- Email: [email protected]
Design and analysis of photonic crystal biperiodic waveguide structure based optofluidic-gas sensor
Optik 126 (2015) 5172–5175
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Design and analysis of photonic crystal biperiodic waveguide structure based optofluidic-gas sensor Ajeet Kumar ∗ , Than Singh Saini, Ravindra Kumar Sinha Department of Applied Physics, Delhi Technological University, Delhi 110 442, India
a r t i c l e
i n f o
Article history: Received 24 October 2014 Accepted 9 September 2015 Keywords: Photonic crystal Biperiodic waveguide Gas sensor Fluid sensor
a b s t r a c t We present a photonic crystal (PhC) biperiodic waveguide (BPW) structure for both gas and fluid sensing based on modulation of refractive index (RI) of supercavities. The proposed structure has been designed by creating a BPW in silicon (Si) substrate with triangular array of air holes. This BPW structure consists of an array of supercavities and only the resonant wavelength is allowed to pass through the waveguide while the rest of the wavelengths are reflected by the structure. The principle of sensing is based on the shift of the resonance wavelength of supercavities when infiltrated by any liquid or gas. The transmission spectra of sensor have been analyzed numerically by finite difference time domain (FDTD) method. The proposed design can be utilized as a gas sensor or fluid sensor by selectively filling the holes of supercavities. For gas sensing application, the sensitivity of the sensor has been found to be 610 nm per refractive index unit (RIU) with minimum detection limit of 0.0001 RIU. Sensitivity of 300 nm/RIU with wider RI detection range of 1.0–1.5 is obtained when the structure is used as a fluid sensor. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction In the recent years, refractive index (RI) based optical micro sensors have gained a high degree of interest because of their application in detection of wide range of physical parameters like humidity, temperature, pressure and force [1–4] as well as chemical and biological parameters [5–7]. Refractive index based sensors can be realized using directional couplers [8], Mach–Zehnder interferometers [9], micro-rings [10] and photonic crystals (PhCs). Among all the devices, PhCs have provided a new platform for the realization of RI optical sensor due to fabrication of low loss waveguides along with high-Q resonant cavities [11,12]. Various RI sensors based on waveguide [13–16], waveguide along with cavity [17,18] and resonant cavities [19–25] have been demonstrated theoretically and experimentally. Although structures employed with resonant cavity in waveguide offer wide measurement range of RI, they have less coupling efficiency from waveguide to cavity which reduces their transmission efficiency [22]. PhC waveguide based optofluidic RI sensors have higher transmission efficiency but they have relatively less sensitivity [15,16]. On the other hand, high Q-cavities based RI gas sensors offer high sensitivity, low detection limit and faster response but with low detection range [26]. All the aforementioned PhC structures can either be used as
∗ Corresponding author. Tel.: +91 7838581069. E-mail address: [email protected] (A. Kumar). http://dx.doi.org/10.1016/j.ijleo.2015.09.157 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
liquid sensor or a gas sensor, however, no PhC structure proposed which provides platform for both gas and liquid sensing with higher sensitivity. In this paper, we have proposed a photonic crystal based biperiodic waveguide (BPW) structure for RI sensing. In our proposed structure, radii of the holes just adjacent to the waveguide have been changed in such a manner that they form an array of supercavities along the waveguide while one supercavity consists of three holes on both sides of the waveguide with radius of central hole modulated. The structure consists of two different lattice constants thus forming a BPW structure. The design consists of triangular array of holes in silicon (Si) substrate as the PBG arises from periodically patterned dielectric lattice Si PhCs and can be utilized for relatively low-loss waveguiding [27] and microcavities with quality factor (Q) as high as 106 [28,29], which is advantageous for both gas and fluid sensing. Initially, RI of all the holes of supercavities has been changed to study the response of structure while in the latter part, RI of only the bigger holes has been changed. In the former case, sensitivity of the structure has been found to be 610 nm/RIU (refractive index unit) with minimum detection limit of order of 10−4 and detection range from 1 to 1.002 while in the latter case sensitivity of 300 nm/RIU has been achieved with wider detection range from 1 to 1.5 and minimum detection limit of 10−3 . The proposed structure can be used for both gas sensing (all the holes of supercavities have been infiltrated) and fluid sensing (bigger holes of supercavities have been filled).
