While performing e.m. simulations, often permittivity or refractive index values are required for materials settings. From what reported in literature, I noted that in Silicon Dioxide the general approximation of epsr= nr2 is not valid, with nr being 1.44 (infrared range) and epsr being 3.89.
Can anyone explain why ?
epsr= nr2 assumed the imaginary part of refractive index is negligible,i.e, the equation is only applicable for very low loss material at optical wavelength.
Dear Zulkifly,
I really thank you for your reply.
I see your answer, but if I apply this concept I'm wondering why silicon is a low loss material, given the approximation is numerically valid, even if actually, as far as I know, doped silicon can take very optically loss, whereas silica gives high loss.
n*=nr-j(ni) ... the equation is only applicable if the imaginary part is much lower than the real part , i.e, ni
n*=nr-j(ni) ... the equation epsr= nr2 is only applicable
if the imaginary part is much lower than the real part , i.e, ni
yes, the theory is known, my question is more related to the practical application of the theory.
Dear Stefano,
in my practical work for index of refraction and permittivity I am using:
http://refractiveindex.info/
I checked for BK-7 and found that all is O.K.
With regards,
Hovik
what was the measured values of the real and imaginary parts of the refractive index of your silicon dioxide?
there are also issues regarding solutions to electromagnetic wave equation.. whether is a vectorial or scalar wave approximation ..the latter assumed the material to be low loss
I didn't, as I have no ellipsometer or prysm coupler. I used tha data available on the web.
I found that Silica has a permittivity of 3.9, with a relative refractive index 1.44. In this case the approximation nr 2= epsr is not valid.
Neverthless, Silicon is reported having a permittivity of 11.68 and a refractive index of 3.48. So, for Silicon the approximation is valid.
So, since both the materials are used for waveguides, I'm wondering about the reason of this, I mean from a practical point of view.
have heard the term material dispersion? Debye's law?
permittivity is used at microwave frequencies... at optical wavelngth the term used is refractive index. ....When you mention silica has a permittivty of 3.9 but a refractive index 1.44 .... i suspect the permittivty values were measured at Rf/microwave frequencies.. if youmeasure at optical wavelength it is most likely the refractive will never be the square root of the the permiitivity obtained at rf/microwave frequencies due to material dispersion
Dear Zulkifly,
I really thank you. So, I search for permittivity measured both in the optical and in the RF domain.
This is very interesting, as my aim is to design a photonic EO modulator, so the same materials (Silica and SIlicon) are used both for waveguiding, so in the optical domain, and as part of a capacitor with an RF electrical field. Therefore, I must take in count the different permittivities when optimizing the optical field and the elctrical field (two different simulators).
I really thank you for your really precious info.
Best regards
Stefano
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