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1T′- and 2H–MoTe 2 are metallic and semiconducting two-dimensional transition metal dichalcogenides, respectively, due to their different atomic structure. It is a pretty attractive work to control the synthesis of MoTe 2 with different phases and study on their different physical properties. In this work, large-area 1T′- and 2H–MoTe 2 films with nanometer thicknesses were successfully prepared. XRD, Raman spectra and SAED pattern all indicate that the two films have different atomic structures. The photoelectric properties and ultrafast photocarrier dynamics were further investigated. The results show that 1T′-MoTe 2 has a higher conductivity than 2H–MoTe 2 by four orders of magnitude, while 2H–MoTe 2 has better photoelectric response with a responsivity of 0.016 A/W at a V ds of 1 V for 532 nm light. Transient absorption spectra reveal that the intraband relaxation time (τ 1) of 1T′-MoTe 2 is ∼6 ps while it is ∼100 ps in 2H–MoTe 2. The interband electron-hole recombination time (τ 2) in 2H–MoTe 2 is longer than that in 1T' phase. Our findings contribute to a deeper understanding of the physical properties of MoTe 2 films with different phases and provide important basis for the applications of 1T′- and 2H–MoTe 2 in various electrical, photoelectric and ultrafast optical devices. Large area 1T ′- and 2H–MoTe 2 films with monoclinic and hexagonal structure respectively are prepared. Experimental results show that there are great differences in electrical, photoelectric and ultrafast optical properties between the two different phase films. [Display omitted] • The method for preparing large-area 1T′- and 2H–MoTe 2 films is introduced. • 1T′-MoTe 2 has high conductivity and 2H–MoTe 2 has good photoelectric response. • The excited carriers in 2H–MoTe 2 have longer lifetime than those in 1T' phase. [ABSTRACT FROM AUTHOR]

As a kind of semiconductors, tellurium has attracted much attention due to its superior optical, electrical, and thermoelectric properties. In particular, a lot of research on low-dimensional tellurium has been performed in recent years, but with relatively few work on its applications in ultrafast photonics. In this work, a femtosecond fiber laser was realized using the prepared tellurium nanowires as the saturable absorber. Tellurium nanowires were synthesized by the hydrothermal method and then deposited on the tapered optical fiber to form the tellurium saturable absorber (Te-SA). The saturable absorption property of Te-SA was tested by the two-arm balance detection method, the saturation intensity and modulation depth were determined to be 8.9 MW/cm2 and 27.0%, respectively. Then, Te-SA was applied in the erbium-doped fiber laser and a stable mode-locked fiber laser with a pulse width of 621 fs was achieved. The results indicate the great potential application of tellurium nanowires in ultrafast photonics. • Tellurium nanorods are prepared by a green and safe hydrothermal method. • The saturable absorber based on the tellurium nanorods shows strong nonlinear optical absorption property. • A mode-locked Er-doped fiber laser with of a pulse width of 621 fs was realized based on the Te-SA. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rigorous mode-locking performance comparison between MoS 2 and MoSe 2 is demonstrated. • Results indicate that MoS 2 has shorter excited state carrier lifetimes than MoSe 2 in visible region. • The MoS 2 and MoSe 2 show excellent performance in achieving femtosecond fiber laser. Recently, MoS 2 and MoSe 2 have attracted significant attention in ultrafast photonics owing to their excellent nonlinear optical properties. The mode-locking performance of them were also investigated intensively. However, so far, there is still no answer to who has better mode-locking performance between MoS 2 and MoSe 2. In this work, we prepared MoS 2 and MoSe 2 nanosheets and the transient absorption spectra indicate that both have excited-state carrier lifetimes of picosecond level in the visible region, and MoS 2 has a shorter recovery time than MoSe 2. The mode-locked Er-doped fiber lasers (EDFL) based on MoS 2 and MoSe 2 were carried out in the same ring laser cavity, which allows for an accurate comparison of their mode-locked performance. The mode-locked EDFL based on MoS 2 saturable absorber (SA) shows a pulse width of 470 fs, which is much shorter than that based on MoSe 2 SA (639 fs). This suggests that MoS 2 has better mode-locking performance than MoSe 2 from the viewpoint of pulse width. [ABSTRACT FROM AUTHOR]

