Wideband-tuneable, nanotube mode-locked, fibre laser

Generation of a switchable and tunable rectangular pulse and multi-pulse with different peak powers at the L-band

A novel, to our knowledge, L-band erbium-doped fiber laser, utilizing a nonlinear optical loop mirror (NOLM) as a mode-locker, is presented in this study. Through precise adjustments of the polarization controllers (PCs), the laser achieves the generation of rectangular pulses with distinct single wavelengths, λ 1=1593n m and λ 2=1571n m, as well as dual-wavelength operation. The laser's operational mode can extend further to include harmonic mode-locking (HML). Furthermore, the investigation reveals the emergence of trapezoidal pulses and low-peak-power rectangular pulses within proximity of the conventional rectangular pulses. Notably, the evolutions of these low-peak-power pulses with the pump power also adhere to the peak power clamping (PPC) effect. Remarkably, the relative positioning of these pulses remains consistent across varying pump power levels or harmonic orders. Intriguingly, the evolution of the trapezoidal pulse with respect to pump power stands in stark contrast to that of the h-shaped pulse.

Wavelength-tunable broadband lasers based on nanomaterials

Nanomaterials are widely used in the fields of sensors, optoelectronics, biophotonics and ultrafast photonics due to their excellent mechanical, thermal, optical, electrical and magnetic properties. Particularly, owing to their nonlinear optical properties, fast response time and broadband operation, nanomaterials are ideal saturable absorption materials in ultrafast photonics, which contribute to the improvement of laser performance. Therefore, nanomaterials are of great importance to applications in wavelength-tunable broadband pulsed lasers. Herein, we review the integration and applications of nanomaterials in wavelength-tunable broadband ultrafast photonics. Firstly, the two integration methods, which are direct coupling and evanescent field coupling, and their characteristics are introduced. Secondly, the applications of nanomaterials in wavelength-tunable broadband lasers are summarized. Finally, the development of nanomaterials and broadband tunable lasers is reviewed and discussed.

Transparent nanopaper for ultrashort pulse generation in the near-infrared region

Transparent nanopaper (T-paper) can be applied in the field of electromagnetic shielding materials, antistatic materials, composite conductive materials, electric pool materials, super capacitors, and thermal management systems. However, this kind of T-paper has not been employed in ultrafast photonics yet. For the first time, to our knowledge, transparent electrical nanopaper is used in fiber lasers, different from the conventional pulsed fiber laser, which operates in the Q-switched regime under low pump power and then in the mode-locked regime under high pump power. Mode-locking is achieved first with a pulse duration of 550 fs under low pump power (166 mW). When further increasing the pump power up to 198 mW, the proposed fiber laser can be converted from a mode-locked to Q-switched state, which is a result of the two-photon absorption effect. The proposed fiber laser based on T-paper can be potentially applied in optical tomography, metrology, spectroscopy, micro-machining technology, and biomedical diagnostics.

0.017 nm, 143 ps passively mode-locked fiber laser based on nonlinear polarization rotation

Mode-locked lasers with ultra-narrow spectral widths and durations of hundreds of picoseconds can be versatile light sources for a variety of newly emergent applications. However, less attention seems to be given to mode-locked lasers that generate narrow spectral bandwidths. We demonstrate a passively mode-locked erbium-doped fiber laser (EDFL) system that relies on a standard fiber Bragg grating (FBG) and the nonlinear polarization rotation (NPR) effect. This laser achieves the longest reported pulse width (to the best of our knowledge) of 143 ps based on NPR and an ultra-narrow spectral bandwidth of 0.017 nm (2.13 GHz) under Fourier transform-limited conditions. The average output power is 2.8 mW, and the single-pulse energy is 0.19 nJ at a pump power of 360 mW.

Full C-band wavelength-tunable, 250 MHz repetition rate mode-locked polarization-maintaining fiber laser

We demonstrate a full C-band wavelength-tunable mode-locked fiber laser with a repetition rate of 250 MHz, representing the highest repetition rate for C-band tunable mode-locked lasers thus far to the best of our knowledge. The polarization-maintaining fiber-based Fabry-Perot cavity enables a fundamental repetition rate of 250 MHz with a semiconductor saturable absorber mirror as a mode-locker. We observed a stable and single soliton mode-locking state with wide tunability of the center wavelength from 1505 to 1561 nm by adjusting the incident angle of a bandpass filter inside the cavity. The wavelength-tunable high-repetition-rate mode-locked laser covering the full C-band is expected to be a compelling source for many frequency-comb-based applications, including high-precision optical metrology, broadband absorption spectroscopy, and broadband optical frequency synthesizers.

Linear-cavity Er-doped fiber mode-locked laser with large wavelength tunability

A linear-type wavelength-tunable all-polarization-maintaining fiber mode-locked laser is proposed for the first time, to our knowledge, and is implemented with an Er-doped fiber and polarization-maintaining fiber components. The tuning range of the center wavelength is from 1533.7 nm to 1565.6 nm. The linear-type configuration makes the proposed laser simpler and more compact, allowing it to achieve the highest repetition rate of 126.5 MHz among C-band wavelength-tunable mode-locked lasers due to its short cavity length. Also, its polarization-maintaining fiber components provide reliable operating robustness. The significant wavelength tunability and high repetition rate of the proposed laser can be expected to make it an attractive resource for various applications, including optical communications, broadband spectroscopic LIDAR, and high-precision ranging.

