Markus Sieger

On-chip integrated mid-infrared GaAs/AlGaAs Mach-Zehnder interferometer.

We report the design, fabrication, and first functional verification of mid-infrared (MIR; 3-12 μm) Mach-Zehnder interferometers (MZIs). The developed MIR-MZIs are entirely chip-integrated solid-state devices based on GaAs/AlGaAs technology waveguide fabricated via conventional optical lithography and reactive ion etching (RIE). Thus, fabricated MIR-MZIs were combined with a broadly tunable quantum cascade laser (tQCL) providing a wavelength coverage of 5.78-6.35 μm. MIR-MZIs have been designed with a waveguide width of 5 μm to ensure single mode behavior, avoiding optically undefined interference patterns. Several structures with different opening angles of the Y-junction were fabricated and tested for maximizing IR radiation throughput. This study demonstrates the feasibility of the very first chip-integrated mid-infrared Mach-Zehnder structures via interference patterns produced by minute amounts of water deposited at different positions of the MIR-MZI structure.

Toward On-Chip Mid-Infrared Sensors

This Feature highlights recent advances on mid-infrared thin-film waveguide technology and on-chip photonics facilitating next-generation label-free chem/bio sensor and assay platforms. Complemented by more recent advancements toward on-chip semiconductor waveguides, it is anticipated that label-free integrated mid-infrared sensing schemes will readily complement existing chem/bio sensor technologies in applications ranging from process monitoring and environmental analysis to biomedical diagnostics and point-of-care devices.

Diamonds Are a Spectroscopist's Best Friend: Thin-Film Diamond Mid-Infrared Waveguides for Advanced Chemical Sensors/Biosensors

The first combination of mid-infrared (MIR) tunable quantum cascade lasers (tQCLs) with thin-film diamond strip waveguides (DSWGs) suitable for advanced chemical sensing/biosensing is demonstrated. The sensing system is composed of thin diamond films grown on surface-passivated Si wafers via chemical vapor deposition (CVD) and microstructured using inductively coupled plasma (ICP) etching, serving as photonic waveguides for radiation emitted by a broadly tunable quantum cascade laser (tQCL) in the spectral regime of 5.78-6.35 μm (1570-1730 cm(-1)). The characterization of the free-standing diamond waveguides reveals excellent transmission properties across a broad MIR band. As a proof of concept, the detection of acetone in D2O via evanescent field absorption is demonstrated achieving a limit of detection (LOD) as low as 200 pL, which indicates a significant sensitivity improvement compared to conventional MIR slab/strip waveguides reported to date. Providing characteristic absorption features within the tuning range of the tQCL, studies using anisaldehyde as an analyte further corroborate the potential of tQCL-DSWG-based chemical sensors/biosensors.

Infrared attenuated total reflection spectroscopy for the characterization of gold nanoparticles in solution.

In situ synthesis of bare gold nanoparticles mediated by stainless steel as reducing agent was monitored via infrared attenuated total reflection (IR-ATR) spectroscopy. Gold nanoparticles were directly synthesized within the liquid cell of the ATR unit taking immediate advantage of the stainless steel walls of the ATR cell. As nanoparticles were formed, a layer of particles was deposited at the SiO2 ATR waveguide surface. Incidentally, the absorption bands of water increased resulting from surface-enhanced infrared absorption (SEIRA) effects arising from the presence of the gold nanoparticles within the evanescent field. Next to the influence of the Au(III) precursor concentration and the temperature, the suitability of IR-ATR spectroscopy as an innovative tool for investigating changes of nanoparticles in solution, including their aggregation promoted by an increase of the ionic strength or via a pH decrease, and for detailing the sedimentation process of gold nanoparticles was confirmed.

Mid-Infrared Spectroscopy Platform Based on GaAs/AlGaAs Thin-Film Waveguides and Quantum Cascade Lasers

The performance and versatility of GaAs/AlGaAs thin-film waveguide technology in combination with quantum cascade lasers for mid-infrared spectroscopy in comparison to conventional FTIR spectroscopy is presented. Infrared radiation is provided by a quantum cascade laser (QCL) spectrometer comprising four tunable QCLs providing a wavelength range of 5-11 μm (1925-885 cm(-1)) within a single collimated beam. Epitaxially grown GaAs slab waveguides serve as optical transducer for tailored evanescent field absorption analysis. A modular waveguide mounting accessory specifically designed for on-chip thin-film GaAs waveguides is presented serving as a flexible analytical platform in lieu of conventional attenuated total reflection (ATR) crystals uniquely facilitating macroscopic handling and alignment of such microscopic waveguide structures in real-world application scenarios.

MBE Growth of Mid-wave Infrared HgCdTe Layers on GaSb Alternative Substrates

GaSb has been studied as a new alternative substrate for growing HgCdTe via molecular beam epitaxy (MBE). Cross-sectional transmission electron microscopy (TEM) studies indicate that MBE-grown CdTe buffer layers on GaSb have much lower misfit dislocation density than comparable layers grown on GaAs. The MBE-grown mid-wave infrared (MWIR) HgCdTe layers on GaSb substrates present material quality comparable to those grown on GaAs substrates, which is one of the state-of-the-art alternative substrates currently used to grow HgCdTe for the fabrication of MWIR detectors and focal plane arrays. Typically, HgCdTe materials grown on GaSb are found to have a rocking curve (double crystal x-ray diffraction) full width at half maximum of ~122 arcsec and an etch pit density of ~mid-106 cm–2. Electron backscatter diffraction mapping shows that the lattice misorientation/misfit dislocations near the HgCdTe/CdTe interface are negligible for GaSb substrates in comparison to GaAs substrates, and that the material quality of the HgCdTe layer on GaSb is determined primarily by the material quality of the CdTe buffer layer. These preliminary results are very encouraging considering that this is a relatively recent research effort, and higher quality MBE-grown HgCdTe materials are expected on GaSb substrates with further optimization of HgCdTe growth conditions as well as further improvements in the growth conditions for CdTe buffer layers.

