Gold standard

A new class of cost-effective broadband absorbers shows promise in capturing light waves outside the visible range.

Visible light accounts for only a little more than 40% of sunlight. Capturing much of it that falls outside this visible range is vital for many applications, including light energy harvesting and photo-detection. Scientists have been developing materials that can absorb incoming light waves without reflection and transmission for over a decade. Most of these materials are costly metamaterials requiring complex and elaborate manufacturing processes.

A finding by Jaspreet Singh at the Indian Institute of Technology (IIT) Ropar may have opened up ways to make a new class of cost-effective broadband absorbers. Singh, a PhD student of Subhendu Sarkar, an Associate Professor of Physics at the institute, has found that easy-to-make nanoporous gold (NPG) films could emerge as an ideal absorber of wavelengths in ultraviolet, visible and near-infrared regions. This will help in the harvesting of energy and sensing applications.

"People in the past elicited such optical response in materials with ordered structures (such as metamaterials). We wanted to see if materials with disordered architecture too can produce this optical response," says Sarkar, who, along with Singh and others, recently published a study in Applied Surface Science (bit.ly/nanoporousgold).

"The material (NPG) is quite cost-effective and is very easy to make," Sarkar says. For this, the scientists first put an adhesive layer of chromium, which was a few nanometres thick. Subsequently, they coated a gold layer of varying thicknesses — 25 nm, 50 nm, 75nm and 100nm — on top of it using a vacuum coating method, commonly used in industrial settings. Atop that, they vacuum-sprayed a layer of gold-silver alloy of a similar thickness. After deposition, the IIT Ropar scientists removed the silver particles by dipping the prepared material briefly in nitric acid. Once washed and air-dried, the broadband absorber was ready to use.

Their subsequent studies showed that the NPG film-based absorber could soak up radiation in ultraviolet, visible and near-infrared ranges with 0.85 absorbance. Absorbance is a measure of how much light is absorbed by a material, the highest theoretical value being 1.
 
The IIT Ropar team also found that the material's light absorption could be tuned by adjusting the thickness of the NPG film, as well as the concentration of acid or the duration of the dip in the acid. The team will now try to substitute the glass substrate with a stretchable polymer.

Pankaj Srivastava, Professor of Physics at IIT Delhi, describes the work as "scientifically sound", but that the test of its significance will be whether the material "has an advantage in terms of its optical properties, or it is opening up a new direction of research in this field, or is cost-effective."