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Abstract
In this article, low-threshold random lasers based on DCM-DEG (DD) gain system with graphene nanosheets are studied. The experiment results show that the threshold of random lasers reduces rapidly when an appropriate amount of graphene nanosheets is added in DD solution. Meanwhile, the quantity and quality of random lasing modes raise significantly. We discussed the potential reasons why the graphene nanosheets can strengthen the sample's random lasing. And, the influence of the graphene nanosheet concentration on the radiation characteristics of random lasers is further studied. When the concentration of graphene nanosheets is 0.088wt%, the lasing threshold of DD samples with graphene nanosheets (GDD) is only about 31.8% of the lasing threshold of DD samples, and the quality of random lasing modes is five times higher than that of the DD sample. To further reduce the lasing threshold, the gold (Au) nanoparticles are added in the mixed solution to form the GDD solution with Au nanoparticles (GGDD). The results show that the lasing threshold of the GGDD sample is about 7.73 µJ/pulse, which is 5.2% of the lasing threshold of the DD sample. This experiment provides a new method to study low-threshold and high-quality random lasers based on graphene.
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1. Introduction
Since Letokhov [1] reported the random laser behavior in 1968, random lasers have attracted much attention from scientists because of its unique physical mechanism and potential application value [2–9]. Compared with traditional lasers, random lasers don't need a strict resonant cavity. The multiple scattering of light in random medium provides the optical feedback for random lasers [2,3]. So far, random lasers have been achieved in diverse materials including semiconductor powders [3,10], biological tissues [11,12], metal nanoparticles [13,14], titanium nitride nanoparticles [15], liquid crystals [16–18] and so on.
Graphene is a two-dimensional (2D) hexagonal honeycomb crystal material composed of carbon atoms with SP2 hybrid orbitals. Graphene can be widely applied to sensors, compound materials and optoelectronic devices [19–23], because of its unique 2D structure and superior mechanical, electrical and optical properties. With the deep research on the unique structure and properties of graphene, the random laser based on active random metamaterials with graphene nanosheets was theoretically predicted by Marini et. al. [24]. Then, the random laser based on vertically oriented graphene network was realized by Chen et. al. [25]. However, their experiment needs advanced nano manufacturing technology. In addition, the radiation characteristics and underlying mechanism of the random lasers enhanced by graphene need to be further studied.
In this paper, a random laser based on DD solution with randomly oriented 2D graphene nanosheets is fabricated, and its radiation characteristics are studied systematically. The radiation characteristics of the samples with and without graphene nanosheets are compared. Then, the influence of the graphene nanosheet concentration on the random laser radiation characteristics is studied. Finally, the threshold of random lasers is further reduced observably by adding Au nanoparticles into GDD solution, and its potential mechanisms are discussed.
2. Sample preparation and experimental setup
In order to prevent graphene nanosheets from precipitating in the DCM (4-dicyanomenthyl-2-methyl-6-p-dimethylaminostyryl-4h-pyran) solution, the diethylene glycol (DEG) with high viscosity coefficient (the viscosity coefficient is 26.1cp at 25°C) is used to be solvent. The DCM dye and graphene nanosheets used in this experiment are provided by Exciton and DK nano, respectively. The scanning electron microscope (SEM) image of graphene nanosheets is shown in Fig. 1(a). The number of layers of the graphene nanosheet is less than 4, and its mean size is about 1.5 µm. The random laser sample is made as follows: firstly, mix the DCM dye and DEG solution as the proportion of 0.3wt%:99.7wt%. Then, graphene nanosheets with different masses are added to the DCM-DEG mixture, and stirred evenly. The concentrations of graphene nanosheets in the DCM-DEG solution are 0.025wt%, 0.05wt%, 0.1wt%, 0.15wt% and 0.2wt%, respectively. Finally, inset the glass capillary with the inner diameter of 0.3 mm into the DCM-DEG solution with graphene nanosheets (GDD). Because of the capillarity, the mixed solution can be siphoned into capillary to form GDD capillary sample. It's worth pointing out that the sample production in this experiment is simple and does not involve advanced nano manufacturing technology.
Fig. 1. (a) The SEM image of graphene nanosheets. (b) Schematic diagram of the experimental setup. $\lambda /2$,P, M, NBS, L and S indicate the half-wave plate, polarizer, mirror, neutral beam splitter, cylindrical lens and sample, respectively.
