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The translation matching method (TMM) was presented here to obtain the refractive index (RI) of chalcogenide films more accurately. It solved the problem of calculation errors of the RI of the film that resulted from using the Swanepoel method after conventional treatments such as illumination, electrical measurement, and annealing. The TMM was suitable for calculating the RI of the film by calibrating the transmission spectrum regardless of whether it had been dealt with. The accuracy of the RI calculation is about 0.1% using the method. Experimentally, the method was used to obtain the RI of three chalcogenide films: they are original homogeneous film and RI nonuniform film treated by annealing and by femtosecond laser direct writing, respectively. The results showed that the decrease in the transmittance and weakened interference effect were effectively overcome and the RI of the three kinds of films was accurately obtained by using the method. This work should provide a useful guideline for obtaining the RI of the transparent optical films. [ABSTRACT FROM AUTHOR]

In this paper, generalized ring Pearcey beams are proposed theoretically and their corresponding generation is demonstrated experimentally by use of the double‐phase method. Generalized ring Pearcey beams evolve into circular or elliptical ring Pearcey beams, depending on the elliptical parameter. It is shown that the autofocusing properties of such beams can be effectively controlled by adjusting the elliptical parameter and the spatial distribution factor. To perform a more comprehensive investigation, the radially polarized forms of the beams are also realized in experiments. It is found that for both cases of vector and scalar, the peak intensity of the beams at the autofocusing plane undergo a dramatic increase as compared with that at the initial plane. Finally, by taking optical trapping as an example, it is proved that generalized ring Pearcey beams could be a productive instrument for industrial applications. [ABSTRACT FROM AUTHOR]

We have experimentally reported aluminum zinc oxide (AZO) as a novel saturable absorber to generate a mode-locked laser operating in dark pulsed regime. The D-shaped fiber was first prepared by side polishing a single-mode fiber to the 1 dB insertion loss. The AZO was dissolved in isopropyl alcohol and then deposited onto the D-shaped fiber by drop casting method. By carefully optimizing the polarization of the cavity, a dark pulse was achieved, which remained stable within the pump power of 65.7–101.35 mW. Pulse width was 220 ns, and the pulse repetition rate was 1.85 MHz. [ABSTRACT FROM AUTHOR]

Modern ultrafast lasers are enabling better operative performances in micro and nano-fabrications of materials, benefited from their ease of use, robustness, and reliability. The purpose of this paper is to bridge the gap between the knowledge of ultrafast optics and the modern industrial applications. The paper introduces main phenomena in ultrafast laser-matter interactions by reviewing relevant theoretical models and simulation techniques. The operation of ultrafast lasers is based on nonlinear optics phenomena, fully described only in the framework of quantum-mechanics. Current trends in the development of modern ultrafast system are given. It also presents a review of the latest advances in ultrafast laser-based manufacturing processes, some of the most interesting industrial applications, and a discussion on future trends and challenges. [ABSTRACT FROM AUTHOR]

Stress birefringence (SBR) of optical components is significant in polarization-dependent and fine optical applications, such as optical lithography. However, the methods generally used for high-resolution birefringence distribution detection are unsuitable for measuring samples with nonplanar surfaces, such as lenses, owing to refraction and aberration. In this work, we propose and verify a method for directly measuring SBR distributions in a lens. An aqueous glycerol solution is used as the refractive index matching liquid to rectify the measuring light path. The retardation level of the matching liquid is confirmed to be considerably smaller than that of the window, thus simplifying the measurement error. The retardation levels of the optical plate and wave plate obtained using this method are consistent with that measured in air (Δ < 0.025° and 0.212°, respectively), as well as with the fast axis angle of the wave plate (Δ < 0.15°). Additionally, the SBR distribution of the lens sample coincides with its physical state and is repeatable with a coefficient of variation of less than 2.585%. This method provides a convenient tool for SBR distribution measurements in a lens, which could be extended to the measurement of optical components exhibiting any surface shape. [ABSTRACT FROM AUTHOR]

We present novel waveguide writing concepts in bulk PMMA. The writing relies on laser induced modification tracks that are completely surrounding a waveguide core. We found the optimal parameters to construct highly reproducible, single-mode waveguides with minimal propagation losses down to 0.6 dB  cm−1. Employing the best geometry, we demonstrate 2D and 3D Y-splitters that are the building blocks for creating complex optical networks such as sensors or lab-on-chip devices in polymer materials. [ABSTRACT FROM AUTHOR]

We report a simple and compact design of a dispersion compensated mode-locked Yb:fiber oscillator based on a nonlinear amplifying loop mirror (NALM). The fully polarization maintaining (PM) fiber integrated laser features a chirped fiber Bragg grating (CFBG) for dispersion compensation and a fiber integrated compact non-reciprocal phase bias device, which is alignment-free. The main design parameters were determined by numerically simulating the pulse evolution in the oscillator and by analyzing their impact on the laser performance. Experimentally, we achieved an 88 fs compressed pulse duration with sub-fs timing jitter at 54 MHz repetition rate and 51 mW of output power with 5.5 × 10−5 [20 Hz, 1 MHz] integrated relative intensity noise (RIN). Furthermore, we demonstrate tight phase-locking of the laser's carrier-envelope offset frequency (ƒceo) to a stable radio frequency (RF) reference and of one frequency comb tooth to a stable optical reference at 291 THz. [ABSTRACT FROM AUTHOR]

