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Abstract Soliton molecules (SMs) play a crucial role in nonlinear optical systems, enriching our understanding of nonlinear science through the study of their interaction dynamics. While passively mode-locked fiber lasers offer an efficient platform for generating diverse types of SMs, the complex internal dynamics of the laser often pose challenges in achieving predetermined temporal separations between SMs. Here, we implement a delayed optical feedback technique within a femtosecond optical parametric oscillator, enabling the generation of SMs with precise and controllable temporal separations. A theoretical model, which models the intracavity iterations of the signal with a simplified Ikeda map, is proposed to study the impact of parametric gain, intracavity feedback delay, and cavity length on the internal separations of the SMs. Our experimental results confirm that adjusting the cavity length allows for producing desired temporal separations within SMs. To reveal the evolution dynamics of the SMs, we further develop a rigorous numerical model using the carrier-resolved Forward Maxwell Equation, which is capable of modeling ultra-broadband complex dynamics based on a single equation without relying on the slowly-varying envelope approximation. The numerical model unveils the rich formation dynamics of the SMs at various separations, which confirms the critical role of the gain window provided by the pump. This work opens up new opportunities for the on-demand generation of SMs and provides valuable insights into the complex dynamics in femtosecond optical parametric oscillator systems with optical delayed feedback.

Bessel beams have multiple applications owing to their propagation-invariant properties, including particle trapping, optical coherence tomography, and material processing. However, traditional Bessel-beam shaping techniques require bulky components, which limits the development of miniaturized optical systems for integration with other devices. Here, we report a novel femtosecond laser direct writing strategy for fabricating mesoscale (from submicrometer to subcentimeter) binary optical elements with microscale resolution. This strategy utilizes femtosecond beams with a long focal depth to increase throughput while reducing the constraints on critical sample positioning. As a demonstration, we manufactured and characterized a 2.2 mm diameter binary axicon. The experimentally measured quasi-Bessel beam intensity distribution and the numerical results were remarkably consistent, demonstrating a suitable tradeoff between the overall size, efficiency, and structural fidelity. Furthermore, a compact Bessel lens containing binary axicons was constructed and successfully used for femtosecond laser mask-less ablation of periodic grating-type surface plasmon polariton excitation units. The demonstrated approach shows significant potential for fabricating customizable integrated optical components.

In this paper, we prove that the third near-infrared (NIR-III) window high-power laser with wavelength in the range of 1600–1800 nm can be obtained by the coherent Raman fiber amplification technique through theoretical and experimental study. Detailed numerical simulation reveals that the nonlinear dynamics of the Raman fiber amplification in the polarization-maintaining double-clad erbium-ytterbium co-doped fiber is similar to that of the Mamyshev oscillator. Through the spectral filtering effect induced by finite Raman gain, we can obtain a high-quality Raman pulse. According to the theoretical results, we design a simple Raman fiber amplification laser and finally obtain a high-quality watt-level NIR-III window laser pulse in which the central wavelength is about 1650 nm and the pulse width can reach 85 fs. The experimental results correspond to the simulation results. Such nonlinear effect is universal in all kinds of fibers, and we think this technology can provide a great contribution to the development of ultrafast fiber lasers.

Soliton molecules in optical resonators have attracted remarkable attention in nonlinear dynamics, driven by their compelling analogies with matter molecules. So far, while extensive research has been conducted on their generation, pulsations, and dissociation behaviors, the investigation of their quasi-periodic dynamics has been relatively limited. Here, we present a systematic exploration of the quasi-periodic dynamics of soliton molecules using advanced balanced optical cross-correlation techniques. The incommensurable quasi-period bifurcations constituted of cascaded Hopf bifurcations are found, providing an unambiguous pathway toward chaotic soliton molecules. The chaotic intramolecular dynamics are analyzed by time series, radio frequency spectra, phase portraits, and Lyapunov exponent analysis. In addition, we reveal an intrinsic frequency entrainment phenomenon experimentally. Such frequency entrainment provides a novel perspective on synchronization in optical resonators, encompassing the competition and interaction of oscillations across multiple temporal scales. Our experimental findings offer clear proof that the gain dynamics serve as the origin of the binding forces between solitons within the molecule, which are well supported by the numerical simulations. By advancing the understanding of sub-femtosecond resolved quasi-period dynamics of optical soliton molecules, this study contributes to the broader field of complex nonlinear dynamics, paving the way for future explorations into the intricate behaviors of solitons within optical resonators and relevant fields.

