We demonstrate strong-field photoelectron emission from gold nanorods driven by femtosecond mid-infrared optical pulses. The maximum photoelectron yield is reached at the localized surface plasmon resonance, indicating that the photoemission is governed by the resonantly-enhanced optical near-field. The wavelength- and field-dependent photoemission yield allows for a noninvasive determination of local field enhancements, and we obtain intensity enhancement factors close to 1300, in good agreement with finite-difference time domain computations.

The active control of matter by strong electromagnetic fields is of growing importance, with applications all across the optical spectrum from the extreme-ultraviolet to the far-infrared. In recent years, phase-stable terahertz fields have shown tremendous potential for observing and manipulating elementary excitations in solids1, 2, 3. In the gas phase, on the other hand, driving free charges with terahertz transients provides insight into ultrafast ionization dynamics4, 5. Developing such approaches for locally enhanced terahertz fields in nanostructures will create new means to govern electron currents on the nanoscale. Here, we use single-cycle terahertz transients to demonstrate extensive control over nanotip photoelectron emission. The terahertz near-field is shown to either enhance or suppress photocurrents, with the tip acting as an ultrafast rectifying diode6. We record phase-resolved sub-cycle dynamics and find spectral compression and expansion arising from electron propagation within the terahertz near-field. These interactions produce rich spectro-temporal features and offer unprecedented control over ultrashort free electron pulses for imaging and diffraction.

We demonstrate ultrafast terahertz (THz) field emission from a tungsten nanotip enabled by local field enhancement. Characteristic electron spectra which result from acceleration in the THz near-field are found. Employing a dual frequency pump–probe scheme, we temporally resolve different nonlinear photoemission processes induced by coupling near-infrared (NIR) and THz pulses. In the order of increasing THz field strength, we observe THz streaking, THz-induced barrier reduction (Schottky effect) and THz field emission. At intense NIR-excitation, the THz field emission is used as an ultrashort, local probe of hot electron dynamics in the apex. A first application of this scheme indicates a decreased carrier cooling rate in the confined tip geometry. Summarizing the results at various excitation conditions, we present a comprehensive picture of the distinct regimes in ultrafast photoemission in the near- and far-infrared.

This article presents a quantum mechanical treatment of strong-field photoelectron emission from nanostructures. The effects of a spatial inhomogeneity of the optical near-field are identified. Furthermore, the importance of electron scattering at the surface is elucidated by contrasting simulations with and without backscattering. Convincing agreement with experimental data under various conditions is demonstrated.

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 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 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.

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.

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.