Nonlinear Absorption Characteristics Research Articles

Nonlinear Optical Saturable Absorption Properties of 2D VP Nanosheets and Application as SA in a Passively Q-Switched Nd:YVO4 Laser

Two-dimensional (2D) violet phosphorus (VP) plays a significant role in the applications of photonic and optoelectronic devices due to its unique optical and electrical properties. The ultrafast carrier dynamics and nonlinear optical absorption properties were systematically investigated here. The intra- and inter-band ultrafast relaxation times of 2D VP nanosheets were measured to be ~6.83 ps and ~62.91 ps using the pump–probe method with a probe laser operating at 1.03 μm. The nonlinear absorption coefficient βeff, the saturation intensity Is, the modulation depth ΔR, and the nonsaturable loss were determined to be −2.18 × 104 cm/MW, 329 kW/cm2, 6.3%, and 9.8%, respectively, by using the Z-scan and I-scan methods, indicating the tremendous saturable absorption property of 2D VP nanosheets. Furthermore, the passively Q-switched Nd:YVO4 laser was realized with the 2D VP nanosheet-based SA, in which the average output power of 700 mW and the pulse duration of 478 ns were obtained. These results effectively reveal the nonlinear optical absorption characteristics of VP nanosheets, demonstrating their outstanding light-manipulating capabilities and providing a basis for the applications of ultrafast optical devices. Our results verify the excellent saturable absorption properties of 2D VP, paving the way for its applications in pulsed laser generation.

Broadband and ascendant nonlinear optical properties of the wide bandgap material GaN nanowires

Gallium nitride (GaN) nanowire, as a type of wide bandgap nanomaterial, has attracted considerable interest because of its outstanding physicochemical properties and applications in energy storage and photoelectric devices. In this study, we prepared GaN nanowires via a facile chemical vapor deposition method and investigated their nonlinear absorption responses ranging from ultraviolet to near-infrared in the z-scan technology under irradiation by picosecond laser pulses. The experiment revealed that GaN nanowires exhibit remarkable nonlinear absorption characteristics attributed to their wide bandgap and nanostructure, including saturable absorption and reverse saturable absorption. When compared to bulk GaN crystals, the nanowires provide a richer and more potent set of nonlinear optical effects. Furthermore, we conducted an analysis of the corresponding electronic transition processes associated with photon absorption. Under high peak power density laser excitation, two-photon absorption or three-photon absorption dominate, with maximum modulation depths of 73.6%, 74.9%, 63.1% and 64.3% at 266 nm, 355 nm, 532 nm, and 1064 nm, respectively, corresponding to absorption coefficients of 0.22 cm/GW, 0.28 cm/GW, 0.08 cm/GW, and 2.82 ×10−4 cm3/GW2. At lower peak energy densities, GaN nanowires demonstrate rare and excellent saturation absorption characteristics at wavelength of 355 nm due to interband transitions, while saturable absorption is also observed at 532 nm and 1064 nm due to band tail absorption. The modulation depths are 85.2%, 41.9%, and 13.7% for 355 nm, 532 nm, and 1064 nm, corresponding to saturation intensities of 3.39 GW/cm2, 5.58 GW/cm2 and 14.13 GW/cm2. This indicates that GaN nanowires can be utilized as broadband optical limiters and high-performance pulse laser modulating devices, particularly for scarce ultraviolet optical limiters, and saturable absorbers for ultraviolet and visible lasers. Furthermore, our study demonstrates the application potential of wide bandgap nanomaterials in nonlinear optical devices.

