Dye-doped polymer liquid crystal (DD-PDLC) random lasers have exhibited promising potential in speckle-free laser imaging, non-polarized flexible displays, biological research, and optical sensing due to their unique features of low spatial coherence, multi-wavelength laser emission, and omnidirectional light output. However, challenges such as high lasing thresholds, poor film uniformity, and insufficient stability persist, primarily arising from fluorescence quenching and dye aggregation-induced gain reduction, photobleaching under prolonged excitation, and inadequate control over liquid crystal alignment and polymer network structures, which lead to unstable light scattering in pure liquid crystal systems. This study demonstrates a systematic optimization strategy for random lasers in PM567 doped poly (methyl methacrylate) (PMMA)/E7 liquid crystal composites, incorporating a patterned sapphire substrate (PSS) with periodic microstructures to enhance feedback efficiency, achieving a quality factor of 2846, a narrow linewidth of approximately 0.2 nm, and a remarkably low threshold of 0.52 mJ/cm2. As a speckle-free random laser source, it exhibits a signal-to-noise ratio (SNR) of 23.37, showing a 16-fold improvement over pumped pulsed lasers (SNR = 1.41) and a 5-fold enhancement compared to white light sources (SNR = 4.57). These advancements have paved the way for the practical application of random lasers in high-resolution speckle-free bioimaging and next-generation optoelectronic devices.

Neuromorphic vision systems are critical for high-efficiency edge intelligence, yet mainstream electrically controlled synaptic devices suffer from irreversible ion migration and poor biomimetic imaging capability. Herein, we propose an all-optically controlled artificial synaptic device based on the photochromic effect of lead zirconate titanate ceramics. By adopting an asymmetric strategy of 395 nm ultraviolet light writing and 808 nm infrared light erasing, it simulates biological synapses' bidirectional plasticity. A biomimetic visual memory imaging system was further constructed, which exhibits long-term paired-pulse facilitation (PPF) beyond short-term plasticity. The PPF index (18%-85%) is flexibly tunable via pulse interval, duration, and background light intensity, realizing dynamic imaging that mimics the brain's memory-forgetting-reconsolidation process. This all-optical scheme offers a highly programmable platform for biomimetic visual systems integrating sensing, memory, and computation.

By leveraging the defect properties of Ga doping, the excellent nonlinear optical (NLO) properties of zinc oxide (ZnO) can be extended to a broadband spectral range. A significant increase in the non-degenerate two-photon absorption coefficient was achieved in the visible region by systematically tuning the pump and probe wavelengths in ultrafast transient absorption spectroscopy. Simultaneously, a high modulation depth was achieved in the Ga-doped ZnO, even under excitation by an extremely low pump fluence. The specific energy level positions of Ga defects with different charge states in ZnO were determined by combining the non-degenerate resonance enhancement effect with photoluminescence spectroscopy. The improved Kerr nonlinearity in the near-infrared region resulting from Ga-related defects was also confirmed by Z-scan measurements. The real part of the figure of merit, evaluated based on the Kerr effect, was significantly superior to that of other wide-bandgap semiconductor materials. This work provides an effective strategy for designing resonance-enhanced NLO responses in wide-bandgap semiconductors via doping, as well as an important reference for developing all-optical switching across a broadband range.

Zinc oxide (ZnO) plays a crucial role in the application of all-optical devices due to its broadband nonlinear optical response. In this work, the broadband nonlinear optical response of Ga-doped ZnO (GZO) was investigated using femtosecond transient absorption spectroscopy (450-800 nm), revealing an ultrahigh modulation depth (similar to 86%) and ultrafast response (similar to 280 fs) at a pump wavelength of 750 nm. Non-degenerate two-photon absorption (TPA) spectra corrected by dispersion revealed significantly stronger TPA in the visible range. An energy level model based on Ga-related defects explained the enhancement mechanism of TPA and the photoluminescence spectrum. This study highlights the significant potential of GZO in future broadband all-optical device applications.

