Theory of high-order harmonic generation from molecules by intense laser pulses

We show that high-order harmonics generated from molecules by intense laser pulses can be expressed as the product of a returning electron wave packet and the photo-recombination cross section (PRCS) where the electron wave packet can be obtained from simple strong-field approximation (SFA) or from a companion atomic target. Using these wave packets but replacing the PRCS obtained from SFA or from the atomic target by the accurate PRCS from molecules, the resulting HHG spectra are shown to agree well with the benchmark results from direct numerical solution of the time-dependent Schr\"odinger equation, for the case of H$_2^+$ in laser fields. The result illustrates that these powerful theoretical tools can be used for obtaining high-order harmonic spectra from molecules. More importantly, the results imply that the PRCS extracted from laser-induced HHG spectra can be used for time-resolved dynamic chemical imaging of transient molecules with temporal resolutions down to a few femtoseconds.Comment: 10 pages, 5 figure

Acoustic far-field hypersonic surface wave detection with single plasmonic nanoantennas

The optical properties of small metallic particles allow us to bridge the gap between the myriad of subdiffraction local phenomena and macroscopic optical elements. The optomechanical coupling between mechanical vibrations of Au nanoparticles and their optical response due to collective electronic oscillations leads to the emission and the detection of surface acoustic waves (SAWs) by single metallic nanoantennas. We take two Au nanoparticles, one acting as a source and the other as a receptor of SAWs and, even though these antennas are separated by distances orders of magnitude larger than the characteristic subnanometric displacements of vibrations, we probe the frequency content, wave speed, and amplitude decay of SAWs originating from the damping of coherent mechanical modes of the source. Two-color pump-probe experiments and numerical methods reveal the characteristic Rayleigh wave behavior of emitted SAWs, and show that the SAW-induced optical modulation of the receptor antenna allows us to accurately probe the frequency of the source, even when the eigenmodes of source and receptor are detuned

Tailored Hypersound Generation in Single Plasmonic Nanoantennas.

Ultrashort laser pulses impinging on a plasmonic nanostructure trigger a highly dynamic scenario in the interplay of electronic relaxation with lattice vibrations, which can be experimentally probed via the generation of coherent phonons. In this Letter, we present studies of hypersound generation in the range of a few to tens of gigahertz on single gold plasmonic nanoantennas, which have additionally been subjected to predesigned mechanical constraints via silica bridges. Using these hybrid gold/silica nanoantennas, we demonstrate experimentally and via numerical simulations how mechanical constraints allow control over their vibrational mode spectrum. Degenerate pump-probe techniques with double modulation are performed in order to detect the small changes produced in the probe transmission by the mechanical oscillations of these single nanoantennas.Fil: Della Picca, Fabricio Leandro. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Laboratorio de Electrónica Cuántica; ArgentinaFil: Berte, Rodrigo. Ministry Of Education Brazil; . Imperial College London; Reino UnidoFil: Rahmani, Mohsen. Imperial College London; Reino UnidoFil: Albella, Pablo. Imperial College London; Reino UnidoFil: Bujjamer, Juan. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Laboratorio de Electrónica Cuántica; ArgentinaFil: Poblet, Martín. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Laboratorio de Electrónica Cuántica; ArgentinaFil: Cortés, Emiliano. Imperial College London; Reino UnidoFil: Maier, Stefan A.. Imperial College London; Reino UnidoFil: Bragas, Andrea Veronica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Laboratorio de Electrónica Cuántica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentin

Comparison of hydrogen (1s) total ionization probabilities by laser pulses of frequency ω = 0.855 au, for several intensities

<p><strong>Figure 4.</strong> Comparison of hydrogen (1s) total ionization probabilities by laser pulses of frequency ω = 0.855 au, for several intensities. Lines correspond to full CVA calculations, while symbols were obtained in the first-order DipA.</p> <p><strong>Abstract</strong></p> <p>We present a detailed study of the ionization probability of H and H_{2}^{+} induced by a short intense laser pulse. Starting from a Coulomb–Volkov description of the process we derive a multipole-like expansion where each term is factored into two contributions: one that accounts for the effect of the electromagnetic field on the free-electron final state and a second factor that depends only on the target structure. Such a separation may be valuable to solve complex atomic or molecular systems as well as to interpret the dynamics of the process in simpler terms. We show that the series expansion converges rapidly, and thus the inclusion of the first few terms is sufficient to produce accurate results.</p

CVA ionization spectra for mathrm{H}_{2}^{+} in the forward condition from (a) 1sσ and <em>R</em> = 2 au, solid line; (b) 2<em>p</em>σ and <em>R</em> = 2 au, dotted line; (c) 1sσ and <em>R</em> = 1.4 au, dashed–dotted line

