This PDF file contains the front matter associated with SPIE Proceedings Volume 12143, including the Title Page, Copyright information, Table of Contents, and Committee Page.

The self-channeling of extremely high power laser beams permits the ignition of plasma filaments in dielectrics, such as air and glasses. If no constraints are imposed by the geometry of the material, the plasma appears as a straight bright line. Here, we show that plasma filaments may be ignited in the vicinity of the interface between two materials in optical fibers, i.e., either the core-cladding or the cladding-air interface. In the latter case, helical plasma filaments lead to the emission of rainbow spiral beams. In perspective, our results may pave the way to a novel approach for micro-structuring optical fibers, and for generating light beams with orbital angular momentum.

All-optical poling was demonstrated for the first time in 1986 in single mode fibers: such nonlinear optical process enabled the introduction of a second-order susceptibility (χ⁽²⁾) in a doped silica fiber. By simply using an intense laser source, alloptical poling, later theoretically described by Stolen and coworkers, permitted the generation of a second harmonic (SH) signal in an otherwise centrosymmetric doped material. More recently, similar experiments have been carried out by exploiting complex beam propagation in multimode fibers. In this work we reveal, for the first time to our knowledge, the 3D spatial distribution of a χ⁽²⁾ nonlinearity written in a graded-index (GRIN) multimode (MM) fiber. In particular, the presence of a doubly-periodic distribution of χ⁽²⁾ is unveiled by means of multiphoton microscopy. The shortest period (tens of micrometers) is due to the beating between the fundamental and the SH beams, and it is responsible for their quasi-phase matching (QPM). Whereas the longest period (hundreds of micrometers) is associated with the periodic evolution, or self-imaging, of the power density of the MM beam along the GRIN MM fiber. The complex modal beating, leading to spatial self-cleaning of the fundamental beam, is thus printed inside the fiber core, and revealed by our measurements. We considered two fibers of similar composition and opto-geometric parameters, and we compared the evolution of the optical poling process with time. Despite the rather similar fiber characteristics, we observed a striking difference in the poling efficiency between the two fibers. Such observation led us to point out the importance of considering the complete fiber fabrication process (both the preform elaboration and the drawing steps) on the final structure and microstructure of optical fibers.

Asymmetries arising from time-symmetry breaking in non-Hermitian potentials are shown to be an effective mechanism to control turbulence in nonlinear systems. The scheme is based on the introduction of a spatiotemporal non-Hermitian potential to control the excitation cascade mechanism responsible for the turbulence. We show the proposal on the complex Ginzburg Landau equation, that covers the universal phenomenon of turbulence in a wide range of disciplines, in particular a fundamental model for laser-like systems. Energy effectively condensates by reducing the energy transfer between transverse modes to high harmonics. The study is performed in one and two-spatial dimensions plus time, both numerically and analytically. The analysis of the turbulent spectrum distributions and the level of condensation of energy into the lower order mode show a crucial dependency on the phase shift between the two quadratures of the complex modulation. The desired turbulence reduction is numerically found for phases close to the ones predicted from the theoretical analysis. Depending on the phase, we are able to either enhance or reduce the spatial instability by breaking the energy cascading symmetry through wave vectors. Therefore, we are able to tailor and control turbulence in the complex Ginzburg Laudau equation and expect these results to have applications in different fields; most straightforward in optics where a non-Hermitian potential may be achieved by manipulating the complex permittivity (representing gain and refractive index).

We study an all-optical Fredkin gate consisting of a Mach-Zehnder interferometer and a phase modulator based on silicon nitride ring resonators. Our study proposes an accurate, deterministic, and energy-efficient all-optical Fredkin gate utilizing the Kerr effect for optical computing. We firstly derive the mathematical model of the proposed controlled-swap gate from the Lugiato-Lefever equation taking into account the third-order optical nonlinearity. In addition, the mechanism of thermo-optical effect in the silicon nitride ring cavity is also included. The phase modulator built by the ring resonator with third-order nonlinearity can introduce a π phase shift to the signal when the Fredkin Gate is in the swap mode. However, it does not affect the overall phase shift of the parallel operation. Besides, the pump power introduces a phase shift to the signal by altering the intensity-dependent refractive index inside the micro-ring cavity. It dominates the switching of the Mach-Zehnder interferometer via cross-phase modulation. Moreover, we investigate the conditions of optical bistability for the silicon nitride cavities that can lead to the malfunctioning of the performance of the Fredkin gate. Lastly, we optimize the Fredkin gate performance by the pump power, pump detuning, and signal-pump wavelength difference sweeps.

