arXiv subjects

Peter Hommelhoff

Publications and source records attributed to Peter Hommelhoff.

At least 37 records · Page 2Linked to original sources

arXiv2022

Few-electron correlations after ultrafast photoemission from nanometric needle tips

Free electrons are essential in such diverse applications as electron microscopes, accelerators, and photo-emission spectroscopy. Often, space charge effects of many electrons are a nuisance. Confined to extremely small space-time dimensions, even two electrons can interact strongly. In this case, the Coulomb repulsion can now be highly advantageous, because it leads to surprisingly powerful electron-electron correlations, as we demonstrate here. We show that femtosecond laser-emitted electrons from nanometric needle tips are highly anti-correlated in energy because of dynamic Coulomb repulsion, with a visibility of $56\,\%$. We extract a mean energy splitting of $3.3\,$eV and a correlation decay time of $82\,$fs. Importantly, the energy-filtered electrons display a sub-Poissonian number distribution with a second order correlation function as small as $g^{(2)} = 0.34$, implying that shot noise-reduced pulsed electron beams can be realized based on simple energy filtering. Even heralded electrons could become available for quantum-enhanced electron imaging protocols. Furthermore, we also reach the strong-field regime of laser-driven electron emission. We gain deep insights into how the electron correlations of the different electron classes (direct vs. rescattered) are influenced by the strong laser fields. Our work levels the field of quantum electron optics, with direct ramifications for shot noise-reduced and quantum electron imaging as well as direct measurements of correlated electrons from inside of strongly correlated matter.

cond-mat.str-el↗

arXiv2022

Experimental considerations in electron beam transport on a nanophotonic chip using alternating phase focusing

Not long after the laser was invented, it has been marked as a candidate source of strong, high-frequency electromagnetic radiation for acceleration of particles. Indeed, while the complex particle accelerator facilities today are an astonishing culmination of decades of work contributed by generations of physicists, engineers, and a host of scientists, new trends and acceleration technologies have been recently proposed and demonstrated. One of these technologies involves the miniaturization of particle accelerators, which is achieved by replacing the radio-frequency electromagnetic fields accelerating the particles with fields in the optical frequency range, using lasers. This entails using nanophotonics structures to provide the required field distribution. Recently, individual elements towards the nanophotonics counterpart of RF accelerators have been demonstrated. Similarly, active electron transport through such a structure has been shown, which was based on the concept of alternating phase focusing. In this contribution, we discuss and augment on the recently-demonstrated principle of alternating phase focusing using optical frequencies, and provide new insights from relevant simulations and experiments. In particular, we show how to identify possible imprecisions and parasitic effects from time delay scans and discuss how the transmission of electrons through the nanometric structure depends on the temporal overlap between electron and laser pulses, and show how the incidence angle of the electron beam can affect the measured transmission of electrons through the structure.

physics.acc-ph↗

arXiv2022

Quantum interference visibility spectroscopy in two-color photoemission from tungsten needle tips

When two-color femtosecond laser pulses interact with matter, electrons can be emitted through various multiphoton excitation pathways. Quantum interference between these pathways gives rise to a strong oscillation of the photoemitted electron current, experimentally characterized by its visibility. In this work, we demonstrate two-color visibility spectroscopy of multi-photon photoemission from a solid-state nanoemitter. We investigate the quantum pathway interference visibility over an almost octave-spanning wavelength range of the fundamental femtosecond laser pulses and their second-harmonic. The photoemission shows a high visibility of 90% +/- 5%, with a remarkably constant distribution. Furthermore, by varying the relative intensity ratio of the two colors, we find that we can vary the visibility between 0 and close to 100%. A simple but highly insightful theoretical model allows us to explain all observations, with excellent quantitative agreements. We expect this work to be universal to all kinds of photo-driven quantum interference, including quantum control in physics, chemistry and quantum engineering.

physics.optics↗

arXiv2022

Demonstration of weak measurements, projective measurements, and quantum-to-classical transitions in ultrafast free electron-photon interactions

How does the quantum-to-classical transition of measurement occur? This question is vital for both foundations and applications of quantum mechanics. Here, we develop a new measurement-based framework for characterizing the classical and quantum free electron-photon interactions and then experimentally test it. We first analyze the transition from projective to weak measurement in generic light-matter interactions and show that any classical electron-laser-beam interaction can be represented as an outcome of a weak measurement. In particular, the appearance of classical point-particle acceleration is an example of an amplified weak value resulting from weak measurement. A universal factor quantifies the measurement regimes and their transition from quantum to classical, where Gamma corresponds to the ratio between the electron wavepacket size and the optical wavelength. This measurement-based formulation is experimentally verified in both limits of photon-induced near-field electron microscopy and the classical acceleration regime using a dielectric laser accelerator. Our results shed new light on the transition from quantum to classical electrodynamics, enabling to employ the essence of wave-particle duality of both light and electrons in quantum measurement for exploring and applying many quantum and classical light-matter interactions.

