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Random laser actions in ultraviolet and visible regions have been demonstrated based on the composites consisting of bio-inspired diatom frustules. Owing to the low optical loss derived from porous network of diatom structures, we report wide spectrum range random lasers arising from GaN film and Rh6G dye via using biological diatoms as scattering centers. Interestingly, both ultraviolet and visible-range random laser actions with very sharp peaks can be easily obtained, with the average length of optics cavity closed to the average size of diatom frustules in both cases, indicating the excellent optical confinement of diatom frustules. It is expected that the first proof of concept shown here can pave an avenue toward future broad-range random lasers and eco-friendly biophotonics devices with high performance and wide spectrum response.
© 2015 Optical Society of America
1. Introduction
Unlike typical lasing systems, the emission of a random laser is provided not by carefully manufactured resonant cavities, but through randomly distributed scatterers within the gain medium [1]. The rigid requirement for the fabrication of laser devices can be relaxed, which enables to open up a great variety for the selection of lasing materials as well as to reduce the cost significantly. Random lasers have been utilized in several kinds of gain media, such as semiconductor powders [2], dyes and scatterers [3], clusters of nanoparticles, and biological tissues [4–6]. Random laser possesses a unique feature that the angular distribution of the emission covers over the complete solid angle; hence, laser action can be observed not only in a specific direction [1–3]. Owing to its simplicity and low spatial coherence, random laser has aroused several potential applications, including high speed, full field imaging and projection [7,8]. In this experiment, we have successfully demonstrated random laser actions in ultraviolet and visible regions using the same bio-inspired scattering centers, which is quite unique compared with all published reports [1–12].
Utilizing bio-inspired materials or biological tissues to generate random laser action have attracted a great deal of attention, because their ubiquitous existence can greatly simplify the fabrication process and reduce the cost. Besides, the bio-comparable and biodegradable optoelectronic devices are environmentally friendly and enabling to achieve the goal of our sustainable planet. Furthermore, random lasers made with biomaterials have practical potential applications, such as medical diagnostic imaging [1,5], which can pave the way for the implementation of low-cost targeted therapy. Indeed, several biomaterials, such as insect wings [6,10], human tissue [4,5], luciferin synthesized by different organisms [13,14], silk fibroin [15] and DNA [16] have been used for the demonstration of random laser action. And many interesting approaches have been implemented to biomaterials for the observation of random laser action [17,18]. In this letter, diatom frustules were used as our lasing scatterers. Diatoms are omnipresent creatures living in the aqueous area, which have been under investigation for more than 200 years. Owing to the high surface area and the porous structure, diatom frustules have been used in the research in optoelectronic including solar cells [19], batteries [20], and novel optical fiber based photonic devices [21]. These two properties of diatomite also imply that diatom frustules can serve as excellent scattering centers for the formation of the close-loop to achieve coherent feedback and to generate random laser action. However, diatom frustules assisted random lasers are extremely rare. Francesca Romana Lamastra et al. [22] have used diatom as light scatterers, but no sharp peaks were shown in the obtained spectra. In this paper, we demonstrated the laser actions at UV and visible light range, using not only laser dye but also semiconductor film as our gain medium, further extending the application and potential usage of diatomite. Our approach shown here is useful to provide an excellent route for the future development of eco-friendly and cost-effective optoelectronic devices with high performance and wide spectrum response.
In order to illustrate our proposed working principle, in this letter we have chosen GaN and Rhodamine 6G (Rh6G) as the gain media. GaN is one of the most interesting and important light-emitting materials, which has been widely studied since 1990s. GaN is a stable and mechanical hard material, with a direct band gap of 3.43 eV lying in UV spectrum. GaN-based materials have found their way in UV light sources and UV laser diodes. Especially, nowadays InGaN/GaN multiple quantum wells play a major role in the lighting industry and change our daily life significantly. Apart from the solid-state gain medium in random lasing device, laser dyes are also very attractive in optical research. Here, we use Rh6G as our gain medium to couple with diatom frustules for the demonstration of visible random laser action.
