Wensong Li

Orange-light passively Q-switched Pr^3+-doped all-fiber lasers with transition-metal dichalcogenide saturable absorbers

We experimentally demonstrated compact orange-light passively Q-switched praseodymium (Pr3+)-doped all-fiber lasers utilizing two-dimensional (2D) transition-metal dichalcogenides (TMDs, WS2 and MoS2) as saturable absorbers (SAs). The filmy TMDs SAs are fabricated by evaporating the composite of TMDs nanosheets and polyvinyl alcohol (PVA), and then inserted into an orange-light Pr3+-doped fiber laser cavity, which is designed by coating dielectric mirrors on fiber ends. Stable passively Q-switched orange lasers at 604 nm are successfully achieved. The passive Q-switching generated the short pulse duration of 435 ns and 602 ns, as well as the tunable repetition rate in the range of 67.3 ~132.2 kHz and 50.8 ~118.4 kHz, respectively. This is, to the best of our knowledge, the shortest-wavelength operation of 2D-material-based pulsed fiber lasers.

Demonstration of patterned polymer-stabilized cholesteric liquid crystal textures for anti-counterfeiting two-dimensional barcodes

We evaluated the feasibility of embedding periodically arranged squares with planar and vertical texture into a background with a developable-modulation (DM) type cholesteric liquid crystal (CLC) fingerprint texture by a two-step ultraviolet-induced polymerization method. Checker-patterned optical diffractive elements, which can be seen as a variation of a two-dimensional (2D) barcode, were first realized and the dependence of diffraction behaviors on incident light polarization and applied voltage were investigated. Taking advantage of the natural randomness and uncontrollable variations of a DM-type fingerprint textures, a polymer-stabilized CLC (PSCLC) graphic symbol with a 2D barcode pattern was then implemented with enhanced anti-counterfeiting features that are difficult to falsify or duplicate. The results indicate that the multiplexing of nonuniform DM-type fingerprint gratings, cross-polarized light readout, and unique polarization diffraction characteristics can improve the level of security.

Interlaced cholesteric liquid crystal fingerprint textures via sequential UV-induced polymer-stabilization

Two types of cholesteric liquid crystal (CLC) fingerprint structures, namely developable-modulation (DM) and growing-modulation (GM), can be contemporaneously prepared in polymer stabilized CLC (PSCLC) composites, to be mutually perpendicular in a repeatable and reversible voltage-switching process when the cell thickness to pitch length ratios (d/P) were appropriately chosen. PSCLC grating structures with interlaced DM/GM fingerprint textures were achieved by varying the applied voltage and sequentially photopolymerizing the dissolved mesogenic monomers through photomasks with stripe and checker patterns. The morphologies of the distinct interlaced PSCLC structures via different UV exposure sequences were investigated under crossed polarizing optical microscopy (POM) and scanning electron microscopy (SEM). The optimized results suggest that DM gratings should be stabilized before GM gratings. Diffraction measurements reveal polarization-dependent properties of the interlaced DM/GM gratings. The intensity redistribution of diffraction orders in orthogonal direction can be achieved by changing the polarization state of incident laser beam.

716 nm deep-red passively Q-switched Pr:ZBLAN all-fiber laser using a carbon-nanotube saturable absorber

We experimentally demonstrated a compact single-wall carbon-nanotube (SWNT)-based deep-red passively Q-switched Pr3+-doped ZBLAN all-fiber laser operating at 716 nm. A free-standing SWNT/polyvinyl alcohol composite film embedded between a pair of fiber connectors was employed as a saturable absorber (SA). The deep-red Q-switched operation is attributed to the combination of implementing a pair of fiber end-facet mirrors to achieve the linear laser resonator and incorporating a SWNT-SA into the cavity as a Q-switcher. Stable short-pulse generation with a duration of 2.3 μs was realized. When gradually increasing the incident pump power, the pulse repetition rate can be linearly tuned from 32.6 to 86.5 kHz, corresponding to a maximum average output power of 1.5 mW and the highest single-pulse energy of 18.3 nJ. To the best of our knowledge, this is the first demonstration of SWNT-based SA for a Q-switched laser at a deep-red wavelength ∼716  nm.

