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10. C. C. Evans, K. Shtyrkova, J. D. B. Bradley, O. Reshef, E. Ippen, and E. Mazur, “Spectral broadening in titanium dioxide waveguides at telecommunication ...
Parabolic opening in atomic layer deposited TiO2 nanobeam operating in visible wavelengths Arijit Bera,∗ Markus H¨ayrinen, Markku Kuittinen, Seppo Honkanen, and Matthieu Roussey Institute of Photonics, University of Eastern Finland Yliopistokatu 7, 80101 Joensuu, Finland ∗ [email protected]

Abstract: We investigate the feasibility of developing a one dimensional photonic crystal cavity on a TiO2 platform operating in the visible. The atomic layer deposition technique is used to finely adjust the parameters of the structure. We present the experimental demonstration of a nanobeam cavity with a quadratically tapered row of holes, in which a parabolic window is opened in order to facilitate the infiltration of gas, liquid, nonlinear material, or quantum emitters. The structure exhibits a photonic band gap between λ = 635 nm and λ = 690 nm and several resonances within the photonic band gap. © 2015 Optical Society of America OCIS codes: (350.4238) Nanophotonics and photonic crystals; (130.3120) Integrated optics devices; (130.5296) Photonic crystal waveguides; (220.4241) Nanostructure fabrication.

References and links 1. M. Gehl, R. Gibson, J. Hendrickson, A. Homyk, A. S¨ayn¨atjoki, T. Alasaarela, L. Karvonen, A. Tervonen, S. Honkanen, S. Zandbergen, B. C. Richards, J. D. Olitzky, A. Scherer, G. Khitrova, H. M. Gibbs, J. Kim, and Y. Lee, “Effect of atomic layer deposition on the quality factor of silicon nanobeam cavities,” J. Opt. Soc. Am. B 29, A55–A59 (2012). 2. A. R. M. Zain, N. P. Johnson, M. Sorel, and R. M. De La Rue, “Ultra high quality factor one dimensional photonic crystal / photonic wire micro-cavities in silicon-on-insulator (SOI),” Opt. Express 16, 12084–12089 (2008). 3. Q. Quan, D. L. Floyd, I. B. Burgess, P. B. Deotare, I. W. Frank, S. K. Y. Tang, R. Ilic, and M. Loncar, “Single particle detection in CMOS compatible photonic crystal nanobeam cavities,” Opt. Express 21, 32225–32233 (2013). 4. F. Liang, N. Clarke, P. Patel, M. Loncar, and Q. Quan, “Scalable photonic crystal chips for high sensitivity protein detection,” Opt. Express 21, 32306–32312 (2013). 5. B. J. M. Hausmann, B. J. Shields, Q. Quan, Y. Chu, N. P. de Leon, R. Evans, M. J. Burek, A. S. Zibrov, M. Markham, D. J. Twitchen, H. Park, M. D. Lukin, and M. Loncar, “Coupling of NV centers to photonic crystal nanobeams in diamond,” Nano Lett. 13, 5791–5796 (2013). 6. G. Ada, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, and M. Chhowalla, “Photoluminescence from chemically exfoliated MoS2 ,” Nano Lett. 11, 5111–5116 (2011). 7. M. Khan, T. Babinec, M. W. McCutcheon, P. Deotare, and M. Loncar, “Fabrication and characterization of highquality factor silicon nitride nanobeam cavities,” Opt. Lett. 36, 421–423 (2011). 8. S. Sergent, M. Arita, S. Kako, K. Tanabe, S. Iwamoto, and Y. Arakawa, “High-Q AlN photonic crystal nanobeam cavities fabricated by layer transfer,” Appl. Phys. Lett. 101, 101106 (2012). 9. H. Zhang, B. Chen, J. F. Banfield, and G. A. Waychunas, “Atomic structure of nanometer-sized amorphous TiO2 ,” Phys. Rev. B 78, 214106 (2008). 10. C. C. Evans, K. Shtyrkova, J. D. B. Bradley, O. Reshef, E. Ippen, and E. Mazur, “Spectral broadening in titanium dioxide waveguides at telecommunication and near-visible wavelengths,” Opt. Express 21, 18582–18591 (2013). 11. T. Alasaarela, L. Karvonen, H. Jussila, A. S¨ayn¨atjoki, S. Mehravar, R. A. Norwood, N. Peyghambarian, K. Kieu, I. Tittonen, and H. Lipsanen, “High quality crystallinity controlled ALD TiO2 for waveguiding applications,” Opt. Lett. 38, 3980–3983 (2013).

