We present performance results of SnO<sub>2</sub> and CuO nanowire gas sensor devices, where single and multi-nanowire device configurations have been employed in order to optimize sensor design. In particular the response to the target gases CO, H<sub>2</sub>, and H<sub>2</sub>S has been measured in dry and humid air; both the SnO<sub>2</sub> and CuO nanowire sensors are able to detect CO in the low ppm concentration range, which is important for environmental monitoring. The CuO multi-nanowire devices show an extraordinary high response to H<sub>2</sub>S with sensitivity in the low ppb concentration. We present our developments of CMOS technology based micro-hotplates, which are employed as platform for gas sensitive thin films and nanowires. Potential heterogeneous integration of nanowires on the micro-hotplate chips as well as an approach towards gas sensor arrays is discussed. We conclude that CMOS integrated multi-nanowire gas sensors are highly promising candidates for the practical realization of multi-parameter sensor devices for indoor and outdoor environmental monitoring.
The employment of nanowires is a very powerful strategy to improve gas sensor performance. We demonstrate a gas
sensor device, which is based on silicon chip-to-chip synthesis of ultralong tin oxide (SnO<sub>2</sub>) nanowires. The sensor
device employs an interconnected SnO<sub>2</sub> nanowire network configuration, which exhibits a huge surface-to-volume ratio
and provides full access of the target gas to the nanowires. The chip-to-chip SnO<sub>2</sub> nanowire device is able to detect a H<sub>2</sub>
concentration of only 20 ppm in synthetic air with ~ 60% relative humidity at room temperature. At an operating
temperature of 300°C a concentration of 50 ppm H<sub>2</sub> results in a sensitivity of 5%. At this elevated temperature the sensor
shows a linear response in a concentration range between 10 ppm and 100 ppm H<sub>2</sub>. The SnO<sub>2</sub>-nanowire fabrication
procedure based on spray pyrolysis and subsequent annealing is performed at atmospheric pressure, requires no vacuum
and allows upscale of the substrate to a wafer size. 3D-integration with CMOS chips is proposed as viable way for
practical realization of smart nanowire based gas sensor devices for the consumer market.
We have realized gas sensor devices, which are based on a single SnO<sub>2</sub>-nanowire or a multiple SnO<sub>2</sub>-nanowire network
as gas sensing components and are very sensitive to the toxic gas H<sub>2</sub>S. The nanowires are fabricated in a two-step
atmospheric pressure synthesis process directly on the Si-chip by spray pyrolysis and subsequent annealing. Exposure of
the single SnO<sub>2</sub>-nanowire sensor H<sub>2</sub>S with a concentration of only 1.4 ppm decreases the resistance by ~ 30%, while the
multiple SnO<sub>2</sub>-nanowire network sensor exhibits a resistance decrease by ~ 90%. The nanowire sensors have
extraordinary sensitivity with resolution limit in the ppb range and are able to measure concentrations well below the
threshold limit value of 10 ppm. Due to their high performance the nanowire based sensors are basically suited for the
realization of smart gas sensing devices for personal safety issues as well as industrial applications.
Various nanostructures with a feature sizes down to 50 nm as well as photonic structures such as waveguides or grating
couplers were successfully replicated into the thermoplastic polymer polymethylpentene employing an injection molding
process. Polymethylpentene has highly attractive characteristics for photonic and life-science applications such as a high
thermal stability, an outstanding chemical resistivity and excellent optical transparency. In our injection molding process,
the structures were directly replicated from 2" silicon wafers that serve as an exchangeable mold insert in the injection
mold. We present this injection molding process as a versatile technology platform for the realization of optical
integrated devices and diffractive optical components. In particular, we show the application of the injection molding
process for the realization of waveguide and grating coupler structures, subwavelength gratings and focusing nanoholes.
