Solvent-stabilized few-layer violet phosphorus and its ultrafast nonlinear optics

Featured with high thermal decomposition temperature and layered structure,violet phosphorus(VP)offers an unparalleled stable allotrope of phosphorus to demonstrate the optoelectronic device and photonics elements with high performance at the nanoscale.Here,we report few-layer and hundreds of nanometer-sized VP with robust stability in different solvents and ambient conditions by ultrasound-assisted liquid phase exfoliation approach.For the first time,the ultrafast carrier dynamics and thirdorder nonlinear optical response of VP were investigated.Sub-picosecond timescale ultrafast carrier dynamic and ultrafast nonlinear saturable absorption of VP were demonstrated.Our findings demonstrated that VP possessed a promising potential for use in ultrafast nonlinear photonic applications such as saturable absorbers and optical switches.

Ultrafast third-order optical nonlinearity of several sandwich-type phthalocyaninato and porphyrinato europium complexes

The third-order optical nonlinearity of two sandwich-type phthalocyaninato and porphyrinato europium com- plexes, including double- and triple-deckers （Eu[Pc（OC5H11）8]2, Eu2（Pc）（TPP）2, Pc=phthalocyanine, TPP=5, 10, 15, 20- tetraphenylporphyrinate）, was investigated by using the femtosecond time-resolved optical Kerr gate method at 830 nm wavelength. Their second-order hyperpolarizability is estimated to be 0.74× 10^-30esu and 3.0× 10^-3esu respectively. This exhibits an evident enhancement in comparison with 0.47×10^-30esu for one-decker Eu（Pc）（acac） （acac=acetylacetonate）, which is also measured under the same conditions. The enhancement is attributed to the introduction of lanthanide metal to the large π-conjugated system, intermacrocycle interaction and two-photon resonance etc.

Table-top optical parametric chirped pulse amplifiers: past and present

The generation of power-and wavelength-scalable few optical cycle pulses remains one of the major challenges in modern laser physics.Over the past decade,the development of table-top optical parametric chirped pulse amplificationbased systems was progressing at amazing speed,demonstrating excellent performance characteristics in terms of pulse duration,energy,peak power and repetition rate,which place them at the front line of modern ultrafast laser technology.At present,table-top optical parametric chirped pulse amplifiers comprise a unique class of ultrafast light sources,which currently amplify octave-spanning spectra and produce carrier-envelope phase-stable,few optical cycle pulses with multi-gigawatt to multi-terawatt peak powers and multi-watt average powers,with carrier wavelengths spanning a considerable range of the optical spectrum.This article gives an overview on the state of the art of table-top optical parametric chirped pulse amplifiers,addressing their relevant scientific and technological aspects,and provides a short outlook of practical applications in the growing field of ultrafast science.

Polarization-dependent ultrafast optical nonlinearities of N,N-dimethylformamide at 400 nm

Ultrafast optical nonlinearities of N,N-dimethylformamide(DMF)are studied by using polarized light at 400 nm.Both nonlinear refraction(NLR)and stimulated Rayleigh-wing scattering(SRWS)depend on the polarization state of incident beam,while two-photon absorption(TPA)changes negligibly with polarization state.The polarization dependence of SRWS originates from that of NLR via self-focusing effect.Third-order susceptibility elements of DMF were determined,and a method to distinguish the multi-photon absorption signal from SRWS in Z-scan is provided.These results are helpful for the nonlinear optical research of the novel materials dissolved in DMF.

Low temperature enhancement of alignment-induced spectral broadening of femtosecond laser pulses

The propagation of femtosecond laser pulses in N2-filled hollow fibers is studied both theoretically and experimentally. The laser pulse aligns the N2 molecules and changes the refractive index, which meanwhile modulates the spectrum of the pulse in turn. The dependence of the spectral modulation on the gas temperature is investigated. We find that both spectral broadening and frequency red-shift are enhanced at low temperature. The degree of enhancement is found to be dependent on the pulse duration. Based on our findings, we propose a method for femtosecond pulse spectral broadening and few-cycle pulse generation via the molecular alignment.

Broadband ultrafast nonlinear optical response of few-layers graphene: toward the mid-infrared regime

Gapless linear energy dispersion of graphene endows it with unique nonlinear optical properties, including broadband nonlinear absorption and giant nonlinear refractive index. Herein, we experimentally observed that fewlayers graphene has obvious nonlinear absorption and large nonlinear refraction, as investigated by the Z-scan technique in the mid-infrared（mid-IR） regime. Our study may not only, for the first time to our knowledge, verify the giant nonlinear refractive index of graphene(～10-7cm2∕W) at the mid-IR, which is 7 orders of magnitude larger than other conventional bulk materials, but also provide some new insights for graphene-based mid-IR photonics,potentially leading to the emergence of several new conceptual mid-IR optoelectronics devices.