#235723 - $15.00 USD Received 5 Mar 2015; revised 13 Apr 2015; accepted 18 May 2015; published 29 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014973 | OPTICS EXPRESS 14973

12. J. D. B. Bradley, C. C. Evans, J. T. Choy, O. Reshef, P. B. Deotare, F. Parsy, K. C. Phillips, M. Loncar, and E. Mazur, “Submicrometer wide amorphous and polycrystalline anatase TiO2 waveguides for microphotonic devices,” Opt. Express 20, 23821–23831 (2012). 13. M. H¨ayrinen, M. Roussey, V. Gandhi, P. Stenberg, A. S¨ayn¨atjoki, L. Karvonen, M. Kuittinen, and S. Honkanen, “Low-loss titanium dioxide strip waveguides fabricated by atomic layer deposition,” J. Lightwave Technol. 32, 208–212 (2014). 14. M. H¨ayrinen, M. Roussey, A. S¨ayn¨atjoki, M. Kuittinen, and S. Honkanen, “Titanium dioxide slot waveguides for visible wavelengths,” Appl. Opt. 54, 2653–2657 (2015). 15. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). 16. B. Wang, M. A. Dundar, R. Notzel, F. Karouta, S. He, and R. W. van der Heijden, “Photonic crystal slot nanobeam slow light waveguides for refractive index sensing,” Appl. Phys. Lett. 97, 151105 (2010). 17. D. Yang, S. Kita, F. Liang, C. Wang, H. Tian, Y. Ji, M. Loncar, and Q. Quan, “High sensitivity and high Q-factor nanoslotted parallel quadrabeam photonic crystal cavity for real-time and label-free sensing,” Appl. Phys. Lett. 105, 063118 (2014). 18. J. Robinson, C. Manolatou, L. Chen, and M. Lipson, “Ultrasmall mode volumes in dielectric optical microcavities,” Phys. Rev. Lett. 95, 143901 (2005). 19. J. Ryckman, and S. Weiss, “Low mode volume slotted photonic crystal single nanobeam cavity,” Appl. Phys. Lett. 101, 071104 (2012). 20. P. Seidler, K. Lister, U. Drechsler, J. Hofrichter, and T. Stferle, “Slotted photonic crystal nanobeam cavity with an ultrahigh quality factor-to-mode volume ratio,” Opt. Express 21, 32468–32483 (2013). 21. C. A. Barrios, “Optical slot-waveguide based biochemical sensors,” Sensors 9, 4751–4765 (2009). 22. V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding and confining light in void nanostructure,” Opt. Lett. 29, 1209–1211 (2004). 23. E. Kuramochi, H. Taniyama, T. Tanabe, K. Kawasaki, Y. Roh, and M. Notomi, “Ultrahigh-Q one-dimensional photonic crystal nanocavities with modulated mode-gap barriers on SiO2 claddings and on air claddings,” Opt. Express 18, 15859–15869 (2010).

1.

