26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Inverse Terahertz Reflection Analysis of Hydration-Dependent Permittivity and Thickness in Soft Contact Lenses

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral ANZOS | Photonics and Optics (ANZCOP)

Speaker

Daniel Tobar (The University of Western Australia)

Description

The performance of soft contact lenses is strongly linked to their hydration dynamics; however, non-contact measurement of hydration in thin hydrated materials remains challenging. Terahertz time-domain spectroscopy (THz-TDS) is highly sensitive to the dielectric relaxation of water, making it a promising tool for probing hydration-dependent changes in soft polymeric materials. However, quantitative interpretation of reflected THz signals from thin hydrated layers is complicated by overlapping internal reflections, frequency-dependent complex permittivity, and unknown thickness.

We present a modular inverse THz reflection framework that simultaneously retrieves the thickness and frequency-dependent permittivity of a single dielectric layer, with specific application to commercial soft contact lenses. The model treats the lens as an absorbing dielectric layer in reflection geometry and simulates the reflected THz waveform using a forward model based on a chosen permittivity description. This simulated waveform is then compared with THz reflection measurements acquired from soft contact lenses during drying. An inverse optimisation procedure is used to recover the thickness and permittivity response that best reproduces the measured THz waveforms. The framework is modular, allowing different dielectric models to be incorporated, including Debye-type relaxations and effective-medium approximations for hydrated polymer systems.

The method is applied to soft contact lenses under controlled hydration conditions, where changes in the retrieved permittivity are used to track water-related dielectric behaviour during drying. Retrieved permittivity changes showed a systematic decrease in water-related dielectric response as the lenses dried. By separating geometric thickness effects from changes in the material permittivity response, the approach provides a pathway for quantifying hydration dynamics in a manner that is difficult to achieve using direct spectral-ratio analysis alone. These results demonstrate the potential of inverse THz reflection modelling as a non-contact tool for characterising thin hydrated materials, with future applications in commercial contact lens evaluation, ocular surface assessment, and other hydration-sensitive polymer systems.