Ultrafast Photonic Applications Research Articles

Fast and Slow Nonlinearities in Epsilon‐Near‐Zero Materials

AbstractNovel materials, with enhanced light–matter interaction capabilities, play an essential role in achieving the lofty goals of nonlinear optics. Recently, epsilon‐near‐zero (ENZ) media have emerged as a promising candidate to enable the enhancement of several nonlinear processes including refractive index modulation and harmonic generation. Here, the optical nonlinearity of ENZ media is analyzed to clarify the commonalities with other nonlinear media and its unique properties. Transparent conducting oxides as the family of ENZ media with near‐zero permittivity in the near‐infrared (telecom) band are focused on. The instantaneous and delayed nonlinearities are investigated. By identifying their common origin from the band nonparabolicity, it is shown that their relative strength is entirely determined by a ratio of the energy and momentum relaxation (or dephasing) times. Using this framework, ENZ materials are compared against the many promising nonlinear media that are investigated in literature and show that while ENZ materials do not radically outpace the strength of traditional materials in either the fast or slow nonlinearity, they pack key advantages such as an ideal response time, intrinsic slow light enhancement, and broadband nature in a compact platform making them a valuable tool for ultrafast photonics applications for decades to come.

Tin Telluride Quantum Dots as a Novel Saturable Absorber for Q‐Switching and Mode Locking in Fiber Lasers

AbstractTin telluride (SnTe) quantum dots (QDs) have attracted considerable interest in the optoelectronic field owing to their favorable properties over conventional QDs, such as a relatively large Bohr radius (95 nm) and low toxicity level. However, till date, the nonlinear optical properties and ultrafast photonics applications of SnTe QDs have remained unexplored. In this study, SnTe QDs with an average diameter of 74 nm (smaller than the Bohr radius of SnTe) are fabricated via a liquid‐phase exfoliation method. Consequently, the nonlinear saturable absorption properties of such SnTe QDs are explored by realizing a modulation depth of 2.2% and saturable intensity of 1.67 GW cm−2. The prepared QDs are then used as saturable absorber (SA) in the erbium‐doped fiber laser ring cavity system. Moreover, Q‐switched and mode‐locked laser pulses with a pulse width of 1.81 µs and 691 fs are generated, respectively. Harmonic mode‐locking with a high repetition rate of 62.1 MHz (fifth order) is experimentally realized. This is the first demonstration, to the authors’ knowledge, of using SnTe QDs‐SA for generating ultrafast laser pulses along with high‐repetition‐rate harmonic mode‐locking pulses. Therefore, this study will establish a new research scope for SnTe QDs in ultrafast photonics, nonlinear photonics, frequency combs, and optical communications.

Ultrashort pulse generation with MXene Ti3C2Tx embedded in PVA and deposited onto D-shaped fiber

A two-dimensional ternary metal carbide or/and nitride known as MXene, own countless merits in terms of electronic and optical properties. Owning to its fast optical-switching capability, strong saturable absorption, good optical damage tolerant, and gapless materials, it attracts a bunch of research efforts in an ultrafast photonics application. Here, the development of MXene-based SA devices for ultrafast laser in an erbium-doped fiber laser (EDFL) cavity was demonstrated. Two MXene Ti3C2Tx-based SAs were prepared, which were a thin film of MXene (MXene-film) and MXene deposited onto D-shaped fiber (MXene-DS), both revealed good nonlinear transmittance properties. The selective etching was used to provide a high-quality MXene powder, and the solution-casting method produced liquid MXene, which was dried to form a thin-film structure, and the liquid MXene was deposited onto the D-shaped fiber structure. The MXene-film and MXene-DS own a modulation depth of 1.22% and 3%, respectively. Later, both SA devices were incorporated into an EDFL cavity, and MXene-film initiated mode-locked (ML) at the low threshold pump of 44 mW, while MXene-DS produce ML at the pump power of 90 mW. The MXene-film generates a 3.64 ps ML with a pulse energy of 6.03 nJ, separately, MXene-DS initiates a 4.6 ps ML with a pulse energy of 7.69 nJ, highest ever reported among MXene-based SA. This demonstration provides a pathway toward high-energy mode-locking applications in the future.

