Nonlinear photonics in silicon germanium waveguides for mid-infrared supercontinuum generation

Abstract

Mid-infrared light (2.5 - 15 um) can be advantageously used for high-sensitivity molecular detection in environment, healthcare, industry and security applications. Various molecules can be detected at trace levels by measuring their strong absorption that is several orders of magnitude stronger in the mid-infrared than in the near-infrared. In recent years, there has been a high demand for compact mid-infrared sensors that can be equipped within cars, drones or even smartphones. These sensors could be realized by relying on a technology that is used in a cost-effective micro-electronics industry. The field of photonics utilizing the complementary metal-oxide-semiconductor technology is referred to as silicon photonics. The envisioned compact (on-chip) mid-infrared sensor consists of a light source, sensing area, and photo-detector. The scope of this thesis has been to develop the first building block of this sensor, which is the mid-infrared supercontinuum light source on a silicon-based chip. Supercontinuum (SC) light is particularly interesting for molecular spectroscopy as it allows for accessing multiple absorption wavelengths at once, enabling the reliable and simultaneous detection of many molecules. Mid-infrared supercontinua on a silicon-based chip have been reported in several platforms on insulating substrates, i.e. silica and sapphire. However, the operation wavelength ranges in these platforms are limited to 3.7 and 5.5 um due to the absorption in silica and sapphire, respectively. The main goals of this thesis have been to explore a silicon-based platform with extended wavelength range deeper in the mid-infrared and to demonstrate a spectrally bright supercontinuum beyond 5.5um. Such a supercontinuum would cover the entire mid-infrared atmospheric absorption band from 4 to 8 um. In addition to high brightness, ultra-fast and high-precision molecular spectroscopy requires high coherence of a supercontinuum. In this context, an additional objective has been to achieve a coherent supercontinuum generation. Germanium is a well-known material in the microelectronics industry which has been suggested for silicon photonics in the mid-infrared owing to its wide transparency window. However, the lattice mismatch between germanium and silicon eventually leads to a large density of threading dislocations at the germanium/silicon interface, which limits the performance of nonlinear optical devices. Our solution to this issue has been to use a silicon germanium-on-silicon platform with 40% of germanium in the alloy. Silicon germanium waveguides buried in silicon, which were not dispersion engineered for supercontinuum, were explored in our group. In these waveguides, L. Carletti demonstrated a promising low loss operation and identified an optimal operation wavelength at around 4um. Based on these initial results; I designed air clad waveguides and optimized their dispersion for supercontinuum generation. The waveguides used in this thesis were fabricated using a technological process developed over the last two decades by our collaborators at the micro and nanotechnology research center CEA-Leti in Grenoble, France. The experiments were then performed at the Laser Physics Centre at Australian National University (ANU) in Canberra, Australia. There, we performed linear and nonlinear measurements using a picosecond and sub-picosecond pump at 4um in wavelength. The experimental results were analyzed using an in-house developed software. The measured supercontinuum spectra/transmission fits and coherence properties have been analyzed using a generalized nonlinear Schrodinger equation solver. Dispersion trimming presented in the fourth chapter has been investigated using a custom-built mode solver. The theory of nonlinear optics used in this thesis is introduced in the first chapter. The second chapter reports a spectrally bright supercontinuum spanning to 8.5um from a dispersion-engineered silicon germanium-on-silicon waveguide. This is a milestone in the mid-infrared silicon photonics since the supercontinuum reached the onset of silicon absorption at 8.5um. In this waveguide, we measured a propagation loss as low as 0.2 dB/cm and more than 10mW on chip supercontinuum power. This is the lowest measured loss and the largest supercontinuum power reported in any silicon-based waveguide in the mid-infrared. The third chapter discusses the coherence of an octave-spanning supercontinuum. Coherence has been numerically analyzed for an experimentally measured supercontinuum. This chapter shows that a high coherence can be achieved in a long waveguide pumped in the anomalous dispersion regime with 200fs pulses. This is possible thanks to the specific dispersion profile with a relatively narrow anomalous dispersion band. In the last chapter, we demonstrate a simple post-fabrication dispersion trimming technique that can be used to optimize dispersion or to shift dispersion from anomalous to all normal. The mid-infrared possesses the fundamental barrier for standard silicon-based platforms including silicon-on-insulator, silicon nitride-on-insulator, and silicon-on-sapphire. The results reported in this thesis clearly establish silicon germanium-on-silicon as a relevant platform for nonlinear silicon photonics in the mid-infrared. The octave-spanning coherent supercontinuum that has been demonstrated paves the way for future mid-infrared molecule sensor on a silicon chip