Polarimetric Balanced Detection: Background-Free Mid-IR Evanescent Field Laser Spectroscopy for Low-Noise, Long-term Stable Chemical Sensing

Abstract

In this work, we introduce polarimetric balanced detection as a new attenuated total reflection (ATR) infrared (IR) sensing scheme, leveraging unequal effective thicknesses achieved with laser light of different polarizations. We combined a monolithic widely tunable Vernier quantum cascade laser (QCL-XT) and a multibounce ATR IR spectroscopy setup for analysis of liquids in a process analytical setting. Polarimetric balanced detection enables simultaneous recording of background and sample spectra, significantly reducing long-term drifts. The root-mean-square noise could be improved by a factor of 10 in a long-term experiment, compared to conventional absorbance measurements obtained via the single-ended optical channel. The sensing performance of the device was further evaluated by on-site measurements of ethanol in water, leading to an improved limit of detection (LOD) achieved with polarimetric balanced detection. Sequential injection analysis was employed for automated injection of samples into a custom-built ATR flow cell mounted above a zinc sulfide multibounce ATR element. The QCL-XT posed to be suitable for mid-IR-based sensing in liquids due to its wide tunability. Polarimetric balanced detection proved to enhance the robustness and long-term stability of the sensing device, along with improving the LOD by a factor of 5. This demonstrates the potential for new polarimetric QCL-based ATR mid-IR sensing schemes for in-field measurements or process monitoring usually prone to a multitude of interferences.

Resolution Limit in Infrared Chemical Imaging

“…Much recent effort has been focused on adding greater molecular information − and speed, , while the image quality is commonly accepted to be optimized by following optical design rules from theoretical models. The emergence of quantum cascade lasers (QCL) is also challenging the conventional performance of IR imaging by offering possibilities for recording and analyses to optimize instrument capability that go beyond Fourier transform IR imaging systems. , The speed and signal-to-noise ratio (SNR) trade-offs in emerging systems are enabling us to ask more fundamental questions on how data quality, optical configuration, and spectral processing may act in concert to maximize information from chemical imaging.…”

“…However, the acquisition of high SNR, high optical fidelity data to realize these ideas has remained challenging. Taking advantage of QCLs, novel techniques such as balanced detection are now enabling high-sensitivity measurements that are pushing the detection limits , for IR spectroscopy. Custom-designed optical configurations are similarly providing high spatial fidelity with greatly reduced speckle effects from laser coherence. , To systematically take advantage of these developments for object recognition by molecular means, a clear definition for achievable resolution is necessary.…”

Mid-infrared dual-comb polarimetry of anisotropic samples

“…These methods nowadays offer high spectral resolutions in laboratory and field applications while providing high optical throughput, sub-mm spot sizes, and sub-s temporal resolution for sensing applications, time-resolved studies, as well as hyperspectral imaging. Of particular interest for structural and chemical analysis of interfaces, aggregates, thin films, and structured materials are polarization dependent QCL-based methods such as AFM-IR 18,19 , IR nanoscopy 20 , antenna-assisted IR nano-spectroscopy 21 , IR microscopy 22,23 , polarimetric IR-ATR 24 , vibrational circular dichroism 25 , far-field optical photothermal IR (O-PTIR) 26 , and IR spectroscopic ellipsometry/polarimetry 10,11,[27][28][29] .…”

Step-Scan Tuning of Vernier Quantum-Cascade Lasers for Rapid Detection of Volatile Organic Molecules

“…Extended-tuning quantum-cascade lasers (QC-XT) are electrically tunable multi-wavelength lasers using the Vernier effect that have the potential to combine rapid, high-resolution spectral scanning with a broad coverage [2]. While such lasers have been developed and commercialized, their potential has not yet been fully explored in previous implementations [3] because a dedicated driving scheme and the corresponding hardware were missing.…”