A. Kumar et al. / Optik 126 (2015) 5172–5175
Fig. 1. Layout of the proposed structure for RI and gas sensing. The structure consists of an array of supercavity (as shown in inset).
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Fig. 4. Computed mode profile of the supercavity for R2 = 0.4 × a at which the resonance wavelength of the supercavity matches with the peak value of Lorentzian response obtained for waveguide.
Fig. 2. Supercavity which has been formed by changing the radius of one hole.
Fig. 5. Transmission spectra obtained for biperiodic structure. Fig. 3. Array of supercavities with different periodicity which lead to the formation of biperiodic structure.
2. Sensor design The structure of the proposed sensor consists of one waveguide in two dimensional (2D) PhC with triangular lattice of circular air holes having periodicity a, as depicted in Fig. 1. The triangular PhC is practically important because it offers large transverse electric (TE) bandgap and can be integrated along with other optoelectonic devices [30]. The waveguide has been formed by removing one row of air holes along the –K direction. The radii of the holes just next to waveguide have been modulated at alternate positions in order to form array of supercavities while one supercavity possess three holes with central hole radius modulated, as shown in Fig. 2. The proposed design for sensor consists of the array of supercavites having different periodicity a = 856 nm, as shown in Fig. 3 and thus creating a biperiodic structure. The structure consists of finite array of 17 × 15 air holes with silicon (n = 3.5) as background material due to its strong potential in the fields of integrated optics and optical sensing [31–34]. The structure has two different size of holes of radii, R1 = 0.35 × a and R2 = 0.4 × a with lattice constant, a = 428 nm. The radius, R2 has been changed in order to get resonant mode in the structure. The light has been coupled at one end of the waveguide and the transmission spectrum has been observed at the other end of waveguide. 3. Simulation and results Numerical analysis has been done using finite difference time domain method (FDTD). Initially computation has been done to calculate resonant mode of supercavity inside the structure. The size of the green hole has been changed to get the resonant mode, as shown in Fig. 2. The calculated results show that the resonant mode has been found for R2 = 0.4 × a, as shown in Fig. 4. The resonance wavelength of the supercavity has been found to be = 1550 nm.
Fig. 6. The magnetic field distribution for biperiodic sensor at resonance wavelength of 1550 nm.
In order to form array of supercavities along the whole waveguide, holes at alternate position have been modified to R2 = 0.4 × a. The calculated transmission spectrum for the BPW structure has been found to have Lorentzian response with maximum transmission of 91.74%, as shown in Fig. 5. It has been found that the BPW structure only allows the resonance wavelength to pass through the waveguide and reflects the remaining wavelengths. The field distribution for the sensor at resonance wavelength of 1550 nm is shown in Fig. 6. The principle involved in RI sensing is based upon the shift of the resonant wavelength of supercavities. Supercavities can be used as a local sensor which when infiltrated with gases or liquids of different RI observe change in their optical properties. Two schemes have been employed to carry out simulation and analysis of the structure. In the first scheme, RI of all the holes of supercavities has been changed while in second case, RI of holes at alternate position has been changed. 3.1. All holes of supercavities have been filled The resonance wavelength of the mode supported by cavity depends upon the local RI in the vicinity of cavity and geometric
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A. Kumar et al. / Optik 126 (2015) 5172–5175
Fig. 7. Normalized transmission spectra of the sensor with five different refractive indices of supercavities.
Fig. 9. Normalized transmission spectra for the sensor with five different refractive indices ranging from n = 1.446 to n = 1.450 in 0.001 increments.
