Ultrafast Photonic Applications Research Articles

Hot electron dynamics in ultrafast multilayer epsilon-near-zero metamaterials

Realizing remarkable tunability in optical properties without sacrificing speed is critical to obtain all optical ultrafast devices. In this work, we investigate the ultrafast temporal behavior of optically tunable epsilon-near-zero (ENZ) metamaterials, operating in the visible spectral range. To perform this the ultrafast dynamics of the hot electrons is acquired by femtosecond pump-probe spectroscopy and studied based on two-temperature model (2TM). We show that pumping with femtosecond pulses changes the effective permittivity of the metamaterial more than 400 %. This significant modulation is more pronounced in ENZ region and we confirm this by the 2TM. The realized ultrafast modulation in effective permittivity, along with the ultrashort relaxation time of 3.3 ps, opens a new avenue towards ultrafast photonic applications.

Nonlinear optical properties of anisotropic two-dimensional layered materials for ultrafast photonics

Abstract The discovery of graphene has intrigued the significant interest in exploring and developing the two-dimensional layered materials (2DLMs) for the photonics application in recent years. Unlike the isotropic graphene, a number of 2DLMs possess the in-plane anisotropic crystal structure with low symmetry, enabling a new degree of freedom for achieving the novel polarization-dependent and versatile ultrafast photonic devices. In this review article, we focus on the typical anisotropic 2DLMs including BP, ReS2, ReSe2, SnS, and SnSe and summarize the recent development of these anisotropic 2DLMs in the pulsed laser and the optical switch applications. First, we introduce the fabrication methods as well as the material characterization of the anisotropic 2DLMs by analyzing the polarized Raman configuration. Second, we discuss the anisotropic nonlinear optical properties of the anisotropic 2DLMs and concentrate on the anisotropic nonlinear absorption response. Next, we sum up state of the art of the anisotropic 2DLMs in the application of pulse lasers and optical switches. This review ends with perspectives on the challenge and outlook of the anisotropic 2DLMs for ultrafast photonics applications.

Graphdiyne‐Polymer Nanocomposite as a Broadband and Robust Saturable Absorber for Ultrafast Photonics

AbstractAs a rising star of all‐carbon nanomaterial, graphdiyne (GDY) has a direct natural bandgap and features strong light–matter interaction, large optical absorption, superior chemical and optical stability, indicating its broad prospects in the field of photonics and optoelectronics. Herein, the broadband nonlinear absorption and transient absorption characteristics of GDY from visible to infrared region has been studied for the first time, and its promising application in ultrafast photonics has been explored. The large nonlinear absorption coefficient (> −1 cm GW−1), low saturation intensity (<13 GW cm−2), and ultrafast relaxation time (<30 ps) of GDY are demonstrated, which indicates the outstanding potential of GDY in photonics among the emerging novel nonlinear optical (NLO) materials. The GDY is mixed with polyvinylpyrrolidone (PVP) to prepare the GDY–PVP nanocomposite, which further improved the stability of GDY. By using the GDY–PVP nanocomposite as saturable absorption material, ultrashort pulse lasers with pulse duration of 385.5 ps and 688 fs are obtained at 1 and 1.5 µm, respectively. This work reveals the excellent nonlinear optical properties of GDY and lays a foundation for its development in advanced nanophotonic devices.

Ultrafast photonics of two dimensional AuTe2Se4/3 in fiber lasers

The exploration of promising nonlinear optical materials, which allows for the construction of high-performance optical devices in fundamental and industrial applications, has become one of the fastest-evolving research interests in recent decades and plays a key role in the development and innovation of optics in the future. Here, by utilizing the optical nonlinearity of a recently synthesized, two dimensional material AuTe2Se4/3 prepared by the self-flux method, a passively mode-locked fiber laser operating at 1557.53 nm is achieved with 147.7 fs pulse duration as well as impressive stability (up to 91 dB). The proposed mode-locked fiber laser reveals superior overall performance compared with previously reported lasers which are more widely studied in the same band. Our work not only investigates the optical nonlinearity of AuTe2Se4/3, but also demonstrates its ultrafast photonics application. These results may stimulate further innovation and advancement in the field of nonlinear optics and ultrafast photonics.

