Open Access
Add to BibSonomyAbstract
We experimentally demonstrate high-Q cavity formation at an arbitrary position on a silicon photonic crystal waveguide by bringing a tapered nanofiber into contact with the surface of the slab. An ultrahigh Q of 5.1 × 105 is obtained with a coupling efficiency of 39%, whose resonant wavelength can be finely tuned by 27 pm by adjusting the contact length of the nanofiber. We also demonstrate an extremely high coupling efficiency of 99.6% with a loaded Q of 6.1 × 103. We show that we can obtain a coupled resonances, which has the potential to be used for slow light generation.
© 2015 Optical Society of America
1. Introduction
Photonic crystal (PhC) devices [1] are prominent candidates for optical signal processing because of their suitability for integration and their potential for strong light-matter interaction. PhCs have been used to realize many devices including slow light waveguides with a large group velocity [2,3], an optical switch and all-optical memories [4,5], a high quantum efficiency photodetector [6], and cavity quantum electro-dynamics devices [7–9]. Furthermore, recent progress on fabrication technologies for CMOS compatible processes has enabled us to fabricate PhC devices using photolithography [10–12], which will accelerate their industrialization. Although the coupling between a PhC nanocavity and waveguides can be well designed, the coupling efficiency between an optical fiber and a PhC/silicon waveguide is usually not high due to the mode mismatch between the waveguide and the optical fiber. In addition, various quantum application is performed in a free-space configuration, which usually suffer from a low collection of light.
To overcome these problems, tapered nanofiber has been used to couple the light evanescently into the PhC devices [13]. A tapered fiber is fabricated by heating and pulling an optical fiber. The narrowest point of the fiber has a sub-micrometer radius; thus light can couple evanescently into the cavity. The coupling efficiency of tapered fiber is as high as 98% for a chalcogenide glass PhC waveguide [14] and 95% for a silicon PhC waveguide [15]. The latest study on the adiabatic fiber-waveguide couplers [16] reports a high coupling efficiency of 97%. This value is even higher even if a grating coupler (maximum 70%) or a spot-size converter (maximum 90%) is used at the silicon wire-waveguide end [17–19].
Recently, Y.-H. Lee et al. reported the demonstration of a reconfigurable fiber-coupled PhC nanocavity [20,21]. In addition to the controllability of the coupling efficiency, this scheme also allowed control of the position and resonant wavelength of the cavity. However, the experimental quality factor (Q) of the cavity was limited to around 104 because of the absorption loss in quantum dot active layer when the cavity was formed on an InGaAsP/InP quantum dot PhC waveguide. Numerical calculations revealed that the coupling efficiency could be as high as nearly 100% with a Q of over 105 [22], but the demonstrated efficiency was only a few percent with a 2D PhC waveguide [21] and 30% with a 1D nanobeam waveguide [23].
Motivated by these highly relevant reports, in this study we formed a reconfigurable cavity on a two-dimensional silicon PhC waveguide with a tapered nanofiber for the simultaneous experimental demonstration of an ultrahigh Q and high coupling efficiency. We will show that a Q of 5.1 × 105 is possible with a coupling efficiency of 39%. We also investigated the tuning performance of the resonant wavelength by changing the length of the contact area with the nanofiber. In addition, we demonstrated all-pass filter type multimode excitation, which may be used for the generation of slow light.
2. Theory of cavity formation
When a nanofiber approaches the top of a PhC waveguide, the effective refractive index of the waveguide increases; thus the cutoff frequency of the waveguide decreases and a modegap cavity is formed. We calculated the band diagram of the fiber-coupled silicon PhC waveguide by using MPB [24] with the following parameters; lattice constant a = 420 nm, waveguide width w = 0.98√3a (W0.98), hole radius r = 0.30a, slab thickness t = 0.50a and refractive index of silicon nSi = 3.47. The radius of the silica fiber was 500 nm (nSiO2 = 1.45). The result is shown in Fig. 1(a). When we reduced the gap between the fiber and the waveguide, the cutoff frequency of the PhC waveguide moved downward. This localized downshift of the cutoff frequency allowed modegap confinement and made it possible to form a cavity [1,25].
