非线性自成像效应及其调控研究

The Talbot effect, a near-field diffraction phenomenon of periodic object, was first observed by H. F. Talbot in 1836 when he illuminated a diffraction grating with a white light source. Recent progresses on fundamental Talbot effect are made with, for example, single photons and entangled photon pairs, waveguide arrays, metamaterials, and electromagnetically induced grating. The effect is more than just an optical curiosity for physicists, and has led to a variety of applications, such as photolithography, optical testing, optical metrology, and array illuminator. The extension to X-ray and terahertz (THz) waves through Talbot interferometers is particularly useful because of the lack of efficient optics for these wavelengths. .Recently, we reported a novel nonlinear Talbot effect in periodically poled LiTaO3 (PPLT) crystals, where the self-imaging is formed by the generated second-harmonic (SH) waves instead of the fundamental input beam. The prerequisite condition to realize the effect is to have a periodic SH pattern, which is fulfilled by the periodic distribution of second-order nonlinear coefficients in the PPLT crystals. Besides no real grating used in the experiment, spatial resolution improvement by a factor of 2, due to frequency doubling, is powerful to high-resolution imaging, compared with the simple input-pump imaging. .In this project, we study the nonlinear Talbot effect in nonlinear photonic crystals (i.e. PPLN) and in LiNbO3 nonlinear waveguide arrays. Also, we will modulate the nonlinear Talbot effect through structure design, electro-optical effect and acousto-optical effect. Moreover, we will develop the imaging technique based on nonlinear Talbot interferometer for observations of the microstructure in PPLN crystals. Such fundamental research will conceptually extend the conventional Talbot effect and thus paves a way for new applications.

Talbot效应,又叫自成像效应,是指光照射到一个具有周期结构的物体时会在后面有限距离处出现物体自身的像。这是一种近场无透镜成像技术。Talbot 效应在实践中已经被广泛地应用于光刻、光学精密测量、光开关等众多领域。在基础研究中，这一概念也被拓展到了量子光学、波色-爱因斯坦凝聚等领域。申请人在国际上率先把非线性光学引入进来,在具有周期非线性系数分布的非线性光子晶体中实现了二次谐波Talbot效应，在成像质量方面有较大的提高。在前期研究基础上，本项目将研究非线性光子晶体和非线性光波导阵列这两个材料体系中的非线性Talbot效应，利用微结构材料设计、电光效应等手段实现对非线性Talbot效应的人工调控，同时探讨基于双光栅Talbot干涉仪的微结构成像技术。通过本课题的研究，可进一步开拓非线性Talbot效应在成像、无损探测、光阵列、光通讯、光计算等领域的应用。

本项目主要研究非线性自成像效应及其调控研究。计划执行期间，基本按申请书中所列的研究内容开展,并且根据目前国内外的研究发展状况以及项目中的新发现进行了很多拓展性研究。（1）在周期极化晶体中系统地研究了非线性自成像效应，包括正负畴产生的不同相位的二次谐波自成像及其叠加，利用准相位匹配技术大大提高了自成像的成像质量，利用声光效应调制非线性自成像效应等。我们在这方面的工作受到OSA的综述期刊Advances in Optics and Photonics的关注，主编Bahaa E. A. Saleh邀请我们写了一篇关于Talbot自成像效应的综述，已经发表；（2）利用非线性光学方法在周期极化晶体中实现超聚焦。我们在非线性Talbot效应的研究中发现了超聚焦效应，其本质是基于干涉衍射的效应。原理可以看作是两种理论的叠加：a. 周期极化晶体中正畴和负畴产生的二次谐波有pi的相位差，会造成二次谐波之间的相消干涉，最终产生突破衍射极限的聚焦点。这种原理类似于子衍射理论；b. 在倍频过程中，相位匹配的要求使得产生的二次谐波自动有带宽限制，这满足了超振子效应。以上两种效应的叠加最终在非线性自成像中形成了突破衍射极限的聚焦点；（3）在光子轨道角动量光束的非线性光学转换方面进行了拓展研究，包括在两维周期极化晶体中实现了光子轨道角动量的复制，在准周期极化晶体中实现了涡旋光束的级联三倍频，在金属纳米孔阵中实现了plasmon增强的涡旋光束倍频等。项目执行期间，在Adv. Opt. Photon.，Phys. Rev. B，Appl. Phys. Lett.，Opt. Lett.，Opt. Express，Sci. Rep.等SCI期刊发表与项目有关的SCI论文13篇。