Nonlinear Saturable Absorption Properties 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.

Saturable absorption properties and ultrafast photonics applications of HfS3.

In this Letter, we focus on investigating the ultrafast photonics applications of two-layer HfS3 nanosheets. We prepared two-layer HfS3 nanosheets and carried out experiments to study their nonlinear saturable absorption properties. The results showed that the two-layer HfS3-based saturable absorber exhibited a modulation depth of 16.8%. Additionally, we conducted theoretical calculations using first principles to estimate the structural and electronic band properties of the two-layer HfS3 material. Furthermore, we utilized the two-layer HfS3 materials as SAs in an erbium-doped fiber cavity to generate mode-locked laser pulses. We measured a repetition frequency of 8.74 MHz, a pulse duration of 540 fs, and a signal-to-noise ratio of 77 dB. Overall, our findings demonstrate that the two-layer HfS3 material can serve as a reliable saturable absorber, possessing properties comparable to currently used two-dimensional materials. This expands the application fields of HfS3 materials and highlights their potential for advanced optoelectronic devices.

Carbon nanodots derived from organo-templated zeolites for femtosecond mode-locked fiber laser

Carbon nanodots (CNDs) have been widely investigated due to their physicochemical and optical properties. However, the nonlinear saturable absorption property and its application in laser technology is rarely reported. Here, we report the CNDs as an ideal saturable absorber (SA) for ultra-stable femtosecond mode-locked fiber laser at a wavelength of 1.5 μm. The mono-sized CNDs of 2 nm are fabricated from a dipropylamine-templated zeolite precursor calcination with a temperature of 380 °C. The nonlinear saturable absorption property of the as-synthesized CNDs is investigated by using a balanced twin-detector measurement system. The modulation depth and saturable intensity are measured to be 33 % and 3.1 MW/cm2, respectively. By inputting the CNDs SA into the erbium-doped fiber laser (EDFL) ring cavity, a highly stable mode-locked EDFL with pulse width of 388 fs and single-to-noise ratio (SNR) of 84.4 dB can be obtained. In addition, stable soliton pairs with single-pulse width of 422 fs and SNR of 84.1 dB can also be achieved. These findings demonstrate that the carbon dots can be used as an ideal SA and has promising applications in ultrafast photonics.

Enhancing saturable absorption in a Au-decorated MoS2/PEDOT:PSS nanocomposite through plasmon resonance and Pauli blocking.

We have effectively demonstrated a technique for substantial alteration of the nonlinear saturable absorption (SA) properties in nanocomposite films (NCF) composed of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) and molybdenum disulfide (MoS2) decorated with gold nanoparticles (AuNPs). This control is achieved by adjusting the AuNP concentration on the MoS2 surface and varying the input pulse energy of the laser. The simple drop-casting method is used to create the nanocomposite films (NCFs) on a glass substrate with different amounts of Au-decorated MoS2. The Kramers-Kronig equations are employed for determining the refractive index and extinction coefficient values of the resulting NCFs. Nonlinear investigations reveal that adding Au-decorated MoS2 to pure PEDOT:PSS alters its optical nonlinearity. Surface plasmon resonance and Pauli blocking have been observed in Au-decorated MoS2/PEDOT:PSS NCFs. This increases NCF's saturable absorption. An open aperture Z-scan method is utilized to study nonlinear optics, with excitation achieved using a nanosecond (ns) pulsed laser operating at a 532 nm wavelength. The findings reveal the noteworthy saturable absorption characteristics of the NCFs.

Role of Pauli blocking for enhancement of saturable absorption in MoS2/PEDOT:PSS nanocomposite films

We have effectively shown a technique for significantly altering the nonlinear saturable absorption (SA) properties of nanocomposite films (NCFs) based on poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) and molybdenum disulfide (MoS2) by regulating MoS2 concentration and input pulse energy of the laser. The NCFs are made using the straightforward drop-cast process on a glass substrate with varying quantities of MoS2. The produced NCFs’ refractive index (n) and extinction coefficient (k) values are determined using the Kramers–Kronig equations. Nonlinear studies show that the optical nonlinearity of pure PEDOT:PSS changes when mixed with MoS2. The Pauli blocking has been observed in MoS2/PEDOT:PSS NCFs. This leads to enhanced SA in NCF. The open-aperture Z-scan approach is used for the nonlinear optical research, and a nanosecond pulsed laser with a wavelength of 532 nm is used for the excitation. The findings obtained show the NCFs’ strong SA qualities.