Large nonlinear optical absorption and refraction of β-In2Se3 thin film from above to below bandgap excitation

Nonlinear optical materials, especially two-dimensional materials, are anticipated to reveal broadband optical nonlinearity for future miniaturized photonic applications. Herein, we report a physical vapor deposition method to produce β-In2Se3 thin film and investigate the broadband nonlinear absorption (β) and refraction (n2) characteristics. The β-In2Se3 semiconductor shows an excellent optical nonlinearity with large β in 102 cm GW−1 scale and n2 in 10−12 cm2 W−1 scale from visible to NIR wavelengths, which are superior to those of metal carbides and nitrides (MXenes) and metal-organic frameworks. This excellent optical nonlinearity makes β-In2Se3 a promising candidate for advanced nanophotonic devices and beyond.

Revealing photoluminescence and nonlinear optical absorption characteristics of PbMo0.75W0.25O4 single crystal for optical limiting applications

Nonlinear absorption properties of PbMo0.75W0.25O4 single crystal fabricated by the Czochralski method were studied. The band gap energy of the crystal was determined as 3.12 eV. Urbach energy which represents the defect states inside the band gap was found to be 0.106 eV. PbMo0.75W0.25O4 single crystal has a broad photoluminescence emission band between 376 and 700 nm, with the highest emission intensity occurring at 486 nm and the lowest intensity peak at 547 nm, depending on the defect states. Femtosecond transient absorption measurements reveal that the lifetime of localized defect states is found to be higher than the 4 ns pulse duration. Open aperture (OA) Z-scan results demonstrate that the PbMo0.75W0.25O4 single crystal exhibits nonlinear absorption (NA) that includes two-photon absorption (TPA) as the dominant mechanism at the 532 nm excitations corresponding to 2.32 eV energy. NA coefficient (β eff ) increased from 7.24 × 10−10 m W−1 to 8.81 × 10−10 m W−1 with increasing pump intensity. At higher intensities β eff tends to decrease with intensity increase. This decrease is an indication that saturable absorption (SA) occurred along with the TPA, called saturation of TPA. The lifetime of the defect states was measured by femtosecond transient absorption spectroscopy. Saturable absorption behavior was observed due to the long lifetime of the localized defect states. Closed aperture (CA) Z-scan trace shows the sign of a nonlinear refractive index. The optical limiting threshold of PbMo0.75W0.25O4 single crystal at the lowest intensity was determined as 3.45 mJ/cm2. Results show that the PbMo0.75W0.25O4 single crystal can be a suitable semiconductor material for optical limiting applications in the visible region.

Sub-nanosecond single mode-locking pulse generation in an intracavity KTP optical parametric oscillator with a few-layer Bi2Te3 topological insulator saturable absorber

By utilizing the ultrasound-assisted liquid phase exfoliation process, Bi2Te3 nanosheets are synthesized and deposited onto a quartz plate to form a saturable absorber (SA), in which nonlinear absorption characteristics around 1 μm are investigated with a home-made Q-switched laser. With the as-prepared Bi2Te3 SA employed, a sub-nanosecond KTiOPO4 (KTP) based intracavity optical parametric oscillator (IOPO) pumped by a dual-loss-modulated Q-switched and mode-locked (QML) laser is successfully realized. With a short duration of sub-nanosecond and a repetition rate of several kilohertz (kHz), the signal wave’s single mode-locking pulse existed underneath a Q-switched envelope can be formed. At an incident pump power of 8.92 W and an acousto-optic modulator (AOM) repetition rate of 2 kHz, a signal wave’s single mode-locking pulse underneath a Q-switching envelope with a measured minimum pulse duration of 520 ps is accomplished. It is the first demonstration of such Bi2Te3 SA utilized in a solid-state QML IOPO laser, to the best of our knowledge. According to the experimental results, this provides an efficient and straightforward approach to acquire sub-nanosecond OPO signal wave pulses. Additionally, the relevant coupled rate equations are established, and the simulation results match the experimental results, revealing the potential of Bi2Te3 as a nanomaterial for optoelectronic applications.

