Sergey V. Yalunin

Quantum coherent optical phase modulation in an ultrafast transmission electron microscope

Coherent manipulation of quantum systems with light is expected to be a cornerstone of future information and communication technology, including quantum computation and cryptography. The transfer of an optical phase onto a quantum wavefunction is a defining aspect of coherent interactions and forms the basis of quantum state preparation, synchronization and metrology. Light-phase-modulated electron states near atoms and molecules are essential for the techniques of attosecond science, including the generation of extreme-ultraviolet pulses and orbital tomography. In contrast, the quantum-coherent phase-modulation of energetic free-electron beams has not been demonstrated, although it promises direct access to ultrafast imaging and spectroscopy with tailored electron pulses on the attosecond scale. Here we demonstrate the coherent quantum state manipulation of free-electron populations in an electron microscope beam. We employ the interaction of ultrashort electron pulses with optical near-fields to induce Rabi oscillations in the populations of electron momentum states, observed as a function of the optical driving field. Excellent agreement with the scaling of an equal-Rabi multilevel quantum ladder is obtained, representing the observation of a light-driven ‘quantum walk’ coherently reshaping electron density in momentum space. We note that, after the interaction, the optically generated superposition of momentum states evolves into a train of attosecond electron pulses. Our results reveal the potential of quantum control for the precision structuring of electron densities, with possible applications ranging from ultrafast electron spectroscopy and microscopy to accelerator science and free-electron lasers.

Attosecond electron pulse trains and quantum state reconstruction in ultrafast transmission electron microscopy

Ultrafast electron and X-ray imaging and spectroscopy are the basis for an ongoing revolution in the understanding of dynamical atomic-scale processes in matter. The underlying technology relies heavily on laser science for the generation and characterization of ever shorter pulses. Recent findings suggest that ultrafast electron microscopy with attosecond-structured wavefunctions may be feasible. However, such future technologies call for means to both prepare and fully analyse the corresponding free-electron quantum states. Here, we introduce a framework for the preparation, coherent manipulation and characterization of free-electron quantum states, experimentally demonstrating attosecond electron pulse trains. Phase-locked optical fields coherently control the electron wavefunction along the beam direction. We establish a new variant of quantum state tomography—‘SQUIRRELS’—for free-electron ensembles. The ability to tailor and quantitatively map electron quantum states will promote the nanoscale study of electron–matter entanglement and new forms of ultrafast electron microscopy down to the attosecond regime.Attosecond electron pulse trains in electron microscopy are demonstrated through the coupling of phase-locked multicolour optical fields with electron pulses. A new variant of quantum state tomography for free-electron ensembles is established.

Controlling the quantum state of free electrons by inelastic optical near-field scattering

Ultrafast transmission electron microscopy is a promising laser-pump electron-probe technique, which allows for studying ultrafast dynamics with both high spatial and temporal resolution [1]. Besides, the high spatial and temporal coherence of the pulsed electron beam enables a coherent manipulation of the free-electron quantum state [2] by inelastic scattering in optical near-fields [3-5]. Upon traversal of an intense optical near-field, the free-electron kinetic energy spectrum develops sidebands that are separated by the photon energy, which can be attributed to a sinusoidal phase modulation of the electron wavefunction. This coherent electron-light interaction opens up the new research field of free-electron quantum optics.

Highly Nonlinear and Ultrafast Optical Phenomena in Metallic Nanostructures

This Chapter presents recent findings on nonlinear ionization and photoemission processes at metallic nanostructures. A particular emphasis is placed on processes which—due to the localized excitation in optical near-fields—exhibit different properties and scalings from their counterparts in the gas phase or at planar surfaces. The Chapter is structured in two parts. The first part discusses various regimes in highly nonlinear photoelectron emission from metallic nanotips, including field-driven photoemission at near- and mid-infrared frequencies, and the control of localized photoemission using intense terahertz transients. In the second part, multiphoton and strong-field ionization of atomic gases in plasmonic antennas and waveguides is presented. It is demonstrated that local ionization is enabled by optical field enhancements in various types of structures. At the same time, fundamental physical limitations preventing efficient high-harmonic generation in nanostructures are discussed.

Quantum coherent interaction of electrons with optical near-fields in an ultrafast electron microscope

We study photon-induced scattering of swift electrons with confined light in an ultrafast transmission electron microscope. The fluence-dependent sideband population in the kinetic energy spectra evidences the quantum coherent manipulation of the electron momenta.

Ultrafast Transmission Electron Microscopy with nanoscale Photoemitters

Ultrafast transmission electron microscopy (UTEM) is a rapidly developing technique which aims to elucidate laser-triggered ultrafast processes at the nanometer scale [1]. In UTEM, ultrafast imaging and spectroscopy with sub-picosecond temporal resolution is achieved by utilizing electron pulses in a stroboscopic laser-pump/electron-probe scheme (Fig 1a). The previously employed flat-surface photocathodes put a severe limit on the achievable spatial resolution due to their intrinsically large electron source size, demanding novel pulsed electron source concepts.

Controlling ultrafast photoelectron dynamics in nanostructure-enhanced THz fields

We present experimental results on a nanostructure streaking scheme employing carrier-envelope-phase-stable THz pulses to control ultrafast photoemission. The tip-enhanced local THz-field allows for field-driven electron dynamics and effective manipulation of trajectories and kinetic energy spectra.

Photoemission at metallic nanostructures: multiphoton and strong-field dynamics

Photoemission from nanostructures offers sub-wavelength field localization and enhancement. Excited by ultrashort pulses, electron emission can be confined and controlled in both time and space. Studies with metallic nanotips have examined the transition to strong-field conditions in photoemission. Reaching deeply into this regime with ultrashort mid-infrared pulses, we generate photoelectrons up to hundreds of electron volts and observe dynamics in which electrons are ejected from the field-enhanced region in less than an optical half cycle. Moreover, single cycle terahertz pulses are shown to yield a novel means of control over the photoemission process.

Confined electron emission with femtosecond timing: nonlinearity, localization, enhancement

The local extraction of electrons from metal nanotips is an essential component of both scanning tunneling microscopes and transmission or scanning electron microscopes based on field emission cathodes. Laser-induced electron emission from sharp tip structures is a prerequisite for equipping such methods with ultrafast temporal resolution. In this paper, recent experiments on femtosecond electron emission from sharp gold tips are discussed. Based on far-field and near-field characterization, confined multiphoton electron emission from the apex is demonstrated. The effective nonlinearity can be tuned by the application of an additional static bias voltage.

Strong-Field Photoelectron Emission From Metal Nanostructures

Photoelectron emission from metallic nanotips is studied experimentally and theoretically in the strong-field regime. The passage from multiphoton to tunnel emission is clearly resolved, and explained in terms of a one-dimensional quantum mechanical treatment.