Resonance Tuning in Plasmonic Nanorods for Enhanced Infrared Spectrum Detecting

Abstract

Surface plasmons are electromagnetic waves propagating along metallic-dielectric interfaces and they have drawn considerable attention in the past ten years since extraordinary optical transmission (EOT) phenomenon was first reported. They can take different forms, from propagating waves to localized electron oscillations. Owning to their peculiar optical properties and particular capability of manipulating light at sub-wavelength scales, a wide range of practical applications have been enabled. Since the near-field of electromagnetic waves can be enhanced dramatically using plasmonic structures with different designs, surface plasmon based waveguides are of special importance to develop sensors and detectors with ul- tra-high sensitivity and figure of merit. Research on surface plasmons covers a broad scope with potential applications rang- ing from waveguiding to sensing. But plasmon assisted infrared detectors are rarely reported especially the ones working under near infrared (NIR) range. Tuning of surface plasmons in the NIR range is critical and essential to develop nano- photonic devices like modulators and sensors. Recently, plasmon assisted devices have drawn increasing interest. In this work, we show the combination of plasmonic nanorod arrays with NIR spectrum detection. Using electron-beam lithography (EBL) and ion milling techniques, gold (Au) nanorod arrays with varying periodicities were fabricated on transparent quartz substrates. By collecting and investigating the transmittance spectra of nanorods, precise tuning of localized surface plasmon resonance (LSPR) in the infrared range can be realized by changing the periodicity of nanorods. Experimental results show that the resonant wavelength of LSPR dips redshifts with increasing array periodicity and the distance of shift can be accu- rately controlled. Simulations show qualitative agreement with experimental results. The influence of waveguide sidewall on device performance is also discussed in detail. In addition, optical properties of nanorods with different heights were com- pared and investigated. By combining silver (Ag) nanorod structures with attenuated total reflection-Fourier transform infra- red (ATR-FTIR) spectroscopy technique, the absorbance of glucose can be effectively enhanced. Therefore, periodical nano- rod arrays can significantly enhance the near field intensity, enabling extensive applications in signal sensing and detecting. Keywords nanorods; surface plasmon resonance; tuning; FTIR; spectrum enhancement