Observation of various bound states based on a graded index multimode fiber saturable absorber

The formation of stable single solitons and various bound states based on a graded index multimode fiber (GIMF) is investigated experimentally for the first time. Taking advantage of the principle of nonlinear multimode interference, a 143.5 cm length of GIMF is bent and two polarization controllers are used to achieve stable passive mode-locking. The spectrum of single solitons centers on 1590.5 nm with a spectral bandwidth of 3.2 nm. 976 fs of pulse duration is obtained at the repetition rate of 14.31 MHz. The maximum output power of 14.572 mW with a corresponding single-pulse energy of 1 nJ is achieved. Meanwhile, all kinds of bound states, including soliton pairs, soliton triplets, soliton quartets, and multiple-pulse bound states with different pulse separations are realized. The bound soliton with two pulses obtained has a fixed pulse interval as the incident pump power is increased. Our results suggest that a GIMF might be employed as an effective saturable absorber for ultrafast photonics applications.

Layered AuTe2Se4/3 for a stable nanosecond Q-switched fiber laser

Q-switched fiber lasers based on saturable absorbers have received some attention in remote sensing, industrial manufacturing and optical communications due to the beam quality, compact structure, flexible application, low cost and good compatibility. In this paper, a stable nanosecond Q-switched fiber laser at 1560.6 nm is achieved by using the layered AuTe2Se4/3. The AuTe2Se4/3 saturable absorber is formed by placing AuTe2Se4/3 between two fiber ferrules. The Q-switched operation is demonstrated due to the optical nonlinearity of AuTe2Se4/3. The repetition rate of the achieved laser covers the range of 105.6 kHz–192.4 kHz. The output power and pulse energy of them are 20.7 mW and 107.58 nJ, respectively. The pulse duration of the stable Q-switched operation maintains at 630.9 ns with the signal to noise ratio of 60 dB. The impressive performance of AuTe2Se4/3 may have applications in ultrafast photonics.

Ultrafast optical switches and pulse lasers based on strong nonlinear optical response of plasmon nanostructures

Nonlinear optical (NLO) effects are ubiquitous in the interaction of light with different materials. However, the NLO responses of most materials are inherently weak due to the small NLO susceptibility and the limited interaction length with the incident light. In plasmonic nanostructures the optical field is confined near the surface of the structures, so that the electromagnetic field is greatly enhanced in a localized fashion by spectral resonance. This effect results in the enhancement of light-matter interaction and NLO response of the material. Ultrafast pulse lasers have been widely used in optical communication, precise measurement, biomedicine, military laser weapons and other important fields due to their excellent performances. Although commercial lasers become very matured, they can achieve ultra-high peak power and ultra-short pulse width and ultra-high repetition rate, but the ultra-fast pulses in the mid-to-far infrared band are seldom studied, so finding a saturable absorber material with excellent performance is of great significance for developing the pulsed lasers. In this paper, we review the recent research progress of the applications of exiton nanostructure in ultrafast optical switches and pulse lasers based on noble metal and non-noble metals. The metallic system mainly refers to gold and silver nanoparticles. For non-noble metals, we mainly introduce our researches of chalcogenide semiconductor, heavily doped oxide and titanium nitride. A variety of wide bandgap semiconductors can exhibit metal-like properties through doping. Since doping can form free carriers, when their size is reduced to a nanometer scale, they will show the characteristics of local surface plasmon resonance, thus realizing ultra-fast nonlinear optical response, and the concentration of doped carriers cannot reach the level of metal carriers, thus being able to effectively reduce the inter-band loss caused by excessively high carriers. Through pump probe detection and Z-scan testing, we found that these plasmonic nanostructures exhibit ultrafast NLO response in tunable resonance bandwidth, which has been utilized as a working material for developing the optical switch to generate the pulsed laser with duration down to a femtosecond range. These results take on their potential applications in ultrafast photonics. Finally, we make a comparison of the pros and cons among different plasmonic materials and present a perspective of the future development.

