Carsten Eschenbaum

Low-threshold conical microcavity dye lasers

We report on lasing in rhodamine 6G-doped, conical polymeric microcavities with high quality factors fabricated on a silicon substrate. Threshold pump energies as low as 3 nJ are achieved by free-space excitation in the quasistationary pumping regime with lasing wavelengths around 600 nm. Finite element simulations confirm that lasing occurs in whispering gallery modes which corresponds well to the measured multimode laser-emission. The effect of dye concentration on lasing threshold and lasing wavelength is investigated and can be explained using a standard dye laser model.

White organic light emitting diodes with enhanced internal and external outcoupling for ultra-efficient light extraction and Lambertian emission

White organic light emitting diodes (WOLEDs) suffer from poor outcoupling efficiencies. The use of Bragg-gratings to enhance the outcoupling efficiency is very promising for light extraction in OLEDs, but such periodic structures can lead to angular or spectral dependencies in the devices. Here we present a method which combines highly efficient outcoupling by a TiO(2)-Bragg-grating leading to a 104% efficiency enhancement and an additional high quality microlens diffusor at the substrate/air interface. With the addition of this diffusor, we achieved not only a uniform white emission, but also further increased the already improved device efficiency by another 94% leading to an overall enhancement factor of about 4.

Direct fabrication of PDMS waveguides via low-cost DUV irradiation for optical sensing

We demonstrate the fabrication of single mode optical waveguides by irradiating polydimethylsiloxane (PDMS) with a low cost Hg lamp through a conventional quartz mask. By increasing the refractive index of the irradiated areas, waveguiding is achieved with an attenuation of 0.47 dB/cm at a wavelength of 635 nm. The refractive index change is stable in ambient air and water for time periods of more than 3 months. The excitation of water-dispersed fluorescent nanoparticles in the evanescent field of the waveguide is demonstrated.

Hybrid lithography: combining UV-exposure and two photon direct laser writing.

We demonstrate a method for the combination of UV-lithography and direct laser writing using two-photon polymerization (2PP-DLW). First a dye doped photoresist is used for UV-lithography. Adding an undoped photoresist on top of the developed structures enables three-dimensional alignment of the 2PP-DLW structures by detecting the spatially varying fluorescence of the two photoresists. Using this approach we show three dimensional alignment by adding 3D structures made by 2PP-DLW to a previously UV-exposed structure. Furthermore, a fluidic system with an integrated total internal reflection mirror to observe particles in a microfluidic channel is demonstrated.

A Biphasic Mercury‐Ion Sensor: Exploiting Microfluidics to Make Simple Anilines Competitive Ligands

Combining the molecular wire effect with a biphasic sensing approach (analyte in water, sensor-dye in 2-methyltetrahydrofuran) and a microfluidic flow setup leads to the construction of a mercury-sensitive module. We so instantaneously detect Hg(2+) ions in water at a 500 μM concentration. The sensor, conjugated non-water soluble polymer 1 (XFPF), merely supports dibutylaniline substituents as binding units. Yet, selective and sensitive detection of Hg(2+) -ions is achieved in water. The enhancement in sensory response, when comparing the reference compound 2 to that of 1 in a biphasic system in a microfluidic chip is >10(3) . By manipulation of the structure of 1, further powerful sensor systems should be easily achieved.

Lab-on-Chip, Surface-Enhanced Raman Analysis by Aerosol Jet Printing and Roll-to-Roll Hot Embossing

Surface-enhanced Raman spectroscopy (SERS) combines the high specificity of Raman scattering with high sensitivity due to an enhancement of the electromagnetic field by metallic nanostructures. However, the tyical fabrication methods of SERS substrates suffer from low throughput and therefore high costs. Furthermore, point-of-care applications require the investigation of liquid solutions and thus the integration of the SERS substrate in a microfluidic chip. We present a roll-to-roll fabrication approach for microfluidics with integrated, highly efficient, surface-enhanced Raman scattering structures. Microfluidic channels are formed using roll-to-roll hot embossing in polystyrene foil. Aerosol jet printing of a gold nanoparticle ink is utilized to manufacture highly efficient, homogeneous, and reproducible SERS structures. The modified channels are sealed with a solvent-free, roll-to-roll, thermal bonding process. In continuous flow measurements, these chips overcome time-consuming incubation protocols and the poor reproducibility of SERS experiments often caused by inhomogeneous drying of the analyte. In the present study, we explore the influence of the printing process on the homogeneity and the enhancement of the SERS structures. The feasibility of aerosol-jet-modified microfluidic channels for highly sensitive SERS detection is demonstrated by using solutions with different concentrations of Rhodamine 6G and adenosine. The printed areas provide homogeneous enhancement factors of ~4 × 106. Our work shows a way towards the low-cost production of tailor-made, SERS-enabled, label-free, lab-on- chip systems for bioanalysis.