Light-induced states in He atoms were characterized using attosecond transient absorption spectroscopy. A 400 as pulse covering the 20-24 eV spectral range serves as the probe pulse, and the effect of a few-cycle near infrared pulse (12 fs, 780 nm) on the absorption spectrum is measured as a function of time delay and near-infrared intensities varying from (5.0 ± 2) × 1010 to (1 ± 0.4) × 1013 W/cm2. Light-induced states resulting from near-infrared coupling of 1s2p to 1s2s, 1s3d, and 1s3s states are observed. Absorption features that likely result from coupling of 1s3p to 1s4s, 1s4d, 1s5s, and 1s5d states are also observed. The light-induced states with the smallest detunings (1s3d and 1s3s) from the dressing frequency may shift to higher frequencies as the dressing intensity is increased. © 2013, Taylor & Francis.

Light-induced states in He atoms were characterized using attosecond transient absorption spectroscopy. A 400 as pulse covering the 20-24 eV spectral range serves as the probe pulse, and the effect of a few-cycle near infrared pulse (12 fs, 780 nm) on the absorption spectrum is measured as a function of time delay and near-infrared intensities varying from (5.0 ± 2) × 1010 to (1 ± 0.4) × 1013 W/cm2. Light-induced states resulting from near-infrared coupling of 1s2p to 1s2s, 1s3d, and 1s3s states are observed. Absorption features that likely result from coupling of 1s3p to 1s4s, 1s4d, 1s5s, and 1s5d states are also observed. The light-induced states with the smallest detunings (1s3d and 1s3s) from the dressing frequency may shift to higher frequencies as the dressing intensity is increased. © 2013, Taylor & Francis.

Featuring extraordinarily curved propagation trajectories, self-accelerating Airy beams have stimulated considerable interests from academic research to practical applications. However, generation of high-quality Airy beams using compact optical devices is still highly desirable. Here we present a design of integrated Airy phase plates (IAPPs) with a miniature size (60 μm × 60 μm × 1.1 μm). In addition, a phase-type Fresnel zone plate (FZP) is incorporated in the phase element to further reduce the system dimension, endowing the IAPPs the ability of directly generating Airy beams without introducing a bulky Fourier transform (FT) lens. The fabrication of IAPPs is facilitated by femtosecond direct laser writing (FsDLW), and the experimentally generated Airy beams are in good agreement with the numerical simulations. Furthermore, the flexible fabrication of IAPPs with different focal lengths is demonstrated, which results in the generation of Airy beams at different Fourier planes as desired. Our design strategy and fabrication methodology of the microscale three-dimensional (3D) phase plates can unfold new capacities of Airy beams for miniaturization applications in fiber optics and in on-chip photonics. [ABSTRACT FROM AUTHOR]

An electric field measurement system with two optical pulse sources down-converts the high-frequency electric signal to the baseband through a probe exploiting the electro optic-effect. From that process, it is feasible to recover the measured frequency. However, the majority of the current studies focus on the case where the difference between the two repetition frequencies is quite small, while only a few involve larger frequency differences. Nevertheless, in a non-laboratory uncontrolled environment such as a field measurement, it is commonly unfeasible to guarantee a small frequency difference imposing further study on frequency recovery methods under larger frequency difference. Spurred by that, in this paper we propose a universal frequency recovery algorithm relying on the Remainder Matching (RM) technique, which is suitable for both large and small frequency differences. In our method, we employ the RM technique to determine the exact remainder combination in the first repetition frequency intervals and calculate the measured frequency. We also present the theoretical basis of RM, along with a summary of the system design rules, which have a significant impact on the performance of the measurement system. Finally, we challenge our RM-based solution on noise and analyze the corresponding false alarm rate metric. Our results demo nstrate that the proposed RM-based frequency recovery algorithm attains an appealing performance. [ABSTRACT FROM AUTHOR]

In this article, the concept of co-locating all metrological time and frequency signals in a single optical channel of a standard, 100-GHz-spaced optical grid is presented and evaluated. The solution is intended for situations where only a narrow optical bandwidth is available in a fiber heavily loaded with standard data traffic. We localized the optical reference signals in the middle of the channel, with signals related to RF reference and time tags shifted ±12.5 GHz apart. In the experimental evaluation with a 260-km-long fiber, we demonstrate that the stability of frequency signals and the calibration of time tags remained at the very same level of stability and accuracy as for systems utilizing separate channels: the fractional long-term instability for the optical frequency reference was below 5 × 10−20, that for the RF reference at the level of 10−17, and the mismatch of the time tag calibration was not more than 10 ps. We also identify possible issues, mainly related to a risk of unwanted Brillouin amplification and scattering. [ABSTRACT FROM AUTHOR]

The nonlinear optical materials with small footprint, low operating energy, ultrafast and large nonlinear optical response are highly required for the signal processing. Here, we investigated the broadband optical nonlinear response of the triangular gold nanoplates on indium tin oxide (ITO) coated glass, and obtained a large optical nonlinear enhancement at its localized surface plasmon resonance (LSPR) region at the wavelength of 780 nm, far from the ENZ (epsilon-near-zero) region of ITO. The measured nonlinear refractive index and nonlinear absorption coefficient can reach up to 8.39×10−15 m2/W and 5.81×10−8 m/W, respectively. The experimental results highlight the noble metals’ potential for nonlinear optics and nanophotonics. [ABSTRACT FROM AUTHOR]