2D materials towards ultrafast photonic applications.

Having accomplished progress in the versatile battlefields of optics, electronics, catalysis, etc., two-dimensional (2D) materials are now venturing and excelling in yet another arena of ultrafast photonics, a rapidly developing field encompassing a large range of important applications including optical modulation through optical limiting/mode-locking, photodetectors, optical communications, integrated miniaturized all-optical devices and so on. Our group has been devoted to building the arsenal of 2D materials with large third-order nonlinearities, including transition metal dichalcogenides (TMDs), carbon nitride, single-element materials from Group 15, 2D hybrids and vdW heterostructures. In particular, we explore their origin of nonlinear optical responses from the aspect of excited state dynamics using time-resolved spectroscopic techniques such as femtosecond transient absorption spectroscopy. In this review, we propose the roadmap for screening 2D materials for ultrafast photonics through focusing on the third-order nonlinear optical properties of 2D materials and corresponding applications, and then performing mechanistic investigations via time-resolved spectroscopy and calculations, which in turn provide feedback to further guide the fabrication of 2D materials. We offer our own insights on the future trends for the development and theoretical calculations of 2D materials/devices in the final part of Perspectives.

Band structure tuning of α-MoO3 by tin intercalation for ultrafast photonic applications.

van der Waals (vdW) transition metal oxides have attracted extensive attention due to their intriguing physical and chemical properties. However, primary drawbacks of these materials are the lack of band structure tunability and substandard optical properties, which severely hinder their implementation in nanophotonic applications. Atomic intercalation is an emerging structural engineering approach for two-dimensional vdW materials to engineer the atomic structure and modify the optical properties, thereby broadening their range of applications. Herein, we synthesized tin-intercalated ultrathin α-MoO3 (Sn-MoO3) nanoribbons via chemical intercalation method and then investigated the broadband nonlinear optics (NLO) of stable few-layer α-MoO3 by performing a Z-scan laser measurement and femtosecond-resolved transient absorption (TA) spectroscopy. Sn-MoO3 showed a stable structure of Mo-O-Sn-O-Mo and a shorter relaxation time than pristine MoO3, indicating the accelerated recombination process of electrons and holes. Furthermore, Sn-MoO3 nanoribbons were used as an optical saturable absorber for ultrafast photonics; a highly stable femtosecond laser with a pulse width of 467 fs was generated from a single-mode fiber in the telecommunication band (1550 nm). These results indicate that atomic intercalation is an effective way to modulate the band structure and nonlinear optical properties of α-MoO3, which hold a great potential in the generation of ultrafast mode-locked laser pulses for optical communication technologies.

2D Ductile Transition Metal Chalcogenides (TMCs): Novel High‐Performance Ag2S Nanosheets for Ultrafast Photonics

AbstractAs a member of the 2D transition metal chalcogenides (TMCs), 2D Ag2S with large absorption coefficient, broadband optical response, extraordinary metal‐like ductility, high plastic deformation strain, unique layered structure, and very low solubility has offered a platform for achieving tremendous success in various fields. Great achievements have been made in exploring the linear optical, mechanical, and photoelectric properties of 2D Ag2S nanosheets. However, the nonlinear optical properties of 2D Ag2S nanosheets and their applications in ultrafast photonics have not been studied. The saturable absorption property of 2D Ag2S nanosheets is experimentally illustrated at the telecommunication band with a large optical modulation depth of 15.5% and a low saturable intensity of 250.4 MW cm−2. Moreover, the 229th harmonic soliton molecule with repetition rate of 1.1 GHz based on 2D Ag2S saturable absorber is realized at the telecommunication band for the first time to the authors' knowledge. Thankfully, 2D Ag2S nanosheets display excellent nonlinear optical properties suggesting that this may open a new door for advanced photonic devices based on 2D Ag2S nanosheets.

