Abstract
Mid-infrared (mid-IR) silicon photonics is expected to lead key advances in different areas including spectroscopy, remote sensing, nonlinear optics or free-space communications, among others. Still, the inherent limitations of the silicon-on-insulator (SOI) technology, namely the early mid-IR absorption of silicon oxide and silicon at λ~3.6 µm and at λ ~8.5 µm respectively, remain the main stumbling blocks that prevent this platform to fully exploit the mid-IR spectrum (λ ~2-20 µm). Here, we propose using a compact Ge-rich graded-index Si1-xGex platform to overcome this constraint. A flat propagation loss characteristic as low as 2-3 dB/cm over a wavelength span from λ = 5.5 µm to 8.5 µm is demonstrated in Ge-rich Si1-xGex waveguides of only 6 µm thick. The comparison of three different waveguides design with different vertical index profiles demonstrates the benefit of reducing the fraction of the guided mode that overlaps with the Si substrate to obtain such flat low loss behavior. Such Ge-rich Si1-xGex platforms may open the route towards the implementation of mid-IR photonic integrated circuits with low-loss beyond the Si multi-phonon absorption band onset, hence truly exploiting the full Ge transparency window up to λ ~15 µm.
Highlights
Research activities on silicon photonics have strongly increased over the last few years, giving birth to a dynamic technological network connecting research hubs, academics and new startup companies, all of them leveraging from the available mature industrial resources of the microelectronics industry
Silicon photonics has been proposed as a compelling solution to address several technical challenges in current biological and chemical sensing systems, taking advantage from the unique mid infrared molecular absorption peaks of several relevant chemical compounds [5, 6]
We demonstrated the potential of Ge-rich graded-index Si1-xGex layers to be used as versatile and compact low-loss mid-IR photonic integrated platforms on Si substrates with an unprecedented transparency bandwidth
Summary
Research activities on silicon photonics have strongly increased over the last few years, giving birth to a dynamic technological network connecting research hubs, academics and new startup companies, all of them leveraging from the available mature industrial resources of the microelectronics industry. We have recently proposed Ge-rich Si1−xGex alloys on graded Si1-xGex layers as an alternative approach for mid-IR integrated photonics. Propagation losses between 1.5 and 2 dB/cm have been obtained at λ ~4.6 μm [25], while broadband Mach–Zehnder interferometers have been demonstrated, working in both quasi-TE and TM polarizations [26] These structures allow to fine tune the refractive index profile, permitting an efficient tailoring of the waveguide properties such as mode confinement and dispersion. One of the relevant features of such Ge-rich Si1−xGex waveguides is their expected wide transparency window, which could potentially extend up to λ = 15 μm, as the refractive index gradient allows to push the optical mode far from the Si substrate where absorption begins to be prohibitive beyond λ ~8.5 μm [5]
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