Slotted photonic crystal nanobeam cavity with parabolic modulated width stack for refractive index sensing Peipeng Xu, Kaiyuan Yao, Jiajiu Zheng, Xiaowei Guan, and Yaocheng Shi* Centre for Optical and Electromagnetic Research, State Key Laboratory for Modern Optical Instrumentation, Zhejiang Provincial Key Laboratory for Sensing Technologies, Zhejiang University, Zijingang Campus, Hangzhou 310058, China *[email protected]
Abstract: We present the design, fabrication, and the characterization of high-Q slotted 1D photonic crystal (PhC) cavities with parabolic-width stack. Their peculiar geometry enables the location of the resonating mode close to the air-band. The majority of optical field distributes in the slotted low-index area and the light matter interaction with the analytes has been enhanced. Cavities with measured Q-factors ~104 have been demonstrated. The refractive index sensing measurement for NaCl solutions with different concentrations shows a sensitivity around 410. Both the achieved Q-factor and the sensitivity are higher than the one reported recently by using 2D slotted PhC cavities. The total size for the sensing part of the present device is reduced to 16.8 × 2.5 μm2. ©2013 Optical Society of America. OCIS codes: (130.3120) Integrated optics devices; (230.5298) Photonic crystals; (280.4788) Optical sensing and sensors.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
R. Daw and J. Finkelstein, “Lab on a chip,” Nature 442(7101), 367 (2006). X. Fan, I. M. White, S. I. Shopova, H. Zhu, J. D. Suter, and Y. Sun, “Sensitive optical biosensors for unlabeled targets: A review,” Anal. Chim. Acta 620(1-2), 8–26 (2008). M. G. Scullion, T. F. Krauss, and A. Di Falco, “Slotted photonic crystal sensors,” Sensors (Basel) 13(3), 3675– 3710 (2013). J. T. Robinson, L. Chen, and M. Lipson, “On-chip gas detection in silicon optical microcavities,” Opt. Express 16(6), 4296–4301 (2008). J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). B. J. Luff, J. S. Wilkinson, J. Piehler, U. Hollenbach, J. Ingenhoff, and N. Fabricius, “Integrated optical MachZehnder biosensor,” J. Lightwave Technol. 16(4), 583–592 (1998). K. Yao and Y. Shi, “High-Q width modulated photonic crystal stack mode-gap cavity and its application to refractive index sensing,” Opt. Express 20(24), 27039–27044 (2012). V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding and confining light in void nanostructure,” Opt. Lett. 29(11), 1209–1211 (2004). A. Di Falco, L. O'Faolain, and T. F. Krauss, “Chemical sensing in slotted photonic crystal heterostructure cavities,” Appl. Phys. Lett. 94(6), 063503 (2009). J. Jágerská, H. Zhang, Z. Diao, N. Thomas, and R. Houdré, “Refractive index sensing with an air-slot photonic crystal nanocavity,” Opt. Lett. 35(15), 2523–2525 (2010). M. G. Scullion, A. Di Falco, and T. F. Krauss, “Slotted photonic crystal cavities with integrated microfluidics for biosensing applications,” Biosens. Bioelectron. 27(1), 101–105 (2011). S. H. Mirsadeghi, E. Schelew, and J. F. Young, “Photonic crystal slot-microcavity circuit implemented in silicon-on-insulator: High Q operation in solvent without undercutting,” Appl. Phys. Lett. 102(13), 131115 (2013). M. Notomi, E. Kuramochi, and H. Taniyama, “Ultrahigh-Q Nanocavity with 1D Photonic Gap,” Opt. Express 16(15), 11095–11102 (2008). E. Kuramochi, H. Taniyama, T. Tanabe, K. Kawasaki, Y.-G. Roh, and M. Notomi, “Ultrahigh-Q onedimensional photonic crystal nanocavities with modulated mode-gap barriers on SiO2 claddings and on air claddings,” Opt. Express 18(15), 15859–15869 (2010). P. B. Deotare, M. W. McCutcheon, I. W. Frank, M. Khan, and M. Lončar, “High quality factor photonic crystal nanobeam cavities,” Appl. Phys. Lett. 94(12), 121106 (2009).
