Structural dynamics of incommensurate charge-density waves tracked by ultrafast low-energy electron diffraction.

We study the non-equilibrium structural dynamics of the incommensurate and nearly commensurate charge-density wave (CDW) phases in 1T- TaS 2 . Employing ultrafast low-energy electron diffraction with 1 ps temporal resolution, we investigate the ultrafast quench and recovery of the CDW-coupled periodic lattice distortion (PLD). Sequential structural relaxation processes are observed by tracking the intensities of main lattice as well as satellite diffraction peaks and the diffuse scattering background. Comparing distinct groups of diffraction peaks, we disentangle the ultrafast quench of the PLD amplitude from phonon-related reductions of the diffraction intensity. Fluence-dependent relaxation cycles reveal a long-lived partial suppression of the order parameter for up to 60 ps, far outlasting the initial amplitude recovery and electron-phonon scattering times. This delayed return to a quasi-thermal level is controlled by lattice thermalization and coincides with the population of zone-center acoustic modes, as evidenced by a structured diffuse background. The long-lived non-equilibrium order parameter suppression suggests hot populations of CDW-coupled lattice modes. Finally, a broadening of the superlattice peaks is observed at high fluences, pointing to a non-linear generation of phase fluctuations.

Real-time spectral interferometry probes the internal dynamics of femtosecond soliton molecules.

Solitons, particle-like excitations ubiquitous in many fields of physics, have been shown to exhibit bound states akin to molecules. The formation of such temporal soliton bound states and their internal dynamics have escaped direct experimental observation. By means of an emerging time-stretch technique, we resolve the evolution of femtosecond soliton molecules in the cavity of a few-cycle mode-locked laser. We track two- and three-soliton bound states over hundreds of thousands of consecutive cavity roundtrips, identifying fixed points and periodic and aperiodic molecular orbits. A class of trajectories acquires a path-dependent geometrical phase, implying that its dynamics may be topologically protected. These findings highlight the importance of real-time detection in resolving interactions in complex nonlinear systems, including the dynamics of soliton bound states, breathers, and rogue waves.

Structural and magnetic characterization of large area, free-standing thin films of magnetic ion intercalated dichalcogenides Mn0.25TaS2 and Fe0.25TaS2.

Free-standing thin films of magnetic ion intercalated transition metal dichalcogenides are produced using ultramicrotoming techniques. Films of thicknesses ranging from 30 nm to 250 nm were achieved and characterized using transmission electron diffraction and x-ray magnetic circular dichroism. Diffraction measurements visualize the long range crystallographic ordering of the intercalated ions, while the dichroism measurements directly assess the orbital contributions to the total magnetic moment. We thus verify the unquenched orbital moment in Fe0.25TaS2 and measure the fully quenched orbital contribution in Mn0.25TaS2. Such films can be used in a wide variety of ultrafast x-ray and electron techniques that benefit from transmission geometries, and allow measurements of ultrafast structural, electronic, and magnetization dynamics in space and time.

Field-driven photoemission from nanostructures quenches the quiver motion.

Strong-field physics, an extreme limit of light-matter interaction, is expanding into the realm of surfaces and nanostructures from its origin in atomic and molecular science. The attraction of nanostructures lies in two intimately connected features: local intensity enhancement and sub-wavelength confinement of optical fields. Local intensity enhancement facilitates access to the strong-field regime and has already sparked various applications, whereas spatial localization has the potential to generate strong-field dynamics exclusive to nanostructures. However, the observation of features unattainable in gaseous media is challenged by many-body effects and material damage, which arise under intense illumination of dense systems. Here, we non-destructively access this regime in the solid state by employing single plasmonic nanotips and few-cycle mid-infrared pulses, making use of the wavelength-dependence of the interaction, that is, the ponderomotive energy. We investigate strong-field photoelectron emission and acceleration from single nanostructures over a broad spectral range, and find kinetic energies of hundreds of electronvolts. We observe the transition to a new regime in strong-field dynamics, in which the electrons escape the nanolocalized field within a fraction of an optical half-cycle. The transition into this regime, characterized by a spatial adiabaticity parameter, would require relativistic electrons in the absence of nanostructures. These results establish new degrees of freedom for the manipulation and control of electron dynamics on femtosecond and attosecond timescales, combining optical near-fields and nanoscopic sources.

Tip-enhanced strong-field photoemission.

Nonlinear photoelectron emission from metallic nanotips is explored in the strong-field regime. The passage between the multiphoton and the optical field emission regimes is clearly identified. The experimental observations are in agreement with a quantum mechanical strong-field model.

Seeded supercontinuum generation with optical parametric down-conversion.

The transition between modulation instability gain and induced soliton fission in nonlinear fiber is experimentally investigated by coherent seeding with the two-color output of an optical parametric oscillator. This approach produces supercontinuum spectra displaying persistent, fine modulation from seeding-induced noise reduction. Numerical simulations support the findings.