Fig. 8. Resonant wavelength shift plotted as a function of ambient index n.
parameters of structure. When all the holes of supercavities have been filled with an analyte, there is change in local RI of the supercavity which shifts the resonance wavelength. The magnitude of the wavelength shift divided by change in RI gives the sensitivity of the device. The shift in resonance wavelength satisfies = o
n no
(1)
where n is the change in RI of supercavities, n is the initial RI of supercavities, is the initial resonance wavelength and is the shift in resonance wavelength when n changes to n + n. The FDTD simulation results show that when the device is operated at resonance wavelength of = 1550 nm, the resonance wavelength shifts about 0.061 nm for n = 0.0001 (as shown in Fig. 7) and hence the device has sensititvity, S = 610 nm/RIU, rendering it suitable for gas sensing. As the full width at half maxima of transmission spectra obtained for BPW structure, as shown in Fig. 5 is around 30 pm, the structure has been able to detect even the smallest change in RI of the order of 10−4 leading to high sensitivity of the structure. In Fig. 8, the resonance wavelength shift = (n) − (air) is plotted as a function of ambient RI, and the shift is 61 pm for n = 10−4 with detection limit of 1.002. 3.2. Only bigger holes of supercavities have been filled In the second scheme, instead of filling all the holes of supercavities only bigger air holes have been infiltrated with an analyte. In this case also, there is change in local RI of the supercavities which shifts the resonance wavelength. The precise simulation results have shown that the sensitivity has been decreased dramatically. In this case, resonance wavelength shifts upto 0.3 nm for n = 0.001, as shown in Fig. 9 with wider measurement range of RI 1.0–1.5. So the sensor’s sensitivity is S = 300 nm/RIU.
Fig. 10. Relation of resonant wavelength and ambient refractive index n.
In PhC waveguide sensor based on dispersive properties of waveguide, for obtaining larger sensitivity, filling fraction of the holes filled with the analyte must be larger. But in this case, by merely filling bigger holes, sensitivity of 300 nm/RIU has been achieved, which in turn means requirement of lesser volume of analyte over conventional waveguides. Also, structures with supercavities are more sensitive to RI changes and when these supercavities are arranged to form a complete waveguide, there is no coupling loss from waveguide to cavity thus offering wider measurement range of RI sensing. Fig. 10 depicts the resonance wavelength shift = (n) − (air) is plotted as a function of ambient RI, and the shift is 0.3 nm for n = 10−3 with measurement range from 1.0 to 1.5. 4. Conclusions We have presented a PhC based BPW structure which provides platform for both gas and fluid sensing. The structure has been implemented by changing the periodicity of holes just next to waveguide and introducing an array of supercavities along the whole waveguide. The transmission spectrum of BPW structure obtained using FDTD method shows that the structure allows only resonant wavelength of supercavity to pass through the waveguide and rest of the wavelengths are reflected. Thus, a Lorentzian response has been obtained for the waveguide instead of the dispersion curve obtained for conventional waveguides. The BPW structure offers several advantages over conventional waveguides or microcavity structures. The BPW structure offers high sensitivity
A. Kumar et al. / Optik 126 (2015) 5172–5175
and wider measurement range for RI sensing. By selective infiltration of holes of supercavity, the same structure can be used either as a gas sensor or a fluid sensor. Another advantage of this structure is that the structure is operated at resonance wavelength of 1550 nm and thus the device can be integrated with other optoelectronic devices used in broadband optical communication. For gas sensing applications, BPW structure offers sensitivity of 610 nm/RIU with minimum detection limit of the order of 10−4 . As an optofluidic sensor, the same structure offers sensitivity of 300 nm/RIU along with wider measurement range of RI from 1.0 to 1.5 and minimum detection limit of 0.001. References [1] H.Y. Fu, H.Y. Tam, L. Shao, X. Dong, P.A. Wai, C. Lu, S.K. Khijwania, Pressure sensor realized with polarization maintaining