Our research group specializes in experimental light-matter interactions and ultrafast optoelectronic technology. Experiments are conducted in Sharp 014A utilizing several laser systems. Recent highlights from the group’s research are shown below.
ULTRA-Strong Light Fields
In ultra-strong light fields, our common understanding of light – matter interactions begins to fail. The speed of the photoelectron becomes relativistic and the magnetic field of light affects the way light interacts with matter. Our recent research results have characterized this progression from the strong- to ultrastrong-field and shown this can have a significant effect on strong field "rescattering physics" that is responsible for high harmonic generation and multielectron ionization for atoms in strong laser fields.
Molecular Ionization of Chloromethane in Strong Fields
The strong and ultrastrong field-molecule interaction is a complex, many-body process involving multiple ionization processes. We present ion yields and molecular fragment energies for the ionization of chloromethane (CH3Cl) in a laser field with intensities spanning from 1014 to 1017 W cm−2. As the laser intensity increases, ionization of CH3Cl is observed to pass from molecular tunneling, to enhanced ionization (EI), to an atomic-like response.
The energy spectra of the ions show no dependence on the intensity and has its source in dissociative molecular ionization. A classical model of an aligned C–Cl ion is used to model the interaction. Following an initial molecular ionization process, our results show EI is a driving influence in the formation of low charge states until ionization become atomic-like and involves tightly bound ion states whose ionization is unaffected by nearest neighbor ions of similar ion charge. Going forward, we expect it would be reasonable to use an atomic response to describe the ionization when a shell gap is reached in the productions of higher charge states for an ion in a strong field molecular ionization process.
This material is based upon work supported by the National Science Foundation under Grant No. 1607321 and No. 1307042.
MeV Photoelectron Spectrometer for Ultraintense Laser Interactions with Atoms and Molecules
Ultrastrong field science encompasses a broad range of topics including the ultimate energy limit of coherent x-ray generation, pair creation, laser initiated nuclear reactions, and high field vacuum-polarization effects to name just a few, all together forming the underpinnings for many ultraintense field investigations. Traditional laser-matter spectroscopy techniques fail to accurately analyze photoelectrons and ions from ultrahigh intensity studies with terawatt and petawatt laser systems.
We present a magnetic deflection spectrometer for ultrahigh intensity laser interactions which provides the high dynamic range and low background event rate necessary for low sample density experiments that are free from Coulomb explosion. The spectrometer is in ultra-high vacuum and employs a rotation stage in the vacuum for the magnet analyzer to select photoelectrons as a function of the emitted into angle from the laser wave vector direction, k.
This material is based upon work supported by the National Science Foundation under Grant No. 1607321.
Controlling Atomic and Molecular Rescattering in Strong / Ultrastrong-fields
Recent studies have shown laser control of ionizing electron wave functions  and the use of elliptically polarized light to control rescattering, which can occur when the electron is driven back into the parent molecular ion by the laser field . When the returning electron rescatters with the parent ion, it may collisionally knock off additional electrons leading to multi-electron nonsequential ionization (NSI) [3, 4], excite inner shell electrons , or release the energy as high harmonic generation (HHG) [6, 7, 8, 9]. In molecular systems, the rescattering electron wave has been used to provide precision measurements of the molecular electron wave functions [10,11] and orbital tomography [12,13,14].
Our recent work investigates the ellipticity dependence of the ultrafast photoionization for Cn+ fragments from methane. The work was featured on the cover of J. Phys. B where the deflection of the returning rescattering electron wave from methane was altered with the laser field polarization to miss the parent molecule. The study extends from the strong field (C+, C2+) at 1014 W/cm2 to the ultrastrong field (C5+) at 1018 W/cm2. The measurements show C+ and C2+ ionization have limited sensitivity to the field polarization. As the laser intensity and corresponding degree of ionization increase (C4+, C5+), the dependence on the field polarization increases. The measurements also show a clear movement from a "strong field" molecule-like response to an "ultrastrong field" atom-like response with the increase in intensity. This material is based upon work supported by the Army Research Office under Award No. W911NF-09-1-0390 and the National Science Foundation under Award No. 0757953.
Dependence of carbon fragments from methane in strong and ultrastrong elliptically polarized laser fields, Ekanayake N, Wen BL, Howard LE, Wells SJ , Videtto M , Mancuso C , Stanev T, Condon Z , LeMar S, Camilo AD, Toth R, Decamp MF, Walker BC, JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, Volume: 44, Article Number: 045604, Published: FEB 28 2011
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Collective Rescattering in High Intensity Laser Fields
Molecules in strong fields exhibit field alignment [1,2,3,4,5,6], stabilization , enhanced ionization [8,9,10,11], dissociation and Coulomb explosion [12,13,14]. When a molecule interacts with a strong laser field (>1013 W/cm2), one or more valance electrons can be stripped away from the parent molecule through molecular field ionization [15,16,17,18,19], an analogue of well known tunneling ionization in atoms . In general however, molecular ionization is more complicated than atomic ionization with charge resonant excitation, nonadiabatic excitation, and enhanced ionization pathways [21,22,23,24]. In the new frontier of ultrastrong field laser science (up to 1020W/cm2) one expects molecules will ionize to higher charge states. At this time, it is not known how this will occur and what role will be played by mechanisms such as enhanced molecular ionization or Coulomb explosion. we begin to address this by measuring fragmentation and intensity dependent ion yields from the linear chain hydrocarbons ethane, butane, and octane.