Single-cavity dual-comb mode-locked lasers, an emerging dual-comb technology, can generate two sets of pulse sequences with excellent mutual coherence, thus eliminating the need for complex active noise reduction systems. We report the generation of dual-wavelength dispersion-managed solitons in a near-zero anomalous-dispersion Er-doped mode-locked fiber laser. Transmissive dual channels are provided by birefringence filtering effect. The dual-wavelength mode-locking centers at 1539.1 and 1578.8 nm with the repetition-rate difference of 420 Hz. Furthermore, 2-hour spectral stability is measured in an open environment, and we analyze the intra-cavity dynamics that influences the long-term stability of the dual-wavelength mode-locked laser operating in the free-running state.

We utilize singular spectrum analysis (SSA) for post-processing distance measurement data obtained by a dual-comb femtosecond laser rangefinder. Signal denoising technique based on SSA allows detection of low amplitude vibrations from time series of distance data with limited signal-to-noise ratio (SNR). Various one-dimensional vibrations, including harmonic, amplitude modulated harmonic, and sinc-function vibrations, with micrometer amplitude comparable with measurement uncertainty of laser rangefinder, have been imposed on the target mirror by a piezo positioner. These vibrations have been extracted by SSA and the obtained vibration amplitudes agree with that imposed on the target very well. A commercial interferometer is used to evaluate the accuracy of SSA. Standard deviations of residuals between the SSA extractions and interferometer measurements are smaller than 0.6 $\mu$m. This technique is promising for complex moving pattern detection in object tracking and remote sensing.

Silicon-vacancy (VSi) centers in silicon carbide (SiC) are expected to serve as solid qubits, which can be used in quantum computing and sensing. As a new controllable color center fabrication method, femtosecond (fs) laser writing has been gradually applied in the preparation of VSi in SiC. In this study, 4H-SiC was directly written by an fs laser and characterized at 293 K by atomic force microscopy, confocal photoluminescence (PL), and Raman spectroscopy. PL signals of VSi were found and analyzed using 785 nm laser excitation by means of depth profiling and two-dimensional mapping. The influence of machining parameters on the VSi formation was analyzed, and the three-dimensional distribution of VSi defects in the fs laser writing of 4H-SiC was established.

WSe2, an intriguing transition metal dichalcogenides (TMDs) material with high-performance electronic and optoelectronics property, is emerging as next-generation photoelectronic devices. In this contribution, by applying WSe2 nanofilm as a saturable absorber in an Er-doped fiber laser, versatile mode-locked operations of conventional soliton, soliton molecules and dual-wavelength mode locked state can be experimentally observed. By properly setting the state of the polarization controller and pump power, the phase difference and pulse separation of soliton molecules can be adjusted. The soliton molecule with four steady phase difference (0, ±π/2, π) are realized in the fiber laser with TMDs based SA for the first time, to our knowledge. Meanwhile, a novel dual-wavelength mode-locked state with a soliton singlet and a soliton molecule is also obtained. Experiments results show that WSe2 is a suitable SA for vector soliton and soliton molecules generation.

Noise in mode-locked lasers has been a central issue for dual-comb metrological applications. In this work, we investigate the laser intensity noise on dual-comb absolute ranging precision. Two different dual-comb schemes based on linear optical sampling (LOS) and nonlinear asynchronous optical sampling (ASOPS) have been constructed. In the LOS scheme, the ranging precision deteriorates with the increase in laser relative intensity noise (RIN). This effect can be corrected by implementing a balanced photo-detection (BPD). In the ASOPS scheme, the experiment shows that the conversion from laser RIN to dual-comb ranging precision is negligible, making a balanced detection unnecessary for ranging precision improvement. The different manners of RIN’s impact on absolute ranging precision are attributed to the distinct cross-correlation signal patterns and the underlying time-of-flight (TOF) extraction algorithms.

