Optical modification of nonlinear crystals for quasi-parametric chirped-pulse amplification.

Quasi-parametric chirped-pulse amplification (QPCPA), which features a theoretical peak power much higher than those obtained with Ti:sapphire laser or optical parametric chirped-pulse amplification, is promising for future ultra-intense lasers. The doped rare-earth ion used for idler dissipation is critical for effective QPCPA, but is usually not compatible with traditional crystals. Thus far, only one dissipative crystal of Sm3+-doped yttrium calcium oxyborate has been grown and applied. Here we introduce optical means to modify traditional crystals for QPCPA applications. We theoretically demonstrate two dissipation schemes by idler frequency doubling and sum-frequency generation with an additional laser. In contrast to absorption dissipation, the proposed nonlinear dissipations ensure not only high signal efficiency but also high small-signal gain. The demonstrated ability to optically modify crystals will facilitate the wide application of QPCPA.

Ultrabroad bandwidth of quasi-parametric amplification beyond the phase-matching limit.

Quasi-parametric amplification (QPA), a variant of optical parametric amplification, can release the phase-matching requirement owing to the introduction of idler dissipation, and thus may support ultrabroad bandwidth. Here we establish the gain-dispersion equation for QPA, which reveals the interplay of signal gain, idler dissipation and phase mismatch. The idler dissipation dramatically enhances the gain bandwidth, which breaks the limit set by phase matching. We theoretically demonstrate that QPA with strong dissipation allows high-efficiency few-cycle pulse amplification in those nonlinear crystals without a magic phase-matching solution.

Sub-picosecond tunable mid-infrared light source for driving high-efficiency optical rectification.

Optical rectification (OR) is a popular way to generate coherent terahertz radiation. Here, we develop a sub-picosecond mid-infrared (mid-IR) light source with a tailored wavelength and pulse duration for enhancing the OR efficiency. Numerical simulations for a LiNbO3-based OR with tilted pulse-front excitation are first conducted to determine the optimal parameters of pump wavelength and pulse duration, demonstrating that the OR efficiency pumped by 4-µm sub-picosecond (0.5-0.6 ps) pulses is approximately twice the value with 0.8-µm pump at the same conditions. Guided by the simulation results, we build a BaGa4Se7-based optical parametric chirped-pulse amplification system with 1030-nm thin-disk pump and broadband mid-IR seeds. The output performances of >200-µJ pulse energy, ∼600-fs pulse duration and 1-kHz pulse repetition rate are achieved in a spectral range tunable from 3.5 to 5 µm. The large energy scalability and high parameter tunability make the light source attractive to high-efficiency OR in various materials.

Serial synthesis of mid-infrared optical parametric amplifiers for enlarging a gain bandwidth.

Broadband optical parametric amplifiers (OPAs) require a group-velocity matching between the signal and the idler. For mid-infrared OPAs, however, the group-velocity matching is usually difficult to meet, rendering a limited gain bandwidth. Here, we report a serial synthesis of bandwidth-limited OPAs to provide a broad gain bandwidth. In a proof-of-principle experiment, two mid-IR OPAs based on KTA crystals with different phase-matching angles are sequentially employed to amplify different spectral regions of a broad seed pulse centered at 3.1 µm. Compared to the traditional two-stage OPA, here the gain bandwidth is nearly doubled, resulting in a much shorter compressed pulse. Such a serial synthesis approach, independent of a nonlinear crystal and an interaction wavelength, particularly suits for enlarging the gain bandwidth of OPAs when broadband amplification is impossible to achieve by a single crystal.

Generation of an ultrashort pulse train through ultrafast parity-time symmetry switching.

We propose a scheme for the direct generation of an ultrashort pulse train as well as the further compression of pulsed lasers based on the nonlinearity inherent to parity-time (PT) symmetric optical systems. Implementation of optical parametric amplification in a directional coupler of χ(2) waveguides enables ultrafast gain switching through pump-controlled breaking of PT symmetry. We theoretically demonstrate that pumping such a PT symmetric optical system with a periodically amplitude-modulated laser enables periodic gain switching, which can directly convert a continuous-wave signal laser into a train of ultrashort pulses. We further demonstrate that by engineering the PT symmetry threshold, an apodized gain switching that enables the production of ultrashort pulses without side lobes. This work suggests a new approach for exploring the non-linearity inherent to various PT symmetric optical structures to extend optical manipulation capabilities.

