Nonlinear Optics using Epsilon Near Zero Materials

该项目将围绕使用ENZ(Epsilon Near Zero)材料的变形表面进行设计，例如ITO作为连接到天线阵列的薄膜衬底。ENZ在材料的介电常数小于1的光谱范围内显示出有趣的特性。由于其独特的性质，ENZ在线性和非线性的许多光学应用中都是非常有用的。例如，在ENZ光谱范围内，通过材料的相位传播很小，消除了对非线性光学过程的相位匹配的需要。另一个例子是，介质内增强的电场强度意味着，通过将其与天线阵列耦合，可以产生需要相对较低的辐照度来观察显著依赖于强度的折射的样品。在这个项目中，将使用Lumerical进行FDTD模拟。从而允许有效地设计变形表面的几何形状，使得其可以被设计成给出定制的光学响应，例如光束操纵。这还将允许调整ITO的厚度，并使天线和ITO之间的耦合达到最佳。ITO薄膜将采用溅射沉积的方法在玻璃上沉积。所沉积的膜的厚度将决定膜可以支持的模式。ENZ支持状态密度较大的光学模式。所关注的模式是费雷尔-贝雷曼模式，辐射体积等离子体的自由空间激发，以及该模式如何与天线共振耦合。制作的样品的光学线性和光学非线性将通过透过率测量来表征。将使用诸如z扫描技术之类的方法来表征光学非线性。许多天线都使用了具有等离子体共振的材料。它们是通过电子束光刻，然后金属蒸发和剥离来制造的。通常使用金和银等材料。它们是一个很受欢迎的选择，因为它们有很大的负介电常数，这使它们成为非常有效的散射体。然而，由于他们的强烈吸收，他们在如何有效地做到这一点上受到限制。早期制造的样品将使用金作为天线阵列，但后来的目标是制造具有耦合到ITO的介质天线的样品。使用介质天线而不是金属天线应该会减少吸收，从而提高效率。该项目的另一个目标是制造能够进行波束控制的亚表面。在ITO上将制造V型天线，以便可以操纵入射高斯光束的相位，以将光束引导到所需的方向。天线形状的双谐振特性允许完全的相位控制。天线相对于电场的方位将决定共振是对称的还是反对称的。对于两种不同的谐振，通过臂的电流近似为流经不同直天线长度的电流。天线将被设计成在保持散射幅度恒定的同时，由于每个天线引入的突然相位变化，在表面上存在相位梯度。

This project will centre around designing metasurfaces which use an ENZ (Epsilon Near Zero) material, such as Indium Tin Oxide (ITO) as a thin film substrate which is coupled to an antenna array. ENZs display interesting properties within the spectral range where the permittivity of the material becomes less than one. ENZs are very useful in many optical applications both linear and nonlinear due to their unique properties. For example, In the ENZ spectral range phase propagation through the material is small removing the need for phase matching of nonlinear optical processes. Another example is how enhanced electric field intensity within the medium means that by coupling it with an antenna array a sample can be produced which requires relatively low irradiance to observe significant intensity dependent refraction. Within this project FDTD simulations will be made using Lumerical. Allowing effective design of the geometries of metasurfaces such that it can be designed to give a tailored optical response such as beam steering for example. This will also allow for the thickness of the ITO to be tuned and the coupling between the antennas and the ITO to be optimal. Thin films of ITO will be deposited on glass using spluttering deposition. The thickness of the film which is deposited will determine the modes which can be supported by the film. ENZs support optical modes with a large density of states. The mode of concern is the Ferrel-Berreman mode, free space excitation of radiative volume plasmons, and how this mode couples with the antenna resonance. The optical linearity and optical nonlinearity of samples which are fabricated will be characterised through transmission measurements. The optical nonlinearity will be characterised using methods such as the z-scan technique. Many have utilised materials with plasmonic resonances for their antennas. These are fabricated by using electron-beam lithography followed by metal evaporation and lift off. Usually using materials such as gold and silver. They are a popular choice because of their large negative permittivity which makes them very effective scatters. However, they suffer from limitations in how efficiently they do this due to their strong absorption. Early samples that are fabricated will use gold for the antenna arrays but later the aim is to fabricate samples which have dielectric antennas coupled to the ITO. Using dielectric antennas as opposed to metallic ones should reduce absorption leading to improved efficiency.Another goal within this project is to fabricate a metasurface which is capable of beam steering. V shaped antennas will be made on the ITO such that the phase across incoming gaussian beam can be manipulated to steer the beam in a desired direction. The double resonance properties of the antenna shape allow for full phase control. The orientation of the antennas with respect to the electric field will dictate whether the resonance is symmetric or antisymmetric. The current through an arm approximates to current through different straight antenna lengths for the two different resonances. The antennas will be designed such that there is a phase gradient across the surface, due to the abrupt phase change each antenna introduces, while keeping the scattering amplitude at a constant.