Reflecting on the Past
Applying Ancient Technologies to Modern Photonics
BY JAMIE KNAPP
NEWPORT CORP. – CORION OPTICAL FILTERS
One source rarely considered when addressing photonics issues is our ancestors: Great technological
discoveries from many millennia ago may
lead to modern day solutions.
A recent example of this involves optimizing gold-based infrared reflectors commonly employed for defense and aerospace instrumentation, imaging optics and
Fourier transform infrared (FTIR) spectroscopy (environmental monitoring).1 In
the latter case, light is transmitted through
the volume of air to be analyzed. The light
is reflected – via gold retroreflector arrays
– back into appropriate analysis instrumentation, where optical absorption is
used to evaluate pollutant content. Open-path environmental FTIR spectroscopy is
used to monitor CO, CO2, NO and SO2
(absorption peaks are in the range of 1580
to 15,000 nm). HF, HCl, H2S, NH3, CH4,
CO2, HCN, C2H4 and C2H2 are measured
using tunable diode laser spectroscopy in
the 1300- to 1700-nm range.
In the open environment, such mirrors
inevitably become soiled and dulled.
When cleaned, the soft gold surfaces are
damaged. Current conventional gold mirrors, which are deposited upon polished
substrates, are therefore oftentimes produced with protective overcoats – common thermal and electron beam-deposited
films include silicon monoxide, zinc sulfide and silicon. Such technologies mandate the use of elevated temperatures; e.g.,
approximately 300 °C. For conventional
metal mirrors, these manufacturing techniques are routine.2
An alternative to costly front-surface
mirrors is replication, a well-established
technology employed to produce high-quality mirrors at a significantly lower
cost. These mirrors, however, consist of a
critical epoxy layer that is sensitive to elevated temperatures above approximately
105 °C. Standard techniques normally employed to create “hardened” gold mirrors
therefore cannot be used.
Unless deposited at elevated temperatures, many protective thin films exhibit
poor adherence to the underlying gold and
have a porous, columnar micromorphol-ogy, limiting their protective properties.
3
There is a critical need, therefore, to develop hard, durable, replicated gold mirrors using means that do not involve the
elevation of temperature. Such a product
must maintain an adequate infrared reflectivity, particularly in the critical 1300- to
1700-nm and 1580- to 15,000-nm ranges.
To address this, one may turn to the
Ancients. Mating metallurgical discoveries
of almost 2600 years ago, together with
state-of-the-art low-temperature thin-film
deposition methods, allows for development of the desired hardened replicated
infrared reflectors.
History
Introduced almost three millennia ago,
coins are an integral part of daily life.
From generation to generation, rulers,
cities and states have issued a countless
number of coins.
Around 670 BC, the ancient Greeks of
Ionia and Lydia – now located in modern-day western Turkey – experimented with
producing standardized preweighed lumps
of electrum, a natural gold and silver alloy
found in local river beds.
4 To minimize
counterfeiting, the lumps were struck with
a chisel to expose their inner core; counterfeits were produced by metal-plating
electrum onto low-value bronze (Figure1).
Unfortunately, this did not slow the innovative counterfeiters who managed to
produce bronze-cored clones. The first
true coin, a piece of metal certified to be
of a guaranteed designated monetary value
by a recognized governmental authority,
was created by King Alyattes of Lydia in
610 BC. The king’s official emblem of the
lion acted as a deterrent to counterfeiters.
Electrum, however, suffers from a variable alloy composition – gold content varied from 45 to 55 percent. Exchange values from coin to coin, therefore, could
vary. To address this, under the rule of the
legendary King Croesus of Lydia, circa
Figure 1. A typical electrum trite from circa 610 BC
bears the emblem of King Alyattes of Lydia. Images
courtesy of Newport Corp.
Figure 2. The gold daric, named for King Darius of
Persia, was created after the defeat of Croesus in
540 BC. This coin bears the image of the king and
was fashioned from a wear-resistant gold alloy.
570 BC, metallurgists developed a means
to divide and purify the gold and silver
from raw electrum.
5 Separate gold and silver coins – 99 percent purity – formed the
first “bi-metallic” currency.
The resultant wealth of this Greek region became too much of a temptation to
the neighboring ancient Persians. In 540
BC, Croesus was defeated and his empire
destroyed. Afterward, King Darius of Persia began striking his new coin (Figure 2).
Known as a gold “daric,” this important
coin featured the king’s image, so it was
vital that it not suffer wear, as was common in Croesus’ previous pure-gold issues. Persian metallurgists created an
alloy that not only maintained the desired
visual brilliance of the coin but that significantly added to its hardness and wear
resistance.
X-ray fluorescence
To unlock the secrets of Darius’ coins,
the nondestructive method of energy-dispersive x-ray fluorescence spectrometry
(XRF) was employed. With this technique,
the sample is irradiated with x-rays, and
re-emitted x-rays have wavelengths that
