Room-Temperature Planar Lasers Based on Water-Dripping

Jun 20, 2017 - The solution-processable colloidal quantum dots (CQDs) attract great interests in small-size laser applications because of the high qua...
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Room-Temperature Planar Lasers Based on Water-Dripping Microplates of Colloidal Quantum Dots Kexiu Rong,† Chengwei Sun,† Kebin Shi,†,‡ Qihuang Gong,†,‡ and Jianjun Chen*,†,‡ †

State Key Laboratory for Mesoscopic Physics, Collaborative Innovation Center of Quantum Matter, Department of Physics, Peking University, Beijing 100871, China, and ‡ Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China S Supporting Information *

ABSTRACT: The solution-processable colloidal quantum dots (CQDs) attract great interests in small-size laser applications because of the high quantum yields and the tunable emission wavelengths. The small CQD lasers based on the microplates are of importance in the highly integrated photonic circuits, and the simple and low-cost manufacturing methods to obtain the CQD microplates are greatly desired and appealing in applications. Here, by employing the simple drop-casting and water-dripping method, the high-quality CQD microplates with various shapes and sizes are experimentally manufactured under the proper solvents and solvent ratio as well as environment temperature. Evidently, this manufacturing method does not require any expensive or special instruments. Because of both the large gain coefficients and the high quality factors of the CQD microplates, the room-temperature planar multi- and single-mode CQD lasers with p-polarized emissions are experimentally realized under low pump thresholds. Moreover, it is demonstrated that the planar CQD microplate laser is easy to be integrated with the waveguides on chips. This simple and low-cost method to manufacture the CQD microplates opens a wide range of possible activities in the area of solid-state small lasers, which are important building blocks for the true integration of optoelectronic circuitry. KEYWORDS: colloidal quantum dots, microplates, planar microlasers, integration, polarization

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crystal cavities,26 the costly and high-vacuum micro/nanofabrication technologies were required. For the WGM cavities by using the microspheres13,15 and the microtoroids,16 numerous fabrication steps with expensive and special instruments were needed. Most recently, the CQD microcavities, which provide both the gains and optical feedbacks, have also been proposed and demonstrated.4−7 For example, by utilizing the complex electrodynamic end-cap trap setup,4 the CQD microdrops were levitated to serve as the WGM microcavities, and the liquid CQD lasers were experimentally demonstrated. By using the special capillary jet technique5 and inkjet-printing method,6 the CQD coffee rings were self-assembled to form the FP5 and WGM6 microcavities, and the CQD lasers were also realized. Two months ago, Handong Sun et al. reported the WGM microbubble laser by drop-casting the highly concentrated CQD/PMMA nanocomposites (∼45 wt %).7 It is noted that the manufacturing processes of these CQD microcavities4−6 (except the microbubble cavities7) are complex and expensive. More importantly, the on-chip integrations of these small CQD lasers with other nanophotonic devices (such as waveguides) are not realized.4−7 Therefore, the development of simple and low-cost manufacturing methods to achieve small CQD lasers

wing to the low-cost fabrications, high quantum yields (QYs ∼ 100%), and tunable emission wavelengths,1−3 the solution-processable colloidal quantum dots (CQDs) have attracted enormous attention in the lasing applications in the past decade.4−26 CQDs can be fabricated by the low-cost wetchemical synthetic method.3 By changing the sizes and the compositions of the CQDs, the CQD lasing wavelengths have already covered the whole range of the visible4−7,9−26 and the near-infrared (NIR)8 spectra. Hence, the CQDs overcome the difficulties encountered by the conventional semiconductor materials in the blue spectrum,6,7,11,14 and they can provide the building blocks for close-packed QD solids.3 Furthermore, the CQDs are very stable, and they can even lase under (or after suffering) bad conditions, such as the aqueous, oxygenic, and thermal environments,27,28 while other gain materials do not work well.29−31 Therefore, a lot of CQDs-based lasers were demonstrated4−26 by employing various cavities, including the Fabry-Pér ot (FP) cavities, 8−12 whispering-gallery-mode (WGM) cavities,13−16 distributed feedback (DFB) cavities,17−21 random cavities,22−25 and photonic crystal cavities.26 These cavities provided the optical feedbacks, while the CQD films8−11,13,15−26 or solutions12,14 were filled to serve as the gain materials. For the FP cavities,8−12 the random cavities,22−25 and the WGM cavity by using the fused silica hollow fibers,14 the bulky sizes made them highly challenging for on-chip integrations. For the DFB cavities17−21 and the photonic © XXXX American Chemical Society

