Titanium dioxide slot waveguides for visible wavelengths Markus Häyrinen,1,* Matthieu Roussey,1 Antti Säynätjoki,1,2 Markku Kuittinen,1 and Seppo Honkanen1 1
Institute of Photonics, University of Eastern Finland, P.O. Box 111, F1-80101 Joensuu, Finland 2
Department of Micro and Nanosciences, Aalto University, Tietotie 3, FI-02150 Espoo, Finland *Corresponding author: [email protected] Received 12 January 2015; revised 25 February 2015; accepted 25 February 2015; posted 25 February 2015 (Doc. ID 232264); published 24 March 2015
We present the first, to our knowledge, experimental demonstration of a titanium dioxide slot waveguide operating in the visible range of light. Ring resonators based on slot waveguides were designed, fabricated, and characterized for λ ≃ 650 nm. The fabrication method includes atomic layer deposition, electron beam lithography, and reactive ion etching. The required narrow slot widths of a few tens of nanometers were achieved by using a conformal atomic layer re-coating technique. This unique feature-size-reduction technique was applied after the final etching step. © 2015 Optical Society of America OCIS codes: (220.4241) Nanostructure fabrication; (130.3120) Integrated optics devices; (230.7370) Waveguides. http://dx.doi.org/10.1364/AO.54.002653
1. Introduction
Slot waveguides were introduced in 2004 by Almeida et al. [1]. They are composed of two rails of high refractive index material separated by a narrow low refractive index gap of a few tens of nanometers. The electric field discontinuity at the high index contrast interfaces leads to a high confinement of the optical field in the slot region, which makes this kind of structure appealing for sensing applications [2,3]. The field discontinuity giving rise to the slot effect is proportional to the square of the refractive index contrast between the slot and the rail. Therefore, the slot waveguide needs to be made out of a material platform that presents a high refractive index contrast. Silicon is one of the most extensively used materials for the fabrication of slot waveguides, and several successful devices, operating in the near-infrared 1559-128X/15/102653-05$15.00/0 © 2015 Optical Society of America
(IR), have been demonstrated [4]. Nevertheless, most of the optical sensors are more efficient in the visible range of the electromagnetic spectrum, especially to perform fluorescence sensing or simply to avoid the absorption peak of water in the near-IR. Since silicon is not transparent at wavelengths below 1 μm, an alternative material has to be used at visible wavelengths. With a high refractive index of n 2.4 at λ 632 nm, titanium dioxide (TiO2 ) is a good candidate for guided-wave applications and sensing. TiO2 films can be grown using atomic layer deposition (ALD) and hence the film thickness and its uniformity can be controlled very accurately. Propagation losses of ALD-TiO2 slab waveguides have been measured to be as low as 2.0–3.5 dB/cm at 633 nm wavelength and less than 1 dB/cm at 1530 nm wavelength [5]. We aim to experimentally demonstrate the feasibility of slot waveguides in amorphous TiO2 deposited by ALD and operating in the visible spectral range. In order to prove the guiding in the slot, a ring resonator structure is designed. Its spectral response 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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2. Fabrication
The fabrication process of the structures is as follows. A TiO2 film (220 nm) is deposited on an oxidized silicon substrate (3.0 μm SiO2 layer) by ALD (Beneq TFS 200). In the used ALD process, the precursors are titanium tetrachloride (TiCl4 ) and water (H2 O), and the deposition temperature is 120°C. With this fabrication temperature, the grown TiO2 film is amorphous. A hard chromium (Cr) mask (50 nm) is deposited using electron beam evaporation (Kurt J. Lesker LAB18). Waveguides are patterned to a hydrogen silsesquioxane resist (HSQ, XR-1541) with electron beam lithography (Vistec EBPG 5000 + ES HR) on top of the chromium layer. The exposed resist is developed with an AZ 351∶H2 O (1∶3) developer. The dry etching of the Cr layer is done using chlorine (Cl2 )-based and oxygen (O2 )-based plasma etching process [54/4 sccm, 5 mTorr, RF power 140 W, and inductively coupled plasma (ICP) power 1500 W] with Plasmalab 100 by Oxford. After Cr etching, the TiO2 layer is dry-etched by using sulfur hexafluoride (SF6 ), O2 , and argon (Ar) 2654
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SiO ed tch r-e Ov e
allows us to determine the effective index of the propagating mode, and then confirm that light is propagating in the slot waveguide forming the ring. Moreover, to our knowledge, this kind of slot microring structure have not yet been demonstrated at visible wavelengths. The potential of ring resonators in sensing has been studied and shown in the near-IR wavelength region by using silicon [3] and silicon nitride [6–8] waveguides. An integrated TiO2 ring resonator has been shown to be well-suited for nano-waveguide applications [9]. In bio-sensing applications the use of visible wavelengths brings advantages. Bio-sensing analysis is often performed in an aqueous environment and the absorption caused by water is more than a thousand times smaller in the visible than in the near-IR wavelength region. Moreover, operating at visible wavelengths provides higher sensitivity and permits the use of fluorescent markers in labelbased sensing methods [10]. TiO2 slot waveguides have been theoretically studied [11] showing the potential for visible wavelengths of such a structure. Silicon carbide slot waveguides have been demonstrated in the visible range for a horizontal slot [12]. When operating at visible wavelengths instead of the near-IR region, the gap in the slot waveguide must be narrower and, to avoid excessive scattering losses due the shorter operation wavelength, stringent requirements are set to the waveguide sidewall roughness. This makes the fabrication more challenging and requires precisely developed nanofabrication processes and a trade-off between ideal device dimensions and limits of the process technology. To achieve the best possible dimensions, we use the re-coating technique presented by Alasaarela et al. [13]. In addition to the feature size reduction, this re-deposition process reduces the sidewall roughness and reduces the propagation losses [14].