Introduction

In recent years, one dimensional nanowaveguide photonic crystal (1D PhC) [1, 2] cavity structures have shown great potential to realize integrated devices for sensing, quantum information processing, and nonlinear optics. 1D nanostrip PhC configurations, also called nanobeam structures, allow a drastic reduction of the device footprint leading to a better suitability for integration with other photonic components. Such devices may find applications in numerous domains of integrated photonics. Due to the strong light-matter interaction occurring in the structure within an extremely small mode volume, the nanobeam cavities are perfectly suitable for on-chip chemical or bio-sensing [3, 4]. The strong light localization provided by the device may help also the optical coupling of the electromagnetic wave to solid-sate quantum emitters embedded within the cavity [5]. Such a structure covered by or fabricated in a nonlinear material can drastically enhance its nonlinear effects due to the huge confinement of light in a very tiny volume, thus reducing the power requirement. Moreover, many of the solid-state active materials, or quantum emitters work predominantly in the visible [6]. In addition, sensors using water-based buffers often suffer from the water absorption peak at the near-infrared, which can be avoided by changing the operating wavelength of the devices to visible. Therefore, developing an efficient nanophotonic platform operating in the visible range of light is of utmost importance. The commonly used semiconductor materials, such as silicon (Si) and gallium arsenide (GaAs), are not transparent at wavelengths below 1 μ m. Therefore, the search for a high refractive index material for visible nanophotonics is focused on the wide band-gap materials such as silicon nitride [7] (Si3 N4 ) and aluminum nitride [8] (AlN) due to their broadband optical transparency from visible to near infrared wavelengths. Indeed, the 1D PhC nanowaveguide cavities with a high quality factor, and operating in the visible, have already been demonstrated with these wide band-gap materials on silicon substrate using an air-bridged type nanobeam structure [7, 8]. #235723 - $15.00 USD Received 5 Mar 2015; revised 13 Apr 2015; accepted 18 May 2015; published 29 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014973 | OPTICS EXPRESS 14974

During the past few years, titanium dioxide (TiO2 ) has appeared as a promising material for visible nanophotonics due to its high refractive index (n = 2.4 at λ = 650 nm, higher than the indices of Si3 N4 and AlN), wide electronic band-gap [9] (Eg = 3.2 eV), and broadband optical transparency. Moreover, the relatively high nonlinear refractive index (n2 = 0.16 × 10−18 m2 /W) and very low two-photon absorption of both crystalline [10] and amorphous [11] TiO2 make it a relevant nonlinear material for the visible range. As an example, low-loss waveguides operating both at visible and near infrared have been demonstrated [12] in TiO2 . In this work, our goal is to demonstrate experimentally a multifunctional device platform on TiO2 , which is based on a nanobeam cavity structure operating in the visible. The basis of the structure is a row of holes on a TiO2 nano-strip waveguide. In particular, we study the effect of an opening directly embedded in a nanobeam cavity on the sensitivity of the device to the refractive index change of the external medium. The opening is destined to help the infiltration of a cover material in the structure. The cover material can be, for example, an analyte (gas or liquid) in case of sensing, or a nonlinear material in case of nonlinear applications. Moreover, we show the importance of the Atomic Layer Deposition (ALD) technique and its unique properties to provide a smooth and uniform device layer and to control, at the nanoscale, the geometrical parameters of the proposed structure. Indeed, working in the visible with nanostructures is a fabrication challenge, since feature sizes as small as 5 nm (in our case) need to be reached. In order to achieve this precision, ALD enables to adjust the structural parameters in a post-fabrication process step. We have recently proved the relevance of this technique by reducing the propagation loss of amorphous TiO2 waveguides to the level of 2.5 dB/cm at λ = 1.55 μ m [13] and by demonstrating a slot waveguide operating at visible wavelengths [14]. 2.

