Mathieu Chauvet

Temporal analysis of open-circuit dark photovoltaic spatial solitons

A theoretical model and experimental results to characterize the time-dependent formation of one-dimensional dark photovoltaic solitons under open-circuit conditions are presented. According to this theory, quasi-steady-state and steady-state solitons can both be obtained. In the quasi-steady-state regime solitons have intensity-independent widths, whereas their formation time is inversely proportional to the intensity, as confirmed by experimental results obtained with LiNbO3 samples. Theory predicts that the response times of steady-state solitons will be given by the dielectric response in the absence of an illuminating field, Td. In the samples used in this research, only a trend toward a steady-state regime was observed, because of the prohibitively high value of Td.

Integrated optofluidic index sensor based on self-trapped beams in LiNbO3

We show that self-trapped beams can form in structured monolithic lithium niobate chips. In particular, they are observed to be unaffected when crossing few hundred microns wide gaps. The technique is employed to fabricate an index sensor constituted of a buried circular optical waveguide crossing a fluidic channel in a lithium niobate substrate. Fluidic channels are realized by precision dicing while the optical waveguides are induced by photorefractive beam self-trapping controlled by the pyroelectric effect. The self-aligning property of this latter waveguides provides a simple fabrication technique of an integrated sensor that accurately measures the refractive index of transparent liquids.

Self-trapped beams for fabrication of optofluidic chips

Design, fabrication and test of an optofluidic device are presented. It is constituted of a circular waveguide crossing a fluidic channel integrated in a monolithic LiNbO3 wafer. The fluidic channel is realized by precision sawing and the optical waveguide is induced by photorefractive beam self-trapping controlled by the pyroelectric effect. The self-aligning property of this latter writing technique allows both, efficient light coupling inside the channel and light collection after channel crossing. It is shown that the refractive index of a liquid present in the fluidic channel can be accurately evaluated by simple monitoring of the light transmitted through the waveguide.

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.

Periodically poled LiNbO3 ridge waveguides on silicon for second-harmonic generation

Nonlinear periodically poled ridge LiNbO3 waveguides have been fabricated on silicon substrates. Components are micromachined with a precision dicing machine and/or by grinding or polishing steps. They show efficient second harmonic generation at telecommunication wavelengths with normalized conversion reaching 600%/W in a 20mm long device. Influence of geometrical non uniformities of waveguides due to fabrication process is asserted. Components characteristics are studied notably their robustness and tunability versus temperature.

3D pseudospectral time domain for modeling second-harmonic generation in periodically poled lithium niobate ridge-type waveguides

We report on an application of a three-dimensional pseudospectral time-domain algorithm that solves with accuracy the nonlinear Maxwell’s equations so as to predict second-harmonic generation in lithium niobate ridge-type waveguides with high index contrast. Characteristics of the nonlinear process such as conversion efficiency and impact of the multimode character of the waveguide are investigated as a function of the waveguide geometry in uniformly and periodically poled media.

SELF-INDUCED WAVEGUIDES CREATED BY SECOND HARMONIC GENERATION IN LITHIUM NIOBATE

In this paper we study second harmonic generation in Lithium Niobate in presence of photorefractive and photovoltaic effect. Our investigation reveals that the application of a strong external bias causes a self focusing effect that efficiently traps both the generated second harmonic and the fundamental beam, resulting in an improved conversion efficiency. The variation of the second harmonic output power during the build up of the space charge field also testifies that the phase relation between the interacting waves is affected by the photorefractive effect, and that the phase matching condition is modified in the part of the crystal shined by the light. Finally, we introduce a numerical model in order to explain the complex dynamic between the photorefractive effect and parametric process. The developed theory is in qualitative agreement with the performed experiments.