quant-ph↗

arXiv2022

Spatio-temporal sampling of near-petahertz vortex fields

Measuring the field of visible light with high spatial resolution has been challenging, as many established methods only detect a focus-averaged signal. Here, we introduce a near-field method for optical field sampling that overcomes that limitation by employing the localization of the enhanced near-field of a nanometric needle tip. A probe field perturbs the photoemission from the tip, which is induced by a pump pulse, generating a field-dependent current modulation that can easily be captured with our electronic detection scheme. The approach provides reliable characterization of near-petahertz fields. We show that not only the spiral wave-front of visible femtosecond light pulses carrying orbital angular momentum (OAM) can be resolved, but also the field evolution with time in the focal plane. Additionally, our method is polarization sensitive, which makes it applicable to vectorial field reconstruction.

physics.optics↗

arXiv2022

Light-field control of real and virtual charge carriers

Light-driven electronic excitation is a cornerstone for energy and information transfer. In the interaction of intense and ultrafast light fields with solids, electrons may be excited irreversibly, or transiently during illumination only. As the transient electron population cannot be observed after the light pulse is gone it is referred to as virtual, while the population remaining excited is called real. Virtual charge carriers have recently been associated with high-harmonic generation and transient absorption, while photocurrent generation may stem from real as well as virtual charge carriers. Yet, a link between the carrier types in their generation and importance for observables up to technological relevance is missing. Here we show that real and virtual carriers can be excited and disentangled in the optical generation of currents in a gold-graphene-gold heterostructure using few-cycle laser pulses. Depending on the waveform used for photoexcitation, real carriers receive net momentum and propagate to the gold electrodes, while virtual carriers generate a polarization response read out at the gold-graphene interfaces. Based on these insights, we further demonstrate a proof of concept of a logic gate for future lightwave electronics. Our results offer a direct means to monitor and excite real and virtual charge carriers. Individual control over each type will dramatically increase the integrated circuit design space and bring closer to reality petahertz signal processing.

physics.optics↗

arXiv2022

Atomic real-space perspective of light-field-driven currents in graphene

When graphene is exposed to a strong few-cycle optical field, a directional electric current can be induced depending on the carrier-envelope phase of the field. This phenomenon has successfully been explained by the charge dynamics in reciprocal space, namely an asymmetry in the conduction band population left after the laser excitation. However, the corresponding real-space perspective has not been explored so far although it could yield knowledge about the atomic origin of the macroscopic currents. In this work, by adapting the nearest-neighbor tight-binding model including overlap integrals and the semiconductor Bloch equation, we reveal the spatial distributions of the light-field-driven currents on the atomic scale and show how they are related to the light-induced changes of charge densities. The atomic-scale currents flow dominantly through the network of the π bonds and are the strongest at the bonds parallel to the field polarization, where an increase of the charge density is observed. The real-space maps of the currents and changes in charge densities are elucidated using simple symmetries connecting real and reciprocal space. We also discuss the strong-field-driven Rabi oscillations appearing in the atomic-scale charge densities. This work highlights the importance of real-space measurements and stimulates future time-resolved atomic-scale experimental studies with high-energy electrons or X-rays, for examples.

physics.optics↗

arXiv2022

Boosting the efficiency of Smith-Purcell radiators using nanophotonic inverse design

The generation of radiation from free electrons passing a grating, known as Smith-Purcell radiation, finds various applications including non-destructive beam diagnostics and tunable light sources, ranging from terahertz towards X-rays. So far, the gratings used for this purpose have been designed manually, based on human intuition and simple geometric shapes. Here we apply the computer-based technique of nanophotonic inverse design to build a 1400nm Smith-Purcell radiator for sub-relativistic 30 keV electrons. We demonstrate that the resulting silicon nanostructure radiates with a 3-times-higher efficiency and 2.2-times-higher overall power than previously used rectangular gratings. With better fabrication accuracy and for the same electron-structure distance, simulations suggest a superiority by a factor of 96 in peak efficiency. While increasing the efficiency is a key step needed for practical applications of free-electron radiators, inverse design also allows to shape the spectral and spatial emission in ways inaccessible with the human mind.

physics.optics↗

arXiv2021

Coulomb interactions and the spatial coherence of femtosecond nanometric electron pulses

