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Higher‐order corner‐bound states in a 2D structure have been found to possess robust and exotic properties beyond the "ordinary" topological edge states, giving rise to a promising applicative potential. For example, the topological nanocavity designed based on the corner states exhibits much better performance than that of the conventional photonic crystal cavity. However, the corner states in the finite Hermitian system are usually coupled with each other, which results in the weakening of their localization. As such, the contradiction between the performance and footprint of topological devices is vexing. Here, it shows that introducing non‐Hermiticity in the higher‐order topological insulators is an effective strategy to enhance the localization of corner states in finite systems. By designing and analyzing the 2D finite Su‐Schriffer‐Heeger optical lattices with two types of non‐Hermitian configuration, the localization degree of higher‐order corner states is found to depend on the gain/loss strength. This is experimentally demonstrated by observing the distribution of corner states in the femtosecond‐laser‐writing loss‐controlled waveguide arrays. This scheme tuning localization of higher‐order corner‐bound states by non‐Hermiticity may offer a new avenue to design robust and compact devices, such as topological nano‐lasers with an ultra‐low threshold. [ABSTRACT FROM AUTHOR]

The fabrication of multicolor luminescent glass with simultaneous controlling of the distribution of emission colors within a monolithic medium has been a tremendous challenge, which, however, could be attractive for diverse photonic applications from optical storage to information encryption. Here, a space‐selective doping method based on a stereolithographic technique is designed, enabling the control of the doping domain of the active luminescence ions, such as Eu3+, Ce3+, Tb3+, and Pr3+, in 3D‐printed single silica glasses. The printed glass shows intense multicolor luminescence, and the interface between the two doping areas is invisible under natural light. By using this technique, multicolor luminescent glass with pre‐designed emission color distribution is fabricated, which enables facile and nondestructive decryption strategies based on photoluminescence under controlled excitation conditions. Besides, the extending of this strategy by using a femtosecond laser for photopolymerization dramatically improves the printing resolution down to the few‐micron scale, enabling the multi‐dimensional optical storage and encryption in miniaturized glass items. This work not only delineates the potential applications of multicolor luminescent glass for multi‐dimensional anti‐counterfeiting and information storage, but also opens up new avenues for prospective applications in sensing, lasers, and displays based on glasses with spatially engineered optical responses. [ABSTRACT FROM AUTHOR]

Emerging 3D photonic circuits would greatly benefit from the ability to integrate skimming waveguides with low loss and controllable inscription depth into photonic circuits. These waveguides allow for the interaction of guiding light directly with external modulation signals and enable programmable photonic circuits. Here, we report the fabrication of a novel photonic-lattice-like skimming waveguide (PLLSW) using femtosecond laser writing. Our method enables fine control of cross-sectional symmetry and writing depth of waveguides, achieving a minimum depth of 1 μm and a low insertion loss of 1 dB. Based on the PLLSW, we demonstrate on-chip light modulation by designing an evanescent-field-type saturable absorber through the coupling of a carbon nanotube film with the PLLSW, which exhibits saturation intensity from 20 to 200 MW/cm2 through the balanced twin-detector measurement. The strong nonlinear optical response of the PLLSW-based saturable absorber is further exploited to drive a Q-switched pulse laser at 1550 nm based on a fiber laser cavity. Our work demonstrates an effective method to integrate nonlinear optical materials into a glass chip for all-optical switching based on 3D waveguides, which holds great potential for the construction of large-scale programmable photonic circuits in the future. [ABSTRACT FROM AUTHOR]

Embedding naked‐eye‐invisible electronic and optical elements in transparent panels is at the heart of enabling mobile transparent accessories by making see‐through smart screens. Here, a novel invisible photonic element, highly see‐through (HST) waveguide, is reported, which is written by femtosecond laser in glass. A general synergistic control of the thermodynamic and dynamic behavior over the matter fluid in the laser irradiated confined region to tune the cross‐section of waveguides and suppress the generation of scattering centers in the waveguides is established. An effective reduction of light leakage (covering red, green, and blue coupled light) by an order of magnitude compared to conventional waveguides is achieved, making it highly see‐through at bright illumination of >100 lux. A general dynamical model based on a frozen‐in shock wave diffusion process is proposed, which is applicable to various glasses regardless of their compositions. Ultra‐wide tuning of HST waveguide mode diameters from 4.9 to 26.5 µm is demonstrated, making it versatile for functionalizing various transparent screens by mode‐matching with fiber sources and integrated planar waveguides of different working wavelengths. [ABSTRACT FROM AUTHOR]

Recently, higher‐order topological insulators (HOTIs), accompanied by topologically nontrivial boundary states with a codimension larger than one, have been extensively explored because of unconventional bulk‐boundary correspondences. As a novel type of HOTIs, very recent works have explored the square‐root HOTIs, where the topologically nontrivial nature of bulk bands stems from the square of the Hamiltonian. In this paper, 2D square‐root HOTIs are experimentally demonstrated in photonic waveguide arrays written in glass using femtosecond laser direct‐write techniques. Edge and corner states are clearly observed at visible light spectra. The dynamical evolutions of topological boundary states are experimentally demonstrated, which verify the existence of photonic corner states in two band gaps. The symmetry‐protected corner states in the photonic square‐root HOTI may have potential applications in information processing and lasing. [ABSTRACT FROM AUTHOR]