Fundamental frequency and second harmonic mode-locked operations in an erbium-doped fiber laser based on Cr2Sn2Te6 saturable absorbers

With the rapid development of ultrafast photonics, the need for optical modulators is expanding. In particular, the optical modulator with a high damage threshold and the ability to generate multiple soliton states. This has prompted an ongoing search for optical modulators based on two-dimensional (2D) materials. Therefore, in this work, a novel 2D material Cr2Sn2Te6 (CSnT) with modulation depth of 6.17% and saturation intensity of 15.97 MW/cm2 is prepared as saturable absorber (SA). The nonlinear optical absorption characteristics and pulse modulation applications of CSnT are showed in the Erbium-doped fiber laser (EDFL). The fundamental frequency mode-locked operation and the second harmonic mode-locked operation are realized simultaneously in the ring cavity. Moreover, CSnT SA has a high damage threshold and high stability, the damage threshold reaches 467 mW and the signal-to-noise ratio (SNR) is higher than 70 dB. The experimental findings demonstrate that the CSnT-based saturable absorber possesses remarkable nonlinear optical properties, indicating its considerable potential in the realm of ultrafast photonics.

Generation of Bright-Dark soliton pairs in mode-locked fiber laser based on WSe2/MoSe2 heterojunction

In this study, the WSe2/MoSe2 heterojunction was fabricated by liquid-phase exfoliation (LPE) method with a modulation depth of 11.39 % and saturable intensity of 0.246 GW/cm2. The WSe2/MoSe2 heterojunction was used as a saturable absorber (SA) to achieve bright-dark soliton pairs modulation in Er-doped fiber laser (EDFL)successfully. Stable bright-dark soliton pairs, with a repetition frequency of 4.459 MHz, were successfully generated using a pump power of 218.8 mW. The central wavelengths were found to be at 1564.16 nm and 1564.98 nm with the peak-to-peak spacing of 0.82 nm. The single-pulse energy was determined to be 0.7 nJ. The experimental results demonstrate the exceptional nonlinear saturable absorption characteristics of the WSe2/MoSe2 heterojunction. These findings offer valuable insights for future research on bright-dark soliton pairs utilizing this material.

Dual-loss-modulated QML erbium-doped fiber laser with microfiber and NPR technique

We demonstrated a dual-loss-modulated Q-switched mode-locked (QML) erbium-doped fiber laser based on the nonlinear polarization rotation (NPR) technique with a microfiber. The microfiber served as both a highly nonlinear medium and a saturable absorber, which together with the NPR technique enables stable QML operation under significant pump power fluctuations. We evaluated the nonlinear properties of microfibers with different waist diameters. A maximum QML envelope energy of 467.15 nJ is achieved, corresponding to an average single-pulse energy of 4.06 nJ beneath the Q-switched envelope, which is 2.35 times higher than the maximum pulse energy attained with dissipative soliton mode-locking under similar pump power. In addition, the QML laser was verified by numerical simulation based on the coupled Ginzburg-Landau equation. The microfiber's customizable size enables significant variations in non-linear and saturable absorption characteristics, creating the potential for QML pulse regulation. This research provides novel insights for designing QML lasers.

Multiple Bound State Soliton Pulses in the All Polarization Maintaining Fiber Laser.

The bound state soliton pulse, a novel mode-locked output state of fiber lasers, has been studied extensively to gain a better understanding of soliton interactions and to explain the mechanism behind the generation of mode-locked pulses. In this particular research, we utilized a self-made saturable absorber (SA) consisting of single-walled carbon nanotubes (SWCNT) in a fully polarization maintaining (PM) erbium-doped fiber optical path. Through this setup, we observed various bound state pulse phenomena, including the double bound state with different phase differences, the bound state formed by two double pulse bound states, the multi-pulse bound state, etc. The abundant bound soliton pulse states demonstrated the excellent nonlinear absorption characteristics of the SA as well as the excellent optical properties of the all-PM fiber laser. It contributed to exploring the relationship between sub pulses and mode-locked pulses in the future. Additionally, due to the strong interaction between bound state solitons and the inherent stability of the PM optical path, there was potential for utilizing this setup as a seed source to enhance the stability of high-power fiber lasers.