Passively mode-locked fiber lasers emit femtosecond pulse trains with excellent short-term stability. The quantum-limited timing jitter of a free running femtosecond erbium-doped fiber laser working at room temperature is considerably below one femtosecond at high Fourier frequency. The ultrashort pulse train with ultralow timing jitter enables absolute time-of-flight measurements based on a dual-comb implementation, which is typically composed of a pair of optical frequency combs generated by femtosecond lasers. Dead-zone-free absolute distance measurement with sub-micrometer precision and kHz update rate has been routinely achieved with a dual-comb configuration, which is promising for a number of precision manufacturing applications, from large step-structure measurements prevalent in microelectronic profilometry to three coordinate measurements in large-scale aerospace manufacturing and shipbuilding. In this paper, we first review the sub-femtosecond precision timing jitter characterization methods and approaches for ultralow timing jitter mode-locked fiber laser design. Then, we provide an overview of the state-of-the-art dual-comb absolute ranging technology in terms of working principles, experimental implementations, and measurement precisions. Finally, we discuss the impact of quantum-limited timing jitter on the dual-comb ranging precision at a high update rate. The route to high-precision dual-comb range finder design based on ultralow jitter femtosecond fiber lasers is proposed. Keywords: Timing jitter, Mode-locked lasers, Dual-comb, Absolute distance measurement

We demonstrate the effective removal of Gordon-Haus (GH) jitter in mode-locked fiber lasers based on the narrow bandpass filtering, verified from large positive dispersion to large negative dispersion. Simulations and experiments indicate that there exists an optimum filtering bandwidth for the near-zero GH jitter along with the constant directly coupled jitter. The corresponding measured root-mean-square timing jitter (10 kHz to 10 MHz) is restrained within 0.6 dB, even though the net dispersion is largely varied by 70% of the intracavity fiber dispersion. The demonstrated effective GH jitter elimination free of the dispersion management gives an easy way for the low-jitter laser design and the related high-precision applications outside the laboratory.

We demonstrate the generation of few cycle mid-infrared (MIR) pulses from a self-compressed optical parametric oscillator (OPO), synchronously pumped by a 105 fs Yb3+ doped fiber laser system. By using a ring cavity design, 4.2-cycle (45.3 fs) MIR pulses at a central wavelength of 3250 nm (with a 10-dB bandwidth of 648 nm) are obtained in a simple, practical, and compact configuration. Detailed theoretical analysis and numerical simulation are performed to better understand the self-compression mechanism in such an OPO cavity. In addition, when pumped at 3.8 W, the OPO delivers idler output power up to 330 mW and the corresponding pump depletion is 74.7%.

After five decades of development, mode-locked lasers have become significant building blocks for many optical systems in scientific research, industry, and biomedicine. Advances in noise measurement and reduction are motivated for both shedding new light on the fundamentals of realizing ultra-low-noise optical frequency combs and their extension to potential applications for standards, metrology, clock comparison, and so on. In this review, the theoretical models of noise in mode-locked lasers are first described. Then, the recent techniques for timing jitter, carrier-envelope phase noise, and comb-line noise measurement and their stabilization are summarized. Finally, the potential of the discussed technology to be fulfilled in novel optical frequency combs, such as electro-optic (EO) modulated combs, microcombs, and quantum cascade laser (QCL) combs, is envisioned.

Optical–optical synchronization between independent mode-locked lasers with attosecond timing precision is essential for arbitrary electric-field waveform generation, subcycle optical pulse synthesis, optical frequency transfer as well as next-generation photon-science facilities, e.g., X-ray free-electron lasers. Long-term stable operation with low timing drift is highly desired for all above applications. Here, we present a five-day uninterrupted timing synchronization between two independent femtosecond Yb-fiber lasers via balanced optical correlation method. The out-of-loop residual timing drift over the entire time frame reaches 733 as rms, corresponding to $1.36\times 10^{{\rm -20}}$ instability at $1.31\times 10^{{\rm 5}}{\rm{\,s}}$. To the best of our knowledge, it is the first characterization of 105 s instability for subfemtosecond optical–optical synchronization based on mode-locked lasers.

We demonstrate that third-order dispersion (TOD) precompensation accompanied with nonlinear compensation can improve the pulse quality of high energy pulses in fiber chirped-pulse amplification (FCPA) system. The negative TOD fiber is implemented as the precompensation of TOD mismatch, resulting small required B-integral, which benefits the pulse quality. The conclusion is given by analytical study and numerical simulations. Based on this method, we obtain high-energy pulses up to 10.4 μJ. The quality of the dechirped pulse is high and >90% of pulse energy is contained in the main pulse. The dechirped pulse duration is ~280 fs, which is nearly the same as the transform-limited pulse of the oscillator. The system also shows a wide tunable range of pulse energy from 5 to 10 μJ with the good pulse quality.

