Photoacoustic Raman Spectroscopy

Gas detection is a cornerstone of environmental monitoring, industrial safety, and medical diagnostics. From tracking emissions in agriculture and industry to analyzing breath for early disease detection, the ability to accurately and reliably sense gases is becoming increasingly vital. Yet, conventional gas detectors, based on catalytic and electrochemical sensing, struggle with selectivity in complex mixtures and degrade in harsh environments.

Optical gas sensors offer a compelling alternative, delivering high sensitivity and leveraging the unique molecular “fingerprints” found in the mid-infrared (MIR) spectral region (λ ~ 4 to 20 µm). These fingerprints arise from molecular vibrations, enabling precise identification of organic and inorganic gases. Despite their promise, current MIR spectroscopic sensors rely on costly quantum cascade lasers and complex heterostructures, limiting their scalability and integration into mobile or wearable platforms. Meanwhile, alternatives like LEDs and MEMS micro-heaters lack sufficient emission power beyond 5 µm.

This PhD research sets out to transform the landscape of gas sensing by exploring two groundbreaking approaches for highly selective, fully integrated “on-chip” gas sensors:

1. Stimulated Raman Spectroscopy: Unlocking Molecular Vibrations with NIR Light

Raman scattering—where photons interact inelastically with molecules—offers a window into vibrational energy states. While spontaneous Raman scattering is inherently weak, stimulated Raman spectroscopy amplifies the signal dramatically by using two laser beams tuned to the vibrational energy difference of the target gas. This technique allows the use of near-infrared (NIR) light, which is readily available in existing photonic platforms, making it a cost-effective and scalable solution.

By harnessing stimulated Raman, this research will pioneer highly selective gas detection using integrated NIR sources, opening doors to compact, low-power sensors suitable for real-world deployment in wearables, mobile devices, and industrial systems.

2. Photoacoustic Spectroscopy: Turning Light into Sound for Ultra-Sensitive Detection

Traditional optical absorption spectroscopy demands long optical paths—often meters to kilometers—to achieve part-per-billion sensitivity. This is impractical for integrated systems. Photoacoustic spectroscopy sidesteps this limitation by converting absorbed light into sound waves. As molecules relax from excited states, they release heat, generating pressure waves when the light is modulated.

This technique is immune to background light interference and doesn’t rely on long optical paths, making it ideal for miniaturized platforms. However, current on-chip implementations lag behind commercial benchtop systems in sensitivity. This PhD will explore novel detector geometries to bridge that gap, pushing the boundaries of what integrated photoacoustic sensors can achieve.

Synergistic Innovation: Combining Raman and Photoacoustic Spectroscopy

The true innovation lies in the strategic combination of stimulated Raman and photoacoustic spectroscopy. By integrating these complementary techniques, this research aims to develop a new class of gas sensors that are not only highly selective and sensitive but also compact, robust, and scalable.