Anti-reflection coatings act as efficient broadband terahertz optics for applications in sensing and imaging...
In everyday use anti-reflection coatings are very familiar, from optical devices, such as glasses and lenses. Essentially they can increase the amount of light that passes through optical instruments by reducing the fraction of light reflected at surfaces. In addition, anti-reflection coatings have applications beyond visible light: for example, in the infrared and terahertz regimes where they are useful for chemical sensing and imaging applications, such as those employed at airport security checks.Collaboratively, between Jing Hua Teng from the A*STAR Institute of Materials Research and Engineering as well as the Institute of Micro-electronics and Osaka University, Japan, they have managed to develop an ultra-thin anti-reflection coating for terahertz waves, which can be applied to almost any surface. As explained by Teng, the nano-fabrication methodology is very straightforward, which only takes one step of photo-lithography, metal deposition and finally 'lift-off'.
Anti-reflection coatings are typically based on interference effects, which requires them to be at least as thick as the wavelength of light. This is very practical for visible light, with wavelengths in the range of hundreds of nanometers. However, it is a serious limitation for infrared and/or terahertz radiation, which has much longer wavelengths of the order of hundreds of microns. Plus these coatings are often only functional over a narrow frequency range, and do not operate across the broad ranges needed for terahertz sensing applications.
The research team developed anti-reflection coatings based on meta-materials, which are metallic structures that are much smaller than the wavelength used. These structures can completely alter the optical properties of a material in a predetermined manner, thus, enabling the generation of a much broader range of optical effects than those that occur naturally. One application of the unusual optical effects is the production of an invisible cloak.
In the new design, the meta-material surface developed by the researchers, was thin strips of Chromium (Cr) fabricated onto a Silicon (Si) surface to form a grating (see image above). Since Si is highly flexible, it is a typical material for terahertz optics. When terahertz light passes through the stripes and into the Si, its phase is changed in the same way as for the much thicker coatings based on interference effects.
It is claimed the meta-surfaces have a distinct advantage in that they can function across an unprecedentedly wide frequency range - 0.06-3 terahertz. The flexibility of the coatings for other wavelengths and/or applications also enhances their commercialisation potential, as commented by Teng - "the beauty of this method is that it is very flexible and can be easily adapted to other metals and substrates." Original article available here
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