photonic crystal fiber-based Sagnac interferometer, Appl. Opt. 47 (2008) 2835–2839. [2] W.J. Bock, J. Chen, T. Eftimov, W. Urbanczyk, A photonic crystal fiber sensor for pressure measurements, IEEE Trans. Instrum. Meas. 55 (2006) 1119–1123. [3] X. Dong, H.Y. Tam, P. Shum, Temperature-insensitive strain sensor with polarization-maintaining photonic crystal fiber based Sagnac interferometer, Appl. Phys. Lett. 90 (2007) 151113–151115. [4] T. Stomeo, M. Grande, A. Qualtieri, A. Passaseo, A. Salhi, D. Biallo, F. Prudenzano, Fabrication of force sensors based on two-dimensional photonic crystal technology, Micro. Eng. 84 (2007) 1450–1453. [5] T. Hasek, R. Wilk, H. Kurt, D. Citrin, M. Koch, Proc. Int. Conf. Teraherz Electonic. (2006) 239. [6] N. Skivesen, A. Têtu, M. Kristensen, J. Kjems, L.H. Frandsen, P.I. Borel, Photoniccrystal waveguide biosensor, Opt. Express 15 (2007) 3169–3176. [7] I.D. Block, L. Chan, T. Cunningham, Photonic crystal optical biosensor incorporating structured low-index porous dielectric, Sens. Actuators, B: Chem. 120 (2006) 187–193. [8] B.J. Luff, B.J. Larris, J.S. Wilkinson, Integrated-optical directional coupler biosensor, Opt. Lett. 21 (1996) 618–620. [9] R.G. Heideman, R.P. Kooyman, J. Greve, Performance of a highly sensitive optical waveguide Mach–Zehnder interferometer immunosensor, Sens. Actuators, B: Chem. 10 (1993) 209–217. [10] K.D. Vos, I. Bartolozzi, E. Schacht, P. Bienstman, R. Baets, Silicon-on-insulator microring resonator for sensitive and label-free biosensing, Opt. Express 15 (2007) 7610–7615. [11] J.D. Joannopoulos, R.D. Meade, J.N. Winn, Photonic Crystals: Molding the Flow of Light, Princeton University Press, Princeton, NJ, 1995. [12] Y. Akahane, T. Asano, B.S. Song, S. Noda, High-Q photonic nanocavity in a twodimensional photonic crystal, Nature 425 (2003) 944–947. [13] S. Xiao, N.A. Mortenson, Proposal of highly sensitive optofluidic sensors based on dispersive photonic crystal waveguides, J. Opt. A: Pure Appl. Opt. 9 (2007) 463–467. [14] S. Xiao, N.A. Mortensen, Highly dispersive photonic band-gap-edge optofluidic biosensors, J. Eur. Opt. Soc., Rapid Publ. 1 (2006) 06026.
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[15] Shruti, R.K. Sinha, R. Bhattacharya, Photonic crystal slab waveguide-based infiltrated liquid sensors: design and analysis, J. Nanophotonics 5 (2011) 053505. [16] H.S. Dutta, S. Pal, Design of a highly sensitive photonic crystal waveguide platform for refractive index based biosensing, Opt. Quant. Electron. 45 (2013) 907–917. [17] X. Wang, Z. Xu, N. Lu, J. Zhu, G. Jin, Ultracompact refractive index sensor based on microcavity in the sandwiched photonic crystal waveguide structure, Opt. Commun. 281 (2008) 1725–1731. [18] X. Wang, N. Lu, Q. Tan, G. Jin, Investigation of biosensor built with photonic crystal microcavity, Chin. Opt. Lett. 12 (2008) 925–927. [19] U. Bog, C.L. Smith, M.W. Lee, S. Tomljenovic-Hanic, C. Grillet, C. Monat, L. O’Faolain, C. Karnutsch, T.F. Krauss, R.C. McPhedran, B.J. Eggleton, High Q microfluidic cavities in silicon-based two-dimensional photonic crystal structures, Opt. Lett. 33 (2008) 2206–2208. [20] S. Tomljenovic-Hanic, C.M. de Sterke, M.J. Steel, Design of high-Q cavities in photonic crystal slab heterostructures by air-holes infiltration, Opt. Express 14 (2006) 12451–12456. [21] P. Domachuk, H.C. Nguyen, B.J. Eggleton, M. Straub, M. Gu, Microfluidic tunable photonic band-gap device, Appl. Phys. Lett. 84 (2004) 1838–1840. [22] D.F. Dorfner, T. Hürlimann, T. Zabel, L.H. Frandsen, G. Abstreiter, Silicon photonic crystal nanostructures for refractive index sensing, Appl. Phys. Lett. 93 (2008) 181103. [23] T. Sünner, T. Stichel, S.H. Kwon, T.W. Schlereth, S. Höfling, Photonic crystal cavity based gas sensor, Appl. Phys. Lett. 92 (2008) 261112. [24] J. Jágerská, H. Zhang, Z. Diao, N.L. Thomas, R. Houdré, Refractive index sensing with an air-slot photonic crystal nanocavity, Opt. Lett. 35 (2010) 2523–2525. [25] J. Dahdah, N. Courjal, F.I. Baida, Analysis of a photonic crystal cavity based on absorbent layer for sensing applications, J. Opt. Soc. Am. B: Opt. Phys. 27 (2010) 305–310. [26] A. Benz, C. Deutsch, M. Brandstetter, A.M. Andrews, P. Klang, H. Detz, W. Schrenk, G. Strasser, K. Unterrainer, Terahertz active photonic crystals for condensed gas sensing, Sensors 11 (2011) 6003–6014. [27] S.Y. Lin, E. Chow, V. Hietala, P.R. Villeneuve, J.D. Joannopoulos, Experimental demonstration of guiding and bending of electromagnetic waves in a photonic crystal, Science 282 (1998) 274–276. [28] B. Song, S. Noda, T. Asano, Y. Akahane, Ultra-high-Q photonic doubleheterostructure nanocavity, Nat. Mater. 