Control Of Electron Dynamics Research Articles

Robust Isolated Attosecond Pulse Generation with Self-Compressed Subcycle Drivers from Hollow Capillary Fibers.

High-order harmonic generation (HHG) arising from the nonperturbative interaction of intense light fields with matter constitutes a well-established tabletop source of coherent extreme-ultraviolet and soft X-ray radiation, which is typically emitted as attosecond pulse trains. However, ultrafast applications increasingly demand isolated attosecond pulses (IAPs), which offer great promise for advancing precision control of electron dynamics. Yet, the direct generation of IAPs typically requires the synthesis of near-single-cycle intense driving fields, which is technologically challenging. In this work, we theoretically demonstrate a novel scheme for the straightforward and compact generation of IAPs from multicycle infrared drivers using hollow capillary fibers (HCFs). Starting from a standard, intense multicycle infrared pulse, a light transient is generated by extreme soliton self-compression in a HCF with decreasing pressure and is subsequently used to drive HHG in a gas target. Owing to the subcycle confinement of the HHG process, high-contrast IAPs are continuously emitted almost independently of the carrier-envelope phase (CEP) of the optimally self-compressed drivers. This results in a CEP-robust scheme which is also stable under macroscopic propagation of the high harmonics in a gas target. Our results open the way to a new generation of integrated all-fiber IAP sources, overcoming the efficiency limitations of usual gating techniques for multicycle drivers.

Orbital perspective on high-harmonic generation from solids

High-harmonic generation in solids allows probing and controlling electron dynamics in crystals on few femtosecond timescales, paving the way to lightwave electronics. In the spatial domain, recent advances in the real-space interpretation of high-harmonic emission in solids allows imaging the field-free, static, potential of the valence electrons with picometer resolution. The combination of such extreme spatial and temporal resolutions to measure and control strong-field dynamics in solids at the atomic scale is poised to unlock a new frontier of lightwave electronics. Here, we report a strong intensity-dependent anisotropy in the high-harmonic generation from ReS2 that we attribute to angle-dependent interference of currents from the different atoms in the unit cell. Furthermore, we demonstrate how the laser parameters control the relative contribution of these atoms to the high-harmonic emission. Our findings provide an unprecedented atomic perspective on strong-field dynamics in crystals, revealing key factors to consider in the route towards developing efficient harmonic emitters.

Residual current under the combined effect of carrier envelope phase and chirp: phase shift and peak enhancement.

We theoretically investigate the residual current of linearly polarized light incident on graphene under the combined effect of carrier envelope phase and chirp. Phase shift and peak residual current enhancement are significantly obtained. Phase shift is the natural result of introducing a linear chirp in the presence of carrier envelope phase. By comparing the residual current integrated along the kx direction for different chirp rates and carrier envelope phases, the enhancement can be observed from two regions, where multiphoton interference is involved. By increasing the chirp rate, the light-graphene interaction turns from a non-perturbative to a perturbative regime. Thus the results of the combined effect can help to find suitable parameters to study regime transition and control of electronic dynamics. We expect that this study contributes to the signal processing at optical frequencies and to the development of optoelectronic integrated device applications.

Electron dynamics and metastable species generation in atmospheric pressure non-equilibrium plasmas controlled by dual LF–RF frequency discharges

In this work, we report an approach to control electron dynamics and metastable species generation and enhance the density of atmospheric pressure non-equilibrium plasmas by using dual-frequency excitation sources. The atmospheric dielectric barrier discharge (DBD) of an α-mode radio frequency (RF = 5 MHz) discharge controlled by a low-frequency (LF = 50 kHz) bias is studied based on a one-dimensional (1D) fluid model. Results show that the variation in amplitude ratio of RF and LF modulates the electron dynamic process, resulting in different spatial distributions of electron and metastable particle densities. Moreover, it is further shown that the electron density is substantially increased when the LF component voltage amplitude is larger than 300 V for the initial setting. The discharge process is characterized by fast Fourier transform of the spatio-temporal evolution of the electron power absorption and discharge current. As the LF is applied, three-wave interactions induced by LF and RF coupling are clearly observed, where the sum and beat frequencies between LF and RF are increased, which results in a substantial increase in the electron density. On the other hand, the high RF harmonics, especially for the fundamental and the third harmonic components, are suppressed when increasing the LF component. This work demonstrates that dual-frequency excitation is efficacious to modulate the electron dynamic behaviors and metastable species generation of atmospheric pressure plasma, which can provide a possible approach of optimizing plasma parameters.