The attenuation or "limitation" of an incoming laser threat to a safe level of radiations using an external element is called optical limitation. The concept of passive laser protection requires the use of materials with nonlinear optical properties that are self-activated when the incident laser threat is above a certain level of intensity. Conjugated polymers are tailored for optical limitation due to their versatility, their non-negligible nonlinear absorption properties and their uniform transmittance characteristics over a broad spectral range, the latter property avoiding color distortions to the observer. Usually, to obtain an optical limiting filter, nanomaterials are dispersed in a polymer host. In this work, we consider polyvinylcarbazole (PVK) as a new polymer host for optical limiting filters. PVK is a high Tg polymer exhibiting a high linear transmittance over a broad spectral range, up to 1.6 μm. The adiabatic bulk polymerization method has been employed to thermally polymerize the monomer in the presence of an initiator. Traces of remaining monomer have qualitatively been monitored using FTIR spectroscopy. Optical limiting experiments have been conducted at the wavelength of 1.06 μm using a Nd:YAG laser source in the nanosecond pulsed regime at a low pulse repetition rate. PVK-based filters present a low nonlinear threshold (i.e. activation of the nonlinear attenuation), a factor of 10 lower than the usual PMMA-based filters. Their global nonlinear attenuation is as high as OD = 2.0.

We report contrasted experimental and theoretical results on second and third harmonic generation from an amorphous silicon nanolayer. We perform this study by analysing the harmonic optical signals generated when fs and ps pulses tuned at 800nm and 1064nm, respectively, interact with the material. We measure second and third harmonic efficiencies as functions of angle of incidence and compare with numerical simulations based on a microscopic hydrodynamic model that accounts for different possible contributions to the nonlinear polarization. We measure and estimate the efficiencies of the nonlinear signals using no assumptions about effective surface or volume nonlinearities, enabling proper explanations about the microscopic origins of SH and TH signals arising from the amorphous silicon nanolayer.

Stoichiometric silicon nitride (Si₃N₄) constitutes a mature platform for integrated photonics. Its pertinent properties, including wide transparency window from the visible to the mid-IR, low propagation loss, and high third-order nonlinearity, are exploited in many linear and nonlinear applications. However, due to the centrosymmetric nature of the Si₃N₄, the absence of the second-order susceptibility (χ⁽²⁾) impedes a realization of three-wave mixing processes as well as the linear electro-optic effect, relevant for many applications on an optical chip. Here, we implement the electric-field poling technique to induce the effective χ⁽²⁾ inside a Si₃N₄ waveguide, thus enabling the linear electro-optic modulation. Using numerical simulations, we estimated the concentration and the diffusion coefficient of the charges responsible for the space-charge electric field formation. In addition, the DC third-order susceptibility of Si₃N₄ previously unknown in the literature is measured using a free-space Mach-Zehnder interferometer.

The Kerr lensing effect influences the amplification of ultra short pulses with high repetition rate. Numerical simulations can estimate this effect and support the optimal design an ultra short laser amplifier. We compare two approaches to simulate laser amplifiers with respect to Kerr lensing effect modeling. These are a Gaussian beam amplification method and the angular spectrum method. In both cases, by Fourier transformation, the beam is considered as a continuous wave beam with a certain spectrum. Then, the Gaussian beam amplification method allows to take into account this spectrum of the beam. This means that the wavelength dependency of stimulated emission cross section and reabsorption cross section can be taken into account. However, a Gaussian beam amplification method does not simulate diffraction effects, which arise from the non-parabolic index of refraction obtained by the Kerr effect. This is an advantage of the angular spectrum method of plane waves. It can model diffraction effects obtained by the Kerr effect. But taking into account the spectrum of the beam is more computational intensive. A parameter which indicates the strength of the Kerr lensing effect is the B-integral. Numerical simulations compare this parameter computed by the Gaussian beam amplification method and the angular spectrum method.

We present the numerical modeling of two different randomization methods of two-dimensional aperiodic photonic lattices based on Mathieu beams, optically induced in a photorefractive media. We numerically study light propagation in such lattices. For both methods, we compare light transport and localization in such lattices along the propagation and for various disorder strengths. For all disorder strengths, a disorder-enhanced light transport is observed. With increasing disorder strength light transport becomes diffusive-like while with further increase of disorder strength, the Anderson localization is observed. For longer propagation distances this transition is more pronounced. The influence of input lattice intensity on the localization effects is studied. We observe more pronounced localization for one of the methods, and different diffraction and localization along different directions, due to the crystal and lattice anisotropy. The difference in light transport and localization between two randomization methods is attributed to various levels of input lattice intensity.

