You can observe effects similar to those produced by a diffraction grating, when you look at a street lamp glowing in the distance through a dense lace curtain. You will see typical rainbow bands, most often four of them – two on both sides of the street lamp, one above and one below it. What you will see may be taken as a manifestation of bending or diffraction of light as a result of obstruction caused by the pattern of threads in the fabric of the lace curtain.
Another type of diffraction gratings are transparent phase diffraction gratings where thin and thick bars substitute for slits and rulings. Research findings of the recent years have been related to a certain type of phase gratings, namely, the so-called ‘subwavelength gratings’ with the grating period smaller than the wavelength of the light for which it is used.
Of special interest are the so-called high-refractive-index contrast gratings (high-contrast grating, HCG). HCGs have peculiar properties that make them suitable for applications for which other types of gratings cannot be used. The most astonishing and at the same time useful property of HCGs is that they give excellent possibilities of manipulating light. In particular, they can operate as near to perfect mirrors with the value of reflectance in excess of 99%. When other parameters are properly controlled, the resultant reflectance may also be close to zero.
Contemporaneously, semiconductor lasers are becoming more and more functional. CD, DVD and Blu-ray players are examples of everyday home electronics in which they are used. Reflective subwavelength gratings have been employed also in this field as thin and efficient mirrors that are an indispensable component of advanced semiconductor lasers called Vertical-Cavity Surface-Emitting Lasers (abbreviated as VCSEL). A subwavelength grating can substitute for the so-called DBR mirror (Distributed Bragg Reflector), which is a structure formed from multiple layers of materials with alternatingly low and high refractive index, commonly used for that purpose. The advantage that HCGs have over them is the breadth of the band of the reflected radiation and higher tolerance of manufacturing-related imperfections. As they are very thin, they make it possible to increase modulation speed of telecommunication lasers, which, for example, improves online data transmission speed.
Subwavelength gratings that produce a reverse effect manifested in substantial reduction of reflectance may serve as antireflective coatings deposited on the surface of optical instruments. Another significant example of their use are anti-reflection coatings for solar cells, which boosts their efficiency. New applications of subwavelength gratings are to be expected in photonics, optoelectronics, and fibreglass technologies.
It is also worth mentioning that natural diffraction gratings occur in the living world in some insects as a result of evolutionary processes. They are structures whose properties are similar to those of subwavelength gratings. In the structure of scales covering wings of Morpho rhetenor butterfly a complicated system of reflective subwavelength gratings that give the wings its intense blue colour and metallic lustre has been discovered. It has been found that the structure of the eye of many moth species includes microscopic rod cells in the cornea, the size of 200 nm. This structure lowers reflectance of the cornea to below 1% across the entire visible spectrum. Moths remain inactive during the day and can fall prey to predators then. Large, luminous eyes may give away their location, and therefore, limiting the reflection of light off their cornea is an evolutionary adaptation to improve their evolutionary fitness. Similar anti-reflective coatings are present on both surfaces of the wings of some cicadas and they are a form of camouflage that facilitates hiding. The mechanism behind how they work has also been explained by their analogy to two dimensional subwavelength gratings.
