Ph.D. Dissertation Defense: Yu-Hsiang Cheng

Monday, August 5, 2013
10:00 a.m.
Room 2460, AVW Bldg.
Maria Hoo
301 405 3681
mch@umd.edu

ANNOUNCEMENT: Ph.D. Dissertation Defense

Name: Yu-Hsiang Cheng

Committee:

Professor Howard M. Milchberg, Chair/Advisor

Professor Christopher C. Davis

Professor Ki-Yong Kim

Assistant Research Scientist John P. Palastro

Professor Amy S. Mullin, Dean's representative

Date/Time: Monday, August 5, 2013 at 10 am

Location: Room 2460, AV Williams Building

Title: HIGH FIELD OPTICAL NONLINEARITIES IN GASES

Abstract:

Optical femtosecond self-channeling in gases, also called femtosecond filamentation, has become an important area of research in high field nonlinear optics. Filamentation occurs when laser light self-focuses in a gas owing to self-induced nonlinearity, and then defocuses in the plasma generated by the self-focused beam. The result of this process repeating itself multiple times is an extended region of plasma formation. Filamentation studies have been motivated by the extremely broad range of applications, especially in air, including pulse compression, supercontinuum generation, broadband high power terahertz pulse generation, discharge triggering and guiding, and remote sensing.

Despite the worldwide work in filamentation, the fundamental gas nonlinearities governing self-focusing had never been directly measured in the range of laser intensity up and including to the ionization threshold. This dissertation presents the first such measurements. We absolutely measured the temporal refractive index change of O2, N2, Ar, H2, D2 and N2O to high-field ultrashort optical pulses with single-shot supercontinuum spectral interferometry, cleanly separating for the first time the instantaneous electronic and delayed rotational nonlinear response in diatomic gases.

As a side product of our measurements, we conclusively showed that a recent claim by several European groups that the optical bound electron nonlinearity saturates and goes negative is not correct. Such a phenomenon would preclude the need for plasma to provide the defocusing contribution for filamentation. Our results show that the ‘standard model of filamentation’, where the defocusing is provided by plasma, is correct.

Finally, we demonstrated that high repetition rate femtosecond laser pulses filamenting in gases can generate long-lived gas density ‘holes’ which persist on millisecond timescales, long after the plasma has recombined. Gas density decrements up to ~20% have been measured. The density hole decay is dominated by thermal diffusion. These density holes will affect all other experiments involving nonlinear high repetition-rate laser pulse energy absorption by gases.

Audience: Graduate  Faculty 

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