Susan Derenko

Mechanical tuning of plasmon resonances in elastic, two-dimensional gold-nanorod arrays

Two-dimensional gold-nanorod arrays (2D-GNA) exhibit distinct resonance peaks in the visible wavelength range that are clearly associated with long and short axis plasmon oscillations. In this paper, we demonstrate a flexible and reproducible way for controlling the plasmon resonance of such 2D-GNAs in-situ, even post-fabrication process, simply by embedding free-standing nanorod arrays into an elastomer thin film. Stretching the polymer film shows the plasmon long-axis resonance to red-shift proportionally to the applied force by as much as 20~nm by increasing the center-to-center distance between individual nanorods. Releasing the load elastically relaxes the stretched polymer film, hence allowing the recording of cyclic load curves while varying the spectral response in-situ. Notably, film stretching along the substrate plane (x-axis) results in a uniaxial distortion of the nanorod lattice. We show how to account for this anisotropic strain in both the experiment and our complementary finite element modelling simulations, which then both match very well. This novel work illustrates both the feasibility and reliability when integrating 2D-GNAs for potential flexible, plasmonic applications.

Local photochemical plasmon mode tuning in metal nanoparticle arrays

We report on the local modification of gold nanoparticle arrays by photochemical deposition of gold from solution. Our method allows to alter the localized surface plasmon resonance (LSPR) in a restricted area by exposure of gold salt (HAuCl4) to light, whereas the expansion of such sections depends on the illumination optics. The geometry parameters of the individual nanoparticles in the modified regions are characterized by SEM and AFM, while the optical properties of distinct array sections are analyzed by means of optical spectroscopy. A blueshift of the surface plasmon resonance wavelength is observed upon the deposition process. An explanation for the blueshift is found by performing calculations using an analytical dipolar interaction model (DIM), which allows us to distinguish the individual contributions of the particle geometry on the one hand and the changes in particle interaction on the other hand. The resulting simulated scattering spectra verify the blueshift of the LSPR, which can be attributed to an increase in aspect ratio of the particles during growth. Since plasmonically active nanoparticle arrays are known to be candidates for sensing applications, this method and the gained understanding can be exploited to fabricate large sensor substrates with local LSPR variations.

Innovative optic sensing concepts towards applications in biotechnology and medicine (presentation video)

The Fraunhofer Institute for Ceramic Technologies and Systems, Branch Materials Diagnostics (IKTS-MD) covers also some fields of biosensing and nanotechnology, from basic research towards applications. This talk will especially address optically based methods for sensing applications: starting from analysis of the fractal dimension of time-resolved auto-fluorescence spectroscopy, to time-resolved luminescence measurements on upconversion phosphors for electron beam monitoring and last a refractive index sensing with a CCD chip technology based on localized SPR sensing. For all discussed methods the possible application will be discussed on examples of demonstrators in the fields of cancer diagnostics, medical surface sterilization process and biosensing.

Plasmonic gradient structures of nanoparticle arrays for optical sensing applications

Driven by a demand for integrated optical sensors in structural health and environmental monitoring we present the application of plasmonic gradient structures as sensor substrate. Therefore, nanoparticle arrays of gold are fabricated by interference lithography, which exhibit localized surface plasmons (LSPs). The plasmonic properties of such nanoparticles can be tuned by altering their size. In our approach, an additional photochemical growth by exposure to HAuCl4 and light is used to manufacture gradients of nanoparticle sizes within the array. These gradients in turn induce different spectral responses depending on the illuminated region of the array gradient. To enable sensing applications, such plasmonic gradient structures are placed as a filter in front of a photodetector to allow detection of transmitted optical signals from different locations of the array. Different applications can be envisioned in this configuration: On the one hand, sensing of wavelength shifts of the illuminating light source can be enabled by comparing the photocurrents generated in adjacent sensor elements. Additionally the application of refractive index measurements is demonstrated with the same detector configuration. The change in extinction of the illuminating light at different wavelengths can be used to obtain an intensity shift at the detector elements. This shift correlates to the change of the spectral resonance conditions in the array gradient upon change of refractive index.