Ultrafast Photonics Research Articles

Coherent terahertz radiation from indium tin oxide film via third-order optical nonlinearity

Coherent terahertz (THz) wave emission from indium tin oxide (ITO) in epsilon-near-zero region had been reported recently as a second-order nonlinear process. However, THz radiation from ITO through third-order nonlinear process has not been reported yet. Here, we demonstrate coherent THz radiation from an ITO film via four-wave mixing, a third-order nonlinear process, under the excitation of two-color pulsed laser fields. Experimentally, an in-line phase compensator is utilized to synthesize the asymmetric optical fields to induce photocurrent in an ITO film and consequently emit THz waves. Our experimental results show that THz radiation from ITO films can be coherently controlled by the relative phase between the two-color laser fields. Such a THz wave generation process can be attributed to the fast intra-band nonlinearity in the conduction band. Our demonstration reveals that the fast nonlinearity in ITO can be used to generate THz waves and promotes the investigation of ITO in ultrafast photonics.

TiO2@Ti3C2Tx heterostructure as an environmentally stable saturable absorber for ultrafast photonics

MXenes, one of the largest families of two-dimensional (2D) materials, provide great potential in photonics, electronics and energy. Especially, MXenes are recently discovered as alternatives to conventional saturable absorbers (SAs). However, the inevitable exposure of metal atoms on surface is likely to react with oxygen or oxygen-containing groups, leading to the severe problem of environmental instability. Here, we developed a partially-controlled oxidation strategy to prepare TiO2@Ti3C2Tx heterostructures. The dense nanosized TiO2 on the surface of heterostructures prevent the further oxidation of the Ti3C2Tx inside, enabling a stable performance in ultrafast photonics. The saturable absorption property of TiO2@Ti3C2Tx heterostructures is verified using the balanced twin-detector measurement system. The modulation depth and saturation intensity were determined to be 14.4% and 2.929 GW/cm2 at 1550 nm, respectively. The Erbium-doped fiber laser (EDFL) with the use of TiO2@Ti3C2Tx SAs exhibits pulse width of 552 fs. More importantly, there is no obvious degradation in saturable absorption property of the TiO2@Ti3C2Tx based EDFL after an exposure in air for more than thirteen months, indicating the excellent environmental stability of the TiO2@Ti3C2Tx heterostructures. This work provides an alternative approach to enhance the long-term performance of Ti3C2Tx optoelectronic devices, which may shed light on the material synthesis and device design of MXenes.

Nanometer-Thin Stacks of MXenes (Ti3C2Tx and Ta4C3Tx) for Applications as Nonlinear Photonic Devices

Two-dimensional (2D) carbides of transition metals, so called “MXenes”, maintain excellent optical properties and unique layer stacking form via hydrogen bonds in contrast to other low-dimensional materials. These promising features have attracted increasingly research interest in the field of ultrafast photonics. In this work, titanium carbide (Ti3C2Tx) and tantalum carbide (Ta4C3Tx), as two promising MXene materials for photonic applications, are experimentally synthesized by the ultrasound-assisted liquid phase exfoliation technique. The morphology of prepared few-layer Ti3C2Tx and Ta4C3Tx has been systematically characterized by transmission electron microscopy and X-ray diffraction analysis. Density functional theory is performed to obtain their electronic band structures, demonstrating metallic properties and verifying their optical absorption at 1 μm. Their tempting optical modulation properties for multi-gigahertz pulsed laser emission are demonstrated by using them as saturable absorbers in a waveguide laser cavity. Particularly, high-performance Q-switched mode-locked lasers with/without laser mirrors are realized based on the hybrid waveguide laser configuration, delivering 1 μm laser pulses with durations as short as 30 ps. The results presented in this work show the great potential of metallic MXenes and waveguide structures for applications in functional photonic devices.

InSb-based saturable absorbers for ultrafast photonic applications.

