Black phosphorus (BP), an emerging narrow direct band-gap two-dimensional (2D) layered material that can fill the gap between the semi-metallic graphene and the wide-bandgap transition metal dichalcogenides (TMDs), had been experimentally found to exhibit the saturation of optical absorption if under strong light illumination. By taking advantage of this saturable absorption property, we could fabricate a new type of optical saturable absorber (SA) based on mechanically exfoliated BPs, and further demonstrate the applications for ultra-fast laser photonics. Based on the balanced synchronous twin-detector measurement method, we have characterized the saturable absorption property of the fabricated BP-SAs at the telecommunication band. By incorporating the BP-based SAs device into the all-fiber Erbium-doped fiber laser cavities, we are able to obtain either the passive Q-switching (with maximum pulse energy of 94.3 nJ) or the passive mode-locking operation (with pulse duration down to 946 fs). Our results show that BP could also be developed as an effective SA for pulsed fiber or solid-state lasers.

Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and Mode-locking laser operation

We propose and demonstrate 1, 1.5, and 2 μm passively Q-switched fiber lasers by exploiting a few-layer Molybdenum sulfide (MoS2) polymer composite as broadband saturable absorber (SA), respectively. The few-layer MoS2 nanosheets are prepared by the liquid-phase exfoliation method, and are composited with polyvinyl alcohol (PVA). The PVA-MoS2 film is sandwiched between two fiber ferrules to form the fiber-compatible SA. The few-layer MoS2 not only shows good transparency from ultraviolet to mid-infrared spectral region, but also possesses the nonlinear saturable absorption. The modulation depth and saturation optical intensity of the PVA-MoS2 film are measured to be 1.6% and 13 MW/cm2 at 1566 nm by the balanced twin-detector technique, respectively. By further inserting the filmy PVA-MoS2 SA into the cavities of Yb-, Er- and Tm-doped fiber lasers, we achieve stable Q-switching operations at 1.06, 1.56, and 2.03 μm, respectively. The output characteristics of the Q-switched pulses at the three wavelengths have been investigated, respectively. The MoS2-based Q-switching enables the large pulse energy of ∼1 μJ with a pulse width of 1.76 μs. This is, to the best of our knowledge, the first demonstration of MoS2-based Q-switched fiber lasers in a wide wavelength range (from 1 to 2 μm). Our results experimentally confirm that the new-type 2-D material, few-layer MoS2, is a promising broadband SA to Q-switch fiber lasers covering all major wavelengths from near- to mid-infrared region.

1-, 1.5-, and 2-μm Fiber Lasers Q-Switched by a Broadband Few-Layer MoS 2 Saturable Absorber

2D Layered Materials: Synthesis, Nonlinear Optical Properties, and Device Applications

We propose and demonstrate a MoS2-based passively Q-switched Er-doped fiber laser with a wide tuning range of 1519.6-1567.7 nm. The few-layer MoS2 nano-platelets are prepared by the liquid-phase exfoliation method, and are then made into polymer-composite film to construct the fiber-compatible MoS2 saturable absorber (SA). It is measured at 1560 nm wavelength, that such MoS2 SA has the modulation depth of ∼ 2% and the saturable optical intensity of ∼ 10 MW/cm(2). By further inserting the filmy MoS2-SA into an Er-doped fiber laser, stable Q-switching operation with a 48.1 nm continuous tuning from S- to C-waveband is successfully achieved. The shortest pulse duration and the maximum pulse energy are 3.3 μs and 160 nJ, respectively. The repetition rate and the pulse duration under different operation conditions have been also characterized. To the best of our knowledge, it is the first demonstration of MoS2 Q-switched, widely-tunable fiber laser.

Widely-tunable, passively Q-switched erbium-doped fiber laser with few-layer MoS2 saturable absorber.

High-performance mode-locked and Q-switched fiber lasers based on novel 2D materials of topological insulators, transition metal dichalcogenides and black phosphorus: review and perspective (invited)

We demonstrated a high-repetition-rate Q-switched fiber laser with topological insulator Bi₂Se₃ absorber. The absorber was made into a film structure by spin-coating method using few-layer Bi₂Se₃ nano-platelets which had regular shape. The uniform film had a low saturable optical intensity of 11 MW/cm(2), which is the lowest saturable optical intensity in the saturable absorbers made by topological insulator till now. By inserting the absorber film into an Erbium-doped fiber laser, a high-repetition Q-switched laser with the repetition rates from 459 kHz to 940 kHz was achieved. The maximum output power was 22.35 mW with the shortest pulse duration of 1.9 μs. To the best of our knowledge, both of the repetition rate and the output power were the highest values among the Q-switched fiber lasers with topological insulator absorber.

High-repetition-rate Q-switched fiber laser with high quality topological insulator Bi₂Se₃ film.

Multi-crystalline fibers of poly (3-hexylthiophene) (P3HT) have been grown on a patterned substrate through annealing the spin-coated film in the solvent of 1,5-pentanediol at 120 °C. The large-area patterns of photoresist grating have been fabricated by interference lithography, enabling growth of crystal fibers in a length scale of centimeters. The produced organic crystal fibers were characterized by microscopic, spectroscopic, and electrical conductivity performances. Mechanisms for the crystallization processes were investigated.

Directional crystallization of polymer molecules through solvent annealing on a patterned substrate.

Organic blends containing heterojunction structures at the interfacial phase have been applied extensively in organic optoelectronic devices to modify charge transfer, separation, and recombination processes. Scaling and controlling the transition domains at the hetero-interface are of crucial importance for deep insights into the involved physics and for architecturing the devices with improved performance. However, it is difficult to recognize and characterize these transition domains directly using the conventional microscopic techniques, in particular when different molecules are dissolved in the same solvent with equal solubility. In this work, we introduce a technique defined as laser-induced selective cross-linking to isolate the interfacial phase from other phases into a directly measurable practicity. Thus, the hetero-domains become visualized and directly measurable. Based on the insolubility of the selectively cross-linked molecules in organic solvents, a lift-off process may remove the uncross-linked or incompletely cross-linked molecules, so that the hetero-domain is more clearly visualized and more precisely measured. A transition domain in a scale of about 200 nm is resolved in the F8BT/PFB blend film between their respectively rich phases after the selective cross-linking of the F8BT molecules by a blue laser. Furthermore, hetero-crosslinking between F8BT and PFB molecules was also resolved by both microscopic and near-field spectroscopic investigations.

Hongwei Li

Papers

High-repetition-rate Q-switched fiber laser with high quality topological insulator Bi₂Se₃ film.

Directional crystallization of polymer molecules through solvent annealing on a patterned substrate.

Laser-induced selective crosslinking for scaling the heterointerfacial domain in polymer blends