Ultrafast Electronic Processes Research Articles

Ultrafast electronic processes in an insulator: The Be and O sites in BeO

The short-time dynamics of amorphous beryllium oxide (a-BeO) has been investigated for electronic excitation/ionization by fast incident electrons, as well as by Ar7+, Ar15+, Xe15+, and Xe31+ ions at velocities of 6–10% the speed of light. Site specific Auger-electron spectra induced by fast heavy ions are the central point of this investigation. Electron induced Auger spectra serve as a reference and electron-energy loss (EELS) spectroscopy as well as resonant inelastic X-ray scattering (RIXS) are invoked for quantitative understanding. For the heavy-ion case, we observe strong variations in the corresponding spectral distributions of Be–K and O–K Auger lines. These are related to local changes of the electron density, of the electron temperature and even of the electronic band structure of BeO on a femtosecond time scale after the passage of highly charged heavy ions.

Ultrafast broadband spectroscopy of crystalline bismuth

Femtosecond spectroscopy in the wavelength range has been used to probe ultrafast electronic and lattice processes in bismuth. The photoresponse of a bismuth crystal is shown to comprise components with relaxation times of , , and . The electron – hole and electron – phonon interaction strengths in bismuth are found to depend significantly on the wave vector in the direction of the Brillouin zone. Comparison of the spectral dependences of the amplitudes of coherent and phonons and the corresponding dependences of the Raman scattering cross sections indicates that these phonon modes differ in generation mechanism. The generation of coherent phonons is mainly due to displacement of the equilibrium position of atoms in the crystal lattice in a nonequilibrium state. This process differs fundamentally from resonance Raman scattering responsible for the coherent excitation of low-symmetry phonon modes.

Identification of ultra-fast electronic and thermal processes during femtosecond laser ablation of Si

Ultra-fast electronic and thermal processes for the energy deposition mechanism during femtosecond laser ablation of Si have been identified by means of atomic force microscopy and Raman scattering techniques. For this purpose, Si targets were exposed with 800-nm, 25-fs Ti:sapphire laser pulses for different laser fluencies in air and under UHV (ultra high vacuum) conditions. Various nano- and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Ultra-fast electronic processes are dominant in the lower-fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower-fluence regime (0.06–0.5 J cm−2 single-shot irradiation under UHV condition and 0.25–2.5 J cm−2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J cm−2 multiple-shot irradiation), and a higher-fluence regime (2.5–3.5 J cm−2 multiple-shot irradiation). Around the ablation threshold fluence, most significant features identified at the Si surface are nanohillock-like structures. The appearance of these nanohillocks is regarded as typical features for fast electronic processes (correlated with existence of hot electrons) and is explained on the basis of Coulomb explosion. The growth of these typical features (nanohillocks) by femtosecond laser irradiation is an element of novelty. At moderate irradiation fluence, a ring-shaped ablation with larger bumps and periodic surface structures is observed and is considered as a footprint of ultra-fast melting. Further increase in the laser fluence, i.e. a higher-fluence regime, resulted in strong enhancement of the thermal process with the appearance of larger islands. The change in surface topography provides an innovative clue to differentiate between ultra-fast electronic processes, i.e. Coulomb explosion (sub-100 fs) at a lower-fluence regime and ultra-fast melting (hundreds of fs) at a moderate-fluence regime, and slow thermal processes (ps time scale) at a higher-fluence regime. These fast electronic and thermal processes are well correlated to structural and crystallographic alterations, inferred from Raman spectroscopy.

