lampros nikolopoulos | Dublin City University (original) (raw)

Papers by lampros nikolopoulos

Contemporary Physics, 2014

AIP Conference Proceedings, 2000

We review recent progress in ab initio approaches to few-photon excitation and ionization of two-... more We review recent progress in ab initio approaches to few-photon excitation and ionization of two-electron systems with emphasis on H− and He. These approaches employ discretized basis sets capable of accounting for correlation as well as for the time evolution of the system under a strong laser pulse. Fundamental aspects of two-photon double ionization of Helium are also discussed.

Springer Series in Chemical Physics, 2012

ABSTRACT Attosecond pulses today are generated at pulse energies leading to intensities sufficien... more ABSTRACT Attosecond pulses today are generated at pulse energies leading to intensities sufficient to induce two-photon transitions in the extreme ultraviolet (XUV) spectral region. Recently, ultra-broadband coherent XUV continua fulfill also the requirements in inducing such processes. Two-XUV-photon ionization is a pivotal tool in attosecond pulse metrology, as well as for XUV-pump-XUV-probe applications targeting the tracking of ultrafast dynamics, providing at the same time spatial selectivity.Based on these developments, this chapter (a) reviews approaches leading to high intensities of attosecond pulse trains and coherent XUV continua; (b) reviews metrology approaches based on two-XUV-photon ionization, showing their importance through comparative studies with existing XUV-IR cross-correlation approaches; and (c) reports the feasibility of XUV-pump-XUV-probe applications at the 1fs resolution level, in an experiment, where atomic coherence is induced in a rich manifold of doubly excited and inner-shell excited autoionizing states, the evolution of which is tracked through double ionization.

Physical Review A

ABSTRACT

A method is presented for the calculation of the double ionization probabilities of a two-elecron... more A method is presented for the calculation of the double ionization probabilities of a two-elecron system in a strong time-dependent laser field. The system wavefunction is expanded on a basis of the zero-order two-electron wavefunctions in the sense that correlation between the electrons is not included in this basis. The electron-electron interaction is treated in an equal footing with the electric field operator in the solution of the time-dependend Schrödinger equation. from the initial state until the wavefunction relaxes. Propagation, and thus correlation effect are fully incorporated, is taken place long after the field is off.

Open Computing Language (OpenCL) is a parallel processing language that is ideally suited for run... more Open Computing Language (OpenCL) is a parallel processing language that is ideally suited for running parallel algorithms on Graphical Processing Units (GPUs). In the present work we report on the development of a generic parallel single-GPU code for the numerical solution of a system of first-order ordinary differential equations (ODEs) based on the OpenCL model. We have applied the code in the case of the Time-Dependent Schr\"odinger Equation of atomic hydrogen in a strong laser field and studied its performance on NVIDIA and AMD GPUs against the serial performance on a CPU. We found excellent scalability and a significant speed-up of the GPU over the CPU device which tended towards a value of about 40 with significant speedups expected against multi-core CPUs.

Physical Review A, 2002

ABSTRACT

New Journal of Physics

We present photoelectron angular distributions resulting from the two-photon direct double ioniza... more We present photoelectron angular distributions resulting from the two-photon direct double ionization of helium, under XUV radiation, by solving the time-dependent (TD) Schrödinger equation. The helium TD wavefunction is expanded in terms of fully correlated multichannel states normalized with incoming-wave boundary conditions. The present study focuses on fields of pulse durations within the subfemtosecond regime and at photon energy of 45 eV where the direct double ionization channel dominates the sequential channel. In addition, at this photon energy, the ejected electrons, resulting from the direct and the sequential path, acquire non-overlapping kinetic energy spectra. Our study reveals a trend for back-to-back ejection asymmetry independently of the kinetic energies of the electrons, thus implying that angular correlations are taking place at the time of the ionization. In addition, for given kinetic-energy sharing, it appears that this asymmetry is developed within an interat...

In view of recent developments in the production of high intensity and subpicosecond pulse durati... more In view of recent developments in the production of high intensity and subpicosecond pulse duration, XUV radiation sources, we discuss a number of novel photoabsorption processes that are expected to provide windows into a variety of hitherto unexplored territory of atomic and molecular structure, involving highly excited states embedded in multiple continua.

