Jan Vogelsang

Proton Structure from the Measurement of 2S-2P Transition Frequencies of Muonic Hydrogen

Proton Still Too Small Despite a protons tiny size, it is possible to measure its radius based on its charge or magnetization distributions. Traditional measurements of proton radius were based on the scattering between protons and electrons. Recently, a precision measurement of a line in the spectrum of muonium—an atom consisting of a proton and a muon, instead of an electron—revealed a radius inconsistent with that deduced from scattering studies. Antognini et al. (p. 417; see the Perspective by Margolis) examined a different spectral line of muonium, with results less dependent on theoretical analyses, yet still inconsistent with the scattering result; in fact, the discrepancy increased. A precision spectroscopic measurement of the proton radius indicates a growing discrepancy with respect to scattering results. [Also see Perspective by Margolis] Accurate knowledge of the charge and Zemach radii of the proton is essential, not only for understanding its structure but also as input for tests of bound-state quantum electrodynamics and its predictions for the energy levels of hydrogen. These radii may be extracted from the laser spectroscopy of muonic hydrogen (μp, that is, a proton orbited by a muon). We measured the 2S1/2F=0-2P3/2F=1 transition frequency in μp to be 54611.16(1.05) gigahertz (numbers in parentheses indicate one standard deviation of uncertainty) and reevaluated the 2S1/2F=1-2P3/2F=2 transition frequency, yielding 49881.35(65) gigahertz. From the measurements, we determined the Zemach radius, rZ = 1.082(37) femtometers, and the magnetic radius, rM = 0.87(6) femtometer, of the proton. We also extracted the charge radius, rE = 0.84087(39) femtometer, with an order of magnitude more precision than the 2010-CODATA value and at 7σ variance with respect to it, thus reinforcing the proton radius puzzle.

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.

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.

Multipass laser cavity for efficient transverse illumination of an elongated volume

A multipass laser cavity is presented which can be used to illuminate an elongated volume from a transverse direction. The illuminated volume can also have a very large transverse cross section. Convenient access to the illuminated volume is granted. The multipass cavity is very robust against misalignment, and no active stabilization is needed. The scheme is suitable for example in beam experiments, where the beam path must not be blocked by a laser mirror, or if the illuminated volume must be very large. This cavity was used for the muonic-hydrogen experiment in which 6 μm laser light illuminated a volume of 7 × 25 × 176 mm3, using mirrors that are only 12 mm in height. We present our measurement of the intensity distribution inside the multipass cavity and show that this is in good agreement with our simulation.

Lifetime and population of the 2S state in muonic hydrogen and deuterium

Radiative deexcitation (RD) of the metastable 2S state of muonic protium and deuterium atoms has been observed. In muonic protium, we improve the precision on lifetime and population (formation probability) values for the short-lived {\mu}p(2S) component, and give an upper limit for RD of long-lived {\mu}p(2S) atoms. In muonic deuterium at 1 hPa, 3.1 +-0.3 % of all stopped muons form {\mu}d(2S) atoms. The short-lived 2S component has a population of 1.35 +0.57 -0.33 % and a lifetime of {\tau}_short({\mu}d) = 138 +32 -34 ns. We see evidence for RD of long-lived {\mu}d(2S) with a lifetime of {\tau}_long({\mu}d) = 1.15 +0.75 -0.53 {\mu}s. This is interpreted as formation and decay of excited muonic molecules.

