Broadband atomic-layer MoS2 optical modulators for ultrafast pulse generations in the visible range

Broadband and ascendant nonlinear optical properties of the wide bandgap material GaN nanowires

Gallium nitride (GaN) nanowire, as a type of wide bandgap nanomaterial, has attracted considerable interest because of its outstanding physicochemical properties and applications in energy storage and photoelectric devices. In this study, we prepared GaN nanowires via a facile chemical vapor deposition method and investigated their nonlinear absorption responses ranging from ultraviolet to near-infrared in the z-scan technology under irradiation by picosecond laser pulses. The experiment revealed that GaN nanowires exhibit remarkable nonlinear absorption characteristics attributed to their wide bandgap and nanostructure, including saturable absorption and reverse saturable absorption. When compared to bulk GaN crystals, the nanowires provide a richer and more potent set of nonlinear optical effects. Furthermore, we conducted an analysis of the corresponding electronic transition processes associated with photon absorption. Under high peak power density laser excitation, two-photon absorption or three-photon absorption dominate, with maximum modulation depths of 73.6%, 74.9%, 63.1% and 64.3% at 266 nm, 355 nm, 532 nm, and 1064 nm, respectively, corresponding to absorption coefficients of 0.22 cm/GW, 0.28 cm/GW, 0.08 cm/GW, and 2.82 ×10-4 cm3/GW2. At lower peak energy densities, GaN nanowires demonstrate rare and excellent saturation absorption characteristics at wavelength of 355 nm due to interband transitions, while saturable absorption is also observed at 532 nm and 1064 nm due to band tail absorption. The modulation depths are 85.2%, 41.9%, and 13.7% for 355 nm, 532 nm, and 1064 nm, corresponding to saturation intensities of 3.39 GW/cm2, 5.58 GW/cm2 and 14.13 GW/cm2. This indicates that GaN nanowires can be utilized as broadband optical limiters and high-performance pulse laser modulating devices, particularly for scarce ultraviolet optical limiters, and saturable absorbers for ultraviolet and visible lasers. Furthermore, our study demonstrates the application potential of wide bandgap nanomaterials in nonlinear optical devices.

Passively mode-locked red Pr:LiYF4 laser based on a two-dimensional palladium diselenide saturable absorber

We report a passively mode-locked Pr:LiYF₄ (Pr:YLF) visible laser using a palladium diselenide (PdSe₂) as a saturable absorber (SA) for the first time, to the best of our knowledge. The nonlinear optical properties of two-dimensional (2D) PdSe₂ nanosheets in the visible band were studied by the open-aperture Z-scan technique. The results indicate the significant saturable absorption properties of PdSe₂ nanosheets in the visible region. Furthermore, the continuous wave mode-locked (CWML) visible laser based on PdSe₂ SA was successfully realized. Ultrashort pulses as short as 35 ps were obtained at 639.5 nm with a repetition rate of 80.3 MHz and a maximum output power of 116 mW. The corresponding pulse energy was 1.44 nJ and peak power was 41.3 W. These results indicate that 2D PdSe₂ SA is a promising high stability passively mode-locked device for ultrafast solid-state visible lasers.

2D Materials Enabled Next-Generation Integrated Optoelectronics: from Fabrication to Applications

2D materials, such as graphene, black phosphorous and transition metal dichalcogenides, have gained persistent attention in the past few years thanks to their unique properties for optoelectronics. More importantly, introducing 2D materials into silicon photonic devices will greatly promote the performance of optoelectronic devices, including improvement of response speed, reduction of energy consumption, and simplification of fabrication process. Moreover, 2D materials meet the requirements of complementary metal-oxide-semiconductor compatible silicon photonic manufacturing. A comprehensive overview and evaluation of state-of-the-art 2D photonic integrated devices for telecommunication applications is provided, including light sources, optical modulators, and photodetectors. Optimized by unique structures such as photonic crystal waveguide, slot waveguide, and microring resonator, these 2D material-based photonic devices can be further improved in light-matter interactions, providing a powerful design for silicon photonic integrated circuits.

