In this paper, we demonstrated a new dye-doped cholesteric liquid crystal (CLC) photonic band edge laser with
emission enhanced by an external cholesteric resonator. As one-dimensional photonic crystal, the 5-&mgr;m dye-doped
cholesteric liquid crystal cell generates circularly polarized laser emission at its photonic band edge. When
sandwiched between two 5-&mgr;m cholesteric liquid crystal mirrors whose reflection band reflects the laser emission
from the central dye-doped CLC laser, the emission can be enhanced by ~800X. In experiment, a second-harmonic
Q-switched Nd-YAG pulsed laser is used to pump the CLC laser assembly at normal incidence. The detected laser
emission is elliptically polarized and is still located at the band edge wavelength of the central CLC cell. The beam
divergence is decreased by ~10X due to an increased cavity length. Theoretical analysis using 4x4 transfer matrix
and scattering matrix has shown that the circular resonator produces transmission peaks based on Fabry-Perot effect
inside reflection band and, moreover, the transmission peak at the band edge of central CLC can be well-preserved.
Both experiment results and simulation results are present in good agreement.
We have obtained a dye-doped chiral photonic crystal (PC) film with reflection band gap much wider than its original
band gap without dye dopants by using multiple-step fabrication processes. Moreover, the dye-doped chiral PC films
using our multiple-step fabrication processes exhibit many oscillations within the broadened reflection band gap. The
abrupt change of the optical density of state (DOS) around the oscillations provides the possibility of generating laser
emission when the dye-doped chiral PC film is pumped by a pulsed laser with wavelength in the absorption region of the
laser dye. Based on this property, we demonstrated random lasers which exhibit different multiple-mode laser
wavelength at different spatial positions. Different from the random lasers induced by the scattering mechanism, the
random lasers from the dye-doped cholesteric polymer film exhibit Gaussian-like beam shape and specific propagation
orientation which is normal to the cholesteric planar surface. It is foreseeable that a high efficiency and high power
broadband laser can be generated using cholesteric polymer films.
Usually when optically pumped, dye-doped cholesteric liquid crystal (CLC) laser generates circularly polarized laser light in the same handedness as the cholesteric helix. On a distributed feedback basis, laser light at photonic band edge comes out from both sides symmetrically. In this work, we incorporated a metallic mirror reflector to the CLC laser on one side so that laser light only emits from single direction and hence the extracted output can be enhanced by ~2-5X. Furthermore, upon reflection the mirror reflector introduces a π phase change. Therefore, two different types of CLC lasers with different polarization states are demonstrated by putting the mirror at different substrates. With a mirror attached at the outer side of the liquid crystal substrate, we obtained a nearly unpolarized CLC laser based on incoherent supposition of two orthogonal circularly polarized beams. With mirror coated at one of the inner surfaces of the liquid crystal cell, we obtained a linearly polarized CLC laser based on coherent combination of two orthogonal circularly polarized beams. For these two cases, the output power and polarization states are compared and the physical mechanism is discussed correspondingly. Moreover, the tuning of the linear polarization direction is demonstrated.
We have investigated the physical and optical properties of the left-handed chiral dopant ZLI-811
mixed in a nematic liquid crystal (LC) host BL006. The solubility of ZLI-811 in BL006 at room
temperature is ~24 wt%, but can be enhanced by increasing the temperature. Consequently, the
photonic band gap of the cholesteric liquid crystal (CLC) mixed with more than 24 wt% chiral
dopant ZLI-811 is blue shifted as the temperature increases. Based on this property, we
demonstrate its applications in thermally tunable band-pass filters and dye-doped CLC lasers. In
addition, we also demonstrated a spatially tunable laser emission by generating a one-dimensional
temperature gradient along the dye-doped cholesteric liquid crystal (CLC) cell. The lasing
wavelength is widely tunable from 577 nm to 670 nm. The lowest excitation energy and maximum
lasing efficiency occur at λ~605 nm which corresponds to the peak fluorescence emission of the
Usually when optically pumped, dye-doped cholesteric liquid crystals (CLC) generate circularly polarized laser light from both directions of the lasing cell along the cholesteric helical axis. In reality, only the laser light from one direction can be utilized. In this paper, we demonstrate a simple method for doubling the laser output of a dye-doped CLC laser. The extracted laser output is nearly doubled. In experiment, we use a 6-ns, frequency-doubled Nd:YAG laser to pump the CLC lasing sample at ~20 degree incident angle. A reflector: a metal mirror or a cholesteric liquid crystal reflector is placed on the backside of the CLC sample. The reflector is in proximity contact with the CLC sample and the laser action occurs only in one direction. For the metal mirror reflector, the two orthogonal circularly polarized beams are mixed by incoherent superposition. While for the cholesteric liquid crystal reflector (same handedness as the lasing cell and highly reflective of the laser light), the enhanced laser output could also be achieved due to further stimulated amplification but the output is dominated by a single polarization state. For both cases the laser output is associated with a loss of coherence. Hence a nearly unpolarized CLC laser or a partially coherent CLC laser with nearly doubled output intensity is obtained.