2. Experiment
The diatomite (Celite® S filter aid, dried, untreated, Sigma-Aldrich) used in this letter was purchased and used without any purification. Diatom frustules are solved into absolute alcohol with a concentration of 3 mg/10 ml. The p-GaN, doped with Mg, thin film used in this study was deposited by metal-organic chemical vapor deposition (MOCVD) process on a sapphire substrate [23], with a thickness of about 1.03 μm. The concentration of Rh6G solution in our experiment is 10−4 M, with absolute alcohol as the solvent.
For the preparation of GaN/diatom sample (diatom as scatterers and GaN as gain medium), the diatom-alcohol solution was spin coated on the GaN/sapphire substrate, and the measurement was performed until the alcohol was vaporized. The similar procedure was followed for the preparation of Rh6G/diatom composite sample. The diatom-alcohol solution was proceeded first on the glass substrate, followed by the Rh6G-alcohol solution. In order to study the lasing behavior, the samples were optically excited by a Q-switched 4th harmonic Nd: YAG laser (266 nm, 3–5 ns pulse, 10 Hz) with the diameter of the beam size about 500 μm, and measured with a Jobin Yvon iHR550 imaging spectrometer system. The pumping-detection geometry possesses that the incident angle is 60 degree to the sample surface in the case of Rh6G/diatom sample, while the incident angle is of about 85 degree in the case of diatom/GaN devices. Also, the measurements on the control group on GaN/sapphire substrate without spin-coated diatom as scatterers were made to stand out the role of diatom frustules in laser action. All the experiments were conducted at room temperature.
3. Results and discussions
Figures 1(a) and 1(b) show scanning electron microscope (SEM) images of the porous diatom frustules. The low magnification SEM image, Fig. 1(a), shows that the diatoms are randomly distributed on the substrate, with the diameters of diatoms of around 7.2 μm. Under a higher magnification, clear radially distributed nano-porous architectures on silica shells can be observed in Fig. 1(b), with the average pore size of around 200 nm. The randomly distributed diatom frustules and the porous structure make diatom a good candidate for photons to scatter. As a result, we presume that diatom frustules can serve as excellent scattering centers. Figure 1(c) shows the X-ray diffraction (XRD) spectrum of GaN substrate used as the gain medium. The diffraction peaks can be well indexed for both GaN film and sapphire substrate. Figure 1(d) shows the photoluminescence (PL) spectra of GaN under different pumping intensity without diatom. The sharp peak on the right is caused by amplified spontaneous emission (ASE) arising from conduction band to acceptor transition [24–26] based on the reason as shown below, and the weak broad UV emission on the left is due to the band-gap transition of GaN film. As we can see in Fig. 1(d), the direct band gap emission is at 361 nm, while conduction band to acceptor emission is at 375 nm, which match well with the values of 3.43 eV and 3.26 eV for band-to-band and conduction band to acceptor transitions, respectively.
Fig. 1 (a) Low magnification scanning electron microscope (SEM) image of diatomite; (b) high magnification SEM image of diatomite; (c) x-ray diffraction pattern of GaN thin film on sapphire substrate; (d) emission spectra of GaN thin film under different excitation energy. ASE represents amplified spontaneous emission.
In order to have a more detailed understanding of the ASE arising from conduction band to acceptor transition, the emission intensity for GaN without adding diatom under different pumping energy is shown in Fig. 2. It is found that the relationship between peak intensity (I) and pump energy can be described well by the expression I = I0 [eAE-1] [27–30], where E is the pumping energy and A is a constant, as shown in the inset of Fig. 2.
Fig. 2 Amplified spontaneous emission spectra of GaN/sapphire substrate under different excitation energy. Inset: emission peak intensity versus pumping energy. Blue spheres denote the experiment data. Dash line represents the exponential fit of the experiment data.
