Optical fibre nanophotonics

Optical fibre nanophotonics
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  Optical Fibre Nanophotonics John Canning Interdisciplinary Photonics Laboratory ( i PL), School of Chemistry, The University of Sydney, Sydney, NSW, Australia  Abstract SummaryTailoring and structuring optical fibres to nanoscale dimensions is rapidly becoming a focus area of research and is important for the eventual success of future in-fibre optical systems and novel technologies. Here,I review one aspect of our work in establishing and pursuing this field: localizing lightfor sensing. Keywords-component; Fiber Bragg grating; acousto-optic effect;  sampled gratings. I.I  NTRODUCTION When we think of opticalfibres, we think of optical  propagation, essentially moving light from one point to another, whether by step-index or diffraction assisted  propagation. The focus on this waveguide properties means that even components, both linear and nonlinear,using optical fibres are largely considered in terms of features on a scale commensurate with the waveguide, or the waveguide mode. Little if any attention is usually paid to sub-wavelength features. In fact, arguably such components are always considered in terms of modification of the wave guiding  properties only. However, recent innovations developing novel Fresnel lenses based on existing curiosities associated with fibre fabrication has transformed our view of optical fibres as solely a means of confinement and transport. The deposition  process of an MCVD system is sufficiently refined to enable the etching of well correlated gradedprofiles at the end of each fibrethat can act to focus light [1]. These features can be less than 100nm in resolution. It was a small step from this innovation to the idea that with sufficient index contrast a  properly designed optical fibre can do more than simply assist  propagation of light, via diffraction,from one point to another  –the Fresnel fibre [2,3] can enable both transportation and focusing of light [3-6], introducing the idea that fibres generally can have multiple capabilities beyond optical transport. In this context, alternative fabrication methods, such as plasma based CVD and structured optical fibre and fibre taper fabrication, have offered straightforward ways in which a fibre can be tailored on the nanoscale. Indeed, feature sizes <10nm have been preserved in tapered photonic crystal fibres [7-9].Structured fibres using air holes have also opened up new  possibilities. Materials that could not previously be integrated,or whose properties prevented mixing with other materials,could now be done so and the collective composite behavior of the fibre enables an optical mode to “see” an equivalent novel material with all-embracing properties. The defining work in this regard was the demonstration of white light generated by selective filling of holes of a photonic crystal structured fibre with three organic dyes [10]. Additional selectivity can be achieved by ion beam and laser processing. The traveling mode was made up of the equivalent of three combined emissions –  blue, green and red -to produce white light. This work introduced several important concepts: (1) selective filling, (2) the composite system acting as unique material,circumventing unwanted interactions between important processes, (3) the integration of materials, such as organic media, into silica which could not be done by other means, and (4), the idea of multiple functionality and the birthof the laboratory-in-a-fibre [11]. More importantly, this work was a “macro” demonstration of how real materials work on the nanoscale – normal optical interactions often see a composite “homogenous” system when the structural features of the system are much smaller than the wavelength. (This is indeed the basis of metamaterials). What comes from all of this is that nanophotonics withina fibre requires an understanding of small features and how to introduce and manipulate them. From this, novel material performance and devices are envisaged.When optical tapers were produced with holes <10nm still  being preserved[9], we are in an extremely