Mid-infrared quasi-parametric chirped-pulse amplification based on Sm:LGN crystals.

We numerically demonstrate highly efficient mid-infrared quasi-parametric chirped-pulse amplification (QPCPA) based on a recently developed Sm3+-doped La3Ga5.5Nb0.5O14 (Sm:LGN) crystal. At pump wavelength around 1 µm, the broadband absorption of Sm3+ on idler pulses can enable QPCPA for femtosecond signal pulses centered at 3.5 or 5 µm, with a conversion efficiency approaching the quantum limit. Due to suppression of back conversion, such mid-infrared QPCPA exhibits robustness against phase-mismatch and pump-intensity variation. The Sm:LGN-based QPCPA will provide an efficient approach for converting currently well-developed intense laser pulses at 1 µm to mid-infrared ultrashort pulses.

Study on Wastewater Demulsification Technology of Crude Oil in Xinjiang Oilfield.

The Second Oil Production Plant of Xinjiang Oilfield produces a large amount of highly emulsified crude oil, which has a serious impact on the subsequent oil-water separation. At present, the concentration of demulsifier has increased to 2000 mg/L, but the demulsification effect is still poor. In this paper, the source and physical properties of highly emulsified crude oil are investigated firstly. The results show that highly emulsified crude oil is composed of three kinds of liquid: (1) conventional water flooding (WF); (2) chemical flooding (CF); (3) fracturing backflow fluid (FB). Among them, high zeta potential, low density difference, high viscosity, and small emulsion particles are responsible for the difficulty in the demulsification of the WF emulsion, while the high pH value is the reason why the CF emulsion is difficult to demulsify. Therefore, systematic experiments were implemented to investigate the optimal demulsification approach towards the three liquids above. As for the WF emulsion, it was necessary to raise the temperature to 70 °C and the concentration of the demulsifier to 200 mg/L. Moreover, it was only necessary to add 200 mg/L of demulsifier to break the CF emulsion after adjusting the pH value to 7, while no extra treatments were needed to break the FB emulsion. We hope this study can provide a new insight for the treatment of emulsions in the later stage of oilfield development.

Efficient generation of mid-infrared few-cycle pulses by the intrapulse difference-frequency generation in YCOB.

Yttrium calcium oxyborate (YCOB) crystals have been widely applied for generating intense near-infrared laser pulses by optical parametric amplification. Here, we show that the YCOB crystals oriented in both the XZ and XY principal planes possess broadband phase-matching property of intrapulse difference-frequency generation in the mid-infrared region. Few-cycle pulses tunable from 2 to 4 µm are experimentally produced by using a 7.5-fs pump laser at 800 nm, in which the conversion efficiency can be as high as 2.5%. With a large-size crystal and high-power pump laser, intrapulse difference-frequency generation based on YCOB may provide a new route for directly producing intense few-cycle mid-infrared pulses.

Demonstration of 85% pump depletion and 10-6 noise content in quasi-parametric chirped-pulse amplification.

Full pump depletion corresponds to the upper limit of the generated signal photons relative to the pump pulse; this allows the highest peak power to be produced in a unit area of ultraintense laser amplifiers. In practical systems based on optical parametric chirped-pulse amplification, however, the typical pump depletion is only ~35%. Here, we report quasi-parametric chirped-pulse amplification (QPCPA) with a specially designed 8-cm-thick Sm:YCOB crystal that highly dissipates the idler and hence improves pump depletion. We demonstrate 56% QPCPA energy efficiency for an 810-nm signal converted from a 532-nm pump, or equivalently 85% pump depletion. As another advantage, such a record high depletion greatly suppresses the parametric superfluorescence noise in QPCPA to only ~1.5 × 10-6 relative to the amplified signal energy. These results pave the way to beyond the ten-petawatt peak power of the currently most intense lasers.

Red-diode-clad-pumped CW and mode-locked Er:ZBLAN fiber laser at 3.5†µm.

