Guide reading
Recently, Professor Hu Xiaopeng and Xie Zhenda of Nanjing University and Professor Cai Xinlun of Sun Yat sen University have jointly developed the preparation technology of high-quality periodically polarized Lithium Niobate Thin film waveguide. The normalized frequency doubling efficiency in the communication band exceeds 80% of the theoretical value.
Research background
Lithium niobate crystal has wide transparent window, low absorption loss, high optical damage resistance, and large nonlinear optics, electro-optic, thermo optic and acousto-optic coefficients. It is the matrix material of integrated quantum communication and quantum computation, classical optical information processing, precise measurement and sensor. In the early development of integrated optics, lithium niobate chips have been used in electro-optic modulation optical information chips by using traditional titanium diffusion and proton exchange processes. In recent years, a historic breakthrough has been made in the preparation of lithium niobate on insulator (lnoi) materials and micro / nano photon structures. The basic obstacles to achieve better optical confinement performance and the effective thickness of lithium niobate film photon chip which is only hundreds of nanometers have been removed. At the end of 2017, Harvard University published a notice on its website entitled "now entering, lithium niobate Valley", in which it emphasized that "lithium niobate means to photonics, and silicon means to electronics." International competition in this field is growing.
Because the thickness of LiNbO 3 film is several hundred nanometers, compared with the traditional weak binding waveguide such as proton exchange, the optical field binding ability is increased by one order of magnitude, so the periodically polarized LiNbO 3 film waveguide can be used to achieve ultra-high efficiency nonlinear frequency conversion. Previously, the Marko Loncar research group of Harvard University developed a periodically polarized lithium niobate film frequency doubled waveguide with a normalized efficiency of 2200-2600% / (w · cm2). Although the efficiency is 20 times that of the traditional proton exchange waveguide, it is only 60% of the theoretical efficiency. The main reason is that the duty cycle of the ferroelectric domain deviates from the optimal value, and the ferroelectric domain is not uniform, so the membrane polarization technology has encountered a bottleneck.
innovation research
Hu Xiaopeng and others used the traditional electric field polarization technology in the polarization of lithium niobate film. In the 1990s, this technology was independently developed by Zhu shining and others of Nanjing University and successfully used in the polarization of bulk lithium niobate crystal. In order to obtain high-quality periodic polarization structure of lithium niobate thin films, the team developed three key technologies: one is to use multi pulse technology to control the transverse expansion of reverse ferroelectric domain during the polarization process, so that the duty cycle is close to the optimal value of 50:50; the other is to use frequency doubling confocal microscopy as a non-destructive observation method of ferroelectric domain morphology, to monitor the transverse expansion of domain in real time; the third is to collect The process of periodic polarization and then etching ridge waveguide is adopted, so that the ridge waveguide with good polarization quality can be etched in the ferroelectric domain area.
Fig. 1 (a) process flow diagram; (b) evolution diagram of transverse growth of reverse ferroelectric domain with the number of high voltage pulses; (c) enlarged diagram of reverse ferroelectric domain observed by frequency doubled confocal microscopy; (d) ridge waveguide is selected in the area with duty cycle close to 50:50 (red dotted box); (E) cross section diagram of lithium niobate film ridge waveguide.
Through the above optimization scheme, a 6 m m long and 4.3 μ m periodic poled LiNbO 3 thin film ridge waveguide was successfully fabricated. This waveguide is used for 1469.6nm frequency doubling of S-band communication, the normalized frequency doubling efficiency is 3061% / (w · cm2), and the measured efficiency is 83% of the theoretical efficiency. In addition, the half height width of the frequency doubling tuning curve is 4nm, which is close to the theoretical half height width of 3.2NM, indicating that the polarization quality is uniform at the length of 6mm waveguide. The development of this technology has laid a solid foundation for the development of high-performance lithium niobate integrated photon chip.
Fig. 2 (a) theoretical, experimental and modified normalized octave efficiency versus fundamental wavelength; and (b) output octave power versus fundamental power.
Recently, this achievement was published online on the title of "optimizing the efficiency of a periodically poled lnoi waveguide using in situ monitoring of the ferroelectric domains" in appl. Physics. Lett. 116, 101104 (2020)). Niu Yunfei, a Ph.D. student from the school of physics, Nanjing University, is the first author of this paper. Lin Chen, Chen Yan, Professor Zhang Yong and Liu Xiaoyue from Sun Yat sen University also made important contributions to this work. Hu Xiaopeng, Xie Zhenda and CAI Xinlun of Sun Yat sen University are the co authors of this paper. Academician Zhu shining carefully guided this work. Relevant research has been supported by Nanjing University excellence program, national key R & D program, natural science fund, Jiangsu Natural Science Fund leading technology basic research project and Guangdong key field R & D program.
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https://doi.org/10.1063/1.5142750
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