Reflectance anisotropy spectroscopy (RAS) allows for in-situ monitoring of reactive ion etching (RIE) of
monocrystalline III-V semiconductor surfaces. Upon use of RAS the sample to be etched is illuminated with broad-band
linearly polarized light under nearly normal incidence. Commonly the spectral range is between 1.5 and 5.5 eV.
Typically the spectrally resolved difference in reflectivity for light of two orthogonal linear polarizations of light is
measured with respect to time - for example for cubic lattices (like the zinc blende structures of most III-V
semiconductors) polarizations along the  and the [-110] direction. Local anisotropies on the etch front cause
elliptical polarization of the reflected light resulting in the RAS signal. The time and photon energy resolved spectra of
RAS include reflectometric as well as interferometric information. Light with wavelengths well above 100 nm (even
inside the material) can be successfully used to monitor surface abrasion with a resolution of some tens of nanometers.
The layers being thinned out act as optical interferometers resulting in Fabry-Perot oscillations of the RAS-signal. Here
we report on RAS measurements assessing the surface deconstruction during dry etching. For low etch rates our
experimental data show even better resolution than that of the (slow) Fabry-Perot oscillations. For certain photon
energies we detect monolayer-etch-related oscillations in the mean reflectivity, which give the best possible resolution in
etch depth monitoring and control, i.e. the atomic scale.
A dynamic microfluidic iris is realized. Light attenuation is achieved by absorption of an opaque liquid (e.g. black ink).
The adjustment of the iris diameter is achieved by fluid displacement via a transparent elastomer (silicone) half-sphere.
This silicone calotte is hydraulically pressed against a polymethylmethacrylate (PMMA) substrate as the bottom
window, such that the opaque liquid is squeezed away, this way opening the iris. With this approach a dynamic range of
more than 60 dB can be achieved with response times in the ms to s regime.
The design allows the realization of a single iris as well as an iris array. So far the master for the molded silicone
structure was fabricated by precision mechanics. The aperture diameter was changed continuously from 0 to 8 mm for a
single iris and 0 to 4 mm in case of a 3 x 3 iris array.
Moreover, an iris array was combined with a PMMA lens array into a compact module, the distance of both arrays
equaling the focal length of the lenses. This way e.g. spatial frequency filter arrays can be realized.
The possibility to extend the iris array concept to an array with many elements is demonstrated. Such arrays could be
applied e.g. in light-field cameras.
AlGaInAsSb-based broad area lasers (BALs) with a monolithically integrated Fourier-optical 4f set-up in a folded-resonator
geometry are realized. The two resonator branches - each one d = 0.825 mm long - are connected through a
dry-etched cylindrical total-internal-reflection (TIR) mirror acting as a Fourier-transform element. Transverse mode
selection (TMS) is achieved by monolithically integrated spatial-frequency filters positioned in the back focal plane of
the mirror (i.e. in the Fourier-transform plane). The whole resonator is gain section (active medium) as well as part of the
TMS 4f set-up at the same time. The integration of TMS within the active BAL chip is shown to be successful. All
employed BAL/TMS type-II heterostructure lasers are MBE-grown on GaSb substrates, designed for an emission
wavelength in the mid-infrared around 2 μm. Different laser samples without any filter elements (no-TMS) and with
filters for the selection of the fundamental transverse mode (#0; TMS0) are prepared and characterized. Just for a proof
of principle also samples for the selection of higher order transverse modes, here exemplarily mode #6 (TMS6) and #8
(TMS8), have been processed and investigated. The free spectral range between the longitudinal modes is found to be
around 0.33 nm corresponding to the BAL's total-resonator length 2d = 1.65 mm (with an effective refractive index
n<sub>eff</sub> ≈ 3.8). This result strongly emphasizes that both resonator branches act together as one entity.
Numerical and experimental results of output dynamics investigations of AR-coated broad area lasers (BALs) above laser threshold are presented. The BALs are subject to feedback from a free-space external Fourier-optical 4<i>f</i>-setup with a <i>spatial</i> reflective filter in the Fourier-plane for transverse mode selection. It is shown theoretically and experimentally that under certain pump current conditions the BALs are operating in a repetitive self-pulsation mode. Pulse duration is approx. 1 ns at repetition rates of 200 to 500 MHz. Using the same setup active mode-locking of a BAL is achieved experimentally. Pulse durations of 103 ps are obtained. The Gaussian-like fundamental and higher order transverse modes up to mode No. 4 can be adjusted while the laser is operating in a mode-locked state. Experimentally, the simultaneous combination of mode-locking, transverse mode selection, and pulse shaping of a BAL in a modified 4<i>f</i>-setup implementing a spectral filter is investigated. Employing an optimized <i>spectral</i> sinc-like function as amplitude and phase filter the mode-locked BAL emits nearly square-shaped pulses with a pulse duration of 705 ps, while running close to the Gaussian-like transverse mode.