Carola Moosmann

Linear and nonlinear optical characterization of aluminum nanoantennas.

We experimentally determine the order of multiphoton induced luminescence of aluminum nanoantennas fabricated on a nonconductive substrate using electron-beam lithography to be 2.11 (±0.10). Furthermore, we optically characterize these nanostructures via linear dark-field microscopy and nonlinear multiphoton laser excitation. We hereby observe different spectral response functions that can be seen as a splitting of peak positions when the antenna arm length is increased to Larm > 150 nm which has not yet been reported for aluminum nanostructures.

Gold nanoantenna resonance diagnostics via transversal particle plasmon luminescence

We perform two-photon excitation confocal experiments on coupled gold nanoantennas and observe time-integrated luminescence spectra that match plasmonic mode emission in the far-field. We show that the transversal particle plasmon mode can be excited, using excitation light that is cross-polarized with respect to the gold luminescence signal and therefore oriented along the long axis of the dipole gold antenna. We provide evidence for losses in polarization information from the excitation channel to the luminescence response due to the nature of the energy and momentum transfer. Finally, we map out the two-photon induced luminescence intensity profile for a fixed excitation wavelength λ and varying antenna arm length L.

Investigating the influences of the precise manufactured shape of dipole nanoantennas on their optical properties

Fabrication of small nanoantennas with high aspect ratios via electron beam lithography is at the current technical limit of nanofabrication and hence significant deviations from the intended shape of small nanobars occur. Via numerical simulations, we investigate the influence of geometrical variations of gap nanoantennas, having dimensions on the order of only a few tens of nanometers. We show that those deviations have a significant influence on the performance of such nanoantennas. In particular, their resonance wavelength as well as the magnitude of absorption and scattering cross section and the electric field distribution in the near field is strongly altered. Our findings are thus of importance for applications based on near field as well as those based on far field interactions with nanoantennas and have to be carefully and individually considered in both cases.

RC-Constant in Organic Photodiodes Comprising Electrodes With a Significant Sheet Resistance

Organic photodiodes provide prospects for the fabrication of arbitrarily shaped photodetectors. However, the enlarged detection area in conjunction with their minuscule absorber layer thickness increases the capacitance of these devices when compared with convential silicon photodiodes. The mandatory transparency of at least one electrode can, so far, only be provided by materials with significant sheet resistances. These factors lead to nonnegligible RC-constants where high frequency signal detection is ultimately RC-limited. In this letter, we devise a method to determine the effective RC-constant for an extended rectangular device comprising electrodes with a significant sheet resistance and show that it is up to 59% smaller than estimated from the geometric device dimensions.

Superresolution optical fluctuation imaging (SOFI) aided nanomanipulation of quantum dots using AFM for novel artificial arrangements of chemically functionalized colloidal quantum dots and plasmonic structures

For single photon experiments or research on novel hybrid structures consisting of several colloidal quantum dots (Qdots) and plasmonic nanoparticles both the precise localization and the optical behavior of the emitters need to be correlated. Therefore, the gap between the high spatial resolution topography information that provides detailed localization of single Qdots and the diffraction limited fluorescence image needs to be overcome. In this paper, we demonstrate the combination of atomic force microscopy (AFM) with wide-field fluorescence microscopy improved by superresolution optical fluctuation imaging (SOFI). With this approach the topography and the superresolution image can be overlaid with sub-diffraction precision. Consequently, we discriminate between single Qdots that are optically active and dark ones. Additionally, the optical time-dependent behavior of molecular emitters can be selectively investigated. This method is, furthermore, useful for an advanced manipulation and characterization toolbox of Qdots in general. In summary, our findings represent an easily adaptable, highly reproducible and comparatively cheap subdiffraction limit imaging method and they facilitate the efficient selection of bright Qdots in a standard lab environment for proof-of-principle nanostructures containing Qdots and for nanomanipulation experiments.

