Structural investigations
The typical XRD pattern of the structures prepared by the pulsed Nd-YAG laser ablation of magnesium target in acetone is given in Fig. 3. The XRD pattern indicates that both MgO and Mg are presented in the sample with a higher percentage of Mg. It is clear from the XRD pattern that the prepared powders are polycrystalline structures inclusive Mg and MgO. The weight percentage of each Mg and MgO structures can be calculated by Rietveld method [25]. The Mg/MgO ratio in the final product synthesized by laser ablation of magnesium target in acetone is 65/35 percentage. The formed phases in the XRD pattern of Mg/MgO microstructures are the same of those are reported by Phuoc et al. and Abrinaei et al [20, 21].
During the ablation process, each laser pulse with quite high energy passes through the acetone above the magnesium target and increases its temperature. When the front part of the laser pulse interacts with the target, it induces a plasma plume on the surface, which is quite massive. Hence, the heat that is transferred between hot confined plasma and the surrounding liquid increases the temperature of the liquid.
The Mg/MgO microstructures could be formed in three stages. In the first stage, after the interplay among magnesium target and laser beam, the high-temperature and high-pressure plasma is generated in the magnesium target and acetone interface. In the next step, the Mg clusters are produced because of successive ultrasonic and adiabatic expansion of the high-temperature and high-pressure magnesium plasma that makes a cold zone of magnesium plume [26, 27]. In this experiment, the interval between two consecutive pulses of Nd-YAG laser is 0.005 s (repetition rate is 200 Hz) and it is much longer than the lifetime of the magnesium plasma plume. Hence, the next laser pulse does not interact with the previous plasma plume. In the third step, the plasma quenches and the produced Mg clusters envisage the acetone and occur some chemical reactions and lead to the formation of Mg/MgO microstructures. Outwardly, Mg clusters generated by laser ablation of Mg in acetone were oxidized in this solvent, possibly under the high-energy conditions via reaction with the oxygenated solvent. The observed color changes in acetone suggested that the solvent was altered or decomposed due to laser heating or reaction with the Mg particles [28].
Optical properties
Figure 4 shows the UV-VIS optical absorption spectrum of the colloidal suspension of the sample in acetone. Obviously, there is a characteristic absorption band at about 410 nm.
The measurement of the energy band gap, Eg, is important in the micro- and nano-materials. There are various methods for calculation of Eg. In this work, the energy band gap was determined using UV-Vis absorption spectrum of Mg/MgO microstructures. The derivative method was applied for measurement of Eg. In this method, the first derivative of the absorbance was evaluated near the fundamental absorption edge, leading to Eg [29]. The energy band gap value is obtained equal to 2.3 eV for Mg/MgO microstructures.
The electron configuration of magnesium is 1 s2 2 s2 2p6 3 s2 that 3 s shell is full. Then, the 3 s shell of Mg would not allow for electrons to increase energy on its own if the 3p band was separated by a gap from the 3 s band. But, in magnesium the energies of 3 s and 3p bands overlap and the 3p band can incorporate six electrons per atom (2 (2 l + 1) =6), overall 3 s and 3p orbitals from a band can incorporate eight electrons. Consequently, the quarter of conduction band in magnesium is full merely. Thus, magnesium is a good conductor while magnesium oxide is close to an ideal insulating ionic solid with a valence band structure dominated by the strong potential of the ionic cores [19].
The calculated band gap of Mg/MgO microstructures synthesized by the laser ablation method in acetone is significantly smaller than the wide band gap energy 7.8 eV expected for the bulk, pure, crystalline MgO. Therefore, it is clear that Mg/MgO microstructures have metallic conduction behavior [18].
FTIR transmittance spectrum in the wave number range of 4000–500 cm−1 for the Mg/MgO microstructures produced by laser ablation of Mg in acetone is presented in Fig. 5.