Handbook for Highly Charged Ion Spectroscopic Research, 2011

Central European Journal of Physics, 2013

ABSTRACT A set of rate equations have been tested against a more robust set of Time-Dependent Den... more ABSTRACT A set of rate equations have been tested against a more robust set of Time-Dependent Density Matrix (TDDM) equations [1] by using them to determine the populations of ion species and autoionising states (AIS) in noble gas atoms when interacting with a strong external field. Two field shapes were tested here — sinusoidal and square - and a variety of pulse characteristics were examined, i.e. intensity, duration and photon energy, for the neon atomic system. It was found that the rate equations were sufficiently accurate only when the external field is way off-resonant with the AIS. Moreover, analytical solutions of the rate equations in the square pulse case agree with the numerical solutions for a time-dependent pulse containing many cycles. An attempt to model a stochastic field was also made and it was found that the use of such a field diminished and broadened the ion yield ratio due to the presence of an added bandwidth.

Springer Proceedings in Physics, 2012

After introducing the importance of non-linear processes in the extreme-ultra-violet (XUV) spectr... more After introducing the importance of non-linear processes in the extreme-ultra-violet (XUV) spectral regime to the attosecond (asec) pulse metrology and time domain applications, we present two successfully implemented techniques with excellent prospects in generating intense asec pulse trains and isolated asec pulses, respectively. For the generation of pulse trains two-color harmonic generation is exploited. The interferometric polarization gating technique appropriate for the generation of intense isolated asec pulses is discussed and compared to other relevant approaches.

Central European Journal of Physics, 2013

ABSTRACT In the present work, we develop a calculational method of solving the scattering equatio... more ABSTRACT In the present work, we develop a calculational method of solving the scattering equations for spherically symmetric potentials by expanding the solutions on Coulomb functions. We utilize a multistep integration scheme together with the standard partial wave analysis in a region where the potential term dominates. The method applies to any physical problem expressed as [∇2 + V(r) + k 2] ψ(r) = 0, while the extension of the method to more general scattering problems is briefly discussed. At present, we demonstrate a two-step Coulomb-fitted integration scheme by calculating the short-range scattering phase shifts for various potentials V(r).

Hyperfine Interactions, 2000

ABSTRACT The interaction between Ps and a strong laser pulse of short duration is studied. Substa... more ABSTRACT The interaction between Ps and a strong laser pulse of short duration is studied. Substantial population is deposited in excited states under few photon excitation. The possible usefulness as a source of excited Ps to spectroscopy and the formation of antihydrogen is discussed. Finally, photoelectron energy spectra are calculated.

Physical Review A - Atomic, Molecular, and Optical Physics, 2005

In anticipation of upcoming two-photon double ionization of atoms and particularly Helium, under ... more In anticipation of upcoming two-photon double ionization of atoms and particularly Helium, under strong short wavelength radiation sources (45 eV), we present quantitative signatures of direct twophoton double ejection, in the photoelectron spectrum (PES) and the peak power dependence, that can be employed in the interpretation of related data. We show that the PES provides the cleanest signature of the process. An inflection (knee) in the laser power dependence of double ionization is also discernible, within a window of intensities which depends on the pulse duration and cross sections PACS numbers: 32.80.Wr

Physical Review A - Atomic, Molecular, and Optical Physics, 2007

We present a method for spectral (bound and continuum) and partial-wave analysis of a three-dimen... more We present a method for spectral (bound and continuum) and partial-wave analysis of a three-dimensional time-dependent wave function, defined on a grid, without projecting onto the field-free eigenstates of the system. The method consists of propagating the time-dependent Schrödinger equation to obtain its autocorrelation function C(t)= after the end of the interaction, at time T , of the system with

High-Power Laser Ablation V, 2004

We review recent developments in the generation and metrology of XUV-attosecond (as) pulses. The ... more We review recent developments in the generation and metrology of XUV-attosecond (as) pulses. The review includes an extended report on the recently achieved 2nd-order autocorrelation measurement of a sub-femtosecond (sub-fs) pulse train.

Physical Review Letters, 2006

A Comment on the Letter by Yasuo Nabekawa et al., Phys. Rev. Lett. 94, 043001 (2005). The authors... more A Comment on the Letter by Yasuo Nabekawa et al., Phys. Rev. Lett. 94, 043001 (2005). The authors of the Letter offer a Reply.

