Abstract
Surface plasmon polaritons (SPP) are exploited due to their intriguing properties for the fabrication and miniaturization of photonic circuits, for surface-enhanced spectroscopy and imaging beyond the diffraction limit. However, excitation of these plasmonic modes by direct illumination is forbidden by energy/momentum conservation rules. One strategy to overcome this limitation relies on diffraction gratings to match the wavevector of the incoming photons with that of propagating SPP excitations. The main limit of the approaches so far reported in the literature is that they rely on highly ordered diffraction gratings fabricated by means of demanding nanolithographic processes. In this work, we demonstrate that an innovative, fully self-organized method based on wrinkling-assisted ion-beam sputtering can be exploited to fabricate large-area (cm2 scale) nanorippled soda lime templates, which conformally support ultrathin Au films deposited by physical deposition. The self-organized patterns act as quasi-one-dimensional (1D) gratings characterized by a remarkably high spatial order, which properly matches the transverse photon coherence length. The gratings can thus enable the excitation of hybrid SPP modes confined at the Au/dielectric interfaces, with a resonant wavelength that can be tuned by modifying the grating period, photon incidence angle, or, potentially, the choice of the thin-film conductive material. Surface-enhanced Raman scattering experiments show promising gains in the range of 103, which are competitive, even before a systematic optimization of the sample fabrication parameters, with state-of-the art lithographic systems, demonstrating the potential of such templates for a broad range of optoelectronic applications aiming at plasmon-enhanced photon harvesting for molecular or biosensing.
Highlights
In the last decades, the field of plasmonics witnessed an outstanding development because of the growing interest in the possibility to manipulate optical fields at the nanoscale, well below the diffraction limit, boosting a very broad range of optoelectronic applications.[1−4] Different so-called plasmonic modes with diverse properties and resonant conditions are found in nature.[5]
Surface plasmon polaritons (SPP) are electromagnetic modes propagating at the interface between a positive and a negative permittivity medium due to resonant oscillations of free carriers.[5−10] Their propagating nature makes them suitable for waveguiding applications crucial for the development of photonic circuits and components, which are not enabled by localized surface plasmons (LSP) supported by subwavelength disconnected nanoparticles.[5,6,11−16] surface plasmon polaritons (SPP) modes can be excited by electromagnetic radiation under selected wavevector coupling conditions, confining the photon energy into subwavelength volumes at the conductive/dielectric interface where the electromagnetic evanescent field is strongly enhanced.[5,6,17]
This peculiar property has recently allowed the highly sensitive near-field detection and imaging of surface plasmon modes with high subwavelength spatial resolution at the surface of atomic two-dimensional (2D) materials in scanning near-field optical microscopy (SNOM)[18−21] and molecular nanoimaging in tip-enhanced Raman scattering (TERS).[22−25] In parallel, highly sensitive biosensing capabilities have been achieved in surface-enhanced spectroscopies such as surface-enhanced Raman spectroscopy (SERS), surface-enhanced coherent anti-Stokes Raman spectroscopy (SECARS), and metal-enhanced fluorescence (MEF), which
Summary
The field of plasmonics witnessed an outstanding development because of the growing interest in the possibility to manipulate optical fields at the nanoscale, well below the diffraction limit, boosting a very broad range of optoelectronic applications.[1−4] Different so-called plasmonic modes with diverse properties and resonant conditions are found in nature.[5]. Nonradiative plasmon decay generating hot electrons into the conductive material can be exploited to enable direct plasmon-enhanced photocatalysis[40,41] and/or hot carrier injection into semiconductors, effectively extending the operating range of photonic devices beyond their band-gap-limited absorption.[42,43] photon−SPP coupling is not allowed by direct illumination because of energy and momentum conservation selection rules; particular strategies have to be adopted, such as EM coupling via subwavelength probes or diffraction gratings, which are able to modulate the wavevector of the incoming photons.[5,6] In particular, diffraction gratings characterized by a very high spatial coherence are typically fabricated via topdown lithographic fabrication methods that, can limit the perspective of large-scale, low-cost applications.[27−29,44−47]
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