Fig. 1 (a) Band diagram of PhC waveguides in contact with a nanofiber. (b) Calculated Hz field profile of an optical cavity created with a nanofiber. The upper and lower figures are views from the top and side, respectively. Light localization is observed at the region where a silica nanofiber is placed on the top of a silicon PhC slab.
To confirm the cavity formation, we calculated the mode profile of the fiber-coupled PhC nanocavity with a 3-D finite difference time domain (FDTD) method [22] by using MEEP [26]. We modeled a bent nanofiber with a curvature radius of R = 125 μm. The cross-sectional radius of the nanofiber was r = 500 nm. When we brought the fiber into contact with a W0.98 PhC waveguide we obtained a mode profile as shown in Fig. 1(b). The Q value was 1.4 × 107 and the mode volume was 1.9(λ/n)3.
3. Experiment
3.1 Demonstration of ultrahigh-Q cavity formation
In this section, we describe our experimental demonstration of cavity formation on a PhC waveguide. For the experiment, we used a dimpled tapered nanofiber [27] that we fabricated with the following processes. First, we fabricated a tapered nanofiber by heating and pulling a standard single-mode optical fiber. Next, we prepared a bare optical fiber with a radius of 62.5 μm as a mold and brought it into contact with the tapered fiber. Then, we heated the contact area to form a dimple in the nanofiber. The fabricated dimpled fiber is shown in Fig. 2(a). This bent and tensed fiber is useful for coupling light into a PhC waveguide from the top of the slab and enables us to control of the size of the contact area precisely. The typical insertion loss of our fiber was 10 dB; however, it can be reduced to almost 0 dB according to a previous report [27].
Fig. 2 (a) Fabricated dimpled fiber. The diameter is about 500 nm and the transmittance is −10 dB. (b) Experimental setup. TLD: Tunable laser diode. VOA: Variable optical attenuator. PC: Polarization controller. PM: Power monitor.
To prepare the PhC waveguide, we used a silicon foundry service to fabricate a silica-clad PhC waveguide with a 248-nm photolithography process [12]. Then the silica cladding was removed by wet etching with 20% hydrogen fluoride. We use the same parameters for the fabrication as those we used for the calculation.
The optical measurement was performed by bringing the dimpled fiber into contact with the PhC waveguide and recording the transmittance spectrum with the setup shown in Fig. 2(b). The measured transmittance spectrum is shown in Fig. 3(a), where the cavity resonances are observed as dips, because we used a side-coupled configuration. The reason for the multiple resonances will be discussed later. When we measured the spectral width of one of the dips, we observed a loaded Ql of 5.1 × 105 as shown in the inset of Fig. 3(a). This resonance had a transmittance T of 61% (i.e. a dip depth of 39%, which we define as a coupling efficiency), with which we obtained an unloaded Qu of 5.7 × 105.
Fig. 3 (a) Transmittance spectrum of a reconfigurable fiber coupled PhC cavity. The vertical axis is normalized with the maximum transmittance of the tapered fiber. Insertion loss of tapered fiber was 4.4 dB and input power was 1.6 μW. (b) Transmittance spectra of the fiber coupled PhC cavity at different input powers Pin. Pin is the power in the nano-tapered fiber immediately before the contact point. (c) Infrared image of a reconfigurable fiber coupled PhC cavity. The upper image is when the cavity is off-resonance. The lower is an image of a cavity on-resonance (Input wavelength of 1538.90 nm). The bright spot at the center of the image is the localized mode.
