Charles Ciret

Measuring the nonlinear refractive index of graphene using the optical Kerr effect method

By means of the ultrafast optical Kerr effect method coupled to optical heterodyne detection (OHD-OKE), we characterize the third-order nonlinear response of graphene and compare it to experimental values obtained by the Z-scan method on the same samples. From these measurements, we estimate a negative nonlinear refractive index for monolayer graphene, n2=-1.1×10-13  m2/W. This is in contradiction to previously reported values, which leads us to compare our experimental measurements obtained by the OHD-OKE and the Z-scan method with theoretical and experimental values found in the literature and to discuss the discrepancies, taking into account parameters such as doping.

Planar achromatic multiple beam splitter by adiabatic light transfer

We introduce a novel achromatic and robust scheme for n-fold multiple beam splitting based on adiabatic light transfer in a planar geometry of coupled waveguides (WGs). The concept is experimentally verified for a one-to-three beam splitter by using a reconfigurable light-induced WG structure at two operating wavelengths. The demonstrated planar-type achromatic beam splitter opens new opportunities for the realization of ultra-high bandwidth on-chip photonic devices.

Dispersive-wave-based octave-spanning supercontinuum generation in InGaP membrane waveguides on a silicon substrate.

We demonstrate the generation of an octave-spanning supercontinuum in InGaP membrane waveguides on a silicon substrate pumped by a 1550-nm femtosecond source. The broadband nature of the supercontinuum in these dispersion-engineered high-index-contrast waveguides is enabled by dispersive wave generation on both sides of the pump as well as by the low nonlinear losses inherent to the material. We also measure the coherence properties of the output spectra close to the pump wavelength and find that the supercontinuum is highly coherent at least in this wavelength range.

Observation of an optical event horizon in a silicon-on-insulator photonic wire waveguide.

We report on the first experimental observation of an optical analogue of an event horizon in integrated nanophotonic waveguides, through the reflection of a continuous wave on an intense pulse. The experiment is performed in a dispersion-engineered silicon-on-insulator waveguide. In this medium, solitons do not suffer from Raman induced self-frequency shift as in silica fibers, a feature that is interesting for potential applications of optical event horizons. As shown by simulations, this also allows the observation of multiple reflections at the same time on fundamental solitons ejected by soliton fission.

Generation of ultra-broadband coherent supercontinua in tapered and dispersion-managed silicon nanophotonic waveguides

The generation of an optimal ultra-broadband supercontinuum (SC) is numerically investigated in tapered and dispersion-managed (DM) silicon nanophotonic waveguides. DM waveguides are structures showing a longitudinally dependent group velocity dispersion that results from the variation of the waveguide width with the propagation distance. A genetic algorithm is used to find the best dispersion map. This allows for the generation of highly coherent supercontinuums that span over 1.14 octaves from 1300 to 2860 nm and 1.25 octaves from 1200 to 2870 nm at the −20  dB level for the tapered and DM waveguides for a 2 μm, 200 fs and 6.4 pJ input pulse. The comparison of these two structures with the usually considered optimal fixed-width waveguide shows that the SC is broader and flatter in the more elaborated DM waveguide, while the high coherence is ensured by the varying dispersion.

Scattering of a cross-polarized linear wave by a soliton at an optical event horizon in a birefringent nanophotonic waveguide.

The scattering of a linear wave on an optical event horizon, induced by a cross-polarized soliton, is experimentally and numerically investigated in integrated structures. The experiments are performed in a dispersion-engineered birefringent silicon nanophotonic waveguide. In stark contrast with copolarized waves, the large difference between the group velocity of the two cross-polarized waves enables a frequency conversion almost independent of the soliton wavelength. It is shown that the generated idler is only shifted by 10 nm around 1550 nm over a pump tuning range of 350 nm. Simulations using two coupled full vectorial nonlinear Schrödinger equations fully support the experimental results.

Observation of two-photon absorption induced soliton fission

Solitons are well known solutions of the nonlinear Schrödinger equation (NLSE). In optics, temporal solitons are localized wavepackets that travel unperturbed in dispersive Kerr media. They can be of different order, with only first order solitons strictly maintain their shape as they propagate, while higher order solitons periodically oscillate along the waveguide. If the propagation deviates from purely Kerr and quadratically dispersive, higher order solitons tend to split into several lower order solitons. This process, often called soliton fission, is the main mechanism underlying supercontinuum generation [1]. In silica fibers, the fission is most often induced by the Raman effect and third-order dispersion. The recent push for optical integration has led to a number of demonstrations of supercontinuum generation in semiconductor nanowaveguides [2]. But with the focus on coherent spectral broadening for frequency comb applications, few studies discuss the reason for the decay of the input soliton. It was shown last year that free carriers can, by blue-shifting the soliton, induce fission in Indium Gallium Phosphide (InGaP) nanowaveguides [3]. Moreover it was suggested that free carrier dispersion drives soliton fission in our recent experiments of supercontinuum generation in silicon wire waveguides [3, 4].

Two-pulse interaction at an optical event horizon in silicon-on-insulator nanophotonic waveguide

Nonlinear interactions occurring in the framework of so-called optical event horizons attract attention from the scientific community as the propagation of waves in nonlinear dispersive media has been proven to be a useful testbed for the study of artificial event horizons [1, 2]. Analogue of optical event horizons occurs when an intense soliton pulse prevents a weak probe wave propagating at a different velocity to penetrate through it. The energy transfer inherent to this nonlinear interaction has been pointed as potentially useful for applications such as efficient frequency converters [3] or all-optical transistors [4]. Though numerous studies have been conducted in photonic crystal fibers, the first demonstrations in integrated structures of optical effects pertaining to event horizons have only been recently reported [5] thanks to the use of high quality dispersion engineered silicon wire waveguides. In this latter work, the soliton pump interacted with a weak CW background, resulting in a poor overall frequency conversion efficiency.

Quantum-like adiabatic light transfer in photo-induced waveguides with longitudinally varying detuning

Besides longitudinally varying coupling constants, the longitudinal variation of the propagation constants leads to an additional parameter for the control of adiabatic light transfer in coupled waveguide systems. Examples are given using waveguides structures recorded with the help of the photorefractive e↵ect and mimicking the quantum processes of Rapid Adiabatic Passage (RAP) and two-state STImulated Raman Adiabatic Passage (two-state STIRAP).

Nonlinear Interactions at an Optical Event Horizon in Nanophotonic Birefringent Waveguides

We study the reflection of a probe wave on an intense pump for both co- and cross-polarized waves. The difference between the polarization dispersion profiles strongly influences the properties of the associated frequency conversion.