(a)
(b)
(c)
(d)
Fig. 1. SEM pictures of the fabricated slot waveguides; (a) tilted view along a slot waveguide; (b) slot waveguide profile for W S 30 nm, filling factor Γ 1∶10; (c) slot waveguide profile for W S 50 nm, filling factor Γ 1∶6; (d) slot waveguide profile for W S 80 nm, filling factor Γ 1∶4.
(25/6/5 sccm, 55 mTorr, RF power 105 W) with Plasmalab 80 by Oxford. After the two dry-etching steps, the remaining Cr layer is removed by a wet-etching solution. Finally the fabricated structure is re-coated with an additional layer (30 nm, total thickness of 250 nm) of TiO2, to reach precisely the targeted slot size. Scanning Electron Microscopy (SEM) images of the fabricated slot waveguides, after the re-deposition, are shown in Fig. 1 and the fabricated ring resonator structure is shown in Fig. 2. Using an ALD re-coating means that the rails of the slot waveguide will become thicker and the gap between the ring and the bus will be reduced, affecting directly the coupling efficiency of the device. The dimensions have then been calculated in order to take into account the additional coating. Nevertheless, it is understandable that the aspect ratio of the slot, in particular, cannot be varied as desired. With the process described previously, we achieved an aspect ratio of 1∶5.5 for the rails. Three fabrication examples are presented in Fig. 1. Figure 1(a) is an SEM picture of a standard slot waveguide in TiO2 after a 30 nm re-coating. One can still observe slight sidewall roughness. This will contribute to propagation loss, especially for visible wavelengths, due to Rayleigh scattering. Figs. 1(b)–1(d) are slot waveguide cross sections for different slot widths (W S ): 30, 50, and 80 nm, respectively. The blue dashed lines show the interface between the TiO2 layer and the SiO2 cladding which is slightly overetched in order to prevent the re-coating from completely filling the slot [Fig. 3(a)]. One can remark that the walls of the 30 nm slot waveguide are a little
(a)
Re-deposited TiO2 layer
Original TiO2 slot rails
(a)
WS
SiO2
WR
Si y
WS = 30 nm
(b)
y
x
WS = 30 nm
(c)
x 2
1
(b) 1
TE slot mode (Ex) neff=1.80 / ng=2.38 y
WS = 80 nm
0
(d)
0.5
TM rail mode (Ey) neff=1.93 / ng=2.48 y
x
WS = 80 nm
Fig. 2. SEM pictures: (a) fabricated ring resonator; (b) zoom-in of the coupling region.
bit tilted (about 5°). One can expect the slot mode to be pushed down in the slot region [15,16], close to the bottom of the slot. Figure 2 is an SEM picture of a slot ring resonator (the radius is r 6 μm). We expect rather high propagation loss along the slot waveguide mainly due to the sidewall roughness that cannot be fully eliminated by the ALD re-coating layer. Consequently, we have chosen a strip waveguide as the bus to bring light to the ring resonator. 3. Theoretical Investigations
The slot waveguide characteristic of the fabricated devices is verified by simulations presented in Fig. 3. In Fig. 3(a), the cross section of the designed and fabricated slot waveguide is presented. Figures 3(b) and 3(c) are the field distributions for the quasi-TE and quasi-TM mode, respectively, for W S 30 nm, and Figs. 3(d) and 3(e) are the field distributions for the quasi-TE and quasi-TM mode, respectively, for W S 80 nm. One can see the slot waveguide behavior of the structure. The modal refractive indices are nTE 1.8 and nTM 1.9 for a 30 nm slot width and nTE 1.5 and nTM 1.7 for an 80 nm slot width. Moreover, one can see a positive effect of the overetching of the oxidized silicon layer. Indeed, it allows the mode to be vertically centered in the slot. As expected, the mode is pushed down in the case of W S 30 nm, due to the tilted sidewalls. The low effective index of the TE mode proves the high confinement of the field inside the low refractive index region. In addition, we have calculated the filling factor (Γ), which is the ratio between the width of the slot and the width of the input nano-waveguide,
(e)
x 2
TE slot mode (Ex) neff=1.50 / ng=1.81
0
1
0
1
TM rail mode (Ey) neff=1.77 / ng=2.4
0.5
0
Fig. 3. (a) Schematic of the TiO2 slot waveguide. Oxide layer thickness is 3.0 μm, the thickness of the additional TiO2 layer is 30 nm, the slot-waveguide rail width is W R 160 nm, the rail height is 250 nm (λ 650 nm), and the slot width is W S (substrate is over-etched in order to compensate for the additional layer thickness of the re-coating); (b) mode profile for W S 30 nm, for TE polarization (sidewalls tilted about 5°); (c) mode profile for W S 30 nm, for TM polarization (sidewalls tilted about 5°); (d) mode profile for W S 80 nm, for TE polarization; (e) mode profile for W S 80 nm, for TM polarization.
