Micro-nano Photonic Devices Based On Lithium Niobate On Insulator

Traditional chip systems with electrons have brought people into the era of information.After decades of booming growth,it is increasingly difficult to improve the storage density and computing speed of information carriers as Moore's law and Shannon's theorems are gradually close to the limit.On the other hand,with the new era of Internet of things?IoT?and 5G emerging,people have higher requirements for signal transmission,modulation and detection.Compared with electrons,photons have shorter response time and have the advantage of large capacity and low crosstalk in information transmission.With the development of micro-nano processing technology,photon-mediated integrated optical chips are gradually coming into practice.Silicon-based photonic chip technology has been relatively mature,and gradually appeared in the large-scale commercial stage.Lithium niobate is awarded as“silicon of photonics”.It is an excellent substrate material for optical chips because of its good response to light,sound,electricity and heat.With the appearance of lithium niobate on insulator?LNOI?,it is possible to bind the light field in the structure of submicron scale,which leads to a revolution in lithium niobate field.In this paper,we first briefly introduce the properties of lithium niobate in various aspects,and then focus on the review of micro-nano devices based on LNOI and introduce their fabrication methods.The existing semiconductor processing technology has made important progress in the patterning,etching and metal film deposition,which lays a solid technological foundation for the direct fabrication of lithium niobate based integrated optical chips in the future.Femtosecond laser is an important method of precision machining because of its extremely high peak power and extremely small heat-affected area.Waveguide and microcavity are important components in photonic chips.In the second chapter we use femtosecond laser to inscribe type II waveguide in bulk lithium niobate.At the same time by combing femtosecond laser and focused ion beam we fabricated microdisk resonators with the quality factor of up to 10⁵ both in lithium niobate and lithium tantalate.Furthermore,the quality factor of the microcavity was improved by an order of magnitude by using femtosecond laser-induced photo-annealing process to restore the damaged lithium niobate lattice.Nonlinear module is also an important building block in the integrated photonic chip.In chapter 4,the nonlinear phase-matching methods in waveguides and microcavities are summarized.In waveguides,the quasi-phase-matching is realized mainly through periodic modulation of nonlinear coefficients or light intensity to obtain efficient frequency doubling output.While for microcavities the phase-matching condition is relatively loose due to strong enhancement of the light field,the nonlinear process can be realized through natural-phase-matching or cyclic-phase-matching.Experimentally,we realized second harmonic generation in the lithium niobate microcavity with the efficiency of about 2.36×10^-6/8)(2 and 1.22×10^-6/8)(2in lithium tantalite microdisk.It is also an important part of the research of integrated optical chips to take advantage of combined materials taking full play to their advantanges.Another important consideration for nonlinear process is the bandwidth.Broadband nonlinear processes have important applications in ultra-short pulse frequency conversion,wavelength division multiplexing systems and even future quantum communication networks.In chapter 5,we theoretically analyze the need to satisfy both phase matching and group velocity matching to achieve broadband frequency conversion.We introduce another dimention of dispersion engineering by thinning the bulk periodically poled lithium niobate to sub-micro scale.In that case group-velocity-matching and phase-matching can be simultaneously fulfilled.We theoretically give the relationship between central wavelength and film thickness.Experimentally,a periodic polarized lithium niobate film with a poling period of 20?m and a thickness of 700nm was prepared.By using fifth-order phase-matching,the second harmonic output with a bandwidth of about 15nm was obtained in the communication band.