Stefan Fasold

Self-suspended micro-resonators patterned in Z-cut lithium niobate membranes

We report on self-suspended micro-resonators patterned in Z-cut lithium niobate on insulator substrates. The fabrication technique consists of two single steps, focused ion beam milling for the micro- and nano-structuring and subsequent SiO2 etching for the realization of thin self-suspended membranes. The fabrication process of a free-standing photonic crystal cavity and a suspended micro-disk is described and the linear and nonlinear optical properties of the micro-resonators are investigated at telecommunication wavelengths. The whispering gallery modes of the micro-disk are measured experimentally and compared to an analytical model. The fundamental transverse-electric polarized mode of the photonic crystal cavity is measured and compared to three dimensional finite difference time domain simulations. Second harmonic generation enhancement due to the field confinement in the cavity mode is demonstrated. These results are promising for the use of Z-cut lithium niobate self-suspended membranes as platforms for highly efficient miniaturized photonic devices for telecommunication applications.

Plasmonic heating with near infrared resonance nanodot arrays for multiplexing optofluidic applications

In this work, we show local laser-induced heating in fluids with gold nanodot arrays prepared by electron-beam lithography that cover resonances in the near infrared spectral range from 750 nm to 880 nm. We utilize two approaches to demonstrate thermal effects, solvent evaporation and flow stop, with a thermosensitive polymer solution. We show that with fluences as low as 4 μJ cm−2, significant heating of the nanostructures occurs in their immediate vicinity. We perform power and wavelength dependent measurements to determine the threshold of the thermal effects. Using wavelengths about 20 nm away from the plasmonic resonance peak, the heating drops drastically, and 30 to 40 nm away, there is mostly no thermal effect. Therefore, working close to the threshold laser power offers the possibility of multiplexed reactions or sensing without cross-talk even though a typical full width at half maximum of a plasmonic resonance spectrum can be as broad as 200 nm. Additionally, comparison with theoretical calculations of heat generation show good agreement with the experimentally determined threshold powers.

Disordered photonic metasurfaces for complex light field control (Conference Presentation)

Optical metasurface can provide control over wavefront, polarization and spectrum of light fields while having just nanoscale thickness, making them promising candidates for flat optical components. Most metasurfaces studied so far consist of two-dimensional subwavelength arrays of designed metallic or dielectric scatterers. Deviations from a periodic, ordered arrangement are usually associated with a deterioration of the optical properties. However, the introduction of controlled disorder also provides interesting opportunities to engineer the optical response of metasurfaces. For example, the introduction of disorder can decrease unwanted anisotropy in the optical response [1], it suppresses scattering into discrete diffraction orders, and it can enhance the metasurfaces’ channel capacity [2]. Here we investigate different types of disordered metasurfaces. We demonstrate that the introduction of rotational disorder at the unit-cell level enables the realization of chiral plasmonic metasurfaces supporting pure circular dichroism and circular birefringence. We show experimentally that the polarization eigenstates of these metasurfaces, which coincide with the fundamental right- and left-handed circular polarizations, do not depend on the wavelength over the spectral range of the metasurface resonances. Thereby, our metasurfaces mimic the behaviour of natural chiral media, while providing a stronger chiral response. Furthermore, we systematically investigate how the introduction of different types of positional disorder influences the complex transmittance spectra of Mie-resonant silicon metasurfaces, showing that disorder provides an independent degree of freedom for engineering their spatial and spectral dispersion. [1] S. S. Kruk et al., Phys. Rev. B 88, 201404(R) (2013). [2] D. Veksler et al., ACS Photonics 2, 661 (2015).

Emission enhancement from MoS 2 monolayers with silicon nanoantennas

Transition-metal-dichalcogenides (TMDs), which exhibit an indirect electronic band gap as bulk crystals, can become direct semiconductors in the monolayer phase [1]. Such monolayer TMDs show unique optical properties arising from the strong two-dimensional confinement of excitons as well as from the reduction in crystal symmetry. However, the strong mismatch in length scale between the sub-nanometer thickness of an atomically thin crystal sheet and the wavelength of propagating infrared or visible light leads to a rather weak light-matter interaction. By tailoring the near-field environment of monolayer TMDs, resonant optical antennas can strongly modify the excitation response [2]. While research efforts targeted at tailoring and enhancing light-matter interactions in monolayer TMDs have so far been limited to plasmonic nanoantennas, here we concentrate on high-index dielectric nanoantennas, which can show negligible intrinsic losses and thus a high radiation efficiency in the visible and near-infrared spectral range.

