Summary of June 17-18, 1999 Laser Processing Consortium Workshop

(Jefferson Lab, Newport News, VA)

As of this meeting the Laser Processing Consortium has been six years on the journey toward a productive FEL facility for R&D. The year since the last meeting has seen much of the critical transition from plans to operating reality. The major task now is to get the user labs productive for good work. Information about the on-going progress of the Jefferson Lab FEL and the user program can be found at www.jlab.org/FEL. The next workshop meeting is planned for January 13-14, 2000.

The major milestones reached for the FEL itself are:

• The FEL accelerator reached its power goals, including full power energy recovery.

• The FEL attained record high power - 700 watts - and exceeded its design goal by reaching 1700 watts subsequently (July 1999). Both are records and will not soon be attained elsewhere.

• All significant aspects are in good agreement with the modeling work done earlier, providing still further confidence in our understanding of the design and operation of high-power FEL.

• FEL users became real, carrying out the first tests on real materials in User Lab 1.

• The first user proposals have been funded: (Tolk et al., Vanderbilt by DOE-BES, Clarke et al., Northrop Grumman by NASA, and Gillman, NSU by ONR.) It is a first step along the path to support as a user facility.

Further milestones can be expected before the January meeting.

• Regular running periods for user tests will now commence, presently budget-limited to 3-4 one-month periods per year. The next operations period is scheduled for Oct. 29 – Nov. 24, 1999.

• The first round of funding for the upgrade is anticipated. The journey to 10 kW IR with a wider wavelength range and 1 kW UV will begin, targeted for turn-on in three years.

• The usefulness of the FEL as a short-pulse Compton scattering x-ray source will be demonstrated.

The recent user tests and most of those planned for the summer run seek to use the laser beam as a tool for removing material. The specific applications deal with pulsed laser deposition of thin film magnetic materials, selective drilling of multi-layer polymer/metal films, precision drilling of metals and intentional beam damage to defense-related materials. In particular, the researchers sought to demonstrate that the FEL can indeed operate in each of the three ablation regimes: long pulse - thermal (as YAG and CO₂ lasers); nanosecond "percussion" (as excimers and Cu Vapor); "ultra-fast" high-peak power (as Ti:sapphire). Only the first was achieved with certainty.

The FEL coming to operational status is attracting growing attention from basic science researchers interested in spectroscopy and related science.

• "SELIM" (Science and Engineering of Laser Interactions with Matter) Harrison - UVa. A collaboration centered at University of Virginia looking at the use of lasers to selectively drive many aspects of the physics and chemistry, using both FEL and an extensive table-top system at UVa. Educational aspects are supported through NSF's IGERT program.

• Chemical Physics (Lehman & Scoles - Princeton, Pate - UVa) - take advantage of FEL's tunability, ps pulse length and high average power to produce otherwise-unavailable beam of molecules which are highly and selectively excited to study gas-phase chemistry related to combustion, atmospheric science, etc. Essentially all the needed hardware is in hand now thanks to a molecular beam scattering apparatus donated by Princeton.

• Terahertz Spectroscopy (Cho & Zhang - Rensselaer). Frequency conversion by solid-state electro-optics makes available a ps-pulse THz beam. THz radiation has unique penetration and absorption characteristics - e.g., to water in living tissues. The average power afforded by the IR FEL significantly increases the scan rates (x1000) possible for imaging applications.
• Multiphoton Ionization in the Mid-IR (Walker - Delaware). Very high intensities yields very strong optical fields facilitating "direct" MPI through virtual states, rather than pumping up a ladder of real states. For a chosen atom (and therefore ionization potential), the FEL's tunability allows different numbers of photons to be addressed, and the high rep rate leads to sufficient signal to detect readily and in detail.
• Materials Physics [Especially of H in Si] (Feldman et al. - Vanderbilt). The importance of hydrogen in Si is familiar in a-Si(H) photovoltaics, but it appears in many other connections as well. Pump/probe and other experiments with an intense, tunable IR source will permit species-specific characterization to develop a detailed level of understanding not yet attained.

The other major group of researchers was interested in both fundamental and applied aspects of materials processing:

• Pulsed Laser Deposition (Horowitz - NRL). Excimer-laser driven PLD is successfully depositing films of cuprate high temperature superconductors for microwave filters, BaSr titanates ferroelectrics for broad-band microwave phase shifters, rare-earth manganates for IR microsensors, indium tin oxide ("ITO") optically transparent anode layers, specialized polymers as the absorber active layer in surface acoustic wave chemical sensors.

• Holographic MOCVD (Gillman - NSU). Projecting the drive laser beam through a holographic filter in photo-MOCVD will directly write a thin-film hologram on the substrate, which can then be used for (e.g.) imaging. The FEL is the most suitable laser for such an undertaking.

• Pulsed Laser Synthesis of C, Si and GaAs (Holloway et al. - W&M). The FEL's high rep rate, potentially a problem for certain material removal applications, may be an advantage for nanotube generation since it offers opportunity to irradiate the emerging plume at a wavelength to which critical precursor species may respond. Further, FEL-based production is an excellent prospect for scale-up if successful.

• FEL Processing of Semiconductors (Gracin et al. - N.C.Central U.). The FEL's wavelength tunability allows for energy deposition in to specific entities in amorphous or polycrystalline semiconductors, potentially providing a means for them to be selectively annealed. Results of preliminary experiments at Duke are encouraging, but the greater average power at JLab is needed.

• Resonant Absorption of a Short Laser Pulse in Doped Dielectric (Lau et al. - Michigan). The extent of absorption at a specific line is modeled as a system response affected by the dopant concentration and absorption line parameters, the laser wavelength and pulse length, and the host optical properties. Energy deposition is maximized by selecting the right combination. Model originally addressed excimer laser damage of quartz optics due to absorption by impurities.

• Microwave-Driven UV Excimer Lamps (Diggs, Ametepe, Manos & Kelley - W&M). Output at fixed input increases with increasing filling gas pressure and decreasing cooling gas temperature. Output is greatest in directions far from the surface normal, because self-absorption is very weak in excimers. Applications of excimer lamps have not taken account of this angular distribution issue.

The next workshop (Jan. 13-14, 2000) is expected to include the results of the first two extended running periods (July and November) as well as reports similar to the above.

Fred Dylla

FEL Program Manager

Jefferson Lab

Michael Kelley, Chair

Laser Processing Consortium

College of William & Mary