On-chip microlasers are desirable to meet the different control requirements and unique demands in different application scenarios. In this work, we obtained the on-chip microlaser by printing pixelated CdSe/ZnS colloidal quantum dots (CQDs), incorporating the quantum dot self-assembly mechanism and the external cavity-free configuration. The spectral purity of the microlaser can be significantly improved by slightly blending polymer into the CQD matrix. The quasitoroid profile was gradually changed to microdisks as the polystyrene (PS) concentration increased from 0 wt.% to 10 wt.%. Specially, when the PS solution varied from 0 wt.% to 1 wt.%, the lasing threshold of 1.4 μJ/mm2 was increased up to 14 μJ/mm2, meanwhile the emission wavelength range showed a 25 nm blue-shift approximately. The easy printing technologies and the low-cost polymer blending method employed in the obtained microlasers will further facilitate the development of printing photonics and electronics, especially in the high-performance microlaser displays and high-precision sensors.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
With the development of full-color laser display technologies [1,2] and wearable optoelectronics [3,4], there have been increasing demand for the high-performance and low-cost on-chip microlasers with excellent color rendering. Due to the great process compatibility with functional polymer, high quantum yield, wide emission wavelength tunability and solution-processable patterning, the CQDs show competing advantages as the gain media in the commercialization of microlasers, especially towards the reliable mass production [5–8].
The traditional CQD microlasers, however, were usually employing external cavity configuration due to its low concentration gain media, which throws a demanding dependence on the quality of the optical cavities in terms of low photoluminescence (PL) coupling efficiencies and the corresponding complicated fabrication processes [9–11]. In recent years, the high concentration gain media used in the CQDs microlasers have given rise to the opportunities for external cavity-free configurations, which were enabled by the single-exciton gain in CQDs films [12–14]. Considering the laser working principle, these configurations require the CQDs microlasers to have intrinsic microcavities, usually induced by the physical profiles of the CQDs films and highly related to the fluidic and viscous properties of the CQDs precursors. For instance, a ring-type CQDs microlaser can be replicated by peeling off the CQDs film from a deliberately designed silicon template, with the aid of ultraviolet-light-curable epoxy . However, the fabrication of high-quality template and the corresponding geometrical replication fidelity put further constraints to the easy realization of microlasers.
Self-assembly of the CQDs provides another promising alternative candidate to produce the CQDs microlasers, to avoid the manufacturing of replication templates. Whispering-gallery mode (WGM) CQDs microbubble laser was self-assembled on the glass substrate during the solvent evaporation after the drop-casting of CQDs/organic nanocomposite solution . During the drying of the solution droplets, tiny bubbles were created along the contact lines serving as the template of the microcavities formation. However, all the bubbles finally dissolved into the solutions within around 1 min, which limited the lifetime of the microbubble laser. Although the introduction of polymethyl methacrylate (PMMA) was able to stabilize the bubble’s shape and extend the lifetime, the poor dimensional uniformity of the CQDs microbubble lasers constrained its application in producing pixelated microlaser array. Besides the verification of PMMA blending on the improved robustness of the microbubble lasers, its influence on optical characteristics still needs to be further investigated for various applications. As a prevailing solution fabrication method, the inkjet printing was also adopted by Wang et al to realize the CdZnS/ZnS alloyed-shell pure CQDs microlasers . The CQDs microlaser obtained in this work had a high lasing threshold (∼67 μJ/mm2) and a specific spectrum, restricting its capabilities as on-chip high-performance optical source or sensor, as well as restricting the versatility of the developed devices.
In the solution processes, the properties of solid materials and solvents in the droplets are critical to the selection of the available printing techniques, such as concentration, viscosity or volatility. Blending of polymers into the CQDs solutions is a common method to modify the kinetics of the ink droplets, meanwhile it also greatly affects the optical characteristics of the final microlaser. The CdSe/ZnS core-shell QDs are optoelectronic nanomaterials well commercialized, due to their high quantum yields, stable chemical properties, and numerous product categories. It is definitely worthwhile to extensively investigate its various physicochemical characteristics as the gain media of the microlasers, especially being processed with the inkjet printing techniques.