MoS2/graphene heterostructure incorporated passively mode-locked fiber laser: from anomalous to normal average dispersion

Two-dimensional (2D) nanomaterials for ultrafast photonic applications have attracted significant attention in recent years. 2D nanocomposites are of great interest because of their capability to combine the merits of each nanomaterial. In this work, we have demonstrated erbium-doped mode-locked fiber lasers that incorporate MoS2/graphene heterostructure based saturable absorbers (SAs) from anomalous to normal average dispersion for the first time. The modulation depth, the saturation intensity, and the non-saturable absorption of the MoS2/graphene heterostructure are measured to be 12.4%, 12.7 MW/cm2 and 28%, respectively. By incorporating this particular MoS2/graphene heterostructure based SA, the mode-locked fiber lasers can produce stable pulse trains at anomalous, near-zero, and normal average dispersion. At an anomalous average dispersion of -0.181 ps2, the Kelly sidebands are found to be superimposed on the optical spectrum, and a stable soliton pulse train has been measured with a signal-to-noise ratio of ∼73 dB in the radio frequency spectrum. At a near-zero average dispersion of -0.082 ps2, a Gaussian-like optical spectrum has been observed where the narrowest pulse width is ∼837 fs. At normal average dispersion of + 0.041 ps2, the steep-edge optical spectrum has been produced, indicating that dissipative solitons have been generated. The obtained results prove that a MoS2/graphene heterostructure is an ideal SA in mode-locked fiber lasers for ultrashort pulse generation from anomalous to normal average dispersion.

Third-order optical nonlinearity of the CuCo0.5Ti0.5O2 nanostructure under 120 fs laser irradiation.

Recently, titanium-based nanostructures with high nonlinear optical properties have found use in ultrafast photonic system applications. Here, we report a study of the third-order nonlinear optical property of the ${{\rm CuCo}_{0.5}}{{\rm Ti}_{0.5}}{{\rm O}_2}$CuCo0.5Ti0.5O2 (CCoTO) nanostructure synthesized via a simple chemical route. The 40-70 nm CCoTO nanoparticles with centrosymmetric crystalline structure show strong absorption in the 325-850 nm wavelength range due to the presence of different crystalline phases and surface vacancies. A Z-scan technique is used to study the electronic third-order nonlinearity of the synthesized nanoparticles, where a low-repetition-rate 120 fs laser source is employed to minimize thermal agitation-related nonlinearity. The CCoTO nanoparticles possess high surface defects due to oxygen- and copper-related vacancies, which are able to enhance the exciton oscillator strength resulting from the high value of third-order optical nonlinearity. The estimated values of nonlinear refractive index (${n_2}$n2) and nonlinear absorption coefficient ($\beta $β) of the CCoTO are $ - {1.24}\; \times \;{{10}^{ - 15}}$-1.24×10-15 and ${3.79} \times {{10}^{ - 11}}$3.79×10-11, respectively, under ${188}\,\,{{\rm GW/cm}^2}$188GW/cm2 incident intensity. The intensity-dependent nonlinear optical property of the synthesized nanoparticles is also studied under different incident laser irradiation (62.7, 93, and ${188}\,\,{{\rm GW/cm}^2}$188GW/cm2). In the two-photon absorption (TPA)-dominated third-order nonlinear optical process, the values of ${n_2}$n2 and $\beta $β of CCoTO are increased with intensifying the incident laser irradiation. The obtained high value of third-order optical nonlinearity of the synthesized nanostructure can be exploited in optical power limiters, pulse power reshaping, and optical switching applications.

