Martin Punke

Dynamic characterization of organic bulk heterojunction photodetectors

The authors report the dynamic properties of bulk heterojunction photodiodes based on a polymer blend system consisting of poly(3-hexylthiophene-2,5-diyl) and the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester. Devices with a high-frequency contact layout were analyzed under continuous wave and pulsed laser illumination (λ=532nm). The organic photodiodes exhibit a pulse response with a full width at half maximum of 11ns to the applied 1.6-ns-long laser pulses. Rise times as small as 1.6ns and fall times <40ns were measured under applied reverse bias.

Nanosecond response of organic solar cells and photodetectors

We examine the impact of various parameters on the transient current density characteristics of organic solar cells and photodetectors by means of numerical simulations. Our self-consistent numerical model treats the dynamics of generated electrons and holes in the framework of a drift-diffusion model. As input parameter for the electric model, the intensity distribution of the incident light is calculated with a transfer-matrix method accounting for interference effects. The results are compared to experimental results. With our approach, we are able to distinguish the influence of different physical effects as they become dominant at different current densities or at different time regimes. This enables us to estimate the electron and hole mobilities separately by fitting the experimental results. Furthermore, space charge effects are identified as being highly important for the transient response of photodetectors.

Laser Diode-Pumped Organic Semiconductor Lasers Utilizing Two-Dimensional Photonic Crystal Resonators

Two-dimensional photonic crystal lasers based on the small molecule organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq 3) doped with 4-Dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) are optically pumped with a conventional low-cost pulsed (In)GaN laser diode. We compare photonic crystal resonators providing first- and second-order distributed feedback and find threshold values of 1.9 and 3.2kW/cm2, respectively. Such inorganic-organic hybrid laser systems open up a way to inexpensive, tunable, and all solid-state lasers in the full visible wavelength range

Optical Data Link Employing Organic Light-Emitting Diodes and Organic Photodiodes as Optoelectronic Components

An optical interconnect solely using organic optoelectronic components is presented. The data link is based on an organic light-emitting diode as the transmitter and an organic photodiode as the receiver. Light is transmitted via a polymer optical fiber coupled to the active components. A digitized audio signal based on the Sony/Philips Digital Interface Format standard at a signal bit rate of 2.8224 Mbit/s (44.1-kHz sampling frequency) is successfully transmitted.

Nanostructuring of organic-inorganic hybrid materials for distributed feedback laser resonators by two-photon polymerization

With two-photon absorption induced polymerization arbitrary three dimensional nano- and microstructures can be patterned directly into photoresists. We report on the fabrication of a low threshold organic semiconductor distributed feedback laser using the technique of two-photon absorption induced polymerization. A surface grating with 400 nm periodicity and 40 nm height modulation was fabricated by two-photon absorption induced polymerization in the organic-inorganic hybrid material ORMOCER. With structuring several stacked layers acting as a planar basis for the nanostructure microscopic substrate tilt can be compensated simply. This enabled us to uniformly nano-structure the surface grating over an area of 200 x 200 microm(2).

Coupling of Organic Semiconductor Amplified Spontaneous Emission Into Polymeric Single-Mode Waveguides Patterned by Deep-UV Irradiation

Single-mode waveguides were fabricated by deep ultraviolet radiation in poly(methyl methacrylate) (PMMA). Using a masking process, the radiation modifies the refractive index of the PMMA forming core and cladding regions for waveguiding. Following the fabrication of the waveguides, the small molecule material aluminum tris(8-hydroxyquinoline) doped with the laser dye DCM is deposited directly onto the waveguide structures. By optical pumping (lambda=355 nm) amplified spontaneous emission was observed at the end facets of the waveguides

Organic semiconductor lasers as integrated light sources for optical sensor systems

We demonstrate the feasibility of organic semiconductor lasers as light sources for lab-on-a-chip systems. These lasers are based on a 1D- or 2D-photonic crystal resonator structure providing optical feedback in the active laser material that is deposited on top, e.g. aluminum tris(8-hydroxyquinoline) (Alq3) doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). We investigated different fabrication methods for the resonator structures, like thermal nanoimprint, UV nanoimprint, and laser interference lithography. Different substrate materials commonly used in lab-on-a-chip systems, e.g. PMMA, Topas, and Ormocer were deployed. By changing the distributed feedback grating periodicity, we demonstrate a tuning range for a single material system of more than 120 nm. The investigated organic semiconductor lasers are optically pumped. External optical pumping provides a feasible way for one-time-use chips. Our recent success of pumping organic lasers with a low-cost laser diode also renders hand-held systems possible. As a further step towards the integration of organic lasers in sensor systems, we demonstrate the coupling of an organic laser into polymeric waveguides which can be combined with microfluidic channels. The integrated organic lasers and the waveguides are both fabricated on the same polished PMMA substrate using thermal nanoimprint lithography and deep-UV modification, respectively. We could demonstrate the guiding of the laser light in single-mode waveguides.

High-repetition-rate white-light pump-probe spectroscopy with a tapered fiber

We have realized a 76 MHz white-light differential transmission spectroscopy system. The technique employs a Ti:sapphire laser oscillator and a tapered fiber to generate a white-light continuum spanning almost the full visible to near-infrared spectral range. Using acousto-optical modulation and subsequent lock-in detection, transient relative transmission changes as small as 10(-5) are detected. The method is applied to study the ultrafast gain dynamics of the active layer of a vertical-external-cavity surface-emitting laser based on a multiple-quantum-well structure.

Photonic Integrated Circuits fabricated by Deep UV and Hot Embossing

We review our work in the field of deep UV modification of methacrylate-based polymers. Planar and rib waveguide structures are presented. A method of integrating polymer waveguides with organic light sources into all-polymer systems is shown.

Optical Digital Audio Interconnect Based on Organic Light Emitting Diodes and Organic Photodiodes

An optical interconnect using solely organic optoelectronic components is presented. Careful optimization of the organic light emitting diodes and photodiodes allows us to successfully transmit a digitized audio signal based on the S/PDIF-format (2.8224 Mbit/s).