(Invited) Silicon Photonics Based on Ge/Sige Quantum Well (QW) Structures and Ge-Rich Materials for Near-IR and Mid-IR
Authors: Ramirez Joan Manel , Vakarin Vladyslav , Chaisakul Papichaya , Serna Samuel , Liu Qiankun , Frigerio Jacopo , Ballabio Andrea , Le Roux Xavier , Vivien Laurent , Isella Giovanni , Cassan Eric , Dubreuil Nicolas , Marris-Morini Delphine 
Published: February 2020

Silicon photonics is a disruptive technology poised to revolutionize several areas of our society. It has often been considered as the natural step forward to overcome some of the inherent bottlenecks of current microelectronics technology. Importantly, photonic integrated circuits can be implemented using CMOS-like fabrication processes, hence benefiting from a mature technology that provides low-cost mass fabrication using advanced tools. A large pool of Si components has been demonstrated, standing for low-loss waveguides, filters, micro-ring resonators, Mach-Zehnder interferometers, array waveguide gratings or photonic crystals, among others [1]. Similarly, the incorporation of Ge in Si photonic platforms has yield the demonstration of several active optical devices such as high-speed optical modulators and photodetectors with unprecedented performance [2]. In addition, strong electro-absorption and electro-refraction has been demonstrated in Ge/SiGe quantum wells showing strong Quantum Confined Stark Effects (QCSE), allowing for efficient optoelectronic modulation and detection at telecom wavelengths [3]. Interestingly, tuning the QW design allows fine tuning of the absorption band edge of devices over a wideband range. Moreover, light emission from the direct bandgap transition HH1-CΓ1 has also been reported in SiGe/Ge quantum wells, opening the route towards the development of light emitting devices fully integrated in a silicon chip [4].
Noticeably, all the above described activities are meant to develop Si-based photonic platforms in the near-infrared wavelength range, where most applications are foreseen. Lately, however, there is an increasing interest in the exploitation of Ge-based photonic platforms at longer wavelengths up to the mid-infrared [5]. Having Ge a transparency window up to 15 µm, there are good prospects for the development of advanced photonic sensing platforms leveraging from the strong mid-IR molecular absorption bands of several chemical and biological substances. Besides, Ge-based platforms have also been proposed as ideal candidates to exploit the nonlinear properties at mid-IR wavelengths, owing to a higher χ(3) nonlinearity and the absence of two-photon absorption beyond λ ≈ 3.15 µm [6]. Important benefits are also expected in data communications, taking advantage from the two atmospheric transparency windows at λ ≈ 3-5 µm and λ ≈ 8-13 µm and the relaxed wavelength regulations put in place.
Thus, in this presentation we will show our recent work on the development of Ge-based photonic devices for telecom and mid-IR applications. For that, a review of the advanced optoelectronic devices demonstrated so far in our group will be presented, including high-speed SiGe/Ge QWs electro-absorption modulators and detectors exploiting the QCSE. Then, further insight on the room temperature luminescence of QWs will be provided, followed by its integration in light emitting diodes in waveguide configuration for planar emission. In addition, a novel mid-IR photonic platform based on Ge-rich graded-index SiGe alloys will be presented. This platform offers an interesting set of advantages over other existing mid-IR approaches such as low-loss propagation and optimal modal area over an unprecedentedly broadband wavelength range [7]. As a consequence, the first demonstration of ultra-wideband mid-IR Mach-Zehnder interferometers operating from λ = 5.5 µm to 8.5 µm was performed using the graded-index SiGe platform [8]. Finally, the third-order nonlinear properties of this platform will be discussed, supported by experimental data on Ge-rich SiGe waveguides using a bidirectional top hat D-scan method [9]. Promising results were obtained, thus providing a promising scenario to develop broadband optical sources based on the supercontinuum generation.

References

[1] D. Thomson et al. ‘Roadmap on silicon photonics’, Journal of Optics, 2016, 18(7), 073003 (2016).
[2] Y. Ishikawa, et al. ‘Germanium for silicon photonics,’ Thin Solid Films, 518(6), S83-S87 (2010).
[3] P. Chaisakul, et al. ‘Integrated germanium optical interconnects on silicon substrates,’ Nature Photonics, 8(6), 482-488 (2014).
[4] P. Chaisakul, et al. ‘Room temperature direct gap electroluminescence from Ge/Si0. 15Ge0. 85 multiple quantum well waveguide,’ Applied Physics Letters, 99(14), 141106 (2011).
[5] R. Soref, ‘Mid-infrared photonics in silicon and germanium,’ Nature photonics, 4(8), 495-497 (2010).
[6] L. Zhang, et al.’Nonlinear Group IV photonics based on silicon and germanium: from near-infrared to mid-infrared,’ Nanophotonics, 3(4-5), 247-268 (2014).
[7] J. M. Ramirez et al. ‘Low-loss Ge-rich Si0.2Ge0.8 waveguides for mid-infrared photonics,’ Opt. Lett. 42(1) 105-108 (2017)
[8] V. Vakarin et al. ‘Ultra-wideband Ge-rich silicon germanium integrated Mach-Zehnder interferometer for mid-infrared spectroscopy,’ Opt. Lett. 42(17), 3482-3485 (2017)
[9] J. M. Ramirez et al. ‘Ge-rich graded-index SiGe waveguides with broadband tight mode confinement and flat anomalous dispersion for nonlinear mid-infrared photonics,’ Opt. Express, 25(6), 6561-6567 (2017)

Figure 1