On-chip plasmonic photonic crystals for near-infrared absorption spectroscopy

Infrared (IR) absorption spectroscopy is widely accepted as a reliable technique for chemical and biological sensing. However, most commercially available IR spectroscopy systems are expensive, bulky bench-top instruments. Demand for developing low cost, portable IR spectroscopy system is continuously growing, especially an on-chip IR spectroscopy that can be conveniently used for field testing is highly desired. In the past years, several on-chip approaches, such as arrayed waveguide grating (AWG), Echelle diffraction grating (EDG), superprism-based photonic crystal, microresonator array, and filter array based on Fabry-Perot (FP) cavities have been reported. However, these techniques suffer from the tradeoff between spectral resolution and operating bandwidth, which limits their practice applications. We demonstrated a new kind of on-chip spectrometer to measure the IR absorption of aromatic compounds, xylene in this paper, achieving wide operating bandwidth over 270nm in near-infrared (NIR) wavelength and a relatively high spectral resolution around 10 nm. The spectrometer uses a band-pass filter array based on narrow-band plasmonic photonic crystals as wavelength-selective components. By measuring the transmitted power of each filter under normal incident from a broadband light source, we can extract the spectroscopy information. The proposed spectrometer has the advantage of ultra-compact size, high throughput (when used with NIR camera), excellent reliability, and wide operating bandwidth which shows great potentials for cost-effective, portable IR spectroscopy system that can be used in a variety of applications such as pollution monitoring and environmental protection.