Q-Switched Erbium-doped Fiber Laser Based on Silicon Nanosheets as Saturable Absorber

Silicon nanosheets-polyvinyl alcohol (Si-PVA) film was prepared and employed as a SA in obtaining an erbium-doped Q-switched laser. To the best of our knowledge, this is the first time that few-layer silicon nanosheets has been used as a SA to achieve a Q-switched erbium-doped fiber laser. We have demonstrated passively Q-switched operation with an average output power of 0.89 mW, a peak pulse power of 6.52 mW, and a pulse width of 2.32 μs with a repetition rate of 58.7 KHz. The results indicate that silicene owns excellent nonlinear properties and extensively potential applications in ultrafast photonics.

SnS Nanosheets for 105th Harmonic Soliton Molecule Generation

AbstractAs a member of the 2D group IV monochalcogenides (MX; M = Sn, Ge; X = S, Se), SnS has attracted great interest due to its outstanding optical, electrical, and optoelectronic properties. Especially, SnS nanosheets material have a large absorption coefficient and high photoelectric conversion efficiency, it can be potentially used in optical modulators, saturable absorbers, solar cells, supercapacitors, and other optical devices. However, the nonlinear optical properties of SnS nanosheets and their applications in ultrafast photonics are seldom studied. In this paper, the nonlinear optical properties of SnS nanosheets have been characterized through a dual‐balance detection system, whose modulation depth is 5.8%. More importantly, 105th harmonic soliton molecule based on SnS saturable absorbers has been realized for the first time to the authors’ knowledge. A compact mode‐locked fiber laser with a pulse duration of 1.02 ps and a repetition rate of 459 MHz is realized near 1.5 µm. It is demonstrated that SnS nanosheets have outstanding nonlinear properties and play an extremely important role in the field of ultrafast photonics.

Passively mode-locked Yb-doped all-fiber oscillator operating at 979 nm and 1032 nm with the single wall carbon nanotubes as SA

We report on an all-fiber passively mode-locked ytterbium-doped (Yb-doped) fiber laser with single polymer composite saturable absorber (SA) based on the single wall carbon nanotubes (SWCNTs) with diameters of 0.8 nm. The SWCNTs are mixed with sodium carboxymethylcellulose (NaCMC) to form SWCNTs SA film. The non-saturable absorption coefficient, modulation depth and saturable intensity of the SWCNTs SA are measured to be 29%, 12% and 0.12 MW/cm2, respectively. By using the SWCNTs as SA, a passively mode-locked Yb-doped fiber laser has been achieved at 979 nm with pulse duration of 228 ps and repetition rate of 15.47 MHz. To our knowledge, this is the first time ever reported in an all-fiber Yb-doped mode-locked fiber laser using SWCNTs as SA at 980 nm, which overcome the difficulties of low gain and serious re-absorption effects in the quasi-three-level system. In order to verify the saturation absorption performance of the SWCNTs, mode-locking experiment in quasi-four-level system is also carried out around 1030 nm and the stable and uniform mode-locked pulses are obtained. A mode-locked fiber laser at 1032 nm is realized with a pulse width of 485 ps and repetition rate of 8.78 MHz. It is sufficiently manifested that SWCNTs with diameters of 0.8 nm have excellent saturable absorption performance as optical mode-locking elements in ultrafast photonic applications.

Passively Q-switched Ytterbium-doped fiber laser based on broadband multilayer Platinum Ditelluride (PtTe2) saturable absorber

Two-dimensional (2D) layered Platinum Ditelluride (PtTe2), a novel candidate of group 10 transition-metal dichalcogenides (TMDs), which provides enormous potential for pulsed laser applications due to its highly stable and strong nonlinear optical absorption (NOA) properties. PtTe2 saturable absorber (SA) is successfully fabricated with firstly demonstrated the passively Q-switched laser operation within a Yb-doped fiber laser cavity at 1066 nm. Few layered PtTe2 is produced by uncomplicated and cost-efficient ultrasonic liquid exfoliation and follow by incorporating into polyvinyl alcohol (PVA) polymer to form a PtTe2-PVA composite thin film saturable absorber. The highest achieved single pulse energy is 74.0 nJ corresponding to pulse duration, repetition rate and average output power of 5.2 μs, 33.5 kHz and 2.48 mW, respectively. This work has further exploited the immeasurable utilization potential of the air stable and broadband group 10 TMDs for ultrafast photonic applications.