Investigating the broadband nonlinear optical response and related dynamic mechanisms in the wide-bandgap semiconductor gallium oxide is crucial for ultrafast photonic applications. In this study, transient absorption spectroscopy was used to probe the metal-doping effect on the bound-electron nonlinear optical response. Fe doping was found to significantly enhance the nondegenerate two-photon absorption, a remarkably large imaginary component of the figure of merit, indicating potential applications for nonlinear absorption-based all-optical switching. Analysis of the optical polarization dependence of carrier absorption, combined with carrier-induced nonlinear refractive effect, demonstrated that Fe doping modulates the carrier lifetime and enables the transformation of phase symbols, establishing mechanisms for the implementation of dual-channel optical switching. An energy-level model based on photoluminescence elucidates the nonlinear optical modulation mechanism of Fe-related defect states on bound electrons and carriers. This study serves as a valuable reference for the design of gallium-oxide-based waveguides and all-optical switching materials.

Fully understanding and modulating the nonlinear absorption in GaN are crucial for designing ultrafast photonic devices. In this work, both the ultra-broadband transient absorption spectra and carrier recombination time in GaN were found to be significantly altered by carbon defects. An energy band model for carbon defect dynamics was established based on transient absorption and photoluminescence spectroscopy. Our model discernibly reveals that CN and tri-carbon in GaN intricately modulate both the absorption spectrum and carrier capture process: The rapid capture of holes by the CN defect significantly reduces the hole recombination time to hundreds of femtoseconds in the near-infrared band. Conversely, the tri-carbon defect exhibited a higher absorption cross section by an order of magnitude than that of free carrier in the visible region with a long carrier recombination time. This work clarifies the modulation mechanisms of complex carbon defects in GaN's nonlinear absorption and provides scientific guidance for designing broadband and integrated ultrafast optical nonlinear devices.

We report on the anisotropic ultrafast optical nonlinearity of Fe-doped SrTiO3 composites (FSTO) using the femtosecond (532 nm, 100 fs) polarization-resolved Z-scan method. The SrTiO3 (STO) and FSTO are first grown via the flame fusion method, and their fundamental optical constants, namely the linear optical absorption and optical band gap value, are then accessed through the optical transmittance measurement. Furthermore, the Zscan results show that the FSTO achieves an enhanced reverse saturation absorption and a significant nonlinear refraction signal compared to the pristine one. More importantly, the Delta T and Delta TP_V of FSTO as a function of the polarization angle theta exhibit oscillation curves with a period of pi/2, which means that the realization of highly polarized optical nonlinearities. In particular, with a further reduction in the repetition frequency of the laser, both the nonlinear absorption and refraction remain unchanged, indicating that the thermal effect is negligible. The maximum nonlinear absorption coefficient beta and nonlinear refractive index n2 of FSTO are determined to be 4.5 x 10_ 11 m/W and 6.4 x 10_ 18 m2/W, respectively, which is a boost of around 7 times compared to that of STO (6.1 x 10_12 m/W / NA). Such enhanced and anisotropic optical nonlinearities are due to the fact that Fe- related defect states give rise to excited state absorption and the crystal orientation is optimized by the Fe element substrate. In addition, the FSTO with enhanced optical nonlinearity has also been demonstrated to exhibit excellent optical limiting performance. The findings presented here indicate that the FSTO can be regarded as a potential candidate for versatile nonlinear optical and photonic devices.

We report the broadband optical nonlinearities and ultrafast carrier dynamics of GO doped with CuS nanoparticles (GO-CuS) following the femtosecond pulse lasers. Both the pure GO and GO-CuS hybrid films are first synthesized by the vacuum filter deposition method and their optical nonlinearities are then examined in the wide wavelength range (400 nm-1064 nm) based on the femtosecond Z-scan technique. The results reveal that, compared to the pristine GO, the introduction of CuS not only makes the GO-CuS hybrid film switched from saturated absorption to reverse saturated absorption possible, but also achieves strong broadband optical nonlinear absorption responses and optical limiting effects. It is demonstrated that the strongest nonlinear absorption coefficient (similar to 3.6 x 10(-7) m/W) appears at 400 nm excitation, which is dozens of times larger than that under the illumination of 1064 nm. By leveraging the femtosecond pump-probe technique, we further study the ultrafast carrier dynamics of the GO-CuS hybrid film carrying the fast and slow relaxation processes. Moreover, the physical mechanisms behind these excellent nonlinear optical responses are elucidated. The findings presented here indicate that the GO-CuS hybrid film can be regarded as a potential promising candidate in various ultrafast photonics devices.

