Density of photon states in dye-doped chiral nematic liquid crystal cells in the presence of losses and gain

Abstract

We calculate the density of photon states (DOS) of the normal modes in dye-doped chiral nematic liquid crystal (LC) cells in the presence of various loss mechanisms. Losses and gain are incorporated into the transmission characteristics through the introduction of a small imaginary part in the dielectric constant perpendicular and along the director, for which we assume no frequency dispersion. Theoretical results are presented on the DOS in the region of the photonic band gap for a range of values of the loss coefficient and different values of the optical anisotropy. The obtained values of the DOS at the photonic band gap edges predict a reversal of the dominant modes in the structure. Our results are found to be in good agreement with the experimentally obtained excitation thresholds in chiral nematic LC lasers. The behavior of the DOS is also discussed for amplifying LC cells providing additional insight to the lasing mechanism of these structures.

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“…Optically pumped lasing of these CLC microunits was characterized on a homemade micro‐photoluminescence system (Figure S6, Supporting Information). Due to the distribution of density of optical states (DOS) induced by the PBG effect of CLC superstructures, [ 48 ] the emission spectra of C6‐CLC microunits were well modulated with more intense emissions at long‐wavelength edge (LWE) than that inside the PBG of CLCs (Figure S7, Supporting Information) at a low pump fluence. Such unique modulation of DOS is favorable for the realization of low‐threshold lasing at the LWE with the embedded C6 molecules providing optical gain and CLC superstructures serving as photonic crystal cavities (Figure S8, Supporting Information).…”

Bioresorbable Microdroplet Lasers as Injectable Systems for Transient Thermal Sensing and Modulation

“…The dependence of lasing threshold on particle size can be attributed to changes in the resonant strength of the CLC cavity and the amount of nonquenched dye available for stimulated emission. 40,41 Analytical forms for lasing threshold originate from the laser rate equations 42 and/or density of state considerations, 41 but result in the following dependence on particle diameter: 40…”

“…The dependence of lasing threshold on particle size can be attributed to changes in the resonant strength of the CLC cavity and the amount of nonquenched dye available for stimulated emission. , Analytical forms for lasing threshold originate from the laser rate equations and/or density of state considerations, but result in the following dependence on particle diameter: E th = Ad + B / d 2 , where E th is lasing threshold, d is particle diameter, and A and B are constants. Increasing the particle size increases the number of periods of the molecular helix within the droplet, thereby enhancing coherent scattering and increasing both the amplitude of the Bragg reflectance and the efficiency of the resonant cavity, Q .…”

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“…S7 ). Such unique modulation of PL emissions can be attributed to the fluctuation of density of optical states (DOS) at different positions of photonic bands of the CLCs [ 38 ], and the resulted higher DOS at band edges can favor the generation of low-threshold laser emissions [ 39 ]. With increasing pump fluence, the PL intensities of the peak at ~637 nm, where the long-wavelength band edge of the CLCs locates, were dramatically amplified ( Figure 2(c) ).…”