We report on a red-diode-clad-pumped continuous-wave (CW) and mode-locked Er:ZBLAN fiber laser at 3.5 µm for the first time. Numerical simulation shows that a heavily-doped Er:ZBLAN fiber is favorable for effective generation of 3.5 µm laser through 658 nm laser diode pumping. Using a 7.0 mol.% Er:ZBLAN fiber, CW output power of 203 mW was experimentally obtained at 3462 nm. By incorporating a home-made semiconductor saturable absorber mirror into the cavity, diode-pumped CW mode-locked 3.5 µm Er:ZBLAN fiber laser was first demonstrated with an average power of 19 mW, a pulse duration of 18.1 ps, and a repetition rate of 46 MHz. The research results show that red-diode-clad-pumping provides a simple and potential scheme for 3.5 µm CW and mode-locked Er:ZBLAN fiber laser.

Semiconductor saturable absorber mirror in the 3-5†µm mid-infrared region.

Semiconductor saturable absorber mirrors (SESAMs) have been regarded as a revolutionary technology for ultrafast mode-locked lasers, producing numerous landmark laser breakthroughs. However, the operating wavelength of existing SESAMs is limited to less than 3 µm. In this study, we create a 3-5 µm mid-infrared (MIR) SESAM by engineering an InAs/GaSb type-II superlattice. Bandgap engineering and the strong coupling between potential wells in a superlattice enable a broadband response of saturable absorption in the 3-5 µm spectral range. Using the fabricated SESAM, we realize a SESAM mode-locked Er:ZBLAN fiber laser at 3.5 µm, which delivers MIR ultrashort pulses with high long-term stability. The breakthrough of SESAM fabrication in the MIR will promote the development of MIR ultrafast coherent sources and related application fields.

Spatiotemporal couplings through a nonlinear phase in broadband optical parametric amplification.

Optical parametric chirped-pulse amplification (OPCPA) is prone to undesired spatiotemporal couplings. This Letter studies a family of OPCPA couplings resulting from the nonlinear phase shift induced by frequency-dependent phase mismatch. These OPCPA couplings manifest as pulse-front deformation, transversely varying pulse duration, and spectrally varying wavefront curvature, which are directly linked with the phase-mismatch dispersion terms. The numerical study in this Letter also reveals that the focused signal intensity severely degrades with increasing signal bandwidth and pump depletion.

2-MW peak-power pulses from a dispersion-managed fluoride fiber amplifier at 2.8 µm.

We report on a scheme of pulse amplification and simultaneous self-compression in fluoride fiber for generating a high-peak-power pulse at 2.8-µm wavelength. We find dispersion management plays a key role for the amplification and self-compression process. Through dispersion management with a Ge rod, pulse amplification and simultaneous pulse self-compression were realized in the small anomalous dispersion region. A 2-MW peak-power pulse was achieved through amplification and self-compression in Er:ZBLAN fiber, with pulse energy of 101 nJ and pulse duration of 49 fs. To the best of our knowledge, this is the highest peak power obtained from fluoride fiber at 2.8 µm, and will benefit a series of applications.

Pushing Optical Switch into Deep Mid-Infrared Region: Band Theory, Characterization, and Performance of Topological Semimetal Antimonene.

The existing pulsed laser technologies and devices are mainly in the infrared spectral region below 3 µm so far. However, longer-wavelength pulsed lasers operating in the deep mid-infrared region (3-20 µm) are desirable for atmosphere spectroscopy, remote sensing, laser lidar, and free-space optical communications. Currently, the lack of reliable optical switches is the main limitation for developing pulsed lasers in the deep mid-infrared region. Here, we demonstrate that topological semimetal antimonene possesses an ultrabroadband optical switch characteristic covering from 2 µm to beyond 10 µm. Especially, the topological semimetal antimonene shows a very low saturable energy fluence (only 3-15 nJ cm-2 beyond 3 µm) and an ultrafast recovery time of ps level. We also demonstrate stable Q-switching in fiber lasers at 2 and 3.5 µm by using topological semimetal antimonene as passive optical switches. Combined with the high environmental stability and easy fabrication, topological semimetal antimonene offers a promising optical switch that extends pulsed lasers into deep mid-infrared region.

Large magnitude, sign controllable, ultrafast group-velocity control via resonant cascaded nonlinearity in tandem.

