4.1 W continuous-wave and broadly wavelength tunable Tm-doped laser in 2.1-2.4 µm spectral region based on vibronic and electronic transitions.

Efficient diode-pumped continuous-wave (CW) and wavelength tunable Tm:YAP lasers based on the vibronic and electronic transitions are investigated. A total maximum output power of 4.1 W is achieved with multi-wavelength output around 2162 nm and 2274 nm, corresponding to a slope efficiency of 29.8% for a 3 at. % Tm:YAP crystal. A maximum output power of 2.48 W with a slope efficiency of 25.4% is obtained at 2146 nm for a 4 at. % Tm:YAP crystal. Using a birefringent filter (BF), the emission wavelengths of the Tm:YAP laser are tuned over spectral ranges of 59 nm from 2115 nm to 2174 nm and 127 nm from 2267 nm to 2394 nm, respectively, which is the first demonstration of wavelength tunable Tm:YAP laser based on the electronic transition 3H4→3H5 and vibronic transition 3F4→3H6, to the best of our knowledge. The results show great potentials of the Tm:YAP crystal for realizing efficient lasers in the spectral range of 2.1-2.4 µm.

Associations between urbanization and the home language environment: Evidence from a LENA study in rural and peri-urban China.

In low- and middle-income countries, urbanization has spurred the expansion of peri-urban communities, or urban communities of formerly rural residents with low socioeconomic status. The growth of these communities offers researchers an opportunity to measure the associations between the level of urbanization and the home language environment (HLE) among otherwise similar populations. Data were collected in 2019 using Language Environment Analysis observational assessment technology from 158 peri-urban and rural households with Han Chinese children (92 males, 66 females) aged 18-24 months in China. Peri-urban children scored lower than rural children in measures of the HLE and language development. In both samples, child age, gender, maternal employment, and sibling number were positively correlated with the HLE, which was in turn correlated with language development.

All-solid-state ultrafast 2 µm laser with 1.83 W output power.

In this paper, a high-power all-solid-state ultrafast 2 µm mode-locked laser is investigated. The particularity of this laser is the simultaneous utilization of two Tm:YAP crystals in the same resonant cavity, independently pumped by two laser diodes. Using a 20% output coupler, pulses with output power as high as 1.83 W are achieved at a wavelength of 1938 nm with a pulse duration of 1.97 ps and a pulse repetition frequency of 100 MHz. To our knowledge, this mode-locked laser achieves the highest output power of any mode-locked Tm:YAP ultrafast laser reported to date. In addition, this paper provides a new approach to solve the problem of low output power due to multi-mode low-brightness laser diode pumping.

General Syntheses of High-Performance Thermoelectric Nanostructured Solids without Post-Synthetic Ligand Stripping.

Ligand-assisted wet chemical synthesis is a versatile methodology to produce controllable nanocrystals (NCs). The post-treatment of ligands is significant for the performance of functional devices. Herein, a method that retains ligands of colloidal-synthesized nanomaterials to produce thermoelectric nanomaterials is proposed, which differs from the conventional methods that strip ligands using multistep cumbersome processes. The ligand-retention method can control the size and dispersity of nanocrystals during the consolidation of the NCs into dense pellets, in which retained ligands are transformed into organic carbon within the inorganic matrices, establishing clear organic-inorganic interfaces. Characterizations of the nonstripped and stripped samples confirm that this strategy can affect electric transport slightly but reduce the thermal conductivity largely. As a result, the materials (e.g., SnSe, Cu2-xS, AgBiSe2, and Cu2ZnSnSe4) with ligands retained achieve higher peak zT and better mechanical properties. This method can be applied to other colloidal thermoelectric NCs and functional materials.

2-µm nonlinear post-compression for generating ∼100-MHz few-cycle laser pulses with watt-level average power.

We firstly report a high pulse repetition rate (101.4 MHz) nonlinear post-compression based on the normal dispersion fiber (NDF) operating in 2-µm wavelength region. With only one-stage NDF-based nonlinear pulse compressor, the 2-µm ultrafast laser pulses are compressed from ∼460 fs down to 70 fs, corresponding to ∼10.4 optical oscillation cycle. With two-stage nonlinear pulse compressor, the input ultrafast laser pulses are further compressed to 28.3 fs (∼4.3 optical oscillation cycle). In each case, the average power of the compressed 2-µm laser pulses exceeds 1 W, which is believed to be the highest average power never achieved at ∼100-MHz pulse repetition rate. The efficiencies of the one-stage and two-stage nonlinear pulse compressors are 64% and 47% respectively.