Carrier–envelope phase-tagged imaging of the controlled electron acceleration from SiO2 nanospheres in intense few-cycle laser fields

Waveform-controlled light fields offer the possibility of manipulating ultrafast electronic processes on sub-cycle timescales. The optical lightwave control of the collective electron motion in nanostructured materials is key to the design of electronic devices operating at up to petahertz frequencies. We have studied the directional control of the electron emission from 95 nm diameter SiO2 nanoparticles in few-cycle laser fields with a well-defined waveform. Projections of the three-dimensional (3D) electron momentum distributions were obtained via single-shot velocity-map imaging (VMI), where phase tagging allowed retrieving the laser waveform for each laser shot. The application of this technique allowed us to efficiently suppress background contributions in the data and to obtain very accurate information on the amplitude and phase of the waveform-dependent electron emission. The experimental data that are obtained for 4 fs pulses centered at 720 nm at different intensities in the range (1–4) × 1013 W cm−2 are compared to quasi-classical mean-field Monte-Carlo simulations. The model calculations identify electron backscattering from the nanoparticle surface in highly dynamical localized fields as the main process responsible for the energetic electron emission from the nanoparticles. The local field sensitivity of the electron emission observed in our studies can serve as a foundation for future research on propagation effects for larger particles and field-induced material changes at higher intensities.

Surface analysis correlated with the Raman measurements of a femtosecond laser irradiated Ca F2

Ultra fast electronic and thermal processes for energy deposition mechanism during femtosecond laser ablation of CaF2 have been identified by means of Atomic Force Microscopy (AFM) and Raman scattering technique. For this purpose, a single crystal CaF2 (111) was exposed with 800nm, 25fs Ti: Sapphire laser pulses at different laser fluences both in air and under UHV condition. Various nano and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Around the ablation threshold fluence, most significant features identified at the CaF2 surface are nanohillock like structures. These nanohillocks are typical features related to the fast electronic processes and are explainable on the basis of Coulomb explosion. At moderate irradiation fluence, bump formation is considered to be due to ultrafast melting. Further increase in the laser fluence resulted into strong enhancement of the thermal process with the appearance of larger humps and craters. These fast electronic and thermal processes are well correlated with the structural and crystallographic alterations inferred from Raman spectroscopy analysis. The nanohillocks appearing at a lower fluence are due to calcium colloid formation (aggregates of metal clusters). At higher fluences and dozes, the compressive as well as tensile stresses along with the presence of calcium carbonate are associated to diffusion, transformation and aggregation of defects which are typical features of thermal processes leading to the growth of larger humps.

High-energy isolated attosecond pulses generated by above-saturation few-cycle fields

The applications of isolated attosecond pulses reported to date, which have demonstrated the great potential of attosecond technology in the investigation of ultrafast electronic processes, have been limited by the low photon flux of the available attosecond sources. We report on the generation of isolated sub-160-as pulses (at a photon energy of ∼30 eV) with a pulse energy, on target, of a few nanojoules. The efficient generation of isolated attosecond pulses in noble gases is produced by 5-fs driving pulses with controlled electric field and peak intensity beyond the gas saturation intensity. The availability of attosecond sources with high peak intensities has potential in opening new avenues for attosecond-pump/attosecond-probe studies of electronic processes in atomic and molecular physics, with interesting prospects in the field of coherent control of electronic motion in complex systems in the attosecond temporal regime. Researchers report the generation of isolated sub-160-attosecond pulses that have photon energies of 30 eV, resulting in an on-target pulse energy of a few nanojoules. The availability of attosecond sources with high peak intensities may open new avenues for attosecond pump/probe studies of electronic processes in atomic and molecular physics.

GaN-based two-dimensional channels: hot-electron fluctuations and dissipation

Ultrafast electronic and phononic processes are investigated in voltage-biased GaN-basedtwo-dimensional channels of interest for heterostructure field-effect transistors. Theaccumulation of non-equilibrium longitudinal optical phonons (hot phonons) is treated forAlGaN/GaN, AlGaN/AlN/GaN, AlGaN/GaN/AlN/GaN, and AlInN/AlN/GaN structuresin terms of the hot-phonon temperature and hot-phonon lifetime. The hot-phononeffect on hot-electron energy relaxation and hot-phonon number relaxation isextracted from an experimental investigation of hot-electron fluctuations and powerdissipation. The measured equivalent hot-phonon temperature is nearly equalto the hot-electron temperature. The hot-phonon lifetime varies in the rangefrom 150 to 800 fs and depends on electron density, temperature, and suppliedelectric power. The experimental dependence of the hot-phonon lifetime on thehot-phonon mode occupancy is unique—neither Raman optical-photon scattering noroptical-phonon-assisted intersubband absorption has, as yet, provided any experimentaldata of this sort.