This study developed a novel frequency-doubled Fabry–Perot cavity method based on a femtosecond laser of 1030 nm, 190 fs, 1 mJ, and 1 kHz. The time interval (60–1000 ps) and attenuation ratio (0.5–0.9) between adjacent sub-pulses of the 515 nm pulse train were able to be easily adjusted, while the efficiency was up to 50% and remained unchanged. Extremely high-quality low-spatial-frequency LIPSS (LSFL) was efficiently fabricated on an indium tin oxide (ITO) film using a pulse train with a time interval of 150 ps and attenuation ratio of 0.9 focused with a cylindrical lens. Compared with the LSFL induced by the primary Gaussian pulse, the uniformity of the LSFL period was enhanced from 481 ± 41 nm to 435 ± 8 nm, the divergence of structural orientation angle was reduced from 15.6° to 3.7°, and the depth was enhanced from 74.21 ± 14.35 nm to 150.6 ± 8.63 nm. The average line edge roughness and line height roughness were only 7.34 nm and 2.06 nm, respectively. The depths and roughness values were close to or exceeded those of resist lines made by the interference lithography. Compared with the common Fabry–Perot cavity, the laser energy efficiency of the pulse trains and manufacturing efficiency were enhanced by factors of 19 and 25. A very colorful "lotus" pattern with a size of 30×28 mm2 was demonstrated, which was covered with high-quality LSFLs fabricated by a pulse train with optimized laser parameters. Pulse trains can efficiently enhance and prolong the excitation of surface plasmon polaritons, inhibit deposition particles, depress ablation residual heat and thermal shock waves, and eliminate high-spatial-frequency LIPSS formed on LSFL, therefore, producing extremely high-quality LSFL on ITO films. [ABSTRACT FROM AUTHOR]

The ability to modulate the nonlinear optical (NLO) response of low‐dimensional metal oxides is highly desirable for diverse applications. It is shown here that the irradiation of a dispersion of 2D oxide (MoO3) nanosheets (NSs) by femtosecond laser pulses enables simultaneous scissoring of the NSs to form well‐dispersed MoO3 quantum dots (QDs) and the modulation of the NLO response in the visible and near‐infrared (NIR) regions. The inversion of the absorptive nonlinearity is observed combined with a larger enhancement of NLO response in the laser‐irradiated MoO3−x NSs and the as‐generated QDs, which is associated with the steady laser‐deprival of oxygen atoms and the resultant localized surface plasmon resonance due to electron doping. By leveraging the sub‐picosecond NLO response of the plasmonic MoO3−x QDs, the development of ultrafast optical switches is demonstrated for ultrashort pulse laser generation based on fiber lasers operating in the NIR regions. Furthermore, the strong field enhancement of these non‐noble metal plasmonic QDs is exploited for the fabrication of substrates for surface‐enhanced Raman scattering, which demonstrate a detection limit down to the part‐per‐billion level. This work delineates an efficient strategy for creating plasmonic oxide QDs with strong NLO response that is attractive for photonic applications. [ABSTRACT FROM AUTHOR]

In recent years, inorganic lead halide perovskite quantum dots have been used in various optoelectronic fields for their excellent luminescence properties, such as narrow emission bands, ultra‐wide tunable emission wavelength, and high quantum efficiency. In this paper, different from luminescence optimization in most research, luminescence degradation of perovskite quantum dots is addressed by femtosecond laser irradiation and successfully used for three‐dimensional data storage in CsPbBr3 quantum dots doped glass. Photoluminescence (PL) degradation can be finely modulated by adjusting the laser parameters. PL degradation mechanism, investigated by optical spectroscopy and morphology characterization, is attributed to laser‐induced decomposition, recrystallization, and defection of CsPbBr3 quantum dots. Laser‐induced PL degradation and the followed PL recovery by heat treatment are repeated for several cycles, showing good reversibility. Multilayer PL degradation patterns are written into the glass and read out without crosstalk, indicating high‐reliability 3D optical storage characteristics. Amazingly, PL degradation can be induced by just a low‐energy single laser pulse with estimated subpicosecond writing time per bit, demonstrating its potential in high‐speed, low‐power consumption 3D optical storage. [ABSTRACT FROM AUTHOR]

