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Abstract
“Naked” ferroferric-oxide nanoparticles (FONPs) synthesized by a femtosecond laser ablation on a bulk stainless steel in liquid were applied to the Nd: YVO4 laser to achieve passive Q-switched pulse laser output. Without the pollution of ligand, the inherent light characteristic of “naked” FONPs was unaffected. The analysis of the morphological characteristics, dominant chemical elements, and phase composition of the FONPs showed that they were mainly composed of Fe3O4, which was spherical with an average diameter of 40 nm. The electron transition and orbital splitting of the iron element’s octahedral center position under the laser-driven were considered the primary mechanisms of saturable absorption of Fe3O4 nanoparticles.
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1. Introduction
With the rapid development of laser technology and nanotechnology [1–3], synthesis of nanoparticles by femtosecond laser ablation in liquid has become a widely researched topic [4–6]. Compared with the chemical synthesis of nanomaterials, laser synthesis technology does not produce ligands because it does not require the participation of chemical reagents in the preparation [7,8]. Therefore, the synthesized “naked” nanoparticles better reveal the native properties of uncontaminated nanoparticles, which are from laser ablation the target material in liquid [9,10]. Laser synthesis technology is a top-down method involving the disciplines of optics, materials, physics, and chemistry. It has a continuous operation mode, is suitable for post-processing, and has a high yield [11,12]. Compared with laser synthesis in air, laser synthesis of nanomaterials in liquid does not pollute the experimental environment, which effectively guarantees the safety of researchers and high-precision instruments [13,14]. The technology of synthesizing nanomaterials by laser ablation in liquid is “green chemistry,” with strong surface activity, simple preparation, and pollution-free production [15,16]. However, the mechanism of nanoparticle generation remains inaccessible [17,18]. The main mechanism of laser synthesis of nanoparticles in air is the Coulomb and phase explosion; the mechanism will also change with the cumulative energy inside the material [19–21]. The mechanism of laser synthesis of nanoparticles in liquid, formed by nucleation, formation, and solidification, is related to the dynamics of laser, liquid, and target material. In the experiment, the differences in material property, shape, chemical composition of the liquid, and laser parameters, will lead to the various morphologies and functions of the nanoparticles [22,23]. The liquid environment of the laser ablation material will induce the special physics in high temperature and high pressure, and the dynamics of cavitation bubbles, which may provide favorable conditions for chemical reactions. Therefore, the nanoparticles are synthesized using the explosive mechanism with the phenomena of plasma and gas-phase chemical reactions in liquid [24,25].
The ferroferric-oxide nanoparticles (FONPs) have remarkable physical properties as transition metal oxides [26,27]. They show considerable potential in various applications, including medical biology, optics, and information fields [28–31]. Additionally, they exhibit strong semiconductor properties owing to the finite size effect [32,33]. The application studies of FONPs in lasers have been reported. In 2019, Li et al. realized a Q-switched pulse laser with FONPs, which were synthesized by chemical co-deposition with an average particle size of 15 nm and deposited onto a thin film for a 1 µm fiber laser [34]. In 2020, Wang et al. deposited FONPs prepared by chemical methods into an excessively tilted fiber grating to achieve passively mode-locked laser output with an average particle size of 54 nm [35]. Yang et al. deposited Fe3O4 nanoparticles with an average particle size of 15 nm into a gold mirror to form a thin film, and a ZBLAN fiber laser with dysprosium-doped exhibited a 3 µm laser output [36]. Cheng et al. obtained Fe3O4 nanoparticles via water-based magnetic fluid and dried them into thin films, which were applied in a laser to achieve Q-switched mode-locked fiber laser output of 1.5 µm pulses. Additionally, the nanoparticles had an average particle size of 10 nm [37]. By 2021, Li et al. synthesized Fe3O4 nanoparticles using thermal decomposition, combined them with chemical agents to form thin films for applying in fiber lasers, and achieved passive Q tuning, respectively, to achieve laser output of three different wavelengths (1 µm, 1.5 µm, and 2 µm); the average particle size was 20 nm [38]. The results show that Fe3O4 nanoparticles can realize Q-switched or mode-locked fiber lasers; however, the Fe3O4 nanoparticles used by the researchers may have been contaminated by the participation of other chemical reagents during the preparation and cannot fully reflect the essential characteristics of Fe3O4 nanomaterial. Furthermore, in terms of the material characteristics of Fe3O4 nanoparticles, only the output parameters of the laser are discussed; the mechanism of saturable absorption is not analyzed.