for both structures and obtained Γ30 1∶10 and Γ80 1∶4. Figure 3 proves that for the 30 nm slot width, with a filling factor of 1∶10, the mode is more confined in the low refractive index area than for the 80 nm slot width having a filling factor of 1∶4. 4. Measurements and Discussion
In order to ensure efficient light coupling into the slot waveguides, the input and output coupling was realized using multimode waveguides (2 μm width) tapered to a single-mode waveguide having the desired width (300 nm) according to the wavelength. Light from a supercontinuum source (NKT Photonics, SuperK COMPACT) was injected with a tapered lens fiber in the device. Using another lensed fiber, the output signal is coupled to an Optical Spectrum Analyzer (OSA, Ando 6315A). The input light is fully unpolarized and unfortunately, it is not possible, in our experimental setup, to polarize it over a large wavelength range, without losing too much power. A picture (top view) of the structure illuminated with a laser diode at λ 640 nm is shown in Fig. 4(a). For this particular wavelength, only the TE polarization is injected in the device. One can see that scattering is observed mainly in a few specific locations, which 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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that these low values are representative of a lossy structure due to several possible factors: due to the bend loss in the ring structure which has a relatively small radius and to the sidewall roughness, this can be observed in the photograph in Fig. 4(a). Nevertheless, the main reason for these low quality-factors is an over-coupling of light in the structure (low F value), due to the short coupling distance between the bus and the ring. Figure 5 presents results extracted from twodimensional (2D) finite-difference time-domain (FDTD) simulations of a slotted ring resonator, where the slot width is varied. Notice that these FDTD simulations use a 2D model of the structure based on an effective index approximation, which does not take into account the tilt of the side walls. Nevertheless, one can see a good agreement in comparison with the mode calculations shown in Fig. 3. From Fig. 5(a), one can see a linear decrease of the group index of the TE polarization and an asymptotic decrease for the TM polarization when the slot width increases. Note that the variation is much larger for the TE mode, which is the slot mode. Moreover, from Fig. 5(b), one can conclude that for small values of W S , the resonances for the TE polarization are stronger, while the opposite is true for high values of W S. Indeed, when the slot width increases, the slot mode becomes lossy, while the rail mode remains well-guided. This is clearly observed in the mode
-1 -2 -3 -4 -5 -6 650
655
660
665
670
Wavelength [nm]
Transmission [dB]
(c)
0 -1 -2 -3 -4 -5 -6 -7 -8 650
660
670
680
690
700
Wavelength [nm] Fig. 4. (a) Photograph of the ring resonator structure when illuminated by a TE-polarized light at λ 640 nm; (b) measured transmission spectra through a 6 μm slotted ring resonator, W S 30 nm; (c) measured transmission spectra through a 6 μm slotted ring resonator, W S 80 nm.
shows that individual defects impart the main contribution to the loss in the device. Moreover, from Fig. 4(a) one can also clearly see light passing through the ring resonator, which is the only part of the device based on a slot waveguide. In the slot ring resonator as well, most of the scattering occurs in a few locations, while other parts of the ring appear to provide light propagation with relatively low scattering loss. The measured transmission spectrum as a function of wavelength is shown in Fig. 4(b) for W S 30 nm, and in Fig. 4(c) for W S 80 nm. By measuring the free spectral range (FSR), and the full-width at half-maximum (FWHM), it is possible to calculate, using Eq. (1), the group index of the mode propagating inside the ring resonator λ2 ; ng 2π · FSR · r
λ ; FWHM
and F
(1)
FSR : FWHM
(2)
We obtain Q30 630 and F 30 6.7 for W S 30 nm and Q80 740 and F 80 5.3 for W S 80 nm. Note 2656
(b)0.9
2.5
0.8
2.4 2.3 2.2 2.1 2.0 1.9
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
1.8 0
20
40
60
80
Slot width WS
(c)
0
100
20
40
60
80
100
690
700
Slot width WS
(d) 0
0 -1 -2 -3 -4 -5 650
-2 -4 -6 -8 -10
654
658
662
Wavelength [nm]
where λ is the wavelength, and r is the radius of the ring. The quality factor (Q) and the finesse (F ) of the ring structure can be calculated using Eq. (2) Q
(a)2.6
Resonance amplitude [arb. u.]
