Subwavelength triangular random gratings

A subwavelength triangular diffractive element, whose local periods are not regular as an ordinary grating but has random values within a certain range, is proposed as a substitute for a common subwavelength antireflection grating. Its diffraction properties are electromagnetically analysed by the finite-difference time-domain method. Such a random grating shows some interesting properties, including suppression of higher diffraction orders. This improves the signal-to-noise ratio of a signal wave. Also, it is found possible for a random grating to retain a diffraction effciency comparable with that of a regular grating, even when some local periods violate the subwavelength condition, provided that the random local period range is properly chosen. This is another advantage of a random grating over the regular grating in the fabrication process, because upper limitation on feature sizes is partially relieved. In addition, application to general random scattering problems is discussed.