Thin-Film Lithium Niobate Technology
Lacking inversion symmetry in the crystal structure of LiNbO3 manifest itself as the material's strong piezoelectric and converse-piezoelectric effect, pyroelectric effect, electro-optic effect, optical nonlinearity and photorefractive effect. In addition, the material exhibits low acoustic loss, and low optical loss in the infrared range. In this work, I investigated the possibility of combining these properties with the highly versatile micro-fabricated structures on a layer transferred LiNbO3 thin film platform to achieve novel devices or improve the performance of existing device technologies. First, I focused on components for multi-frequency RF front-end applications. Specifically, I demonstrated LiNbO3 thin-film contour mode resonators (CMRs) for multi-frequency wide bandwidth RF filters. The resonators exhibit mechanical Q ranging from 1,000 to over 3,000, and kt2 ranging from 4% to 15%. The highest kt2 xQ demonstrated is 194. Along with the high Q and high kt2 , comes with the low motional impedance, which is important for achieve low filter insertion loss. In addition, the resonator frequencies are defined by photolithography, thus enabling devices with different frequencies to be integrated on a single chip. On a single chip, I demonstrated resonators with frequencies ranging from 400MHz to 2GHz. As the achievable bandwidth of the filter is ultimately limited by the electro-mechanical coupling factor (kt2 ) of the resonators, while the roll-off is determined by resonator quality factor (Q ). Therefore, LiNbO3 thin-film CMR technology shows great potential in achieving filters with wide bandwidth, steep roll-off and low insertion loss. In the meantime, I also investigated low temperature coefficient of frequency (TCF) LiTaO3 thin-film resonators for oscilla- tor applications, another key component for multi-frequency RF front-ends. As LiTaO3 also exhibits low acoustic loss and strong piezoelectric effect, LiTaO3 CMRs also have high mechanical Q and kt2 , which is important to achieve low oscillator phase noise, jitter and good power handling capability. With the capability of multi-frequency integration, I demonstrated resonators with frequencies ranging from 500MHz to 900MHz and a TCF as low as 16ppm/K and Q as high as 3200. In addition, these devices exhibit low motional impedance (10-20), which is important for the power handling for oscillator application. While commercial markets demand multi-frequency band-select duplexer and diplexer filters, with fractional bandwidth (BW) ranging from 3% to 10%, steep filter roll-off and low insertion loss; components for reconfigurable RF front-ends are actively pursued for defense applications. In this work, frequency programmable RF MEMS resonators for reconfigurable RF front-ends is investigate. Exploiting tip based piezoelectric domain engineering in LiNbO3 thin film, the mechanical resonance coupled by the electrode transducers can be programmed. Therefore, resonator frequency can be "written" and "erased". Particularly, a 681MHz resonance peak is "written" on the programmable resonator and "erased". Finally, LiNbO3 thin-film disk resonators are demonstrated exhibiting intrinsic optical quality factor of 484k, which is highest demonstrated from thin-film LiNbO3 resonators to date. Taking advantage of the high quality factor, acousto-optic modulation is demonstrated. As a demonstration of concept, electrical field is applied to the photonic disk resonator through a metal probe, and causes modulation of the optical carrier. A 35dBm beat note between the optical carrier and the modulated side band was generated from the photodetector. The thin-film LN platform can open up paths to many novel and high performance devices. Presented here are building blocks for RF and photonic applications. Combin- ing mechanical resonators with optical ones, acousto-optic modulator is also demonstrated. In parallel, leveraging piezoelectric domain engineering, reconfigurable RF front-end can also be achieved. Moving forward, other novel devices (chip-scale optical frequency combs, high efficiency chip scale optical frequency doubling and THz generation) can be achieved leveraging other multi-domain coupling properties of LiNbO3 with the building blocks demonstrated here.
thin-film lithium niobate; contour mode resonator; photonic resonator
Bhave, Sunil A.
Pollock, Clifford Raymond; Lal, Amit
Ph.D. of Electrical Engineering
Doctor of Philosophy
dissertation or thesis