Nanowire laser measures the refractive index of its surroundings
From vineyards to oil refineries, refractometers are common quality-control tools. Grape growers gauge the ripeness of their fruit from the refractive index of the juice, petroleum engineers use refractive-index sensors to monitor the hydrocarbon content of refined fuels, and beekeepers use them to measure the water content of honey. In biomedicine, the sensors can detect cancer or other abnormal cells, which typically have higher than normal refractive indices.
High-tech sensors rarely get simpler than the one devised by the University of Michigan’s Xudong Fan and his collaborators: It’s a 204-nm-diameter, 15-μm-long cadmium sulfide wire that acts as an optical cavity. As illustrated in the figure, when the nanowire is excited with an external light source, it lases at a wavelength that depends on the refractive index of its surroundings.
Fan and his colleagues tested their sensor in mixtures of ethanol and toluene; changing the relative concentration changed the refractive index. They found that the lasing linewidth is sufficiently narrow to distinguish variations in refractive index as small as 1.4 × 10−3.
Additionally, because the reflectivity of the wire’s ends depends on the difference between the refractive indices of the wire and the liquid, so does the output laser intensity. Consequently, the lasing intensity could also serve as a proxy for the liquid environment’s refractive index. Although intensity measurements are somewhat less precise than wavelength measurements, they wouldn’t require an interferometer.
Cells have been shown to readily internalize nanowires. Thus, say Fan and his colleagues, their nanowire laser has the potential to sense the refractive indices of individual cells. (X. Wu et al., Appl. Phys. Lett. 111, 031112, 2017 .)