Dual-Wavelength Spectrum-Shaped Mid-Infrared Pulses and Steering High-Harmonic Generation in Solids

Mid-infrared (MIR) ultra-short pulses with multiple spectral-band coverage and good freedom in spectral and temporal shaping are desired by broad applications such as steering strong-field ionization, investigating bound-electron dynamics, and minimally invasive tissue ablation. However, the existing methods of light transient generation lack freedom in spectral tuning and require sophisticated apparatus for complicated phase and noise control. Here, with both numerical analysis and experimental demonstration, we report the first attempt, to the best our knowledge, at generating MIR pulses with dual-wavelength spectral shaping and exceptional freedom of tunability in both the lasing wavelength and relative spectral amplitudes, based on a relatively simple and compact apparatus compared to traditional pulse synthesizers. The proof-of-concept demonstration in steering the high-harmonic generation in a polycrystalline ZnSe plate is facilitated by dual-wavelength MIR pulses shaped in both spectral and temporal domains, spanning from 5.6 to 11.4 μm, with multi-microjoule pulse energy and hundred- milliwatt average power. Multisets of harmonics corresponding to different fundamental wavelengths are simultaneously generated in the deep ultraviolet region, and both the relative strength of individual harmonics sets and the spectral shapes of harmonics are harnessed with remarkable freedom and flexibility. This work would open new possibilities in exploring femtosecond control of electron dynamics and light–matter interaction in composite molecular systems.

Theoretical investigation of electron dynamics driven by laser pulses in graphene nanoribbons

Control of electron dynamics in graphene nanoribbons (GNRs) is critically important for the future applications of graphene-based nanoelectronic devices. Based on real-time simulations, we investigate the manipulation of electronic transport by femtosecond laser pulses in armchair GNRs. The simulation results show that the multiphoton absorption process can drive the photon-excited electron flow along a GNR. The corresponding transferred charges are determined by the order of multiphoton absorption and depend on the central frequency and energy distribution of few-cycle laser pulses. Furthermore, the transferred charge can be controlled by the carrier-envelope phase of pulses; this dependence vanishes as the duration time of laser pulses increases. These results also provide insights into the manipulation of electron dynamics using lasers and the design of optoelectronic devices based on graphene materials.

Raman Interferometry between Autoionizing States to Probe Ultrafast Wave-Packet Dynamics with High Spectral Resolution.

Photoelectron interferometry with femtosecond and attosecond light pulses is a powerful probe of the fast electron wave-packet dynamics, albeit it has practical limitations on the energy resolution. We show that one can simultaneously obtain both high temporal and spectral resolution by stimulating Raman interferences with one light pulse and monitoring the modification of the electron yield in a separate step. Applying this spectroscopic approach to the autoionizing states of argon, we experimentally resolved its electronic composition and time evolution in exquisite detail. Theoretical calculations show remarkable agreement with the observations and shed light on the light-matter interaction parameters. Using appropriate Raman probing and delayed detection steps, this technique enables highly sensitive probing and control of electron dynamics in complex systems.

Spin current in chemical reactions

Development in attosecond technologies has been realizing real-time control of electronic dynamics. As a useful means for real-time monitoring of radical bond-rearrangement reactions, we introduce spin flux to track the dynamics of spin density in them. As an illustrative example, we show the spin flux in the course of the basic radical reaction H· + H2. It is demonstrated that spin flux induces spin-polarization in the molecular target (H2) to weaken the covalent bond, thus leading to possible bond cleavage. The mechanism shown here is in harmony with the three-stage mechanism in radical reactions by Nagase et al.

Generation and characterization of isolated attosecond pulses at 100 kHz repetition rate

The generation of coherent light pulses in the extreme ultraviolet (XUV) spectral region with attosecond pulse durations constitutes the foundation of the field of attosecond science. Twenty years after the first demonstration of isolated attosecond pulses, they continue to be a unique tool enabling the observation and control of electron dynamics in atoms, molecules, and solids. It has long been identified that an increase in the repetition rate of attosecond light sources is necessary for many applications in atomic and molecular physics, surface science, and imaging. Although high harmonic generation (HHG) at repetition rates exceeding 100 kHz, showing a continuum in the cutoff region of the XUV spectrum, was already demonstrated in 2013, the number of photons per pulse was insufficient to perform pulse characterization via attosecond streaking, let alone to perform a pump-probe experiment. Here we report on the generation and full characterization of XUV attosecond pulses via HHG driven by near-single-cycle pulses at a repetition rate of 100 kHz. The high number of 10 6 XUV photons per pulse on target enables attosecond electron streaking experiments through which the XUV pulses are determined to consist of a dominant single attosecond pulse. These results open the door for attosecond pump-probe spectroscopy studies at a repetition rate 1 or 2 orders of magnitude above current implementations.