Plasmonic nanofocusing – grey holes for light

Abstract Improving the resolution and sensitivity in all-optical microscopy and spectroscopy is inevitably one of the most important challenges in contemporary optical and nanoscience. Here, we discuss a novel approach, plasmonic nanofocusing, towards broadband, coherent all-optical microscopy with ultrahigh temporal and spatial resolution. The conceptual idea is to launch radially symmetric surface plasmon polariton modes onto the shaft of a sharp, conical metal taper. While propagating towards the apex of the pointed taper, the spatial extent of the plasmonic mode gradually shrinks, from several microns in diameter to a spot size of less than 10 nm at the pointed apex of the conical taper. Concomitantly, the local field amplitude of the plasmon mode gradually increases, resulting in a pronounced field enhancement at the apex and – thus – a bright and spatially isolated coherent light source with dimensions far below the diffraction limit. In this review, we characterize the optical properties of such three-dimensional conical metal tapers and demonstrate nanofocusing of radially symmetric plasmon modes. We use this nanolight source for coherent light scattering spectroscopy and demonstrate the sensitivity enhancement resulting from the pronounced spatial field confinement. It is shown that such off-resonant plasmonic nanoantennas facilitate the creation of nanofocused light spots with few-cycle time resolution. As a first application of this ability to nanolocalize ultrashort plasmon wavepackets, we demonstrate remotely-triggered multiphoton-induced photoemission from the very apex of the taper and implement this novel ultrafast electron gun in a point-projection electron microscope. Our results not only indicate the favourable optical properties of this plasmonic nanolens but also suggest that it may find interesting applications in ultrafast scanning optical spectroscopy and might enable new types of ultrafast electron holography and scanning tunnelling microscopy.

Long-lived electron emission reveals localized plasmon modes in disordered nanosponge antennas

We report long-lived, highly spatially localized plasmon states on the surface of nanoporous gold nanoparticles—nanosponges—with high excitation efficiency. It is well known that disorder on the nanometer scale, particularly in two-dimensional systems, can lead to plasmon localization and large field enhancements, which can, in turn, be used to enhance nonlinear optical effects and to study and exploit quantum optical processes. Here, we introduce promising, three-dimensional model systems for light capture and plasmon localization as gold nanosponges that are formed by the dewetting of gold/silver bilayers and dealloying. We study light-induced electron emission from single nanosponges, a nonlinear process with exponents of n≈5...7, using ultrashort laser pulse excitation to achieve femtosecond time resolution. The long-lived electron emission process proves, in combination with optical extinction measurements and finite-difference time-domain calculations, the existence of localized modes with lifetimes of more than 20 fs. These electrons couple efficiently to the dipole antenna mode of each individual nanosponge, which in turn couples to the far-field. Thus, individual gold nanosponges are cheap and robust disordered nanoantennas with strong local resonances, and an ensemble of nanosponges constitutes a meta material with a strong polarization independent, nonlinear response over a wide frequency range.

Ultrafast coherent dynamics of Rydberg electrons bound in the image potential near a single metallic nano-object (Presentation Recording)

Image potential states are well established surface states of metallic films [1]. For a single metallic nanostructure these surface states can be localized in the near-field arising from illumination by a strong laser field. Thus single metallic nanostructures offer the unique possibility to study quantum systems with both high spatial and ultrafast temporal resolution. Here, we investigate the dynamics of Rydberg states localized to a sharp metallic nanotaper. For this purpose we realized a laser system delivering few-cycle pulses tunable over a wide wavelength range [2]. Pulses from a regenerative titanium:sapphire amplifier generate a white light continuum, from which both a proportion in the visible and in the infrared are amplified in two non-collinear optical parametric amplification (NOPA) stages. Difference frequency generation (DFG) of both stages provides pulses in the near-infrared. With a precisely delayed sequence of few-cycle pulses centered around 600 nm (NOPA#1 output) and 1600 nm (DFG output) we illuminate the apex of a sharply etched gold tip. Varying the delay we observe an exponential decay of photoemitted electrons with a distinctly asymmetric decay length on both sides, indicating the population of different states. Superimposed on the decay is a clearly discernible quantum beat pattern with a period of <50 fs, which arises from the motion of Rydberg photoelectrons bound within their own image potential. These results therefore constitute a step towards controlling single electron wavepackets released from a gold tip opening up fascinating perspectives for applications in ultrafast electron microscopy [3]. [1] Hofer, U. et al. Science 277, 1480 (1997) [2] Vogelsang, J., Robin J. et al. Opt. Express 22, 25295 (2014) [3] Petek, H. et al. ACS Nano 8, 5 (2014)

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.