Novel nano- and micro-textures for highly efficient outcoupling in white organic light emitting diodes

We demonstrate two approaches to significantly enhance the outcoupling in organic light emitting diodes. Nanostructures in combination with a microlens array or spherical texturing lead to efficiency enhancement factors of ~4 and ~3.7, respectively.

Lasing in dye-doped high-Q conical polymeric microcavities

We report on lasing in conical microcavities, which are made out of the low-loss polymer poly (methyl methacrylate) (PMMA) doped with the dye rhodamine 6G, and directly fabricated on silicon. Including a thermal reflow step during fabrication enables a significantly reduced surface roughness, resulting in low scattering losses of the whispering gallery modes (WGMs). The high cavity quality factors (above 2·106 in passive cavities) in combination with the large oscillator strength gain material enable lasing threshold energies as low as 3 nJ, achieved by free-space excitation in the quasistationary pumping regime. Lasing wavelengths are detected in the visible wavelength region around 600 nm. Finite element simulations indicate that lasing occurs in fundamental TE/TM cavity modes, as these modes have - in comparison to higher order cavity modes - the smallest mode volume and the largest overlap with the gain material. In addition, we investigate the effect of dye concentration on lasing wavelength and threshold by comparing samples with four different concentrations of rhodamine 6G. Observations are explained by modifying the standard dye laser model.

Roll-to-roll production of a microfluidic platform and its functionalization by means of digital printing technologies for gas and fluid sensors (Conference Presentation)

The individualized functionalization of mass-produced microstructures is still challenging for the process technology. Here, a rroll-to-roll based process hot embossing is presented for the production of microfluidic structures by means of hot embossing is presented. The resulting microfluidic channels are functionalized modified with different materials. Thereby, digital printing technologies such as aAerosoljet or inkjet are used. This approach allows for mass production of microfluidic channels and their the individualized individual functionalizationfunctionalization of mass produced microfluidic channels. The encapsulation of the channels also takes placeis realized in an R2R-based thermal bonding process without adding any solvent or adhesive. Taking account ofUsing this approach, several sensor systems for gas and / or fluid detection could be demonstrated. Surface -eEnhanced Raman Scattering scattering (SERS) with amplification enhancement factors of up to 107 [1] is demonstrated by printing gold nanoparticles into the microfluidic channel. We evaluate the printed SERS structures using solutions of rhodamine 6G and adenosine as exemplary analytes. Furthermore, these channels could be functionalized with different fluorescent organic semiconductors. Their fluorescence intensity is quenched in the presence of a nitroaromatic compounds. By using different materials simultaneously, we are able to measure a fingerprint like pattern of different analytes, which we evaluated by means ofusing pattern recognition algorithms. This method can be used both in the gas phase (electronic nose) and in fluids (electronic tongue) for the detection of nitroaromatic compounds [2,3]. With the opto-electronic nose, we were able to reach detections limits below 1ppb. [1] A. Habermehl et al, Sensors 17, 2401 (2017). [2] N. Bolse et al, Flexible and Printed Electronics 2, 024001 (2017) [3] N. Bolse et al, ACS Omega 2 (10), 6500-6505 (2017)

Comparing roll-to-roll and laser-assisted hot embossing for micro- and nanofabrication

We demonstrate the suitability of two cost efficient technologies, namely roll-to-roll hot embossing and laser-assisted hot embossing, to fabricate arrays of structures in the microscale down to the sub-100 nm range. We therefore employ polymers with a relatively moderate glass transition temperature, e.g., cyclic olefin copolymer (COC) and polystyrene (PS). We compare the two replication processes regarding their precision and cost using different 1D and 2D nanostructure gratings and microfluidic channels. All nickel shims used for the replication are fabricated in combination of electron beam or UV lithography and nickel electroforming. The replicated structures are used in different applications. The nanopillar arrays are coated with gold and integrated in the hot embossed microfluidic channels for lab-on-a-chip (LoC) surface-enhanced Raman analysis. We evaluate the as-fabricated 2D nanopillar arrays for surface-enhanced Raman spectroscopy (SERS) using solutions of rhodamine 6G as exemplary analytes. The influence of the geometrical parameters like diameter and pitch of the polymer structures as well as the influence of the gold layer thickness are discussed. 1D-gratings will be used as resonators for organic distributed feedback (DFB) lasers. Both elements, the SERS chips and the organic DFB lasers as tunable excitation source can be combined in the future to form one Raman-on-Chip optofluidic platform for sensitive detection of low-concentrated analytes in water.