Ultrafast Coupling of Optical Near Fields to Low-Energy Electrons Probed in a Point-Projection Microscope.

We report the first observation of the coupling of strong optical near fields to wavepackets of free, 100 eV electrons with <50 fs temporal resolution in an ultrafast point-projection microscope. Optical near fields are created by excitation of a thin, nanometer-sized Yagi-Uda antenna, with 20 fs near-infrared laser pulses. Phase matching between electrons and near fields is achieved due to strong spatial confinement of the antenna near field. Energy-resolved projection images of the antenna are recorded in an optical pump-electron probe scheme. We show that the phase modulation of the electron by transverse-field components results in a transient electron deflection while longitudinal near-field components broaden the kinetic energy distribution. This low-energy electron near-field coupling is used here to characterize the chirp of the ultrafast electron wavepackets, acquired upon propagation from the electron emitter to the sample. Our results bring direct mapping of different vectorial components of highly localized optical near fields into reach.

In-line interferometer for broadband near-field scanning optical spectroscopy.

We present and investigate a novel approach towards broad-bandwidth near-field scanning optical spectroscopy based on an in-line interferometer for homodyne mixing of the near field and a reference field. In scattering-type scanning near-field optical spectroscopy, the near-field signal is usually obscured by a large amount of unwanted background scattering from the probe shaft and the sample. Here we increase the light reflected from the sample by a semi-transparent gold layer and use it as a broad-bandwidth, phase-stable reference field to amplify the near-field signal in the visible and near-infrared spectral range. We experimentally demonstrate that this efficiently suppresses the unwanted background signal in monochromatic near-field measurements. For rapid acquisition of complete broad-bandwidth spectra we employ a monochromator and a fast line camera. Using this fast acquisition of spectra and the in-line interferometer we demonstrate the measurement of pure near-field spectra. The experimental observations are quantitatively explained by analytical expressions for the measured optical signals, based on Fourier decomposition of background and near field. The theoretical model and in-line interferometer together form an important step towards broad-bandwidth near-field scanning optical spectroscopy.

CeIV -GdIII Mixed Oxides as Hosts for ErIII -Based Upconversion Phosphors.

A family of ErIII and ErIII -YbIII based nanophosphors, hosted in monophasic oxidic CeIV -GdIII binary solid solutions, was prepared. The samples were formulated with a constant ErIII content as the activator, with the eventual addition of YbIII as a sensitizer. The amorphous Ce0.94-x Gdx Er0.06 (OH)CO3 ⋅H2 O and Ce0.94-x Gdx Er0.05 Yb0.01 (OH)CO3 ⋅H2 O precursors were prepared by following the urea method to obtain monodispersed spheres of tunable size ranging from 30 to 450 nm. After being decomposed at 1273 K under an atmosphere of air, the precursors of 200 nm in diameter evolved into monophasic polycrystalline particles preserving the parent shape and size. The role of the composition of the binary matrices in the emission properties was evaluated for two different excitation wavelengths (976 nm and 780 nm) based on the upconversion (UC) emission spectra and their dependence on the incident power. The yield of the UC process is discussed in the framework of established and novel alternative mechanisms. The number of vacancies and mainly the symmetry of the ErIII environment play major roles in the deactivation pathways of the UC emission mechanisms. However, the colours obtained by employing bare CeIV or GdIII hosts are preserved in the related monophasic CeIV -rich or GdIII -rich binary hosts.

Ultrafast Electron Emission from a Sharp Metal Nanotaper Driven by Adiabatic Nanofocusing of Surface Plasmons.

We report photoelectron emission from the apex of a sharp gold nanotaper illuminated via grating coupling at a distance of 50 µm from the emission site with few-cycle near-infrared laser pulses. We find a fifty-fold increase in electron yield over that for direct apex illumination. Spatial localization of the electron emission to a nanometer-sized region is demonstrated by point-projection microscopic imaging of a silver nanowire. Our results reveal negligible plasmon-induced electron emission from the taper shaft and thus efficient nanofocusing of few-cycle plasmon wavepackets. This novel, remotely driven emission scheme offers a particularly compact source of ultrashort electron pulses of immediate interest for miniaturized electron microscopy and diffraction schemes with ultrahigh time resolution.

High passive CEP stability from a few-cycle, tunable NOPA-DFG system for observation of CEP-effects in photoemission.

The investigation of fundamental mechanisms taking place on a femtosecond time scale is enabled by ultrafast pulsed laser sources. Here, the control of pulse duration, center wavelength, and especially the carrier-envelope phase has been shown to be of essential importance for coherent control of high harmonic generation and attosecond physics and, more recently, also for electron photoemission from metallic nanostructures. In this paper we demonstrate the realization of a source of 2-cycle laser pulses tunable between 1.2 and 2.1 µm, and with intrinsic CEP stability. The latter is guaranteed by difference frequency generation between the output pulse trains of two noncollinear optical parametric amplifier stages that share the same CEP variations. The CEP stability is better than 50 mrad over 20 minutes, when averaging over 100 pulses. We demonstrate the good CEP stability by measuring kinetic energy spectra of photoemitted electrons from a single metal nanostructure and by observing a clear variation of the electron yield with the CEP.