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26908
16. M. W. McCutcheon and M. Lončar, “Design of a silicon nitride photonic crystal nanocavity with a quality factor of one million for coupling to a diamond nanocrystal,” Opt. Express 16(23), 19136–19145 (2008). 17. Y. Gong and J. Vučković, “Photonic crystal cavities in silicon dioxide,” Appl. Phys. Lett. 96(3), 031107 (2010). 18. B.-H. Ahn, J.-H. Kang, M.-K. Kim, J.-H. Song, B. Min, K.-S. Kim, and Y.-H. Lee, “One-dimensional parabolicbeam photonic crystal laser,” Opt. Express 18(6), 5654–5660 (2010). 19. Q. Quan and M. Lončar, “Deterministic design of wavelength scale, ultra-high Q photonic crystal nanobeam cavities,” Opt. Express 19(19), 18529–18542 (2011). 20. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96(20), 203102 (2010). 21. D. R. Lyde, ed., Handbook of Chemistry and Physics (CRC Press, 1997–1998). 22. K. F. Palmer and D. Williams, “Optical properties of water in the near infrared,” J. Opt. Soc. Am. 64(8), 1107– 1110 (1974). 23. C. Monat, P. Domachuk, and B. J. Eggleton, “Integrated optofluidics: A new river of light,” Nat. Photonics 1(2), 106–114 (2007).
1. Introduction Optical biosensors have attracted considerable interest for lab-on-a-chip applications due to their advantages such as small size, biocompatibility, and lack of the need for fluorescent labels [1–3]. Many micro-photonic devices have been proposed to realize optical biosensors, such as ring resonators [4], surface plasmons resonators [5], Mach-Zehnder interferometer [6], and photonic crystals (PhC) [7]. As far as the use of PhC resonators as refractive index sensors is concerned, strong light matter interaction for the optical fields with the analytes are preferred. However, in typical PhC cavities, the optical mode is strongly confined in the high index material to achieve high Q. Almeida et al. proposed “slot-waveguide” structures whereby the discontinuity of the optical field at a dielectric interface was exploited to support a propagation mode in the lowindex narrow gap [8]. Such concept has attracted people’s attention in the sensing community widely especially in two-dimensional (2D) PhC cavities [9–12]. Such 2D slotted PhC cavities optical sensors enjoy extremely small active sensing volume. However, the footprint for the sensing part is always large with ~60 × 60 μm2 [12], which is not conductive to be densely assembled for sensor arrays. In the present paper, we combined the concept of the one-dimensional (1D) PhC nanobeam cavities [13–15] and the “slot waveguide”. We presented the design, fabrication, and the characterization of high-Q slotted 1D PhC cavities with parabolic-width stack. The fabricated device shows a Q-factor near 104 which can be further improved by optimizing the fabrication processes since the calculated Q-factor is ~106. The measurements for NaCl solutions with different concentrations show that a sensitivity around 410 nm/RIU can be achieved. With a comparable sensitivity to the sensors based on 2D slotted PhC cavities [10, 12], the size for the sensing part is only 16.8 × 2.5 μm2, which is attractive for the realization of on-chip sensor arrays. 2. Device design and analysis Several designs of high Q 1D PhC cavities have been proposed by modulating the air-hole size or lattice constant [15–17]. For such designs, nanometer level dimension control is required to achieve precise patterning. The fabrication is very challenging due to the proximity effect caused by the electron scattering in the e-beam resist. To ease the fabrication, an adiabatic parabolic width tapering concept [7, 18] was utilized in this paper to obtain high Q-factor and natural coupling between the cavity and the feeding waveguides [19]. Figure 1(a) shows the schematic for the 1D slotted stack cavity considered in this work. The slotted stack cavity is formed by introducing a slot between two periodic arrays of dielectric stacks. Figure 1(c) is the zoomed in picture of the framed part in (a) with annotation. The widths (Wy(i)) of the dielectric blocks are quadratically modulated (Wy(i) = Wy(0) + i2(Wy(imax)Wy(0))/imax2, from the center to both sides, i increases from 0 to imax),while Wx keeps unchanged. Thus, a Gaussian type confinement has been achieved [20].The band diagrams of the period slotted stack with Wy = 3.0a (width of the central stack) and Wy = 6.0a (width of the edge stack) for TE polarization were given in Fig. 1(d). The three dimensional ðnite
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26909
difference time domain (3D-FDTD) method with Bloch boundary conditions was utilized for the simulations. As expected, the band for the structure with Wy = 3.0a was higher than the one for Wy = 6.0a. The resonant frequency of the cavity mode [the black dot in Fig. 1(d)] is slightly lower than the dielectric band edge of the stacks with Wy = 3.0a. The Silicon-on-Insulator (SOI) platform has been used in this work. The refractive index and the thickness of the silicon and silica layer are 3.46, 1.44 and 220 nm, 2 μm, respectively. Figure 1(b) shows the intensity profile (|Ey|2) calculated by 3D FDTD method. To keep the resonance of the cavity mode near 1550 nm, we choose the following parameters: lattice constant a = 410 nm, Wx = 0.4a, imax = 20, Slot Width = 200 nm. The optimal structure supports a fundamental mode at λ = 1546.5 nm with a Qtot of 3.73 × 106 and the effective mode volume of 0.217(λ/nc)3 (defined as V =
dV ε
2
2
E / (ε E ) max ).The coupling Q is
calculated by monitoring the power coupled into the waveguide mode [17]. By defining quality factor with Qcoupling = ωU/P (ω is the frequency of the mode, U is the total energy of the mode, P is the power coupled to the feeding waveguide), we obtain Qcoupling = 8.883 × 106, and Qrad = 1/(1/Qtot-1/Qcoupling) = 6.43 × 106. The majority of electric field is strongly localized in the slotted low-index area, which indicates a strong interaction between the analytes and the cavity mode.
