Two-dimensional (2D) materials are crystals with one to a few layers of atoms and are being used in many fields such as optical modulator, photodetector, optical switch, and ultrafast lasers. Their exceptional optoelectronic and nonlinear optical properties make them as a suitable saturable absorber for laser cavities. This review focuses on the recent progress in ultrafast laser use 2D materials as a saturable absorber. 2D materials traditionally include graphene, topological insulators, transition metal dichalcogenides, as well as new materials such as black phosphorus, bismuthene, antimonene, and MXene. Material characteristics, fabrication techniques, and nonlinear properties are also introduced. Finally, future perspectives of ultrafast lasers based on 2D materials are also addressed.

Recent progress in ultrafast lasers based on 2D materials as a saturable absorber

Based on the saturable absorption feature of a two-dimensional (2D) nano-material, antimonene, the passively Q-switched operation for solid-state laser was realized for the first time For the 946 and 1064 nm laser emissions of the Nd:YAG crystal, the Q-switched pulse widths were 209 and 129 ns, and the peak powers were 148, 177 W, respectively For the 1342 nm laser emission of the Nd:YVO4 crystal, the Q-switched pulse width was 48 ns, giving a peak power of 2817 W Our research shows that antimonene can be used as a stable, broadband optical modulating device for a solid-state laser, which will be particularly effective for long wavelength operation

Passively Q-switched Nd 3+ solid-state lasers with antimonene as saturable absorber

In this paper, we report a 946nm double Q-switched laser side pumped by an 808-nm pulse laser diode (LD). A layered tungsten diselenide (WSe2) saturable absorber (SA) together with an MgO doped LiNbO3 electro-optic (EO) modulator is applied to double Q-switch the Nd: YAG laser, producing trains of nanosecond-duration pulses with 500 Hz repetition rate. Such WSe2 saturable absorbers are fabricated by chemical vapor deposition (CVD) in a hot wall chamber and then embedded into a resonant mirror. The achieved pulse energy of double Q-switched laser at 946 nm is approximately 2.63 mJ with 10.8 ns pulse width and the peak power is round 244 kW, corresponding to the beam quality factors of M2
x = 3.846,M2
y = 3.861. Monolayer WSe2 nanosheets applied in the experiment would be a promising SA for passive Q-switching operation.

946 nm Nd: YAG double Q-switched laser based on monolayer WSe 2 saturable absorber

Fiber lasers are widely used in industrial processing, sensing, medical and communications applications due to their simple structure, good stability and low cost. With the rapid development of fiber lasers and the sustained improvement of industrial laser quality requirements, researchers in ultrafast optics focus on how to get laser pulses with high output power and narrow pulse duration. Q-switched technology is one of the most effective techniques to generate ultrashort pulses. In this paper, a tungsten diselenide saturable absorber with 16.82% modulation depth is prepared by chemical vapor deposition. Experimental results show that when the pump power changes from 115.7 mW to 630 mW, the all-fiber laser can achieve a stable Q-switched pulse output. The repetition rate of the output pulse varies from 80.32 kHz to 204.2 kHz, the pulse duration is 581 ns, the maximum output power is 17.1 mW and the maximum pulse energy is 83.7 nJ. Results in this paper show that tungsten diselenide can be applied to ultrafast optics, which is a kind of saturable absorption material with excellent properties.

http://iopscience.iop.org/article/10.1088/1555-6611/aa9e38/pdf

Q-switched all-fiber laser with short pulse duration based on tungsten diselenide

Longitudinally diode-pumped continuous-wave dual-wavelength Nd:YAG laser at 1116 and 1123 nm is achieved with maximum output power of 8.18 W and slope efficiency of 42.3%. Using WS
 2
 as saturable absorber, passive Q-switching of the dual-wavelength laser is also achieved with a maximum average output power of 0.41 W, the shortest pulse width of about 0.64 μs at pulse repetition rate of 100.9 kHz. The present Q-switched laser operation leads to maximum pulse energy of 4.06 μJ and pulse peak power of 6.35 W. A 2-D material WS
 2
 has been experimentally confirmed to be very promising as Q switcher at 1.1 μm, for the first time to our knowledge.

