Subwavelength focusing of light reflected from a silver surface with a periodic structure

It is demonstrated that the reflection of light from the surface of silver with a periodic structure in the form of a one-dimensional grating can lead to subwavelength focusing of light near the surface. Under these conditions, the diffraction limit on spatial resolution is given by the expression $\Delta x=a_0/2$, where $a_0$ is the grating period. For short grating periods (about $10$ nm), this limit is much smaller than the classical diffraction limit $\Delta z=\lambda/2n$ (where $n$ is the refractive index of an infinite optical medium and $\lambda$ is the wavelength). It is shown that, with an increase in the depth of the grating profile, the field strength of the diffracted waves near the silver surface becomes significantly higher than that of the specularly reflected wave.