Transmittance and reflectance of crystalline quartz and high-and low-water content fused silica from 2 Am to 1 mm

James B. Heaney, K. P. Stewart, and Georg Hass

The transmittances and reflectances of cultured crystalline quartz, Suprasil, Suprasil W, and Infrasil werecompared over the wavelength region from 2 to 1000 ,am. The high-water content of Suprasil and the low-water content of cultured crystalline quartz, Suprasil W, and Infrasil were determined by their transmit-tances measured at 2.73 ,um where water content causes high absorption in optical materials. The fact thatthe fused silicas, both with high- and low-water content, had identical far-IR transmittances and that theirtransmittances were greatly inferior to that of crystalline quartz led to the conclusion that their inferiortransmittance is due to their amorphous structure and not to their water content.

1. IntroductionNatural crystalline quartz has long been recognized

as an extremely useful mineral for electronic and opticalapplications. Cultured crystalline quartz, grown fromselected seed material, has been produced to eliminatetrace impurities and growth irregularities found in thenatural minerals whose presence interferes with theirapplications in science and industry. The opticalproperties of crystalline quartz, both natural and cul-tured, have been studied over a wavelength region ex-tending from the UV to the far IR by many investiga-tors.1-5 The same is true for amorphous fused silica,or fused quartz, which can now be produced as a ho-mogeneous material of high purity in almost any desiredsize. However, its IR optical properties have not beencharacterized over as broad a wavelength region, orsample types, as those of crystalline quartz.6-8 Bothcrystalline quartz and the amorphous fused silicas areexcellent optical materials for use from the UV to thenear IR but become strongly absorbing at -4 Aim. Ithas been reported that the transmittance of crystallinequartz increases again at -50 gim, whereas the amor-phous fused silicas remain strongly absorbing up to'150 Iim. This broad region of high absorptivity forfused silica, from -4 to -150 gim, limits its utility as aless expensive far-IR optical substitute for crystallinequartz. The physical and mechanical superiority, but

The first two authors are with NASA Goddard Space Flight Center,Greenbelt, Maryland 20771; G. Hass is at 7728 Lee Avenue, Alexan-dria, Virginia 22308.

Received 19 August 1983.

optical inferiority, of fused silica compared with crys-talline quartz for far-IR optical applications has beenreported by Hutchinson.8

Cultured crystalline quartz is virtually free of water,whereas fused silica is commercially available in formscontaining water in widely varying amounts. Thispaper will present the measured transmittances andreflectances of cultured crystalline quartz and fusedsilica, with high and negligible water content, over thewavelength region from 2 to 1000 ,um.

II. Experimental TechniquesThe cultured crystalline quartz specimens chosen for

measurement had been studied previously in the VUVregion of the spectrum by Hass and Hunter.1 Theyfound that optical cultured crystalline quartz can benonuniform in transmittance across a single piece.However, this nonuniformity, caused by extremelysmall amounts of impurities, is restricted to wavelengthsshorter than 1800 A and has no effect on its UV, visible,and IR optical properties.

Three cuts, X, Y, and Z, of the hexagonal uniaxialquartz crystal were measured. The orientation of thesecuts in the crystal is shown in Fig. 1.1 The Z axis is theoptical axis of the crystal and is orthogonal to the planecontaining the Z-cut. The three axes X, X', and X"that pass through the corners of the hexagon that formsthe section perpendicular to the optical axis are iden-tified as the electrical axes and a plane section cut fromthe crystal perpendicular to any one of these axes isknown as an X-cut.9 The Y-cut samples are obtainedfrom crystal sections that are orthogonal to any one ofthe axes Y, Y', and Y" that are perpendicular to thecrystal faces and are known as the mechanical axes. 9

The Y axes intersect the X axes at an angle of 30°, andthe Y-cut is also known as the 300 cut. This method of

15 December 1983 / Vol. 22, No. 24 / APPLIED OPTICS 4069

M2

TO DETECTOR

Z CUT

Y CUT

REF.

Fig. 2. Specular reflectometer for relative measurements at near-normal incidence.

