Integrated hybrid plasmonic-photonic structures for lab-on-chip sensing
Plasmonic nanostructures have been used in different applications such as molecular sensing, surface enhanced Raman spectroscopy (SERS), localized heating for dense magnetic recording, energy harvesting, and cancer therapy. Plasmonic nanoparticles (nanoresonators) localize the lightwave beyond the diffraction limit, and can provide ultrahigh field enhancements. The coupling of the lightwave to the localized resonance modes of plasmonic structures is not very efficient using free-space optics. In this research, we have utilized the potentials of integrated photonics and plasmonics in a unified platform. The proposed hybrid platform benefits from the best of the integrated photonics and plasmonics. Integrated photonic waveguides and microresonators provide a low-loss means of steering and trapping the lightwave on the chip. When combined with plasmonic structures, they can provide new potentials, addressing the needs in applications such as lab-on-chip sensing. Fig. 1a shows the scanning electron micrograph (SEM) of a hybrid plasmonic-photonic waveguide structure that consists of a plasmonic gold nanorod integrated with a Silicon Nitride photonic ridge waveguide, fabricated using nanolithography techniques. The input lightwave couples to the waveguide mode and excites the localized resonance mode of the plasmonic nanoresonator. Fig. 1b shows the darkfield image of an array of gold nanorods on a waveguide, where the very bright scattered signal shows efficient coupling to the localized resonance modes. We have demonstrated a coupling efficiency of better than 10% to each individual plasmonic nanoresonator.
Figure
1. (a) The scanning electron micrograph (SEM) of a hybrid plasmonic-photonic waveguide structure consisting of a gold nanorod and a Silicon Nitride waveguide. (b) Darkfield microscope image of an array of gold nanorods integrated on a ridge waveguide and excited near the resonance wavelength.
On-chip Localized Surface Plasmon Resonance (LSPR) Sensing
It has been shown that by using individual plasmonic nanoresonators, LSPR sensing can be carried out for detection of very small changes in biological or chemical analytes. However, the lack of an efficient coupling mechanism to individual plasmonic nanoparticles, limits the SNR and the detection limits. The large coupling efficiency to each individual plasmonic nanoresonator in our hybrid plasmonic-photonic waveguide structure can address these issues. Figure 2a demonstrates the schematic of LSPR sensing using the hybrid plasmonic-photonic waveguide structure. It can be seen from Fig. 2b that by introducing an analyte, the resonance redshifts. Figure 2c shows the LSPR sensing measurement results using different analytes of calibrated refractive indices. The linear regression analysis shows a large sensitivity of 168nm/RIU. The proposed sensor structure is very compact, and several of these resonators can be used in a multiplexed scheme, each functionalized for a particular target analyte.
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2. (a) On-chip LSPR sensing using the hybrid plasmonic-photonic waveguide structure. (b) Resonance shift due to the introduction of an analyte. (c) Sensitivity measurement results.