[2] Distributed feedback lasing using Bragg reflection of a periodic structure instead of external mirrors was first proposed in 1971,[3] predicted theoretically with cholesteric liquid crystals in 1978,[4] achieved experimentally in 1980,[5] and explained in terms of a photonic band gap in 1998.
[8] For light circularly polarized with the same handedness, this regular modulation of the refractive index yields selective reflection of the wavelength given by the helical pitch, allowing the liquid-crystal laser to serve as its own resonator cavity.
[15] Tuning of the output wavelength is achieved by smoothly varying the helical pitch: as the winding changes, so does the length scale of the crystal.
[5][20][21][22] By applying a temperature gradient perpendicular to the direction of emission varying the location of stimulation, frequency may be selected across a continuous spectrum.
[23] Similarly, a quasi-continuous doping gradient yields multiple laser lines from different locations on the same sample.
[24] If a defect is introduced into the liquid crystal to disturb the periodicity, a single allowed mode may be created inside of the photonic bandgap, reducing power leeching by spontaneous emission at adjacent frequencies.