High-fidelity photodetection enables the transfer of the low noise inherent to optical oscillators to the microwave domain. However, when photodetecting optical signals of the highest timing stability, photodiode flicker (1/f) noise can dominate the resulting timing jitter at timescales longer than ∼1 ms. With the goal of improving femtosecond-level timing fidelity when transferring from the optical to microwave domain, we vary the duty cycle of a train of optical pulses and show that the photodetector flicker phase noise on a photonically generated 1 GHz microwave signal can be reduced by ∼10 dB under ultrashort pulse illumination, reaching as low as −140/f dBc/Hz. In addition, a strong correlation between amplitude and phase flicker noise is found, implying a single baseband noise source can modulate both quadratures of the microwave carrier. These findings expand the limits of the ultimate timing stability that can be transferred from optics to electronics. [ABSTRACT FROM AUTHOR]

In attosecond and strong-field physics, the acquisition of few-cycle laser sources open up a new area. We report a nonlinear pulse compression technology combining multi-thin-plate spectral broadening and nonlinear self-compression that generates a 0.52 mJ, good spatial quality characteristics and a spectral bandwidth supporting a 14 fs Fourier transform limited duration at 1 kHz repetition rate and at a center wavelength of 1.9 μm. The total energy transfer efficiency is up to 83%. Pulse to pulse stability of the energy output is 0.7% (RMS). The pulse duration is near 3 optical cycles. This pulse compression approach can be a key-enabling technology for the next generation of extreme photonics, attosecond research and coherent x ray-science, and it also can be further extended to mid-infrared lasers with longer wavelengths and higher peak power. [ABSTRACT FROM AUTHOR]

A whispering-gallery mode microsphere resonator based on optical fiber with an open microcavity is demonstrated, which is created by use of femtosecond laser micromachining and fusion splicing techniques. When a microsphere is placed into the open microcavity, whispering-gallery mode can be excited through evanescent field coupling. The device can be constructed in either single-mode fiber or multimode fiber, with the insertion loss of ∼13.2 and ∼21.0 dB, and the quality factor of 4.81X103 and 2 X105 respectively. Because of its features of compact structure, low cost, easy fabrication, convenient operation and high durability, the device is expected to have many photonics applications. [ABSTRACT FROM AUTHOR]

Linear optical sampling (LOS) is a powerful technique to circumvent the electronic bottleneck arising in optical modulation analyzer (OMA). To extend the range of operation wavelength for the LOS-based OMA, the fiber optical frequency comb (FOFC) is proposed to replace the narrowband mode-locked fiber laser (NMFL) as an optical sampling source. However, this procedure reduces the signal-to-noise ratio (SNR), due to the constraints of the unideal response of balanced photodetector (BPD) and the limited power per FOFC tone. Here, we investigate the biased balance detection (BBD) scheme to overcome such issue. Our simulation results show that, the characterization performance of the FOFC-based LOS with the help of the BBD scheme is almost the same as that of the NMFL-based LOS, when the PDM-QPSK/16QAM/64QAM signals are set as the signal under test (SUT). Meanwhile, we find that the dynamic range of SUT input power is determined by the BPD imbalance and the FOFC power. By increasing either the average power of the FOFC from 4 to 10 mW or the imbalance of the BPD from 1.03 to 1.04, we can enhance the dynamic range of SUT input power by 8.8 mW and 6.6 mW, respectively. Finally, we develop the equipment and experimentally verify the BBD scheme by a precise characterization of 32 GBaud PDM-QPSK signal. [ABSTRACT FROM AUTHOR]

Fiber Bragg gratings (FBGs) are essential optical components, which due to their design flexibility offer numerous prospects for a wide range of applications in fiber laser, sensor, and telecommunication technologies. Here we demonstrate that a phase mask interferometer driven by a deep-ultraviolet femtosecond laser enables inscription of FBGs with top-hat spectral reflection bandwidth. FBGs with the bandwidth of 2 nm have been achieved with inscription of several superimposed narrow FBGs with bandwidth less than 0.2 nm. The induced refractive index modulation profile of the superimposed gratings has been investigated with two measurement methods, namely, optical frequency domain reflectometry and the layer-peeling method. The analysis has shown that the spatial modulation index profile follows a sinc profile and has a very narrow central peak of less than 0.4 mm. Importantly, the FBGs provide low group delay values in comparison to their chirped counterparts. Additionally, the small center structure makes such gratings ideal for fiber sensing with high local resolution. The demonstrated FBG inscription method, developed initially to fabricate optical reflectors for infrared laser systems, can be translated to other applications, such as biophotonics, telecommunications, sensing and astrophysics. [ABSTRACT FROM AUTHOR]

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 μm. This technique is promising for complex moving pattern detection in object tracking and remote sensing. [ABSTRACT FROM AUTHOR]

