Can degenerate modes have different resonant frequencies in photonic crystal cavities?

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shpongle
Can two degenerate modes have different resonant frequencies? I am asking question in context of resonance in photonic crystal cavities.

I am reviewing a paper (I have attached it).

Figure 2 (a) and (b) of this paper show modal profile for 2x2PCRR (photonic crystal ring resonator). In this figure two degenerate modes - quadruple0 and quadruple90 exist at different resonant frequencies!

This is possible? I thought degenerate modes have same energy and resonant frequency values.
 
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I read "On the other hand, a pair of doubly degenerate modes at" to mean that each of the wavelengths exhibits a double degeneracy, and that there are a pair of wavelengths.

I agree that degeneracy refers to multiple modes with the same energy; the phrasing of the article is confusing.
 
Many thanks for taking interest in reading the paper.

I am using using CST Microwave Studio, a commercial EM solver to simulate a simple photonic crystal given in figure 2(b) of this paper. The top crystal with a smaller cavity.

My question is:

Why do Quad0 and Quad90 occur at different wavelengths ? Quad90 is a degenerate mode of Quad0, shouldn't they occur at same wavelengths?

Also, out of curiosity, why do modes degenerate?