Design of the nanobeam cavity

We first designed a nano-strip waveguide in TiO2 (n = 2.4 at λ = 650 nm, for amorphous ALD-TiO2 ), supporting fundamental quasi-TE and quasi-TM modes at 650 nm wavelength with modal indices nTE = 1.997 and nTM = 2.083, respectively. The width and height of the strip waveguide are both set to 300 nm. The designed nanobeam cavity consists of one row of holes periodically arranged along the nano-strip waveguide. The period is set to p = 180 nm, in order to open a photonic band gap in the visible. To achieve a smooth modal transition between the nano-waveguide and the PhC, the diameter of the holes of the input (output) PhC is linearly increased (decreased) until (from) the center of the device by 2 nm steps from d = 94 nm until d = 134 nm. This kind of a taper, running all along the structure, is called Gaussian-type [15]. Sensitivities of 69 nm/RIU and 83 nm/RIU have been experimentally demonstrated by F. Liang et al. [4] and Q. Quan et al. [3] in 2013, with similar structures in silicon but without an opening and operating around λ = 1550 nm. In the past few years, double nanobeam [16] and quadrabeam [17] structures with nano-slots in between, have been proposed as efficient platforms for sensing applications. Moreover, slotted nanobeam devices have been numerically studied [18] and experimentally demonstrated [19, 20] on SOI platforms operating in near IR range. In order to maximize the overlap of the resonant modes with the cover material, we propose here the opening of a parabolic window directly in the PhC lattice [Fig. 1(a)]. The opening spans over 24 periods and is centered on the PhC structure. At its extremities, its width is w = 5 nm, and at the center, its maximum width is w = 75 nm. In addition to the light confinement provided by the slot waveguide, we expect an increase in liquid infiltration into the structure [21]. It will enhance the overlap between the confined light and the analyte thereby offering substantial improvement of sensitivity over the nanobeam structures without an opening. Moreover, the parabolic shape of the opening enables smooth coupling between the cavity mode and the slot waveguide mode.

#235723 - $15.00 USD Received 5 Mar 2015; revised 13 Apr 2015; accepted 18 May 2015; published 29 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014973 | OPTICS EXPRESS 14975

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Fig. 1. (a) Schematics of the proposed nanobeam cavity (only a few holes are presented, while the real structure has 42 holes) with open window. The period p is set to 180 nm, the diameters d are varied from 94 nm to 134 nm and the maximum width of the opening is w = 75 nm. (b) FDTD simulated transmission spectra for both quasi-TE (blue curve) and quasi-TM (red curve) polarizations, (c) Field distribution (Ex ) in the XZ plane for the resonant wavelength λ = 651 nm for the quasi-TE polarization, and (d) the Ex field amplitude distributions along the two observation lines [Fig. 1(a)] at λ = 651 nm, revealing the presence of the slot modes within the opening.

Using the Finite Difference Time Domain (FDTD) method, we have calculated the transmission spectrum of the structure for the quasi-TE (blue curve) and the quasi-TM (red curve) polarizations, shown in Fig. 1(b). The resonant modes appear at λ = 651 nm, λ = 673.8 nm and λ = 691.2 nm, for the quasi-TE polarization, and at λ = 666.4 nm and λ = 691.6 nm, for the quasi-TM polarization. The field distribution for the quasi-TE resonance at λ = 651 nm (Ex

#235723 - $15.00 USD Received 5 Mar 2015; revised 13 Apr 2015; accepted 18 May 2015; published 29 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014973 | OPTICS EXPRESS 14976

field) is presented in Fig. 1(c). From this field distribution, one can remark that the mode is confined within the opening. The presence of the slot mode [22] confined within the parabolic opening is evident from Fig. 1(d), where the Ex field profiles are shown along the two observation lines (red and blue dashed lines in Fig. 1(a)). Owing to the confinement of the cavity modes mainly within the cover material in the opening, the structure can be used to enhance the sensitivity in chemical and bio-molecule sensing with easier liquid infiltration and greater interaction of light with the analyte. To give an estimation of the sensitivity, we simulate the structure while varying the refractive index of the cover material in the range 1.3 to 1.4, which corresponds to most of the water based bio-analytes. We obtain a simulated refractive index sensitivity of 219 nm/RIU (refractive index unit) with this structure which is a significant improvement compared to the nanobeam structures without any opening [3, 4]. Table 1 presents a comparison (based on FDTD simulations) of different types of openings in terms of their sensitivities (S) to a refractive index change of the cover material and in terms of the quality factor (Q). Here we compare our proposed nanobeam geometry with a parabolic opening to the Gaussian-type nanobeam without an opening, with a rectangular opening (having the so-called slot waveguide), and with an elliptic opening. From this table, it is evident that creating an open window in the middle of the nanobeam decreases drastically the quality factor of the structure. Due to this opening, the index contrast inside the PhC is reduced thereby reducing the quality factor. One can also note that, compared to a vertically symmetric nanobeam structure (PhC membrane) [23], the quality factor is much less. The use of different materials as upper and lower cladding (air and SiO2 respectively) breaks the symmetry in the vertical direction (y-axis). The use of SiO2 with refractive index of 1.45 (at λ = 650 nm) is reducing the index contrast between the core and the substrate material, which affects the quality factor significantly. Nevertheless, one can observe at the same time a significant increase of the sensitivity due to the significant increase in overlap of light with analyte in the opening window. Table 1. Comparison between different openings types. wmax denotes the width of the opening at its center.

