High-power 100 W Kerr-lens mode-locked ring-cavity femtosecond Yb:YAG thin-disk oscillator.

High-power femtosecond pulses delivered at a high-repetition rate will aid machining throughput and improve signal-to-noise ratios for sensitive measurements. Here we demonstrate a Kerr-lens mode-locked femtosecond Yb:YAG ring-cavity thin-disk oscillator with a multi-pass scheme for the laser beam. With four passes through the thin disk, 175-fs pulses were delivered from the oscillator at an average power of 71.5 W and a repetition rate of 65.3 MHz. The corresponding intra-cavity peak power of 110 MW is ample for intra-cavity nonlinear conversion into more exotic wavelength ranges. With six passes, the average output power reached 101.3 W. To the best of our knowledge, this is the highest average output power of any mode-locked ring laser. These results confirm the viability of using multi-pass configuration on a thin-disk ring oscillator for high-throughput femtosecond applications.

Generation of femtosecond optical vortices with multiple separate phase singularities from a Kerr-lens mode-locked Yb:KGW oscillator.

Femtosecond optical vortices with a phase singular point have diverse applications such as microscopic particles manipulation, special-structure micro-processing and quantum information. Raising the number of singularity points can provide additional dimensions of control. Here we report for what we believe is the first time the generation of femtosecond optical vortices with multiple (two and five) singularities directly from a laser oscillator. The average powers and pulse durations of the resulting vortex pulses are several hundred milliwatts and less than 300 fs, respectively. This work represents an innovate way for obtaining femtosecond multi-vortices, opening the way to the further studies of optical vortex crystals and their applications.

Self-started Kerr-lens mode-locked thin-disk oscillator.

Kerr-lens mode-locking (KLM) has been widely used in thin-disk oscillators to generate high-power femtosecond pulses. Here we demonstrate a Kerr-lens mode-locked Yb:YAG thin-disk oscillator that can be self-started under two configurations. The first can deliver 13-W, 235-fs pulses at a repetition rate of 103 MHz; the second delivers 49 W at a repetition rate of 46.5 MHz, whose corresponding pulse energy of 1.05 µJ is, to the best of our knowledge, the highest energy ever obtained in self-started Kerr-lens mode-locked oscillators. A new method to initiate KLM in the form of optical perturbation in a thin-disk oscillator has also been demonstrated.

Sub-100-fs Kerr-lens mode-locked Yb:YAG ring-cavity thin-disk oscillator.

Ultrafast ring-cavity thin-disk oscillators combine high output power with the flexibility of generating output either unidirectionally or bidirectionally. Here, we report a Kerr-lens mode-locked ring-cavity Yb:YAG thin-disk oscillator delivering unidirectional 89-fs pulses by inducing additional spectral broadening with nonlinear plates. This is the shortest pulse duration for a ring-cavity mode-locked thin-disk oscillator. Bidirectional mode-locking was also realized. These results lay the foundation for the more efficient generation of high-order harmonics at MHz repetition rates and high-power dual frequency combs.

Two-dimensional metallic carbon allotrope with multiple rings for ion batteries.

Two-dimensional (2-D) materials, especially carbon allotropes, have larger storage capacity and faster diffusion rate due to their unique structures and are usually used in ion batteries. Recently, a new stable two-dimensional carbon allotrope, namely PAI-graphene, was reported by first-principles calculations. Due to its lightweight and multiple-ring structure, great stability and excellent properties, here, we theoretically reveal the excellent performance of PAI-graphene as an anode material for Li-/Na-ion batteries. Our results show that PAI-graphene has intrinsic metallicity before and after adsorption of Li/Na, which ensures that it has good conductivity when working as an electrode material. In addition, PAI-graphene exhibits quite low open circuit voltage (0.342-0.190 V for Li, 0.339-0.233 V for Na) and diffusion barrier (0.34 eV for Li, 0.17 eV for Na), which indicates its superiority as an anode material. Most noteworthily, the Na storage capacity of PAI-graphene is up to 1674 mA h g-1, which is much higher than that of most 2-D anode materials. Thus, we believe that PAI-graphene can be an outstanding anode material with outstanding performance.

Pentagonal B2C monolayer with extremely high theoretical capacity for Li-/Na-ion batteries.

Recently, two-dimensional (2-D) materials with a Penta-atomic-configuration such as Penta-graphene have received considerable attention because of their potential applications in electronics, spintronics and ion batteries. Previously, Penta-graphene has been proposed as an excellent anode material for Li-/Na-ion batteries with a high theoretical capacity (1489 mA h g-1). Here, based on the first-principles calculations, we report that a new 2-D material namely Penta-B2C can become another excellent anode material with even higher theoretical capacity for Li-/Na-ion batteries than Penta-graphene. Our results demonstrate that Li/Na atoms can be stably adsorbed on Penta-B2C. Meanwhile, Penta-B2C shows metallic conductivity during the adsorption. Most strikingly, the theoretical capacities of Penta-B2C are as high as 1594 for Li and 2391 mA h g-1 for Na, which are superior to those of the most known 2-D anode materials. Especially, the Na theoretical capacity of Penta-B2C sets a new record among known 2-D anode materials. In addition, Penta-B2C possesses relatively low open-circuit voltage and a low diffusion barrier for ions, which are vital for anode materials. These results highly promise that Penta-B2C can be an excellent anode material with a fast charge/discharge rate and extremely high theoretical capacity for Li-/Na-ion batteries.

Broadband, few-cycle mid-infrared continuum based on the intra-pulse difference frequency generation with BGSe crystals.

