to be described herein is a method for adjusting an index of refraction of a lanthanum modified lead - zirconate - titanate ( plzt ) optical coating which has been deposited on an optical body , for example , a lens , the index of refraction of the coating being changed by heat treating the coated optical body in a controlled atmosphere . a specific composition of the plzt has a zr / ti ratio in the range around 65 / 35 and with lanthanum substitution in the range around 8 - 9 %. the plzt is preferably sputtered to form the film or coating on a surface of the optical body . the initial index of refraction of the plzt coating is relatively high ≅ 2 . 5 . it has been determined that at a heat treating temperature of about 800 ° c the refractive index can be changed . in this invention , the optical body being coated is of an optical material able to withstand a temperature excursion to 800 ° for a period of hours . one specific example of a suitable material is glass . electro - optic ceramic material of the quaternary ( pb 1 - x la x ) ( zr y ti 1 - y ) o 3 system commonly known as plzt has been intensively investigated in recent years . the high optical transmission and high refractive index in the visible to near infrared region , large linear and quadratic electro - optic effect , and the availability of materials possessing a range of desirable properties make plzt extremely useful for electro - optic applications . epitaxial growth of plzt monocrystalline thin film oxides is generally obtained by chemical vapor transport and liquid phase epitaxy methods . the refractive index of plzt is strongly dependent on the la content and the zr / ti ratio . this makes the medium suitable for the in - diffusion or out - diffusion technique to achieve refractive index modification . modification of refractive index of plzt is possible because pbo is a component in plzt that can be readily out - diffused as well as in - diffused at a temperature of 800 ° c . first analyzing the case of out - diffusion , plzt may be designated as ( pbo ) 1 - x . sup .. la x ( zr y ti . sub . ( 1 - y )) 1 - x / 4 o 2 + x . if the concentration of pbo for a given plzt sample is altered , the material after diffusion may be designated as from this formulation , it can be expected that a small change of dν is equivalent to a change of - 2dx . from known results on the refractive index dependence on x ( la concentration ), we have ## equ1 ## following the analysis of j . r . carruthers , i . p . kaminov and l . w . stulz , &# 34 ; diffusion kinetics and optical wave - guiding properties of out - diffused layers in linbo 3 and litao 3 &# 34 ;, appl . optics , 13 , 2333 ( 1974 ), we may define the concentration of pbo , relating to variation of ν by c = - δν ( m . sub . pbo / m . sub . plzt ) ρ . sub . plzt = - δν × 0 . 567 equation 3 where m &# 39 ; s are the molecular weights and ρ the density . fick &# 39 ; s second law of diffusion is defined by ## equ2 ## where y is the distance from surface and t the time . the diffusion constant d is temperature t dependent as which defines the activation energy q d , with r = 1 . 99 cal /° k mol . the vaporization flux j . sub . ν is given by where ierfc is the integral of the error function complement . alternatively , experimental results indicate that a reasonable fit could be obtained using exponential dependence . at the surface , the vaporization flux is related to the equilibrium vapor pressure p l in atm and temperature t in ° k by the langmuri relation , assuming the molecules evaporate into vacuum , at this point , we can assess the technique to evaluate the pertinent coefficients with a view of achieving an understanding of the diffusion process in plzt . the measurable quantities are : ( 1 ) δn ( o , t ) by brewster angle technique on sample surface , ( 2 ) δn ( y , t ) by interference microscope measurements and , ( 3 ) j . sub . ν by direct measurement of weight loss . we obtain d by the δn ( o , t ) and j . sub . ν measurements . measuring δn ( y , t ) also yields j . sub . ν and d . in table 1 are listed the physical properties , of the component materials , relevant to the diffusion process . this table indicates that at a temperature at or below 800 ° c , only pbo will be affected . table 1 . ______________________________________physical properties of componentmaterials of plzt vaporization melting density temperature atmaterial temp . ° c gm / cc 10 . sup .. sup .