Hot-carrier tunable abnormal nonlinear absorption conversion in quasi-2D perovskite

Controlling the high-power laser transmittance is built on the diverse manipulation of multiple nonlinear absorption (NLA) processes in the nonlinear optical (NLO) materials. According to standard saturable absorption (SA) and reverse saturable absorption (RSA) model adapted for traditional semiconductor materials, the coexistence of SA and RSA will result in SA induced transparency at low laser intensity, yet switch to RSA with pump fluence increasing. Here, we observed, in contrast, an unusual RSA to SA conversion in quasi-two-dimensional (2D) perovskite film with a low threshold around 2.6 GW cm−2. With ultrafast transient absorption (TA) spectra measurement, such abnormal NLA is attributed to the competition between excitonic absorption enhancement and non-thermalized carrier induced bleaching. TA singularity from non-thermalized “Fermi Sea” is observed in quasi-2D perovskite film, indicating an ultrafast carrier thermalization within 100 fs. Moreover, the comparative study between the 2D and 3D perovskites uncovers the crucial role of hot-carrier effect to tune the NLA response. The ultrafast carrier cooling of quasi-2D perovskite is pointed out as an important factor to realize such abnormal NLA conversion process. These results provide fresh insights into the NLA mechanisms in low-dimensional perovskites, which may pave a promising way to diversify the NLO material applications.

Flexible wavelength-, pulse-controlled mode-locked all-fiber laser based on a fiber Lyot filter

In this paper, we report a flexible wavelength-, pulse-controlled mode-locked all-fiber laser based on a novel fiber optic Lyot filter. The wavelength, pulse duration and spectral bandwidth of passive mode-locked lasers can be tuned by controlling the polarization controller. The proposed Lyot filter was constructed by a single-mode fiber insertion between two polarization-maintaining fibers. The filter bandwidth and laser output tunability were based on the birefringence characteristics of the polarization-maintaining fibers. This all-fiber laser is simple and stable and can be used for various applications where width-tunable or wavelength-tunable pulses are necessary.

Wavelength and bandwidth tunable filter and its application in a dissipative soliton fiber laser

A wavelength and bandwidth tunable filter and its application in a dissipative soliton (DS) Yb-doped fiber laser are demonstrated. The spectral filter consisting of six cascaded temperature-sensitive long-period fiber gratings is designed and fabricated for the first time, to the best of our knowledge. The corresponding spectral characteristics of the filter are also characterized with temperature. Its 3-dB bandwidth can be adjusted from 4.84 to 11.02 nm, and the center wavelength is continuously adjustable from 1036 to 1045 nm. The sensitivity of the variation of the center wavelength and the linearity of the center wavelength variation are 32 pm/°C and 99.53%, respectively. This home-made spectral filter has two notable features: i) with regard to the tunable spectrum, the 3-dB bandwidth of the spectrum filter can be unchanged; ii) with regard to the spectral tunability, the 3-dB bandwidth of the spectral filter can also be quantitatively changed as needed by changing the heating mode. The home-made spectral filter is used in the DS fiber laser to further realize the continuous adjustment of DS with a tuning accuracy of 0.03 nm by a step size of 1°C. Such a wavelength-tunable DS fiber laser greatly enhances the design flexibility of the coherent anti-Stokes Raman scattering source.

Passively mode-locked erbium-doped fiber laser based on a nanodiamond saturable absorber

In this paper, we propose nanodiamond (ND) material as a saturable absorber (SA) to generate short pulses from a mode-locked erbium-doped fiber laser (EDFL). The ND-SA is fabricated by the drop-casting method using polyvinyledenedifluoride-trifluoroethylene as a host polymer and methyl ethel ketone as a solvent liquid. The SA, which possesses 20% ND concentration, has a 5.46% modulation depth with 2000MW/cm² saturation intensity. Sequentially, the performance of the EDFL is investigated after integrating an ND-SA within the laser ring. The results reveal that the presented ND-SAs produce stable ultrashort laser pulses. Moreover, the fabricated ND film is a promising solid film for many photonic schemes. The proposed mode-locked EDFL-based ND-SA starts a mode-locking operation at a pumped power of 116 mW. The generated mode-locked pulses have a pulse duration of 0.84 ps, a repetition rate of 1.93 MHz, and a power of 0.517 mW, at a pumped power of 187 mW. Finally, to the best of our knowledge, this is the first time that the ND-SA has been used as a mode locker within the EDFL as a thin film and with the suggested fabrication method.

Controllable Synthesis of Narrow-Gap van der Waals Semiconductor Nb2GeTe4 with Asymmetric Architecture for Ultrafast Photonics

Ultrafast photonics has become an interdisciplinary topic of great consequence due to the spectacular progress of compact and efficient ultrafast pulse generation. Wide spectrum bandwidth is the key element for ultrafast pulse generation due to the Fourier transform limitation. Herein, monoclinic Nb₂GeTe₄, an emerging class of ternary narrow-gap semiconductors, was used as a real saturable absorber (SA), which manifests superior wide-range optical absorption. The crystallization form and growth mechanism of Nb₂GeTe₄ were revealed by a thermodynamic phase diagram. Furthermore, the Nb₂GeTe₄-SA showed reliable saturation intensity and larger modulation depth, ascribed to a built-in electric field driven by the asymmetric crystal architecture confirmed via X-ray diffraction, polarized Raman spectra, and scanning transmission electron microscopy. Based on the Nb₂GeTe₄-SA, femtosecond mode-locked operation with good overall performance was achieved by a properly designed ring cavity. These results suggest that Nb₂GeTe₄ shows great promise for ultrafast photonic applications and arouse interests in exploring the intriguing properties of the ternary van der Waals material family.

Broadband Nonlinear Optical Absorption Induced by Bandgap Renormalization in CVD-Grown Monolayer MoSe2

Optical modulation on ultrashort time scales is both of central importance and an essential operation for applications in photonics and optoelectronics. Here, with a giant bandgap renormalization due to a high density of carrier injected by a femtosecond pulse, we realize an expected broadband saturable absorption in chemical vapor deposition grown monolayer transition-metal dichalcogenide MoSe₂. Our findings reveal the band edge shift from ∼1.53 to ∼0.52 eV under the pump excitation of 0.80 eV, which is induced by the nonequilibrium occupation of electron-hole states after a Mott transition as well as the increase of carrier temperature.

