We present several integrated technologies on Silicon, from visible to mid-infrared, for particulate matter and gas detection. We present new concepts to detect in the visible particulate matter with a high sensitivity and a discrimination of both particle sizes and refractive indices. For gas detection, mid-infrared technologies developments include on one hand, microhotplate thermal emitters, as a cheap solution for gas sensing, eventually enhanced by plasmonics, and on the other hand quantum cascade lasers-based photoacoustic sensors, for high precision measurement, and for which the integration on Silicon is pushed forward for a reduction of costs.
Since many important molecules have strong “fingerprints” in the mid-infrared (mid-IR, between 3μm and 15μm), this wavelength spectrum is currently gaining significant attention for applications ranging from pollution detection, quality control in the food industry, early cancer diagnosis, security and safety [1, 2]. Molecular sensing devices in the mid-IR are currently being developed and are in the process of commercialization. An appealing approach is to create molecular sensing devices in the mid-IR based on low cost integrated mid-IR chips. A key building block is a high brightness integrated broadband light source that would allow the detection of several molecules characterized by distinct absorption lines in parallel. Such an integrated broadband source, referred to as a supercontinuum source, has been already demonstrated in the mid-IR on a chalcogenide chip . However, demonstrating mid-IR supercontinuum on group IV materials, in order to exploit the advantages of reliable CMOS fabrication technology, remains a challenge. So far, numerous CMOS-compatible supercontinuum sources have been demonstrated in silicon nitride-on-insulator [4, 5], silicon-on-insulator [6, 7], silicon germanium-on-insulator  and silicon-on-sapphire platforms . However, these sources are limited up to 3.5µm and 6µm due to the absorption in the silica and sapphire substrate, respectively. More recently, the silicon germanium-on-silicon platform [10, 11], emerged as an attractive platform for mid-IR photonics, with transparency potentially extending up to 15μm depending on the Ge content .
Here we report experimentally the first octave spanning supercontinuum generation from a SiGe waveguide in the actual mid-IR with 5mW on-chip power exceeding that produced so far in any other Si-based platform (0.15mW in SiGe/SiO2  and ~1mW in silicon-on-sapphire ). Our 4.25µm x 2.70µm cross-section air-clad SiGe-on-Si waveguide has been designed and manufactured to achieve single mode operation at 4µm, low anomalous dispersion and strong fundamental TE mode confinement in the core nonlinear material (~96% at 4μm). Losses as low as 0.4dB/cm were measured between 3.8 and 5µm. The achieved supercontinuum covered more than an octave between 2.95 and 6.0µm was generated by pumping a 7cm long waveguide with ~ 200fs pulses at 4.15µm and 63MHz repetition rate, as delivered by a Miropa-fs optical parameter amplifier. These results were supported by simulations and the related generation of a high brightness supercontinuum establishes silicon germanium-on-silicon as a promising platform for integrated nonlinear photonics in the mid-IR, with the potential to extend the operating range to beyond 8μm.
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With the recent progress in integrated silicon photonics technology and the recent development of efficient quantum cascade laser technology (QCL), there is now a very good opportunity to investigate new gas sensors offering both very high sensitivity, high selectivity (multi-gas sensing, atmosphere analysis) and low cost thanks to the integration on planar substrate. In this context, we have developed singlemode optical waveguides in the mid-infrared based on Silicon/Germanium alloy integrated on silicon. These waveguides, compatible with standard microelectronic technologies present very low loss in the 3300 – 1300 cm<sup>-1</sup> range. This paper presents the design, technological realization, and characterization of array waveguide grating devices specifically developed for the simultaneous detection of several gas using arrays of QCL sources. Gas sensing generally requires a tunable source continuously covering the whole operational range of the QCL stack. With this objective, specific design has been adopted to flatten the optical transfer function of the whole multiplexers. Samples devices around 2235cm<sup>-1</sup> were realized and tested and showed results in good agreement with the modeling, flat transmission over a full 100 cm<sup>-1</sup> operational range were obtained with a peak-to-valley modulation of -5dB were experimentally measured. These devices will be soon associated with QCL arrays in order to provide integrated, powerful, multi wavelength, laser sources in the 2235 cm<sup>-1</sup> region applicable to NO, CO, and CO<sup>2</sup> multi-gas sensor.
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.
