Integrated photonic crystals platform for biosensing

Abstract

Photonic crystals have been shown to be a promising technology for improving the performance of light emitting diodes, solar cells and optical communication components. More recently there has been interest in the application of photonic crystals for bio‐chemical sensing since they provide the potential benefits of high sensitivity, label free, real time detection with low limit of detection. Optical sensing mechanisms such as Surface Plasmon Resonance (SPR), and Evanescent Field (EF) sensing methods are currently popular. These are all sensitive to small changes in refractive index (RI) of part of the device. To date SPR methods provide the highest level of sensitivity but have the disadvantage of requiring an expensive gold coating.

AROMA Sensor:
As a high sensitivity, low cost alternative to conventional SPR methods, this thesis investigates a new concept for bio‐chemical sensing recently developed at Southampton, which uses vertical projection of leaky transmitted modes of a photonic crystal as the sensing method. We call this Angle Resolved Out‐coupled Mode Analysis (AROMA). This method is highly sensitive to small changes in refractive index at the sidewalls of the holes of a photonic crystal resulting in a strong angular shift of an out coupled beam of light. Changes in RI causes a shift in the projected spot position that can be recorded by a CCD/ CMOS camera.

Sensor performance is shown to far exceed normal SPR. Simulation and experimental results demonstrate a sensitivity of 10 degree/RIU from a non‐optimised sensor and simulation results indicate an improved sensitivity of 6500 degree/RIU by optimising the sensor operating point. Responsivity of the sensor was investigated by sequentially depositing a series of sub nm ZnO layers, and was found to be highly linear.

Photonic crystal coupler and system integration:
Apart from the sensor, a new concept for light coupling is developed and optimised. We extend photonic crystal technology to create a combined light coupler / splitter component allowing arbitrary N‐channel light coupling to a simple slab waveguide device. The coupler is combined with multiple sensors to make a fully functional multi‐channel (4‐12 channels) sensor operating at 785nm.    This is integrated into a high refractive index (n=1.7) Silicon Oxynitride (SiON) slab waveguide deposited onto a transparent borosilicate glass substrate. The aim for the slab waveguide was to mimic the refractive index of available polymer materials so that the entire system could eventually be fabricated on a flexible polymer substrate by nanoimprint lithography.

Design and modelling:
This thesis describes the design and optimisation of each component (sensor, coupler and slab waveguide), presenting in depth background physics and rigorous design methods for each component. 3D models were developed based on Rigorous Coupled Wave Analysis (RCWA) and Finite‐Difference Time‐Domain (FDTD) methods. RCWA models allowed accurate prediction and optimisation of light coupling and projection angles for any selected operating wavelength. FDTD methods allowed careful analysis of the interaction between the light field in the slab waveguide and materials placed in the holes. It also predicts the far‐field projected beam pattern for the sensor.
Applications:
Capability to detect (dry) monolayer coatings was proven for a simple self‐assembled monolayer molecule coating (p‐tolyltrichlorosilane (TTCS)) and also deoxyribonucleic acid (DNA) was successfully detected, close to physiological levels. To achieve this a complex hybridisation process was developed. Sensor response as a function of self‐assemble molecule (SAM) length and distance from the sidewalls was investigated in detail by using reversible chains of long chain charged molecules (lysine, poly‐lysine, bovine serum albumin protein). A detector surface with a layer of poly‐lysine‐g‐PEG was successfully replaced by a poly‐lysine molecule with larger molecule weight. Sequentially additional bovine serum albumin protein binding with the Polylysine was detected.
Capability to detect biomolecules in an aqueous environment is intrinsically difficult for most bio‐sensors. By fabricating the device on a transparent glass substrate, and designing the device to project light backwards through the substrate, it became possible to detect small changes in refractive index for liquids placed on the exposed top surface with no detriment to the readout method. The bulk sensitivity of the sensor for liquids was evaluated by measuring a sequence of glucose solutions with increasing concentrations. A highly linear response was again observed.

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Identifiers

ORCID for James Gates: orcid.org/0000-0001-8671-5987

ORCID for Tasmiat Rahman: orcid.org/0000-0002-6485-2128

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