The mid-Infrared wavelength range (2-20 µm), so-called fingerprint region, contains the very sharp vibrational and rotational resonances of many chemical and biological substances. Thereby, on-chip absorption-spectrometry-based sensors operating in the mid-Infrared (mid-IR) have the potential to perform high-precision, label-free, real-time detection of multiple target molecules within a single sensor, which makes them an ideal technology for the implementation of lab-on-a-chip devices.
Benefiting from the great development realized in the telecom field, silicon photonics is poised to deliver ultra-compact efficient and cost-effective devices fabricated at mass scale. In addition, Si is transparent up to 8 µm wavelength, making it an ideal material for the implementation of high-performance mid-IR photonic circuits. The silicon-on-insulator (SOI) technology, typically used in telecom applications, relies on silicon dioxide as bottom insulator. Unfortunately, silicon dioxide absorbs light beyond 3.6 µm, limiting the usability range of the SOI platform for the mid-IR. Silicon-on-sapphire (SOS) has been proposed as an alternative solution that extends the operability region up to 6 µm (sapphire absorption), while providing a high-index contrast. In this context, surface grating couplers have been proved as an efficient means of injecting and extracting light from mid-IR SOS circuits that obviate the need of cleaving sapphire. However, grating couplers typically have a reduced bandwidth, compared with facet coupling solutions such as inverse or sub-wavelength tapers. This feature limits their feasibility for absorption spectroscopy applications that may require monitoring wide wavelength ranges. Interestingly, sub-wavelength engineering can be used to substantially improve grating coupler bandwidth, as demonstrated in devices operating at telecom wavelengths.
Here, we report on the development of fiber-to-chip interconnects to ZrF4 optical fibers and integrated SOS circuits with 500 nm thick Si, operating around 3.8 µm wavelength. Results on facet coupling and sub-wavelength engineered grating coupler solutions in the mid-IR regime will be compared.
We present an innovative and efficient technique for post-fabrication trimming of silicon photonic integrated circuits
(PICs). Our approach exploits the high photosensitivity of chalcogenide glasses (ChGs) to induce local and permanent
modifications of the optical properties and spectral responses of ChG-assisted silicon devices. We experimentally
demonstrate the potential of this technique on ring resonator filters realized on a silicon-on-insulator platform, for which
post-fabrication treatments enable to counteract the strong sensitivity to technological tolerances. Photosensitive ChGassisted
silicon waveguides were realized by deposition of a As2S3 chalcogenide layer on top of conventional silicon
channel waveguides. A resonant wavelength shift of 6.7 nm was achieved, largely exceeding the random resonance
spread due to fabrication tolerances. Neither the ChG layer deposition, nor the trimming process introduces appreciable
additional losses with respect to the bare silicon core waveguide. Performances of the trimming technique, such as speed
and saturation effects, as well as nonlinear behavior and infrared writing issues are investigated and experimentally
characterized.
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