X CUT1.00

0.90

0.80

0.70

C 0.60

2' 0.50

. 0.40

0.30

0.20

0.10

0.0 o | ;_ 1 1 XX0 I | I I I I I , I I X.! J I I I I I

1 10 100 1000WAVELENGTH (pm)

Fig. 3. Degree of polarization P and principal transmittances forparallel k, and crossed k2 grid type polarizers.

desired wavelength range. The KRS-5 substrate + gridwas useful over the wavelength range from 2 to 25 gim;the kapton substrate + grid was used at wavelengthslonger than 25 gim. Since the beam emerging from theinterferometer/spectrometer has significant polariza-tion that is spectrally variant, the linear polarizers wereplaced in a fixed orientation to maximize the outputsignal, and the samples were rotated to position theiroptical axes either parallel (E ray) or perpendicular (Oray) to the electric vector vibration direction of thelinearly polarized output beam.

Water content in SiO2 produces a strong absorptionat 2.73 gim. Therefore, transmittance measurementsat this wavelength were used to establish the relativewater content in the various SiO2 materials studied.Crystalline quartz showed no decrease in transmittanceat 2.73 gim, and Suprasil W and Infrasil exhibited anegligible transmittance loss, but the transmittance of3-mm thick Suprasil decreased to almost zero at thiswavelength. This clearly showed the tremendous dif-ference in water content of the crystalline quartz andthe three types of amorphous fused silica.

Ill. ResultsFigure 4 contains the reflectance and transmittance

of 3-mm thick crystalline quartz over the wavelengthrange from 2 to 1000 ,um for both the 0 and E rays.These results were composed from separate measure-

Fig. 1. Idealized quartz crystal showing the orientation of the X-,Y-, and Z-cuts relative to the optical axis.'

identifying the crystal cuts is related to their piezo-electric properties which are discussed more completelyby Terman.9

Each of the X- and Y-cut sections contain the opticalaxis parallel to their surfaces, and a polarizing micro-scope was used to identify the directions parallel andperpendicular to the optical axis. Consequently, theX- and Y-cut samples were oriented in a linearly pola-rized beam to establish the directions of the ordinaryO and extraordinary E rays during transmittance andreflectance measurements. The X- and Y-cuts areoptically identical, and, because they are also birefrin-gent, they are valuable for use as Babinet-Soleil com-pensators and quarterwave plates in ellipsometry.

Suprasil, a relatively high-water content fused silica,and Suprasil W and Infrasil, containing negligibleamounts of water, were selected for the measurementsto determine the effect of water content on the IR op-tical properties of amorphous fused quartz. (Thesethree silicas were supplied by Amersil, Inc., Sayreville,N.J.)

Transmittance and reflectance measurements weremade over the wavelength range from 2 to 1000 gmusing a Nicolet model 8000HV Fourier transformspectrometer. The samples were located in a f/2.3beam in the sample compartment for transmittancemeasurements. Figure 2 contains a sketch of thespecular reflectometer that was alternately placed in thesample compartment. Reflectance was measured rel-ative to freshly evaporated aluminum standards whosereflectance values are well known over an extended re-gion in the IR.10

Evaporated grid type polarizers were used whenevermeasurements were made on X- or Y-cut crystallinequartz samples. Figure 3 presents the polarizationcharacteristics of the two polarizers used to cover the

4070 APPLIED OPTICS / Vol. 22, No. 24 / 15 December 1983

X -

POLARIZER

M a

>T FROM INTERFEROMETER

1000WAVELENGTH IpWu)

Fig. 4. Transmittance and reflectance of cultured crystalline quartz,3 mm thick, for both 0 and E rays, over the wavelength range from

2 to 1000 Am.

ments of reflectance and transmittance performed onX- and Y-cut crystalline quartz samples. Reflectancemeasurements were performed at an incidence angle of-10°. The X- and Y-cuts that contain the optical axisparallel to their surfaces were measured in two orthog-onal directions of a linearly polarized beam. Trans-mittance and reflectance were found to be identical forboth cuts. Rotation of the samples 900 around the in-cident axis in the linearly polarized beam establishedthe 0- and E-ray reflectances and transmittances.Similar measurements with a Z-cut sample producedresults identical to those found for the 0 ray of the X-and Y-cut samples.

