Efficient photorefractive effect triggered by pyroelectricity in magnesium doped LiNbO3 films.

An efficient photorefractive effect triggered by pyroelectricity is demonstrated in slab waveguides constituted of magnesium oxide (MgO)-doped LiNbO3 film on insulator. A microwatt-level continuous wave guided at 532 nm is self-trapped to form a 10 µm FWHM beam triggered by only a few degrees of temperature increase of the sample. A fast self-focusing response time on the order of milliseconds is measured for milliwatts of injected beam, more than two orders of magnitude faster than in the undoped LiNbO3 film. Long lived 2-D induced waveguides are found to be written in the films.

Opto-microfluidic coupling between optical waveguides and tilted microchannels in lithium niobate.

This work presents a reconfigurable opto-microfluidic coupling between optical waveguides and tilted microfluidic channels in monolithic lithium niobate crystal. The light path connecting two waveguide arrays located on opposite sides of a microfluidic channel depends on the refractive index between the liquid phase and the hosting crystal. As a result, the optical properties of the flowing fluid, which is pumped into the microfluidic channel on demand, can be exploited to control the light pathways inside the optofluidic device. Proof-of-concept applications are herein presented, including microfluidic optical waveguide switching, optical refractive index sensing, and wavelength demultiplexing.

Supervised learning of soliton X-junctions in lithium niobate films on insulator.

In this Letter, the first implementation, to our knowledge, of X-junctions between photorefractive soliton waveguides in lithium niobate-on-insulator (LNOI) films is reported. The experiments were performed on 8 µm thick films of congruent undoped LiNbO3. Compared with bulk crystals, the use of films reduces the soliton formation time, allows more control over the interaction between the injected soliton beams, and opens a route to integration with silicon optoelectronics functions. The created X-junction structures show effective supervised learning, directing the signals propagated inside the soliton waveguides into the output channels highlighted by the control assigned by the external supervisor. Thus, the obtained X-junctions have behaviors analogous to biological neurons.

Birefringence phase-matched direct third-harmonic generation in a ridge optical waveguide based on a KTiOPO4 single crystal.

Birefringence phase-matched third-harmonic generation at 1594 nm is performed for the first time in a KTiOPO4 single crystal micrometric ridge waveguide. The energy conversion efficiency reaches 3.4% for a pump energy as low as 2 µJ over a pulse duration of 15 ps at a repetition rate of 10 Hz. Strong agreements between theory and experiments for both phase-matching and conversion efficiency is obtained, which let us envision future triple photon generation quantum experiments.

Linear and nonlinear optical properties of co-sputtered Ge-Sb-Se amorphous thin films.

Amorphous Ge-Sb-Se thin films were co-sputtered from ${{\rm GeSe}_4}$GeSe4 and ${{\rm Sb}_2}{{\rm Se}_3}$Sb2Se3 targets. Depending on the film composition, linear optical properties were studied by ellipsometry. The Kerr coefficient and two-photon absorption coefficient were estimated using Sheik-Bahae's formalism for co-sputtered films of ${{\rm GeSe}_4} {\text -} {\rm Sb}_2{{\rm Se}_3}$GeSe4-Sb2Se3 compared to ${{\rm GeSe}_2}{\text -}{\rm Sb}_2{{\rm Se}_3}$GeSe2-Sb2Se3 pseudo-binary system and ${{\rm As}_2}{{\rm Se}_3}$As2Se3 as reference. The Kerr coefficient was found within the range of $4.9 {\unicode {x2013}}- 21 \times {10^{ - 18}}$4.9--21×10-18. Quantitatively by means of a figure of merit at 1.55 µm, thin films with compositions of ${{\rm Ge}_7}{\rm Sb}_{25}{\rm Se}_{68}$Ge7Sb25Se68 and ${{\rm Ge}_9}{\rm Sb}_{20}{\rm Se}_{71}$Ge9Sb20Se71 having an estimated Kerr coefficient of about ${10.1} \times {10^{ - 18}}\;{{\rm m}^2}{{\rm W}^{ - 1}}$10.1×10-18m2W-1 and ${13.4} \times {10^{ - 18}}\;{{\rm m}^2}{{\rm W}^{ - 1}}$13.4×10-18m2W-1 should be considered for the future nonlinear optical integrated platforms. Such compositions being close to ${({{\rm GeSe}_4})_{50}}{({{\rm Sb}_2}{{\rm Se}_3})_{50}}$(GeSe4)50(Sb2Se3)50 pseudo-binary (i.e., ${\rm Ge}_{7.5}{\rm Sb}_{25.0}{\rm Se}_{67.5}$Ge7.5Sb25.0Se67.5) provides just the trade-off between a high Kerr coefficient and low optical losses related to two-photon absorption.

