| Whispering gallery mode(WGM)microcavities,which benefit from their ultra-high quality(Q)factor and small mode volume(V),can significantly enhance the interaction of light and matter,making them important in both fundamental physics and photonic applications.In recent years,the WGM microcavity has been widely used and developed in the fields of nonlinear optics,optical sensing,cavity mechanics and cavity quantum electrodynamics.Because its light enhancement characteristics are very suitable for the study of nonlinear optics,it has opened up a new path to study nonlinear characteristics under low pump optical power.Nonlinear research hotspots based on WGM microcavities include some basic nonlinear effects such as Pockels and Kerr effects,in addition to harmonic generation,four-wave mixing effects,and stimulated scattering.In addition,more complex nonlinear processes can be achieved by cascading nonlinear effects.As an excellent nonlinear optical material,lithium niobite has large second-order nonlinear coefficients,a very wide transparent window,and relatively high refractive indices.The development of lithium niobate on insulator(LNOI)technology has made it a great candidate in the field of integrated optics.In recent years,research based on LNOI has become a new hot spot,and the etching process has matured.Basic nonlinear process studies,such as second harmonic generation,sum frequency generation and parametric down conversion based on LNOI integrated WGM microcavity,have been reported.After cascading multiple nonlinear and electro-optic modulation,third harmonic,four-wave mixing,and optical frequency comb can also be realized.Although the dry etching process has been broken through,various processes have their own characteristics and scopes of application.Focused ion beam(FIB)etching and reactive ion etching(RIE)have been studied in this paper.The advantage of the FIB etching method is that it can be patterned by direct writing.By FIB etching,we obtained a LNOI microdisk with a high quality factor(10~5).Large-area preparation of photonic devices requires the use of electron beam lithography to prepare a mask,followed by RIE.Based on this technique,we have made gratings,waveguides and microrings on LNOI.In the LNOI microdisk,we obtained experimental high-efficiency second harmonic generation(SHG),cascaded third harmonic generation and cascaded four-wave mixing(FWM)effects.In the telecommunications band,efficient second harmonic in the visible range can be obtained with only a few milliwatts of pump power.After increasing the pump power,we achieved third harmonic by cascading nonlinear processes.When double pumping is used,effectiveFWM is achieved via quadratic nonlinear processes of SHG and difference-frequency generation.The cascading process effectively compensates for the shortcoming of the small third-order nonlinear coefficient.The FWM can also be used in optical parametric amplification.Through these nonlinear processes,the phase matching conditions and the mode coupling theory in cavity are further verified.Our research has greatly enriched the way of frequency conversion on integrated lithium niobate chips.In addition,we have also fabricated a double-layer lithium niobate microdisk whose mode energy is mainly confined outside the microcavity.The double-layer microdisk can achieve large mode overlapping with the ambient environment,as well as strong electric field and gradient force at surface.The exterior WGM possesses high quality factors in exceed of10~5 without metallic loss.The geometry is based on the horizontal slot-waveguide structure of two vertically stacked crystalline microdisks made of lithium niobate on insulator nanofilms.The loss is reduced by use of the crystalline nanofilm at sub-nm rms surface roughness for low scattering and absorption.The exterior slot WGMs based on the demonstrated configuration and material can be highly favored in various applications including optical sensing,surface optical nonlinearity,and optomechanics. |
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