Chalcogenide glass (ChG) optical fibers have excellent transmission performance in the mid-infrared range, and microstructured devices prepared on ChG fibers play an important role in the field of infrared sensing and optical integration. In this study, high-quality chalcogenide optical fibers were prepared, and grating structures were produced on their end surfaces. An easy-to-use FC cable was first prepared and polished to high flatness with the surface roughness of less than 0.08 μm. 1- and 2-D diffraction gratings were then prepared on the end surfaces of the chalcogenide fibers via femtosecond laser line-by-line direct writing technology. The ChG fiber end surface gratings prepared with optimized femtosecond laser parameters exhibited high diffraction efficiency and clear diffraction pattern, indicating that they had excellent optical performance. The performance of the grating was tested by using a laser source with wavelengths covering the visible to the near-infrared regions. This measurement showed that the first-order diffraction efficiency of the grating on the end face could reach close to 50% at 1550 nm. [ABSTRACT FROM AUTHOR]

Second harmonic generation is an efficient way to create coherent radiation at wavelengths that are not accessible with standard laser sources. In this work we demonstrate second harmonic generation from 1550 to 775 nm in a polymer photonic integrated circuit via the hybrid integration of a periodically poled lithium niobate crystal. The bulk crystal is inserted in an on-chip free-space section between two waveguide coupled GRIN lenses. Fiber to fiber conversion efficiencies were 0.03%/W for a continuous wave laser source and 100%/W for a femtosecond laser source. Furthermore, third and fourth harmonic light at 517 and 388 nm was observed. [ABSTRACT FROM AUTHOR]

Metal nanomaterials have attracted increasing attention due to their outstanding nonlinear optical and photonic properties, making them as potential saturable absorber (SA) candidates for realizing ultrafast photonic devices. In this article, we demonstrate the generation of mode-locked dual-wavelength pulse trains in an Ag nanoplates (AgNPTs)-based Yb-doped all-fiber laser for the first time to the best of our knowledge. The AgNPTs are synthesized by seed-mediated growth and then integrated into a fiber ferrule by optical deposition, which serve as SA in the ring laser cavity. The plasmonic properties of such nanoplates are measured by absorption spectrophotometry and their nonlinear optical properties are characterized by Z-scan. The measured nonlinear saturable absorption of the AgNPTs-based SA is 6.4%. In our laser, the dual-wavelength synchronous mode-locking is achieved at the center wavelengths of 1031.92 and 1033.24 nm with 3-dB spectral bandwidth of 0.52 and 0.46 nm, respectively, where 293-ps pulse trains with a repetition rate of 11.43 MHz are obtained at the pump power of 350 mW. The results demonstrate that the solution-processed AgNPTs are promising SA candidates for achieving stable and low-cost pulsed laser sources which could be used for spectroscopy and ultrafast photonics. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and perovskites hold substantial promise for various optoelectronic applications such as light emission, photodetection, and energy harvesting. However, each of these materials possesses certain limitations that can be overcome by synergistically combining them to form heterostructures, thereby unveiling intriguing optical properties. In this study, we present an uncomplicated technique for crafting a van der Waals (vdW) heterojunction comprising monolayer WS 2 and a Ruddlesden-Popper (RP) perovskite, namely (TEA) 2 PbI 4 . By utilizing ultrafast transient absorption (TA) spectroscopy, we explored the charge carrier dynamics within the WS 2 /(TEA) 2 PbI 4 heterostructure. Our findings uncover a type-II band alignment in the heterostructure, facilitating rapid (within 260 fs) hole transfer from WS 2 to the perovskite and leading to the formation of interlayer excitons (IXs) with a much longer lifetime (728 ps). This strategic approach has the potential to contribute to the development of hybrid systems aimed at achieving high-performance optoelectronic devices.

The generation of high-repetition rate (frep ≥ 10 GHz) ultra-broadband optical frequency combs (OFCs) at 1550 nm and 1310 nm is investigated by seeding two types of highly nonlinear fibers (HNLFs) with 10 GHz picosecond pulses at the pump wavelength of 1550 nm. When pumped near the zero dispersion wavelength (ZDW) in the normal dispersion region of a HNLF, 10 GHz flat-topped OFC with 43 nm bandwidth within 5 dB power variation is generated by self-phase modulation (SPM)-based OFC spectral broadening at 26.5 dBm pump power, and 291 fs pulse trains with 10 GHz repetition rate are obtained at 18 dBm pump power without complicated pulse shaping methods. Furthermore, when pumped in the abnormal dispersion region of a HNLF, OFCs with dispersive waves around 1310 nm are studied using a common HNLF and fluorotellurite fibers, which maintain the good coherence of the pump light at 1550 nm. At the same time, sufficient tunability of the generated dispersive waves is achieved when tuning the pump power or ZDW.

Using coupled electromagnetic and hydrodynamic calculations, we elucidate theoretically the topographic transition from a random metallic surface to a periodic sub-wavelength grating by ultrashort laser ablation. The origin of this transition lies in the successive selection of hybrid surface waves scattered by random nanoholes. Contrary to the common belief that surface plasmon polaritons play the dominant role in the process and define the grating periodicity, we show that both quasi-cylindrical and surface plasmon waves are involved, whereas the diversity in the resulting spacings λ/2–λ (λ is the laser wavelength) is the manifestation of a broad frequency overlap of these waves, controlled by their relative phase shifts with respect to the plasmonic counterparts. The topography evolution imposes the dominant contribution to the surface sub-wavelength pattern by selecting the appropriate wave character from plasmonic modes to evanescent cylindrical waves. With the radiation dose, the grating periodicity exhibits a pronounced blue shift due to reinforced dipole–dipole coupling between the nanoholes and surface curvatures in the laser-processed area. This allows the creation of regular patterns with tunable periodicity.