4 (2005) 207–210. [29] T. Asano, B.S. Song, S. Noda, Analysis of the experimental Q factors (∼1 million) of photonic crystal nanocavities, Opt. Express 14 (2006) 1996–2002. [30] C. Jamois, R. Wehrspohn, J. Schilling, F. Muller, R. Hillebrand, W. Hergert, Siliconbased photonic crystal slabs: two concepts, IEEE J. Quantum Electron. 38 (2002) 805–810. [31] T. Krauss, R. De La Rue, Photonic crystal in the optical regime-past, present and future, Prog. Quantum Electron. 23 (1999) 51–96. [32] I.M. White, X. Fan, On the performance quantification of resonant refractive index sensors, Opt. Express 16 (2008) 1020–1028. [33] N. Mortensen, S. Xiao, J. Pedersen, Liquid-infiltrated photonic crystals: enhanced light-matter interactions for lab-on-a-chip applications, Microfluid. Nanofluid. 4 (2008) 117–127. [34] D. Dorfner, T. Zabel, T. Hürlimann, N. Hauke, L. Frandsen, U. Rant, G. Abstreiter, J. Finley, Photonic crystal nanostructures for optical biosensing applications, Biosens. Bioelectron. 24 (2009) 3688–3692.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Design and analysis of photonic crystal biperiodic waveguide structure based optofluidic-gas sensor Ajeet Kumar ∗ , Than Singh Saini, Ravindra Kumar Sinha Department of Applied Physics, Delhi Technological University, Delhi 110 442, India
a r t i c l e
i n f o
Article history: Received 24 October 2014 Accepted 9 September 2015 Keywords: Photonic crystal Biperiodic waveguide Gas sensor Fluid sensor
a b s t r a c t We present a photonic crystal (PhC) biperiodic waveguide (BPW) structure for both gas and fluid sensing based on modulation of refractive index (RI) of supercavities. The proposed structure has been designed by creating a BPW in silicon (Si) substrate with triangular array of air holes. This BPW structure consists of an array of supercavities and only the resonant wavelength is allowed to pass through the waveguide while the rest of the wavelengths are reflected by the structure. The principle of sensing is based on the shift of the resonance wavelength of supercavities when infiltrated by any liquid or gas. The transmission spectra of sensor have been analyzed numerically by finite difference time domain (FDTD) method. The proposed design can be utilized as a gas sensor or fluid sensor by selectively filling the holes of supercavities. For gas sensing application, the sensitivity of the sensor has been found to be 610 nm per refractive index unit (RIU) with minimum detection limit of 0.0001 RIU. Sensitivity of 300 nm/RIU with wider RI detection range of 1.0–1.5 is obtained when the structure is used as a fluid sensor. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction In the recent years, refractive index (RI) based optical micro sensors have gained a high degree of interest because of their application in detection of wide range of physical parameters like humidity, temperature, pressure and force [1–4] as well as chemical and biological parameters [5–7]. Refractive index based sensors can be realized using directional couplers [8], Mach–Zehnder interferometers [9], micro-rings [10] and photonic crystals (PhCs). Among all the devices, PhCs have provided a new platform for the realization of RI optical sensor due to fabrication of low loss waveguides along with high-Q resonant cavities [11,12]. Various RI sensors based on waveguide [13–16], waveguide along with cavity [17,18] and resonant cavities [19–25] have been demonstrated theoretically and experimentally. Although structures employed with resonant cavity in waveguide offer wide measurement range of RI, they have less coupling efficiency from waveguide to cavity which reduces their transmission efficiency [22]. PhC waveguide based optofluidic RI sensors have higher transmission efficiency but they have relatively less sensitivity [15,16]. On the other hand, high Q-cavities based RI gas sensors offer high sensitivity, low detection limit and faster response but with low detection range [26]. All the aforementioned PhC structures can either be used as
