a naive answer

you need to have an idea of the possible source of absorption.

for instance, if optical phonons are present singularities will occur at the corresponding frequencies. between 400 and 700 nm it is more electronic excitations that will play a role.

below are two references that help me (at lower energies)

hope it helps

regards

[27] F. J. Garcia de Abajo and P. M. Echenique, Phys. Rev. B

46, 2663 (1992).

[28] Palik and Hunter Handbook of Optical Constants in Solids

(Academic Press, New York, 1985).

In case of a thin film, it is necessary to measure the reflectance and transmitance (or reflectance from both sides of a film) and then use our method :

1. Kudykina T.A.,Boundary Conditions in Case of Electromagnetic Wave Absorption

physica status solidi,  1990, v.(b)160, N2

2.Kudykina T.A., Lisitsa M.P.  Размерные зависимости оптических параметров тонких пленок Ge, Si, Se, Te. Оптоэлектроника и полупроводниковая техника. 1997, вып.32., 106-114.

3.  Kudykina T.A.  Optical peculiarities of thin absorbing films. SPIE, 1998,v. 3573

Is it in a thin film? If so, you can use ellipsometric data to simply find out both n and k. Further details in:

-https://www.researchgate.net/publication/263974257_UltravioletVisible_Plasmonic_Properties_of_Gallium_Nanoparticles_Investigated_by_Variable-Angle_Spectroscopic_and_Mueller_Matrix_Ellipsometry

and

-http://photonics.intec.ugent.be/education/IVPV/res_handbook/v2ch27.pdf

Best

Article Ultraviolet−Visible Plasmonic Properties of Gallium Nanopart...

Its a nano-Sphere.. I have got a book which is Handbook of Optical constants of Solids by Edward Palik...most of the properties in lumeric FDTD defined by this book... very good book..Thank you very much to all i am really appreciate your reply...especially Juan Marcoz you always reply my questions i appreciate it man... 

Not always, just sometimes, no problem, hehe. Yes, talking about optical properties Palik is a widely accepted reference.

but the other people not even some time ..so i am using that refrence..hehehe  I am new in this field my background is of power electronics and solid state physics.  

Recently i define Calcite in FDTD it shows the 0.98 RMS error. i have put data from the book of Palik.. But material data is not matched with FDTD model... 

@Tamara Kudykina  my question is about refractive index and what you provide me the links please look yourself before you suggest to anyone...

i depending on the SPR  simulation program to evaluate any refractive index real and imaginary of sensing layer >>>because the the absorption of sensing layer depending on the thickness so if I know thickness I can get the real and imaginary part

thanks a lot Ali... i have  sorted out now... thanks for co-operation

Please read the paper :

https://arxiv.org/ct?url=https%3A%2F%2Fdx.doi.org%2F10.1051%2F0004-6361%253A20078468&v=b861e42f