2D noncarbon materials-based nonlinear optical devices for ultrafast photonics [Invited]

Ultrafast lasers play an important role in a variety of applications ranging from optical communications to medical diagnostics and industrial materials processing. Graphene and other two-dimensional（2D） noncarbon materials, including topological insulators（TIs）, transition metal dichalcogenides（TMDCs）, phosphorene, bismuthene, and antimonene, have witnessed a very fast development of both fundamental and practical aspects in ultrafast photonics since 2009. Their unique nonlinear optical properties enable them to be used as excellent saturable absorbers（SAs） that have fast responses and broadband operation, and can be easily integrated into lasers. Here, we catalog and review recent progress in the exploitation of these 2D noncarbon materials in this emerging field. The fabrication techniques, nonlinear optical properties, and device integration strategies of 2D noncarbon materials are first introduced with a comprehensive view. Then, various mode-locked/Q-switched lasers（e.g., fiber, solid-state, disk, and waveguide lasers） based on 2D noncarbon materials are reviewed. In addition, versatile soliton pulses generated from the mode-locked fiber lasers based on 2D noncarbon materials are also summarized. Finally, future challenges and perspectives of 2D materials-based lasers are addressed.

Ultrafast optical nonlinearity of blue-emitting perovskite nanocrystals

Perovskite nanocrystals(NCs) have strong nonlinear optical responses with a number of potential applications,ranging from upconverted blue-lasing to the tagging of specific cellular components in multicolor fluorescence microscopy. Here, we determine the one-photon linear absorption cross section of two kinds of blue-emitting perovskite NCs, i.e., CsPbCl_3 and CsPb(Cl_(0.53)Br_(0.47))_3, by utilizing femtosecond transient absorption spectroscopy. The wavelength-dependent nonlinear refraction and two-photon absorption have been measured at wavelengths from 620 to 720 nm by performing Z-scan measurements. The nonlinear optical responses of CsPb(Cl_(0.53)Br_(0.47))_3 are much more pronounced than those of CsPbCl_3 due to the larger structural destabilization of the former.

All-polarization-maintaining figure-eight Er-fiber ultrafast laser with a bidirectional output coupler in the loss-imbalanced nonlinear optical loop mirror

In this Letter, we report on a novel architecture for a self-starting mode-locked figure-eight erbium-doped fiber laser using a loss-imbalanced nonlinear optical loop mirror(NOLM) with a bidirectional output coupler. An allpolarization-maintaining structure is adopted. A 2 × 2 optical coupler with a splitting ratio of 50:50 is used at the junction to form an NOLM. Another coupler with a splitting ratio of 10:90 is introduced at one end of the fiber loop. The 10:90 coupler plays two roles: power attenuator and bidirectional output coupler. This architecture can achieve both large modulation depth and good self-starting ability simultaneously. With this architecture,the self-starting mode-locking operation is achieved easily with pump power above the threshold. The clockwise and counter-clockwise mode-locked output powers are 10.1 and 10.3 mW, respectively, with the repetition rate of 3.63 MHz. The spectral bandwidths of the clockwise and counter-clockwise mode-locked output pulses are 7.4 and 2.9 nm, and the corresponding pulse widths of the direct outputs are 530.6 fs and 1.55 ps, respectively.

Efficient generation of third harmonics in Yb-doped femtosecond fiber laser via spatial and temporal walk-off compensation

On the basis of a home-made femtosecond Yb-doped fiber laser, we designed a compact and efficient third harmonic generation scheme by a simple compensation plate of β-Ba B2O4 crystal. The compensation plate is optimized through its thickness and cutting angle to reverse both spatial and temporal walk-off. By optimizing the parameters of the compensation plate and incident light intensity, a maximum output of 2.23 W with a repetition rate of 1 MHz at 345 nm is obtained, which implies a conversion efficiency of 23% from the infrared to ultraviolet.

Two-channel, dual-beam-mode, wavelength-tunable femtosecond optical parametric oscillator

Optical vortices,which carry orbital angular momentum,offer special capabilities in a host of applications.A single-laser source with dual-beam-mode output may open up new research fields of nonlinear optics and quantum optics.We demonstrate a dual-channel scheme to generate femtosecond,dualwavelength,and dual-beam-mode tunable signals in the near infrared wavelength range.Dual-wavelength operation is derived by stimulating two adjacent periods of a periodically poled lithium niobate crystal.Pumped by an Yb-doped fiber laser with a Gaussian(lp?0)beam,two tunable signal emissions with different beam modes are observed simultaneously.Although one of the emissions can be tuned from1520 to 1613 nm with the Gaussian(ls?0)beam,the other is capable of producing a vortex spatial profile with different vortex orders(ls?0 to 2)tunable from 1490 to 1549 nm.The proposed system provides unprecedented freedom and will be an exciting platform for super-resolution imaging,nonlinear optics,multidimensional quantum entanglement,etc.

Femtosecond laser trapping dynamics of two-photon absorbing hollow-core nanoparticles

We investigate femtosecond laser trapping dynamics of two-photon absorbing hollow-core nanoparticles with different volume fractions and two-photon absorption(TPA)coefficients.Numerical simulations show that the hollow-core particles with low and high-volume fractions can easily be trapped and bounced by the tightly focused Gaussian laser pulses,respectively.Further studies show that the hollow-core particles with and without TPA can be identified,because the TPA effect enhances the radiation force,and subsequently the longitudinal force destabilizes the trap by pushing the particle away from the focal point.The results may find direct applications in particle sorting and characterizing the TPA coefficient of single nanoparticles.