0
Transmission [dB]
Transmission [dB]
(b)
INPUT
TE polarized light
Group index ng
OUTPUT
Transmission [dB]
(a)
666
670
-12 650
660
670
680
Wavelength [nm]
Fig. 5. 2D FDTD simulations of the ring resonators; (a) dependence of the group index (ng ) on the slot width (W S ) for TE polarization [blue squares and solid line (linear fit)] and TM polarization [red circles and solid line (exponential fit)]; (b) dependence of the average intensity of the resonance peaks on W S for TE (blue squares) and TM (red circles) polarizations. (c) Transmission spectra for the TE (blue curves) and TM (red curves) polarization through a ring resonator of radius 6 μm for a slot width of 30 nm; (d) transmission spectra for the TE (blue curves) and TM (red curves) polarization through a ring resonator of radius 6 μm for a slot width of 80 nm.
simulations [Figs. 3(b)–3(d)]. Figures 5(b) and 5(c) present the transmission spectra for W S 30 nm and W S 80 nm, respectively, for the two polarizations. One can observe a good match of these results compared to the experimental spectra in Fig. 4. The group index values can be extracted from the experiments [Fig. 4] and simulations [Figs. 3 and 5]. We denote by ng and n~ g , the theoretical and the experimental group indices, respectively. For the W S 30 nm slot waveguide we obtain n~ 30 g 2.2 and for W S 80 nm, n~ 80 g 2.42. We obtain theoretically 80 n30 g 2.21 and ng 2.40, from the spectra presented in Fig. 5. The excellent match between the theoretical and experimental mode properties proves the existence of a slot mode in the ring resonators. 5. Conclusion
We have designed, fabricated, and characterized TiO2 ring resonators composed of slot waveguides for the visible wavelength range. Conformal redeposition, which is one of the main benefits of the ALD technique, was used for the feature size reduction. In this way, a sufficiently narrow slot size was achieved. This kind of a ring resonator structure may open up interesting possibilities, in particular at the visible wavelengths, for waveguide biosensor applications. This work was 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). References 1. V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding and confining light in void nanostructure,” Opt. Lett. 29, 1209–1211 (2004). 2. C. Barrios, “Ultrasensitive nanomechanical photonic sensor based on horizontal slot-waveguide resonator,” IEEE Photon. Technol. Lett. 18, 2419–2421 (2006). 3. T. Claes, J. Molera, K. De Vos, E. Schachtb, R. Baets, and P. Bienstman, “Label-free biosensing with a slot-waveguidebased ring resonator in silicon on insulator,” IEEE Photon. J. 1, 197–204 (2009).
4. A. Säynätjoki, L. Karvonen, T. Alasaarela, X. Tu, T. Y. Liow, M. Hiltunen, A. Tervonen, G. Q. Lo, and S. Honkanen, “Low-loss silicon slot waveguides and couplers fabricated with optical lithography and atomic layer deposition,” Opt. Express 19, 26275–26282 (2011). 5. T. Alasaarela, T. Saastamoinen, J. Hiltunen, A. Säynätjoki, A. Tervonen, P. Stenberg, M. Kuittinen, and S. Honkanen, “Atomic layer deposited titanium dioxide and its application in resonant waveguide grating,” Appl. Opt. 49, 4321–4325 (2010). 6. K. B. Gylfason, C. F. Carlborg, A. Kazmierczak, F. Dortu, H. Sohlström, L. Vivien, C. A. Barrios, W. van der Wijngaart, and G. Stemme, “On-chip temperature compensation in an integrated slot-waveguide ring resonator refractive index sensor array,” Opt. Express 18, 3226–3237 (2010). 7. C. A. Barrios, K. B. Gylfason, B. Sánchez, A. Griol, H. Sohlström, M. Holgado, and R. Casquel, “Slot-waveguide biochemical sensor,” Opt. Lett. 32, 3080–3082 (2007). 8. C. A. Barrios, B. Sánchez, K. B. Gylfason, A. Griol, H. Sohlström, M. Holgado, and R. Casquel, “Demonstration of slot-waveguide structures on silicon nitride/silicon oxide platform,” Opt. Express 15, 6846–6856 (2007). 9. J. T. Choy, J. D. B. Bradley, P. B. Deotare, I. B. Burgess, C. C. Evans, E. Mazur, and M. Lončar, “Integrated TiO2 resonators for visible photonics,” Opt. Lett. 37, 539–541 (2012). 10. T. Nuutinen, P. Karvinen, J. Rahomaki, and P. Vahimaa, “Resonant waveguide grating (RWG): Overcoming the problem of angular sensitivity by conical, broad-band illumination for fluorescence measurements,” Anal. Methods 5, 281–284 (2013). 11. G. Testa and R. Bernini, “Slot and layer-slot waveguide in the visible spectrum,” J. Lightwave Technol. 29, 2979–2984 (2011). 12. G. Pandraud, A. Neira, E. Margallo-Balbas, C.-K. Yang, and P. Sarro, “Demonstration of pecvd SiC–SiO2–SiC horizontal slot waveguides,” IEEE Photon. Technol. Lett. 22, 398–400 (2010). 13. T. Alasaarela, A. Säynätjoki, T. Hakkarainen, and S. Honkanen, “Feature size reduction of silicon slot waveguides by partial filling using atomic layer deposition,” Opt. Eng. 48, 080502 (2009). 14. M. Häyrinen, M. Roussey, V. Gandhi, P. Stenberg, A. Säynätjoki, 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). 15. A. Säynätjoki, T. Alasaarela, A. Khana, L. Karvonen, P. Stenberg, M. Kuittinen, and S. Honkanen, “Angled sidewalls in silicon slot waveguides conformal filling and mode properties,” Opt. Express 17, 21066–21076 (2009). 16. A. Säynätjoki, B. Bai, A. Tervonen, J. Turunen, and S. Honkanen, “Enhanced vertical confinement in angled-wall slot waveguides,” Opt. Rev. 17, 181–186 (2010).