Efficient Excited-State Deactivation of the Gly-Phe-Ala Tripeptide via an Electron-Driven Proton-Transfer Process

Ab initio electronic-structure calculations indicate a mechanism for efficient excited-state deactivation of a low-energy conformer of the Gly-Phe-Ala tripeptide. The particularly short excited-state lifetime can explain the unexpected absence of this conformer in resonant two-photon ionization spectra. It is suggested that these ultrafast electronic deactivation processes provide specific conformers of peptides with a high degree of photostability.

Laser-induced currents along molecular wire junctions

The treatment of the previous paper is extended to molecular wires. Specifically, the effect of electron-vibrational interactions on the electronic transport induced by femtosecond omega+2omega laser fields along unbiased molecular nanojunctions is investigated. For this, the photoinduced vibronic dynamics of trans-polyacetylene oligomers coupled to macroscopic metallic leads is followed in a mean-field mixed quantum-classical approximation. A reduced description of the dynamics is obtained by introducing projective lead-molecule couplings and deriving an effective Schrodinger equation satisfied by the orbitals in the molecular region. Two possible rectification mechanisms are identified and investigated. The first one relies on near-resonance photon-absorption and is shown to be fragile to the ultrafast electronic decoherence processes introduced by the wire's vibrations. The second one employs the dynamic Stark effect and is demonstrated to be highly efficient and robust to electron-vibrational interactions.

Robust Ultrafast Currents in Molecular Wires through Stark Shifts

A novel way to induce ultrafast currents in molecular wires using two incident laser frequencies, omega and 2omega, is demonstrated. The mechanism relies on Stark shifts, instead of near-resonance photon absorption, to transfer population to the excited states and exploits the temporal profile of the field to generate phase-controllable transport. Calculations in a trans- polyacetylene oligomer coupled to metallic leads indicate that the mechanism is highly efficient and robust to ultrafast electronic dephasing processes induced by vibronic couplings.

Isolated sub-100‐aspulse generation via controlling electron dynamics

A new method to coherently control the electron dynamics is proposed using a few-cycle laser pulse in combination with a controlling field. It is shown that this method not only broadens the attosecond pulse bandwidth, but also reduces the chirp, then an isolated 80-as pulse is straightforwardly obtained and even shorter pulse is achievable by increasing the intensity of the controlling field. Such ultrashort pulses allow one to investigate ultrafast electronic processes which have never be achieved before. In addition, the few-cycle synthesized pulse is expected to manipulate a wide range of laser-atom interactions.

Tracing Ultrafast Interatomic Electronic Decay Processes in Real Time and Space

Advances in laser pump-probe techniques open the door for observations in real time of ultrafast electronic processes. Particularly attractive is the visualization of interatomic processes where one can follow the energy transfer from one atom to another. The interatomic Coulombic decay (ICD) provides such a process which is abundant in nature. A wave packet propagation method now enables us to trace fully ab initio the electron dynamics of the process in real time and space, taking into account all electrons of the system and their correlations. The evolution of the electronic cloud during the ICD process in NeAr following Ne2s ionization is computed and analyzed. The process takes place on a femtosecond time scale, and a surprisingly strong response is found already in the attosecond regime.

Fast electronic and thermal processes in femtosecond laser ablation of Au

Velocity distribution, pulse width dependence studies, and two-pulse correlation measurements have been used to study the possibility of the occurrence of ultrafast electronic and thermal ablation processes in Au exposed to ultrashort laser pulses in the femtosecond to picosecond time domain. Three distinct different velocity groups (5.5, 1.5, and 0.25eV) have been observed and can be attributed to two ultrafast electronic processes (Coulomb explosion and rapid plasma formation) and a thermal process. The buildup of a rapid plasma favors the laser energy absorption around 400fs after the beginning of the laser-matter interaction.

