This article demonstrates a bistable optical valve in a photonic liquid crystal fiber using the thermal hysteresis effect of the phase transition between the cholesteric phase and the blue phase (BP). The attenuation is due to various scattering losses in different phases. Both cholesteric and BPs can exist stably at room temperature (RT) and can also be switched to each other using temperature-control processes. The transmission spectrum and the intensity of the guided light can be controlled with various extents of scattering loss. For optical communications, this device can be manipulated over a loss difference of 10 dB at RT and insensitive to the polarization of light.
Liquid crystal random fiber lasers (LC-RFLs) exhibit low spatial coherence, extraordinary tunability, and high flexibility and are therefore promising for use in imaging and related applications. They nevertheless suffer from ineffective emission along the fiber axis. This work develops an end-emitting LC-RFL that is based on a pump-induced gain–loss structure, which effectively modulates the optical feedback, thereby providing directional random lasing along the fiber. The laser emission comprises ∼10 000 transverse spatial modes that are mutually incoherent, resulting in a negligible speckle contrast of ∼0.7% (far below the speckle-perception threshold of human eyes). The developed LC-RFL is used to perform speckle-free full-field imaging in a setting with strong optical crosstalk in a multimode fiber. Our findings support the potential widespread use of LC-RFL as a spatially incoherent, flexible laser source.
This paper reports on the observation and detailed investigation of the bidirectional random lasing emitted from an active twisted nematic liquid crystal, of which the polarization states are asymmetric (non-parallel to each other). In such a laser, the liquid crystal acts as a random distributed feedback cavity with an inherently built-in polarization rotator. While propagating in the anisotropic cavity, the polarization of the dye-emitted light rotates with the gently twisted optic axis. The output polarization states are, therefore, parallel to each of the surface alignment directions. The mode stability and electrical switchability of the laser were also examined. Moreover, correlations between the lasing threshold, alignment direction, dye concentration, and film thickness were established. The findings not only suggest an optimum concentration for low-threshold random lasing operation but also disclose the threshold inversion behavior of nematic random lasers.
Photonic crystals enable modulation of light waves in space, time, and frequency domains; in particular, chiral photonic crystals are uniquely suitable for polarization rotation and switching of complex vector fields. Current development of chiral photonic crystals, nevertheless, are still confronted with limitations of one form or the other such as large optical losses, limited or absence of tunability, narrow operation bandwidth, and/or insufficient optical thickness for practical implementation. In this work, we show that cholesteric liquid crystals as 1D tunable chiral photonic crystals are promising alternatives to not only address all these issues and deficiencies but also enable new photonic applications in wider temporal and spectral realms. Our work entails a detailed study of the dynamical evolution of cholesteric helical self-assembly and defect formation in the bulk of thick cholesteric liquid crystals under various applied electric field conditions and a thorough exploration of how applying fields of vastly different frequencies can eliminate and/or prevent the formation of unremovable defects and to control the alignment of cholesteric helices in the entire bulk. We have developed a dual-frequency field assembly technique that enables robust room-temperature fabrication of stable well-aligned cholesteric liquid crystals to unprecedented thickness (containing thousands of grating periods) demanded by many photonic applications. The resulting chiral photonic crystals exhibit useful much-sought-after capabilities impossible with other existing or developing chiral photonic crystals—compactness (single, flat, millimeter-thick optical element), high transmission, dynamic tunability, large polarization rotation, and various switching/modulation possibilities for ultrafast and continuous-wave lasers in the visible, near- and mid-infrared regimes.
Bi0.9Pb0.1FeO3 (BPFO) films were grown on SrRuO3 (SRO)/SrTiO3 (STO) substrates. The surface morphology of BPFO films is highly dependent on that of the SRO layer. Though the step height of STO (100) substrate is equal to one unit cell of STO crystal, the height and width of steps on the surface of SRO and BPFO are larger, which supports a step bunching growth mode on both the SRO layer and BPFO films. At zero bias voltage, the BPFO film exhibits a natural dipole polarization toward the SRO layer, which is believed to be due to the negative charge accumulation at the BPFO/SRO interface, and manifests of 71° and 109° but 180° domain walls. Doping of Pb distorted the BPFO crystal lattice to near cubic that weakens the electric anisotropy and forms a two-step flipping process. To complete a 180° dipole flipping procedure, the dipole moment first rotates 71° to adjacent states followed by a 109° rotation to the final 180° state.
This work demonstrates a multi-stable variable optical attenuator (VOA) that is fabricated by infiltrating a photonic crystal fiber (PCF) with a liquid crystal (LC) gel. Varying the cooling rate or biasing the electric field during gelation yields various degrees of scattering. Therefore, LC gel-filled PCFs with various transmittances can be realized. At a wavelength of 1550 nm, an attenuation rate of -33.4 dB/cm is obtained at a cooling rate of 30°C/min and a biasing voltage of 400 V during gelation. The proposed all-in-fiber VOA exhibits tunable attenuation and multiple stable states at room temperature.
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