Title: Ultrafast Mid-IR Nonlinear Optics in Gas-filled Hollow-core Photonic Crystal Fibers
Supervisors:
Principal supervisor, Associate Professor Morten Bache, DTU Fotonik
Co-supervisor Professor Ole Bang, DTU Fotonik
Evaluation Board:
Associate Professor Jesper Lægsgaard, DTU Fotonik (Chairman)
Associate Professor Nicolas Joly, University of Erlangen-Nüremberg, Germany
Associate Professor Fabio Biancalana, Heriot-Watt University, United Kingdom
Master of the Ceremony:
Professor Peter Uhd Jepsen, DTU Fotonik
Abstract:
Invention of hollow-core fiber has been proven an ideal medium to study light-gas interaction. Tight confinement of light inside hollow-core fiber allows unremitting and tailored interaction between light and gas over long distances. In this work, we used a special kind of hollow-core fiber - hollow-core anti-resonant (HC-AR) fiber to study the various nonlinear effects filled with Raman free noble gas. One of the main striking features of HC-AR fiber is that ∼99.99% light can be guided inside the central hollow-core region, which significantly enhances damage threshold level. HC-AR fiber can sustain 10s of μJ pulse energies, tolerate mutiple-watts of average powers, provide a clean spatial mode profile and give flexible beam handling and delivery. It also offers relatively low-loss, broadband guidance, and low anomalous group-velocity dispersion (GVD). Both the dispersion and nonlinearity can be tuned by simply changing the pressure of the gas while at the same time providing extremely wide transparency ranges. In this thesis, we propose several low-loss broadband guidance HC-AR fibers and investigate soliton-plasma dynamics using HC-AR fiber filled with noble gas in the mid-IR.
The combined action of self-focusing self-phase modulation (SPM) and anomalous GVD allows strong soliton self-compression down to sub-single cycle duration inside HC-AR fiber. The peak intensity at the maximum temporal compression can reach over 1014 W/cm2 which is sufficient to ionize the gas and form a plasma.
We investigate numerically soliton-plasma interaction in a noble-gas-filled silica HC-AR fiber pumped in anomalous dispersion regime at 3.0μm. We observe multiple soliton self-compression stages due to distinct stages where either the self-focusing or the self-defocusing nonlinearity dominates. Specifically, the parameters may be tuned so the competing plasma self-defocusing nonlinearity only dominates over the Kerr self-focusing nonlinearity around the soliton self-compression stage, where the increasing peak intensity on the leading pulse edge initiates a competing self-defocusing plasma nonlinearity acting nonlocally on the trailing edge, effectively preventing soliton-formation there. As the plasma switches off after the self-compression stage, self-focusing dominates again, initiating another soliton self-compression stage in the trailing edge. This process is accompanied by supercontinuum generation spanning 1-4 μm. We also demonstrate coherence of the supercontinuum and find that the spectral coherence drops as the secondary compression stage is initiated.