Machine learning and applications in ultrafast photonics

Recent years have seen the rapid growth and development of the field of smart photonics, where machine-learning algorithms are being matched to optical systems to add new functionalities and to enhance performance. An area where machine learning shows particular potential to accelerate technology is the field of ultrafast photonics — the generation and characterization of light pulses, the study of light–matter interactions on short timescales, and high-speed optical measurements. Our aim here is to highlight a number of specific areas where the promise of machine learning in ultrafast photonics has already been realized, including the design and operation of pulsed lasers, and the characterization and control of ultrafast propagation dynamics. We also consider challenges and future areas of research. The potential of machine-learning application to the field of ultrafast photonics is reviewed, with key examples including pulsed lasers, and control and characterization of ultrafast propagation dynamics.

Temperature and Pulse-Energy Range Suitable for Femtosecond Pulse Transmission in Si Nanowire Waveguide

We experimentally measured the femtosecond pulse transmission through a silicon-on-insulator (SOI) nanowire waveguide under different temperatures and input pulse energy with a cross-correlation frequency-resolved optical gating (XFROG) measurement setup. The experimental results demonstrated that the temperature and pulse energy dependence of the Si photonic nanowire waveguide (SPNW) is interesting rather than just monotonous or linear, and that the suitable temperature and pulse-energy range is as suggested in this experiment, which will be valuable for analyzing the practical design of the operating regimes and the fine dispersion engineering of various ultrafast photonic applications based on the SPNWs. The research results will contribute to developing the SPNWs with photonic elements and networks compatible with mature complementary metal–oxide–semiconductors (CMOS).

Vanadium disulfide for ultrafast photonic application

The investigation of two-dimensional (2D) nonlinear optical materials offers a promising way to construct the high-performance optical devices in fundamental and industrial applications because of their rich distinct optoelectronic properties. Herein, by utilizing the liquid exfoliation method, vanadium disulfide (VS2) nanosheets are prepared and the thickness is measured to be 3.16 nm. In addition, we have fabricated the VS2-based optical device and the nonlinear optical property is characterized with modulation depth of 23.97%. By using VS2 as saturable absorber, a high stable passively mode-locking Er-doped fiber laser is obtained with pulse duration of 169 fs and the largest average output power of 70.5 mW. The slope efficiency is up to 7.9%. In comparison to recent results of mode-locking fiber lasers with 2D materials, the VS2-based fiber laser demonstrates better performance. To the best of our knowledge, this is the first example of using VS2 for generating femtosecond mode-locked laser pulse. Our experimental results not only reveal VS2 ultrafast photonics application, but also advance the high-performance applications for information science and nonlinear optics.

Zero-Dimensional MXene-Based Optical Devices for Ultrafast and Ultranarrow Photonics Applications.

In recent years, MXene has become a hotspot because of its good conductivity, strong broadband absorption, and tunable band gap. In this contribution, 0D MXene Ti3C2Tx quantum dots are synthesized by a liquid exfoliation method and a wideband nonlinear optical response from 800 to 1550 nm is studied, which have a larger nonlinear absorption coefficient β of –(11.24 ± 0.14) × 10–2 cm GW–1. The carrier dynamic processes of 0D MXene are explored with ultrahigh time resolution nondegenerate transient absorption (TA) spectroscopy, which indicates that the TA signal reaches its maximum in 1.28 ps. Furthermore, 0D MXene is used to generate ultrashort pulses in erbium or ytterbium‐doped fiber laser cavity. High signal‐to‐noise (72 dB) femtosecond lasers with pulse durations as short as 170 fs with spectrum bandwidth of 14.8 nm are obtained. Finally, an ultranarrow fiber laser based on 0D MXene is also investigated and has a full width at half maximum of only 5 kHz, and the power fluctuation is less than 0.75% of the average power. The experimental works prove that 0D MXene is an excellent SA and has a promising application in ultrafast and ultranarrow photonics.

ZrSe2 nanosheet as saturable absorber for soliton operations within an Er-doped passive mode-locked fiber laser.