To confirm the localization of the mode, we increased the input power and observed the influence of the thermo-optic (TO) effect, which is caused by multiphoton absorption if the light is strongly localized. As the result in Fig. 3(b) shows, clear TO bistability was observed, which is strong evidence of light localization. Another direct proof of the localization was the observation of the localized mode from the top of the slab achieved by using an infrared image sensor. Figure 3(c) contains images showing an input laser light when the cavity is on resonance and off resonance. We clearly observed a localized light in the contact region of the nano-tapered dimpled fiber.
Next, we describe the polarization dependence. Since only the transverse electric field (TE) mode exhibits a photonic band-gap, the observation of the polarization dependence is the third proof of the excitation of the mode-gap resonances. We show the transmittance spectra at two different polarizations (90 deg) in Fig. 4(a), where we observed resonances only when we excited the cavity in the TE mode.
Fig. 4 (a) Transmittance spectra of TE and TE polarized light. The vertical axis is normalized with the maximum transmittance of the tapered fiber. Insertion loss of tapered fiber was 9.3 dB and input power was 20 μW. (b) Enlarged views of a mode with maximized coupling efficiency.
Finally, we attempted to maximize the coupling efficiency by carefully controlling the polarization. The coupling efficiency is defined by the dip depth of the normalized transmittance spectrum. We defined it in this way, because the input light will completely enter the cavity when the dip depth is at maximum (critical coupling condition). A maximized coupling efficiency of 96.4% was obtained for a mode with a Ql of 4.9 × 104 at wavelength of 1538.29 nm, and a coupling efficiency of 99.6% was achieved for a mode with Ql of 6.1 × 103 at a wavelength of 1535.37 nm [Fig. 4(b)]. Such a high coupling efficiency with an optical fiber is attractive for various applications including switching and quantum optics.
3.2 Tuning the resonant wavelength
Next, we demonstrate control of the resonant wavelength of the cavity by changing the length of the contact area between a nanofiber and a PhC waveguide. We moved the xyz translation stage and shifted the PhC waveguide 100 nm downwards from the initial contact position [Fig. 5(a)]. As the stage moves downwards, the contact area between the nanofiber and PhC waveguide gradually decreases. This changes the size of the contact area and results in a blue shift of the resonant wavelength because of the shorter cavity length. The experimental result is shown in Fig. 5(b). The wavelength tuning resolution is about 27 pm when the position is changed for 100 nm for the mode with the highest Q. This corresponds to a wavelength tuning senstivity of 0.27 pm/nm. Although the resonant wavelength shift is also observed at different modes, some are less sensitive because their modes penetrate less towards the nanofiber.
Fig. 5 (a) Schematic illustration of how we change the size of the contact area. (b) Spectral response of the resonant wavelength tuning using the method in (a).
The high Q and the high/direct coupling characteristics of this cavity, along with the wavelength tuning ability and the re-location ability, are extremely useful if we want to use such devices for strong and weak coupling experiments in, for example, cavity quantum electron-dynamics [28], in particular when we have a wavelength tuning precision of several tens of pm.
4. Demonstration of coupled cavity by utilizing waveguide disorder
Finally, we focus on the TE-like mode and discuss the reason for the observed excitation of multi-resonant peaks in Fig. 4(a). There are at least 28 peaks in 6.6-nm wavelength range centered at 1537 nm. The wavelength separation between the peaks is too small to explain in terms of the excitation of different longitudinal modes of a Fabry-Pérot cavity.
To clarify the origin of these peaks, we measured the surface of the PhC waveguide with a scanning electron microscope (SEM). The SEM image is shown in Fig. 6. We found that the surface of the PhC waveguide is not perfectly flat, but has bumps at a high of about 15 nm. These bumps result in a small fluctuation in the potential well along the waveguide, thus, multiple cavities are formed. The peaks at shorter wavelengths tend to have a lower Q (the average Q at shorter wavelengths is 2.8 × 104, in contrast to 1.4 × 105 at longer wavelengths), which we believe is due to the different spectral distances from the mode-gap of the continuum propagation mode of the waveguide. The multiple scattering that occurs in a disordered PhC waveguide has been well studied by a number of groups in connection with the Anderson localization of light, and the characteristics of the obtained spectrum are indeed close to those reported in these studies [29–32].
