Spatiotemporal light bullets and supercontinuum generation in β-BBO crystal with competing quadratic and cubic nonlinearities

Second harmonic generation from symmetry breaking stimulated by mixed organic cations in zero-dimensional hybrid metal halides

Mixing cations with different chemical properties to induce the generation of asymmetric structures is a new approach for tuning the optical properties of hybrid organic-inorganic metal halides (HOIMHs). In this study, zero-dimensional (C₉N₃H₁₅)(C₉H₁₃SO)MBr₆ (M = Bi/Sb, [C₉N₃H₁₅]²⁺ = [(C₄N₂H₁₀)(C₅NH₅)]²⁺ and [C₉H₁₄SO]⁺ = [CH₃(C₆H₄)OS(CH₃)₂]⁺) are synthesized. Two different cations cause both compounds to crystallize in the polar space group P2₁2₁2₁, thus resulting in significant phase matchable second harmonic generation under a 1064 nm laser excitation. Thus, (C₉N₃H₁₅)(C₉H₁₃SO)BiBr₆ and (C₉N₃H₁₅)(C₉H₁₃SO)SbBr₆ exhibit intensities that are approximately 1.8 and 1.7 times that of KH₂PO₄, respectively. The results of density functional theory calculations show that both (C₉N₃H₁₅)(C₉H₁₃SO)BiBr₆ and (C₉N₃H₁₅)(C₉H₁₃SO)SbBr₆ exhibit direct bandgaps of 2.95 and 2.81 eV, respectively. Additionally, because of the distortion of the inorganic octahedra, (C₉N₃H₁₅)(C₉H₁₃SO)SbBr₆ exhibited bright yellow emission at room temperature, which is attributed to ns² fluorescence emission. We believe that the symmetry of the HOIMH crystal structure can be broken by introducing spatially differentiated bifunctional organic cations, which consequently enables even-order nonlinear activities.

Cascaded nonlinearity induced spatial domain effects in a high power femtosecond optical parametric oscillator

We have investigated the effect of cascaded optical nonlinearity on the spatial beam properties of a femtosecond optical parametric oscillator (OPO). The OPO was operated with a tunable phase mismatch by varying the angle of the nonlinear crystal. The cascaded nonlinearity induced self-focusing and defocusing changed resonator's stability and impacted mode properties. With tuning of a phase mismatch, the calculated parabolic part of cascaded nonlinearity lens focal length changes from f ∼ 30 mm (D ∼ 33 m^-1 at Δθ ∼ -0.5^o) to infinity and back to f ∼ -110 mm (D ∼ -9 m^-1 at Δθ ∼ 0.9^o) in the LBO nonlinear crystal. Such high power nonlinear lenses in a cavity operated near its stability limit promoted the generation of axially asymmetric or pass-to-pass unstable resonator modes. It was shown that phase mismatched optical parametric oscillation changes the physical character of the resonator from linear to ring-like with two nonlinear crystals having two different focusing powers. Calculations showed that the QCN induced spatial nonlinear phase should lead to severe longitudinal chromatic aberrations for broad spectrum pulses. A numerical simulation in XYZ spatial domain and calculations using ABCD matrix approach confirmed the physical mechanisms underlying the experimental results and allowed for the interpretation of the observed effects.

Generalized nonlinear Schrödinger equations describing the Second Harmonic Generation of femtosecond pulse, containing a few cycles, and their integrals of motion

An interaction of laser pulse, containing a few cycles, with substance is a modern problem, attracting attention of many researches. The frequency conversion is a key problem for a generation of such pulses in various ranges of frequencies. Adequate description of such pulse interaction with a medium is based on a slowly evolving wave approximation (SEWA), which has been proposed earlier for a description of propagation of the laser pulse, containing a few cycles, in a medium with cubic nonlinear response. Despite widely applicability of the frequency conversion for various nonlinear optics problems solutions, SEWA has not been applied and developed for a theoretical investigation of the frequency doubling process until present time. In this study the set of generalized nonlinear Schrödinger equations describing a second harmonic generation of the super-short femtosecond pulse is derived. The equations set contains terms, describing the pulses self-steepening, and the second order dispersion (SOD) of the pulse, a diffraction of the beam as well as mixed derivatives. We propose the transform of the equations set to a type, which does not contain both the mixed derivatives and time derivatives of the nonlinear terms. This transform allows us to derive the integrals of motion of the problem: energy, spectral invariants and Hamiltonian. We show the existence of two specific frequencies (singularities in the Fourier space) inherent to the problem. They may cause an appearance of non-physical absolute instability of the problem solution if the spectral invariants are not taken into account. Moreover, we claim that the energy preservation at the laser pulses propagation may not occur if these invariants do not preserve. Developed conservation laws, in particular, have to be used for developing of the conservative finite-difference schemes, preserving the conservation laws difference analogues, and for developing of adequate theory of the modulation instability of the laser pulses, containing a few cycles.

Spatiotemporal continuum generation in polariton waveguides

We demonstrate the generation of a spatiotemporal optical continuum in a highly nonlinear exciton-polariton waveguide using extremely low excitation powers (2-ps, 100-W peak power pulses) and a submillimeter device suitable for integrated optics applications. We observe contributions from several mechanisms over a range of powers and demonstrate that the strong light-matter coupling significantly modifies the physics involved in all of them. The experimental data are well understood in combination with theoretical modeling. The results are applicable to a wide range of systems with linear coupling between nonlinear oscillators and particularly to emerging polariton devices that incorporate materials, such as gallium nitride and transition metal dichalcogenide monolayers that exhibit large light-matter coupling at room temperature. These open the door to low-power experimental studies of spatiotemporal nonlinear optics in submillimeter waveguide devices.

