Shine light on a molecule and it will generally scatter elastically. But a few photons will bounce off with a different wavelength, having transferred some of their energy to the molecule. This is known as Raman scattering and it involves the excitation of vibrational or rotational transitions — a very handy feature for vibrational spectroscopy, as the wavelength shifts act as molecular fingerprints, allowing for the identification of molecules in a sample. Now, two studies in Nature Photonics have shown that a related technique has potential to improve imaging capabilities — from the single-molecule level all the way up to intact tissues.

Credit: NPG

Raman scattering and its close cousin, stimulated Raman scattering, enable another spectroscopic technique known as coherent anti-Stokes Raman scattering (CARS), which uses photons of different colours and yields a coherent signal. CARS has been used and studied extensively over the past forty years, but it seems that there is still room for innovation. For instance, CARS is useful for imaging biological tissues, but it does have its limitations — particularly in terms of speed. It is also hampered by distortions owing to the non-resonant background that arises due to electronic excitations that accompany the resonant vibrational modes.

Charles H. Camp Jr and colleagues (Nature Photon. 8, 627–634; 2014) made use of this non-resonant background to amplify weak Raman signals and, by combining this with three-colour excitation, they succeeded in producing high-speed chemical images of liver tissues (pictured). Their technique is expected to find application in fundamental research and clinical studies — and may even lead to further developments in in vivo imaging.

At the other end of the spectrum, probing single molecules in ambient conditions presents a different set of challenges and requires another approach. Amplification of the weak Raman signal from a single molecule can be achieved with surface-enhanced Raman scattering using plasmonic antennas, as Steven Yampolsky and co-workers have shown (Nature Photon. 8, 650–656; 2014). The team attached ethylene molecules to two gold nanospheres in a dumbbell-like structure and enclosed it in silica. With their dumbbell antenna, they were able to use the CARS signal to record the motion of the vibrational wave packet of single molecules in real time. Reaching such sensitivity at room temperature is quite remarkable. CARS may be old, but it is certainly capable of learning some new tricks.