The information burst in modern society poses new challenges to information preservation technology, both in storage efficiency, capacity, and security. Multi‐dimensional optical data storage based on the femtosecond laser direct writing technique is a promising way to face these challenges. Herein, a new optical storage method based on femtosecond laser‐induced local darkening of CdSe quantum dots doped glass is reported. Photoluminescence image and spectrum indicate that highly luminescent well‐crystallized CdSe quantum dots can be amorphized by a single femtosecond laser pulse and reconstructed by thermal treatment, resulting in reversible luminescence modulation, in which the luminous efficacy of CdSe quantum dots in glass matrix was well recovered after each cycle of writing and erasing. Complicated patterns written inside the glass and multi‐layer information are also successfully inscribed and well demonstrated. Data bit could be written by just one 380 fs single pulse with pulse energy as low as 100 nJ. The present work provides a potential way for three‐dimensional optical information storage and data encryption with low power consumption, fast writing speed, large capacity, and high security. [ABSTRACT FROM AUTHOR]

Er‐activated fiber lasers are critical for many significant applications, including fiber communication, gas sensing, lidar and military. Although the design of suitable fiber materials has improved the performance of Er‐activated fiber lasers, extension of their operation wavebands still remains a significant challenge. Here, a compact all‐fiber laser is proposed and demonstrated, which can support lasing beyond 1630 nm, based on an Er‐activated hybridized glass fiber. The collaborative theoretical analysis and experimental characterization indicate that various distinct structure units can form in the hybridized fiber. This leads to the notable inhomogeneous spectral broadening. By using this hybridized fiber, a tunable fiber laser device is constructed and the wavelength can be rationally tuned in the range from 1609 to 1631 nm by mechanically stretching the fiber Bragg gratings (FBGs) manufactured by the point‐by‐point femtosecond laser inscription. Furthermore, a new fiber sensor system based on this seldom exploration waveband is built and efficient ethylene detection is demonstrated. The results highlight that rational hybridization mediated short‐ and medium‐structure modification strategy, demonstrated here in Er‐activated fiber laser, can be extended to other rare‐earth glass fiber systems for the exploration of fiber devices at new wavebands. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically show that the field-free molecular alignment can be controlled by shaping the femtosecond laser pulse with a periodic phase step modulation, involving the maximum degree and temporal structure of the molecular alignment. We show that the molecular alignment can be completely suppressed or reconstructed as that by the transform-limited laser pulse, the temporal structure of the alignment transient can be controlled with a desired shape, and the molecular alignment and antialignment for any temporal structure can be switched. Furthermore, we also show that both the degree and direction of the molecular alignment at a fix time delay can be continuously modulated. [ABSTRACT FROM AUTHOR]

This work discusses the response of Na to both high-energy electrons and femtosecond-laser (fs-laser) pulses in the soda-lime glass. The evidence for different responses of Na to high-energy electron irradiation between glasses with and without fs-laser irradiation suggests that the chemical and/or physical states of Na in the fs-laser irradiated glass are different from those in the original glass. Fs-laser pulses in the glass may be able to neutralize Na, which may form clusters. These results suggest that close attention should be paid to the defects associated with Na when optical or physical data are interpreted in fs-laser irradiated Na glasses. [ABSTRACT FROM AUTHOR]

In this paper, the effect of the irradiation pulse number on the morphology of the near-wavelength ripples perpendicular to the laser polarization was investigated. The results show that the ripple spacing first fast decreases with the increased irradiation pulse number and finally reaches a stable value. The change of ripple morphology with different incidence angles was also studied. The ripples are found keeping always perpendicular to the laser polarization and have respective ripple spacings of 650, 480 and 390 nm upon the laser irradiation with pulse number N = 16 and fluence of 1.33 J/cm at the incidence angles of 0°, 30° and 60°. The change of the ripple spacing with different excitation levels was also studied. The theory for interference of the incident wave with the surface plasmon polariton wave (SPP) and the Sipe-Drude theory are both employed to analyze the experimental results. Detailed calculations show that the Sipe-Drude theory works better for explaining the main features of the ripples on silicon wafer surface. We suggest that the interaction of the incident wave with some form of surface electromagnetic wave except SPP is most possibly responsible for the ripple formation on silicon surface. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) complex nanostructures on the surface of ZnO crystal are fabricated by the interference of three 800 nm fs laser beams. The 2D nanostructures exhibit a great enhancement of UV emission excited by infrared fs laser with central wavelengths ranging from 1,200 nm to 2,000 nm. We propose that the defect states in the band gap of 2D nanostructures induced by 800 nm fs laser ablation cause the great enhancement of UV emission. We make theoretical calculations and explain well with the experimental results. [ABSTRACT FROM AUTHOR]

We theoretically and experimentally demonstrate the quantum coherent control of the resonance-mediated two-photon absorption in rare-earth ions by the phase-shaped femtosecond laser pulse. Our theoretical results show that the resonance-mediated two-photon absorption can be effectively controlled, but the control efficiency depends on the laser repetition rate in real experiment due to the long lifetime and the short decoherence time of the excited state, and the larger laser repetition rate yields the lower control efficiency. These theoretical results are experimentally confirmed in glass sample doped with Er3+ by utilizing the femtosecond lasers with low repetition rate of 1 kHz and high repetition rate of 80 MHz. [ABSTRACT FROM AUTHOR]