The transverse coherence of electrons is of utmost importance in high resolution electron microscopes, point-projection microscopes, low-energy electron microscopy and various other applications. Pulsed versions of many of these have recently been realized, mostly relying on femtosecond laser-triggering electron emission from a sharp needle source. We here observe electron interference fringes and measure how the interference visibility becomes reduced as we increase the electron bunch charge. Due to the extremely strong spatio-temporal confinement of the electrons generated here, we observe the visibility reduction already at average electron bunch charges of less than 1 electron per pulse, owing to the stochastic nature of the emission process. We can fully and quantitatively explain the loss of coherence based on model simulations. Via the van Cittert-Zernike theorem we can connect the visibility reduction to an increase of the effective source size. We conclude by discussing emittance, brightness and quantum degeneracy, which have direct ramifications to many setups and devices relying on pulsed coherent electrons.

physics.optics↗

arXiv2021

Quantum-coherent light-electron interaction in an SEM

The last two decades experimentally affirmed the quantum nature of free electron wavepackets by the rapid development of transmission electron microscopes into ultrafast, quantum-coherent systems. In particular, ultrafast electron pulses can be generated and timed to interact with optical near-fields, yielding coherent exchange of the quantized photon energy between the relativistic electron wavepacket and the light field. So far, all experiments have been restricted to the physically-confining bounds of transmission electron microscopes, with their small, millimeter-sized sample chambers. In this work, we show the quantum coherent coupling between electrons and light in a scanning electron microscope, at unprecedentedly low electron energies down to 10.4 keV, so with sub-relativistic electrons. Scanning electron microscopes not only afford the yet-unexplored electron energies from ~0.5 to 30 keV providing optimum light-coupling efficiencies, but they also offer spacious and easily-configurable experimental chambers for extended and cascaded optical set-ups, potentially boasting thousands of photon-electron interaction zones. Our results unleashes the full potential of quantum experiments including electron wavepacket shaping and quantum computing with multiple arithmetic operations and will allow imaging with low-energy electrons and attosecond time resolution.

physics.optics↗

arXiv2021

Electronic coherence and coherent dephasing in the optical control of electrons in graphene

Electronic coherence is of utmost importance for the access and control of quantum-mechanical solid-state properties. Using a purely electronic observable, the photocurrent, we measure an electronic coherence time of 22 +/- 4 fs in graphene. The photocurrent is ideally suited to measure electronic coherence as it is a direct result of quantum path interference, controlled by the delay between two ultrashort two-color laser pulses. The maximum delay for which interference between the population amplitude injected by the first pulse interferes with that generated by the second pulse determines the electronic coherence time. In particular, numerical simulations reveal that the experimental data yield a lower boundary on the electronic coherence time and that coherent dephasing masks a lower coherence time. We expect that our results will significantly advance the understanding of coherent quantum-control in solid-state systems ranging from excitation with weak fields to strongly driven systems.

cond-mat.mes-hall↗

arXiv2021

Electron phase space control in photonic chip-based particle acceleration

Particle accelerators are essential tools in science, hospitals and industry. Yet, their costs and large footprint, ranging in length from meters to several kilometres, limit their use. The recently demonstrated nanophotonics-based acceleration of charged particles can reduce the cost and size of these accelerators by orders of magnitude. In this approach, a carefully designed nano-structure transfers energy from laser light to the particles in a phase-synchronous manner, thereby accelerating them. However, so far, confinement of the particle beam in the structure over extended distance has been elusive; it requires complex control of the electron beam phase space and is mandatory to accelerate particles to the MeV range and beyond with minimal particle loss. Here, we demonstrate complex electron phase space control at optical frequencies in the 225 nanometre narrow channel of a record-long photonic nanostructure. In particular, we experimentally show alternating phase focusing, a particle propagation scheme for, in principle, arbitrarily long minimal-loss transport. We expect this work to enable MeV electron beam generation on a photonic chip, with direct ramification for new forms of radiotherapy and compact light sources, and other forms of electron phase space control resulting in narrow energy or zeptosecond-bunched beams, for instance.

physics.acc-ph↗

arXiv2021

Imprinting the quantum statistics of photons on free electrons

The fundamental interaction between free electrons and light stands at the base of both classical and quantum physics, with applications in free-electron acceleration, radiation sources, and electron microscopy. Yet, to this day, all experiments involving free-electron light interactions are fully explained by describing the light as a classical wave, disregarding its quantum nature. Here, we observe quantum statistics effects of photons on free-electron-light interactions. We demonstrate interactions passing continuously from Poissonian to super-Poissonian and up to thermal statistics, unveiling a surprising manifestation of Bohr's Correspondence Principle: the transition from quantum walk to classical random walk on the free-electron energy ladder. The electron walker serves as the probe in non-destructive quantum detection, measuring the photon-correlation ${g^{(2)} (0)}$ and higher-orders ${g^{(n)} (0)}$. Unlike conventional quantum-optical detectors, the electron can perform both quantum weak measurements and projective measurements by evolving into an entangled joint-state with the photons. Our findings suggest free-electron-based non-destructive quantum tomography of light, and constitute an important step towards combined attosecond-temporal and sub-A-spatial resolution microscopy.