All-fiber passively Q-switched 604 nm praseodymium laser with a Bi 2 Se 3 saturable absorber

We experimentally demonstrated a simple passively Q-switched praseodymium (Pr³⁺)-doped all-fiber laser at 604 nm with a Bi₂Se₃ saturable absorber (SA). A Bi₂Se₃/polyvinyl alcohol composite film is sandwiched between two ferrules to construct a fiber-compatible Q-switcher. Two fiber end facet mirrors build a compact-linear resonator. The repetition rate of the achieved 604 nm Q-switching pulse can be widely tuned from 86.2 to 187.4 kHz, and the pulse duration can be as narrow as 494 ns. To the best of our knowledge, this is the shortest operation wavelength of a Bi₂Se₃-based pulsed all-fiber laser at 604 nm.

Dynamic cholesteric liquid crystal superstructures photoaligned by one-step polarization holography

A convenient approach to modulate the fingerprint textures of methyl red (MR) doped cholesteric liquid crystals by asymmetric photoalignment in the green-light waveband is presented, resulting in the generation of voltage-controllable helical superstructures. The interaction between the MR molecules and the incident light polarization determines the initial twisted planar geometry, providing a multivariant control over the stripe directions of fingerprint textures by applying a proper electric field. The key factors for precise manipulation of fingerprint stripes in a predictable and rewritable manner are analyzed theoretically and investigated experimentally, which involves the alignment asymmetry, the ratio of cell gap to natural pitch length, and the chirality of chiral dopant. Dynamic periodic fingerprint textures in shapes of dashed curve and dashed line are further demonstrated by utilizing a facile one-step polarization holography process using two beams with orthogonal circular and orthogonal linear polarizations, respectively. It is believed that the practical approach described in this study would enrich the research contents of self-assembled hierarchical superstructures using soft liquid crystal building blocks.A convenient approach to modulate the fingerprint textures of methyl red (MR) doped cholesteric liquid crystals by asymmetric photoalignment in the green-light waveband is presented, resulting in the generation of voltage-controllable helical superstructures. The interaction between the MR molecules and the incident light polarization determines the initial twisted planar geometry, providing a multivariant control over the stripe directions of fingerprint textures by applying a proper electric field. The key factors for precise manipulation of fingerprint stripes in a predictable and rewritable manner are analyzed theoretically and investigated experimentally, which involves the alignment asymmetry, the ratio of cell gap to natural pitch length, and the chirality of chiral dopant. Dynamic periodic fingerprint textures in shapes of dashed curve and dashed line are further demonstrated by utilizing a facile one-step polarization holography process using two beams with orthogonal circular and orthogonal line...

Investigation on the effect of output mirror transmission in WS 2 -based red-light passively Q-switched Pr:ZBLAN all-fiber lasers

We report the experimental investigation of visible passively Q-switched Pr3+-doped all-fiber lasers with tungsten disulfide (WS2) saturable absorber, where red-light short-pulse generations from different output mirror transmissions are systemically characterized. The proposed simple and compact all-fiber linear cavity was constructed by a fiber-pigtail-based blue laser-diode pump, a Pr3+-doped fluorozirconate glass active fiber, and the fiber end-facet mirrors. Integrating a free-standing layered WS2 film into the laser cavity initiated the Q-switching operation. Stable microsecond-duration output pulses with kilohertz repetition rates are achieved, corresponding to a few mW/nJ average output power and single-pulse energy. The comparisons on red-light Q-switched output parameters for output transmissions of both ∼40% and ∼80% are performed. This work could provide a useful guideline to manipulate the output performance of visible pulsed all-fiber lasers for various practical applications.

Growth and characterization of zeolitic imidazolate framework-8 nanocrystalline layers on microstructured surfaces for liquid crystal alignment

Zeolitic imidazolate framework-8 (ZIF-8) nanocrystalline layers were grown on carboxylate-terminated sol–gel films patterned with surface-relief microstructures. The coverage of ZIF-8 nanocrystals was significantly affected by the surface morphology, like the size and interspace of microstructures. Its vertically aligning ability to a typical nematic liquid crystal (LC) E7 was verified and its applications as a hybrid aligned nematic (HAN) LC cell-based voltage-dependent light modulator and a switchable diffraction grating were demonstrated.