Introduction

External control of the lasing action as well as random lasing is an interesting phenomenon in recent years which can be achieved using electric filed, temperature as well as magnetic field [1,2,3]. This ability to achieve controllable random lasing via external fields such as magnetic field needs efficient magnetic media or nanoparticles (NPs) such as TiO2 and Fe3O4 NPs to manipulate the distribution of Fe3O4 NPs, which in turn alters the formation of the coherent loops and properties of laser action [4]. Also, random lasing action by ZnO NPs film deposited on a NiFe thin film can result in lasing mode switching [5]. The magnetic switching of random laser (RL) is attributed to the direction and polarization dependent emission of light in the ferromagnetic nematic liquid crystals in an external magnetic field [3]. The main difference between this type of laser and the rest of the lasers remains in the mechanism of confining the light within the active medium [6]. From a random laser physics perspective, the combination of feedback by multiple scattering and gain leads to occurrence of lasing. Narrow spectral emission features (typical line width of features < 1 nm) are found in most random laser experiments Nevertheless, the origin of narrow spectral features, called “spikes”, remains a subject of great controversy in the multiple scattering community [7].

However, there are several approaches to break through the bottlenecks of mirrors absence or cavities in RL in comparison to the conventional lasers. Here, we classify the related studies into four main categories: wavelength manipulation, mode control, directional confinement, and threshold abatement. Wavelength manipulation is one of the most crucial parts of random lasers. There are two kinds of tuning designs, which are known as preprocess and post process. The tuning strategies of preprocess include modifying the absorption condition, size of scatterers, and geometry of photonic crystals. Further, external parameters such as optics, electricity, temperature, and deformation can realize wavelength tunability after the fabrication of disordered nanostructures, so-called post process. Further, people drive persistent endeavors to control the random lasing modes. Mode-locking and single-mode random lasers, the longstanding scientific goals, have been demonstrated by the pumping scheme, Raman gain, intentional defect sites, and bioinspired photonic structure. Mode-transition can also be accomplished by altering the pumping condition, modulating the concentration of gain media or scatterers, and the mechanically induced reformation. Although inherent angle-free emissions of random lasers are useful for some specific applications, a directional output is also highly desired. Several directional confinements have been introduced into random lasing systems, such as low-dimensional cavities, optical waveguides, and customized pump profiles. The strategies for lowering the lasing threshold have been extensively studied, e.g., optimization of the mean free path and particle size or fine-tuning the concentration or refractive index between scattering and gain media [4].

On the other hand, plasmonic RL is another type of lasers, which introduce to reduce the lasing threshold and enhance the coherency of lasing process [8]. In this kind of RL, the gain heavily depends on the strength of scattering medium where the light interacts with these disturbed amplification media in such systems [9]. Fluorescence resonance energy transfers and surface plasmon resonance have also been highly addressed. In another situation, if the hot electrons depleted their energy in the metal film, the temperature of plasmonic nanostructures will increase significantly within minutes and then gradually saturated with the environment, which is called thermoplasmonic effects of plasmonic nanostructures [10,11,12,13,14,15]. The thermoplasmonic effects have been widely used in the fields related to energy harvesting [16,17,18,19,20], lithography [21], photothermal therapy [22,23,24,25,26,27,28], solar desalination [29, 30], and photo detections [31, 32].

Now, ferrous ferric oxide nanoparticles (Fe3O4 NPs) are chosen to develop the magnetic controlled RL. R6G laser dye is selected from various laser dyes as the gain media because of its exceptional chemical stability and high efficiency. Through disorder-induced scatterings, the scattered photons can form coherent loops to trigger laser action. With an applied magnetic field, the concentration of Fe3O4 NPs can be manipulated, enabling control of the formation of coherent loops. In this report, we use the combination of plasmonic and magnetic NPs to obtain controllable RL in external magnetic field with good efficiency.

Experimental setup

Rhodamine powder (C28H31N2O3Cl) with a molecular weight of 479.03 g/Mol was purchased from Chem Supply Australia Pty Ltd and dissolved in ethanol (99.99%), While Au gold pure bar and wire were used as electrodes in an electrical exploding wire (EEW) technique [33]. Furthermore, Fe3O4 nanoparticles of purity 99.5%, average particle size of 20 nm, was diluted in distilled water, used as received. Dye powder of 0.191 g was dissolved in (40 ml) of ethanol to produce a solution of the R6G dye with concentration (1 × 10–2) M. Different concentrations of Rh6G dye solution were prepared according to the following formula [34].

$$ c = \frac{W}{Mw}\frac{1000}{V} $$
(1)

where C represents the concentration of the dye in (M = mole/l), W describes the weight of the dye powder in g, Mw, refers to the molecular weight of the dye in g/mole, and V is the volume of the solvent in ml.