Sb-related III-V compounds have recently gained great research interest owing to their excellent optical and electrical characteristics, which provide many possibilities in photonics and electronics. This study investigated the application of InSb films in ultrafast photonics. An InSb film was fabricated on the tapered zone of a microfiber, and its saturation intensity, modulation depth, and non-saturable loss were determined as 119.8 MW cm-2, 23.5%, and 27.3%, respectively. The structure of the electronic band and density of states of InSb were theoretically calculated. Notably, mode-locked and Q-switched fiber lasers were realised by incorporating the InSb-microfiber device into two different Er-doped fiber cavities. In the Q-switching state, the narrowest pulse duration was measured as 1.756 μs with a maximum single-pulse energy of 221.95 nJ and a signal-to-noise ratio of 60 dB. In the mode-locking operation, ultrafast lasers with a high signal-to-noise ratio (70 dB), a pulse width as narrow as 265 fs and a repetition rate of 49.51 MHz were acquired. Besides, the second-harmonic mode-locked state was built with an output power of 13.22 mW. In comparison with the reported laser performance with 2D materials as saturable absorbers, the InSb-based mode-locked and Q-switched fiber lasers proposed herein exhibit better comprehensive performance.

Investigations of the nonlinear optical properties of ZrGeTe4 nanosheets and their application in ultrafast photonics

We prepared ZrGeTe4 nanosheets and investigated their nonlinear optical properties. ZrGeTe4 nanosheets as a saturable absorber were applied in an Er-doped fiber laser for conventional soliton and harmonic mode-locking pulses generation.

High-performance γ-MnO 2 Dual-Core, Pair-Hole Fiber for Ultrafast Photonics

Manganese dioxide (MnO 2 ) is a widely used and well-studied 3-dimensional (3D) transition metal oxide, which has advantages in ultrafast optics due to large specific surface area, narrow bandgap, multiple pores, superior electron transfer capability, and a wide range of light absorption. However, few studies have considered its excellent performance in ultrafast photonics. γ-MnO 2 photonics devices were fabricated based on a special dual-core, pair-hole fiber (DCPHF) carrier and applied in ultrafast optics fields for the first time. The results show that the soliton molecule with tunable temporal separation (1.84 to 2.7 ps) and 600-MHz harmonic solitons are achieved in the experiment. The result proves that this kind of photonics device has good applications in ultrafast lasers, high-performance sensors, fiber optical communications, etc., which can help expand the prospect of combining 3D materials with novel fiber for ultrafast optics device technology.

Ultrafast fluorescence depolarisation in green fluorescence protein tandem dimers as hydrophobic environment sensitive probes.

Advances in ultra-fast photonics have enabled monitoring of biochemical interactions on a sub nano-second time scale. In addition, picosecond dynamics of intermolecular energy transfer in fluorescent proteins has been observed. Here, we present the development of a genetically encoded fluorescent sensor that can detect changes in hydrophobicity by monitoring ultrafast fluorescence depolarisation. Our sensor is composed of a pair of dimeric enhanced green fluorescent proteins (dEGFPs) linked by a flexible amino-acid linker. We show dimerisation is perturbed by the addition of glycerol which interferes with the hydrophobic interaction of the two proteins. Time-resolved fluorescence anisotropy revealed a systematic attenuation of ultrafast fluorescence depolarisation when the sensor was exposed to increasing glycerol concentrations. This suggests that as hydrophobicity increases, dEGFP pairing decreases within a tandem dimer. Un-pairing of the protein fluorophores dramatically alters the rate of energy transfer between the proteins, resulting in an increase in the limiting anisotropy of the sensor.

Broadband ultrafast photogenerated carrier dynamics induced by intrinsic defects in <inline-formula><tex-math id="Z-20231102113631">\begin{document}$\boldsymbol\beta$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20231173_Z-20231102113631.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20231173_Z-20231102113631.png"/></alternatives></inline-formula>-Ga<sub>2</sub>O<sub>3</sub>