To investigate the above phenomenon in more detail, we monitored the mode profile of these localized modes with an infrared camera from top of the PhC waveguide slab as shown in Fig. 7. When we input light with a wavelength of 1534.33 nm, we observe multiple bright spots along the PhC waveguide, which is evidence of multiple localizations at different positions. Although a closer investigation of the coupling between the cavities is needed to clarify the characteristics as a coupled resonator optical waveguide (CROW) [33,34], the measured image of the multiple bright spots suggests that the device may be used as an all-pass filter (APF) type coupled cavity system [35], which is useful for demonstrating optical buffers and slow light. Since our system excite the cavity in a side-coupled configuration, the transmittance spectrum with multiple peaks and the localization of light at different positions (with single input wavelength) are strong evidences of the APF resonance. It should be noted that these resonances can be excited with high coupling through a tapered fiber, which is the advantage of this scheme.
Fig. 7 Images of a fiber-coupled PhC cavity observed from the top of the slab. (a) Visible CCD image. (b) Infrared camera image at 1550 nm input (no resonance). (c) As (b) but at 1534.33 nm, which is in resonance with the structure.
4. Summary
In summary, we demonstrated experimentally high-Q cavity formation on a Si PhC waveguide with a tapered nanofiber, and obtained an ultrahigh Q of 5.1 × 105 at a coupling efficiency of 39%. These high values were obtained because we employed a silicon PhC waveguide as a platform. We also demonstrated high coupling efficiency and obtained an extremely high 96.4% transmittance for a mode with a Ql of 4.9 × 104 and an even higher 99.6% for a mode with a Ql of 6.1 × 103. We also achieved resonant wavelength control with a resolution of 27 pm.
In addition, we discussed about the coupled-cavity resonances we observed, originated from the surface disorder of a PhC waveguide. It may allow us to study optical buffering and slow light that have high coupling with optical fiber.
Acknowledgment
This work was supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan for the Photon Frontier Network Program.
References and links
1. T. Tanabe, M. Notomi, E. Kuramochi, A. Shinya, and H. Taniyama, “Trapping and delaying photons for one nanosecond in an ultrasmall high-Q photonic-crystal nanocavity,” Nat. Photonics 1(1), 49–52 (2007). [CrossRef]
2. B. Corcoran, C. Monat, C. Grillet, D. J. Moss, B. J. Eggleton, T. P. White, L. O’Faolain, and T. F. Krauss, “Green light emission in silicon through slow-light enhanced third-harmonic generation in photonic-crystal waveguides,” Nat. Photonics 3(4), 206–210 (2009). [CrossRef]
3. T. Baba, T. Kawaaski, H. Sasaki, J. Adachi, and D. Mori, “Large delay-bandwidth product and tuning of slow light pulse in photonic crystal coupled waveguide,” Opt. Express 16(12), 9245–9253 (2008). [CrossRef] [PubMed]
4. K. Nozaki, T. Tanabe, A. Shinya, S. Matsuo, T. Sato, H. Taniyama, and M. Notomi, “Sub-femtojoule all-optical switching using a photonic-crystal nanocavity,” Nat. Photonics 4(7), 477–483 (2010). [CrossRef]
