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Embedded optical micro/nano-fibers for stable devices Na Lou,1 Rajan Jha,2 Jorge Luis Domínguez-Juárez,1 Vittoria Finazzi,1 Joel Villatoro,1 Gonçal Badenes,1 and Valerio Pruneri1,3 1
ICFO–Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860, Castelldefels (Barcelona), Spain School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Samantapuri, Bhubaneswar 751013, India 3 ICREA–Institucio Catalana de Recerca i Estudis Avançats, 08010, Barcelona, Spain *Corresponding author:
[email protected]
2
Received October 22, 2009; revised December 15, 2009; accepted January 7, 2010; posted January 22, 2010 (Doc. ID 118945); published February 11, 2010 We present a technique to embed silica micro and nanofibers in low-index material (Teflon) using an inexpensive and straightforward fabrication process based on spin coating. The optical properties of the silica micro/nano-fibers have been investigated when they are bare or completely or partially embedded. Optical degradation occurs in bare fibers with diameters smaller than twice the wavelength of the guided light, thus making protection through embedding necessary. Our results also show that completely embedded fibers do not degrade over a long time, while partially embedded fibers can preserve the large evanescent waves without undergoing considerable degradation, which would be further reduced or even become negligible with functional overlayers. The results represent a step forward toward the development of durable and stable devices based on optical micro/nano fibers. © 2010 Optical Society of America OCIS codes: 230.3990, 280.4788, 060.2370.
Since the demonstration of subwavelength photonic wires by Tong et al. in 2003 [1], considerable attention has been paid to their properties and applications; see, for example, [2]. The fabrication of subwavelength-diameter optical fibers (also known as optical micro/nanofibers) can be carried out; for example, by simply tapering conventional optical fiber down to dimensions smaller than the wavelength of the guided light. The appeal of optical micro and nanofibers (MNFs) is their low-loss transmission, easy coupling to fiber components, and large and strong evanescent waves [1,2]. The latter property makes MNFs attractive for efficient light coupling to ultrasmall cavities [3] or for trapping and manipulating cold atoms [4]. Large evanescent waves also make possible the interrogation of ambient changes, since the interaction of the external medium with the evanescent part of the guided mode propagating along the MNF is quite strong [5]. So far, several groups have reported different highly sensitive sensors for such parameters as hydrogen gas, refractive index, and humidity, and even for molecular absorption [6–11]. The advantage of the MNF evanescentwave sensors is that their sensitivity can be enhanced by simply reducing the fiber diameter. This is so because as the fiber structure becomes thinner it becomes more interactive with the surroundings and therefore more sensitive to ambient changes. However, when the dimensions of an optical MNF are comparable with the guided wavelength, the MNF undergoes mechanical and optical degradation [2,12]. The degradation is more prominent as the MNF becomes thinner and thinner. To develop durable and functional sensors and other devices based on optical MNFs, adequate protection is highly desirable, if not necessary. The protection of the MNFs must not sacrifice their large evanescent fields, and at the same time it must warrant low-loss transmission and good mechanical strength. To this end different attempts 0146-9592/10/040571-3/$15.00
have been reported so far, but most of them do not fulfill all the aforementioned requirements. In most of the cases the MNFs simply lie on the protecting material or are fully embedded in low-index materials [2,12–17]. The embedding is typically carried out by depositing drops of polymer on the MNF, which usually allows the formation of bubbles that induce considerable scattering losses [2,12]. Here we report on the optical-mechanical properties of MNFs partially or fully embedded in double layers of Teflon that are deposited by inexpensive and well-established spin-coating techniques to ensure homogeneity and controllable thickness. Low- and high-index substrates can be used with this technique. Our studies reveal that MNFs can be made with long-term stability, thus holding promise for the development of durable and functional MNF devices. To fabricate the MNFs we tapered standard singlemode fiber using a computer-controlled tapering station. The heating source was a flame torch produced by an appropriate mixture of oxygen and butane. The translation stages (VT-80 Micos) used for drawing the fiber had resolution of 1 m. A collection of samples with diameters ranging from 0.75 to 3 m, each with a length of 5 mm, were fabricated. The typical loss induced by the fabrication process was around 0.02 dB, which was measured at = 1550 nm (wavelength of the optical source). The degradation over time of bare MNFs with different diameters was studied first (in a conventional laboratory). To do so we launched light from a highly stable distributed-feedback-laser diode (S3FC1550 Thorlabs) operating at a peak wavelength of 1550 nm with 0.6 nm spectral linewidth and 1.5 mW of maximum power. The samples were kept straight during the time the measurements lasted. Figure 1 shows the loss change as a function of time of three samples, one with diameter larger than , one with diameter equal to , and another one with diameter © 2010 Optical Society of America
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Fig. 1. (Color online) Loss change as a function of time observed in three bare optical microfibers with diameters indicated in the figure. The operating wavelength was 1550 nm. The time 0 is when the fabrication of the MNF ends.
