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We report random lasing in colloidal quantum dots (CQDs) doped disordered polymer. The CdSe/ZnS core-shell CQDs are dispersed in hybrid polymer including two types of monomers with different rates of polymerization. After UV curing, spatially localized random resonators are formed owing to long range refractive-index fluctuations in inhomogeneous polymer with gain. Upon the optical excitation, random lasing action is triggered above the threshold of 7mJ/cm2. Through the investigation on the spectral characteristics of random laser, the wavelengths of random lasers strongly depend on pump position, which confirms that random laser modes originate from spatially localized resnonators. According to power Fourier transform of emission spectrum, the average size of equivalent micro resonators is attributed to be 50 μm. The proposed method provides a facile route to develop random lasers based on CQDs, showing potential applications on random fiber laser and laser displays.
© 2016 Optical Society of America
1.Introduction
In the past decades, random lasers in disorder gain materials have attracted a great deal of attention of researchers. In contrast to traditional lasers emitted from regular resonators with reflectors, the random lasers are formed by multiple light scattering or refractive index fluctuations [1–4]. The optical modes of random lasers are complex and can be Anderson localized or otherwise confined in space or extended, depending on the mean free path of the sample [5–8]. Various disordered gain media have been demonstrated, ranging from semiconductor powders to π-conjugated polymers, dye-doped liquid crystals, rare earth-doped nanopowders, photonic crystals and biological tissues [9–16]
Colloidal quantum dots (CQDs) have some irreplaceable advantages, i.e. high quantum yields, good color purity, broad excitation spectra, and wavelength tunability of the photoluminescence (PL) emission [17]. Therefore, CQDs are expected to serve as gain materials embed within a variety of structures to realize amplified spontaneous emission (ASE) and lasing, such as distributed feedback structure [18–20], microring resonators [21,22] and chiral nematic liquid crystals [23,24].” However, the CQDs based random laser has rarely been reported. To the best of our knowledge, there are only two kinds of random lasers reported based on CQDs: 1. Random lasers generated from inhomogeneous and close-packed CQD film on a rough micron-scale groove or a glass substrate [25,26]; 2. Random lasers generated from colloidal quantum dots triggered by the laser dye in a quantum dot-dye solution [27,28]. Both these two kinds of CQDs random lasers were difficult to subsequent applications, as well as long term stability.
In this work, we report a novel random laser media based on CQD doped inhomogeneous polymeric matrices. The system is composed of a capillary tube fully filled with CQDs doped acrylic resin and NOA65. The different polymerization rate of two monomers increases inhomogeneous degree and refractive-index fluctuation of the disorder medium, leading to formation of spatially localized random resonators. The single-shot spectra are collected to analyse the spectral characteristics of random laser; the results of which show that peak wavelengths of laser are relatively stable and strongly depend on excited position and the excited length. The power Fourier transform (PFT) analysis is employed to derive the average equivalent diameters of resonators. Our work provides a facile route to produce random laser, which may develop a prototype on CQDs based random fiber lasers and laser displays.
2.Experimental details
The red-emitting CQDs were provided by Tianjin Nanocomy Technology Co. Ltd, China, with a CdSe core and a ZnS shell structure. The quantum yield of quantum dots in toluene was 70% (using Rhodamine 6G as reference). The CQDs doped inhomogeneous polymer (CQD-DIP) and CQDs doped homogeneous polymer (CQD-DHP, without NOA65) with weight ratio of the CQDs, photonitiator (Irgcure 184, available from Ciba Specialty Chemicals Inc.), NOA65 (norland optical adhesive 65, NOA65) and acrylic resin of 2:2:20:76 and 2:2.5:0:95.5 were prepared, respectively. Here CQD-DHP was used as a reference sample to determine the function of disorder structure. The prepared mixtures were filled into capillary tubes with an inner diameter of 300 μm. Next, the samples in capillary tube were exposed by an UV lamp (20mW/cm2) for two minutes to make sure a complete solidification. The two ends of the capillary tubes were sealed by curing with optical glue. All the operations were performed in an argon environment because the quantum yield of CQDs would be dramatically reduced by oxygen and moisture in the air. The refractive indexes of acrylic resin and NOA65 after curing were 1.44 and 1.52, respectively.
The setup of PL emission testing system is shown in Fig. 1(a). The pump laser is generated by a frequency doubled Nd:YAG pulse laser (wavelength: 532 nm, repetition rate: 10 Hz, pulse duration: 8 ns, Quanta-Ray). A λ/2 waveplate for 532 nm and a polarizer are placed in front of the exit of pump laser to adjust the pump energy per pulse incident on the sample. Each pulse is divided into two parts by using a beam splitter, of which the small part is monitored by an energy meter. The excitation beam is focused on the capillary tube by a cylindrical lens. The length of the pump stripe on the sample is 5 mm, the width is fixed at 1 mm.
