Graham J. Sharp

Design and process development of a photonic crystal polymer biosensor for point-of-care diagnostics

In this work, we report advances in the fabrication and anticipated performance of a polymer biosensor photonic chip developed in the European Union project P3SENS (FP7-ICT4-248304). Due to the low cost requirements of point-ofcare applications, the photonic chip is fabricated from nanocomposite polymeric materials, using highly scalable nanoimprint- lithography (NIL). A suitable microfluidic structure transporting the analyte solutions to the sensor area is also fabricated in polymer and adequately bonded to the photonic chip. We first discuss the design and the simulated performance of a high-Q resonant cavity photonic crystal sensor made of a high refractive index polyimide core waveguide on a low index polymer cladding. We then report the advances in doped and undoped polymer thin film processing and characterization for fabricating the photonic sensor chip. Finally the development of the microfluidic chip is presented in details, including the characterisation of the fluidic behaviour, the technological and material aspects of the 3D polymer structuring and the stable adhesion strategies for bonding the fluidic and the photonic chips, with regards to the constraints imposed by the bioreceptors supposedly already present on the sensors.

Asymmetric split H-shape nanoantennas for molecular sensing

In this paper we report on a very sensitive biosensor based on gold asymmetric nanoantennas that are capable of enhancing the molecular resonances of C-H bonds. The nanoantennas are arranged as arrays of asymmetric-split H-shape (ASH) structures, tuned to produce plasmonic resonances with reflectance double peaks within the mid-infrared vibrational resonances of C-H bonds for the assay of deposited films of the molecule 17β-estradiol (E2), used as an analyte. Measurements and numerical simulations of the reflectance spectra have enabled an estimated enhancement factor on the order of 105 to be obtained for a thin film of E2 on the ASH array. A high sensitivity value of 2335 nm/RIU was achieved, together with a figure of merit of approximately 8. Our experimental results were corroborated using numerical simulations for the C-H stretch vibrational resonances from the analyte, superimposed on the plasmonic resonances of the ASH nanoantennas.

Mapping the sensitivity of split ring resonators using a localized analyte

Split ring resonator (SRR) based metamaterials have frequently been demonstrated for use as optical sensors of organic materials. This is made possible by matching the wavelength of the SRR plasmonic resonance with a molecular resonance of a specific analyte, which is usually placed on top of the metal structure. However, systematic studies of SRRs that identify the regions that exhibit a high electric field strength are commonly performed using simulations. In this paper we demonstrate that areas of high electric field strength, termed “hot-spots,” can be found by localizing a small quantity of organic analyte at various positions on or near the structure. Furthermore, the sensitivity of the SRR to the localized analyte can be quantified to determine, experimentally, suitable regions for optical sensing.

NIL fabrication of a polymer-based photonic sensor device in P3SENS project

We present the most recent results of EU funded project P3SENS (FP7-ICT-2009.3.8) aimed at the development of a low-cost and medium sensitivity polymer based photonic biosensor for point of care applications in proteomics. The fabrication of the polymer photonic chip (biosensor) using thermal nanoimprint lithography (NIL) is described. This technique offers the potential for very large production at reduced cost. However several technical challenges arise due to the properties of the used materials. We believe that, once the NIL technique has been optimised to the specific materials, it could be even transferred to a kind of roll-to-roll production for manufacturing a very large number of photonic devices at reduced cost.

Negative index fishnet with nanopillars formed by direct nano-imprint lithography

In this paper we demonstrate the ability to fabricate fishnets by nanoimprinting directly into a pre-deposited three layer metal–dielectric–metal stack, enabling us to pattern large areas in two minutes. We have designed and fabricated two different fishnet structures of varying dimensions using this method and measured their resonant wavelengths in the near-infrared at 1.45 μm and 1.88 μm. An important by-product of directly imprinting into the metal–dielectric stack, without separation from the substrate, is the formation of rectangular nanopillars that sit within the rectangular apertures between the fishnet slabs. Simulations complement our measurements and suggest a negative refractive index real part with a magnitude of 1.6. Further simulations suggest that if the fishnet were to be detached from the supporting substrate a refractive index real part of 5 and FOM of 2.74 could be obtained.

