Optical Materials. Section 18. Materials

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Ezra Pitts
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1 18-1 Optical Materials Sectio 18 Materials

2 Idex of Refractio Some commo idices: vacuum 1.0 helium hydroge air water 1.33 fused silica 1.46 plastics borosilicate crow glass 1.51 crow glass 1.52 light flit glass 1.57 dese barium crow 1.62 dese flit 1.72 diamod m 5 m m 10 m m m The idex of refractio is usually quoted relative to air istead of vacuum.

3 Dispersio The refractive idex is a fuctio of wavelegth. The idex icreases at the absorptio bads. This is aomalous dispersio. Betwee the absorptio bads the idex decreases with wavelegth. ( ) Absorptio Bad 18-3 Optical materials Geerally oe of the absorptio bads occurs i the ultraviolet ad oe occurs i the ifrared. The ormal part of the curve is used i the visible. The idex for blue wavelegths is higher tha the idex for red wavelegths.

4 Differet Materials Idex Wavelegth ( m) N-LaSF31 N-SF6 N-LaK8 N-F2 N-BK7 N-FK51

5 Dispersio Curves Idex of refractio is commoly measured ad reported at the specific wavelegths of elemetal spectral lies. Over the visible spectrum, the dispersio of the idex of refractio for optical glass is about 0.5% (low dispersio) to 1.5% (high dispersio) of the mea value of the idex. 1 d - 1 F d C d - C F - C F (H) m d (He) m C (H) m I (Hg) m h (Hg) m F' (Cd) m g (Hg) m e (Hg) m D (Na) m C' (Cd) m r (He) m t (Hg) m 18-5 Refractivity: 1 d Pricipal Dispersio: Partial Dispersio F d C C Dispersio: d d 0

6 Abbe Number Abbe umber (or reciprocal relative dispersio) is the sigle umber used to characterize the dispersio of the idex of a optical material: V d 1 F C Refractivity Pricipal dispersio Typical values of the Abbe umber for optical glass rage from 25 to 65. Low -values idicate high dispersio. Relative partial dispersio ratio or P-value gives the fractio of the total idex chage that occurs betwee the d ad C wavelegths d - C. P P dc, d F C C 18-6 Due to the flatteig of the dispersio curve, P d,c < 0.5. P-values ca also be defied for other sets of wavelegths: P XY, X F Y C

7 Glass Map The glass map plots idex of refractio versus Abbe umber. By traditio, the Abbe umber icreases to the left, so that dispersio icreases to the right. The glass lie is the locus of ordiary optical glasses based o silico dioxide. FK51 80 Crow Glass 70 Rare Earth Glass BK7 Fused Silica Water SK16 BaK4 60 LaK8 LaF21 KzFS4 PMMA 50 LaSF31 Flit Glass KzFS5 40 F2 LLF1 LaSF46 SF6 SF2 Polystyree Polycarboate Dispersio d

8 Copyright SCHOTT 2018 North Joh America, E. Greivekamp Ic. Glass Map Schott North America, Ic. 18-8

9 Glass Map - Cotiued The lie at separates the glasses ito the two primary classificatios: crow glass (low dispersio) ad flit glass (high dispersio). The additio of lead oxide icreases the dispersio ad the idex ad moves the glass up the glass lie. To icrease the idex without chagig the dispersio, barium oxide is added. The rare earth glasses are lathaum oxide based (istead of silico dioxide) ad provide high idex ad low dispersio. Glasses away from the glass lie are softer ad more difficult to polish. They are also susceptible to staiig. They ca be very expesive just for the raw material. Low idex glasses are less dese ad geerally have better blue trasmissio Glasses have bee recetly reformulated to elimiate lead ad arseic. Lead is replaced with other elemets, especially titaium. The ew or evirometally safe glasses usually carry a N, S or E prefix (depedig o the maufacturer).

10 Glass Code The six-digit glass code specifies the idex ad the Abbe umber: abcdef 1. abc de. f d For example BK7 (the sigle most commo optical glass): d The ehaced glass code also ecodes the desity of the glass i g/cm 3 : For BK7: The glass desity is 2.51 g/cm 3

11 Glass Data * Schott Glass Techologies Ic. desigatio. Equivalet glasses ca also be obtaied from Ohara Corp. ad Hoya Corp. Material Code d F C P Rel $ N-FK51* N-BK LLF N-BaK N-KzFS N-F N-SK N-SF N-KzFS N-LaK N-LaF N-SF N-LaSF31A N-LaSF46A Fused Silica PMMA Polycarboate Polystyree Water The properties of a idividual sample, especially for the plastic materials ad water, ca vary from these catalog values. For precisio systems, the measured idices of the actual glass should be used i fial desigs. The listed idices are measured relative to air ( ), ad the idices should be corrected for use i vacuum.

12 Other Glass Properties I additio to idex data at various wavelegths, the glass catalog lists other materials properties importat for a desig such as coefficiets of thermal expasio, temperature coefficiets of refractive idex, iteral trasmissio as a fuctio of wavelegth, several mechaical properties, ad chemical resistace values. Chemical properties (low is good): CR Climate resistace, water vapor (Scale 1-4) FR Stai resistace (Scale 0-5) SR Resistace to acids (Scale 1-4 plus 51-53) AR Alkali resistace (Scale 1-4) PR Phosphate resistace (Scale 1-4) α = Temperature coefficiet of expasio T g = Glass trasitio temperature C p = Specific heat = Thermal coductivity = Desity HK = Koop Hardess τ i = Iteral trasmissio ( 5 mm ad 25 mm thick) d/dt = Temperature coefficiet of refractive idex Coefficiets of the dispersio formula Approx. Glass Cost (2014): N-BK7 N-LaK8 N-FK51 $11/lb $85/lb $280/lb N-LaSF31 $380/lb 18-12

15 Copyright SCHOTT 2018 North Joh America, E. Greivekamp Ic. N-BK7 Schott North America, Ic

17 Other Optical Materials Fused Silica or Fused Quartz: Pure SiO 2 ; ot crystallie (amorphous) Good UV trasmissio to about 200 m (depeds o purity) Used for microlithography leses Low thermal expasio: BK7 = 7.1x10-6 / C Fused Silica = 5.5x10-7 / C Note: Fused silica has a extremely high chage of idex with temperature eve though it is dimesioally very stable. May ot be a good choice for thick widows. Low Expasio Materials: Partially crystallized glasses glass ceramics CER-VIT ULE Zerodur = 0 +/- 1x10-7 / C 18-17

18 More Optical Materials Crystallie Materials: Some are birefriget. Others are polycrystallie proe to scatterig. Ofte the oly way to obtai trasmissio i the IR. Examples: Si m ad m Ge m ZS m ZSe m CaF m (Fluorite) NaCl m LiF m CsBr m CsI m A high idex implies high Fresel reflectios (36% for Ge) require AR coatig. Some of the materials are sesitive to water vapor. Some provide excellet trasmissio i the UV ad the IR. Some are very soft ad fragile hard to polish. For IR applicatios, some of the materials may be sigle-poit diamod tured Plastics: Usually molded; ca also be cast. Idex chages with water absorptio. Most commo: Acrylic (PMMA) (Plexiglas) Polystyree Polycarboate