In difference in refractive index of two materials

Inthe recent years, the interest of studying the optical thin films ispermanently growing.

1-10 Optical thin film coatings have variety ofuses in different areas of day –to –day life and find its application in thefields like instrumentation, optical diagnostics3,display technologies,avionics5-11, interferometry, spectrophotometers and holography 12-21etc. The optical thin film filters are a key factor to tune the white lightwith respect to the selective range of wavelength for the device or instrument.All the filters basically work on interference principle of light during vacuumevaporation of dielectric materials in the form of layers. These types ofdielectric materials have certain advantages like variable bandwidth 22,23over a large range, random selection 24- 26ofwavelengthregion and high transmission because of the non-absorbing nature of thedielectric materials26-28.

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In present days, most of the filters are designedand constructed using stack of different materials in an alternative fashion ofindices of refraction and thickness. In general, all the thin film coatings arefabricated by using conventional techniques with high and low index layers, whichworks on the basis of difference in refractive index of two materials and correspondingthickness of each layer. Recently, there has been a growing interest ininhomogeneous, graded index51andrugate thin films44-48,which are characterized by continuous variation of refractive index orthickness or both throughout the film. The advantages of such graded or rugateoptical thin films are too abruptthe discontinuity of the index of medium andcoated film as well as the outer layer index to the air. In alternativehomogenous layers, there is large variation of refractive index mismatch atboundaries of glass to coated layer and final layer to the exit medium, whichin turn develop higher harmonics at the regions adjacent to stop band.

Elimination of these higher order harmonics by using graded index designspromotes thesuppression of high frequencies 25,26,  higher laser damage threshold 20-24,51,high temperature stability 27, smaller residual stress28,29,better adhesion30, 31 and wear resistance 29,32 of thefilm. Allthe optical display systems usually use dichroic mirrors with color splittingand combining for projection displays.4,33,34 The color splitting isstrongly related to the incidentangles of the light falling on the substrate.

34Toget the uniform color performanceof the mirror, it is essentialto vary the filmthickness gradually throughout the surface. Many methods have already been usedfor performing the gradient mirror coatings. One of the possible method toperform thegradation of the thin film is use of shaper 35- 40Shapersduringthe process of deposition.

However,designing of shapersand Shapers is a challengingtask especiallyfor small size of dielectric optical components. By using the Shaper,maintainingthe uniformity of the layer with the shaper from end to end is complex. It hasbeen reported that it is difficult to obtain the thickness distribution as wellas gradation along the mirror surface in the rotating planetary holder withoutcorrection of the Shaper34, 49- 51. Toovercome the above problem, an optimal method with modified holder withoutcorrecting the Shaper is adopted in this work. The rotatory jig holder waschanged with flat jig holderusing mechanical fitting to hold the substrate attilt angles. The modified mechanical holder consists of a specially designed hollowjig, which is resting on the spring adjusted portion.

By adjustingthe height ofthe spring, tilt angle of the substrate can be varied. With these variations,the incident angles across the surface of the mirror and consistent colorperformance across the large surface of the mirrorwas observed. Moreover, themodified coating jig is more flexible to provide gradient mirror coating withdifferent thicknesses.

The optical performance of the mirror ischaracterizedusing the Gonio spectrometer and the roughness gradient in the mirror wasmeasured using the Profilometer.   

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