Detection of gain enhancement in laser-induced fluorescence of rhodamine B lasing dye by silicon dioxide nanostructures-coated cavity

In this work, nanostructured silicon dioxide films are deposited by closed-field unbalanced direct-current (DC) reactive magnetron sputtering technique on two sides of quartz cells containing rhodamine B dye dissolved in ethanol with 10‒5 M concentration as a random gain medium. The preparation conditions are optimized to prepare highly pure SiO2 nanostructures with a minimum particle size of about 20 nm. The effect of SiO2 films as external cavity for the random gain medium is determined by the laser-induced fluorescence of this medium, and an increase of about 200% in intensity is observed after the deposition of nanostructured SiO2 thin films on two sides of the dye cell.


Introduction
Scattering varies with the average radius of the particle, index of refraction, scatterers density, and the incident wavelength. If the particle size is small compared to the wavelength, Rayleigh scattering (in all directions) occurs, whereas bigger particles reflect and refract light [1,2]. If there are many scatterers randomly distributed inside the medium, the incident light wave will undergo scattering, and the scattered light will have a random path [3]. If the light waves have a long path inside the medium longer than the straight path of the waves, it will leave the medium without scattering. This is known as "multiple scattering", and it is not restricted to light waves but it's applied to all waves in nature [4][5][6]. There is an important parameter in multiple scattering; that is the mean free path (l) which is defined as the average distance for a light wave transferred between two scattering events [6] and given by [7,8] scatt scatt 1 l N σ = . (1) where N scatt and σ scatt are the volume density and cross section of the scattering particles. If the random amplification media have high scatterer' density, the mean free path of light will be short, and the interference effects of the scattered waves will be present alongside the gain of the active medium. The unification of interference and gain in a system results in a new laser output [9].
Improving the active medium of the dye laser has attracted the interest of many researches and studies. The first attempts started in the 1960 and continued up to now [10]. Rhodamine B (RB) dye is considered as a common typical material that belongs to the xanthene group and operates in the visible range [11]. The lasing emission of RB dye also depends on the type of solvent used for dissolving this dye to form its solution. For example, the central lasing wavelength is increased by about 43 nm when changing the solvent from ethanol to ethylene glycol. In general, the lasing emission is ranging within 570 nm -682 nm [12]. The unique properties of this dye have attracted the attention of researchers in several military, medical, and industrial applications The fluorescence lifetime of this dye is about 4.08 × 10 -9 s, which means that the possibility of inter-system crossing is very weak (< 10 7 s -1 ) [13]. Using rhodamine dye as an active media of dye laser, it is possible to obtain continuous or pulsed laser output according to the pumping method in which a wide tunable wavelengths range can be obtained [14,15].
Random laser has received much attention as a new development of laser physics and technology [16]. Its stimulated emission sources are different from those found in ordinary conventional lasers. The random laser is essentially built on insulating and semiconducting media [17]. It is characterized by being an "open resonator", i.e., it does not need an external optical cavity. Also, the feedback mechanism is provided using the so-called "scatterers" or "the tiny cavities" [18,19]. Random laser is characterized by its small size, simplicity, low cost, and applicability [20][21][22].
he interference between the scattered waves occurs when there is a high density of scatterers [23]. Since the transferring waves are exactly on the same distance, they maintain the same essential phase relationship even though the path is formed by a large number of scattering events [24,25]. If the scattered waves are in phase with each other, constructive interference occurs and thus forming closed paths of light in the scattering media [26,27]. These closed paths provide the coherent feedback mechanism [28][29][30]. The scatterers within the laser media contribute in trapping light photons due to multiple scattering; in doing so, the light path inside these media will be long (long time interval), and this is one reason for a decrease in the threshold intensity [31,32].
In this work, nanostructured silicon dioxide films are deposited by closed-field unbalanced direct-current (DC) magnetron sputtering technique on two sides of quartz cells containing RB dye dissolved in ethanol with 10 -5 M concentration as a random gain medium. The preparation conditions are optimized to prepare highly pure SiO 2 nanostructures with as minimum as possible particle size. The effect of SiO 2 films as external cavity for the random gain medium is studied by the laser-induced fluorescence of this medium.

