Fiber guiding at the Dirac frequency beyond photonic bandgaps

Light trapping within waveguides is a key practice of modern optics,both scientifically and technologically.Photonic crystal fibers traditionally rely on total internal reflection(index-guiding fibers)or a photonic bandgap(photonic-bandgap fibers)to achieve field confinement.Here,we report the discovery of a new light trapping within fibers by the so-called Dirac point of photonic band structures.Our analysis reveals that the Dirac point can establish suppression of radiation losses and consequently a novel guided mode for propagation in photonic crystal fibers.What is known as the Dirac point is a conical singularity of a photonic band structure where wave motion obeys the famous Dirac equation.We find the unexpected phenomenon of wave localization at this point beyond photonic bandgaps.This guiding relies on the Dirac point rather than total internal reflection or photonic bandgaps,thus providing a sort of advancement in conceptual understanding over the traditional fiber guiding.The result presented here demonstrates the discovery of a new type of photonic crystal fibers,with unique characteristics that could lead to new applications in fiber sensors and lasers.The Dirac equation is a special symbol of relativistic quantum mechanics.Because of the similarity between band structures of a solid and a photonic crystal,the discovery of the Dirac-point-induced wave trapping in photonic crystals could provide novel insights into many relativistic quantum effects of the transport phenomena of photons,phonons,and electrons.

Coherent Random Lasing Realized in Polymer Vesicles

We have demonstrated the realization of a coherent vesicle random lasing(VRL)from the dye doped azobenzene polymer vesicles self-assembled in the tetrahydrofuran-water system,which contains a double-walled structure:a hydrophilic and hydrophobic part.The effect of the dye and azobenzene polymer concentration on the threshold of random laser has been researched.The threshold of random laser decreases with an increase in the concentration of the pyrromethene 597(PM597)laser and azobenzene polymer.Moreover,the scattering of small size group vesicles is attributed to providing a loop to boost the coherent random laser through the Fourier transform analysis.Due to the vesicles having the similar structure with the cell,the generation of coherent random lasers from vesicles expand random lasers to the biomedicine filed.

Wide tunable laser based on electrically regulated bandwidth broadening in polymer-stabilized cholesteric liquid crystal

Electrically responsive photonic crystals represent one of the most promising intelligent material candidates for technological applications in optoelectronics. In this research, dye-doped polymer-stabilized cholesteric liquid crystals(PSCLCs) with negative dielectric anisotropy were fabricated, and mirrorless lasing with an electrically tunable wavelength was successfully achieved. Unlike conventional liquid-crystal lasers, the proposed laser aided in tuning the emission wavelength through controlling the reflection bandwidth based on gradient pitch distribution. The principal advantage of the electrically controlled dye-doped PSCLC laser is that the electric field is applied parallel to the helical axis, which changes the pitch gradient instead of rotating the helix axis, thus keeping the heliconical structure intact during lasing. The broad tuning range(～110 nm) of PSCLC lasers,coupled with their stable emission performance, continuous tunability, and easy fabrication, leads to its numerous potential applications in intelligent optoelectronic devices, such as sensing, medicine, and display.

Band-gap-tailored random laser

A band-gap-tailored random laser with a wide tunable range and low threshold through infrared radiation is demonstrated. When fluorescent dyes are doped into the liquid crystal and heavily doped chiral agent system,we demonstrate a wavelength tuning random laser instead of a side-band laser, which is caused by the combined effect of multi-scattering of liquid crystal(LC) and band-gap control. Through rotating the infrared absorbing material on the side of the LC cell, an adjustable range for random lasing of 80 nm by infrared light irradiation was observed.

Temperature Sensitivity of Polymer Fiber Microlasers

There are many kinds of materials or methods used to make optical microcavities,and they have many different geometric structures.And electrospinning technique has become a very convenient and easy one to prepare polymer fiber.Based on this situation,PM597-doped polymer solution was prepared into high-performance fibers with different diameters by electrospinning technology in our work.In order to better study the temperature sensing of polymer fiber whispering gallery mode,we have placed it on two different substrates with gold and aluminum.A 532 nm pulsed laser beam was used to excite a single fiber in the radial direction,then the whispering gallery mode(WGM)laser was observed and the distribution of WGM was determined by theoretical calculations.The threshold of samples on aluminum substrate is 0.4μJ.In addition,it is found that the samples on aluminum substrate performed better in temperature sensing,and the value is 0.13 nm/℃.As a result,WGM polymer fiber microcavities on aluminum substrate made by electrospinning technology have very broad development prospects in biosensing,optical pump lasers and other applications.