Optical materials with a high refractive index, such as silicon, are receiving increasing attention in the photonics research community. When structured on the nanoscale, these materials display optical Mie-type resonances with low loss and effective manipulation of light. High-refractive-index nanostructures are therefore key ingredients in the design of novel nanophotonic devices and have great technological potential, ranging from surface coloration to ultra-thin optical devices. The performance of such optical devices is directly linked to the electromagnetic near field of high-refractive-index nanostructures. Unfortunately, the optical near field is extremely challenging to measure, and it involves dedicated near-field instruments. Here, we demonstrate that Raman spectroscopy – a technique available in laboratories worldwide – can be turned into a versatile near-field instrument for high-refractive-index nanophotonics Raman spectroscopy measures the inelastic scattering of photons due to phonons and its intensity scales dramatically with the electric near-field intensity. We exploit this property to measure the near-field enhancement produced by two important classes of optical resonances in high-refractive-index nanostructures; Fabry-Pérot and Mie resonances. We realize these resonances in judiciously sized silicon thin films and nanodisk arrays, respectively, and provide experimental characterization using a novel Raman setup with tunable excitation wavelength. An intuitive theoretical model based on the enhancement of the stored electric energy corroborates our experimental findings.
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