Recent: Chloromethane Ionization with Time-of-Flight Technology
My area of research lies in experimental light-matter interactions and ultrafast opto-electronic technology. Experiments are conducted in Sharp 014A utilizing several laser systems. The equipment and lab set up can be found below, along with some recent highlights and posters.
ULTRA-Strong Light Fields
In ultrastrong 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.
Limits of Strong Field Rescattering in the Relativistic Regime
Rescattering between a photoelectron and parent ion is an essential physical process supporting two decades of advances in x-ray radiation generation, multielectron dynamics, and the nascence of attosecond science. Recombination with the parent ion in higher intensity collisions requires taking into account the presence of relativistic effects. Via a strong field quantum description and Monte Carlo semiclassical approach, recollision in the relativistic regime is investigated. We find the relativistic recollision energy cutoff is independent of the ponderomotive potential Up, in contrast to the well-known 3.2Up scaling. The relativistic recollision energy cutoff is determined by the ionization potential of the atomic system and achievable with non-negligible recollision flux before entering a “rescattering free” interaction.
We show the ultimate energy cutoff and highest rescattering flux is best realized by intense, short wavelength lasers and cannot exceed a few thousand Hartree, indicating that hard x-rays via recollision-induced high-order harmonic generation can be extended to photon energies of 60 keV and setting a boundary for recollision based attosecond physics.
This material is based upon work supported by the National Science Foundation under Grants No. 1607321 and No. 1307042 and the Program of Introducing Talents of Discipline to Universities Grant No. B12024.
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 coherenet 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 terrawatt 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 & 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 slides shown here capture 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 is 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.
MeV Photoelectrons from Ultrastrong Fields
Recent photoelectron measurements from ultrastong field interactions with atoms are shown in the above photoelectron spectrum for the ionization of argon at 1019W/cm2. The photoelectrons have the highest energy observed from the photoionization of a single atom by optical frequency photons. In any sense of the definition this interaction is highly nonperturbatuve with the light-atom interaction resulting in the absorption of nearly one-million photons during ionization. Comparisions with relativistic calculations show the two major features in the photoelectron spectrum come from the ionization of the inner shell electrons (dashed line calculation) peaked at 0.6 MeV and the low energy, valence electrons at less than 200 keV. The calculation including both the valence and inner shell electrons is shown as a solid line in the figure.
Lorentz Force Deflection of Strong Field Rescattering
The above figure is a snapshot of the ionizing electron from an atom. Normally, in laser fields the electron leaves the atom and ionizes along the electric field direction. As the light wave oscillates, the photoelectron may then recollides with the atom as it quivers. As the laser strength increases (b-c), the outgoing electron experiences a force not only from the electric field, but also the magnetic field of light. This results in a deflection of the electron away from the atom by several nanometers (z-direction in figure). The impact of the deflection is that both radiation and multielectron ionization from strong laser fields and rescattering may be suppressed. This can occur at intensities as low as 1014 W/cm2 for mid-IR light (2 to 5 micron wavelengths) or as high as 1018 W/cm2 for UV radiation.
Sharp 014A Laboratory houses four optical tables in a HEPA filtered environment and is home for several laser systems and a growing number of experimental chambers. The experiments can be broken down into two parts: optical technology and the physics of light or photoexcitation dynamics. In this page, recent projects are highlighted.
The optical technology part of the program currently involves the development of compact terawatt amplifier with an industrial partner ALTOS, Inc. The goal of this project is to build a laser system at the level of 1 microcent per watt of peak power. This project is being funded by the Delaware Research Partnership (DRP). The DRP encourages industrial partnerships with the University of Delaware to best meet the needs of both parties and serve the community. The laser (shown at the left) is composed of two 10 Hz repetition rate, unstable oscillator pump lasers that are frequency doubled in KTP and used to pump a 4-pass bowtie Ti:sapphire amplifier. The system currently operates at a conversion efficiency of about 35 % green to red and produces, at maximum power, pulses with 0.28 J of energy. The final goal of this system is to be able to amplify light from the 5 mJ level to 300 mJ level across a bandwidth of about 20 THz.