Diode-pumped Tm3+,Ho3+ co-doped GAGG mode-locking laser near the 2.1µm wavelength region.

In this paper, a laser diode (LD) pumped passive mode-locking Tm,Ho:GAGG laser based on a semiconductor saturable absorber mirror (SESAM) is reported. By adjusting the group delay dispersions inside the laser cavity and transmissions of the output couplers (OCs), a shortest pulse duration of 10.84 ps at 2089.9 nm is achieved, the average output power is 33.17 mW and the laser runs at a 83.01 MHz repetition rate. A maximum average output power of 66.43 mW is also obtained at 2089.9 nm with a pulse duration of 16.56 ps by using an OC of 3%. To the best of our knowledge, this is the first report on the mode-locking Tm,Ho:GAGG laser.

Exciton-induced mid-infrared optical nonlinearity of wide bandgap hexagon boron nitride.

In this Letter, an exciton absorption assumption is made to explain the mid-infrared saturable absorption performance of the 2D h-BN nanosheet. The exciton binding energy of ∼0.4 eV corresponds to the light wavelength around 3 µm, matching well with the experimental results. Experimentally, the h-BN saturable absorber (SA) shows a modulation depth of 5.3% in the wavelength region of 3 µm. By employing the h-BN SA in an Er:Lu2O3 laser, laser pulses with a pulse duration of 252 ns are realized at a repetition rate of 169 kHz, corresponding to a pulse energy of 3.55 µJ and peak power of 14 W. The exciton absorption assumption will help obtain a better understanding of the nonlinear optical dynamics in 2D materials from a new perspective.

Sub-60-fs, compact 1100-nm fiber laser system based on double-pass pre-chirp managed amplification.

An ytterbium-doped, single-stage, double-pass nonlinear fiber amplification system was fabricated for amplifying an 1100-nm mode-locking fiber laser. Pre-chirp managed amplification (PCMA) was applied in realizing the nonlinear amplification process with an all-polarization-maintaining (PM) fiber construction. The system can deliver 19.8-nJ, 58.7-fs, 24.4-MHz amplified signal pulses with a 10-dB spectral range spanning from 1049 nm to 1130 nm. Further experimental investigations were conducted in exploring the dynamics of the double-pass nonlinear amplification process. This compact 1100-nm ultrafast fiber laser can be implemented for multi-photon microscopy (MPM) with deep penetration depth.

Raman Linewidth Contributions from Four-Phonon and Electron-Phonon Interactions in Graphene.

The Raman peak position and linewidth provide insight into phonon anharmonicity and electron-phonon interactions in materials. For monolayer graphene, prior first-principles calculations have yielded decreasing linewidth with increasing temperature, which is opposite to measurement results. Here, we explicitly consider four-phonon anharmonicity, phonon renormalization, and electron-phonon coupling, and find all to be important to successfully explain both the G peak frequency shift and linewidths in our suspended graphene sample over a wide temperature range. Four-phonon scattering contributes a prominent linewidth that increases with temperature, while temperature dependence from electron-phonon interactions is found to be reversed above a doping threshold (ℏω_{G}/2, with ω_{G} being the frequency of the G phonon).

Investigation of Nonlinear Optical Modulation Characteristics of MXene VCrC for Pulsed Lasers.

We report the surface morphology and the nonlinear absorption characteristics of MXene VCrC nanosheets prepared by the liquid-phase exfoliation method. The self-made MXene VCrC was applied as a saturable absorber in the Tm:YAP laser experiments, performing excellent Q-switching optical modulation characteristics in infrared range. With this absorber, a stable passively Q-switched 2 µm laser was achieved. Under an incident pump power of 3.52 W, a maximum output power of 280 mW was obtained with a T = 3% output coupler at a repetition frequency of 49 kHz. The corresponding pulse energy and peak power were 5.7 µJ and 6.6 W, respectively. The shortest pulse duration was 658 ns at the repetition rate of 63 kHz with a T = 1% output coupler.

