Cavity optomechanics: SERS boost

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  • Published: Dec 1, 2015
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Cavity optomechanics: SERS boost

Exploiting optomechanics

An illustration of light-mediated detection of a molecule. Credit: N. Antille, EPFL

Cavity optomechanics is a recent and growing research field exploring the interaction between light and mechanical objects in a wide range of physical systems, from kilometre-long interferometers down to high Q resonators at the micro- and nano-scale. It is now shown that the tools and concepts of cavity optomechanics can be exploited at the molecular scale to overcome the sensitivity limitations of SERS, surface enhanced Raman scattering, according to work undertaken at École Polytechnique Fédérale de Lausanne (EPFL).

SERS is increasingly the powerful "go to" technique for single molecule studies and can lead to dramatic enhancement of the Raman scattered signals, a surprising observation first made by Emory and Nie in a landmark experiment in 1997. Yet, to date, the underlying physics is not fully understood, preventing a systematic improvement of its sensitivity. Now, researchers at EPFL have demonstrated that a light-induced force can amplify the sensitivity of SERS and the spatial resolution of TERS, a related scanning-tip technique for molecular imaging. The team provides details in the journal Nature Nanotechnology. SERS, as we know, extends Raman spectroscopy using plasmonic “hot-spots” in which molecules can be placed to allow their internal vibrational modes to be observed as a shift in scattered laser light. Metallic nanoparticles act as a kind of antenna bringing the light into relief on the molecular scale; this boosts the signal by up to ten orders of magnitude. Nevertheless, the amount of Raman-scattered laser light is also limited by the relevant molecular vibrational modes being frozen at room temperature.

Theoretical solution

Now, two members of Tobias J. Kippenberg's team at EPFL have found a theoretical solution to this problem, showing that SERS can be pushed even further in sensitivity and resolution. The key in overcoming the weak vibrations is the cloud of oscillating electrons, the plasmon, which can exert a force on the vibrations of the molecule being investigated. "This insight was possible by uncovering an analogy between SERS and the very different and much larger systems used in cavity optomechanics," team member Christophe Galland told SpectroscopyNOW. Following this new framework, Philippe Roelli and Galland were able to determine the exact conditions needed for this light-induced force to drive the molecule's vibrations to larger amplitudes. As the light-force amplifies the vibrations of the molecule, the interaction between the molecule and the confined laser light grows stronger at the same time. This, the team found, can increase significantly the signal that SERS picks up, well beyond what was achievable even with the best of previously known mechanisms.


The team points out that this novel enhancement mechanism has not been considered before and they call it dynamical backaction amplification of molecular vibrations, referring to a phenomenon first demonstrated in 2005 that allows amplification (or cooling) of mechanical modes via radiation pressure. In the current paper the team explains how they obtain this result: "We first map the system onto the canonical Hamiltonian of cavity optomechanics, in which the molecular vibration and the plasmon are parametrically coupled. We express the vacuum optomechanical coupling rate for individual molecules in plasmonic 'hot-spots' in terms of the vibrational mode's Raman activity and find it to be orders of magnitude larger than for microfabricated optomechanical systems."

"Our work offers specific guidelines for designing more efficient metallic nanostructures and excitation schemes for SERS," explains Roelli. "It can push the limits of the technique in sensitivity and resolution." In so doing, this research opens up new directions for science in the control of molecular vibrations with light, with potential applications ranging from biology and chemistry to quantum technologies.

"The next step is to provide unambiguous experimental evidence for the backaction amplification mechanism," explains Galland. "Although there are already striking observations from other groups, which originally triggered our interest in SERS and its link with optomechanics, we want to perform new experiments with precisely defined plasmon properties, excitation wavelength and molecular vibrational modes, in order to demonstrate that backaction amplification alone can explain the additional signal enhancement. Indeed, the challenge is to rule out other potential sources of enhancement, such as chemical modification of the molecule in contact to metals, or direct heating by absorption of laser light."

He adds that there are a range of potential applications of this work, namely in understanding recently observed puzzling SERS and TERS phenomena reported by Zhang and colleagues in Nature in 2013 for instance. The research also has the potential to improve our undestanding of quantum superposition. Galland adds that, "More generally, we provide specific guidelines for the design of SERS experiments, which depart from widely accepted ideas. Our work makes specific predictions concerning optimal choice of excitation wavelength and the employed plasmon resonance. Beyond increasing SERS sensitivity, our proposed scheme would allow exciting only one specific vibrational mode of a molecule. This would bring the molecular vibrations out of thermal equilibrium, thus enabling the measure of much finer spectroscopic signatures or the study of the internal dynamics of the molecule and the redistribution of energy within different vibrational modes."

Related Links

Nature Nanotech 2015, online: "Molecular cavity optomechanics as a theory of plasmon-enhanced Raman scattering"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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