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Institute of Photonics, University of Eastern Finland, P.O. Box 111, F1-80101 Joensuu, Finland 2
Department of Micro and Nanosciences, Aalto University, Tietotie 3, FI-02150 Espoo, Finland *Corresponding author: [email protected] Received 12 January 2015; revised 25 February 2015; accepted 25 February 2015; posted 25 February 2015 (Doc. ID 232264); published 24 March 2015
We present the first, to our knowledge, experimental demonstration of a titanium dioxide slot waveguide operating in the visible range of light. Ring resonators based on slot waveguides were designed, fabricated, and characterized for λ ≃ 650 nm. The fabrication method includes atomic layer deposition, electron beam lithography, and reactive ion etching. The required narrow slot widths of a few tens of nanometers were achieved by using a conformal atomic layer re-coating technique. This unique feature-size-reduction technique was applied after the final etching step. © 2015 Optical Society of America OCIS codes: (220.4241) Nanostructure fabrication; (130.3120) Integrated optics devices; (230.7370) Waveguides. http://dx.doi.org/10.1364/AO.54.002653
1. Introduction
Slot waveguides were introduced in 2004 by Almeida et al. [1]. They are composed of two rails of high refractive index material separated by a narrow low refractive index gap of a few tens of nanometers. The electric field discontinuity at the high index contrast interfaces leads to a high confinement of the optical field in the slot region, which makes this kind of structure appealing for sensing applications [2,3]. The field discontinuity giving rise to the slot effect is proportional to the square of the refractive index contrast between the slot and the rail. Therefore, the slot waveguide needs to be made out of a material platform that presents a high refractive index contrast. Silicon is one of the most extensively used materials for the fabrication of slot waveguides, and several successful devices, operating in the near-infrared 1559-128X/15/102653-05$15.00/0 © 2015 Optical Society of America
(IR), have been demonstrated [4]. Nevertheless, most of the optical sensors are more efficient in the visible range of the electromagnetic spectrum, especially to perform fluorescence sensing or simply to avoid the absorption peak of water in the near-IR. Since silicon is not transparent at wavelengths below 1 μm, an alternative material has to be used at visible wavelengths. With a high refractive index of n 2.4 at λ 632 nm, titanium dioxide (TiO2 ) is a good candidate for guided-wave applications and sensing. TiO2 films can be grown using atomic layer deposition (ALD) and hence the film thickness and its uniformity can be controlled very accurately. Propagation losses of ALD-TiO2 slab waveguides have been measured to be as low as 2.0–3.5 dB/cm at 633 nm wavelength and less than 1 dB/cm at 1530 nm wavelength [5]. We aim to experimentally demonstrate the feasibility of slot waveguides in amorphous TiO2 deposited by ALD and operating in the visible spectral range. In order to prove the guiding in the slot, a ring resonator structure is designed. Its spectral response 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
2653
2. Fabrication
The fabrication process of the structures is as follows. A TiO2 film (220 nm) is deposited on an oxidized silicon substrate (3.0 μm SiO2 layer) by ALD (Beneq TFS 200). In the used ALD process, the precursors are titanium tetrachloride (TiCl4 ) and water (H2 O), and the deposition temperature is 120°C. With this fabrication temperature, the grown TiO2 film is amorphous. A hard chromium (Cr) mask (50 nm) is deposited using electron beam evaporation (Kurt J. Lesker LAB18). Waveguides are patterned to a hydrogen silsesquioxane resist (HSQ, XR-1541) with electron beam lithography (Vistec EBPG 5000 + ES HR) on top of the chromium layer. The exposed resist is developed with an AZ 351∶H2 O (1∶3) developer. The dry etching of the Cr layer is done using chlorine (Cl2 )-based and oxygen (O2 )-based plasma etching process [54/4 sccm, 5 mTorr, RF power 140 W, and inductively coupled plasma (ICP) power 1500 W] with Plasmalab 100 by Oxford. After Cr etching, the TiO2 layer is dry-etched by using sulfur hexafluoride (SF6 ), O2 , and argon (Ar) 2654
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
2
SiO ed tch r-e Ov e
allows us to determine the effective index of the propagating mode, and then confirm that light is propagating in the slot waveguide forming the ring. Moreover, to our knowledge, this kind of slot microring structure have not yet been demonstrated at visible wavelengths. The potential of ring resonators in sensing has been studied and shown in the near-IR wavelength region by using silicon [3] and silicon nitride [6–8] waveguides. An integrated TiO2 ring resonator has been shown to be well-suited for nano-waveguide applications [9]. In bio-sensing applications the use of visible wavelengths brings advantages. Bio-sensing analysis is often performed in an aqueous environment and the absorption caused by water is more than a thousand times smaller in the visible than in the near-IR wavelength region. Moreover, operating at visible wavelengths provides higher sensitivity and permits the use of fluorescent markers in labelbased sensing methods [10]. TiO2 slot waveguides have been theoretically studied [11] showing the potential for visible wavelengths of such a structure. Silicon carbide slot waveguides have been demonstrated in the visible range for a horizontal slot [12]. When operating at visible wavelengths instead of the near-IR region, the gap in the slot waveguide must be narrower and, to avoid excessive scattering losses due the shorter operation wavelength, stringent requirements are set to the waveguide sidewall roughness. This makes the fabrication more challenging and requires precisely developed nanofabrication processes and a trade-off between ideal device dimensions and limits of the process technology. To achieve the best possible dimensions, we use the re-coating technique presented by Alasaarela et al. [13]. In addition to the feature size reduction, this re-deposition process reduces the sidewall roughness and reduces the propagation losses [14].
