We have synthesized and characterized three types of perovskite alkaline niobate nanowires: NaNbO 3 , KNbO 3 , and LiNbO 3 (XNbO 3 ). All three types of nanowires exhibit strong nonlinear response. Confocal imaging has been employed to quantitatively compare the efficiency of synthesized nanowires to generate second harmonic signal and to show that LiNbO 3 nanowires exhibit the strongest nonlinear response. We also investigated the polarization response of the second harmonic generation (SHG) signal in all three types of alkaline nanowires for the two geometries tractable by our optical trapping setup. The SHG signal is highly influenced by the nanowire crystallinity and experimental geometry. We also demonstrate for the first time wave-guiding of SHG signal in all three types of alkaline niobate nanowires. By carefully examining nonlinear properties of (XNbO 3 ) nanowires we suggest which type of wires are best suited for the given application.

Nonlinear microscopes have seen an increase in popularity in the life sciences due to their molecular and structural specificity, high resolution, large penetration depth, and volumetric imaging capability. Nonetheless, the inherently weak optical signals demand long exposure times for live cell imaging. Here, by modifying the optical layout and illumination parameters, we can follow the rotation and translation of noncentrosymetric crystalline particles, or nanodoublers, with 50 μs acquisition times in living cells. The rotational diffusion can be derived from variations in the second harmonic intensity that originates from the rotation of the nanodoubler crystal axis. We envisage that by capitalizing on the biocompatibility, functionalizability, stability, and nondestructive optical response of the nanodoublers, novel insights on cellular dynamics are within reach.

Geometrical effects in optical nanostructures on nanoscale can lead to interesting phenomena such as inhibition of spontaneous emission, 1,2 high-reflecting omnidirectional mirrors, structures that exhibit low-loss-waveguiding, 3 and light confinement. 4,5 Here, we demonstrate a similar concept of exploiting the geometrical effects on nanoscale through precisely fabricating lithium niobate (LiNbO 3 ) nanocones arrays devices. We show a strong second harmonic generation (SHG) enhancement, shape and arrangement dependent, up to 4 times bigger than the bulk one. These devices allow below diffraction limited observation, being perfect platforms for single molecule fluorescence microscopy 6 or single cell endoscopy. 7 Nanocones create a confined illumination volume, devoid from blinking and bleaching, which can excite molecules in nanocones proximity. Illumination volume can be increased by combining the SH enhancement effect with plasmon resonances, excited thanks to a gold plasmonic shell deposited around the nanostructures. This results in a local further enhancement of the SH signal up to 20 times. The global SH enhancement can be rationally designed and tuned through the means of simulations. KEYWORDS: Second harmonic generation, lithium niobate, nanocones, enhancement, plasmons, single molecule detection N owadays, frequency conversion is a common tool used to create visible coherent light for the frequencies where no alternative laser sources are available. At the heart of this phenomenon are typically nonlinear birefringent crystals, which respond to the interaction with sufficiently high light intensities with nonlinear optical processes. Efficient second order nonlinear optical interactions, 8 such as second harmonic generation (SHG), sum frequency generation, and difference frequency generation, occur only in noncentrosymmetric crystals, which are crystals that do not display inversion symmetry in their unit cell. Over the years, it has been realized that for the highest efficiency of the SHG process, employed crystals with dimensions much larger than the impinging laser wavelength have to be phased matched. On the other hand, in nanoscale materials, which has one of the structure dimensions much smaller than the coherent length, the phase matching condition can usually be ignored. 9 Additionally, thanks to their limited size, light-emitting nanoscale probes have been proposed as the ideal platform for single molecule fluorescence imaging 6 and single molecule endoscopy. 7 SHG nanostructures perfectly match the requirements for these applications, having already been used as local excitation sources. 10,11 They were also used as probes for the cell and in vivo imaging ensuring relatively low intensity illumination of cells that avoids cell damage and being free from photobleaching, photoblinking, and dye saturation effects. 12,13 Today, a reliable fabrication method allowing for parallel and high signal-to-noise ratio measurement is still missing.In principle and, as we are going to demonstrate, ...