Light‐guided spectral sculpting in chiral azobenzene‐doped cholesteric liquid crystals for reconfigurable narrowband unpolarized light sources

Precise manipulation of Bragg reflection in cholesteric liquid crystals (CLCs) is essential for advancing reconfigurable optics. However, existing photo-responsive material-doped CLC technologies that rely on single-wavelength photoisomerization encounter several challenges, including slow response times, limited tunability, inadequate spatial control, and instability caused by pitch variations due to diffusion. Here, we present a robust dual-wavelength photoisomerization method to simultaneously achieve <i>trans</i>-to-<i>cis</i> and <i>cis</i>-to-<i>trans</i> photoisomerization of chiral azobenzene-doped CLCs, which enables broadband, reversible, and spatially addressable control over the Bragg reflection spectrum. By employing counterpropagating laser beams at 405 nm and 532 nm, we precisely control the <i>trans</i>-<i>cis</i> isomerization dynamics of azobenzene chiral dopants, achieving spectral shifts exceeding 100 nm primarily through reversible modulation of the helical pitch of the CLCs. Furthermore, manipulating the intensity ratio and geometry of the excitation beams allows for tailored pitch gradients, reflection bandwidths, and central wavelengths with remarkable fidelity. Our approach enhances pitch boundaries and reduces molecular diffusion, facilitating the micrometer-scale patterning of optical textures, which surpasses traditional single-wavelength methods. Additionally, we present an innovative narrowband spectral filtering technique by sequentially transmitting light through pitch-selective CLC regions under circular polarization control. This reconfigurable manipulation strategy paves the way for developing programmable photonic systems, including adaptive optics, diffractive optics, and tunable displays.