Experimental studies of ultrahigh intensity light-matter interactions are also underway in the laboratory. Currently, we are interested in measurements of how very intense light interacts with atoms. As atoms are exposed to the laser fields, we observe the products from this excitation process. The chamber used to measure the ions from the interaction is shown to the right. This chamber measures the interaction of light with single atoms in a vacuum less than one-trillionth of an atmosphere (<1E-13). The intensity of the light in the focus of the laser beam is about 10,000 trillion W/cm2
The experimental apparatus has resulted from graduate and undergraduate students working to gain insights into the new and exciting physics. The exploratory research experience is a unique educational opportunity relevant to many careers including optical communications technology and fundamental studies in atomic, molecular, optical, or plasma physics. Skills learned in the program range from machining and computer aided drafting and design (CADD) to quantum mechanics and numerical calculations of the Schrodinger equation.
The research laboratory is in room 014A in Sharp Laboratory on the academic mall of the University of Delaware. The graduate student offices are in 014B of Sharp Laboratory. This is a picture of the north side of 014A just after it was renovated. My sincere thanks to all who helped in the planning and construction. A very nice job was done!
Oscillator Physics: The oscillator is pumped continuously with 527 nm green light. A Ti:Sapphire crystal is used as the lasing medium and emits a range of wavelengths that superimpose to create an ultra-short mode-locked pulse. Due to the high intensities in the crystal, a Kerr lens affects the laser cavity. The pulse then travels through a set of prisms to compensate for the dispersion in the Ti:Sapphire crystal. These prisms assist in mode-locking the oscillator by changing the phases of the various wavelengths of the pulse. Sometimes the prisms need to be slightly adjusted to maintain mode-locking. The pulse then travels back through the crystal gaining more energy. Since the pulse intensity is much greater than the continuous beam it is more focused by the Kerr lens can be discriminated by an intracavity hard aperture which primarily blocks continuous beam. The final result is a laser that creates an ultrafast short pulse train that seeds the laser system.
Oscillator Design: The oscillator’s 2.2 mm long Ti:Sapphire crystal is cut at Brewster’s angle and is cooled to 18°C to prevent thermal lensing and damage to the crystal. The construction uses a folded cavity with fused silica prisms. The optimal pulse dispersion compensation with no power loss occurs when the beam enters both prisms 3 mm from their tips. A slight blue tuning occurs in the prisms to assist with mode-locking. Sometimes small adjustments need to be made in the second prism and the hard aperture (normal width marking of ~0.9) to maintain mode-locking. When operating with CW and the aperture fully open, the power is typically 160 mW for 2.2 Watts from the 527 nm green pump. The mode-locked pulse train serves as reference timing signal and the seed pulse for the laser system. The mode-locked pulses are 20 femtoseconds at 76 MHz, with a center wavelength at 800 nm and a full width half max (fwhm) of 70 nm. They have an average power of 210 mW when pumped with 2.2 Watts of 527nm green light.
Stretcher: The purpose of the stretcher is to expand the temporal length of the pulses, decreasing their peak power allowing them to travel through the optics and crystals without damaging them. The stretcher consists of two 1200 groove/mm gratings in parallel configuration, a large concave mirror and a retro-reflector. The pulse enters the chamber and hits the first grating causing the different wavelengths of light to be reflected at different angles. The pulse is then reflected off of a series of mirrors to the second grating. From here the pulse travels to the retro-reflector which changes the height of the beam in the chamber. The pulse then travels back through the grating and mirror setup but can now exit the chamber due to the retro-reflector. A diode measures the pulse train to provide the synchronization clock for the laser. This synchronization signal is sent to the Pockels cell driver which will inject one of the stretched pulses into the regenerative amplifier (regen). After the pulse leaves the stretcher it goes through a faraday isolator to prevent feedback from the regen to the oscillator. The result is the 20 femtosecond pulses coming out of the oscillator are stretched to about 400 picoseconds due to the difference in optical path length of the various colors.
