This investigation reports for the first time a novel phenomenon, called band-tunable color cone lasing emission (CCLE),
based on a single-pitched one-dimensional photonic crystal-like dye-doped cholesteric liquid crystal (DDCLC) cell. The
lasing wavelength in the CCLE pattern is distributed continuously at 676.7-595.6 nm as the oblique angle increases
continuously from 0° to 50° relative to the helical axis. The variation of the lasing wavelength in the CCLE with the
oblique angle is consistent with that of the wavelength at the long-wavelength edge (LWE) of the CLC reflection band
(CLCRB) with the oblique angle. Simulation results obtained utilizing Berreman's 4×4 matrix method show that, at each
oblique angle, the associated group velocity and density of photonic state (DOS) are near zero and large at the shortwavelength
edge (SWE) and LWE of the CLCRB, respectively, and are in good agreement with experimental results.
The particularly strong lasing ring emission at a cone angle of ~35° can be explained to be likely due to a special effect
that, under the condition of overlap between the LWE of the CLCRB measured at 35° and the SWE of the CLCRB
measured at 0°, the LWE and SWE fluorescence propagating along 35° and 0°, respectively, may indirectly enhance
each other due to individual enhanced rate of spontaneous emission. Furthermore, the lasing band of the CCLE can be
tuned from long-wavelength (deep red~orange) to short-wavelength (orange~green) regions by changing the
concentration of the chiral or by the photo-irradiation on a DDCLC cell with a photoisoemerizable chiral dopant.
Kinetics of the photoexcitation of azo-dye doped liquid crystals (ADDLCs) is observed by the improved
attenuated total reflection leading a pump light into a coupling prism, namely pumped ATR (PATR)
experiment. The technique of ATR could be applied on an LC cell based on that the guide mode is
overcome by the usage of an isotropic surface, which is also a usual way to observe the reaction involving
the adsorption effect. The evanescent wave of the pump light induces the photoexcitation of azo dye at the
limited effective depth, and the evanescent wave of the probe light detects the dynamic behavior in real
time. The time response of the photoexcitation in ADDLCs in the pumped ATR experiment is less than 2ms
and believed the reaction occur in the local region of 100-nm. This result is a witness of that the reaction in
this local region is free of the influence of the elastic continuum of the whole LC cell. With the help of the
local sensitivity and local disturbance, the detailed reactions during the photoexcitation process are clearly
observed and the dynamics equation are expressed to illustrate the oscillation phenomena of LC directors as
the pump light irradiates on this region.
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