In the FTIR spectrum, the stretching bands of the superficial OH groups are seen in the region between 4000 and 3500 cm−1. The absorption band around 3448 cm−1 is broad and corresponds to the –OH-group-stretching vibration that is dedicated to –OH-stretching mode of residue water and absorbed acetone on the surface of Mg/MgO microstructures. This peak shows the presence of hydroxyl groups at low coordination sites or defects [30]. The absorption bands around 2926 and 2851 cm−1 are due to surface OH stretch appearing from hydroxyl groups in dissociated state or C–H stretch of organic residue [31], where these peaks are due to surface OH stretch in-phase and out of phase, respectively. The peak at 1672 cm-1 was attributed to the bending vibration of the water molecule. An absorption peak at 1457 cm−1 is assigned to νa (C–O) + δ (OC = O) modes. Thus, these contain signatures of adsorption and chemisorption of water and acetone. Tow sharp absorption peaks at 1107 and 970 cm−1 are devoted to C–O/C–O stretching modes. While the band at 861 cm−1 corresponds to ν (Mg–O) + δ (O–C = O), a peak at 673 cm−1 is ascribed to bending mode of O = C = O or vibration of water. The two peaks at 450 cm-1 and 515 cm-1 affirmed the presence of Mg-O vibrations [32].
Morphological properties
The morphology of the structures examined by scanning electron microscopy (SEM) is shown in Fig. 6.
The SEM image shows that the Mg/MgO microstructures are constructed of particles in the nearly square shape. The obvious SEM picture of Mg/MgO structures was obtained after about 10,000 times grandiosity. The lines appeared in this image related to aluminum foil on which colloidal solution was dried.
The corresponding particle size distribution of the mean sizes is shown in Fig. 7. As shown in the figure, the size distribution width becomes narrow from 1 to 1.5 μm.
Phuoc et al. prepared Mg/MgO nanoparticles in acetone and isopropanol by Nd: YAG (λ = 1064 nm) laser ablation of a magnesium target with distribution sizes were ranged from 15 to 20 nm up to 50 – 100 nm [20]. Abrinaei et al. applied the Nd-YAG (λ = 1064 nm) and cooper vapor laser beam and formed the Mg/MgO spherical and plate-like nanostructures and cubic microstructures with distribution sizes were ranged from 80 to 100 nm and 1–1.1 μm, respectively [21]. As a comparing the results, it approved the results of previous work’s author that showed the parameters of laser and liquid medium in laser ablation experiment significantly alter the shape and size of resultant products [21].
Nonlinear optical properties
NLO parameters, the nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of the colloidal Mg/MgO structures were obtained by the following relationships [23]:
$$ {T}_{norm}= L n\left(1+{q}_0\left( z, t\right)\right)/{q}_0\left( z, t\right) $$
(1)
$$ {q}_0\left( z, t\right)=\beta I{L}_{eff}/\left(1+{z}^2/{z}_0^2\right) $$
(2)
$$ {L}_{e ff}=\left(1-{e}^{-\alpha L}\right)/\alpha $$
(3)
$$ \alpha =-\left(1/ L\right) L n\left( I/{I}_0\right) $$
(4)
$$ \left|\varDelta {T}_{p- v}\right|=0.406{\left(1- S\right)}^{0.25}\left(2\pi /\lambda \right){n}_2 I{L}_{eff} $$
(5)
Where Tnorm is the normalized transmission in the open-aperture Z-scan setup and q0(z,t) is a dimensionless factor, ∆Tp-v is the normalized difference between the peak and the valley in the curve of normalized transmittance (Tnorm) versus location of the sample (z). In these equations, λ is the wavelength of radiation (532 nm), I is the intensity of radiation, S is the fraction of radiation detected by the detector (the transmittance of the aperture), α is the linear absorption coefficient, L refers to the sample length (1 mm), and Leff is an effective sample thickness, which was measured by OL setup shown in Fig. 2.
Optical limiting
In the recent years, the selection of NLO materials in which increase influence of light leads to significant decreases in transmittance is taken into consideration. These materials are used for optical power limiters devices. Investigating of new materials as the optical limiter is important for the protection of a person’s eye and optical sensors from laser irradiation. In this work, the OL experiment was carried out by locating the sample at focus location and evaluating the transmitted power thru the aperture for several incident laser powers. By the OL consideration, the critical power of the laser beam at which the nonlinearity starts to affect the transmission can be measured. It is evident that the materials are suitable for OL applications that possess the lower OL threshold.
The experimental setup for OL measurements is shown in Fig. 2. In the OL configuration, the aperture is not used. The sample is put near the focal plane of the lens and the input power is changed after crossing the sample. A 50% beam splitter divides the initial power into the half. The power meter 1 is used to measure the input power. The output power of the transmitted beam through the Mg/MgO solution is measured by power meter 2.
