Optics & Photonics News

nickel by electrodeposition, and he measured their complex refractive indices. Alexander E. Outerbridge of the U.S. Mint developed and patented a similar deposition process.

From fiction to fact

In the early 20th century, scientists learned that thin sheets of silver could transmit white light as blue. George Gabriel Stokes was the first to note a decrease in the reflectivity of silver in the ultraviolet.

So is Scotty’s transparent aluminum possible? Yes, it is. Two modern-day examples include aluminum oxynitride (AlON) and a transparent aluminum film created by FLASH laser bombardment.

Even for relatively thick
silver films, there was
a “window” between
Scientists began developing AlON,
an aluminum-based ceramic, in 1981. It
was covered heavily in the
press, which has led many
to believe that it may have
inspired the writers of Star
Trek IV. Surmet Corpora-
tion (Mass., U.S.A.) com-
mercially produces AlON
by packing, compressing
and curing aluminum oxy-
nitride powder into a mold.
AlON is an optically clear
material that is four times
harder than fused silica
glass, stable up to 1,200 °C
and strong enough to stop a

AlON is four times harder than fused silica glass, stable up to 1,200 °C and strong enough to stop a .50 caliber bullet.

316 and 326 nm in the ultraviolet, which has been exploited to produce ultraviolet bandpass filters. Copper was reported to transmit visible light at sufficient thickness, giving white light a bluish or pinkish appearance.

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Shortly after Stokes’ discoveries, R.W. Wood demonstrated that alkali metals are transparent in the ultraviolet, approaching the visible; for example, lithium has a long-wave cutoff at 205 nm, sodium at 210, potassium at 315 nm and rubidium at 360. Cesium is transparent to visible light, extending up to 440 nm. Unfortunately, alkali metals are highly reactive. Even when they are sandwiched between plates of UV transmitting material, they tend to pull into droplets and develop holes. Nevertheless, several different groups have used them to make ultraviolet transmitting filters with thick-nesses of a single micron.

.50 caliber bullet. Production costs limit its use to high-performance military applications such as armored windows, battlefield optics and round windows covering missile sensors.

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In 2008, scientists at Oxford University, U.K., created a truly transparent form of aluminum by bombarding the metal with the FLASH (free-electron laser in Hamburg) laser. The laser emits a short pulse that knocks out a core electron from each aluminum atom without disrupting the crystal structure. This turns the metal sheet nearly invisible to extreme ultraviolet radiation. Although still in development, this material has potential applications in planetary science and nuclear fusion. t

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More recently, thin films of metals and semiconductors have been used as extreme ultraviolet transmitting films. Aluminum transmits from 8 to 32 nm, indium from 73 to 112 nm and bismuth from 50 to 75 nm.

Stephen R. Wilk ( swilk@comcast.net) is with Bodkin Design and Engineering in Newton, Mass., U.S.A.

[ References and Resources ]

At high pressure, sodium is transparent in visible light and essentially becomes an insulator. New research has shown that sodium can also transmit red light at 1. 9 Mbar and is predicted to be completely transparent in visible light at about 3 Mbar.