Bulletin of the American Physical Society
2013 Joint Meeting of the APS Division of Atomic, Molecular & Optical Physics and the CAP Division of Atomic, Molecular & Optical Physics, Canada
Volume 58, Number 6
Monday–Friday, June 3–7, 2013; Quebec City, Canada
Session U3: High Harmonic and Science |
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Chair: Francois Legare, Centre Energie Materiaux Telecommunications Room: 202 |
Friday, June 7, 2013 10:30AM - 10:42AM |
U3.00001: A phase coherent dual-comb source in the VUV based on intracavity high harmonic generation David Carlson, Tsung-Han Wu, R. Jason Jones Intra-cavity high harmonic generation (iHHG) utilizing femtosecond enhancement cavities (fsEC's) has been established as an efficient route for generation of femtosecond frequency combs in the vacuum and extreme ultraviolet spectral regions. To enable more robust direct frequency comb spectroscopy of complex atomic and molecular structure in the VUV, we have developed a novel dual-comb system. Two phase-coherent fiber based frequency combs are up converted to the VUV using iHHG. Dual-comb spectroscopy has already been established in the IR as a powerful spectroscopic method that does not require high-resolving power components to isolate individual fs comb modes. In such systems, the second phase coherent frequency comb acts as a local oscillator, enabling one to directly measure individual comb components through the unique heterodyne beat frequency of each comb mode. By allowing the detection of individual comb components in the VUV and XUV for the first time, our system will also enable a systematic study of its noise properties, offering insight to the underlying physics of iHHG that affects the XUV comb coherence. We demonstrate the current dual-comb VUV source as a first step towards precision dual-comb spectroscopy of atomic and molecular systems in the VUV. [Preview Abstract] |
Friday, June 7, 2013 10:42AM - 10:54AM |
U3.00002: Direct Measurement of the XUV Frequency Comb Coherence Craig Benko, Thomas K. Allison, Arman Cingoz, Dylan C. Yost, Jun Ye We present the first demonstration of XUV radiation with phase coherence capable of reaching sub-kHz resolution. The XUV comb is produced by frequency up conversion of a near-infrared frequency comb by intra-cavity high harmonic generation (HHG). Using an 80 W Yb:fiber fs frequency comb, we simultaneously pump two femtosecond enhancement cavities to reach intensities suitable for HHG. The harmonics are out-coupled from the cavities using sapphire plates placed at Brewster angle for the pump laser. We developed an interferometer capable of operating in the XUV and measured a heterodyne beat between the two sources at different harmonics. Despite being insensitive to common-mode pump laser noise, the heterodyne beats will reveal any noise added by the intra-cavity HHG process. This will allow us to probe the fundamental limit on the coherence properties of HHG. [Preview Abstract] |
Friday, June 7, 2013 10:54AM - 11:06AM |
U3.00003: Controlling generation process of an attosecond electron wave packet in a molecule Hiromichi Niikura When a molecule is exposed to intense laser fields, tunnel ionization may occur from not only the highest molecular orbital but also the lower electronic states. This process creates electron wave packet motion in a molecule. Using unaligned molecules, we have demonstrated that the attosecond electron wavepacket motion can be mapped onto the 2D high-harmonic spectra measured as a function of the delay between the two-color laser pulses in 2011. Here we extend this approach to control and identify which electronic states are included in the electron wave packet. We combine a molecular alignment technique with the two-color field approach. Using a lineally polarized laser pulse, we create a rotational wave packet that exhibits field-free alignment. Then we probe the electron wave packet by using the orthogonally polarized 800 nm and 400 nm pulses. By aligning molecules, we control the tunnel ionization probability of each electronic state. We observe that the 2D high-harmonic spectra generated by the two-color fields depends on the molecular alignment angle. We show that the potion of the electronic states responsible for the high harmonic emission, thus, generation process of the attosecond electron wave packet can be controlled. [Preview Abstract] |
Friday, June 7, 2013 11:06AM - 11:18AM |
U3.00004: Probing Attosecond Electron Dynamics in Helium with Attosecond Transient Absorption Michael Chini, Xiaowei Wang, Yan Cheng, Yi Wu, Di Zhao, Dmitry Telnov, Shih-I Chu, Zenghu Chang Recent advances in the generation, characterization, and application of isolated attosecond pulses and few-cycle femtosecond lasers have given experimentalists the necessary tools for dynamic measurements on fast-evolving quantum systems. In this work, attosecond transient absorption spectroscopy with ultrabroadband isolated attosecond pulses is used to elucidate the ultrafast dynamics -- on timescales shorter than the laser cycle -- in the laser-dressed singly-excited states of the helium atom, demonstrating for the first time that the attosecond transient absorption technique allows for state-resolved and simultaneous measurement of bound and continuum state dynamics. Subcycle phenomena are observed in prototypical quantum mechanical processes such as the AC Stark and ponderomotive energy level shifts, Rabi oscillations and electromagnetically-induced absorption and transparency, and two-color multi-photon absorption to dipole-forbidden states of the atom. Furthermore, dynamic interference oscillations, corresponding to quantum path interferences involving bound and free electronic states of the atom, are observed for the first time in an optical measurement. The ultrabroadband isolated pulses with durations as short as 67 attoseconds were characterized using the PROOF technique. [Preview Abstract] |
