On-chip near-infrared spectroscopy of CO2 using high resolution plasmonic filter array

Xinyuan Chong, Erwen Li, Kenneth Squire, Alan X. Wang
2016 Applied Physics Letters  
We report an ultra-compact, cost-effective on-chip near-infrared spectroscopy system for CO 2 sensing using narrow-band optical filter array based on plasmonic gratings with a waveguide layer. By varying the periodicity of the gratings, the transmission spectra of the filters can be continuously tuned to cover the 2.0 lm sensing window with high spectral resolution around 10 nm. Our experimental results show that the on-chip spectroscopy system can resolve the two symmetric vibrational bands of
more » ... ibrational bands of CO 2 at 2.0 lm wavelength, which proves its potential to replace the expensive commercial IR spectroscopy system for on-site gas sensing. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4953261] Carbon dioxide (CO 2 ) is the most important greenhouse gas and the major product of combustion. Detecting CO 2 is critical for many practical applications such as combustion control 1 and pollution monitoring. 2 Of all existing gas sensor technologies, infrared (IR) absorption is widely used as a simple and reliable technique for both detection and identification of hazardous and greenhouse gases. IR spectroscopy relies on the optical absorption of molecular vibration bands, which represent the signatures of various gas molecules. Generally, existing IR detection techniques can be divided into two categories: spectroscopy and non-dispersive techniques. Non-dispersive techniques, as the name implies, do not have wavelength dispersive component, such as grating. Therefore, it will not provide spectroscopy information about the sample. In contrast, due to the wavelength dispersive components, spectroscopy techniques are able to detect multiple samples simultaneous by the unique spectral feature of the sample. In contrast, spectroscopy system is much more complicated, which contains wavelength dispersive components. It allows the spectroscopy techniques to detect multiple samples simultaneous by the unique spectral feature of the sample. In the mid-infrared (MIR, 2.5-25 lm) region, both of these sensing techniques offer remarkably high sensitivity by probing the fundamental vibrational and rotational transitions of CO 2 molecules. In the near-infrared (NIR, 0.8-2.5 lm) region, the absorption comes from the overtones of the fundamental bands, and is usually two to three orders of magnitude lower than that of the MIR, which results in relatively low sensitivity. In addition, the absorption bands of various gases are close to each other or even partially overlapped in the NIR region, which makes it difficult for non-dispersive techniques. Therefore, NIR spectroscopy can provide unprecedented specificity and multiplex gas sensing capabilities compared with non-dispersive techniques. 3-6 However, existing IR spectroscopy systems such as Fourier transform infrared (FTIR) spectroscopy are bulky, expensive, and power-hungry table-top equipment that can only be used in the laboratory. Therefore, there is a strong need to develop miniaturized on-chip NIR spectroscopy for on-site or portable gas sensing. Particularly for CO 2 sensing, there are two major absorption bands in the NIR region: 1.57 lm and 2.0 lm. Because of the matured optical telecommunication industry, optoelectronic devices at 1.57 lm window such as light sources, spectrometers, and photodiodes are cost-effective, miniaturized, and widely available. Unfortunately, the sensitivity at 1.57 lm is extremely low ($1.8 Â 10 À3 cm À1 ), and it requires an ultra-long absorption path length that can only be provided by multi-pass gas cells 7,8 to achieve high sensitivity. On the other hand, 2.0 lm wavelength can provide 70Â stronger absorption (0.135 cm À1 ). Some recent work has been published on 2.0 lm CO 2 NIR spectroscopy due to the availability of the light sources. 6,9-11 Nevertheless, they utilize either expensive distributed-feedback (DFB) lasers or bulky FTIR spectrometers. Therefore, ultra-compact spectrometers 12,13 still remain a grand challenge and research focus. In this paper, we develop a simple and cost-effective plasmonic narrow-band filter array on a single chip, covering the wavelength range from 1990 nm to 2040 nm. Plasmonic filters are actually based on the well-known phenomena called "extraordinary optical transmission" (EOT). 14-16 By exploiting plasmonic nanostructures, such as nanohole or nanoslit arrays, efficient conversion between photons and plasmons can be controlled at subwavelength scale, leading to very high optical transmission, which provides alternative solutions to traditional optical processes such as color filtering and spectral imaging. 17-23 Compared with existing plasmonic filters at visible and UV wavelength ranges, we focused on the design of NIR wavelength filter array with high spectral resolution. By measuring the transmitted power of each filter, the spectroscopy information can be extracted. Most of the reported plasmonic filters are used for color filters with relatively low resolution (>50 nm). Due to the compactness and low cost of this filter array, it can potentially replace the expensive and bulky FTIR spectroscopy systems for mobile NIR gas sensing. The configuration of the plasmonic filter is shown in Figure 1(a) , which consists of a gold (Au) thin film with periodic nano-slits on a quartz (n sub ¼ 1.45) substrate. A layer of UV cured SU-8 (n SU-8 ¼ 1.57) photoresist covers the Au thin film, serving as a waveguide layer. Transverse magnetic (TM)
doi:10.1063/1.4953261 fatcat:4t2bspuijzaytbghzgnnowpsay