A team of Spain-based researchers has developed a highly sensitive detector for identifying molecules via their infrared vibrational “fingerprint”. The new detector converts incident infrared light into ultra-confined "nanolight" in the form of phonon polaritons within the detector´s active area. This mechanism serves two crucial purposes: it improves the detector's overall sensitivity and enhances the vibrational fingerprint of nanometer-thin molecular layer placed on top of the detector, allowing this molecular fingerprint to be more easily detected and analyzed.
The compact design and room-temperature operation of the device hold promise for developing ultra-compact platforms for molecular and gas sensing applications.
Molecules have "fingerprints" of sorts, or rather unique features that can be used to differentiate them. Each type of molecule, when illuminated with the right light, vibrates at a characteristic frequency (its resonance frequency, which typically occurs at infrared frequencies) and strength. Similar to what can be done with human fingerprints, one can exploit this information to distinguish different types of molecules or gases from each other. That can also help protect from potential dangers, by identifying poisonous and dangerous substances or gases instead of criminals.
Infrared fingerprint spectroscopy is a conventional approach that uses infrared reflection or transmission spectra to identify different molecules. However, the small size of organic molecules compared to the infrared wavelength results in a weak scattering signal, making it challenging to detect small quantities of material. In recent years, this limitation has been addressed using Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy. SEIRA spectroscopy leverages infrared near-field enhancement provided by rough metal surfaces or metallic nanostructure to amplify the molecular vibrational signals. The main advantage of SEIRA spectroscopy is its ability to measure and study minute material quantities.
Recently, phonon polaritons—coupled excitations of electromagnetic waves with atomic lattice vibrations—particularly hyperbolic phonon polaritons in thin layers of hexagonal boron nitride (h-BN), have emerged as promising candidates for boosting the sensitivity of SEIRA spectroscopy. However, SEIRA spectroscopy remains a far-field technique that requires bulky equipment, such as light sources, SEIRA substrates, and typically nitrogen-cooled infrared detectors. This reliance on large instruments limits its potential for miniaturization and on-chip applications.
In its recent work, the team of researchers has successfully demonstrated the first on-chip phononic SEIRA detection of molecular vibrations. This result was made possible through the joint experimental efforts of Nanogune and ICFO researchers, along with theoretical support from the groups of Dr. Alexey Nikitin at the Donostia International Physics Center and Prof. Luis Martín-Moreno at the Instituto de Nanociencia y Materiales de Aragón (CSIC- Universidad de Zaragoza). The researchers employed ultra-confined HPhPs to detect molecular fingerprints in nanometer-thin molecular layers directly in the photocurrent of a graphene-based detector, eliminating the need for traditional bulky IR detectors.
In the longer-term, the team believes that on-chip infrared detectors operating at room temperature could enable rapid molecular identification, potentially integrated into smartphones or wearable electronics, which would offer a platform for compact sensitive, room-temperature infrared spectroscopy.