Figure 2a represents the FESEM image of the ZnO microspheres to show the microspheres with rough surfaces and average diameters of about 1 μm. Further magnification from the microsphere’s surface in Fig. 2b signifies that the individual microsphere comprises nano-protrusions with average diameters of ~ 30 nm, which are uniformly grown very closely spaced.
Figure 2 Field emission scanning electron microscopy (FESEM) images of the ZnO microspheres: (a) at scale bar 1 μm and (b) 200 nm from the surface of a microsphere. Full size image
Figure 3 shows the evolution of the emission spectra as a function of the excitation pump energy. At low pump pulse energy, spontaneous Raman scattering and fluorescence exhibit themselves as a broad spectrum. Once the pump energy reaches a certain threshold, the first Raman signal at a wavelength of 578 nm with a bandwidth of about 0.4 nm begins to grow. By gradually increasing the pump energy, the intensity of the single Raman signal increases, and the emission background decreases. The exponential growth of the Raman photons above the threshold due to the SRS process is responsible for this enhancement37. As the increase in the pump energy is ongoing, the second Raman signal appears at 580 nm wavelength. One can conclude that when the intensity of the first Raman signal becomes sufficient, this signal can act as a pump to produce the next Stokes by SRS. Without previous Raman signal, it is impossible to obtain a higher Raman signal in a cascaded SRS process38. As shown in Fig. 3, at higher pump energy, the third Raman signal manifests itself at the wavelength of 583 nm due to the generation of higher Raman signals at the higher pump energies3.
Figure 3 Evolution of the emission spectrum of Rhodamine 6G containing ZnO microspheres as a function of the pump energies. Full size image
Observation of the cascaded SRS effect requires both high Raman gain of the medium and high pump power6. Using intense pump energy supports the required gain and is a key factor for the cascaded SRS effect occurrence. In addition, strong multiple elastic scattering increases the path length, and nonlinear interaction of photons results in the decreased threshold and favors Raman amplification19. Therefore, a proper selection of the scatterer centers to provide effective multiple elastic scattering is essential.
The increased surface area of microspheres due to porous surface structures provides high scattering efficiency. Furthermore, each nano-protrusion on the surface can act similar to a reflector for multiple elastic scattering. Consequently, efficient multiple elastic scattering requirements could be satisfied using the mentioned microspheres’ specific morphology as scatterer centers.
Figure 4 shows the variations of the linewidths versus the input pump energy for the first and second stokes signals. A slightly observed broadening in both Raman stokes with increasing the pump energy is attributed to the more intense nonlinear effects such as self phase modulation (SPM) at higher pump energies4,5,27.
Figure 4 The linewidth (FWHM) of 1st Stokes (blue squares) and 2nd Raman Stokes (red circles) as a function of the input pump energy. Full size image
In Fig. 5, the spontaneous Raman spectrum of Rhodamine 6G was measured and compared with the emission spectrum of Rhodamine 6G containing ZnO microspheres. All three signals coincide with Raman signals of Rhodamine 6G dye, which provides evidence for the random Raman lasing process and confirms that the lasing modes originate from Rhodamine 6G Raman resonance.
Figure 5 (a) Emission spectrum from Rhodamine 6G containing ZnO microspheres at 109 mJ/pulse pump energy. (b) The Raman spectrum of pure Rhodamine 6G dye. The arrows show the correspondence of the emission spectrum of Rhodamine 6G dye-containing ZnO microspheres and the Raman signals of pure Rhodamine 6G dye. Full size image
Figure 6 shows the output Raman intensity of the first signal (578 nm) versus pump energy. A clear threshold for the Raman signal is observed to estimate the value for the transition from spontaneous scattering to SRS to be about 60 mJ/Pulse. The evolution of Raman signal intensity versus pump energy shows a nonlinear behavior. It is because the SRS intensity depends on both pump and random laser intensities11,21. Generally, output intensity in random lasers due to the absence of a stationary cavity is influenced by the multiple elastic scattering, which is occurred randomly at different directions39,40. Particularly, the fluctuations in the output intensity are further in solution random lasers because the Brownian motion of particles makes different configurations of scatterers at each excitation pulse41. On the other hand, in a dye solution, the reabsorption process due to the overlap of emission and absorbance spectrums of the dye is a factor of loss where many photons are absorbed before they leave the sample42.
Figure 6 Output intensity versus incident pump energy at the first Raman signal (578 nm) from Rhodamine 6G solution containing ZnO microspheres. Full size image
Figure 7a,b show the evolution of the Raman signals under multiple pulses at fixed energies of 70 and 82 mJ/pulse, respectively. As can see in Fig. 7a, at an excitation energy of 70 mJ/pulse, the Raman signal with a wavelength of 578 nm and a bandwidth of 0.4 nm reproduces well as the same as Fig. 3, for each excitation pulse. A similar result is observed at higher pump energy of 82 mJ/pulse, and Raman signals with wavelengths of 578 and 580 nm with relative linewidths of 0.7 and 0.5 nm are appeared, respectively. However, the intensity of Raman modes changes from pulse to pulse due to the dependence of SRS generation on scattering and absorption of photons37. Moreover, in a disordered solution medium, the distribution of scattering centers varied frequently due to the free movements of particles results in the partial variation of scattering intensity.
Figure 7 The behavior of Raman signals at independent single shots at (a) 70 mJ/pulse and (b) 82 mJ/pulse in Rhodamine 6G solution containing ZnO microspheres. Full size image
This experimental result reveals that the wavelengths of Raman signals are unchangeable from one pump pulse to another while only their emission intensity changes slightly.
http://feeds.nature.com/~r/srep/rss/current/~3/6n7_Z47safU/s41598-021-01354-8