Figure 1(b) is the schematic diagram of the experimental setup. In the experiment, a linearly polarized double-frequency Nd:YAG pulse laser with the wavelength of 532 nm, frequency of 10 Hz and pulse width of 8 ns is used as the pump source. The energy of the pump light can be manipulated by adjusting the energy attenuator composed of a half wave plate ($\lambda /2$) and a polarizer (P). Then, the light is divided into two beams with equal energy by the neutral beam splitter (NBS). One beam is used as a reference light and the energy is recorded by an energy meter. The other beam is used as an excitation light which passes through the cylindrical lens (L) to form an excitation band with a length of 10 mm and a width of 0.2 mm at the capillary sample. The light emitted from the sample is collected by an optical fiber spectrometer with a resolution of 0.13 nm, and its data is recorded by a laptop.
3. Experimental results and discussion
In order to prove that adding graphene nanosheets into DD solution can promote the random lasing, we compared the emission spectra of the GDD sample and DD sample, as shown in Fig. 2(a). When the pump energy P is fixed at 91.2 µJ/pulse, the emission spectrum of the DD sample is a spontaneous emission spectrum. The central wavelength is about 647.5 nm, and the full width at half maximum (FWHM) is about 60 nm, which indicates that the pump energy of 91.2 µJ/pulse is lower than the lasing threshold of the DD sample. However, at the top of the emission spectrum of the GDD sample, many discrete peaks are observed and their FWHM is only about 0.5 nm. The appearance of these discrete peaks indicates that the random laser occurs in the GDD sample. Therefore, we can conclude that adding graphene nanosheets into DD solution can promote the generation of random lasers and reduce the random lasing threshold. Figure 2(b) shows the structure of the GDD sample. In this gain system, DCM dye molecules provide the gain for the optical amplification. Randomly distributed graphene nanosheets localize a large number of photons in the gain system through multiple scattering, which can extend the residence time of photons in the gain system. Thus, graphene nanosheets can provide the optical feedback for the formation of random lasers. When the energy gain of light in the gain system is greater than the energy loss, the random laser occurs.
Fig. 2. (a) The emission spectrum of the DD sample (black line) and the GDD sample (red line), when the pump energy $P = 91.2\mu J/pulse$. The concentration of graphene nanosheets is 0.1wt%. (b) The schematic drawing of the structure of the GDD sample.
Figure 3(a) shows the emission spectrum of the GDD sample as a function of the pump energy, when the densities of graphene nanosheets are 0.1wt%. Figure 3(b) shows the peak intensity and FWHM of the corresponding emission spectrum as a function of the pump energy. Results show that when the pump energy is low, the emission spectrum is the spontaneous emission spectrum of DCM dye molecules, and the FWHM is wider than 60 nm. When the pump energy exceeds a certain value, many discrete peaks appear at the top of the spontaneous emission spectrum, and these peaks’ FWHM is less than 0.5 nm. Because of the appearance of these peaks, the peak intensity of the emission spectrum also increases sharply, forming a distinct inflection point as shown in Fig. 3(b). The pump energy at this point is called the lasing threshold of the random laser. The experimental results show that the lasing threshold of the GDD sample is about 51.3 µJ/pulse. For comparison, the emission spectrum of the DD sample as the function of pump energy is studied as shown in Fig. 3(c). Some weak spikes are observed at the top of the spectrum when the pump energy is big enough. This may be because the inner surface of the capillary is slightly rough, which provides the optical scattering feedback for the formation of spikes. We found no spikes in the open system under the same pump energy. Figure 3(d) shows the peak intensity and FWHM of the corresponding emission spectrum as a function of the pump energy. The lasing threshold of the DD sample is about 148.2 µJ/pulse, which is about three times larger than that of the GDD sample. By comparing Fig. 3(a) and Fig. 3(c), we found that the random lasing spectrum of the GDD sample has more modes and the discrete peaks are more obvious. These mean that the quality of the random laser from the GDD sample is much better than that of the DD sample, which will be discussed in detail latter.
Fig. 3. (a, c) The emission spectrum of the GDD sample and DD sample as a function of the pump energy. (b, d) The peak intensity and FWHM of the corresponding emission spectrum as a function of the pump energy.
