Nonlinear Refraction, Excited State Absorption, and Multiphoton Absorption of 1,3-Thiazolium-5-thiolate Mesoionic Compound.

We synthesized the mesoionic compound 2-(4-chlorophenyl)-3-methyl-4-(4-methylphenyl)-1,3-thiazole-5-thiolate and measured its refractive and absorptive nonlinear optical response in different temporal and spectral regimes. The experiments were performed by using the Z-scan technique with two pulsed light sources: the second harmonic (at 532 nm) of a mode-locked and Q-switched Nd-YAG laser (100 ps, 10 Hz) and a Ti: Sapphire laser system (100 fs, 1 kHz) operating at 800 nm. The observation and characterization of nonlinear refraction, two- and three-photon absorption, and excited state absorption of the mesoionic compound dissolved in dimethyl sulfoxide, in different concentrations, are presented and discussed with basis on the population redistribution in a three-energy-level model that allows the determination of the parameters which characterize the nonlinear response.

Intensity correlation scan (IC-scan) technique to characterize the optical nonlinearities of scattering media.

Light scattering, whether caused by desired or spurious elements, is considered one of the main phenomena that present great challenges for the nonlinear (NL) optical characterization of turbid media. The most relevant disturbing factor is the random deformation suffered by the spatial intensity distribution of the laser beam due to multiple scattering. In this work, we report the intensity correlation scan (IC-scan) technique as a new tool to characterize the NL optical response of scattering media, by taking advantage of light scattering to generate speckle patterns sensitive to wavefront changes induced by the self-focusing and self-defocusing effects. Peak-to-valley transmittance curves, with a higher signal-to-noise ratio, are obtained by analyzing the spatial intensity correlation functions of the different speckle patterns, even in very turbid media where conventional NL spectroscopy techniques fail. To demonstrate the potential of the IC-scan technique, the NL characterization of colloids that contain a high concentration of silica nanospheres as scatterers, as well as gold nanorods, which act as NL particles and light scatterers, was performed. The results show that the IC-scan technique is more accurate, precise and robust to measure NL refractive indices in turbid media, overcoming limitations imposed by well-established Z-scan and D4σ techniques.

Phosphotellurite glass and glass-ceramics with high TeO2 contents: thermal, structural and optical properties.

Phosphotellurite based glasses have interesting features such as low characteristic temperatures, high glass forming ability, high thermal stability against crystallization and a broad transparency window from ultraviolet (UV) to near-infrared (NIR), which makes them promising materials for photonic applications. In this work, phosphotellurite binary glasses, having a composition (100 - x)TeO2 - xBa(PO3)2 with x varying from 1 to 20 mol%, were synthesized by the conventional melt-quenching method in covered gold crucibles under air. Optical, physical and structural properties of the new glass samples were investigated by differential scanning calorimetry, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, linear optical absorption from UV to NIR, IR transmittance, and optical limiting experiments. Transparent glass-ceramics in the visible range were obtained for phosphotellurite samples containing 2, 4 and 6 mol% of Ba(PO3)2 and the phase crystallization was investigated through Rietveld analysis and transmission electron microscopy. The incorporation of Ba(PO3)2 into the TeO2 network drastically increases the thermal stability against devitrification and helps to shift the infrared multiphonon absorption edge to longer wavelengths. Nonlinear measurements performed with a picosecond laser at 532 nm indicate large effective nonlinear absorption coefficients for all samples. In summary, the dependence of the spectroscopic properties on the compositions of the samples revealed promising transparent glass and glass-ceramics for photonic applications.

Replica Symmetry Breaking in the Photonic Ferromagneticlike Spontaneous Mode-Locking Phase of a Multimode Nd:YAG Laser.

We demonstrate the replica symmetry breaking (RSB) phenomenon in the spontaneous mode-locking regime of a multimode Q-switched Nd:YAG laser. The underlying mechanism is quite distinct from that of the RSB recently observed in random lasers. Here, there is no random medium and the phase is not glassy with incoherently oscillating modes as in random lasers. Instead, in each pulse a specific subset of longitudinal modes are activated in a nondeterministic way, whose coherent oscillation dominates and frustrates the others. The emergence of RSB coincides with the onset of ultrashort pulse generation typical of the spontaneous mode-locking regime, both occurring at the laser threshold. On the other hand, when high losses are introduced, RSB is inhibited and only the amplified stimulated emission with replica symmetry is observed. Our results disclose the only theoretically predicted photonic phase with RSB that remained unobserved so far.

Nonlinear polarization instability in cubic-quintic plasmonic nanocomposites.

We report a study of the nonlinear birefringence induced in a metal-dielectric nanocomposite due to the contributions of third- and fifth-order optical nonlinearities. A theoretical model describing the evolution of the light polarization state of a confined laser beam propagating through the nonlinear medium is developed with basis on a pair of coupled dissipative cubic-quintic nonlinear differential equations related to the two orthogonal polarizations of the optical field. As a proof-of-principle experiment we demonstrate the control of the light beam polarization in a silver-nanocolloid by changing the silver nanoparticles volume fraction, f, and the light intensity. A large nonlinear phase-shift (~20π) was observed using a 9 cm long capillary filled with silver nanoparticles suspended in carbon disulfide. Experiments using colloids with 1.0×10-5≤f≤4.5×10-5 and maximum light intensities of tens of MW/cm2 are performed. In addition, we demonstrate that the modulation instability is highly sensitive to the quintic nonlinearity contribution performed showing good agreement with the experimental results.

Guiding and confinement of light induced by optical vortex solitons in a cubic-quintic medium.

The observation of confinement and guiding of light induced by an optical vortex soliton (OVS) in a cubic-quintic (defocusing-focusing) medium is reported. The waveguide effect induced by the bright region of the vortex beam, is mainly due to the defocusing nonlinearity that allows the guiding of a probe beam propagating inside of the OVS dark region. The experimental results are corroborated by numerical simulations based on the cubic-quintic nonlinear Schrödinger equation, showing excellent agreement.

Measurements of the nonlinear refractive index in scattering media using the Scattered Light Imaging Method--SLIM.

The Scattered Light Imaging Method (SLIM) was applied to measure the nonlinear refractive index of scattering media. The measurements are based on the analysis of the side-view images of the laser beam propagating inside highly scattering liquid suspensions. Proof-of-principle experiments were performed with colloids containing silica nanoparticles that behave as light scatterers. The technique allows measurements with lasers operating with arbitrary repetition rate as well as in the single-shot regime. The new method shows advantages and complementarity with respect to the Z-scan technique which is not appropriate to characterize scattering media.

An optimization procedure for the design of all-optical switches based on metal-dielectric nanocomposites.

We present a procedure to optimize the performance of all-optical switches based on metal-dielectric nanocomposites. The management of constructive and destructive interference between the third-, fifth- and seventh-order susceptibilities allowed characterization of optimal conditions for ultrafast switching with reduced losses. Proof-of-principle experiments with metal-colloids are reported to validate the method.

Spatial phase modulation due to quintic and septic nonlinearities in metal colloids.

We report spatial self- and cross-phase modulation effects in metal-dielectric nanocomposites (MDNCs) whose nonlinear (NL) response is dominated by the quintic or septic refractive nonlinearity. The MDNCs consist of silver nanoparticles (NPs) suspended either in acetone or carbon disulfide and their effective NL susceptibilities are controlled by adjusting the volume fraction occupied by the NPs and the incident laser intensity. A theoretical treatment based on the Maxwell-Garnett model was developed to include contributions up to the seventh-order susceptibility showing a very good agreement with the experimental results.