Resonant cascaded nonlinearity (RCN) induced by optical parametric amplification (OPA) in a chirped quasi-phase-matching chip can be applied to control the group velocity of ultrafast lasers. However, the group delay produced in a single-stage OPA is limited to the pulse duration, and its sign cannot be altered. In this study, we propose a tandem RCN configuration with multiple OPA stages that can produce large-magnitude and sign-controllable group delays. The group delay produced in the multi-stage configuration is shown to be a linear superposition of each single-stage group delay. By virtue of the byproduct idler in the OPA process, the signal-group delay can be altered from positive to negative (and vice versa) with the same chip structure and pump condition. In the numerical simulation with two OPA stages, both a positive and negative group delay of six-fold pulse duration were achieved for 100-fs pulses at 1550 nm. A much larger group delay can be achieved by increasing the number of OPA stages.

Few-cycle pulses tunable from 3 to 7 µm via intrapulse difference-frequency generation in oxide LGN crystals.

An ultrashort mid-infrared (IR) source beyond 5 µm is crucial for a plethora of existing and emerging applications in spectroscopy, medical diagnostics, and high-field physics. Nonlinear generation of such sources from well-developed near-IR lasers, however, remains a challenge due to the limitation of mid-IR crystals. Based on oxide La3Ga5.5Nb0.5O14 (LGN) crystals, here we report the generation of femtosecond pulses tunable from 3 to 7 µm by intrapulse difference-frequency generation of 7.5 fs, 800 nm pulses. The efficiency and bandwidth dependences on pump polarization and crystal length are studied for both Type-I and Type-II phase-matching configurations. Maximum pulse energy of ∼10nJ is generated at 5.2 µm with a conversion efficiency of ∼0.14%. Because of the few-cycle pump pulse duration, the generated mid-IR pulses are as short as about three cycles. These results, to the best of our knowledge, represent the first experimental demonstration of LGN in generating mid-IR ultrashort pulses.

Generation of a mid-infrared femtosecond vortex beam from an optical parametric oscillator.

Mid-infrared femtosecond vortex beams generated by optical parametric oscillators (OPO) are reported for the first time, to the best of our knowledge. Order-tunable femtosecond Hermite-Gauss beams from the first to sixth order are produced from a synchronously pumped OPO and then converted into the corresponding first through sixth-order femtosecond vortex beams by a cylindrical lens mode converter. By slightly tuning the cavity length, the wavelength of the vortex beam can be continuously tunable in the range from 2323 to 2382 nm, and the pulse duration can be changeable from ${\sim}{400}\;{\rm fs}$∼400fs to ${\sim}{1.1}\;{\rm ps}$∼1.1ps. The work provides a flexible and reliable way to generate mid-infrared femtosecond vortex beams, and is of special significance for expanding the wavelength range of femtosecond vortex beams and their application fields.

Facile fabrication of 2D stanene nanosheets via a dealloying strategy for potassium storage.

In this work, a facile dealloying strategy was developed for the large scale fabrication of 2D stanene nanosheets under ambient conditions. The obtained stanene nanosheets exhibited typical 2D structure with a thickness of 4 nm and a lateral size of several micrometers. Furthermore, the 2D stanene nanosheets displayed outstanding performance in potassium storage.

Few-layer tin-antimony nanosheets: a novel 2D alloy for superior lithium storage.

A novel 2D alloy, free-standing few-layer SnSb nanosheets, is fabricated via a liquid-phase exfoliation approach. The resultant few-layer SnSb possesses ultrathin features (1-4 nm), large aspect ratio, largely exposed surfaces and a precise stoichiometric ratio between Sn and Sb (1 : 1). These few-layer SnSb nanosheets are systematically investigated for lithium storage, and exhibit a high reversible capacity of 694 mA h g-1, high rate capability and good cycling performance, as we expected.

Sum-frequency generation with femtosecond conical refraction pulses.

In this Letter, we study short-wavelength conical refraction (CR) via sum-frequency generation (SFG) in the femtosecond regime, a previously unaddressed topic. Based on biaxial crystal of KGd(WO4)2 whose dispersion of optical-axis orientation is negligible in near-IR, conventional femtosecond lasers at 800 and 1054 nm are transformed into CR beams, respectively. Femtosecond CR beams at 454 nm are generated via SFG with the near-IR CR beams. While the generated sum-frequency ring is typically incomplete, a full-ring distribution can be achieved by adopting Type-II SFG with a large phase mismatch. We find that the femtosecond sum-frequency ring under various phase-matching conditions evolves as typical CR beams.