Nonlinear Optics Pushed to the Edge

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Science  02 May 2014:
Vol. 344, Issue 6183, pp. 483-484
DOI: 10.1126/science.1253531

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Nonlinear optics describes processes in which coherent photons change their properties, such as their frequency, when they pass through a medium. This field traces back to the demonstration of frequency doubling (1)—generating photons with twice the frequency of the input laser light—via second-harmonic generation [(SHG); see the figure, panel A]. Because SHG is a second-order parametric process, the symmetry of the medium must lack inversion symmetry (2). Interfaces between bulk (or three-dimensional) materials can meet this criterion, which has made SHG microscopy an indispensable tool for their study (3). On page 488 of this issue, Yin et al. (4) open a new dimension in second-harmonic microscopy with the optical visualization of the edge modes in two-dimensional (2D), atomically thick materials (5).