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CX2SA > SCIENC 22.11.05 05:22l 86 Lines 3979 Bytes #999 (0) @ WW
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Subj: LEDs work like butterflies'
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LEDs work like butterflies' wings
=================================
When scientists developed an efficient device for emitting light, they hadn't
realised butterflies have been using the same method for 30 million years.
Fluorescent patches on the wings of African swallowtail butterflies work in a
very similar way to high emission light emitting diodes (LEDs).
These high emission LEDs are an efficient variation on the diodes used in
electronic equipment and displays.
The University of Exeter, UK, research appears in the journal Science.
In 2001, Alexei Erchak and colleagues at the Massachusetts Institute of
Technology (MIT) demonstrated a method for building a more efficient LED.
Most light emitted from standard LEDs cannot escape, resulting in what
scientists call a low extraction efficiency of light.
Ingenious design
----------------
The LED developed at MIT used a two-dimensional (2D) photonic crystal - a
triangular lattice of holes etched into the LED's upper cladding layer - to
enhance the extraction of light.
And layered structures called Bragg reflectors were used to control the
emission direction. These high emission devices potentially offer a huge step
up in performance over standard types.
Pete Vukusic and Ian Hooper at Exeter have now shown that swallowtail
butterflies evolved an identical method for signalling to each other in the
wild.
Swallowtails belonging to the Princeps nireus species live in eastern and
central Africa. They have dark wings with bright blue or blue-green patches.
The wing scales on these swallowtails act as 2D photonic crystals, infused with
pigment and structured in such a way that they produce intense fluorescence.
Pigment on the butterflies' wings absorbs ultra-violet light which is then re
-emitted, using fluorescence, as brilliant blue-green light.
Performance-enhancing bugs
--------------------------
Most of this light would be lost were it not for the pigment being located in a
region of the wing which has evenly spaced micro-holes through it.
This slab of hollow air cylinders in the wing scales is essentially mother
nature's version of a 2D photonic crystal.
Like its counterpart in a high emission LED, it prevents the fluorescent colour
from being trapped inside the structure and from being emitted sideways.
The scales also have a type of mirror underneath them to upwardly reflect all
the fluorescent light that gets emitted down towards it. Again, this is very
similar to the Bragg reflectors in high emission LEDs.
"Unlike the diodes, the butterfly's system clearly doesn't have semiconductor
in it and it doesn't produce its own radiative energy," Dr Vukusic told the BBC
News website "That makes it doubly efficient in a way.
"But the way light is extracted from the butterfly's system is more than an
analogy - it's all but identical in design to the LED."
Dr Vukusic agreed that studying natural designs such as this could help
scientists improve upon manmade devices.
"When you study these things and get a feel for the photonic architecture
available, you really start to appreciate the elegance with which nature put
some of these things together," he said.
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