Paris Panagiotopoulos

Carrier field shock formation of long-wavelength femtosecond pulses in single-crystal diamond and air

We numerically demonstrate the formation of carrier field shocks in single-crystal diamond and air for a wide variety of input conditions using two different electric field propagation models. We investigate the impact of numerous physical effects on the carrier wave shock. It is shown that, in many cases, a field shock is essentially unavoidable and therefore extremely important in the propagation of intense long-wavelength pulses in weakly dispersive nonlinear media. We found that single-crystal diamond is the ideal nonlinear medium for carrier wave shock formation using mid-IR laser pulses carrying moderate energies.

Simulations of 10 μm filaments in a realistically modeled atmosphere

We numerically study the formation and propagation dynamics of 10 μm filaments in the atmosphere. We investigate filament formation of multi-Joule 100 fs and 1 ps duration pulses over propagation lengths of 100 m and find that the carrier self-steepening regularization mechanism predicted for 4 μm wavelengths still holds. In our study we include multiple physical effects ignored in the past, such as rotational Raman and avalanche ionization. In addition we incorporate in our simulations the most detailed dispersive model available for atmospheric air, which includes multiple species, such as O2, N2, Ar, CO2, CO, and CH4, for a variety of humidity levels. Results presented here are expected to have a significant impact in the wider field of nonlinear optics where the use of mid-infrared lasers is rapidly growing.

Extreme events in resonant radiation from three-dimensional light bullets

We report measurements that show extreme events in the statistics of resonant radiation emitted from spatiotemporal light bullets. We trace the origin of these extreme events back to instabilities leading to steep gradients in the temporal profile of the intense light bullet that occur during the initial collapse dynamics. Numerical simulations reproduce the extreme valued statistics of the resonant radiation which are found to be intrinsically linked to the simultaneous occurrence of both temporal and spatial self-focusing dynamics. Small fluctuations in both the input energy and in the spatial phase curvature explain the observed extreme behavior.

Numerical Simulation of Ultra-Short Laser Pulses

This chapter contains the numerical models used to describe ultra-short pulse propagation while taking a brief look on their derivation from the Maxwell equations. The main models described here are the unidirectional pulse propagation equation (UPPE) and the modified Kadomtsev–Petviashvili equation (MKP), while a brief look is taken at the main envelope counterpart, the nonlinear envelope equation (NEE) model. The chapter concludes with the presentation of various applications of the UPPE and the MKP in the study of carrier wave shocks in various realistically model media, and a special case of the propagation properties of the two-dimensional XY Airy beam and its radial symmetric version, the ring-Airy beam, in the linear and nonlinear regimes.

Simulation of LWIR TW ultrashort pulses over kilometer ranges in the atmosphere

We have identified major paradigm shifts relative to near-IR filamentation when high power multiple terawatt laser pulses are propagated at mid-IR and long-IR wavelengths within key atmospheric transmission windows. Individual filaments at near-IR (800 nm) wavelengths typically persist only over tens of centimeters, despite the whole beam supporting them being sustained over about a Rayleigh range. In the important mid-IR atmospheric window (3.2 - 4 μm) optical carrier wave self-steepening (carrier shocks) tend to dominate and modify the onset of long range filaments. These shocks generate bursts of higher harmonic dispersive waves that constrain the intensity growth of the filament to well below the traditional ionization limit, making long range low loss propagation possible. For long wavelength pulses in the 8-12 μm atmospheric transmission window, many-electron dephasing collisions from separate gas species act to dynamically suppress the traditional Kerr self-focusing lens and leads to a new type of whole beam self-trapping over multiple Rayleigh ranges. This prediction is key, since strong linear diffraction at these wavelengths are the major limitation and normally requires large launch beam apertures. We will present simulation results that predict multiple Rayleigh range propagation paths for whole beam self-trapping and will also discuss some recent efforts to extend the HITRAN linear atmospheric transmission/refractive index database to include nonlinear responses of important atmospheric molecular constituents.

Helical microfibers created by photopolymerization with light fields possessing orbital angular momentum (Conference Presentation)

Photopolymerization, the process of using ultraviolet light to activate polymerization within resins, is a powerful approach to create arbitrary, transparent micro-objects with a resolution below the diffraction limit. Such microstructures have been optimized for optical manipulation and are finding application elsewhere, including micro-optics, mechanical microstructures and polymer crystallography. Furthermore, due to self-focusing, photopolymerization can form a waveguide, which develops into an optical fibre as long as submillimeters. Importantly, to date virtually all photopolymerization studies have been performed with incident light fields possessing planar wavefronts and simply exploit the beam intensity profile. Here we investigate photopolymerization of ultraviolet curing resins with a light field possessing orbital angular momentum (OAM). We show that the annular vortex beam breaks up via modulation instability into the m-microfibers, depending on the azimuthal index m of an incident optical vortex. These microfibers exhibit helical structures with chirality determined by the sign of m and mirror the helical nature of the incident vortex beam wavefront. We have developed a numerical model based on the Beam Propagation Method that captures the key experimental observations for a variety of optical vortices characterized by their azimuthal index m. This research opens up a range of new vistas and has broad consequences for the fields of structured light, new approaches to writing novel mesoscopic structures and applications such as in detecting or sorting the OAM mode (e.g. photonic lanterns) in areas including optical communications and manipulation.

Long range robust multi-terawatt MWIR and LWIR atmospheric light bullets

There is a strong push worldwide to develop multi-Joule femtosecond duration laser pulses at wavelengths around 3.5-4 and 9-11μm within important atmospheric transmission windows. We have shown that pulses with a 4 μm central wavelength are capable of delivering multi-TW powers at km range. This is in stark contrast to pulses at near-IR wavelengths which break up into hundreds of filaments with each carrying around 5 GW of power per filament over meter distances. We will show that nonlinear envelope propagators fail to capture the true physics. Instead a new optical carrier shock singularity emerges that can act to limit peak intensities below the ionization threshold leading to low loss long range propagation. At LWIR wavelengths many-body correlations of weakly-ionized electrons further suppress the Kerr focusing nonlinearity around 10μm and enable whole beam self-trapping without filaments.

Spatiotemporal Light Bullets in Bulk Media

Three-dimensional light bullets in bulk media are identified, through their specific energy density flux, as polychromatic Bessel-like pulses. We also study resonant radiation emission with applications in rogue event studies and THz detection.