Research Of Bound States In The Continuum Enhanced Second-Harmonic Generation In Thin-Film Lithium Niobate

When monochromatic light with frequency ν is incident into the nonlinear medium,the light with frequency 2ν will be output if appropriate conditions.This is the second harmonic generation(SHG),which is also called optical frequency doubling.The optical second harmonic is a very important concept in nonlinear optics.It has good application in the field of microscopic imaging and extended laser wavelength.But the nonlinear optical effects are generally weak,it needs to choose appropriate material and achieve more demanding conditions to realize efficient second harmonic conversion.The lithium niobate(LN)that has appreciable second-order nonlinearity coefficient and is optical transparency in the broad spectral range from visible to middle infrared,plays a leading role in the nonlinear optical materials.Thus,the LN is suitable for efficient second harmonic generation.But traditional bulk LN devices require considerable interaction length and satisfy phase match for efficient second harmonic generation,it restricts the application in practical production.The thin-film lithium niobate on insulator(LNOI)has already industrialized with the technological breakthrough.The thin-film LNOI can provide higher optical mode confinement in comparison to bulk LN devices to decrease interaction length,also can avoid second harmonic conversion efficiency decline due to phase mismatch to some extent.And the thin-film LN is beneficial to promote integration level.Therefore,it has more practical significance to do research into SHG on the thin-film LNOI platform.Nowadays,researchers have conducted some studies to enhance second harmonic generation on LNOI.But,the conversion efficiency of second harmonic generation is still limited.We think about combining thin-film LN with the bound states in the continuum(BIC),and utilize the high quality factor resonance of the bound states in the continuum to realize the enhancement of the second harmonic generation.In this work we design a periodic structure in z-cut thin-film LN to support the symmetryprotected bound states in the continuum.Because of the bound states in the continuum,the mode distribution can be better limited and radiation loss of pump wave can be inhibited,which produce higher near-field enhancement in nanostructure.When the symmetry has been broken,the symmetry-protected bound states in the continuum degrade into quasi-bound states in the continuum(Q-BIC),the mode quality factor decreases,thereby matching with the line width of pump wave that make power coupling in the nonlinear process more easily,in addition,it is keep high quality factor yet.The main research work contains that: 1.We use the broadband supercontinuum laser source to measure the transmission spectra of the LN metasurface to get the dispersion bands,and analyze the correspondence between the resonance characteristic in transmission spectra and the mode of the bound states in the continuum,indicate that the high quality resonance of transmission spectra appears due to the bound states in the continuum 2.We lead the femtosecond laser to the structure area to evaluate the enhancement of the second harmonic generation and analyze the dispersion bands of second harmonic.Then we find that the dispersion bands of second harmonic signal match with witch of the transmission spectra,and the second harmonic in the structural area has significantly increased.Finally,we studied the far-field diffraction image characteristics of the second harmonic signal.The results of this work will provide reference for engineering compact coherent light source in a broad wavelength range.