Seyfollah Toroghi

Post-fabrication voltage controlled resonance tuning of nanoscale plasmonic antennas.

Voltage controlled wavelength tuning of the localized surface plasmon resonance of gold nanoparticles on an aluminum film is demonstrated in single particle microscopy and spectroscopy measurements. Anodization of the Al film after nanoparticle deposition forms an aluminum oxide spacer layer between the gold particles and the Al film, modifying the particle-substrate interaction. Darkfield microscopy reveals ring-shaped scattering images from individual Au nanoparticles, indicative of plasmon resonances with a dipole moment normal to the substrate. Single particle scattering spectra show narrow plasmon resonances that can be tuned from ~580 to ~550 nm as the anodization voltage increases to 12 V. All observed experimental trends could be reproduced in numerical simulations. The presented approach could be used as a general postfabrication resonance optimization step of plasmonic nanoantennas and devices.

Cascaded plasmon resonant field enhancement in nanoparticle dimers in the point dipole limit

Cascaded field enhancement in silver dimer nanostructures is investigated using a dipole-dipole interaction model. Field enhancement spectra are evaluated as a function of the particle size difference and inter-particle spacing. We observe three distinct regimes of cascaded field enhancement: hindered cascading, multiplicative cascading, and the ultimate cascading limit, depending on the dimer interaction strength. Multiplicative cascading at small inter-particle spacing leads to analytic expressions for the ultimate internal and external field enhancement factors. For silver dimers in a host with index 1.5, we obtain a maximum internal field enhancement of 2.9 × 103, a factor of 75 larger than that of an isolated silver nanoparticle.

Heterogeneous plasmonic trimers for enhanced nonlinear optical absorption

A dramatic enhancement of the thermally induced nonlinear optical response in compositionally heterogeneous plasmonic trimers is reported. It is demonstrated numerically that the nonlinear absorption performance of silver nanoparticle dimers under pulsed illumination can be enhanced by more than two orders of magnitude through the addition of only 0.1 vol. % of gold in the dimer gap. The nonlinear absorption performance of the resulting Ag-Au-Ag trimer exceeds the peak performance of isolated gold nanoparticles by a factor 40. This dramatic effect is enabled by cascaded plasmon resonance, resulting in extreme field concentration in the central nanoparticle of the trimer. The observed localized heat-generation, large optical response, and a predicted response time below 1 ns make these structures promising candidates for use in nonlinear optical limiting and optical switching.

Second-harmonic generation in single-mode integrated waveguides based on mode-shape modulation

Second-harmonic generation is demonstrated using grating-assisted quasi-phase matching, based on waveguide-width modulation or mode-shape modulation. Applicable to any thin-film integrated second-order nonlinear waveguide, the technique is demonstrated in compact lithium niobate ridge waveguides. Fabricated devices are characterized with pulsed-pumping in the near-infrared, showing second-harmonic generation at a signal wavelength of 784 nm and propagation loss of 1 dB/cm.

Cascaded field enhancement in plasmon resonant dimer nanoantennas compatible with two-dimensional nanofabrication methods

Cascaded field enhancement is demonstrated in asymmetric plasmon resonant dimer nanoantennas consisting of shape-tuned ellipsoidal nanoparticles. The nanoparticles that make up the dimer have identical thickness, suggesting that the presented approach can be used to design cascaded dimer antennas compatible with standard two-dimensional top-down nanofabrication tools such as electron beam lithography and nano-imprint lithography. Cascaded excitation is achieved by modification of the in-plane particle aspect ratios in a way that keeps the resonance frequency of the individual particles fixed while significantly changing their polarizability. The achievable field enhancement is evaluated as a function of the particle volume ratio and spacing.

A Model of Lasing Action in a Quasi-Four-Level Thin Active Media

A self-consistent numerical model has been developed for simulating lasing properties of a typical thin-disk laser in detail. The temperature-dependent form of the Boltzmann occupation factors, absorption and stimulated-emission cross sections, and thermal conductivity of the Yb:YAG crystal as a quasi-four-level atomic system have been utilized for obtaining various effective operating variables. A Monte Carlo ray-tracing-based code and 2-D finite-element analysis (FEA) with the ANSYS package have been employed to calculate the absorption power and temperature distribution inside the crystal, respectively. Rate equations have also been included in order to obtain other lasing properties. These equations predict that characteristics of the laser are affected by the Boltzmann occupation factors of the pump and the laser states simultaneously. Based on the results, optical pumping efficiency has been examined as a function of output coupler reflectivity, number of the pump beam passes, and temperature.

Cascaded plasmon resonances multi-material nanoparticle trimers for extreme field enhancement

Optical field enhancement in coupled plasmonic nanostructures has attracted significant attention because of field enhancement factors that significantly exceed those observed in isolated nanostructures. While previous studies demonstrated the existence of such cascaded field enhancement in coupled nanospheres with identical composition, this effect has not yet been studied in systems containing multiple materials. Here, we investigate the polarization-dependent optical response of multi-material trimer nanostructures composed of Au nanoparticles surrounded by two Ag nanoparticles as a function of nanoparticle size and inter-particle spacing. We observe field enhancement factors that are ten times larger than observed in isolated Au nanoparticles.

Design of cascaded plasmon resonances for ultrafast nonlinear optical switching

The optical properties of cascaded plasmon resonant metallic nanocomposites are investigated. Plasmon resonances and their related field distributions are numerically evaluated in two-dimensional arrays of spherical silver nanoparticles embedded in a dielectric host. The field distributions in structures with identical particle sizes indicate the presence of a largely dipolar particle response, with a small multipole resonance contribution at high frequency. However, in arrays consisting of particles with dissimilar sizes, an additional coupled mode appears in which the dipole moment in adjacent particles is found to be anti-parallel. For increasing size-dissimilarity a higher electric field enhancement is observed inside the small metal nanospheres, indicative of a cascaded field enhancement effect. This effect may be used to enhance the nonlinear optical response of an effective medium composed of particles with engineered size dispersion and particle placement.

Taking cascaded plasmonic field enhancement to the ultimate limit in silver nanoparticle dimers

Cascaded optical field enhancement in coupled plasmonic nanostructures has attracted significant attention because of field enhancement factors that dramatically exceed those observed in isolated nanostructures. While previous studies demonstrated the existence of cascaded enhancement, little work has been done to identify the requirements for achieving maximum field enhancement. Here, we investigate cascaded field enhancement in silver nanosphere dimers as a function of volume ratio and center-to-center separation, and show the requirements for achieving the ultimate cascading limit in nanoparticle dimers. We observe field enhancements that are a factor 75 larger than observed in isolated silver nanoparticles.

Numerical calculation of temperature-dependent absorption in end pumped Yb:YAG thin disk laser

In this paper, the dependence of absorption power on the temperature in an end pumped Yb:YAG quasi-three level thin disk laser is calculated. Here, we have used the temperature-dependent form of Boltzmann occupation factors, absorption cross section and thermal conductivity of the Yb:YAG crystal. A Monte Carlo ray tracing code developed by our group and a 2D finite element analysis (FEA) with the ANSYS package are used to calculate the absorption power and the temperature distribution inside the Yb:YAG thin disk laser respectively. An iterative method used for combination of these two methods to achieve the temperature-dependent absorption power.