Fig. 1. (a) Schematics of slotted width modulated stack cavity. (b) Electric field intensity (|Ey|2) calculated by 3D FDTD method for the cavity with Slot Width = 200 nm, a = 410 nm, Wx = 0.4a and Wy parabolically modulated from Wy(0) = 3.0a in the center to Wy(20) = 6.0a on either side. (c) Zoomed in picture of the framed part in (a) with annotations. (d) TE band structure of periodic stacks with Wy = 3.0a (red lines) and Wy = 6.0a (blue lines).The black dots indicates the resonant frequency. (e)Wavelength shifts and Qtot variances over different background refractive indexes. (f) Influence of the slot width on the Q-factor and sensitivity of slotted stack cavity sensor.
Toward a quantitative understanding of the response to changes of the refractive index, the resonant wavelengths for the cavity with different ambient analytes have been calculated
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26910
using 3D-FDTD. The device is covered with a PMMA layer for our simulations, which is consistent with our following experiment. Figure 1(e) shows the resonant wavelength and Qfactor varies with different refractive indices of the covering analytes. A red shift of the resonance is clearly visible as the refractive index of the ambient is increased. We find a linear dependence of the resonance on the refractive index, which is preferred for sensing applications. From the numerical simulations, we find that the width of the slot highly influences the sensing performances. Figure 1(f) shows the Q-factor and the sensitivity vary as the function of the slot width. Here the sensitivity is defined as the resonant wavelength shifts induced by the change of one refractive index unit (RIU).We can find the sensitivity increases with the slot width, while the Q-factor decreases exponentially by expanding the slot width. To achieve high Q-factor while keep an attractive sensitivity, we choose a trade-off value of the Slot Width = 200 nm. For the cavity with Slot Width = 200 nm, a = 410 nm, Wx = 0.4a and Wy parabolically modulated from Wy(0) = 3.0a in the center to Wy(20) = 6.0a on either side, we can obtain a Q-factor = 3.73 × 106 and a sensitivity = 437 nm/RIU. 3. Fabrication and optical characterization of the device The devices were fabricated on a silicon-on-insulator (SOI) wafer (SOITEC Inc.) with a silicon layer of 220 nm and a buried oxide layer of 2 μm. A positive-tone electron beam resist film (PMMA 950K) was used as the etching hard mask. The film with a thickness of 310 nm was spin coated onto the SOI wafer at 4000 rpm and then baked on a hot plate at 180 °C for 20 minutes. Resists were exposed using an e-beam lithography tool (Raith150 II) at 20KV acceleration voltage and developed in a MIBK: IPA (1:3) mixture followed by a de-ionized (DI) water rinse. Then the pattern was transferred to the 220 nm silicon layer by inductively coupled plasma (ICP) etcher using a gas mixture of SF6 and C4F8. To remove the remaining resists, the samples were soaked and agitated in 1-Methyl-2-pyrrolidone (NMP), then followed by acetone in ultrasonic cleaner for 5 minutes, and finally rinsed in DI water. The grating couplers with the period of 645 nm with the duty cycle of 50:50 are fabricated on both the input/output waveguides by another overlay exposure followed by a shallow etching (70 nm). Figure 2(a) shows a scanning electron microscope micrograph of the fabricated device. One should note that Fig. 2(a) only includes left half of the device (the right half is symmetrical to the left) to give a clear view for the details. The enlarged views for the sensing cavity, the input grating coupler, and the coupling region between the ridge/slot waveguides are given in Fig. 2(b)-2(d), respectively. A ridge taper with length of 200 μm was utilized for the coupling between the 10 μm wide grating coupler to the 700 nm wide ridge waveguide. Then the 700 nm wide ridge waveguide is butt coupled to the 200 nm slot waveguide [see Fig. 2(d)]. The calculated coupling efficiency between the ridge/slot waveguides is ~84%.Another slot taper with length of 300 μm was introduced to expand the width of the slot waveguide to the width of the cavity region (2.46 μm) while keeping the slot width unchanged. Finally, the whole device was covered by a 500 nm PMMA layer to increase the coupling efficiency of the grating coupler. This PMMA layer was also used to define the sensing window. To evaluate the sensing potential for the cavity, a sensing window was opened over the cavity by another overlay exposure. Thus only the cavity part was exposed to the ambient, whereas the rest was still covered with the PMMA film.