We demonstrate for the first time the generation of octave-spanning mid-infrared using a BGSe nonlinear crystal. A Cr:ZnS laser system delivering 28-fs pulses at a central wavelength of 2.4 µm is used as the pump source, which drives the intra-pulse difference frequency generation inside the BGSe crystal. As a result, a coherent broadband mid-infrared continuum spanning from 6 to 18 µm has been obtained. It shows that the BGSe crystal is a promising material for broadband, few-cycle mid-infrared generation via frequency down conversion with femtosecond pump sources.

Ti2P monolayer as a high performance 2-D electrode material for ion batteries.

Electrical conductivity, storage capacity and ion diffusion ability are three crucial parameters for battery electrode materials. However, rare existing two-dimensional (2-D) electrode materials can achieve high performances in all these parameters. Here, we report that a 2-D transition-metal phosphide, the Ti2P monolayer, is a promising superior electrode material which realizes high performances in all the parameters mentioned above. The Ti2P monolayer has a stable honeycomb crystal structure. It has a metallic electronic structure with Li/Na adsorption, which ensures good electrical conductivity during the battery operation. We find that Li/Na can chemically bond to the Ti2P substrate, with specific charge exchanges. Our results show the Li/Na capacity in the Ti2P monolayer is about 846 mA h g-1, which is much higher than that of the graphite anode. Remarkably, the Li/Na diffusion barrier on the Ti2P monolayer is only 12-16 meV, which is lower than that in all 2-D anode materials proposed till now. Our work highly promises that theTi2P monolayer can serve as a superior anode material for Li-ion/Na-ion batteries by providing good electrical conductivity, high storage capacity and ultrafast ion diffusion.

High-order femtosecond vortices up to the 30th order generated from a powerful mode-locked Hermite-Gaussian laser.

Femtosecond vortex beams are of great scientific and practical interest because of their unique phase properties in both the longitudinal and transverse modes, enabling multi-dimensional quantum control of light fields. Until now, generating femtosecond vortex beams for applications that simultaneously require ultrashort pulse duration, high power, high vortex order, and a low cost and compact laser source has been very challenging due to the limitations of available generation methods. Here, we present a compact apparatus that generates powerful high-order femtosecond vortex pulses via astigmatic mode conversion from a mode-locked Hermite-Gaussian Yb:KGW laser oscillator in a hybrid scheme using both the translation-based off-axis pumping and the angle-based non-collinear pumping techniques. This hybrid scheme enables the generation of femtosecond vortices with a continuously tunable vortex order from the 1st up to the 30th order, which is the highest order obtained from any femtosecond vortex laser source based on a mode-locked oscillator. The average powers and pulse durations of all resulting vortex pulses are several hundred milliwatts and <650 fs, respectively. In particular, 424-fs 11th-order vortex pulses have been achieved with an average power of 1.6 W, several times more powerful than state-of-the-art oscillator-based femtosecond vortex sources.

Binary pentagonal auxetic materials for photocatalysis and energy storage with outstanding performances.

Since the discovery of penta-graphene, two-dimensional (2-D) pentagonal-structured materials have been highly expected to have desirable performance because of their unique structures and accompanied physical properties. Hence, based on the first-principles calculations, we performed a systematical study on the structure, stability, mechanical and electronic properties, and potential applications on carbon-based pentagonal materials with binary compositions, namely, Penta-CnX6-n (n = 1, 2, 4, 5; X = B, N, Al, Si, P, Ga, Ge, As). We found that eleven out of thirty-two Penta-CnX6-n have good stability and can be further studied. Among them, two materials, namely, Penta-C4P2 and Penta-C5P are metallic, and others are indirect band gap semiconductors, whose band gaps calculated by the HSE06 functional are in the range of 1.37-6.43 eV, covering the infrared-visible-ultraviolet regions. Furthermore, we found that metallic Penta-CnX6-n can become promising anode materials for Na-ion batteries (NIBs) with high storage capacity, while some semiconducting Penta-CnX6-n can become excellent water splitting photocatalysts. In addition, Penta-C4P2 and Penta-C2Al4 were found to have obvious in-plane negative Poisson's ratio (NPR) of -0.083 and -0.077, respectively. More interestingly, we found that Penta-C2Al4 exhibits a peculiar in-plane half negative Poisson's ratio (H-NPR) with the fundamental mechanism clarified. These outstanding performances endow binary pentagonal materials with excellent application prospects.

Prediction of two-dimensional CP3 as a promising electrode material with a record-high capacity for Na ions.

Borophene with a maximum Li/Na capacity of 1984 mA h g-1 (nanoscale 2016, 8, 15 340-15 347) has shown the highest capacity among two-dimensional (2-D) anode materials identified so far. Herein, we report the record break for Na-ion using a newly proposed 2-D material, namely, CP3. We fully investigated Li- and Na-ion adsorption and diffusion processes on a CP3 monolayer. We found that the material can enable stable Li/Na adsorption considering charge accumulation on CP3 surfaces. The ion diffusion barriers for Li and Na were identified to be 98 meV and 356 meV, respectively. These values were comparable or smaller than those of the typical high-capacity electrode materials such as borophene. Most remarkably, the maximum Na capacity in CP3 monolayer can reach up to 2298.9 mA h g-1, which breaks the value recorded using borophene (1984 mA h g-1). Our work highly promises that the 2-D CP3 material could serve as an outstanding electrode material for Na-ion batteries with an extremely high storage capacity.