In this work, we aim to investigate wavelength-dependent tunable optical nonlinearities and concomitant beam shaping of graphene oxide-ZnFe 2 O 4 (GO-ZFO) composite following the femtosecond pulse lasers with the wavelength of 532 nm and 800 nm. To begin with, narrow bandgap semiconductor ZnFe 2 O 4 (ZFO) nanoparticles are decorated on the GO nanosheet with the hydrothermal method and then characterized by a variety of material characterization methods. Subsequently, the femtosecond Z-scan results reveal that, compared to the pristine GO, the GO-ZFO composite features nonlinear optical absorption switching from saturated absorption to reverse saturated absorption with 532 nm laser excitation, whereas its nonlinear refraction converts from self-focusing effect to self-defocusing with 800 nm wavelength illumination, thus leading to tunable nonlinear optical responses. More specifically, the strongest nonlinear absorption (refraction) coefficients are given to be 1.1 × 10 −9 m/W (2.9 × 10 −16 m 2 /W) and 7.1 × 10 −10 m/W (−1.6 × 10 −16 m 2 /W), respectively, with 532 nm and 800 nm laser excitations, which correspond to approximately several times of the pure GO. In principle, these exotic optical nonlinearities of the GO-ZFO composite may be due to the broadband absorption and narrow bandgap of doped ZFO and photoinduced electron and energy transfer between the GO and ZFO. Femtosecond pump-probe measurements reveal that the formation of the GO-ZFO heterojunction effectively induces a prolonged (39.7 ps) carrier decay lifetime following photoexcitation. Such dynamics indicate that the interfacial interaction hinders immediate recombination, thus leading to the enhanced optical nonlinearities. Furthermore, we find that beam shaping can be achieved by leveraging the optical nonlinearities of the GO-ZFO composite, in which the input Gaussian beams are reshaped into doughnut ones within the micrometer scale. The findings presented here demonstrate that the GO-ZFO composite can be regarded as an excellent nonlinear optical competitor, highlighting the versatility and potential in broadband nonlinear photonics devices and super-resolved imaging.

Herein, intensive visible and near infrared upconversion (UC) emission in 0.1-1.0 mol% trivalent neodymium ion (Nd3+) doped GdVO4 single crystals were investigated under continuous wave lasers at 808 nm and 1064 nm, wherein thermal phonon-assisted excitation and electron-phonon coupling activities play crucial roles in the one-photon UC process. The intensity of the one-photon UC increases exponentially with temperature and varies with varying Nd3+ ions concentration and wavelength, demonstrating strong potential for use in high-sensitivity temperature sensors. Polarization dependent UC emission spectra were analyzed based on the phonon vibration modes and directions in Nd3+ ions doped GdVO4 single crystals. This study comprehensively analyzes the effects of temperature variation and polarization effects on the one-photon pumped UC luminescence of Nd3+ ions, providing theoretical insights for advancing Nd3+ doped lasers and optoelectronic devices.

Optical nonlinear response and its dynamics of wide-bandgap materials are key to realizing integrated nonlinear photonics and photonic circuit applications. However, those applications are severely limited by the unavailability of both dispersion and dynamics of nonlinear refraction (NLR) via conventional measurements. In this work, the broadband NLR dynamics with extremely high sensitivity (lambda/1000) can be obtained from absorption spectroscopy in GaN:C using the refraction-related interference model. Both the absorption and refraction kinetics are found to be significantly modulated by the C-related defects. Especially, we demonstrate that the refractive index change Delta n of GaN:C is negative and can be used to realize all-optical switching applications owing to the large NLR and ultrafast switching time. The NLR under different non-equilibrium carrier distributions originates from the capture of electrons by C-N(+) defect state, while the absorption modulation originates from the excitation of tri-carbon defects. We believe that this work provides a better understanding of the GaN:C nonlinear properties and an effective solution to broadband NLR dynamics of transparent thin films or heterostructure materials. (c) 2024 Chinese Laser Press

The color of the diamond obtained with the B2S3 additive changes from yellow to light blue and the resistivity of the synthesized diamond drops from 3.89 × 10 Ω cm to 2.51 × 10−1 Ω cm with the introduction of Ti/Cu into the synthetic cavity.

