一、题目：热控特异电磁介质与光学微操控技术的理论研究
二、主讲人：俞昕宁
三、时间：2020年12月10日（周四），上午09:30
四、报告方式：钉钉在线, 请扫码进群
内容摘要：
特异电磁介质(Metamaterials)是一种人造亚波长单元阵列成的材料。传统材料的电磁特性源自原子和分子，而特异电磁介质给予我们设计亚波长尺度的“原子”[1]。因此特异电磁介质能够实现诸如电磁波隐身、负折射材料、零折射材料、超分辨率等等性质[2-7]。除了特殊电磁特性，为了增加其在设计和应用中的灵活性与自由性，可调控特异电磁介质也被提出并实现[8-11]。研究生期间，我在微波频段研究了温度调控磁性特异电磁介质以及其在零折射率的电磁特性。基于等效介质理论、Mie散射理论和多重散射理论[12; 13]，我们得到了三角晶格阵列成的单晶钇铁石榴石(YIG)柱的温度、饱和磁化强度与等效折射率关系。我们将YIG柱构成等腰直角三角形，通过调节温度实现了等效折射率从负折射到零折射到正折射的连续调控[14]。随后，我们通过设计和优化磁性特异电磁介质实现了阻抗匹配的零折射材料, 多重散射理论的数值模拟表明该体系可以实现电磁波的高效率无相位延迟下的传输[15]。基于零折射磁性特异电磁介质设计不同的输出界面能够实现不同的相位控制，调控电磁波前。
电磁学领域另一个研究热点是光学微操控技术。从1970年Ashkin的开创性工作[16]以来历经半个世纪的发展，光学微操控技术在物理、化学和生物等领域得到了广泛应用[17-20]。尤其是1986年诞生的单光束梯度力捕获技术，也就是光镊技术，成为了20世纪90年代以来化学和生物的单分子实验中不可或缺的技术手段，Ashkin也因此于2018年获得诺贝尔物理学奖[21]。光镊诞生三十年以来，研究者普遍采用的几何光学模型[22]和偶极模型[23]都是近似模型，只适用于特别大或小的微粒受到的光力分解，对于近几年在光学实验中广泛关注的Mie粒子(r≈λ)无能为力。广义Mie散射理论虽然能够严格求解任意尺寸粒子在光场中受到的光力，但是其将电磁场用分波系数展开的方法隐藏了场量与光力的关系，导致无法将光力分解为梯度力和散射力[24-26]。为此，我们根据发展完成的局域解析方法[27; 28]，将平面波作用于粒子受到的光力从偶极模型(l=1阶)，推广到任意阶次l，给出了任意阶次光力与电磁场关系的解析式[29; 30]。
我们利用极化效应调控三束平面波构成的光晶格，基于平面波的多极展开法将分解得到的梯度力、散射力与光场模式进行比较，得到光力与光场之间的内在联系。我们发现不同极化下梯度力和散射力的模式一致，但是量级不同，从而导致总光力的模式不同。由此我们可以改变极化控制光力的捕获和释放过程[31]。同样，基于多极展开法，我们将贝塞尔光束作用于球形粒子的光力分解为梯度力和散射力，并分析贝塞尔光束的电磁场量与光力的关系。我们严格证明了径向光力是梯度力，而角向和纵向光力是散射力[32]。此外，基于Mie 理论，我发展了求解任意波矢量为复数电磁场的分波系数的解析方法。该方法避免了之前工作中使用的投影法需要计算数值积分的繁琐过程[33; 34]，可以用于计算衰逝场、表面等离激元场等等[35]。
主讲人简介：
俞昕宁，男，1988年生，浙江宁波人。现为复旦大学物理系博士后(数学系流动站进站)。
2011年毕业于浙江师范大学物理系，学士学位。2015年毕业于浙江师范大学光学系，硕士学位，研究电磁波在特异电磁介质的传播，导师刘士阳教授。2019年毕业于复旦大学物理系，博士学位，研究光学微操控技术，导师林志方教授。2019年至今，复旦大学博士后，继续研究光学微操控技术，导师林志方教授，合作导师吴泉水教授。获2016–2017 学年复旦大学优秀学生，2019年上海市‘超级博士后’资助。
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