John Justice

Emerging optofluidic technologies for point-of-care genetic analysis systems: a review

This review describes recently emerging optical and microfluidic technologies suitable for point-of-care genetic analysis systems. Such systems must rapidly detect hundreds of mutations from biological samples with low DNA concentration. We review optical technologies delivering multiplex sensitivity and compatible with lab-on-chip integration for both tagged and non-tagged optical detection, identifying significant source and detector technology emerging from telecommunications technology. We highlight the potential for improved hybridization efficiency through careful microfluidic design and outline some novel enhancement approaches using target molecule confinement. Optimization of fluidic parameters such as flow rate, channel height and time facilitates enhanced hybridization efficiency and consequently detection performance as compared with conventional assay formats (e.g. microwell plates). We highlight lab-on-chip implementations with integrated microfluidic control for “sample-to-answer” systems where molecular biology protocols to realize detection of target DNA sequences from whole blood are required. We also review relevant technology approaches to optofluidic integration, and highlight the issue of biomolecule compatibility. Key areas in the development of an integrated optofluidic system for DNA hybridization are optical/fluidic integration and the impact on biomolecules immobilized within the system. A wide range of technology platforms have been advanced for detection, quantification and other forms of characterization of a range of biomolecules (e.g. RNA, DNA, protein and whole cell). Owing to the very different requirements for sample preparation, manipulation and detection of the different types of biomolecules, this review is focused primarily on DNA–DNA interactions in the context of point-of-care analysis systems.

Low-threshold lasing in novel microdisk geometries

We have fabricated arrays of semiconductor microdisks with diameters from 3.5-9 /spl mu/m using wet chemical etching in combination with epitaxial lift-off and metallic bonding techniques. The microdisks are supported on glass for higher optical confinement and to allow novel annular geometries. The optically pumped microdisks have high-Q (>2000) modes, low-pump thresholds (<20 /spl mu/W) and multimode lasing at liquid nitrogen temperatures. Successive modes are excited by temperature tuning the bandgap.

High aspect ratio nano-fabrication of photonic crystal structures on glass wafers using chrome as hard mask

Wafer-scale nano-fabrication of silicon nitride (Si x N y ) photonic crystal (PhC) structures on glass (quartz) substrates is demonstrated using a thin (30 nm) chromium (Cr) layer as the hard mask for transferring the electron beam lithography (EBL) defined resist patterns. The use of the thin Cr layer not only solves the charging effect during the EBL on the insulating substrate, but also facilitates high aspect ratio PhCs by acting as a hard mask while deep etching into the Si x N y . A very high aspect ratio of 10:1 on a 60 nm wide grating structure has been achieved while preserving the quality of the flat top of the narrow lines. The presented nano-fabrication method provides PhC structures necessary for a high quality optical response. Finally, we fabricated a refractive index based PhC sensor which shows a sensitivity of 185 nm per RIU.

Integrated plasmonic circuitry on a vertical-cavity surface-emitting semiconductor laser platform

Integrated plasmonic sources and detectors are imperative in the practical development of plasmonic circuitry for bio- and chemical sensing, nanoscale optical information processing, as well as transducers for high-density optical data storage. Here we show that vertical-cavity surface-emitting lasers (VCSELs) can be employed as an on-chip, electrically pumped source or detector of plasmonic signals, when operated in forward or reverse bias, respectively. To this end, we experimentally demonstrate surface plasmon polariton excitation, waveguiding, frequency conversion and detection on a VCSEL-based plasmonic platform. The coupling efficiency of the VCSEL emission to waveguided surface plasmon polariton modes has been optimized using asymmetric plasmonic nanostructures. The plasmonic VCSEL platform validated here is a viable solution for practical realizations of plasmonic functionalities for various applications, such as those requiring sub-wavelength field confinement, refractive index sensitivity or optical near-field transduction with electrically driven sources, thus enabling the realization of on-chip optical communication and lab-on-a-chip devices.

Lithographically Defined, Room Temperature Low Threshold Subwavelength Red-Emitting Hybrid Plasmonic Lasers.

