A core component of all scanning near-field optical microscopy (SNOM) systems is the optical probe, which has evolved greatly but still represents the limiting component for the system. Here, we introduce a new type of optical probe, based on a Fractal Fibre which is a special class of photonic crystal fibre (PCF), to directly address the issue of increasing the optical throughput in SNOM probes. Optical measurements through the Fractal Fibre probes have shown superior power levels to that of conventional SNOM probes. The results presented in this paper suggest that a novel fibre design is critical in order to maximize the potential of the SNOM.
© 2007 Optical Society of America
During the last quarter of a century, the SNOM has undergone numerous improvements to the point where several commercial companies supply these instruments in various forms worldwide -. A core component of all SNOM systems is the optical probe, which has evolved greatly but still represents the limiting component for the system. In order to capture non-diffraction limited optical images, the SNOM utilizes an optical probe, with a sub-wavelength aperture, to deliver an optical signal to or receive optical information from a sample of interest. If the probe is very close to the sample, the resolution is commensurate with the size of this aperture. The optical probe can be manufactured using a number of different techniques -, but typically these probes are fabricated using optical fibres that are tapered down to nanometer-scale tips and the outside is coated with a thin layer of metal . The key problem with this type of probe is the excessive loss that occurs, which effectively limits the applications of the microscope . Low power throughput means slow scanning speeds. The high loss arises firstly from attenuation through the sub-wavelength aperture, which is unavoidable, and secondly from the interaction of light with the metal coating in the tapered region of the tip. We intend to address the latter of these mechanisms by reducing the interaction between the transmitted light and the probes’ metal coating. Using a combination of PCF technology and a new type of fibre called a Fractal Fibre (special class of PCF), we will present a new type of ultra high throughput probe that will maximize the potential of the SNOM.
2. Fractal fibre concept
When standard optical fibre (eg. SMF-28e) is used to produce tapered metal-coated SNOM probes, dopant diffusion occurs during the tapering process due to the high temperatures involved and there is a substantial change in fibre geometry . This can be observed schematically in Fig. 1(a).
As a result, the modal field will spread across the whole fibre and there is an interaction between the field and the metal coating, leading to strong attenuation, as shown in Fig. 1(b). However, the interaction of the modal field and the metal coating can be reduced, and the throughput optical power can be increased, if a single material PCF is used instead of a doped fibre, as depicted in Fig. 1(c). In this case the holes scale down with the taper, and the dopant diffusion problem is eliminated due to the absence of any fibre dopants. The tapering of PCFs post manufacture is well understood -, with non-SNOM applications ranging from microfluidics , to supercontinuum generation , couplers  and low-loss transition to conventional optical fibers .
In 1975, Benoít Mandelbrot coined the term Fractal, and published his ideas two years later, which describe structures that include shapes that are recursively constructed or self-similar, that is, a shape that appears similar at all scales of magnification and is therefore often referred to as ‘infinitely complex’ . The novel fibre, presented here for the first time, is based on the Fractal pattern and was designed to maximize the optical throughput of near-field probes. Standard PCFs are typically manufactured using a technique of stacking silica capillary tubes of equal size in a hexagonal lattice pattern . However, the Fractal fibre consists of a series of rings of holes whose cross-sectional area increases with increasing distance from the center core region in such a manner that the average effective index of the fibre decreases with increasing distance from the core region. The fibre requires a π/n rotation of the ring of holes, where n is the number of holes in each ring, with respect to each consecutive ring of holes so that the holes are not aligned. A schematic representation of the cross-section of the Fractal fibre is shown in Fig. 2(a).
When the Fractal fibre is tapered to form a near-field probe, the inner ring of holes collapses into the core region and the modal field is then confined by successive rings of holes, which can be observed schematically in Fig. 2(b). The guided light will essentially experience the same difference index in each region and consequently the light guiding properties of the fibre are also essentially the same as the diameter of the fibre is reduced. The elements around the core region are typically disposed with tapering which forms an arrangement that has a cross-sectional shape similar to a fractal pattern. The motivation behind the Fractal fibre is to maintain constant optical properties, or modal effective index, for as long a length as possible while the fibre is being tapered as the outer diameter is being reduced in size.