Ultrafast Fiber Lasers with Low-Dimensional Saturable Absorbers: Status and Prospects

Wide-spectral saturable absorption (SA) in low-dimensional (LD) nanomaterials such as zero-, one-, and two-dimensional materials has been proven experimentally with outstanding results, including low saturation intensity, deep modulation depth, and fast carrier recovery time. LD nanomaterials can therefore be used as SAs for mode-locking or Q-switching to generate ultrafast fiber laser pulses with a high repetition rate and short duration in the visible, near-infrared, and mid-infrared wavelength regions. Here, we review the recent development of emerging LD nanomaterials as SAs for ultrafast mode-locked fiber laser applications in different dispersion regimes such as anomalous and normal dispersion regimes of the laser cavity operating in the near-infrared region, especially at ~1550 nm. The preparation methods, nonlinear optical properties of LD SAs, and various integration schemes for incorporating LD SAs into fiber laser systems are introduced. In addition to these, externally (electrically or optically) controlled pulsed fiber laser behavior and other characteristics of various LD SAs are summarized. Finally, the perspectives and challenges facing LD SA-based mode-locked ultrafast fiber lasers are highlighted.

MXenes: Synthesis, Optical Properties, and Applications in Ultrafast Photonics

Recently, 2D materials are in great demand for various applications such as optical devices, supercapacitors, sensors, and biomedicine. MXenes as a kind of novel 2D material have attracted considerable research interest due to their outstanding mechanical, thermal, electrical, and optical properties. Especially, the excellent nonlinear optical response enables them to be potential candidates for the applications in ultrafast photonics. Here, a review of MXenes synthesis, optical properties, and applications in ultrafast lasers is presented. First, aqueous acid etching and chemical vapor deposition methods for preparing MXenes are introduced, in which the storage stability and challenges of the existing synthesis techniques are also discussed. Then, the optical properties of MXenes are discussed specifically, including plasmonic properties, optical detection, photothermal effects, and ultrafast dynamics. Furthermore, the typical ultrafast pulsed lasers enabled by MXene-based saturable absorbers operated at different wavelength regions are summarized. Finally, a summary and outlook on the development of MXenes is presented in the perspectives section.

Watt-level ultrafast bulk laser with a graphdiyne saturable absorber mirror

Few-layered graphdiyne (GDY) was successfully fabricated and applied as a saturable absorber to generate a watt-level ultrafast solid-state bulk laser. The maximum output power of up to 1.27 W was obtained with a pulse width of 23 ps and a repetition rate of 92.9 MHz, using Nd:YVO₄ crystal as a gain medium. To the best of our knowledge, this is the first application of GDY as a mode locker in all-solid-state bulk lasers. These results indicate the promising potential of GDY for producing high-power ultrafast lasers.

Towards visible-wavelength passively mode-locked lasers in all-fibre format

Mode-locked fibre lasers (MLFLs) are fundamental building blocks of many photonic systems used in industrial, scientific and biomedical applications. To date, 1-2 μm MLFLs have been well developed; however, passively mode-locked fibre lasers in the visible region (380-760 nm) have never been reported. Here, we address this challenge by demonstrating an all-fibre visible-wavelength passively mode-locked picosecond laser at 635 nm. The 635 nm mode-locked laser with an all-fibre figure-eight cavity uses a Pr/Yb codoped ZBLAN fibre as the visible gain medium and a nonlinear amplifying loop mirror as the mode-locking element. First, we theoretically predict and analyse the formation and evolution of 635 nm mode-locked pulses in the dissipative soliton resonance (DSR) regime by solving the Ginzburg-Landau equation. Then, we experimentally demonstrate the stable generation of 635 nm DSR mode-locked pulses with a pulse duration as short as ~96 ps, a radio-frequency signal-to-noise ratio of 67 dB and a narrow spectral bandwidth of <0.1 nm. The experimental results are in excellent agreement with our numerical simulations. In addition, we also observe 635 nm noise-like pulse operation with a wide (>1 nm) and modulated optical spectrum. This work represents an important step towards miniaturized ultrafast fibre lasers in the visible spectral region.