Disorder-enabled pure circular dichroism in bilayer plasmonic metasurfaces

In optical metasurfaces, disorder, e.g. in the arrangement or shape of its building blocks, is usually associated with a deterioration of the optical properties due to an increase of incoherent scattering. However, more recently, researchers have started recognizing the introduction of controlled disorder as a strategy to diversify engineering options of metasurfaces. For example, the introduction of disorder can be used to decrease unwanted anisotropy in their optical response [1] and enhance the information density encoded in wavefront shaping metasurfaces [2].

A Green's function based analytical method for forward and inverse modeling of quasi-periodic nanostructured surfaces

We present an efficient Greens function based analytical method for forward but particularly also for the inverse modeling of light scattering by quasi-periodic and aperiodic surface nanostructures. In the forward modeling, good agreement over an important texture amplitude range is achieved between the developed formalism and exact rigorous calculations on the one hand and angle resolved light scattering measurements of complex quasi-periodic SiO2-Au nanopatterned interfaces on the other hand. Exploiting our formalism, we demonstrate for the first time how the inverse problem of quasi-periodic surface textures for a desired multiresonant absorption response can be expressed in terms of coupled systems of multivariate polynomial equations of the height profiles Fourier amplitudes. A good estimate of the required surface profile can thus be obtained in a computationally cheap manner via solving the multivariate polynomial equations. In principle, the inverse modeling formalism introduced here can be impl...

Enhancement of light-matter interaction in MoS 2 monolayers by resonant nanoparticles

Transition metal dichalcogenides (TMDs) are class of materials, which have recently attracted a high degree of scientific attention. In these materials, the adjacent crystal layers are mutually attached only by weak Van-der-Waals forces, enabling the preparation of TMD monolayers with thicknesses below one nanometer. Interestingly, TMDs, which are indirect semiconductors in their bulk form, become direct semiconductors in the monolayer limit. These monolayers show pronounced excitonic emission lines and a strong second-order nonlinearity, making TMDs very interesting for a variety of optical applications.

Diffractive optical elements made from photonic metamaterials

The excitation of surface plasmon polaritons in metallic nanostructures significantly enhances light-matter interactions at sub-wavelength scales. This enables novel optical effects that rely on artificial materials, so-called metamaterials. Metamaterials are made from densely packed and sufficiently small nanostructured unit cells. The purpose of metamaterials is to act comparable to bulk materials but with effective properties that can be tailored by the geometry of the unit cells. However, recently it became obvious that many appealing applications are already in reach by employing an ultrathin layer of metamaterial. Exploiting the metamaterials’ primary optical properties in the form of their dispersive complex reflection and transmission coefficients, single functional layers instead of bulk metamaterials are already sufficient for achieving sophisticated device properties. In particular, if the unit cells change across the functional layer, a new class of devices can be perceived that shape the light in the far-field according to predefined patterns. These metamaterial layers, usually referred to as metasurfaces, possess major advantages when compared to traditional optical elements. Here, we provide an overview of the burgeoning field of research that explores metasurfaces to affect an incident field spatially and spectrally in a deterministic way to enable functional diffractive optical elements.

Relaxation time mapping of single quantum dots and substrate background fluorescence

We experimentally investigated the role of background signal in time resolved photoluminescence experiments with single quantum dots on substrates. We show that the background fluorescence signal from thin gold films fabricated by electron-beam evaporation and from Al2O3 layers fabricated by atomic layer deposition have to be taken into consideration in experiments on the single photon level. Though all investigated components can be distinguished by their photoluminescence decay rates, the presence of the background signal prevents the observation of photon antibunching from single quantum dots. Moreover, a single quantum dot acts as a hot spot enabling the plasmon supported fluorescence enhancement of gold.