In our measurements, we find the molecular fragment ions, C+, and C+2 are created in an intensity window from 1014 W/cm2 to 1015 W/cm2 and have intensity dependent yields similar to the molecular fragments CmHn+ and CmHn+2. The figure (C+ and C+4 fragments from methane (solid circle), ethane (open circle), butane (invert triangle) and octane (upright triangle). The calculated atomic tunneling (ADK) C+ and C+4 tunneling ionization yield is also shown (bold line)) shows the yield of C+, which is suprisingly independent of the molecular parent chain length from methane to octane. Higher charge states, such as C+4 also shown in the figure, however are sensitive to the parent ion size and can show an order of magnitude increase in the yield between methane and butane. This observation is believed to be the first observation of the onset of "collective rescattering" in molecules where rescattering ionization is the result of rescattering electron interactions from adjacent nuclei. Such a mechanism is the prelude to the collective response seen in clusters and plasmas. This material is based upon work supported by the Army Research Office under Award No. W911NF-09-1-0390 and the National Science Foundation under Award No. 0757953.
Ionization of ethane, butane, and octane in strong laser fields, Palaniyappan S, Mitchell R, Ekanayake N, Watts AM, White SL, Sauer R, Howard LE, Videtto M, Mancuso C, Wells SJ, Stanev T, Wen BL, Decamp MF, Walker BC, PHYSICAL REVIEW A Volume: 82, Article Number: 043433, Published: OCT 26 2010
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Angular Distributions for Photoelectrons from Ultrastrong Field – Atom Interactions
The angular distibutions of the photoionization (shown just above the spectrum as inserts) have been measured and are currently being analyzed to provide insight into the fundamentals of the ionization and propagation of the photoelectrons. At this time, the highest energy photoelectrons (> 0.3 MeV) are in agreement with our relativistic calculations shown in the figure. These higher energy photoelectrons come from the inner shell of the atom and experience the highest intensities in the laser focus.
The lower energy photoelectron angular distributions ( < 0.2keV ) are not well understood at this time. Coming from the ionization of the valence shell, they are not in agreement with an independent electron picture of the ionization and collective, multielectron effects are believed to play a significant role for these lower energy photoelectrons. This material is based upon work supported by the Army Research Office under Award No. W911NF-09-1-0390 and the National Science Foundation under Award No. 0757953.
Photoionization by an ultraintense laser field: Response of atomic xenon, DiChiara AD, Ghebregziabher I, Waesche JM, Stanev T, Ekanayake N, Barclay LR, Wells SJ, Watts A, Videtto M, Mancuso CA, Walker BC, PHYSICAL REVIEW A, Volume: 81, Article Number: 043417, Published: APR 2010
The Collective Molecular to Atomic Transition in Ultrastrong Fields
When molecules or clusters are exposed to strong fields, the atoms and molecules begin to field ionize and then undergo complex enhanced ionization and resonant excitation in the field that further excites and ionizes the system. We have begun to address questions on how ionization proceeds for molecules and clusters in ultra-strong, relativistic fields.
For example, whether Coulomb and collective molecular mechanisms play a dominant role as the molecule ionizes to higher charge states; or if molecular characteristics are eventually lost with the atom-field interaction becoming dominant. Our experiments (results shown below) show C+2 and C+3 ions from methane are produced through a molecular response, however, as one proceeds to C+4 ions and removes the last valence electron, the ionization mechanism reflects both molecular and atomic character.
The ionization of the inner shell 1s electrons is relativistic and the C+5 ions from methane are produced entirely from an atomic-like response in an ultra-intense field, including cross-shell rescattering ionization and a photoelectron spectrum in excellent agreement with an atomic model of the ionization.
The image shown here captures the essence of our current understanding. In the strong field, The outer valence electrons involve longer-range interactions, which at 1014W/cm2 fields are comparable to the bond distances and on the order of angstroms. This distance is when the Coulomb field equals the laser field. Inner shell electrons involve short-range interactions and field intensities that are correspondingly higher. The distances, which are sub-Angstrom, result in an interaction much like a free atom/ion-field interaction since the molecular character length scale is much greater than the interaction length at these intensities.
This material is based upon work supported by the Army Research Office under Award No. W911NF-09-1-0390 and the National Science Foundation under Award No. 0757953.