Conference Agenda

We present a self-mixing (SM) approach for methane detection in the mid-infrared (Mid-IR) region using a Quantum Cascade Laser (QCL) operating within the 7669–7679 nm spectral window. This range encompasses two strong methane absorption lines centered at 7673 nm and 7678 nm. Unlike conventional Mid-IR gas sensing architectures that require cooled photodetectors, the SM technique leverages the intrinsic sensitivity of the QCL to optical feedback, enabling photodetector-free operation and significantly reducing power and system complexity.

A mechanical chopper modulates the optical feedback by periodically interrupting the external cavity return beam, producing a square-wave modulation of the QCL terminal voltage, typically on the order of tens of millivolts. This signal is subsequently amplified and measured using a lock-in amplifier with extended integration time, enhancing detection sensitivity while suppressing broadband noise.

The SM signal is susceptible to spurious phase noise caused by environmental instabilities that perturb the optical path length, manifesting as non-absorption-related fluctuations in the QCL terminal voltage. To suppress these effects, we introduce mechanical vibration of the external cavity mirror up to several tens of micrometres. The frequency of the vibration is in the order of 100s Hz. This vibration produces multiple complete cycle phase shifts that, due to the lock-in amplifier’s filtering, are effectively averaged out, preserving the absorption-dependent signal component.

Experimental validation was conducted using two aluminum hollow waveguides of 7.5 cm and 25 cm lengths, provided by the University of Ulm. Methane concentrations of 1300 ppm and 477 ppm were tested. The system was calibrated for both path lengths, and measurements revealed a linear dependence between the terminal voltage swing and optical absorption, consistent with theoretical models for QCLs under weak to moderate optical feedback regimes.

The Limit of Detection (LOD) achieved with the 25 cm waveguide was 50 ppm at the 7673 nm line, with a calculated Limit of Quantitation (LOQ) of approximately 150 ppm. Owing to the generalizability of the SM mechanism, this sensing strategy can be extended to other gas species and Mid-IR spectral regions, considering the absorption coefficient of the targeted line(s) and provided that the laser operation under feedback remains unaffected.

In summary, we demonstrate the viability of a QCL-based SM sensor for low-cost, compact, and energy-efficient methane detection in the Mid-IR. Ongoing work aims to eliminate mechanical components through novel self-mixing architectures employing all-electronic or integrated optical modulation schemes.

Helicobacter pylori infection is a major cause of gastritis, peptic ulcers, and gastric cancer1,2, highlighting the need for accessible, rapid, and accurate diagnostic tools. Conventional methods such as endoscopy, biopsy, and culture provide high accuracy but are invasive, costly, and require specialized personnel, limiting their use in routine or decentralized healthcare settings. Non-invasive alternatives like stool antigen and serological tests are easier to perform but may lack the sensitivity and specificity required for reliable detection or fail to distinguish active from past infections. The ¹³C-urea breath test (UBT) is a well-established, non-invasive approach that leverages isotopic ratio measurements of exhaled ¹³CO₂/¹²CO₂ following ingestion of ¹³C-labeled urea. However, traditional implementations using FTIR3 or IRMS4,5 are typically bulky and impractical for point-of-care applications. In this work, we present a miniaturized mid-infrared (MIR) sensor platform that integrates two quantum cascade lasers (QCLs) emitting at 2310 cm⁻¹ and 2770 cm⁻¹ with substrate-integrated hollow waveguide (iHWG) gas cells of 3 cm and 9 cm lengths. This configuration enables precise, real-time detection of the ¹³CO₂/¹²CO₂ isotopic ratio in exhaled breath samples. The system achieves detection limits below 30 ppm for ¹³CO₂ using minimal sample volumes (~480 μL) with a linearity R² = 0.998 across clinically relevant isotopic ratios. This approach enhances isotopic selectivity and minimizes spectral interferences, while the miniaturized iHWGs significantly reduce the overall system footprint compared to traditional FTIR-based setups. These results demonstrate the feasibility of a portable, MIR-based breath analysis platform for H. pylori detection, paving the way for decentralized and potentially home-based diagnostics. Future work will focus on integrating it into user-friendly diagnostic devices.

Surface-enhanced Raman spectroscopy (SERS) enables the detection of vibrational spectra from highly diluted molecules adsorbed onto rough, plasmonically active coin metal surfaces. It offers sensitivities comparable to fluorescence spectroscopy while providing structural information at the same time. Despite its potential in pharmaceutical, environmental and quality control applications, SERS is rarely used in routine analysis, where mass spectrometry, UV/VIS spectroscopy, and fluorescence detection remain standard. This is largely due to challenges related to the stability, reproducibility, universality, and adsorption behavior of molecules on SERS substrates.

To address these limitations, we developed a pressure-stable microsensor that integrates a silver-based SERS substrate with a platinum electrode, enabling simultaneous electrochemical manipulation and real-time SERS detection—a technique known as spectroelectrochemistry. The incorporation of electrochemical control enhances signal intensity, activates the SERS substrate, regulates analyte adsorption, facilitates desorption, and provides additional structural insights through spectral modulation.

We demonstrate the performance of this sensor through applications in HPLC, including the separation and detection of classical SERS model dyes and B vitamins. These examples highlight the synergistic benefits of combining SERS with spectroelectrochemistry, marking a significant step toward the routine use of SERS in analytical chemistry.