3. Fractal fibre fabrication
The fabrication of the Fractal fibre posed a host of new challenges that had not previously been dealt with by other fibre fabrication facilities. As previously mentioned, standard PCFs are typically produced from stacking silica capillary tubes of similar size in a lattice structure. The production of the Fractal fibre had the complex challenge of trying to incorporate capillary tubes with significantly varied internal holes sizes. Although the hole collapse rates were different from the center to the outer edges of the preform, the drawing rates were maintained with scaling for the preform and cane, similar to normal PCF fabrication. However, in fibre form, the internal hole collapse rates were dissimilar to standard PCF fabrication. An intricate capillary stacking procedure was implemented in order to produce the Fractal preform. A picture of the stacked capillary tubes can be observed in Fig. 3(a) and the subsequently fused preform and fibre cane are shown in Figs. 3(b) and (c), respectively.
Interstitial gap fillers helped to relax the pressure operating range used during production. After numerous trials with increasingly more sophisticated designs, we found the only practical way to remove the interstitial (unwanted) holes without compromising the remaining hole dimensions was not during fibre drawing but in the preform phase using a custom designed differential pressure control system that created a vacuum on the interstitial holes whilst maintaining a positive pressure within the capillary (required) holes. In the final stage of the manufacturing process, the first Fractal fibre was drawn from the fibre cane to have an outer diameter of 125 micrometers. No drilling techniques were necessary, which is a major improvement of previous methods for making highly complex fibres. It also marks the first complex design layout manufactured by modified stacking methods that is not a regular structure. The fabrication of this fibre formed the foundation for the successful production of an ultra-high throughput optical probe.
4. Fabrication and characterization of fibre probes
Carbon-dioxide laser-based pulling methods were used to taper and break optical fibres to tip diameters in the order of 50 nm -. This technique is the industry standard which is used for the production of optical fibre-based near-field probes. In addition to the Fractal fibre, a single-mode fibre and PCF were also tapered in order to accurately compare the optical throughput properties of the each probe. The cross-section of the single-mode fibre, PCF and Fractal fibre can be observed in Figs. 4(a), 4(b) and 4(c), respectively.
The single-mode, PCF and Fractal fibres were each tapered using a custom-built carbon dioxide laser-based pulling system. For each sample, a 5 cm length of fibre was stripped of its acrylate coating using methylene chloride and then cleaned with ethanol. The partially stripped fibre was then held under tension between two pulling arms and a carbon dioxide laser was used to heat the fibre. The motorized arms pulled the fibre in opposite directions until the fibre thinned and finally snapped to form two individual tapered optical fibres with overall taper lengths of approximately 1.25 mm and tip diameters in the order of 50 nm. All of the fibre tapers described in this paper were fabricated at room temperature and no additional gas was used to pressurize the holes during the tapering.
The tapering of the each of the fibres was performed using identical conditions in a controlled manner whereby the longitudinal internal holes of the PCF and Fractal fibres remained open to within a few micrometers from the tip of each probe . The single-mode fibre obviously has no internal holes but the external geometry of the probe was found to be identical to that of the PCF and Fractal probes. In an untapered single-mode optical fibre, light is confined predominantly to the central core region because of the difference between the refractive indices of the core and the cladding. As the optical fibre is tapered, the core reduces in size and diffraction effects force more light to propagate in the cladding until guidance is provided predominantly by the large index difference at the cladding-air interface . Further tapering would force an increasing fraction of light to propagate in air and increase the effective aperture size at the end of the taper, so a metal coating is applied to confine the light within the conical dielectric waveguide. However the metal also gives rise to large absorption and significant optical loss. Ultimately the light escapes the probe through a sub-wavelength aperture at the end of the tapered region but with fractional transmission ranging from 10-5 to 10-1 for aperture diameters of 50nm to 5000nm, respectively . Hence, minimizing the interaction of the propagating modal field with the metal coating is essential for enhancing the optical throughput of SNOM probes. When tapered, the structure of the Fractal fibre effectively provides a continuous isolating annular air gap between the light propagating in the inner region of the fibre and the metal-coated outer region of the fibre. Preliminary numerical modelling has confirmed that this strategy decreases the loss to the metal and has shown that is most effective when the gap is relatively close to the edge of the fibre.
