Klaus Huska

Terahertz emission from lateral photo-Dember currents.

The photo-Dember effect is known as source of impulsive THz radiation after excitation with femtosecond optical pulses. The origin of the emission is the ultrafast separation of electron and holes in strong carrier gradients due to different diffusion coefficients. For a simple semiconductor surface the time dependent polarization is oriented perpendicular to the excited surface which complicates efficient out coupling of THz radiation. We investigate a new scheme for generating strong carrier gradients parallel to the surface. In that case the photo-Dember currents are oriented parallel to the surface and the generated THz radiation can be easily out coupled. This concept can be scaled up so that multiple phase coherent photo-Dember currents contribute to the THz emission. These passive THz emitters reach electric field amplitudes comparable to high-efficiency externally biased photoconductive emitters.

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.

Design of plasmonic grating structures towards optimum signal discrimination for biosensing applications

Sensors based on surface plasmon resonances (SPRs) have proven themselves as promising devices for molecular investigations - still there is potential to determine the geometrical parameter set for optimal sensing performance. Here we propose a comprehensive design rule for one-dimensional plasmonic grating structures. We present an analytical approach, which allows for estimation of the grating parameters for best SPR coupling efficiency for any geometry and design wavelength. On the example of sinusoidal gratings, we expand this solution and discuss numerically and experimentally, how the grating modulation depth can be refined to achieve optimal signal resolution. Finally, we propose a benchmark factor to assess the sensor performance, which can be applied to any sensing scheme utilizing resonances, allowing for comparison of different technological platforms.

Intense terahertz generation based on the photo-Dember effect

We demonstrate a new scheme for generating THz radiation based on the photo-Dember effect in lateral geometry. By micro-structuring a semiconductor surface we achieve strongly enhanced THz emission comparable to high-efficiency externally biased photoconductive emitters.

Computational investigation of silicon thin-film solar cells withgrating structures fabricated by holographic lithography

Light trapping due to rough interfaces is a common and industrially applied technique to enhance cell performance in silicon thin-film solar cells. The induced scattering enhances the absorption and consequently the conversion efficiency of the device. Periodic structures promise to further enhance the light trapping, allowing a beneficial reduction of the absorber layer thickness. In this work, solar cells with transparent front contacts with a two-dimensional (2D) grating structure produced by holographic lithography are investigated. The grating structures are characterized by various means and the results are used to calibrate finite-difference time-domain (FDTD) simulations. With the computational method, the influence of the grating height on the solar cell performance is investigated.

New THz emitter device concept based on lateral photo-dember currents

A new concept for the generation of terahertz (THz) radiation is presented. Instead of accelerating carriers in an external bias field as state-of-the-art THz emitters generate THz radiation [1], the THz radiation is generated by lateral Photo-Dember currents induced by inhomogeneous optical excitation [2]. We prove the principle in the simplest geometry on GaAs.