In this work we show the prospect of using periodic refractive index modulation – photonic crystals – to control the spatial dispersion curvature in a way that could potentially counteract the Kerr effect, thus suppressing filamentation. Numerical simulations by beam propagation method indicate that fine control of the spatial dispersion can be achieved by tuning the photonic crystal geometry and projecting to the appropriate Bloch mode by chirping the longitudinal period. Fabrication of such photonic crystals poses some unique challenges due to the small feature size and large-scale requirement. Bessel beam writing is shown to be a valid method for the fabrication of such photonic crystals.

Random fiber lasers utilizing Rayleigh backscattering possess a stochastic dynamics of the output power right above the threshold with narrow spectral lines appearing in random parts of the generation spectrum. We study the role of the random feedback in formation of these modes. We show that the generation of the localized modes does not require purely random feedback, and those may appear if both Rayleigh backscattering and point-like reflectors with tunable reflectance, based on a Sagnac fiber mirror are included into the laser cavity.

In this paper, we study THz generation by optical rectification in GaSe crystal pumped under normal incidence below and around the bandgap. We compared the generated THz signal from 3 crystals with different thicknesses (1 mm, 0.1 mm and 0.02 mm) when pumped with femtosecond laser pulse whose wavelength is tuned from 800 down to 540 nm. With the 1-mm thick crystal, we observe a pronounced THz emission peak when the pumping photon energy is 1.77 eV because of better phase matching conditions. We also notice that the efficiency improvement is more pronounced for higher THz frequencies. The experimental data are nicely fitted by our OR generation model. Since the 2 other crystals are thin, we don’t observe phase matching impact on the signal. When pumping around the bandgap, we detect another, smaller, THz emission peak, due to nonlinearity enhancement this time.

We study the propagation of coherent broadband light through laser induced periodic surface structures (LIPSS) fabricated in diamond, by comparing the performance of the laser generated nanostructures with sinusoidal ideal structures in the near-IR. The simulations were carried out for a variety of LIPSS morphologies, including nanostructure aspect ratio and the irregularities arising from the LIPSS technique. The results show that these irregularities produce substantial diffraction compared to the ideal structures, meaning that their use for some optical applications may be limited.

Supercontinuum generated in optical fibers due to various nonlinear processes has a very broad spectrum and it is one of the most important and analyzed nonlinear optics phenomena, which has many important applications. Although a wide variety of supercontinuum generation in PCFs research has been done, research in which the length of a short PCF is changed in small steps has not been performed. Since extremely rapid spectrum extension in femtosecond regime of supercontinuum generation occurs in the first few centimeters of the PCF, comprehensive knowledge of spectrum evolution is necessary to improve efficiency of this nonlinear processes. In this paper we present our experimental of light generation in short PCF, when fiber length was changed in small steps using ~ 110 fs duration and 1030 nm wavelength pulses pumping.

Chiral molecules are typically optically active with inherently non-superimposable mirror images. Henceforth, these molecules can exhibit differences in absorption between left-and-right circularly polarized light. This difference in light absorption is terminologically called circular dichroism. In this paper, a comparative theoretical and spectroscopic study on the one-photon absorption circular dichroism (OPCD) and two-photon absorption circular dichroism (TPCD) of an optically active chiral ketone over the far and near-ultraviolet region is presented. As molecular transitions associated with OPCD and TPCD obey different selection rules with feasibly different spectral features, the two processes are thus complementary. The strengths and weaknesses of employing one and two-photon circular dichroism of R-(+)-3-Methylcyclopentanone to investigate optical activity are discussed. Moreover, the basic theory of OPCD and TPCD is also shown and compared against corresponding spectroscopic results.

In this paper, we present a study on nonlinear optical effects in two types of core-shell nanoparticles with different claddings - thiol-termination ligand and sodium citrate/PVP claddings. Nanoparticles were dissolved in Dichloromethane and studied using the Z-scan method. As laser sources we used a tunable laser with 150 fs pulse width and 500 kHz pulse repetition rate. To study what processes lead to refractive index changes we used the polarization-resolved Z-scan method. A strong thermal response was observed for spherical nanoparticles. Both nanoparticles had similar spectral dispersion for Two-photon absorption, but spherical had a much stronger Kerr response.