Carbon nanomaterials as nonlinear optical modulators for pulsed laser generation at near-infrared region

In this study, carbon nanoparticles (CNPs) were experimentally investigated as novel saturable absorbers (SAs) capable of delivering strong ultrashort pulses in a passively Q-switched fiber laser. Carbon nanoparticles produced by candle burning and laser ablation can act as saturable absorbers (SAs). In addition, the nonlinear saturable absorption properties of the two synthetic CNPs in telecommunication region (1.5 μm) are investigated. Nonlinear optical measurements show that both SAs have good saturable absorption properties with average modulation depths of about ∼1 and 5 % at 1560 nm. These candle soot carbon nanoparticles (CSC-np) and laser ablation carbon nanoparticles (LAC-np) based SAs, can be used as a building blocker for the generating of stable Q-switched pulses in the near-infrared region. The shortest pulse duration measured with CSC-np and LAC-np are 5.62 and 3.87 μm, respectively. The pulse energy, peak power and SNR were also compared with both SA types at the highest pump power. The damage threshold and long-term stability of the CSC-cn SA and LAC-np were also explored. To the best of the author's knowledge, it is the first comparison of performance and long-term stability of EDFL based on two experimental techniques CSC-np and LAC-np based SAs. These results indicate that LAC-np SA provide have several advantages, including a high damage threshold, good saturable absorption, and optimum stability over a long time compared to CSC-np SA.

Nonlinear optical characteristics of germanium and its application for generating bound state solitons within Er-doped fiber laser

Recently, germanium (Ge) was also considered as the two-dimensional (2D) material saturable absorber (SA) because of its excellent nonlinear saturable absorption properties. However, the research of Ge SA Er-doped fiber laser is still rare. In this work, bound state solitons were demonstrated in Ge SA Er-doped fiber laser. To be best of our knowledge, no previous work has ever achieved that. Three kinds of bound state solitons with 9.77 MHz fundamental frequency were investigated, indicating that Ge has excellent nonlinear optical characteristics to promote the development of bound state soliton.

CrPS4 Nanoflakes as Stable Direct-Band-Gap 2D Materials for Ultrafast Pulse Laser Applications

Two-dimensional (2D) materials have attracted considerable attention due to their potential for generating ultrafast pulsed lasers. Unfortunately, the poor stability of most layered 2D materials under air exposure leads to increased fabrication costs; this has limited their development for practical applications. In this paper, we describe the successful preparation of a novel, air-stable, and broadband saturable absorber (SA), the metal thiophosphate CrPS4, using a simple and cost-effective liquid exfoliation method. The van der Waals crystal structure of CrPS4 consists of chains of CrS6 units interconnected by phosphorus. In this study, we calculated the electronic band structures of CrPS4, revealing a direct band gap. The nonlinear saturable absorption properties, which were investigated using the P-scan technique at 1550 nm, revealed that CrPS4-SA had a modulation depth of 12.2% and a saturation intensity of 463 MW/cm2. Integration of the CrPS4-SA into Yb-doped fiber and Er-doped fiber laser cavities led to mode-locking for the first time, resulting in the shortest pulse durations of 298 ps and 500 fs at 1 and 1.5 µm, respectively. These results indicate that CrPS4 has great potential for broadband ultrafast photonic applications and could be developed into an excellent candidate for SA devices, providing new directions in the search for stable SA materials and for their design.

NbS2-nanosheet-based saturable absorber for 1.5 µm and 2 µm ultrafast fiber lasers

Niobium disulfide is a kind of transition metal dichalcogenides which has the advantages of large modulation depth, short excited carrier recovery time and high carrier mobility, theoretically being very suitable for the applications in ultrafast photonics. Up to now, there are few reports on the application of NbS2 in ultrafast optical devices, especially in the devices with long working wavelength, such as 2 µm. In this work, few-layer NbS2 nanosheets were prepared by liquid exfoliation method and the NbS2-SA was prepared by covering the nanosheets on tapered fiber. The device NbS2-SA exhibits good nonlinear saturable absorption property with a modulation depth of 10.2 % at 1550 nm. Based on the NbS2-SA, both 1.5 µm and 2 µm ultrafast fiber lasers are realized. The 1.5 µm mode-locked fiber laser has a pulse width of 753 fs and the 2 µm mode-locked fiber laser has an estimated pulse width of 5.77 ps and a maximum output power of 5.56 mW. All the results prove that NbS2 is an excellent broadband saturable absorption material.