Mid to far infrared is an important wavelength band for detection of substances. Incandescent sources are often used in
infrared spectroscopy because they are simple and cost effective. They are however broadband and quasi isotropic. As a
result, the total efficiency in a detection system is very poor. Yet it has been shown recently that thermal emission can be
designed to be directional and/or monochromatic. To do so amounts to shape the emissivity. Any real thermal source is
characterized by its emissivity, which gives the specific intensity of the source compared to the blackbody at the same
temperature. The emissivity depends on the wavelength and the direction of emission and is related to the whole
structure of the source (materials, geometry below the wavelength-scale...). Emissivity appears as a directional and
chromatic filter for the blackbody radiation. Playing with materials and structure resonances, the emissivity can be
designed to optimize the properties of an incandescent source. We will see how it is possible to optimize a plasmonic
metasurface acting as an incandescent source, to make it directional and quasi monochromatic at a chosen wavelength.
We will target a CO2 detection application to illustrate this topic.
To engineer a cheap, portable and low-power optical gas sensor, incandescent sources are more suitable than expensive quantum cascade lasers and low-efficiency light-emitting diodes. Such sources of radiation have already been realized, using standard MEMS technology, consisting in free standing circular micro-hotplates. This paper deals with the design of such membranes in order to maximize their wall-plug efficiency. Specification constraints are taken into account, including available energy per measurement and maximum power delivered by the electrical supply source. The main drawback of these membranes is known to be the power lost through conduction to the substrate, thus not converted in (useful) radiated power. If the membrane temperature is capped by technological requirements, radiative flux can be favored by increasing the membrane radius. However, given a finite amount of energy, the larger the membrane and its heat capacity, the shorter the time it can be turned on. This clearly suggests that an efficiency optimum has to be found. Using simulations based on a spatio-temporal radial profile, we demonstrate how to optimally design such membrane systems, and provide an insight into the thermo-optical mechanisms governing this kind of devices, resulting in a nontrivial design with a substantial benefit over existing systems. To further improve the source, we also consider tailoring the membrane stack spectral emissivity to promote the infrared signal to be sensed as well as to maximize energy efficiency.
Metal-Insulator-Metal (MIM) and Insulator-Metal (IM) sub-wavelength arrays are studied to perform filtering in Visible (VIS) and Near-Infrared (NIR) respectively. We investigate the MIM sub wavelength pattern using CMOS compatible materials like silicon nitride (SiN) core and aluminum (Al) metal for visible color filtering, and IM sub wavelength array with the same materials for near- infrared filtering using Rigorous Coupled Wave Analysis (RCWA). Transmission as high as 50 % is observed for VIS-filters, while for NIR filters maximum transmissions of 80% is observed. Metallic absorption in Infrared is significantly reduced using IM structure. Enhancement in Infrared transmission by factor of 1.5 is observed upon using IM structure instead of MIM structure. Blue shift in transmission spectra is observed with increase in roundness of the patch corners. Angular tolerance of ± 20° in incidence is observed for the arrays studied.
Hole arrays metallic filters can be made independent to polarization at normal incidence. However they may lose this
property for a non-normal incidence, being dependent to both polar and azimuthal incident angles. These variations of
the filter characteristics according to light orientation and polarization are not desirable for most optical applications.
Yet, for specific geometric parameters, high-stability can be obtained for cruciform-holes Ag-SiO<sub>2</sub> filters. In this article, we propose a review of cross-holes metallic filters, working with CMOS-compatible materials in the visible range. We find out the main geometrical parameters impacting the filters sensitivity to the incident angles and polarization and link their role to spectral stability. We give proper design rules to realize stable filters which may lead to optical sensors with very low spectral variations whatever the incidence and the polarization of the source.
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.
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.
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
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.
Compact photonic crystal mirrors (PCM) formed in 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. These mirrors provide high-efficiency and broadband reflectivity (stop-band superior to 400nm), when involving
two slow Bloch modes of the crystal. They allow also for an accurate control of the polarization.
These PCMs can be used in new photonic devices, where they replace DBR mirrors. The authors report on the
demonstration of a compact and highly selective (Q>1000) tunable filter at 1.55&mgr;m, using a Fabry-Perot resonator
combining a bottom micromachined 3-pair-InP/air-gap Bragg reflector with a top InP/air PCM. Micromechanical tuning
of the device via electrostatic actuation is also demonstrated over a 20nm range for a maximum 4V tuning voltage. The
active version of this device is also considered: a PCM-VCSEL is studied, combining a solid 40 quarter wavelength
InP/InGaAlAs DBR with a top PCM. First experimental results show a high Q-factor (around 2000) compatible with a
laser regime. We finally demonstrate in this paper a vertical-cavity Fabry-Perot filter with ultimate compactness,
associating two PCMs.
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.