Hi Abdul,

Polyelectrolte multilayer films might be an alternative:

Reversibly erasable nanoporous anti-reflection coatings from polyelectrolyte multilayers, Nature Materials 1, 59 - 63 (2002) 

Yours,

Andreas

In brief, among the best materials that act as antireflection coatings , in particular for optical devices are silicon nitride, silica (SiO2), Titania and zinc oxide.

There several techniques to fabricate them , which mainly depend on the type of application.

I hope this can help.

soluble Teflon AF !!!, n=1.28

In my opinion the biggest effect is not refraction but surface topographie.I would go that way or maybe a combination. Try to diffuse the incoming light by rough surface in nano or micro scale. Plus maybe high absorption of light = black

Refraction and smoothness are two significant factor! Average roughness no more than 1 nm - good!

TiN is a very good choice as an ARC if you have the facility.

low refractive index material viz. Al2O3, MgF2, and SiO2 single layer thin films

Thanks 

Dear Keyur Patel and Aadhityan Arivazhagan

If I understand well, then you have a Ti-metal (or substrate with opaque Ti coating), overcoated with a SiO2 layer. As Ti has a refractive index of about 2.37 (middle of visible spectrum), you will need a material with a refractive index of SQRT(2.37*R.I. of incoming medium) or 1.54 to have AR properties in the middle of the visible spectrum. SiO2 comes close (R.I. 1.45) and is closer than Al2O3 (1.68). There will be some reflection still (middle of the visible spectrum) and there will be rising reflection for lower and higher wavelengths. Such a single coating is called a V-coat AR-coating. If you need AR over a broader spectrum, you will need to work with minimum a 3-layer system (maybe even 4 layers). (e.g. a thin layer of TiO2 - which you probably already have as a native oxide - a half-wave layer of Al2O3 and a quarter wave layer of SiO2). In any case you will need to determine the wavelength (or range) over which you want AR ... 

Material of antireflection coatings is depend on refractive index of substrate. For Si or LiJO3 (1.6) enough SiO2 (1.4). For SiO2 or glass/quartz (1.4 - 1.5)  will be required n

Thanks Sir Paul Lippens 

Before reading your suggestion, yesterday, I have deposited Ti (first layer) as reflecting layer, SiO2 (2nd layer) as a dielectric and then mixture of TiO2 and SiO2 (3rd layer) to get good absorption on glass substrate. But still there is reflection.

May be I need some more description about half-wave layer of Al2O3 and a quarter wave layer of SiO2? Half-wave layer and Quarter-wave layer means their thickness? please

Dear Abdul,

I just read your response to Paul Lippens, and I am not sure to understand what you are trying to accomplish.  Do you want a black coating on glass? (low reflection with high absorption?)

Dear Abdul,

I referred to the general theory of optical thin films (see e.g. book of Angus McLeod). Suppose your design wavelength is 550 nm (about middle of the visible spectrum) and you want antireflection for that wavelength (maybe even for a range of wavelengths below or above this 550 nm). Then your design wavelength is 550 nm; a quarter wave thin film of SiO2 then will have a physical thickness of 95 nm (t = lambda/(4*n) with n = 1.45). A half-wave thickness of Al2O3 is a physical thickness t = 164 nm (t = lambda/(2*n) with n = 1.68). In any case your outermost coating must be a low index coating (if you want to sputter it, your only option is SiO2; in case you can do thermal evaporation a better choice is MgF2 with n = 1.32). This must be a quarter wave thickness. However, the ideal index of that coating should be SQRT(2.37*1) or 1.54, hence SiO2 already has a too low index. In that sense, it is a good idea to put some TiO2 in the SiO2 coating. Ideally you need 9% TiO2 in SiO2 (0.09 * 2.45 + 0.91 * 1.45 = 1.54). This would give you zero reflectance for the design wavelength. Introducing an Al2O3 layer of quarter wave between the Ti and the top coat (mix of 9% TiO2 and SiO2) will make the AR spectrum broader.

Paul,

What you explained would work if Ti was a dielectric layer (purely real refractive index), but making an antireflection coating on a thick reflective metal is more complex, and usually requires a `black-layer coating` made of a few layers of dielectric and metal (very thin semitransparent layers of metal).  It could be just 3 layers (diel/thin-metal/diel) on Ti.  But I am not sure that it is what Abdul needs.

I think black carbon coating is the required answer. This will give you maximum absorption and no reflection.

I suggest SiNx as ARC, I have used it before in thin film solar cell and I've had positive results

Thanks to all friends

 Respected Daniel Poitras,

I am sorry for very late reply.

Yes Sir, this is my final requirement to deposit a black coating on glass or may be on some other substrates.

Now, I am trying to use textured surfaces to attain high absorption. 

I shall appreciate your expert suggestions.

Thanks

Textured surfaces contribute to scattering, but not absorption.

OK Thanks Elina Yu. Ladilina,

I shall consider this and appreciate your suggestion.

i am also working on on the comparison of ARC material.

texturing involves roughening the  surface of a substrate to scatter or redirect light. The most common texturing method is wet chemical etching in an alkaline solution, usually KOH or NaOH. Immersing the substrates e.g (single-crystalline silicon) in the solution leads to anisotropic etching; the resulting surface morphology is dependent on the orientation of the wafer and the relative etch rate between different crystal planes .i.e  depend on the substrate used. I think this link will help   M. A. Green et al. Very high efficiency silicon solar cells - science and technology. IEEE Trans. Electron Devices 46(10) (1999), pp. 1940-1947.

Thanks a lot brother Ismail Ibrahim

I would suggest a coating structure that looks like this:

Thick Ti/SiO2/thin Ti (or better, Cr, Ni, Inconel)/SiO2

The thickness values of SiO2 layers and thin metal (Ti,Cr...) layer would have to be optimize to lower the reflectance.  Such systems can reach very low reflectance and can look very black.  For even lower reflectance, you can deposit it on a rough substrate or add one more pair thin-metal/SiO2.

A few questions:

• what level of reflectance do you want?
• what is the wavelength range?
• Where do you want the absorption to occur? (in the thick Ti layer? elsewhere?)

I attached 2 papers from my group where we show black layer coating designs on reflective susbtrates.

Article Black Layer Coatings for the Photolithographic Manufacture o...

Article Versatile computer program for absorbing optical thin film systems

Thanks a lot 

Prof. Daniel Poitras

• Our target is to totally overcome the reflection.
• Wavelength range is from 300 nm to 2500 nm.

We also have tried some textured surfaces and got a some good results according to our requirement.

Thanks for your help