Abstract Standing on the potential for high-speed modulation and switching in the terahertz (THz) regime, all-optical approaches whose response speeds mainly depend on the lifetime of nonequilibrium free carriers have attracted a tremendous attention. Here, we establish a novel bi-direction THz modulation experiment controlled by femtosecond laser for new functional devices. Specifically, time-resolved transmission measurements are conducted on a series of thin layers Bi2Se3 films fabricated straightforwardly on Al2O3 substrates, with the pump fluence range from 25 μJ/cm2 to 200 μJ/cm2 per pulse. After photoexcitation, an ultrafast switching of THz wave with a full recovery time of ~10 ps is observed. For a longer timescale, a photoinduced increase in the transmitted THz amplitude is found in the 8 and 10 quintuple layers (QL) Bi2Se3, which shows a thickness-dependent topological phase transition. Additionally, the broadband modulation effect of the 8 QL Bi2Se3 film is presented at the time delays of 2.2 ps and 12.5 ps which have a maximum modulation depth of 6.4% and 1.3% under the pump fluence of 200 μJ/cm2, respectively. Furthermore, the absorption of α optical phonon at 1.9 THz shows a time-dependent evolution which is consistent with the cooling of lattice temperature.

Abstract The layered MoS2 has recently attracted significant attention for its excellent nonlinear optical properties. Here, the ultrafast nonlinear optical (NLO) absorption and excited carrier dynamics of layered MoS2 (monolayer, 3–4 layers, and 6–8 layers) are investigated via Z-scan and transient absorption spectra. Our experimental results reveal that NLO absorption coefficients of these MoS2 increase from–27 × 103 cm/GW to–11 × 103 cm/GW with more layers at 400-nm laser excitation, while the values decrease from 2.0 × 103 cm/GW to 0.8 × 103 cm/GW at 800 nm. In addition, at high pump fluence, when the NLO response occurs, the results show that not only the reformation of the excitonic bands, but also the recovery time of NLO response decreases from 150 ps to 100 ps with an increasing number of layers, while the reductive energy of A excitonic band decreases from 191.7 meV to 51.1 meV. The intriguing NLO response of MoS2 provides excellent potentials for the next-generation optoelectronic and photonic devices.

The dynamic response of magnetic order to optical excitation at sub‐picosecond scale has offered an intriguing alternative for magnetism manipulation. Such ultrafast optical manipulation of magnetism has become a fundamental challenging topic with high implications for future spintronics. Here, this study demonstrates such manipulation in Co2FeSi films grown on flexible polyimide substrate, and demonstrates how the magneto‐optical interaction can be modified by using strain engineering which in turn triggers the excitation of both dipolar and exchange spin waves modes. Furthermore, Gilbert damping and spin‐orbit coupling in Co2FeSi can both be tuned significantly by altering the magnitude and type of applied strain, suggesting an appealing way to manipulate spin wave propagation. These results develop the optical manipulation magnetism into the field of spin wave dynamics, and open a new direction in the application of spin orbitronics and magnonics devices using strain engineering. [ABSTRACT FROM AUTHOR]

We present experiments on cell cultures and brain slices that demonstrate two‐photon optogenetic pH sensing and pH‐resolved brain imaging using a laser driver whose spectrum is carefully tailored to provide the maximum contrast of a ratiometric two‐photon fluorescence readout from a high‐brightness genetically encoded yellow‐fluorescent‐protein‐based sensor, SypHer3s. Two spectrally isolated components of this laser field are set to induce two‐photon‐excited fluorescence (2PEF) by driving SypHer3s through one of two excitation pathways—via either the protonated or deprotonated states of its chromophore. With the spectrum of the laser field accurately adjusted for a maximum contrast of these two 2PEF signals, the ratio of their intensities is shown to provide a remarkably broad dynamic range for pH measurements, enabling high‐contrast optogenetic deep‐brain pH sensing and pH‐resolved 2PEF imaging within a vast class of biological systems, ranging from cell cultures to the living brain. [ABSTRACT FROM AUTHOR]

A novel few-mode fiber (FMF) based saturable absorber (SA) that works on the principle of nonlinear multimode interference has been proposed. To optimize the FMF parameters for the fabrication of SA, which is a combination of FMF with two single mode fibers (SMFs), a detailed numerical simulation is performed. The developed SMF-FMF-SMF structure exhibits a modulation depth and saturation intensity of 30.6% and 11.01 MW/cm2, respectively. By inserting the fabricated SA in an all-normal dispersion Ytterbium doped cavity, we have been able to generate a laser pulse duration of 300 fs with an average power/pulse energy of 2.4 mW/64.8 pJ and a repetition rate of 37 MHz as an output. Thus, the proposed SA helps in generating stable pulses from the laser with enhanced output characteristics. We envisage that the fabricated SA would play an indispensable role in the development of ultrafast fiber lasers. [ABSTRACT FROM AUTHOR]

We report on a novel waveguide-based polarization beam splitter (PBS) fabricated by femtosecond (fs) laser in lithium niobate (LiNbO3) crystals. This monolithic PBS is composed of tailored waveguide configurations with different guiding properties in which linearly polarized light along extraordinary refractive index (ne) and ordinary index (no) can be well separated. At the wavelength of 1064 nm, polarization extinction ratio (PER) can reach to 16.60 dB and 16.18 dB for ne and no polarizations, with insertion loss (IL) of 3.86 dB and 4.15 dB respectively. Our work may pave a new way for designing high-performance PBS and creating compact polarization conversion systems in integrated photonics and quantum photonics. [ABSTRACT FROM AUTHOR]

