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Photodetectors are one of the most critical components for future optoelectronic systems and it undergoes significant advancements to meet the growing demands of diverse applications spanning the spectrum from ultraviolet (UV) to terahertz (THz). 2D materials are very attractive for photodetector applications because of their distinct optical and electrical properties. The atomic-thin structure, high carrier mobility, low van der Waals (vdWs) interaction between layers, relatively narrower bandgap engineered through engineering, and significant absorption coefficient significantly benefit the chip-scale production and integration of 2D materials-based photodetectors. The extremely sensitive detection at ambient temperature with ultra-fast capabilities is made possible with the adaptability of 2D materials. Here, the recent progress of photodetectors based on 2D materials, covering the spectrum from UV to THz is reported. In this report, the interaction of light with 2D materials is first deliberated on in terms of optical physics. Then, various mechanisms on which detectors work, important performance parameters, important and fruitful fabrication methods, fundamental optical properties of 2D materials, various types of 2D materials-based detectors, different strategies to improve performance, and important applications of photodetectors are discussed.
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We report a self-starting mode-locked all-polarization-maintaining (PM) holmium (Ho)-doped fiber oscillator operating at â¼2.08 µm based on nonlinear polarization evolution (NPE). The oscillator is configured as a linear cavity structure with two output ports exhibiting completely different pulse characteristics. One output port of the oscillator can deliver a stable, clean soliton-like pulse with a pulse duration of 439â fs and an average power of 7.5â mW at a fundamental repetition rate of 61.67â MHz. In contrast, the other port delivers a low-quality pulse with a complex structure. Numerical simulations reveal that the pulse difference between the two ports is mainly caused by the nonlinear optical interactions between the slow-axis and fast-axis modes in the PM fibers. Furthermore, the obtained clean pulses show significant improvements in relative intensity noise and power stability compared to complex pulses. Our study can help researchers obtain high-quality, stable pulses from PM-NPE mode-locked fiber oscillators.
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Three-micrometer mid-infrared (MIR) femtosecond pulse sources with a high repetition rate (HRR) have potential applications in a number of fields such as biological imaging, optical frequency combs, and gas detection. In this paper, by optimizing the fiber length and the cavity structure, we demonstrated a highly stable, self-starting mode-locked fluoride fiber laser (MLFFL) with a fundamental repetition rate of â¼165â MHz and a signal-to-noise ratio (SNR) of 90â dB. As far as we know, this stands as the highest fundamental repetition rate ever acquired directly from an ultrafast MLFFL in the >2.5â µm MIR region. Stable 352-fs pulses at 2795â nm with an average output power of 392â mW and a low integrated relative intensity noise (RIN) of 0.018% [10â Hz, 10â MHz] were generated. The root mean square (RMS) power fluctuation is 0.17% over 2â h, which indicates excellent oscillator stability. This high-performance laser offers a practicable scheme both for scaling the repetition frequency in MIR MLFFLs and high-precision ultrafast applications at longer wavelengths.
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We report a tunable spatiotemporally mode-locked large-mode-area Er:ZBLAN fiber laser based on the nonlinear polarization rotation technique. A diffraction grating is introduced to select the operating wavelength. Under the spectral and spatial filtering effects provided by the grating and spatial coupling respectively, stable ps-level spatiotemporally mode-locked pulses around 2.8â µm with a repetition rate of 43.4â MHz are generated. Through a careful adjustment of the grating, a broad wavelength tuning range from 2747 to 2797â nm is realized. To the best of our knowledge, this is the first wavelength-tunable spatiotemporally mode-locked fiber laser in the mid-infrared region.
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We experimentally investigate the generation of h-shaped pulse in an all-polarization-maintaining (PM) and all-normal-dispersion (ANDi) mode-locked fiber laser. The generated pulse is demonstrated to be a unitary pulse, instead of a noise-like pulse (NLP). Furthermore, by employing an external filtering system, the obtained h-shaped pulse can be resolved into rectangular-shaped pulses, chair-like pulses, and Gaussian pulses. The authentic AC traces with a double-scale structure of unitary h-shaped pulses and chair-like pulses are observed on the autocorrelator. The chirp of h-shaped pulses is also proved similar to that of DSR pulses. To the best of our knowledge, this is the first time that the existence of unitary h-shaped pulse generation has been confirmed. Moreover, our experimental results reveal the close relationship of formation mechanisms of dissipative soliton resonance (DSR) pulses, h-shaped pulses, and chair-like pulses, which helps to unify the essences of such "DSR-like" pulses.
