Salim Boutami

Broadband and compact 2-D photonic crystal reflectors with controllable polarization dependence

Two-dimensional (2-D) compact photonic crystal reflectors on suspended InP membranes were studied under normal incidence. We report the first experimental demonstration of 2-D broadband reflectors (experimental stopband superior to 200 nm, theoretical stopband of 350 nm). They are based on the coupling of free space waves with two slow Bloch modes of the crystal. Moreover, they present a very strong sensitivity of the polarization dependence, when modifying their geometry. A compact (50/spl times/50 /spl mu/m/sup 2/) demonstrator was realized and characterized, behaving either as a broadband reflector or as a broadband transmitter, depending on the polarization of the incident wave. Experimental results are in good agreement with numerical simulations.

Highly selective and compact tunable MOEMS photonic crystal Fabry-Perot filter

The authors report a compact and highly selective tunable filter using a Fabry-Perot resonator combining a bottom micromachined 3-pair-InP/air-gap Bragg reflector with a top photonic crystal slab mirror. It is based on the coupling between radiated vertical cavity modes and waveguided modes of the photonic crystal. The full-width at half maximum (FWHM) of the resonance, as measured by microreflectivity experiments, is close to 1.5nm (around 1.55 microm). The presence of the photonic crystal slab mirror results in a very compact resonator, with a limited number of layers. The demonstrator was tuned over a 20nm range for a 4V tuning voltage, the FWHM being kept below 2.5nm. Bending of membranes is a critical issue, and better results (FWHM=0.5nm) should be obtained on the same structure if this technological point is fixed.

Photonic Crystal-Based MOEMS Devices

In this paper, a new class of microoptoelectromechanical system (MOEMS) devices combining photonic crystals (PCs) formed in in-plane waveguiding membranes (in-plane 1-D or 2-D large contrast modulation of the optical index) and a multilayer stack (1-D vertical modulation of the optical index) according to the so-called 2.5-D micronanophotonics approach is reported. The operation of the devices is based on the resonant coupling between radiated optical modes and slow Bloch modes waveguided in the particular membranes of the stack, which are laterally patterned to form a PC. Use of high-index contrast PC gratings result in enhanced lateral compactness of the devices. The MOEMS functionality is achieved via micromechanical subwavelength vertical displacement of some of the suspended membranes. Recent demonstrations of devices (including tunable filters and surface-emitting microsources) operating along these principles are presented

Vertical microcavities based on photonic crystal mirrors for III-V/Si integrated microlasers

The on-coming photonic layer of CMOS integrated circuits needs efficient light sources to treat and transmit the flow of data. We develop new configurations of III-V/Si vertical cavity lasers coupled to silicon optical waveguides using mirror/coupler based on photonic crystals. These devices can be fabricated using fully CMOS-compatible technological steps. Using this approach, the optical gain is provided by the III-V material, while all the remaining part of the optical cavity is in silicon. The output coupling to the sub-µm waveguides of the CMOS optical layer can then be inherently optimised since the laser mirror/coupler and the Si output waveguides will be realised together during the same fabrication step. It has been demonstrated that photonic crystals membrane can act as very efficient reflectors (PCM-mirrors) for vertical microresonators. In this communication, the design of a vertical cavity microlaser based on these PCM-mirrors will be presented. We will show that high Q-factors (>10000) along with strong vertical and lateral confinements can be achieved. As a first demonstration, experimental results on silicon PhC-mirrors and associated vertical cavities will be discussed, showing Q factors larger than 2000. Finally, theoretical results on the coupling between such cavities and a silicon micro-waveguide will be presented.

Mid-infrared octave spanning supercontinuum generation to 8.5 μm in silicon-germanium waveguides

Efficient on-chip molecule and bio-agent detection can be achieved by accessing strong molecular absorption lines in the mid-infrared, but it requires high output power broadband mid-IR sources. Here, we report supercontinuum generation in an air-clad Si0.6Ge0.4/Si waveguide that emits a broad spectrum spanning from 3.0xa0μm to 8.5xa0μm. These waveguides have anomalous dispersion and low propagation loss (<0.4u2009u2009dB/cm) in the mid-IR, which leads to a supercontinuum output with a high average power of more than 10xa0mW on-chip. The realization of broadband mid-IR sources with high spectral brightness makes the SiGe-on-Si platform promising for a wide range of applications.

Photonic crystal slab mirrors for an ultimate vertical and lateral confinement of light in vertical Fabry Perot Cavities

Vertical Fabry Perot cavities (VFPCs) have enabled the realization of devices of great interest, like filters, photodetectors, VCSELs. In traditional VFPCs, the optical feedback is provided by two distributed Bragg Mirrors (DBRs). However, DBRs present two major drawbacks: they are generally rather thick mirrors, and they do not allow for a very high control on the lateral losses of the VFPC. We propose the use of a novel type of mirror, the photonic crystal slab mirror (PCM) which is able to overcome these limitations. In fact, we demonstrate that PCMs are ultra-thin single-layer mirrors that exhibit a very high reflectivity, and that allow also for a very tight control of the lateral velocity of photons, by a convenient engineering of the PCM Bloch modes. This concept will lead to the realization of ultra-compact and highly resonant VFPCs, interesting for VCSELs, non-linear optics-based devices, imaging, highly sensitive detectors, or 3D optical communication routing.