Waterproof coatings for high-power laser cavities

With the ever-increasing laser power and repetition rate, thermal control of laser media is becoming increasingly important. Except for widely used air cooling or a bonded heat sink, water cooling of a laser medium is more effective in removing waste heat. However, how to protect deliquescent laser media from water erosion is a challenging issue. Here, novel waterproof coatings were proposed to shield Nd:Glass from water erosion. After clarifying the dependence of the waterproof property of single layers on their microstructures and pore characteristics, nanocomposites that dope SiO₂ in HfO₂ were synthesized using an ion-assisted co-evaporation process to solve the issue of a lack of a high-index material that simultaneously has a dense amorphous microstructure and wide bandgap. Hf_0.7Si_0.3O₂/SiO₂ multifunctional coatings were finally shown to possess an excellent waterproof property, high laser-induced damage threshold (LIDT) and good spectral performance, which can be used as the enabling components for thermal control in high-power laser cavities.

Filamentation-free self-compression of mid-infrared pulses in birefringent crystals with second-order cascading-enhanced self-focusing nonlinearity

We experimentally demonstrate virtually lossless, filamentation-free and energy-scalable more than three-fold self-compression of mid-infrared laser pulses at 2.1 μm in a birefringent medium (β-BBO crystal), which stems from favorable interplay between the second-order cascading-enhanced self-phase modulation and anomalous group velocity dispersion. By choosing an appropriate input beam diameter and intensity, the self-compression down to sub-30 fs pulse widths with gigawatt peak power is achieved without the onset of beam filamentation and associated nonlinear losses due to the multiphoton absorption, yielding the energy throughput greater than 86%.

Planar light bullets under conditions of second-harmonic generation

We study solutions to second-harmonic-generation equations in two-dimensional media with anomalous dispersion. The analytical solution is obtained in an approximate form of the planar spatiotemporal two-component soliton by means of the averaged Lagrangian method. It is shown that a decrease in the amplitudes of both soliton components and an increase in the value of the transverse coordinate are accompanied by an increase in their temporal duration. Within this variational approach, we have managed to find a stability criterion for the light bullet and a period of oscillations of soliton parameters. Then, we use the obtained form as an initial configuration to carry out the direct numerical simulation of soliton dynamics. We demonstrate stable propagation of spatiotemporal solitons undergoing small oscillations predicted analytically for a long distance. The formation of a two-component light bullet is shown when we launch a pulse only at the fundamental frequency. In addition, we investigate the phase and group-velocity mismatch effects on the propagation of pulses.

Cross-polarized, multi-octave supercontinuum generation

The generation of superoctave spectra from the interaction of intense ultrashort optical pulses and cubic nonlinearity is the result of interplay between the dispersion and nonlinearity of a material and various propagation effects. The cubic nonlinearity can be enhanced when it is combined with a quadratic-cascaded nonlinearity, relaxing the requirement on the laser's peak intensity for supercontinuum (SC) generation. In this Letter, we demonstrate and compare the generation of an SC driven from cubic and cascaded quadratic nonlinearities at an anomalous and zero dispersion wavelength (ZDW). We show the filament-free SC generation of femtosecond mid-infrared pulses by harvesting cascaded quadratic nonlinearity and, at ZDW, requires a lower threshold peak intensity and results in a higher power spectral density for the newly generated spectral components. The presented method is a suitable approach for generating multi-octave spectra from low peak-power, high average-power oscillators or a suitable seed for optical parametric amplifiers and multi-octave field synthesizers.

Second-order cascading-assisted filamentation and controllable supercontinuum generation in birefringent crystals

We experimentally investigate filamentation and supercontinuum generation in a birefringent medium (BBO crystal), in the self-focusing regime where intrinsic cubic nonlinearity is either enhanced or reduced by the second-order cascading due to phase-mismatched second harmonic generation. We demonstrate that the supercontinuum spectral extent is efficiently controlled by varying the phase mismatch parameter. In the range of negative phase mismatch, we achieve full control of the blue-shifted spectral broadening, which is very robust and independent on the input pulse energy. In the range of positive phase mismatch, both the blue-shifted and the red-shifted spectral broadenings are controlled simultaneously, however showing a certain dependence on the input pulse energy. The results are interpreted in terms of complex interplay between the self-phase-matched second harmonic generation, which is a process inherent to narrow ultrashort pulsed laser beams and concurrent self-steepening processes which arise from cubic and cascaded-quadratic nonlinearities.

Odd harmonics-enhanced supercontinuum in bulk solid-state dielectric medium

We report on generation of ultrabroadband, more than 4 octave spanning supercontinuum in thin CaF₂ crystal, as pumped by intense mid-infrared laser pulses with central wavelength of 2.4 μm. The supercontinuum spectrum covers wavelength range from the ultraviolet to the mid-infrared and its short wavelength side is strongly enhanced by cascaded generation of third, fifth and seventh harmonics. Our results capture the transition from Kerr-dominated to plasma-dominated filamentation regime and uncover that in the latter the spectral superbroadening originates from dramatic plasma-induced compression of the driving pulse, which in turn induces broadening of the harmonics spectra due to cross-phase modulation effects. The experimental measurements are backed up by the numerical simulations based on a nonparaxial unidirectional propagation equation for the electric field of the pulse, which accounts for the cubic nonlinearity-induced effects, and which reproduce the experimental data in great detail.