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An all-polarization-maintaining (PM) mode-locked fiber laser based upon nonlinear polarization evolution (NPE) that operates around 976â nm is presented. The NPE-based mode-locking is realized using a special section of the laser which comprises three pieces of PM fibers with specific deviation angles between the polarization axes and a polarization-dependent isolator. By optimizing the NPE section and adjusting the pump power, dissipative soliton (DS) pulses with a pulse duration of â¼6 ps, a spectral bandwidth of >10â nm and a maximum pulse energy of 0.54 nJ are generated. Self-starting, steady mode-locking operation is achievable within a pump power range of â¼2 W. Moreover, by incorporating a segment of passive fiber into the appropriate location in the laser resonator, an intermediate regime between stable single-pulse mode-locking and noise-like pulse (NLP) is realized in the laser. Our work expands the dimension of the research on the mode-locked Yb-doped fiber laser operating around 976â nm.
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By combining nonlinear polarization rotation (NPR) and semiconductor saturable absorber, we report a hybrid mode-locked Er:ZBLAN fiber oscillator at 2.8â µm. Stable 325-fs mode-locked pulses with an average power of 131â mW and a record signal-to-noise ratio of 79â dB at the fundamental frequency of 55.4â MHz are generated. Numerical simulations are carried out based on the modified coupled nonlinear Schrödinger equations, and offer new insights into the underlying dynamics of pulse generation. The simulations indicate that compared with Er:ZBLAN fiber lasers mode-locked by NPR alone, the hybrid mode-locked Er:ZBLAN fiber oscillator allows a wider range and a lower threshold of the pump power while maintaining the ultrashort pulse width. Moreover, we numerically demonstrate that the hybrid mode-locked oscillator is less sensitive to the variation of polarization states, which will increase its robustness against environmental disturbance. This is the first time that the hybrid mode-locking technique is applied in the mid-infrared, opening up new opportunities for the development of stable ultrafast mid-infrared laser sources and practical applications outside the laboratory.
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As the critical device of microwave photonics and optical communication, the low-loss and high-efficiency optical phase shifter has attracted intense attention in photonic integrated circuits. However, most of their applications are restricted to a particular band. Little is known about the characteristics of broadband. In this paper, an SiN-MoS2 integrated broadband racetrack phase shifter is demonstrated. The coupling region and the structure of the racetrack resonator are elaborately designed to improve the coupling efficiency at each resonance wavelength. The ionic liquid is introduced to form a capacitor structure. Then, the effective index of the hybrid waveguide can be efficiently tuned by adjusting the bias voltage. We achieve a phase shifter with a tunable range covering all the WDM bands and even up to 1900â nm. The highest phase tuning efficiency is measured to be 72.75â pm/V at 1860â nm, and the corresponding half-wave-voltage-length product is calculated as 0.0608â V·cm.
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We report an all-fiber 2.8-µm ultra-short pulse master oscillator power amplifier (MOPA) system seeded by a soliton self-frequency shift from a mode-locked thulium-doped fiber laser. This all-fiber laser source delivers 2.8-µm pulses with an average power of 3.42 W, a pulse width of 115 fs, and a pulse energy of 45.4 nJ. We demonstrate, to the best of our knowledge, the first femtosecond watt-level all-fiber 2.8-µm laser system. A 2.8-µm pulse seed was obtained via the soliton self-frequency shift of 2-µm ultra-short pulses in a cascaded silica and passive fluoride fiber. A novel, to the best of our knowledge, high-efficiency and compact home-made end-pump silica-fluoride fiber combiner was fabricated and used in this MOPA system. Nonlinear amplification of the 2.8-µm pulse was realized, and soliton self-compression was observed accompanied by spectral broadening.
