Nonlinear Optical Phenomena in Hybrid TeO2-Si3N4 Waveguides

Abstract

Over the last few decades nonlinear integrated optical devices have emerged as an enabling technology for a number of important applications in communications, computing, sensing, medical, and defense and security systems. It all has been possible through the advancement of micro and nano-fabrication techniques and the development of nonlinear optical materials that can leverage the mature complementary metal-oxide semiconductor (CMOS) infrastructure. Among the materials explored for integrated nonlinear photonics silicon nitride (Si3N4) is one of the most suitable and widely used. Despite its maturity and prominence, there are a few challenges that persist. One of them is the difficulty in the fabrication of thick low-loss Si3N4 waveguides suitable for nonlinear applications using conventional wafer-scale methods. In particular, a waveguide thickness of at least 700 nm is needed to attain anomalous dispersion which is critical for efficient nonlinear processes. However, fabricating such waveguides through the preferred method of low-pressure chemical vapor deposition is challenging due to cracks developing during deposition caused by the differences in tensile stress between the Si3N4 layer and substrate. Tellurium oxide (TeO2) is among the oxide glasses with the most attractive optical properties. TeO2 has a relatively large linear refractive index, wide transparency, high nonlinearity, high Raman gain, high acousto-optic figure of merit, negligible nonlinear losses, and high rare earth dopants solubility making it a good candidate for linear, nonlinear, and active optical devices. This work presents a study of nonlinear optical phenomena in a hybrid TeO2-coated Si3N4 platform. The platform is based on a thin commercial foundry Si3N4 which avoids the need for customized fabrication processes and highly nonlinear TeO2 that is deposited at a low temperature by reactive radio frequency magnetron sputtering which is compatible with back-end-of the line CMOS processing. Importantly, the required anomalous dispersion is attained by adding the TeO2 coating which also helps to enhance waveguide nonlinearity owing to its higher nonlinearity than Si3N4. In addition, the TeO2 can host rare-earth dopants for waveguide amplifiers and lasers, offering the potential for monolithic linear, nonlinear, and active functionalities in the same platform. Chapter 1 presents an overall discussion on material platforms that have been studied for integrated nonlinear photonics in comparison to Si3N4 and TeO2. Chapter 2 covers the theoretical background of nonlinear optics, discusses the main nonlinear processes, nonlinear integrated devices under study, and their applications, and introduces the hybrid TeO2-Si3N4 platform. Chapter 3 presents an analytical and numerical study on waveguide nonlinearity enhancement and dispersion engineering. The results presented show enhancement of the nonlinear parameter for TeO2-Si3N4 waveguides of up to three times that of stoichiometric Si3N4 and calculated anomalous dispersion at near-infrared wavelengths for 400-nm Si3N4 coated with varying TeO2 thicknesses. Chapter 4 provides an experimental proof-of-concept for the hybrid TeO2 on a 400-nm-thick Si3N4 platform as a candidate for monolithic linear, nonlinear and active photonic circuits. Dispersion measurements results are presented showing anomalous dispersion for a 400 nm thick, 1.6 µm wide Si3N4 strip waveguide coated with a 424 nm TeO2 film, with values of ⁓25 and ⁓78 ps/nm∙km at 1552 nm for the fundamental transverse electric and transverse magnetic modes, respectively. Microring resonator frequency combs with up to 8 comb lines covering a 1000-nm wavelength span for a pump power of 251 mW coupled into the bus waveguide are presented. Multimode lasing and a net gain of 2.8 dB in an Er-doped TeO2-coated Si3N4 microdisk resonator and a 6.7 cm long paperclip waveguide, respectively, are also reported. Chapter 5 presents experimental results on supercontinuum generation where an octave-spanning supercontinuum covering wavelengths from 940 to 1930 nm is demonstrated at a low peak pump power of just 258 W for 100 fs pulses centered at 1565 nm. Chapter 6 covers a comprehensive analytical and numerical study of Raman amplification in the hybrid platform. The results are promising and show high potential for on-chip Raman amplification at reasonable powers, device footprint, and lower losses that can be reached by improving fabrication methods and waveguide designs. For example, for a projected loss of 0.01 dB/cm which is lower than our current figure but higher than the state-of-the-art Si3N4 loss a net gain of up to 10 dB is shown for 1 W pump power in a 1.3 m long TeO2-coated Si3N4 spiral waveguide. Chapter 7 summarizes and discusses the results presented and provides propositions for future work.