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Chip-based mid-infrared frequency combs

Periodic Reporting for period 1 - MIRCOMB (Chip-based mid-infrared frequency combs)

Reporting period: 2016-09-01 to 2018-08-31

Mid-infrared (Mid-IR) range (i.e. the spectral window 2.5—10 μm) is considered as the golden mine for molecular spectroscopy, where most materials feature strong absorptions due to molecular transitions, e.g. hydrogen bonds (N-H, O-H and C-H) show absorptions in the range 2.5—4.0 μm (4000—2500 cm-1), and the absorption strengths are usually 10 to 1000 times greater than those in the visible and near-infrared ranges. Triggered by a significant number of applications such as pharmaceutical, environmental or medical breath analysis, mid-IR spectroscopy has attracted substantial attention in the past decade. The development of mid-IR spectroscopy is mainly focusing on the creation of robust and coherent mid-IR sources, while a compact and simple system configuration is also required for potential applications beyond the laboratory frame.

MIRCOMB aimed to demonstrate a coherent and broadband mid-IR frequency comb based on integrated and compatible photonic platforms. A mid-IR frequency comb is a series of lasers with equal spacing between frequencies in the mid-infrared range. The innovative approach was to transplant the Kerr frequency comb technology into the mid-IR spectral range, which enables coherent optical frequency combs with high compactness. This approach was invented by the host EPFL group and has found great success in the near-infrared telecommunication range in the past decade, but still remains quite unexplored in the mid-IR range. Moreover, we have investigated the approach of photonic chip-based coherent supercontinuum process that also represents an efficient and compact solution to the mid-IR frequency comb generation. The project consists of two primary phases: (phase I) mid-IR frequency comb generation in photonic chip-based silicon nitride platform; and (phase II) mid-IR dual comb spectroscopy.
Objectives of MIRCOMB are the following:
1. Fabrication of high Q on-chip silicon nitride microresonators with mid-IR compatibility
2. Demonstration of mid-IR optical frequency combs and solitons in silicon nitride microresonators
3. Compact Mid-IR dual comb spectroscopy system

Overall, the project has fully achieved its objectives and milestones for the period. In particular, we have accomplished the following milestones:
1. Photonic chip-based large-cross-section silicon nitride platform, which enables mid-IR nano-photonic waveguides with ultra-low loss, as well as mid-IR high-Q microresonators (Qin > 600,000);
2. Mid-IR Kerr frequency comb at 2.5 μm in high-Q silicon nitride microresonators;
3. A diverse of soliton dynamics in optical microresonators, including soliton switching, Raman effects and breather solitons;
4. Coherent and broadband mid-IR frequency comb in the first principle region 2.5—4.0 μm, in large-cross-section silicon nitride waveguides via the supercontinuum process;
5. Mid-IR dual comb spectroscopy from chip-based silicon nitride nano-photonics waveguides.

Deliverables of MIRCOMB include:
1. Publication of mid-IR frequency combs via coherent supercontinuum process in large-cross-section silicon nitride waveguides;
2. Publications on the study of soliton dynamics;
3. Proof-of-concept experiments on mid-IR dual-comb spectroscopy.
MIRCOMB implemented a chip-based mid-IR frequency comb, and demonstrated mid-IR dual comb spectroscopy from a chip-scale nano-photonic platform.

While mid-IR frequency comb technology is a hot topic in the community of spectroscopy, MIRCOMB represents a significant contribution with its results been published in several journal articles and widely disseminated at international conferences.

The impact of this project include:
1. MIRCOMB has been situated in a high-impact and fast developing field and thus attracted great attention; and
2. The results and objectives we have accomplished within this project have in return provided strong impacts to the community, which will contribute to the development of this field, not only at present but also in the future.

In particular, the silicon nitride nano-photonic waveguides that are developed within MIRCOMB can extend the commercial market of using such chip-based nano-photonic devices for ultrafast laser operation, particularly the supercontinuum process.

To the European Union, MIRCOMB has also made contributions to the EU H2020 program in excellence research, proving that Europe continues to produce world class science, and being amongst the first to advance optical frequency comb technologies.