There is no absorption band at 2.73 gim in the trans-mittance spectrum to indicate the presence of water inthe material. (Spectral resolution was -2 X 10-3 giMor 4 cm- at X = 2.73 gim.) For these 3-mm thick sam-ples, the transmittance goes to zero at -4.5 gim and re-mains at zero up to 40 gim. The only feature of signif-icance in the far-IR portion of the transmittance spec-trum is an absorption band centered at -78 gm ob-served in the 0-ray component. This feature is absentfrom the E-ray transmittance spectrum.

Thickness related interference maxima and minimaoccurring at very far-IR wavelengths were of no prac-tical consequence and are not plotted in Fig. 4 andsubsequent figures.

The dominant features of the reflectance spectrumare the sharply defined reststrahlen bands. A com-prehensive discussion of the IR lattice structure ofnatural crystalline quartz that causes these bands hasbeen published by Spitzer and Kleinman.2 The re-flectance data in Fig. 4 and subsequent figures includecontributions from the front and back surfaces ofplane-parallel plates. The combined transmittance andreflectance data indicate that absorption is very low incrystalline quartz for wavelengths longer than 100 gimand diminishes as the 1000-gm region is approached.

The transmittance and reflectance of Suprasil, 1.5mm thick, are presented in Fig. 5 over the wavelengthrange from 2 to 1000 gim. The strong absorption bandat 2.73 gim in the transmittance spectrum indicatesclearly that this fused silica specimen contains consid-erably more absorbed water than the cultured crystal-

TAVEL H SURASIL (1.5z80

60

10 1 1 0 'I l l00 ' l0l

IIJ T \ .SUPRASIL {1.5 mm

WAVELENGTH {"ml

Fig. 6. Transmittance and reflectance of SuprasilW 1.5 mm thick, eovthe wavelength range from 2 to 1000 tm.

1.1 I~~~~~~~~~~~~~~~~~ IU RAIIW (.

lin qurt sape ise bv.Spa b

40 -i e

bc. useful I T

I- 201I1I0I

WAVELENGTH (,-I

Fig. . Transmittance and reflectance of Suprasil, 1.5 mm thick, oe

ve the wavelength range from 2 to 1000 m.

bnds qat ap21ls discuse bve notasishapl100rt. U nli te crystalline quartz a h oweerus

side bands seen in Fig. 4 that are characteristic of theSiO2 crystal lattice are missing in the amorphousSi820

Figure 6 contains the transmittance and reflectanceof Suprasil W. 1.5 mm thick, over the wavelength rangefrom 2 to 1000 Ani. Suprasil W is a relatively waterfreeversion of Suprasil, and the lack of a strong absorptionband at 2.73 Aum confirms this. Except for this oneobvious difference, the measured transmittance andreflectance curves for Suprasil W are essentially iden-tical to those of Suprasil. This is also true for the onsetof far-IR transmittance in the vicinity of 1oo Aum, whichis the same for both materials.

The transmittance and reflectance of Infrasil arepresented in Fig. 7 over the wavelength range from 2 to1000 AuM. Infrasil and Suprasil W were developed aswaterfree versions of Suprasil. Infrasil's far-UVtransmittance is inferior to that of Suprasil and SuprasilW. but its low-water absorption in the vicinity of 2.73,um makes Infrasil a good material for prisms, sub-strates, or windows for the visible and near IR. Themore expensive Suprasil W which has even less water

15 December 1983 / Vol. 22, No. 24 / APPLIED OPTICS 4071

0

0

4~u0

20 o40

.0 I~~~~~1 10 1 0

WAVELENGTH t-im

Fig. 7. Transmittance and reflectance of Infrasil,1.5 mm thick, overthe wavelength range from 2 to 100010m.

100

80

2

= 60

20.40

20

AUARTZ CRYSTAL CUTSUPRASIL 1.5 mm} ,1 I 3

--- SUPRASI L W1 5-mm a

10 100WAVELENGTH lrny

Fig. 8. Transmittances of cultured crystalline quartz, 1 and 3 mmthick, Suprasil and Suprasil W, both 1.5 mm thick, compared over the

wavelength range from 2 to 1000 m.

