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 (ß) 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 ß 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 ß 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.

Metallic MXene Saturable Absorber for Femtosecond Mode-Locked Lasers.

2D transition metal carbides, nitrides, and carbonitides called MXenes have attracted much attention due to their outstanding properties. However, MXene's potential in laser technology is not explored. It is demonstrated here that Ti3 CN, one of MXene compounds, can serve as an excellent mode-locker that can produce femtosecond laser pulses from fiber cavities. Stable laser pulses with a duration as short as 660 fs are readily obtained at a repetition rate of 15.4 MHz and a wavelength of 1557 nm. Density functional theory calculations show that Ti3 CN is metallic, in contrast to other 2D saturable absorber materials reported so far to be operative for mode-locking. 2D structural and electronic characteristics are well conserved in their stacked form, possibly due to the unique interlayer coupling formed by MXene surface termination groups. Noticeably, the calculations suggest a promise of MXenes in broadband saturable absorber applications due to metallic characteristics, which agrees well with the experiments of passively Q-switched lasers using Ti3 CN at wavelengths of 1558 and 1875 nm. This study provides a valuable strategy and intuition for the development of nanomaterial-based saturable absorbers opening new avenues toward advanced photonic devices based on MXenes.

Anomalous Raman scattering and lattice dynamics in mono- and few-layer WTe2.

Tungsten ditelluride (WTe2) is a layered material that exhibits excellent magnetoresistance and thermoelectric behaviors, which are deeply related with its distorted orthorhombic phase that may critically affect the lattice dynamics of this material. Here, we report comprehensive characterization of Raman spectra of WTe2 from bulk to monolayer using experimental and computational methods. We find that mono and bi-layer WTe2 are easily identified by Raman spectroscopy since two or one Raman modes that are observed in higher-layer WTe2 are greatly suppressed below the noise level in the mono- and bi-layer WTe2, respectively. In addition, the frequency of in-plane A1(7) mode of WTe2 remains almost constant as the layer number decreases, while all the other Raman modes consistently blueshift, which is completely different from the vibrational behavior of hexagonal metal dichalcogenides. First-principles calculation validates experimental results and reveals that anomalous lattice vibrations in WTe2 are attributed to the formation of tungsten chains that make WTe2 structurally one-dimensional.

Common features of orientational order at the temperature of maximum density for various water models: molecular dynamics study.

Canonical ensembles for liquid water were obtained from molecular dynamics simulations at various temperatures using the TIP5P, TIP4P-FQ, TIP4P, and SPC/E water models at a fixed density of 1 g/cm3. From these ensembles, it was found that the distributions of the orientational order parameter q of these models showed similar patterns as temperature changed except that the distributions were shifted relative to each other by the difference of their temperature of maximum density (TMD). The four models exhibited similar distributions and average values of orientational order around their respective TMDs, and these common features were investigated in detail especially. The current study suggests that the unique microscopic configuration of water molecules cause TMD phenomenon in any reasonable water model. This finding provides a useful tool in the development of new water potentials by offering guidelines to predict the TMD, avoiding troublesome isothermal-isobaric ensemble simulations.

The structure of water near platinum and its significance in water-adsorbent system: molecular dynamics study.

Water structure around hydrophilic adsorbents is expected to differ from normal water structure. In some cases, a change of water structure seems to significantly affect phenomena involved in water-adsorbent systems. To investigate this, we have performed molecular dynamic simulations of three systems, water between (111) surfaces of Pt, water between (100) surfaces of Pt, and pure water at 298 K. For the analysis of water structure, we concentrated on the five-membered ring (R5) structure and six-membered ring (R6) structure, which are the most probable in nature. For both (111) and (100) of Pt, the ratio of hexameric ring/pentameric ring (R6/R5) in semibound regions was higher than that of bulk water, as expected. The function of adsorbent ceramics was explained in terms of the change of water structure when they are used as biomaterials, support materials, etc. The obscure mechanisms for the effects of medicine stones, antioxidants, etc., were also discussed from this point of view.