Realizing zT>2 in Environment-Friendly Monoclinic Cu2 S-Tetragonal Cu1.96 S Nano-Phase Junctions for Thermoelectrics.

Phase-junction nanocomposites, made of nanograins with the same composition but different phases, offer a platform to optimize the physiochemical performance of materials. Herein, we demonstrate a straightforward strategy to synthesize Cu2-x S phase-junction nanocomposites by retaining surface 1-dodecanethiol (DDT) ligands, in contrast to the traditional method that strips the ligands. As a result, phase junctions between a conventional monoclinic (m) phase and an unconventional metastable tetragonal (t) phase are obtained. The significantly improved power factor is obtained due to the doping of the t-phase. The phase-junction interfaces reduce thermal conductivity. Finally, surface regulation of phase junctions pushes the peak zT to 2.1 at 932 K, being the highest reported for environment-friendly metal sulfides. This work provides a paradigm to optimize thermoelectric performance by controlling phase junctions through surface-ligand tuning.

Investigation on the optical nonlinearity of the layered magnesium-mediated metal organic framework (Mg-MOF-74).

The wavelength-related optical nonlinearities of few-layer Mg-MOF-74 nanosheets were investigated in the wavelength region around 1.08, 1.94, and 2.85 µm by the closed aperture Z-scan, open aperture Z-scan and I-scan method. Under the excitation of 100-µJ laser pulses, the nonlinear refractive index (n2) of -7.7 ± 2.6, -131 ± 5 and 4.9 ± 0.2 cm2/W were obtained, respectively. The wavelength-related optical nonlinearity of the Mg-MOF-74 nanosheet was also investigated. In 2.85 µm wavelength region, the Mg-MOF-74 nanosheets shows a stable saturable absorption property with a modulation depth of 8% and a saturation intensity of 170 mJ/cm2. In the 1.08 and 1.94 µm wavelength regions, we can observe that the Mg-MOF-74 transits from saturable absorption regime to reverse saturable absorption regime with the increasing incident laser intensity. Employed as a saturable absorber in a Er:Lu2O3 laser, Mg-MOF-74 nanosheet shows a thickness-related laser modulation performance. The shortest laser pulse of 284-ns was achieved under a repetition rate of 116 kHz with a 6-nm-thick Mg-MOF-74 nanosheet, which corresponds to a pulse energy of 3.2 µJ and a peak power of 11.4 W.

Theoretical and experimental investigations on doubly Q-switched Tm:YAP laser with EOM and Sb2Te3 nanosheets.

In this paper, two-dimensional material Sb2Te3 nanosheets are fabricated and the optical nonlinear response is investigated. A laser diode (LD) end-pumped doubly Q-switched Tm:YAP laser with electro-optic modulator (EOM) and Sb2Te3 nanosheets based saturable absorber (SA) is presented. The shortest pulse duration of 38 ns is achieved at the pulse repetition frequency of 100 Hz, corresponding to the highest peak power of 111.8 kW. The double Q-switching technique shows the advantages of pulse duration compression and peak power improvement. The coupled rate equations for the doubly Q-switched laser are developed and the corresponding numerical simulation agrees with the experimental results. We believe that the Sb2Te3 is a potential nanomaterial for the application in optoelectronic field.

Ultrahigh Thermal Conductivity of θ-Phase Tantalum Nitride.

Extracting long-lasting performance from electronic devices and improving their reliability through effective heat management requires good thermal conductors. Taking both three- and four-phonon scattering as well as electron-phonon and isotope scattering into account, we predict that semimetallic θ-phase tantalum nitride (θ-TaN) has an ultrahigh thermal conductivity (κ), of 995 and 820 W m^{-1} K^{-1} at room temperature along the a and c axes, respectively. Phonons are found to be the main heat carriers, and the high κ hinges on a particular combination of factors: weak electron-phonon scattering, low isotopic mass disorder, and a large frequency gap between acoustic and optical phonon modes that, together with acoustic bunching, impedes three-phonon processes. On the other hand, four-phonon scattering is found to be significant. This study provides new insight into heat conduction in semimetallic solids and extends the search for high-κ materials into the realms of semimetals and noncubic crystal structures.

