Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - INsPIRE (Chip-scale INtegrated Photonics for the mid-Infra REd)

Teaser

Mid-infrared (mid-IR) spectroscopy is a nearly universal way to identify chemical and biological substances, as most of the molecules have their vibrational and rotational resonances in the mid-IR wavelength range. Commercially available mid-IR systems are based on bulky and...

Summary

Mid-infrared (mid-IR) spectroscopy is a nearly universal way to identify chemical and biological substances, as most of the molecules have their vibrational and rotational resonances in the mid-IR wavelength range. Commercially available mid-IR systems are based on bulky and expensive equipment, while lots of efforts are now devoted to the reduction of their size down to chip-scale dimensions. The demonstration of mid-IR photonic circuits on silicon chips will benefit from reliable and high-volume fabrication to offer high performance, low cost, compact, low weight and power consumption photonic circuits, which is particularly interesting for mid-IR spectroscopic sensing systems that need to be portable and low cost. In this context, the INsPIRE project addresses a new route towards key advances in the development of chip-scale integrated circuits on silicon for the mid-IR wavelength range. The original idea is to combine the wide transparency window of Ge to provide wideband low-losses optical waveguide, with active devices using non-linear effects in Ge-rich SiGe materials. The objectives of the project are far beyond the state of the art, by targeting the monolithic integration of passive and active devices for operation in the 3 to 15 µm wavelength range. As a main cornerstone we will demonstrate an optical photonic circuit relying on a mid-IR light emitter combined with a mid-IR spectrometer and a detector array. The integration will be performed using Ge-rich-SiGe waveguides allowing the extension of the wavelength range up to 15 µm. Such demonstration, which will constitute a breakthrough for establishing chip-scale circuits for the mid-IR photonics, requires a deep knowledge and understanding of Ge/SiGe optical properties. In particular, second- and third-order nonlinear optical properties of Ge/SiGe structures will be investigated in a wide spectral range from 3 to 15 µm.

Work performed

From the beginning of the project major steps have been successfully passed towards the successful development of chip-scale integrated circuits on silicon for the mid-IR wavelength range based on Ge-rich SiGe integrated platform. Among the main results we can list the following major achievements:
• Broadband Ge-rich SiGe waveguide have been demonstrated. A flat propagation loss characteristic of (2±1) dB/cm over a wavelength span from λ = 5.5 μm to 8.5 μm (limited only by the characterization set-up) is observed.
• The Ge-rich SiGe platform has been tested as a mid-IR photonic chip-scale sensor, based on the use of the evanescent component of the guided optical mode to probe specific molecular absorption features of the surrounding cladding environment. As a proof of concept, we monitored the absorption spectral patterns of a standalone photoresist spin-coated onto spiral Ge-rich SiGe waveguides. A significant increase of the waveguide optical loss at the spectral window of 5.8-6.2 µm is identified and correlated with the inherent photoresist absorption. The ability of this platform to sense small concentrations of methane gas has also been discussed.
• A set of passive devices has been demonstrated, building a complete integrated mid-IR platform : Mach Zehnder interferometer working from 5.5 to 8.5 µm wavelength, both in TE and TM polarization have been demonstrated as well as the first resonators working in the 8 µm range, based either on Fabry-Perot cavity using Bragg grating mirrors or racetrack resonators
• Fourier transform spectrometers have been demonstrated, in the spatial heterodyne configuration. Operation from 5 to 8.5 µm is demonstrated. A new thermally-tuned spatial heterodyne spectrometer has been proposed and demonstrated, to overcome the resolution/bandwidth tradeoff of classical approaches.
• Finally efficient tailoring of the chromatic dispersion curves of these waveguides in the mid-IR is expected. Thus waveguides have been designed for the exploitation of non-linear effects, achieving broadband flat anomalous dispersion for the quasi-TM optical mode with γmax = 14 ps/nm/km over ~ 1.4 octave spanning (λ ~ 3 - 8 µm).


In the different cases, new concepts have been proposed and demonstrated to adapt the photonics devices to the specificities of the mid-IR range (especially the wide spectral range when multispecies absorption spectroscopy is targeted). These results pave the way towards the demonstration of compact, portable, label-free and highly sensitive photonic integrated sensors based on Ge-rich SiGe circuits.

Final results

Mid-IR spectroscopy is a nearly universal way to identify chemical and biological substances. In the so-called “fingerprint” region, most molecules have their vibrational and rotational resonances. Mid-IR spectroscopy thus provides powerful informations for performing non-intrusive diagnostics of composite systems. The mid-IR region also contains two important windows (3-5µm and 8-13 µm) in which the atmosphere is relatively transparent. These wavelength ranges can be exploited to detect small traces of environmental and toxic vapours in a variety of applications including defense, security and industrial solutions. In this context the INsPIRE project already made key advances, beyond the state of the art for operation from 3 to 15 µm wavelength.
Among the remaining challenges, the exploitation of the non-linear effects to build active devices, and the developpement of optical modulators will be targeted in the last period of the project.

Website & more info

More info: https://minaphot.c2n.universite-paris-saclay.fr/en/funded_projects/inspire/.