Exploiting evanescent field interaction with magnetic copper ferrite@reduced functionalized graphene oxide for soliton mode-locked pulses generation

This study explored the potential of the CuFe2O4@rFGO nanocomposite as a saturable absorber (SA) material for use in a mode-locked thulium/holmium doped fiber laser (THDFL). The CuFe2O4@rFGO nanocomposite was synthesized using a simple hydrothermal method. To facilitate the coating of the arc-shaped fiber and enable its use as an SA, two varying concentrations of CuFe2O4@rFGO nanocomposite (5 and 10 mg/ml) were prepared. The SA with 5 mg/ml CuFe2O4@rFGO displays nonlinear absorption characteristics with a saturation intensity of 0.26 MW/cm2 and a modulation depth of 36.65%. In contrast, the 10 mg/ml CuFe2O4@rFGO-based SA has a saturation intensity of 1.14 MW/cm2 and a modulation depth of 44.12%. By interacting with the evanescent field of light within the arc-shaped fiber, the CuFe2O4@rFGO nanocomposite enables the generation of stable mode-locked pulses in the THDFL system. The 5 mg/ml CuFe2O4@rFGO-based SA produced mode-locked pulses with a pulse duration of 1.445 ps, a repetition rate of 12.43 MHz, and a signal-to-noise ratio (SNR) of approximately 54 dB. On the other hand, the 10 mg/ml based SA generated mode-locked pulses with a repetition rate of 12.14 MHz, an SNR of 57 dB, and a pulse duration of 1.336 ps. This finding will open new avenues for CuFe2O4@rFGO research in photonics applications.

Two types of ultrafast mode-locking operations from an Er-doped fiber laser based on germanene nanosheets

As a member of Xenes family, germanene has excellent nonlinear saturable absorption characteristics. In this work, we prepared germanene nanosheets by liquid phase exfoliation and measured their saturation intensity as 0.6 GW/cm2 with a modulation depth of 8%. Then, conventional solitons with a pulse width of 946 fs and high-energy noise-like pulses with a pulse width of 784 fs were obtained by using germanene nanosheet as a saturable absorber for a mode-locked Erbium-doped fiber laser. The characteristics of the two types of pulses were investigated experimentally. The results reveal that germanene has great potential for modulation devices in ultrafast lasers and can be used as a material for creation of excellent nonlinear optical devices to explore richer applications in ultrafast photonics.Graphical

Ultrafast optical nonlinearity in natural van der Waals heterostructure nanosheets of franckeite

Franckeite, a natural van der Waals heterostructure with excellent physicochemical characteristics, has shown great potential for high-performance optoelectronic device. Here, the few-layer franckeite nanosheets have been prepared by liquid phase-exfoliation method successfully, and the nonlinear optical absorption and carrier dynamics behaviors of the nanosheets have been studied experimentally. The franckeite nanosheets show wavelength- and intensity-dependent nonlinear absorption characteristics, and exhibit ultrafast carrier dynamic process. The ultrafast optical nonlinearity in franckeite nanosheets has been further validated via an Er3+-doped fiber laser to deliver stable femtosecond pulse. The experimental results reveal that franckeite heterostructure nanosheets can be an ideal candidate in the design and manufacture of ultrafast nonlinear optoelectronic devices.