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.

In-line interferometric femtosecond stimulated Raman scattering spectroscopy.

We present in-line interferometric femtosecond stimulated Raman scattering (II-FSRS), a new method to measure the spectral Raman intensity and phase over a broad spectral range, potentially in a single shot. An analytic model is developed, that excellently reproduces the measured spectra. Additionally, the performance of II-FSRS is directly compared in experiments to two established techniques, namely femtosecond stimulated Raman scattering and femtosecond Raman induced Kerr-effect spectroscopy.

Interferometric background reduction for femtosecond stimulated Raman scattering loss spectroscopy.

We present a purely optical method for background suppression in nonlinear spectroscopy based on linear interferometry. Employing an unbalanced Sagnac interferometer, an unprecedented background reduction of 17 dB over a broad bandwidth of 60 THz (2000 cm(-1)) is achieved and its application to femtosecond stimulated Raman scattering loss spectroscopy is demonstrated. Apart from raising the signal-to-background ratio in the measurement of the Raman intensity spectrum, this interferometric method grants access to the spectral phase of the resonant χ(3) contribution. The spectral phase becomes apparent as a dispersive lineshape and is reproduced numerically with a simple oscillator model.

Noise reduction of supercontinua via optical feedback.

The impact of delayed optical feedback on the supercontinuum noise properties is investigated numerically and experimentally. The supercontinuum is generated by coupling femtosecond laser pulses into a microstructured fiber within a ring resonator, which introduces the optical feedback. The power noise and spectral amplitude noise properties of this feedback system are numerically and experimentally compared with single-pass supercontinuum generation. In a demonstrative experiment via optical feedback the power noise could be reduced by 15 dB and the spectral amplitude noise could be reduced by up to 28 dB.

Adjustment of supercontinua via the optical feedback phase--experimental verifications.

The manipulation of a supercontinuum via delayed optical feedback is investigated experimentally. The supercontinuum is generated in a microstructured fiber and a feedback ring resonator introduces the optical feedback and leads to the formation of different regimes of nonlinear dynamics. Via the feedback phase the optical spectrum and the regimes of nonlinear dynamics can be adjusted systematically. The impact of delay detuning on two different length scales, namely on a sub-wavelength scale and on a larger scale in the order of 10 µm are discussed. Additionally, the adjustment of the optical spectrum without changing the regime of nonlinear dynamics is demonstrated.

Adjustment of supercontinua via the optical feedback phase - numerical investigations.

The delay and phase dependent behavior of a system for supercontinuum generation by using a microstructured fiber within a synchronously pumped ring resonator is presented numerically. The feedback introduced by the resonator led to an interaction of the supercontinuum with the following femtosecond laser pulses and thus to the formation of a nonlinear oscillator. Via the feedback phase different regimes of nonlinear dynamics, such as steady state, period multiplication, limit cycle and chaos can be adjusted systematically. The spectrum within one regime of nonlinear dynamics can additionally be modified independently from the regime of nonlinear dynamics.

Experimental investigations on nonlinear dynamics in supercontinuum generation with feedback.

A system for supercontinuum generation by using a photonic crystal fiber within a synchronously pumped ring cavity is presented. The feedback led to an interaction of the generated supercontinuum with the following femtosecond laser pulses and thus to the formation of a nonlinear oscillator. The nonlinear dynamical behavior of this system was investigated experimentally and compared with numerical simulations. Steady state, period doubling and higher order multiplication of the repetition rate as well as limit cycle and chaotic behavior were observed in the supercontinuum generating system.

Nonlinear dynamics of femtosecond supercontinuum generation with feedback.

We numerically study the impact of feedback on supercontinuum generation within a microstructured fiber inside a ring resonator, synchronously pumped with femtosecond pulses. In certain parameter ranges we observe a steady-state oscillator-like operation mode of the system. Depending on pump power also period doubling up to chaos is shown by the system. Even with the inclusion of realistic pump noise as perturbation, the periodic behavior was still achievable in numerical modeling as well as in a first experimental verification.

A route to sub-diffraction-limited CARS Microscopy.

We theoretically investigate a scheme to obtain sub-diffraction-limited resolution in coherent anti-Stokes Raman scattering (CARS) microscopy. We find using density matrix calculations that the rise of vibrational (Raman) coherence can be strongly suppressed, and thereby the emission of CARS signals can be significantly reduced, when pre-populating the corresponding vibrational state through an incoherent process. The effectiveness of pre-populating the vibrational state of interest is investigated by considering the excitation of a neighbouring vibrational (control) state through an intense, mid-infrared control laser. We observe that, similar to the processes employed in stimulated emission depletion microscopy, the CARS signal exhibits saturation behaviour if the transition rate between the vibrational and the control state is large. Our approach opens up the possibility of achieving chemically selectivity sub-diffraction-limited spatially resolved imaging.