Kerr-lens mode-locked Pr3+:LuLiF4 laser

We demonstrate the Kerr-lens mode-locked Pr³⁺:LuLiF₄ (Pr:LLF) laser pumped by a blue laser diode (LD). By theoretical calculation of the group velocity dispersion in the laser gain, the compensation was employed for the realization of the continuous-wave mode-locked laser at the wavelength of 604 nm with the pulse width of 1.1 ps which, to the best of our knowledge, is the shortest pulse width in the Pr³⁺ ion doped crystal lasers pumped with LDs. It can be believed that the present Pr:LLF laser should provide some inspiration for the development of the blue LD pumped visible lasers, especially in the mode-locking laser operation.

Large optical nonlinearity of ITO/Ag/ITO sandwiches based on Z-scan measurement

Indium tin oxide (ITO)-based sandwich structures with the insertion of silver (ITO/Ag/ITO) show large nonlinear optical enhancement of both nonlinear refraction and saturable absorption. Here optical nonlinearity is measured using a Z-scan experiment with a 1310 nm pulsed laser at normal incidence. The nonlinear refractive index (n₂=15.43×10^-16  m²/W) for an ITO/Ag/ITO sandwich with a 14 nm silver interlayer and the nonlinear absorption coefficient (β=-648×10^-11  m/W) for an ITO/Ag/ITO sandwich with a 10 nm silver interlayer are about 18 and 16 times greater than that of a single-layer ITO, respectively. Meanwhile, a large figure of merit and modulation depth values hinted that ITO/Ag/ITO sandwiches are promising saturable absorber materials to switch continuous laser waves into laser pulses. These novel nonlinear optical properties make ITO/Ag/ITO sandwiches a promising candidate for all-optical modulation devices at optical communication wavelengths.

Diode-pumped 915-nm Pr:YLF laser passively mode-locked with a SESAM

A diode-pumped, passively mode-locked laser emitting at 915 nm with a praseodymium-doped yttrium lithium fluoride (Pr:YLF) crystal was demonstrated for the first time, to the best of our knowledge. Utilizing two polarization-combined blue pumping laser diodes (LDs) and a semiconductor saturable absorber mirror (SESAM), stable continuous-wave (CW) mode-locking operations were achieved with a maximum average output power of 408 mW and a slope efficiency of 10.8%. Laser pulse durations of 15 ps were obtained with a spectral full width at half maximum (FWHM) of 0.15 nm and a repetition rate of 1.53 GHz.

Near-infrared absorbers based on the heterostructures of two-dimensional materials

Although the conductance and dielectric function of graphene can be tuned by applying external voltage, the tunability is less than 3%. Hybridizing graphene with other two-dimensional transition metal dichalcogenides (TMDs) can improve the adjustability and tunability of the optical properties of graphene-based structures at near-infrared frequencies. In this paper, we theoretically compute the dielectric function of graphene-MoTe₂-graphene and graphene-MoTe₂-graphene heterostructures utilizing the quantum electrostatic heterostructure (QEH) model, which is an ab-initio method. Utilizing the QEH results, we propose a hyper crystal (HC) absorber at near-infrared frequencies. Hence, we use the transfer matrix method to investigate our proposed absorber analytically. Moreover, we simulate the graphene-TMD-graphene (G-TMD-G) absorbers by the numerical finite difference time domain method. The results of the numerical solution are consistent with those of the analytical method. Due to the dependency of the Fermi level of graphene on the direct bandgap of the TMDs, the dielectric function of the G-TMD-G heterostructure can be tuned and enhanced further by changing the number of TMD layers. Finally, we demonstrate that the full absorption of the heterostructures can be achieved at different frequencies for transverse magnetic polarization. Since the thicknesses of the layers in the HC are lower than the wavelength of the light, no diffracted bands are ubiquitous, and the absorption can be observed for a wide range of incidence angles and bandwidths at near-infrared frequencies. Because of utilizing graphene-based HCs, in addition to the feasibility of design compared to the complex metasurfaces, the absorption bandwidth is significant for a wide range of incidence angles. This kind of HC absorber can be used in the design of sensitive optical devices, such as tunable filters, detectors, and photovoltaic applications.