Single photon MIR upconversion detector at room temperature with a PPLN ridge waveguide.

In this paper we describe an upconversion detector in the mid infrared (around 3.5 µm). We take advantage of the PPLN ridge waveguide technology to achieve single photon detection at room temperature on a single spatial mode. With a pump power of 192 mW we obtain a detection efficiency of 0.4% for 22k dark count per second, which corresponds to a noise equivalent power of 3.0 fW · Hz-1/2 and an internal conversion efficiency of 85 %/W of pump.

Phase-matched second-harmonic generation in a flux grown KTP crystal ridge optical waveguide.

Type II second-harmonic generation was performed in a 15.8-mm-long KTiOPO4 (KTP) micrometric ridge waveguide with an average transversal section of 38 µm2. Theoretical predictions are compared with experiments. Strong agreements are obtained for both phase-matching wavelengths and second-harmonic intensity. This work opens wide perspectives for integrated parametric optics.

Light coupling with a nonlinear prism.

We propose the use of a prism with nonlinear optical properties to improve the prism-coupling method. The principle is based on the inscription of an adapted waveguide inside this prism by beam self-trapping to enhance the coupling efficiency and stability. The experimental demonstration is realized with a prism diced from a LiNbO3 wafer to couple light into a resonator.

Fast-beam self-trapping in LiNbO(3) films by pyroelectric effect.

We report light-beam self-trapping triggered by the pyroelectric effect in an isolated ferroelectric thin film. Experiments are performed in an 8-µm-thick congruent undoped LiNbO(3) film bonded onto a silicon wafer. Response time two orders of magnitude faster than in bulk LiNbO(3) is reported. The original underlying physics specific of this arrangement is discussed.

Use of quasi-local photorefractive response to generated superficial self-written waveguides in lithium niobate.

We report the formation of surface self-written waveguides by means of surface pyrolitons in lithium niobate. By a specific orientation of the crystal axis the quasi-local slow photorefractive response of lithium niobate was used to induce a self-confined beam exactly at the crystal-air interface. The mode profile of the photo-induced waveguide is strongly asymmetric due to the interface presence.

Low-power plasmon-soliton in realistic nonlinear planar structures.

We study the propagation of nonlinear waves in layered nonlinear dielectric/linear dielectric/metal planar structures. We develop vector models that describe the light propagation in such configurations and allow us to obtain both one- and two-dimensional solutions. We compute the nonlinear dispersion relation and the field profiles, and estimate losses. We use our models to design realistic structures, in terms of linear and nonlinear properties, which support soliton waves with a plasmon tail at low peak power around or below 1 GW/cm(2). These results open the way for potential observation of such states in chalcogenide waveguides associated with silica and metal films. In the proposed structures, the nonlinearity confines the field in both transverse directions. A recordable plasmonic part of the field extends in air.

Writing and probing light-induced waveguides thanks to an endlessly single-mode photonic crystal fiber.

We demonstrate writing and probing of light-induced waveguides in photorefractive bulk LiNbO3 crystal using an endlessly single-mode photonic crystal fiber. The optical waveguides are written at visible wavelengths by slightly raising the ferroelectric crystal temperature to benefit from the pyroelectric-driven photorefractive effect and the guiding properties are investigated at telecom wavelengths using the same photonic crystal fiber. End butt coupling with this photonic crystal fiber enables writing and probing of optical waveguides due to the self-alignment properties of spatial solitons.

Adiabatic self-focusing in media with spatially variable nonlinearity.

