Designing Tunable Miniaturized Spectroscopic Gas Sensor Using Optofluidic Hollow-Core Photonic Crystal Fiber M. Ebnali-Heidari 1 , F. Koohi-Kamali 1 , A. Ebnali-Heidari 1 , and M. K. Moravvej-Farshi 2 , Boris T. Kuhlmey 3 1 Faculty of Engineering, Shahrekord University, Shahrekord 8818634141, Iran 2 Advanced Devices Simulation Lab (ADSL), Faculty of Electrical and Computer Engineering, Tarbiat Modares University (TMU), Tehran, Iran. 3 Institute of Photonics and Optical Science (IPOS) and School of Physics, University of Sydney, Camperdown, New South Wales 2006, Australia Abstract: A tunable optofluidic slow-light hollow-core photonic crystal fiber (HPCF), applicable to miniaturized spectroscopic gas sensors is proposed. We present numerical results showing how this optofluidic structure can be used for gas detection. OCIS codes: (300.0300) General; (000.0000) General Optical absorption spectroscopy is very suitable for gas detection. In miniaturized spectroscopic gas sensors, due to the short absorption path length, absorption detection may not be sensitive enough. To overcome this problem in the context of miniature absorption spectroscopy devices, photonic crystal (PhC) based gas sensors have been proposed. In particular, miniaturized devices based on slow-light PhC waveguides and Hollow core PCFs (HPCFs) offer a fascinating solution for index sensing and absorption spectroscopy [1-2]. The slow-light regime increases a device’s effective optical length leading to enhancing weak light–matter interactions. In this paper, we have taken advantage of the possibilities offered by optofluidic approach for designing a tunable slow-light HPCFs for absorption gas sensing. A similar technique with different application has already been demonstrated for PhCs and solid core PCFs [3-4]. Cross-section of the proposed structure is represented in Fig. 1(a). To achieve a tunable slow- light modes near the band edge [5], the selected air- holes forming the first ring of cladding are infiltrated by fluid with refractive index n f (pink circles). Poynting flux of defect mode near the band edge is shown in Fig. 1(b). Figure 1(c) shows the calculated dispersion curve for guided modes of the un-infiltrated HPCF (solid curves) and the corresponding modes of the proposed HPCF infiltrated with n F =1.3, 1.4, and 1.5. The comparison reveals that propagating mode confined within the core of the infiltrated PCF is redshifted with respect to the un-infiltrated case. Figure 1(d) illustrates plots of the spectra for the absorbance enhancement factor and the group index for gas samples with imaginary part of n′′=0.001, and n′′=0.01, for the case that the selected air-hole (pink circles) being infiltrated by fluid with refractive index of n F =1.5. As seen in Fig. 1(d), the group index for the large n′′ gas sample n g decreases to its saturation values. However, as shown in this figure for small n′′ the proposed structure is thus most useful for detection of weakly absorbing or dilute gases. The advantage of the proposed structure is possibility of tuning of desired slow light region of the guided mode corresponding to an absorption line of target gas sample. Fig. 1 (a) Cross section of the HPCF (b) Poynting flux of defect mode near the band edge (c) Influence of fluid infiltration to the dispersion curve (d) (left panel) the spectra for the absorbance enhancement factor (right panel) the group index of the guided mode of the proposed structure. [1] N. A. Mortensen, et al. “Liquid-infiltrated photonic crystals,” Microfluid. & Nanofluid. 4, 117, 2008. [2] Cubillas, Ana M., et al. "Photonic crystal fibres for chemical sensing and photochemistry." Chemical Society Reviews 42.22 2013. [3] M. Ebnali-Heidari, et al. "Dispersion engineering of slow light PhC using microfluidic infiltration," Optics express,. 17,. 1628-1635, 2009 [4] M . Ebnali-Heidari, et al. “Proposal for Supercontinuum Generation by Optofluidic Infiltrated PCFs”. IEEE, JSTQE, 20, 5, 2014 [5] A. F. Oskooi, et al. “Zero–group velocity modes in chalcogenide holey photonic-crystal fibers," Optics express, 17, 10082-10090, 2009.