Control of Electron Motion on an Attosecond Timescale
AMOP Seminar
3:30 pm
Jorgensen Hall Room: 145
Speaker: Jean Marcel Ngoko Djiokap
Abstract:
Technological advances 17 years ago in producing new extreme ultraviolet coherent light sources with attosecond (1 as=10-18s) duration have created a new research field, namely, attosecond physics. A main goal of attosecond physics is to control electron motion on its natural (attosecond) timescale, in order to probe bond formation and breaking in molecules during chemical reactions. A milestone toward achieving such goal is the experimental realization of isolated, few-cycle, linearly-polarized attosecond pulses with stable and tunable carrier-envelope phase (CEP). However, the low intensity of these existing sources is a major obstacle towards inducing and controlling electron motion via nonlinear effects. Use of circularly or elliptically polarized attosecond light opens the possibility of investigating effects that are not accessible with linearly-polarized pulses.
In this talk, after briefly introducing the physical mechanisms at the basis of attosecond pulse generation, I will focus on our numerical and analytical methods for the investigation of ultrafast ionization processes in atoms and molecules of astrophysical interest, with emphasis on two-electron processes in which electron correlations play a key role. Enabled by the broad bandwidth of attosecond pulses, the first unusual effect we predicted in double ionization of helium by an intense few-cycle elliptically polarized attosecond pulse is the nonlinear dichroism [1]. The other effect is the novel electron phenomenon of electron vortices [2] in attosecond photoionization of atoms and molecules (cf. Fig.1), which provides a dramatic example of wave-particle duality. Our predictions of electron matter-wave vortices [2,3], which have now been observed experimentally [4], have already opened a new interdisciplinary area in physics.
Biography: Dr. J. M. Ngoko Djiokap is since 2014 a research assistant professor in the Department of Physics and Astronomy at the University of Nebraska-Lincoln. Since earning his Ph.D. at the Université catholique de Louvain (Belgium) in 2010, he has made several breakthroughs in theoretical attosecond physics and strong-field physics, with a focus on two-electron processes. He is interested in general on laser-matter interactions and collisional processes, and is working on numerical and analytic theoretical descriptions of two-electron atomic and molecular processes.
References: [1] J.M. Ngoko Djiokap, N.L. Manakov, A.V. Meremianin, S.X. Hu, L.B. Madsen, and A.F. Starace, Phys. Rev. Lett. 113, 223002 (2014). [2] J.M. Ngoko Djiokap, S.X. Hu, L.B. Madsen, N.L. Manakov, A.V. Meremianin, and A.F. Starace, Phys. Rev. Lett. 115, 113004 (2015). [3] J.M. Ngoko Djiokap, A.V. Meremianin, N.L. Manakov, S.X. Hu, L.B. Madsen, and A.F. Starace, Phys. Rev. A 94, 013408 (2016). [4] D. Pengel, S. Kerbstadt, D. Johannmeyer, L. Englert, T. Bayer, and M. Wollenhaupt, Phys. Rev. Lett. 118, 053003 (2017).
Abstract:
Technological advances 17 years ago in producing new extreme ultraviolet coherent light sources with attosecond (1 as=10-18s) duration have created a new research field, namely, attosecond physics. A main goal of attosecond physics is to control electron motion on its natural (attosecond) timescale, in order to probe bond formation and breaking in molecules during chemical reactions. A milestone toward achieving such goal is the experimental realization of isolated, few-cycle, linearly-polarized attosecond pulses with stable and tunable carrier-envelope phase (CEP). However, the low intensity of these existing sources is a major obstacle towards inducing and controlling electron motion via nonlinear effects. Use of circularly or elliptically polarized attosecond light opens the possibility of investigating effects that are not accessible with linearly-polarized pulses.
In this talk, after briefly introducing the physical mechanisms at the basis of attosecond pulse generation, I will focus on our numerical and analytical methods for the investigation of ultrafast ionization processes in atoms and molecules of astrophysical interest, with emphasis on two-electron processes in which electron correlations play a key role. Enabled by the broad bandwidth of attosecond pulses, the first unusual effect we predicted in double ionization of helium by an intense few-cycle elliptically polarized attosecond pulse is the nonlinear dichroism [1]. The other effect is the novel electron phenomenon of electron vortices [2] in attosecond photoionization of atoms and molecules (cf. Fig.1), which provides a dramatic example of wave-particle duality. Our predictions of electron matter-wave vortices [2,3], which have now been observed experimentally [4], have already opened a new interdisciplinary area in physics.
Biography: Dr. J. M. Ngoko Djiokap is since 2014 a research assistant professor in the Department of Physics and Astronomy at the University of Nebraska-Lincoln. Since earning his Ph.D. at the Université catholique de Louvain (Belgium) in 2010, he has made several breakthroughs in theoretical attosecond physics and strong-field physics, with a focus on two-electron processes. He is interested in general on laser-matter interactions and collisional processes, and is working on numerical and analytic theoretical descriptions of two-electron atomic and molecular processes.
References: [1] J.M. Ngoko Djiokap, N.L. Manakov, A.V. Meremianin, S.X. Hu, L.B. Madsen, and A.F. Starace, Phys. Rev. Lett. 113, 223002 (2014). [2] J.M. Ngoko Djiokap, S.X. Hu, L.B. Madsen, N.L. Manakov, A.V. Meremianin, and A.F. Starace, Phys. Rev. Lett. 115, 113004 (2015). [3] J.M. Ngoko Djiokap, A.V. Meremianin, N.L. Manakov, S.X. Hu, L.B. Madsen, and A.F. Starace, Phys. Rev. A 94, 013408 (2016). [4] D. Pengel, S. Kerbstadt, D. Johannmeyer, L. Englert, T. Bayer, and M. Wollenhaupt, Phys. Rev. Lett. 118, 053003 (2017).