Designable Spectrometer-Free Index Sensing Using Plasmonic Doppler Gratings

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 plasmonic Doppler gratings (PDGs), which provide broadband and azimuthal angle dependent grating periodicity. Different from LSPR sensors, PDG index sensors are 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 inplane 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 = 1.00~1.52) and small index variation (n = 1.3330~1.3650). 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.

Normal Doppler Frequency Shift in Negative Refractive‐Index Systems

“…Last but not least, it is worth noting that the Doppler effect has wide applications in various realms including the laser cooling, the tunable control of the transition frequency of semiconductor emitters, and the design of plasmonic Doppler gratings for azimuthal angle resolved nanophotonic applications such as color sorters or refractive index sensors . Then the revealed phenomenon of superlight Doppler effects in negative refractive‐index systems may enrich these useful applications, as well as the exploration of non‐locality in photonics and plasmonics.…”

Spectrometer‐free Optical Hydrogen Sensing Based on Fano‐like Spatial Distribution of Transmission in a Metal−Insulator−Metal Plasmonic Doppler Grating

“…To obtain the transmission intensity angle profiles at different hydrogen concentrations in Figure 3b, we use a filter to select the slit area with a single-pixel width, each pixel corresponds to one intensity value and the pixel coordinate on the slit can be transformed into an azimuthal angle (Figure S4, Supporting Information). [46,47] Upon the absorption of hydrogen gas, changes in the transmission intensity were observed at several specific azimuthal angles. To clearly visualize the effect on the transmission and remove the influence of source fluctuation, we plotted the relative change, [22] Q i = (X i −X 0 )/X 0 , as a function of azimuthal angle in Figure 3c.…”

Spectrometer-Free Graphene Plasmonics Based Refractive Index Sensor

“…These components are usually bulky and thus not suitable for integration [7]. Therefore, novel designs that can discard these bulky elements have to be sought after, using spectrometer-free conceptions [7,8]. In [7] the spectrometer-free design is realized by measure of the transmission intensity change, while in [8], the sensor produces different reflection images as responses to the RI change.…”

“…Therefore, novel designs that can discard these bulky elements have to be sought after, using spectrometer-free conceptions [7,8]. In [7] the spectrometer-free design is realized by measure of the transmission intensity change, while in [8], the sensor produces different reflection images as responses to the RI change. However, in [7] the measured optical intensity is usually more sensitive to noise from the source and detector than to the resonance peak.…”

“…However, in [7] the measured optical intensity is usually more sensitive to noise from the source and detector than to the resonance peak. In [8], to get and analyze the reflection image, a CCD array and post-image processing are required which also increases the complexity of the system. Generally speaking, the key to the spectrometer-free design consists in finding another response to the RI change.…”