Phase matching or quasi-phasematching(QPM)is of significant importance to the conversion efficiency of second harmonic generation(SHG)in artificial nonlinear crystals like lithium niobate(LN)crystal or microstructured nonlinear crystals like periodic-poled lithium niobate(PPLN)crystals.In this paper,we propose and show that the incident angle of pump laser light can be harnessed as an alternative versatile tool to engineer QPM for high-efficiency SHG in a PPLN crystal,in addition to conventional means of period adjusting or temperature tuning.A rigorous model is established and analytical solution of the nonlinear conversion efficiency under the small and large signal approximation theory is obtained at different incident angles.The variation of phase mismatching and walk-off length with incident angle or incident wavelength are also explored.Numerical simulations for a PPLN crystal with first order QPM structure are used to confirm our theoretical predictions based on the exact analytical solution of the general large-signal theory.The results show that the narrow-band tunable SHG output covers a range of 532 nm–552.8 nm at the ideal incident angle from 0°to 90°.This theoretical scheme,fully considering the reflection and transmission at the air-crystal interface,would offer an efficient theoretical system to evaluate the nonlinear frequency conversion and help to obtain the maximum SHG conversion efficiency by selecting an optimum incident wavelength and incident angle in a specially designed PPLN crystal,which would be very helpful for the design of tunable narrow-band pulse nanosecond,picosecond,and femtosecond laser devices via PPLN and other microstructured LN crystals.
Nonlinear holography has recently emerged as a novel tool to reconstruct the encoded information at a new wavelength,which has important applications in optical display and optical encryption.However,this scheme still struggles with low conversion efficiency and ineffective multiplexing.In this work,we demonstrate a quasi-phasematching(QPM)-division multiplexing holography in a three-dimensional(3D)nonlinear photonic crystal(NPC).3D NPC works as a nonlinear hologram,in which multiple images are distributed into different Ewald spheres in reciprocal space.The reciprocal vectors locating in a given Ewald sphere are capable of fulfilling the complete QPM conditions for the high-efficiency reconstruction of the target image at the second-harmonic(SH)wave.One can easily switch the reconstructed SH images by changing the QPM condition.The multiplexing capacity is scalable with the period number of 3D NPC.Our work provides a promising strategy to achieve highly efficient nonlinear multiplexing holography for high-security and high-density storage of optical information.
Quasi-phase-matching(QPM)has become one of the most common approaches for increasing the efficiency of nonlinear three-wave mixing processes in integrated photonic circuits.Here,we provide a study of dispersion engineering of QPM second-harmonic(SH)generation in stoichiometric silicon nitride(Si3 N4)waveguides.We apply waveguide design and lithographic control in combination with the all-optical poling technique to study the QPM properties and shape the waveguide dispersion for broadband spectral conversion efficiency inside Si3 N4 waveguides.By meeting the requirements for maximal bandwidth of the conversion efficiency spectrum,we demonstrate that group-velocity matching of the pump and SH is simultaneously satisfied,resulting in efficient SH generation from ultrashort optical pulses.The latter is employed for retrieving a carrier-envelope-offset frequency of a frequency comb by using an f-2 f interferometric technique,where supercontinuum and SH of a femtosecond pulse are generated in Si3N4 waveguides.Finally,we show that the waveguide dispersion determines the QPM wavelength variation magnitude and sign due to the thermo-optic effect.
EDGARS NITISSBORIS ZABELICHOZAN YAKARJUNQIU LIURUI NING WANGTOBIAS J.KIPPENBERGAND CAMILLE-SOPHIE BRÈS
The quasi-phase-matching(QPM)technique has drawn increasing attention due to its promising applications in areas such as nonlinear frequency conversion for generating new laser light sources.In this paper,we will briefly review the main achievements in this field.We give a brief introduction of the invention of QPM theory,followed by the QPM-material fabrication techniques.When combing QPM with the solid-state laser techniques,various laser light sources,such as single-wavelength visible lasers and ultraviolet lasers,red–green–blue three-fundamentalcolor lasers,optical parametric oscillators in different temporal scales,and passive mode-locking lasers based on cascaded second-order nonlinearity,have been presented.The QPM technique has been extended to quantum optics recently,and prospects for the studies are bright.
Based on the second-order nonlinearity, we present a bidirectional tunable all-optical switch at C-band by introducing backward quasi-phase-matching technique in Mg-doped periodically poled lithium niobate （MgO：PPLN） waveguide with a nano-structure called multiple resonators. Two injecting forward lights and one backward propagating light interact with difference frequency generations. The transmission of forward signal and backward idler light can be modulated simultaneously with the variation of control light power based on the basic ＂phase shift＂ structure of a single resonator. In this scheme, all the results come from our simulation. The speed of this bidirectional optical switch can reach to femtosecond if a femtosecond laser is used as the control light.