In this work, we systematically characterized the CdSe/ZnS CQDs microlasers obtained by inkjet printing of the CQD solutions after blending polymer with different concentrations, in terms of the structure formation and optical characteristics. The microlasers were finally formed by the self-assembly of the CQDs and polymer as hybrid ink droplets, which provides a demonstration of the future low-cost mass production by inkjet printing for the flat-panel laser displays.
2. Fabrication and measurement
2.1 Device fabrication
In the previous work , the distributed Bragg reflector (DBR) substrate was chosen in order to obtain the high reflectivity (99.5%) for the acquisition of the lasing signal. In our work, the fluorinated ethylene propylene (FEP) sheets were used because FEP has high transmittance at the lasing wavelength, which can enhance the signal collection in our measurement setup. Meanwhile, the FEP substrate has a low refractive index (RI ∼1.33) and excellent hydrophobic effect, which has been used in our previous works . The 60 μm nozzle diameter was used here for the inkjet printer, which is a critical factor to determine the droplet volume (∼1.13 × 105 μm3), drive voltage as well as the dimension of the microlaser. The CQDs solutions were prepared in toluene by blending gain medium CdSe/ZnS core-shell QDs (8 nm, quantum yield: 82%) and PS (RI ∼1.59) used as cavity host. The pure PS solution was printed as control sample. The distorted microdisk morphology was observed by optical microscope, as shown in Fig. 1(b), which probably results from the non-uniform surface tension in the various directions in each droplet, possibly a potential candidate in the directional emission . On the contrary, printing of the pure CQDs dispersion toluene solution (30 mg/mL) can produce a symmetrical quasitoroid microlaser, as shown in Fig. 1(k). In order to modify the lasing behaviors, the PS solutions with concentrations of 0.5 wt.%, 1 wt.%, 2 wt.%, 5 wt.%, and 10 wt.%, respectively, were mixed with pure CQDs solution by the volume ratio of 1:1 as hybrid ink droplets.
2.2 Measurement setup
The excitation and detection of the on-chip CQDs microlaser were conducted by a micro-photoluminescence (μ-PL) system, as shown in Fig. 2. The frequency-doubled Nd:YAG laser (λ = 532 nm, PNG-002025-040, Nanolase Corp.) was chosen as the excitation source, with a pulse width of 0.5 ns and pulse frequency of 10 Hz. A variable neutral density (ND) filter was used to adjust the power of pulsed laser. The pump source (300-μm-diameter light spot) was focused on the CQDs microlaser through a planoconvex lens with a focal length of 2 cm, after the free-space transmission. The lasing signal was detected by a multimode fiber (Thorlabs, FP600ERT, core diameter: 200 μm), and the input aperture was aligned with the bottom-side edge of the microlaser through optical microscope system (Eclipse TE2000-U, Nikon) to minimize the PL background. A grating spectrometer (MS7504, Solar TII) was used to record the lasing spectra of the CQDs microlasers, where the exposure time of the detector was set to be 1 s. The device under test (DUT) was adhered to the surface of a silicone film, sandwiched by the FEP substrate and an organic glass plate to maintain the flatness of the devices.
3. Results and discussion
Based on the above measurement setup, we have measured the optical characteristics of the printed CQDs microlasers with different PS blending concentrations, in terms of optical morphologies, spectra characteristics, lasing thresholds, and emission wavelength ranges. Figures 1(c) ̴1(n) show the optical morphologies and corresponding scattering light spots variations of printed CQDs microlasers. Figures 1(c) ̴1(e) and 1(i) ̴1(k) show the microdisk profiles gradually evolved to the quasitoroid structures, as the PS blending concentration decreases. It reveals that the blending of PS solution did not change the dominant self-assembly process of the CQDs, even under a relatively large concentration (up to 10 wt.%). The excess polymer had a tendency to migrate into the central area of the droplet, which could result in the similar dispersion of CQDs and lead to the reduced radius of the quasitoroid microlaser. This could also weaken the output intensity of the scattering light due to the decreased CQDs concentration, even under a high pump intensity (42.3 μJ/mm2). In our experiments, once the concentrations of the PS solution were larger than 2 wt.%. the lasing behavior could hardly be observed, as shown in Figs. 1(f) ̴1(h). When the diluted PS solution (less than 2 wt.%) was blended, the self-assembly of CQDs could maintain the formation of well-constructed quasitoroid. As shown in Figs. 1(l) ̴1(n), the variations of scattering light spots demonstrated the structure formation of the CQDs microlasers.