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26911
Fig. 2. (a) SEM micrograph of the device (left half). (b)(c)(d) SEM images show the fabricated slotted stack cavity, input grating coupler, and the coupling region connecting the strip/slot waveguides. (e) Measured transmission spectrum of the slotted stack cavity in vacuum. The inset shows the fit to Lorentzian lineshape for the resonance (Q = ~9200).
TE polarized light from a tunable laser (Agilent 81940A) was launched to the cavity through a grating coupler. Another grating coupler was used for collecting the transmitted optical signal into the power meter (Agilient 81618A). Figure 2(e) presents the transmission spectrum of the slotted stack PhC cavity normalized by the peak resonance transmission. Lorentzian fitting of the resonance reveals a measured quality factor Q~9200 and the extinction ratio larger than 10 dB. The quality factor is smaller than the simulated value due to the fabrication imperfections and the material loss. Figure 3(a) shows the measured transmission spectrum when the device is immersed in NaCl solution with different concentrations. The ratio of the refractive index change to the concentration variation for the aqueous solution of NaCl is 0.0018 refractive index units (RIU)/1% at 20°C [21].In our experiment, the concentration for the NaCl solution varies from 0% to 25% with a step of 5% and the refractive index changes from 1.333 to 1.378 with a step of 0.009, correspondingly. From Fig. 3(a), one sees that the resonant wavelength increases as the concentration of the NaCl solution increases. The Q-factor of the cavity immersed in NaCl solution with 25% concentration is about 4700. Such a degradation of Q-factor may come from the water absorption loss [22]. After each measurement, the chip was rinsed with deionized water and agitated on a 90 °C hot plate for 20 mins to remove the residuals. For each measurement with different NaCl concentrations, the resonance for the cavity covered with DI water is used as the reference for calibration. This helps to avoid any influence of variations in incidental factors (e.g., temperature variation). According to the resonant wavelengths measurement for the cavity covered with different NaCl solutions, one can find that the sensitivity of device is about 410 nm/RIU with a linear fitting [Fig. 3(b)], which indicates good agreement to the simulations (~413 nm/RIU). This value is 1.5 times to the one achieved by the cavity without the slot as expected [7]. Both the achieved Q-factor and the sensitivity are higher than the one reported recently by using 2D slotted PhC cavities [12]. Besides, the sensing part (16.8 × 2.5 μm2) is only 1/100 of the one reported in ref. 12 (larger than 60 × 60 μm2). To be used in a practical context, our compact device can be combined with microfluidic channels, which suggests strong potential for lab-on-a-chip applications [23]. Furthermore, we did find our device supports the high order mode, this high order mode could be well separated from the fundamental mode with its intensity much smaller (~15 dB)
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26912
than the fundamental one. From Fig. 1(e), we can also find the Q-factors maintain a higher than 106 over a wide wavelength range, indicating a very large sensing range of the device. This is especially advantageous compared to the whispering mode based cavities with its sensing range inevitably limited by the free spectral range (FSR).
Fig. 3. (a) Measured transmission responses of the slotted stack photonic crystal cavity covered the aqueous NaCl solutions with different concentrations. (b) The resonant wavelength λ shifts as the concentration varies.
4. Conclusion In conclusion, we have presented a high-Q slotted PhC nanobeam cavity with parabolic-width stack for refractive index sensing. By employing parabolic modulated width stack without any reflected mirror, high Q-factor near 104 has been achieved. The transmission spectrum measurement for the cavities covered by NaCl solutions with different concentration shows a sensitivity of 410 nm/RIU, which is comparable with the one based on 2D slotted PhC cavities. The small footprint of the sensing part (16.8 × 2.5 μm2) suggests a strong potential in on-chip biochemical sensing arrays. Acknowledgments We thank Jianwei Tang for valuable discussions. This work was partially supported by the National Nature Science Foundation of China (Grant No. 61377023), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY13F050002), and the Program of Zhejiang Leading Team of Science and Technology Innovation.
