Surmounting The Great Silica Integration Problem

The Team [From Left to Right]: 
Masood Naqshbandi, John Canning, Brant Gibson, Melissa Nash, Maxwell Crossley

Authors: John Canning¹, Brant Gibson²

Affiliation:
¹interdisciplinary Photonics Laboratory (iPL), School of Chemistry, The University of Sydney, Australia,
²School of Physics, The University of Melbourne, Australia

The global interconnectedness we all share today, primarily via the optical internet, comes from one material: silica. It makes up the global web that carries communication to most corners containing bipeds. Only silica, with a sprinkling of germanium and a few trace dopants, has offered both an extraordinary toughness, chemical inertness and a super-extraordinary transparency that enables this to happen, aided by silicate-based amplifiers lightly doped with erbium to kick these signals along. So it’s no wonder a Nobel Prize was awarded to acknowledge this impact, literally the backbone of the Glass Age [1]. And its existence continues to fire the imagination and yearning for something even greater – an intelligent web that can emulate a living organism with sensors along its paths providing data on the environment and a distributed intelligence analysing, sifting and even making decisions based on this data [2].

These sensors promise to span remote intelligent energy monitoring, machinery control, remote telemedicine and teaching and more. But in order to do this it has become clear that the very properties that enabled the global village – robustness, stability and reliability – are the same that limits the transformation of the optical web from a passive transport medium to an active, dynamic intelligent network. For this, silica must have added functionality at least in places along the grid. For the time being, all solutions explore externalisation onto alternative material platforms, the majority of which are simply incompatible making integration with the web one of the most prolific research areas across all disciplines.

This begs the question: why cannot silica be functionalised? The answer comes down to simple thermodynamics involved with the traditional chemical bond – silica processing involves extraordinarily high temperatures: 1900 °C plus before it can be melted and drawn into fibre. Very little material can survive these conditions – some of the rare earths are fortunately, enabling critical amplification to exist; but organic or carbon systems, which increasingly underpin sensors and new technologies such as diamond based photonics [3], are not.

At the interdisciplinary Photonics Laboratories at The University of Sydney, Australia, we believe we have come up with a solution: using nanoparticles held together by intermolecular forces [4]. Unlike chemical bonds, intermolecular forces are universal with most cases being attractive at the same temperature, namely 25 °C. To exploit and demonstrate the potential of this approach, we take a novel fabrication avenue that nearly everyone has some experience with, albeit often without realising it – evaporative self-assembly.

Figure 1: A batch of self-assembled wires approximately 10 μm by 5 cm long fabricated from 20 nm silica nanoparticles on a glass substrate. An aspect ratio > 50 000 is easily demonstrated.

Watching a coffee drop evaporate [5] can be like sticking needles into one’s eyes but if you watch carefully enough you’ll see a relatively wonderful example of physics in action – convective flow directing particles to the outer rim so that the brown spot becomes clear in the middle. When intermolecular forces are thrown in by replacing the coffee with silica nanoparticles, packing constraints take place and the steadily shrinking drop experiences very high radial stresses – cracks form, and after a couple of bifurcations, uniform cracking is obtained to produce silica wires (Figure 1). In further work, by controlling the evaporation conditions using laser processing, a very high degree of directionality is possible improving uniformity of wires and more [6]. Intermolecular forces are often seen as weak and at the molecular level this is often the case – but unlike the chemical bond they are additive so the more of it there is the stronger a material.

Since these forces are universal and operate at room temperature – this means we can mix in almost anything and have done so using organic dyes to dope the wires during their fabrication. Importantly, in collaboration with the School of Physics at the University of Melbourne, mixed nano-particle self-assembly has allowed a wire to be fabricated containing nanodiamonds. Some of these nanodiamonds themselves have nitrogen vacancy defect sites which emit single photons (Figure 2). With little blinking observed and good thermal stability, diamond is one of the most ideally suited material systems for single photon generation. We have now been able to integrate this into silica itself, clearly demonstrating the potential of our approach for enabling the tools for quantum techniques into the global web.

Figure 2. (a) A scanning confocal map of the photoluminescence from nitrogen-vacancy (NV) defect centres within the nanodiamond-embedded silica microwire sample (image is taken from the top surface of the microwire; scale bar corresponds to 10 μm). (b) Single photon emission detected from the particle shown in the zoomed region of (a) where the scale bar corresponds to 2 μm. The inset shows the photostable emission from the single emitter.

The silica nanoparticle platform, held together by intermolecular forces, allows total integration of new materials into existing silica communications and sensor networks. This hybrid material has the potential to open up a vast field for compositional control of other organic, inorganic and biological molecules and species within silica waveguides (or any other nanoparticle platform) for applications in opto-electronics (for example, graphite), photovoltaics (for example, customised porphyrins and metals), plasmonics and metamaterials (for example, metals) and novel optical circuitry (for example, magnetic materials).

References:
[1] 2009 Nobel Prize in Physics, Charles Kao. Link to Nobel Prize 2009.
[2] J. Canning, “Optical sensing: the last frontier for enabling intelligence in our wired up world and beyond”, Photonic Sensors, SpringerOpen, 2 (3), 193-202, (2012). Abstract.
[3] Mark P. Hiscocks, Kumaravelu Ganesan, Brant C. Gibson, Shane T. Huntington, François Ladouceur, and Steven Prawer, “Diamond waveguides fabricated by reactive ion etching”, Optics Express, 16, 19512-19519 (2008). Abstract.
[4] Masood Naqshbandi, John Canning, Brant C. Gibson, Melissa M. Nash & Maxwell J. Crossley, “Room temperature self-assembly of mixed nanoparticles into photonic structures”, Nature Communications, 3, 1188 (2012). Abstract.
[5] Robert D. Deegan, Olgica Bakajin, Todd F. Dupont, Greb Huber, Sidney R. Nagel and Thomas A. Witten, “Capillary flow as the cause of ring stain from dried liquid drops”, Nature 389, 827–829 (1997). Abstract.
[6] J. Canning, H. Weil, M. Naqshbandi, K. Cook, and M. Lancry, “Laser tailoring surface interactions, contact angles, drop topologies and the self-assembly of optical microwires”, To appear in Opt. Mat. Express (2013).