Two-dimensional materials have extensively promoted the development of ultrafast photonics in the past decade. In our work, the saturable absorption properties of ZrSe2 were presented. The saturation intensity, modulation depth, and nonlinear absorption coefficient of the ZrSe2 saturable absorber (SA) are about 13.14MW/cm2, 6.09%, and 1.85∗10-1cm/GW. In the experiment, based on the ZrSe2 SA, two types of solitons were recorded. A conventional soliton with a pulse width of 985 fs and a three-pulse bound state soliton have been obtained. Our experiment reveals that ZrSe2 can be employed for generating multiple ultrafast soliton generations and possess promising application in ultrafast photonics.

Noise amplification in all-normal dispersion fiber supercontinuum generation and its impact on ultrafast photonics applications

Highly coherent and low-noise supercontinuum (SC) sources based on nonlinear spectral broadening of femtosecond pulses in all-normal dispersion (ANDi) fibers are attractive for many applications in ultrafast photonics. By simulating a real nonlinear pulse compression experiment, we numerically investigate the impact of shot noise and technical pump laser fluctuations on the quality and stability of single-cycle pulse generation and other multi-shot experiments based on the manipulation of the SC spectral phase. We find that for pump pulse durations of less than 600 fs, input relative intensity noise < 1 %, and correctly chosen fiber lengths, the initial fluctuations of the pump laser are at most amplified by a factor of three. We also show that the usual strong correlation between SC coherence and quality of the compressed pulses collapses in the presence of technical noise, and that in this situation the coherence is not a useful figure of merit to quantify pulse quality, noise amplification, or decoherence due to incoherent nonlinear dynamics. Our results highlight the very limited impact of technical pump laser noise on ANDi SC generation and are of practical relevance for many ultrafast photonics applications that require high-quality, low-noise SC sources.

Photon-generated carrier transfer process from graphene to quantum dots: optical evidences and ultrafast photonics applications

Graphene/III–V semiconductor van der Waals (vdW) heterostructures offer potential access to physics, functionalities, and superior performance of optoelectronic devices. Nevertheless, the lack of a bandgap in graphene severely restricts the controllability of carrier properties and therefore impedes its applications. Here, we demonstrate the engineering of graphene bandgap in the graphene/GaAs heterostructure via C and Ga exchange induced by the method of femtosecond laser irradiation (FLI). The coupling of the bandgap-opened graphene with GaAs significantly enhances both the harvest of photons and the transfer of photon-generated carriers across the interface of vdW heterostructures. Thus, as a demonstration example, it allows us to develop a saturable absorber combining a delicately engineered graphene/GaAs vdW heterostructure with InAs quantum dots capped with short-period superlattices. This device exhibits significantly improved nonlinear characteristics including <1/3 saturation intensity and modulation depth 20 times greater than previously reported semiconductor saturable absorber mirrors. This work not only opens the route for the future development of even higher performance mode-locked lasers, but the significantly enhanced nonlinear characteristics due to doping-induced bandgap opening of graphene by FLI in the vdW heterostructures will also inspire wide applications in photonic and optoelectronic devices.

All-polarization-maintaining, all-normal-dispersion mode-locked fiber laser with spectral filtering in a nonlinear optical loop mirror.

The spectral filtering effect is essential to dissipative dynamics in an all-normal-dispersion (ANDi) mode-locked fiber laser. In this study, we numerically and experimentally demonstrate the spectral filtering process of a nonlinear optical loop mirror (NOLM). Taking advantage of the 40/60 NOLM's spectral filtering ability, we designed a novel all-polarization-maintaining ANDi mode-locked fiber laser without using a separate spectral filter. The NOLM functions as an artificial saturable absorber and a spectral filter in an ANDi cavity. During mode locking, we observed that the NOLM decreased the spectral width of the pulse from 5.46 to 4.38 nm. The fiber laser generated 509-fs compressed pulses at the repetition rate of 13.4 MHz. Our work provides a promising novel and compact ANDi fiber laser for ultrafast photonic applications.