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26913
Abstract: We present the design, fabrication, and the characterization of high-Q slotted 1D photonic crystal (PhC) cavities with parabolic-width stack. Their peculiar geometry enables the location of the resonating mode close to the air-band. The majority of optical field distributes in the slotted low-index area and the light matter interaction with the analytes has been enhanced. Cavities with measured Q-factors ~104 have been demonstrated. The refractive index sensing measurement for NaCl solutions with different concentrations shows a sensitivity around 410. Both the achieved Q-factor and the sensitivity are higher than the one reported recently by using 2D slotted PhC cavities. The total size for the sensing part of the present device is reduced to 16.8 × 2.5 μm2. ©2013 Optical Society of America. OCIS codes: (130.3120) Integrated optics devices; (230.5298) Photonic crystals; (280.4788) Optical sensing and sensors.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
R. Daw and J. Finkelstein, “Lab on a chip,” Nature 442(7101), 367 (2006). X. Fan, I. M. White, S. I. Shopova, H. Zhu, J. D. Suter, and Y. Sun, “Sensitive optical biosensors for unlabeled targets: A review,” Anal. Chim. Acta 620(1-2), 8–26 (2008). M. G. Scullion, T. F. Krauss, and A. Di Falco, “Slotted photonic crystal sensors,” Sensors (Basel) 13(3), 3675– 3710 (2013). J. T. Robinson, L. Chen, and M. Lipson, “On-chip gas detection in silicon optical microcavities,” Opt. Express 16(6), 4296–4301 (2008). J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). B. J. Luff, J. S. Wilkinson, J. Piehler, U. Hollenbach, J. Ingenhoff, and N. Fabricius, “Integrated optical MachZehnder biosensor,” J. Lightwave Technol. 16(4), 583–592 (1998). K. Yao and Y. Shi, “High-Q width modulated photonic crystal stack mode-gap cavity and its application to refractive index sensing,” Opt. Express 20(24), 27039–27044 (2012). V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding and confining light in void nanostructure,” Opt. Lett. 29(11), 1209–1211 (2004). A. Di Falco, L. O'Faolain, and T. F. Krauss, “Chemical sensing in slotted photonic crystal heterostructure cavities,” Appl. Phys. Lett. 94(6), 063503 (2009). J. Jágerská, H. Zhang, Z. Diao, N. Thomas, and R. Houdré, “Refractive index sensing with an air-slot photonic crystal nanocavity,” Opt. Lett. 35(15), 2523–2525 (2010). M. G. Scullion, A. Di Falco, and T. F. Krauss, “Slotted photonic crystal cavities with integrated microfluidics for biosensing applications,” Biosens. Bioelectron. 27(1), 101–105 (2011). S. H. Mirsadeghi, E. Schelew, and J. F. Young, “Photonic crystal slot-microcavity circuit implemented in silicon-on-insulator: High Q operation in solvent without undercutting,” Appl. Phys. Lett. 102(13), 131115 (2013). M. Notomi, E. Kuramochi, and H. Taniyama, “Ultrahigh-Q Nanocavity with 1D Photonic Gap,” Opt. Express 16(15), 11095–11102 (2008). E. Kuramochi, H. Taniyama, T. Tanabe, K. Kawasaki, Y.-G. Roh, and M. Notomi, “Ultrahigh-Q onedimensional photonic crystal nanocavities with modulated mode-gap barriers on SiO2 claddings and on air claddings,” Opt. Express 18(15), 15859–15869 (2010). P. B. Deotare, M. W. McCutcheon, I. W. Frank, M. Khan, and M. Lončar, “High quality factor photonic crystal nanobeam cavities,” Appl. Phys. Lett. 94(12), 121106 (2009).
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26908
16. M. W. McCutcheon and M. Lončar, “Design of a silicon nitride photonic crystal nanocavity with a quality factor of one million for coupling to a diamond nanocrystal,” Opt. Express 16(23), 19136–19145 (2008). 17. Y. Gong and J. Vučković, “Photonic crystal cavities in silicon dioxide,” Appl. Phys. Lett. 96(3), 031107 (2010). 18. B.-H. Ahn, J.-H. Kang, M.-K. Kim, J.-H. Song, B. Min, K.-S. Kim, and Y.-H. Lee, “One-dimensional parabolicbeam photonic crystal laser,” Opt. Express 18(6), 5654–5660 (2010). 19. Q. Quan and M. Lončar, “Deterministic design of wavelength scale, ultra-high Q photonic crystal nanobeam cavities,” Opt. Express 19(19), 18529–18542 (2011). 20. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96(20), 203102 (2010). 21. D. R. Lyde, ed., Handbook of Chemistry and Physics (CRC Press, 1997–1998). 22. K. F. Palmer and D. Williams, “Optical properties of water in the near infrared,” J. Opt. Soc. Am. 64(8), 1107– 1110 (1974). 23. C. Monat, P. Domachuk, and B. J. Eggleton, “Integrated optofluidics: A new river of light,” Nat. Photonics 1(2), 106–114 (2007).