∗ Corresponding author. Tel.: +91 7838581069. E-mail address: [email protected] (A. Kumar). http://dx.doi.org/10.1016/j.ijleo.2015.09.157 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
liquid sensor or a gas sensor, however, no PhC structure proposed which provides platform for both gas and liquid sensing with higher sensitivity. In this paper, we have proposed a photonic crystal based biperiodic waveguide (BPW) structure for RI sensing. In our proposed structure, radii of the holes just adjacent to the waveguide have been changed in such a manner that they form an array of supercavities along the waveguide while one supercavity consists of three holes on both sides of the waveguide with radius of central hole modulated. The structure consists of two different lattice constants thus forming a BPW structure. The design consists of triangular array of holes in silicon (Si) substrate as the PBG arises from periodically patterned dielectric lattice Si PhCs and can be utilized for relatively low-loss waveguiding [27] and microcavities with quality factor (Q) as high as 106 [28,29], which is advantageous for both gas and fluid sensing. Initially, RI of all the holes of supercavities has been changed to study the response of structure while in the latter part, RI of only the bigger holes has been changed. In the former case, sensitivity of the structure has been found to be 610 nm/RIU (refractive index unit) with minimum detection limit of order of 10−4 and detection range from 1 to 1.002 while in the latter case sensitivity of 300 nm/RIU has been achieved with wider detection range from 1 to 1.5 and minimum detection limit of 10−3 . The proposed structure can be used for both gas sensing (all the holes of supercavities have been infiltrated) and fluid sensing (bigger holes of supercavities have been filled).
A. Kumar et al. / Optik 126 (2015) 5172–5175
Fig. 1. Layout of the proposed structure for RI and gas sensing. The structure consists of an array of supercavity (as shown in inset).
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Fig. 4. Computed mode profile of the supercavity for R2 = 0.4 × a at which the resonance wavelength of the supercavity matches with the peak value of Lorentzian response obtained for waveguide.
Fig. 2. Supercavity which has been formed by changing the radius of one hole.
Fig. 5. Transmission spectra obtained for biperiodic structure. Fig. 3. Array of supercavities with different periodicity which lead to the formation of biperiodic structure.
2. Sensor design The structure of the proposed sensor consists of one waveguide in two dimensional (2D) PhC with triangular lattice of circular air holes having periodicity a, as depicted in Fig. 1. The triangular PhC is practically important because it offers large transverse electric (TE) bandgap and can be integrated along with other optoelectonic devices [30]. The waveguide has been formed by removing one row of air holes along the –K direction. The radii of the holes just next to waveguide have been modulated at alternate positions in order to form array of supercavities while one supercavity possess three holes with central hole radius modulated, as shown in Fig. 2. The proposed design for sensor consists of the array of supercavites having different periodicity a = 856 nm, as shown in Fig. 3 and thus creating a biperiodic structure. The structure consists of finite array of 17 × 15 air holes with silicon (n = 3.5) as background material due to its strong potential in the fields of integrated optics and optical sensing [31–34]. The structure has two different size of holes of radii, R1 = 0.35 × a and R2 = 0.4 × a with lattice constant, a = 428 nm. The radius, R2 has been changed in order to get resonant mode in the structure. The light has been coupled at one end of the waveguide and the transmission spectrum has been observed at the other end of waveguide. 3. Simulation and results Numerical analysis has been done using finite difference time domain method (FDTD). Initially computation has been done to calculate resonant mode of supercavity inside the structure. The size of the green hole has been changed to get the resonant mode, as shown in Fig. 2. The calculated results show that the resonant mode has been found for R2 = 0.4 × a, as shown in Fig. 4. The resonance wavelength of the supercavity has been found to be = 1550 nm.