Pressing challenges in halide perovskite photovoltaics—from the atomic to module level

Carlo A.R. Perini is a post-doctoral researcher at Georgia Institute of Technology, Energy Materials Lab. His studies focus on the development and understanding of interface passivation of 3D metal halide perovskites using vapor deposition routes. He has a PhD in Physics at Politecnico di Milano, in collaboration with the Italian Institute of Technology (IIT), focused on the study of energy efficient, scalable, and environmentally friendly deposition routes for lead halide perovskites. Tiarnan A. S. Doherty is a PhD student in the Cavendish laboratory at the University of Cambridge working on nanoscale structure property relationships in metal-halide perovskites. He received the gold graduate student award at the Fall Materials Research Society meeting. Samuel D. Stranks is a University Lecturer in Energy and Royal Society University Research Fellow at the University of Cambridge. His group's research focuses on the optical and electronic properties of emerging semiconductors for low-cost, transformative electronics applications including light-harvesting (e.g., photovoltaic) and light-emission (e.g., LED) devices. He received the 2018 Henry Moseley Award and Medal from the Institute of Physics, the 2019 Marlow Award from the Royal Society of Chemistry, and is a co-founder of Swift Solar, a startup developing lightweight perovskite PV panels. Juan-Pablo Correa-Baena is an Assistant Professor and Goizueta Junior Faculty Chair in the School of Materials Science and Engineering at Georgia Institute of Technology. His group focuses on the understanding and control of electronic dynamics at the nanoscale for low-cost semiconductors, such as halide perovskites. He completed his PhD at the University of Connecticut, followed by a postdoctoral fellowship at EPFL, Switzerland, then a US Department of Energy postdoctoral fellowship at MIT, USA. Robert L. Z. Hoye is a Lecturer (Assistant Professor) at Imperial College London. He leads the Energy Materials and Devices group, which is working on developing defect-tolerant semiconductors and their application in optoelectronic devices. This includes tandem photovoltaics, indoor solar cells, and light-emitting diodes. Robert has received the Young Engineer of the Year Award from the Royal Academy of Engineering, the Sir Henry Royce Medal from the Institution of Engineering and Technology, and the Rising Star Award from the Waterloo Institute for Nanotechnology.

Femtosecond Laser Induced Resonant Tunneling in an Individual Quantum Dot Attached to a Nanotip

Quantized nano-objects offer a myriad of exciting possibilities for manipulating electrons and light that impact photonics, nanoelectronics, and quantum information. In this context, ultrashort laser pulses combined with nanotips and field emission have permitted renewing nano-characterization and control electron dynamics with unprecedented space and time resolution reaching femtosecond and even attosecond regimes. A crucial missing step in these experiments is that no signature of quantized energy levels has yet been observed. We combine in situ nanostructuration of nanotips and ultrashort laser pulse excitation to induce multiphoton excitation and electron emission from a single quantized nano-object attached at the apex of a metal nanotip. Femtosecond induced tunneling through well-defined localized confinement states that are tunable in energy is demonstrated. This paves the way for the development of ultrafast manipulation of electron emission from isolated nano-objects including stereographically fixed individual molecules and high brightness, ultrafast, coherent single electron sources for quantum optics experiments.

Ripple period adjustment on SiC surface based on electron dynamics control and its polarization anisotropy

In this paper, the Mach Zehnder interferometer is utilized to generate a collinear dual optical path with 800-nm center wavelength, 1 kHz repetition rate, 120-fs pulse duration and an energy ratio of 1:1 to scan 6H-SiC crystal at a scanning speed of 500 μm/s, resulting in laser-induced periodic surface structure (LIPSS). From the perspective of instantaneous electron density level, variation of the period of low spatial frequency LIPSS (LSFL) with interpulse delay time and polarization angle is studied. The scanning electron microscope (SEM) characterization of the line-scanned ablation zone combined with fast Fourier transformation (FFT) spectrum analysis shows that the period of LSFL decreases with the increase of delay time, and remains stable when the delay exceeds 240 fs. Meanwhile, the period of LSFL increases with the increase of polarization angle under the same delay time, showing an anisotropy that dependent on the polarization angle. The variation trend of the maximum electron density level with delay time calculated by electron rate equation is roughly consistent with the periodic variation trend. Interaction between incident light and transient metal-like SiC surface is simulated based on finite-difference time-domain method (FDTD). The simulation results show that the surface of the sample has a periodic light field enhancement structure controlled by the transient electron density, which explains the phenomenon that the period decreases with increasing delay time. It is further confirmed that the formation of LSFL is due to the interference of incident light and surface plasmon, and the period of LSFL can be controlled by adjusting the electron density level by delay time. The periodic polarization dependence of LSFL may be related to the relative orientation change of the laser pulse front tilt (PFT) and laser polarization.

