We demonstrate an essentially dispersion-free and diffraction-limited focusing of few-cycle laser pulses through all-reflective microscope objectives. By transmitting 6-fs-pulses from a Ti:sapphire oscillator through an all-reflective 0.5 NA objective, we reach a focus with a beam diameter of 1.0 μm, preserving the time structure of the pulses. The temporal and spatial pulse profile is recorded simultaneously using a novel tip-enhanced electron emission autocorrelator, indicating a focal volume of these pulses of only 1.8 μm^3. We anticipate that the demonstrated technique is of considerable interest for inducing and probing optical nonlinearities of individual nanostructures.

We compare single- and double-sided excitation methods of adiabatic surface plasmon polariton (SPP) wave superfocusing for scattering-type metallic near-field scanning optical microscopy (s-NSOM). Using the results of full 3D finite difference time domain analyses, the differences in field enhancement factors are explained and reveal the mode selectivity of a conical NSOM tip for adiabatic SPP superfocusing. Exploiting the mode-symmetric nature of the tip further, we also show that it is possible to selectively confine either the electric or magnetic field at the NSOM tip apex, by simply adjusting the relative phase between the SPP waves in the double-sided excitation approach.

The problem of highly nonlinear photoemission from a metal surface is considered using analytical and numerical approaches. Descriptions are found which cover both the weak-field and the strong-field regimes and the transition between them. The results of a time-dependent perturbation theory are in very good agreement with those from more numerically involved schemes, including a variational version of the Floquet method and a Crank-Nicolson-like numerical scheme. The implemented Crank-Nicolson variant uses transparent boundary conditions and an incident plane-wave state in the metal. Both numerical approaches give very similar results for weak and intermediate fields, while in the strong-field regime the Crank-Nicolson scheme is more effective than the Floquet method. We find an enhancement in the effective nonlinearity in the weak-field regime, which is caused by surface scattering of the final state. The presented theory also covers angular emission probabilities as a function of light intensity and explains an increase toward forward emission with growing field strength.

We explore imaging of local electromagnetic fields in the vicinity of metallic nanoparticles using a grating-coupled scattering-type near-field scanning optical microscope. In this microscope, propagating surface plasmon polariton wavepackets are launched onto smooth gold tapers where they are adiabatically focused toward the nanometer-sized taper apex. We report two-dimensional raster-scanned optical images showing pronounced near-field contrast and demonstrating sub-30 nm resolution imaging of localized surface plasmon polariton fields of spherical and elliptical nanoparticles. By comparison to three-dimensional finite-difference time domain simulations, we conclude that virtually background-free near-field imaging is achieved. The microscope combines deep subwavelength resolution, high local field intensities and a straightforward imaging contrast, making it interesting for a variety of applications in linear and nonlinear nanospectroscopy.

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.

Ultra-fast nano-optics is a comparatively young and
rapidly growing field of research aiming at probing, manipulating
and controlling ultrafast optical excitations on nanometer
length scales. This ability to control light on nanometric length
and femtosecond time scales opens up exciting possibilities for
probing dynamic processes in nanostructures in real time and
space. This article gives a brief introduction into the emerging research
field of ultrafast nano-optics and discusses recent progress
made in it. A particular emphasis is laid on the recent experimental
work performed in the authors’ laboratories. We specifically
discuss how ultrafast nano-optical techniques can be used to
probe and manipulate coherent optical excitations in individual
and dipole-coupled pairs of quantum dots, probe the dynamics
of surface plasmon polariton excitations in metallic nanostructures,
generate novel nanometer-sized ultrafast light and electron
sources and reveal the dipole interaction between excitons
and surface plasmon polaritons in hybrid metal-semiconductor
nanostructures. Our results indicate that such hybrid nanostructures
carry significant potential for realizing novel nano-optical
devices such as ultrafast nano-optical switches as well as surface
plasmon polariton amplifiers and lasers.
Two-dimensional finite difference time domain (FDTD) simulation
of the spatio-temporal evolution of a 10 fs light pulse at
a center wavelength of 810 nm propagating through a tapered,
perfectly conducting metal-coated fiber probe of 100 nm aperture
diameter. The field intensity |Ex(x, y, t)|2 is displayed on
a logarithmic intensity scale at four different instants in time.
After t =approx. 14 fs the pulse center reaches the aperture, generating
directly below it an ultra-short near-field spot of light.