quant-ph↗

arXiv2021

Two-color coherent control in photoemission from gold needle tips

We demonstrate coherent control of photoemission from a gold needle tip using a two-color laser field. The relative phase between a fundamental field and its second harmonic imprints a strong modulation on the emitted photocurrent with up to 96.5 % contrast. The contrast as a function of the second harmonic intensity can be described by three interfering quantum pathways. Increasing the bias voltage applied to the tip reduces the maximum achievable contrast and modifies the weights of the involved pathways. Simulations based on the time-dependent Schrödinger equation reproduce the characteristic cooperative signal and its dependence on the second harmonic intensity, which further confirms the involvement of three emission pathways.

physics.optics↗

arXiv2021

Adiabaticity parameters for the categorization of light-matter interaction -- from weak to strong driving

We investigate theoretically and numerically the light-matter interaction in a two-level system (TLS) as a model system for excitation in a solid-state band structure. We identify five clearly distinct excitation regimes, categorized with well-known adiabaticity parameters: (1) the perturbative multiphoton absorption regime for small driving field strengths, and four light field-driven regimes, where intraband motion connects different TLS: (2) the impulsive Landau-Zener (LZ) regime, (3) the non-impulsive LZ regime, (4) the adiabatic regime and (5) the adiabatic-impulsive regime for large electric field strengths. This categorization is tremendously helpful to understand the highly complex excitation dynamics in any TLS, in particular when the driving field strength varies, and naturally connects Rabi physics with Landau-Zener physics. In addition, we find an insightful analytical expression for the photon orders connecting the perturbative multiphoton regime with the light field-driven regimes. Moreover, in the adiabatic-impulsive regime, adiabatic motion and impulsive LZ transitions are equally important, leading to an inversion symmetry breaking of the TLS when applying few-cycle laser pulses. This categorization allows a deep understanding of driven TLS in a large variety of settings ranging from cold atoms and molecules to solids and qubits, and will help to find optimal driving parameters for a given purpose.

quant-ph↗

arXiv2021

Coherent scattering of an optically-modulated electron beam by atoms

Recent technological advances allowed the coherent optical manipulation of high-energy electron wavepackets with attosecond precision. Here we theoretically investigate the collision of optically-modulated pulsed electron beams with atomic targets and reveal a quantum interference associated with different momentum components of the incident broadband electron pulse, which coherently modulates both the elastic and inelastic scattering cross sections. We show that the quantum interference has a high spatial sensitivity at the level of Angstroms, offering potential applications in high-resolution ultrafast electron microscopy. Our findings are rationalized by a simple model.

physics.atom-ph↗

arXiv2020

Charged particle guiding and beam splitting with auto-ponderomotive potentials on a chip

We report guiding and manipulation of charged particle beams by means of electrostatic optics based on a principle similar to the electrodynamic Paul trap. We use hundreds of electrodes fabricated on planar substrates and supplied with static voltages to create a ponderomotive potential for charged particles in motion. Shape and strength of the potential can be locally tailored by the electrodes' layout and the applied voltages, enabling the control of charged particle beams within precisely engineered effective potentials. We demonstrate guiding of electrons and ions for a large range of energies (from 20 to 5000 eV) and masses (5E-4 to 131 atomic mass units) as well as electron beam splitting as a proof-of-concept for more complex beam manipulation. Simultaneous confinement of charged particles with different masses is possible, as well as guiding of electrons with energies in the keV regime, and the creation of highly customizable potential landscapes, which is all hard to impossible with conventional electrodynamic Paul traps.

physics.app-ph↗

arXiv2020

Sub-cycle temporal evolution of light-induced electron dynamics in hexagonal 2D materials

Two-dimensional materials with hexagonal symmetry such as graphene and transition metal dichalcogenides} are unique materials to study light-field-controlled electron dynamics inside of a solid. Around the $K$-point, the dispersion relation represents an ideal system to study intricately coupled intraband motion and interband (Landau-Zener) transitions driven by the optical field of phase-controlled few-cycle laser pulses. Based on the coupled nature of the intraband and interband processes, we have recently observed in graphene repeated coherent Landau-Zener transitions between valence and conduction band separated by around half an optical period of ~1.3 fs [Higuchi et al., Nature 550, 224 (2017)]. Due to the low temporal symmetry of the applied laser pulse, a residual current density and a net electron polarization are formed. Here we show extended numerical data on the temporal evolution of the conduction band population of 2D materials with hexagonal symmetry during the light-matter interaction, yielding deep insights to attosecond-fast electron dynamics. In addition, we show that a residual ballistic current density is formed, which strongly increases when a band gap is introduced. Both, the sub-cycle electron dynamics and the resulting residual current are relevant for the fundamental understanding and future applications of strongly driven electrons in two-dimensional materials, including graphene or transition metal dichalcogenide monolayers.

physics.optics↗