Jouni Mäkitalo

Second-Harmonic Generation Imaging of Metal Nano-Objects with Cylindrical Vector Beams

We introduce an imaging technique based on second-harmonic generation with cylindrical vector beams that is extremely sensitive to three-dimensional orientation and nanoscale morphology of metal nano-objects. Our experiments and second-harmonic field calculations based on frequency-domain boundary element method are in very good agreement. The technique provides contrast for structural features that cannot be resolved by linear techniques or conventional states of polarization and shows great potential for simple and cost-effective far-field optical imaging in plasmonics.

Metamaterials with tailored nonlinear optical response.

We demonstrate that the second-order nonlinear optical response of noncentrosymmetric metal nanoparticles (metamolecules) can be efficiently controlled by their mutual ordering in an array. Two samples with minor change in ordering have nonlinear responses differing by a factor of up to 50. The results arise from polarization-dependent plasmonic resonances modified by long-range coupling associated with metamolecular ordering. The approach opens new ways for tailoring the nonlinear responses of metamaterials and their tensorial properties.

Boundary element method for surface nonlinear optics of nanoparticles

We present the frequency-domain boundary element formulation for solving surface second-harmonic generation from nanoparticles of virtually arbitrary shape and material. We use the Rao-Wilton-Glisson basis functions and Galerkins testing, which leads to very accurate solutions for both near and far fields. This is verified by a comparison to a solution obtained via multipole expansion for the case of a spherical particle. The frequency-domain formulation allows the use of experimentally measured linear and nonlinear material parameters or the use of parameters obtained using ab-initio principles. As an example, the method is applied to a non-centrosymmetric L-shaped gold nanoparticle to illustrate the formation of surface nonlinear polarization and the second-harmonic radiation properties of the particle. This method provides a theoretically well-founded approach for modelling nonlinear optical phenomena in nanoparticles.

Second-harmonic generation from metal nanoparticles: resonance enhancement versus particle geometry.

We demonstrate that optical second-harmonic generation (SHG) from arrays of noncentrosymmetric gold nanoparticles depends essentially on particle geometry. We prepare nanoparticles with different geometrical shapes (L and T) but similar wavelengths for the polarization-dependent plasmon resonances. In contrast to recent interpretations emphasizing resonances at the fundamental frequency, the T shape leads to stronger SHG when only one, instead of both, polarization component of the fundamental field is resonant. This is explained by the character of plasmon oscillations supported by the two shapes. Our numerical simulations for both linear and second-order responses display unprecedented agreement with measurements.

Particle plasmon resonances in L-shaped gold nanoparticles

We present an extensive experimental and theoretical study of the particle plasmon resonances of L-shaped gold nanoparticles. For the small characteristic size of the particles, we observe more higher-order resonances than previously from related shapes, and show that a short-wavelength resonance arises from the particle arm width and is not the suggested volume plasmon. We interpret the resonances through the local vector electric field in the structure and by fully taking into account the particle symmetry.

Second-harmonic generation imaging of semiconductor nanowires with focused vector beams.

We use second-harmonic generation (SHG) with focused vector beams to investigate individual vertically aligned GaAs nanowires. Our results provide direct evidence that SHG from oriented nanowires is mainly driven by the longitudinal field along the nanowire growth axis. Consequently, focused radial polarization provides a superior tool to characterize such nanowires compared to linear polarization, also allowing this possibility in the native growth environment. We model our experiments by describing the SHG process for zinc-blende structure and dipolar bulk nonlinearity.

Multipolar second-harmonic emission with focused Gaussian beams

We show that electric-dipole-allowed surface second-harmonic (SH) generation with focused Gaussian beams can be described in terms of Mie- type multipolar contributions to the SH signal. In contrast to the traditional case, where Mie multipoles arise from field retardation across nanoparticles, the multipoles here arise from the confined source volume and the tensorial properties of the SH response. We demonstrate this by measuring strongly asymmetric SH emission into reflected and transmitted directions from a nonlinear thin film with isotropic surface symmetry, where symmetric emission is expected using traditional formalisms based on plane-wave excitation. The proposed multipole approach provides a convenient way to explain the measured asymmetric emission. Our results suggest that the separation of surface and bulk responses, which have dipolar and higher-multipolar character, respectively, may be even more difficult than thought. On the other hand, the multipolar approach may allow tailoring of focal conditions in order to design confined and thin nonlinear sources with desired radiation patterns.

Three-dimensional winged nanocone optical antennas

Optical antennas convert propagating far-field radiation into localized near-fields. Perhaps the simplest optical antenna is a sharp metal tip, which is commonly utilized in near-field microscopy and spectroscopy. Usually, the coupling of far-field light to the near-field requires either a field polarized along the tip axis (longitudinal field) or a surface grating on the tip. Here, we demonstrate that efficient transformation of incident far-field radiation to localized energy at the cone apex can be achieved using nanocones with wings oriented perpendicular to the tip axis.

Tip-enhanced Raman scattering from bridged nanocones.

We present two silver nanocones separated by 450 nm, well beyond the typical gap spacing of coupled nanoantennas, and connected by a metal bridge to facilitate plasmonic coupling between them. The tip-enhanced Raman scattering from crystal violet molecules is found to be almost an order of magnitude higher from the bridged cones than from individual cones. This result is supported by local-field calculations of the two types of structures. The bridged nanocones are easily fabricated by a nanoimprint-based process, thus offering a faster and simpler approach compared to other fabrication techniques.

Fabrication of Ion-Shaped Anisotropic Nanoparticles and their Orientational Imaging by Second-Harmonic Generation Microscopy.

Ion beam shaping is a novel and powerful tool to engineer nanocomposites with effective three-dimensional (3D) architectures. In particular, this technique offers the possibility to precisely control the size, shape and 3D orientation of metallic nanoparticles at the nanometer scale while keeping the particle volume constant. Here, we use swift heavy ions of xenon for irradiation in order to successfully fabricate nanocomposites consisting of anisotropic gold nanoparticle that are oriented in 3D and embedded in silica matrix. Furthermore, we investigate individual nanorods using a nonlinear optical microscope based on second-harmonic generation (SHG). A tightly focused linearly or radially-polarized laser beam is used to excite nanorods with different orientations. We demonstrate high sensitivity of the SHG response for these polarizations to the orientation of the nanorods. The SHG measurements are in excellent agreement with the results of numerical modeling based on the boundary element method.