Contemporary Physics, 2014

AIP Conference Proceedings, 2000

We review recent progress in ab initio approaches to few-photon excitation and ionization of two-... more We review recent progress in ab initio approaches to few-photon excitation and ionization of two-electron systems with emphasis on H− and He. These approaches employ discretized basis sets capable of accounting for correlation as well as for the time evolution of the system under a strong laser pulse. Fundamental aspects of two-photon double ionization of Helium are also discussed.

Springer Series in Chemical Physics, 2012

ABSTRACT Attosecond pulses today are generated at pulse energies leading to intensities sufficien... more ABSTRACT Attosecond pulses today are generated at pulse energies leading to intensities sufficient to induce two-photon transitions in the extreme ultraviolet (XUV) spectral region. Recently, ultra-broadband coherent XUV continua fulfill also the requirements in inducing such processes. Two-XUV-photon ionization is a pivotal tool in attosecond pulse metrology, as well as for XUV-pump-XUV-probe applications targeting the tracking of ultrafast dynamics, providing at the same time spatial selectivity.Based on these developments, this chapter (a) reviews approaches leading to high intensities of attosecond pulse trains and coherent XUV continua; (b) reviews metrology approaches based on two-XUV-photon ionization, showing their importance through comparative studies with existing XUV-IR cross-correlation approaches; and (c) reports the feasibility of XUV-pump-XUV-probe applications at the 1fs resolution level, in an experiment, where atomic coherence is induced in a rich manifold of doubly excited and inner-shell excited autoionizing states, the evolution of which is tracked through double ionization.

Physical Review A

ABSTRACT

A method is presented for the calculation of the double ionization probabilities of a two-elecron... more A method is presented for the calculation of the double ionization probabilities of a two-elecron system in a strong time-dependent laser field. The system wavefunction is expanded on a basis of the zero-order two-electron wavefunctions in the sense that correlation between the electrons is not included in this basis. The electron-electron interaction is treated in an equal footing with the electric field operator in the solution of the time-dependend Schrödinger equation. from the initial state until the wavefunction relaxes. Propagation, and thus correlation effect are fully incorporated, is taken place long after the field is off.

Open Computing Language (OpenCL) is a parallel processing language that is ideally suited for run... more Open Computing Language (OpenCL) is a parallel processing language that is ideally suited for running parallel algorithms on Graphical Processing Units (GPUs). In the present work we report on the development of a generic parallel single-GPU code for the numerical solution of a system of first-order ordinary differential equations (ODEs) based on the OpenCL model. We have applied the code in the case of the Time-Dependent Schr\"odinger Equation of atomic hydrogen in a strong laser field and studied its performance on NVIDIA and AMD GPUs against the serial performance on a CPU. We found excellent scalability and a significant speed-up of the GPU over the CPU device which tended towards a value of about 40 with significant speedups expected against multi-core CPUs.

Physical Review A, 2002

ABSTRACT

New Journal of Physics

We present photoelectron angular distributions resulting from the two-photon direct double ioniza... more We present photoelectron angular distributions resulting from the two-photon direct double ionization of helium, under XUV radiation, by solving the time-dependent (TD) Schrödinger equation. The helium TD wavefunction is expanded in terms of fully correlated multichannel states normalized with incoming-wave boundary conditions. The present study focuses on fields of pulse durations within the subfemtosecond regime and at photon energy of 45 eV where the direct double ionization channel dominates the sequential channel. In addition, at this photon energy, the ejected electrons, resulting from the direct and the sequential path, acquire non-overlapping kinetic energy spectra. Our study reveals a trend for back-to-back ejection asymmetry independently of the kinetic energies of the electrons, thus implying that angular correlations are taking place at the time of the ionization. In addition, for given kinetic-energy sharing, it appears that this asymmetry is developed within an interat...

In view of recent developments in the production of high intensity and subpicosecond pulse durati... more In view of recent developments in the production of high intensity and subpicosecond pulse duration, XUV radiation sources, we discuss a number of novel photoabsorption processes that are expected to provide windows into a variety of hitherto unexplored territory of atomic and molecular structure, involving highly excited states embedded in multiple continua.

