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INsPIRE Résumé de rapport

Project ID: 639107
Financé au titre de: H2020-EU.1.1.

Periodic Reporting for period 2 - INsPIRE (Chip-scale INtegrated Photonics for the mid-Infra REd)

Reporting period: 2016-10-01 à 2018-03-31

Summary of the context and overall objectives of the project

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 to the end of the period covered by the report and main results achieved so far

The first objective of the project was to characterize the loss of Ge-rich SiGe alloys in the mid-IR, as only few data have been reported in the literature regarding the loss of these materials in this wavelength range. As a main achievement we demonstrated low-loss Ge-rich SiGe (with 80% of germanium) waveguides on graded SiGe substrates operating in the mid-infrared wavelength range. Propagation losses as low as (1.5 ± 0.5) dB/cm and (2 ± 0.5) dB/cm were first measured at λ = 4.6 µm for the quasi-TE and quasi–TM polarizations, respectively [1]. A flat propagation loss characteristic of (2±1) dB/cm was then demonstrated over a wavelength span from λ = 5.5 µm to 8.5 µm [2]. Broadband mid-IR Mach–Zehnder interferometers were then demonstrating, working from 5.5 to 8.5 µm wavelength (the wavelength range is only limited by the characterization set-up)[3]. It has been shown that the wideband operation originates from the refractive index gradient in the structures, which is a key advantage of the platform developed with INsPIRE project.
As a second objective, we explored non-linear (NL) effects in Ge-rich SiGe materials, as no experimental values were reported on SiGe alloys at the beginning of the project. The first 3rd order NL experimental characterization of Ge-rich Si1-xGex waveguides has thus been performed, with Ge concentrations x ranging from 0.7 to 0.9. The characterization performed using a bi-directional top hat D-Scan method, at 1580 nm has been used to model NL properties of Ge-rich SiGe materials in the mid-IR. [4] A strong increase of the non-linear refractive index when the Ge concentration is larger than 80% is thus obtained, confirming the opportunity of using Ge-rich SiGe materials for efficient non-linear devices. To exploit the this NL effects for efficient devices such as broadband supercontinuum, the waveguide design has then been optimized to obtain simultaneously a good mode overlap with the Ge-rich region and a broadband flat anomalous dispersion over a wide wavelength range from λ = 3 µm to 8 µm. From these results it has been possible to estimate the NL parameter (γeff) of such waveguides, suggesting a promising performance for broadband supercontinuum generation [5].

[1] J.M. Ramirez et al, Optics Letters 42 (1) 105 (2017).
[2] J.M. Ramirez, et al, Under submission
[3] V. Vakarin, et al, Optics Letters Vol. 42 n°17, 3482-3485 (2017)
[4]S. Serna, et al Scientific report, 7, 14692 (2017)
[5] J.M. Ramirez, Optics Express 25 (6) 6561 (2017).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

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. In the first period of the project a new and unique mid-IR set-up was created to characterize photonic integrated circuits (PIC), and the first demonstrations of ultra-wideband operation already put forward the potential of the graded Ge-rich SiGe waveguides as fundamental blocks for mid-infrared photonic integrated circuits. The demonstration of broadband mid-IR Mach–Zehnder interferometers paves the way for further realization of complete mid-IR integrated spectrometers. In parallel, the investigation of NL effect, suggesting a promising performance for broadband supercontinuum generation, allow to consider the demonstration of complete mid-IR photonic integrated circuits by the end of the project. Among the key challenges, integrated resonators and sensing part will be the next building blocks to be developed in the coming period.
As a long term perspective, the integration of the mid-IR systems on silicon chips will benefit from high-volume and low-cost fabrication in microelectronic. High performance, low cost, small size, low weight and low power consumption systems are thus expected.