Joanna M. Atkin

Light on the Tip of a Needle: Plasmonic Nanofocusing for Spectroscopy on the Nanoscale

The efficiency of plasmonic nanostructures as optical antennas to concentrate optical fields to the nanoscale has been limited by intrinsically short dephasing times and small absorption cross sections. We discuss a new optical antenna concept based on surface plasmon polariton (SPP) nanofocusing on conical noble metal tips to achieve efficient far- to near-field transformation of light from the micro- to the nanoscale. The spatial separation of the launching of propagating SPPs from their subsequent apex confinement with high energy concentration enables background-free near-field imaging, tip-enhanced Raman scattering, and nonlinear nanospectroscopy. The broad bandwidth and spectral tunability of the nanofocusing mechanism in combination with frequency domain pulse shaping uniquely allow for the spatial confinement of ultrashort laser pulses and few-femtosecond spatiotemporal optical control on the nanoscale. This technique not only extends powerful nonlinear and ultrafast spectroscopies to the nanoscale but can also generate fields of sufficient intensity for electron emission and higher harmonic generation.

Femtosecond Nanofocusing with Full Optical Waveform Control

The simultaneous nanometer spatial confinement and femtosecond temporal control of an optical excitation has been a long-standing challenge in optics. Previous approaches using surface plasmon polariton (SPP) resonant nanostructures or SPP waveguides have suffered from, for example, mode mismatch, or possible dependence on the phase of the driving laser field to achieve spatial localization. Here we take advantage of the intrinsic phase- and amplitude-independent nanofocusing ability of a conical noble metal tip with weak wavelength dependence over a broad bandwidth to achieve a 10 nm spatially and few-femtosecond temporally confined excitation. In combination with spectral pulse shaping and feedback on the second-harmonic response of the tip apex, we demonstrate deterministic arbitrary optical waveform control. In addition, the high efficiency of the nanofocusing tip provided by the continuous micro- to nanoscale mode transformation opens the door for spectroscopy of elementary optical excitations in matter on their natural length and time scales and enables applications from ultrafast nano-opto-electronics to single molecule quantum coherent control.

Nano-optical imaging and spectroscopy of order, phases, and domains in complex solids

The structure of our material world is characterized by a large hierarchy of length scales that determines material properties and functions. Increasing spatial resolution in optical imaging and spectroscopy has been a long standing desire, to provide access, in particular, to mesoscopic phenomena associated with phase separation, order, and intrinsic and extrinsic structural inhomogeneities. A general concept for the combination of optical spectroscopy with scanning probe microscopy emerged recently, extending the spatial resolution of optical imaging far beyond the diffraction limit. The optical antenna properties of a scanning probe tip and the local near-field coupling between its apex and a sample provide few-nanometer optical spatial resolution. With imaging mechanisms largely independent of wavelength, this concept is compatible with essentially any form of optical spectroscopy, including nonlinear and ultrafast techniques, over a wide frequency range from the terahertz to the extreme ultraviolet. The past 10 years have seen a rapid development of this nano-optical imaging technique, known as tip-enhanced or scattering-scanning near-field optical microscopy (s-SNOM). Its applicability has been demonstrated for the nano-scale investigation of a wide range of materials including biomolecular, polymer, plasmonic, semiconductor, and dielectric systems. We provide a general review of the development, fundamental imaging mechanisms, and different implementations of s-SNOM, and discuss its potential for providing nanoscale spectroscopic including femtosecond spatio-temporal information. We discuss possible near-field spectroscopic implementations, with contrast based on the metallic infrared Drude response, nano-scale impedance, infrared and Raman vibrational spectroscopy, phonon Raman nano-crystallography, and nonlinear optics to identify nanoscale phase separation (PS), strain, and ferroic order. With regard to applications, we focus on correlated and low-dimensional materials as examples that benefit, in particular, from the unique applicability of s-SNOM under variable and cryogenic temperatures, nearly arbitrary atmospheric conditions, controlled sample strain, and large electric and magnetic fields and currents. For example, in transition metal oxides, topological insulators, and graphene, unusual electronic, optical, magnetic, or mechanical properties emerge, such as colossal magneto-resistance (CMR), metal–insulator transitions (MITs), high-T C superconductivity, multiferroicity, and plasmon and phonon polaritons, with associated rich phase diagrams that are typically very sensitive to the above conditions. The interaction of charge, spin, orbital, and lattice degrees of freedom in correlated electron materials leads to frustration and degenerate ground states, with spatial PS over many orders of length scale. We discuss how the optical near-field response in s-SNOM allows for the systematic real space probing of multiple order parameters simultaneously under a wide range of internal and external stimuli (strain, magnetic field, photo-doping, etc.) by coupling directly to electronic, spin, phonon, optical, and polariton resonances in materials. In conclusion, we provide a perspective on the future extension of s-SNOM for multi-modal imaging with simultaneous nanometer spatial and femtosecond temporal resolution.

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.

Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2.

The insulator-metal transition (IMT) of vanadium dioxide (VO2) has remained a long-standing challenge in correlated electron physics since its discovery five decades ago. Most interpretations of experimental observations have implicitly assumed a homogeneous material response. Here we reveal inhomogeneous behaviour of even individual VO2 microcrystals using pump-probe microscopy and nanoimaging. The timescales of the ultrafast IMT vary from 40±8 fs, that is, shorter than a suggested phonon bottleneck, to 200±20 fs, uncorrelated with crystal size, transition temperature and initial insulating structural phase, with average value similar to results from polycrystalline thin-film studies. In combination with the observed sensitive variations in the thermal nanodomain IMT behaviour, this suggests that the IMT is highly susceptible to local changes in, for example, doping, defects and strain. Our results suggest an electronic mechanism dominating the photoinduced IMT, but also highlight the difficulty to deduce microscopic mechanisms when the true intrinsic material response is yet unclear.

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.

Photocurrent spectroscopy of low-k dielectric materials: Barrier heights and trap densities

Measurements of photoinduced current have been performed on thin films of porous low-k dielectric materials comprised of carbon-doped oxides. The dielectric films were deposited on silicon surfaces and prepared with a thin gold counterelectrode. From the spectral dependence of the photoinduced current, barrier heights for the dielectric∕silicon and dielectric∕gold interface were deduced. Transient currents were also found to flow after the photoexcitation was abruptly stopped. An estimate of the density of shallow electron traps within the low-k material was obtained from the measurement of the net charge transported from this detrapping current. A density of traps in the range of 6×1016traps∕cm3 was inferred for the low-k films, far exceeding that observed by the same technique for reference dielectric films of pure SiO2. This behavior was also compatible with photocurrent I‐V measurements on the low-k dielectric films and SiO2 reference sample.

Charge trapping at the low-k dielectric-silicon interface probed by the conductance and capacitance techniques

Trap states close to the interfaces in thin films of porous low-k dielectric materials are expected to affect interfacial barriers with contacts and consequently electrical leakage and reliability in these materials. These interfacial traps were investigated using capacitance and conductance measurements in metal/insulator/silicon capacitor structures composed of carbon-doped oxide low-k dielectric films with gold counterelectrodes. The measurements yielded information on the charge state of the low-k dielectric and an estimated density of traps near the Si interface of 2×1011 cm−2 eV−1, considerably greater than in typical SiO2 films. The effects of temperature and annealing were also investigated. An activation energy of 0.36±0.04 eV for trap filling and emptying was inferred.

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.