• Two-dimensional PdTe 2 is a novel saturable absorption material which has been rarely reported. • Few-layer PdTe 2 nanosheets are prepared by the simple and effective ultrasonic liquid exfoliation method. • Yb-doped mode-locked fiber laser with a pulse width of 887.2 ps is realized using PdTe 2 /PVA as the saturable absorber. In this work, two-dimensional few-layer palladium ditelluride (PdTe 2) nanosheets are produced by the simple and effective ultrasonic liquid exfoliation method and used as novel saturable absorber in an Yb-doped mode-locked fiber laser. The results show that the prepared flexible PdTe 2 /PVA composite film has a modulation of 3.7% and a saturation intensity of 77.6 MW/cm2. The obtained Yb-doped mode-locked fiber laser based on the film shows a pulse width of 887.2 ps with a repetition and central wavelength of 8.921 MHz and 1069 nm, respectively. So far, there has been few reports on PdTe 2 as saturable absorbers to realize mode-locked fiber lasers. The results suggest that two-dimensional PdTe 2 has great potential applications in nonlinear optics and ultrafast photonics. [ABSTRACT FROM AUTHOR]

Femtosecond laser, compared to continuous wave (c.w.) or nanosecond laser, has shown great promise in diversities of frontier applications because of its extremely short pulse width, high peak power and low thermal effect. In order to obtain femtosecond laser output, the design of suitable saturable absorber (SA) materials as a nonlinear modulation device is a key issue for a given laser system. However, the previous SA materials are limited in inorganic semiconductors, which have some constraints such as narrow working wavelength ranges, complex preparation process and high cost. By contrast, organic semiconductors have high molecular design freedom, good flexibility, simple preparation process, and low cost, and would act as an ideal SA material. Here, we report a novel strategy to tune and realize the femtosecond laser out by employing an organic small molecule SA of (Z)-2-(4-(pyrene-1-yl) phenyl)-3-(4-(1,2,2-triphenylvinyl) phenyl) acrylonitrile (Py-TPE). We show that Py-TPE has an ultrafast excited carrier relaxation time less than 20 ps, shorter than most of inorganic semiconductor materials. More importantly, by depositing such Py-TPE on the surface of tapered fiber as an SA device in Er-doped fiber laser system, an ultrafast fiber laser output with a pulse width as narrow as 447 fs and good stability was realized. This is the first report, to the best of our knowledge, to achieve femtosecond laser by taking advantage of the Py-TPE with ultrafast excited carrier relaxation time. These results demonstrate that Py-TPE is a novel organic saturable absorption material with great potential application in ultrafast laser and photonics. • Py-TPE shows an ultrafast excited carrier relaxation time (<74.8 fs) in the near-infrared region. • The use of Py-TPE SA in fiber lasers results in an exceptionally low threshold (30 mW) for mode-locking due to its low saturation intensity (20.61 MW/cm2). • The Er-doped mode-locked fiber laser based on Py-TPE SA has pretty narrow pulse width (447 fs) and good stability. [ABSTRACT FROM AUTHOR]

Novel layered bismuth oxysulfide (Bi 2 O 2 S) has shown great potential in photoelectric detection, photocatalysis, and photochemical cells due to its excellent properties such as excellent environmental stability, high carrier mobility, and favorite band gap (about 1∼1.5 eV). These excellent physical properties also make it advantageous for applications in the field of ultrafast photonics. However, to date, there has been no reported research on Bi 2 O 2 S in ultrafast fiber lasers. In this work, we report a facile method to synthesize Bi 2 O 2 S nanosheets by a combination of the hydrothermal and liquid-phase exfoliation (LPE) methods with good crystallinity and thickness less than 10 nm. Further, stable mode-locked pulses are achieved by applying the Bi 2 O 2 S SA in Er-doped and Tm-doped fiber lasers, respectively. The Er-doped mode-locked fiber laser has a pulse width of 453.6 fs and the optical spectrum is stable, with a maximum output power of 5.1 mW in the mode-locked state. The obtained Tm-doped mode-locked fiber laser based on Bi 2 O 2 S has an estimated pulse width of 2.69 ps and a maximum output power of 7.13 mW. Our results provide new opportunities for the development of Bi 2 O 2 S together with its application in ultrafast photonics. • The application of novel Bi 2 O 2 S nanosheets in ultrafast fiber lasers are investigated for the first time. • The Er-doped mode-locked fiber laser based on Bi 2 O 2 S has the narrowest pulse width. • The Bi 2 O 2 S based Tm-doped mode-locked fiber laser has a high output power and a large slope efficiency. [ABSTRACT FROM AUTHOR]