Experiment
A p-type silicon wafer of 10 cm in diameter and 300 μm in thickness was used as the target to be sputtered and maintained carefully on the cathode. It was cleaned by hydrofluoric acid (HF), ethanol, and distilled water, dried, and then used for deposition process. Highly-pure argon and oxygen gases were used as discharge and reactive gases, respectively. The deposition process was performed on quartz cells. Before using them in sputtering experiments, these cells were first cleaned with ethanol to remove any oil layers or residuals may exist on their surfaces, which should be rinsed and washed with distilled water to remove ethanol, and then dried completely before being kept in clean case or placed inside the vacuum chamber. More details on the deposition system can be found in previous works [33,34].
The RB laser dye solutions were prepared by dissolving the required amount of the dye in the ethanol. This amount of the dye (W) was weighed using a precise digital balance of 10 -4 g sensitivity (Matter Company) and could be calculated as [35] Photonic Sensors 82 where M w is the molecular weight of the dye (g/mole), v is the volume of the solvent (ml), and c is the molar concentration (mole/liter). A high concentration of 10 -2 mole/liter of RB dye solutions was prepared and then diluted to concentrations of 10 -3 mole/liter, 5×10 -3 mole/liter, 10 -4 mole/liter, 5×10 -4 mole/liter , 10 -5 mole/liter, 5×10 -5 mole/liter, and 10 -6 mole/liter, respectively.
The operating conditions of the system were divided into two groups: constant and variable. The constant operating conditions include vacuum pressure, current limiting resistance, discharge voltage, discharge current, cooling temperature, cooling water flow rate, and deposition time. The variable operating conditions included gas pressure, inter-electrode distance, gas mixing ration, and gas flow rate. Varying discharge voltage was almost possible during the operation. In addition, turning the cooling system off would raise the temperature of either electrode to 40 ℃ -45 ℃ with circulating water, while stopping the circulation of water would raise electrode temperature more (up to 150 ℃).
The quartz cell to be coated was completely immersed in the generated plasma. Big efforts were made to obtain uniform spatial distribution of plasma column in order to ensure homogeneous growth of prepared films. This could be done by controlling operation conditions and parameters, mainly the inter-electrode distance, gas flow, and discharge current.
The samples prepared in this work were characterized in order to determine their structural and spectral characteristics. The measurements and characterization tests include film thickness measurements, X-ray diffraction (XRD) patterns, atomic force microscopy (AFM), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), ultraviolet-visible (UV-Vis) spectroscopy, and fluorescence measurements.
The X-ray diffraction patterns were recorded by a Shimadzu 6000 X-Ray Diffractometer system. This system measured the diffraction intensity as a function of Bragg's angle. The radiation source was Cu(kα) with wavelength of 1.5406 Å, current of 30 mA, and voltage of 40 kV. The scanning angle (2θ) could be varied in the range of 20 degree -60 degree with a speed of 4 deg/min.
The SEM measurements were performed using MIRA3 Tescan FE-SEM instrument on samples prepared at the optimum conditions in order to introduce the prepared nanostructures as well as determine the nanoparticle size of the prepared samples. This instrument provided an EDS test to determine the content of elements in the final sample.
The FTIR measurements were carried out by FTIR spectrometer (SHIMADZU FTIR-8400S) on KBr pellets containing the prepared samples. These measurements were performed in the spectral range from 400 cm -1 to 4000 cm -1 .
Transmission and absorption spectra of the prepared samples were recorded in the spectral range 200 nm -800 nm with an optical resolution of about 0.2 nm by using a computer-controlled UV-Vis spectrophotometer (K-MAC SpectraAcademy SV-2100). This measuring process could be precisely controlled by suitable software installed on a personal computer. The fluorescence spectra were recorded using F96 Fluorescence Spectrophotometer unit (Shanghai LengGuang Tech. Co., Ltd.) in the emission wavelengths range 180 nm -900 nm, with xenon continuous-wave (CW) lamp as the excitation source.
In order to introduce the amplification activity in the prepared samples, the K-MAC SpectraAcademy SV-2100 instrument was used for amplified spontaneous emission (ASE) measurements after replacing the light source with a laser of 532 nm wavelength as an intensive light source required for the excitation of the prepared samples. The laser beam was incident on an empty face of the cell at 90° with respect to the line between the coated faces and the photodetector. The optimum conditions to prepare silicon dioxide nano films on the external walls of the quartz cell were inter-electrode distance of 4 cm, Ar:O 2 gas mixing ratio of 70:30, total gas pressure of 0.08 torr, discharge voltage of 2.5 kV, discharge current of 35 mA, anode temperature of 150 ℃, and cathode temperature of 27 ℃ (room temperature). The thickness of the prepared films is determined by laser interference fringes method to be 118 nm and 121 nm for two sides of the cubic quartz cells coated with SiO 2 films after three hours deposition time.  Figure 2 shows the absorption and transmission spectra of the samples prepared at different deposition times (2 h, 2.5 h, 3 h, 3.5 h, and 4 h) in the spectral range 200 nm -800 nm. It is clear that the sample prepared at shorter deposition time shows lower absorbance than those prepared at longer times. This is a typical behavior because a decrease in the deposition time leads to the growth of films with lower thickness as lower numbers of particle layers are grown on the substrate. Accordingly, the transmittance is decreased with increasing deposition time for the same reason. According to the observable stability of transmittance and absorbance values of the prepared Photonic Sensors 84 samples over the spectral range of these measurements, these samples can be considered as optically homogeneous, and this is a very important feature for the applications to use such samples. Figure 3 shows the XRD pattern of the SiO 2 film sample prepared after deposition time of 3 hours. It is clearly observed that the prepared sample is amorphous, which is a characteristic of nanostructures; therefore, calculations of the structural parameters are not performed as no specific peak could be analyzed.  Figure 4 shows the SEM image for the SiO 2 film sample prepared after deposition time of 3 hours. The minimum particle size for each sample is determined to be 20 nm. The sample shows aggregation as large grains are formed. As the structural purity of the silicon dioxide (SiO 2 ) nanostructured thin films to be deposited on the dye cells in this work is highly required, the contents of composing elements (i.e., silicon and oxygen) in the final product are determined by the EDS measurements, as shown in Fig. 5. All samples show reasonably high peaks for both elements with lower peaks belonging to the elements which may be contained in the structure of the quartz substrates. However, the structures of these samples can be considered sufficiently pure. This structural purity is very important for these nanoparticles to play the role of scattering centers (scaterrers) for the random gain media. Weight percentage contents of silicon and oxygen in the final product are obtained from EDS results for the prepared samples to be 31.03 wt.% and 58.84 wt.%, respectively.  Figure 6 shows the 2D and 3D AFM images for the nanostructured SiO 2 thin films deposited on the dye cells after deposition time of 3 hours. As can be seen, the aggregation on the surface of prepared sample is sufficiently low. The average diameter of particles is 91.83 nm.

