Surface plasmon polariton enhances silicon light emission

Schematic diagram of the Au-SiN_x waveguide.

Professor Yidong Huang's group from Tsinghua University, China, has recently reported experimental results of enhanced spontaneous emission (SE) from silicon nanoparticles embedded in silicon nitride, which verified the feasibility of enhancing the silicon light emission with the help of surface plasmon polariton (SPP) propagating on the metal waveguide. It is published in Chinese Optics Letters, Volume 11, 2013 (/col/abstract.cfm?uri=col-11-2-022401).

For future high-performance processors, the mass data transmission is one of the major technique bottlenecks, and a promising solution is on-chip optical interconnection, which could monolithically integrate both the electronic and photonic devices on a silicon chip. Up to now, silicon-based devices such as modulators, waveguides, and detectors have been successfully realized, but the emission efficiencies of silicon light sources are still as low as 10^-5-10^-2. Since 2006, Prof. Huang's group has engaged in the research on the plasmonic enhancement for silicon light emission.

SPP is an electromagnetic excitation of TM mode propagating along the metal-dielectric interface. Especially, SPP has smaller mode volume than that of optical wave by 1/10 at the resonance frequency with corresponding larger density of state by over 10 times. So, the SE rate could be significantly enhanced by SPP mode. In this work, the researchers prepared amorphous silicon nanoparticles in silicon-rich silicon nitride (SiN_x) by plasma enhanced chemical vapor deposition (PECVD) and fabricated a thin gold film of 15-25nm on top of the SiN_x layer by magnetron sputtering. With the steady-state photoluminescence, the SPP resonance of the Au-SiN_x waveguide was then observed; furthermore, with the time-resolved photoluminescence (TRPL), both of the average SE lifetimes before and after coating gold film were measured, indicating that the SE rate was enhanced about threefold.

Instead of measuring photoluminescence intensity, TRPL spectrum is adopted to obtain the SE lifetimes in this work. Because SE rate is inversely proportional to SE lifetime, the SE-rate variation could be directly measured. Despite the enhancement of only ~3, which is still not enough for a practical device, this work verified the possibility of applying plasmonic enhancement on silicon light source.

Although the measured enhancement is much lower than the theoretical prediction (>100), according to the latest work of this group, they have found the physical limitations to obtain ultrahigh plasmonic enhancement, which include the propagation loss of SPP mode and the spectral broadening of emission linewidth. Now, they are still working on these subjects to achieve a high-efficiency silicon light source in the future.

COL封面故事: 表面等离子激元可提高纳米硅材料的发光效率

金-氮化硅波导的结构示意图

未来高性能处理器中，海量数据的传输是提升处理能力的瓶颈，最为可能的技术途径是基于硅衬底实现电子与光子器件融合的光电集成回路。目前，硅基器件已经能够实现光子的调控、传输和探测，唯有硅光源研究进展缓慢，效率仍然很低(10^-5-10^-2)。

清华大学电子工程系黄翊东教授课题组利用金属波导增强包埋在氮化硅母体中纳米硅点发光效率，验证了借助表面等离子激元(Surface Plasmon Polariton, SPP)提高纳米硅材料发光效率的可行性，为未来实现片上集成硅光源奠定了基础。该研究成果将发表在Chinese Optics Letters 2013年第2期上(/col/abstract.cfm?uri=col-11-2-022401)。

面等离子激元是一种沿着金属-介质界面传播的TM偏振电磁激励。在谐振频率处，SPP的模式体积极小(小于普通光波模式的1/10)，态密度极高(比普通光波模式高10倍以上)。因此借助SPP，有可能提高发光材料的自发辐射速率。在该研究工作中，研究人员利用等离子化学气相沉积(PECVD)生长出含有非晶态纳米硅点的富硅氮化硅(SiN_x)材料，然后利用磁控溅射制备出15-25 nm厚的金纳米薄膜。通过稳态光致发光谱，观察到了金-;氮化硅波导的SPP;谐振峰，验证了SPP;模式的存在；通过时间分辨光致发光谱分别测试了制备金纳米薄膜前、后硅材料发光的平均自发辐射寿命，结果表明自发辐射速率可以提高约3倍。

该研究制备出了带有金属波导结构的富硅氮化硅材料，并且不同于以往研究中通过测量光强变化间接研究自发辐射速率改变的方法，采用了时间分辨光致发光谱测量了自发辐射寿命，由于自发辐射速率反比于寿命，因此更为直接地研究了自发辐射速率的改变。尽管目前的实验结果只获得了约3倍的增强，对于真正实用的器件还需进一步提高，但该研究证明了借助表面等离子激元是增强硅材料发光效率的可能途径。

目前实验中获得的增强倍数远远低于理论的预期(>100)，该研究小组最新的研究工作表明SPP模式的传播损耗和发光材料发光谱的展宽是获得极高增强倍数的制约因素。在后续的研究工作中，研究人员将针对减小发光线宽和降低传输损耗开展工作，以期获得更高效率的硅光源。