(a)
(b)
(c)
(d)
Fig. 1. SEM pictures of the fabricated slot waveguides; (a) tilted view along a slot waveguide; (b) slot waveguide profile for W S 30 nm, filling factor Γ 1∶10; (c) slot waveguide profile for W S 50 nm, filling factor Γ 1∶6; (d) slot waveguide profile for W S 80 nm, filling factor Γ 1∶4.
(25/6/5 sccm, 55 mTorr, RF power 105 W) with Plasmalab 80 by Oxford. After the two dry-etching steps, the remaining Cr layer is removed by a wet-etching solution. Finally the fabricated structure is re-coated with an additional layer (30 nm, total thickness of 250 nm) of TiO2, to reach precisely the targeted slot size. Scanning Electron Microscopy (SEM) images of the fabricated slot waveguides, after the re-deposition, are shown in Fig. 1 and the fabricated ring resonator structure is shown in Fig. 2. Using an ALD re-coating means that the rails of the slot waveguide will become thicker and the gap between the ring and the bus will be reduced, affecting directly the coupling efficiency of the device. The dimensions have then been calculated in order to take into account the additional coating. Nevertheless, it is understandable that the aspect ratio of the slot, in particular, cannot be varied as desired. With the process described previously, we achieved an aspect ratio of 1∶5.5 for the rails. Three fabrication examples are presented in Fig. 1. Figure 1(a) is an SEM picture of a standard slot waveguide in TiO2 after a 30 nm re-coating. One can still observe slight sidewall roughness. This will contribute to propagation loss, especially for visible wavelengths, due to Rayleigh scattering. Figs. 1(b)–1(d) are slot waveguide cross sections for different slot widths (W S ): 30, 50, and 80 nm, respectively. The blue dashed lines show the interface between the TiO2 layer and the SiO2 cladding which is slightly overetched in order to prevent the re-coating from completely filling the slot [Fig. 3(a)]. One can remark that the walls of the 30 nm slot waveguide are a little
(a)
Re-deposited TiO2 layer
Original TiO2 slot rails
(a)
WS
SiO2
WR
Si y
WS = 30 nm
(b)
y
x
WS = 30 nm
(c)
x 2
1
(b) 1
TE slot mode (Ex) neff=1.80 / ng=2.38 y
WS = 80 nm
0
(d)
0.5
TM rail mode (Ey) neff=1.93 / ng=2.48 y
x
WS = 80 nm
Fig. 2. SEM pictures: (a) fabricated ring resonator; (b) zoom-in of the coupling region.
bit tilted (about 5°). One can expect the slot mode to be pushed down in the slot region [15,16], close to the bottom of the slot. Figure 2 is an SEM picture of a slot ring resonator (the radius is r 6 μm). We expect rather high propagation loss along the slot waveguide mainly due to the sidewall roughness that cannot be fully eliminated by the ALD re-coating layer. Consequently, we have chosen a strip waveguide as the bus to bring light to the ring resonator. 3. Theoretical Investigations
The slot waveguide characteristic of the fabricated devices is verified by simulations presented in Fig. 3. In Fig. 3(a), the cross section of the designed and fabricated slot waveguide is presented. Figures 3(b) and 3(c) are the field distributions for the quasi-TE and quasi-TM mode, respectively, for W S 30 nm, and Figs. 3(d) and 3(e) are the field distributions for the quasi-TE and quasi-TM mode, respectively, for W S 80 nm. One can see the slot waveguide behavior of the structure. The modal refractive indices are nTE 1.8 and nTM 1.9 for a 30 nm slot width and nTE 1.5 and nTM 1.7 for an 80 nm slot width. Moreover, one can see a positive effect of the overetching of the oxidized silicon layer. Indeed, it allows the mode to be vertically centered in the slot. As expected, the mode is pushed down in the case of W S 30 nm, due to the tilted sidewalls. The low effective index of the TE mode proves the high confinement of the field inside the low refractive index region. In addition, we have calculated the filling factor (Γ), which is the ratio between the width of the slot and the width of the input nano-waveguide,
(e)
x 2
TE slot mode (Ex) neff=1.50 / ng=1.81
0
1
0
1
TM rail mode (Ey) neff=1.77 / ng=2.4
0.5
0
Fig. 3. (a) Schematic of the TiO2 slot waveguide. Oxide layer thickness is 3.0 μm, the thickness of the additional TiO2 layer is 30 nm, the slot-waveguide rail width is W R 160 nm, the rail height is 250 nm (λ 650 nm), and the slot width is W S (substrate is over-etched in order to compensate for the additional layer thickness of the re-coating); (b) mode profile for W S 30 nm, for TE polarization (sidewalls tilted about 5°); (c) mode profile for W S 30 nm, for TM polarization (sidewalls tilted about 5°); (d) mode profile for W S 80 nm, for TE polarization; (e) mode profile for W S 80 nm, for TM polarization.
