nickel by electrodeposition, and he measured their complex refractive indices.
Alexander E. Outerbridge of the U.S.
Mint developed and patented a similar
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
than fused silica
glass, stable up
to 1,200 °C and
to stop a .50
316 and 326 nm in
the ultraviolet, which
has been exploited to
bandpass filters. Copper
was reported to transmit
visible light at sufficient
thickness, giving white
light a bluish or pinkish appearance.
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.
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
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 ( email@example.com) is with
Bodkin Design and Engineering in Newton,
[ 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.
>> T. Turner. “Transparent silver and other
metallic films,” Proc. Royal Soc. London
81, 301 (1908).
>> B. Nagler. “Turning solid aluminum transparent by intense soft X-ray photoionization,” Nat. Physics 5, 693 (2009).
>> D. Pile. “Transparent sodium” Nat. Photonics 3, 250 (2009).
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