The optical signature of an SCM is a circular-polarization-sensitive stopband. The center wavelength and the width of this stopband depend on the direction of the wavevector of an incident circularly polarized plane wave. Most significantly, the stopband is exhibited when the incident plane wave’s handedness is the same as the structural handedness of the SCM, but not otherwise. The stopband is best seen when the thickness of the SCM exceeds a certain number of helical pitches.^{6}^{,}^{9}^{–}^{11} When dissipation is small enough to be ignored, $\epsilon \u0333r(z)$ is positive definite, and the variations of $\epsilon a,b,c$ with respect to the free-space wavelength $\lambda 0$ are also small enough to be ignored, the circular-polarization-sensitive stopband can be delineated as^{4}Display Formula
$\lambda 02\Omega \u2208{[\epsilon c,\epsilon d]cos1/2\u2009\theta ,\epsilon c<\epsilon d,[\epsilon d,\epsilon c]cos1/2\u2009\theta ,\epsilon c>\epsilon d,$(4)
where $\theta $ is the angle of incidence with respect to the $z$-axis and Display Formula$\epsilon d=\epsilon a\epsilon b\epsilon a\u2009cos2\u2009\chi +\epsilon b\u2009sin2\u2009\chi .$(5)
Provided that $Re[\epsilon \sigma ]\u226b|Im[\epsilon \sigma ]|$ for all $\sigma \u2208{a,b,c}$, the estimates provided by Eq. (4) can be used with $\epsilon c$ replaced by $|\epsilon c|$ and $\epsilon d$ by $|\epsilon d|$. The exhibition of the circular-polarization-sensitive stopband is called the circular Bragg phenomenon.