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Developement of compact single-cycle light sources

Periodic Reporting for period 1 - CoSiLiS (Developement of compact single-cycle light sources)

Reporting period: 2019-04-01 to 2021-03-31

Ultrashort optical pulses play a vital role in a wide range of industrial and scientific applications including telecommunication networks, biological imaging, spectroscopy, metrology such as LiDAR, and advanced material processing solutions. In the last two decades, silicon photonics, that is the integration of complex optical systems using optical waveguide structures which are fabricated on a silicon wafer using standard semiconductor mass fabrication processes have revolutionized the field of photonics. Over the last five years increasing efforts have been put forward to build ultra-short pulsed lasers and frequency combs with silicon photonics technology. One of the most promising avenues are so called dissipative Kerr solitons (DKSs), which form in high-Q dielectric optical microresonators that are coherently driven with a continuous wave or long pump laser. The interplay of anomalous chromatic dispersion and the Kerr-type third order nonlinearity stabilizes a short soliton pulse. Many systems of optical microresonators have been shown to support soliton generation so far. Most importantly, several planar integrated optical waveguide systems such as silicon nitride, lithium niobate, tantalum oxide and a range of III-V semiconductor waveguide systems have been shown to support DKS generation. The pulse duration and spectral envelope of such soliton microcombs is determined by the dispersion landscape of the supporting microresonator system. Embedded into such a background, the project “Development of compact single-cycle light sources (CoSiLiS)” aimed to tap the potential of soliton microcombs to generate ultrashort single-cycle light fields and demonstrate super-octave spanning and single-cycle pulse generation directly from a photonic microresonator.
Planar integrated waveguide systems allow to engineer the chromatic dispersion by tuning of the waveguide cross section, which limits the flexibility to flatten the dispersion landscape sufficiently and to achieve ultra-broadband and single cycle DKS. Hence, the CoSiLiS project set out to investigate the best possible dispersion engineering techniques and work was focussed on the investigation of different coupled resonator geometries with the aim to find the most promising system for ultra-broadband and single cycle pulse generation. During the project period different approaches to advanced dispersion engineering were tested and implemented with the goal to custom tailor the chromatic dispersion of microresonator systems to increase spectral bandwidth of DKS for ultra-short pulse generation beyond the state-of-the-art in integrated photonic dispersion engineering Results from soliton generation in these systems include the ability to control the optical bandwidth of solitons and dispersive waves. Also soliton generation in new systems of coupled resonators were investigated and reveal the ability to in-situ control the soliton spectral envelope and the constructive and destructive interference between the soliton and radiated dispersive waves. Results of CoSiLiS have been presented in numerous conferences and were subject of a publication in the prestigious journal Nature Physics.
In the past few years and during the project time frame, there has been rapid development on the research of Kerr microresonator frequency combs and soliton pulse generation, in which the EPFL host group continuously played a pioneering and leading role.

First, broadband nano/micro-photonics-based frequency combs have now been demonstrated in different platforms and in different wavelength ranges. In CoSiLiS, we contributed to demonstrating the broadband frequency comb generation with advanced dispersion engineering on both concentric and nonconcentric coupled microring resonator systems and have demonstrated direct electrical control over the spectral envelope of the resulting soliton microcomb.

Secondly, our work within CoSiLiS on soliton generation in the fully hybridized supermodes non-concentric coupled resonators revealed a wide range of novel nonlinear dynamics that spurred a flurry of follow-up projects in the EPFL host group and in the Kerr microcomb community.
Microscope image of a concentrically coupled microring resonator for advanced dispersion engineering