1. Introduction Optical biosensors have attracted considerable interest for lab-on-a-chip applications due to their advantages such as small size, biocompatibility, and lack of the need for fluorescent labels [1–3]. Many micro-photonic devices have been proposed to realize optical biosensors, such as ring resonators [4], surface plasmons resonators [5], Mach-Zehnder interferometer [6], and photonic crystals (PhC) [7]. As far as the use of PhC resonators as refractive index sensors is concerned, strong light matter interaction for the optical fields with the analytes are preferred. However, in typical PhC cavities, the optical mode is strongly confined in the high index material to achieve high Q. Almeida et al. proposed “slot-waveguide” structures whereby the discontinuity of the optical field at a dielectric interface was exploited to support a propagation mode in the lowindex narrow gap [8]. Such concept has attracted people’s attention in the sensing community widely especially in two-dimensional (2D) PhC cavities [9–12]. Such 2D slotted PhC cavities optical sensors enjoy extremely small active sensing volume. However, the footprint for the sensing part is always large with ~60 × 60 μm2 [12], which is not conductive to be densely assembled for sensor arrays. In the present paper, we combined the concept of the one-dimensional (1D) PhC nanobeam cavities [13–15] and the “slot waveguide”. We presented the design, fabrication, and the characterization of high-Q slotted 1D PhC cavities with parabolic-width stack. The fabricated device shows a Q-factor near 104 which can be further improved by optimizing the fabrication processes since the calculated Q-factor is ~106. The measurements for NaCl solutions with different concentrations show that a sensitivity around 410 nm/RIU can be achieved. With a comparable sensitivity to the sensors based on 2D slotted PhC cavities [10, 12], the size for the sensing part is only 16.8 × 2.5 μm2, which is attractive for the realization of on-chip sensor arrays. 2. Device design and analysis Several designs of high Q 1D PhC cavities have been proposed by modulating the air-hole size or lattice constant [15–17]. For such designs, nanometer level dimension control is required to achieve precise patterning. The fabrication is very challenging due to the proximity effect caused by the electron scattering in the e-beam resist. To ease the fabrication, an adiabatic parabolic width tapering concept [7, 18] was utilized in this paper to obtain high Q-factor and natural coupling between the cavity and the feeding waveguides [19]. Figure 1(a) shows the schematic for the 1D slotted stack cavity considered in this work. The slotted stack cavity is formed by introducing a slot between two periodic arrays of dielectric stacks. Figure 1(c) is the zoomed in picture of the framed part in (a) with annotation. The widths (Wy(i)) of the dielectric blocks are quadratically modulated (Wy(i) = Wy(0) + i2(Wy(imax)Wy(0))/imax2, from the center to both sides, i increases from 0 to imax),while Wx keeps unchanged. Thus, a Gaussian type confinement has been achieved [20].The band diagrams of the period slotted stack with Wy = 3.0a (width of the central stack) and Wy = 6.0a (width of the edge stack) for TE polarization were given in Fig. 1(d). The three dimensional ðnite
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26909
difference time domain (3D-FDTD) method with Bloch boundary conditions was utilized for the simulations. As expected, the band for the structure with Wy = 3.0a was higher than the one for Wy = 6.0a. The resonant frequency of the cavity mode [the black dot in Fig. 1(d)] is slightly lower than the dielectric band edge of the stacks with Wy = 3.0a. The Silicon-on-Insulator (SOI) platform has been used in this work. The refractive index and the thickness of the silicon and silica layer are 3.46, 1.44 and 220 nm, 2 μm, respectively. Figure 1(b) shows the intensity profile (|Ey|2) calculated by 3D FDTD method. To keep the resonance of the cavity mode near 1550 nm, we choose the following parameters: lattice constant a = 410 nm, Wx = 0.4a, imax = 20, Slot Width = 200 nm. The optimal structure supports a fundamental mode at λ = 1546.5 nm with a Qtot of 3.73 × 106 and the effective mode volume of 0.217(λ/nc)3 (defined as V =
dV ε
2
2
E / (ε E ) max ).The coupling Q is
calculated by monitoring the power coupled into the waveguide mode [17]. By defining quality factor with Qcoupling = ωU/P (ω is the frequency of the mode, U is the total energy of the mode, P is the power coupled to the feeding waveguide), we obtain Qcoupling = 8.883 × 106, and Qrad = 1/(1/Qtot-1/Qcoupling) = 6.43 × 106. The majority of electric field is strongly localized in the slotted low-index area, which indicates a strong interaction between the analytes and the cavity mode.
