Dr. Jan Renger

Nanoparticles capable of efficiently generating nonlinear optical signals, like second harmonic generation, are attracting a lot of attention as potential background-free and stable nano-probes for biological imaging. However, second harmonic nanoparticles of different species do not produce readily distinguishable optical signals, as the excitation laser mainly defines their second harmonic spectrum. This is in marked contrast to other fluorescent nano-probes like quantum dots that emit light at different colors depending on their sizes and materials. Here, we present the use of resonant plasmonic nanoparticles, combined with broadband phase-controlled laser pulses, as tunable sources of multicolor second harmonic generation. The resonant plasmonic nanoparticles strongly interact with the electromagnetic field of the incident light, enhancing the efficiency of nonlinear optical processes. Because the plasmon resonance in these structures is spectrally narrower than the laser bandwidth, the plasmonic nanoparticles imprint their fingerprints on the second harmonic spectrum. We show how nanoparticles of different sizes produce different colors in the second harmonic spectra even when excited with the same laser pulse. Using these resonant plasmonic nanoparticles as nano-probes is promising for multicolor second harmonic imaging while keeping all the advantages of nonlinear optical microscopy.

Nanoantennas are well-known for their effective role in fluorescence enhancement, both in excitation and emission. Enhancements of 3-4 orders of magnitude have been reported. Yet in practice, the photon emission is limited by saturation due to the time that a molecule spends in singlet and especially triplet excited states. The maximal photon stream restricts the attainable enhancement. Furthermore, the total number of photons emitted is limited by photobleaching. The limited brightness and observation time are a drawback for applications, especially in biology. Here we challenge this photon limit, showing that nanoantennas can actually increase both saturation intensity and photostability. So far, this limit-shifting role of nanoantennas has hardly been explored. Specifically, we demonstrate that single light-harvesting complexes, under saturating excitation conditions, show over a 50-fold antenna-enhanced photon emission stream, with 10-fold more total photons, up to 108 detected photons, before photobleaching. This work shows yet another facet of the great potential of nanoantennas in the world of single-molecule biology.