We present a theoretical formalism to describe the amplification of two monochromatic waves counter-propagating in a rare-earth-doped optical fiber amplifier. Interaction of the waves through a dynamical population inversion grating inscribed in the active fiber by the waves during their amplification results in a strong power transfer from one wave to another providing a preferable amplification of one wave at the expense of another. In this sense, the effect is similar to stimulated Brillouin scattering and is expected to be observed with both pumped and unpumped rare-earth-doped fibers possessing a finite polarizability difference between the excited and ground states.

The human eye, sensors and photoreceptors can be damaged if they are submitted to laser beams with high intensities and low divergence. In the military field, an optical sensor can be exposed beyond its capacities and be damaged by pulsed laser radiations. In order to have a protection against such threats, passive optical limiting is used1. It is based on nonlinear effects which allow to reduce the intensity of aggressive laser radiations. Nowadays, the development of nonlinear filters for optical limiting is a challenge in order to have protection against threats caused by a pulsed laser beam. As a matter of fact, tactical lasers are a threat toward sensing devices between hundreds of meters and some kilometers.^2,3

This work has for purpose the synthesis of methacrylate-based hybrid polymer systems in order to study their nonlinear properties for the optical limiting in the near infrared part of the electromagnetic spectrum. Several polymers have been selected with specific properties, such as a high transmittance (>80%) over a broad spectrum and a significant third order nonlinear susceptibility χ³. Poly(methyl methacrylate) (PMMA) is considered as the standard host polymer material because of its high transparency up to 1.6μm but also due to its good accessibility. However, to avoid some issues (e.g. exothermy of the polymerization and Tromsdorff-Norish effect during the radical polymerization) regarding the use of methyl methacrylate (MMA) and to improve the nonlinear properties like the nonlinear and damage thresholds, new polymers or hybrid systems have been synthesized. In the present work, a thermoset matrix is studied corresponding to a mix between a reactive oligomer and a monomer both with methacrylate functions. The use of an oligomer and a monomer with a high boiling temperature as precursors allows to prevent the monomer evaporation, resulting in the formation of bubbles in the final material, related to the reaction exotherm.

The application of two-photon absorption (TPA) for local nonequilibrium charge generation in the semiconductor microelectronic structures improves the 3D-spatial resolution of optical testing techniques (such as OBIC and similar), as compared to those, based on single-photon absorption (SPA). In this paper we discuss the results of laser single-event effect (SEE) simulation in digital potentiometer AD8400, using the TPA of tightly focused femtosecond laser radiation. The experiments were performed at the setup, which includes the tunable optical parametric amplifier with output wavelength in 900…1200 nm region. The results were compared to those obtained by SPA technique on the same experimental setup.

In photonic reservoir computing, semiconductor lasers with delayed feedback have been used to efficiently solve difficult and time-consuming problems. The injection of data in these systems is often performed optically into the reservoir. Based on simulations, we show that the performance depends heavily on the way that information is encoded in this optical injection signal. In the simulations, we compare various input configurations consisting of Mach-Zehnder modulators and phase modulators for injecting the signal. We observe far better performance in our results, see also [Bauwens et al, Opt. Express 30, 13434 (2022)], on a one-step ahead time-series prediction task when modulating the phase of the injected signal rather than only modulating its amplitude.

Pulse propagation through hollow-core fibers (HCFs) filled with noble gas is a stable and efficient technique for pulse self-compression. The scalability of soliton dynamics in gas-filled HCFs, varying over a large range of energies, from sub-μJ to above mJ, allows to tune the energy of the generated few-cycle pulses too a great extent. Scaling relations can be used to produce propagation dynamics and effects that are invariant and essentially identical for multiple sets of input conditions. But, for the same input soliton order, the scaling relations derived under different dispersion conditions, such as different gas pressure, result in somewhat different scaling laws. This leads to an ambiguity in the compression factor and compression length for any particular soliton order N. It is thus necessary to find an accurate soliton order which can describe the self-compression dynamics over different dispersion conditions. We numerically simulate soliton self-compression in an argon gas-filled HCF across a wide range dispersion conditions and present an accurate soliton order for better understanding of the self-compression behavior. We introduce an effective soliton order Neff, for explaining the behavior of soliton dynamics in systems with high third order dispersion (TOD). This provides us with universal scaling laws for generating high-energy few-cycle pulses, which are critical for generating single and trains of attosecond pulses, as well as electron and ion acceleration strategies in intense laser pulses.