Reflecting on the Past
BY JAMIE KNAPP
NEWPORT CORP. – CORION OPTICAL FILTERS
One source rarely considered when addressing photonics issues is our ancestors: Great technologicaldiscoveries from many millennia ago maylead to modern day solutions.
A recent example of this involves optimizing gold-based infrared reflectors commonly employed for defense and aerospace instrumentation, imaging optics andFourier transform infrared (FTIR) spectroscopy (environmental monitoring).1 Inthe latter case, light is transmitted throughthe volume of air to be analyzed. The lightis reflected – via gold retroreflector arrays– back into appropriate analysis instrumentation, where optical absorption isused to evaluate pollutant content. Open-path environmental FTIR spectroscopy isused to monitor CO, CO2, NO and SO2
(absorption peaks are in the range of 1580to 15,000 nm). HF, HCl, H2S, NH3, CH4,CO2, HCN, C2H4 and C2H2 are measuredusing tunable diode laser spectroscopy inthe 1300- to 1700-nm range.
In the open environment, such mirrorsinevitably become soiled and dulled.When cleaned, the soft gold surfaces aredamaged. Current conventional gold mirrors, which are deposited upon polishedsubstrates, are therefore oftentimes produced with protective overcoats – common thermal and electron beam-depositedfilms include silicon monoxide, zinc sulfide and silicon. Such technologies mandate the use of elevated temperatures; e.g.,approximately 300 °C. For conventionalmetal mirrors, these manufacturing techniques are routine.2
An alternative to costly front-surfacemirrors is replication, a well-establishedtechnology employed to produce high-quality mirrors at a significantly lowercost. These mirrors, however, consist of acritical epoxy layer that is sensitive to elevated temperatures above approximately105 °C. Standard techniques normally employed to create “hardened” gold mirrorstherefore cannot be used.
Unless deposited at elevated temperatures, many protective thin films exhibitpoor adherence to the underlying gold andhave a porous, columnar micromorphol-ogy, limiting their protective properties. 3There is a critical need, therefore, to develop hard, durable, replicated gold mirrors using means that do not involve theelevation of temperature. Such a productmust maintain an adequate infrared reflectivity, particularly in the critical 1300- to1700-nm and 1580- to 15,000-nm ranges.
To address this, one may turn to theAncients. Mating metallurgical discoveriesof almost 2600 years ago, together withstate-of-the-art low-temperature thin-filmdeposition methods, allows for development of the desired hardened replicatedinfrared reflectors.
Introduced almost three millennia ago,coins are an integral part of daily life.From generation to generation, rulers,cities and states have issued a countlessnumber of coins.
Around 670 BC, the ancient Greeks ofIonia and Lydia – now located in modern-day western Turkey – experimented withproducing standardized preweighed lumpsof electrum, a natural gold and silver alloyfound in local river beds. 4 To minimizecounterfeiting, the lumps were struck witha chisel to expose their inner core; counterfeits were produced by metal-platingelectrum onto low-value bronze (Figure1).
Unfortunately, this did not slow the innovative counterfeiters who managed toproduce bronze-cored clones. The firsttrue coin, a piece of metal certified to beof a guaranteed designated monetary valueby a recognized governmental authority,was created by King Alyattes of Lydia in610 BC. The king’s official emblem of thelion 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, couldvary. To address this, under the rule of thelegendary King Croesus of Lydia, circa
570 BC, metallurgists developed a meansto divide and purify the gold and silverfrom raw electrum. 5 Separate gold and silver coins – 99 percent purity – formed thefirst “bi-metallic” currency.
The resultant wealth of this Greek region became too much of a temptation tothe neighboring ancient Persians. In 540BC, Croesus was defeated and his empiredestroyed. Afterward, King Darius of Persia began striking his new coin (Figure 2).
Known as a gold “daric,” this importantcoin featured the king’s image, so it wasvital that it not suffer wear, as was common in Croesus’ previous pure-gold issues. Persian metallurgists created analloy that not only maintained the desiredvisual brilliance of the coin but that significantly added to its hardness and wearresistance.
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, andre-emitted x-rays have wavelengths that
Figure 1. A typical electrum trite from circa 610 BCbears the emblem of King Alyattes of Lydia. Imagescourtesy of Newport Corp.
Figure 2. The gold daric, named for King Darius ofPersia, was created after the defeat of Croesus in540 BC. This coin bears the image of the king andwas fashioned from a wear-resistant gold alloy.
Applying Ancient Technologies to Modern Photonics