On-chip sensing of volatile organic compounds in water by hybrid polymer and silicon photonic-crystal slot-waveguide devices

Abstract

ABSTRACT Lab-on-chip integrated infrared spectroscopy and sensing with hybrid polymer and silicon photonic crystal slot waveguides is demonstrated for the specific and selective identification of volatile organic compounds, xylene and toluene, in water. A 300 micron long photonic crystal slot waveguide was demonstrated that enabled the detection of 100ppb xylene in water by near-infrared absorption signatures, with five times higher sensitivity on an order of magnitude smaller length scale. The on-chip absorption spectroscopy, determined by Beer-Lambert absorption law, is enabled by the combined effects of slow light and high electric field intensity enhancement in photonic crystal slot waveguides. Keywords: photonic crystal waveguide, photonic crystal slot waveguide, on-chip absorption, infrared absorption spectroscopy. *swapnajit.chakravarty@omegaoptics.com; phone 1 512 996-8833; fax 1 512-873-7744; omegaoptics.com 1. INTRODUCTION Infrared (IR) absorption measurements are the simplest label free techniques for detection and identification of substances based on their unique spectral signatures. It is widely used in applications in organic [1] and inorganic chemistry, studies of polymer degradation in forensic analysis [2], water content or moisture characterization in agricultural and food products [3], fuel quality control [4], hydrocarbon processing [5] and refining and petroleum characterization [6]. On-chip infrared absorption spectroscopy provides an opportunity to make a new generation of sensors that recognize substances based on their unique mo lecular absorption signatures. However, in order to provide the same or similar sensitivity as bench-top sensors, it is necessary to have the same effective absorption path length. We previously demonstrated that photonic crystal slot waveguides provide the combined benefits of slow light and high electric field enhancement in a narrow low index slot which increase the effective absorptio n path length on miniature length scales that can be shrunk to dimensions of a chip which can thus be fabricated by planar lithography [7-8]. Furthermore, since the dispersive properties of photonic crystal slot waveguides are determined by Maxwell’s electromagnetic wave equations which are independent of frequency, designs are therefore valid across a wide wavelength range as long as the material of the waveguide is transparent. Since silicon is transparent in the wavelength range 1.2-6P m, the silicon platform can be used for the fabrication of chip-integrated near-infrared absorption spectroscopy sensors. The slow light bandwidth is however small ~10nm over which the group index is greater than 20, hence multiple photonic crystal slot waveguides each cove ring a portion of the infrared spectrum n eed to be fabricated on the chip to create a complete absorption spectroscopy device. Furthermore, the group index changes continuously as a function of wavelength from the transmission band edge of the photonic crystal slot waveguide. Since it is necessary to ensure that the slow light bandwidth of the photonic crystal slot waveguide coincides with the absorption peak of the material to be detected, small errors in device fabrication can cause the slow light bandwidth to deviate leading to reduction in slow light effect and hence a reduction in sensitivity. In this paper, we present designs of photonic crystal slot waveguide devices with a nearly constant group index over a range of wavelength that results in a fabrication tolerant design to ensure the sensitivity of the device at the absorption