Plasmonic nanostructures are metal devices that enhance local electromagnetic fields in a material by coupling incoming light with “plasmons” (oscillations of electron density in a metal). The optical properties of these materials are directly related to so-called localized surface plasmon resonances, which depend on the collective excitations of the electrons. They are increasingly being used in chemical sensors and in vivo imaging, and such applications exploit the “secondary light” emitted by these nanostructures. This light is of a different wavelength than the light used to excite the sample, but until now, researchers were unsure as to its exact nature.

Thanks to new pulsed laser excitation experiments on aqueous suspensions of gold nanorods as a model system, a team led by David Cahill is now saying that resonant electronic Raman scattering may be used to describe the light emission. Fluorescence occurs in certain materials, such as organic dyes or phosphors, that absorb light of one wavelength and then emit it at a different wavelength. However, in Raman scattering, the wavelength of light used to excite a sample is instantly shifted to a different colour by being scattered by molecular vibrations or electron-hole pairs in the material.

Raman scattering model is correct

In their experiments, the Illinois team excited gold nanorods using laser light at a wavelength of 488 nm and 785 nm and analysed the spectra produced from the samples. The researchers found that light emission was enhanced at the electromagnetic plasmonic resonance of the rods.

Light emission from plasmonic nanostructures at wavelengths shorter than the wavelength of light used to excite the sample is usually described as the simultaneous absorption of two photons followed by fluorescence, explains team member Jingyu Huang. “However, we have found that models that describe the emission as Raman scattering from electron-hole pairs can successfully predict how the light emitted depends on the laser power, the time between two closely spaced laser pulses, and the duration of a laser pulse.

Better understanding the mechanisms behind secondary light emission will be very useful for biological imaging techniques that rely on analysing this type of light, he adds.

Cahill says that his team now plans to study lithographically fabricated plasmonic nanostructures. “This is so that we can better understand aspects of the light emission that elude us at present – for example, the disagreement between the observed and predicted light emission at very high laser powers,” he told nanotechweb.org.

The current work is detailed in PNAS doi: 10.1073/pnas.1311477111.

Further reading

Nanoparticles slow down cancer cells (Apr 2013)
Medical imaging goes for gold (Apr 2007)
Seeing biomarkers with the naked eye (Aug 2011)