Daniel R. Solli

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.

Fast light, slow light, and phase singularities: a connection to generalized weak values.

We demonstrate that Aharonov-Albert-Vaidman weak values have a direct relationship with the response function of a system, and have a much wider range of applicability in both the classical and quantum domains than previously thought. Using this idea, we have built an optical system, based on a birefringent photonic crystal, with an infinite number of weak values. In this system, the propagation speed of a polarized light pulse displays both superluminal and slow light behavior with a sharp transition between the two regimes. We show that this systems response possesses two-dimensional, vortex-antivortex phase singularities. Important consequences for optical signal processing are discussed.

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.

Stimulated supercontinuum generation extends broadening limits in silicon

We demonstrate that stimulated supercontinuum generation alleviates restrictions on spectral broadening in silicon waveguides. At telecommunications wavelengths, two-photon and free-carrier absorption typically deplete the pump before large broadening factors can be achieved. However, broadening via modulation instability (MI) can be enhanced by seeding, which also substantially improves the energy efficiency of spectral broadening in media with nonlinear loss. Coherent seeding also generates a stable output spectrum, in contrast to conventional approaches where broadening starts from noise. The combination of self-phase modulation and stimulated modulation instability generates broadening factors in excess of 40-fold at moderate intensity levels, with >15-times better energy efficiency.

Real-time detection of soliton interactions in a few-cycle femtosecond oscillator

Mode-locked lasers are a paramount example of dissipative nonlinear systems that support bound-states of multiple sohtons [1, 2]. However, rapid dynamic interactions between sohton complexes with pico-to femtosecond separation are mostly inaccessible to standard laser characterization techniques, including scanning interferometric autocorrelation, temporally-averaging spectroscopy or single-shot measurements at low repetition rates.

The start of femtosecond mode-locking and transient soliton dynamics captured with real-time spectroscopy

We resolve the startup of femtosecond Kerr-lens mode-locking with real-time spectroscopy at 90MHz frame-rate and identify dynamics that are masked in time-averaged acquisition. Specifically, we observe nonlinear pulse beatings and transient stages of multi-soliton operation.

Strong-Field Photoemission from Metallic Nanotips

Nonlinear photoemission from single nanostructures is investigated over a broad wavelength range in the near- and mid-infrared. The field enhancement at a nanotip apex and mid-infrared excitation enable sub-cycle dynamics, where electrons are ejected from the near-field within a fraction of the optical half-cycle. The implications of this field-driven acceleration are studied via photoemission spectrocopy and numerical simulations.

Controlling ultrafast photoelectron dynamics in nanostructure-enhanced THz fields

We present experimental results on a nanostructure streaking scheme employing carrier-envelope-phase-stable THz pulses to control ultrafast photoemission. The tip-enhanced local THz-field allows for field-driven electron dynamics and effective manipulation of trajectories and kinetic energy spectra.

Ultrafast Single-Shot Measurements in Modulation Instability and Supercontinuum

The real-time measurement of ultrafast noisy processes is challenging because it requires single-shot resolution, broadband fidelity and long-record length. It is especially difficult to measure fluctuations in the optical supercontinuum, a white light source that can span over an octave in bandwidth.

Mid-infrared Photoelectron Emission and Acceleration at Metallic Nanotips

We present localized photoemission from metallic nanotips using few-cycle pulses at near- and mid-infrared wavelengths ranging from 0.8-8µm. Photoelectron energies up to hundreds of eV are observed, and a sub-cycle acceleration regime is reached.