Novel nanomaterials based saturable absorbers for passive mode locked fiber laser at 1.5μm

Compared with continuous wave lasers, ultrafast lasers have the advantages of ultra-short pulse width and ultra-high peak power, and have significant applications in optical communications, medical diagnostics, and precision machining. Saturable absorber (SA) technology is the most effective technique for the generation of ultra-fast lasers, which are based on artificial SAs and natural SAs. Among them, the semiconductor saturable absorber mirror has become the most commonly used form at present. Recently, basic research and application of nanomaterials such as carbon nanotubes (CNTs) and graphene have been developed rapidly. Researchers have found that nanomaterials exhibit extraordinary characteristics in ultrafast photonics, such as the low saturation intensity of CNTs, zero-band gap of graphene, and extremely high modulation depth of the topological insulator nano-films. Since graphene was first reported as an SA in 2009, many other nanomaterials have been successively explored, resulting in the rapid development of novel nanomaterial-based SAs. In this paper, we classified the nanomaterials used in SA mode-locking technology at 1.5μm and reviewed their research progress with a particular focus on nonlinear optical properties, integration strategies, and applications in the field of ultrafast photonics.

PbS Quantum Dots Saturable Absorber for Dual-Wavelength Solitons Generation

PbS quantum dots (QDs), a representative zero-dimensional material, have attracted great interest due to their unique optical, electronic, and chemical characteristics. Compared to one- and two-dimensional materials, PbS QDs possess strong absorption and an adjustable bandgap, which are particularly fascinating in near-infrared applications. Here, fiber-based PbS QDs as a saturable absorber (SA) are studied for dual-wavelength ultrafast pulses generation for the first time to our knowledge. By introducing PbS QDs SA into an erbium-doped fiber laser, the laser can simultaneously generate dual-wavelength conventional solitons with central wavelengths of 1532 and 1559 nm and 3 dB bandwidths of 2.8 and 2.5 nm, respectively. The results show that PbS QDs as broadband SAs have potential application prospects for the generation of ultrafast lasers.

Black Phosphorus/Polymers: Status and Challenges

As a newly emerged mono-elemental nanomaterial, black phosphorus (BP) has been widely investigated for its fascinating physical properties, including layer-dependent tunable band gap (0.3-1.5 eV), high ON/OFF ratio (10⁴ ), high carrier mobility (10³ cm² V^-1 s^-1 ), excellent mechanical resistance, as well as special in-plane anisotropic optical, thermal, and vibrational characteristics. However, the instability caused by chemical degradation of its surface has posed a severe challenge for its further applications. A focused BP/polymer strategy has more recently been developed and implemented to hurdle this issue, so at present BP/polymers have been developed that exhibit enhanced stability, as well as outstanding optical, thermal, mechanical, and electrical properties. This has promoted researchers to further explore the potential applications of black phosphorous. In this review, the preparation processes and the key properties of BP/polymers are reviewed, followed by a detailed account of their diversified applications, including areas like optoelectronics, bio-medicine, and energy storage. Finally, in accordance with the current progress, the prospective challenges and future directions are highlighted and discussed.

Generating narrow bandwidth pulses in a hybrid mode-locked fiber laser

We demonstrate for the first time, to our knowledge, an all-fiber erbium-doped mode-locked laser in which mode-locking (ML) is realized by the combination of nonlinear polarization rotation and a saturable dynamic filtering effect, thereby generating nearly transform-limited ultrashort pulses with a pulse duration and spectral width of 45.2 ps and 0.0775 nm, respectively. The laser achieves both ML and harmonic ML by increasing the pump power. Simultaneously, the filtering function is maintained by the saturable dynamic induced grating (SDIG) throughout the power-modulation process. Furthermore, numerical simulations are used to analyze the pulse energy evolution in the cavity, revealing the advantages of hybrid ML in decreasing the pulse duration and time-bandwidth product under narrow filtering status. This work proposes a practical method to achieve ultrafast laser pulses with a narrow bandwidth, solving the problem that the SDIG has a hard time realizing a stable ML sequence.

Computational Bounds to Light-Matter Interactions via Local Conservation Laws

We develop a computational framework for identifying bounds to light-matter interactions, originating from polarization-current-based formulations of local conservation laws embedded in Maxwell's equations. We propose an iterative method for imposing only the maximally violated constraints, enabling rapid convergence to global bounds. Our framework can identify bounds to the minimum size of any scatterer that encodes a specific linear operator, given only its material properties, as we demonstrate for the optical computation of a discrete Fourier transform. It further resolves bounds on far-field scattering properties over any arbitrary bandwidth, where previous bounds diverge.

Optical fibres with embedded two-dimensional materials for ultrahigh nonlinearity

Nonlinear optical fibres have been employed for a vast number of applications, including optical frequency conversion, ultrafast laser and optical communication^1-4. In current manufacturing technologies, nonlinearity is realized by the injection of nonlinear materials into fibres^5-7 or the fabrication of microstructured fibres^8-10. Both strategies, however, suffer from either low optical nonlinearity or poor design flexibility. Here, we report the direct growth of MoS₂, a highly nonlinear two-dimensional material¹¹, onto the internal walls of a SiO₂ optical fibre. This growth is realized via a two-step chemical vapour deposition method, where a solid precursor is pre-deposited to guarantee a homogeneous feedstock before achieving uniform two-dimensional material growth along the entire fibre walls. By using the as-fabricated 25-cm-long fibre, both second- and third-harmonic generation could be enhanced by ~300 times compared with monolayer MoS₂/silica. Propagation losses remain at ~0.1 dB cm^-1 for a wide frequency range. In addition, we demonstrate an all-fibre mode-locked laser (~6 mW output, ~500 fs pulse width and ~41 MHz repetition rate) by integrating the two-dimensional-material-embedded optical fibre as a saturable absorber. Initial tests show that our fabrication strategy is amenable to other transition metal dichalcogenides, making these embedded fibres versatile for several all-fibre nonlinear optics and optoelectronics applications.

