Photonic Crystal Defects with Increased Surface Area for Improved ...
Photonic Crystal Defects with Increased Surface Area for Improved Refractive Index Sensing 1
Christopher Kang1,*, Christopher Phare2, and Sharon M. Weiss1,2,3 Interdisciplinary Graduate Program in Materials Science, Vanderbilt University, Nashville, TN 37235, USA 2 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA 3 Department of Electrical Engineering and Computer Science, Vanderbilt University, 37235, USA Corresponding author: [email protected]
Yurii A. Vlasov and Solomon Assefa IBM T.J. Watson Research Center, Yorktown Heights, NY 10536, USA
Abstract: Photonic crystal cavities with tunable surface area via multiple-hole defects were investigated for increased resonance wavelength shifts upon exposure to variable-index analytes. Sensitivity was improved by 20% compared to simulated solid L3 cavities. ©2010 Optical Society of America OCIS codes: (230.5298) Photonic crystals; (230.5750) Resonators; (130.6010) Sensors.
1. Introduction Photonic crystal (PhC) slabs with defects are a promising platform for refractive index-change sensors due to their high field concentration, which allows reduced analyte volumes, and sharp spectral resonances that enable the detection of small refractive index perturbations in the defects. The sensitivity of photonic crystal and other sensor platforms is characterized by the wavelength shift of a distinct spectral feature as a function of the refractive index change of analyte exposed to the sensor. Previously, it has been shown that high surface area porous materials, especially those with resonant spectral features such as porous silicon waveguides, provide increased sensitivity compared to conventional sensor platforms with limited surface area that rely on evanescent wave detection [1]. By combining high quality factor PhC cavities with high surface area multiple-hole defects (MHD) [2-3], we present an improved platform for achieving high sensitivity refractive index sensing. In this paper, we show for the first time the experimental realization of MHD cavities and their optical response when exposed to variable index analyte. Complementary FDTD simulations are also presented. 2. Simulation and fabrication PhC slab devices were fabricated on a silicon-on-insulator substrate with device layer thickness of 220 nm and buried oxide thickness of 2 µm. In order to demonstrate the shift in resonance wavelength with increased surface area, various designs of MHD cavities with 3 holes were fabricated at different MHD hole diameters. PhC patterns were transferred to the silicon surface using electron beam lithography and reactive ion etch, as described in [4]. Polymer spot-size converters were implemented on the devices in order to increase the coupling efficiency of the input/output light [4]. Multiple devices were laid out on a chip, and the input/output facets were formed by cleaving the chip on both sides. Devices were simulated with freely available finite-difference time-domain (FDTD) code with subpixel averaging implemented for increased accuracy [5]. A transmission experiment consisting of a MHD L3 cavity coupled to a W1 PhC waveguide was set up as shown in Fig. 1a, using a three-dimensional simulation space with a resolution of 24 grid points per lattice constant (a = 410 nm). A perfectly matched layer (PML) with a thickness of a was implemented at each extent of the 3D space in order to minimize the effect of reflections on the cavity mode. A single Gaussian pulse centered at the normalized frequency of 0.26(c/a) and with a width of 0.1(c/a) was excited at the left end of the W1 waveguide and allowed to propagate in time down the waveguide. The waveguide flux at the right end of the waveguide was computed to analyze the guided mode interaction with the cavity. The resonant cavity mode appears as a sharp dip in the transmission spectrum. 3. Results and Discussion Passive measurements of the 3-hole MHD design were carried out to first examine the effect of increased surface area on the resonance mode, as shown in Figure 1a. The resonance is blue-shifted for increasing MHD hole diameter, due to the decreasing effective index of the mode. The transmission spectrum of the MHD L3 cavities was then measured for a single MHD device (70 nm diameter) after exposure to 5 solvent solutions with varying mixtures of isopropyl alcohol (IPA) and methanol. The analyte mixtures were prepared with concentrations of 10%/90%, 30%/70%, 50%/50%, 70%/30%, and 90%/10% of IPA/methanol by weight, respectively. The shift in
refractive index between each solution was calculated to be 0.01152 RIU. Due to the quickly evaporating nature of the solvent solution, drops of mixture were continuously placed on top of the PhC devices during each measurement, covering the entire chip. Figure 2 shows the measurement and simulation results for the refractive index sensing experiment, indicating a device sensitivity of 115 nm/RIU and 65 nm/RIU, respectively. The devices simulated with FDTD with 3 MHD holes inside the cavity were found to have a 20% improvement over a similar simulation with reduced resolution of 8 grid points per lattice constant. A reduced resolution in this case was used due to the absence of small features. We anticipate improving the sensitivity further using high Q-factor designs that utilize more surface area through the addition of more MHD holes inside the cavity.