Q-Switched Dual-Wavelength Laser at 1116 and 1123 nm Using WS2 Saturable Absorber

We demonstrate a passively Q-switched Nd: YAG quasi-three-level laser operating at 946 nm using MoS2 as saturable absorber. A maximum average output power of 210 mW is achieved at an absorbed pump power of 6.67 W with a slope efficiency of about 5.8%. The shortest pulse width and maximum pulse repetition frequency are measured to be 280 ns and 609 kHz, respectively. The maximum pulse energy and maximum pulse peak power are therefore estimated to be about 0.35 μJ and 1.23 W, respectively. This work represents the first MoS2-based Q-switched laser operating at 0.9 µm spectral region.

MoS 2 -based passively Q-switched diode-pumped Nd:YAG laser at 946 nm

The physical mechanism of improving the photoelectric performance of InGaN/AlGaN‐based near UV light‐emitting diode (LED) with convex quantum barrier and staggered quantum well (QW) is studied by numerical simulation. The simulation results indicate that the voltage–current characteristics of the LED structure with convex quantum barrier and staggered QW are effectively improved compared with the traditional multiple quantum well (MQW) structure, and its electroluminescence (EL) intensity and light output power are significantly improved. The main physical mechanisms are: on the one hand, the convex quantum barrier with a lower average Al component can reduce the polarization electric field at the interface between the quantum barrier and QW as well as the effective potential barrier of holes, improve the spatial separation of electron and hole wave functions, promote the injection efficiency of carriers, and improve the uniformity of carrier distribution in the MQWs active region; On the other hand, staggered QWs can provide stronger carrier confinement effect and further increase the overlap of electron and hole wave functions, so as to improve the carrier radiative recombination efficiency; In a word, this work provides a valuable reference for obtaining high‐performance near‐ultraviolet LED.

Improved Performance of InGaN/AlGaN Multiple‐Quantum‐Well Near‐UV Light‐Emitting Diodes with Convex Barriers and Staggered Wells

In this paper, we present a novel AlGaN-based multiple-quantum-well (MQW) deep ultraviolet light-emitting diode (DUV-LED) structure with two parts linearly graded barriers. The simulation result shows that at a current of 50mA, the light output power (LOP) of the DUV-LED with two parts linearly graded barrier MQWs have significant improvement as compared to stationary barriers. The electroluminescence (EL) spectrum and radiative recombination rate of novel DUV-LEDs are also larger more than twice that of the conventional quantum well structure. The reason is that the injection efficiency of holes is increased which helps improve the hole and electron concentration in the active area. Meanwhile, the electric field is also decreased by using two parts linearly graded quantum barriers, and according to reduce the electric field the quantum-confined Stark effect (QCSE) and the bend of the energy band get relieved. This article is protected by copyright. All rights reserved. 

Performance of Improvement of AlGaN‐Based Deep UV Light‐Emitting Diode with Two Parts Linearly Graded Barriers

Enhanced high electron mobility transistor featuring a lattice matched AlInGaN/GaN heterojunction and composite gate structure

The serious electron leakage and poor transport of hole injection layers in deep‐ultraviolet (DUV) light‐emitting diodes (LEDs) lead to an imbalance between the hole and electron currents, which reduces the device's performance. Herein, a new DUV LED structure is designed. The aluminum composition of the p‐type electron‐blocking layer (p‐EBL) gradually decreases from 0.95 to 0.75 along the growth direction, replacing the traditional bulk p‐EBL. When the injection current is 100 mA, the optical power of this structure is about 32% higher than that of the traditional structure. In addition, when the p‐EBL adopts the opposite gradient (the Al composition gradually increases from 0.75 to 0.95), the device performance suddenly drops dramatically. The related devices are simulated by Silvaco TCAD software. The results show that the performance outputs of the two opposite aluminum gradient growth modes are completely opposite. The proposed p‐EBL with a gradual step‐down Al composition along the growth direction helps to improve the efficiency of hole injection into multiple quantum wells (MQWs) and reduce the level of electron leakage. This work provides an effective solution for increasing the light output power of high‐performance DUV LEDs.

Efficient Carrier Confinement in AlGaN‐Based Deep‐Ultraviolet Light‐Emitting Diodes with a Composition‐Graded Electron‐Blocking Layer

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