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We report a hybrid mode-locked fiber laser at 2.8â µm based on a large-mode-area Er:ZBLAN fiber. Reliable self-starting mode-locking is achieved via the combination of nonlinear polarization rotation and a semiconductor saturable absorber. Stable mode-locked pulses with a pulse energy of 9.4 nJ and a pulse duration of 325 fs are generated. To the best of our knowledge, this is the highest pulse energy directly generated from a femtosecond mode-locked fluoride fiber laser (MLFFL) to date. The measured M2 factors are below 1.13, indicating a nearly diffraction-limited beam quality. Demonstration of this laser provides a feasible scheme for the pulse energy scaling of mid-infrared MLFFLs. Moreover, a peculiar multi-soliton mode-locking state is also observed, in which the time interval between the solitons varies irregularly from tens of picoseconds to several nanoseconds.
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A photonic crystal fiber utilizing surface plasmon resonance (PCF-SPR) sensor based on refractive index (RI) control of magnetic fluid (MF) is designed. The air holes of the sensor are arranged in a hexagonal shape, and the optical field transmission channels on both sides of the central air hole can effectively confine the energy of the optical field. We use MF as the sensing medium, and coat the inner wall of the central air hole with gold. It can effectively stimulate the SPR effect to achieve the purpose of magneto-refractive modulation. We study the sensing characteristics of the proposed sensor by finite element analysis. The results show that the highest refractive index sensitivity reaches 19520 nm/RIU in the RI range of 1.42-1.435 and the maximum figure of merit (FOM) is 374.3 RIU-1. In addition, the magnetic field and the temperature response characteristics of the designed sensor are also investigated. In the magnetic field range of 50-130 Oe, the magnetic field sensitivity is 590 pm/Oe. In the temperature range of 24.3-144.3 °C, the temperature sensitivity is only -29.7 pm/â. The proposed sensor has significant advantages such as stable structure, high sensitivity, easy integration, resistance to electromagnetic interference and can be used for weak magnetic magnitude detection. It has wide application prospects in industrial production, military, and medical equipment.
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We demonstrate the generation of soliton and dissipative soliton in an ultrafast thulium (Tm) doped fiber laser based upon cross-phase modulation (XPM) induced mode-locking. The mode-locking is realized by periodically modulating the 2-µm signal through XPM that is activated by an injected 1.5-µm pulsed laser. Such a mechanism enables the laser to be mode-locked in various operation regimes without any real or artificial saturable absorbers. Thanks to the XPM pulling effect, the wavelength of the Tm-doped fiber laser can be tuned by adjusting the repetition frequency of the 1.5-µm pulsed laser. The maximum tuning ranges achieved in this work for the soliton and dissipative soliton regimes are respectively 11 nm and 15 nm. The outcomes of this work not only provide a continuously and controllably wavelength-tunable ultrafast laser but also offer a passively synchronized dual-color fiber laser system, which is promised for many important applications such as Raman spectroscopy, nonlinear frequency conversion systems, and multi-color pump-probe systems.
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We demonstrate a wavelength-tunable, sub-200â fs, and watt-level thulium-doped ultrafast fiber oscillator with a fundamental frequency repetition rate of 509.7â MHz. The wavelength can be tuned between 1918.5â nm and 2031â nm by adjusting the intra-cavity waveplates. When the wavelength is tuned to below 2000â nm, the average output power exceeds 1â W. The oscillator provides a maximum average power of 1.314â W (corresponding to a pulse energy of 2.58â nJ) and a highest peak power of 12.5â kW at 1940â nm. Such a high-power, tunable 2-µm mode-locked fiber laser is an ideal light source candidate for a variety of applications, such as frequency metrology, molecular spectroscopy, and ultrafast pump-probe spectroscopy.
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Gradient-based optimization combined with the adjoint method has been demonstrated to be an efficient way to design a nano-structure with a vast number of degrees of freedom. However, most inverse-designed photonic devices are applied as linear photonic devices. Here, we demonstrate the nonlinear optical response in inverse-designed integrated splitters fabricated on a SiN platform. The splitting ratio is tunable under different incident powers. The thermo-optical effect can be used as an effective approach for adjusting the nonlinear optical response threshold and modulation depth of the device. These promising results indicate the great potential of inverse-designed photonic devices in nonlinear optics and optical communications.
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We demonstrate a low-power, compact micro-ring phase shifter based on hybrid integration with atomically thin two-dimensional layered materials, and experimentally establish a low-loss silicon nitride platform. Using a wet transfer method, a large-area few-layer MoS2 film is hybrid integrated with a micro-ring phase shifter, leading to a tuning efficiency of 5.8 pm V-1 at a center wavelength of 1545.294â nm and a half-wave-voltage-length product as low as 0.09â V cm. Our device is designed to provide a hybrid-integration-based active phase modulation scheme for integrated optical communication networks with large-cross-section silicon nitride waveguides.