Carbon nanoparticles (CNPs) as a saturable absorber for a passively Q-switched erbium (Er3+) doped fiber laser

The carbon nanoparticles (CNPs) are demonstrated to be capable of acting as a saturable absorber (SA) for the Q-switching of an erbium doped fiber laser (EDFL). The Q-switched EDFL based on CNPs-SA emits at a central wavelength of 1566 nm with a 3 dB spectral bandwidth of 1.72 nm. The radio-frequency (RF) spectrum has a signal-to-noise ratio (SNR) of 42 dB at the fundamental repetiton rate, indicating a good pulse stability. The stable pulse operation can be maintained under the pump power of ranging from 28 to 272 mW. The pulse repetition frequency increases from 12.26 to 73.30 kHz, and the pulse narrows down from 23.32 to 6.27 μs, correspondingly. In addition, we investigate the nonlinear saturable absorption property of CNPs, and measure the SA’s damange threshold as well as the modulation depth. The synthesized CNPs are also characterized by Field Emission Scanning Electron Microscopy (FE-SEM), Energy dispersive X-rays (EDX), Raman, Photoluminescence (PL), X-rays diffraction (XRD), and Fourier Transform Infrared (FTIR) spectroscopy. These experimental results reveal that CNPs-based SAs can be equivalent or even superior to the widely used nanoparticle SAs, and will have potential applications for pulsed laser sources.

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.

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.

Realization of harmonic mode-locking in thulium-doped fiber laser using In2(SO4)3-based saturable absorber

Harmonic mode-locking (HML) is realized in a thulium-doped fiber laser (TDFL) using indium sulfate (In2(SO4)3) as a saturable absorber (SA). The In2(SO4)3 has nonlinear saturable absorption properties with the nonlinear optical modulation depth of 13.3 % and the saturation power of 0.11 kW at 1930 nm. The fundamentally mode-locked pulses operate at 1964.44 nm have the 3-dB bandwidth of 4.70 nm, the pulse width of 1.266 ps, and the pulse repetition rate of 25.37 MHz. The maximum resonance frequency of 228 MHz is achieved, which corresponds to the 9th harmonic. The supermode suppression level (SMSL) up to 63 dB is realized. The spectral and temporal characteristics of the output pulses are investigated as a function of harmonic order. The experimental results demonstrated that In2(SO4)3 can be a promising mode-locker for generating harmonic ultrafast pulses in 2 μm waveband.

Bi2O2Te Nanosheets Saturable Absorber‐Based Passive Mode‐Locked Fiber Laser: From Soliton Molecules to Harmonic Soliton

AbstractAs a recent addition to the emerging 2D bismuth oxychalcogenides (Bi2O2X, where X = S, Se, and Te), atomically‐thin bismuth oxytellurium (Bi2O2Te) exhibits unique thermoelectric and photoelectric properties. In this study, high‐quality Bi2O2Te nanosheets (NSs)‐based saturable absorber (SA) with a modulation depth of 8.2% is developed using the liquid phase exfoliation (LPE) method and is successfully applied to a passively mode‐locked Er‐doped fiber laser (EDFL) for the first time (with reference to the available literature). Due to the excellent nonlinear saturable absorption property of Bi2O2Te NSs, various switchable and stable mode‐locking states can be realized in the same EDFL, including fundamental frequency conventional soliton (CS), soliton molecules, and harmonic mode‐locking (HML) states. Among them, the CS pulse at 7.48 MHz (fundamental frequency) with a high signal‐to‐noise ratio (SNR) of 67.25 dB is obtained. The 2nd, 3rd, and 4th‐order soliton molecules are observed with a soliton pulse separation of ≈6.2 ps. Moreover, for HML, a maximum repetition rate of 1.78 GHz (i.e., corresponding to the 238th‐order harmonic) is obtained. The results reveal that Bi2O2Te NSs demonstrate excellent nonlinear optical modulation properties and can serve as a fundamental basis for promotion of academic research and innovation of engineering applications involving ultrafast photonics.

The nonlinear optical property of germanium saturable absorber and its application in normal dispersion Er-doped fiber laser

Recently, germanium (Ge) has been considered as the two-dimensional (2D) material saturable absorber (SA) due to its excellent nonlinear saturable absorption properties. However, previous reports explored the nonlinear saturable absorption properties of Ge SA in anomalous dispersion Er-doped fiber (EDF) laser. To date, the mode-locked operation achieved by normal dispersion EDF laser based on Ge SA has not yet been reported until now. In this work, Ge SA with saturation intensity of 11.69 MW/cm2 and modulation depth of 6.17 % was used to demonstrate a normal dispersion EDF laser. Morphological and nonlinear saturable absorption properties of Ge were also investigated. To the best of our knowledge, the mode-locked pulse with repetition rate of 6.66 MHz were achieved in normal dispersion EDF laser based on Ge SA for the first time. Compared with previous work about Ge SA in anomalous dispersion regime, our result proves that Ge also has excellent optical performance in normal dispersion regime.