Mode-locked ytterbium-doped fiber laser based on offset-spliced graded index multimode fibers

We developed a passively mode-locked ytterbium-doped fiber laser based on offset-spliced graded index multimode fibers (OS-GIMFs), where OS-GIMFs play the role of saturable absorbers. The modulation depth of the saturable absorber is 15.28%, which significantly improves the performance of saturable absorber by adopting offset-spliced method. A stable mode-locked pulse train at 1040 nm with a 27.32 MHz repetition rate is generated when pump power reaches the threshold. The output pulse-width is a ∼11 ps and can be compressed to 980 fs by using a pair of gratings. The fiber laser is superior in terms of low cost, high damage threshold, and easy fabrication; thus, it has potential for application in ultrafast photonics and industrial processing.

Sensitive saturable absorber and optical switch of epsilon-near-zero medium

We investigated the nonlinear absorption and optical switching properties of a typical Epsilon-near-zero (ENZ) thin film made of indium tin oxide (ITO) in the near-infrared region. ITO has a low saturable absorption intensity with the advantages of electric field enhancement. The Q-switching performance in the near-infrared region was demonstrated with the ITO as a passive Q-switcher in the bulk lasers for the first time, to the best of our knowledge. This work shows that ENZ materials have promising applications in ultrafast photonics and may be helpful for the development of further nonlinear optoelectronic devices.

Generation of Bright/Dark Pulses in an Erbium-Doped Fiber Laser Mode-Locked With Glycerin

A passively mode-locked Erbium-doped fiber laser using glycerin as a saturable absorber (SA) has been proposed and experimentally demonstrated. The glycerin-SA is made of two fiber ferrules matched into a cannula, and the gap between them is filled with glycerin. Such an SA incorporated into the cavity leads to stable mode-locking operation. The generation of both bright and dark pulses is observed. The bright pulse is centered at 1568.92 nm, with a repetition rate of 23.08 MHz, the maximum output power of 33.5 mW, duration of 3.57 ps, and a signal-to-noise ratio of 55.6 dB, which exhibits good performance on self-starting and stability. Moreover, the dark pulses with the same repetition rate are achieved by adjusting the polarization state properly. The dark pulses are contributed to generating the domain-wall solitons induced by the orthogonal polarization coupling. The effectiveness of glycerin-SA as a mode-locker is validated. Considering its superiority in terms of low cost, easy fabrication, and non-volatile feature, we believe that glycerin-SA can be found many practical applications in ultrafast photonics.

Synthesis of high quality silver nanowires and their applications in ultrafast photonics.

Silver nanowires are widely used in catalysts, surface enhanced Raman scattering, microelectronic equipment, thin film solar cells, microelectrodes and biosensors for their excellent conductivity, heat transfer, low surface resistance, high transparency and good biocompatibility. However, the optical nonlinearity of silver nanowires has not been further explored yet. In this paper, three silver nanowire samples with different concentrations are produced via a typical hydrothermal method. Their applications to fiber lasers are implemented to prove the optical nonlinearity of silver nanowires for the first time. Based on three kinds of silver nanowires, the mode-locked operation of fiber lasers is successfully realized. Moreover, the fiber laser based on the silver nanowire with a concentration of 2 mg/L demonstrates the shortest pulse duration of 149.3 fs. The experiment not only proves the optical nonlinearity of silver nanowires, but also has some enlightenment on the selection of the optimum concentration of silver nanowires in the consideration of ultrashort pulse output.