# Double-Sided Illumination Grating-Coupled Surface Plasmon Resonance Sensors Using Direct Optical Discs

## TL;DR

Researchers used commercial optical discs as low-cost materials for surface plasmon resonance sensors, finding that front-side illumination and silver coatings improve performance.

## Contribution

The study introduces optical discs as affordable substrates for GC-SPR sensors and compares the effects of illumination and metal choice on sensor performance.

## Key findings

- CD-R discs showed the highest refractive index sensitivity among the tested optical discs.
- Front-side illumination produced stronger and more reproducible SPR excitation than back-side illumination.
- Silver-coated gratings provided sharper resonance responses compared to copper-coated ones.

## Abstract

What are the main findings?
Compared BD-R, DVD-R, and CD-R as low-cost grating substrates for GC-SPR.Identified SP dispersion modes of each optical disc consistent with SPR resonances.Front versus back illumination changes resonance clarity and spectrum shape.Demonstrated Ag and Cu as plasmonic materials for low-cost GC-SPR platforms.RI sensitivity depends on period: CD-R highest sensitivity.

Compared BD-R, DVD-R, and CD-R as low-cost grating substrates for GC-SPR.

Identified SP dispersion modes of each optical disc consistent with SPR resonances.

Front versus back illumination changes resonance clarity and spectrum shape.

Demonstrated Ag and Cu as plasmonic materials for low-cost GC-SPR platforms.

RI sensitivity depends on period: CD-R highest sensitivity.

What are the implications of the main findings?
Provide guidelines for suitable grating structures and metal choice in GC-SPR.Recommend front-side illumination for robust and reproducible GC-SPR sensing.Highlight back-side limits: substrate loss/interference reduces resonance clarity.Demonstrate RCWA as a predictive tool for GC-SPR sensor design.

Provide guidelines for suitable grating structures and metal choice in GC-SPR.

Recommend front-side illumination for robust and reproducible GC-SPR sensing.

Highlight back-side limits: substrate loss/interference reduces resonance clarity.

Demonstrate RCWA as a predictive tool for GC-SPR sensor design.

Commercial optical discs are used as low-cost grating substrates for fabricating grating-coupled surface plasmon resonance (GC-SPR) sensors, and the effects of front-side and back-side illumination are systematically compared. Three different discs were used as grating substrates with grating periods (Λ) of 322 ± 5.2 nm for BD-R, 805 ± 7.5 nm for DVD-R, and 1.582 ± 0.013 µm for CD-R. Silver (Ag) and copper (Cu) films were deposited by magnetron sputtering to form plasmonic gratings. The shallow grating height of BD-R supported continuous metal coverage, while the deeper DVD-R and CD-R grooves resulted in a less continuous layer. Plasmonic responses were measured using wavelength-modulated SPR spectroscopy and predicted with rigorous coupled wave analysis (RCWA). Ag-coated gratings produced sharper and more clearly identifiable resonances than Cu-coated gratings, which exhibited broader due to stronger damping. Front-side illumination produced stronger and more reproducible SPR excitation across all disc types, whereas back-side illumination resulted in more complex spectra as light propagates through the polycarbonate layer. Refractive index sensitivities based using Ag-coated discs of 394, 321, and 290 nm/RIU were obtained for CD-R, BD-R, and DVD-R, respectively. The results clarify the influence of fabrication strategy, illumination geometry, and disc grating geometry on resonance quality and sensitivity in low-cost optical disc-based GC-SPR sensors.