Evolution of polarization dependent microstructures induced by high repetition rate femtosecond laser irradiation in glass.

We report the observation of an anomalous polarization dependent process in an isotropic glass induced by long time stationary irradiation of a high repetition rate near-infrared femtosecond laser. Two distinctive types of polarization dependent microstructures were induced at different irradiation stages. At early stage (a few seconds), a dumbbell-shaped structure elongated perpendicularly to the laser polarization formed at the top of the modified region, which was later erased by further irradiation. At later stage (above 30 s), bubbles filled with O2 formed by the irradiation, which were distributed along the laser polarization at a distance far beyond the radius of the laser beam. Based on a simple modeling of light reflection on boundaries, a thermal accumulation process was proposed to explain the formation and evolution of the dumbbell-shaped microstructure. The possible factors responsible for polarization dependent distribution of bubbles are discussed, which needs further systematic investigations. The results may be helpful in the development of femtosecond laser microprocessing for various applications.

Long persistent phosphors--from fundamentals to applications.

Owing to the unique mechanism of photoelectron storage and release, long persistent phosphorescence, also called long persistent luminescence or long lasting afterglow/phosphorescence, plays a pivotal role in the areas of spectroscopy, photochemistry, photonics and materials science. In recent years, more research has focused on the manipulation of the morphology, operational wavebands and persistent duration of long persistent phosphors (LPPs). These desired achievements stimulated the growing interest in designing bio-labels, photocatalysts, optical sensors, detectors and photonic devices. In this review, we present multidisciplinary research on synthetic methods, afterglow mechanisms, characterization techniques, materials system, and applications of LPPs. First, we introduce the recent developments in LPPs for the synthesis of nanoparticles from the aspects of particle sizes, monodispersity and homogeneity based on the urgent application of bio-imaging. In the later sections, we present the possible mechanisms, which involve the variation of trap distribution during the trapping and de-trapping process, complicated photo-ionization reaction of trap site levels and impurity centers together with their corresponding migration kinetics of carriers. Meanwhile, we emphasize the characterization techniques of defects, used to qualitatively or quantitatively describe the types, concentrations and depths of the traps. This review article also highlights the recent advances in suggested LPPs materials with a focus on the LPPs' hosts and optically active centers as well as their control, tuning and intrinsic links. We further discuss the classification of LPPs based on the different emission and excitation wavebands from the ultraviolet to the near-infrared region along with an overview of the activation mode of afterglow. Afterwards, we provide an exhibition of new products towards diverse application fields, including solar energy utilization, bio-imaging, diagnosis, and photocatalysts. Finally, we summarize the current achievements, discuss the problems and provide suggestions for potential future directions in the aforementioned parts.

Hybrid coordination-network-engineering for bridging cascaded channels to activate long persistent phosphorescence in the second biological window.

We present a novel "Top-down" strategy to design the long phosphorescent phosphors in the second biological transparency window via energy transfer. Inherence in this approach to material design involves an ingenious engineering for hybridizing the coordination networks of hosts, tailoring the topochemical configuration of dopants, and bridging a cascaded tunnel for transferring the persistent energy from traps, to sensitizers and then to acceptors. Another significance of this endeavour is to highlight a rational scheme for functionally important hosts and dopants, Cr/Nd co-doped Zn1-xCaxGa2O4 solid solutions. Such solid-solution is employed as an optimized host to take advantage of its characteristic trap site level to establish an electron reservoir and network parameters for the precipitation of activators Nd(3+) and Cr(3+). The results reveal that the strategy employed here has the great potential, as well as opens new opportunities for future new-wavelength, NIR phosphorescent phosphors fabrication with many potential multifunctional bio-imaging applications.

Anisotropically Enhanced Nonlinear Optical Properties of Ensembles of Gold Nanorods Electrospun in Polymer Nanofiber Film.

Polymeric nanofibers containing gold nanorods (GNRs) are aligned in a uniform orientation through electrospinning. The dispersive and absorptive parts of the third-order optical nonlinear optical refractive index of the composite film measured by polarization dependent z-scan method are demonstrated to be anisotropically enhanced. Anisotropic optical response of the aligned GNRs and its connection with the ultrafast electron dynamics are discussed in light of the results of resonant femtosecond pump-probe experiments. The significant appearance of anisotropic nonlinear optical properties of ensembles of GNRs is attributed to the sensitive excitation of longitudinal surface plasmon resonance (LSPR) of highly aligned GNRs. For the macroscopic applications of ensembles of GNRs, such as passive mode-locking and all-optical switching, the experimental results demonstrate that the alignment of GNRs through electrospinning should be very high efficient, and economic.

