Janne Laukkanen

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.

Dipole limit in second-harmonic generation from arrays of gold nanoparticles

We present a multipolar tensor analysis of second-harmonic generation from arrays of noncentrosymmetric gold nanoparticles. In contrast to earlier results, where higher multipoles and symmetry-forbidden signals arising from sample defects play a significant role, the present results are completely dominated by symmetry-allowed electric-dipole tensor components. The result arises from significant improvement in sample quality, which suppresses the higher-multipole effects and enhances the overall response by an order of magnitude. The results are a prerequisite for metamaterials with controllable nonlinear properties.

Experimental demonstration of the generation of the longitudinal E-field component on the optical axis with high-numerical-aperture binary axicons illuminated by linearly and circularly polarized beams

We analytically and numerically show that by introducing asymmetry into axicon design it becomes possible to generate the longitudinal E-field component on the optical axis for linearly and circularly polarized incident beams. Binary axicons with high numerical aperture (NA) are fabricated in three configurations?axisymmetric and spiral, and bi-axicon?by electron beam lithography. Experimental measurements for the near-field diffraction of the most common and easy to implement incident beams?linearly and circularly polarized?are conducted. The experimental results agree with the theoretical analysis.

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.

Chiral coupling in gold nanodimers

Second-harmonic generation from T-shaped gold nanodimers with nanogaps has different efficiencies for left- and right-hand circularly polarized fundamental light. The difference arises from the chiral symmetry breaking of the dimers due to nonorthogonal mutual orientations of the horizontal and vertical bars of the T shape and depends on the gap size. Unexpectedly, the smallest gap with presumably the strongest coupling gives rise to only a small difference. All results can be explained by considering the distribution of the polarized local field in the structure.

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.

Subwavelength cylindrical grating by holistic phase-mask coordinate transform

A periodic grating with an integer number of periods is fabricated at the resist-coated wall of a cylinder by exposing a circularly symmetrical planar high index phase mask to a cylindrical wave. This extends the spatial coherence features easily achievable in a planar 2D space to the 3D space of cylindrical waves and elements.

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.

Lasing in microdisks of ultrasmall diameter

It is demonstrated by calculations and experimental results that room-temperature lasing can be obtained at the ground-state optical transition of InAs/InGaAs/GaAs quantum dots in optical microcavities with a record-small diameter of 1.5 μm. In 1-μm cavities, lasing occurs at the wavelength of one of the whispering-gallery modes within the band corresponding to the first excited-state optical transition.

Scatterometer for characterization of diffractive optical elements

Diffractive optical elements can control light beyond the capabilities of traditional optical components, which offers great opportunities for numerous applications. However, modern functionalities are related to even smaller feature sizes, which causes remarkable challenges for the structure measurements in the nanoscale. The current methods are not optimal for the characterization of diffractive structures. Optical scatterometry, however, would be a good tool for non-destructive characterization, and in particular, for inline process control of the fabrication. In this paper, detailed information about a goniometric scatterometer built at MIKES (Centre for Metrology and Accreditation), basics of the inverse problem approach for solving the parameters of the structure and first measurements for confirming the functionality of the scatterometer as well as the capabilities of it are discussed.