Re: [Meep-discuss] Negative Permittivity?

[Ardavan Oskooi]

On 01/03/2018 01:07 PM, Priscilla Kelly wrote:


I would like to probe a material with a pulse whose frequency has an epsilon = 
-.3 + .002i. I have tested the system with an epsilon of 1 and everything works 
as expected.


A negative, frequency-independent epsilon will yield 
exponentially-growing fields as a consequence of violating causality 
(due to Kramers-Kronig relations). For details, see the related 
discussion in the documentation 
. 
If you want to model materials with negative epsilon, you must add 
dispersion.
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[Meep-discuss] Negative Permittivity?

[Priscilla Kelly]

Hello, 

I would like to probe a material with a pulse whose frequency has an epsilon = 
-.3 + .002i. I have tested the system with an epsilon of 1 and everything works 
as expected. 

I changed epsilon like so:

(define-param r -.3)
(define-param i .002)
(set! default-material (make medium (epsilon r) (D-conductivity (/ (* 2 pi fcen 
i) r

where fcen is the frequency of the incoming pulse. 

However, when I look at the output field I am getting +nan.0+nan.0is for every 
time step. 

I just saw in a post from 2014 
(https://www.mail-archive.com/meep-discuss@ab-initio.mit.edu/msg05168.html) 
that the real part of
permittivity may never be negative. 

Has this been fixed? Is this a failure in how I set up the problem, or is the 
meep code not capable of handling negative permittivity values?

Thank you,
Priscilla
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