The availability of laser pulses with femtosecond duration allows materials to be subjected to higher light intensity than ever before, opening the door to the study of laser/material interactions in a new regime. Various microstructures induced by femtosecond laser have been reported, such as color centers, voids, and refractive index change. In this article, we review our recent research findings on the femtosecond laser induced microstructures in glasses, which are not only helpful for understanding the interaction process between the ultrafast laser and the glassy materials, but also valuable for the fabrication of integrated optical elements. [ABSTRACT FROM AUTHOR]

We report on the preparation of Ga/Al nanoalloys embedded glass-ceramic by heat treatment as well as femtosecond laser irradiation. The Ga/Al nanoalloys embedded glass-ceramic exhibits intense extinction in the visible region, as well as superior optical nonlinearities. The third-order nonlinear optical properties of the glass-ceramics were investigated by employing Z-scan technique. The maximum nonlinear extinction coefficient β and nonlinear refraction γ value reached 1.14 × 10−11 m/W and 2.01 × 10−17 m2/W, respectively. Three-dimensional Ga/Al nanoalloys embedded structures were also achieved in glass by scanning the focused femtosecond laser beam. These results may find applications for the fabrication of ultrafast optical switches and diffractive optical elements. [ABSTRACT FROM AUTHOR]

We report on the synthesis of nanocrystalline zirconia in liquid using femtosecond laser ablation. Nanocrystalline cubic zirconia has been prepared by femtosecond laser ablation of zirconium in ammonia, while nanocrystalline tetragonal and monoclinic zirconia was synthesized in water. The physical and chemical mechanisms of the formation of nanocrystalline metastable zirconia are discussed. The intrinsic properties of femtosecond laser ablation in liquid and OH may be responsible for the synthesis of cubic zirconia. It is suggested that the femtosecond laser pulse can create higher temperature and pressure conditions at a localized area in the liquid than the nanosecond laser pulse and the cooling is also faster in the femtosecond laser ablation process, which determined the difference between the products synthesized with femtosecond and nanosecond-pulsed laser ablation. [ABSTRACT FROM AUTHOR]

Abstract: A 120fs Ti–sapphire laser was used to fabricate waveguides in YAG crystal. A 7mm long waveguide was written at a position of 100μm below the surface, which shows multimode propagation at 633nm with optical attenuation of about 0.2dB/mm. The light guiding occurs in the region around the visible laser-damaged region, indicating that the light guiding area is induced by stress. The waveguide exhibited strong birefringence property with maximum magnitude of about 1.5×10−5. Infrared and Raman spectroscopy analysis indicate there is no change in chemical composition in laser-modified zone. [Copyright &y& Elsevier]

In this article, the chemical and structural changes inside soda-lime glasses induced by femtosecond (fs) laser pulsing have been reported, based on transmission electron microscopy and electron energy loss spectroscopy studies. Under fs-laser interaction, Na-rich phases are formed, and Na nanoparticles are also precipitated around the Na-rich phases. These findings demonstrate how powerful and efficient the fs-laser pulsing and interaction can be in making novel microstructures in soda-lime silicate glass, and they bridge the gap between the macroscale property changes and nanometer-scale structures. [ABSTRACT FROM AUTHOR]

Actuators containing sensors are developed for various applications. However, it is still challenging to equip miniaturized high‐precision sensors with actuators at the micrometer scale to achieve micro‐environment detection. Here, a two‐in‐one processing strategy to construct a sensor‐equipped magnetic actuator (SEMA) is proposed. The design of magnetic actuators is continuously optimized to achieve less resistance and faster acceleration. Besides, the temperature‐dependent properties of semiconductor quantum dots (QDs) are used to fabricate the temperature‐sensitive unit (TSU), with a sensitivity of up to 111 pm °C−1. Finally, as a proof of concept, the real‐time temperature detection of arbitrary micro‐area in microchannels is achieved. This method of equipping sensors with microactuators will bring many potential applications in microenvironment sensing, micro‐mechanical manipulating, and microcargo transporting. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically demonstrate that the (2+1+1) resonance enhanced multi-photon ionization photoelectron spectroscopy in sodium atom can be effectively controlled by shaping femtosecond laser pulse with a π phase step modulation in weak laser field, involving its total photoelectron energy, maximal photoelectron intensity, and spectroscopic bandwidth. Our results show that the total photoelectron energy can be suppressed but not enhanced, the maximal photoelectron intensity can be enhanced and also suppressed, and the photoelectron spectroscopy can be tremendously narrowed. These theoretical results can provide a feasible scheme to achieve the high-resolution photoelectron spectroscopy and study the excited state structure in atomic and molecular systems. [ABSTRACT FROM AUTHOR]

