A 23-watt single-frequency vertical-external-cavity surface-emitting laser.

We report on a single-frequency semiconductor disk laser which generates 23.6 W output power in continuous wave operation, at a wavelength of 1013 nm. The high output power is a result of optimizing the chip design, thermal management and the cavity configuration. By applying passive stabilization techniques, the free-running linewidth is measured to be 407 kHz for a sampling time of 1 ms, while undercutting 100 kHz in the microsecond domain.

4.35 kW peak power femtosecond pulse mode-locked VECSEL for supercontinuum generation.

We report a passively mode-locked vertical external cavity surface emitting laser (VECSEL) producing 400 fs pulses with 4.35 kW peak power. The average output power was 3.3 W and the VECSEL had a repetition rate of 1.67 GHz at a center wavelength of 1013 nm. A near-antiresonant, substrate-removed, 10 quantum well (QW) gain structure designed to enable femtosecond pulse operation is used. A SESAM which uses fast carrier recombination at the semiconductor surface and the optical Stark effect enables passive mode-locking. When 1 W of the VECSEL output is launched into a 2 m long photonic crystal fiber (PCF) with a 2.2 µm core, a supercontinuum spanning 175 nm, with average power 0.5 W is produced.

Terahertz radiation at 0.380 THz and 2.520 THz does not lead to DNA damage in skin cells in vitro.

The question whether nonionizing electromagnetic radiation of low intensity can cause functional effects in biological systems has been a subject of debate for a long time. Whereas the majority of the studies have not demonstrated these effects, some aspects still remain unclear, e.g., whether high-frequency radiation in the terahertz range affects biological systems. In particular for frequencies higher than 0.150 THz, investigations of the ability of radiation to cause genomic damage have not been performed. In the present study, human skin cells were exposed in vitro to terahertz radiation at two specific frequencies: 0.380 and 2.520 THz. Power intensities ranged from 0.03-0.9 mW/cm(2) and the cells were exposed for 2 and 8 h. Our goal was to investigate whether the irradiation induced genomic damage in the cells. Chromosomal damage was not detected in the different cell types after exposure to radiation of both frequencies. In addition, cell proliferation was quantified and found to be unaffected by the exposure, and there was no increase in DNA damage measured in the comet assay for both frequencies. For all end points, cells treated with chemicals were included as positive controls. These positive control cells clearly showed decreased proliferation and increased genomic damage. The results of the present study are in agreement with findings from other studies investigating DNA damage as a consequence of exposure to the lower frequency range (<0.150 THz) and demonstrate for the first time that at higher frequencies (0.380 and 2.520 THz), nonionizing radiation does not induce genomic damage.

Evolution of multi-mode operation in vertical-external-cavity surface-emitting lasers.

The longitudinal multi-mode emission in a vertical-external-cavity surface-emitting laser is investigated using both single shot streak camera measurements and interferometric measurement techniques. For this, the laser is operated in the single- and two-color emission regime using both an etalon and a free-running configuration without etalon, respectively. The laser emission is analyzed with respect to pump power and output coupling losses for a long and for a short resonator. We observe a steep increase of emission bandwidth close to the laser threshold and monitor the transition between longitudinal single- and multi-mode operation. Additionally, the results indicate that a stable two-color operation is related to a sufficiently high number of oscillating longitudinal modes within each color.

Efficient terahertz en-face imaging.

In this work, we develop a pulsed terahertz imaging system in reflection geometry, where due to scanning of the terahertz beam neither the sample nor the emitter and detector have to be moved. We use a two mirror galvanoscanner for deflecting the beam, in combination with a single rotationally symmetric focusing lens. In order to efficiently image planar structures, we develop an advanced scanning routine that resolves all bending effects of the imaging plane already during measurement. Thus, the measurement time is reduced, and efficient imaging of surfaces and interfaces becomes possible. We demonstrate the potential of this method in particular for a plastic-metal composite sample, for which non-destructive evaluation of an interface is performed.