Low loss SiGe graded index waveguides for mid-IR applications

Abstract

In the last few years Mid InfraRed (MIR) photonics has received renewed interest for a variety of commercial, scientific and military applications. This paper reports the design, the fabrication and the characterization of SiGe/Si based graded index waveguides and photonics integrated devices. The thickness and the Ge concentration of the core layer were optimized to cover the full [3 - 8 µm] band. The developed SiGe/Si stack has been used to fabricate straight waveguides and basic optical functions such as Y-junction, crossings and couplers. Straight waveguides showed losses as low as 1 dB/cm at λ = 4.5 µm and 2 dB/cm at 7.4 µm. Likewise straight waveguides, basic functions exhibit nearly theoretical behavior with losses compatible with the implementation of more complex functions in integrated photonics circuits. To the best of our knowledge, the performances of those Mid-IR waveguides significantly exceed the state of the art, confirming the feasibility of using graded SiGe/Si devices in a wide range of wavelengths. These results represent a capital breakthrough to develop a photonic platform working in the Mid-IR range.

Efficient 52 µm wavelength fiber-to-chip grating couplers for the Ge-on-Si and Ge-on-SOI mid-infrared waveguide platform

“…Considerable research is geared towards the integration and miniaturization of various optical components for the midIR on these platforms, such as AWGs and PCGs [11][12][13][14], grating couplers and low loss waveguides [6,[15][16][17][18][19][20], ring resonators [12,21,22] and quantum cascade laser (QCL) source integration [24]. Packaging, as well as wafer-level testing of these optical devices requires the implementation of efficient fiber-to-chip grating couplers.…”

Ge-rich graded-index Si_1-xGex waveguides with broadband tight mode confinement and flat anomalous dispersion for nonlinear mid-infrared photonics

“…Later on, Si 1-x Ge x alloys gained attention due to their ability to fine-tune the refractive index and bandgap at will, being able to provide complex graded refractive index profiles to efficiently accommodate the guided optical mode [13]. As a result, competitive propagation losses as low as 1 dB/cm for λ = 4.5 µm and 2 dB/cm for λ = 7.4 µm were obtained by ramping the Ge concentration in the Si 1-x Ge x alloy up to ≈ 40 % [14]. More recently, low-loss Ge-rich Si 1-x Ge x waveguides with a top Ge concentration up to 80 % were also demonstrated at λ = 4.6 µm [15].…”

“…Thus, we used an h core = 6 µm as yielded a good balance between tight mode confinement, good mode overlap with the Ge-rich region and an expected low dislocation density upon fabrication by state-of-the-art deposition techniques. In that regard, suitable techniques that have already provided notable results are reduced pressure chemical vapor deposition (RPCVD) or low energy plasma enhanced chemical vapor deposition (LEPECVD), among others [14,15].…”

Integrated photonics for infrared spectroscopic sensing

“…Passive photonic components need to be made from optically transparent materials in the mid-IR. Since silica becomes opaque at wavelengths longer than 4 μm, mid-IR integrated resonators demonstrated to date either involve special suspended designs [1][2][3][4][5][6][7][8][9] or new material platforms, such as silicon-on-sapphire [10][11][12][13][14][15][16], silicon-on-nitride [17][18][19][20], or germanium-on-silicon [21][22][23][24][25][26]. Active optoelectronic devices rely on narrow-bandgap semiconductors to enable operation at mid-IR wavelength, although lattice mismatch of these semiconductors generally prohibit their epitaxial growth and integration on silicon, and therefore heterogeneous integration of III-V semiconductors or graphene have been demonstrated as an alternative route to resolve the challenge [27][28][29][30].…”