Received: April 6, 2017 Published: June 20, 2017 A

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octane to hexane, the surface of the CQD film is flat, but many defects are observed because of the large evaporation rate of the mixed solvent. For the large solvent ratio of octane to hexane, the defect number decreases but the surface of the CQD film becomes not flat (forming the concentric rings) due to the small evaporation rate of the mixed solvent.33 Consequently, the proper volume ratio (1:5) of octane to hexane is chosen in the experiment. Moreover, the environment temperature can also influence the evaporation rate of the mixed solvent. The higher the environment temperature, the larger the evaporation rate of the mixed solvent. For the volume ratio of 1:5, the highquality CQD films can be obtained at the environment temperature between 20 and 28 °C. The dark-field optical image of the drop-casted CQD film (after baking) under the white-light illumination is displayed in Figure 1c. Here, no obvious scattered light is observed, indicating that the CQD film is uniform and nearly has no defects. Because of the high quantum yields of the CQD film, a red uniform background of the photoluminescence (PL) emission is clearly observed, as shown in Figure 1c. It should be pointed out that the dark points in Figure 1c,d come from the charge coupled device (CCD), and the dark points remain unchanged when the imaging areas of the sample are changed. During the evaporation of the mixed solvent, there are residual strains in the CQD film.11,22,23 To crack the CQD film to micrometer areas, the surface tension of the water drop34 is used to release the residual strains in the CQD film. This is the second step. By dripping a water drop on the CQD film, the CQD film cracks around the three-phase (CQDs−water−air) contact line, as shown in Figure 1b. The dark-field optical image of the CQD film after cracking is displayed in Figure 1d. It is observed that the CQD film is cracked, and some crackenclosed areas are peeled off from the glass substrate. These peeled CQD areas become the free-floating CQD microplates on the surface of the water drop, as schematically depicted in Figure 1b. Characterizations of CQD Microplates. In order to characterize and utilize the CQD microplates, the CQD microplates are transferred to the glass substrate and indium−tin−oxide (ITO) substrate (see Fabrication section). The dark-field optical image of one CQD microplate on the glass substrate is displayed in Figure 2a, and the area of this CQD microplate is about 40 × 30 μm2. Also, it is observed that the red PL emission from the body of the CQD microplate is quite weak and uniform, while the light at the microplate edges is very bright and smooth. This phenomenon reveals that the CQD microplates may have high qualities, such as uniform surfaces, smooth edges, and nearly no defects. Compared to other methods to obtain gain microplates,31,35−39 this simple method does not need any expensive or special instruments, and a large number of CQD microplates with various shapes can be manufactured, as displayed by Figure S1 in Supporting Information. Although the shapes of the CQD microplates are uncontrollable, the number (∼1 × 105) of the CQD microplates is very large. Hence, the desired microplates can be easily found in the experiment. Moreover, similar to other gain microplates,35−42 the positions of the CQD microplates can also be micromanipulated by a probe connected to a 3D translation stage. To provide more evidence about the high qualities of the CQD microplates, the surface morphologies of the CQD microplates are measured. The atomic-force microscopy (AFM) image of the CQD microplate on the glass substrate

with easy on-chip integrations and enhanced lasing performances are urgent and important in practical applications. In the letter, the high-quality CQD microplates are experimentally manufactured by using a simple and low-cost method, which only comprises the drop-casting and waterdripping steps in the ambient environment (atmospheric temperature and pressure). Obviously, this manufacturing method does not require any expensive or special instruments. By carefully choosing the proper solvents and solvent ratio as well as environment temperature, the CQD microplates after the water-dripping step possess high qualities, such as flat surfaces, smooth end facets, and nearly no defects. Because of both the large gain coefficients and the high quality factors of the CQD microplates, the low-threshold planar CQD lasers as well as the easy on-chip integration with the waveguide are experimentally demonstrated at room temperature. By choosing the CQD microplate with a very irregular shape, the single-mode CQD laser is also achieved. Moreover, the lasing emissions from the CQD microplates are measured to be linearly polarized, and the transverse magnetic (TM) waveguide mode dominates the resonances in the CQD microplates although the transverse electric (TE) waveguide mode has the largest effective refractive index and confinement factor. The underlying physics of this abnormal phenomenon is explained.

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RESULTS AND DISCUSSION Manufacture of CQD Microplates. The simple and lowcost method to manufacture the CQD microplates includes two main steps, as schematically depicted in Figure 1a,b. In the first

Figure 1. Two main steps to manufacture the CQD microplates in the ambient environment. Schematics of the (a) drop-casting and (b) water-dripping steps. Dark-field optical images of the CQD film after the (c) drop-casting and (d) water-dripping steps.

step, the mixed solvent of octane and hexane containing CQDs is drop-casted on the glass substrate (see Fabrication section), and a close-packed CQD film2,3,32 with a large area (20 × 20 mm2) is self-stacked, as shown in Figure 1a. Here, the added octane in the CQD solution (hexane) is used to uniformly spread the CQDs across the whole glass substrate due to the different chain lengths of octane and hexane. It is found that the qualities of the CQD film, such as the uniformity and flatness, are closely related to the solvent ratio as well as the environment temperature. Under the small solvent ratio of B