Fig. 6. The magnetic field distribution for biperiodic sensor at resonance wavelength of 1550 nm.
In order to form array of supercavities along the whole waveguide, holes at alternate position have been modified to R2 = 0.4 × a. The calculated transmission spectrum for the BPW structure has been found to have Lorentzian response with maximum transmission of 91.74%, as shown in Fig. 5. It has been found that the BPW structure only allows the resonance wavelength to pass through the waveguide and reflects the remaining wavelengths. The field distribution for the sensor at resonance wavelength of 1550 nm is shown in Fig. 6. The principle involved in RI sensing is based upon the shift of the resonant wavelength of supercavities. Supercavities can be used as a local sensor which when infiltrated with gases or liquids of different RI observe change in their optical properties. Two schemes have been employed to carry out simulation and analysis of the structure. In the first scheme, RI of all the holes of supercavities has been changed while in second case, RI of holes at alternate position has been changed. 3.1. All holes of supercavities have been filled The resonance wavelength of the mode supported by cavity depends upon the local RI in the vicinity of cavity and geometric
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A. Kumar et al. / Optik 126 (2015) 5172–5175
Fig. 7. Normalized transmission spectra of the sensor with five different refractive indices of supercavities.
Fig. 9. Normalized transmission spectra for the sensor with five different refractive indices ranging from n = 1.446 to n = 1.450 in 0.001 increments.
Fig. 8. Resonant wavelength shift plotted as a function of ambient index n.
parameters of structure. When all the holes of supercavities have been filled with an analyte, there is change in local RI of the supercavity which shifts the resonance wavelength. The magnitude of the wavelength shift divided by change in RI gives the sensitivity of the device. The shift in resonance wavelength satisfies = o
n no
(1)
where n is the change in RI of supercavities, n is the initial RI of supercavities, is the initial resonance wavelength and is the shift in resonance wavelength when n changes to n + n. The FDTD simulation results show that when the device is operated at resonance wavelength of = 1550 nm, the resonance wavelength shifts about 0.061 nm for n = 0.0001 (as shown in Fig. 7) and hence the device has sensititvity, S = 610 nm/RIU, rendering it suitable for gas sensing. As the full width at half maxima of transmission spectra obtained for BPW structure, as shown in Fig. 5 is around 30 pm, the structure has been able to detect even the smallest change in RI of the order of 10−4 leading to high sensitivity of the structure. In Fig. 8, the resonance wavelength shift = (n) − (air) is plotted as a function of ambient RI, and the shift is 61 pm for n = 10−4 with detection limit of 1.002. 3.2. Only bigger holes of supercavities have been filled In the second scheme, instead of filling all the holes of supercavities only bigger air holes have been infiltrated with an analyte. In this case also, there is change in local RI of the supercavities which shifts the resonance wavelength. The precise simulation results have shown that the sensitivity has been decreased dramatically. In this case, resonance wavelength shifts upto 0.3 nm for n = 0.001, as shown in Fig. 9 with wider measurement range of RI 1.0–1.5. So the sensor’s sensitivity is S = 300 nm/RIU.
Fig. 10. Relation of resonant wavelength and ambient refractive index n.