An optical fiber in-line Fabry--Perot interferometric device is presented for ultrasensitive strain sensing by use of vernier effect. The device consists of two parallel structured Fabry--Perot cavities in cascaded connection, formed by two pairs of femtosecond laser inscribed in-fiber reflection mirrors. The cavity lengths and their difference can be accurately controlled, which enables an extremely large amplification factor when vernier effect is used and the strain sensitivity achieved is ∼236.32 pm/μϵ. Moreover, the temperature cross sensitivity realized is as low as ∼29.2 nϵ/°C. The device is featured with ultra-compact size, robust structure, easy fabrication, convenient operation and good reproducibility, which makes it highly attractive in many sensing applications where extremely large strain sensitivity is critically required. [ABSTRACT FROM AUTHOR]

In recent years, femtosecond laser direct writing technology has made great progress in integrated quantum photonic circuits due to its advantages of applicability to an extensive range of materials and true 3-D ability. However, a waveguide prepared via a femtosecond laser shows a non-circular cross section and high birefringence, which seriously restricts the development of polarization encoded quantum photonic integrated chips towards greater integration and higher functions. Here, we propose an amplitude-phase double shaping method to reduce waveguide birefringence, which involves using a cylindrical lens and slits. A waveguide prepared using this method shows a circularity cross-section that reaches up to 97.6% and a birefringence that is as low as 1.49 × 10−6. Based on such an ultralow birefringence waveguide, a polarization independent Hong-Ou-Mandel quantum interferometer was fabricated. Interference visibilities for identical photon pairs in different polarization states were found to be more than 95% with a standard deviation of 0.6%. The ultra-low birefringence single-mode waveguide and the polarization-independent interferometer realized using this approach will play an important role in large-scale polarization encoding integrated quantum photonic chips. [ABSTRACT FROM AUTHOR]

Ultrafast electron diffraction (UED) has evolved to be a powerful tool for the study of structural dynamics with subpicosecond temporal resolution and subatomic spatial resolution. Recently, there have been intense research efforts toward femtosecond timing jitter and stability for improving the temporal resolution of UEDs, however, so far there has been no work showing long‐term (e.g., >1 h) stable timing for MeV‐level UED systems. In this article, a comprehensive timing synchronization method, based on optical‐radiofrequency synchronization and THz streaking, is demonstrated to maintain sub‐10‐fs long‐term timing stability for radiofrequency‐gun‐based MeV‐level UED, which results in 5.5 fs root‐mean‐square timing drift maintained over 4600 s. With high electron energy and low timing drift, the demonstrated capability is an important step toward studying ultrafast phenomena in samples with low scattering power, such as volatile gases and 2D materials. [ABSTRACT FROM AUTHOR]

Recent advancements in photonic-electronic integration push towards denser multichannel fiber to silicon photonic chip coupling solutions. However, current packaging schemes based on suitably polished fiber arrays do not provide sufficient scalability. Alternatively, lithographically-patterned fused silica glass interposers have been proposed, allowing for the integration of fanout waveguides between a dense array of on-chip silicon waveguides and a cleaved fiber ribbon. In this paper, we propose the use of femtosecond laser inscription for the fabrication of the fused silica glass interposer, allowing for a monolithic integration of waveguides and V-grooves for fiber alignment. The waveguides obtained by Femtosecond Laser Direct Writing (FLDW) have a propagation loss of 0.88 dB/cm at 1550 nm. The mode-field diameter is 12.8 $\pm$ 0.4 $\mu$ m, allowing for a coupling loss of 1.24 $\pm$ 0.32 dB when coupling to a standard single mode optical fiber, passively aligned to the fused silica waveguide by insertion in a V-groove created by Femtosecond Laser Irradiation followed by Chemical Etching (FLICE). The average surface roughness of the etched waveguide facet is 160 $\pm$ 5 nm. Scattering loss when coupling to fiber is reduced by use of an index-matching adhesive for fiber fixation. A polished out-of-plane coupling mirror at an angle of 41.5 $^{\circ }$ injects the light into standard grating couplers, providing a quasi-planar fiber-to-chip package. The excess loss of the proposed solution is limited to 2 dB per interface, including mirror, waveguide and fiber coupling losses. [ABSTRACT FROM AUTHOR]

Patch-clamp technique is the gold standard for cellular electrophysiological measurements, which is capable of measuring single ion transport events across the cell membrane. However, the measurement possesses significant complexities, and it requires a high level of expertise, while its experimental throughput is nevertheless considerably low. Here, we suggest and experimentally demonstrate a laser-assisted method for performing cellular electrophysiological measurements. Femtosecond laser pulses, coupled to an optical microscope, are used to form a sub-micrometer hole on a thin polymer membrane separating two electrodes, where a nearby cell is subsequently placed onto the hole by negative pressure. Afterwards, the cell is punctured using subsequent laser exposure, revealing the cell membrane over the hole for electrophysiological recording. This system could be used to increase the output amount of the electrophysiological measurements substantially. [ABSTRACT FROM AUTHOR]