Hybrid plasmonic lasers provide deep subwavelength optical confinement, strongly enhanced light-matter interaction and together with nanoscale footprint promise new applications in optical communication, biosensing, and photolithography. The subwavelength hybrid plasmonic lasers reported so far often use bottom-up grown nanowires, nanorods, and nanosquares, making it difficult to integrate these devices into industry-relevant high density plasmonic circuits. Here, we report the first experimental demonstration of AlGaInP based, red-emitting hybrid plasmonic lasers at room temperature using lithography based fabrication processes. Resonant cavities with deep subwavelength 2D and 3D mode confinement of λ2/56 and λ3/199, respectively, are demonstrated. A range of cavity geometries (waveguides, rings, squares, and disks) show very low lasing thresholds of 0.6-1.8 mJ/cm2 with wide gain bandwidth (610 nm-685 nm), which are attributed to the heterogeneous geometry of the gain material, the optimized etching technique, and the strong overlap of the gain material with the plasmonic modes. Most importantly, we establish the connection between mode confinements and enhanced absorption and stimulated emission, which plays critical roles in maintaining low lasing thresholds at extremely small hybrid plasmonic cavities. Our results pave the way for the further integration of dense arrays of hybrid plasmonic lasers with optical and electronic technology platforms.

Ageing studies on red-emitting VCSELs for polymer optical fibre applications

The paper reports on the development of VCSELs emitting near to 650 nm for low cost links employing PMMA based polymer optical fibre (POF). AlGaInP/AlGaAs VCSELs are fabricated on wafers from two different MOCVD sources. Accelerated ageing tests are done for the two types of devices.

Vertical-Cavity Surface-Emitting Lasers With Integrated Excitation of Surface Plasmon Polariton Modes

We report the excitation of surface plasmon polaritons (SPPs) on a thin Au layer integrated on top of the mirror of a vertical-cavity surface-emitting laser (VCSEL) along with their subsequent extraction to air. Gratings etched into the Au layer are used to couple the light in to and out of the metal film. The polarization properties of the optical emission from out-coupling gratings placed 5 mu m away from the central optical aperture are used to confirm the coupling of the VCSEL light into and out of the SPP modes. The result paves the way to compact integrated plasmonic devices.

Monolithic integration of wavelength-scale diffractive structures on red vertical-cavity lasers by focused ion beam etching

We report the fabrication and characterization of wavelength-scale diffractive optical elements etched directly on the surface of red (660 nm) vertical-cavity surface-emitting lasers. The structures were fabricated by focused ion beam etching. Linear and two-dimensional (2-D) grating configurations were investigated. Each showed excellent suppression of the zeroth-order diffracted beam. Compared to the power from an unetched laser, /spl sim/22% of the emission was coupled into the first order for linear gratings and 12% for the 2-D structures. Polarization was independent of grating orientation for grating pitches as small as 1/spl lambda/. Threshold current increases of 35%-40% were measured.

Development of photonic crystal structures for on-board optical communication

We present designs for sharp bends in polymer waveguides using colloidal photonic crystal (PhC) structures. Both silica (SiO2) sphere based colloidal PhC and core-shell colloidal PhC structures having a titania (TiO2) core inside silica (SiO2) shells are simulated. The simulation results show that core-shell Face Centered Cubic (FCC) colloidal crystals have a sufficient refractive index contrast to open up a bandgap in the desired direction when integrated into polymer waveguides and can achieve reflection <70% for the appropriate plane. Different crystal planes of the FCC structure are investigated for their reflection and compared with the calculated bandstructure. Different techniques for fabrication of PhC on rectangular seed layers namely slow sedimentation; spin coating and modified doctor blading are discussed and investigated. FCC and Random FCC silica structures are characterized optically to show realisation of (001) FCC.

Dual resonance approach to decoupling surface and bulk attributes in photonic crystal biosensor.

A sub-wavelength grating-based photonic crystal sensor is designed to excite two spectrally and spatially different guided mode resonances that have distinctive electric field distributions. We present and validate the uni-polarized dual resonance approach to separating bulk index perturbations from surface-binding events in a single measurement by monitoring the resonance wavelength shifts. This self-referencing method will reduce errors in the measurement of biomolecule binding events on sensor surfaces in a perturbed environmental background.