In this study, the “naked” FONPs synthesized using femtosecond laser ablation on a bulk stainless steel in liquid were applied to a CW laser with Q-switched pulse output for the first time. The saturable absorption of FONPs was analyzed from the material characteristics. The morphology, color, phase, and chemical compositions were discussed to confirm the existence of Fe3O4 in the FONPs. Moreover, the saturable absorption of FONPs was first analyzed in terms of atomic orbit. Owing to the finite size effect and inverse spinel crystals of FONPs, iron extra-nuclear electrons are very active. The transition (L-edge) manifests at the core of the octahedron when the electron clouds of the iron atomic orbit absorb energy to a threshold. Then, the electrons in the valence band are subsequently shifted towards the conduction band; the FONPs’ saturable absorption will be achieved.
2. Experimental setup
The experimental setup used for synthesis of nanoparticles by femtosecond laser ablation in liquid is shown in Fig. 1, including a laser, collimation white-light source (LED), beam transmission, image monitoring (CCD), and a three-dimensional translation stage. The real-time monitoring system composed of a LED and a CCD was mainly used for monitoring the processes of the femtosecond laser ablation. Gaussian femtosecond laser beams with a center wavelength of 780 nm were output by a commercial Ti: Sapphire laser (Spitfire, Spectra Physics). The maximum repetition rate of the pulse laser was 1 kHz, and the pulse duration was 230 fs. The pulse energy of the incident laser beam could be finely adjusted by rotating a double wave-plate (DWP), and the laser beam was perpendicularly incident to the flat iron block in deionized water.
The flat stainless steel block was set in a vessel with deionized water, which can be moved by a 3D translation stage. The FONPs were synthesized in the deionized water by laser beam ablation of the stainless steel block in deionized water. The round flat stainless steel block used in this experiment was a standard industrial stainless steel 1Cr13 with a thickness of 2 mm and a diameter of 30 mm. The height of the deionized water in a 100 ml container is about 6 mm, and the upper surface of the stainless steel sample is about 4 mm above the water surface. Continuous fold-back straight-line ablation was adopted, and the linear speed was 0.3 mm/s.
The diameters of the FONPs synthesized by laser ablation were measured using a ZEISS SPRA 55 field-emission scanning electron microscope (FE-SEM) operated in the secondary electron imaging mode. The chemical composition was characterized using an energy dispersive spectrometer (EDS) attached to a ZEISS SPRA 55. The structural characteristics of the FONPs were obtained by X-ray diffraction (XRD).
3. Results and discussion
The FONPs synthesized by femtosecond laser synthesis in liquid were kept in the vessel, and then the colloidal solution was closed with the lid. It was naturally air-dried to powder form in the super-clean laboratory at 18°C, and this time period was about six months. For the experiment, the powdered nanoparticles were then diluted with alcohol into a colloidal solution with a concentration of 0.05 mg/ml. Five milliliters of the colloidal solution was sucked with a graduated plastic pipette and dropped onto a microscope slide with 1 mm thickness, and then it was naturally air-dried about 2 days in the super-clean laboratory at 18°C. After being dried, the FONPs were absorbed on the surface of a microscope slide. Finally the saturable absorber (SA) was prepared. In the experiment, the SA was a microscope slide covered with FONPs.
The CW laser used in the Q-switched experiment was in the linear flat cavity in Fig. 2(a), which included an Nd: YAG laser crystal, a SA, and laser mirrors M1 and M2. The gain medium with an Nd3+ concentration of 0.7% was cut into dimensions of 17 mm in length and 3 × 3 mm2 in cross-section, and both of its light-passing faces were coated with 1064 nm anti-reflection films. Diachronic mirror M1 (R = ∞) was coated with films of 808 nm high transmission and 1064 nm high reflection, and M2 (R = 100 mm) focused a 1064 nm beam on the region of FONPs (SA), which was used to output laser with a 15% transmittance. The length of the laser cavity was 120 mm in our experiment; the laser spot diameter on the surface of the SA was 150 µm. A photodetector (PD) was employed behind M2 to detect the signals from the laser.
Fig. 2. a) Schematic diagram of passively Q-switched Nd: YVO4 laser by the FONPs; b) The pulse trains and single pulse profiles of the Q-switched laser.
After shaping by the lenses, an 808 nm fiber-coupled diode laser (LD) was used to pump an Nd: YVO4 crystal, which was cooled at 20°C by water in the meantime. With an 808 nm CW-laser pump, the laser mirror (M2) outputs a 1064 nm pulse laser, as shown in Fig. 2(b). The repetition rate and pulse duration were measured to be 100 kHz and 1.2 µs by a fast photodetector (DET10 A/M, Thorlabs, Inc., USA) and a digital oscilloscope (TDS5000B, Tektronix, Inc., USA), respectively.
Figure 3 illustrates the surface morphologies of the FONPs made by laser synthesis at a laser fluence of 59.2 J/cm2, where the femtosecond laser power is 850 mW and the continuous ablation duration is about 4 hours. The FONPs are spherical, and the average diameter is about 40 nm from the size distribution. In Table 1, the FONPs ablated by a femtosecond laser in liquid contain the main chemical elements of iron, carbon, and chromium. There are also other elements in the FONPs, such as oxygen and nitrogen, which could be generated from air during the oxidation.
