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Controlling the evanescent field within platform waveguide technologies underpins waveguide nanophotonics and is critical to optimising the interaction with integrated specialised materials or devices under test. Unfortunately, this interaction is often small since the evanescent field is a fraction of the total optical field. Here we propose and demonstrate, through simulation and experiment, how the waveguide evanescent field can be enhanced substantially by using high index interface layers, which draw out the optical field in the probe vicinity taking advantage of field localisation. This can be further enhanced by extended resonant and gallery modes within the channels of a structured cylindrical waveguide. Several orders of magnitude increased sensitivity with minimal added insertion loss is obtained using self-assembled layers of TiO2 (B) nanoparticles and porphyrin within a silica structured optical fibre. The combination of novel photonics with specialty material integration highlights the potential scope for physics, chemistry, sensing and materials research.
©2011 Optical Society of America
1. Introduction
Driven by telecommunications, silicon and silica are the two key material platforms for waveguide device research. Novel functionality using more suitable materials often demands integration into or onto these platforms. Given limits in refractive index, for these platforms to play a key role in sensing and biodiagnostics, control, manipulation and enhancement of the evanescent field is essential. For example, evanescent field spectroscopy (EFS) using optical waveguides, both conventional [1] and structured [2] promises to become a central tool in chemical sensing, biodiagnostics and nano-thin film research. The combination with waveguides offers selective excitation or detection of targeted species at an interface. An example is conventional fluorescence imaging of proteins, DNA and other species used in biosensor assays and larger volumes, has to contend with substantial background signal from their complex environment [3]. Interface detection using the evanescent regime offers an alternative that can provide selectivity whilst permitting complicated spectral and temporal interrogation techniques; it also reduces overall absorption within a sample, preventing thermally induced damage, a problem limiting sample preservation as well as live cell-imaging [4]. Lab-on-a-chip [5] and lab-in-a-fibre [6] technology benefit from this potential selectivity enabling multiple functionality and devices in confined areas. There are, however, key challenges restricting this approach.
Given the configuration scope possible, to narrow this discussion we focus on structured optical fibres that have micro or nano channels where the core traveling modes have an evanescent field directly within surrounding channels. This avoids secondary processes in conventional fibres such as cladding mode evanescent field excitation through long period gratings [1]. By having long interaction lengths, the poor optical penetration within the channel (few % typically in silica) is overcome. Unprecedented sensitivity is possible - a previously postulated near IR band associated with charge transfer between the porphyrin dichloro[5,10,15,20-tetra(heptyl)porphyrinato]tin(IV) and the silica surface of the channels was observed within a fibre over 90 cm long [2]. Unfortunately, for many applications that rely on overlap with the evanescent field (chemical sensing, biosensing, optoelectronic devices) these lengths become impractical. Solutions include enhancing the evanescent interaction using resonating modes within planar ring waveguides [7,8] and the use of direct mode-channel overlap within bandgap waveguides [9–13]. The latter, however, are extremely sensitive to perturbations from microbending, temperature and strain, which shift dispersion, and therefore the bands, affecting interactions. Insertion losses are high, reducing cavity quality and limiting remote or distributed sensing. On the other hand, the presence of bandgap dispersion within solid core structured waveguides [14] has been exploited in integrated form: e.g. polarisation dispersion-matched ultra-narrow resonances in silicon photonic crystal circuits [15] are promising for biosensing and single molecule detection [16,17]. Therefore, for many applications the field at the interface (e.g. selective surfaces in biodiagnostics), where the sample collects or is processed, is important. The evanescent regime is also important for exciting plasmon resonances in integrated metal layers and particles, which in turn have highly localised and intensified evanescent fields of their own, reducing the input field threshold for applications such as catalysis [18] and fluorescence sensitivity [19].
2. Enhancing the Evanescent Field
The use of optical impedance matching (arising from different velocities of light in different refractive index media) to enable localised evanescent optical fields to reach high intensities within small channels is a recent proposal, using moderate index silicon slot waveguides [20,21] and within holes inside low index silica structured fibres [22–24]. Interestingly, edge localisation in silica based optical fibres obtained by simulation has not been observed - for example, the optical mode spread across the sub wavelength holes in [23,24] is similar to that demonstrated over much larger sized holes in a Fresnel fibre [25], the result of cladding induced coherent scattering rather than interface edge localisation. Nonetheless, numerical simulation (vector wave expansion using ABC_FDM [26] and finite elements based COMSOL [27]), predicts that the smaller the hole the substantially larger the field within the hole becomes [22,27]. In practice, holes as small as 10 nm have been achieved [28,29], a fascinating result given these are approaching the local nanocavity dimensions of the structure in the intermediate regime between rigid tetrahedral imposed local order and long range glassy randomness. For many applications, however, such as chemical sensing and biodiagnostics, holes or channels have to be sufficiently large to overcome rate limiting steps such as diffusion (Brownian or Fickian), occasionally kinetic related impediments (electro-osmosis and electrophoresis when charged particles are involved) and other effects [30]. On the other hand, there is clearly a potential advantage to exploiting the accumulation, or enhancement, of evanescent field, improving sensitivity and efficiency. Therefore, the approach we report here avoids subwavelength holes. Rather, it is based on enhancing the sensitivity of robust structured waveguides generally by depositing and integrating a higher index nanolayer to draw out the guided optical light to enhance the evanescent field interactions but simultaneously minimise additional confinement losses. Additionally, we use TiO2 (B) given it has better bio-, electrochemical and catalytic activity [31–33] compared with Si and SiO2, making it important for biosensors. As well as being compatible with silica, this approach has much broader implications for nanophotonics and nanotechnology generally as will be seen.