5. E. Kuramochi, K. Nozaki, A. Shinya, K. Takeda, T. Sato, S. Matsuo, H. Taniyama, H. Sumikura, and M. Notomi, “Large-scale integration of wavelength-addressable all-optical memories on a photonic crystal chip,” Nat. Photonics 8(6), 474–481 (2014). [CrossRef]
6. T. Tanabe, H. Sumikura, H. Taniyama, A. Shinya, and M. Notomi, “All-silicon sub-Gb/s telecom detector with low dark current and high quantum efficiency on chip,” Appl. Phys. Lett. 96(10), 101103 (2010). [CrossRef]
7. T. Yoshie, A. Scherer, J. Hendrickson, G. Khitrova, H. M. Gibbs, G. Rupper, C. Ell, O. B. Shchekin, and D. G. Deppe, “Vacuum Rabi splitting with a single quantum dot in a photonic crystal nanocavity,” Nature 432(7014), 200–203 (2004). [CrossRef] [PubMed]
8. D. Englund, A. Faraon, I. Fushman, N. Stoltz, P. Petroff, and J. Vucković, “Controlling cavity reflectivity with a single quantum dot,” Nature 450(7171), 857–861 (2007). [CrossRef] [PubMed]
9. A. Laucht, F. Hofbauer, N. Hauke, J. Angele, S. Stobbe, M. Kaniber, G. Böhm, P. Lodahl, M.-C. Amann, and J. J. Finley, “Electrical control of spontaneous emission and strong coupling for a single quantum dot,” New J. Phys. 11(2), 023034 (2009). [CrossRef]
10. K. K. Mehta, J. S. Orcutt, O. Tehar-Zahav, Z. Sternberg, R. Bafrali, R. Meade, and R. J. Ram, “High-Q CMOS-integrated photonic crystal microcavity devices,” Sci Rep 4, 4077 (2014). [CrossRef] [PubMed]
11. M. Shinkawa, N. Ishikura, Y. Hama, K. Suzuki, and T. Baba, “Nonlinear enhancement in photonic crystal slow light waveguides fabricated using CMOS-compatible process,” Opt. Express 19(22), 22208–22218 (2011). [CrossRef] [PubMed]
12. Y. Ooka, T. Tetsumoto, A. Fushimi, W. Yoshiki, and T. Tanabe, “CMOS compatible high-Q photonic crystal nanocavity fabricated with photolithography on silicon photonic platform,” Sci. Rep.5, 11312 (2015) (in press).
13. K. Srinivasan, P. Barclay, M. Borselli, and O. Painter, “Optical-fiber-based measurement of an ultrasmall volume high-Q photonic crystal microcavity,” Phys. Rev. B 70(8), 081306 (2004). [CrossRef]
14. C. Grillet, C. Smith, D. Freeman, S. Madden, B. Luther-Davies, E. Magi, D. Moss, and B. Eggleton, “Efficient coupling to chalcogenide glass photonic crystal waveguides via silica optical fiber nanowires,” Opt. Express 14(3), 1070–1078 (2006). [CrossRef] [PubMed]
15. P. E. Barclay, K. Srinivasan, M. Borselli, and O. Painter, “Probing the dispersive and spatial properties of photonic crystal waveguides via highly efficient coupling from fiber tapers,” Appl. Phys. Lett. 85(1), 4 (2004). [CrossRef]
16. T. G. Tiecke, K. P. Nayak, J. D. Thompson, T. Peyronel, N. P. De Leon, V. Vuletic, and M. D. Lukin, “Efficient fiber-optical interface for nanophotonic devices,” Optica 2(2), 70–75 (2015). [CrossRef]
17. C. Li, H. Zhang, M. Yu, and G. Q. Lo, “CMOS-compatible high efficiency double-etched apodized waveguide grating coupler,” Opt. Express 21(7), 7868–7874 (2013). [CrossRef] [PubMed]
18. V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28(15), 1302–1304 (2003). [CrossRef] [PubMed]
