Masood Naqshbandi

Room temperature self-assembly of mixed nanoparticles into photonic structures

Manufacturing complex composites and structures using incompatible materials is central to next-generation technologies. In photonics, silica offers passivity, low loss and robustness, making it the ideal material platform for optical transport. However, these properties partly stem from the high-temperature processing conditions necessary for silica waveguide fabrication, restricting the functionalisation of waveguides to robust inorganic dopants. This means for many sensor and active device applications, large numbers of materials are excluded. These include many organic and carbon systems such as dyes and diamond. Here we propose using intermolecular forces to bind nanoparticles together at room temperature and demonstrate the room-temperature self-assembly of long microwires (length ~7 cm, width ~10 μm) with and without rhodamine B. Further we report on mixed self-assembly of silica and single-photon-emitting nitrogen-vacancy-containing diamond nanoparticles, opening up a new direction in material science.

Measurement of Fluorescence in a Rhodamine-123 Doped Self-Assembled “Giant” Mesostructured Silica Sphere Using a Smartphone as Optical Hardware

The blue OLED emission from a mobile phone was characterised, revealing a sharp emission band centred at λ = 445 nm with a 3dB bandwidth Δλ ∼ 20 nm. It was used to excite Rhodamine 123 doped within a “giant” mesostructured silica sphere during fabrication through evaporative self-assembly of silica nanoparticles. Fluorescence was able to be detected using a standard optical microscope fitted with a green transmission pass filter and cooled CCD and with 1 ms exposure time demonstrating the potential of mobile platforms as the basis for portable diagnostics in the field.

Laser tailoring surface interactions, contact angles, drop topologies and the self-assembly of optical microwires

UV laser irradiation (λ = 193 nm), below and above damage thresholds, is used to both alter and pattern the surface properties of borosilicate slides to tune and control the contact angle of a water drop over the surface. Large variation exceeding 25° using laser processing alone, spanning across both sides of the original contact angle of the surface, is reported. An asymmetric contact angle distribution, giving rise to an analogous ellipsoidal-like drop caplet, is shown to improve convective self-assembly of silica nanoparticles into straighter optical microwires.

Manipulating and controlling the evanescent field within optical waveguides using high index nanolayers

We propose and demonstrate, through simulation and experiment, how the interaction of an optical field within a waveguide designed for chemical sensing and, more generally, evanescent field spectroscopy can be enhanced substantially by strategic deposition of high index surface layers. These layers draw out the optical field in the vicinity of probing and take advantage of field localisation through optical impedance matching. Localisation of the evanescent field to the inner layer in turn is accompanied by whispering gallery modes within the channels of a structured cylindrical waveguide, further enhancing sensitivity. A novel demonstration based on self-assembled layers made up of TiO2 within a structured optical fibre is demonstrated, using a simple porphyrin as the spectroscopic probe. This technique offers optimisation of the limitations imposed on practical waveguide sensors that are highly sensitive but nearly always at the expense of low loss. The principles have potential ramifications for nanophotonics more generally and these are discussed.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.

Measurement of Rhodamine B absorption in self-assembled silica microwires using a Tablet as the optical source

A simple demonstration utilising the optical light source of an Android tablet to obtain the absorption spectra of Rhodamine dye stained self-assembled silica microwires is demonstrated. The spectrum is collected using a portable Spectrometer. This highlights the potential of tablet technology as portable optical hardware in its own right and we discuss how to potentially achieve complete integration of spectrometer onto the tablet.

Controlled fabrication of macroscopic mesostructured silica spheres for potential diagnostics and sensing applications

We report on fabricating macro-scale mesostructured silica spheres on superhydrophobic surfaces from silica nanoparticles via evaporative self-assembly. The problem of cracking and shattering of the dried macro-spheres has been addressed by using dopants such as graphite, porphyrins and rhodamine-B as well as increasing the evaporation time by using anti-freeze. Limits in size reduction associated with hand-held deposition are overcome using ink-jet deposition — spheres < 100 μm are routinely produced. Silica spheres doped with porphyrin and fluorescent dyes demonstrate an ability of functionalising these spheres to improve biocompatibility and allow fluorescent diagnostics.

Porphyrin-doped solgel-lined structured optical fibers for local and remote sensing

We constructed a type of sensor by depositing a solgel layer within the interior holes of a silica-structured fiber and, subsequently, coating this with an acid-responsive porphyrin. Protonation of the porphyrin by an acidic gas (HCl in this case), is detected by a large change in the visible spectrum. Compared to previous work on a liquid-core sensor in a structured optical fiber, the signal-to-noise ratio of this gas sensor shows a reduced signal strength, but the detection rate is increased about fortyfold.

Remote gaseous acid sensing within a porphyrin-doped TiO2 sol-gel layer inside a structured optical fibre

A porphyrin containing sol-gel layer has been deposited within the interior of the channels of a silica structured optical fibre. Gaseous HCl detection based on protonation of the porphyrin and observed as a change in the spectrum is demonstrated. This system is compared to previous work based on an acid sensor within a liquid-core fibre. The signal-to- noise of this type of fibre system shows a higher level of sensitivity than the liquid-core and has a forty-fold acid diffusion rate increase due to the different medium for acidification.

Magnetic induction-induced resistive heating of optical fibers and gratings

Magnetic induction heating of optical fibers packaged with a steel plate is studied using a fiber Bragg grating. The dependence on the induced wavelength shift with magnetic field is obtained for a commercially available induction heater. More than a 300°C temperature rise is observed within seconds. The potential of magnetic induction as an efficient and rapid means of modulating devices and as a novel approach to potential optical based magnetic field and current sensing is proposed and discussed. The extension of the ideas into micro and nanophotonics is described.

Acoustically modulated long period grating sensor for simultaneous viscosity and density measurement

Acoustic waves, generated by exploiting the acousto-optic effect within silica optical fibres, were used to enhance the sensitivity of a long period grating. Most of the physical parameters measured by using an LPG involve simple linear and passive transformations of spectral shifts. However, by adding a temporal element using acoustic waves, parameters that require dynamic assessment become accessible. In this work we demonstrate the measurement of fluid viscosity by measuring the rise time and acousto-optic efficiency. We show results of sensor characterization and suggest the possibility of monitoring a chemical reaction in real time.