In2S3 nanoflakes-based saturable absorber for multi-state soliton generation

Attributing to their excellent photoelectric properties, metal sulfides are potential alternatives for femtosecond photonics. Here, indium sulfide nanoflakes are employed as saturable absorbers. The morphology and chemical composition of In2S3 nanoflakes are characterized. An In2S3 nanoflakes-based SA is fabricated by the method of evanescent light deposition with modulation depth of 2.7 % and saturable intensity of 5 MW/cm2. Stable mode locking with central wavelength of 1530 nm, 3 dB bandwidth of 4.9 nm, pulse duration of 733 fs and fundamental repetition rate of 8.18 MHz is obtained. By carefully adjusting of polarization controller and pumping power, harmonic mode locking with repetition rate of 538 MHz could be achieved. By changing the length of single mode fiber and altering the angle of PC, harmonic soliton molecules centered at 1560 nm are generated with spectral modulation period of 1.3 nm and pulse interval of 4.96 ps. The multi-state soliton generation of the laser confirms the excellent nonlinear absorption characteristics of In2S3 in the communication band, which could be expected to enable more practical instrumentations for spectroscopy, optical sensing and optoelectronics, etc.

Role of defects on the nonlinear optical properties of La doped Bi2WO6 nanostructures for optical device applications

Lanthanum (La) doped Bismuth tungstate (Bi2WO6) nanostructures was successfully synthesized by chemical method and investigated its third order nonlinear optical properties. The effect of La doping on the optical, structure, chemical composition and morphology of the nanostructures were confirmed by X-ray diffraction (XRD), UV–Visible spectroscopy, Raman, X-ray Photoelectron spectroscopy (XPS) and Transmission electron microscopy (TEM). The results show that the synthesized samples are highly crystalline and the replacement of Bi3+ by La3+ produces a high degree of Bi2WO6 lattice distortion. The addition of La dopant shifts the absorption edge towards higher wavelength and a decrease in the band gap values was observed. The nonlinear optical properties of the nanostructures were studied by an open and closed aperture Z-scan using a continuous wave Diode Pumped Solid State (DPSS) laser at 532 nm. For all samples the open aperture configuration have shown reverse saturable absorption behavior and the closed aperture scan showed self-defocusing effect. The nonlinear absorption and refraction characteristics are found to increase upon La doping.

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.

Photothermal property investigation of V2CTx MXene and its use for all-optical modulator

MXenes, as novel two-dimensional materials, have garnered significant attention in the field of lasers and photonics owing to their outstanding electronic and optical properties. The results pertaining to the photothermal properties and nonlinear absorption characteristics of V2CTx MXene presented herein demonstrate the application of photothermal effect-based polarization rotation for all-optical signal modulation. In our analysis, first, the nonlinear absorption coefficient of a V2CTx thin film was measured using an open-aperture z-scan technique. The estimated nonlinear absorption coefficient was approximately −9.26 × 103 cm/GW at 1560 nm. Next, density functional theory calculations were conducted to determine the electronic band structure and optical absorption spectrum of V2CTx MXene. Following this, photothermal properties of V2CTx MXene were characterized in terms of the photothermal effect-based polarization rotation using 980 nm control and 1550 nm probe beams. The power-dependent temperature slope and conversion efficiency of the phase shift were measured to be 0.15 °C/mW and 0.02 π/mW, respectively. Finally, an all-optical modulator was experimentally implemented using the photothermal effect-based polarization rotation. The rise and fall time constants were estimated to be approximately 311 and 321 μs, respectively.

All-fiber wavelength-tunable mode-locked cylindrical vector beams laser based on multimode interference

Cylindrical vector beams (CVBs) are of increasing concern due to their polarization and axially symmetrical distribution of field intensity. In this paper, we report for the first time an all-fiber single- and dual-wavelength tunable CVBs fiber laser using the nonlinear optical saturation absorption characteristics and filtering properties of nonlinear multimode interference (NL-MMI). In the experiment, the central wavelength of mode-locked CVBs fiber laser can be tuned from 1541 nm to 1564 nm at single-wavelength and from 1542.8 and 1562.925 nm to 1546.52 and 1563.86 nm at dual-wavelength. This NL-MMI-based wavelength-tunable ultrashort pulse CVBs fiber laser has excellent development prospects in optical communication.