interesting regime. The fact is in silica, nanocavities already exist which are not much smaller than this –that is, we are capable of producing features that must be subject to the same local molecular strains that determine the local structure of the glass. In fact, at these scales, we are no longer in an amorphous regime with detectable intermediate range order becomingrecognisable. To imagine a future where we attempt to manipulate and position structure within amorphous materials on these scales, then we can see that the local strains and variations which are considered homogenous in the bulk media can quickly become a dominantchallenge. The intermediate zone and how it may affect short range order and long range disorder has the  potential to play significant roles in properly understanding glass, and its transformation to crystalline polymorphs, on the macro scale, and indeed play a role in elucidating the enigmatic nature of a glass.3-D optical nano-localisation maysoonbe observable.II.S MALL HOLES AND OPTICAL LOCALISATION In 2006, we reported an interesting theoretical observation: as the hole size of a Fresnel structured fibre is made smaller, light builds up in the hole with sharp boundary features[12]. This is due to the accumulation of theevanescent field within a thin glass capillary such that it exceeds the balanced optical mode within the glass itself and can be explained by the fact that light travel faster in the hole than in the glass, a form of optical impedance matching(not dissimilar to the impedance  matching in microstripline design). This observation was supported a year later by work from Bath university examining a small hole within a photonic crystal fibre, much the same experiment but smaller still[13]. Although experimental results were not quite conclusive(since the light extended beyond the hole,looking remarkably familiar to the hole localisation obtained within Fresnel fibre with a much larger hole suggesting diffractive effects are involved [3-5] –and that they inadvertedly made a similar Fresnel fibre!), the theoretical results supported the idea of optical impedance matching which was first described in silicon-on-oxide (SOI) slotwaveguides[14-15], where the much higher index enabled the direct observation of this over short lengths. In fibres, the short lengths of our tapers combined with the low silica index, meant we were unable to readily observe the localisation directly.Nevertheless, the fact that the optical field can be manipulated in this way means the  potential existsfor unprecedented beam shaping on the nanoscale and for unusual devices based on coupling of holes. This is an exciting but very challenging area to work inwhen the index contrast with air is quite low -it is likely much of the progress in this regard will initially be done with higher index semiconductors using planar configurations  before translating across to silica fibres.Alternative use of higher index glasses is possible though the issue of loss, quality control, and other effects needs consideration.III. DEPOSITING NANOLAYERSINTO OPTICAL FIBRES One of the key challenges in exploiting optical evanescent waves and interface localisation is the low refractive index of silica. We have seen that to observe localisation, small holes are required. For many applications, however, such as chemical and bio-sensing, holes have to be sufficiently large to overcome rate limiting steps such as diffusion (Brownian or Fickian),occasionallykinetic related impediments (electro-osmosis and electrophoresis when charged particles are involved) and potentially even van der Waals forces when holesare really small. Below about 1  m, these effects can prevent any intake of a sample, for example. On the other hand,there is clearly a potential advantage to exploiting the accumulation of evanescent field, improving sensitivity and efficiency. How to best access this opportunity? This localisation of the optical field at the interface is much stronger in materials with higher refractive indices (as has been observed with silicon [14-15]). Fortunately, structured optical fibres allow novel approaches to resolve