Suspended mid-infrared fiber-to-chip grating couplers for SiGe waveguides

Silicon photonics has taken great importance owing to the applications in optical communications, ranging from short reach to long haul. Originally dedicated to telecom wavelengths, silicon photonics is heading toward circuits handling with a broader spectrum, especially in the short and mid-infrared (MIR) range. This trend is due to potential applications in chemical sensing, spectroscopy and defense in the 2-10 μm range. We previously reported the development of a MIR photonic platform based on buried SiGe/Si waveguide with propagation losses between 1 and 2 dB/cm. However the low index contrast of the platform makes the design of efficient grating couplers very challenging. In order to achieve a high fiber-to-chip efficiency, we propose a novel grating coupler structure, in which the grating is locally suspended in air. The grating has been designed with a FDTD software. To achieve high efficiency, suspended structure thicknesses have been jointly optimized with the grating parameters, namely the fill factor, the period and the grating etch depth. Using the Efficient Global Optimization (EGO) method we obtained a configuration where the fiber-to-waveguide efficiency is above 57 %. Moreover the optical transition between the suspended and the buried SiGe waveguide has been carefully designed by using an Eigenmode Expansion software. Transition efficiency as high as 86 % is achieved.

Multispectral Detection with Metal-Dielectric Filters: An Investigation in Several Wavelength Bands with Temporal Coupled-Mode Theory

Multispectral infrared (IR) detection is of great interest to enhance our ability to gather information from a scene. Filtering is a low-cost alternative to the complex multispectral device architectures to which the IR community has devoted much attention. Multilayer dielectric filters are standard in industry, but they require changing the thickness of at least one layer to tune the wavelength. Here, we pursue an approach based on apertures in a metallic layer of fixed thickness, in which the filtered wavelengths are selected by varying the aperture geometry. In particular, we study filters made of at least one sheet of resonating apertures in metal embedded in dielectrics. We will discuss two interesting problems that arise when one attempts to design such filters. First, metallic absorption must be taken into account. Second, the form and size of the pattern is limited by lithography. We will present some design examples and an attempt at explaining the filtering behavior based on the temporal coupled mode theory. That theory models the filter as a resonator interacting with the environment via loss channels. The transmission is solely determined by the loss rates associated with those channels. This model allows us to give a general picture of the filtering performance and compare their characteristics at different wavelength bands.

Control of mode volume and radiation dynamics of a slow-light-mode in a quasi-3D photonic crystal configuration

We propose a new approach to realise surface addressable active photonic crystal devices. High Q-factor and low optical volume can be achieved combining lateral control of the mode size by a local modulation of the planar photonic crystal parameters, and vertical confinement assisted by a Bragg reflector. The low Q-factor of a 1D PC band edge mode can be increased up to 40000, while the optical mode volume is limited at the wavelength scale. Experimental results on laser operation achieved using this strategy in the case of an InP-based PC membrane bonded onto a Si/SiO2 Bragg reflector will be presented.

Compact photonic devices based on 1D and 2D photonic crystal broadband reflectors

1D and 2D compact photonic crystal reflectors on suspended InP membranes are theoretically and experimentally studied under normal incidence. They are based on the coupling of free space waves with slow Bloch modes of the crystal. We first present monomodal 1D photonic crystal reflectors. Then, we focus on multimodal 1D reflectors, which involve two slow Bloch modes of the crystal, and thus present broadband high-efficiency characteristics. 2D broadband reflectors were also investigated. They allow for an accurate control on the polarization dependence of the reflection. A compact (50 μm x 50 μm) demonstrator was realized and characterized, behaving either as a broadband reflector or as a broadband transmitter, depending on the polarization of the incident wave (experimental stop-band superior to 200nm, theoretical stop-band of 350nm). These photonic crystal slabs can be used in new photonic devices as reflectors, where they can replace multilayer Bragg mirrors. The authors report a compact and highly selective tunable filter using a Fabry-Perot resonator combining a bottom micromachined 3-pair-InP/air-gap Bragg reflector with a top photonic crystal slab mirror. It is based on the coupling between radiated vertical cavity modes and waveguided modes of the photonic crystal. The full-width at half maximum (FWHM) of the resonance, as measured by microreflectivity experiments, is close to 1.5nm (around 1.55 μm). The presence of the photonic crystal slab mirror results in a very compact resonator, with a limited number of layers. The demonstrator was tuned over a 20nm range for a 4V tuning voltage, the FWHM being kept below 2.5nm.