Tobias Wienhold

On-chip microlasers for biomolecular detection via highly localized deposition of a multifunctional phospholipid ink

We report on a novel approach to realize on-chip microlasers, by applying highly localized and material-saving surface functionalization of passive photonic whispering gallery mode microresonators. We apply dip-pen nanolithography on a true three-dimensional structure. We coat solely the light-guiding circumference of pre-fabricated poly(methyl methacrylate) resonators with a multifunctional molecular ink. The functionalization is performed in one single fabrication step and simultaneously provides optical gain as well as molecular binding selectivity. This allows for a direct and flexible realization of on-chip microlasers, which can be utilized as biosensors in optofluidic lab-on-a-chip applications. In a proof-of-concept we show how this highly localized molecule deposition suffices for low-threshold lasing in air and water, and demonstrate the capability of the ink-lasers as biosensors in a biotin-streptavidin binding experiment.

Diffusion driven optofluidic dye lasers encapsulated into polymer chips

Lab-on-a-chip systems made of polymers are promising for the integration of active optical elements, enabling e.g. on-chip excitation of fluorescent markers or spectroscopy. In this work we present diffusion operation of tunable optofluidic dye lasers in a polymer foil. We demonstrate that these first order distributed feedback lasers can be operated for more than 90 min at a pulse repetition rate of 2 Hz without fluidic pumping. Ultra-high output pulse energies of more than 10 μJ and laser thresholds of 2 μJ are achieved for resonator lengths of 3 mm. By introducing comparatively large on-chip dye solution reservoirs, the required exchange of dye molecules is accomplished solely by diffusion. Polymer chips the size of a microscope cover slip (18 × 18 mm(2)) were fabricated in batches on a wafer using a commercially available polymer (TOPAS(®) Cyclic Olefin Copolymer). Thermal imprinting of micro- and nanoscale structures into 100 μm foils simultaneously defines photonic resonators, liquid-core waveguides, and fluidic reservoirs. Subsequently, the fluidic structures are sealed with another 220 μm foil by thermal bonding. Tunability of laser output wavelengths over a spectral range of 24 nm on a single chip is accomplished by varying the laser grating period in steps of 2 nm. Low-cost manufacturing suitable for mass production, wide laser tunability, ultra-high output pulse energies, and long operation times without external fluidic pumping make these on-chip lasers suitable for a wide range of lab-on-a-chip applications, e.g. on-chip spectroscopy, biosensing, excitation of fluorescent markers, or surface enhanced Raman spectroscopy (SERS).

Densely Packed Microgoblet Laser Pairs for Cross-Referenced Biomolecular Detection

Microgoblet laser pairs are presented for cross‐referenced on‐chip biomolecular sensing. Parallel readout of the microlasers facilitates effective mutual filtering of highly localized refractive index and temperature fluctuations in the analyte. Cross‐referenced detection of two different types of proteins and complete chemical transducer reconfiguration is demonstrated. Selective surface functionalization of the individual lasers with high spatial accuracy is achieved by aligned microcontact stamping.

Pump spot size dependent lasing threshold in organic semiconductor DFB lasers fabricated via nanograting transfer

Optically excited organic semiconductor distributed feedback (DFB) lasers enable efficient lasing in the visible spectrum. Here, we report on the rapid and parallel fabrication of DFB lasers via transferring a nanograting structure from a flexible mold onto an unstructured film of the organic gain material. This geometrically well-defined structure allows for a systematic investigation of the laser threshold behavior. The laser thresholds for these devices show a strong dependence on the pump spot diameter. This experimental finding is in good qualitative agreement with calculations based on coupled-wave theory. With further investigations on various DFB laser geometries prepared by different routes and based on different organic gain materials, we found that these findings are quite general. This is important for the comparison of threshold values of various devices characterized under different excitation areas.

Efficient free-space read-out of WGM lasers using circular micromirrors.

We report on efficient free-space read-out of whispering-gallery mode microlasers using circular micromirrors. A ten-fold improvement in detection efficiency can be achieved by directing emission from all azimuthal angles of the cavity to the detector.

High-Q polymer resonators with spatially controlled photo-functionalization for biosensing applications

We demonstrate the applicability of polymeric whispering gallery mode resonators fabricated on silicon as biosensors. Optical measurements on the passive resonators in the visible spectral range yield Q-factors as high as 1.3×107. Local, covalent surface functionalization, is achieved by spatially controlled UV-exposure of a derivative of the photoreactive crosslinker benzophenone. Protein detection is shown using the specific binding of the biotin-streptavidin system.

Microoptical device for efficient read-out of active WGM resonators

Whispering-gallery mode (WGM) resonators are known to offer outstanding properties for applications in photonics and telecommunication. Despite their promising performance, one major obstacle for the use of WGM resonators in industrial products is the need of expensive components and high-precision setups for their operation, requiring a controlled lab environment. For industrial applications technically simpler and more robust realizations are desired. Active WGM resonators utilize an optical gain medium for light amplification within the resonator and may be operated as lasers. They offer several advantages over their passive counterparts, such as cheap pump sources, free space excitation of resonator modes, and potentially narrower line widths. However, collection of the light emitted from the resonator still bears several challenges. Emission occurs in plane of the resonator and radiation is emitted isotropically along the circumference. Thus, detectors positioned in plane of the resonator may collect only a limited angular segment of the resonators light emission. We report on a microoptical device which is integrated on the resonator chip and redirects all in-plane emission of active WGM resonators into a defined off-plane direction. Redirected light can easily be collected using a standard detector. Contrary to other approaches our microoptical device does not decrease the quality factor (Q factor) of the resonator. As light from all angular segments of the resonator is collected, the detected signal-to-noise ratio is expected to be largely improved. Our microoptical device therefore offers a promising approach towards mass-producible integration of active WGM resonators, e. g. into a Lab-on-a-Chip, for sensor applications, where smallest possible frequency shifts need to be read out by a highly sensitive detector.

Integrated dye lasers for all-polymer photonic Lab-on-a-Chip systems

Basierend auf integrierten Farbstofflasern wurden zwei optische Lab-on-a-Chip Systeme entwickelt. Zur effizienten Anregung von Fluoreszenzmarkern wurden optofluidische Farbstofflaser mit verteilter Ruckkopplung (DFB Laser) untersucht. Fur die markerfreie Molekuldetektion wurden Mikrokelchlaser entwickelt, die auf Flustergaleriemoden basieren. Besonderes Augenmerk lag auf einer moglichen Grosserienfertigung der Chips als kostengunstige Einwegartikel und auf einer einfachen Handhabung.

Vertically stacked all-polymer whispering-gallery mode lasers for biosensing applications

We report on the fabrication of polymeric whispering-gallery mode (WGM) lasers. Our approach enables high packing density by vertical stacking and multiplexed readout of resonators and lends itself to signal referencing or multi-target sensing.

Free-space read-out of WGM lasers using circular micromirrors

We report on efficient free-space read-out of whispering-gallery mode microlasers using circular micromirrors. A ten-fold improvement in detection efficiency can be achieved by directing emission from all azimuthal angles of the cavity to the detector.