Tailoring Multi-Walled Carbon Nanotubes into Graphene Quantum Sheets

Transformation of carbon nanotubes (CNTs) into sub-10 nm pieces is highly required but remains a great challenge. Herein, we report a robust strategy capable of mechanically tailoring pristine multi-walled carbon nanotubes (MWCNTs) into graphene quantum sheets (M-GQSs) with an extremely high yield of up to 44.6 wt %. The method combines silica-assisted ball-milling and sonication-assisted solvent exfoliation and therefore enables reproducible high-yield production of M-GQSs directly from MWCNTs. Remarkable solvent diversity and extraordinary solvability (up to 7 mg/mL) are demonstrated facilitating the solution processing of the M-GQSs. The M-GQSs are essentially monolayers with intrinsic curvature, which could be determinative to their outstanding performances in both dispersions and thin films. Besides the excitation wavelength-, concentration-, and solvent-dependent photoluminescence in dispersions, the solid-state fluorescence and exceptional nonlinear saturation absorption (NSA) in thin films are demonstrated. Particularly, NSA with relative modulation depth up to 46% and saturation intensity down to 1.53 MW/cm² are achieved in M-GQS/poly(methyl methacrylate) hybrid thin films with a loading content of merely 0.2 wt %. Our method opens up a new avenue toward conversion and utilization of CNTs.

Enhanced saturable absorption in the laser-treated free-standing carbon nanotube films

We demonstrate an increase of optical transmittance and saturable absorption of laser-treated free-standing single-walled carbon nanotube (SWNT) films. The combined acid and low-power non-destructive laser treatment ensures an enhancement of linear transmittance across the visible range and double-digit increase of the saturable absorption of femtosecond laser radiation at 795 nm. The saturable absorption coefficient and the ratio of saturable to non-saturable losses increase by 26% and 35%, correspondingly, while the saturation intensity decreases by 20% because of the treatment. Our analysis indicates that with the performed treatment one can significantly improve the nonlinear optical properties of free-standing SWNT-based ultrafast saturable absorbers.

Nonlinear optical properties of arsenic telluride and its use in ultrafast fiber lasers

We report the first investigation results of the nonlinear optical properties of As2Te3. More specifically, the nonlinear optical absorption properties of the prepared α-As2Te3 were investigated at wavelengths of 1.56 and 1.9 μm using the open-aperture (OA) Z-scan technique. Using the OA Z-scan technique, the nonlinear absorption coefficients (β) of α-As2Te3 were estimated in a range from (- 54.8 ± 3.4) × 104 cm/GW to (- 4.9 ± 0.4) × 104 cm/GW depending on the irradiance of the input beam at 1.56 μm, whereas the values did from (- 19.8 ± 0.8) × 104 cm/GW to (- 3.2 ± 0.1) × 104 cm/GW at 1.9 μm. In particular, the β value at 1.56 μm is an order of magnitude larger than the previously reported values of other group-15 sesquichalcogenides such as Bi2Se3, Bi2Te3, and Bi2TeSe2. Furthermore, this is the first time report on β value of a group-15 sesquichalcogenide at a 1.9-μm wavelength. The density functional theory (DFT) calculations of the electronic band structures of α-As2Te3 were also conducted to obtain a better understanding of their energy band structure. The DFT calculations indicated that α-As2Te3 possess sufficient optical absorption in a wide wavelength region, including 1.5 μm, 1.9 μm, and beyond (up to 3.7 μm). Using both the measured nonlinear absorption coefficients and the theoretically obtained refractive indices from the DFT calculations, the imaginary parts of the third-order optical susceptibilities (Im χ(3)) of As2Te3 were estimated and they were found to vary from (- 39 ± 2.4) × 10-19 m2/V2 to (- 3.5 ± 0.3) × 10-19 m2/V2 at 1.56 μm and (- 16.5 ± 0.7) × 10-19 m2/V2 to (- 2.7 ± 0.1) × 10-19 m2/V2 at 1.9 μm, respectively, depending on the irradiance of the input beam. Finally, the feasibility of using α-As2Te3 for SAs was investigated, and the prepared SAs were thus tested by incorporating them into an erbium (Er)-doped fiber cavity and a thulium-holmium (Tm-Ho) co-doped fiber cavity for both 1.5 and 1.9 μm operation.

2D materials towards ultrafast photonic applications

Two-dimensional materials are now excelling in yet another arena of ultrafast photonics, including optical modulation through optical limiting/mode-locking, photodetectors, optical communications, integrated miniaturized all-optical devices, etc.

Q-switched ytterbium-doped fiber laser based on evanescent field interaction with lutetium oxide

We demonstrated lutetium oxide (${\textrm{Lu}_2}{\textrm{O}_3}$Lu₂O₃) deposited onto D-shaped fiber producing Q-switched ytterbium-doped fiber laser (YDFL) with an operating wavelength of 1037 nm. D-shaped fiber ${\textrm{Lu}_2}{\textrm{O}_3}$Lu₂O₃ as a saturable absorber (SA) was prepared using a polishing-wheel technique by polishing 2 times to establish an excellent evanescent field interaction between material and light on the surface of the polished region. The SA was deployed into a YDFL to generate Q-switching. The proposed D-shaped fiber ${\textrm{Lu}_2}{\textrm{O}_3}$Lu₂O₃ initiated pulses as short as 3.6 µs, with the highest repetition rate of 65.8 kHz. Stability of the SA is proven, as it produced stable pulses within the pump power of 99 to 133 mW with an SNR of 62.13 dB. Q-switched YDFL generates pulses with an output power of 0.93 to 1.99 mW and pulse energy of 17 to 30 nJ. We obtained a laser cavity with the optical-to-optical efficiency of 3.33%, which was the highest among D-shaped fiber-deposited SA materials in YDFL. Therefore, ${\textrm{Lu}_2}{\textrm{O}_3}$Lu₂O₃ deposited onto D-shaped fiber can be deployed as an SA in YDFL for a portable Q-switched laser source.

Fiber all-optical light control with low-dimensional materials (LDMs): thermo-optic effect and saturable absorption

In this paper, we review the recent studies on all-optical light control based on two main nonlinear mechanisms in LDMs: the thermo-optic effect and saturable absorption. The compactness of LDMs makes them the ideal medium for all-optical control systems. Many all-optical devices are demonstrated based on the properties of thermo-optic effects and saturable absorption. The materials characteristics and fabrication and the future prospects for all-optical control will also be shown.