Electron choreography at the attosecond time scale

In this work, we report on coherent control of electron dynamics in atoms via attosecond pulse-shaping. We show that the photoelectron emission from argon gas produced by absorption of an attosecond pulse train (APT) made of odd and even harmonics can be manipulated along the direction of polarization of the light by tuning the spectral components (amplitude and phase) of the pulse. In addition, we show that APTs produced with a two-color (400- plus 800 nm) femtosecond driving field exhibit high temporal tunability, which is optimized for an intensity ratio between the two colors in the range of 0.1 to 5%.

Betatron radiation polarization control by using an off-axis ionization injection in a laser wakefield acceleration.

Tunable X-ray sources from a laser-driven wakefield have wide applications. However, due to the difficulty of electron dynamics control, currently the tunability of laser wakefield-based X-ray sources is still difficult. By using three-dimensional particle-in-cell simulations, we propose a scheme to realize controllable electron dynamics and X-ray radiation. In the scheme, a long wavelength drive pulse excites a plasma wake and an off-axis laser pulse with a short wavelength co-propagates with the drive pulse and ionizes the K-shell electrons of the background high-Z gas. The electrons can be injected in the wakefield with controllable transverse positions and residual momenta. These injected electrons experience controllable oscillations in the wake, leading to tunable radiations both in intensity and polarization.

Outer Van Allen belt trapped and precipitating electron flux responses to two interplanetary magnetic clouds of opposite polarity

Abstract. Recently, it has been established that interplanetary coronal mass ejections (ICMEs) can dramatically affect both trapped electron fluxes in the outer radiation belt and precipitating electron fluxes lost from the belt into the atmosphere. Precipitating electron fluxes and energies can vary over a range of timescales during these events. These variations depend on the initial energy and location of the electron population and the ICME characteristics and structures. One important factor controlling electron dynamics is the magnetic field orientation within the ejecta that is an integral part of the ICME. In this study, we examine Van Allen Probes (RBSPs) and Polar Orbiting Environmental Satellites (POESs) data to explore trapped and precipitating electron fluxes during two ICMEs. The ejecta in the selected ICMEs have magnetic cloud characteristics that exhibit the opposite sense of the rotation of the north–south magnetic field component (BZ). RBSP data are used to study trapped electron fluxes in situ, while POES data are used for electron fluxes precipitating into the upper atmosphere. The trapped and precipitating electron fluxes are qualitatively analysed to understand their variation in relation to each other and to the magnetic cloud rotation during these events. Inner magnetospheric wave activity was also estimated using RBSP and Geostationary Operational Environmental Satellite (GOES) data. In each event, the largest changes in the location and magnitude of both the trapped and precipitating electron fluxes occurred during the southward portion of the magnetic cloud. Significant changes also occurred during the end of the sheath and at the sheath–ejecta boundary for the cloud with south to north magnetic field rotation, while the ICME with north to south rotation had significant changes at the end boundary of the cloud. The sense of rotation of BZ and its profile also clearly affects the coherence of the trapped and/or precipitating flux changes, timing of variations with respect to the ICME structures, and flux magnitude of different electron populations. The differing electron responses could therefore imply partly different dominant acceleration mechanisms acting on the outer radiation belt electron populations as a result of opposite magnetic cloud rotation.

Symmetry breaking in the Stark Control of Electrons at Interfaces (SCELI).

Ultrafast control of electron dynamics is essential for future innovations in nanoelectronics, catalysis, and molecular imaging. Recently, we developed a general scheme (Stark Control of Electrons at Interfaces or SCELI) to control electron dynamics at interfaces [A. J. Garzón-Ramírez and I. Franco, Phys. Rev. B 98, 121305 (2018)] that is based on using few-cycle lasers to open quantum tunneling channels for interfacial electron transfer. SCELI uses the Stark effect induced by non-resonant light to create transient resonances between a donor level in material B and an acceptor level in material A, resulting in B → A electron transfer. Here, we show how SCELI can be employed to generate net charge transport in ABA heterojunctions without applying a bias voltage, a phenomenon known as laser-induced symmetry breaking. The magnitude and sign of such transport can be controlled by simply varying the time asymmetry of the laser pulse through manipulation of laser phases. In particular, we contrast symmetry breaking effects introduced by manipulation of the carrier envelope phase with those introduced by relative phase control in ω + 2ω laser pulses. The ω + 2ω pulse is seen to be far superior as such pulses exhibit a larger difference in field intensity for positive and negative amplitudes. The results exemplify the power of Stark-based strategies for controlling electrons using lasers.