Opening type None

Schematics

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Sensitivity 95 nm/RIU

Q factor 35,000

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1000

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850

As we can see from Table 1, all types of openings are very similar in term of sensitivity. Although, the rectangular opening can offer a higher Q-factor, it will be difficult for the analyte to infiltrate the slot region having a width of 30 nm. As a consequence, the sensitivity will be reduced substantially. Our choice of a parabolic opening is thus based on the fact that by slowly increasing the width of the opening, one can adiabatically couple light from the nanobeam structure to the slot mode. In this case, the start and end of the opening should be as small as possible, thus we choose w = 5 nm as the opening width. In the middle of the cavity, the opening should be as large as possible in order to increase the liquid infiltration. We choose w = 75 nm as the opening width in the middle. However, the inner corrugation of the opening due to the presence of the holes remains. The resulting structure is thus the opening of a corrugated slot waveguide with a parabolic variation of the slot width.

#235723 - $15.00 USD Received 5 Mar 2015; revised 13 Apr 2015; accepted 18 May 2015; published 29 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014973 | OPTICS EXPRESS 14977

3.

Fabrication and characterization

In order to facilitate the coupling of light from a lensed fiber into and out-of the 300 nm wide nano-strip waveguide, we taper it into a 2 μ m wide waveguide at both ends. The fabrication process of the nanobeam cavity includes atomic layer deposition (ALD), electron beam lithography (EBL) and reactive ion etching (RIE) tools. We want to point out that the structure is fabricated in two main steps. First, the etching of the structure and second, the adjustment of the parameters using a re-coating made by ALD. This technique is used here since the targeted dimensions could not be reached directly during the first step. At the beginning, the TiO2 thin film is 20 nm thinner than the targeted value (300 nm). After the etching step, the radii of the holes as well as the width of the strip should be 20 nm and 40 nm smaller, respectively. At the second step, 20 nm TiO2 is re-coated on the structure to adjust the targeted parameters precisely. A 280 nm thin film of TiO2 is deposited on top of an oxidized silicon substrate (2 μ m thick oxide layer) by ALD (TFS 200 by Beneq). In the ALD process, TiCl4 and H2 O are used as precursors at 120◦ C process temperature needed for the deposition of amorphous TiO2 . A chromium (Cr) layer (50 nm) is used as a hard mask between TiO2 and hydrogen silsesquioxane (HSQ, XR-1541) resist layer to which the structure is patterned by using EBL (Vistec EBPG 5000+ ES HR). The patterned structure is developed with an AZ351:H2 O developer. The inductively coupled plasma (ICP)-RIE etching of Cr in Cl2 /O2 environment and RIE etching of TiO2 in SF6 /O2 /Ar environment are performed, by using Plasmalab 100 and Plasmalab 80 by Oxford, respectively. The remaining Cr is removed by wet etching. After the final etching step, a 20 nm re-coating of TiO2 is done by ALD to reach the targeted parameters. Figure 2(a) shows the SEM pictures of the whole fabricated device and zoom-in pictures of one of the tapers and the open window in the middle of the structure after re-coating. The use of Cr mask and high radio frequency (RF) power for TiO2 etching lead to a non-negligible surface roughness on the sidewalls of the structure, as one can see from the SEM pictures [Fig. 2(a)]. In spite of these issues, one can observe a good agreement between the designed geometry and the fabricated sample, which proves the applicability of ALD to precisely tune the parameters of the TiO2 structures at the nanoscale. The fabricated sample is