this issue. One possible approach is to deposit a higher refractive index material along the inside wall of each channel. This has theadvantage of drawingthe optical field closer to the holes and at the same time allowing a field enhancement at the interface. What about leakage and confinement losses that may increase in such a configuration? To appreciate the potential of this approach, consider the simulation of a simple 3-ring photonic crystal fibre with and without an inner layer of TiO 2 . The results are shown in figure 1–details of the simulation can be obtained in other published proceedings [16]. From the cross section of each case, there is clearly a significantly enhanced Figure 1. Simulation of field confinement within (a) a simple 2-ring structured optical fibre; (b) the same fibre with a 155nm layer of refractive index n = 2.6; and (c) cross-section of simulations showing enhanced optical localisation of light particularly near the high index surfaces (orange dashed). 1050-5-10-10 -5 0 5 10 -8-24-40-56-72  x  (  m)      y       (           m      )   1050-5-10-10 -5 0 5 10 -8-24-40-56-72   1050-5-10-10 -5 0 5 10 -8-24-40-56-72  x  (  m)      y       (           m      ) (a)   1050-5-10-10 -5 0 5 10 -8-24-40-56-72  x  (  m)      y       (           m      )   1050-5-10-10 -5 0 5 10 -8-24-40-56-72   1050-5-10-10 -5 0 5 10 -8-24-40-56-72  x  (  m)      y       (           m      ) (a) 1050-5-10-10 -5 0 5 10 -4-12-20-28  x  (  m)      y       (           m      )   1050-5-10-10 -5 0 5 10 -4-12-20-28  x  (  m)      y       (           m      ) (b)   1050-5-10-10 -5 0 5 10 -4-12-20-28  x  (  m)      y       (           m      )   1050-5-10-10 -5 0 5 10 -4-12-20-28  x  (  m)      y       (           m      ) (b) Light is drawn into the ringand holes holehole   Light is drawn into the ringand holes holehole (c)  increase in optical field within the holes when a layer is used (despite an effectively larger core and greater leakage –the TiO 2 supports some light).Within the first 100nm away from interface regions of the hole (where most of the evanescent field is confined), there is more than an order of magnitude increase in potential interaction with a sample under test.We recently reportedsupporting experimental evidence to confirm this idea and the degree of improvement[16]. A rathernovel approach to forming the TiO 2 layer was developed –self-assembly of TiO 2 nanoparticleson the ultra smooth surface of the silica channels of a photonic crystal fibre through van der Waals forces.The spectroscopic probe used to test the idea was a deposited porphyrin compound-5,10,15,20-tetra(4-carboxyphenyl)porphyrin (TCPP). A brief summary of the results are shown in figure 2.The blue Soret B band is barely detected and there is no evidence of the weaker Q bands without the layer. By using the layer, the Soret band is off scale whilst the Q bands are readily detected –the predicted more than an order of magnitude increase in detection is achieved. For both cases the measured concentrations of introduced  porphyrin were within a factor of two of each other. Recalling that the input and output ends of the fibre are coupled to standard telecommunications fibres, the net increase in  background loss is small, showing that this approach is  potentially practical for both chemical and bio sensing.We recently used asimilar approach–this time forming the TiO 2 layer by sol gel -was used to enhance both the sensitivity and response time of an acid sensor [17]. IV.D ISCUSSIONS AND CONCLUSION It is clear that there is enormous scope for working down to nanoscale dimensions in optical fibres, whetherthey areall– solid structures,such as contemporary Bragg fibres being suggested for coloredclothing [18],or air structured fibres of various materials to exploit many of the concepts I have introduced here and more.In fact, we can conclude that optical fibre nanophotonics is key to enabling extensive multi-functionality and multiple devices in the one waveguide, a  precursor tocomprehensivelab-in-a-fibre technology.A CKNOWLEDGMENT Australian Research Council (ARC) and the Department of Innovation Industry Science and Research (DIISR) grants are gratefully acknowledged. Many colleagues have been involved in aspects of this work over the years; in particular Whayne Padden, Cicero Martelli, Claire Rollinson, Brant Gibson, Danijel Boskovic, Shane Huntington, and Max Crossley. R  EFERENCES[1]J. Canning, K. Sommer, S. Huntington, A.L.G. Carter, “Silica based fibre Fresnel lens”, Opt. Commun., 199, pp.375-381, 2001.