Dissertation Defense: Michael Hastings, "Long Wave Infrared Pulse Compression and Nonlinear Propagation of Nontrivial Optical Waveforms"

Abstract

In this proposed dissertation new frontiers in laser physics are explored with an emphasis on long-wave infrared (LWIR) pulse propagation in gaseous media in the extreme nonlinear optics regime. Numerical experiments are used to study physics which the results from these simulations should motivate experiments. The study comprises of four main primary research objectives:

1. Terawatt-level few-cycle pulse compression of LWIR using a gas-filled multi-pass cell.
2. Formation of X-waves from LWIR pulses in Xenon and Air.
3. Probing the transition from nonlinearly dominated (gKPE) to dispersion dominated (NLSE) physics by engineering a monotonically increasing dispersion profile using the Sellmeier equation parameters.
4. Synthesizing exotic optical waveforms from harmonics of a LWIR fundamental wavelength to generate terahertz radiation mediated by multi-color pulse filamentation.

Starting from currently available CO₂ laser pulses I show by using a gas-filled multi-pass cell, a laser pulse with a central wavelength in the LWIR can be compressed in a controlled way to sub-100 fs duration and contain multiple terawatts of peak power at the output. The simulated gas-filled multi-pass cell is filled with the second most abundant isotope of CO₂, which allows for long distance, low-loss propagation in the compression chamber. By using a multi-pass cell filamentation is avoided, so that laser induced damage of mirrors is avoided and the resulting ultrashort pulse can be extracted from the cell. The compression is caused by self-compression, mediated by the balance of anomalous dispersion of the propagation medium and the normal dispersion produced by self-phase modulation (SPM). The cavity mirrors act to maximize SPM while avoiding filamentation.

Using the sub-100 fs duration pulse generated by the gas-filled multi-pass cell as an input, numerical experiments showed that a broadband, multi-harmonic spanning X-wave forms during optical filamentation collapse in Xenon and Air. A self-contained X-wave pulse is generated during regularization of optical field shocking in the form of a dispersive wave being shed out the back of the pulse.

Next, the transition that marks the origination point of the X-wave is studied further. It is determined that by engineering a monotonically increasing dispersion profile so that the GVD becomes steeper and more curved the transition from gKPE to NLSE can be moved to longer wavelengths. With sufficiently large GVD (dispersion dominated vs nonlinearity dominated) the multi-harmonic X-wave reverts to a common single-harmonic X-wave.

Finally, exotic waveforms, akin to those found in RF communication (square, triangle, sawtooth, etc.), are simulated in Xenon and shown to generate THz efficiently as the carrier-envelope phase offset is tuned and number of harmonics increases. This results in an even broader spectrum that includes the X-wave. The spectral extent from the multi-color pulse filamentation starting from a LWIR fundamental stretches remarkably from the THz all the way to the deep UV.