Session P7: High Harmonics and Ultrafast Processes in Strong Fields

Characterization of induced nanoplasmonic fields in time-resolved photoemission: a classical trajectory approach applied to gold nanospheres

Attosecond time-resolved spectroscopy is being extended from the study of the electronic dynamics in atoms and molecules to the investigation of electron propagation and collective electronic (plasmonic) effects near solid surfaces [1,2] and nanoparticles [1,3]. We simulated streaked photoelectron energy spectra as a function of the time delay between ionizing single attosecond XUV and streaking IR pulses, within a classical-trajectory Monte-Carlo-sampling approach. For the examples of streaked photoemission form 5 and 50 nm radius gold nanospheres, we discuss the imprint of sub-infrared-cycle plasmonic and electronic dynamics on streaked photoelectron spectra [4]. [1] U. Thumm, Q. Liao, E. M. Bothschafter, F. S\"{u}{\ss}mann, M. F. Kling, and R. Kienberger, p. 387, Handbook of Photonics, Vol. 1, (Wiley 2015) [2] Q. Liao and U. Thumm, Phys. Rev. A 92, 031401(R) (2015). [3] J. Li, E. Saydanzad, and Uwe Thumm, Phys. Rev. A 94, 051401(R) (2016). [4] E. Saydanzad, J. Li, and Uwe Thumm, submitted to Phys. Rev. A.   

New Quantum Diffusion Monte Carlo Method for strong field time dependent problems

We have recently formulated the Quantum Diffusion Quantum Monte Carlo (QDMC) method for the solution of the time-dependent Schr{\”o}dinger equation when it is equivalent to the reaction-diffusion system coupled by the highly nonlinear potentials of the type of Shay. Here we formulate a new Time Dependent QDMC method free of the nonlinearities described by the constant stochastic process of the coupled diffusion with transmutation. As before two kinds of diffusing particles (color walkers) are considered but which can further also transmute one into the other. Each of the species undergoes the hypothetical Einstein random walk progression [1] with transmutation. The progressed particles transmute into the particles of the other kind before contributing to or annihilating the other particles density. This fully emulates the Time Dependent Schr{\”o}dinger equation for any number of quantum particles. The negative sign of the real and the imaginary parts of the wave function is handled by the ``spinor" densities carrying the sign as the degree of freedom. We apply the method for the exact time-dependent observation of our discovered two-electron Langmuir configurations in the magnetic and circularly polarized fields. [1] A. Einstein, Ann. d Phys. {\bf 17}, 549, (1905)   

Control of threshold enhancements in harmonic generation by atoms in a two-color laser field with orthogonal polarizations

Threshold phenomena (or channel-closing effects) are analyzed in high-order harmonic generation (HHG) by atoms in a two-color laser field with orthogonal linearly polarized components of a fundamental field and its second harmonic [1]. We show that the threshold behavior of HHG rates for the case of a weak second harmonic component is sensitive to the parity of a closing multiphoton ionization channel and the spatial symmetry of the initial bound state of the target atom, while for the case of comparable intensities of both components, suppression of threshold phenomena is observed as the relative phase between the components of a two-color field varies. A quantum orbit analysis as well as phenomenological considerations in terms of Baz' theory of threshold phenomena [2] are presented in order to describe and explain the major features of threshold phenomena in HHG by a two-color field. [1] M.V. Frolov \textit{et al.}, Phys. Rev. A \textbf{93}, 023430 (2016). [2] A.I. Baz', Zh. Eksp. Teor. Fiz. \textbf{33}, 923 (1957) [Sov. Phys. JETP \textbf{6}, 709 (1958)].   

Femtosecond Photoelectron Imaging of Dissociating and Autoionizing States in Oxygen

Time-resolved photoelectron spectra from molecular oxygen have been recorded with high energy and time resolution using a velocity map imaging (VMI) spectrometer. High harmonics were used to prepare neutral Rydberg states converging to the $c ^4\Sigma^{-}_{u}$ ionic state. These states display both autoionization and predissociation.\footnote{H. Timmers, N. Shivaram, and A. Sandhu, Phys. Rev. Lett. \textbf{107}, 173001 (2012)} A femtosecond laser pulse centered at 780 nm was used to probe the system, ionizing both the excited molecular states and the predissociated neutral atomic fragments. Electrons were collected in the 0-3 eV range using a VMI spectrometer and their spectra were reconstructed using a Fast Onion-peeling algorithm. By looking at IR modification to the electron spectrum, new features are observed which could originate from long-range columbic interactions or previously unobserved molecular decay channels. Ongoing studies extend this technique to other systems exhibiting non-adiabatic dynamics.   

