Vasily Kravtsov

Plasmonic nanofocused four-wave mixing for femtosecond near-field imaging

Femtosecond nonlinear optical imaging with nanoscale spatial resolution would provide access to coupled degrees of freedom and ultrafast response functions on the characteristic length scales of electronic and vibrational excitations. Although near-field microscopy provides the desired spatial resolution, the design of a broadband high-contrast nanoprobe for ultrafast temporal resolution is challenging due to the inherently weak nonlinear optical signals generated in subwavelength volumes. Here, we demonstrate broadband four-wave mixing with enhanced nonlinear frequency conversion efficiency at the apex of a nanometre conical tip. Far-field light is coupled through a grating at the shaft of the tip, generating plasmons that propagate to the apex while undergoing asymptotic compression and amplification, resulting in a nonlinear conversion efficiency of up to 1 × 10(-5). We apply this nonlinear nanoprobe to image the few-femtosecond coherent dynamics of plasmonic hotspots on a nanostructured gold surface with spatial resolution of a few tens of nanometres. The approach can be generalized towards spatiotemporal imaging and control of coherent dynamics on the nanoscale, including the extension to multidimensional spectroscopy and imaging.

Hybrid Tip-Enhanced Nanospectroscopy and Nanoimaging 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, their physical properties are strongly influenced by nanoscale heterogeneities in the form of edges, twin boundaries, and nucleation sites. Using combined tip-enhanced Raman scattering and photoluminescence (PL) nanospectroscopy and nanoimaging, we study the associated effects on the excitonic properties in monolayer WSe2 grown by physical vapor deposition. With ∼15 nm spatial resolution, we resolve 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 spectroscopy and imaging with nanomechanical strain control 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.

Group delay and dispersion in adiabatic plasmonic nanofocusing

We study the decrease in group velocity of broadband surface plasmon polariton propagation on a conical tip, using femtosecond time-domain interferometry. The group delay of (9±3) fs measured corresponds to a group velocity at the apex of less than 0.2c. The result agrees in general with the prediction from adiabatic plasmonic nanofocusing theory, yet is sensitive with respect to the exact taper geometry near the apex. This, together with the sub 25 fs(2) second-order dispersion observed, provides the fundamental basis for the use of plasmons for broadband slow-light applications.

Nanofocused Plasmon-Driven Sub-10 fs 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 less than 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 μm tip–sample distance. On the basis of nume...

Variable-Temperature Tip-Enhanced Raman Spectroscopy of Single-Molecule Fluctuations and Dynamics

Structure, dynamics, and coupling involving single-molecules determine function in catalytic, electronic or biological systems. While vibrational spectroscopy provides insight into molecular structure, rapid fluctuations blur the molecular trajectory even in single-molecule spectroscopy, analogous to spatial averaging in measuring large ensembles. To gain insight into intramolecular coupling, substrate coupling, and dynamic processes, we use tip-enhanced Raman spectroscopy (TERS) at variable and cryogenic temperatures, to slow and control the motion of a single molecule. We resolve intrinsic line widths of individual normal modes, allowing detailed and quantitative investigation of the vibrational modes. From temperature dependent line narrowing and splitting, we quantify ultrafast vibrational dephasing, intramolecular coupling, and conformational heterogeneity. Through statistical correlation analysis of fluctuations of individual modes, we observe rotational motion and spectral fluctuations of the molecule. This work demonstrates single-molecule vibrational spectroscopy beyond chemical identification, opening the possibility for a complete picture of molecular motion ranging from femtoseconds to minutes.

Control of Plasmon Emission and Dynamics at the Transition from Classical to Quantum Coupling

With nanosecond radiative lifetimes, quenching dominates over enhancement for conventional fluorescence emitters near metal interfaces. We explore the fundamentally distinct behavior of photoluminescence (PL) with few-femtosecond radiative lifetimes of a coupled plasmonic emitter. Controlling the emitter-surface distance with subnanometer precision by combining atomic force and scanning tunneling distance control, we explore the unique behavior of plasmon dynamics at the transition from long-range classical resonant energy transfer to quantum coupling. Because of the ultrafast radiative plasmon emission, classical quenching is completely suppressed. Field-enhanced behavior dominates until the onset of quantum coupling dramatically reduces emission intensity and field enhancement, as verified in concomitant tip-enhanced Raman measurements. The entire distance behavior from tens of nanometers to subnanometers can be described using a phenomenological rate equation model and highlights the new degrees of freedom in radiation control enabled by an ultrafast radiative emitter near surfaces.

Femtosecond Near-Field Imaging with Plasmonic Nanofocused Four-Wave Mixing

Combining broadband plasmonic nano-focusing and femtosecond pulse-shaping we demonstrate spatio-temporal nano-imaging based on four-wave mixing, resolving the coherent electron dynamics at a rough Au edge with few-fs temporal and 10’s nm spatial resolution.