19. K. Shiraishi, H. Yoda, A. Ohshima, H. Ikedo, and C. S. Tsai, “A silicon-based spot-size converter between single-mode fibers and Si-wire waveguides using cascaded tapers,” Appl. Phys. Lett. 91(14), 141120 (2007). [CrossRef]
20. M.-K. Kim, I.-K. Hwang, M.-K. Seo, and Y.-H. Lee, “Reconfigurable microfiber-coupled photonic crystal resonator,” Opt. Express 15(25), 17241–17247 (2007). [CrossRef] [PubMed]
21. M.-K. Kim, J.-Y. Kim, J.-H. Kang, B.-H. Ahn, and Y.-H. Lee, “On-demand photonic crystal resonators,” Laser Photon. Rev. 5(4), 479–495 (2011). [CrossRef]
22. I. Karnadi, J.-Y. Kim, B.-H. Ahn, H.-J. Lim, and Y.-H. Lee, “Efficient photon collection from reconfigurable photonic crystal slab resonator operating at short wavelengths,” J. Opt. Soc. Am. B 29(10), 2669–2674 (2012). [CrossRef]
23. H.-J. Lim, C.-M. Lee, B.-H. Ahn, and Y.-H. Lee, “Dual-rail nanobeam microfiber-coupled resonator,” Opt. Express 21(6), 6724–6732 (2013). [CrossRef] [PubMed]
24. M. I. T. Photonic Bands, (MPB) is a free software package for computing the band structures (dispersion relations) and electromagnetic modes of periodic dielectric structure. MPB was developed at MIT, http://abi-nitio.mit.edu/wiki/index.php/MIT_Photonic_Bands.
25. Y. Takahashi, H. Hagino, Y. Tanaka, B. S. Song, T. Asano, and S. Noda, “High-Q nanocavity with a 2-ns photon lifetime,” Opt. Express 15(25), 17206–17213 (2007). [CrossRef] [PubMed]
26. MEEP is a free finite-difference time-domain (FDTD) simulation software package developed at MIT, http://abi-nitio.mit.edu/wiki/index.php/Meep
27. C. P. Michael, M. Borselli, T. J. Johnson, C. Chrystal, and O. Painter, “An optical fiber-taper probe for wafer-scale microphotonic device characterization,” Opt. Express 15(8), 4745–4752 (2007). [CrossRef] [PubMed]
28. H. Sumikura, E. Kuramochi, H. Taniyama, and M. Notomi, “Ultrafast spontaneous emission of copper-doped silicon enhanced by an optical nanocavity,” Sci Rep 4, 5040 (2014). [CrossRef] [PubMed]
29. V. Savona, “Electromagnetic modes of a disordered photonic crystal,” Phys. Rev. B 83(8), 085301 (2011). [CrossRef]
30. S. Mazoyer, J. P. Hugonin, and P. Lalanne, “Disorder-induced multiple scattering in photonic-crystal waveguides,” Phys. Rev. Lett. 103(6), 063903 (2009). [CrossRef] [PubMed]
31. M. Patterson and S. Hughes, “Interplay between disorder-induced scattering and local field effects in photonic crystal waveguides,” Phys. Rev. B 81(24), 245321 (2010). [CrossRef]
32. H. Thyrrestrup, S. Smolka, L. Sapienza, and P. Lodahl, “Statistical theory of a quantum emitter strongly coupled to Anderson-localized modes,” Phys. Rev. Lett. 108(11), 113901 (2012). [CrossRef] [PubMed]
33. M. Notomi, E. Kuramochi, and T. Tanabe, “Large-scale arrays of ultrahigh-Q coupled nanocavities,” Nat. Photonics 2(12), 741–747 (2008). [CrossRef]
34. A. Melloni, A. Canciamilla, C. Ferrari, F. Morichetti, L. O’Faolain, T. F. Krauss, R. D. La Rue, A. Samarelli, and M. Sorel, “Tunable delay lines in silicon photonics: Coupled resonators and photonic crystals, a comparison,” IEEE Photonics J. 2(2), 181–194 (2010). [CrossRef]
35. F. Xia, L. Sekaric, and Y. Vlasov, “Ultracompact optical buffers on a silicon chip,” Nat. Photonics 1(1), 65–71 (2007). [CrossRef]