The spin effects on the propagation characteristic of circularly polarized electromagnetic (EM) wave in high density strongly magnetized plasma are discussed based on the the classical hydrodynamical model of relativistic spin plasma. The dielectric coefficients for right-hand circularly polarized (RCP) and left-hand circularly polarized (LCP) waves are obtained. Results show that the spin effects can affect the propagation characteristic of circularly polarized EM wave dramatically. Provided the spin effect is strong enough, LCP waves can also propagate in the magnetized over-dense plasma, while RCP waves may not. The strength of spin effects can be enhanced by increasing the plasma density or/and EM wave intensity.

GaN is a one of promising materials for nonlinear optical applications. In this work, the broadband nonlinear optical response and potential applications for all-optical switching (AOS) are evaluated in low-defect GaN. In the pump-probe experiments, the ultrafast optical switching times are consistent with pulse widths accompanied with relative weak free-carrier absorption response, and the modulation contrast can reach ∼60% by varying the polarization orientations between the pump and probe lights. In the visible region, the broadband two-photon absorption effect exhibits excellent values for the imaginary part of figure of merit (FOM), providing the possibility of AOS based on nonlinear absorption (magnitude). While in the near-infrared region and under the presence of three-photon absorption, not only the real part of FOM based on Kerr effect is evaluated, but also the maximum light intensity for the usage of AOS based on nonlinear refraction (phase) is determined. The broadband nonlinear optical and AOS features in low-defect GaN will be highly favorable for the applications in the field of integrated nonlinear photonics and photonic circuits.

In this study, ultrafast third-order optical nonlinearity of ZnO crystals with different crystal planes and doping under femtosecond linearly/radially polarized light (330 fs, 532 nm) excitation is investigated. First, the band gap of ZnO crystals is analyzed using an ultraviolet- visible spectroscopy absorption spectrum. Next, the third-order optical nonlinearity of the ZnO crystals under femtosecond linearly/radially polarized light is measured using the Z-scan technique. The results show that the nonlinear saturation absorption effect and self-focusing effect are observed in ZnO crystals with different crystal planes and doping under the excitation of linearly and radially polarized lights owing to the electron transition in the defect state of the ZnO crystals. The third-order optical nonlinearity of ZnO[101] is the strongest for femtosecond linearly polarized light excitation, owing to the third- order optical nonlinearity of near-resonance enhancement caused by the narrowed energy bandgap in ZnO crystals. However, under the excitation of radially polarized light, ZnO[110] has the strongest third- order optical nonlinearity, probably owing to the combination of the narrow energy bandgap and the anisotropic nonlinearity caused by the axisymmetric polarization of the femtosecond vector laser. This study has potential application value in saturable absorbers, ultrafast light field regulation, and super-resolution imaging.

Crystallization of diamond with different nitrogen concentrations was carried out with a FeNiCo–C system at pressure of 6.5 GPa. As the nitrogen concentration in diamond increased, the color of the synthesized diamond crystals changed from colorless to yellow and finally to atrovirens (a dark green). All the Raman peaks for the obtained crystals were located at about 1330 cm −1 and contained only the sp 3 hybrid diamond phase. Based on Fourier transform infrared results, the nitrogen concentration of the colorless diamond was < 1 ppm and absorption peaks corresponding to nitrogen impurities were not detected. However, the C-center nitrogen concentration of the atrovirens diamond reached 1030 ppm and the value of A-center nitrogen was approximately 180 ppm with a characteristic absorption peak at 1282 cm −1 . Furthermore, neither the NV 0 nor the NV − optical color center existed in diamond crystal with nitrogen impurities of less than 1 ppm by photoluminescence measurement. However, Ni-related centers located at 695 nm and 793.6 nm were observed in colorless diamond. The NE8 color center at 793.6 nm has more potential for application than the common NV centers. NV 0 and NV − optical color centers coexist in diamond without any additives in the synthesis system. Importantly, only the NV − color center was noticed in diamond with a higher nitrogen concentration, which maximized optimization of the NV − /NV 0 ratio in the diamond structure. This study has provided a new way to prepare diamond containing only NV − optical color centers.