Few-layer Ti3CN MXene for ultrafast photonics applications in visible band

Ti3CN, as a typical hetero-MXene, has attracted tremendous attention for its unique properties. However, its ultrafast photonics applications are still rare. Here, the few-layer Ti3CN MXene was successfully prepared by selective etching and molecular delamination technique. The nonlinear optical response of few-layer Ti3CN MXene at 640 nm was studied using the open-aperture Z-scan technique. The as-prepared Ti3CN MXene sample exhibited excellent nonlinear saturable absorption characteristics, resulting in the nonlinear absorption coefficient β of −4.05 × 10-2 cm/GW, which was one order of magnitude larger than that of black phosphorus (BP) and molybdenum disulfide (MoS2). For the optical modulation applications of few-layer Ti3CN MXene, passively Q-switched (PQS) solid-state visible lasers based on Ti3CN saturable absorber (SA) at 522 nm, 607 nm, 639 nm, and 721 nm were successfully realized. Furthermore, a Ti3CN-based stable passively mode-locked Pr:YLF red laser was also successfully achieved with a pulse duration of 30 ps, and the corresponding repetition rate was 73.1 MHz. The optical modulation device based on few-layer Ti3CN MXene shows good performance. Our work demonstrates that the tremendous prospects of the few-layer Ti3CN MXene as a visible optical modulation device in ultrafast photonics applications.

Annealing duration dependent optical, nonlinear optical, and optical limiting properties of rare-earth doped glasses embedded with gold nanoparticles

In this work, the Eu2O3 doped B2O3 glass specimen embedded with gold nanoparticles (NPs) was designed by the process of melt-quenching and subjected for thermal treatment to understanding the efficacy of thermal dependency on the optical and nonlinear optical (NLO) characteristics. Linear optical studies have revealed a decrease trend in the energy band gap as the function of annealing duration, indicating the increase of non-bridging oxygen concentration in the glass host. The NLO properties were assessed in near-infrared region utilizing 150 fs laser pulses delivered at the rate of 80 MHz. The aperture free Z-scan measurements exhibited an increased nonlinear absorption coefficient with annealing duration. Further, the closed aperture Z-scan data depicted an increase trend in the positive nonlinear refraction with annealing duration and eventually reduced at the greater duration of annealing. The optical limiting studies demonstrated similar behaviour to the nonlinear absorption characteristics of studied glass samples. These results are related to the local field stimulated by surface plasmons of gold NPs near the Eu3+ ions during the exposure of glass to high fluence laser beam. Our results indicate that B2O3 glasses embedded with Au NPs heat-treated for 30 h at 450 °C are good candidates for nonlinear photonic, optoelectronic, and optical power limiting applications in photonics.

Nonlinear optical absorption characteristics of PbMoO4 single crystal for optical limiter applications

Molybdate materials take great interest due to their photocatalytic and optoelectronic applications. In this report, PbMoO4 single crystal, one of the member of molybdate materials, is grown by Czochralski technique and the change of nonlinear absorption characteristic depending on the input intensity was reported. Linear absorption analysis revealed the band gap energy and Urbach energy as to be 3.12 and 0.52 eV, respectively. Nonlinear absorption characteristics of the PbMoO4 single crystal was examined with the open aperture (OA) Z-scan experiments at 532 nm excitation wavelength under various input intensities. Fitting results of the OA Z-scan experiments indicated that PbMoO4 single crystal has nonlinear absorption (NA) behavior, and NA coefficient (βeff) increased from 7.11 × 10−8 to 1.96 × 10−7 m/W with increasing input intensity. This observation was associated with the increase of the contribution of the free carrier absorption to the NA with the generation of more excited electrons with increasing input intensity. At the 532 nm excitation wavelength (2.32 eV), the dominant mechanisms were revealed as one photon and free carrier absorptions. The optical limiting threshold of the PbMoO4 single crystal was obtained to be 4.91 mJ/cm2. The reported results indicated that PbMoO4 single crystal can be a good optical limiter in the visible wavelength region due to its effective NA behavior.