Martin R. Hofmann

Medical THz imaging: an investigation of histo-pathological samples

We present a THz investigation of histo-pathological samples including the larynx of a pig and a human liver with metastasis. Our measurements show that different types of tissue can be clearly distinguished in THz transmission images, either within a single image or by a comparison of images obtained for different frequency windows. This leads to the problem that images obtained for different frequencies inherently have a different spatial resolution. An image obtained from two such images by a simple mathematical operation may contain artefacts. We discuss measures to deal with this problem. Furthermore, we investigate the possibility of improving the spatial resolution of THz images. Finally, we present a cw THz imaging system based on a photomixer and an external cavity semiconductor laser that allows for simultaneous two-mode operation. The cw system is less expensive and more compact than conventional time-domain imaging systems.

Four-wave mixing and direct terahertz emission with two-color semiconductor lasers

We have observed four-wave mixing in a semiconductor laser configured to emit on two wavelengths simultaneously. The four-wave mixing sidebands exist up to 4 THz stemming from a modulation of the carrier plasma at the difference frequency of the two laser modes. In addition, we were able to generate and detect tunable THz radiation at this difference frequency from the laser device itself suggesting a scheme for a tunable THz source.

Ultrafast (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser for the 1.3 μm wavelength regime

(GaIn)(NAs) vertical-cavity surface-emitting lasers for room-temperature emission at 1.3 μm wavelength are designed and grown by metal-organic vapor-phase epitaxy using dimethylhydrazine and tertiarybutylarsine. Room-temperature operation at wavelengths up to 1.285 μm is achieved with low optical pumping thresholds between 1.6 and 2.0 kW/cm2. Stimulated emission dynamics after femtosecond optical pumping are measured and compare favorably with results on (GaIn)As/Ga(PAs)-based structures.

Optical spin manipulation of electrically pumped vertical-cavity surface-emitting lasers

We analyze the potential for the spin manipulation of vertical-cavity surface-emitting lasers (VCSELs) by operating them electrically and injecting additional spin-polarized carriers by polarized optical excitation. The output polarization of the VCSELs can be easily controlled by the spin orientation of the optically injected carriers when the injection current does not exceed the threshold current.

Ultrafast spin-induced polarization oscillations with tunable lifetime in vertical-cavity surface-emitting lasers

We report spin-induced polarization oscillations in vertical-cavity surface-emitting lasers above threshold and at room temperature. The oscillation frequency is 11.6 GHz, which is significantly higher than the modulation bandwidth of less than 4 GHz in the device. The oscillation frequency is determined by an additional resonance frequency in birefringence containing microcavities, which is potentially much higher than the conventional relaxation oscillation frequency. The damping of the oscillations can be controlled by the current, allowing for oscillation lifetimes much longer than the spin lifetime in the device as well as for short bursts potentially interesting for information transmission.

Fourier-transform external cavity lasers

We explore the potential of a new laser resonator design that uses an intracavity Fourier transformation and allows for multi-color operation, gain extension and intracavity second harmonic generation (SHG). First, purely electronically controlled wavelength tuning is demonstrated using liquid crystal displays (LCDs) and digital mirror devices. Moreover, the new design can be applied to different lasers as shown on the examples of a laser diode and a Thulium-doped fiber laser in the mid-infrared. Furthermore, we demonstrate the simultaneous control of multiple gain media within one single external cavity. In addition, tunable emission in the 490 nm range is obtained using intracavity SHG with one single control parameter. Finally, we unambiguously prove simultaneous multi-wavelength operation with variable wavelength spacings and apply it to the generation of THz difference frequencies.

Linewidth enhancement factor and optical gain in (GaIn)(NAs)/GaAs lasers

Experimental results on the linewidth enhancement factor α of 1.3 μm (GaIn)(NAs) lasers are presented and analyzed on the basis of a comparison with theoretical data obtained from a microscopic model. Our experimental data are obtained from the shift of the Fabry–Perot modes with injection current using an approach to eliminate temperature-dependent artifacts. At the emission wavelength at threshold we find a value of 2.5 for α which clamps for varying injection current.

Electron spin injection into GaAs from ferromagnetic contacts in remanence

We demonstrate electrical spin injection into a (GaIn)As∕GaAs light-emitting diode from the remanent state of ferromagnetic contacts in perpendicular geometry. Using a Fe∕Tb multilayer structure with perpendicular magnetic anisotropy and a reverse-biased Schottky contact, we achieve a circular polarization degree of the emitted light of 0.75% at 90K.

Two-colour diode lasers for generation of THz radiation

Direct emission of terahertz (THz) radiation out of a two-colour semiconductor laser is reported and analysed. The geometry of the tunable two-colour laser is described together with the physical mechanisms responsible for the emission at the THz difference frequency of the two pump colours used. On the basis of this analysis, different possibilities are suggested to increase the emitted THz intensity towards application relevant levels.

Validation of optical coherence tomography in vivo using cryostat histology

We aimed to validate for the first time optical coherence tomography (OCT) measurements of epidermal thickness (ET) using cryopreparation for histology. OCT assessments of ET were performed on healthy skin using the algorithms as follows: first, peak-to-valley analysis of the A-scan (ET-OCT-V), second, line-traced image analysis of the B-scan (ET-OCT-IA). Histology was performed using cryostat sections which were also evaluated using the image analysis (ET-Histo). We selected 114 samples, including B-scans and corresponding histology, for method comparison between ET-OCT-IA and ET-Histo. Forty-two A-scans were available for method comparison between ET-OCT-V and ET-Histo. Bland and Altman plots revealed a marked bias with wide 95% limits of agreement for ET-OCT-V versus ET-Histo. Comparison of ET-OCT-IA versus ET-Histo revealed only a slight bias and narrow 95% limits of agreement. A-scan analysis for ET determination is linked to significant limitations and lacks agreement with histology. By contrast, we observed satisfactory agreement between ET-OCT-IA and ET-Histo indicating that both methods can be utilized interchangeably. OCT using the line-traced image analysis of the B-scan appears to be a valid and relatively practicable method for the determination of ET in vivo. Furthermore, the comparisons with the in vivo OCT profiles demonstrate that cryostat sectioning provides a better preservation of relative and absolute dimensions of skin layers than paraffin embedding.