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We report a mode-locked high-power all-polarization-maintaining Er/Yb-doped large-mode-area fiber oscillator based on a bias nonlinear amplifying loop mirror (NALM). The oscillator can generate â¼1-nJ femtosecond pulses without dispersion compensation. By inserting a Martinez-type compensator to provide normal dispersion, it can generate >10-nJ picosecond dissipative solitons (DSs). The measured M2 factors are below 1.5, indicating a good beam quality. When the cavity dispersion is tuned to be â¼0.704 ps2, the oscillator can deliver chirped DSs with an average power as high as 690 mW at a repetition rate of 49.86â MHz, corresponding to a pulse energy of â¼13.8 nJ. The pulse after compression has a near Fourier-limited width of â¼2 ps. Successful demonstration of this laser provides a robust scheme for improving the performance of ultrafast fiber lasers in average power and pulse energy.
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We report a high-average-power mid-infrared ultrafast laser system consisting of a fluoride fiber mode-locked oscillator and a nonlinear amplifier. A backward pumping scheme was used in the amplifier to simultaneously realize pulse amplification and self-compression. The input signal polarization was demonstrated to play an important role in the self-compression process. Through the optimization of input polarization, a 4.13 W average-power 59 fs pulse at 2.8 µm was achieved, with an estimated pulse energy of 42.2 nJ and a peak power of 715â kW. To the best of our knowledge, this is the highest average-power pulse with sub-100-fs duration generated from a mid-infrared fiber laser system to date.
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Silicon Photomultiplier (SiPM) is a sensor that can detect low-light signals lower than the single-photon level. In order to study the properties of neutrinos at a low detection threshold and low radioactivity experimental background, a low-temperature CsI neutrino coherent scattering detector is designed to be read by the SiPM sensor. Less thermal noise of SiPM and more light yield of CsI crystals can be obtained at the working temperature of liquid nitrogen. The breakdown voltage (Vbd) and dark count rate (DCR) of SiPM at liquid nitrogen temperature are two key parameters for coherent scattering detection. In this paper, a low-temperature test is conducted on the mass-produced ON Semiconductor J-Series SiPM. We design a cryogenic system for cooling SiPM at liquid nitrogen temperature and the changes of operating voltage and dark noise from room to liquid nitrogen temperature are measured in detail. The results show that SiPM works at the liquid nitrogen temperature, and the dark count rate drops by six orders of magnitude from room temperature (120 kHz/mm2) to liquid nitrogen temperature (0.1 Hz/mm2).
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We demonstrated a mode-locked fiber laser based on a novel photonic device that combined optical microfiber coupler (OMC) and saturable absorption materials. The stable ultrafast laser was formed based on the interaction between the deposited Indium Antimonide (InSb) and the evanescent field on OMC. Different from optical microfiber (OM), OMC can directly output the mode-locked laser without additional beam splitting devices, which further improves the integrated characteristics of the fiber laser. The pulse duration of the output pulse is 405 fs at the central wavelength of 1560 nm. To the best of our knowledge, this is the first time that optical microfiber coupler based saturable absorber (OMC-SA) for mode-locked fiber laser is demonstrated.
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We report a compact, self-starting dispersion-managed mode-locked thulium-doped fiber oscillator that delivers 2.6 nJ pulses at 2 µm with a repetition rate of 250 MHz. The average output power and spectral bandwidth of the pulses reach impressive values of 648 mW and 103 nm, respectively. The generated pulses are near linearly chirped, capable of linearly compressing to 74 fs in a normal dispersion fiber after power attenuation. Using a nonlinear fiber compression scheme can even compress the pulses to 29 fs (4.3-cycle). The remaining pulse energy is 1.15 nJ, and the corresponding peak power is estimated as 39.4 kW. To the best of our knowledge, this is the first demonstration of nonlinearly compressing the pulse of a 2 µm fiber oscillator to the sub-5 cycle regime. Such a few-cycle fiber laser could be an ideal candidate source for short-wavelength mid-infrared frequency metrology and molecular spectroscopy applications.