3. The Concept and Simulation
As a demonstration, we consider one of the most promising waveguide configurations used in optical sensing research – the so-called structured optical fibre, such as a “photonic crystal” silica fibre, where propagation is determined largely by effective step-index total internal reflection [12], though dispersive effects from the periodic structure are always present [14]. A schematic of a simple two ring structure is shown in Fig. 1 – for practical purposes the bulk of the optical field is confined by the first ring of holes. Propagation and the waveguide mode field distribution, including the evanescent field within the holes, are evaluated numerically using a full vectorial algorithm for 2-D structures, successfully used to design various structured and diffractive fibres [http://code.google.com/p/polymode/]. The algorithm solves Maxwell’s equations based on the adjustable boundary condition – Fourier decomposition method (ABC_FDM) [26]. Finite differences are used in the radial direction while the Fourier decomposition method used in the angular direction helps speed up computational time, permitting faster turnaround on a high precision desktop computer.
Fig. 1 Simulation of field confinement within (a) a simple 2-ring structured optical fibre; (b) the same fibre with a 155 nm 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).
Figure 1(a) shows the numerical simulation of the fibre guided mode – the optical field intensity is plotted on log scale to exaggerate the distribution. The bulk of the optical field lies within the core defined by the first ring of holes whilst the second ring prevents any further leakage loss. Importantly, only ~3% of the light is in the exponentially decaying evanescent field inside the first ring of holes. Because of the low refractive index contrast between silica and air (Δn ~0.45) no edge localisation of the optical field is observed. Of that evanescent light, ~50% is within the first (100-130) nm of the hole, signifying ~1.5% overlap with a sample thickness d ~(100-150) nm. Despite long fibre interaction lengths, this represents an inefficient design for most sensors – multiplexed sensors, for example, would benefit from higher interaction efficiencies.
In Fig. 1(b) we consider the addition of a high index layer deposited onto the holes. The layer is chosen to have a thickness d ~150nm to ensure that sufficient evanescent field is drawn out. It is considered to be made up of TiO2 in its rutile form (n = 2.6) since the fabrication of these is realisable and the index is greater than silica. Further, for biosensing applications, TiO2 films have high bioactivity, whereas SiO2 has very low or none depending on formation [31]. There are a number of interesting features observed in Fig. 1(b) including strong localization of the optical field both within the layer and at the surface region of the inner holes and some of the outer holes. This field appears uniform at the edges of the layers, indicative of ring resonator modes being established. The resonance modes on the channel surfaces, in particular, show the fine structure characteristic of ring resonant modes, including standing whispering gallery modes [34] that will further enhance the interactions with material on these surfaces. These ring layers deposited into the channel constitute long cylindrical waveguides, or extended ring resonators, which can support multiple ring modes. In principle this allows unprecedented sensitivity blending orthogonal ring resonance with infinite longitudinal propagation, a novel waveguide format – an extended version, for example, of the principles used to enhance florescence based biosensing with cylindrical cavities [35]. To illustrate more clearly the enhanced optical localization observed in this fibre design, especially away from the core, a cross-section of the simulated optical field with and without the layers is shown in Fig. 1(c). There is between 2 and 3 dB higher signal, indicating twice as much light at the inner surface of the channel closest to the core, most of which is due to resonant enhancement by the ring layers, with much higher interrogation on the other side of the holes and in ring 2 – in total, the simulation predicts a few orders of magnitude increased sensitivity. Considering experimental variations, at least an order of magnitude increase in sensitivity may be anticipated using this approach. The observed resonant light in some of the outer ring is indicative of ring resonator coupling.