In this paper, we report on a compact frequency conversion scheme based on high-power photonic crystal fiber (PCF) laser at optical communication band using a multigrating MgO-PPLN crystal. Temperature dependence of wave-vector mismatch, group velocity, central wavelength, and conversion efficiency was theoretically analyzed. Temperature tuning characteristics were experimentally obtained by propagating 240-fs pulses with a 126-MHz repetition rate in a 10-mm-long MgO-PPLN crystal. We also observed and characterized second-harmonic generation (SHG), third-harmonic generation (THG), and fourth-harmonic generation processes in the same crystal. Maximum SHG and THG conversion efficiency values of 27.5% and 4.3% were achieved at 100 °C and 77 °C, respectively.

Usually, only focused femtosecond (fs) lasers at near-infrared (NIR) range can induce photodamage to transparent cells, making it difficult to treat large amount of cells by such optical methods for photostimulation. In this study, we clarify the mechanism of photodamage to cells that are co-cultured with gold nanorods (GNRs) by fs laser. The pulse duration and repetition rate of the fs laser play a key role in cell damage suggesting that the heat accumulation contributes to the major part for the cell damage rather than the high peak power which mainly determines the efficiency of multiphoton excitation. We further show that cellular Ca2+ can also be released in this scheme, but the process is more sensitive to peak power. Our results can provide a large-scale GNR-mediated photostimulation for cell signaling modulation.

Negative curvature hollow-core fibers (NC-HCFs) are useful as gas sensors. We numerically analyze the single-mode performance of NC-HCFs. Both single-ring NC-HCFs and nested antiresonant fibers (NANFs) are investigated. When the size of the cladding tubes is properly designed, higher-order modes (HOMs) in the fiber core can be coupled with the cladding modes effectively and form high-loss supermodes. For the single-ring structure, we propose a novel NC-HCF with hybrid cladding tubes to enable suppression of the first two HOMs in the core simultaneously. For the nested structure, we find that cascaded coupling is necessary to maximize the loss of the HOMs in NANFs, and, as a result, NANFs with five nested tubes have an advantage in single-mode guidance performance. Moreover, a novel NANF with hybrid extended cladding tubes is proposed. In this kind of NANF, higher-order mode extinction ratios (HOMERs) of 105 and even 106 are obtained for the LP11 and LP21 modes, respectively, and a similar level of 105 for the LP02 modes. Good single-mode performance is maintained within a broad wavelength range. In addition, the loss of the LP01 modes in this kind of NANF is as low as 3.90 × 10−4 dB/m.

Femtosecond laser ranging has drawn great interest in recent years, particularly based on an asynchronous optical sampling implementation where a pair of femtosecond lasers are used. High precision absolute ranging either relies on tightly-phase-locked optical frequency combs (a dual-comb setup) or multiple averaging of the measurements from two free-running femtosecond lasers. The former technique is too complicated for practical applications, while the latter technique does not apply to moving targets. In this report, we propose a new route to utilizing a powerful singular spectrum analysis (SSA) filtering method to improve femtosecond laser ranging precision for moving targets with acceleration. The SSA method is capable of separating complex patterns in signals without a priori knowledge of the dynamical model. Here, we utilize the basic SSA filter to extract the target trajectory in the presence of measurement noise both in numerical simulation and in the absolute ranging experiment based on a pair of free-running femtosecond lasers. The experimentally-achieved absolute ranging uncertainty of a moving target is well below 110 nm at a 200-Hz update rate by applying the basic SSA filter. This method paves the way to the practical applications of femtosecond absolute ranging for dynamic objects.

The position effect of spectral filter (SF) in an all-normal-dispersion fiber laser is numerically studied and the dependence of the available bandwidth range of the SF on its position in the laser cavity is examined. The results have shown that the output pulses with high energy and strong peak power is obtained when the SF with an appropriate spectral bandwidth is placed at the optimal positions in the laser cavity.