High harmonic generation spectroscopy of laser induced phase transitions in strongly correlated systems

We study theoretically high harmonic generation in the 1D Fermi-Hubbard model in the quantum tunneling regime with intense mid-infrared (MIR) and THz fields. This is the first theoretical study of high harmonic generation in strongly correlated solids, with electron correlation effects explicitly taken into account with no additional approximations beyond the Fermi-Hubbard Hamiltonian itself. We find that the mechanism of harmonic emission in strongly correlated solids is distinct from the one observed in semiconductors and dielectrics. The mechanism relies on the production of doublon-hole pairs via Landau-Dykhne type tunneling and is inherently linked to an insulator-to-metal phase transition. We show that high harmonic generation can be used to time this phase transition with the accuracy of a few femtoseconds, i.e. within the fraction of the single oscillation of the driving electromagnetic field. Our work opens the window for the investigation of ultrafast phase transitions in the condensed phase using high harmonic spectroscopy.   

Orientation dependent high harmonic generation from solids

We present a theoretical study of the generation of high harmonic radiation in an MgO crystal driven by a strong mid-infrared laser field. We find that the resulting harmonic spectra depend on the relative orientation of the crystal and the laser polarization, exhibiting two-, four- or six-fold rotation symmetry around different crystal axes. Similar orientation-dependent harmonic spectra have recently been experimentally observed in MgO [Y. You, \textit{et al.}, Anisotropic high-harmonic generation in bulk~crystals, \textit{Nature Physics} (2016)], and were explained using a real-space picture. We show that the orientation dependence in MgO can also be explained using a momentum-space picture, in terms of different band curvatures and coupling strengths along different polarization directions and orientations. Furthermore, we show that the primary and secondary plateau in the harmonic spectrum exhibit very different orientation dependences, and we explain this using the momentum space picture.   

Harmonic generations in fractal-poled LiNbO3 at mid-infrared wavelengths

We report harmonic generation in a LiNbO$_{\mathrm{3}}$-based photonic crystal by mid-infrared femtosecond laser pulses. We observe harmonics generated at different driver frequencies below the band gap, extending up to 11$^{\mathrm{th}}$ order for a 4 um driver. We interpret the results by solving coupled wave equations, which include cascade nonlinear processes with the aid of quasi phase matching. The harmonics driven by 4 um consist of collinear harmonic comb and non-collinear spatially separated visible colors. The conversion efficiency is measured to be 13-16 {\%}.   

Laser waveform control of extreme ultraviolet high harmonic generation in solids

Solid-state high-order harmonic generation (HHG) process has emerged as a novel method to produce attosecond pulses and to probe electronic structure of bulk materials. Much of these applications need time-domain information such as the phase delay between different harmonics at the sub-cycle level. Here, we use few cycle driving pulses to control the generation process in time-domain through carrier-envelope phase (CEP) setting. We find that the XUV harmonic spectrum from MgO depends strongly to the CEP setting. Our analysis based on quantum model shows that such dependence originates from the delay between harmonics at the sub-cycle level. Experimental results show that the delay depends on the strength of the laser field. These features are consistent to the emission from driven multi-band non-linear current. Thus, our results suggest a new approach to study multi-band electron dynamics in strongly driven periodic solids with unprecedented time resolution. The time-domain information we reveal is critical for attosecond pulse metrology based on solid-state HHG.   

Discontinuities in the Electromagnetic Fields of Vortex Beams from the Complex Source/Sink Model

An analytical discontinuity is reported in what was thought to be the discontinuity-free exact nonparaxial vortex beam phasor generated from the complex source/sink model\footnote{A. April, ``Ultrashort, Strongly Focused Laser Pulses in Free Space," in Coherence and Ultrashort Pulse Laser Emission, edited by F. J. Duarte (InTech, 2010) Chap. 16, pp. 355-382.}. This discontinuity appears for all odd values of the orbital angular momentum mode. Such discontinuities in the phasor lead to nonphysical discontinuities in the real electromagnetic field components. We will illustrate the source of the discontinuities, and provide graphical evidence of the discontinuous real electric fields for the first and third orbital angular momentum modes. In addition, a simple means of avoiding these discontinuities will be presented.