- 4 torr______________________________________ pbo 890 9 . 5 550la . sub . 2 o . sub . 3 2215 6 . 5 2000 tio . sub . 2 1850 4 . 3 1000 zro . sub . 2 2700 5 . 6 2200______________________________________ it is believed , from the indicated properties , that at a temperature of 800 ° c or below , out - diffusion of any component except pbo is not appreciable . based on this observation , in - diffusion has been performed also by placing pbo materials in the diffusion chamber . the observed recovery of the refractive index and weight increase under this condition further verifies that pbo is the active element involved in the diffusion process . the weight loss due to out - diffusion on plzt samples is a function of temperature . the diffusion time is calculated based on the time a peak temperature with a small addition to take into account the temperature rise and fall for each heating cycle . these results give the following data shown in table 2 . table 2 . ______________________________________diffusion temperature and vaporization fluxsample diffusion jtreated temperature gm / cm . sup .. sup .- 2 / sec p . sub . l ( atm ) ______________________________________plzt 800 ° c ( thin ) 41 × 10 . sup .. sup .- 9 1 . 9 × 10 . sup .. sup .- 9 800 ° c ( thick ) 25 . 6 × 10 . sup .. sup .- 9 1 . 2 × 10 . sup .. sup .- 9 750 ° c 5 × 10 . sup .. sup .- 9 0 . 23 × 10 . sup .. sup .- 9 700 ° c 1 . 3 × 10 . sup .. sup .- 9 0 . 059 × 10 . sup .. sup .- 9plzt 800 ° ( thin ) - 9 . 4 × 10 . sup .. sup .- 9 0 . 45 × 10 . sup .. sup .- 9 + pbo______________________________________ the refractive index n at the surface of out - diffused and subsequently in - diffused samples were measured after each diffusion cycle by the brewster angle technique . the result from one sample is shown in fig1 . fig1 shows surface refractive index n f changes versus cycles of out - diffusion and in - diffusion of plzt . in the specific example shown by the curve of this figure , each heating cycle consists of a 20 minute temperature rise time , 40 minutes at 800 ° c , and 5 minutes of temperature fall time , except the first seven cycles where only rapid heating and cooling were used for sample normalization . the original sample weight is 7 gm with a surface area of 5 . 7cm 2 . the vacuum at the vacuum pump is 3 - 8 × 10 . sup . - 6 mmhg . to evaluate that t 1 / 2 dependence of n at the surface of the sample , the results are replotted as shown in fig2 . fig2 plots the surface refractive index versus square root of diffusion cycles . this gives δn / t 1 / 2 which is related to j . sub . ν by ## equ3 ## from these results we obtain δn / t 1 / 2 to be 0 . 0061 sec 1 / 2 . this combined with j . sub . ν = 41 × 10 . sup . - 9 yields the refractive index gradient has been measured in a bulk thick sample using a zeiss jamin - lebedeff transmission interference microscope . the out - diffused sample was first sliced perpendicular to the out - diffused surface and then polished to optical flat . care was taken to avoid any turned down at the edge . the interference pattern at the surface of the out - diffused sample was photographed with a 5460 ° a light source and the fringe lines of the photograph observed . the center of gravity of each fringe line was traced and the surface refractive index profile was then obtained . the diffusion depth extended to ≅ 0 . 1 millimeter . the thickness of the sample used was 114 . 3 μm . the refractive index variation at the surface was given from equations 2 , 3 , and 6 to yield ## equ4 ## the measured value of ( dn / dy ) = 7 . 7 cm . sup . - 1 . therefore , for this particular sample with j . sub . ν = 1 . 2 × 10 . sup . - 9 , we obtain this checks well with the d value obtained earlier since they are measured for different samples . for the optical coating applications , the coating thicknesses are approximately 1 / 4to a few wavelengths of the optical beam , and the refractive index of the coating is nearly uniform throughout the out - diffused layer . in the above description i have described a method for adjusting the index of refraction of a plzt coating which has been deposited on an optical body by heat treating the plzt coating in a controlled atmosphere whereby a single coating material can be used yet providing an adjustable index to match a desired value .