Electrically tuneable optofluidic metasurface for the dynamic control of light

The dynamic control of light at the nanoscale remains a longstanding challenge in photonics. Metasurfaces, composed of two-dimensional arrays of subwavelength scatterers (“meta-atoms”), provide a new paradigm for addressing this challenge, enabling exceptional wavefront control. Making meta-atoms active enables devices whose properties can be manipulated post-fabrication, unlocking a much broader range of applications. This thesis presents a novel tuneable metasurface architecture, the Optofluidic tunable metasurface (o-TM), to dynamically control the properties of light. A reflection phase shift of 1.75π is achieved across an ultra-low voltage window (±3 V), and the device is applied to a biosensing application, where the detection of a planktonic bacteria is demonstrated.

The active material of choice is indium tin oxide (ITO), a transparent conducting oxide. The first part of this work provides a comprehensive characterisation of ITO’s photonic properties, employing the Drude-Lorentz model to describe the effect of charge carriers on the complex refractive index of the material. Nanostructures are fabricated directly into the ITO thin films, demonstrating their potential for both passive and active photonic applications. Theoretical and computational models supplement all experimental findings.

Building on the understanding of ITO as a photonic material, several tuneable metasurface prototypes are developed and analysed through simulations and experiments. Tuneability is achieved by combining an electrolyte component with the ITO layer, forming an electrolyte-gated transistor that induces a space-charge layer at the interface. This approach enables significant index tuning, similar to that observed at the epsilon-near-zero (ENZ) regime, but with significantly reduced optical losses. Moreover, the incorporation of a liquid component introduces an additional degree of freedom, paving the way for novel human-technology interfaces or novel biosensing geometries.

The spectral, phase, and switching-speed performance of the o-TM are demonstrated, alongside the development of an all-pass filter variant, which enables pure phase tuning with very little amplitude variation. The effectiveness of the device is evidenced through a biosensing experiment, where a strain of E. coli is successfully detected. By sweeping the applied voltage, detection is achieved without the need for a spectrometer, offering a cost-effective, compact, and robust alternative sensing technique.

The o-TM has far-reaching applications in optical computing, biosensing, and display technology, to name a few. This work establishes ITO as a practical tuneable material, achieving a unity-order refractive index shift in the visible-NIR range, crucially without any non-linear ENZ effects. I hope that this research will inspire the future integration of ITO and optofluidics, laying the foundation for high-performance tuneable metasurfaces with broad cross-disciplinary applications.