Lead sulfide nanoparticles for dual-wavelength ultrashort pulse generation

Nanoparticle materials have many potential applications in the area of electronics and optoelectronics due to their unique and versatile properties. In particular, lead sulfide nanoparticles (PbS NPs) have shown excellent ultrafast photonics and can be applied to communication systems because of their low bandgap, high thermal damage threshold and stability. The wavelength division multiplexor (WDM) technique is vital to fiber optical communication, which allows the transmission of many different-wavelength signals in one fiber cable. However, PbS NPs for multi-wavelength pulse generation has not been reported until now. In this work, PbS NPs have been investigated and successfully applied in an Er-doped fiber laser as a saturable absorber (SA) to generate a dual-wavelength short pulse for the first time. A picosecond-level ultrashort pulse at center wavelengths of 1545 and 1585 nm can be achieved simultaneously or respectively. It is worth mentioning that the two wavelengths are separated up to 40 nm, which can significantly expand the optical communication capacity. The result suggests that PbS NPs as smart nonlinear optical components have wide applications in optical communications, short-pulse lasers, and even high-performance photodectors.

Heavily Doped Semiconductor Colloidal Nanocrystals as Ultra-Broadband Switches for Near-Infrared and Mid-Infrared Pulse Lasers

Heavily self-doped semiconductors can be designed to be used in advanced photonics due to both fabrication and functional advantages. Ultrafast response, strong optical nonlinearity, broadband wavelength range, and accessibility of integration are major challenges for ultrafast all-optical photonics to operate in the infrared wavelength range. Here, solution-processed Cu_1.8Se semiconductor nanocrystals (NCs) demonstrate an ultrafast response (about 360-520 fs), strong optical nonlinearity (as large as -1.4 × 10³ cm GW^-1), and broadband (from 800 to 3000 nm) nonlinear optical absorption in the near-infrared and mid-infrared wavelength ranges. The ultrafast response and larger optical nonlinearity may be triggered by the plasma ground-state bleaching in the strong surface electromagnetic filed. Stable Q-switched lasers in Er-doped fiber laser, Tm-doped fiber laser, and Ho/Pr-codoped ZBLAN fiber laser are operated, respectively. These findings indicate that Cu_1.8Se NCs are prospective nonlinear materials for ultrafast response and broadband pulse laser.

Ionic Liquid Gated Carbon Nanotube Saturable Absorber for Switchable Pulse Generation

Materials with electrically tunable optical properties offer a wide range of opportunities for photonic applications. The optical properties of the single-walled carbon nanotubes (SWCNTs) can be significantly altered in the near-infrared region by means of electrochemical doping. The states' filling, which is responsible for the optical absorption suppression under doping, also alters the nonlinear optical response of the material. Here, for the first time we report that the electrochemical doping can tailor the nonlinear optical absorption of SWCNT films and demonstrate its application to control pulsed fiber laser generation. With a pump-probe technique, we show that under an applied voltage below 2 V the photobleaching of the material can be gradually reduced and even turned to photoinduced absorption. Furthermore, we integrated a carbon nanotube electrochemical cell on a side-polished fiber to tune the absorption saturation and implemented it into the fully polarization-maintaining fiber laser. We show that the pulse generation regime can be reversibly switched between femtosecond mode-locking and microsecond Q-switching using different gate voltages. This approach paves the road toward carbon nanotube optical devices with tunable nonlinearity.

Ultrafast and broadband optical nonlinearity in aluminum doped zinc oxide colloidal nanocrystals

Heavily doped oxide semiconductors can be tailored for widespread application in near-infrared (NIR) and mid-infrared (mid-IR) wavelength ranges because of both functional and fabrication advantages. Here, the ultrafast and broadband nonlinear saturable absorption of Al-doped zinc oxide nanocrystals (AZO NCs) is investigated by using the Z-scan technique and the pump-probe technique. The nonlinear absorption coefficient is as high as -1.90 × 10³ cm GW^-1 in the wide infrared (IR) wavelength range (from 800 to 3000 nm). Furthermore, a maximum optically induced refractive index of -1.85 × 10^-1 cm² GW^-1 in the dielectric region and 2.09 × 10^-1 cm² GW^-1 in the metallic region leads to an ultrafast nonlinear optical response (less than 350 femtoseconds). Mode-locked fiber lasers at 1064 nm and 1550 nm as well as Q-switched fiber lasers near 2000 nm and 3000 nm prove the use of employing AZO NCs as a broadband and ultrafast nonlinear optical device, which provides a valuable strategy and intuition for the development of nanomaterial-based photonic and optoelectronic devices in the NIR and mid-IR wavelength ranges.

Synthesis of high quality silver nanowires and their applications in ultrafast photonics

Silver nanowires are widely used in catalysts, surface enhanced Raman scattering, microelectronic equipment, thin film solar cells, microelectrodes and biosensors for their excellent conductivity, heat transfer, low surface resistance, high transparency and good biocompatibility. However, the optical nonlinearity of silver nanowires has not been further explored yet. In this paper, three silver nanowire samples with different concentrations are produced via a typical hydrothermal method. Their applications to fiber lasers are implemented to prove the optical nonlinearity of silver nanowires for the first time. Based on three kinds of silver nanowires, the mode-locked operation of fiber lasers is successfully realized. Moreover, the fiber laser based on the silver nanowire with a concentration of 2 mg/L demonstrates the shortest pulse duration of 149.3 fs. The experiment not only proves the optical nonlinearity of silver nanowires, but also has some enlightenment on the selection of the optimum concentration of silver nanowires in the consideration of ultrashort pulse output.

