We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission using near-ultraviolet laser triggering with a photon energy of 3.1 eV and compare it to DC-field emission. A carbon-nanotube is brought in close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes we deduce an upper limit of the effective source radius both in laser-triggered and DC-field emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain (0.80±0.05) nm in laser-triggered and (0.55±0.02) nm in DC-field emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laser-induced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for time-resolved electron imaging techniques are discussed.Coherent electron sources are central to studying microscopic objects with highest spatial resolution. They provide electron beams with flat wavefronts that can be focused to the fundamental physical limit given by matter wave diffraction [1]. Currently, time-resolved electron based imaging is pursued with large efforts, both in realspace microscopy [2, 3] and in diffraction [4,5]. However, the spatial resolution in time-resolved electron microscopy is about two orders of magnitude worse than its DC counterpart [6], which reaches below 0.1Å [7]. Combining highest spatial resolution with time resolution in the picosecond to (sub-) femtosecond range requires spatially coherent electron sources driven by ultrashort laser pulses. Although laser-driven metal nanotips promise to provide coherent electron pulses with highest time resolution, a quantitative study of their spatial coherence has been elusive. Here we demonstrate that photoemitted electrons from a tungsten nanotip are highly coherent.So far no time-resolved electron based imaging instrument fully utilizes the coherence capabilities provided by nanotip electron sources. Meanwhile, nanotips operated in DC-field emission are known and employed in practical applications for almost half a century for their paramount spatial coherence properties [8]. Thence, highest resolution microscopy as well as coherent imaging, such as holography and interferometry, have long been demonstrated in DC-field emission [1, 9, 10]. Here we investigate whether these concepts can be inherited to laserdriven nanotip sources by comparing the spatial coherence of photoemitted electron beams to their DC counterparts. This would enable time-resolved high resolution imaging, but may also herald fundamental studies based on the generation of quantum degenerate electron beams [11].The spatial coherence of electron sources is commonly quantified by means of their effective source radius r eff . It eq...

Carbon nanotubes exhibit a wealth of unique physical properties. By virtue of their exceptionally low mass and extreme stiffness they provide ultrahigh-quality mechanical resonances, promise long electron spin coherence times in a nuclear-spin free lattice for quantum information processing and spintronics, and feature unprecedented tunability of optical transitions for optoelectronic applications. Excitons in semiconducting single-walled carbon nanotubes could facilitate the upconversion of spin, mechanical or hybrid spin-mechanical degrees of freedom to optical frequencies for efficient manipulation and detection. However, successful implementation of such schemes with carbon nanotubes has been impeded by rapid exciton decoherence at non-radiative quenching sites, environmental dephasing and emission intermittence. Here we demonstrate that these limitations may be overcome by exciton localization in suspended carbon nanotubes. For excitons localized in nanotube quantum dots we found narrow optical lines free of spectral wandering, radiative exciton lifetimes and effectively suppressed blinking. Our findings identify the great potential of localized excitons for efficient and spectrally precise interfacing of photons, phonons and spins in novel carbon nanotube-based quantum devices.

Raman spectroscopy reveals chemically specific information and provides label-free insight into the molecular world. However, the signals are intrinsically weak and call for enhancement techniques. Here, we demonstrate Purcell enhancement of Raman scattering in a tunable high-finesse microcavity, and utilize it for molecular diagnostics by combined Raman and absorption imaging. Studying individual single-wall carbon nanotubes, we identify crucial structural parameters such as nanotube radius, electronic structure and extinction cross-section. We observe a 320-times enhanced Raman scattering spectral density and an effective Purcell factor of 6.2, together with a collection efficiency of 60%. Potential for significantly higher enhancement, quantitative signals, inherent spectral filtering and absence of intrinsic background in cavity-vacuum stimulated Raman scattering render the technique a promising tool for molecular imaging. Furthermore, cavity-enhanced Raman transitions involving localized excitons could potentially be used for gaining quantum control over nanomechanical motion and open a route for molecular cavity optomechanics.