The high dye concentration of 10–2 M was diluted via the dilution method to obtain different concentrations of dye (10−3, 10−4, 5 × 10–5, 2 × 10–5, 10–5, 5 × 10–6 and 10−6 M), according to the formula [33]:

$$ C_{1} \times V_{1} = C_{2} \times V_{2} ; $$
(2)

where C1 and V1 are the molarity and volume of the concentrated solution, C2 and V2 denote the molarity and volume of the diluted solution.

The absorption and emission spectra were taken for these different concentrations of the dye by the UV–Visible spectrometer to select the best concentration, which will be used in work.

The preparation of the gold nanoparticles AuNPs by the EEW technique was done as illustrated in Fig. 1. The Au plate was used as a cathode and the Au wire (0.3 mm diameter) has been utilized as an anode terminal. Terminals have been connected to a power supply of 82 V and 100 A. A hard plastic holder has held the Au plate and fixed in a glass vessel with 60 ml of distilled water with magnetic stirring for all the preparation time. The electrical circuit stayed open to the point where the contact has been made manually between the wire and plate for quite a short duration, which results in an explosion through the wire, and then many explosions were made. This metal vapor creates a lower resistance path, allowing an even higher current to flow. Then an electric arc is formed that turns the vapor into plasma, resulting in a bright flash of light. The plasma is allowed to extend freely, creating a shock wave. Electromagnetic radiation is released in tandem with the shock wave. The shock wave pushes liquid, gaseous and plasmatic metal outwards, breaking the circuit and ending the process [33].

Fig. 1
figure 1

Experimental setup to fabricate AuNPs

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In addition, Fe3O4 nanoparticles were prepared in five different concentrations via a dilution process of 5 mg in 20 ml of distilled water. After preparation, X-ray diffractometer and the transmission electron microscope (TEM) were used to investigate, the size and shape of the produced NPs. UV–VIS absorption spectra of the samples were recorded.

To obtain a gain medium for our random laser, different concentrations of Au NPs and Fe3O4 NPs were mixed with the R6G dye (1 × 10–5) M as the best dye concentrations due to its luminescence spectra. The mixing ratio was (4:2), where (4 ml) of the R6G dye was mixed with (2 ml) from each concentration of the AuNPs. The prepared samples were stirred at room temperature in an ultrasonic bath for about 15 min to obtain the best homogeneity of the R6G dye molecules with AuNPs. Through this mixing process, five samples of this mixture were obtained, after which the UV–visible and the emission spectra were investigated for the five solutions. For the third group of samples, mixtures of these NPs as Au 10% + Fe3O4 90%, Au 30% + Fe3O4 70%, Au 50% + Fe3O4 50%, Au 70% + Fe3O4 30%, and Au 90% + Fe3O4 10% were prepared as S1 to S5 samples. These mixtures were mixed with the best concentration of the R6G (10–5 M) to prepare the final gain media.

Finally, all of the samples were examined by the random laser experimental setup with and without an external magnetic field of 35 mT.

Results and discussion

TEM images of the prepared NPs are depicted in Fig. 2 for Au NPs and Fe3O4 in (a), and (b), respectively. As shown in Fig. 2, there are less than 50 nm NPs sizes with overall circular shape and for the magnetic ones, surely there are single domain ones based on this size based on some important reports [34]. Based on this fact that for Fe3O4 NPs less than 20 nm, we have superparamagnetic state and from 20 to 50 nm, we can estimate single domain construction.

Fig. 2
figure 2

TEM image of a Au NPs and b Fe3O4 NPs; the scale bar set to 100 nm and 1 µm respectively

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Furthermore, absorption spectra of Au and Fe3O4 NPs and their mixture are shown in Fig. 3a, b confirming the localized surface Plasmon resonance of the gold NPs in different concentrations and the interband absorption of the magnetic NPs plus the mixture of these resonances in Fig. 3c.