4. Synthesis and deposition of TiO2 layers and porphyrin within a structured fiber
4.1. Structured optical fiber
For the actual experimental work, the structured optical fibre shown in the scanning electron microscope (SEM) image of Fig. 2 was used. This fibre was assembled from Heraeus F300 silica capillaries and tubes, fused together on a glass lathe under combined pressure (capillaries) and vacuum (in-between capillaries), before being draw down to a diameter of ~125 μm using two draws on an optical fibre draw tower at ~1800 °C. Custom pressure control kept the holes open both on the lathe and on the tower. Introducing a slight aperiodicity in the structure by having the holes increase in size outwards, is thought to account for the low measured propagation loss <6 dB/km despite having only 3 rings [36]. Lengths of 20 cm were used for the samples under test.
Fig. 2 SEM image of the core cross-section of a structured optical fibre with 3 rings of holes. Optical guidance is dominated by the two inner rings.
4.2. TiO2 synthesis
A novel approach to the incorporation of TiO2 layers was carried out after fibre fabrication. It involves the self-assembly of TiO2 nanoparticles into low scattering films through van der Waals forces and crystal sheet formation – for this uniform size nanoparticles were required. Those obtained commercially were found to vary substantially so these were fabricated directly by hydrolysis of small quantities of titanium isopropoxide, Ti{OCH(CH3)2}4, suspended in isopropanol and added to ethanol [37]. By avoiding an additional hydrothermal reaction, larger particles, chosen to ensure that there was a sufficiently thick layer to generate optical localisation of the evanescent field within the holes as calculated by simulation, could be obtained. Dynamic light scattering (DLS) measurements and transmission electronic microscopy (TEM) images confirmed that uniform TiO2 nanoparticles on the order of (155 ± 20) nm were obtained. Figure 3(a) shows a monoclinic crystal configuration consistent with a partially hydrolysed form akin to so-called TiO2 (B) [38]. TiO2 (B) forms thin nano-sheet layers at low temperatures but the use of a stronger hydrothermal process (NaOH) and temperatures above 100-150 °C leads to these sheets rolling up to produce solid nanowires [39]. With even higher temperatures, these wires convert to the anastase structural polymorph of TiO2 and eventually rutile. By using low temperature preparation, sheets of metastable TiO2 (B) also have enhanced electrochemical and catalytic properties [32,33] relative to the other polymorphs. Therefore, despite a slightly lower effective refractive index (n ~2.5), the solution containing these monoclinic crystals was selected for insertion into the structured optical fibres to form a layer approximately 155 nm thick through flushing and evaporative self-assembly on the surface channels at room temperature, itself a novel procedure.
Fig. 3 TEM images of (a) crystal of TiO2 showing evidence of a monoclinic unit cell and (b) similar crystal coated with TCPP.
4.3. Porphyrin spectroscopic probe
The spectroscopic probe chosen for these experiments was 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (TCPP), where the carboxylic groups should attach well to the TiO2 and SiO2, although it is probably likely to be better with TiO2 (B). For all preparation conditions, the solvent employed was ethanol (EtOH) and the porphyrin concentration in EtOH was [TCPP] = 1.5 x 10−3 M. This was reduced to [TCPP] ~7.5 x 10−4 M after dilution and mixing with the TiO2/EtOH solution (TCPP: TiO2 = 1:1). After stirring, particles were filtered and examined under TEM – Fig. 3(b) shows evidence of attached porphyrin on a single crystal. Interestingly, as well as precipitating on the crystals, the titania generally catalysed precipitation of excess porphyrins out of solution, observed both as brown aggregates which upon precipitation led to increased solution transparency. To verify attachment, a corresponding red-shift, Δλ ~(3-10) nm, is observed for both Soret B and Q bands in the spectra for various concentrations, shown in Fig. 4 . The shift to longer wavelengths is consistent with J aggregation (side by side alignment) [40] rather than H aggregation (face to face) which is observed on much smaller sized nanoparticles <40 nm [41]. The general broadening of the bands reflects the size distribution of the particles.
5. Proposed Experiments
The method chosen to make a high index layer within the air channels of a structured optical fibre is one based on TiO2 (B) forming sheets [37], through a novel low temperature flushing and evaporative self-assembly of the relatively large (155nm) monoclinic crystals onto the inner surface of our structured optical fibre channels. A reasonable expectation is that if the quality of film formation or coverage is poor, then given that these particles are commensurate in dimension with typical probe wavelengths (10-40% of visible and near IR wavelengths), substantial Mie scattering should occur, translating to readily detected propagation losses. Good film formation, on the other hand, should show little increase in loss, other than coupling losses with input and output fibres whilst showing improved sensitivity through an enhanced evanescent field. Therefore, there are three key experiments to verify simultaneously good film formation and the proposed model of enhancement in our structured optical fibre. The structured fibre with the higher index layer coated with porphyrin should show much greater signal sensitivity and detection than the fibre sample with no layer but with channels coated similarly with porphyrin. Potentially, failed film formation will lead to large scattering that will undermine such a result. Therefore, a second sample with porphyrins mixed onto the TiO2 prior to insertion should act as a reference from which the absolute amount of scatter may be determined, since the attached porphyrin will prevent extended TiO2 layers from forming. Thus the experiments involve the separate optical interrogation of the following:
- (1) 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (TCPP) only filled fibre;
- (2) TCPP coated TiO2 particles obtained after mixing prior to insertion into fibre. In this case, the porphyrins binding to the surface will prevent film formation leading to large scattering losses; and
- (3) TCPP and TiO2 particles inserted into fibre without mixing (no coating). Film formation becomes possible leading to lower scattering losses.