In the effective index method, which models photonic crystal fibers by means of a step-index waveguide analogy, two critical parameters are the effective index of the cladding and the effective core radius. It has been shown that the use of an effective core radius as a function of the relative air hole diameter, or also of the relative wavelength, can improve the accuracy of this method. We show, by comparison with a rigorous finite-difference frequency-domain method, that the reported improved fully vectorial effective index methods have commonly adopted a radius of the equivalent circular unit cell, which does not give the best accurate effective cladding index as compared with the use of an equivalent circular unit cell having the same area as the hexagonal unit cell. Furthermore, by defining both the radius of the equivalent circular unit cell and the effective core radius as a function of the relative air hole diameter, and the relative wavelength, we believe that the fully vectorial effective index method can be further enhanced in terms of accuracy of both the effective cladding index and the modal index. OCIS codes: 060.2310, 060.2400, 000.4430.

We report an experimental investigation on the irradiation of copper thin films with high repetition rate femtosecond laser pulses (1040 nm, 50 MHz), in ambient air and liquid water. We observe a novel, striking phenomenon of chromatic copper oxides (CuO and Cu2O) annuli generation. The characteristic features of the chromatic copper oxide annuli are studied by exploiting micro-Raman spectroscopy, optical and scanning electron microscopies. In the case of irradiation in water, the seldom investigated effects of the immersion time, tw, after irradiation with a fixed number of pulses are analyzed, and an intriguing dependence of the color of the chromatic annuli on tw is observed. This remarkable behavior is explained by proposing an interpretation scenario addressing the various processes involved in the process. Our experimental findings show that Cu2O nanoparticles (size of ≈20 nm) and Cu2O nanocubes (nanocube edges of ≈30, ≈60 nm) can be effectively generated by exploiting high repetition rate laser-assisted oxidation. [ABSTRACT FROM AUTHOR]

Mitochondrial oxidative flashes (mitoflashes) are oxidative burst events in mitochondria. It is crosslinked with numerous mitochondrial molecular processes and related with pivotal cell functions such as apoptosis and respiration. In previous research, mitoflashes were found as spontaneous occasional events. It would be observed more frequently if cells were treated with proapoptotic chemicals. We show that multiple mitoflashes can be initiated by a single femtosecond-laser stimulation that was tightly focused on a diffraction-limited spot in the mitochondrial tubular structure. The mitoflash events triggered by different photostimulations are quantified and analyzed. The width and amplitude of mitoflashes are found very sensitive to stimulation parameters including laser power, exposure duration, and total incident laser energy. This study provides a quantitative investigation on the photostimulated mitoflashes. It may thus demonstrate such optical method to be a promising technique in future mitochondrial research. [ABSTRACT FROM AUTHOR]

We demonstrate the effective removal of Gordon-Haus (GH) jitter in modelocked fiber lasers based on the narrow bandpass filtering, verified from large positive dispersion to large negative dispersion. Simulations and experiments indicate that there exists an optimum filtering bandwidth for the near-zero GH jitter along with the constant directly coupled jitter. The corresponding measured root-mean-square timing jitter (10 kHz to 10 MHz) is restrained within 0.6 dB, even though the net dispersion is largely varied by 70% of the intracavity fiber dispersion. The demonstrated effective GH jitter elimination free of the dispersion management gives an easy way for the low-jitter laser design and the related high-precision applications outside the laboratory. [ABSTRACT FROM AUTHOR]

Optical–optical synchronization between independent mode-locked lasers with attosecond timing precision is essential for arbitrary electric-field waveform generation, subcycle optical pulse synthesis, optical frequency transfer as well as next-generation photonscience facilities, e.g., X-ray free-electron lasers. Long-term stable operation with low timing drift is highly desired for all above applications. Here, we present a five-day uninterrupted timing synchronization between two independent femtosecond Yb-fiber lasers via balanced optical correlation method. The out-of-loop residual timing drift over the entire time frame reaches 733 as rms, corresponding to 1.36 × 10−20 instability at 1.31 × 105 s. To the best of our knowledge, it is the first characterization of 105 s instability for subfemtosecond optical–optical synchronization based on mode-locked lasers. [ABSTRACT FROM AUTHOR]