In order to further study the influence of graphene nanosheets on the random lasing of GDD samples, we studied the radiation characteristics of GDD samples with different concentrations of graphene nanosheets as a function of pump energy, as shown in Fig. 4(a, c). Figure 4(b, d) show the peak intensity and FWHM of the corresponding emission spectrum as a function of the pump energy. When the concentration of graphene nanosheets is 0.05wt% (shown in Fig. 4(a, b)), the random lasing threshold is about 63.6 µJ/pulse, which is much smaller than that of the DD sample (148.2µJ/pulse, shown in Fig. 3(c, d)). When the concentration of graphene nanosheets is 0.1wt%, the random lasing threshold is further reduced to about 51.3 µJ/pulse (shown in Fig. 3(a, b)), which is only 34.6% of the lasing threshold of the DD sample. This is mainly because the scattering intensity of the mixed solution are enhanced with the increase of the concentration of graphene nanosheets. However, when the concentration of graphene nanosheets is further increased to 0.15wt%, the threshold of random laser increases to 127.2 µJ/pulse on the contrary (shown in Fig. 4(c, d)). This is mainly because the absorption loss of the mixed solution is enhanced with the increase of the concentration of graphene nanosheets. In addition, graphene nanosheets occupy the positions originally belonging to DCM dye molecules, which reduces the gain coefficient of the mixed solution. In order to get the lowest lasing threshold of GDD samples, the experiment data are fitted by the Bezier function as shown in Fig. 4(e). The black rectangle in Fig. 4(e) is the experiment data, and the black solid line is the Bezier fitting curve. From Fig. 4(e), we can find that with the concentration of graphene nanosheets increases from 0wt% (without graphene nanosheets) to 0.088wt%, the threshold of random laser decreases from 148.2 µJ/pulse to 47.2 µJ/pulse. When the concentration of graphene nanosheets further increases from 0.088wt% to 0.2wt%, the random laser threshold increases from 47.2 µJ/pulse to 151.3 µJ/pulse. Therefore, we can manipulate the threshold of random laser by adjusting the concentration of graphene nanosheets. When the concentration of graphene nanosheets is 0.088wt%, the random lasing threshold is the smallest, and the threshold is about 31.8% of the lasing threshold of the DD sample.
Fig. 4. (a, c) The emission spectrum of GDD samples with different concentrations of graphene nanosheets as a function of the pump energy. The concentrations of graphene nanosheets are (a) 0.05wt%, (c) 0.15wt%, respectively. (b, d) The peak intensity and FWHM of the corresponding emission spectrum as a function of the pump energy. (f) The threshold of the random lasers as a function of the concentration of graphene nanosheets.
The quality factor of the random laser mode can be expressed as [15]:
In this formula, n (n = 1,2,3…) marks the random laser modes;${\lambda _n}$ is the peak wavelength at the nth random laser mode;${I_{pn}}({\lambda _n})$ is the peak intensity at the wavelength ${\lambda _n}$; ${I_{sn}}({\lambda _n})$ is the spontaneous emission intensity at the wavelength ${\lambda _n}$. The value of ${\eta _n}$ is between 0 and 1. ${\eta _n} = 0$ indicates that there is no coherent random laser mode (i.e. no discrete peak) in the spectrum. The closer the ${\eta _n}$ to 1, the better the quality of the random laser mode (i.e. the discrete peaks are more distinct).
By calculating the quality factor of the random laser modes of the GDD sample when the pump energy is 91.2 µJ/pulse (green line in Fig. 3(a)) and the quality factor of the random laser modes of the DD sample when the pump energy is 171.3 µJ/pulse (pink line in Fig. 3(c)), we got that the highest quality factor of the random laser modes in the two spectra is about 0.22 and 0.044, respectively. The existence of graphene nanosheets improves the quality of the random laser mode by 5 times.
In order to deeply understand the enhancement effect of the graphene nanosheets, we calculated the equivalent cavity pathlength of the random laser through the power Fourier transform of the emission spectrum from the DD sample and GDD sample, as shown in Fig. 5. The insets in Fig. 5(a) and Fig. 5(b) are the corresponding emission spectrum, respectively. The equivalent cavity pathlength of a random laser is proportional to the residence time of photons in the gain medium. That is to say, the longer the equivalent cavity pathlength, the stronger the gain and amplification of photons. This is beneficial for the generation of random lasers. The equivalent cavity pathlength can be expressed as [26,27]:
In this equation, $m$ is the series of Fourier harmonics; ${d_m}$ is the Fourier component and $n$ is the refractive index of the system.
Fig. 5. The power Fourier transform of the corresponding emission spectrum. The inset of (a) shows the emission spectrum of the DD sample, when the pump energy is 171.3 µJ/pulse. The inset of (b) shows the emission spectrum of the GDD ample, when the concentration of graphene nanosheets is 0.1wt%, and the pump energy is 91.2 µJ/pulse.
It can be seen from Fig. 5(a) that the first harmonic component ${d_1} = 6.86\mathrm{\mu} \mathrm{m}$ of the emission spectrum from the DD sample is about 6.86 µm. According to Fig. 5(b), the first harmonic component ${d_1} = 6.86\mathrm{\mu} \mathrm{m}$ of the emission spectrum from the GDD sample is about 23.25 µm. The refractive index n of DEG is 1.447. According to formula (2), the equivalent cavity pathlength of the DD sample and GDD sample is 14.88 µm and 50.45 µm, respectively. The existence of graphene nanosheets increases the equivalent cavity pathlength of the GDD sample by 3.4 times, which is an important reason for the significant reduction of random lasing threshold of the GDD sample.
