Second-Harmonic Enhancement with Mie Resonances in Perovskite Nanoparticles

Second-harmonic generation (SHG) in nanostructures gives rise to many applications such as lab-on-a-chip and imaging by frequency doubling. However, the SHG signal decreases with volume, and the conversion efficiency is limited. Thus, means to enhance nonlinear signals at the nanoscale are needed. For instance, while plasmonic nanostructures offer a high enhancement due to the strong confinement of the electromagnetic field, they have high losses and the fabrication methods are difficult. In this work, we propose to enhance the SHG by using the intrinsic scattering properties of an all-dielectric perovskite nanostructure. We demonstrate the Mie scattering resonances of individual barium titanate (BaTiO₃) nanoparticles with diameters between 200 and 250 nm. We distinguish contributions of the magnetic dipole and magnetic quadrupole. Then, we use the Mie resonances to achieve an SHG enhancement of 4 orders of magnitude within the same nanoparticle. Our results suggest that a strong increase of the SHG signal can be obtained without using plasmonic or hybrid nanostructures. We show a straightforward way of enhancing low optical signals within a single material, which will facilitate the study of other nonlinear phenomena at the nanoscale

Deep Tissue Imaging with Highly Fluorescent Near-Infrared Nanocrystals after Systematic Host Screening

Photoluminescent inorganic nanoparticles are attractive as bioimaging contrast agents because they do not degrade by photobleaching and do not suffer from concentration quenching as clinically applied organic dyes. Here, for the first time, a large variety of oxide, phosphate, and vanadate nanocrystals doped with Nd³⁺ are systematically examined and compared as down-converting photoluminescent contrast agents to understand underlying physical properties and to identify the brightest composition. These inorganic crystals are particularly attractive for bioimaging in the near-infrared (NIR) window, where absorption and scattering by human tissue are reduced drastically. Through close control of their crystal size, the resulting fluorescence properties are quantitatively compared under NIR excitation. Most interestingly, BiVO₄ doped with Nd³⁺ is shown to be the most efficient composition. Its application as a photoluminescent NIR imaging contrast agent is demonstrated <i>ex vivo</i> with chicken skeletal muscle and bovine liver tissues. Under a harmless laser power density (0.2 W/cm²), fluorescent BiVO₄ particles could be clearly detected at an injection depth of 20 mm by a simple commercial camera

Enhancing Guided Second-Harmonic Light in Lithium Niobate Nanowires

We experimentally demonstrate practical approaches to enhance second-harmonic (SH) generation in individual lithium niobate nanowires (NWs) with a sub-micrometer cross-section and length up to tens of micrometers. We establish that parametric interactions of guided modes propagating along the NW determine the SH output power, which can be therefore controlled by the NW length. We show that the SH power is increased by about 84 times at wavelengths corresponding to modal phase-matching. Importantly, at non-phase-matched wavelengths the SH power can be improved by a factor of up to 9.3 by adjusting the NW length with a focused ion beam. We also characterize SH emission directionality, which can be further tailored for applications in integrated optical circuits and nonlinear microscopy

Polar Second-Harmonic Imaging to Resolve Pure and Mixed Crystal Phases along GaAs Nanowires

In this work, we report an optical method for characterizing crystal phases along single-semiconductor III–V nanowires based on the measurement of polarization-dependent second-harmonic generation. This powerful imaging method is based on a per-pixel analysis of the second-harmonic-generated signal on the incoming excitation polarization. The dependence of the second-harmonic generation responses on the nonlinear second-order susceptibility tensor allows the distinguishing of areas of pure wurtzite, zinc blende, and mixed and rotational twins crystal structures in individual nanowires. With a far-field nonlinear optical microscope, we recorded the second-harmonic generation in GaAs nanowires and precisely determined their various crystal structures by analyzing the polar response for each pixel of the images. The predicted crystal phases in GaAs nanowire are confirmed with scanning transmission electron and high-resolution transmission electron measurements. The developed method of analyzing the nonlinear polar response of each pixel can be used for an investigation of nanowire crystal structure that is quick, sensitive to structural transitions, nondestructive, and on-the-spot. It can be applied for the crystal phase characterization of nanowires built into optoelectronic devices in which electron microscopy cannot be performed (for example, in lab-on-a-chip devices). Moreover, this method is not limited to GaAs nanowires but can be used for other nonlinear optical nanostructures

Far-Field Imaging for Direct Visualization of Light Interferences in GaAs Nanowires

The optical and electrical characterization of nanostructures is crucial for all applications in nanophotonics. Particularly important is the knowledge of the optical near-field distribution for the design of future photonic devices. A common method to determine optical near-fields is scanning near-field optical microscopy (SNOM) which is slow and might distort the near-field. Here, we present a technique that permits sensing indirectly the infrared near-field in GaAs nanowires via its second-harmonic generated (SHG) signal utilizing a nonscanning far-field microscope. Using an incident light of 820 nm and the very short mean free path (16 nm) of the SHG signal in GaAs, we demonstrate a fast surface sensitive imaging technique without using a SNOM. We observe periodic intensity patterns in untapered and tapered GaAs nanowires that are attributed to the fundamental mode of a guided wave modulating the Mie-scattered incident light. The periodicity of the interferences permits to accurately determine the nanowires’ radii by just using optical microscopy, i.e., without requiring electron microscopy