An optical medium whose nonlinearity can be spatially adjusted is considered to study beam reshaping. The concept is applied to perform adiabatic self-focusing of broad beams. Experimental results are obtained in a photorefractive lithium niobate crystal where the self-focusing nonlinearity is controlled over propagation by a temperature gradient. As a demonstration, gradual self-focusing is shown to transform an incoming beam into an output circular spot 10 times smaller over a 2 cm long crystal submitted to a 30 °C temperature gradient. Once formed, the adiabatic self-focused beam has inscribed a funnel waveguide in the crystal.

Observation of photorefractive simultons in lithium niobate.

Spatial and temporal locking of fundamental and second harmonic pulses was realized by means of photorefractive nonlinearity and highly mismatched harmonic generation. Due to the presence of both phase-locked and unlocked second harmonic pulses, a twin simultonic state was observed. Simultonic filamentation occurring at high pumping rates allowed us to determine a relation between the simulton's waist and its intensity.

Surface-wave pyroelectric photorefractive solitons.

Surface-wave solitons, induced by the pyroelectric effect, are formed at the interface between a photorefractive ferroelectric medium and a linear medium. These optical solitons are trapped in both transverse dimensions and are efficiently attracted to the interface. The asymmetric shape of the nonlinear index change formed under the charge saturation regime is responsible for the surface-wave solitons' formation. Experimental demonstrations are performed in a lithium niobate sample with moderate temperature change. The phenomenon is successfully explained through numerical simulations.

Pyroliton: pyroelectric spatial soliton.

The concept of optical beam self-trapping in pyroelectric photorefractive medium is presented. We show that the temperature controlled spontaneous polarisation of ferroelectric crystals produces an optical nonlinearity that can lead to formation of 2-D spatial soliton named pyroliton. Experimental demonstrations performed in lithium niobate crystals illustrate that efficient self-trapping occurs either for ordinary or extraordinary polarisation under moderate temperature increase. For instance, a 15 microm diameter pyroliton can be formed with a 10 degree temperature raise.

Kerr spatial solitons in chalcogenide waveguides.

Kerr spatial solitons are observed in slab chalcogenide waveguides at near-IR wavelengths. Waveguides are realized either by electron-beam evaporation or rf sputtering of a Ge-Sb-S compound deposited on oxidized silicon wafer. The Kerr coefficient of the thin film is evaluated to be 5 x 10(-18) m(2)/W from the experimentally required soliton power at 1.5 mum. Limitations due to material photosensitivity are revealed.

Complete spatial and temporal locking in phase-mismatched second-harmonic generation.

We experimentally demonstrate simultaneous phase and group velocity locking of fundamental and generated second harmonic pulses in Lithium Niobate, under conditions of material phase mismatch. In phase-mismatched, pulsed second harmonic generation in addition to a reflected signal two forward-propagating pulses are also generated at the interface between a linear and a second order nonlinear material: the first pulse results from the solution of the homogeneous wave equation, and propagates at the group velocity expected from material dispersion; the second pulse is the solution of the inhomogeneous wave equation, is phase-locked and trapped by the pump pulse, and follows the pump trajectory. At normal incidence, the normal and phase locked pulses simply trail each other. At oblique incidence, the consequences can be quite dramatic. The homogeneous pulse refracts as predicted by material dispersion and Snell's law, yielding at least two spatially separate second harmonic spots at the medium's exit. We thus report the first experimental results showing that, at oblique incidence, fundamental and phase-locked second harmonic pulses travel with the same group velocity and follow the same trajectory. This is direct evidence that, at least up to first order, the effective dispersion of the phase-locked pulse is similar to the dispersion of the pump pulse.

Impact of tensorial nature of the electro-optic effect on vortex beam propagation in photorefractive media.

Influence of the anisotropic tensorial electro-optic effect of LiNbO(3):Fe photorefractive defocusing medium on propagation of a vortex beam is numerically and experimentally investigated. Characteristic behaviors are depicted by varying light polarization, sign of vortex angular momentum and propagation directions.

Assessment method for photo-induced waveguides.

A method to probe the guiding characteristics of waveguides formed in real-time is proposed and evaluated. It is based on the analysis of the time dependent light distribution observed at the exit face of the waveguide while progressively altering its index profile and probed by a large diameter optical beam. A beam propagation method is used to model the observed dynamics. The technique is applied to retrieve the properties of soliton-induced waveguides.