Feb 2, 2012
New-look nanocrescents magnify surface plasmon resonance peak and improve tunability
For devices based on localized surface plasmon resonance (LSPR), increasing the local field enhancement (LFE) and the density of hot spots is a key issue. For in vivo biological applications, the ability to tune the LSPR peak to the "biological window" is also required. A standalone metallic nanocrescent is a plasmonic structure exhibiting high LFE and some LSPR tunability, but it is not perfect. To make improvements, a new structure – multilayer Au/Dielectric/Au nanocrescents adhered to a dielectric cylinder – has been presented by researchers at Shanghai Jiao Tong University, China, and University of California-Berkeley, US. 3D FE simulations show that the structure has stronger LFE and larger hot-spot areas compared with a standalone Au nanocrescent. In addition, by adjusting the cylinder's size and material, the LSPR peak can be tailored to the visible, the NIR or even the MIR regime with little increase in radiation loss.
The increased LFE and hot-spot areas are associated with the increased tips in the multilayer structure and the intra-particle coupling among the Au crescents. Results also show that further intensified LFE can be achieved using semiconductor materials instead of dielectrics, due to the charge transport at the metal-semiconductor interface.
Advantages of cylinder-based tuning
Compared with changing the size of the independent metallic crescent, the unique cylinder-based tuning technique has several advantages. First, the plasmon band can be tuned over a wider range. For example, the plasmon peak can be tuned into the NIR regime for in vivo detection, and even into the MIR regime for catching some specific targets. Second, the band tuning is more precise since the cylinder can be fabricated by photolithography, which is suitable for preparing reproducible devices with well controlled parameters. Finally, tuning by changing the cylinder material rather than the size resolves the conflict between the red-shift of the resonance peak and a decrease in LFE, as an increase in radiation loss can be fully avoided when the size of the crescent is fixed.
All of these benefits suggest that the multilayer Au/dielectric/Au nanocrescent structure has valuable potential in a variety of applications such as diagnostics and in vivo nanoscale spectroscopy.
Having performed the numerical study, the team is now working on the fabrication of the multilayer Au/dielectric/Au nanocrescent structure.
More details can be found in the journal Nanotechnology.
About the author
Kaiyu Wu is an MSc student in the School of Microelectronics, Shanghai Jiao Tong University. His research focuses on plasmonic devices and their biological applications. Xiulan Cheng is associate professor in bioelectronics at the School of Microelectronics, Shanghai Jiao Tong University. Luke P Lee is the Arnold and Barbara Silverman Distinguished Professor, Bioengineering Director of the Biomedical Institute for Global Healthcare Technology, and Co-Director of the Berkeley Sensor & Actuator Center (BSAC). He leads the BioPOETS group at BSAC.