Fig. 1. (a) Schematics of slotted width modulated stack cavity. (b) Electric field intensity (|Ey|2) calculated by 3D FDTD method for the cavity with Slot Width = 200 nm, a = 410 nm, Wx = 0.4a and Wy parabolically modulated from Wy(0) = 3.0a in the center to Wy(20) = 6.0a on either side. (c) Zoomed in picture of the framed part in (a) with annotations. (d) TE band structure of periodic stacks with Wy = 3.0a (red lines) and Wy = 6.0a (blue lines).The black dots indicates the resonant frequency. (e)Wavelength shifts and Qtot variances over different background refractive indexes. (f) Influence of the slot width on the Q-factor and sensitivity of slotted stack cavity sensor.
Toward a quantitative understanding of the response to changes of the refractive index, the resonant wavelengths for the cavity with different ambient analytes have been calculated
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26910
using 3D-FDTD. The device is covered with a PMMA layer for our simulations, which is consistent with our following experiment. Figure 1(e) shows the resonant wavelength and Qfactor varies with different refractive indices of the covering analytes. A red shift of the resonance is clearly visible as the refractive index of the ambient is increased. We find a linear dependence of the resonance on the refractive index, which is preferred for sensing applications. From the numerical simulations, we find that the width of the slot highly influences the sensing performances. Figure 1(f) shows the Q-factor and the sensitivity vary as the function of the slot width. Here the sensitivity is defined as the resonant wavelength shifts induced by the change of one refractive index unit (RIU).We can find the sensitivity increases with the slot width, while the Q-factor decreases exponentially by expanding the slot width. To achieve high Q-factor while keep an attractive sensitivity, we choose a trade-off value of the Slot Width = 200 nm. For the cavity with Slot Width = 200 nm, a = 410 nm, Wx = 0.4a and Wy parabolically modulated from Wy(0) = 3.0a in the center to Wy(20) = 6.0a on either side, we can obtain a Q-factor = 3.73 × 106 and a sensitivity = 437 nm/RIU. 3. Fabrication and optical characterization of the device The devices were fabricated on a silicon-on-insulator (SOI) wafer (SOITEC Inc.) with a silicon layer of 220 nm and a buried oxide layer of 2 μm. A positive-tone electron beam resist film (PMMA 950K) was used as the etching hard mask. The film with a thickness of 310 nm was spin coated onto the SOI wafer at 4000 rpm and then baked on a hot plate at 180 °C for 20 minutes. Resists were exposed using an e-beam lithography tool (Raith150 II) at 20KV acceleration voltage and developed in a MIBK: IPA (1:3) mixture followed by a de-ionized (DI) water rinse. Then the pattern was transferred to the 220 nm silicon layer by inductively coupled plasma (ICP) etcher using a gas mixture of SF6 and C4F8. To remove the remaining resists, the samples were soaked and agitated in 1-Methyl-2-pyrrolidone (NMP), then followed by acetone in ultrasonic cleaner for 5 minutes, and finally rinsed in DI water. The grating couplers with the period of 645 nm with the duty cycle of 50:50 are fabricated on both the input/output waveguides by another overlay exposure followed by a shallow etching (70 nm). Figure 2(a) shows a scanning electron microscope micrograph of the fabricated device. One should note that Fig. 2(a) only includes left half of the device (the right half is symmetrical to the left) to give a clear view for the details. The enlarged views for the sensing cavity, the input grating coupler, and the coupling region between the ridge/slot waveguides are given in Fig. 2(b)-2(d), respectively. A ridge taper with length of 200 μm was utilized for the coupling between the 10 μm wide grating coupler to the 700 nm wide ridge waveguide. Then the 700 nm wide ridge waveguide is butt coupled to the 200 nm slot waveguide [see Fig. 2(d)]. The calculated coupling efficiency between the ridge/slot waveguides is ~84%.Another slot taper with length of 300 μm was introduced to expand the width of the slot waveguide to the width of the cavity region (2.46 μm) while keeping the slot width unchanged. Finally, the whole device was covered by a 500 nm PMMA layer to increase the coupling efficiency of the grating coupler. This PMMA layer was also used to define the sensing window. To evaluate the sensing potential for the cavity, a sensing window was opened over the cavity by another overlay exposure. Thus only the cavity part was exposed to the ambient, whereas the rest was still covered with the PMMA film.
