基于介电常数近零模式与间隙表面等离激元强耦合的增强非线性光学效应

Enhanced nonlinear optical effects based on strong coupling between epsilon-near-zero mode and gap surface plasmons

非线性光学效应在光通信、光探测、量子信息等领域发挥着举足轻重的作用,然而天然材料的光学非线性响应通常很弱.本文利用氧化铟锡(ITO)薄膜激发的介电常数近零模式,与金属-介质-金属结构激发的间隙表面等离激元发生强耦合,在近红外波段实现宽带(约1000 nm)的增强非线性光学效应,其非线性折射率n_(2)的最大值可达3.02 cm^(2)/GW,与之前报道的单层ITO薄膜的非线性折射率相比,增大了接近3个数量级.因此,可在低功率光照下得到显著的折射率变化,可望应用于全光存储、全光开关等纳米光子器件的设计.

Optical nonlinear effect plays an important role in optical communication,optical detection,quantum information and other areas.However,it is constrained by the weakness of the nonlinear optical response of the common materials.The enhancement of the optical nonlinear response on a nanoscale becomes a critical challenge.Over the years,several ways to enhance the optical nonlinear effects have been suggested.In fact,these technologies can slightly enhance the optical nonlinear response.Recently,some research groups focused on the materials with vanished permittivity,which is called epsilon-near-zero(ENZ)material,showing that it can exhibit large optical nonlinearity due to the field enhancement in the material of this type.However,the ENZ material only holds a large optical nonlinear response in a limited spectral range.In order to overcome this limitation,here in this paper we report the ENZ mode which is excited by the ITO film and strongly coupled to the gap surface plasmons excited by the metal-dielectric-metal structure.To acquire the nonlinear refractive index n2,we first calculate the ITO permittivity through the Drude-Lorentz model and find the wavelength of the ENZ material.Then we calculate the time-dependent electron temperature and lattice temperature of ITO by the two-temperature model.According to the elevated electron temperature,we can calculate the plasma frequencyω_(p),and by taking it into the Drude-Lorentz model,we can obtain a new permittivity of ITO compared with the initial one.Finally,we can calculate the variation of the refractive index△n,and the nonlinear refractive index n_(2)=△n/I_(0).In this paper,our coupled structure exhibits a broadband(~1000 nm bandwidth)enhancement of the nonlinear optical effect in the near-infrared spectrum,a maximum nonlinear refractive index n_(2) as large as 3.02 cm^(2)·GW^(-1),which is nearly 3 orders larger than the previously reported nonlinear refractive index of bare ITO film.As a result,it is possible to realize a dramatically large variation of nonlinear refractive index under a low-power optical field.It is expected to be used in the nano photonic devices such as optical storage,all-optical switches,etc.