Various optical color centers in natural and synthetic diamond have attractive great attention of researched; to further investigate the effects of additive on diamond properties is of practical significance. In this work, diamond crystallization was investigated by temperature gradient growth method with Ge additive at a set pressure of 6.5 GPa and synthesis temperature ranging from 1290 to 1310 °C. Scanning electron microscope (SEM), and spectral characterization including Fourier transform infrared spectroscopy (FTIR) and photoluminescence (PL) spectra were used to investigate the surface growth mechanism of diamond. It was noticed that the visible inclusions gradually reduced in the as-grown diamond, accompanying with the increase of Ge additive in the synthesis cavity. The (111) surface of the synthesized diamond followed the layer growth pattern from the SEM results. According to the measured FTIR results, the nitrogen impurity concentration values of the synthesized diamond crystals decreased from 500 to 90 ppm, confirming that Ge showed its own ability to remove nitrogen impurity in the growth stage of diamond. Additionally, the nickel-related and nitrogen-related optical color centers of the obtained diamond were characterized by PL spectra. Characteristic peaks due to nickel-related optical color centers located at 515 or 535 nm, and Characteristic peaks attributing to nitrogen-related optical color centers located at 575 or 637 nm. We believe that the research is of great significance for the application of diamond in optical and quantum information science fields.

We report two-photon excited ultrafast carrier trapping and recombination in n-type β-Ga2O3 crystals by using femtosecond transient absorption spectroscopy. The broadband absorption spectra arising from the defect are polarization dependent, especially, two absorption peaks can be observed by subtracting the absorption transients under two probe polarizations. We attribute these observed defect-related absorption features to optical transitions from the valence band to different charge states of a native defect (such as gallium vacancies). A model for carrier capture by multilevel of a single defect is proposed to interpret the data, wherein holes are captured more efficiently than electrons by the defects, and the absorption cross sections for the defects are at least ten times larger than that for free carriers. Our results reveal the potential applications of β-Ga2O3 in ultrafast and broadband optoelectronic devices.

A nitrogen-doped diamond crystal with (111) orientation was synthesized with an NaN3 additive in the FeNi-C system at a pressure of 6.5 GPa and a temperature of 1,310°C, using the temperature gradient growth (TGG) method. Spectroscopic properties such as the absorption spectrum and the Raman spectrum as well as the Fourier transform infrared (FTIR) spectrum were studied. FTIR spectroscopy of the C-N vibrational modes at 1,344 and 1,130 cm−1 suggested a nitrogen content of 310 ppm. Its nonlinear optical (NLO) response was investigated using the Z-scan technique under the femtosecond regime. Due to the presence of nitrogen defects, the synthesized crystal performed large nonlinear absorption under both 800- and 532-nm wavelength excitations. However, intrinsic diamond only experiences nonlinear refraction under these two wavelength excitations. Its broadband NLO properties indicated that nitrogen-doped diamond crystals were suitable for the application of ultrafast optical devices.

The remarkably reversible thermochromic luminescence behavior and the rare nonlinear optical (NLO) properties of the [Ag55(MoO4)6(C[triple bond, length as m-dash]C t Bu)24(CH3COO)18(CH3COO)]·2H2O ({Ag55Mo6} for short) nanocluster reported were investigated experimentally. The important contributions of Ag+, C[triple bond, length as m-dash]C- ions and MoO4 2- groups to the NLO properties were proved by further density functional theory (DFT) calculations.