A cholesteric liquid crystal (CLC) formed by chiral molecules represents a self-assembled one-dimensionally periodic helical structure with pitch p in the submicrometer and micrometer range. Because of the spatial periodicity of the dielectric permittivity, a CLC doped with a fluorescent dye and pumped optically is capable of mirrorless lasing. An attractive feature of a CLC laser is that the pitch p and thus the wavelength of lasing λ¯ can be tuned, for example, by chemical composition. However, the most desired mode to tune the laser, by an electric field, has so far been elusive. Here we present the realization of an electrically tunable laser with λ¯ spanning an extraordinarily broad range (>100 nm) of the visible spectrum. The effect is achieved by using an electric-field-induced oblique helicoidal (OH) state in which the molecules form an acute angle with the helicoidal axis rather than align perpendicularly to it as in a field-free CLC. The principal advantage of the electrically controlled CLCOH laser is that the electric field is applied parallel to the helical axis and thus changes the pitch but preserves the single-harmonic structure. The preserved single-harmonic structure ensures efficiency of lasing in the entire tunable range of emission. The broad tuning range of CLCOH lasers, coupled with their microscopic size and narrow line widths, may enable new applications in areas such as diagnostics, sensing, microscopy, displays, and holography.
Flow-assisted self-healing of the helical structure in a cholesteric liquid crystal