6. Experiments and Results
In practice, the samples are inserted under air pressure (3 atm) and room temperature into the structured optical fibre until solution is detected upon exit. Each sample is passed through leaving behind a deposited layer. To separate attached material from loss aggregates additional flushing with ethanol, and drying with N2, is carried out. A schematic of the optical interrogation configuration used is shown in Fig. 5 . A broadband white light source (Oriel Hg-Xe lamp) is coupled into standard telecommunications fibre before being coupled into the optical fibre under test. The output is collected with another telecommunications fibre which then couples into an optical spectrum analyser (OSA – Ando AQ6315A).
Fig. 5 Schematic of the optical interrogation setup. The spectrum within the sample fibre under test is collected using a broadband Hg-Xe white light source and optical spectrum analyser (OSA).
The experimental results are shown in Fig. 6 . In the first experiment (1), where the TCPP is inserted directly into the fibre with only a few per cent field in the evanescent tails within the channels, detection of the TCPP porphyrin is minimal over this length of fibre – some of the Soret B band may be seen but otherwise there is no unambiguous detection of the weaker Q bands. In the second experiment (2), where the TCPP is mixed with the nanoparticles prior to insertion, the B band is readily detected and evidence of the Q bands is observed. This increase in detection is clearly related to more optical field overlapping within the layer of TiO2 which contains the porphyrin. However, this has come at the expense of a large increase in background loss – more than double (>100%) - consistent with Mie scattering, anticipated on the basis that the attached porphyrin prevents sheet formation. In the final experiment (3), where the TCPP is introduced after the TiO2, the largest signal detection is achieved. The signal is readily observed and the background loss has not increased substantially (<25%) consistent with uniform layer formation. In contrast to the previous experiments, the Soret absorption bands are all detected clearly and unambiguously and the B band signal is stronger than that of experiment (2) despite detection residing mainly in the evanescent regime. This is consistent with the simulated optical localisation at the edges which is higher than within the high index regions (as shown in Fig. 1) and a reduced background loss. Using a Gaussian fitting in the linear regime to study the integrated area of the blue Soret B band, with the curve of (1) as the reference line, the increase in B signal is more than 60 times for (3) over (2), which in turn is ~1014 more sensitive than when no TiO2 is used and virtually no B band is detected.
Fig. 6 Transmission spectrum of 3 structured fibre samples referenced to pristine sample: (black) – with TCPP only; (purple) – with TCPP mixed with TiO2 nanoparticles; and (green) with TiO2 layer and TCPP. Details in the text.
7. Conclusion
The use of high index nanolayers is an effective approach to enhancing the evanescent field within structured waveguides by more than several orders of magnitude, through edge and resonance localisation, although some contribution through stronger attachment with a more active TiO2 (B) surface is expected. The self-assembled TiO2 layer within the cylindrical channel of a structured optical fibre creates a high index ring waveguide capable of supporting novel extended resonant modes, including whispering gallery modes. The increased insertion loss was negligible for the best case whilst a dramatic increase in sensitivity is obtained. This agreement with simulation demonstrates that it is possible to model with a reasonable degree of accuracy evanescent field nanophotonics using novel materials. The results point to the feasibility of remote optical interrogation. TiO2, in particular TiO2 (B), has important surface properties of benefit to a range of chemical and biosensor applications – our results point to a simple approach in readily integrating it into structured optical fibres. More generally, the principles outlined here can be varied to suit many other applications, research and components technologies, including 2 and 3-D structured waveguides for example, which may require different material hosts. Improvements in material deposition within optical fibres, will lead to greater control of dimensions and material quality to ensure even greater improvements in sensitivity are still yet possible whilst maintaining low loss.
Acknowledgments
This work was funded by several projects: Australian Research Council (ARC) Discovery Projects (DP0770692, DP0879465), and Department of Industry, Innovation, Science and Research (DIISR) International Science Linkage (CG130013). W. Padden acknowledges funding from an ARC Linkage Project (LP 0990871). The authors thank Lorenzo Costanzo and Tze Sum for technical assistance.
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