The construction of functional photonic structures in transparent solids for various applications, such as 3D displays, optical information processing, and tunable lasers, is of great scientific and technological importance. Here, self‐assembled crystallite‐based grating nanostructures are created in an unconventional multicomponent glass with an ultrafast laser. The novel nanogratings are organized as periodically assembled crystalline and amorphous phases, exhibiting strong polarization‐dependent birefringence. The Ta2O5 component in the glass strongly contributes to the creation of nanogratings. Furthermore, a picosecond laser rather than a femtosecond laser is established to more suitably create nanogratings in the target glass, proving the critical role of thermal accumulation during nanoscale crystallization. Finally, nanogratings are demonstrated to be broadband variable near‐infrared optical attenuators with a high attenuation ratio, indicating the potential application of nanogratings in optical information processing at communication wavelengths. These findings provide new directions for fabricating nanogratings in functional glasses for advanced integrated photonics and offer information for revealing the mechanism of nanograting formation. [ABSTRACT FROM AUTHOR]

Broadband optical amplification is crucially important for expanding the transmission capacity of optical communication system and resolving the issue of internet traffic. A major challenge is the development of broadband gain materials. Herein, a heterogeneity engineering strategy is proposed for constructing all-inorganic transparent composite with tunable and ultrabroadband luminescence. The composite is characterized by the glass matrix homogeneously embedded with highly dense nanocrystalline domains with the optimal crystallinity of ∼63.7%. Notably, the formation dynamics and phase configuration of the composite can be controlled. As a result, the interesting optical response with tunable and ultrabroadband near-infrared luminescence from 1000 to 1800 nm can be realized. In addition, the embedded high density crystalline domains gift the composite with the excellent antilaser damage behavior and three-dimensional smooth active waveguide can be directly written by using femtosecond laser. This unique feature allows the creation of the integrated waveguide array device and the potential for broadband optical amplification is demonstrated. Results suggest that the proposed heterogeneity engineering strategy can not only be extended for exploitation of active composite system, but also development of three-dimensional broadband photonic devices with high integration level. [ABSTRACT FROM AUTHOR]

Highlights: [•] We reviewed the proposed mechanisms of nanomaterial generation by femtosecond laser ablation in solution. [•] The factors affecting the final structures of nanomaterials were reviewed. [•] The approaches to control the structures of nanomaterials, and to tune the size and size distribution were reviewed. [•] The nanomaterials prepared by femtosecond laser ablation in solution and their potential applications were reviewed. [Copyright &y& Elsevier]

Abstract: We fabricated lead nanocrystals inside a specially designed borosilicate glass by two parallel approaches, thermal stimulation and femtosecond laser irradiation. Thermal treatment can precipitate lead nanocrystals inside whole glass, while femtosecond laser irradiation can induce lead nanocrystals within selective area of glass. The fabricated nanocomposite exhibits large third-order optical nonlinearity and ultrafast response time, which is suitable for fabrication of ultrafast optical switches. Furthermore, the femtosecond laser induced refractive index difference between lead crystalline and glass is about 0.13, which is enough to fabricate optical devices such as photonic crystals. [Copyright &y& Elsevier]

Upconversion luminescence of lanthanide ion doped nanocrystals under ultrashort laser pulse excitation can find significant applications in areas like super-resolution nonlinear microscopic imaging and attracts increasing attention. Though major excitation mechanisms, such as excited state absorption (ESA), energy transfer upconversion (ETU) and multi-photon absorption (MPA), have been analyzed and discussed intensively, quantitative differentiation of their respective contributions with convincing experimental proof has seldom been reported however. Here we report experimental demonstration of a simple but effective way to quantitatively identify simultaneous MPA from other mechanisms like ESA and ETU involved in the Er3+ doped NaYF 4 nanocrystals in aluminosilicate glass ceramics under femtosecond laser field excitation. Comparative studies using laser pulses of the same spectral bandwidth but different time duration or repetition rate are performed and convincing experimental evidences for their respective contribution weights are obtained. The dependence of the MPA excitation efficiency on the spectral bandwidth of the femtosecond laser pulse is also explored for maximizing the upconversion luminescence yield. And predictions from a theoretical model agree well with experimental observations. • Demonstrating an effective way to identify multi-photon absorption (MPA) from other mechanisms, like ESA or ETU. • The MPA is found to play a dominant role in exciting the green upconversion emission. • The MPA excitation efficiency can be maximized by optimizing the spectral bandwidth of the femtosecond laser. [ABSTRACT FROM AUTHOR]

We report on square-shape distribution of ZnO crystals in glass by simultaneous femtosecond laser irradiation at multiple spots with the application of a spatial light modulator. Raman spectra and Raman mapping were used to characterize the formation of ZnO crystals and distribution of ZnO crystals, respectively. The shapes of the laser-modified region and the distribution of ZnO crystals were compared between the cases of one spot irradiation and multiple spot irradiation. The elemental distributions of the laser-modified region were investigated by an Electron Probe Microanalyzer. [ABSTRACT FROM AUTHOR]