Metamaterial fishnet structure formed from nanoimprint lithography

We report on the fabrication and characterisation of fishnet structures of various dimensions on a polymer layer. The fabrication process causes metal-dielectric-metal rectangular pillars to be compressed to the bottom of fishnet structures. The metamaterial structures are fabricated using nanoimprint lithography, allowing large areas to be patterned quickly and good reproducibility through multiple use of the nanoimprint stamp. A tri-layer comprising of silver (Ag) and magnesium fluoride (MgF2) was deposited on a thick polymer layer, in this instance PMMA, before being directly imprinted by a stamp. When the metal-dielectric layered pillars are imprinted to a sufficient depth in the PMMA below the fishnet, distinct resonance peaks can be measured at both visible and near-infrared frequencies. The precise wavelength of the resonant peak at near-infrared and its Q-factor can be changed by altering the physical dimensions and number of metal and dielectric layers of the fishnet respectively. The response viewed at visible frequencies is due to the pillars that sit in the PMMA, below the fishnet. Silver and magnesium fluoride layers that comprise the suppressed pillars are crushed during the imprinting process but still allow for light to be transmitted. Despite imprinting directly into multiple metal and dielectric layers, high quality structures are observed with a minimum feature size as low as 200 nm. Resonance peaks are measured experimentally in reflectance using an FTIR spectrometer with a calcium fluoride (CaF2) beam-splitter and a visible wavelength range spectrometer with a silicon (Si) detector.

Fabrication of 70nm split ring resonators by nanoimprint lithography

We report on the fabrication of 70 nm wide, high resolution rectangular U-shaped split ring resonators (SRRs) using nanoimprint lithography (NIL). The fabrication method for the nanoimprint stamp does not require dry etching. The stamp is used to pattern SRRs in a thin PMMA layer followed by metal deposition and lift-off. Nanoimprinting in this way allows high resolution patterns with a minimum feature size of 20 nm. This fabrication technique yields a much higher throughput than conventional e-beam lithography and each stamp can be used numerous times to imprint patterns. Reflectance measurements of fabricated aluminium SRRs on silicon substrates show a so-called an LC resonance peak in the visible spectrum under transverse electric polarisation. Fabricating the SRRs by NIL rather than electron beam lithography allows them to be scaled to smaller dimensions without any significant loss in resolution, partly because pattern expansion caused by backscattered electrons and the proximity effect are not present with NIL. This in turn helps to shift the magnetic response to short wavelengths while still retaining a distinct LC peak.

Array metasurfaces for biomedical sensing at infra-red wavelengths

Detection and identification of biomedically significant molecules is an important application in infra-red (IR) spectroscopy. This presentation will consider some of the significant features of the different alternative building-block elements that can be used in array metasurfaces for enhanced detection sensitivity. The presentation will also address techniques and issues associated with the deposition and localisation of biological and organic chemical molecular material for detection and measurement using IR spectroscopy.

Trimming of silicon-on-insuiator micro-ring resonators by laser irradiation

Silicon on Insulator (SOI) is a well-established photonic platform that is of interest to photonic integrated circuits (PICs) due to its compatibility with existing fabrication processes used in the manufacture of electronic integrated circuits. Although SOI offers a high refractive index contrast that enables strong optical confinement of the propagating optical mode in the silicon layer, the performance of the photonic circuit components (such as, for example, ring resonators) is significantly dependant on fabrication tolerance. This can result in ring resonances that substantially deviate from the designed ones. It is therefore advantageous to be able to trim or tune such components post-processing, so as to adjust a resonance to the desired wavelength.

Progress in metamaterial fishnet formed by nanoimprint lithography and asymmetric split ring resonators (A-SRRs) for sensing

Metamaterial structures fabricated using nanoimprint lithography, allow large areas to be patterned quickly through multiple use of a nanoimprint stamp. The fabrication process produces fishnet structures of various dimensions on a polymer layer with metal-dielectric-metal compressed rectangular pillars. Simulations show negative refractive index and good figures of merit. However both of these figures could be increased by removal from the underlying polymer structure. We also report on the use of asymmetric spilt ring resonators used to detect small quantities of hormone estradiol by tuning the plasmonic resonances near the molecular resonance of the C-H stretch at nominally 3.31 microns.