Jul 17, '17Sensing the nanoscale with visible light, and the fundamentals of disordered waves [Science 365, 6341 (2017)]
We cannot see atoms with the naked eye because they are so small relative to the wavelength of light. This is an instance of a general rule in optics – light is insensitive to features which are much smaller than the optical wavelength. However, a new experiment by Hanan Herzig Sheinfux and Dr. Yaakov Lumer from the group of Prof. Moti Segev from the Technion, in collaboration with Dr. Guy Ankonina and Prof. Guy Bartal (Technion), and Prof. Azriel Genack (City University of New York), shows that even features that are more than 100 times smaller than the wavelength can still be sensed by light. This work examines a stack of nanometrically thin layers. The exact thickness of the layers is purposely random, and ordinarily would not affect the optical properties of the sample. But in this experiment a 2 nm (~6 atoms) thickness increase in one of 180 layers is enough to noticeably change the reflected intensity at a specific angle of incidence.
The combined effect of all the random variations in all of the layers manifests an important physical phenomenon called Anderson localization, but in a regime where it was believed to have vanishingly small effects. In 1958, Anderson predicted a transition from a conductor to an insulator would occur in electronic system as the disorder increases. The phenomenon is a general wave phenomenon but is particularly hard to observe when the random features of a sample are much smaller than the wavelength. However, the localization effect found here near the critical angle is surprisingly potent and sensitive to minute variations in the structure and may lead to new applications in sensing.
Figure: Schematic of experimental setup.