characterized by coupling light from a supercontinuum laser source (SuperK Compact, NKT Photonics) to the input of the sample with a tapered lens fiber (Oz Optics). The output signal is collected from the other end of the chip by another lensed fiber, which is connected to an optical spectrum analyzer (OSA, Ando AQ 6315A). The in and out coupling fibers are held on XYZ linear stages with piezo-electric actuators enabling nanometer scale movements, and the sample is placed in between, on a YZ linear stage. The measured transmission spectrum is presented in Fig. 2(b). The spectral response shows a photonic band-gap starting at λ = 635 nm. The supercontinuum source used in the experiment is fully unpolarized. It means that both quasi-TE and quasi-TM polarizations propagate in the structure. When observing the simulated spectra [Fig. 1(b)], it is evident that the intensities of the two polarizations are superimposed to obtain the measured spectrum [Fig. 2(b)]. The position of the photonic band-gap as well as the position of the resonance at λ = 651 nm are in good agreement with the simulated spectra. One can observe a slight shift of the other resonances that can be explained by a tiny variation of the geometry compare to the simulated one. We point out that light is guided through the structure only after the re-deposition of a 20 nm TiO2 layer by ALD. Before this step, the waveguide cross-section was too small and the hole diameters were too large to allow light propagation through the structure at the desired wavelength range. Also note that, the higher intensity level of the peak at λ = 651 nm compared to the simulation is due to the use of a lensed fiber for coupling, leading to a decrease of the coupling efficiency at longer wavelengths. In our case, the coupling is optimized for λ = 650 nm. The experimental Q value is, for the peak at λ = 651,

#235723 - $15.00 USD Received 5 Mar 2015; revised 13 Apr 2015; accepted 18 May 2015; published 29 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014973 | OPTICS EXPRESS 14978

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Fig. 2. (a) SEM pictures of the whole fabricated device and zoom-ins on one of the tapers and the opening window in the middle of the structure. The hole diameter is varied from 94 nm to 134 nm by steps of 2 nm. The opening window covers 24 holes and its largest width is w = 75 nm. (b) the measured transmission spectrum of the device with unpolarized input.

Q = 275. This lower value, compared to the theoretical one, may be due to the resolution of the optical spectrum analyzer set at 5 nm, in order to maintain a reasonable signal to noise ratio. 4.

Conclusion

In conclusion, we have shown the potential offered by ALD in conformal re-coating of nanostructures in order to finely tune their structural parameters. This technique is used to demonstrate experimentally a nanobeam structure in a TiO2 platform operating in the visible. We introduce a new device configuration composed of a nanobeam cavity in which a parabolic opening, acting as a slot waveguide, is created to allow an enhanced interaction of the confined light with the cover material. The field enhancement within the cover material, due to the slot effect, shows promise of this 1D PhC configuration in applications such as liquid and gas sensing.

#235723 - $15.00 USD Received 5 Mar 2015; revised 13 Apr 2015; accepted 18 May 2015; published 29 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014973 | OPTICS EXPRESS 14979

Acknowledgments This research is supported by the Finnish Funding Agency for Technology and Innovation (TEKES) through the EAKR projects ALD-nano-medi and Nanobio (grants 70011/12 and 70005/14) and the Academy of Finland (grants 272155 and 250968).

#235723 - $15.00 USD Received 5 Mar 2015; revised 13 Apr 2015; accepted 18 May 2015; published 29 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014973 | OPTICS EXPRESS 14980