Handbook for Highly Charged Ion Spectroscopic Research, 2011

Central European Journal of Physics, 2013

ABSTRACT A set of rate equations have been tested against a more robust set of Time-Dependent Den... more ABSTRACT A set of rate equations have been tested against a more robust set of Time-Dependent Density Matrix (TDDM) equations [1] by using them to determine the populations of ion species and autoionising states (AIS) in noble gas atoms when interacting with a strong external field. Two field shapes were tested here — sinusoidal and square - and a variety of pulse characteristics were examined, i.e. intensity, duration and photon energy, for the neon atomic system. It was found that the rate equations were sufficiently accurate only when the external field is way off-resonant with the AIS. Moreover, analytical solutions of the rate equations in the square pulse case agree with the numerical solutions for a time-dependent pulse containing many cycles. An attempt to model a stochastic field was also made and it was found that the use of such a field diminished and broadened the ion yield ratio due to the presence of an added bandwidth.

Springer Proceedings in Physics, 2012

After introducing the importance of non-linear processes in the extreme-ultra-violet (XUV) spectr... more After introducing the importance of non-linear processes in the extreme-ultra-violet (XUV) spectral regime to the attosecond (asec) pulse metrology and time domain applications, we present two successfully implemented techniques with excellent prospects in generating intense asec pulse trains and isolated asec pulses, respectively. For the generation of pulse trains two-color harmonic generation is exploited. The interferometric polarization gating technique appropriate for the generation of intense isolated asec pulses is discussed and compared to other relevant approaches.

Central European Journal of Physics, 2013

ABSTRACT In the present work, we develop a calculational method of solving the scattering equatio... more ABSTRACT In the present work, we develop a calculational method of solving the scattering equations for spherically symmetric potentials by expanding the solutions on Coulomb functions. We utilize a multistep integration scheme together with the standard partial wave analysis in a region where the potential term dominates. The method applies to any physical problem expressed as [∇2 + V(r) + k 2] ψ(r) = 0, while the extension of the method to more general scattering problems is briefly discussed. At present, we demonstrate a two-step Coulomb-fitted integration scheme by calculating the short-range scattering phase shifts for various potentials V(r).

Hyperfine Interactions, 2000

ABSTRACT The interaction between Ps and a strong laser pulse of short duration is studied. Substa... more ABSTRACT The interaction between Ps and a strong laser pulse of short duration is studied. Substantial population is deposited in excited states under few photon excitation. The possible usefulness as a source of excited Ps to spectroscopy and the formation of antihydrogen is discussed. Finally, photoelectron energy spectra are calculated.

Physical Review A - Atomic, Molecular, and Optical Physics, 2005

In anticipation of upcoming two-photon double ionization of atoms and particularly Helium, under ... more In anticipation of upcoming two-photon double ionization of atoms and particularly Helium, under strong short wavelength radiation sources (45 eV), we present quantitative signatures of direct twophoton double ejection, in the photoelectron spectrum (PES) and the peak power dependence, that can be employed in the interpretation of related data. We show that the PES provides the cleanest signature of the process. An inflection (knee) in the laser power dependence of double ionization is also discernible, within a window of intensities which depends on the pulse duration and cross sections PACS numbers: 32.80.Wr

Physical Review A - Atomic, Molecular, and Optical Physics, 2007

We present a method for spectral (bound and continuum) and partial-wave analysis of a three-dimen... more We present a method for spectral (bound and continuum) and partial-wave analysis of a three-dimensional time-dependent wave function, defined on a grid, without projecting onto the field-free eigenstates of the system. The method consists of propagating the time-dependent Schrödinger equation to obtain its autocorrelation function C(t)= after the end of the interaction, at time T , of the system with

High-Power Laser Ablation V, 2004

We review recent developments in the generation and metrology of XUV-attosecond (as) pulses. The ... more We review recent developments in the generation and metrology of XUV-attosecond (as) pulses. The review includes an extended report on the recently achieved 2nd-order autocorrelation measurement of a sub-femtosecond (sub-fs) pulse train.

Physical Review Letters, 2006

A Comment on the Letter by Yasuo Nabekawa et al., Phys. Rev. Lett. 94, 043001 (2005). The authors... more A Comment on the Letter by Yasuo Nabekawa et al., Phys. Rev. Lett. 94, 043001 (2005). The authors of the Letter offer a Reply.