The plot of output power versus input power for Mg/MgO microstructures synthesized by laser ablation of magnesium target in acetone is shown in Fig. 8. A threshold is attained at 20 mW of the input power. After 20 mW, the output power stabilized against the input power. At a low incident power up to 20 mW, the output power alters linearly with a ratio of I/I0 = 0.89. By using the Eq. (4), the linear absorption coefficient for these microstructures is obtained: α = 1.16 cm-1.
The plot of output power versus input power for Mg/MgO nanostructures synthesized by laser ablation of magnesium target in isopropanol is shown in Fig. 9. A threshold is reached at 20 mW of the input intensity with slight variation in the output intensity for larger amounts of the input intensities. At a low incident power up to 20 mW, the output power alters linearly with a ratio of I/I0 = 0.88. By using the Eq. (4), the linear absorption coefficient for nanostructures prepared by laser ablation of magnesium target in isopropanol is obtained: α = 1.27 cm-1.
The OL results confirm that structures prepared by laser ablation of Mg target in acetone and isopropanol are good candidates for OL at 532 nm pulsed lasers.
Closed- aperture Z-scan
The closed-aperture normalized transmittance curve for microstructures synthesized in acetone is shown in Fig. 10. For a sample that exhibits both nonlinear absorption and refraction, their contribution to the far-field beam profile and Z-scan transmittance is coupled. However, it is simple to eliminate the nonlinear absorption contribution to the closed-aperture data. To determine the correct value, the normalized closed-aperture Z-scan data should be divided by the open-aperture Z-scan data to retrieve n2.
Self-focusing and self-defocusing of laser radiation in nonlinear media is a well-known effect that occurs due to the nonlinear index of refraction n2. Self-focusing is the effect of positive n2 while self-defocusing occurs when n2 is negative. As shown in the Fig. 10, the curves exhibited a peak-to-valley shape indicating a negative value of the nonlinear refractive index, n2, that shows Mg/MgO microstructures act as a self-defocusing material.
In Fig. 10, the solid curve shows the theoretical fit to the experimental data. The nonlinear refractive index can be measured by fitting the experimental data with the Eq. (5). The nonlinear refractive index of the synthesized structures in acetone is obtained equal to 8.2 × 10-13 cm2/W. Here, the value of aperture linear transmission, S, is 0.3.
The closed-aperture normalized transmittance curve for nanostructures formed in isopropanol is shown in Fig. 11. As shown in the Fig. 11, the curves exhibited a peak-to-valley shape indicating a negative value of the nonlinear refractive index, n2, that shows Mg/MgO nanostructures act as a self-defocusing material.
The solid curve shows the theoretical fit to the experimental data, In Fig. 11. The nonlinear refractive index can be measured by fitting the experimental data with the Eq. (5). The nonlinear refractive index of the synthesized nanostructures in isopropanol media is obtained equal 2.2 × 10-12 cm2/W.
Open-aperture Z-scan
The open-aperture Z-scan allows measuring the nonlinear absorption coefficient, β. When the sample is located far from the focal point, the laser radiation intensity is low and T(z) is close to 1. The intensity becomes higher as the sample moves closer to the lens focal point. As the result of the positive nonlinear absorption, T(z) becomes smaller and reaches the minimum at the focal point. In the case of negative nonlinear absorption, the reverse picture occurs.
Figure 12 illustrates the open-aperture Z-scan curve of colloidal Mg/MgO microstructures measured by Z-scan setup. It is seen that the open-aperture transmittance has a minimum transmittance. The minimum transmittance confirms the presence of reverse saturation absorption in Mg/MgO microstructures. A fit of the Eq. (1) to the experimental data (solid curve) is depicted in Fig. 12, and yields the value of the nonlinear absorption coefficient β = 1.15 × 10−8 cm/W. The open-aperture z-scan data for Mg/MgO microstructures synthesized in acetone was fitted with two-photon absorption (2PA) theoretical curve.
The open-aperture normalized transmittance curve for nanostructures formed in isopropanol is shown in Fig. 13. As shown in the Fig. 13, the open-aperture transmittance has a minimum transmittance. The minimum transmittance confirms the presence of reverse saturation absorption in Mg/MgO nanostructures. A fit of the Eq. (1) to the experimental data (solid curve) is depicted in Fig. 13, and yields the value of nonlinear absorption coefficient β = 1.03 × 10−7 cm/W. The open-aperture z-scan data for Mg/MgO nanostructures synthesized in isopropanol was fitted to 2PA theoretical curve, properly.