Optical-waveguiding Properties Of Single-crystal LiNbO₃ Microwires And Nanowires Synthesized By Hydrothermal Method

One-dimensional (ID) micro/nanostructures have been well-recognized from fundamental research to various practical applications as they have low dimensionality, tight optical confinement, and excellent waveguiding capabilities. Among them, nonlinear crystal microwires and nanowires (MNWs) exhibit high nonlinearity and benefit from negligible two-photo absorption and free-carrier effects, as well as a higher damage threshold, so they have potential in future fast response and high bandwidth photonic devices. Lithium niobate (LiNbO3, LN), one of the most important nonlinear crystals, has an excellent ferroelectricity, piezoelectric effect, Pockels effect, photoelasticity, and nonlinear optical polarizability. Moreover, it has been widely used in waveguiding, optical switching, optical modulation, and second harmonic generation. Recently, single-crystal LN MNWs have been synthesized and are expected to be very promising nanomaterials in various future applications. However, the synthesis methods for LN MNWs are still under improvement. Therefore, it is important to develop suitable MNW fabrication approaches and characterize the surface morphology and optical quality for future optical applications.In this work, we synthesized high quality single-crystal LN MNWs by using the hydrothermal method. Besides, we studied the optical-waveguiding properties of the MNWs, including the propagation loss and waveguide coupling abilities.In the first part of this work (Chapter 2), we introduced how to synthesis LN MNWs by using the hydrothermal method and we characterized the as-synthesized LN MNWs by using scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The obtained MNWs were 100 nm to 2 μm in diameter and 10 μm to 60 μm in length. The cross-section is quadrangular and the surface quality is excellent.A propagation-distance-dependent output measurement was introduced to characterize the propagation loss in LN MNWs in the second part of this work (Chapter 3). Typical losses of 0.15 dB/μm at a wavelength of 722 nm and 0.18 dB/μm at a wavelength of 532 nm in an 830-nm-diameter LN MNW were obtained by using the system. Compared with previous work on barium boron oxide (BBO) MNWs, the propagation losses of the LN MNWs obtained here are much lower, indicating that the LN MNWs synthesized using the hydrothermal method exhibit better propagating quality as nonlinear optical waveguides.We studied the optical waveguide coupling abilities of the LN MNWs in the third part of this work (Chapter 4). We cut a 980-nm-diameter,42-μm-long LN MNW into two parts and adjusted the coupling angle to approximately 100°. Then, the fiber taper was coupled to one part with coupling angle of 98°, we observed that light coming out from the other part still possessed a definite amount of intensity. This finding demonstrates that LN MNWs could couple light with a wide angle and possess easy interconnecting capabilities.Our work shows that LN MNWs possess excellent optical-waveguiding properties. Besides that their propagation losses are low, they can also be coupled with wide-angle and interconnected easily. As LN MNWs are attracting growing interest for future micro/nanoscale photonic devices, this work can serve as a reference for future LN-MNW-based applications in areas ranging from optical modulation to wavelength conversion and near-field imaging.