Slawa Schmidt

Carrier-envelope phase effects on the strong-field photoemission of electrons from metallic nanostructures

The carrier-envelope phase of laser fields at metal tips can affect the generation and motion of strong-field emitted electrons. Observed variations in the width of plateau-like photoelectron spectra characteristic of the sub-cycle regime may lead to the control of coherent electron motion at metallic nanostructures on ultrashort lengths and timescales.

Adiabatic nanofocusing on ultrasmooth single-crystalline gold tapers creates a 10-nm-sized light source with few-cycle time resolution.

We demonstrate adiabatic nanofocusing of few-cycle light pulses using ultrasharp and ultrasmooth single-crystalline gold tapers. We show that the grating-induced launching of spectrally broad-band surface plasmon polariton wavepackets onto the shaft of such a taper generates isolated, point-like light spots with 10 fs duration and 10 nm diameter spatial extent at its very apex. This nanofocusing is so efficient that nanolocalized electric fields inducing strong optical nonlinearities at the tip end are reached with conventional high repetition rate laser oscillators. We use here the resulting second harmonic to fully characterize the time structure of the localized electric field in frequency-resolved interferometric autocorrelation measurements. Our results strongly suggest that these nanometer-sized ultrafast light spots will enable new experiments probing the dynamics of optical excitations of individual metallic, semiconducting, and magnetic nanostructures.

Ultrasmall bullets of light--focusing few-cycle light pulses to the diffraction limit.

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 µm3. We anticipate that the demonstrated technique is of considerable interest for inducing and probing optical nonlinearities of individual nanostructures.

Wave front adaptation using a deformable mirror for adiabatic nanofocusing along an ultrasharp gold taper

We describe and demonstrate the use of an adaptive wave front optimization scheme for enhancing the efficiency of adiabatic nanofocusing of surface plasmon polariton (SPP) waves along an ultrasharp conical gold taper. Adiabatic nanofocusing is an emerging and promising scheme for controlled focusing of far field light into nanometric volumes. It comprises three essential steps: SPP excitation by coupling far field light to an SPP waveguide, SPP propagation along the waveguide and adiabatic SPP nanofocusing towards a geometric singularity. For commonly used complex waveguide geometries, such as, e.g., conical metal tapers, a realistic modeling and efficiency optimization is challenging. Here, we use a deformable mirror to adaptively control the wave front of the incident far field light. We demonstrate an eight-fold enhancement in nanofocusing efficiency and analyze the shape of the resulting optimized wave front. The introduced wave front optimization scheme is of general interest for guiding and controlling light on the nanoscale.

Distinguishing between ultrafast optical harmonic generation and multi-photon-induced luminescence from ZnO thin films by frequency-resolved interferometric autocorrelation microscopy.

The nonlinear optical properties of thin ZnO film are studied using interferometric autocorrelation (IFRAC) microscopy. Ultrafast, below-bandgap excitation with 6-fs laser pulses at 800 nm focused to a spot size of 1 µm results in two emission bands in the blue and blue-green spectral region with distinctly different coherence properties. We show that an analysis of the wavelength-dependence of the interference fringes in the IFRAC signal allows for an unambiguous assignment of these bands as coherent second harmonic emission and incoherent, multiphoton-induced photoluminescence, respectively. More generally our analysis shows that IFRAC allows for a complete characterization of the coherence properties of the nonlinear optical emission from nanostructures in a single-beam experiment. Since this technique combines a very high temporal and spatial resolution we anticipate broad applications in nonlinear nano-optics.

Controlling the Motion of Strong-Field, Few-Cycle Photoemitted Electrons in the Near-Field of a Sharp Metal Tip

The real-time probing of electron motion in solid nanostructures or the visualization of nanoplasmonic field dynamics may come into reach using electron pulses generated by strong-field tunneling from sharp gold tips irradiated by few-cycle laser pulses. The acceleration of the ultrashort electron wavepackets in the near field of the sharp gold tips introduces new possibilities of steering and control of electron wavepackets by light, which is expected to pave the way towards such ultrafast probing. Here we discuss the motion of these highly accelerated electrons in the near-field and demonstrate how the carrier-envelope phase admits a new control mechanism for their motion.

Strong-field photoemitted electrons from metallic tips show carrier-envelope phase effects

Summary form only given. Sharp nanometer-sized metallic tips recently emerged as a test bed for exploring strong-field phenomena such as high-harmonic generation and photoemission [1-4]. When being illuminated with few-cycle laser pulses of sufficient field strength, optical field enhancement at the tip apex results in tunnelling of electrons out of the tip. The acceleration of these electrons within the local near-field gradient can be so strong that the typical quiver motion of the electrons in an oscillating laser field is fully suppressed [3,4]. These sub-cycle electrons traverse the near field with a decay length of only a few nm within less than one half cycle of the laser field. Hence their motion is expected to sensitively depend on the carrier-envelope phase (CEP) of few-cycle driving pulses, enabling steering and controlling of electron motion around metallic nanoparticles by CEP variation.Here, we show for the first time how the CEP of such few-cylce pulses affects the acceleration of strong-field emitted electrons in the near-field of sharp nanometer-sized gold tips. Gold tips, etched to an apex radius of down to 5 nm are irradiated with 16-fs pulses (2.6 cycles) at a wavelength of 1.65 μm from a noncollinear optical parametric amplifier (NOPA) system followed by difference frequency generation (Fig. 1a). The combination of frequency conversion stages ensures that the pulses have a highly stable CEP with residual phase fluctuations of ~66 mrad as measured in an f-to-2f interferometer over a time span of 10 min. The CEP is controlled via a pair of fused silica wedges, and the energy spectra of the emitted and accelerated electrons are measured as a function of CEP using a photo-electron spectrometer (PES).The recorded kinetic energy spectra (Fig. 1b) show a clear modulation of the spectral width with the CEP. The red and black circles indicate the highand low energy cutoff and are plotted in Fig. 1c together with fitted sinecurves. They display an inversely phased 2π-periodicity, leading to a periodic narrowing and broadening of the spectra. The same periodicity is found in the total electron yield (Fig. 1d). The measurements agree well with simulations, tracing the marked periodic modulation of the high-energy cutoff to the variation of the maximum field amplitude with CEP and its effect on the near-field acceleration. We believe that such a field-driven control of the electron motion in the near field of solid state nanostructures can be seen as a new form of quantum electronics, paving the way towards the generation, measurement, and application of attosecond electron pulses.

Strong field acceleration of attosecond electron pulses emitted by an individual metallic nanostructure

We report on the observation of strong near-field acceleration of attosecond electron pulses emitted from a sharp nanometer-sized gold tip. Kinetic energy spectra extending over tens of eV and varying qualitatively with laser wavelength and intensity are explained in terms of the spatiotemporal electron dynamics in the strong field gradient at the tip apex.