Georg Bastian

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.

Optimized Design of Plasmonic MSM Photodetector

We present an optimized design for a plasmonic metal-semiconductor-metal photodetector with interdigitated electrodes with subwavelength dimensions and a single GaInNAs quantum well (QW) as an absorbing layer. The excitation of surface plasmons at the metal-semiconductor interface leads to a strong field enhancement near the metallic electrodes. This results in an increased absorption in the QW, allowing both fast electrical response of the photodetector and high quantum efficiencies. With a grating periodicity of 820 nm and electrode finger width of 460 nm a 16-fold increase in the absorption of p-polarized light in the QW is achieved in comparison to the case without electrodes.

Laser speckle reduction via colloidal-dispersion-filled projection screens

We use projection screens filled with colloidal dispersions to reduce laser speckle in laser projection systems. Laser light is multiply scattered at the globules of the colloidal dispersions internal phase, which do Brownian movement. The integration time of the human eye causes a perception of a reduced laser speckle contrast because of temporal averaging. As a counteracting effect, blurring of projected images occurs in the colloidal dispersion, which degrades image quality. We measure and compare speckle reduction and blurring of three different colloidal dispersions filled into transmission screens of different thicknesses. We realized a high speckle contrast reduction at simultaneously low blurring with a thin screen filled with a highly scattering colloidal dispersion with forward-peaked scattering. We realize speckle contrast values below 3% at acceptable blurring.

Organic semiconductor distributed feedback laser fabricated by direct laser interference ablation

We use a pulsed, frequency tripled picosecond Nd:YAG laser for holographic ablation to pattern a surface relief grating into an organic semiconductor guest-host system. The resulting second order distributed feedback lasers exhibit laser action with laser thresholds being comparable to those obtained with resonators structured by standard lithographic techniques. The details of the interference ablation of tris-(8-hydroxyquinoline) aluminum (Alq(3)) doped with the laser dye 4- dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) are presented and discussed. Lasing action is demonstrated at a wavelength of 646.6 nm, exploiting second order Bragg reflection in a relief grating with a period of 399 nm.

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.

Numerical simulation tool for synthetic speckle pattern images and their intensity-based integration under variable conditions for metrology applications

In many metrological applications speckle can be used to determine surface properties where several complex effects and parameters are of concern. However, an analytical modelling of the decorrelation of speckle patterns created e.g. by two wavelength illumination of a surface is possible only for Gaussian-distributed surface height values and in illumination regimes where the separation of the two illumination wavelengths is much smaller than the mean value of the two wavelengths. Depending on the fabrication process of the illuminated surface the statistical distribution of the surface height values can differ significantly from a Gaussian distribution. We present a numerical simulation tool for the synthetic generation of laser-speckle patterns which a non-imaging observer (e.g. a bare CCD-chip) detects in the farfield of an illuminated surface. We investigate for which illumination wavelengths separation the analytical model is valid with a centre wavelength of 650 nm and compare simulated speckle images originating from surfaces with equal- and Gaussian-distributed height values under two-wavelength illumination. We show which differences from the analytical model for the correlation of the two emerging speckle images occur and consider the implications for an experimental situation.

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.

Speckle phenomena in pulsed broad-area vertical-cavity surface-emitting laser emission under different driving and illumination conditions

We have investigated the speckle contrast using different configurations: we place the screen either in the imaged near-field or in the far-field of the VCSELs emission whereas the CCD is placed in the near-or the far-field of the screen. The measured speckle contrast is strongly dependent on both the configuration and the driving conditions of the VCSEL.

Distribution and Emission Properties of Fluorescing Nanospheres on 2D Photonic Crystal Slabs

This paper investigates the quantum-optical interplay of photonic crystals (PCs) with localized emitters. Results show that the local density of states in the PC unit cell causes a position dependent modification of the spontaneous emission rate.

Nanostructured metallic electrodes for optoelectronic devices

Recently, arrays of subwavelength slits or holes in metal films have attracted a lot of interest due to the anomalously high transmission of light. The properties of the transmission resonances can be controlled by the periodicity and the width and height of the slits/holes in the array. Our aim is to employ the plasmonic effects in the design of metal-semiconductor-metal (MSM) photodetectors and light emitting diodes.