The ultra-wide bandgap semiconductor gallium oxide <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> with enhanced resistance to the irradiation and temperature is favorable for high-power and high-temperature optoelectronic devices. <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> also exhibits great potential applications in the field of integrated photonics because of its compatibility with the CMOS technique. However, a variety of intrinsic and extrinsic defects and trap states coexist in <i>β</i>-Ga<sub>2</sub>O<sub>3</sub>, including vacancies, interstitials, and impurity atoms. The defect-related carrier dynamics in <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> not only adversely affect the optical and electrical properties, but also directly limit the performance of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> based devices. Therefore, a comprehensive understanding of the carrier transportation and relaxation dynamics induced by intrinsic defects is very important. Supercontinuum-probe spectroscopy can provide a fruitful information about the carrier relaxation processes in different recombination mechanisms, and thus becomes an effective way to study the defect dynamics. In this work, we study the dynamics of carrier trapping and recombination induced by intrinsic defects in pristine <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> crystal by using wavelength-tunable ultrafast transient absorption spectroscopy. The broadband absorption spectra induced by the intrinsic defects are strongly dependent on the polarization of pump pulse and probe pulse. Particularly, two absorption peaks induced by the two defect states can be extracted from the transient absorption spectra by subtracting the absorption transients under two probe polarizations. The observed defect-induced absorption features are attributed to the optical transitions from the valence band to the different charge states of the intrinsic defects (such as gallium vacancy). The data are well explained by a proposed carrier capture model based on multi-level energies. Moreover, the hole capture rate is found to be much greater than that of the electron, and the absorption cross-section of the defect state is at least 10 times larger than that of free carrier. Our findings not only clarify the relationship between intrinsic defects and photogenerated carrier dynamics, but also show the importance in the application of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> crystals in ultrafast and broadband photonics.

Pioneering experiments on optical solitons in fibres

Fifty years ago, the concept of solitons in optical fibres was proposed numerically by F. Tappert and A. Hasegawa from Bell Labs. Seven years later in 1980, experiments confirmed that dispersion and the Kerr nonlinearity could combine to yield a coherent ultrashort pulse able to propagate without distortion. The implications of this discovery go well beyond the sector of telecommunications as initially suggested, and solitons now play a major role in modern ultrafast nonlinear photonics.

Материалы для сверхбыстрой терагерцовой фотоники

Designing of the ultrafast terahertz photonics devices requires materials with the biggest nonlinear refractive index coefficient n2 and the lowest settling time of the nonlinear response τ in the terahertz spectral range. In the present study, we show that the ratio n2/τ for media with the vibrational nonlinearity in the terahertz range is determined by the square of the medium thermal expansion coefficient and the fifth power of its dominative stretching vibrational mode frequency. These findings are based on the theory of the media vibrational nature nonlinear polarization response to incident THz radiation. We provide estimated values of the n2/τ ratio for a group of liquids and crystal materials. According to our evaluations, n2/τ value for alpha-pinene in the terahertz spectral range is about 10^6 cm^2/J and is almost the biggest one compared to materials with different nonlinearity types in various spectral ranges.

Mode-locking pulse generation based on lead-free halide perovskite CsCu2I3 micro-rods with high stability

A CsCu2I3 SA-based passively mode-locking fiber laser was demonstrated. The fiber laser could operate stably for at least five months, which will potentially unlock the pathways for ultrafast photonics based on lead-free halide perovskites.

Investigation of charge carrier dynamics in a Ti3C2Tx MXene for ultrafast photonics applications

Ti3C2Tx MXene was synthesized through LiF/HCl etching, and its structural and optical properties were examined. Additionally, ultrafast transient absorption spectroscopy was employed to investigate the excited state dynamics and electronic structure across the femtosecond to nanosecond timescales.

Flexible optical limiters based on Cu3VSe4 nanocrystals.

Optical limiters are greatly needed to protect eyes and sensitive optoelectronic devices such as photodetectors and sensors from laser damage, but they are currently plagued by low efficiency. In this work, we utilized Cu3VSe4 nanocrystals (NCs) to enhance laser protection performance, and they exhibit higher saturation intensity and broader nonlinear spectral response extending into the near IR region than the C60 benchmark. A flexible optical limiter goggle prototype based on the NCs significantly attenuated the incident laser beam, with Z scan and I scan measurements demonstrating a giant nonlinear absorption coefficient β value of 1.0 × 10-7 m W-1, a large optical damage threshold of 3.5 J cm-2, and a small starting threshold of 0.22 J cm-2. Transient absorption spectroscopy disclosed that the origin of the excellent nonlinearity was associated with quasi-static dielectric resonance behavior and a large TPA cross-section of 3.3 × 106 GM was measured for Cu3VSe4 NCs, suggesting the potential of intermediate bandgap (IB) semiconductors as alternatives to plasmonic noble metals for ultrafast photonics. Hence, optical limiters based on such semiconductors offer new avenues for laser protection in optoelectronic and defense fields.