Experimental demonstration of soliton-plasmon coupling in planar waveguides (Conference Presentation)

Merging the fields of plasmonics and nonlinear optics authorizes a variety of fascinating and original physical phenomena. In this study, we specifically study the combination of the strong light confinement ability of surface plasmon polaritons (SPP) with the beam self-trapping effect that occur in nonlinear optical Kerr medium. Although this idea of plasmon-soliton has been the subject of several theoretical or numerical articles, no experimental evidence has been revealed yet. One reason is that in the proposed configurations the requested nonlinear refractive index change amplitude to generate a plasmon-soliton is too high to be reached in available material. Another limitation is due to the large propagation losses associated with plasmons. In the present study, a proper architecture has been designed and then fabricated allowing the first experimental observation of hybrid coupling between a spatial optical soliton and a SPP in a metal-Kerr dielectric structure. nTo be able to trigger the nonlinearity at moderate light power and simultaneously to allow propagation over several millimetres distance, a metal-dielectric structure was designed. It consists of a four-layer planar geometry made of a transparent Kerr dielectric layer placed on a lower refractive index medium, with on its top surface a thin dielectric layer covered by a metallic film deposited on top. The Kerr medium is a 3µm thick chalcogenide film (Ge28.1Sb6.3Se65.6) with a high refractive index deposited by RF magnetron sputtering on an oxidized silicon substrate. The thickness of the thin SiO2 layer is 10 nm while the top gold layer is 30 nm. Samples are about 5-6 mm along propagation direction (z-axis). nAs shown by numerical simulations, the designed planar nonlinear waveguide with its top silica and gold layer supports a fundamental TE mode profile at NIR wavelengths whose transverse profile along y (perpendicular to the layers) is not affected by the metal layer while the TM mode is clearly localized near the SiO2-metal-chalcogenide interfaces due to its plasmonic part. The estimated nonlinear parameter γ of the TM mode is nearly three times larger than the TE one. Consequently, in nonlinear regime an enhanced self-focusing effect is expected for this TM wave. Experiments are performed with a tunable optical parametric oscillator emitting 200 fs pulses at 1.55 µm with a repetition rate of 80 MHz. The experimental analysis consists in injecting a typical 4 × 30 μm2 (FWHM in x-y cross section) elliptical laser beam into the waveguide and monitoring the output beam spatial profile evolution versus light power. Different arrangements are tested that unambiguously reveal the plasmon-soliton coupling. For instance, experiments are conducted with and without the metallic layer and for both TE and TM polarizations. In addition, different positions on the sample of the metal part with several lengths chosen between 0.1 to 2mm are tested. Additional experiments are in progress to analyze the beam evolution with near-field scanning microscopy and simulations of the beam propagation in the full structure are developed to reach a better and fully quantitative description of the observed phenomena.

Beam self-action in planar chalcogenide waveguides

We present a new experimental technique based on the analysis of beam self-action to measure optical nonlinearity in planar waveguides. This technique is applied to analyze the nonlinear properties of slab chalcogenide waveguides that can develop Kerr induced self-focusing or self-defocusing, depending upon the waveguide structure and composition. Optical nonlinearity in chalcogenide waveguide is studied in the 1200 nm to 1550 nm wavelength range in femtosecond regime. Results of the proposed technique compare favorably with n2 values obtained with the Z-scan technique. In addition, beam self-trapping in the chalcogenide waveguides due to material photosensitivity is also observed.

Resonance measurements techniques of optical whispering gallery mode mini-disc resonators for microwave photonics applications

The aim of this work is to compare advantages and disadvantages of different techniques for coupling a mini-discoptical- resonator to determine quality factor of its resonance. Optical fiber coupled to a resonator consists in a mini disc with whispering gallery modes at its circumference. We choose to work with three materials and design compact miniresonators. Fused silica is found to be suitable for these applications thanks to its hardness in the range 6-7 and the behavior to mechanical shocks, despite its sensitivity to water pollution. With its tetragonal crystal and a good behavior with risk of water pollution, Calcium fluoride is a good candidate despite sensitivity to mechanical shocks. Magnesium fluoride is the third material used. As a critical step, taper coupling is set with a 20nm resolution positioning system. Miniresonator is excited from a system equipped with a tunable laser diode with a tunability from 1490 to 1640 nm and a linewidth narrower than 300kHz. Light is coupled into the microsphere either from glass or fiber prism or with fiber taper via evanescent field. We have also used a single frequency 660nm laser diode with a linewidth narrower than 100kHz which can be tuned about 10pm to test a single resonant peak. Both sources are used with either a tapered fiber or a filed fiber. Resonance is observed and quality factor of the resonators is found to be in the range of 108.