Primary processes underlying the photostability of isolated DNA bases: adenine.

The UV chromophores in DNA are the nucleic bases themselves, and it is their photophysics and photochemistry that govern the intrinsic photostability of DNA. Because stability is related to the conversion of dangerous electronic to less-dangerous vibrational energy, we study ultrafast electronic relaxation processes in the DNA base adenine. We excite adenine, isolated in a molecular beam, to its pipi* state and follow its relaxation dynamics using femtosecond time-resolved photoelectron spectroscopy. To discern which processes are important on which timescales, we compare adenine with 9-methyl adenine. Methylation blocks the site of the much-discussed pisigma* state that had been thought, until now, minor. Time-resolved photoelectron spectroscopy reveals that, although adenine and 9-methyl adenine show almost identical timescales for the processes involved, the decay pathways are quite different. Importantly, we confirm that in adenine at 267-nm excitation, the pisigma* state plays a major role. We discuss these results in the context of recent experimental and theoretical studies on adenine, proposing a model that accounts for all known results, and consider the relationship between these studies and electron-induced damage in DNA.

Ultrafast electronic relaxation processes in porous silicon

We have studied ultrafast relaxation processes in porous silicon using a femtosecond pump and probe pulse-correlation technique at 440-nm wavelength. We have observed photoinduced absorption with a response time of the order of 5 ps. In addition to this fast relaxation process, we have also observed a strong induced absorption change on millisecond timescales. From the observation of the picosecond response, the relevant carrier dynamics is clarified as follows: carriers are excited in Si microcrystals and then rapidly thermalize to the surface states within 5 ps. The observation of the strong absorption change on millisecond timescales suggests the possibility of fabricating optical logic gates or all-optical modulators using porous silicon.

Femtosecond time-resolved ionization spectroscopy of ultrafast internal-conversion dynamics in polyatomic molecules: Theory and computational studies

A framework for the theoretical description of two-pulse time-resolved ionization spectroscopy of ultrafast excited-state dynamics of polyatomic molecules is developed. The radiation–matter interaction as well as intramolecular couplings in the excited-state manifold are treated nonperturbatively by solving the time-dependent Schrödinger equation. The numerical solution is based on a discretization of the ionization continua which becomes particularly efficient for ultrashort laser pulses. With this method converged computations of ionization signals become possible even for complex molecular systems. Computer simulations are performed for a model system representing three-dimensional non-Born–Oppenheimer excited-state dynamics on conically intersecting potential-energy surfaces (the S1 and S2 surfaces of pyrazine). The dependence of the observable time-resolved ionization signals (total ion yield as well as photoelectron spectrum) on the properties of the laser pulses (carrier frequency and pulse duration) is explored. It is demonstrated that ultrafast electronic decay processes as well as coherent vibrational motion in excited states can be monitored by pump–probe ionization with suitable pulses. The dependence of the time-resolved ionization signals on properties of the cation (ionization potentials and potential-energy surfaces) is also discussed.

Femtosecond spectroscopy in amorphous silicon and silicon-germanium alloys

The methods of femtosecond optical spectroscopy have been used to study the ultrafast electronic processes in undoped alloys of amorphous hydrogenated silicon prepared by rf glow discharge. The transient changes in reflectivity and transmission are studied as a function of wavelength, carrier density and alloy composition. The results are explained by taking into account hot carrier relaxation in the extended states, trapping into weakly localized bandtail states and thermalization in those bandtail states.

Initial stages of trapping in a-Si:H observed by femtosecond spectroscopy.

We report the first femtosecond spectroscopic investigation of ultrafast electronic processes in $a$-Si:H. By tuning the probe wavelength in a wide range around the optical gap, we are able to follow the time evolution of carrier trapping in band-tail states. At moderate injected carrier density, the band-tail states are populated by multiple trapping. As more carriers are injected, the shallow band-tail states saturate and deeper band-tail states capture carriers by direct trapping, which is a faster process (1-2 ps) than multiple trapping (10 ps).