2D GeP-based photonic device for near-infrared and mid-infrared ultrafast photonics

Abstract Germanium phosphide (GeP), a rising star of novel two-dimensional (2D) material composed of Group IV–V elements, has been extensively studied and applied in photonics thanks to its broadband optical absorption, strong light–matter interaction and flexible bandgap structure. Here, we show the strong nonlinear optical (NLO) properties of 2D GeP nanoflakes in the broadband range with open-aperture Z-scan technique to explore the performance of 2D GeP microfiber photonic devices (GMPDs) in near-infrared (near-IR) and mid-infrared (mid-IR) ultrafast photonics. Our results suggest that employing the GMPD as an optical device in an erbium-doped fiber laser (EDFL) system results in ultrashort pulses and rogue waves (RWs) at 1.55 μm. Likewise, by the incorporation of GMPD into a thulium-doped fiber laser (TDFL) system, stable ultrashort pulse operation is also achieved at 2.0 μm. We expect these findings to be an excellent GMPD that can be applied in mode-locked fiber lasers to open up new avenues for its development and application in ultrafast photonics.

GeSe Evanescent Field Saturable Absorber for Mode-Locking in a Thulium/Holmium Fiber Laser

A mode-locked Thulium-Holmium doped fiber laser (THDFL) with a Germanium Selenide (GeSe) saturable absorber (SA) is demonstrated for operation in the 2.0 μm wavelength region. The SA device is fabricated by drop-casting the GeSe onto an arc-shaped fiber which is then incorporated into the THDFL to induce mode-locked pulses. Stable mode-locking is attained at 1908.78 nm, with pulse duration of 1.67 ps and output power of 2.74 mW at a maximum pump power of 476 mW. The results of this work show that the GeSe can be a viable alternative for ultrafast photonics applications in the 2.0 μm. region.

Few-Layer Bismuthene for Coexistence of Harmonic and Dual Wavelength in a Mode-Locked Fiber Laser.

Bismuthene, as a novel two-dimensional (2D) material, has attracted extensive attention because of its outstanding properties including narrow band gap, stability at room temperature, nonlinear optical transmission, and so on. In this paper, the physical characteristic, nonlinear optical response, and ultrafast photonics application of few-layer bismuthene are studied experimentally. By the balanced twin-detector measurement method, the saturable absorption property of few-layer bismuthene with a modulation depth of 2.5% and saturable intensity of 110 MW/cm2 at the optical communication band (C-band) is illustrated. Dependent on a few-layer bismuthene saturable absorber, an all-fiber ultrashort pulse laser is fabricated and the proposed fiber laser can operate with coexistence of harmonic mode-locking and dual-wavelength mode-locking. The different laser generations of harmonic and dual wavelength depend on the saturable absorption of few-layer bismuthene, the suitable birefringence and nonlinearity strength in the laser cavity. The results suggest that the ultrashort pulse laser obtained based on few-layer bismuthene could be applied to the field of pump-probe experiments and tunable terahertz radiation generation potentially.

Facile Synthesis of 2D Tin Selenide for Near‐ and Mid‐Infrared Ultrafast Photonics Applications

Facile Synthesis of 2D Tin Selenide for Near‐ and Mid‐Infrared Ultrafast Photonics Applications

Temporal fine structure of all-normal dispersion fiber supercontinuum pulses caused by non-ideal pump pulse shapes.

We experimentally investigate the spectro-temporal characteristics of coherent supercontinuum (SC) pulses generated in several implementations of silica and soft-glass all-normal dispersion (ANDi) photonic crystal fibers optimized for pumping with Erbium (Er):fiber femtosecond laser technology. We characterize the resulting SC using time-domain ptychography, which is especially suitable for the measurement of complex, spectrally broadband ultrashort pulses. The measurements of the ANDi SC pulses reveal intricate pulse shapes, considerable temporal fine structure, and oscillations on time scales of < 25 femtoseconds, which differ from the smoothness and simplicity of temporal profiles obtained in numerical simulations and observed in previous experiments. We link the measured complex features to temporal sub-structures of the pump pulse, such as pre- and post-pulses and low-level pedestals, which are common in high pulse energy ultrafast Er:fiber systems. We also observe spectro-temporal structures consistent with incoherent noise amplification in weakly birefringent fiber samples. Our results highlight the importance of the pump source and polarization-maintaining (PM) fibers for high-quality SC generation and have practical relevance for many ultrafast photonics applications employing ANDi fiber-based SC sources.