Fe3O4 nanoparticles as a saturable absorber for giant chirped pulse generation

Fe₃O₄ nanoparticles (FONPs) are magnetic materials with a small band gap and have well-demonstrated applications in ultrafast photonics, medical science, magnetic detection, and electronics. Very recently, FONPs were proposed as an ideal candidate for pulse generation in fiber-based oscillators. However, the pulses obtained to date are on the order of microseconds, which is too long for real application in communication. Here, we report the use of FONPs synthesized by a sol-hydrothermal method and used as a saturable absorber (SA) to achieve nanosecond pulses in an erbium-doped fiber laser (EDFL) for the first time. The proposed fiber laser is demonstrated to have a narrow spectral width of around 0.8 nm and a fixed fundamental repetition rate (RPR) of 4.63 MHz, whose spectra and pulse dynamics are different from the mode-locked lasers reported previously. It is demonstrated that the proposed fiber laser based on a FONP SA operates in the giant-chirp mode-locked regime. The most important result is the demonstration of a pulse duration of 55 ns at an output power of 16.2 mW, which is the shortest pulse based on FONPs for EDFLs reported to date. Our results demonstrate that the FONP dispersion allows for an excellent photonic material for application in ultrafast photonics devices, photoconductive detectors, and optical modulators.

Versatile Mode-Locked Operations in an Er-Doped Fiber Laser with a Film-Type Indium Tin Oxide Saturable Absorber

We demonstrate the generation of versatile mode-locked operations in an Er-doped fiber laser with an indium tin oxide (ITO) saturable absorber (SA). As an epsilon-near-zero material, ITO has been only used to fashion a mode-locked fiber laser as an ITO nanoparticle-polyvinyl alcohol SA. However, this type of SA cannot work at high power or ensure that the SA materials can be transmitted by the light. Thus, we covered the end face of a fiber with a uniform ITO film using the radio frequency magnetron sputtering technology to fabricate a novel ITO SA. Using this new type of SA, single-wavelength pulses, dual-wavelength pulses, and triple-wavelength multi-pulses were achieved easily. The pulse durations of these mode-locked operations were 1.67, 6.91, and 1 ns, respectively. At the dual-wavelength mode-locked state, the fiber laser could achieve an output power of 2.91 mW and a pulse energy of 1.48 nJ. This study reveals that such a proposed film-type ITO SA has excellent nonlinear absorption properties, which can promote the application of ITO film for ultrafast photonics.

Nanotube mode-locked, wavelength and pulsewidth tunable thulium fiber laser

Mode-locked oscillators with highly tunable output characteristics are desirable for a range of applications. Here, with a custom-made tunable filter, we demonstrate a carbon nanotube (CNT) mode-locked thulium fiber laser with widely tunable wavelength, spectral bandwidth, and pulse duration. The demonstrated laser's wavelength tuning range reached 300 nm (from 1733 nm to 2033 nm), which is the widest-ever that was reported for rare-earth ion doped fiber oscillators in the near-infrared. At each wavelength, the pulse duration can be regulated by changing the filter's bandwidth. For example, at ~1902 nm, the pulse duration can be adjusted from 0.9 ps to 6.4 ps (the corresponding output spectral bandwidth from 4.3 nm to 0.6 nm). Furthermore, we experimentally and numerically study the spectral evolution of the mode-locked laser in presence of a tunable filter, a topic that has not been thoroughly investigated for thulium-doped fiber lasers. The detailed dynamical change of the mode-locked spectra is presented and we observed gradual suppression of the Kelly sidebands as the filter's bandwidth is reduced. Further, using the polarization-maintaiing (PM) cavity ensures that the laser is stable and the output laser's polarization extinction ratio is measured to exceed 20 dB.

Broadband polarization-insensitive saturable absorption of Fe2O3 nanoparticles

The synthesis and functionalization of transition-metal oxides are one of the most active research areas in advanced materials. As a typical transition-metal oxide, iron oxide has been widely used in lithium-ion batteries, gas sensors, and for water treatment. Herein, we synthesized Fe₂O₃ nanoparticles by a co-precipitation method that is inexpensive and non-toxic. The Fe₂O₃ nanoparticles exhibited broadband saturable absorption. Furthermore, thin Fe₂O₃ polyvinyl alcohol films were prepared to realize Q-switched operations in a ytterbium-doped fibre laser, an erbium-doped fibre laser, and a thulium-doped fibre laser. Attributed to the polarization-insensitive feature of the saturable absorber, Q-switched cylindrical vector beams were also generated based on mode coupling and selection in two-mode fibre lasers. Such Fe₂O₃ nanoparticles show great promise for use in Q-switching applications of infrared fibre lasers and cylindrical vector lasers.

Graphene-based all-optical multi-parameter regulations for an ultrafast fiber laser

Ultrafast lasers with tunable capabilities of pulse duration and spectrum have widespread applications in telecommunication, spectroscopy, and nonlinear optical bio-imaging. However, traditional mechanical and electrical tuning methods are still challenging for precise and stable controlling. Based on graphene's photo-thermal effect, we tune the bandwidths and wavelengths of chirped fiber Bragg gratings with flexible graphene-coating approaches. By inserting the fabricated devices into an ultrafast fiber laser cavity, durations and wavelengths of the generated pulses can be all-optically tuned with sensitivities of 470 fs/mW and 2.9 pm/mW, separately. Such an optical-controlled method provides a compact and precise way to regulate various laser properties.

Measurement of complex optical susceptibility for individual carbon nanotubes by elliptically polarized light excitation

The complex optical susceptibility is the most fundamental parameter characterizing light-matter interactions and determining optical applications in any material. In one-dimensional (1D) materials, all conventional techniques to measure the complex susceptibility become invalid. Here we report a methodology to measure the complex optical susceptibility of individual 1D materials by an elliptical-polarization-based optical homodyne detection. This method is based on the accurate manipulation of interference between incident left- (right-) handed elliptically polarized light and the scattering light, which results in the opposite (same) contribution of the real and imaginary susceptibility in two sets of spectra. We successfully demonstrate its application in determining complex susceptibility of individual chirality-defined carbon nanotubes in a broad optical spectral range (1.6-2.7 eV) and under different environments (suspended and in device). This full characterization of the complex optical responses should accelerate applications of various 1D nanomaterials in future photonic, optoelectronic, photovoltaic, and bio-imaging devices.