Fig. 3. a) The SEM image of FONPs; b) The size distribution of nanoparticles; c) The spherical nanoparticles of a FONP; d) The XRD results of FONPs.
Table 1. Actual chemical compositions of FONPs by femtosecond laser synthesis in liquid
The structural characteristics of the FONPs are obtained by X-ray diffraction (XRD) in Fig. 3(d), and it is noted that a series of characteristic peaks appeared at the points of 220, 311, 113, 400, 422 and 440, which were corresponding to 2θ values of 30.1°, 35.5°, 42.1°, 44.5°, 53.4°, and 62.6°.
The diffraction peaks are matched with the inverse spinel structure phase of Fe3O4 except 113 by the standard card JCPDS card no. 79-0419 [39,40]. The peak of 113 corresponds to the crystal properties of γ- Fe2O3. The FONPs obtained by laser synthesis are black, and γ- Fe2O3 is brown. There are still some small peaks, which may correspond to the oxides of chromium and a little bit of iron oxides. The above results confirm the presence of the crystalline phase of Fe3O4.
In Fig. 4, Fe3O4 has an inverse spinel pattern with alternating octahedral and tetrahedral layers. In a unit cell, tetrahedral (A) sites are populated by Fe3+ ions, and octahedral (B) sites contain both Fe3+ and Fe2+ ions [41–43]. The center of a crystal structure is occupied by the octahedral structure, where the electrons are fragile. Under the condition of changes from the external physical states such as temperature field and laser field, etc., the electrons are delocalized from the original orbit [44–47]. The results from scanning transmission electron microscopy showed the changes in the electronic state of Fe ions under 2p → 3d transitions (L- edge) under externally excited occurrence [48–50]. Moreover, 3d atomic orbits in the vicinity of the L-edge transitions were sensitive to splitting under the disturbance of external physical fields [47,51,52]. The results from the scanning z-scan technique showed the nonlinear optical properties of the FONPs, such as two-photon absorption and the Kerr coefficient, and the coefficients were (1.6 ± 0.8) cm/GW and - (3.5 ± 1.5) × 10−14 cm2/W, respectively [53–55]. Therefore, as a transition metal oxide, the atomic orbit of the FONPs is unstable depending on the condition of the external physical fields, and the optical properties induced by the electron transitions are apparent. Thus, it has been a challenge to determine the fundamental properties of FONPs, such as their energy band gap, optical properties, and electronic structure, owing to their nonstoichiometric chemical composition.
Fig. 4. Schematic of a positive transient differential transmission in L-edge domains from the octahedral (B) site of a unit cell of Fe3O4. The right inset is the band gap structures of L-edge domains. The left inset is the atomic orbital structures of the octahedral site.
The FONPs synthesized using femtosecond laser ablation in liquid were inserted into the linear cavity of the CW laser. Subsequently, as the loss began to increase, laser energy further accumulated in the cavity [56,57]. When the laser energy was large enough to reach a threshold, the atomic orbital transition (L-edge) appeared on the octahedral (B) site of a crystalline structure, and the iron 3d orbit may have split, as shown in Fig. 4. The escaping electrons filled the band gap of FONPs, and the energy band gap closed. At this time, the laser passed through the FONPs in the cavity, where the gain and loss were almost equal, and the output laser from M2 was stable. The energy gap of the “naked” FONPs with an average particle size of 40 nm was about 0.75 eV [52,58,59], which is less than the photon energy (1.16 eV) of the laser pulse used to stimulate the orbit dynamics of FONPs. With the output of the laser, the electron-hole pairs of FONPs were recombined owing to the decrease in laser energy in the cavity [60–62]. Subsequently, the opening of the band gap could slow the carrier’s recombination. The laser could not pass through the FONPs, and energy recommenced to accumulate in the cavity. Thus, the FONPs had a saturated absorption function to achieve Q-switched pulse laser output.
4. Conclusions
In summary, “naked” FONPs were synthesized using femtosecond laser ablation in liquid, and they were applied to a linear cavity with Q-switched pulse laser output. The analysis of the SEM and XRD verified the phase Fe3O4 in the FONPs, which are spherical with an average diameter of 40 nm. In addition, the changes of electron transition were discussed through the results of scanning transmission electron microscopy and Z-scanning technology. As the energy in the cavity increases to a threshold, the band gap of FONPs was filled. At the moment, a laser of 1064 nm passed through the FONPs and achieves output. The energy band gap restarts to open after the decrease in laser energy in the cavity. Thus, the FONPs have a saturable absorption.
Funding
the Key R&D Project in Shaanxi Province of China (2023-ZDLGY-37).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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