Henning Fouckhardt

Deep wet etching of fused silica glass for hollow capillary optical leaky waveguides in microfluidic devices

We report on a technology for the fabrication of hollow capillary optical leaky waveguides in fused silica glass. The fabrication process is based on lithography, wet chemical etching and aligned direct bonding. We have developed a single-layer photoresist soft mask which allows for channel etch depths up to 33 µm in fused silica glass. To our knowledge, such etch depths have never been achieved before in fused silica glass with single-layer soft etch masks. Aligned direct glass-glass bonding is used for the first time to obtain channels with almost circular profiles with diameters between 16 and 66 µm. Capillary optical leaky waveguides embedded into microfluidic devices can be used, for example, for capillary electrophoresis and hyper Rayleigh scattering.

Fourier-optical transverse mode selection in external-cavity broad-area lasers: experimental and numerical results

Broad-area lasers (BALs) with external Fourier-optical cavities with spatial filter for transverse mode selection have been studied experimentally and theoretically. The transverse mode structure of BALs is modeled using a three mirror cavity approach. The model accounts for gain saturation, carrier diffusion, and anti-guiding effects. Near- and far-field distributions are calculated and compared to experimental results. Transverse mode selection is achieved for BALs with a residual front facet reflectivity of 10% at low pump currents. For BALs having a very low front facet reflectivity of 0.001%, transverse modes can be selectively excited up to pump currents more than 200% above laser threshold. Calculations show that BALs having a 0.001%-antireflection coating with an external Fourier-optical cavity high above threshold can operate in a self-Q-switched-like mode with pulse durations of 2-4 ns and repetition rates of 100-200 MHz. Experimental results obtained with a hybrid integrated-optical external cavity for transverse mode selection are also presented.

Advantages and disadvantages of sulfur passivation of InAs/GaSb superlattice waveguide photodiodes

In this work, the influence of ammonium sulfide (NH4)2S passivation on waveguide based mid-infrared InAs/GaSb superlattice photodetectors (2–5 µm wavelength) has been studied. The current–voltage characteristics for reverse as well as for forward bias of passivated samples have been examined. The advantages of this have been the reduction of the reverse leakage current and the increase of zero bias resistance. As a disadvantage the decrease of the photoresponsivity after sulfur passivation has been found. Furthermore, it has been observed that the passivation solution does not only passivate the surface of GaSb, but it also reacts with entire GaSb layers and can destroy the devices.

Fourier-optical selection of higher order transverse modes in broad area lasers

A partially and a highly antire ection coated broad area laser are operated in an external cavity Fourier-optical 4f set-up to experimentally investigate transverse mode selection. The external cavity consists of a lens and a spatial frequency filter. Running freely the lasers show non-stationary filamentation. Placing the spatial filter unit directly onto the optical axis gives cw fundamental mode operation and a transverse shift of the spatial filter in the plane of the active region allows for selective excitation of higher order modes.

Intracavity stabilization of broad area lasers by structured delayed optical feedback.

The influence of structured delayed optical feedback (SDOF) on a broad area laser is investigated experimentally. SDOF is realized with a miniature-sized convex external mirror. The setup takes into account the small time scales involved in semiconductor laser dynamics by employing short external resonator lengths. Careful choice of the feedback parameters leads to a narrow single-lobe farfield even at high pump currents. The experimental results confirm earlier microscopic dynamic simulations by O. Hess et al. predicting that SDOF might be capable of stabilizing the emission of broad area lasers.

An improved shooting approach for solving the time-independent Schrödinger equation for III/V QW structures

Abstract An improved shooting approach is presented and used to solve the one-dimensional Schrodinger equation in a simple one-band effective mass model whose applicability to the InAs/GaSb broken gap system is shown by experimental results. The derived numerical results are compared with numerical data from the ordinary shooting approach.

Deep reactive ion etching of GaSb in Cl2/Ar-plasma discharges using single-layer soft mask technologies

Technologies for GaSb dry etching employing Cl2/Ar-plasma discharges are reported. Etch rates higher than 2 μm min−1 are achieved in a conventional reactive ion etching (RIE) chamber. To our knowledge these are the highest etch rates reported in GaSb dry-etch technology with conventional RIE. Also, three different single-layer soft mask processes are described and compared with respect to suitability for deep RIE of GaSb. Soft masks have many advantages over (metal) hard masks, such as easy and inexpensive processing, low pinhole density and high etching reproducibility. The well known AZ5214E resist from Clariant allows for GaSb etch depths of up to 6.4 μm keeping dimensional accuracy. Using TI 35ES photoresist, developed by MicroChemicals, GaSb etch profiles of up to 51 μm depth are obtained revealing considerable dimensional stability. This photoresist material is reported in the literature for the first time. By applying optimized GaSb dry-etch parameters, SU-8–50, an epoxy-type resist, developed by MicroChem Corp., will show an outstanding resist mask lifetime of up to 420 min, if SU-8–50 is spin-coated to a thickness of 80 μm. Thus it is possible to achieve GaSb etch depths beyond 100 μm. Deep dry etching of GaSb can be exploited to fabricate fiber or capillary connections, or to create substrate windows for backside-illuminated photodetectors for the mid-infrared (MIR) wavelength range.

Deep fused silica wet etching using an Au-free and stress-reduced sputter-deposited Cr hard mask

A technological process for wet chemical etching of microfluidical/microoptical elements in fused silica using a Cr hard mask is presented. The Cr layer is fabricated using sputter deposition under different Ar pressures to reduce the intrinsic stress in the hard mask. Stress reduction was achieved using an Ar pressure of 4.8 × 10−3 mbar, whereas compressive and tensile stress are produced by lower or higher pressures, respectively. An etch depth of 104 µm was achieved without detachment of the mask. The overall lithography process and etching defects caused by non-optimal fabrication parameters are also discussed. Special attention is given to the pressure during sputtering and the resulting mask quality in different stress regimes.

Deep dry etching of GaAs and GaSb using Cl2/Ar plasma discharges

Deep dry etch processes for GaAs and GaSb are investigated. Reactive ion etching in Cl2/Ar plasma discharges is used to pattern GaAs and GaSb with a single-layer soft mask resist. Soft masks have many advantages over (metal) hard masks such as easy and inexpensive processing, low pinhole density, and high etching reproducibility. Using TI35ES, an image reversal resist developed by MicroChemicals, GaAs etch profiles of up to 25 μm depth and GaSb structures of more that 50 μm depth are obtained revealing considerable dimensional stability. Even for etch durations of more than 50 min the resist can be easily removed from the sample surface after the etch process. Roughnesses of etched surfaces of less than 1 nm (rms, measured by atomic force microscopy) are obtained after reactive ion etching for both materials. Photoluminescence measurements reveal that the surface stoichiometry is conserved during the etching step. Deep dry etching of GaAs and GaSb can be exploited for example to fabricate fiber or capilla...

Multitude of glass surface roughness morphologies as a tool box for dosed optical scattering

Glass surface roughness can be considered positive for certain applications in optics. Results of maskless (i.e., lithography-free) reactive-ion-etch runs are given, where (depending on etch parameters) self-masking leads to very different surface morphologies. The latters variety allows for virtually any desired scattered visible light power portion, a result that we term dosed scattering. The surfaces are characterized, for example, by the root-mean-square roughness delta(rms), while their optical scattering is described by the diffuse transmission as well as the Harvey parameters. Since the wavelengths lambda in glass are around delta(rms), the shapes of the individual scatterers account for local refraction adding up to the usual scattering.