Surface Plasmon Polaritons Unveil Spin of Light
When photons with different polarizations (left or right circular polarization) are incident on the SREC, they will be transformed to SPPs in different routes (path 1 and path 2). Thus, information transformation from spin encoding to route encoding is realized.
With the rapid development of quantum technologies, optical circuits become more and more complex. Usually, several classical and quantum optical devices are needed in an optical circuit, which requires high stability. Free-space optics cannot meet the requirement of quantum information for further development any more. However, photonic integrated circuits (PICs) inherit the advantages of both integrated electronic circuits and free-space optics, such as low power consumption, high stability, high integration density, strong anti-jamming capability, and so on. So PICs attract more and more attention in recent years. In free-space optics, information is usually encoded on the two spin eigenstates of photons , whereas the routes of waveguide are used for encoding information in PICs. Therefore, in order to communicate information between free-space optics and PICs, it is essential that information is transformed between these two encoding schemes.
The research group, led by Prof. Guangcan Guo, from Key Lab of Quantum Information, University of Science and Technology of China, proposed a novel method to transform information from spin encoding to route encoding. It is known that the photon spin state, i.e., the polarization of light, could affect the properties of the excited surface plasmon polaritons (SPPs). By manipulating the structure of the medium and the spin state of the excitation light, the propagation mode of SPPs can be controlled. Based on this mechanism, a spin-route encoding convertor (SREC) with special array structure on the center was designed, so that SPPs could be steered to a particular waveguide according to the spin states of the excitation light. For example, SPPs can be output from the left (right) waveguide with the input of left (right) polarization. Accordingly, the transformation from spin encoding to route encoding is realized. It is reported in Chinese Optics Letters Vol. 12, No. 7, 2014 (/col/abstract.cfm?uri=col-12-7-072401).
This method is based on the properties of SPPs depending on the excitation polarization. SPPs can break the diffraction limit and propagate along nanostructure. In addition, the SREC can be directly integrated on the chip, which can make the circuit more compact. It thus has a potential application in integrated photonics.
The SREC has proved to be theoretically feasible by this work. Future work will be focused on the experimental verification. Because the current micro-nano processing technology can meet the precision requirement of the designed SREC, and the optical detection sensitivity can reach single photon level, the transformation of information from spin encoding to route encoding is expected to be experimentally realizable.
利用表面等离子体激元读懂光子自旋
图片说明：不同偏振的光子（左旋偏振和右旋偏振）入射到自旋-路径编码转化器后，分别被转换为不同路径（路径1和路径2）中的表面等离子体激元，从而实现光子偏振信息到表面等离子体路径信息的转换。
随着量子信息技术的飞速发展，光路的复杂程度越来越高，通常在一个光路中需要级联多个经典的和量子的光学干涉装置，这对光路的稳定性要求较高。自由空间光学已经不能满足量子信息发展的需求，而集成光学既具有集成电路的高稳定性、高集成度等优点，又有自由空间光学的低功耗、高处理速度和抗干扰性强等优点，越来越受到人们的关注。在自由空间光学中，往往采用光子的两个本征自旋态来编码信息，而在集成光学中，经常采用波导路径来编码信息。为了让两者之间能够进行信息传递，必须对这两种不同编码方式的信息进行相互转化。
中国科学技术大学郭光灿院士课题组提出了一种从自旋编码转换为路径编码的信息处理方法。光子的自旋态，即光的偏振，会影响所激发的表面等离子体激元（SPPs）的性质。通过调节介质结构和激发光子的自旋态，可以得到不同传播模式的SPPs。根据这个原理，设计特定的阵列介质结构，使不同自旋态光子激发的SPPs耦合到不同的波导路径中，如输入左旋偏振态的光子可以在左边波导中输出SPPs，输入右旋偏振态的光子则在右边波导中输出SPPs，从而实现信息从自由空间光学的自旋编码与集成光学的路径编码之间的转换。该方法是基于SPPs的性质对光子偏振的依赖性，SPPs能够突破衍射极限而束缚在微纳结构中传播，而且自旋-路径编码转换器可以直接集成在芯片上，提高器件的集成度，这在集成光学的研究中具有潜在的应用价值。相关研究成果发表在Chinese Optics Letters 2014年第7期上（http://www.opticsinfobase.org/col/abstract.cfm?uri=col-12-7-072401）。
该工作从理论上证明了自旋-路径编码转换器的可行性，接下来将从实验上观察它的效果。现在的微纳加工技术可以满足所设计的自旋-路径编码转换器的精度要求，实验探测技术也可以达到单光子级别，因此在实验上实现信息从自旋编码到路径编码是有可能的。