smaller than . From the figure it can be seen that the transmission decreases with time at rates of ⬃0.01, ⬃0.14, and ⬃0.33 dB/ h for the 2, 1.5, and 0.75 m diameter optical microfibers, respectively. We believe that the light is lost owing to surfaces roughnesses and nanocracks that grow with time. These roughnesses and cracks are responsible for scattering of the light guided along the fiber length and hence to loss of light. It was observed that the degradation was minimal and even negligible when the samples had diameters ⬎2. The results shown in Fig. 1 suggest that protection of the MNFs becomes essential when their dimensions are comparable with or smaller than the wavelength of the guided light. To avoid degradation of the MNFs without affecting their optical properties, we sandwiched them in two layers of DuPont Teflon AF 1601 in a configuration depicted in Fig. 2. Teflon was selected as the protecting material because it has a refractive index of 1.31 (at = 1550 nm). Moreover, it has extremely low solubility in most chemicals and is easy to process us-
Fig. 2. (Color online) Illustration of (a) fully embedded and (b) partially embedded optical microfibers and (c) embedded micro/nano-fiber stack (c). 1, wafer or substrate; 2, Al layer; 3, 4, the two layers of Teflon; 5, optical micro/nano fiber.
ing spin coating. The original 18% Teflon AF solution (DuPont 601S2-100-18) was further diluted in the same solvent (3M Flourinert Electronic Liquid FC40) down to 12%. To enhance the adhesion of Teflon to the wafer, a 10-nm-thick layer of aluminum was first sputtered on the wafer. An underlying layer of Teflon was first deposited on the substrate (layer 3 in Fig. 2) with sufficiently large thickness 共 ⬃ 4 m兲 to avoid optical interaction (coupling) between the MNF and the substrate. This layer was baked at 50° C for 5 min in order to partially remove the solvent and also to improve the adhesion to the substrate. Over the thick layer a thinner one with thicknesses between 1.5 to less than 4 m was spin coated (layer 4 in Fig. 2). Such a thin layer allowed us to partially or totally embed the MNFs. We would like to point out that the MNFs were not fixed to the substrate during the spin-coating processes but were gently immersed in the thin Teflon layer when it was still fresh. To do so, a homemade setup was used with which we handled the samples with utmost care and precision. Once the MNFs were embedded, the Teflon layers were again baked for 10 min to remove the solvent. The technique just described allows depositing uniform Teflon layers that do not shrink or contain bubbles. Moreover, the Teflon adhesion to the underlying Al layer or the adhesion between Teflon layers is quite strong; for example, the layers cannot be peeled off easily. Figure 3 shows an scanning-electron-microscope image of a section of the partially embedded MNF. The thin layer of Teflon was partially etched to see the thick layer (the layer beneath the MNF). The image revealed uniform layers with no bubbles. The insertion losses of fully and partially embedded MNFs were similar and typically around 2 dB. The transmission as a function of time was also measured for embedded MNFs with the same procedure described above. Figure 4 shows the loss change over time observed in a 1 m diameter fiber when it was completely or partially embedded in Teflon. It can be observed that the fully embedded microfiber does not exhibit any sign of degradation, even after 60 h. However, this geometry would not allow access to the evanescent optical wave, making the configuration not useful for several applications, such as optical sensing. On the contrary, with the partially embedded MNFs one can access the evanescent waves for different applications, although the fiber degrades at a rate of ⬃0.01 dB/ h (similar to that of the bare 2 m diameter MNF). Moreover, when a liquid (e.g., water) is placed on the top of the device no change in optical
Fig. 3. (Color online) SEM image of a partially embedded optical microfiber close to the thinnest section. The arrow shows the underlaying layer of Teflon.