Ultrathin gold nanowires as broadband saturable absorbers for ultrashort pulsed lasers

Ultrathin gold nanowires (UGNWs) with a diameter of ⩽2 nm have attracted much attention because of their high aspect ratios, unusual physical properties and potential applications in nanoelectronics, sensors and photonics. Despite recent advances on UGNWs and the related devices, nonlinear optical properties of UGNWs and their application on photonic devices (e.g. ultrashort pulsed lasers) remain largely unexplored. Here we show that UGNWs can be used as broadband saturable absorbers (SAs) for constructing ultrashort pulsed lasers with an operating wavelength range from near-infrared to mid-infrared (MIR). UGNWs are prepared by using a silane-mediated approach, and then mixed with polyvinyl alcohol to form the UGNWs film. The UGNWs film not only show strong absorption from visible to MIR spectral region (over 25 μm), but also possess the nonlinear saturable absorption properties in the spectral region. As the UGNWs film SA is inserted into an erbium or thulium doped silica fiber laser cavity, stable passively mode-locked lasers at 1562.2 or 1970 nm with a pulse width of a few hundreds of femtoseconds is obtained, respectively. Especially, by inserting the UGNWs film SA into a holmium/praseodymium codoped fluoride fiber laser cavity, stable passively mode-locked laser at 2864 nm is achieved. Our results show that the UGNWs film are promising SAs for constructing broadband ultrashort pulsed lasers.

Diode-pumped passively Q-switched Yb[formula omitted]:LuYLaVO4 laser with GaAs saturable absorber

A passively Q-switched (PQS) Yb3+:LuYLaVO4 laser based on a GaAs saturable absorber is reported. The nonlinear saturable absorption property of GaAs has been studied, the saturation fluence, modulation depth and non-saturation loss were characterized to be 3.3 MW/cm2, 4.5% and 54.7%, respectively. In the PQS laser operation, the highest pulse repetition rate, shortest pulse width, largest single pulse energy and highest pulse peak power were measured to be 40 kHz, 4 ns, 16.7 μJ and 4.2 kW. The peak power achieved nearly three orders of magnitude improvement compared with the former Yb3+:LuYLaVO4 PQS lasers.

Large-scale few-layered MoS2 as a saturable absorber for Q-switching operation at 2.3 µm.

In this Letter, the fabrication of large-scale (50.8 mm in diameter) few-layered MoS2 with physical vapor deposition on sapphire is described. Open-aperture Z-scan technology with a home-made excitation source at 2275 nm was applied to explore its nonlinear saturable absorption properties. The as-grown few-layered MoS2 membrane possessed a modulation depth of 17% and a saturable intensity of 1.185 MW cm-2. As a consequence, the deposited MoS2 membrane was exploited as a saturable absorber to realize a passively Q-switched Tm:YAP laser for the first time, to the best of our knowledge. Pulses as short as 316 ns were generated with a repetition rate of 228 kHz, corresponding to a peak power of 5.53 W. Results confirmed that the two-dimensional layered MoS2 could be beneficial for mid-infrared photonic applications.

Saturable absorption property and laser modulation performance of zeolitic imidazolate framework-67 (ZIF-67) at 2.85 µm

In this paper, we prepared zeolitic imidazolate framework-67 (ZIF-67) nanomaterials through the liquid-phase exfoliation method and investigated its nonlinear saturable absorption property at ∼ 3 µm by Z-scan and I-scan method. As a saturable absorber (SA) in the Er:Lu2O3 laser (2.85 µm), the materials show excellent passively Q-switched performance. Under the maximum pump power of 7.3 W, 256-ns laser pulses were delivered at repetition rate of 140-kHz, which corresponds to an average output power of 0.9 W. The experimental results indicated that the ZIF-67 SA is a potential SA candidate in passively Q-switched laser at ∼ 3 µm.

Nanosecond Q-switched laser with PEDOT: PSS saturable absorber.

We demonstrate deployment of the nonlinear saturable absorption property of the organic material poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) for pulse generation in the near-infrared region. The saturable absorber (SA) film was made using a straightforward process of depositing a layer of the PEDOT: PSS material onto a polyvinyl alcohol (PVA) film. The prepared SA was inserted into an erbium-doped fiber laser cavity as a Q-switcher to produce laser pulses with a maximum pulse rate of 92.75 kHz, minimum pulse duration of 912 ns, and highest pulse energy of 222.83 nJ. Results showed that PEDOT: PSS/PVA SA could become a promising SA for various laser applications. To our knowledge, this is the first time that PEDOT: PSS/PVA has been utilized as a SA to produce a stable Q-switched laser in 1.55 µm.