We present a surface emitting GaAs/AlGaAs laser diode beam steering device based on the surface mode emission (SME) technique. The SME-laser diodes operate in a single-mode and show efficient surface emission into a single beam with a minimum beam divergence of 0.11 degree(s). We demonstrate a large digital beam steering from this device, which is achieved by mode switching. The special SME-structure supports two single-mode emission wavelengths, which are spaced by 8.38 nm. Digital switching between these two modes by a proper current pulse sequence leads to a steering of the surface emitted single-beam by 4.9 degree(s). Presently the beam steering frequency is limited to a maximum value of 0.15 MHz. An optimized device design for a continuous steering of the single, surface emitted beam is proposed.
First results from surface mode emitting (SME)-laser diodes utilizing a first-order grating are presented. The use of a first-order grating instead of a third-order grating strongly improves the radiation characteristics of surface emitting SME-laser diodes. Although a real single mode operation from SME-laser diodes is not yet achieved, the tunability of the main emission wavelength by changes of the waveguide thickness is clearly demonstrated. The crucial feature of the SME-technique is that it provides a high flexibility, when processing surface emitting laser diodes with desired radiation pattern and wavelength emission characteristics. These features are discussed to demonstrate the high application potential of the SME-laser diodes.
Frequency tuning of a vertical-cavity surface-emitting laser (VCSEL) achieved through the monolithical integration of a modulator diode is reported. Current injection into the modulator diode locally changes the refractive index. This in turn leads to a shift of the Fabry-Perot- resonances of the microcavity. The experimental results show a gradient of a frequency blue- shift up to 0.93 GHz/mA by the modulator current. The maximum obtained frequency shift was 14 GHz at 15 mA modulator current. The useful tuning range is at present restricted to approximately 40 mA modulator current due to thermal effects. The onset of these effects leads to a bending over of the frequency shift from a Plasma effect dominated regime to a thermally dominated regime and in turn to a frequency red-shift. A simple theoretical model considering Plasma effect and Joule effect agrees well with the experimental data and predicts a maximum value for the gradient of frequency shift of 1.15 GHz/mA with the given structure.
Based on a frequency tunable twin-guide (TTG) InGaAs/GaAs multiple quantum well (MQW) laser structure, we developed a novel design concept for a surface emitting laser device enabling spatial beam steering. Utilizing a change in the refractive index of the parallel monolithically integrated modulator diode due to carrier injection, we observe a continuous emission frequency (wavelength) shift up to (Delta) f equals 85 GHz ((Delta) (lambda) equals -0.35 nm). For this preliminary structure the experimental results are consistent with our model calculations. Based on the theoretical model, for an optimized device a tuning range of (Delta) f equals 1600 GHz ((Delta) (lambda) >= 5 nm) is expected. For the novel surface emitting device design, we make use of an additional structure on top of the TTG laser including a second waveguide and a grating. This will enable a wavelength dependent surface emission angle, i.e., continuous beam steering, by coupling the laser and the surface mode. A calculational model was developed to estimate the steering characteristics in dependence on the dielectric device structure including mode guiding and the surface grating shape.
A novel surface emitting laser diode beam steering device based on the excitation and emission of surface modes is presented. This new device is a longitudinally segmented wavelength tunable GaAs/AlGaAs double heterostructure laser diode, which is modified to allow a coupling of the laser mode to a transverse electric polarized surface mode. This results in surface emission with very narrow beam divergence. A steering of the surface emitted farfield pattern is achieved by a variation of the emission wavelength: By electrically changing the emission wavelength from 877.54 nm to 879.13 nm the dominant surface emitted peak is steered by 0.4 degree(s).
It is shown for the first time that strongly directional emission of defined polarization can be achieved from conventional AlGaAs/GaAs double heterostructure surface emitting light emitting diodes (LEDs) via coupling to surface plasmons. By microstructuring the surface, LEDs were fabricated with a beam divergence of less than 4 deg and an increased quantum efficiency. It is demonstrated that the surface plasmon excitation and emission mechanism has the potential to improve the performance of LEDs. In addition, it is shown that this principle can be applied to laser diodes. A controlled outcoming of laser light by a surface plasmon coupling is demonstrated.