To assist in determining the exit point of the propagating optical field, which can be directly related to loss, each probe was immersed in a fluorescent solution which had a refractive index greater than that of the silica cladding (1.458 ± 0.001). The solution can be used to precisely determine when the optical modes(s) of each probe are no longer internally supported within the core guiding region and ‘leak’ into the surrounding medium. Blue light, with a wavelength of emission at 488 nm, was equally coupled into each of the three different probes and the fluorescent solution was excited to emit green light when interacting with the 488nm source. Each sample was imaged with a 24 bit color CCD camera and the exit point for the propagating light in the three difference fibre samples was determined by examining the point at which the solution fluoresced. The goal of this characterization method was to locate the point at which light leakage occurs from each tip. The greater the distance between the exit point of the propagating field and the tip, the greater the interaction with the metal coating and hence, greater optical absorption.
The three different probe samples can be observed in Fig. 5, where (a) corresponds to a single-mode (conventional) probe, (b) shows a PCF probe and (c) depicts the Fractal fibre probe.
Figure 5(a) shows a substantial amount of optical leakage from the single-mode probe. The surrounding solution is fluorescing at the point where the light is not confined within the optical probe. This exit point for the light occurs at approximately 40 micrometers from the probe tip and it is clear that the pumped blue light does not propagate through to the end of the probe.
By observing this image, it is clear that a metal coating is required in order to enable propagation within the remaining length of the probe tip. Figure 5(b) shows that a greater fraction of the launched blue light propagates through to the PCF probe tip. This can be confirmed by the bright blue point at the tip of the probe. The transmission properties of the PCF probe are greatly enhanced compared to the conventional single-mode taper. However, there is some optical leakage of the propagating light within a few microns from the tip. This is most likely due to some diffraction effects as the hole-to-hole pitch within the tapered hexagonal air-silica lattice approaches the 1st order Bragg condition.
The Fractal probe, shown in Fig. 5(c), resulted in a substantial increase in the optical throughput compared to that of the PCF and conventional single-mode probes. This can clearly be observed by the fact that almost all of the optical power in the probe, blue light, propagates to the tip. The power at the tip was so intense that it almost saturated the CCD camera. As the Fractal fibre is reduced in diameter during the tapering process, the modal field remains confined within the core region as the inner ring of holes collapses and the modal field is then confined by successive rings of holes. The guided light essentially experiences the same effective index difference in each region and consequently the light guiding properties of the fibre are also essentially the same as the diameter of the fibre is reduced, which can be clearly noted in Fig. 5(c). The strong confinement through the Fractal Fibre probe, and to a lesser extent the PCF probe, can be clearly observed by the high numerical aperture of the probe tip. The high numerical aperture of the Fractal Fibre probe also indicates that the collection ability of this type of probe is greater than that of the PCF probe and substantially greater than that of the single-mode probe. There is however a small fraction of power lost within a few microns of the probe tip. This is a direct result of the Fractal fibre only possessing three rings of eight holes. Future versions of this fibre will incorporate a greater number of rings with a greater number of holes per ring. A more complex version of the Fractal fibre will yield even higher optical throughput which may remove the need to coat the probes with metal. This extremely important finding stems directly from the novel cross-section geometry of the Fractal fibre.
When tapered to form an optical probe, it has been shown that the Fractal fibre structure enables light to be confined almost all the way to the tip of the probe. The Fractal fibre represents a completely innovative step in the development of unique fibre structures which are designed to minimize loss. This fibre has applications far beyond those indicated here, including a reduction in the strict criteria for drawing that are normally imposed for standard fibres, leading to cheaper and faster manufacturing in the photonics industry. The Fractal Fibre has many special features still to be investigated, for example, extremely low bend loss compared to traditional fibres.
The fabrication of ultra high throughput optical probes directly allows scanning probe microscopy to further access the fields of Nanotechnology and Biotechnology. Low power probes that limit imaging speeds restrict the applicability of these SNOM techniques. Ultra high optical throughput Fractal probes have the real potential of making a significant contribution to the field. Fractal probes have the capability to enable the use of near field microscopy for the examination of biological processes in real time in addition to high speed nano-engineering procedures.
This project is proudly supported by the International Science Linkages programme established under the Australian Government’s innovation statement Backing Australia’s Ability. The authors would also like to acknowledge J. Digweed, J. Zagari and B. Ashton for their assistance with fibre preparation and useful discussions.
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