Nanoholes help make optical devices

A photosensitive solid-liquid compound made by filling nanoholes in aluminium oxide or polymer films with a solution of dye could be used to make efficient rewriteable recording media and all-optical switches. The device works by changing colour when ultraviolet (UV) light is applied to it. The nanoholes prevent the liquid from flowing, which safeguards the written information.

Some liquids have excellent optical properties not seen in solids, such as large refractive index change, efficient magneto-optical effects and high optical nonlinearity. However, liquids are rarely used in optical devices because they are difficult to handle.

Now, Mitsunori Saito and colleagues at Ryukoku University in Japan have overcome this problem by confining a dye solution in nanoholes that are between 50 and 200 nm wide. The liquid is photochromic – it changes colour when UV light is applied to it by undergoing isomerisation.

Although photochromism has been exploited in liquid-based devices before now, such as in photonic switches and diffraction gratings, the dyes were dispersed in polymer or glass matrices. Unfortunately, the resulting solids have slow response times because it is difficult to deform the photochromic molecules in a rigid matrix.

In the new device, confining the solution in nanoholes allows the researchers to handle the compound like a solid, while allowing photochromic isomerisation to take place. What is more, because the nanostructure is smaller than the wavelength of light, the compound acts as a metamaterial with uniform optical properties. The nanoholes also prevent the solution from flowing once an image has been optically written, therefore preventing it from being erased. Saito and colleagues made their device by allowing a solution of spirobenzopyran to percolate into columnar nanoholes in a film made of anodic alumina, polyvinylidenedifluoride or polycarbonate. This film was sandwiched between two glass plates. When exposed to UV light, the transparent film turns blue or violet depending on the matrix. The film becomes transparent again if visible light is applied to it.

“One can therefore write a pattern by either irradiating the transparent film with UV light or by irradiating the coloured film with visible light,” Saito told nanotechweb.org. “We can then erase the written pattern by irradiating with UV or visible light.” He says that the device could be used in dynamic holographic recording and all-optical switching.

The researchers now plan to investigate other photochromic dyes and study how molecular structure and size affect the photochromism in the nanoholes. They hope to create a novel compound with improved photochromic functions too. “We are also looking for porous materials with smaller holes (10 nm or smaller) to look at the effect of free volume on photochromism,” added Saito.