The inset of Fig. 3 shows a superposition of the emission derived from pure GaN sample and GaN/diatom composite. Compared with the pure GaN, more sharp and narrow peaks appear from the diatom/GaN composite. Meanwhile, the emission intensity of diatom/GaN is enhanced and tends to be stronger than that of the pure GaN sample. To have a better understanding of the emission properties, we have demonstrated enlarged emission spectra from the GaN/diatom sample under different excitation energy, as shown in Fig. 3. As the pumping energy increases, more and more narrower peaks with full width at half maximum (FWHM) less than 1 nm emerge and superpose on the original background emission peak centered at around 375 nm. Also, the peak intensity drastically increases with increasing excitation energy. It is worth nothing that the lasing mode shows slightly random fluctuations under every different pumping condition, it reflects the inherent nature of random laser action. Figure 4 shows the FWHM narrowing and the peak intensity surging with increasing excitation energy. At low pumping energy, for example 111 μJ, the FWHM is of about 7.2 nm. However, with larger excitation pulse energy, for example 201 μJ, the FWHM decreases to about 0.65 nm and the peak intensity is about 44 times larger. As shown in Fig. 4, the behavior of the peak intensity emitted from GaN/diatom sample cannot be finely fitted by the exponential dependence as described in Fig. 2 (inset), and hence the sharp peaks observed here cannot be attributed to the mechanism of ASE. Indeed, the existence of the pronounced threshold behavior for both FWHM and emission intensity provides a clear signature for the occurrence of laser action. Because of the difference in oscillator strength between band-to-band and conduction band to acceptor transitions, random laser action only occurs around 375 nm, instead of 365 nm. In 1962, Rashba et al. [31] first illustrated the weakly bound excitons of defects can generate giant oscillator strength many orders larger compared with that of the free excitons [32]. Thus, the conduction band to acceptor transition can generate light with higher degree of coherence than that of the band-to-band transition, which makes the random laser action occur only at 375 nm. This behavior has also been observed in several previous works [33].
Fig. 3 Random laser spectra with various excitation energies derived from GaN/diatom composite. Inset: large-scale emission spectra comparison between the pure GaN sample and GaN/diatom composite. Red line denotes the emission spectrum of pure GaN sample. Blue line represents the emission spectrum of diatom/GaN composite. Both spectra are measured under the same excitation energy of 176 μJ.
Fig. 4 Emission peak intensity and full width at half maximum (FWHM) versus excitation energy of GaN/diatom sample. Red spheres and Blue stars represent the experiment data of peak intensity and FWHM, respectively. Dash line is the exponential fit for the amplified spontaneous emission.
For the occurrence of the laser action, Fabry-Perot (FP) or whispering gallery mode (WGM) will not be considered to be the dominant mechanism. As we can see from Fig. 1(b), there are plenty of nano-cavities lying in the disk-like diatom structure. This kind of structure does not form highly oriented vertical nanowire arrays with flat facets in both ends to serve as optical cavities for FP resonances, nor spherical or hexagonal shape cavities to confine the light for WGMs [34,35]. However, the diatom frustules can be randomly distributed into the gain medium, making the close-loop path (optical cavity) be provided by the multiple scattering of light [2]. The length of the cavity L can be calculated by the following formula: L = λ2/nΔλ, where the wavelength difference Δλ can be derived from the two nearest lasing peaks, λ is the wavelength of the highest lasing peak (~375 nm) and n is the effective refractive index (~1.4). The physical reasoning of this formula is that the coherence feedback in a closed-loop can be generated if standing waves of light are obtained in the closed-loop. The boundary condition of standing waves in the closed-loop requires that the number of wavelengths fit into the length of the closed-loop, which represents the length of a cavity. Hence, L = mnλ, where m is an integer, n is the refractive index and λ is the wavelength of wave in vacuum. Therefore, we know that Δ(1/λ) = 1/nL. By using the approximation Δ(1/λ) ~- Δλ/λ2, we can easily obtain the formula for free spectral range (FSR) = Δλ = λ2/nL. A more rigorous derivation of this formula can be found in the book [36]. This formula is an analogy from the FSR formula of a ring resonator [37,38], since random laser and ring resonator both form a close-loop to generate coherence feedback. By using the parameter above, the cavity length of the diatom and GaN composites is estimated to be 108.54 μm. This value meets well with the average size of diatom frustules, which reconfirms the above conclusion that the random laser action due to the multiple scattering of the skeleton of diatom is the dominant mechanism in this laser system.
