Joonas Lehtolahti

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.

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.

Surface lattice resonances in second-harmonic generation from metasurfaces

We investigate second-harmonic generation from arrays of metal nanoparticles as a function of the incident angle of light into the structure. The generated signal is significantly affected by the surface-lattice resonances of the structure.

Spectral control in anisotropic resonance-domain metamaterials.

We introduce a concept to control the spectral and dichroic properties of metamaterials. The approach is based on anisotropic metal nanoparticles and on varying their mutual orientation in a periodic lattice. Even seemingly inconsequential changes in particle ordering strongly modify the dichroic properties of the arrays and result in either very narrow resonances or ultrabroad extinction ranges. The results arise from long-range diffractive coupling between the particles, as determined by the dependence of the array unit cell size on particle ordering.

Coherence modulation by deterministic rotating diffusers

We design rotating diffusers with deterministic complex-amplitude transmission functions, which give rise to tailored spatial coherence modulation when transilluminated by an axially incident coherent Gaussian beam. Mathematical expressions are derived for the immediate diffuser output as well as for the far-field response. An experimental demonstration is given using a diffuser fabricated by lithographic techniques.

Detection of partial polarization of light beams with dipolar nanocubes.

We confirm experimentally that the degree and state of polarization of a random, partially polarized electromagnetic beam can be obtained by probing the field with a nanoscatterer. We use a gold nanocube on silicon substrate as a local scatterer and detect the polarization characteristics of the scattered far field, which enables us to deduce the state of partial polarization of the field at the nanoprobe site. In contrast to previous beam characterization methods where spatial resolution is limited by the pixel size of the detector, the accuracy of the current technique is specified by the particle size. Our work is the first step towards polarization-state detection of random optical near fields for which the use of nanoprobes is required.

Human color vision provides nanoscale accuracy in thin-film thickness characterization

We study how accurately a naked human eye can determine the thickness of thin films from the observed color. Our approach is based on a color-matching experiment between thin-film samples and a simulated color field shown on an LCD monitor. We found that the human color observation provides an extremely accurate evaluation of the film thickness, and is comparable to sophisticated instrumental methods. The remaining color differences for the matched color pairs are close to the literature value for the smallest visually perceivable color difference.

Metamaterials with Tailorable Nonlinear Optical Properties

Second-order nonlinear processes such as second-harmonic generation (SHG) require noncentrosymmetric structures. The development of second-order metamaterials, however, has been hampered by symmetry breaking due to sample defects and shape distortions. The resulting outcomes can be interpreted in terms of effective higher-multipole (magnetic and quadrupole) effects that strongly modify the radiative properties of the samples.

Enhancement of second-harmonic generation from metasurfaces by reduced number of particles

We use second-harmonic generation to investigate arrays of V-shaped nanoparticles with different particle number densities. Due to lattice interactions, the response is enhanced 8-fold by reducing the number density without increasing losses.

Enhancement of second-harmonic generation from metasurfaces through surface lattice resonances(Conference Presentation)

The optical properties of metal nanostructures result from their localized surface plasmon resonances (LSPRs). The exact spectral positions of LSPRs depend on the geometry of the particles and optical parameters of the surrounding material. In addition, when the nanoparticles are arranged in an array (metasurface) the LSPRs associated with isolated particles are affected by the presence of other array members. As a result, sharp spectral features related to surface lattice resonances (SLRs) can be observed. SLRs are sensitive to the angle of incidence of incoming light due to their association with the appearance and disappearance of diffraction orders in the optical response. The resonances lead to strong local fields (hot spots) in the proximity of the particles. Such hot spots are particularly important when one wants to enhance nonlinear optical effects, for example second-harmonic generation (SHG). In the case of SHG, the sample needs to be also non-centrosymmetric, which can be fulfilled by using, e.g., V-shaped nanoparticles. In this paper, we show that SHG from arrays of V-shaped gold nanoparticles can be enhanced when the angle of incidence is matching optimal conditions for the excitation of SLRs. Our sample consists of an array of V-shaped nanoparticles fabricated by standard electron-beam lithography and lift-off techniques. The gold particles are distributed in the array with a spacing of 500 nm and have the same dimensions to obtain a resonance close to fundamental wavelength for the polarization along the symmetry axis. The SHG experiments were performed in transmission mode with the incident beam weakly focused on the samples. Polarizers and a half-wave plate were used to control the polarizations of the fundamental and second-harmonic beams. The SHG signal was collected by a photon-counting system for varying incident angles (rotation with respect to the symmetry axis (y) of the sample or orthogonal to it (x)). SLRs can modify LSPRs leading to visible features in the SHG response. When the incident angle is increased, a redshift towards fundamental wavelength is observed together with narrowing of the resonance. Such improvement in the quality of the resonance results in stronger SHG. Thus, the sample needs to be rotated significantly to meet the optimal conditions for SHG enhancement, which can be as high as by a factor of four (projection x-z) or a factor of 10 (projection y-z) comparing to SHG measured at normal incidence. The maximum enhancement of the SHG signal is related to the SLRs which occur near incident angles that allow diffraction orders to propagate along the sample plane (in air or in the substrate). Our investigation of the role of SLRs in SHG from metasurfaces shows that such resonances lead to prominent features in the angle-dependent SHG responses, which results in the enhancement of SHG by a factor of up to ten. In order to achieve the optimal conditions of SHG enhancement the sample needs to be rotated from normal incidence to match the angle that allows diffraction order to propagate in the substrate.