Results and discussions
Certain applications of nanostructures, such as photodetectors, energy conversion devices, gas sensing, and ultra-hard coatings, require as high as possible surface area. Therefore, the prepared samples can be efficiently used for such applications.
In order to introduce the effect of nanostructured SiO 2 thin films deposited on two sides of the dye where n s is the number of scatterers, and σ s is the scattering cross-section of the individual scatterer. Due to anisotropy in scattering, the scattering mean free path (l s ) is not sufficient to describe this effect. Therefore, the average distance that the light travels before its direction of propagation is randomized is alternatively considered. This distance is known as transport mean free path (l t ) defined by the following relation as [37] where g = <cosθ> is the average cosine of scattering angle θ (so-called the anisotropy parameter). For isotropic scattering (e.g., Rayleigh scattering from particle of size << λ ), g = 0 or l t = l s , hence [38,39]: where λ is the excitation wavelength, and W is the spectral scattering width of the ray reflected from the sample. Nanostructures, in general, are described with very high surface roughness and hence very large number of single particles. Therefore, the value of l s is minimized according to (3). The transport mean free path (l t ) is accordingly minimized, and the light is amplified within a very small distance as traversing through the random gain medium. The amplification effect is subsequently maximized. An increase of about 200% in laser-induced fluorescence intensity of RB dye is observed after the deposition of nanostructured SiO 2 thin films on two sides of the dye cell.

Conclusions
In concluding remarks, highly-pure nanostructured silicon dioxide films with the minimum particle size of about 20 nm are deposited by the closed-field unbalanced dc magnetron sputtering technique on two sides of quartz cells containing RB dye dissolved in ethanol with 10 -5 M concentration to act as external cavity scatterers for this random gain medium. This role is determined by the laser-induced fluorescence of the dye sample, and an increase of about 200% in its intensity is observed after the deposition of nanostructured SiO 2 thin films. These results can be successfully used to detect the gain enhancement in such media for spectroscopic and photonic applications.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/ licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.