Toward attosecond control of electron dynamics in two-dimensional materials

Attosecond motion of strongly driven electrons encodes information on intense laser–solid interactions, leading to material- and laser-dependent photoemission and high-harmonic generation (HHG). Here, we investigate sub-cycle control over the electron dynamics in two-dimensional (2D) materials via adjusting the relative phase of two-color pulses. Electrons in 2D solids are sensitive to the exact shape of the optical field, showing properties similar to those of isolated atoms. Accompanied by spectra modulations, the resultant harmonic yields are enhanced and the cutoff energy increases to a ratio as large as 24% when an optimized phase is applied. Different from the atomic-like HHG, however, the cutoff energy is linearly dependent on the maximum strength of the light electric field, in line with that shown in conventional bulk solids. Thus, two-dimensional materials provide a unique platform where both bulk and atomic electron dynamics can be investigated. Our work suggests a simple way to generate optimized harmonic emission with existing experimental laser technology and offers a powerful tool for analyzing attosecond quantum dynamics during laser–solid interactions.

High-order harmonic generation in aligned molecules by combination of weak XUV and intense IR laser pulses

We present the response of the electron to a weak XUV pulse in the presence of an intense IR 800 nm laser field. Simulations are performed on an aligned molecular ion H2+ by numerically solving the corresponding time-dependent Schrödinger equation. It is found that high order harmonic generation (HHG) spectra exhibit signature of harmonic humps, which is shown to be sensitive to the XUV pulse polarization and frequency, and the time delay between the two pulses. Harmonic humps correspond to the perturbative XUV absorption. The enhancement of harmonic humps in IR fields allows to control electron dynamics by XUV-IR combined fields in HHG processes.

Electron dynamics in strained graphene

This paper presents a theoretical description of the effects of strain induced by out-of-plane deformations on charge distributions and transport on graphene. A review of a continuum model for electrons using the Dirac formalism is complemented with elasticity theory to represent strain fields. The resulting model is cast in terms of scalar and pseudo-magnetic fields that control electron dynamics. Two distinct geometries, a bubble and a fold, are chosen to represent the most commonly observed deformations in experimental settings. It is shown that local charge accumulation regions appear in deformed areas, with a peculiar charge distribution that favors occupation of one sublattice only. This unique phenomenon that allows to distinguish each carbon atom in the unit cell, is the manifestation of a sublattice symmetry broken phase. For specific parameters, resonant states appear in localized charged regions, as shown by the emergence of discrete levels in band structure calculations. These findings are presented in terms of intuitive pictures that exploit analogies with confinement produced by square barriers. In addition, electron currents through strained regions are spatially separated into their valley components, making possible the manipulation of electrons with different valley indices. The degree of valley filtering (or polarization) for a specific system can be controlled by properly designing the strained area. The comparison between efficiencies of filters built with this type of geometries identifies extended deformations as better valley filters. A proposal for their experimental implementations as component of devices, and a discussion for potential observation of novel physics in strained structures are presented at the end of the paper.

Control of electron dynamics, radical and metastable species generation in atmospheric pressure RF plasma jets by Voltage Waveform Tailoring

Atmospheric pressure capacitively coupled radio frequency discharges operated in He/N2 mixtures and driven by tailored voltage waveforms are investigated experimentally using a COST microplasma reference jet and by means of kinetic simulations as a function of the reactive gas admixture and the number of consecutive harmonics used to drive the plasma. Pulse-type ‘peaks’-waveforms, that consist of up to four consecutive harmonics of the fundamental frequency (f = 13.56 MHz), are used at a fixed peak-to-peak voltage of 400 V. Based on an excellent agreement between experimental and simulation results with respect to the DC self-bias and the spatio-temporal electron impact excitation dynamics, we demonstrate that Voltage Waveform Tailoring allows for the control of the dynamics of energetic electrons, the electron energy distribution function in distinct spatio-temporal regions of interest, and, thus, the generation of atomic nitrogen as well as helium metastables, which are highly relevant for a variety of technological and biomedical applications. By tuning the number of driving frequencies and the reactive gas admixture, the generation of these important species can be optimised. The behaviour of the DC self-bias, which is different compared to that in low pressure capacitive radio frequency plasmas, is understood based on an analytical model.