Tellurium@Selenium core-shell hetero-junction: Facile synthesis, nonlinear optics, and ultrafast photonics applications towards mid-infrared regime

Tellurium (Te) is a promising material for photonics applications due to its intriguing advantages, such as excellent optical nonlinearity, larger saturable absorption, strong thermo-optic effect, and superior UV–Vis photo-response. However, the limited optical absorption and uncertain stability of Te has greatly limited its applications in mid-infrared and higher band optical regimes. In this contribution, Te@Se core-shell hetero-junction was constructed by epitaxial growth Se on Te nanotubes (NTs), which exhibits strong broadband linear and nonlinear optical response. Compared to pristine Te NTs and current mainstream nanomaterials, Te@Se shows stronger nonlinear absorption and lower saturation intensity at 800 nm. By employing Te@Se as a saturable absorber device of erbium-doped fiber and thulium-doped fiber lasers, the high-power mode-locked pulses with very stable operation in near- and mid- infrared regime can be acquired. These results suggest that Te@Se hetero-junction is a promising material with infinite potential for new generation ultrafast nano-photonics.

In2Se3 nanosheets for harmonic mode-locked fiber laser

Two-dimensional materials with a sheet structure have excellent optical, electrical and mechanical properties, and have attracted much attention in recent years, especially In2Se3 (the N-type semiconductor compound), which has a rapid development in the fields of materials science and optical communication. In this paper, the nonlinear saturation absorption characteristics of In2Se3 are studied. The In2Se3 nanosheet dispersion can be used in ultrafast photonics applications. The nonlinear absorption is measured by power dependent method, and the modulation depth and saturation intensity are 3.8% and 246.6 MW cm−2, respectively. More importantly, In2Se3 is used as a saturable absorber (SA) in a passively mode-locked erbium-doped fiber laser. The proposed mode-locked fiber laser is demenstrated with a center wavelength of 1529.4 nm, a fundamental frequency of 5.9 MHz, a spectral width of 3.96 nm, a pulse width of 1.38 ps, and a signal-to-noise ratio of 55 dB. For the first time, harmonic mode-locking with a high-repetition rate of 431 MHz is achieved when the pump power is 360 mW corresponding to 73rd-order harmonic mode locking. It can be seen that In2Se3 is indeed a new excellent photonic material, which can be used in fiber optic communication, SAs photonics, laser material processing and light modulators.

Mode-Locked Tm Fiber Laser With a Tapered GIMF SA Based on Nonlinear Multimode Interference Effect

We experimentally demonstrate a passively mode-locked all-fiber Tm laser with an all-fiber saturable absorber (SA) which operates based on the nonlinear multimode interference (NL-MMI) effect in a tapered graded-index multimode fiber (GIMF). The tapered GIMF SA demonstrates the nonlinear characteristics of modulation depth of 21.15%, and saturation fluence of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$89.04~\mu \text{J}$ </tex-math></inline-formula> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . We obtain stable fundamentally mode-locking operation at a pump threshold of 120 mW. The output soliton pulses have a center wavelength, spectral width, pulse duration and repetition rate of 1935 nm, 2.1 nm, 1.9 ps and 18.79 MHz, respectively. This is a simple, low-cost, stable, and convenient laser oscillator with many potential applications in eye-safe ultrafast photonics.

Layered iron pyrite for ultrafast photonics application

Abstract Two-dimensional (2D) transition metal dichalcogenide materials have attracted much attention in recent years due to their excellent electro-optical properties. FeS2, the ideal composition of iron pyrite, is a 2D transition metal dichalcogenide which has been potentially used in the electronic, optical, and chemical fields. On the other hand, the narrow band gap of FeS2 (≈0.96 eV) makes it very suitable and promising for the ultrafast application in near-infrared regimes. However, the potential application of FeS2 in laser technology has not been explored till now. Ultrashort pulse lasers have great applications in industry and science because of its stability, ease of operation, and portability. Passively mode-locked fiber lasers using 2D materials (such as MoS2, CuS2, and WS2) as saturable absorber are intensively investigated. Here, layered FeS2 has been characterized systematically. It is successfully applied in ultrafast photonics and plays a key component in the passively mode-locked laser for the first time. The single pulse can be obtained with 1.7-ps pulse duration, 1.89-nm spectral width, and fundamental repetition of 6.4 MHz at 1563 nm central wavelength. Through controlling the pump power, the evolution of the pulse train can be observed, which can be transformed from single pulse to bound states. Also, the harmonic mode-locked fiber laser is observed with the pump power high enough.