Sensitivity improvement in fluorescence‐based particle detection

Microfluidic flow cytometers are highly interesting candidates for biomedical point‐of‐care applications. However, the sensitivity, reliability, and throughput of these systems must be improved to provide the full functionality of established flow cytometric systems. One proposed method to improve fluorescence detection systems is to use spatial modulation techniques. We derive the noise‐related statistics and calculate the coefficient of variation for a detection system with and without spatial modulation. We measure the noise properties of a nonmodulated microfluidic fluorescence particle detection system and analyze the possible performance gains using spatial modulation.

Particle detection from spatially modulated fluorescence signals

Flow cytometry relies on the detection of cells selectively stained with fluorescence markers. Optically they can be detected as fluorescence particles. The use of microuidics offers a wide range of benefits over traditional flow cytometer designs but when replacing expensive components with inexpensive counterparts the sensitivity of the instrument suffers. To increase the sensitivity of the detection system, spatial modulation has been proposed. Spatial modulation is implemented via _ne pitched shadow masks close to the microuidic channel which generate a signal pattern when a fluorescent particle passes by. Using a _ne pitch and long total length for the pattern a high spatial resolution and long total exposure time are combined. Particle detection from local maxima is not directly possible with spatially modulated signals due to the jagged pulse shape. We compare the performance of different approaches for particle detection from local maxima. Matched filtering and the derivative of the correlation signal provide either a good peak-signal-to-noise ratio (PSNR) or a high spatial resolution. But both approaches suffer from low dynamic range due to side maxima. We derive the solution for a minimum- mean-square-error (MMSE) filter which transforms the modulated pulse shape into a target pulse shape with a single strong maximum. We investigate the performance of the MMSE filter and find that it provides tunable suppression of noise and side maxima along with a high spatial resolution. The use of the MMSE filter therefore is an ideal choice for particle detection from spatially modulated signals.

Fluorescence particle detection using microfluidics and planar optoelectronic elements

Detection of fluorescent particles is an integral part of flow cytometry for analysis of selectively stained cells. Established flow cytometer designs achieve great sensitivity and throughput but require bulky and expensive components which prohibit mass production of small single-use point-of-care devices. The use of a combination of innovative technologies such as roll-to-roll printed microuidics with integrated optoelectronic components such as printed organic light emitting diodes and printed organic photodiodes enables tremendous opportunities in cost reduction, miniaturization and new application areas. In order to harvest these benefits, the optical setup requires a redesign to eliminate the need for lenses, dichroic mirrors and lasers. We investigate the influence of geometric parameters on the performance of a thin planar design which uses a high power LED as planar light source and a PIN-photodiode as planar detector. Due to the lack of focusing optics and inferior optical filters, the device sensitivity is not yet on par with commercial state of the art flow cytometer setups. From noise measurements, electronic and optical considerations we deduce possible pathways of improving the device performance. We identify that the sensitivity is either limited by dark noise for very short apertures or by noise from background light for long apertures. We calculate the corresponding crossover length. For the device design we conclude that a low device thickness, low particle velocity and short aperture length are necessary to obtain optimal sensitivity.

Longitudinal and transversal optical antenna plasmon resonance spectra from two-photon laser excitation

We record two-photon luminescence (TPL) emission spectra for two-arm nanoantennas of width 20 nm, height 30 nm and variable arm length of 25 to 65 nm, with a 20 nm gap between the arms, fabricated on an indium tin oxide (ITO) covered glass substrate using electron beam lithography and gold evaporation. Using a single excitation laser wavelength of 810 nm for all characterization experiments, the TPL emission is identified as a plasmon relaxation. To record the spectra, single structures are placed in the laser focus by making use of a raster scanning piezo stage. An oil immersion objective lens (100 ×, NA 1.46) is used both for the excitation and detection channel. A single-photon-counting avalanche photodiode detects the plasmon emission intensity, the response spectrum is observed with an electron multiplying CCD camera. The TPL nature of the excitation process is verified by recording emission versus excitation power curves [1].