The lasing spectra of the printed CQD microlasers were characterized using a 1200 l/mm grating spectrometer. As shown in Fig. 3, the spectra purities were significantly improved after the PS blending, which facilitate the implementations of the high-precision sensing. The discrete lasing peaks with nearly equal interval indicated the WGM lasing of the CQDs microlasers [20,21]. It reveals that the PS blending can be able to suppress the generation of the multimodes, primarily because of the reduced toroid edge widths and defective surface profiles. In the pure CQDs microlaser, multimode overlapping can be observed, which is mainly originated from the wide edge size and smooth surface morphology of the quasitoroid.
Figure 4 shows the typical scanning electron microscope (SEM) images of a pure CQDs microlaser (Fig. 4(a)) and 1 wt.% PS-blended CQDs microlaser (Fig. 4(c)). It is observed that ~9 μm edge width of the pure CQDs quasitoroid microlaser was decreased down to approximately 4.5 μm when the PS solution with the concentration of 1 wt.% was incorporated into the pure CQDs ink. Meanwhile, a few cracks are found to distribute around the PS-blended CQDs quasitoroid microlaser surface in comparison with the morphology of the pure CQDs microlaser. Both the deformation phenomena dominate the change of the lasing spectrum characteristics of the CQDs microlasers. Zoomed-in SEM images further verified these changes in the structure formation, as shown in Fig. 4(b) and Fig. 4(d). For the PS-blended CQDs microlasers, the lasing spectra were recorded under the pump intensity of 34.4 μJ/mm2 for 2 wt.% PS blending, 19 μJ/mm2 for 1 wt.%, 17.4 μJ/mm2 for 0.5 wt.%, and 13.3 μJ/mm2 for 0 wt.%, respectively. In the characterized spectra, the full width at half maximum (FWHM) was estimated to be around 0.11 nm, denoted as δλ, which was principally limited by the spectrometer resolution. The slight FWHM variations were negligible for CQDs microlasers with different blending concentrations. Resulting from effective RI changes induced by PS blending, the free spectrum range (FSR) of 0.82 nm for 1 wt.% PS-blended CQDs microlaser was decreased down to 0.75 nm for 0.5 wt.% blending. For the pure CQDs microlaser, the appearance of numerous competitive modes disturbed the estimation of this value. On the other hand, the intensified competition of the multimodes deteriorated the spectrum stability of the pure CQDs microlaser. In contrast with the case of pure CQDs, the PS-blended CQDs microlaser has better photostability in terms of the lasing wavelength position and emission intensity due to the relatively discrete lasing mode distribution. Given the advantage of the stable PL property of the core-shell CQD nanoparticle, the printed CQDs microlasers had good resistance to the environment fluctuation and the photobleaching effect, etc. This point was also verified in our previous work . The analysis of spectra characteristics demonstrated the effects of PS blending on the emission purities of printed CQDs microlasers.
