arXiv subjects

Abdoulaye Ndao

Publications and source records attributed to Abdoulaye Ndao.

27 records · Page 2Linked to original sources

arXiv2021

Electro-optically modulated polarization mode conversion in lithium niobate ridge waveguides

Lithium niobate on insulator (LNOI) waveguides, as an emerging technology, have proven to offer a promising platform for integrated optics, due to their strong optical confinement comparable to silicon on insulator (SOI) waveguides, while possessing the versatile properties of lithium niobate, such as high electro-optic coefficients. In this paper, we show that mode hybridization, a phenomenon widely found in vertically asymmetric waveguides, can be efficiently modulated in an LNOI ridge waveguide by electro-optic effect, leading to a polarization mode converter with 97% efficiency. Moreover, the proposed device does not require tapering or periodic poling, thereby greatly simplifying the fabrication process. It can also be actively switched by external fields. Such a platform facilitates technological progress of photonic circuits and sensors.

physics.optics↗

arXiv2021

Broadband optical power limiter metalens

In recent years, the need for high-power laser is of great interest for different applications ranging from direct-laser processing, light detection, medicine, and lighting. However, high-power lasers with high intensities give rise to fundamental problems for optical detectors and imaging systems with low threshold damage, which still need reliable solutions. Here, we report and numerically demonstrate a hybrid system that synergically combines a broadband optical power limiter with a transmittance difference between on-state (70^o C) and off-state (25^o C) about 62.5%, and a diffraction-limited broadband metalens from 1534 nm to 1664 nm. Such a metalens power limiter could be used in any system requiring an intermediate focal plane in the optical path to the detector from damage by exposure to high-intensity lasers.

physics.optics↗

arXiv2020

Efficient coupling between slanted bound states in the continuum cavity and plasmonic nanoantennas induced local field enhancement

Over the last few years, optical nanoantennas are continuously attracting interest owing to their ability to efficiently confine, localize resonance, and significantly enhanced electromagnetic fields at subwavelength scale. However, such strong confinement can be further enhanced by using an appropriate combination of optical nanoantenna and Slanted Bound states in the continuum cavities. Here, we propose to synergistically bridge the plasmonic nanoantenna and high optical quality-factor cavities to numerically demonstrate three orders of magnitude local intensity enhancement. The proposed hybrid system paves a new way for applications requiring highly confined fields such as optical sensing, optical trapping, nonlinear optics, quantum optics, etc.

physics.optics↗

arXiv2018

Suspended single mode microdisk lasers

Miniature semiconductor lasers have attracted a large amount of interest owing to their potential as highly integrated components in photonic circuits or in sensors. Particularly, microdisk lasers exploiting whispering gallery modes have been regarded as an important candidate because of their relatively small footprint and low threshold. However, it has been challenging for microdisk to operate under single mode operation and to lase in a preselected mode. We report subwavelength microdisk resonators suspended in air with connecting bridges and propose a simple method using the number and symmetry of bridges to enhance or reduce wave confinement in the whispering gallery cavity. Moreover, a suitable choice of bridges increases the quality factor of microdisks compared to microdisks resonator without bridges. Using this method, we demonstrate single mode lasing of preselected modes at telecommunication wavelength.

physics.app-ph↗

arXiv2018

All-dielectric metasurface cylindrical lens

Conventional optical components have been proposed to realize high-quality line focusing with uniform intensity distribution such as cylindrical lenses, segmented wedge-arrays, or a combination of prisms and spherical mirrors. However, numerous factors such as the manufacturing tolerances of conventional lenses or the need for precise alignment of the lenses cause wavefront aberrations that impact the performance of optical systems. These aforementioned limitations of conventional optical components affect the uniformity of the intensity distribution. Here, we numerically and experimentally demonstrate an integrable planar all-dielectric cylindrical lens for uniform line focusing. The lens has a NA of 0.247 and a measured uniformity of 0.92% at 800 nm.

physics.optics↗

arXiv2017

Plasmonless polarization-selective metallic Semi-coaxial Aperture Arrays in the visible range

In this letter, we perform numerical and experimental studies of the optical response of an original configuration based on enhanced transmission through guided mode based metamaterials. The proposed structure is inspired by annular aperture array (AAA) where the cylindrical symmetry is broken in order to acquire polarization-sensitive metasurfaces. The experimental results, which are in good agreement with numerical simulations, demonstrate that the structure acts as a polarizer exhibiting an extinction ratio of (15:1) with a maximum transmission coefficient up to 85% which is more efficient than what it is expected with a typical plasmonic resonance

physics.optics↗

arXiv2017

Quantification of the transmission properties of metasurfaces illuminated by finite-size beams

The aim of this letter is to {present analytical method to} quantitatively address the influence of a focusing illumination on the transmission /reflection properties of a {metasurface illuminated by a finite-size beam}. In fact, most theoretical and numerical studies are performed by considering an infinite periodic structure illuminated by a plane wave. In practice, one deals with a finite-size illumination and structure. Combination of the angular spectrum expansion with a monomodal modal method is performed to determine the beam size needed to acquire efficient properties of a Metasurface that behaves as Anisotropic Plate MAP Interesting results show that the beam-size can be as small as 5X5 periods to recover the results of a plane wave. Other results also show that the beam-size could be used as an extrinsic parameter to enhance the MAP performance and to finely adjust its expected properties (birefringence and/or transmission coefficient).

physics.optics↗

arXiv2017

Brillouin spectroscopy of optical microwires

We describe an easy-to-implement technique that allows for a complete experimental characterization of sub-wavelength diameter tapered optical fibers. Our method is based on a direct and fast numerical analysis of the backward Brillouin spectrum measured using a highly sensitive single-ended heterodyne coherent detection. It can be performed in situ without any manipulation nor optical alignment of optical microfibers. Sensitivity as high as a few nanometer for fiber diameter ranging from 500 nm to 1.2 micron is reported. This new method may help for the design and characterization of optical fiber tapers widely used in many applications such as optical sensing, atom trapping, quantum optics, and plasmonics.

physics.optics↗

arXiv2017

From parabolic-trough to metasurface-concentrator

Metasurfaces are promising tools towards novel designs for flat optics applications. As such their quality and tolerance to fabrication imperfections need to be evaluated with specific tools. However, most such tools rely on the geometrical optics approximation and are not straightforwardly applicable to metasurfaces. In this Letter, we introduce and evaluate, for metasurfaces, parameters such as the intercept factor and the slope error usually defined for solar concentrators in the realm of ray-optics. After proposing definitions valid in physical optics, we put forward an approach to calculate them. As examples, we design three different concentrators based on three specific unit cells and assess them numerically. The concept allows for the comparison of the efficiency of the metasurfaces, their sensitivities to fabrication imperfections and will be critical for practical systems.

physics.optics↗