In order to further reduce the random laser threshold, Au nanospheres with the diameter of 50 nm are dispersed in the GDD mixed solution to make a GDD capillary sample containing Au nanoparticles (GGDD). Noble metal nanoparticles, which have the localized surface plasmon resonance (LSPR) characteristic under the excitation of light, are often used in the study of random lasers and optical sensors, especially the Au and Ag nanoparticles [9,10,28]. Figure 6(a) shows the emission spectrum of GGDD samples as a function of the pump energy. Figure 6(b) shows the peak intensity and FWHM of the corresponding emission spectrum as a function of the pump energy. Results show that the lasing threshold of GDD samples is further reduced by adding Au nanoparticles. It can be seen from Fig. 6(b) that the lasing threshold is about 7.73 µJ/pulse, which is about 15.1% of the GDD sample and is about 5.2% of the DD sample.
Fig. 6. (a) The emission spectrum of the GGDD sample as a function of the pump energy. Inset shows the LSPR of Au nanoparticles. (b) The peak intensity and FWHM of the corresponding emission spectrum as a function of the pump energy. The number density of the Au nanoparticles is $7.908 \times {10^{10}}/ml$. The concentration of graphene nanosheets is 0.1wt%.
The schematic diagram of Au nanoparticles enhancing the random lasing from GGDD samples is shown in the inset of Fig. 6(a). The LSPR of the single Au nanoparticle and the LSPR coupling between the Au nanoparticle and graphene significantly enhance the electric field intensity in the LSPR area. These greatly improve the fluorescence amplification efficiency of DCM dye molecules [29]. As we know, the fluorescence intensity of dye molecules can be expressed as ${i_F} = \alpha {I_0}{\varphi _F}$. In the formula, $\alpha {I_0}$ represents the total number of photons absorbed by the fluorescent substance per unit time and volume; ${I_0}$ represents the intensity of the incident light; ${\varphi _F} = k_r^s{\tau _s}$ represents the fluorescence quantum yield; $k_r^s$ is the radiation decay rate constant of the electrons from the excited state to the ground state; ${\tau _s}$ is the lifetime of the excited state electrons. According to the classical electromagnetic theory, the incident light intensity ${I_0}$ is proportional to the square of the electric field (${E^2}$). Therefore, the fluorescence intensity of the dye molecule is proportional to the square of the local electric field around the fluorescent substance. The LSPR of Au nanoparticles enhances the fluorescence intensity of DCM molecules, which further reduces the random lasing threshold. In addition to the LSPR, Au nanoparticles also provide scatter for the random laser formation. According to the previous studies [30,31], the scattering intensity provided by Au nanoparticles under these experimental conditions is weak. Therefore, the enhanced action of Au nanoparticles on random lasers mainly comes from the LSPR of Au nanoparticles.
4. Conclusion
In conclusion, the radiation characteristics of the random laser based on DCM-DEG solution with 2D graphene nanosheets are studied. By comparing the radiation characteristics of DD samples and GDD samples, we found that graphene nanosheets with appropriate concentration can promote the realization of random lasers. The lasing threshold is reduced significantly and the quality of the random laser mode is improved obviously. When the concentration of graphene nanosheets is 0.088wt%, the lasing threshold of GDD samples is the smallest (47.2 µJ/pulse), which is about 31.8% of the lasing threshold of DD samples. The existence of graphene nanosheets improves the quality of the random laser mode by least 5 times. The potential reasons are discussed in detail. The graphene nanosheet network localizes a large number of photons in the gain system to form local modes by multiple scattering, which provides optical feedback for the generation of random lasers. Finally, by adding Au nanoparticles into the GDD mixed solution, the lasing threshold of the GGDD sample is further reduced to 7.37 µJ/pulse, which is 15.1% of the lasing threshold of GDD sample and 5.2% of the lasing threshold of DD sample. This research provides a new way for the realization of low-threshold and high-quality random lasers based on 2D graphene nanosheets. Next, we will further study the dynamically controllable random lasers based on the graphene characteristics including the photo-thermal expansion [32,33], plasmon [34,35] and so on.
Funding
National Key Research and Development Program of China (2019YFA0705000); National Natural Science Foundation of China (11974218, 12104268, 12192254, 12274268); Local Science and Technology Development Project of the Central Government (YDZX20203700001766).
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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