Friday, June 7, 2013 11:18AM - 11:30AM |
U3.00005: Probing molecular dynamics using attosecond transient absorption Yan Cheng, Michael Chini, Xiaowei Wang, Yang Wang, Yi Wu, Zenghu Chang Transient absorption experiments using isolated attosecond pulses have recently been used to uncover fast dynamics in laser-perturbed bound and autoionizing state wavepackets in atoms. Application of this technique to the study of fast dynamics in molecules requires unique attosecond light sources. In particular, since chemical reaction dynamics are driven primarily by valence electron motion, attosecond pulses with low photon energies in the VUV are needed. Here, we apply attosecond transient absorption spectroscopy to the study of vibrational wavepackets in hydrogen molecules. Laser-induced dynamics in the vibrational wavepacket excited by the attosecond pulse through $D\leftarrow X$ vibronic transitions (14-16 eV) are revealed by delay-dependent changes in the vibrational state absorption features. The isolated attosecond pulses produced with generalized double optical gating with low photon energies present a promising route to ``filming'' chemical reactions in real time. [Preview Abstract] |
Friday, June 7, 2013 11:30AM - 11:42AM |
U3.00006: Probing Ultrafast Internal Conversion in Ethylene Using Vacuum Ultraviolet Pulses Travis Wright, Champak Khurmi, Daniel Slaughter, Nobuhiko Kuze, Ali Belkacem Molecules with the ability to quickly dissipate energy when exposed to light play a fundamental role in processes such as vision and DNA photostability. The motion that these large molecules execute to achieve radiationless de-excitation is often centered about a single carbon-carbon double bond. Ethylene is the smallest molecule with a carbon-carbon double bond, and exhibits internal conversion within 50 to 100 femtoseconds of excitation. Using intense vacuum ultraviolet (VUV) light produced from a high harmonics source, we perform pump-probe spectroscopy on ethylene with both arms containing VUV light. By collecting both photoelectrons and photoions, we can gain new insight into the time evolution of ethylene's excited state. [Preview Abstract] |
Friday, June 7, 2013 11:42AM - 11:54AM |
U3.00007: Circularly Polarized Attosecond Pulses and Atto-Magnetism Kaijun Yuan, Andre Bandrauk We present a method for producing a single circularly polarized attosecond pulse by an intense few cycle elliptically polarized laser pulse combined with a mid-infrared laser pulse from numerical solutions of the time dependent Schr\"{o}dinger equation for aligned H$_{2}^{+}$. Simulations show that in the intense mid-infrared laser field with frequency 62.5 THz (wavelength $\lambda =$ 4800nm) and intensity $\sim$10$^{14}$W/cm$^{2}$, a single circularly polarized 114 as pulse can be generated by an elliptically polarized 400nm and 5x10$^{14}$W/cm$^{2}$ laser pulse with ellipticity 0.59. With such generated ultrashort circularly polarized pulses, localized ``spinning'' electron wave packet can be created on the attosecond time scale and the sub-nanometer molecular scale, leading to time dependent electronic currents and attosecond magnetic fields inside molecules. It is shown that with an intense circularly polarized attosecond UV laser pulse with intensity 10$^{16}$W/cm$^{2}$, a strong magnetic field with several tens of Teslas can be induced.\\[4pt] [1] K.J. Yuan and A.D. Bandrauk, J. Phys. B 45, 074001 (2012); Phys. Rev. Lett. 120, 023003 (2013). [Preview Abstract] |
Friday, June 7, 2013 11:54AM - 12:06PM |
U3.00008: ABSTRACT WITHDRAWN |
Friday, June 7, 2013 12:06PM - 12:18PM |
U3.00009: Ellipiticity of higher order harmonics Yuqing Xia, Agnieszka Jaron-Becker High-order harmonic generation (HHG) results from the extreme distortion of an electron wave function in a system in the presence of a strong laser field. Since both the ionization and electron recombination steps of HHG process are dependent on the particular symmetry of the active orbital and its orientation with respect to the laser field, HHG provides a unique probe of the electronic properties and structure of a molecule. We investigate in detail how the information is encoded in the intensities and phases of the harmonics. We calculate the spectra and the ellipticity of harmonics including the contributions from all orbitals using Time-Dependent Density Functional Theory (TDDFT) method. The results are compared with calculations within ``Strong Field Approximation'' (SFA) as well as with experiments. We investigate relative contributions from different active orbitals and in particular if it is possible to identify each orbital's contribution. [Preview Abstract] |
Friday, June 7, 2013 12:18PM - 12:30PM |
U3.00010: High-Order Harmonic-Generation Spectroscopy with an Elliptically Polarized Laser Field M.V. Frolov, N.L. Manakov, T.S. Sarantseva, Anthony F. Starace Analytic formulas describing high-order harmonic generation (HHG) by atoms in an intense laser field with small ellipticity are obtained quantum mechanically in the tunneling limit [1]. The results show that factorization of the HHG yield in terms of an electron wavepacket and the photorecombination cross section (PRCS) is valid only for $s$ states of a bound atomic electron, whereas the HHG yield for $p$ states involves two different atomic parameters. For the latter case, elliptic HHG spectroscopy enables one to retrieve both the energy and angular dependence of the PRCS of the target atom, as we illustrate for the case of HHG by Xe in a mid-infrared laser field.\\[4pt] [1]~M.V. Frolov, N.L. Manakov, T.S. Sarantseva, and A.F. Starace,~Phys. Rev. A~\textbf{86}, 063406 (2012). [Preview Abstract] |
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