Nicolas Godbout

Z-scan measurement of the nonlinear refractive index of graphene

Han Zhang, Stéphane Virally, Qiaoliang Bao, Kian Ping Loh, Serge Massar, Nicolas Godbout, and Pascal Kockaert OPERA-photonics, Université libre de Bruxelles, 50 Av. F. D. Roosevelt, CP 194/5, B-1050 Bruxelles, Belgium Engineering Physics Department, École polytechnique de Montréal, P.O. Box 6079, Station Centre-ville, Montréal (Québec), H3C 3A7 Canada Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Laboratoire d’information quantique, CP 225, Université libre de Bruxelles, 50 Av. F. D. Roosevelt, B-1050 Bruxelles, Belgium ∗Corresponding author: hzhang@ulb.ac.be, smassar@ulb.ac.beUnder strong laser illumination, few-layer graphene exhibits both a transmittance increase due to saturable absorption and a nonlinear phase shift. Here, we unambiguously distinguish these two nonlinear optical effects and identify both real and imaginary parts of the complex nonlinear refractive index of graphene. We show that graphene possesses a giant nonlinear refractive index n(2)≃10(-7) cm(2) W(-1), almost 9 orders of magnitude larger than bulk dielectrics. We find that the nonlinear refractive index decreases with increasing excitation flux but slower than the absorption. This suggests that graphene may be a very promising nonlinear medium, paving the way for graphene-based nonlinear photonics.

Dynamics of parabolic pulses in an ultrafast fiber laser

We report experimental and numerical results on the dynamics and propagation of parabolic pulses in a passively mode-locked ytterbium-doped fiber laser. Experimental data and numerical simulations are shown to match. Particular importance is attached to the pulse-shaping process in the different sections of the resonator. The paramount role of the gain fiber and saturable absorber in the laser is explicated.

Prospective for biodegradable microstructured optical fibers.

We report fabrication of a novel microstructured optical fiber made of biodegradable and water soluble materials that features approximately 1 dB/cm transmission loss. Two cellulose butyrate tubes separated with hydroxypropyl cellulose powder were codrawn into a porous double-core fiber offering integration of optical, microfluidic, and potentially drug release functionalities.

Ultranarrow Faraday rotation filter at the Rb D1 line.

We present a theoretical and experimental study of the ultranarrow bandwidth Faraday anomalous dispersion optical filter operating at the rubidium D1 line (795 nm). This atomic line gives better performance than other lines for key figures of merit, e.g., simultaneously 71% transmission, 445 MHz bandwidth, and 1.2 GHz equivalent-noise bandwidth.

Dual-modality needle probe for combined fluorescence imaging and three-dimensional optical coherence tomography

To the best of our knowledge, we present the first needle probe for combined optical coherence tomography (OCT), and fluorescence imaging. The probe uses double-clad fiber (DCF) that guides the OCT signal and fluorescence excitation light in the core and collects and guides the returning fluorescence in the large-diameter multimode inner cladding. It is interfaced to a 1310 nm swept-source OCT system that has been modified to enable simultaneous 488 nm fluorescence excitation and >500 nm emission detection by using a DCF coupler to extract the returning fluorescence signal in the inner cladding with high efficiency. We present imaging results from an excised sheep lung with fluorescein solution infused through the vasculature. We were able to identify alveoli, bronchioles, and blood vessels. The results demonstrate that the combined OCT plus fluorescence needle images provide improved tissue differentiation over OCT alone.

Double-clad fiber coupler for endoscopy.

We present a double-clad fiber coupler (DCFC) for use in endoscopy to reduce speckle contrast, increase signal collection and depth of field. The DCFC is made by fusing and tapering two all silica double-clad fiber (DCF) and allows achromatic transmission of >95% of core illumination (1265nm - 1325nm) as well as collection of >42% of inner cladding diffuse light. Its potential for endoscopy is demonstrated in a spectrally encoded imaging setup which shows speckle reduction by a factor 5, increased signal collection by a factor 9 and enhanced depth of field by 1.8 times. Separation by the DCFC of single- and multi-mode signals allows combining low-speckle reflectance images (25.5 fps) with interferometrically measured depth profiles (post-processed) for of small three-dimensional (3D) features through an all-fiber low loss instrument.

Multiuser quantum key distribution using wavelength division multiplexing

Quantum cryptography, or more specifically, quantum key distribution (QKD), has attracted a lot of attention in the recent years with the discovery that it can provide absolute secrecy for communications. We propose a new architecture for implementing a fiber-based network of quantum key distribution using optical wavelength division multiplexing in the fiber. We discuss the advantages over previous proposals and we report experimental work demonstrating the feasibility of the proposed architecture.

Entanglement-enhanced probing of a delicate material system

Probing of delicate systems demands high sensitivity from limited probe energy. Quantum optical techniques to beat the standard quantum limit (SQL) offer a practical advantage for these measurements. Here we report the first entanglement-enhanced measurement of a delicate material system. We non-destructively probe an atomic spin ensemble by near-resonant Faraday rotation, a measurement that is limited by probe-induced scattering in quantum memory and spin-squeezing applications. We use narrowband, atom-resonant NooN states to beat the standard quantum limit of sensitivity, both on a per-photon and a per-damage basis.

Characterization of thermal poling in silica glasses by current measurements

Abstract We present herein a charge migration model that predicts the electric field distribution within a Suprasil™ silica planar sample after poling. The predicted distribution is in agreement with the currents measured at the electrodes during the poling and depoling processes. This electric field is responsible for an apparent second-order non-linear coefficient originating from the third-order one. Numerical values obtained from fits of the experimental curves with the model equations are compatible with the values obtained by optical characterization reported in the literature.

Asymmetric double-clad fiber couplers for endoscopy.

We present an asymmetric double-clad fiber coupler (A-DCFC) exploiting a disparity in fiber etendues to exceed the equipartition limit (≤50% extraction of inner cladding multi-mode light). The A-DCFC is fabricated using two commercially available fibers and a custom fusion-tapering setup to achieve >70% extraction of multi-mode inner cladding light without affecting (>95% transmission) single-mode light propagation in the core. Imaging with the A-DCFC is demonstrated in a spectrally encoded imaging setup using a weakly backscattering biological sample. Other applications include the combination of optical coherence tomography with weak fluorescent or Raman scattering signals.