In order to further demonstrate the effects of polymer blending on lasing behaviors of printed CQDs microlasers, the lasing thresholds were characterized and compared for these samples. During the experiments, the emission intensities of the sharpest peaks marked in Fig. 3 at the different pump intensities were extracted to determine the lasing thresholds of CQDs microlasers. By fitting the inflection points in the emission intensity vs pump intensity plots, as shown in Fig. 5, the lasing thresholds of CQDs microlasers were measured to be 14 μJ/mm2 for 2 wt.% blending, 9.6 μJ/mm2 for 1 wt.%, 5.7 μJ/mm2 for 0.5 wt.%, and 1.4 μJ/mm2 for 0 wt.%, respectively. As mentioned in our previous work, the uncertainty of the threshold intensity for all experiments was within ± 0.28 μJ/mm2 . The maximum value was about 10 times the threshold of the pure CQDs microlaser, which was caused by the reduced CQDs gain medium concentration. It is encouraging to note that the minimum threshold (5.7 μJ/mm2) of the printed PS-doped CQDs quasitoroid microlasers was 10 times lower than that of the microlaser reported by Wang et al . It illustrated that few cracks existing in the surface of PS-blended CQDs microlaser did not seriously deteriorate lasing signal transmission (Fig. 4(d)). In addition, it could also be helpful to improve out-of-plane emission required by the laser display application. Herein, the cracks are probably due to the mechanical properties of the phenyl-group structure in the PS polymer under the large internal stress within the CQDs film. If the PS-blended CQDs microlaser is used as the bio-chemical sensor, the cracking issue will lead to emission wavelength fluctuation due to the environmental solution immersion . In order to improve the performance of device, this issue could be gradually eliminated by choosing other flexible optical polymer (e.g., PMMA ) and optimizing the concentration of hybrid ink droplet. The SEM images shown in Fig. 4(e) and Fig. 4(f) had supplied a proof for this hypothesis. The lasing spectrum of the PMMA-blended CQDs microlaser with concentration of 1 wt.% PMMA was characterized under the pump intensity of 36.5 μJ/mm2, as presented in Fig. 6(a). It can be found that there were still some high-order modes appeared in the local regions comparing with the case of PS-blended CQDs microlaser (Fig. 3(b)). It is because that the approximately 4.5 μm-width toroid resonator with a smooth morphology supported the propagation of the high-order modes. To better suppress the generation of the multimodes in the PMMA-blended CQDs microlaser as a high-precision sensor, the concentration of the PMMA solution would need to be carefully optimized in combination with the corresponding printing process.
Figure 6(b) shows the tunability of lasing spectra from CQDs microlasers with different PS concentrations. It is easily found that red-shift appeared in the lasing wavelength range as the PS blending concentration decreases, which is due to large RI difference between PS and CQD, as well as the corresponding increased effective RI of the CQDs/PS nanocomposite film . Furthermore, the purity of lasing spectrum can be accordingly modified as mentioned before. When the PS polymers with the concentrations of 0.5 wt.% and 1 wt.% were blended into the CQDs matrix, the overall lasing spectrum happened to be approximately 20 nm and 25 nm blue-shifts. The spectrum shifts triggered by polymer blending verified the selectivity of lasing region in the printed CQDs microlaser, which provides a feasible pathway to the realization of color-purity-dependent applications based on the printed on-chip CQDs micro-source. In order to demonstrate the reproducibility of the experiment results, we estimated the characteristics of the four CQDs microlaser under the same printing condition. Through the SEM measurement, the deviations of the diameters in the printed CQDs microlasers were within ± 2 μm. Given the effect from the residual CQDs ink around the nozzle, the lasing spectra of the CQDs microlasers existed differences in some extent, especially for the pure CQDs microlaser. Accordingly, the deviations of the lasing thresholds were measured to be within ± 3 μJ/mm2.
In this work, we have demonstrated an on-chip pixelated CdSe/ZnS CQDs microlaser on the FEP substrate utilizing the inkjet printing technique. To modify the optical characteristics of pure CQDs microlaser, PS solutions with different concentrations were introduced to the pure CQDs matrix to investigate the improvement on the structure formation and lasing characteristics. Increasing the polymer concentration, the quasitoroid profiles of microlasers were gradually changed into microdisk structures, meanwhile the lasing thresholds were increased. Moreover, the PS blending also improved purity of lasing spectrum by decreasing the geometrical sizes of quasitoroid resonators. Due to the reduced effective RI of the CQDs/PS composite film, the increase of the FSR and blue-shift of emission wavelength range were quantitatively observed in the spectra characterizations. The analysis on the optical characteristics of CQDs microlaser verified patterning ability of the hybrid ink droplet in the printing technique. The successful demonstration of pixelated on-chip PS-doped CdSe/ZnS CQDs microlaser extends the application scope of the inkjet printing technique. On the other hand, the development of high-performance CQDs microlaser will further accelerate the progress of practical microlaser-based displays and sensors.
National Natural Science Foundation of China (NSFC) (61805104, 61435006, 61525502); Open Project of Wuhan National Laboratory for Optoelectronics (2018WNLOKF015); The Science and Technology Planning Project of Guangdong Province (2017B010123005); Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X121).
The authors declare that there are no conflicts of interest related to this article.
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