Hybrid thin-film lithium niobate-silicon nitride photonics
Date
2020
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Lithium niobate (LiNbO3) has been a widely popular electro-optic (EO) material since 1970. Large electro-optic coefficients and lower third-order nonlinearity compared to other III-V materials (e.g., InP, Si) make LiNbO3 an ideal candidate for active photonic devices. Although indium phosphide (InP) and silicon (Si) platform-based foundries are already established, a LiNbO3-based foundry has yet to be established. Recent developments in crystal ion-sliced (CIS) thin-film lithium niobate on insulator (TFLNOI) have enabled a new class of electro-optic modulators with tighter mode confinement, a compact footprint, ultra-high bandwidth, and low switching voltages. With this advance in thin-film technology, photonic integrated circuits (PICs) in the LiNbO3 platform can now be realized, paving the way for future LiNbO3 platform-based foundries. However, LiNbO3 suffers from troublesome micro-structuring in comparison to silicon-based materials, and etching of LiNbO3 is not compatible with foundry fabrication. To address this challenge, a hybrid material system combining the electro-optic properties of TFLNOI with the ultra-low propagation loss of silicon nitride (Si3N4) has been proposed and developed. An etchless LiNbO3 modulator fabrication is possible using this hybrid platform. In addition, this hybrid platform addresses critical figures of merit, such as size, weight, power consumption, cost, and performance (SWaP-CP). This dissertation discusses the theory, design, simulation, fabrication, and characterization of different essential PIC devices on a foundry-compatible hybrid Si3N4-LiNbO3 platform. ☐ The main building blocks of PICs, such as a single transverse electric (TE) mode optical waveguide, compact waveguide bending, 1×2 Multi-mode Interference (MMI), Mach-Zehnder Interferometer (MZI) and high-quality factor (Q) micro-ring resonator are demonstrated in this novel hybrid platform (Si3N4-LiNbO3). Then using the electro-optic properties of the TFLNOI, we show a record-low half-wave voltage MZI modulator, a single-arm phase modulator, and a high-performance micro-ring modulator. ☐ The hybrid Si3N4-LiNbO3-based waveguide is formed by loading a Si3N4 strip on an EO material of X-cut thin-film LiNbO3. A low-voltage operable electro-optic modulator is critical for applications ranging from data transmission to analog photonic links. Using this proposed hybrid platform, we have demonstrated a subvolt EO modulator, a milestone in LiNbO3-based EO modulators. A single drive push-pull configuration Mach-Zehnder modulator with an interaction region length of 2.4 cm demonstrates a DC half-wave voltage of only 0.875 V, which corresponds to a modulation efficiency per unit length of 2.11 V·cm. The best device demonstrates a static extinction ratio of ~27 dB, 3 dB EO bandwidth of at least 29 GHz, and an on-chip optical loss of ~1.53 dB. ☐ The developed hybrid Si3N4-LiNbO3 micro-ring resonator exhibits a high intrinsic quality factor of 1.85 × 105, a resonance extinction ratio of ~27 dB within the optical C-band, and direct current (DC) tunability of 1.78 pm/V near the 1550 nm wavelength. The micro-ring modulator’s DC and radio-frequency (RF) performance are significantly improved by modifying the electrical and optical designs. The fabricated device has a DC tunability and an intrinsic quality factor of 2.9 pm/V and 1.3 × 105, respectively. The modified micro-ring modulator exhibits enhanced modulation efficiency (>10 dB) at modulation frequencies that match the racetrack’s optical free spectral range (FSR). This is the first example of an FSR-coupled micro-ring modulator in this hybrid platform. ☐ Finally, an alternative method (bonding) is demonstrated to realize this hybrid platform to allow for selectively placing the TFLNOI on PICs of other materials (e.g., Si, Si3N4, InP). In this dissertation, a gradual mode transition structure is proposed to efficiently guide the mode from a Si3N4 waveguide to a hybrid Si3N4-LiNbO3 waveguide. The proposed transition structure is applied to demonstrate a hybrid Si3N4-LiNbO3 micro-ring resonator using the bonded method.
Description
Keywords
Analog photonics, Lithium niobate, Modulator, Ring resonator, Silicon photonics, Thin film