Sönke Klinkhammer

Plastic lab-on-a-chip for fluorescence excitation with integrated organic semiconductor lasers

Laser light excitation of fluorescent markers offers highly sensitive and specific analysis for bio-medical or chemical analysis. To profit from these advantages for applications in the field or at the point-of-care, a plastic lab-on-a-chip with integrated organic semiconductor lasers is presented here. First order distributed feedback lasers based on the organic semiconductor tris(8-hydroxyquinoline) aluminum (Alq3) doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyril)-4H-pyrane (DCM), deep ultraviolet induced waveguides, and a nanostructured microfluidic channel are integrated into a poly(methyl methacrylate) (PMMA) substrate. A simple and parallel fabrication process is used comprising thermal imprint, DUV exposure, evaporation of the laser material, and sealing by thermal bonding. The excitation of two fluorescent marker model systems including labeled antibodies with light emitted by integrated lasers is demonstrated.

Continuously tunable solution-processed organic semiconductor DFB lasers pumped by laser diode

The fabrication and characterization of continuously tunable, solution-processed distributed feedback (DFB) lasers in the visible regime is reported. Continuous thin film thickness gradients were achieved by means of horizontal dipping of several conjugated polymer and blended small molecule solutions on cm-scale surface gratings of different periods. We report optically pumped continuously tunable laser emission of 13 nm in the blue, 16 nm in the green and 19 nm in the red spectral region on a single chip respectively. Tuning behavior can be described with the Bragg-equation and the measured thickness profile. The laser threshold is low enough that inexpensive laser diodes can be used as pump sources.

Voltage-controlled tuning of an organic semiconductor distributed feedback laser using liquid crystals

Voltage-controlled continuous tuning of the laser wavelength of an organic distributed feedback laser is demonstrated by incorporation of liquid crystals (LCs) in the top cladding layer. Orientation of the LCs and hence the modal refractive index are controlled by applying a lateral electrical field. Laser emission shifts by 4 nm at an applied voltage of 675 V. The device showed lasing thresholds of about 286 nJ per pulse. The tuning behaviour is analyzed by implementation of a voltage-dependent spatial LC director orientation profile in a slab waveguide model and solving the Bragg condition to obtain the voltage-dependent lasing wavelength.

Ink-Jet-Printed Organic Semiconductor Distributed Feedback Laser

Ink-jet-printed organic distributed feedback (DFB) lasers are realized by employing light-emitting copolymer and suitable organic solvents to meet the demands of printability and optical amplification in a nanopatterned conjugated polymer slab waveguide. We demonstrate the accurate lateral positioning of ink-jet-printed patches of the gain material on a polymer substrate with 500×500 µm2 grating areas. We also printed patches of large lateral dimension of 6 mm2 on a silica grating. The high uniformity of the film thickness leads to a laser wavelength variation of less than 3 nm over the whole area.

Strongly confined, low-threshold laser modes in organic semiconductor microgoblets

We investigate lasing from high-Q, polymeric goblet-type microcavities covered by an organic semiconductor gain layer. We analyze the optical modes in the high-Q cavities using finite element simulations and present a numerical method to determine the cutoff thickness of the gain layer above which the whispering gallery modes are strongly confined in this layer. Fabricated devices show reduced lasing thresholds for increasing gain layer thicknesses, which can be explained by a higher filling factor of the optical modes in the gain layer. Furthermore, reduced lasing threshold is accompanied by a red-shift of the laser emission.

Optical spectroscopy with organic semiconductor lasers

We report on the fabrication of large-scale surface gratings by laser interference lithography and reactive ion etching on which we evaporated a thin film of the organic semiconductor tris(8-hydroxyquinoline) aluminum doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyril)-4H-pyrane. We created a thickness gradient by using a rotating shadow mask evaporation technique. This allowed us to continuously tune the emission wavelength from 606 nm to 661 nm on a single substrate. After encapsulation, we demonstrated the usefulness of such low-cost and tunable organic semiconductor lasers by conducting simple fluorescence excitation and transmission spectroscopy measurements using a minimal amount of additional optical components.

Nanograting transfer for light extraction in organic light-emitting devices

Damage-free transfer of nanogratings as waveguide mode extraction elements onto the top of top-emission organic light-emitting devices(OLEDs) was realized with the assistance of cyclic olefin copolymer molds. Photoluminescence and electroluminescence measurements together with transfer matrix calculations were used to identify the extracted waveguide modes. The presented one-step, wet-free, and etchless nanotransfer approach can avoid damage on the top-emission OLEDstructure and does not adversely affect the OLED performance. It offers the possibility for the independent optimization of OLEDfabrication and nanofabrication, and has potential applications in organic optoelectronic devices, especially top-emission OLEDs.

Integration of organic semiconductor lasers and waveguides into PMMA based microfluidic lab-on-a-chip systems

We present poly(methyl methacrylate) (PMMA) based microfluidic systems for biophotonic sensing applications. The fabrication of the devices is based on thermal nanoimprinting and thermal bonding of polymer substrates. We integrate and encapsulate distributed feedback (DFB) organic semiconductor lasers based on tris(8-hydroxyquinoline) aluminium (Alq3) doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyril)-4H-pyrane (DCM). Deep ultraviolet (DUV) lithography is used to induce waveguides into the PMMA, guiding the laser light to embedded microfluidic channels. Light of the integrated lasers is efficiently coupled into the waveguides by introducing a topographical step.

Integrated photonic lab-on-chip systems for biomedical applications

Currently, one may find a wide variety of approaches for integrated lab-on-chip systems developed for applications in the biomedical field. Our contributions within the area of polymer based photonic systems are presented here. We are utilizing mass production techniques and head for lab-on-a-chip systems with solely optical and fluidic interfaces, avoiding electrical interconnects. Fluidic structures are implemented in the chips mainly by using the same technologies, which are chosen to create the optical elements. While photonic structures may require dimensions in the sub-100 nm range, microfluidic channels are more than one order of magnitude above this regime. Nevertheless, our approach allows for a limited number of process steps by simultaneous multiscale fabrication. Organic semiconductor lasers are generated by evaporating a thin film of photoactive material on top of a distributed feedback (DFB) grating. Gratings are replicated by hot embossing into poly(methyl methacrylate) (PMMA) bulk material. The lasing wavelength in the visible light regime of the on-chip lasers is selected by altering the thickness of the vacuum deposited organic semiconductor active material or the DFB grating period. Waveguides are monolithically integrated in PMMA via photodegradation through deep ultraviolet irradiation. The coupling of laser light into these waveguides is optimized. Hence, laser light is guided to an interaction zone with a biological sample in the microfluidic channel on chip. Micro-optical cavities are designed and processed to be functionalized for detecting biological binding events in the channel. Surface functionalization, e.g. by Dip-Pen Nanolithography, is carried out for integrated label-free detection as well as for fluorescence excitation.

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.