Attainment of spatial resolutions far below diffraction limits by means of optical methods constitutes a challenging task. Here, we design nonlinear nanorulers that are capable of accomplishing approximately 1 nm resolutions by utilizing the mechanism of plasmon-enhanced second-harmonic generation (PESHG). Through introducing Au@SiO2 (core@shell) shell-isolated nanoparticles, we strive to maneuver electric-field-related gap modes such that a reliable relationship between PESHG responses and gap sizes, represented by "PESHG nanoruler equation", can be obtained. Additionally validated by both experiments and simulations, we have transferred "hot spots" to the film-nanoparticle-gap region, ensuring that retrieved PESHG emissions nearly exclusively originate from this region and are significantly amplified. The PESHG nanoruler can be potentially developed as an ultrasensitive optical method for measuring nanoscale distances with higher spectral accuracies and signal-to-noise ratios.

devices, [6] and Raman, [7] fluorescence, [8,9] even infrared [10] spectroscopy. Nonetheless, the lack of high-efficiency substrates with uniformly distributed hotspots greatly hinders the widespread and practical application of plasmon-enhanced spectroscopies (PESs), such as plasmon enhanced second harmonic generation (PESHG) and surface-enhanced Raman scattering (SERS). Hence, constructing hotspot platforms with high-efficiency is of great importance. At present, 0D, 1D, and 2D hotspots based on different types of metal nanostructures are the most frequently used enhancement structures, such as 0D single isolated nanospheres, [11] 1D nanowires [12] or nanorods, [13,14] and 2D nanoarrays, [15][16][17][18] synthesized using various nanofabrication techniques. However, these low spatial dimensions limit the number of hotspots that can be generated and thus impede further improvements in the detection sensitivity of PESs.Due to the higher density of hotspots and larger surface area for adsorbing probe molecules, 3D hotspot platforms can give stronger and more stable SERS signals than that of 0D, 1D, or 2D nanostructures. However, the rational design and simple construction of 3D SERS substrates with controllable and reproducible nanoscale hotspots and stronger Raman enhancement factors via a facile and green approach is a longstanding challenge and numerous efforts have been devoted to their fabrication. Ag NP decorated cactus-like 3D SERS substrates [19] and 3D hot-junction formation of gold nanoparticles on 2D silicate nanoplatelets [20] had been designed, which exhibited good reproducibility and considerable SERS signal amplification. Also, it had been demonstrated that SERS enhancements of 3 orders of magnitude could be generated during evaporation of a drop containing pure Ag NPs solution compared with the dried substrate. [21] Unfortunately, the SERS signals were not stable and rapidly decreased after the complete evaporation of water. To alleviate this drawback, aqueous glycerol was added in the Ag NPs sol to slow the evaporation, [22] which extended the duration of maximum Raman signal. However, the Raman signal kept decreasing with time due to charge exchange among aggregated Ag NPs. Considering this charge

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Surface plasmonic systems provide extremely efficient ways to modulate light-matter interaction in photon emission, light harvesting, energy conversion and transferring, etc. Various surface plasmon enhanced luminescent behaviors have been observed and investigated in these systems. But the origin of an avalanche-like photoluminescence, which was firstly reported in 2007 from Au and subsequently from Ag nanowire arrays/monomers, is still not clear. Here we show, based on systematic investigations including the excitation power/time related photoluminescent measurements as well as calculations, that this avalanche-like photoluminescence is in fact a result of surface plasmon assisted thermal radiation. Nearly all of the related observations could be perfectly interpreted with this concept. Our finding is crucial for understanding the surface plasmon mediated thermal and photoemission behaviors in plasmonic structures, which is of great importance in designing functional plasmonic devices.