Fig. 1 (a) Experimental setup of the measurement of the PL spectrum of the samples. The inset is a photograph of the sample excited at 10 mJ/cm2. A filter for 532 nm is placed in front of the camera to block the pump light; (b) absorption (black line) and PL spectra of the CQD-DIP before (red line) and after (green line) curing. The emission spectra of the sample are obtained under the excitation of a continuous-wave pump laser with a wavelength of 532 nm at the power of 10 mW; (c) Micro-image of the capillary samples: the picture on the top is the CQD-DHP, the image at bottom is the CQD-DIP.
Figure 1(b) shows the absorption spectrum of the CQD-DIP (black line) and the PL spectrum of the CQD-DIP before (red line) and after (green line) curing. The PL peaks of the two samples are both at 625 nm. The full width at half maximum (FWHM) of sample is attributed to 33 nm. The PL intensity of CQD-DIP matrix decreases approximately by 20% compared with that before curing.
Figure 1(c) is the micro-image of the samples in capillary: the picture on top is the CQD-DHP, which shows a homogenous medium. The image at bottom is the CQD-DIP, which exhibits a distinct disorder structure. A lot of air holes with a size ranging from a few microns to dozens of micron are dispersed in hybrid polymer. This inhomogeneity is formed owing to different rate of photo polymerization of acrylic resin and NOA65 under the UV radiation. It is worth mentioning that the refractive index of capillary tube 1.46, and the refractive index of polymer is ranging from 1.44 to 1.52. Therefore, total reflection may exist at polymer-capillary interface, which have positive effects on generation of random lasers.
3. Results and discussion
3.1 Spectral characteristics of CQD-DIP and CQD-DHP
Figure 2(a) indicates the PL spectra (single-shot of the laser pulse) evolution of CQD-DIP as pump energy increases from 5 mJ/cm2 to 12 mJ/cm2. The PL spectra are measured by a fiber spectrometer (HR4000, resolution ~0.04 nm, Ocean Optics Inc.). As the increase of pump energy, a piecewise linear growth of emission intensity shows a threshold behaviour, as shown in the inset of Fig. 2(a). The threshold of CQD-DIP is 7mJ/cm2. At low pumping energy, the spectrum consisted of a single broad spontaneous emission peak with an 8 nm linewidth, the increments in emission intensity are small. When the incident pumping energy exceed 7mJ/cm2, many discrete peaks emerge and superimpose on the main emission band. The linewidths of these peaks are 0.15 nm. With the increase of pump energy, these discrete peaks increase in number and in intensity rapidly. All the behaviour of the sharp discrete peaks demonstrates that the emission is a typical random lasing action.
Fig. 2 Spectra of emission from CQD-DIP (a) and CQD-DHP (b) when the excitation intensity ranging from 5mJ/pulse to 14mJ/Pulse. The insets are the emission intensities evolution with increased pump energy; Spectra of random laser in different pump positions (c) and (d) under a intensity of 7.5mJ/Pulse. The insets are the enlarged images of corresponding spectra. The vertical lines guide the peak positions of random lasers.
As a reference sample, the single-shot PL spectra evolution of CQD-DHP as pump energy increases from 5 mJ/cm2 to 14 mJ/cm2 is shown in Fig. 2(b). The increment in the emission intensity with the increase of pump energy is small and exhibits a linear growth, as shown in the inset of Fig. 2(b). No discrete peaks appear in spectra. The steady growth of emission intensity can be attribute to an ASE process [29]. By comparing the results of two samples, it is demonstrated that disordered structure introduced by NOA65 effectively increases the light path length and forms spatially localized random resonators in the active medium, due to refractive index fluctuations, resulting in simultaneously laser appearance. Only CQD-DIP sample has random lasing action, so the following results are all based on CQD-DIP.