To further explore the availability for producing random laser action in the other spectrum by using diatom, we have performed the similar experiment for Rh6G and diatom composite. Figure 5 shows the absorption spectrum of 10−4 M Rh6G-ethanol solution with the absorption peak wavelength of about 532 nm. The spectrum shows that Rh6G absorbs photons lie in the blue and green light range, which can fairly support the as-seen orange color of Rh6G-ethanol solution in Fig. 5 (inset). The emission spectra of Rh6G/diatom sample under different pumping energy are shown in Fig. 6. We can clearly see the emergence of several very sharp peaks with FWHM less than 1 nm above a threshold pumping energy, as shown in the inset of Fig. 6 (left inset), the pumping energy of the threshold is around 90 μJ. The lasing mode is not strictly fixed under different pumping energy, and the central peak wavelength is fluctuating in the range of 538-540 nm, which lies in visible light range.
Fig. 5 Absorption spectrum of 10−4 M Rh6G-ethanol solution. Inset: Bottle of 10−4 M Rh6G-ethanol solution.
Fig. 6 Lasing spectra of Rh6G/diatom sample under different pumping energy. The left inset shows the emission peak intensity versus pumping energy. The right inset is the multi-mode random lasing spectrum of Rh6G/diatom sample, with pumping energy of 120 μJ.
An enlarged scale of the lasing spectra with the pumping energy of 120 μJ is shown in Fig. 6 (right inset). Apparently, all these behaviors indicate the occurrence of random laser action in the Rh6G/diatom composite. There is a pillar that lies in the range of 529-535 nm in these spectra, which stems from the second harmonic signal from our 266 nm pulse laser and cannot be filtered because of the limitation of our instruments. We therefore have also demonstrated random laser action in the visible light range based on Rh6G/diatom composite. We can also calculate the cavity length L in the Rh6G/diatom composite, with the wavelength of the highest lasing peak λ (~539 nm) and the wavelength difference Δλ (~1.79 nm). The length of the close-loop path is of about 115.78 μm, which is closed to that of the diatom/GaN composite. This result implies that the optics cavities in both diatom/GaN and diatom/Rh6G composites exhibit the similar properties. It is believed that the random laser action assisted by diatom can be obtained by replacing with appropriate gain media to achieve a wide spectrum range covering from UV to visible-range radiation.
4. Conclusions
To sum up, we have successfully demonstrated the random laser actions in ultraviolet and visible regions based on the composites consisting of diatom frustules and GaN/sapphire and Rh6G laser dye, respectively. Owing to the porous structure of diatom frustules, the light can be easily scattered to achieve the close-loop for the formation of coherent feedback in both samples, with the length of the closed-loop about 100 μm in both cases. As a result, low optical loss random lasers assisted by diatom frustules can be obtained. Also, by changing appropriate gain media, random laser spectra can be extended to include different wavelengths by using different gain media. Our results therefore demonstrate that diatomite can serve as an alternative template for the realization of a wide range random laser action. The approach shown here not only pave the way for the implementation of future broad-range random lasers, but also should be able to provide a useful route for the creation of eco-friendly and cost-effective optoelectronic devices with high performance and wide spectrum response.
Acknowledgment
Y.-C. Chen and C.-S. Wang contributed equally to this work. This work was supported by Ministry of Science and Technology and Ministry of Education of Taiwan under the grant number of MOST 99-2119-M-002-019-MY3 and 104 R 89 0932 respectively.
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