Nanoplasmonic magneto-optical isolator

Fig. 1. (a) Nanoplasmonic isolator which consists of a cylindrical cavity with radius R placed close to an MDM waveguide with width W. The metal and MO material are shown with blue and orange colors, respectively. The structure is under a static magnetic field in the z direction. (b) Cross-sectional view of the structure at the z=0 plane.

Optical isolators are nonreciprocal devices that allow light to pass in one direction but block light in the opposite direction. They are typically used to prevent unwanted back reflections into optical oscillators such as lasers, and to suppress crosstalk between different optical devices. Nonreciprocal optical elements such as circulators and isolators are essential for the realization of integrated optical circuits. The design of nonreciprocal components requires breaking the time reversal symmetry. This can be achieved through the use of nonlinear materials, materials with time-dependent properties, and magneto-optical materials. However, since the magneto-optical response of natural materials is weak at optical wavelengths, designing nonreciprocal devices that are based on magneto-optical materials results in bulky structures that are much larger than the wavelength. The advent of silicon photonics and photonic crystals has reduced the size of nonreciprocal optical components down to wavelength scale.

To further decrease the size down to the subwavelength scale, one needs to beat the diffraction limit. Nanoscale metallic structures that support surface plasmon polaritons can be used to achieve subwavelength scale optical components, because they can beat the diffraction limit. Combining metallic and magneto-optical materials can therefore pave the way for highly compact nonreciprocal plasmonic elements. One of the promising ways to engineer integrated plasmonic circuits is to employ metal-dielectric-metal waveguides. Several different nanoscale plasmonic components based on metal-dielectric-metal waveguides have been proposed, including filters, couplers, sensors, switches, and rectifiers.

The research group led by Professor Georgios Veronis from Louisiana State University introduced an extremely compact nanoplasmonic isolator. The isolator consists of a plasmonic resonator placed close to a metal-dielectric-metal plasmonic waveguide with the material filling the waveguide and the resonator being a magneto-optical material （Fig. 1）. Their research results are published in Chinese Optics Letters, Volume 19, No. 8, 2021 (Vahid Foroughi Nezhad, Chenglong You, and Georgios Veronis, Nanoplasmonic magneto-optical isolator [Invited]).

The structure introduced in this work is subject to an externally applied static magnetic field. The cavity mode without magneto-optical activity splits into two modes when magneto-optical activity is present. In addition, the metal-dielectric-metal waveguide leads to a second resonance due to the geometrical asymmetry caused by the waveguide. When magneto-optical activity is present, the cavity becomes a traveling wave resonator. Traveling wave modes do not couple equally into the forward and backward propagating waveguide modes due to momentum matching. As a result, the transmission of the structure depends on the direction of the incident light. When light is incident from one direction, it is mostly absorbed, whereas, when it is incident from the opposite direction, it is mostly transmitted. The proposed structure therefore operates as an optical isolator.

The isolation ratio of an isolator is defined as the ratio of the transmitted optical power for light incident from the direction which allows light to pass, to the transmitted optical power for light incident from the opposite direction which blocks light. In addition, the insertion loss of an isolator is defined as the loss of optical power, when light incident from the direction which allows light to pass is transmitted through the isolator. The researchers showed that the proposed isolator has large isolation ratio and small insertion loss. They also showed that there is a tradeoff between the isolation ratio and the insertion loss of the isolator. This work will pave the way for fully functional nanoscale integrated optical circuits.

纳米等离激元磁光隔离器

光隔离器件是一种只允许光沿一个方向传播而在相反方向阻挡光通过的非互易光无源器件，常用于防止光学振荡器（例如激光振荡器）中不需要的反向放射光，并能抑制不同光学设备之间的串扰。环行器、隔离器等非互易光学元件是实现集成光路的关键器件。

非互易元件可以通过使用非线性材料、具有时间相关特性的材料和磁光材料来打破时间反演对称性。然而，由于自然材料的磁光响应在光波段较弱，基于磁光材料设计的非互易器件体积会大于波长的体积结构。硅光子学和光子晶体的出现将非互易光学元件的尺寸缩小到了波长范围。

为了将非互易光学元件的尺寸进一步减小到亚波长尺度，需要突破衍射极限。金属纳米结构的表面等离激元能够突破光学衍射极限，从而可用于实现亚波长级光学元件。因此，将金属和磁光材料结合能够更好地获得高度紧凑型非互易等离激元元件。此外，金属-电介质-金属波导也有望用于构造等离激元集成电路。目前，已经提出了几种基于金属-电介质-金属波导的纳米等离激元元件，包括滤波器、耦合器、传感器、开关和整流器。

美国路易斯安那州立大学的Georgios Veronis教授团队推出了一种高度紧凑型纳米等离激元磁光隔离器。该隔离器由一个放置在金属-电介质-金属等离激元波导附近的等离激元谐振器组成，波导中填充材料和谐振器材料均为磁光材料。相关研究成果发表在Chinese Optics Letters 2021第19卷第8期上（Vahid Foroughi Nezhad, Chenglong You, and Georgios Veronis. Nanoplasmonic magneto-optical isolator [Invited]）。

该工作中，隔离器的结构受到外部施加的静磁场的影响。当存在磁光活性时，无磁光活性的腔模分裂为两种模式。同时，由于波导的几何不对称性，金属-电介质-金属波导会导致第二次谐振。当存在磁光活性时，腔体成为行波谐振器。由于动量匹配，行波模式不能平均耦合到正向和反向传播的波导模式中。此外，在该结构中当光从一个方向入射时，它大部分被吸收，而从相反方向入射时，它大部分被透射，结构中光的透射取决于光的入射方向。因此，该结构可用于光隔离器。

隔离器的隔离比是指从允许光通过的方向入射的光的传输光功率与从相反方向入射的光的传输光功率之比。此外，当从允许光通过的方向入射的光通过隔离器时，隔离器的插入损耗即为光功率损耗。Georgios Veronis教授团队发现，该隔离器具有较大的隔离比和较小的插入损耗。同时还发现，隔离器的隔离比和插入损耗之间存在折衷关系，并指出该研究将为全功能纳米级集成光学电路的发展奠定基础。