An Ultrabroadband Mid-Infrared Pulsed Optical Switch Employing Solution-Processed Bismuth Oxyselenide

Pulsed lasers operating in the mid-infrared (3-25 µm) are increasingly becoming the light source of choice for a wide range of industrial and scientific applications such as spectroscopy, biomedical research, sensing, imaging, and communication. Up to now, one of the factors limiting the mid-infrared pulsed lasers is the lack of optical switch with a capability of pulse generation, especially for those with wideband response. Here, a semiconductor material of bismuth oxyselenide (Bi₂ O₂ Se) with a facile processibility, constituting an ultrabroadband saturable absorber for the mid-infrared (actually from the near-infrared to mid-infrared: 0.8-5.0 µm) is exhibited. Significantly, it is found that the optical response is associated with a strong nonlinear character, showing picosecond response time and response amplitude up to ≈330.1% at 5.0 µm. Combined with facile processibility and low cost, these solution-processed Bi₂ O₂ Se materials may offer a scalable and printable mid-infrared optical switch to open up the long-sought parameter space which is crucial for the exploitation of compact and high-performance mid-infrared pulsed laser sources.

Coexistence of dissipative soliton and stretched pulse in dual-wavelength mode-locked Tm-doped fiber laser with strong third-order dispersion

Mode-locked lasers with strong high order dispersion exhibit rich nonlinear dynamics. Here we numerically and experimentally demonstrate coexistence of dissipative soliton (DS) and stretched pulse (SP) in a dual-wavelength mode-locked Tm-doped fiber laser with strong third-order dispersion (TOD), where the DS and SP show completely different pulse duration and peak power. Wavelength-dependent feature of the net cavity group-velocity dispersion (GVD) leaded by the strong TOD plays a key role for the coexistence patterns. To our best knowledge, this is the first demonstration of the coexistence of different mode-locked pulse regimes with strong laser cavity TOD.

Reverse Saturable Absorption Induced by Phonon-Assisted Anti-Stokes Processes

In materials showing reverse saturable absorption (RSA), optical transmittance decreases at intense laser irradiation. One approach to application of these materials is to protect the sensors or human eyes from laser damage. To date, research has mainly concentrated on thin films and suspensions of graphite and its nanostructure (including nanotubes, graphene, and graphene oxides), which are mainly used as an optical limiter for nanosecond laser pulses. Moreover, thin individual pieces of semiconductor usually exhibit increased transmittance due to saturable absorption when the laser energy (E_laser ) is higher than the band gap (E_B ). Here, it is shown that indirect gap semiconductor WSe₂ exhibits high RSA on exposure to a femtosecond laser under E_laser > E_B near band gap excitation, which is attributed to the longitudinal optical phonon-assisted anti-Stokes transition by the annihilation of phonons and the absorption of photons. An optical limiting threshold (≈21.6 mJ cm^-2 ) lower than those reported for other optical-limiting materials currently for femtosecond laser at 800 nm is observed.

Nonlinear Optics with 2D Layered Materials

2D layered materials (2DLMs) are a subject of intense research for a wide variety of applications (e.g., electronics, photonics, and optoelectronics) due to their unique physical properties. Most recently, increasing research efforts on 2DLMs are projected toward the nonlinear optical properties of 2DLMs, which are not only fascinating from the fundamental science point of view but also intriguing for various potential applications. Here, the current state of the art in the field of nonlinear optics based on 2DLMs and their hybrid structures (e.g., mixed-dimensional heterostructures, plasmonic structures, and silicon/fiber integrated structures) is reviewed. Several potential perspectives and possible future research directions of these promising nanomaterials for nonlinear optics are also presented.

Tungsten diselenide for all-fiber lasers with the chemical vapor deposition method

Two-dimensional materials have become the focus of research for their photoelectric properties, and are employed as saturable absorption materials. Currently, the challenge is how to further improve the modulation depth of saturable absorbers (SAs) based on two-dimensional materials. In this paper, three kinds of WSe2 films with different thicknesses are prepared using the chemical vapor deposition method. The nonlinear optical responses of the WSe2 films including the nonlinear saturable absorption and nonlinear refractive index are characterized by the double-balanced detection method and Z-scan experiments. Different modulation depths are successfully obtained by controlling the thickness of the WSe2 films. We further incorporate them into an all-fiber laser to generate mode-locked pulses. The mode-locked fiber lasers with a pulse duration of 185 fs, 205.7 fs and 230.3 fs are demonstrated when the thickness of the WSe2 films is measured to be 1.5 nm, 5.7 nm and 11 nm, respectively. This work provides new prospects for WSe2 in ultrafast photonic device applications.

Three-dimensional Dirac semimetal thin-film absorber for broadband pulse generation in the near-infrared

In this Letter, the transient nonlinear absorption of three-dimensional (3D) topological Dirac semimetal Cd₃As₂ thin film was characterized in the near-infrared band. By performing broadband pump-probe measurements, we experimentally proved that molecular beam epitaxy (MBE) grown Cd₃As₂ exhibits strong and tunable saturable absorption effects across 1-2 μm. By further inserting the Cd₃As₂ film into the cavities of Tm- and Er-doped fiber lasers, we obtained stable mode-locked operations at 1.96 and 1.56 μm. Our results experimentally establish that Cd₃As₂ is a promising broadband saturable absorber (SA) for pulsed lasers in the infrared range.

All-fiber radially/azimuthally polarized lasers based on mode coupling of tapered fibers

We demonstrate a mode converter with an insertion loss of 0.36 dB based on mode coupling of tapered single-mode and two-mode fibers, and realize all-fiber flexible cylindrical vector lasers at 1550 nm. Attributing to the continuous distribution of a tangential electric field at taper boundaries, the laser is switchable between the radially and azimuthally polarized states by adjusting the input polarization. In the temporal domain, the operation is controllable among continuous-wave, Q-switched, and mode-locked statuses by changing the saturable absorber or pump strength. The duration of Q-switched radially/azimuthally polarized laser spans from 10.4/10.8 to 6/6.4 μs at the pump range of 38 to 58 mW, while that of the mode-locked pulse varies from 39.2/31.9 to 5.6/5.2 ps by controlling the laser bandwidth. The proposed laser combines the features of a cylindrical vector beam, a fiber laser, and an ultrafast pulse, providing a special and cost-effective source for practical applications.

