Janne Simonen

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.

Rotation of microparticles with Bessel beams generated by diffractive elements

Abstract We show that imaging a non-diverging Bessel beam by a spherical lens leads to the generation of a diverging Bessel beam. Expressions for the projections of the Umov-Poynting vector for a two-dimensional TE-polarized Bessel beam and a three-dimensional paraxial linearly polarized Bessel beam are derived. A fifth-order Bessel beam is produced using a single optical element-a 16-level phase-only diffractive helical axicon fabricated using electron beam lithography. This beam was successfully used to trap and rotate 5-10 μm diameter yeast particles and polystyrene beads of diameter 5 μm.

Strong second-harmonic generation in silicon nitride films

We observe strong second-harmonic generation from silicon nitride films prepared on fused silica substrates by plasma enhanced chemical vapor deposition. The components of the second-order nonlinear optical susceptibility tensor of the films are calibrated against quartz crystal. The dominant component has the magnitude of 2.5 pm/V, almost two orders of magnitude larger than reported for Si3N4, and about three times larger than for the traditional nonlinear crystal of potassium dihydrogen phosphate. The results indicate that silicon nitride has great potential for second-order nonlinear optical devices, especially in on-chip nanophotonics.

Nanoimprint fabrication of gold nanocones with ~10 nm tips for enhanced optical interactions

We show that nanoimprint lithography combined with electron-beam evaporation provides a cost-efficient, rapid, and reproducible method to fabricate conical nanostructures with very sharp tips on flat surfaces in high volumes. We demonstrate the method by preparing a wafer-scale array of gold nanocones with an average tip radius of 5 nm. Strong local fields at the tips enhance the second-harmonic generation by over 2 orders of magnitude compared with a nonsharp reference.

Color-tunable electroluminescence from white organic light-emitting devices through coupled surface plasmons

The authors report color-tunable electroluminescence from white organic light-emitting devices (WOLEDs) through coupling of surface plasmons in a metal/insulator/metal (MIM) structure. The MIM structure was fabricated by depositing Ag and 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP) films on the Ag cathode of a WOLED. The transmission wavelength through the MIM structure depends on the thickness of the middle BCP layer and can be tuned in the visible range. Therefore, the broadband emission from WOLEDs is selectively transmitted, and color-tunable EL emission was obtained. Blue, green, and red light emissions were observed when the BCP layer thicknesses are 70, 100, and 130nm, respectively.

Diffractive shaping of excimer laser beams

Abstract We address the problem of shaping the intensity distribution of a highly directional partially coherent field, such as an excimer laser beam, by means of diffractive optics. Our theoretical analysis is based on modelling the multi-transverse-mode laser beam as a Gaussian Schell-model beam. It is shown numerically that a periodic element, which is unsuitable for the shaping of a coherent laser beam, works well with an excimer laser beam because of its partial spatial coherence. The conversion of an approximately Gaussian excimer laser beam into a flat-top beam in the Fourier plane of a lens is demonstrated with a diffractive beam shaper fabricated as a multilevel profile in SiOl by electron-beam lithography and proportional reactive-ion etching.

Third- and second-harmonic generation microscopy of individual metal nanocones using cylindrical vector beams

We demonstrate third- (THG) and second-harmonic generation (SHG) microscopy of individual silver nanocones using tightly focused cylindrical vector beams (CVBs). Although THG is expected to be a weaker process than SHG, the yield for THG with radial polarization was higher than for SHG. We also found an excellent correlation between the imaging properties of THG and SHG, suggesting that both are governed by the same overall features of the individual nanocone. We also found that the transverse spatial resolution of THG with CVBs, particularly RP, exceeds that of SHG. Our work establishes the potential of THG microscopy with CVBs for structure-sensitive imaging of three-dimensional (3D) metal nano-objects.

Broadband infrared mirror using guided-mode resonance in a subwavelength germanium grating

We demonstrate a broadband mirror for the IR wavelength region comprising a subwavelength grating made of germanium. We design and optimize the guided-mode resonances in the structure for TM-polarized incident light by rigorous electromagnetic simulations. The grating structure is realized by nanoimprint lithography and dry etching. The reflectivity of the mirror is over 95% for the wavelength range between 2245 and 3080nm.

Plasmonic crystal for efficient energy transfer from fluorescent molecules to long-range surface plasmons

Corrugated metallic thin film structures that do not support short-range surface plasmon modes but do support long-range modes are discussed. The coupling efficiency of the energy of excited fluorescent molecules to long-range modes is theoretically calculated using the rigorous coupled wave approach. The obtained maximum coupling efficiency is found to be 55%, more that two times higher than the efficiency of uncorrugated metallic thin films.

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.