Most electronic devices have glass or plastic covers that protect against dust, moisture, and other environmental contaminants, but light reflection from these surfaces can make information displayed on the screens difficult to see. To address this problem, a method was developed for reducing the surface reflections from glass surfaces to nearly zero by etching tiny nanoscale features into them.
Whenever light encounters an abrupt change in refractive index (how much a ray of light bends as it crosses from one material to another, such as between air and glass), a portion of the light is reflected. The nanoscale features have the effect of making the refractive index change gradually from that of air to that of glass, thereby avoiding reflections. The ultra-transparent nanotextured glass is antireflective over a broad wavelength range (the entire visible and near-infrared spectrum) and across a wide range of viewing angles. Reflections are reduced so much that the glass essentially becomes invisible.
This “invisible glass” could not only improve the user experience for consumer electronic displays, but also enhance the energy-conversion efficiency of solar cells by minimizing the amount of sunlight lost to reflection. It could also be a promising alternative to the damage-prone antireflective coatings conventionally used in lasers that emit powerful pulses of light, such as those applied to the manufacture of medical devices and aerospace components.
To texture the glass surfaces at the nanoscale, an approach called self-assembly was used — the ability of certain materials to spontaneously form ordered arrangements on their own. In this case, the self-assembly of a block copolymer material provided a template for etching the glass surface into a “forest” of nanoscale cone-shaped structures with sharp tips — a geometry that almost completely eliminates the surface reflections. Block copolymers are industrial polymers (repeating chains of molecules) found in many products including shoe soles, adhesive tapes, and automotive interiors.
To quantify the performance of the nanotextured glass surfaces, the amount of light transmitted through and reflected from the surfaces was measured. Experimental measurements of surfaces with nanotextures of different heights showed that taller cones reflect less light; for example, glass surfaces covered with 300-nanometer-tall nanotextures reflect less than 0.2 percent of incoming red-colored light (633-nanometer wavelength). Even at the near-infrared wavelength of 2,500 nanometers and viewing angles as high as 70 degrees, the amount of light passing through the nanostructured surfaces remains high — above 95 and 90 percent, respectively.
In another experiment, the performance of a commercial silicon solar cell without a cover was compared with a conventional glass cover, and with a nanotextured glass cover. The solar cell with the nanotextured glass cover generated the same amount of electric current as the one without a cover. The nanotextured glass also was exposed to short laser pulses to determine the intensity at which the laser light begins to damage the material. Measurements revealed the glass can withstand three times more optical energy per unit area than commercially available antireflection coatings that operate over a broad wavelength range.