Fig. 3
figure 3

Absorption spectra of a Au NPs, b Fe3O4 NPs, c mixture of Au and Fe3O4 NPs and Emission spectra of d Au NPs and e mixture of Au and Fe3O4 NPs in Rh6G medium

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To start random lasing efficiency, the emission spectrum of each group of NPs in the Rh6G solution was recorded as depicted in Fig. 3d and e. The emission intensity of each group of samples at the best concentrations of the dye medium and at different pumping intensities from 20 to 60 micro joules were recorded, confirming that the better gain and coherency in the random lasing was found at a higher pumping energy.

As outlined in Fig. 3e, a lower full width at half maximum (FWHM) for the mixture sample can be seen due to the plasmonic effect of the gold NPs with the Fe3O4 neighboring ones which is confirmed by thermoplasmonic effect in these kinds of NPs.

In the next step, the data of the five different NPs mixtures in different pumping energies were collected. The random laser response of the different mixtures samples pumped by 2.3 mJ, without and with external magnetic field is shown in Fig. 4a and b, respectively.

Fig. 4
figure 4

The lasing emission spectra of Au-Fe3O4 mixture samples at higher pumping energy equal to 2.3 mJ a without MF and b with MF for five different samples (S1 to S5)

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In the main samples without the external magnetic field, higher concentrations of the mixture of Au and Fe3O4 NPs resulted in enhanced intensity on lasing and more spikes in the main emission peak (Fig. 4a) which is a common occurrence in the random media.

The laser emission spectra of the S1 to S5 samples (Au(10%) + Fe3O4(90%), Au(30%) + Fe3O4(70%), Au(50%) + Fe3O4(50%), Au(70%) + Fe3O4(30%), Au(90%) + Fe3O4(10%)) were recorded under an external magnetic field normal to the cell plane.

In the fixed applied magnetic field of the order of 35 mT, number of spikes diminished in all of the five samples; however, a remarkable change occurred in the FWHM of the main emission lasing peak. This change in the emission intensity which arises from the single domain nature of Fe3O4 NPs of sizes below 50 nm (according to the TEM image of NPs) as well as the FWHM under external MF are also presented in Fig. 4. Change in the random lasing emission intensity under external magnetic field also confirms in Fig. 5. As shown in this figure, the lasing emission heatmap at the same wavelength and in the Maximum intensity region for five samples with and without magnetic fields. Decrease in the maximum intensity is the first output of this external field effect, but in the second glance, decrease in the number of spikes is appear. In addition, as it can be seen, for S1 and S2 samples (with 10 and 30 percent of plasmonic NPs), we have the lasing intensity in the same order for both categories; but when we apply MF, the two other samples as S3 and S4, without MF, shows different lasing intensity and by MF, because gathering the magnetic NPs in the direction of external MF, they show the same order of lasing. Until the maximum concentration of magnetic NPs in the S5 sample (with 90 percent of Fe3O4), we have enhancement in the lasing due to the larger scattering mean free path. Furthermore, the intensity variation as a function of applied magnetic field, from 0 to 35 mT, in S5 (90%Au + 10%Fe3O4) sample is shown in Fig. 5c.

Fig. 5
figure 5

The lasing emission’ Heatmap at the same wavelength and in the Maximum intensity region for a without magnetic field, b with magnetic field and c Intensity variation as a function of applied magnetic field in S5 (90%Au + 10%Fe3O4) sample

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Indeed, there are two main phenomena in the change of RL action in the external magnetic field effect which can be explained by the ordered single domain magnetic NPs in the field and possible by the transmissivity of the dye in the experience of the applied magnetic field.

Furthermore, it is obvious that by using an external magnetic field, the coherency percentage of the RL decreased and tends to disappear because of the fixed direction of the magnetic field which can affect the coherency loops due to NPs ordering direction in the dye medium. If one can use circular magnetic field, the ability to control the coherent loop becomes available.

Conclusion

Overall, the electrical exploding wire technique is a simple and efficient technique to fabricate magneto-plasmonic mixture nanostructures as scatterers points for the random lasing applications. In the main samples containing a mixture of Fe3O4 and Au nanoparticles, without the external magnetic field, more lasing efficiency was detected with the higher concentrations due to the plasmonic effect of the gold NPs with more spikes in the main emission peak. Using the applied magnetic field at 35 mT, the number of spikes declined and the FWHM of the main emission peak increased from 6 to 9 nm. These results can be used to introduce a new kind of a tunable random lasing gain media under the external magnetic field.