Toward on-chip mid-infrared chem/bio sensors using quantum cascade lasers and substrate-integrated semiconductor waveguides

In this contribution, we discuss progresses on ultra-sensitive chemical sensing in the mid-infrared (MIR; 3-20 μm) spectral regime by combining microfabricated GaAs/Al0.2Ga0.8As waveguides and sensing structures with quantum cascade lasers (QCL). Modern epitaxial grown methods including molecular beam epitaxy (MBE) and metal–organic vapor-phase epitaxy (MOVPE) were applied facilitating epitaxial growth of on-chip MIR GaAs/Al0.2Ga0.8As (6 μm core / 6 μm cladding) semiconductor slab waveguides, which were then structured with reactive ion etching (RIE) and/or focused ion beam milling (FIB) for establishing a variety of substrate-integrated GaAs/Al0.2Ga0.8As waveguide geometries. A distributed feedback (DFB) QCL lasing at a wavelength of 10.3 μm was combined with planar waveguide slabs and strip waveguides, respectively. Exemplary detection of acetic anhydride on strip waveguides (50 μm waveguide width) result in a limit of detection (LOD) of 0.05 pL, which is among the most sensitive direct evanescent field absorption measurements with substrate-integrated waveguides using MIR sensing systems reported to date. The first mid-infrared Mach-Zehnder interferometers (MIR-MZI) was recently design, fabricated, and functionally verified using a broadly tunable quantum cascade laser (tQCL) providing access to a spectral window of 5.78-6.35 μm. Finally, the development of first MIR ring resonators via microfabrication is shown providing an outlook toward next-generation miniaturized MIR sensor devices based on substrate-integrated semiconductor waveguides.

A novel chemometric classification for FTIR spectra of mycotoxin-contaminated maize and peanuts at regulatory limits

ABSTRACT The rapid identification of mycotoxins such as deoxynivalenol and aflatoxin B1 in agricultural commodities is an ongoing concern for food importers and processors. While sophisticated chromatography-based methods are well established for regulatory testing by food safety authorities, few techniques exist to provide a rapid assessment for traders. This study advances the development of a mid-infrared spectroscopic method, recording spectra with little sample preparation. Spectral data were classified using a bootstrap-aggregated (bagged) decision tree method, evaluating the protein and carbohydrate absorption regions of the spectrum. The method was able to classify 79% of 110 maize samples at the European Union regulatory limit for deoxynivalenol of 1750 µg kg–1 and, for the first time, 77% of 92 peanut samples at 8 µg kg–1 of aflatoxin B1. A subset model revealed a dependency on variety and type of fungal infection. The employed CRC and SBL maize varieties could be pooled in the model with a reduction of classification accuracy from 90% to 79%. Samples infected with Fusarium verticillioides were removed, leaving samples infected with F. graminearum and F. culmorum in the dataset improving classification accuracy from 73% to 79%. A 500 µg kg–1 classification threshold for deoxynivalenol in maize performed even better with 85% accuracy. This is assumed to be due to a larger number of samples around the threshold increasing representativity. Comparison with established principal component analysis classification, which consistently showed overlapping clusters, confirmed the superior performance of bagged decision tree classification. GRAPHICAL ABSTRACT

Portable Infrared Laser Spectroscopy for On-site Mycotoxin Analysis

Mycotoxins are toxic secondary metabolites of fungi that spoil food, and severely impact human health (e.g., causing cancer). Therefore, the rapid determination of mycotoxin contamination including deoxynivalenol and aflatoxin B1 in food and feed samples is of prime interest for commodity importers and processors. While chromatography-based techniques are well established in laboratory environments, only very few (i.e., mostly immunochemical) techniques exist enabling direct on-site analysis for traders and manufacturers. In this study, we present MYCOSPEC - an innovative approach for spectroscopic mycotoxin contamination analysis at EU regulatory limits for the first time utilizing mid-infrared tunable quantum cascade laser (QCL) spectroscopy. This analysis technique facilitates on-site mycotoxin analysis by combining QCL technology with GaAs/AlGaAs thin-film waveguides. Multivariate data mining strategies (i.e., principal component analysis) enabled the classification of deoxynivalenol-contaminated maize and wheat samples, and of aflatoxin B1 affected peanuts at EU regulatory limits of 1250 μg kg−1 and 8 μg kg−1, respectively.

Optimizing the design of GaAs/AlGaAs thin-film waveguides for integrated mid-infrared sensors

Optical simulations of GaAs/AlGaAs thin-film waveguides were performed for investigating the dependence of the modal behavior on waveguide geometry and the resulting analytical sensitivity. Simulations were performed for two distinct mid-infrared wavelengths, thereby demonstrating the necessity of individually designed waveguide structures for each spectral regime of interest. Hence, the modal behavior, sensitivity, and intensity of the evanescent field were investigated via modeling studies at 1600 and 1000  cm−1, thereby confirming the utility of such simulations for designing mid-infrared sensors based on thin-film waveguide technology.