Regen: The regen has a ring cavity design that uses two Ti:Sapphire rods with a 22 mm effective path as the gain medium. The beam enters the regen, is reflected off of a thin film polarizer and goes through a half wave plate which rotates the polarization of the light. Injection seed pulses from the oscillator trigger the Pockels cell (8 Volts) which acts as another half wave plate effectively trapping the pulse in the cavity for 17-18 round trips. The regen cavity is pumped by the green light from a Surelite laser with 0.2 Watts at 10 Hz. This Surelite laser is divided once it enters the cavity to pump both crystals equally from each side to ensure that the pulse is properly amplified. The pulse is amplified from a few hundred picojoules to 5 mJ at a much slower repetition rate of 10 Hz. Once the pulse is sufficiently amplified, the Pockels cell switches back to 0 V allowing the pulse exit the cavity via the thin film polarizer. In the regen cavity TEM00 gain is strongly favored and consequently gain narrowing occurs reducing the bandwidth to 30 nm. A transmission etalon counteracts this effect by decreasing the intensity around 795 nm, restoring pulse bandwidth to 55 nm.
Multipass: The multipass amplifier is designed in a bowtie shape that passes the beam through the gain medium a total of 5 times. The gain medium is a Ti:Sapphire crystal 15 mm in diameter and 21 mm in length which is pumped equally from both sides by a Quanta Ray Nd:YAG laser from Spectra Physics at 10 W. The crystal is mounted on a copper stage with 20°C circulating water to extract heat, counteracting thermal-lensing. Micro-lenses are implemented to refine the pump lasers to the best possible mode quality. As a result, the multipass amplifier can amplify 6 mJ pulses to 290 mJ at 10 Hz.
Compressor: The compressor is designed to be the exact opposite of the stretcher, making the pulses shorter to increase their peak power. The compressor can also be adjusted to account for any dispersion in the laser system. It consists of two 1500 groove/mm gratings mounted in parallel with a center to center separation of 40 cm. The beam is expanded to a 2 cm FWHM diameter before entering the compressor to prevent air breakdown and damage to the optics. Due to the efficiency of the gratings and other factors the compressor has a throughput of 67%. Once focused, the final result is a 40-45 femtosecond pulse with 6 Trillion Watts of energy.
Ultrafast lasers have increased laboratory light intensities to 10^20 times the sunlight on earth, making it increasingly relevant to carry out photoelectron experiments at ultrahigh intensities. Photoelectrons at ultrahigh intensities can involve inner shell excitations, laser acceleration, multielectron dynamics, X-ray radiation, Lorentz Deflection, and Nonlinear Thomson radiation.
We present measurements and theory for the interaction of ultraintense fields with neon and xenon. Intensity dependent yields for photoelectrons with 75KeV, 250KeV and 500 KeV are measured in the intensity range of 10^17 W/cm^2 to 10^19 W/cm^2 and show excellent agreement with a semiclassical 3D model of ionization. Shown are the polar and azimuthal angular distribution of the photoelectron final states at ultrahigh intensity from highly charged ions, out to 1 MeV.
The research in this group investigates the science of atoms, molecules, and light. Fundamental questions we address include:
1) Atom interactions with ultra-intense laser fields
2) Laser technology for high power, ultrafast lasers.
The interaction of atoms with laser intensities from 1017 W/cm2 to 1019 W/cm2 gives rise to very high-energy particles and radiation. The electrons ripped off the atom by ultra-intense lasers will have kinetic energies exceeding their rest mass. They will travel near the speed of light and have millions of electron volts of energy. The dynamics are relativistic and the physics is only now beginning to be understood.
Intense laser interactions and measurements of the fast dynamics for processes in atoms and molecules require lasers that can generate very short pulses of light with high peak intensities. This research laboratory is also involved in laser and optical technology development with some of the highest peak power lasers in the world. These efforts are aimed to meet the scientific demands for high repetition rate, high peak power, ultrashort pulse duration light sources with wavelengths between 300 eV and 0.5 eV. Recent advances in beam shaping technology allow amplification to the terawatt peak power level with a spatial beam profile better than what is available with many 5 milliwatt HeNe lasers.
The research in high intensity physics has significant overlap with commercial interests. These range from laser induced breakdown spectroscopy (LIBS) to high data rate communications in optical fibers. The two posters below relate to the broad application and role of lasers and optical technology in society.