Interfacial Charge Transfer and Ultrafast Photonics Application of 2D Graphene/InSe Heterostructure

Interface interactions in 2D vertically stacked heterostructures play an important role in optoelectronic applications, and photodetectors based on graphene/InSe heterostructures show promising performance nowadays. However, nonlinear optical property studies based on the graphene/InSe heterostructure are insufficient. Here, we fabricated a graphene/InSe heterostructure by mechanical exfoliation and investigated the optically induced charge transfer between graphene/InSe heterostructures by taking photoluminescence and pump-probe measurements. The large built-in electric field at the interface was confirmed by Kelvin probe force microscopy. Furthermore, due to the efficient interfacial carrier transfer driven by the built-in electric potential (~286 meV) and broadband nonlinear absorption, the application of the graphene/InSe heterostructure in a mode-locked laser was realized. Our work not only provides a deeper understanding of the dipole orientation-related interface interactions on the photoexcited charge transfer of graphene/InSe heterostructures, but also enriches the saturable absorber family for ultrafast photonics application.

CoS nanosheets for generation of vector soliton and bound solitons in nonlinear optical fiber system

Transition metal chalcogenides (TMCs) with two-dimensional (2D) structures are of great interest for their extraordinary electrical, optical, magnetic, and mechanical properties. In this work, a novel cobalt sulfide (CoS) nanosheets (NSs) based saturable absorber (SA) with a modulation depth of 6.4 % is fabricated through liquid phase exfoliation (LPE) method, and successfully applied to a passive mode-locked erbium-doped fiber laser (PML-EDFL) for the first time as far as we know. By adjusting the pump power and optimizing the orientation of the intra-cavity polarization controller, a stable conventional solitons (CS) pulse with a duration of 905 fs and signal-to-noise ratio (SNR) of 58.5 dB at fundamental repetition frequency of 4.03 MHz, a pair of bound solitons with time interval of 4.9 ps between two 1.8 ps pulses are generated in proposed PML-EDFL. Meanwhile, the vector characteristics of generated soliton pulse are investigated, and polarization-locked vector solitons are observed. The results reveal that the CoS NSs have superior polarization insensitive nonlinear optical modulation properties and will find extensive application potential in the ultrafast photonics.

A remarkable third-order nonlinear optical behavior of single-crystal bromide organic-inorganic lead halide perovskite

The optical and structural aspects of methylamine lead bromide (MAPbBr3) single crystal (SC) are investigated. Based on Kramers–Kronig (KK) equations, the optical and dielectric parameters are investigated. The optical phonons Longitudinal optical (LO = 678 cm−1) and Transversal optical (TO = 578 cm−1) phonon modes are associated with the optical interaction with lattices in the visible region for MAPbBr3 SC. The Z-scan method is used to investigate the third-order nonlinear coefficients of SC hybrid organic–inorganic lead halide perovskites, MAPbBr3 with the 2.19 eV optical band gap energy. According to the nonlinear optical (NLO) properties, MAPbBr3 has a high third-order NLO response. The two-photon absorption (2PA) coefficient is found to be 3 × 10-4 cm W−1 using the transmission open aperture Z-scan technique. The nonlinear refraction (NLR) is obtained in the order of 10-9 cm2 W−1 utilizing a close aperture Z-scan, which revealed self-defocusing phenomena. This research shows that MAPbBr3 is a practical and low-priced nonlinear material for ultrafast photonics applications.

Broadband Nonlinear Response and Ultrafast Photonics Applications in Few-Layer MBene

