ELECTRIC FIELD TUNING A SPECTRUM OF NEMATIC LIQUID CRYSTAL LASING WITH THE USE OF A PERIODIC SHADOW MASK

Lasing has been observed in the dye-doped isotropic and nematic phases of liquid crystals in a plane layer placed between one substrate with a uniform transparent electrode and another substrate with a system of parallel chromium (nontransparent) electrodes. The latter has a periodicity of 15 μm and plays a double role: (i) in both the phases it operates as a shadow mask providing a spatial modulation of the pump beam and consequently, a gain of the material and (ii) in the nematic phase it additionally creates a Bragg resonator structure with spatial modulation of the refraction index when an electric voltage is applied across the transparent and non-transparent electrodes. The resonator operates at high order Bragg resonance modes of numbers m = 71–79 for three lasing dyes studied. With that simple cell, a voltage induced tuning of the spectral positions of lasing lines over 25 nm has been demonstrated without using holographic or other complicated techniques. The mechanism of the observed phenomenon is discussed using modeling of the liquid crystal reorientation and optical properties in the spatially periodic electric field.