Slotted photonic crystal biosensors

Optical biosensors are increasingly being considered for lab-on-a-chip applications due to their benefits such as small size, biocompatibility, passive behaviour and lack of the need for fluorescent labels. The light guiding mechanisms used by many of them result in poor overlap of the optical field with the target molecules, reducing the maximum sensitivity achievable. This thesis presents a new platform for optical biosensors, namely slotted photonic crystals, which engender higher sensitivities due to their ability to confine, spatially and temporally, the peak of optical mode within the analyte itself. Loss measurements showed values comparable to standard photonic crystals, confirming their ability to be used in real devices. A novel resonant coupler was designed, simulated, and experimentally tested, and was found to perform better than other solutions within the literature. Combining with cavities, microfluidics and biological functionalization allowed proof-of-principle demonstrations of protein binding to be carried out. High sensitivities were observed in smaller structures than most competing devices in the literature. Initial tests with cellular material for real applications was also performed, and shown to be of promise. In addition, groundwork to make an integrated device that includes the spectrometer function was also carried out showing that slotted photonic crystals themselves can be used for on-chip wavelength specific filtering and spectroscopy, whilst gas-free microvalves for automation were also developed. This body of work presents slotted photonic crystals as a realistic platform for complete on-chip biosensing; addressing key design, performance and application issues, whilst also opening up exciting new ideas for future study

Ultra-high Q/V hybrid cavity for strong light-matter interaction

The ability to confine light at the nanoscale continues to excite the research community, with the ratio between quality factor Q and volume V, i.e., the Q/V ratio, being the key figure of merit. In order to achieve strong light-matter interaction, however, it is important to confine a lot of energy in the resonant cavity mode. Here, we demonstrate a novel cavity design that combines a photonic crystal nanobeam cavity with a plasmonic bowtie antenna. The nanobeam cavity is optimised for a good match with the antenna and provides a Q of 1700 and a transmission of 90%. Combined with the bowtie, the hybrid photonic-plasmonic cavity achieves a Q of 800 and a transmission of 20%, both of which remarkable achievements for a hybrid cavity. The ultra-high Q/V of the hybrid cavity is of order of 106 (λ/n)−3, which is comparable to the state-of-the-art of photonic resonant cavities. Based on the high Q/V and the high transmission, we demonstrate the strong efficiency of the hybrid cavity as a nanotweezer for optical trapping. We show that a stable trapping condition can be achieved for a single 200 nm Au bead for a duration of several minutes (ttrap > 5 min) and with very low optical power (Pin = 190 μW)

Slotted photonic crystal cavities with integrated microfluidics for biosensing applications

We demonstrate the detection of dissolved avidin concentrations as low as 15 nM or 1 mu g/ml using functionalized slotted photonic crystal cavities with integrated microfluidics. With a cavity sensing surface area of approximately 2.2 mu m(2), we are able to detect surface mass densities of order 60 pg/mm(2) corresponding to a bound mass of approximately 100 ag. The ultra-compact size of the sensors makes them attractive for lab-on-a-chip applications where high densities of independent sensing elements are desired within a small area. The high sensitivity over an extremely small area is due to the strong modal overlap with the analyte enabled by the slotted waveguide cavity geometry that we employ. This strong overlap results in larger shifts in the cavity peak wavelength when compared to competing approaches. (C) 2011 Elsevier B.V. All rights reserved.</p

Valve controlled fluorescence detection system for remote sensing applications

We demonstrate a microfluidics-based fluorescence detection device where the filters, source, detector, and electronically controlled valves are embedded into a Polydimethylsiloxane (PDMS)-based microfluidic chip. The device reported here has been specifically designed for chlorophyll a fluorescence sensing in autonomous systems, such as oceanic applications. In contrast to a monolithic approach, the modular approach made the fabrication of this device simpler and cheaper. For fluorescence detection, an InGaN/GaN LED is used as the excitation source to specifically excite chlorophyll a; a metal-dielectric Fabry-Perot filter was used to extinguish out-of-band excitation. A simple Si photodiode is used as detector and provided with a thermally evaporated CdS emission filter to block the excitation source. This filter combination provides an excellent solution to the difficult problem of combining high-rejection excitation and emission filters in an integrated thin-film format. Furthermore, the metal-dielectric filter provides a much broader angular response than a comparable multilayer Bragg mirror, which is a key advantage in the integrated format. We use a novel paraffin wax-based valve design affords low power single-use actuation, between 0.5 and 1 J per actuation and withstands 0.6 bar differential pressure, which provides better performance than its previously reported counterparts. The remote valve-controlled operation of the fluorescence detection system is demonstrated, illustrating the measurement of a chlorophyll a solution, with a detection limit of 340 mu M and subsequent valve-controlled flushing of the measurement reservoir.</p