Theoretical and experimental investigations on Nb2CTxMXene Q-switched Tm:YAP laser at 2µm for the nonlinear optical response.

In this paper, the Nb2CTxMXene nanosheets were fabricated and the corresponding microstructures were investigated. The nonlinear optical response was illustrated by open aperture Z-scan and I-scan methods. The ground and the excited state absorption cross-sections of 2D Nb2CTxMXene were also investigated. As the saturable absorber (SA), the Nb2CTxMXene was applied in the passively Q-switched Tm:YAP laser. 1.96µs Q-switched pulses with 3.97 W peak power were achieved at the repetition frequency of 80 kHz. Further theoretical model was built by using the coupled rate equations in simulating the dynamic process of the passively Q-switched Tm:YAP laser. The numerical simulation results are fundamentally in agreement with the experimental results, which proves the Nb2CTxMXene can be a good potential nanomaterial for further optoelectronic applications.

The Investigation on Ultrafast Pulse Formation in a Tm-Ho-Codoped Mode-Locking Fiber Oscillator.

We experimentally investigate the formation of various pulses from a thulium-holmium (Tm-Ho)-codoped nonlinear polarization rotation (NPR) mode-locking fiber oscillator. The ultrafast fiber oscillator can simultaneously operate in the noise-like and soliton mode-locking regimes with two different emission wavelengths located around 1947 and 2010 nm, which are believed to be induced from the laser transition of Tm3+ and Ho3+ ions respectively. When the noise-like pulse (NLP) and soliton pulse (SP) co-exist inside the laser oscillator, a maximum output power of 295 mW is achieved with a pulse repetition rate of 19.85-MHz, corresponding to a total single pulse energy of 14.86 nJ. By adjusting the wave plates, the fiber oscillator could also deliver the dual-NLPs or dual-SPs at dual wavelengths, or single NLP and single SP at one wavelength. The highest 61-order harmonic soliton pulse and 33.4-nJ-NLP are also realized respectively with proper design of the fiber cavity.

Saturable absorption characteristics of Bi2Se3 in a 2 µm Q-switching bulk laser.

We investigate the saturable absorption properties of Bi2Se3 in a bulk laser operating at 2 µm wavelength region. The Bi2Se3 saturable absorber (SA) is prepared with the liquid-phase exfoliation method, which gives a saturable input flux of 4.3 mJ/cm2, a modulation depth of ∼10%, and a non-saturable absorption of 10.2%. With the Bi2Se3 saturable absorber, a passive Q-witching Tm:YAG ceramic laser is realized with a shortest pulse duration of 355 ns, a single pulse energy of 6.76 µJ and peak power of 19 W. We believe that this is the first report on Bi2Se3 Q-switched 2 µm bulk laser.

27 W 2.1 µm OPCPA system for coherent soft X-ray generation operating at 10 kHz.

We developed a high power optical parametric chirped-pulse amplification (OPCPA) system at 2.1 µm harnessing a 500 W Yb:YAG thin disk laser as the only pump and signal generation source. The OPCPA system operates at 10 kHz with a single pulse energy of up to 2.7 mJ and pulse duration of 30 fs. The maximum average output power of 27 W sets a new record for an OPCPA system in the 2 µm wavelength region. The soft X-ray continuum generated through high harmonic generation with this driver laser can extend to around 0.55 keV, thus covering the entire water window (284 eV - 543 eV). With a repetition rate still enabling pump-probe experiments on solid samples, the system can be used for many applications.

Passively Q-switched Er:Lu2O3 laser with MXene material Ti4N3Tx (T = F, O, or OH) as a saturable absorber.

In this Letter, we report a titanium nitride (Ti4N3Tx) passive Q-switched Er:Lu2O3 laser. The homemade two-dimensional Ti4N3Tx saturable absorber shows excellent passive Q-switching performance around a 2.85 µm wavelength region. Under the absorbed pump power of 7.4 W, the passive Q-switching laser yields a maximum output power of 0.778 W at a pulse repetition rate of 113.7 kHz, corresponding to a single pulse energy of 6.84 µJ and peak power of 24.57 W.