<p><strong>Figure 9.</strong> CVA ionization spectra for \mathrm{H}_{2}^{+} in the forward condition from (a) 1sσ and <em>R</em> = 2 au, solid line; (b) 2<em>p</em>σ and <em>R</em> = 2 au, dotted line; (c) 1sσ and <em>R</em> = 1.4 au, dashed–dotted line. The laser parameters are the same as used in figure <a href="http://iopscience.iop.org/0953-4075/46/17/175603/article#jpb471071f7" target="_blank">7</a>. Curves (b) and (c) are shifted in energy.</p> <p><strong>Abstract</strong></p> <p>We present a detailed study of the ionization probability of H and H_{2}^{+} induced by a short intense laser pulse. Starting from a Coulomb–Volkov description of the process we derive a multipole-like expansion where each term is factored into two contributions: one that accounts for the effect of the electromagnetic field on the free-electron final state and a second factor that depends only on the target structure. Such a separation may be valuable to solve complex atomic or molecular systems as well as to interpret the dynamics of the process in simpler terms. We show that the series expansion converges rapidly, and thus the inclusion of the first few terms is sufficient to produce accurate results.</p

Idem figure 1 with ω = 1.71 au (Γ = 34.2)

<p><strong>Figure 2.</strong> Idem figure <a href="http://iopscience.iop.org/0953-4075/46/17/175603/article#jpb471071f1" target="_blank">1</a> with ω = 1.71 au (Γ = 34.2).</p> <p><strong>Abstract</strong></p> <p>We present a detailed study of the ionization probability of H and H_{2}^{+} induced by a short intense laser pulse. Starting from a Coulomb–Volkov description of the process we derive a multipole-like expansion where each term is factored into two contributions: one that accounts for the effect of the electromagnetic field on the free-electron final state and a second factor that depends only on the target structure. Such a separation may be valuable to solve complex atomic or molecular systems as well as to interpret the dynamics of the process in simpler terms. We show that the series expansion converges rapidly, and thus the inclusion of the first few terms is sufficient to produce accurate results.</p

Electronic spectra for ionization of H_{2}^{+}(1sσ) as a function of the electron energy, for emission in the forward direction

<p><strong>Figure 7.</strong> Electronic spectra for ionization of H_{2}^{+}(1sσ) as a function of the electron energy, for emission in the forward direction. We have considered laser pulses of <em>N</em> = 1, 7, and 27 cycles, with frequencies ω = 0.855 and 1.71 au, and an amplitude <em>F</em>₀ = 0.05 au. Full line: CVA; dotted line: DipA.</p> <p><strong>Abstract</strong></p> <p>We present a detailed study of the ionization probability of H and H_{2}^{+} induced by a short intense laser pulse. Starting from a Coulomb–Volkov description of the process we derive a multipole-like expansion where each term is factored into two contributions: one that accounts for the effect of the electromagnetic field on the free-electron final state and a second factor that depends only on the target structure. Such a separation may be valuable to solve complex atomic or molecular systems as well as to interpret the dynamics of the process in simpler terms. We show that the series expansion converges rapidly, and thus the inclusion of the first few terms is sufficient to produce accurate results.</p

Ionization spectra for H from 2s for laser pulses with ω = 0.855 au, <em>F</em>₀ = 0.05 au and <em>N</em> = 7 cycles

<p><strong>Figure 6.</strong> Ionization spectra for H from 2s for laser pulses with ω = 0.855 au, <em>F</em>₀ = 0.05 au and <em>N</em> = 7 cycles. Full line (green): CVA; dotted line (black): DipA; dashed–dotted line (blue): second-order calculations.</p> <p><strong>Abstract</strong></p> <p>We present a detailed study of the ionization probability of H and H_{2}^{+} induced by a short intense laser pulse. Starting from a Coulomb–Volkov description of the process we derive a multipole-like expansion where each term is factored into two contributions: one that accounts for the effect of the electromagnetic field on the free-electron final state and a second factor that depends only on the target structure. Such a separation may be valuable to solve complex atomic or molecular systems as well as to interpret the dynamics of the process in simpler terms. We show that the series expansion converges rapidly, and thus the inclusion of the first few terms is sufficient to produce accurate results.</p

Electron spectra for ionization of H(1s) as a function of the ejected electron energy for a laser pulse with <em>N</em> = 1 (up), <em>N</em> = 7 (middle) and <em>N</em> = 27 (bottom) cycles

<p><strong>Figure 1.</strong> Electron spectra for ionization of H(1s) as a function of the ejected electron energy for a laser pulse with <em>N</em> = 1 (up), <em>N</em> = 7 (middle) and <em>N</em> = 27 (bottom) cycles. Laser frequency ω = 0.855 au, <em>F</em>₀ = 0.05 au (Γ = 17.1). Full line: CVA; dotted line: DipA; full line with circles: TDSE results.</p> <p><strong>Abstract</strong></p> <p>We present a detailed study of the ionization probability of H and H_{2}^{+} induced by a short intense laser pulse. Starting from a Coulomb–Volkov description of the process we derive a multipole-like expansion where each term is factored into two contributions: one that accounts for the effect of the electromagnetic field on the free-electron final state and a second factor that depends only on the target structure. Such a separation may be valuable to solve complex atomic or molecular systems as well as to interpret the dynamics of the process in simpler terms. We show that the series expansion converges rapidly, and thus the inclusion of the first few terms is sufficient to produce accurate results.</p