3.2 Excitation position
Next, we record the shot-to-shot spectra of random lasers from two different excited positions to study the influence of excited position on random lasers. The applied pump energy is fixed at 7.5 mJ/cm2, which is a little higher than the threshold so that only a few of resonators can lase. In such weak pump condition, as the pump stripe is fixed on one position of the sample, the emitting peak wavelengths are relatively stable though some fluctuations in intensity, as shown in Figs. 2(c) and 2(d). To give a clear demonstration, the spectra are amplified and ploted in the inset, as shown in the inset of Figs. 2(c) and 2(d), where the vertical lines guide the peak wavelengths under different excitations. Obviously, these peak wavelengths are stable, which demonstrates that the excited spatially localized resonators are stable. The intensity fluctuations between different pulse can be attributed to mode competition, which is very common in random laser under a photo-pumping regime [30,31]. On the other hand, the peak wavelengths shift dramatically when the excited positions are shifted compared with Figs. 2(c) and 2(d). The various disorder structure and CQDs concentration result in the formation of different resontanors in different areas. Therefore, the emitting peak wavelengths, representing the modes of micro resonators, change from one position to another. In a word, the random lasers are generated from many spatially localized micro resonators, and represent the modes of these micro cavities.
3.3 Excitation length
The influence of the pump length (under the same pump energy of 8 mJ/cm2) on random laser is determined experimentally. According to Letokhov’s light diffusion model, it is easy to infer that there is a critical excitation length above which gain becomes larger than loss, and random laser emerges [6]. The pump stripe length is varied from 0.5 mm to 5.0 mm, controlled by an adjustable mount. As shown in Fig. 3(a), the critical excitation length to realize lasing is 2.4mm: when the pump stripe length exceeds this value, sharp peaks emerge; below that value, the emission intensity is very low and no obvious sharp peak can be observed. Moreover, the spectral position and intensity of discrete peaks vary with pump stripe length, which illustrates that the number and mode of spatially localized resonators depend on applied excitation length.
Fig. 3 (a) Spectral evolution with increased pump stripe length (pump stripe with tunable length in the range of 0~5.0 mm and here the pump intensity is 8 mJ/Pulse), the emission intensities are represented by different color. (b) Spectral evolution with changing the observation angle with pump intensity is 10 mJ/Pulse), the emission intensities are represented by different color. The inset illustrates the experimental setup. (c) The power Fouier transform spectrum for random laser spectrum (inset).
3.4 Detection angle
Unlike a conventional laser with a certain emission direction, most random laser can be observed in all directions [6]. The emission spectra from different observation angles are measured under a fixed pump energy of 10 mJ/cm2. The setup of such experiment is given in the inset of Fig. 3(b). The spectral positon of sharp peaks are different between different detection angles as shown in Fig. 3(b). Because the modes in scattering system is defined when the excitation length, position and pump energy are fixed. The detected light signals with different observation angles are generated from the ensembles of modes. Therefore, there are obvious variations in the relative intensity of the modes because escape probability of each mode changes with emission angle [32]. Meanwhile, the emission spectra show considerable variation with the change of detection angle. Moreover, the excited volume in the direction along the capillary is much bigger than that in other directions. Therefore, in this direction, random lasers show the highest intensity; as the observation angle increases, the emission intensity and the number of the sharp peaks decline gradually.
3.5 PFT results
The PFT is an effective method to deduce the size of the spatially localized resonators [33]. For long range disorder amplifying material, a ring-like resonator model has been applied successfully to explain the underlying mechanism of the random laser action [4]. In this model, spatially localized resonators are formed by total internal reflections caused by refractive index fluctuation. And the PFT of the random lasing spectrum is a useful and simple method to estimate the diameter of spatially localized ring-like resonator. The inset of Fig. 3(c) gives a typical random laser spectrum of our system. The corresponding PFT is shown in the Fig. 3(c). The periodicity of PFT is attributed to the formation of equivalent ring-like resonator in the disordered system. In our system, the average spatial periodicity length Δd, extracted from the PFT spectrum is about 37 μm. Then one can calculate the average equivalent resonator diameter D = 50 μm by using the relation of ∆d = nD/2, where n = 1.5 is the effective refractive index at the lasing wavelength [4].
4.Conclusions
In summary, we have demonstrated random lasing actions in CQDs doped inhomogeneous polymer. Owing to refractive-index fluctuation caused by long-range disorder, spatially localized resonators are formed in our system. Random lasing action occurred under the excitation of 532nm, the threshold of which is 7mJ/cm2. Single shot spectra are recorded to analyse the laser modes in different pump position and the results show the stability of the wavelength of emitted laser. From the PFT of random laser spectrum, the average equivalent resonator diameter in this system is deduced to be 50 μm. Our work provides an extremely simple method to realize the random laser with CQDs as gain material, and it opens the way to further work on random lasers using this inhomogeneous system, especially in random fiber laser and laser amplifier.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 61271066) and the Foundation of Independent Innovation of Tianjin University (Grant No. 60302070).
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