Nonlinear optical response during the electron transition process originated from 3D spin-orbit splitting in NiO nanosheets

NiO, a 3d transition-metal oxide with the strong electron correlation, has attracted great physical attention due to the spin-orbit splitting of 3d electrons. By taking advantage of electron transition process originated from 3d spin-orbit splitting, it may be applied to many photonics areas by linear or nonlinear optical response. To further broaden the photonics applications of NiO, we originally explore the nonlinear optical response, saturable absorption, during the electronic transition due to 3d spin-orbit splitting under a strong optical field and successfully applied in the ultrafast photonics as a mode-locker for the generation of visible laser pulses, which is the result of dynamic balancing process by the electron transition arising from ground state (³A_2g) to excited state (¹E_g) of spin-orbit splitting in the Ni²⁺ 3d configurations. With the NiO nanosheet film for saturable absorption, we experimentally realize a pulsed visible laser at a wavelength of 640.3 nm for the first time to our knowledge. These results indicate that the study of electron transition process generated by 3d spin-orbit splitting in 3d transition-metal oxides should be helpful for the development of ultrafast photonics and related devices design.

Photo-induced excitonic structure renormalization and broadband absorption in monolayer tungsten disulphide

Atomically thin transition metal dichalcogenides (TMDCs) have emerged as a new class of two-dimensional (2D) material for novel optoelectronic applications. In particular, 2D TMDCs are viewed as intriguing and appealing materials to construct Q-switching and mode-locked modulators, due to their broadband saturable absorption even of photon energy below their excitonic energies. However, the dynamics and mechanism of saturable absorption inside TMDCs has yet to be investigated. In this paper, the relaxation dynamics of monolayer tungsten disulphide (WS₂) was investigated considering different excitonic transitions. WS₂ illustrates dramatic changes in optical responses when excited by intense laser pulses, which are characterized by the broadband photo-induced nonresonance absorption and the giant excitonic bands renormalization process. The experimental results show that strong photo-induced restructuring of excitonic bands has picosecond lifetime and full recovery of optical responses takes hundreds of picosecond. Additionally, our observations reveal that heavy renormalization and overlap of excitonic bands are induced by strong many-body Coulomb interactions. Moreover, the broadband absorption feature of WS₂ opens up new applications in broadband saturable absorbers and ultrafast photonic devices.

Emerging photonic architectures in two-dimensional opto-electronics

This review summarizes recent developments in opto-electronic device architectures comprising van der Waals two-dimensional materials for enhanced light–matter interactions.

High-power self-mode-locked Pr:YLF visible lasers

We demonstrate efficient self-mode-locked green and red lasers with repetition rates of tens and hundreds of megahertz in a Pr:YLF crystal. Using double-end blue-diode-pumped geometry, more than 0.68 W average output power at 522 nm and more than 1.44 W at 639 nm are obtained, which are believed to be the highest average output powers for mode-locked praseodymium lasers operating in the visible wavelength region.

Visible laser emission from a praseodymium-doped fluorozirconate guided-wave chip

We report visible continuous-wave laser emission at 636 nm from a praseodymium-doped fluorozirconate glass guided-wave chip laser. This ultra-fast laser inscribed gain chip is demonstrated to be a compact and integrated laser module. The laser module, pumped by 442 nm GaN laser diodes, generates >8  mW lasing output with a beam quality of Mxy2∼1.15×1.1(±0.1). To the best of our knowledge, this is the first visible laser emission from a glass-based waveguide chip laser.