaging and -spectroscopy

Nanoscale forces play an important role in different scanning probe microscopies, most notably atomic force microscopy (AFM). In contrast, in scanning near-field optical microscopy (SNOM) a light-induced coupled local optical polarization between tip and sample is typically detected by scattering to the far field. Measurements of the optical gradient force associated with that optical near-field excitation would offer a novel optical scanning probe modality. Here we provide a generalized theory of optical gradient force nano-imaging and -spectroscopy. We quantify magnitude and distance dependence of the optical gradient force and its spectral response. We show that the optical gradient force is dispersive for single particle electronic and vibrational resonances, distinct from recent claims of its experimental observation. In contrast, the force can be absorptive for collective resonances. We provide a guidance for its measurements and distinction from competing processes such as thermal expansion.

cond-mat.mes-hall↗

arXiv2015

Phase transition in bulk single crystals and thin films of VO2 by nano-infrared spectroscopy and imaging

We have systematically studied a variety of vanadium dioxide (VO2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO2 with sub-grain-size spatial resolution (~20 nm), we show that epitaxial strain in VO2 thin films not only triggers spontaneous local phase separations but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain and symmetry dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches.

cond-mat.str-el↗

arXiv2015

Amplitude- and phase-resolved nano-spectral imaging of phonon polaritons in hexagonal boron nitride

Phonon polaritons are quasiparticles resulting from strong coupling of photons with optical phonons. Excitation and control of these quasiparticles in 2D materials offer the opportunity to confine and transport light at the nanoscale. Here, we image the phonon polariton (PhP) spectral response in thin hexagonal boron nitride (hBN) crystals as a representative 2D material using amplitude- and phase-resolved near-field interferometry with broadband mid-IR synchrotron radiation. The large spectral bandwidth enables the simultaneous measurement of both out-of-plane (780 cm-1) and in-plane (1370 cm-1) hBN phonon modes. In contrast to the strong and dispersive in-plane mode, the out-of-plane mode PhP response is weak. Measurements of the PhP wavelength reveal a proportional dependence on sample thickness for thin hBN flakes, which can be understood by a general model describing two-dimensional polariton excitation in ultrathin materials.

cond-mat.mes-hall↗