How to find Resonance frequency of rectangualr cavity from FDTD time domain data

Hello,
I have written 3D FDTD code with Mur ABC which gives correct results for time domain data.
Now I want to compute the resonance frequency of a rectangular cavity using this 3D FDTD code.
I donot know how I will do that.

Would someone please advise me the steps I need to follow now to calculate the rectangular cavity resonance frequency.
Any code or help on how the time domain data need to be processed is highly appreciated. Thank you.

Hi
you can use from energy curve to find resonant frequencies in a cavity.
So,you have to follow these steps:
1. recored E field components in one point at cavity,in time domain.
2. use from energy formula,to get energy in time domain.
3. use FFT to convert energy from time domain to frequency domain.
4. plot energy in frequency domain versus frequency range.
from the plot,you can find resonant frequencies of modes.
look at the attachment (section 2.5 and figure 4,5,6,...)
good luck
*ghasem*