Molecular coherence corresponds to a situation where an ensemble of molecules is prepared in a superposition state and vibrates in unison; this can lead to remarkable results. One example is a technique termed molecular modulation, which allows ultrafast laser pulse shaping and non-sinusoidal field synthesis via coherent Raman generation. Another example is molecular (Raman) spectroscopy, which is a powerful tool for chemical sensing, identification, and mapping. This technique relies on rich and specie-specific, fingerprint-like, spectra of molecular vibrations, which are read out optically. Spontaneous Raman micro-spectroscopy is the simplest to implement, but has two significant drawbacks. Firstly, spontaneous scattering is inefficient, therefore requiring long acquisition times; secondly, the technique is limited by optical diffraction, preventing its usage in studying nanoscale structures.<br/>In our work, we solve the first problem by using molecular coherence. When laser pulses excite molecules of a sample into a coherent state, Raman signal generation is enhanced multi-fold. The molecular excitation can occur through a Raman, pump-Stokes process, or by a direct infrared, single-photon process. Aside from details such as transition selection rules and molecular symmetry considerations, the two types of molecular coherence work comparably well and result in similarly large signal enhancement factors. The second limitation of the traditional Raman spectroscopy, the optical diffraction limit, is circumvented by using a near-field technique where a metalized scanning tip acts like a plasmonic nano-antenna and allows efficient collection of spectroscopic signal from a nanometer-scale hot spot. Such tip-enhanced Raman technique was recently used, for example, for direct readout of nucleic acid sequences.<br/>These two approaches – the coherent laser signal generation and tip-enhanced detection – when combined, will enhance our ability to perform molecular spectroscopic measurements at the single virus level, and will enable chemical mapping of various bio-agents and pathogens, aiding in rapid and sensitive detection, diagnostics, and drug development.

Molecular coherence-based laser spectroscopic tools for biophotonics

ACS Fall 2021

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