Wideband Ge-Rich SiGe Polarization-Insensitive Waveguides for Mid-Infrared Free-Space Communications

Inverse design of mid-infrared diamond waveguide beam splitter

Diamond is a supreme material for mid-infrared (MIR) integrated photonics as it has a transparency window up to 20 µm that covers the entire fingerprint region. However, its relatively low refractive index poses a challenge in designing an MIR diamond functional device with both small footprint and high transmission efficiency. Here we propose and demonstrate the inverse design of an MIR diamond waveguide beam splitter operating at the wavelength of 15 µm with a small footprint of ∼15 µm × ∼15 µm and a total transmission efficiency above 95%. Our work paves a new avenue for the design of compact and high-efficiency MIR diamond photonic devices.

Broadband 2 × 2 multimode interference coupler for mid-infrared wavelengths

Beam splitters are core components of photonic integrated circuits and are often implemented with multimode interference couplers. While these devices offer high performance, their operational bandwidth is still restrictive for sensing applications in the mid-infrared wavelength range. Here we experimentally demonstrate a subwavelength-structured 2×2 multimode interference coupler with high performance in the 3.1-3.7µm range, doubling the bandwidth of a conventional device.

Suspended germanium waveguides with subwavelength-grating metamaterial cladding for the mid-infrared band

In recent years, sensing and communication applications have fueled important developments of group-IV photonics in the mid-infrared band. In the long-wave range, most platforms are based on germanium, which is transparent up to ∼15-µm wavelength. However, those platforms are limited by the intrinsic losses of complementary materials or require complex fabrication processes. To overcome these limitations, we propose suspended germanium waveguides with a subwavelength metamaterial lateral cladding that simultaneously provides optical confinement and allows structural suspension. These all-germanium waveguides can be fabricated in one dry and one wet etch step. A propagation loss of 5.3 dB/cm is measured at a wavelength of 7.7 µm. These results open the door for the development of integrated devices that can be fabricated in a simple manner and can potentially cover the mid-infrared band up to ∼15 µm.

Ultra-wideband Ge-rich silicon germanium mid-infrared polarization rotator with mode hybridization flattening

In this work we investigate the implementation of ultra-wideband polarization rotator in the mid-infrared spectral region. A new design method of the rotation section is proposed, yielding a polarization rotator with an extinction ratio of at least 15 dB in a wavelength range of 2 µm. For a spectral range wider than 3.8 µm, an extinction ratio of at least 10 dB is achieved for this design. The device is 1660 µm long and the associated insertion loss is below 1.2 dB on the full operational wavelength range. The influence of geometrical parameters with respect to the design method to obtain such a broadband behavior is discussed. Finally, to increase the tolerance to fabrication errors, a tapered rotator design is proposed. Such a device can support up to ± 100 nm fabrication errors and still guarantees remarkable broadband behavior. To the best of our knowledge, this is the first time an integrated polarization rotator is designed to operate for the wavelength range of 4 to 9 µm with a bandwidth exceeding 2 µm.

Broadband integrated racetrack ring resonators for long-wave infrared photonics

Long-wave infrared photonics is an exciting research field meant to revolutionize our daily life by means of key advances in several domains including communications, imaging systems, medical care, environmental monitoring, or multispectral chemical sensing, among others. For this purpose, integrated photonics is particularly promising owing to its compactness, mass fabrication, and energy-efficient characteristics. We present in this Letter, for the first time to the best of our knowledge, broadband integrated racetrack ring resonators operating within the crucial molecular fingerprint region. Devices show an operation bandwidth of Δλ≈900  nm with a central wavelength of λ≈8  μm, a quality factor of Q≈3200, and an extinction ratio of ER≈10  dB around the critical coupling condition. These resonant structures establish the basis of a new generation of integrated building blocks for long-wave infrared photonics that opens the route towards miniaturized multitarget molecule detection systems.

On-chip Bragg grating waveguides and Fabry-Perot resonators for long-wave infrared operation up to 8.4 µm

Taking advantage of unique molecular absorption lines in the mid-infrared fingerprint region and of the atmosphere transparency window (3-5 µm and 8-14 µm), mid-infrared silicon photonics has attracted more research activities with a great potential for applications in different areas, including spectroscopy, remote sensing, free-space communication and many others. However, the demonstration of resonant structures operating at long-wave infrared wavelengths still remains challenging. Here, we demonstrate Bragg grating-based Fabry-Perot resonators based on Ge-rich SiGe waveguides with broadband operation in the mid-infrared. Bragg grating waveguides are investigated first at different wavelengths from 5.4 µm up to 8.4 µm, showing a rejection band up to 21 dB. Integrated Fabry-Perot resonators are then demonstrated for the first time in the 8 µm-wavelength range, showing Q-factors as high as 2200. This first demonstration of integrated mid-infrared Fabry-Perot resonators paves the way towards resonance-enhanced sensing circuits and non-linear based devices at these wavelengths.