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26911
Fig. 2. (a) SEM micrograph of the device (left half). (b)(c)(d) SEM images show the fabricated slotted stack cavity, input grating coupler, and the coupling region connecting the strip/slot waveguides. (e) Measured transmission spectrum of the slotted stack cavity in vacuum. The inset shows the fit to Lorentzian lineshape for the resonance (Q = ~9200).
TE polarized light from a tunable laser (Agilent 81940A) was launched to the cavity through a grating coupler. Another grating coupler was used for collecting the transmitted optical signal into the power meter (Agilient 81618A). Figure 2(e) presents the transmission spectrum of the slotted stack PhC cavity normalized by the peak resonance transmission. Lorentzian fitting of the resonance reveals a measured quality factor Q~9200 and the extinction ratio larger than 10 dB. The quality factor is smaller than the simulated value due to the fabrication imperfections and the material loss. Figure 3(a) shows the measured transmission spectrum when the device is immersed in NaCl solution with different concentrations. The ratio of the refractive index change to the concentration variation for the aqueous solution of NaCl is 0.0018 refractive index units (RIU)/1% at 20°C [21].In our experiment, the concentration for the NaCl solution varies from 0% to 25% with a step of 5% and the refractive index changes from 1.333 to 1.378 with a step of 0.009, correspondingly. From Fig. 3(a), one sees that the resonant wavelength increases as the concentration of the NaCl solution increases. The Q-factor of the cavity immersed in NaCl solution with 25% concentration is about 4700. Such a degradation of Q-factor may come from the water absorption loss [22]. After each measurement, the chip was rinsed with deionized water and agitated on a 90 °C hot plate for 20 mins to remove the residuals. For each measurement with different NaCl concentrations, the resonance for the cavity covered with DI water is used as the reference for calibration. This helps to avoid any influence of variations in incidental factors (e.g., temperature variation). According to the resonant wavelengths measurement for the cavity covered with different NaCl solutions, one can find that the sensitivity of device is about 410 nm/RIU with a linear fitting [Fig. 3(b)], which indicates good agreement to the simulations (~413 nm/RIU). This value is 1.5 times to the one achieved by the cavity without the slot as expected [7]. Both the achieved Q-factor and the sensitivity are higher than the one reported recently by using 2D slotted PhC cavities [12]. Besides, the sensing part (16.8 × 2.5 μm2) is only 1/100 of the one reported in ref. 12 (larger than 60 × 60 μm2). To be used in a practical context, our compact device can be combined with microfluidic channels, which suggests strong potential for lab-on-a-chip applications [23]. Furthermore, we did find our device supports the high order mode, this high order mode could be well separated from the fundamental mode with its intensity much smaller (~15 dB)
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26912
than the fundamental one. From Fig. 1(e), we can also find the Q-factors maintain a higher than 106 over a wide wavelength range, indicating a very large sensing range of the device. This is especially advantageous compared to the whispering mode based cavities with its sensing range inevitably limited by the free spectral range (FSR).
Fig. 3. (a) Measured transmission responses of the slotted stack photonic crystal cavity covered the aqueous NaCl solutions with different concentrations. (b) The resonant wavelength λ shifts as the concentration varies.
4. Conclusion In conclusion, we have presented a high-Q slotted PhC nanobeam cavity with parabolic-width stack for refractive index sensing. By employing parabolic modulated width stack without any reflected mirror, high Q-factor near 104 has been achieved. The transmission spectrum measurement for the cavities covered by NaCl solutions with different concentration shows a sensitivity of 410 nm/RIU, which is comparable with the one based on 2D slotted PhC cavities. The small footprint of the sensing part (16.8 × 2.5 μm2) suggests a strong potential in on-chip biochemical sensing arrays. Acknowledgments We thank Jianwei Tang for valuable discussions. This work was partially supported by the National Nature Science Foundation of China (Grant No. 61377023), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY13F050002), and the Program of Zhejiang Leading Team of Science and Technology Innovation.
#197176 - $15.00 USD Received 5 Sep 2013; revised 24 Oct 2013; accepted 24 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026908 | OPTICS EXPRESS 26913