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the optical micro/nano fibers are completely embedded the degradation is fully eliminated, but one has no access to the evanescent waves. This problem can be overcome with partially embedded microfibers, which degrade at a rate by approximately 0.01 dB/ h. Although already low the degradation can be made practically negligible with the addition of the functional layers (e.g., biolayers), which will eventually form the sensing structure. It is important to point out that microfluidics channels can be formed on the embedded optical microfibers. Therefore we believe that the work presented here is an essential step required for the development of durable and functional devices based on optical micro or nanofibers. Fig. 4. (Color online) Loss as a function of time observed in a 1-m-thick optical fiber when it was partially embedded (solid line) and totally embedded in Teflon (dotted line). The measurements were carried out at 1550 nm. The time 0 was when the embedded process ended.
loss was observed for the completely embedded MNF, while it was significant for a partially embedded MNF. We further investigated the behavior of partially embedded MNF when an index-matching oil or acetone (refractive index of 1.42 and 1.36, respectively) was placed on the sensitive region of the device (Fig. 5). The transmission at 1550 nm dropped to zero with the oil, while it was reduced to about 20% with the acetone. This is a further evidence of the presence of an evanescent interaction that depends on the surrounding material. A full recovery was also observed when the liquids were removed, suggesting that partially embedded MNFs can be reused. In conclusion, we have reported on the properties of optical fiber MNFs when they are bare or partially or completely embedded into Teflon layers. Our studies revealed that bare optical microfibers degrade over time when their diameters are similar to or smaller than the wavelength of the light they guide. The degradation can be prevented by embedding the MNFs in low-index polymer such as Teflon through straightforward controlled fabrication processes. If
Fig. 5. (Color online) Transmission as a function of time observed in a partially embedded 1 m diameter MNF when oil or acetone (inset) was placed on it.
This work was carried out with the financial support of the Spanish Ministerio de Educación y Ciencia through grant TEC2006-10665/MIC and the Spanish Ministry of Public Works through the project SOPROMAC P41/08. The authors also acknowledge funding by the “Ramón y Cajal” program. References 1. L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Maxwell, and E. Mazur, Nature 426, 816 (2003). 2. G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. Sessions, E. Koukharenko, X. Feng, G. Murugan, J. Wilkinson, and D. Richardson, Adv. Opt. Photon. 1, 107 (2009). 3. C. Grillet, C. Monat, C. L. Smith, B. J. Eggleton, D. J. Moss, S. Frédérick, D. Dalacu, P. J. Poole, J. Lapointe, G. Aers, and R. L. Williams, Opt. Express 15, 1267 (2007). 4. G. Sagué, E. Vetsch, W. Alt, D. Meschede, and A. Rauschenbeutel, Phys. Rev. Lett. 99, 163602 (2007). 5. M. Sumetsky, in Advanced Photonic Structures for Biological and Chemical Detection, X. Fan, ed. (Springer, 2009). 6. J. Villatoro and D. Monzón-Hernández, Opt. Express 13, 5087 (2005). 7. L. Zhang, F. Gu, J. Lou, X. Yin, and L. Tong, Opt. Express 16, 13349 (2008). 8. M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and D. J. DiGiovanni, J. Lightwave Technol. 24, 242 (2006). 9. X. Guo and L. Tong, Opt. Express 16, 14429 (2008). 10. F. Xu, V. Pruneri, V. Finazzi, and G. Brambilla, Opt. Express 16, 1062 (2008). 11. F. Warken, E. Vetsch, D. Meschede, M. Sokolowski, and A. Rauschenbeutel, Opt. Express 15, 11952 (2007). 12. F. Xu and G. Brambilla, Jpn. J. Appl. Phys. Part 1 47, 6675 (2008). 13. C. Caspar and E.-J. Bachus, Electron. Lett. 25, 1506 (1989). 14. P. Polynkin, A. Polynkin, N. Peyghambarian, and M. Mansuripur, Opt. Lett. 30, 1273 (2005). 15. G. Vienne, Y. Li, and L. Tong, IEEE Photon. Technol. Lett. 19, 1386 (2007). 16. L. Xiao, M. D. Grogan, S. G. Leon-Saval, R. Williams, R. England, W. J. Wadsworth, and T. A. Birks, Opt. Lett. 34, 2724 (2009). 17. Y. Wu, Y. J. Rao, Y. H. Chen, and Y. Gong, Opt. Express 17, 18142 (2009).