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(b)
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Figure 1. (a) Normalized transmission of different diameter MHD cavities for the 3-hole design shown in (b), (b) fabricated L3 cavity with 85 nm diameter 3-hole MHD, (c) Z-cut of PhC waveguide and defect epsilon (black: Silicon, white: solvent solution) as simulated in FDTD,
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(b)
Figure 2. Results of sensitivity experiment for: (a) fabricated devices, with resonance shown for 5 solvent mixture solutions, and (b) comparison with FDTD simulation, with linear regression for both sets of data. [1] G. Rong, A. Najmaie, J. E. Sipe, and S. M. Weiss, “Nanoscale porous silicon waveguides for label-free DNA sensing,” Biosens. Bioelectron. 23, 1572 (2008). [2] C. Kang and S. M. Weiss, “Photonic crystal with multiple-hole defect for sensor applications,” Opt. Express 16, 18188 (2008). [3] C. Kang and S. M. Weiss, "Photonic crystal defect tuning for optimized light-matter interaction," Proc. SPIE 7031, 70310G (2008). [4] S.J. McNab, N. Moll, and Y.A. Vlasov, “Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides,” Opt. Express 11, 2927 (2003). [5] A. Farjadpour, D. Roundy, A. Rodriguez, M. Ibanescu, P. Bernel, J. D. Joannopoulos, S. G. Johnson and G. Burr, “Improving accuracy by subpixel smoothing in FDTD,” Opt. Letters 31, 2972-2974 (2006).
Christopher Kang1,*, Christopher Phare2, and Sharon M. Weiss1,2,3 Interdisciplinary Graduate Program in Materials Science, Vanderbilt University, Nashville, TN 37235, USA 2 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA 3 Department of Electrical Engineering and Computer Science, Vanderbilt University, 37235, USA Corresponding author: [email protected]
Yurii A. Vlasov and Solomon Assefa IBM T.J. Watson Research Center, Yorktown Heights, NY 10536, USA
Abstract: Photonic crystal cavities with tunable surface area via multiple-hole defects were investigated for increased resonance wavelength shifts upon exposure to variable-index analytes. Sensitivity was improved by 20% compared to simulated solid L3 cavities. ©2010 Optical Society of America OCIS codes: (230.5298) Photonic crystals; (230.5750) Resonators; (130.6010) Sensors.