The interaction of intense femtosecond laser pulses with transparent materials is a topic that has caused great interest of scientists over the past two decades. It will continue to be a fascinating field in the coming years. This is because many challenging fundamental problems have not been solved, especially concerning the interaction of strong, ultra-short electromagnetic pulses with matter, and also because potential advanced technologies will emerge due to the impressive capability of the intense femtosecond laser to create new material structures and hence functionalities. When femtosecond laser interacts with matter, a large amount of energy will be released during an ultra-short period of time, resulting in extremely high energy intensity. This opens the avenue to explore new light–matter interacting phenomena, investigate the details of the dynamical processes of the light–matter interaction, and fabricate various integrated micro-devices. In recent years we have witnessed exciting development in understanding and applying femtosecond laser induced phenomena in transparent materials. The interaction of femtosecond laser pulses with transparent materials relies on non-equilibrium process with photon beams and this provides new access to create materials and micro-devices that cannot be obtained by other means. Understanding of the physical mechanisms of many induced phenomena is extremely challenging. The aim of this review is to present a critical overview of the current state of the art in studying femtosecond laser induced various phenomena in transparent materials, including their physical and chemical mechanisms, the applications and limitations as well as the future research trends. The first part of the review presents the basics of femtosecond laser systems, important parameters influencing the femtosecond laser interaction with transparent materials, and a brief description of various energy transfer processes in materials during femtosecond laser irradiation. The second part will give an account on various phenomena such as multiphoton excited upconversion luminescence, long lasting phosphorescence, formation of color centers, valence state change, precipitation of nanoparticles and nanocrystals, microvoids, polarization-dependent and periodic surface structures, refractive index change, polymerization and air-bubble formation. The third part describes recently observed “anomalous” phenomena such as induced birefringence, nanogratings, nanovoid arrays, migration of ions, nonreciprocal photosensitivity, high pressure crystalline phase, and their underlying mechanisms, and their potential prospects as a new tool for photonic technology development. The final part points out the major challenges and future research trends in this promising field. [ABSTRACT FROM AUTHOR]

In this paper, we report the birefringence effect of surface self-assembly periodic nanostructures induced on 6H-SiC by femtosecond laser irradiation. Birefringence characteristic (e.g. cross-polarized image), measured by cross polarized microscopy, was found to be controlled by both single pulse energy and scanning velocity. Comparing birefringence measurement results of nanostructures and morphology characterization by Scanning electron microscopy, it is shown that ∼200 nm-period deep-subwavelength periodic ripples (DSWR) plays a dominating role in the birefringence effect. Raman spectra show that the change of retardance with pulse energy and scanning velocity is most possibly caused by the thickness variation of DSWR. Finally, a light attenuator based on a single layer of DSWR structure on 6H-SiC surface was constructed and tested by light source of 800 nm to have a tunable attenuating ratio of 69–100%. [ABSTRACT FROM AUTHOR]

A deep-ultraviolet (UV) nonlinear optical BaAlBO 3 F 2 crystal was space-selective precipitated in stoichiometric 50BaF 2 –25Al 2 O 3 –25B 2 O 3 glass by using a 500 kHz femtosecond pulse laser, which was confirmed by X-ray diffraction analysis (XRD). The distribution of BaAlBO 3 F 2 crystals in glass was analyzed by Raman spectra and Raman mapping. The second-harmonic generation (SHG) intensity of BaAlBO 3 F 2 crystals in glass could be tuned by changing the laser average power and exposure time. Absorption spectra were used to investigate the transmittance of the glass sample before and after the femtosecond laser irradiation. [ABSTRACT FROM AUTHOR]

We report on an abnormal migration phenomenon of alkali ions in calcium silicate glasses, induced by 250 kHz, 800 nm femtosecond laser irradiation. The diameter of the laser-affected zone is not dependent on the glass-transition temperature of glass samples. Elemental analysis by Electro-Probe Microanalyzer demonstrates that K ions migrate to the periphery of the laser-affected zone, while Cs ions show an opposite tendency in the two alkali ions coexisting glass. The possible mechanism on the observed abnormal phenomenon is discussed. [ABSTRACT FROM AUTHOR]

We report on microstructure modification and local luminescence change in oxyfluoride tellurite glasses induced by a 1030 nm, 500 kHz femtosecond laser. Tellurium nanoparticles are precipitated in the laser-modified region based on X-ray diffraction analysis and transmission electron microscopy observation. Micro-Raman spectra and energy dispersive spectroscopy show that migration of ions (Te 4 + , Zn 2 + and O 2− ) and coordination state transformation of Te 4 + ions occurred after the irradiation. The luminescence property of Er 3 + -doped glass also changes in the irradiated zone. The mechanisms of above phenomena are interpreted. During the 500 kHz femtosecond laser irradiation, photo-ionization process plays a significant role in the precipitation of Te crystalline phase. The creation of sharp temperature gradient results in modification of element distribution, which leads to the coordination change of Te 4 + ions and luminescence change of Er 3 + ions. These results are of reference value to fabricate integrated optical waveguide amplifiers and lasers in oxyfluoride tellurite glasses. [ABSTRACT FROM AUTHOR]

