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PT symmetry in optics beyond the paraxial approximation.

The concept of the PT symmetry, originating from the quantum field theory, has been intensively investigated in optics, stimulated by the similarity between the Schrödinger equation and the paraxial wave equation governing the propagation of light in guiding structures. We go beyond the paraxial approximation and demonstrate, solving the full set of the Maxwell's equations for the light propagation in deeply subwavelength waveguides and periodic lattices with balanced gain and loss, that the PT symmetry may stay unbroken in this setting. Moreover, the PT symmetry in subwavelength guiding structures may be restored after being initially broken upon the increase of gain and loss. Critical gain/loss levels, at which the breakup and subsequent restoration of the PT symmetry occur, strongly depend on the scale of the structure.