Coherent supercontinuum light expanding from 1.4 to 4 lm is generated in a 4 cm long tapered fluorotellurite microstructured fiber (MF) pumped by a 1980 nm femtosecond fiber laser. The spectral broadening in the tapered fluorotellurite MF is caused by self-phase modulation, the Raman soliton, and red-shifted dispersive wave generation. Our results show that tapered fluorotellurite MFs are promising nonlinear medium for generating coherent broadband mid-infrared supercontinuum light. [ABSTRACT FROM AUTHOR]

All-optical pulse generation opens up a field for ultrawideband (UWB) applications. However, controllable pulse width and pulse type are still challenging. Here, we present a theoretical model and stimulated results of monocycle and doublet waveforms generation using programmable optical photon echo progress. We synthesized instantaneously monocycle and doublet waveforms by adjustment of pulse width, pulse amplitude, pulse position, and time interval of subpulses. We verified the possible application of the proposed method to design U.S. Federal Communications Commission-compliant UWB waveforms, and therefore, it may provide an avenue for waveform generation. [ABSTRACT FROM AUTHOR]

Tunable mid-infrared dispersive waves are generated in a birefringent fluorotellurite microstructured fiber (FTMF) pumped by a 1560 nm femtosecond fiber laser. The FTMF have two zerodispersion wavelengths (ZDWs) for each polarization axis. The second ZDWs for the fast and slow axes of the FTMF are 2224 and 2042 nm, respectively. As the pump laser is polarized along the fast (or slow) axis of the FTMF, tunable mid-infrared dispersive waves from 2680 to 2725 nm (or from 2260 to 2400 nm) are generated in the FTMF when the Raman soliton meets the second zerodispersion wavelength of the fast (or slow) axis with increasing the pump power. Our results show that the designed FTMFs are promising nonlinear media for generating tunable mid-infrared light sources. [ABSTRACT FROM AUTHOR]

A strip/slot hybrid horizontal silicon nitride slot waveguide is designed to provide an ultraflat and low dispersion. By optimizing the height and width of the structure, an ultraflat and low dispersion of ~0 ± 7 ps/nm/km over 812 nm wavelength range (from 1137 to 1949 nm) can be achieved. The waveguide with a 20-nm conformal overlayer has chromatic dispersion within ±1 ps/nm/km over 682-nm bandwidth. So the flatness is 0.0015, which is the lowest flatness in near-infrared regime of this kind of waveguide to our knowl. edge. The influence of the waveguide sidewall to dispersion is also discussed. [ABSTRACT FROM AUTHOR]

We demonstrate a simple passive method to generate broadband chirped super-Gaussian pulses from high-gain ytterbium-doped fiber amplification system without obvious bandwidth degradation. By use of the inherent gain shaping effect of fiber amplifiers, pulses can recover the initial Super-Gaussian profile and bandwidth with large gain and optical signal-to-noise ratio (OSNR). In this method, fiber amplifier itself works as a spectral filter with flexible transmission profiles and gain, which can be optimized by pump power and fiber length. A spectrally resolved numerical model with amplified spontaneous emission (ASE) is developed for broadband ytterbium-doped double-clad fiber pulse amplifiers. In a proof-of-principle two-stage amplification system, with a shorter fiber preamplifier working in the gain-narrowing region as a pre-shaper, broadband Super-Gaussian pulses resemble themselves after the second amplification stage with the largest total gain and OSNR. Experimental results quantitatively confirm the numerical predictions. The proposed gain shaping method also provides a new way to compensate the pulse deformation and bandwidth narrowing in high-gain fiber chirped pulse amplification systems. [ABSTRACT FROM PUBLISHER]

For decades, lasers have been a daily tool in most biological research for fluorescent excitation by confocal or multiphoton microscopy. More than 20 years ago, cell photodamage caused by intense laser stimulation was noticed by generating reactive oxygen species, which was then thought as the main damage effect by photons. In this study, we show that laser stimulation can induce autophagy, an important cell lysosomal pathway responding to immune stimulation and starvation, without any biochemical treatment. Two different types of laser stimulations are found to be capable of activating autophagy: continuous scanning by continuous-wave visible lasers and a short-time flash of femtosecond laser irradiation. The autophagy generation is independent from wavelength, power, and scanning duration of the visible lasers. In contrast, the power of femtosecond laser is very critical to autophagy because the multiphoton excited Ca2+ dominates autophagy signaling. In general, we show here the different mechanisms of autophagy generation by such laser stimulation, which correspond to confocal microscopy and cell surgery, respectively. Those results can help further understanding of photodamage and autophagy signaling. [ABSTRACT FROM AUTHOR]