In this letter, the design and characterization of an all-fiber femtosecond laser is reported that generates three wavelengths at 1030 nm, 1550 nm, and 1900 nm from a single cavity. The cavity is based on Er-doped fiber and it is mode-locked by a fiber coupled saturable absorber mirror. We take advantage of fiber optics nonlinearity to generate 1030 nm and 1900 nm wavelengths which can be amplified using well-known Yb-doped and Tm-doped amplifiers. The third output of the laser can be optimized to generate femtosecond pulses at a wide range of central wavelength from 1700 nm to 2000 nm. Since all three outputs of the laser are being generated from the same cavity, they can be easily synchronized to be used in various pump and probe applications. This approach reduces the size, power consumption and cost of the laser compared to three individual mode-locked cavities. The laser can be packaged in a 20 cm $\times20$ cm $\times20$ cm form factor. [ABSTRACT FROM AUTHOR]

We present experimental and numerical investigations of high-energy mid-infrared filamentation with multi-octave-spanning supercontinuum generation (SCG), pumped by a 2.4 μm, 250 fs Cr:ZnSe chirped-pulse laser amplifier. The SCG is demonstrated in both anomalous and normal dispersion regimes with YAG and polycrystalline ZnSe, respectively. The formation of stable and robust single filaments along with the visible-to-mid-infrared SCG is obtained with a pump energy of up to 100 μJ in a 6-mm-long YAG medium. To the best of the authors' knowledge, this is the highest-energy multi-octave-spanning SCG from a laser filament in a solid. On the other hand, the SCG and even-harmonic generation based on random quasi-phase matching (RQPM) are simultaneously observed from the single filaments in a 6-mm-long polycrystalline ZnSe medium with a pump energy of up to 15 μJ. The numerical simulations based on unidirectional pulse propagation equation and RQPM show excellent agreement with the measured multi-octave-spanning SCG and even-harmonic generation. They also reveal the temporal structure of mid-infrared filaments, such as soliton-like self-compression in YAG and pulse broadening in ZnSe. [ABSTRACT FROM AUTHOR]

We report on the design and fabrication of straight and S-curved waveguides in neodymium-doped yttrium aluminate (Nd:YAP) crystal by using direct femtosecond laser writing. These S-curved channel waveguides are based on the hexagonal optical-lattice-like and depressed cladding geometry. For each type waveguide, S-curves with three lateral offsets of 50 μm, 100 μm, and 150 μm are implemented. These waveguides are with good guiding properties and low bending losses. With S-curved waveguide as laser cavity, dual-wavelength, 31.6 GHz waveguide laser operating at 1064 nm and 1079 nm have been demonstrated based on MoS2 as a saturable absorber. This work paves the way to develop new on-chip ultrafast laser sources based on femtosecond laser inscribed S-curved waveguides. [ABSTRACT FROM AUTHOR]

Highly nonlinear multimode optical integrated devices hold great promise for the emerging large-capacity mode division multiplexing (MDM) technology, and they correspondingly enable multimode optical signal processing at nodes of the MDM network. Here, we propose a highly nonlinear multimode organic-silicon slot waveguide (HN-OSSW) whose nonlinearity is greatly improved by the intensive optical field confinement in the nano-gaps filled with highly nonlinear organic material. Specifically, the achieved nonlinear coefficients of the HN-OSSW under TE0 and TE1 modes are estimated higher than 7000 W−1m−1 and 5800 W−1m−1, respectively. Also, by phase mismatch optimization, this nanostructure exhibits superior intramode four-wave mixing conversion efficiency but ignorable intermode crosstalk. Coupling efficiency of the mode multiplexer and the optimal nonlinear interaction length are discussed in detail. Based on the HN-OSSW, we then numerically demonstrate a multimode all-optical hexadecimal addition and subtraction for sixteen phase-shift keying (16PSK) signals with the operation rate of 1.28Tb/s. Six different logic operations including A+B-C, A+C-B, B+C-A, A+B, A-B and B-A are simultaneously obtained with good performance evaluations, indicating the promising prospect of the HN-OSSW applied in ultrafast multimode signal processing. [ABSTRACT FROM AUTHOR]

A lithium niobate (LiNbO3) single chip two dimensional (2D) electric field sensor has been proposed, fabricated and experimentally demonstrated. Two perpendicular electrodes are designed and fabricated on an optical waveguide asymmetric dual parallel Mach-Zehnder interferometer (ADMZI). Experimental results demonstrate that the sensor can be used to detect the two field components of a transient pulsed electric field simultaneously. The minimum/maximum detectable electric fields for the sensor ${z}$ and ${y}$ axes are 5 kV/m, 140 kV/m and 10 kV/m, 106 kV/m respectively for measuring the nanosecond and lightning electromagnetic pulse (EMP) in the time domain. In addition, the frequency response deviation is ±2.5 dB for the ${y}$ axis from 10 kHz to 1 GHz and for the ${z}$ axis from 100 kHz to 1 GHz. [ABSTRACT FROM AUTHOR]

Here in we present the fabrication and performance of waveguide tapers produced by femtosecond laser induced element redistribution in modified phosphate glasses. More particularly, it is demonstrated that by controlling the scan velocity during the writing process it is possible to adequately tune both the size of the modified area and the refractive index contrast to produce waveguides that can cope with mode field diameters in the range of 7–16 μm. In addition, we fabricated tapered structures through the induction of an acceleration in the laser scanning velocity, resulting in a device that can efficiently convert a wide range of mode fields. The fine control achieved over the index contrast in the range of 10−3 and 10−2 would allow the production of a wide variety of tapers that could potentially be used to couple numerous photonic devices. [ABSTRACT FROM AUTHOR]

