Optical Science and Engineering ETDs

Publication Date

Winter 12-15-2021

Abstract

Filamentation in air is a phenomenon that has been extensively investigated for the last two decades. At sufficiently high intensity, even air is a nonlinear medium. These intensities are reached with ultrashort pulses (50 to 100 fs) of more than 1 J energy, which self-focus in air, reach ionizing intensities of oxygen and nitrogen, creating a plasma that defocuses the beam. The air filament is a self-induced waveguide resulting from a balance of focusing and defocusing. In this work new techniques were developed to visualize and analyze this phenomenon through its emission, in particu- lar the UV emission of the nitrogen cation. Contrary to popular belief, this emission does not proceed to be instantaneous upon ionization of nitrogen, but is delayed by tens of ps with respect to the ionizing/propagating short pulse. The time resolved emission of the nitrogen cation is very complex and was analyzed through pump- probe spectroscopy, the pump being the filamenting pulse at 800nm, and the probe its second harmonic. It was found that the medium within the filament volume provided transient optical amplification at the wavelengths of the rotational-vibrational lines of N2+ . The temporal behavior of the gain itself is complex, since it repeats itself at fixed time intervals (“revival times”). The wavelength of these lines is also time dependent, because they are Stark shifted in the expanding plasma. This study suggests that the filament creates a “laser in the sky”, which could be invaluable in amplifying weak return signals in remote sensing.

Degree Name

Optical Science and Engineering

Level of Degree

Doctoral

Department Name

Optical Science and Engineering

First Committee Member (Chair)

Dr. Jean-Claude Diels

Second Committee Member

Dr. Ladan Arissian

Third Committee Member

Dr. Francesca Cavallo

Fourth Committee Member

Dr. Paul Schwoebel

Keywords

ultrafast, ultrashort, air lasing, spectroscopy, filaments, revivals

Document Type

Dissertation

Language

English

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