Wednesday, March 12, 2014

TODAY! Seminar: "Femtosecond laser filamentation in air: the roles of optical nonlinearity and plasma generation" by Dr. Yu-hsin Chen 3.12.14/11:00am-12:00pm/CREOL 102

Seminar: "Femtosecond laser filamentation in air: the roles of optical nonlinearity and plasma generation" by Dr. Yu-hsin Chen
Wednesday, March 12, 2014 11:00 AM to 12:00 PM
CREOL Room 102

Dr. Yu-hsin Chen

An intense femtosecond laser pulse propagating in a gas may collapse into one or multiple “filaments” when its peak power exceeds the critical power (5 – 10 GW in air) for nonlinear self-focusing. In atmosphere, the laser intensity is typically ~ 1013 W/cm2 in the filament, leaving a weakly-ionized plasma channel which can extend meters in length with a diameter of < 100 μm. While it has been generally accepted that laser filamentation is the consequence of self-focusing-induced beam collapse stabilized by plasma generation and de-focusing, neither the field-induced nonlinearity nor the plasma generation had been directly measured. This uncertainty has given rise to recent controversy about whether plasma generation does indeed counteract the positive nonlinearity, as an alternate theory suggests that the stabilization mechanism is contributed by saturation of optical nonlinearity.
For a basic understanding of femtosecond filamentation and for applications, the focusing and defocusing mechanisms – nonlinear self-focusing and ionization – must be understood. By employing a single-shot, time-resolved technique based on spectral interferometry to study the constituents of air, it is found that the rotational responses in O2 and N2 are the dominant nonlinear effect in filamentary propagation when the laser pulse duration is longer than ~ 100 fs. Furthermore, we find that the instantaneous nonlinearity scales linearly up to the ionization threshold, suggesting that an ionization-free, negative stabilization of filamentation does not exist. This is confirmed by space-resolved plasma density measurements in meter-long filaments using optical interferometry with a grazing-incidence probe laser pulse.

Dr. Yu-hsin Chen received the B.S. degree in electrical engineering and the M.S. in electro-optical engineering in 2000 and 2002, respectively, both from National Taiwan University. He obtained his Ph.D. degree in electrical engineering from University of Maryland, College Park in 2011. Then he worked as a postdoctoral researcher at Lawrence Livermore National Laboratory.
Dr. Chen’s research interests are in ultrafast lasers, nonlinear optics, high-intensity laser-plasma interactions, and laser acceleration of charged particles. He has won 2012 Marshall N. Rosenbluth Outstanding Doctoral Thesis Award in plasma physics, for his work on femtosecond laser filamentation in atmosphere.

For additional information:
Dr. Martin C. Richardson
Pegasus Professor and University Trustee Chair, Northrop Grumman Prof of X-ray Photonics; Prof of Optics; Director Townes Laser Institute

mcr @ creol. ucf . edu

No comments: