  The question of how to continuously manipulate a photonic system between a
radiating state and a bound state is an important challenge in photonics, as
its solution promises broad technological relevance for optical sensors,
modulators, switches and displays. Existing approaches utilise the inherent
wave-nature of electromagnetic fields to their advantage, and are commonly
identified as bound states in the continuum (BICs), as they resemble singular
bound states embedded in a band of radiating states. Although quasi-BICs have
been demonstrated for numerous symmetric periodic photonic crystals, their
existence so far has been limited to narrow spectral ranges, and their
application in switches or modulators needs large changes of the refractive
index. Here, we show that the incidence of two guided symmetric substrate waves
of opposite phase onto a symmetric periodic film waveguide enables the
excitation of self-stabilising BICs which are electrically switchable in a
broad optical range, with small changes of the refractive index. An
experimental verification of the concept shows a switching contrast C=700 at a
wavelength of 532 nm and C=1000 at a wavelength of 632.8 nm.
