Single-molecule optical absorption imaging by nanomechanical photothermal sensing at room temperature

TL;DR

This paper introduces nanomechanical photothermal microscopy, a technique that detects single-molecule absorption at room temperature by measuring temperature-induced frequency shifts in silicon nitride drums, surpassing traditional optical limits.

Contribution

The authors develop a nanomechanical sensing method that directly measures absorption via temperature-sensitive substrates, overcoming scattering and shot noise limitations of conventional optical absorption microscopy.

Findings

Achieved a sensitivity of 45 fW/√Hz for absorption detection.

Demonstrated detection of single molecules with SNR > 60.

Successfully imaged and analyzed non-fluorescent nanoparticles and molecules.

Abstract

Absorption microscopy is a powerful technique, enabling the detection of single non- fluorescent molecules at room temperature. So far, the molecular absorption has been probed optically via the attenuation of a probing laser. The sensitivity of optical probing is not only restricted by background scattering, but it is fundamentally limited by laser shot noise. Here, we present nanomechanical photothermal microscopy, which overcomes the scattering and shot noise limit by detecting the sample absorption directly with a temperature sensitive substrate. We use nanomechanical silicon nitride drums, whose resonant frequency detunes with local heating. Individual Au nanoparticles with diameters from 10 nm - 200 nm and single molecules (Atto 633) are scanned with a 305 {\mu}W heating laser with a peak irradiance of 330 {\mu}W/{\mu}m2. Using stress-optimized drums, we achieve a sensitivity of 45 fW/sqrt(Hz), which results in a signal-to-noise ratio of >60 for a single molecule. Our method has important consequences for a wide range of applications, such as imaging, absorption analysis and spectrochemical analysis of non-fluorescent samples.