for both structures and obtained Γ30 1∶10 and Γ80 1∶4. Figure 3 proves that for the 30 nm slot width, with a filling factor of 1∶10, the mode is more confined in the low refractive index area than for the 80 nm slot width having a filling factor of 1∶4. 4. Measurements and Discussion
In order to ensure efficient light coupling into the slot waveguides, the input and output coupling was realized using multimode waveguides (2 μm width) tapered to a single-mode waveguide having the desired width (300 nm) according to the wavelength. Light from a supercontinuum source (NKT Photonics, SuperK COMPACT) was injected with a tapered lens fiber in the device. Using another lensed fiber, the output signal is coupled to an Optical Spectrum Analyzer (OSA, Ando 6315A). The input light is fully unpolarized and unfortunately, it is not possible, in our experimental setup, to polarize it over a large wavelength range, without losing too much power. A picture (top view) of the structure illuminated with a laser diode at λ 640 nm is shown in Fig. 4(a). For this particular wavelength, only the TE polarization is injected in the device. One can see that scattering is observed mainly in a few specific locations, which 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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that these low values are representative of a lossy structure due to several possible factors: due to the bend loss in the ring structure which has a relatively small radius and to the sidewall roughness, this can be observed in the photograph in Fig. 4(a). Nevertheless, the main reason for these low quality-factors is an over-coupling of light in the structure (low F value), due to the short coupling distance between the bus and the ring. Figure 5 presents results extracted from twodimensional (2D) finite-difference time-domain (FDTD) simulations of a slotted ring resonator, where the slot width is varied. Notice that these FDTD simulations use a 2D model of the structure based on an effective index approximation, which does not take into account the tilt of the side walls. Nevertheless, one can see a good agreement in comparison with the mode calculations shown in Fig. 3. From Fig. 5(a), one can see a linear decrease of the group index of the TE polarization and an asymptotic decrease for the TM polarization when the slot width increases. Note that the variation is much larger for the TE mode, which is the slot mode. Moreover, from Fig. 5(b), one can conclude that for small values of W S , the resonances for the TE polarization are stronger, while the opposite is true for high values of W S. Indeed, when the slot width increases, the slot mode becomes lossy, while the rail mode remains well-guided. This is clearly observed in the mode
-1 -2 -3 -4 -5 -6 650
655
660
665
670
Wavelength [nm]
Transmission [dB]
(c)
0 -1 -2 -3 -4 -5 -6 -7 -8 650
660
670
680
690
700
Wavelength [nm] Fig. 4. (a) Photograph of the ring resonator structure when illuminated by a TE-polarized light at λ 640 nm; (b) measured transmission spectra through a 6 μm slotted ring resonator, W S 30 nm; (c) measured transmission spectra through a 6 μm slotted ring resonator, W S 80 nm.
shows that individual defects impart the main contribution to the loss in the device. Moreover, from Fig. 4(a) one can also clearly see light passing through the ring resonator, which is the only part of the device based on a slot waveguide. In the slot ring resonator as well, most of the scattering occurs in a few locations, while other parts of the ring appear to provide light propagation with relatively low scattering loss. The measured transmission spectrum as a function of wavelength is shown in Fig. 4(b) for W S 30 nm, and in Fig. 4(c) for W S 80 nm. By measuring the free spectral range (FSR), and the full-width at half-maximum (FWHM), it is possible to calculate, using Eq. (1), the group index of the mode propagating inside the ring resonator λ2 ; ng 2π · FSR · r
λ ; FWHM
and F
(1)
FSR : FWHM
(2)
We obtain Q30 630 and F 30 6.7 for W S 30 nm and Q80 740 and F 80 5.3 for W S 80 nm. Note 2656
(b)0.9
2.5
0.8
2.4 2.3 2.2 2.1 2.0 1.9
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
1.8 0
20
40
60
80
Slot width WS
(c)
0
100
20
40
60
80
100
690
700
Slot width WS
(d) 0
0 -1 -2 -3 -4 -5 650
-2 -4 -6 -8 -10
654
658
662
Wavelength [nm]
where λ is the wavelength, and r is the radius of the ring. The quality factor (Q) and the finesse (F ) of the ring structure can be calculated using Eq. (2) Q
(a)2.6
Resonance amplitude [arb. u.]
