Designable spectrometer-free index sensing using plasmonic Doppler gratings

TL;DR

This paper introduces a fully designable plasmonic Doppler grating (PDG) sensor that enables broadband, angle-dependent index sensing without the need for spectrometers, allowing precise detection of environmental index changes.

Contribution

The work presents a novel, fully designable PDG platform that translates index changes into observable optical features, eliminating the need for spectrometers and enabling flexible, on-chip sensing.

Findings

Successfully detected large and small index variations.

Accurately quantified tiny, nonlinear water-ethanol index changes.

Demonstrated on-site, single-color index sensing without a spectrometer.

Abstract

Typical nanoparticle-based plasmonic index sensors detect the spectral shift of localized surface plasmon resonance (LSPR) upon the change of environmental index. Therefore, they require broadband illumination and spectrometers. The sensitivity and flexibility of nanoparticle-based index sensors are usually limited because LSPR peaks are usually broad and the spectral position cannot be freely designed. Here, we present a fully designable index sensing platform using a plasmonic Doppler grating (PDG), which provides broadband and azimuthal angle-dependent grating periodicities. Different from LSPR, the PDG index sensor is based on the momentum matching between photons and surface plasmons via the lattice momentum of the grating. Therefore, index change is translated into the variation of in-plane azimuthal angle for photon-to-plasmon coupling, which manifests as directly observable dark bands in the reflection image. The PDG can be freely designed to optimally match the range of index variation for specific applications. In this work, we demonstrate PDG index sensors for large (n = 1.00 to 1.52) and small index variation (n = 1.3330 to 1.3650). The tiny and nonlinear index change of water-ethanol mixture has been clearly observed and accurately quantified. Since the PDG is a dispersive device, it enables on-site and single-color index sensing without a spectrometer and provides a promising spectroscopic platform for on-chip analytical applications.