Active control of rogue waves for stimulated supercontinuum generation.

We demonstrate that rogue waves provide a powerful tool to actively control a nonlinear system with minimal effort. Specifically, optical rogue waves--rare, bright flashes of broadband light arising in subthreshold supercontinuum generation--are initiated by an exceedingly weak stimulus. Using this effect, we produce an optically switchable, ultrastable, and bright supercontinuum with greatly enhanced coherence.

Broadband optical near-field microscope for nanoscale absorption spectroscopy of organic materials.

Conjugated organic materials in the solid state are generally amorphous or polycrystalline, with local order only achieved in mesoscopic domains with size ranging from a few tens to a few hundreds of nanometres. Understanding the interplay between mesoscopic order and macroscopic behaviour of these materials calls for a spatially resolved study of their optical properties. Near-field scanning optical microscopy allows one in principle to beat the diffraction limit in optical imaging. A quantitative measurement of nanoscale absorption spectra is, however, complicated by the difficulty of obtaining broadband near-field illumination with sufficiently high intensity. Here we demonstrate a near-field spectrometer with 100-nm spatial resolution based on an ultrabroadband Ti : sapphire oscillator coupled to an aperture-based near-field scanning optical microscopy, enabling structural phase-selective imaging of organic materials at the nanoscale. In polycrystalline phtalocyanine films we can distinguish between the crystalline and the amorphous phase, thus providing previously unavailable information on their mesoscopic texture.

Optical rogue waves.

Recent observations show that the probability of encountering an extremely large rogue wave in the open ocean is much larger than expected from ordinary wave-amplitude statistics. Although considerable effort has been directed towards understanding the physics behind these mysterious and potentially destructive events, the complete picture remains uncertain. Furthermore, rogue waves have not yet been observed in other physical systems. Here, we introduce the concept of optical rogue waves, a counterpart of the infamous rare water waves. Using a new real-time detection technique, we study a system that exposes extremely steep, large waves as rare outcomes from an almost identically prepared initial population of waves. Specifically, we report the observation of rogue waves in an optical system, based on a microstructured optical fibre, near the threshold of soliton-fission supercontinuum generation--a noise-sensitive nonlinear process in which extremely broadband radiation is generated from a narrowband input. We model the generation of these rogue waves using the generalized nonlinear Schrödinger equation and demonstrate that they arise infrequently from initially smooth pulses owing to power transfer seeded by a small noise perturbation.

Grating-coupling of surface plasmons onto metallic tips: a nanoconfined light source.

We describe and demonstrate a new nanometer-scale broadband light source. It is based on the grating-coupled excitation of surface plasmon polaritons (SPPs) on the shaft of a sharp conical metal taper with a tip radius of few tens of nanometers. Far-field excitation of linear nanoslit gratings results in the resonant generation of SPPs traveling over more than 10 mum to the tip apex and converging to an intense radiative local light spot. Such nanofabricated tips are expected to find various applications in nanospectroscopy, overcoming problems with background illumination in apertureless microscopy.

Localized multiphoton emission of femtosecond electron pulses from metal nanotips.

Intense multiphoton electron emission is observed from sharp (approximately 20 nm radius) metallic tips illuminated with weak 100-pJ, 7-fs light pulses. Local field enhancement, evidenced by concurrent nonlinear light generation, confines the emission to the tip apex. Electrons are emitted from a highly excited nonequilibrium carrier distribution, resulting in a marked change of the absolute electron flux and its dependence on optical power with the tip bias voltage. The strong optical nonlinearity of the electron emission allows us to image the local optical field near a metallic nanostructure with a spatial resolution of a few tens of nanometers in a novel tip-enhanced electron emission microscope.

Femtosecond light transmission and subradiant damping in plasmonic crystals.

We report the first observation of subradiance in plasmonic nanocrystals. Amplitude- and phase-resolved ultrafast transmission experiments directly reveal the coherent coupling between surface plasmon polaritons (SPPs) induced by periodic variations in the dielectric function. This interaction results in the formation of plasmonic band gaps and coupled SPP eigenmodes with different symmetries, as directly shown by near-field imaging. In antisymmetric modes, radiative SPP damping is strongly suppressed, increasing the SPP lifetime from 30 fs to more than 200 fs. The findings are analyzed within a coupled resonance model.

Demonstration of superluminal effects in an absorptionless, nonreflective system.

We present an experimental and theoretical study of a simple, passive system consisting of a birefringent, two-dimensional photonic crystal and a polarizer in series, and show that superluminal dispersive effects can arise even though no incident radiation is absorbed or reflected. We demonstrate that a vector formulation of the Kramers-Kronig dispersion relations facilitates an understanding of these counterintuitive effects.