Controllable gene expression is always a challenge and of great significance to biomedical research and clinical applications. Recently, various approaches based on extra-engineered light-sensitive proteins have been developed to provide optogenetic actuators for gene expression. Complicated biomedical techniques including exogenous genes engineering, transfection, and material delivery are needed. Here we present an all-optical method to regulate gene expression in targeted cells. Intrinsic or exogenous genes can be activated by a Ca2+-sensitive transcription factor nuclear factor of activated T cells (NFAT) driven by a short flash of femtosecond-laser irradiation. When applied to mesenchymal stem cells, expression of a differentiation regulator Osterix can be activated by this method to potentially induce differentiation of them. A laser-induced "Ca2+-comb" (LiCCo) by multi-time laser exposure is further developed to enhance gene expression efficiency. This noninvasive method hence provides an encouraging advance of gene expression regulation, with promising potential of applying in cell biology and stem-cell science. [ABSTRACT FROM AUTHOR]

Femtosecond lasers have been advancing Biophotonics research in the past two decades with multiphoton microscopy, microsurgery, and photodynamic therapy. Nevertheless, laser irradiation is identified to bring photodamage to cells via reactive oxygen species (ROS) generation with unclear mechanism. Meanwhile, currently in biological researches, there is no effective method to provide controllable ROS production precisely, which originally is leaked from mitochondria during respiration and plays a key role in a lot of important cellular processes and cellular signaling pathways. In this study, we show the process of how the tightly focused femtosecond-laser induces ROS generation solely in mitochondria at the very beginning and then release to cytosol if the stimulus is intense enough. At certain weak power levels, the laser pulses induce merely moderate Ca2+ release but this is necessary for the laser to generate ROS in mitochondria. Cellular original ROS are also involved with a small contribution. When the power is above a threshold, ROS are then released to cytosol, indicating photodamage overwhelming cellular repair ability. The mechanisms in those two cases are quite different. Those results clarify parts of the mechanism in laser-induced ROS generation. Hence, it is possible to further this optical scheme to provide controllable ROS generation for ROS-related biological researches including mitochondrial diseases and aging. [ABSTRACT FROM AUTHOR]

A chirp-tunable femtosecond 10 W, 42MHz photonic-crystal-fiber oscillator-amplifier system that is capable of delivering sub-60 fs light pulses at 1040 nm is used to demonstrate high-efficiency terahertz radiation generation via optical rectification in GaP crystals only a few millimeters in length. The optimization of the chirp of the fiber-laser pulses is shown to radically enhance the terahertz output, indicating one possible way to more efficiently use these extended nonlinear crystals in compact fiber-pumped terahertz radiation sources. [ABSTRACT FROM AUTHOR]

A modified Z-scan method was used to achieve polarization-resolved measurement of nonlinear properties of undoped bulk GaP excited by a femtosecond laser at 1040 nm. The optical Kerr nonlinearities of (100)-,(110)-, and (111)-cut GaP slabs were characterized experimentally at room temperature. The open aperture Z-scan curves demonstrate that three-photon absorption in GaP dominates the nonlinear absorption processes under the experimental conditions. In addition, experimental results show that both three-photon absorption and Kerr nonlinearity of undoped GaP possess the same anisotropy and crystal orientation dependence. [ABSTRACT FROM AUTHOR]

Vertical micro-holes were fabricated inside a photosensitive glass (FOTURAN) by focused femtosecond laser (λ = 775 nm) writing, followed by heat treatment and wet chemical etching in 8% hydrofluoric acid solution for 50 min. The micro-holes were analyzed by optical and scanning electron microscopy, and was found they own circular cross-section and clear edge. At present, micro-holes with aspect ratio of about 7 is achieved. By varying the incident laser fluence in a range of 2.3–36.2 J/cm2 and the laser writing velocity in 100–1000 μm/s, the influences of femtosecond laser parameters on the formation of micro-holes are characterized as that: writing velocities hardly affect the micro-hole diameter, while relatively lower laser fluences result in smaller diameter, and the cross-section is more circular in this case. The possible reason for this phenomenon is discussed. [ABSTRACT FROM AUTHOR]