Tailored surface birefringence by femtosecond laser assisted wet etching.

Surface texturing is demonstrated by the combination of wet etching and ultrafast laser nanostructuring of silica glass. Using potassium hydroxide (KOH) at room temperature as an etchant of laser modified glass, we show the polarization dependent linear increase in retardance reaching a threefold value within 25 hours. The dispersion control of birefringence by the etching procedure led to achromatic behaviour over the entire visible spectral range. The mechanism of enhanced KOH etching selectivity after femtosecond laser exposure is discussed and correlated to the formation of various laser-induced defects, such as silicon-rich oxygen deficiency and color centers.

Airy beams generated by ultrafast laser-imprinted space-variant nanostructures in glass.

We demonstrate a technique to generate accelerating Airy beams with a femtosecond laser-imprinted space variant birefringent structure in silica glass. Our approach enables the generation of dual Airy beams with polarization sensitive beam deflection. The produced beam is used for the glass scribing. After the glass-breaking process, a spontaneous self-detachment of a fiber-like structure occurs that can be exploited as an alternative way for fabricating glass cantilevers.

Seemingly unlimited lifetime data storage in nanostructured glass.

We demonstrate recording and retrieval of the digital document with a nearly unlimited lifetime. The recording process of multiplexed digital data was implemented by femtosecond laser nanostructuring of fused quartz. The storage allows unprecedented parameters including hundreds of terabytes per disc data capacity, thermal stability up to 1000 °C, and virtually unlimited lifetime at room temperature. We anticipate that this demonstration will open a new era of eternal data archiving.

Polarization sensitive camera by femtosecond laser nanostructuring.

A polarization imaging device based on a femtosecond laser nanostructured birefringent array is demonstrated. The device enables instant measurement of the distribution of the Stokes vectors in the visible spectrum. Polarimetric measurements with radially and circularly polarized light distributions are demonstrated.

Femtosecond versus picosecond laser machining of nano-gratings and micro-channels in silica glass.

The ability of 8 picosecond pulse lasers for three dimensional direct-writing in the bulk of transparent dielectrics is assessed through a comparative study with a femtosecond laser delivering 600 fs pulses. The comparison addresses two main applications: the fabrication of birefringent optical elements and two-step machining by laser exposure and post-processing by chemical etching. Formation of self-organized nano-gratings in glass by ps-pulses is demonstrated. Differential etching between ps-laser exposed regions and unexposed silica is observed. Despite attaining values of retardance (>100 nm) and etching rate (2 µm/min) similar to fs pulses, ps pulses are found unsuitable for bulk machining in silica glass primarily due to the build-up of a stress field causing scattering, cracks and non-homogeneous etching. Additionally, we show that the so-called "quill-effect", that is the dependence of the laser damage from the direction of writing, occurs also for ps-pulse laser machining. Finally, an opposite dependence of the retardance from the intra-pulse distance is observed for fs- and ps-laser direct writing.

Extraordinary anisotropy of ultrafast laser writing in glass.

The unusual dependence of femtosecond laser writing on the light polarization and direction of raster scanning is demonstrated in silica and chalcogenide glasses. Two different mechanisms contributing to the observed anisotropy are identified: the chevron-shaped stress induced by the sample movement and the pulse front tilt of ultrashort light pulse. Control of anisotropies associated with the spatio-temporal asymmetry of an ultrashort pulse beam and scanning geometry is crucial in the ultrafast laser machining of transparent materials.

Twisting light with micro-spheres produced by ultrashort light pulses.

We propose a radial polarizer based on light refraction on a transparent isotropic sphere. We demonstrate theoretically and experimentally that the circularly polarized light impinging on the sphere produces double charged optical vortex. The method is applied to generate optical vortices on a small scale using hollow micro-spheres produced by femtosecond laser in fused silica.

Photosensitivity control of an isotropic medium through polarization of light pulses with tilted intensity front.

We present the first experimental evidence of anisotropic photosensitivity of an isotropic homogeneous medium under uniform illumination. Our experiments reveal fundamentally new type of light induced anisotropy originated from the hidden asymmetry of pulsed light beam with a finite tilt of intensity front. We anticipate that the observed phenomenon, which enables employing mutual orientation of a light polarization plane and pulse front tilt to control interaction of matter with ultrashort light pulses, will open new opportunities in material processing.