In PhC waveguide sensor based on dispersive properties of waveguide, for obtaining larger sensitivity, filling fraction of the holes filled with the analyte must be larger. But in this case, by merely filling bigger holes, sensitivity of 300 nm/RIU has been achieved, which in turn means requirement of lesser volume of analyte over conventional waveguides. Also, structures with supercavities are more sensitive to RI changes and when these supercavities are arranged to form a complete waveguide, there is no coupling loss from waveguide to cavity thus offering wider measurement range of RI sensing. Fig. 10 depicts the resonance wavelength shift = (n) − (air) is plotted as a function of ambient RI, and the shift is 0.3 nm for n = 10−3 with measurement range from 1.0 to 1.5. 4. Conclusions We have presented a PhC based BPW structure which provides platform for both gas and fluid sensing. The structure has been implemented by changing the periodicity of holes just next to waveguide and introducing an array of supercavities along the whole waveguide. The transmission spectrum of BPW structure obtained using FDTD method shows that the structure allows only resonant wavelength of supercavity to pass through the waveguide and rest of the wavelengths are reflected. Thus, a Lorentzian response has been obtained for the waveguide instead of the dispersion curve obtained for conventional waveguides. The BPW structure offers several advantages over conventional waveguides or microcavity structures. The BPW structure offers high sensitivity
A. Kumar et al. / Optik 126 (2015) 5172–5175
and wider measurement range for RI sensing. By selective infiltration of holes of supercavity, the same structure can be used either as a gas sensor or a fluid sensor. Another advantage of this structure is that the structure is operated at resonance wavelength of 1550 nm and thus the device can be integrated with other optoelectronic devices used in broadband optical communication. For gas sensing applications, BPW structure offers sensitivity of 610 nm/RIU with minimum detection limit of the order of 10−4 . As an optofluidic sensor, the same structure offers sensitivity of 300 nm/RIU along with wider measurement range of RI from 1.0 to 1.5 and minimum detection limit of 0.001. References [1] H.Y. Fu, H.Y. Tam, L. Shao, X. Dong, P.A. Wai, C. Lu, S.K. Khijwania, Pressure sensor realized with polarization maintaining photonic crystal fiber-based Sagnac interferometer, Appl. Opt. 47 (2008) 2835–2839. [2] W.J. Bock, J. Chen, T. Eftimov, W. Urbanczyk, A photonic crystal fiber sensor for pressure measurements, IEEE Trans. Instrum. Meas. 55 (2006) 1119–1123. [3] X. Dong, H.Y. Tam, P. Shum, Temperature-insensitive strain sensor with polarization-maintaining photonic crystal fiber based Sagnac interferometer, Appl. Phys. Lett. 90 (2007) 151113–151115. [4] T. Stomeo, M. Grande, A. Qualtieri, A. Passaseo, A. Salhi, D. Biallo, F. Prudenzano, Fabrication of force sensors based on two-dimensional photonic crystal technology, Micro. Eng. 84 (2007) 1450–1453. [5] T. Hasek, R. Wilk, H. Kurt, D. Citrin, M. Koch, Proc. Int. Conf. Teraherz Electonic. (2006) 239. [6] N. Skivesen, A. Têtu, M. Kristensen, J. Kjems, L.H. Frandsen, P.I. Borel, Photoniccrystal waveguide biosensor, Opt. Express 15 (2007) 3169–3176. [7] I.D. Block, L. Chan, T. Cunningham, Photonic crystal optical biosensor incorporating structured low-index porous dielectric, Sens. Actuators, B: Chem. 120 (2006) 187–193. [8] B.J. Luff, B.J. Larris, J.S. Wilkinson, Integrated-optical directional coupler biosensor, Opt. Lett. 21 (1996) 618–620. [9] R.G. Heideman, R.P. Kooyman, J. Greve, Performance of a highly sensitive optical waveguide Mach–Zehnder interferometer immunosensor, Sens. Actuators, B: Chem. 10 (1993) 209–217. [10] K.D. Vos, I. Bartolozzi, E. Schacht, P. Bienstman, R. Baets, Silicon-on-insulator microring resonator for sensitive and label-free biosensing, Opt. Express 15 (2007) 7610–7615. [11] J.D. Joannopoulos, R.D. Meade, J.N. Winn, Photonic Crystals: Molding the Flow of Light, Princeton University Press, Princeton, NJ, 1995. [12] Y. Akahane, T. Asano, B.S. Song, S. Noda, High-Q photonic nanocavity in a twodimensional photonic crystal, Nature 425 (2003) 944–947. [13] S. Xiao, N.A. Mortenson, Proposal of highly sensitive optofluidic sensors based on dispersive photonic crystal waveguides, J. Opt. A: Pure Appl. Opt. 9 (2007) 463–467. [14] S. Xiao, N.A. Mortensen, Highly dispersive photonic band-gap-edge optofluidic biosensors, J. Eur. Opt. Soc., Rapid Publ. 1 (2006) 06026.
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