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.
The Mid-IR spectral range (2.5 μm up to 12 μm) has been considered as the paradigm for innovative silicon photonic devices. In less than a decade, chemical sensing has become a key application for Mid-IR silicon photonic devices because of the growing potential in spectroscopy, materials processing, chemical and biomolecular sensing, security and industry applications. Measuring in this spectral range, usually called molecule fingerprint region, allows to address a unique combination of fundamental absorption bands orders of magnitude stronger than overtone and combination bands in the near IR. This feature provides highly selective, sensitive and unequivocal identification of the chemicals.<p> </p> Progress in Cascade Laser technology (QCL and ICL) allows to select emission wavelengths suitable to target the detection of specific chemicals. With these sources, novel spectroscopic tools allowing real-time in-situ detection of gasses down to traces are nowadays commercially available.<p> </p> Mid-IR Si photonics has developed a novel class of integrated components leading to the integration at chip level of the main building blocks required for chemical sensing, i.e. the source, the PICs and the detector. Three main directions of improvement can be drawn: i) extend the range of wavelengths available from a single source, ii) move beam handling and routing from discrete optics to PICs and iii) investigate detection schemes for a fully integrated on-chip sensing.<p> </p> This paper reviews recent key achievements in the miniaturization and the co-integration of photonics devices at chip and packaging level to address cost, size and power consumption. Perspectives on potential applications will also be presented.
Photoacoustic (PA) spectroscopy is among the most sensitive techniques used to monitor chemical emission or detect gas traces. In the mid-infrared, where most of gases of interest have their strongest absorption lines, this technique takes advantage of the high optical power and room temperature operation of quantum cascade lasers (QCL). We have recently demonstrated that centimeter-size PA cells can compete, with bulky commercial systems for gas sensing without any compromises on performances. We demonstrate a new step towards cost reduction, extreme integration, and mass deployment of such PA sensors with a miniaturized silicon PA-cell fabricated on standard CMOS tools. The design, fabrication and characterizations of this new sub-centimeter PA cell built on a silicon platform are presented. First, the component has been designed using a detailed physical model, accounting for viscous and thermal losses, and metamodel-based optimization techniques. Second, it has been fabricated on our 200 mm CMOS pilot line. Several wafers have been released and diced. Single chips have then been assembled with commercial capacitive microphones and finally characterized on our reference gas bench. The photoacoustic simulations and the acoustics experiments are in a good agreement. The tiny PA cell exhibits a sensitivity down to the ppm level for CO<sub>2</sub> at 2300 cm<sup>-1</sup>, as well as for CH<sub>4</sub> at 3057 cm<sup>-1</sup> even in a gas flow. Taking advantage of the integration of QCLs on Si and photonic circuitry, the silicon PA cell concept is currently being extended towards a fully integrated multigas detector.
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.
The linear and nonlinear optical response of SiGe waveguides in the mid-infrared are experimentally measured. By cutback measurements we find the linear losses to be less than 1.5dB/cm between 3μm and 5μm, with a record low loss of 0.5dB/cm at a wavelength of 4.75μm. By launching picosecond pulses between 3.25μm and 4.75μm into the waveguides and measuring both their self-phase modulation and nonlinear transmission we find that nonlinear losses can be significant in this wavelength range due to free-carrier absorption induced by multi-photon absorption. This should be considered when engineering SiGe photonic devices for nonlinear applications in the mid-IR.
We present a scheme for the realization of high performances, large tuning range, fully integrated and possibly low cost mid infrared laser source based on quantum cascade lasers and silicon based integrated optics. It is composed of a laser array and a laser combiner. We show that our metal grating approach gives many advantages for the fabrication yield of those laser arrays. We show the results of such a fabrication at 1350 cm<sup>-1</sup> with 60 cm<sup>-1</sup> tuning range. The silicon is a low cost option for the size consuming combiner. In the development of the SiGe platform, we present the loss measurement set up and we show losses below 1dB/cm at 4.5μm.
We report on the advanced optical characterizations of microfabricated solid immersion lenses with 2-μm diameter,
operating at λ= 642 nm. The main feature, the spot size reduction, has been investigated by applying a focused Gaussian
beam of NA = 0.9. Particular illuminating beams, e.g., Bessel-Gauss beams of the zeroth and the first order, a doughnutshape
beam and its decompositions, i.e. two-half-lobes beams, have also been used to influence the shape of the
immersed focal spot. Detailed optical characterizations have been conducted by measuring the amplitude and phase
distributions with a high-resolution interference microscope (HRIM) in volume around the focal spot. The immersion
effect of the SiO<sub>2</sub> solid immersion lens leads to a spot-size reduction of approximately 1.5 which agrees well with theory.
Particularly shaped incident beams exhibit a comparable size reduction of the immersed spots. Such structured focal
spots are of significant interest in optical trapping, lithography, and optical data storage systems.