A real-time lidar with 0.1 mega-hertz update rate and few-micrometer resolution incorporating dispersive Fourier transformation and instantaneous microwave frequency measurement is proposed and demonstrated. As a femtosecond laser pulse passing through an all-fiber Mach–Zehnder interferometer, where the detection light beam is inserted into the optical path of one arm, the displacement is encoded to the frequency variation of the temporal interferogram. To deal with the challenges in storage and real-time processing of the microwave pulse generated on a photodetector, we turn to microwave photonic signal processing. A carrier wave is modulated by the time-domain interferogram using an intensity modulator. After that, the frequency variation of the microwave pulse is uploaded to the first order sidebands. Finally, the frequency shift of the sidebands is turned into transmission change through a symmetric-locked frequency discriminator. In experiment, a real-time ranging system with adjustable dynamic range and detection sensitivity is realized by incorporating a programmable optical filter. Standard deviation of 7.64 μm, overall mean error of 19.10 μm over 15 mm detection range and standard deviation of 37.73 μm, overall mean error of 36.63 μm over 45 mm detection range are realized, respectively. [ABSTRACT FROM AUTHOR]

Midinfrared (MIR) is an attractive spectral region for many applications ranging from vibrational spectroscopy to attosecond physics. However, in this spectral region, the dispersion management techniques are not as mature as the ones in the near-infrared (NIR) or visible, which is a key ingredient of ultrafast laser technology. In this manuscript, the transfer of dispersion management schemes for optical parametric (chirped pulse) amplifiers (OP(CP)As) from NIR to MIR is discussed. Among those, a scheme based on Martinez-type grating stretcher and a bulk compressor is proposed and numerically analyzed. As a case study, the proposed scheme is applied numerically to a 2-µm pumped two-stage optical parametric amplifier (OPA) system at 4.06 µm. In the model OPA system, the seed pulses are generated by white-continuum in bulk crystal and then amplified via zinc germanium phosphide (ZGP)-based OPAs. Parametric gain bandwidth of the ZGP nonlinear crystal is numerically studied. According to the numerical simulations, 3-mm crystal supports pulses with 26.5 fs pulse duration (FWHM) at 4.06 µm. With the proposed dispersion management scheme, the seed pulses are first stretched inside a Martinez grating stretcher to 1.4-1.6 ps for efficient amplification in OPA stages and then compressed down to the transform-limit inside CaF2 bulk material. [ABSTRACT FROM AUTHOR]

In the past three decades, ultrafast pulse laser technology has greatly progressed and applied widely in many subjects, such as physics, chemistry, biology, materials, and so on. Accordingly, as well as for future developments, to measure or characterize the pulses temporally in femtosecond domain is indispensable but still challenging. Based on the operation principles, the measurement techniques can be classified into three categories: correlation, spectrogram, and spectral interferometry, which operate in time-domain, time-frequency combination, and frequency-domain, respectively. Here, we present a mini-review for these techniques, including their operating principles, development status, characteristics, and challenges. [ABSTRACT FROM AUTHOR]

Although higher order Poincaré (HOP) beams are attractive due to their unique advantages and various efforts have been made for the generation of HOP sphere beams, the current generation technologies suffer from both limited output power and narrow bandwidth, preventing further practical applications. In this paper, we report on a femtosecond optical parametric oscillator (OPO) which can deliver high average power, on-demand arbitrary HOP sphere beams tunable across a wide spectral range in the near-infrared. The desired beam mode can be achieved by manipulating the geometric phase inside the OPO cavity to map the polarization to orbital angular momentum (OAM). Pumped by a Gaussian beam from an Yb-fiber laser, the generalized vector vortex femtosecond signal beams with spectral coverage from 1367 to 1650 nm are obtained. The device can produce over 200 mW structured beam signal power cross the complete tuning range with as much as 377 mW at 1480 nm for 2 W of input pump power. The proposed configuration, which shows superiorities of high average output power, flexible wavelength tunability and complete mode controlling property, may have direct implications in applications such as quantum optics and material processing. [ABSTRACT FROM AUTHOR]

The infrared (IR) is a strategically important band for numerous applications from environment to biochemical imaging. Here a novel IR optical component integrates more than 2000 plano-concave microlenses on the zinc selenide (ZnSe) surface within a footprint of $5\times 5$ mm2 is proposed by a combined method of femtosecond laser direct irradiation and chemical wet etching. Microlenses with diameter from 15 to $30~\mu \text{m}$ are realized by controlling the laser power and etching time. The uniform close-packed microlens arrays (MLAs) with minimized volume and high transparency at wavelengths of $0.76\sim 22\mu \text{m}$ are realized. The MLAs have relatively good topography and surface quality. The modulation transfer function (MTF), imaging test and optical-focusing test confirm the outstanding IR imaging. Owing to the merits of micro-size, light weight, high integration and excellent IR imaging performance, the as-fabricated MLA is promising in cutting-edge mid-IR and far-IR application, such as IR sensing system, imaging system, and so on. [ABSTRACT FROM AUTHOR]