0
Transmission [dB]
Transmission [dB]
(b)
INPUT
TE polarized light
Group index ng
OUTPUT
Transmission [dB]
(a)
666
670
-12 650
660
670
680
Wavelength [nm]
Fig. 5. 2D FDTD simulations of the ring resonators; (a) dependence of the group index (ng ) on the slot width (W S ) for TE polarization [blue squares and solid line (linear fit)] and TM polarization [red circles and solid line (exponential fit)]; (b) dependence of the average intensity of the resonance peaks on W S for TE (blue squares) and TM (red circles) polarizations. (c) Transmission spectra for the TE (blue curves) and TM (red curves) polarization through a ring resonator of radius 6 μm for a slot width of 30 nm; (d) transmission spectra for the TE (blue curves) and TM (red curves) polarization through a ring resonator of radius 6 μm for a slot width of 80 nm.
simulations [Figs. 3(b)–3(d)]. Figures 5(b) and 5(c) present the transmission spectra for W S 30 nm and W S 80 nm, respectively, for the two polarizations. One can observe a good match of these results compared to the experimental spectra in Fig. 4. The group index values can be extracted from the experiments [Fig. 4] and simulations [Figs. 3 and 5]. We denote by ng and n~ g , the theoretical and the experimental group indices, respectively. For the W S 30 nm slot waveguide we obtain n~ 30 g 2.2 and for W S 80 nm, n~ 80 g 2.42. We obtain theoretically 80 n30 g 2.21 and ng 2.40, from the spectra presented in Fig. 5. The excellent match between the theoretical and experimental mode properties proves the existence of a slot mode in the ring resonators. 5. Conclusion
We have designed, fabricated, and characterized TiO2 ring resonators composed of slot waveguides for the visible wavelength range. Conformal redeposition, which is one of the main benefits of the ALD technique, was used for the feature size reduction. In this way, a sufficiently narrow slot size was achieved. This kind of a ring resonator structure may open up interesting possibilities, in particular at the visible wavelengths, for waveguide biosensor applications. This work was 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). References 1. V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding and confining light in void nanostructure,” Opt. Lett. 29, 1209–1211 (2004). 2. C. Barrios, “Ultrasensitive nanomechanical photonic sensor based on horizontal slot-waveguide resonator,” IEEE Photon. Technol. Lett. 18, 2419–2421 (2006). 3. T. Claes, J. Molera, K. De Vos, E. Schachtb, R. Baets, and P. Bienstman, “Label-free biosensing with a slot-waveguidebased ring resonator in silicon on insulator,” IEEE Photon. J. 1, 197–204 (2009).
4. A. Säynätjoki, L. Karvonen, T. Alasaarela, X. Tu, T. Y. Liow, M. Hiltunen, A. Tervonen, G. Q. Lo, and S. Honkanen, “Low-loss silicon slot waveguides and couplers fabricated with optical lithography and atomic layer deposition,” Opt. Express 19, 26275–26282 (2011). 5. T. Alasaarela, T. Saastamoinen, J. Hiltunen, A. Säynätjoki, A. Tervonen, P. Stenberg, M. Kuittinen, and S. Honkanen, “Atomic layer deposited titanium dioxide and its application in resonant waveguide grating,” Appl. Opt. 49, 4321–4325 (2010). 6. K. B. Gylfason, C. F. Carlborg, A. Kazmierczak, F. Dortu, H. Sohlström, L. Vivien, C. A. Barrios, W. van der Wijngaart, and G. Stemme, “On-chip temperature compensation in an integrated slot-waveguide ring resonator refractive index sensor array,” Opt. Express 18, 3226–3237 (2010). 7. C. A. Barrios, K. B. Gylfason, B. Sánchez, A. Griol, H. Sohlström, M. Holgado, and R. Casquel, “Slot-waveguide biochemical sensor,” Opt. Lett. 32, 3080–3082 (2007). 8. C. A. Barrios, B. Sánchez, K. B. Gylfason, A. Griol, H. Sohlström, M. Holgado, and R. Casquel, “Demonstration of slot-waveguide structures on silicon nitride/silicon oxide platform,” Opt. Express 15, 6846–6856 (2007). 9. J. T. Choy, J. D. B. Bradley, P. B. Deotare, I. B. Burgess, C. C. Evans, E. Mazur, and M. Lončar, “Integrated TiO2 resonators for visible photonics,” Opt. Lett. 37, 539–541 (2012). 10. T. Nuutinen, P. Karvinen, J. Rahomaki, and P. Vahimaa, “Resonant waveguide grating (RWG): Overcoming the problem of angular sensitivity by conical, broad-band illumination for fluorescence measurements,” Anal. Methods 5, 281–284 (2013). 11. G. Testa and R. Bernini, “Slot and layer-slot waveguide in the visible spectrum,” J. Lightwave Technol. 29, 2979–2984 (2011). 12. G. Pandraud, A. Neira, E. Margallo-Balbas, C.-K. Yang, and P. Sarro, “Demonstration of pecvd SiC–SiO2–SiC horizontal slot waveguides,” IEEE Photon. Technol. Lett. 22, 398–400 (2010). 13. T. Alasaarela, A. Säynätjoki, T. Hakkarainen, and S. Honkanen, “Feature size reduction of silicon slot waveguides by partial filling using atomic layer deposition,” Opt. Eng. 48, 080502 (2009). 14. M. Häyrinen, M. Roussey, V. Gandhi, P. Stenberg, A. Säynätjoki, 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). 15. A. Säynätjoki, T. Alasaarela, A. Khana, L. Karvonen, P. Stenberg, M. Kuittinen, and S. Honkanen, “Angled sidewalls in silicon slot waveguides conformal filling and mode properties,” Opt. Express 17, 21066–21076 (2009). 16. A. Säynätjoki, B. Bai, A. Tervonen, J. Turunen, and S. Honkanen, “Enhanced vertical confinement in angled-wall slot waveguides,” Opt. Rev. 17, 181–186 (2010).
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