Abstract: Bismuth nanoparticles are directly precipitated inside a bismuth germinate glass by irradiation with a 1030nm, 500kHz femtosecond laser, which are confirmed by means of X-ray diffraction, transmission electron microscope and Raman spectra. In addition, the results of electron probe microanalyzer indicate that the elements distribution around the focal point is induced simultaneously. We suggest that the precipitation of Bi nanoparticles and redistribution of ions with different diffusion coefficients should mainly be ascribed to the heat accumulation effect and thermal diffusion. The results provide a novel approach to produce Bi nanoparticles inside glass and have promising applications for the fabrication of photonic devices. [Copyright &y& Elsevier]

Highlights: [•] Abnormal elements redistribution behavior was observed in tellurite glass. [•] The refractive index and Raman intensity distribution changed significantly. [•] The relative glass composition remained unchanged while the glass density changed. [•] First time report on the abnormal element redistribution behavior in glass. [•] The glass network structure determines the elemental redistribution behavior. [Copyright &y& Elsevier]

Abstract: Localized micro-structure and luminescence property are modified in Ag+-doped phosphate glass with high repetition rate femtosecond laser pulses. Confocal fluorescence spectra show that the luminescence intensity due to fs-laser induced Ag0 gradually increases and then decreases with the decreasing distance from the center of the laser-affected zone. In the inner structure, the intensity ratio of two Raman bands at 708cm−1 and 1210cm−1 decreases with the more close to the center of the laser-affected zone. A possible mechanism is proposed to explain the observed phenomena. [Copyright &y& Elsevier]

Abstract: Transparent glasses imbedded with functional nanocrystals have attracted considerable interest and been studied widely in the past several decades, due to their applications in optical devices, such as optical amplifiers, nonlinear optical devices, optical storage, 3D displays, etc. Femtosecond (fs) laser irradiation induced crystallization of glass has been proved to be a powerful tool to realize the space-selective precipitation of various nanocrystals inside glasses. In this review, we highlight our recent research achievements on the fs laser induced precipitation of nanocrystals inside glasses, including nonlinear optical crystal, upconversion luminescence crystal, high refractive index crystal, and metal nanoparticles. These studies not only help to understand the interaction process between fs laser and glassy materials, but also have potential applications in the fabrication of micro-optical devices. [Copyright &y& Elsevier]

Abstract: We theoretically demonstrate that the terahertz pulse generation via optical rectification in ZnTe crystal can be effectively controlled by making use of the shaped femtosecond laser pulses with a square phase modulation. We show that, by precisely controlling these parameters characterizing the square phase modulation, the terahertz spectrum is periodically modulated and the terahertz pulses are split into multiple subpulses, and the modulation period of the terahertz spectrum and the relative intensity and time interval of these terahertz subpulses can be continuously tuned. Furthermore, we utilize the power spectrum of the shaped femtosecond laser pulses to explain the physical control mechanism of the shaped and tunable terahertz pulse generation. [Copyright &y& Elsevier]

Abstract: We report on a simple method for fabricating ultra-small nanodiamonds (NDs) with size tunability and uniformity by femtosecond laser ablation in solution. Photoluminescence (PL) is tailored in a broad range from ultra-violet (UV) to red region through adjusting the carbon sources or solutions. The NDs also emit multiphoton luminescence. Comprehensive discussions based on the anomalous PL behavior and well-defined absorption and excitation peaks suggest that UV emission originates from radiative recombination through intrinsic direct transitions between highly localized π states and blue or green emission originates from radiative recombination through defect energy trapping states. [Copyright &y& Elsevier]

Abstract: The success in construction of three-dimensional micro optical components or devices inside transparent materials is highly dependent on the ability to modify materials’ local structure. Especially, the realization of space-selective manipulation of element distribution is highly desirable since most of optical parameters such as refractive index and luminescence are closely related to element distribution. Up to present, the only way to control selective element distribution is local melting of glass. Here, we reported, for the first time to our knowledge, the success in realization of space-selective manipulation of element distribution in glassy state region (i.e., un-melted region) inside glass with the irradiation of high repetition rate femtosecond laser. Confocal fluorescence spectra and micro-Raman spectra show that the luminescence distribution of Cu+ ions and the glass network structure can be controlled with femtosecond laser irradiation, revealing the potential applications of this technique in the fabrication of functional waveguides and integrated optical devices. [Copyright &y& Elsevier]

Abstract: We report on the precipitation of Si nanocrystals inside a borosilicate glass by using an 800nm, 250kHz femtosecond laser irradiation, which was confirmed with X-ray diffraction, Raman spectra and transmission electron microscopy analyses. Refractive index profile reveals that the refractive index of the Si nanocrystals precipitated region increased up to 8.7% in comparison with that of the unirradiated area, leading to a large diffraction efficiency of the fabricated dot structure. Furthermore, the third-order optical nonlinearity of the Si nanocrystals precipitated glass is greatly enhanced based on the Z-scan measurement. These results may find applications for the fabrication diffractive optical devices and optical switches. [Copyright &y& Elsevier]