2D MBenes, the latest derivatives of bulk ternary or quaternary transition metal boride (MAB) phases, have gradually attracted increasing research attention as novae in the flatland. However, the fabrication of MBenes is still a great challenge at present, leading to the lack of reported MBene applications in the photonic field (e.g., ultrafast lasers). Hence, we successfully fabricated few-layer Mo4/3B2–xTz MBenes by selectively etching nanolaminated 3D parent boride (Mo2/3Y1/3)2AlB2. Density functional theory calculations were used to theoretically analyze its electronic and optical properties. By employing the Z-scan technique, the nonlinear absorption characteristics of Mo4/3B2–xTz MBenes from the visible to mid-infrared regime were revealed for the first time. Compared with mainstream 2D materials and common MXenes, Mo4/3B2–xTz has more potential as a nonlinear optical (NLO) material, such as stronger nonlinear absorption (about 100 times of graphene) and lower saturation intensity (about 1% of black phosphorus). Furthermore, by integrating Mo4/3B2–xTz as a saturable absorber (SA) into three similar gain fiber-based laser resonators, broadband stable ultrafast pulses were obtained with pulse durations of 358.7 ps (1 μm), 582.3 fs (1.5 μm), and 2.31 ps (2 μm). Innovation in MBenes as SA will allow the design of novel photonic systems and devices with broadband functionality, opening a promising paradigm for the conscious design of high-performance NLO materials.

InP/ZnSeS/ZnS Core–Shell Quantum Dots as Novel Saturable Absorbers in Mode‐Locked Fiber Lasers

AbstractIn this study, red light emitting InP/ZnSeS/ZnS quantum dots (QDs) are prepared by thermal injection method. The results show that the InP/ZnSeS/ZnS QDs saturable absorbers (SA) have good nonlinear saturable absorption properties with modulation depth of 24.2% and saturation intensity of 0.08 KW cm−2. Then the QD SAs are applied to the erbium‐doped fiber laser (EDFL) ring cavity system, and a mode‐locked laser pulse with a pulse width of 635 fs is generated. In addition, the pulse width is determined by the carrier recovery time, which is closely related to the defect density in the material. A large number of defects are eliminated in the QDs by hydrogen fluoride (HF) treatment, and the pulse width is reduced from 635 to 450 fs. Results of time‐resolved photoluminescence and ultrafast transient absorption spectroscopy (TAS) show that HF treatment indeed reduces defects in InP/ZnSeS/ZnS QDs, indicated by the decrease of the carrier recovery time. This is different from the reported 2D SA materials, which usually reduce the carrier recovery time by increasing the defect density. This study will inspire new applications of QDs in ultrafast photonics and nonlinear optics.

Single crystalline BiOCl nanosheets with oxygen vacancies for ultrafast mode-locking operation

In this work, we synthesized the single-crystalline square BiOCl nanosheets with oxygen vacancies via the facile solvothermal route. The prepared BiOCl nanosheets were deposited on a tapered microfiber as a saturable absorber. Via the evanescent filed interaction with the BiOCl saturable absorber, a soliton mode-locked Er-doped fiber laser was demonstrated for the first time, producing a pulse duration of 990 fs at a central wavelength of 1563 nm with a FWHM bandwidth of 3.2 nm. The signal-to-noise ratio was 36 dB at the fundamental repetition rate of 7.9 MHz. Our results confirmed that the BiOCl nanosheets with abundant oxygen vacancies could be applied for the ultrafast photonics.

Mo4/3B2T2-based saturable absorber for generating conventional and bound-state solitons in erbium-doped fibre laser

As a new two-dimensional (2D) material, Mo4/3B2T2 has attracted great attention due to its characteristic electrical and optical properties. However, the nonlinear optical properties of Mo4/3B2T2 and its applications in the ultrafast photonics aspect have not been reported. In this work, we first demonstrate the nonlinear saturable absorption properties of 2D material Mo4/3B2T2. The modulation depth and saturation absorption intensity of Mo4/3B2T2 are 4.5 % and 534 MW/cm2, respectively. Herein, conventional soliton and bound-state solitons were generated in a fibre laser in the 1.5-μm region after inserting the 2D material Mo4/3B2T2 into the laser cavity as a saturable absorber (SA). A highly stable conventional soliton is observed at 1560.57 nm with a 3 dB bandwidth of 3.12 nm. The frequency rate, pulse width, and signal-to-noise ratio were 7.59 MHz, 1.08 ps, and 65.83 dB, respectively. In addition, the bound-state solitons are obtained at a pump power of 320 mW. The central wavelength, pulse interval and spectral modulation period of the bound-state soliton are 1560.9 nm, 6.1 ps and 1.33 nm, respectively. These results suggest that 2D material Mo4/3B2T2 can be used as a promising SA in ultrafast and ultra-narrow photonics.