[2]J. Canning, “Diffraction-Free Mode Generation and Propagation in Optical Waveguides”, Opt. Comm., 207(1-6) pp. 35-39 2002.[3]J. Canning, “Fresnel Optics Inside Optical Fibres”,in Photonics Research Developments, Nova Science Publishers, US, 2008, Chapter 5.[4]J. Canning, E. Buckley, K. Lyytikainen, “Propagation inair by field superposition of scattered light within a Fresnel fibre”, Opt. Lett. 28, (4),  pp. 230-2332, 2003.[5]J. Canning, E. Buckley, K. Lyytikainen, “All-Fibre Phase-Aperture Zone Plate Fresnel Lenses”, Electron. Lett., 39, (3), pp.311-312, 2003.[6]J. Canning, E. Buckley, K. Lyytikainen, “Multiple Source Generation using Air-Structured Optical Waveguides for Optical Field Shaping and Transformation Within and Beyond the Waveguide”, Opt. Express, 11 (4), pp.347-358, 2003.[7]B. C. Gibson, S. T. Huntington, S. Rubanov, P. Olivero, K. Digweed, J. Canning, J. Love, “Exposure and characterization of nanostructured hole arrays hole arrays in tapered photonic crystal fibers using a combined FIB/SEM technique”, Opt. Express, 13(22), pp.9023-9028, 2005.[8]C.M. Rollinson, S.M. Orbons, S.T. Huntington, B.C. Gibson, J. Canning, J.D. Love, A. Roberts, D.N. Jamieson, “Metal-free scanning optical microscopy with a fractal fibre probe”, Optics Express, 17 (3), 1772-1780, 2009.[9]C. M. Rollinson, S.T. Huntington, B.C. Gibson, J. Canning, “Fractal fibre for enhanced throughput SNOM probes”, in Trends in Photonics 2010(Ed. J. Canning), Research Signpost, 2010.Chapter 12.[10]J. Canning, M. Stevenson, T.K. Yip, S.K. Lim, C. Martelli, “White light sources based on multiple precision selective micro-filling of structured optical waveguides”, Opt. Express, 16 (20), 15700-15708, 2008.[11]J. Canning, “New Trends in Structured Optical Fibres for Telecommunications and Sensing”, 5 th International Conference on Optical Communications and Networks and the 2 nd International Symposium on Advances and Trends in Fiber Optics and Applications (ICOCN/ATFO 2006), Chengdu, China, 2006.[12]C. Martelli, J. Canning, “Fresnel fibres for sensing”, Optical Fiber Sensors Conference (OFS 2006), postdeadline, Cancun Mexico, (2006)[13]G. S. Wiederhecker, C. M. B. Cordeiro, F. Couny, F. Benabid, S. A. Maier, J. C. Knight, C. H. B. Cruz & H. L. Fragnito, “Field enahcement within an opticla fibre with a subwavelength hole”, Nat. Phot., 1, pp. 115-117, 2007.[14]V.R. Almeida, Q.F. Xu, C.A. Barrios, M. Lipson, “Guiding and confining light in voidnanostructure”,Opt. Lett. 29, pp.1209–1211, 2004.[15]Q.F. Xu, V.R.Almeida, R.R.Panepucci, M.Lipson, “Experimental demonstration ofguiding and confining light in nanometer-size low-refractive-index material”,Opt. Lett. 29,pp. 1626–1628,2004.[16]J. Canning, W. Padden, D. Boskovic, M. Naqshbandi, L. Costanzo, G. Huyang, H. de Bruyn, T. H. Sum, M. J. Crossley, “Comparing porphyrin absorption within a structured optical fibre with and without a TiO 2 deposited layer”, Asia Pacific Optical Sensors (APOS 2), Guanzhou, China, 2010.[17]G. Huyang, J. Canning, M. Aslund, D. Stocks, T. Khoury, M. J. Crossley, “Inline Remote Acid Sensing Using an Optical Fibre Porphyrin Micro-Cell Reactor”, Sensors Topical Meeting, Advanced Photonics: OSA Optics & Photonics Congress, Germany, 2010.[18]P. Brown, B. Kokuoz, B. Ellerbrock, K. Stevens, J. Ballato, “Novel approaches to complex optical fibres”,inTrends in Photonics 2010(Ed. J. Canning), Research Signpost, 2010.Chapter 1. 4005006007000369   a   b  s  o  r   b  a  n  c  e   (   d   B   ) wavelength (nm)  TCPP TCPP on TiO 2  layer  Figure 2. Absorption of 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (TCPP) inside a silica photonic crystal fibre without (black) a TiO 2 layer and with a TiO 2 layer(blue) [colour diagram].
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