Abstract: Permanent microscale bubbles with varied size and number density are induced in borosilicate glasses by adjusting the focusing depth (FD) of a tightly focused femtosecond laser. With continuously increasing of the focusing depth, the average size of generated bubbles experiences an increase–decrease process. However, the number density of generated bubbles experiences an opposite changing process compared to the change of the size. The possible mechanism for the bubble generation and changing with the focusing depth has been discussed. [Copyright &y& Elsevier]

Abstract: With the irradiation by femtosecond (fs) laser with high repetition rate, GeS2 micro/nano-crystalline formation and microstructural modification occurred in pseudo-binary GeS2–In2S3 glass, while almost no similar change was observed in GeS2 glass. The addition of In2S3 is beneficial for the precipitation of GeS2 micro/nano-crystals. It is expected that functional micro/nano-crystals can be controllably prepared in chalcogenide glasses by fs laser irradiation through glass composition design. [Copyright &y& Elsevier]

Abstract: Transparent glasses containing copper nanoparticles are promising materials for ultrafast all-optical switches in the THz region due to their wide range of resonant absorption frequencies, ultrafast time response, as well as large third-order nonlinear optical coefficients associated with the surface plasmon resonance (SPR) in the visible region. In this paper, three dimension controllable precipitation of copper nanoparticles inside a borosilicate glass by irradiation of femtosecond laser pulses is studied. According to the designed program, different patterns made up of copper nanoparticles can be induced inside the glass sample. Absorption spectra are used to confirm the precipitation of copper nanoparticles. The precipitated nanoparticles can be space-selectively “dissolved” by the second time femtosecond laser irradiation. The involved mechanisms are discussed. [Copyright &y& Elsevier]

Abstract: We report on highly localized precipitation of copper nanoparticles in borate glasses by irradiation with femtosecond laser pulses. The area in the vicinity of the laser focal point of an 800nm femtosecond laser inside the glass sample became red due to the precipitation of copper nanoparticles after the femtosecond laser irradiation. The precipitation of copper nanoparticles was confirmed by transmission electron microscopy images, electron energy-loss spectra and absorption spectra. The average power of the femtosecond laser and the irradiation time are two important factors related to the formation of the precipitates. The mechanism of the precipitation of copper nanoparticles is discussed. [Copyright &y& Elsevier]

Abstract: Localized microstructures were induced in a germanate glass by irradiation with a focused 800nm, 250kHz femtosecond laser pulses. Compared with unirradiated glass, an increase in intensity and a frequency shift of the Raman scattering bands have been observed in a ring-shaped region inside the laser-modified structure. After annealing the sample near the glass transition temperature, the ring-shaped contrast due to enhanced Raman scattering faded away. We propose that the increase in Raman scattering band intensity may be due to local compression of the glass induced by the femtosecond laser pulses irradiation. [Copyright &y& Elsevier]

Abstract: The carbon nanodots (CNDs) were synthesized by a simple method of femtosecond laser ablation of bagasse in ethanol. Strong optical limiting effects of as-prepared CNDs to 800nm femtosecond laser pulses were observed with the threshold of 74mJ/cm2. The strong two photon absorption of CNDs is responsible for the optical limiting response. The nonlinear coefficient was determined by the open-aperture Z-scan technique. [Copyright &y& Elsevier]

Abstract: We report the elemental redistribution in a Bi-doped oxyfluoride glass induced by a 250-kHz femtosecond laser. Our results show that the relative concentration of the network modifier Bi ions in the modified region is higher than that in the unmodified regions. However, the relative concentration of the network modifier Ca ions is opposed to that of Bi ions. This is the first time to observe the differential distribution of relative concentration between the network modifiers. These results are important to practical application in the fabrication of waveguide lasers and amplifiers in active ion-doped glasses. [Copyright &y& Elsevier]

Abstract: Water-soluble CdTe quantum dots (QDs) with green and yellow emission are prepared in aqueous solution using thioglycolic acid (TGA) as stabilizing agent. The as-obtained CdTe QDs are characterized by absorption and photoluminescence spectra. Strong emission is observed when the CdTe QDs are excited by an 800nm femtosecond laser. The ln–ln relations between the integral intensity of CdTe QDs and the excitation power are investigated, and the results indicate that the upconversion luminescence is a two-photon assisted process via a virtual energy state. [ABSTRACT FROM AUTHOR]

Abstract: α-Bi2O3 nanoparticles have been produced by femtosecond laser ablation in ethanol at room temperature. Their crystal structure, crystal phase, shape and size distribution, and photophysical property are investigated using X-ray diffraction, Raman, transmission electron microscopy (TEM), high-resolution TEM and UV–vis spectroscopy. The α-Bi2O3 nanoparticles produced by laser ablation with two particle size population distributions having the mean particle size of ∼10 and ∼60nm exhibited a good photocatalytic activity on the photodegradation of indigo carmine under 365nm light emitting diode irradiation. [ABSTRACT FROM AUTHOR]