Wavelength and pulse duration tunable ultrafast fiber laser mode-locked with carbon nanotubes

Ultrafast lasers with tunable parameters in wavelength and time domains are the choice of light source for various applications such as spectroscopy and communication. Here, we report a wavelength and pulse-duration tunable mode-locked Erbium doped fiber laser with single wall carbon nanotube-based saturable absorber. An intra-cavity tunable filter is employed to continuously tune the output wavelength for 34 nm (from 1525 nm to 1559 nm) and pulse duration from 545 fs to 6.1 ps, respectively. Our results provide a novel light source for various applications requiring variable wavelength or pulse duration.

Bismuth Telluride nanocrystal: broadband nonlinear response and its application in ultrafast photonics

We come up with a hybrid liquid exfoliation method to prepare bismuth telluride nanocrystals efficiently and cost-effectively. The nonlinear transmittance of the nanocrystals has been characterized with Z-scan technique, which can manifest its broadband saturable absorption behavior experimentally. The as-fabricated nanocrystals were integrated onto fiber end facet to form a fiber compatible nonlinear absorption device with optical deposition method, which was then used to modulate the fiber laser with different cavity configurations to deliver pulsed laser successfully. The noise-like pulse and dissipative soliton have been obtained with wavelength centered at 1562 nm and 1068 nm, respectively. These results confirm the effectiveness of the hybrid liquid exfoliation method to prepare bismuth telluride into nanocrystals, and the broadband nonlinear optical response and ultrafast photonics application potential of the nanocrystals.

Buildup dynamics of dissipative soliton in an ultrafast fiber laser with net-normal dispersion

Taking advantage of technology of spatio-temporal reconstruction and dispersive Fourier transform (DFT), we experimentally observed the buildup dynamics of dissipative soliton in an ultrafast fiber laser in the net-normal dispersion regime. The soliton buildup dynamics were analyzed in both the spectral and temporal domains. We firstly revealed that the appearing of the spectral sharp peaks with oscillation structures during the mode-locking transition is caused by the formation of structural dissipative soliton. The experimental results were explained by the numerical simulations. These findings would give some new insights into the dissipative soliton buildup dynamics in ultrafast fiber lasers.

Facile one-step fabrication of upconversion fluorescence carbon quantum dots anchored on graphene with enhanced nonlinear optical responses

A novel nanocomposite hybrid, carbon quantum dots (CQD)/graphene oxide (GO), which combines the favorable optical properties of both its components, is synthesized by a facile one-step electrochemical method. Transmission electron microscopy, Raman spectroscopy, UV-vis spectroscopy, and fluorescence studies show that the CQDs uniformly attach on the GO surface, which enables highly efficient energy transfer between CQDs and GO. The nonlinear optical and optical limiting (OL) performances are investigated by the open-aperture Z-scan technique in the nanosecond regime using a laser with a wavelength of 532 nm. The as-prepared CQD/GO composite offers a significantly improved OL performance compared with GO because of the charge/energy transfer process between the CQDs and GO. The main contributors to the enhanced OL effect in the CQD/GO hybrid are a combination of nonlinear scattering and increased nonlinear absorption resulting from efficient charge/energy transfer at the CQD/GO interface.

A novel nanocomposite hybrid, carbon quantum dots (CQD)/graphene oxide (GO), which combines the favorable optical properties of both its components, is synthesized by a facile one-step electrochemical method.

Two-photon saturable absorption properties and laser Q-switch application of carbon quantum dots

In this Letter, high-quality carbon quantum dots (C-QDs) with an average size of 15 nm are synthesized by using a solvothermal method. A C-QD saturable absorber mirror (SAM) was prepared, characterized, and employed as an ultrafast optical switch successfully in a 1.0 μm solid-state laser. The saturable absorption effect (at 1 μm) far away from the linear absorption band of the C-QDs could be attributed to two-photon saturable absorption, which has a native characteristic of wavelength selectivity. The modulation depth (ΔT) and saturable energy intensity (ϕ_s) of the C-QD-SA was measured to be about 4% and 15.34  W/mm², respectively. By using this SA, a Q-switched Nd:GdVO₄ laser at 1 μm were first realized with the shortest pulse width of 66.8 ns and a maximum repetition rate of 1.13 MHz, respectively. The results indicate that C-QDs may found to be a decent carbon SA material for the high-repetition-rate pulsed laser applications.

High-repetition-rate, multi-pulse all-normal-dispersion fiber laser

In this Letter, a fiber laser that exploits the dissipative Faraday instability as a pulse-generating mechanism is presented, and its dynamics are studied numerically. The proposed laser operates in the all-normal-dispersion regime and produces a train of quasi-parabolic pulses, with a repetition rate that can be controlled depending on the cavity dispersion and nonlinearity, ranging from 10 to 50 GHz. It exploits a lumped amplification scheme, which can be potentially realized with rare-earth gain media. The issues concerning the stability of the pulses are discussed, and the differences with similar pulsed lasers are highlighted. In particular, the transition from the ordered multi-pulse regime proposed here to the random pulse operation mode already studied in the literature is discussed.

Generation of vector dissipative and conventional solitons in large normal dispersion regime

We report the generation of both polarization-locked vector dissipative soliton and group velocity-locked vector conventional soliton in a nanotube-mode-locked fiber ring laser with large normal dispersion, for the first time to our best knowledge. Depending on the polarization-depended extinction ratio of the fiber-based Lyot filter, the two types of vector solitons can be switched by simply tuning the polarization controller. In the case of low filter extinction ratio, the output vector dissipative soliton exhibits steep spectral edges and strong frequency chirp, which presents a typical pulse duration of ~23.4 ps, and can be further compressed to ~0.9 ps. In the contrastive case of high filter extinction ratio, the vector conventional soliton has clear Kelly sidebands with transform-limited pulse duration of ~1.8 ps. Our study provides a new and simple method to achieve two different vector soliton sources, which is attractive for potential applications requiring different pulse profiles.