WAVELENGTH (uum}35 40 50 75 100 200 1000

100 I l

QUARTZ CRYSTAL 11

40

20MEASURED5/

2 0

20

300 250 200 150 100 50 0WAVENUMBERS 6cm I

Fig. 9. Measured far-IR transmittance of cultured crystalline quartz,1 mm thick, compared with transmittances calculated from opticalconstants published by Loewenstein et al.4 (indicated by circles and

squares).

absorption at 2.73 gim is a preferred optical material foruse from the UV to the near IR.

Except for a slightly lower transmittance at 2.73 gim,the measured transmittance and reflectance spectra ofInfrasil over the wavelength range from 2 to 1000 ,gm areidentical to that of Suprasil W. Suprasil, Suprasil W,and Infrasil have far-IR transmittances that are iden-tical.

A slight variation of approximately +0.02 was ob-served in the measured reststrahlen reflectance peakheights among the various fused silicas. Reflectancemeasurements performed on several additional Suprasilsamples yielded reststrahlen peak values that alsovaried within this +0.02 range. However, reflectancemeasurements of the front and back surfaces of eachpolished Suprasil, Suprasil W, and Infrasil sampleagreed within the limits of reflectance measurementprecision, +0.005, over the wavelength range from 2 to25 gim. This slight variance in reststrahlen reflectanceamong fused silica samples was treated as a propertyunique to individual samples and of no consequence forthis study.

A comparison of the IR transmittances of culturedcrystalline quartz (1- and 3-mm thick) Suprasil (1.5mm) and Suprasil W (1.5 mm) over the wavelengthrange from 2 to 1000 gm is presented in Fig. 8. Al-though the water content of Suprasil and Suprasil Wsamples differs greatly, their far-IR transmittances areidentical. The waterfree cultured crystalline quartz hasa far-IR transmittance cuton that is sharper and occursat a shorter wavelength then the transmittance of theamorphous fused silica.

These results are sufficient to demonstrate that thefar-IR transmittance of crystalline quartz is superior tothat of the amorphous fused silicas because of its crys-talline structure and not because of the water contentof the fused silicas.

Finally, the measured far-IR transmittance of crys-talline quartz, 1 mm thick, presented in this paper iscompared in Fig. 9 to 0 and E transmittances calculatedfrom optical constants published by Loewenstein et al. 4The agreement is very good over the entire region withthe exception of the measured strength of the absorp-tion line at 78.4 ,um (127.5 cm- 1) in the 0-ray spec-trum.

This paper was presented at the Annual Meeting ofthe Optical Society of America, Tucson, Ariz., Oct.1982.

References1. G. Hass and W. R. Hunter, Appl. Opt. 17, 2310 (1978).2. W. G. Spitzer and D. A. Kleinman, Phys. Rev. 121, 1324

(1961).3. E. E. Russell and E. E. Bell, J. Opt. Soc. Am. 57, 341 (1967).4. E. V. Loewenstein, D. R. Smith, and R. L. Morgan, Appl. Opt. 12,

398 (1973).5. K. D. Cummings and D. B. Tanner, J. Opt. Soc. Am. 70, 123

(1980).6. C. M. Randall and R. D. Rawcliffe, Appl. Opt. 6, 1889 (1967).7. T. J. Parker, J. E. Ford, and W. G. Chambers, Infrared Phys. 18,

215 (1978).8. I. H. Hutchinson, Infrared Phys. 22, 117 (1982).9. F. E. Terman, Electronic and Radio Engineering (McGraw-Hill,

New York, 1955).10. G. Hass, J. B. Heaney, and W. R. Hunter, in Physics of Thin

Films, Vol. 12, G. Hass, M. H. Francombe, and J. L. Vossen, Eds.(Academic, New York, 1982), pp. 1-51.

4072 APPLIED OPTICS / Vol. 22, No. 24 / 15 December 1983

. . ; | .