Contrary to periodic structures, disordered ones do not have true band-gaps. But still, if the scattering is strong enough, most wavelengths can not propagate inside a disordered structure either, due to a phenomenon known as "Anderson Localization". If we change slowly the incident wavelength (shown by the white sinusoidal in the bottom corner), we see that most wavelengths penetrate very little, but for some there is a randomly generated mode inside the system to couple to. These modes are very narrow in frequency, so even by scanning the wavelength very slowly, they appear and disappear in a blink.
<a href="//commons.wikimedia.org/wiki/User:Berto" title="User:Berto">Jacopo Bertolotti</a>
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