1. Introduction Photonic crystal (PhC) slabs with defects are a promising platform for refractive index-change sensors due to their high field concentration, which allows reduced analyte volumes, and sharp spectral resonances that enable the detection of small refractive index perturbations in the defects. The sensitivity of photonic crystal and other sensor platforms is characterized by the wavelength shift of a distinct spectral feature as a function of the refractive index change of analyte exposed to the sensor. Previously, it has been shown that high surface area porous materials, especially those with resonant spectral features such as porous silicon waveguides, provide increased sensitivity compared to conventional sensor platforms with limited surface area that rely on evanescent wave detection [1]. By combining high quality factor PhC cavities with high surface area multiple-hole defects (MHD) [2-3], we present an improved platform for achieving high sensitivity refractive index sensing. In this paper, we show for the first time the experimental realization of MHD cavities and their optical response when exposed to variable index analyte. Complementary FDTD simulations are also presented. 2. Simulation and fabrication PhC slab devices were fabricated on a silicon-on-insulator substrate with device layer thickness of 220 nm and buried oxide thickness of 2 µm. In order to demonstrate the shift in resonance wavelength with increased surface area, various designs of MHD cavities with 3 holes were fabricated at different MHD hole diameters. PhC patterns were transferred to the silicon surface using electron beam lithography and reactive ion etch, as described in [4]. Polymer spot-size converters were implemented on the devices in order to increase the coupling efficiency of the input/output light [4]. Multiple devices were laid out on a chip, and the input/output facets were formed by cleaving the chip on both sides. Devices were simulated with freely available finite-difference time-domain (FDTD) code with subpixel averaging implemented for increased accuracy [5]. A transmission experiment consisting of a MHD L3 cavity coupled to a W1 PhC waveguide was set up as shown in Fig. 1a, using a three-dimensional simulation space with a resolution of 24 grid points per lattice constant (a = 410 nm). A perfectly matched layer (PML) with a thickness of a was implemented at each extent of the 3D space in order to minimize the effect of reflections on the cavity mode. A single Gaussian pulse centered at the normalized frequency of 0.26(c/a) and with a width of 0.1(c/a) was excited at the left end of the W1 waveguide and allowed to propagate in time down the waveguide. The waveguide flux at the right end of the waveguide was computed to analyze the guided mode interaction with the cavity. The resonant cavity mode appears as a sharp dip in the transmission spectrum. 3. Results and Discussion Passive measurements of the 3-hole MHD design were carried out to first examine the effect of increased surface area on the resonance mode, as shown in Figure 1a. The resonance is blue-shifted for increasing MHD hole diameter, due to the decreasing effective index of the mode. The transmission spectrum of the MHD L3 cavities was then measured for a single MHD device (70 nm diameter) after exposure to 5 solvent solutions with varying mixtures of isopropyl alcohol (IPA) and methanol. The analyte mixtures were prepared with concentrations of 10%/90%, 30%/70%, 50%/50%, 70%/30%, and 90%/10% of IPA/methanol by weight, respectively. The shift in
refractive index between each solution was calculated to be 0.01152 RIU. Due to the quickly evaporating nature of the solvent solution, drops of mixture were continuously placed on top of the PhC devices during each measurement, covering the entire chip. Figure 2 shows the measurement and simulation results for the refractive index sensing experiment, indicating a device sensitivity of 115 nm/RIU and 65 nm/RIU, respectively. The devices simulated with FDTD with 3 MHD holes inside the cavity were found to have a 20% improvement over a similar simulation with reduced resolution of 8 grid points per lattice constant. A reduced resolution in this case was used due to the absence of small features. We anticipate improving the sensitivity further using high Q-factor designs that utilize more surface area through the addition of more MHD holes inside the cavity.
(a)
(b)
(c)
Figure 1. (a) Normalized transmission of different diameter MHD cavities for the 3-hole design shown in (b), (b) fabricated L3 cavity with 85 nm diameter 3-hole MHD, (c) Z-cut of PhC waveguide and defect epsilon (black: Silicon, white: solvent solution) as simulated in FDTD,
(a)
(b)
Figure 2. Results of sensitivity experiment for: (a) fabricated devices, with resonance shown for 5 solvent mixture solutions, and (b) comparison with FDTD simulation, with linear regression for both sets of data. [1] G. Rong, A. Najmaie, J. E. Sipe, and S. M. Weiss, “Nanoscale porous silicon waveguides for label-free DNA sensing,” Biosens. Bioelectron. 23, 1572 (2008). [2] C. Kang and S. M. Weiss, “Photonic crystal with multiple-hole defect for sensor applications,” Opt. Express 16, 18188 (2008). [3] C. Kang and S. M. Weiss, "Photonic crystal defect tuning for optimized light-matter interaction," Proc. SPIE 7031, 70310G (2008). [4] S.J. McNab, N. Moll, and Y.A. Vlasov, “Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides,” Opt. Express 11, 2927 (2003). [5] A. Farjadpour, D. Roundy, A. Rodriguez, M. Ibanescu, P. Bernel, J. D. Joannopoulos, S. G. Johnson and G. Burr, “Improving accuracy by subpixel smoothing in FDTD,” Opt. Letters 31, 2972-2974 (2006).
