We demonstrate the utility of optical second harmonic generation (SHG) polarimetry to perform structural characterization of noncentrosymmetric, single-crystalline II-VI semiconducting nanowires, nanobelts, and nanoflakes. By analyzing anisotropic SHG polarimetric patterns, we distinguish between wurtzite and zincblende II-VI semiconducting crystal structures and determine their growth orientation. The crystallography of these nanostructures was then confirmed via transmission electron microscopy measurements performed on the same system. In addition, we show that some intrinsic material properties such as nonlinear coefficients and geometry-dependent optical in-coupling coefficients can also be determined from the SHG experiments in WZ nanobelts. The ability to perform SHG-based structural characterization and crystallographic study of II-VI semiconducting single-crystalline nanomaterials will be useful to correlate structure-property relationships of nanodevices on which transmission electron microscopy measurements cannot be typically performed.

Semiconductor nanostructures such as nanowires and nanoribbons functioning as Fabry-Pérot (F-P)-type optical cavities and nanolasers have attracted great interest not only for their potential use in nanophotonic systems but also to understand the physics of light-matter interactions at the nanoscale. Due to their nanoscale dimensions, new techniques need to be continuously developed to characterize the nature of highly confined optical modes. Furthermore, the inadequacy of typical far-field photoluminescence experiments for characterizing the nanoscale cavity modes such as parity and order has precluded efforts to obtain precise information that is required to fully understand these cavities. Here, we utilize a modified Young's interference method based on angle-resolved microphotoluminescence spectral technique to directly reveal the parity of F-P cavity modes in CdS nanostructures functioning as waveguides and nanolasers. From these analyses, the mode order can be straightforwardly obtained with the help of numerical simulations. Moreover, we show that the Young's technique is a general technique applicable to any F-P type cavities in nanoribbons, nanowires, or other photonic and plasmonic nanostructures.

We have investigated second harmonic generation (SHG) from Ag-coated LiNbO 3 (LN) core-shell nanocuboids and found that giant SHG can occur via deliberately designed double plasmonic resonances. By controlling the aspect ratio, we can tune fundamental wave (FW) and SHG signal to match the longitudinal and transverse plasmonic modes simultaneously, and achieve giant enhancement of SHG by 3×10 5 in comparison to a bare LN nanocuboid and by about one order of magnitude to the case adopting only single plasmonic resonance. The underlying key physics is that the double-resonance nanoparticle enables greatly enhanced trapping and harvesting of incident FW energy, efficient internal transfer of optical energy from FW to SHW, and much improved power to transport the SHG energy from the nanoparticle to the far-field region. In this work we will address this problem by insightful design. The most important thing is to look for a nanoparticle with double SPR modes. For this purpose, we turn to nanoparticles of anisotropic geometry, among which nanorods have attracted great attention due to their dimension induced wavelength tunability and polarization sensitivity [11][12][13]. One can control the aspect ratio of nanorods easily and obtain double SPR modes simultaneously, which are associated with the longitudinal and transverse SPR modes [14,15]. Moreover, the optical cross-sections of nanorods are much higher than those of nanospheres, and this has attributed nanorods promising features in wide applications like low-threshold surface plasmon amplification [16,17] and ultrafast optical devices [18]. The two SPR modes can be designed to enable their wavelengths matched to the absorption bands and emission bands of fluorescent molecules absorbed in gold nanorods [19] for enhancing their fluorescence intensity. It would be expected that this double-resonance mechanism should also play a very good role in enhancing SHG intensity of nanostructured materials.To illustrate and confirm this hypothesis, we propose and design an anisotropic nanoparticle, an Ag-coated LiNbO 3 (LN) core-shell nanocuboid [Ag-LN in Fig. 1(a) [23,24]. This approach is efficient equally for nonlinear optical problem solutions.Assume a plane wave FW at the frequency of ω 1 is incident upon a nonlinear particle. The FW electric field 1

The coupling effect between two gold nanospheres (GNSs) is investigated for second-harmonic generation (SHG) in this paper. When the two GNSs approach each other, the coupling effect becomes stronger, and it consequently leads to stronger enhancement of SHG. A strong coupling area between two GNSs is also found. We propose a physical quantity, the coupling strength to describe the coupling effect on SHG enhancement and show that it can fully illustrate the relationship between the SHG enhancement and the gap of two GNSs. Moreover, further calculations reveal the existence of hot spots of both fundamental wave (FW) and excited second harmonic wave in the intermediate region of two GNSs. Our study can not only help to understand the coupling effect on SHG precisely but also offer some possibility to design a nonlinear plasmonic nano-ruler.

We present design of an all-optical diode in a metal-dielectric structure where plasmonic attenuation and quasi-phase-matching (QPM) is harnessed to improve its performance greatly. Due to the anti-symmetric design of the nonlinear susceptibility, different incident direction will ignite different plasmonic nonlinear process, which either compensates plasmonic attenuation sufficiently or accelerates it seriously. As a result, unidirectional output of plasmonic signal is achieved. This designed all-optical diode shows advantages of low power consumption, short sample length, high isolation contrast, wide acceptance of structural and initial conditions, and tunable unidirectionality, and becomes of practical interest.