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attributed to the multibeam interference in the high-index CQD layer (Figure S3 in Supporting Information). Pumped by a continuous-wave (CW) laser (λp = 400 nm), the PL peak of the CQD microplate appears around 650 nm with a full width at half-maximum (fwhm) about 50 nm, as depicted by the red line in Figure 2d. These properties of the CQD microplates are consistent with that of the high-quality CQD films. It should be pointed out that the optical properties of the CQD film and microplates are very stable in water. The PL spectrum and PL efficiency from the CQD microplates do not change as the microplates are dispersed in water for 2 h. The reason is that the organic quantum dots are closely stacked to form the CQD film, and the water cannot penetrate into the CQD film and microplates. Therefore, the high-quality CQD microplates are very stable even suffering the terrible preparation conditions, such as baking, water steeping, cracking, and substrate transferring in the ambient environment. Multimode Lasing from the CQD Microplate. To observe the lasing emissions, the CQD microplates on the glass substrate are pumped by a picosecond laser at room temperature (see Measurement section). Here, the spot diameter (fwhm) of the pump beam is about 165 μm. The spectrum of the pump beam is displayed by the violet line in Figure 2d. It is observed that the CQD microplates have a strong absorption at the pump wavelength (λp = 430 nm). The emission spectra of a square-like CQD microplate (side length ≈ 60 μm) under different pump fluences are displayed in Figure 3a. At a low pump fluence (P ≤ 170 μJ/cm2), the emission is very weak (black line in Figure 3a), and the measured PL spectrum from the CQD microplate is similar to that in Figure 2d, as shown by the inset in Figure 3a. Hence, the line width (fwhm) of the weak emission is Δλ ≈ 50 nm. When the pump fluence reaches P = 210 μJ/cm2, several resonant peaks can be observed, as depicted by the red line in Figure 3a. The reason is that the low-loss resonant modes obtain enough gains in the CQD microplate under this pump fluence, which verifies that the CQD microplate is an optical microcavity. As the pump fluence further increases, the intensities of the resonant peaks increase sharply, as shown by the red, blue, and green lines in Figure 3a. The high-loss resonant modes can obtain sufficient gains under high pump fluences, and thus, it is also noticed that the number of the resonant peaks increases with the pump fluence in the gain range of the CQDs (610− 650 nm). This gain range is located on the left side of the PL peak (λ = 650 nm), indicating the quasi-type-II CQDs.23,43 The free spectral range (FSR) of the microcavity is calculated to be about 1.3 nm. This small FSR is attributed to that a large number of resonant modes obtain sufficient gains in the CQD microplate. The FSR can be increased by decreasing the size and symmetry of the CQD microplate, which can reduce the number of the low-loss resonant modes.38,44−46 In addition, the line width of the resonant peak (λ = 626.6 nm) is measured to be about Δλ = 0.5 nm, and the corresponding quality factor of the optical microcavity equals Q = λ/Δλ ≈ 1250. This quality factor (Q ≈ 1250) is greater than that of most of the FP cavities5,8−11 and DFB cavities,18−21 and it is even comparable to that of the WGM cavities fabricated by the costly and special instruments.6,14,15 Furthermore, this quality factor (Q ≈ 1250) is comparable to the best results in the previous works by employing other gain microplates.31,37,39−41 Hence, the CQD microplates are high-quality optical microcavities, and lasing can be realized at low pump fluences in the CQD microplates.

Figure 2. Surface morphologies and optical properties of the CQD microplates after the substrate transferring. (a) Dark-field optical image of the CQD microplate on the glass substrate. (b) AFM image of the CQD microplate on the glass substrate. (c) SEM image of the end facets of the CQD microplate on the ITO substrate. (d) Absorption (black line) and PL (red line) spectra of the CQD microplate. The violet line denotes the spectrum of the picosecond pump laser (λp = 430 nm).

is depicted in Figure 2b. It can be seen that the CQD microplate comprises the sharp and smooth edges as well as the flat surface. The thickness of the CQD microplate is about 780 nm, and its surface roughness is about 10 nm (Figure S2 in Supporting Information). Thus, the surface of the CQD microplate is quite flat at the optical level. It should be pointed out that the flatness of the CQD microplate manufactured by this simple method is much better than that of most of the previous reports.5,6,16,22−25 The scanning electron microscopy (SEM) image of one CQD microplate on the ITO substrate is displayed in Figure 2c. Here, the ITO substrate is used as the conductor in the SEM measurement. The SEM image also reveals that the surface of the CQD microplate is quite flat. More importantly, the SEM image shows that the end facets of the CQD microplate are very sharp and smooth. Based on the high-resolution SEM image of the CQD microplates (nonordered layer) and the short evaporation time (∼2 min) in the film formation,32 the CQD microplates are the glassy solids. Because of the high refractive index of the CQD microplate (n = 1.73), the PL emissions from the CQDs can be well confined in the planar glass-CQDs-air waveguide.18,21,36 Moreover, nearly total internal reflection can occur at the end facets of the microplate.31,35−41 Hence, the CQD microplates manufactured by this proposed method are appropriate candidates for the on-chip optical microcavities, which have been verified by the perovskite,35,40 PbI2,36 ZnO,37 CdS,38 and organic crystal31,39,41 microplates in recent years. However, most of the previous gain microplates31,35−39 were manufactured by the costly and high-vacuum vapor phase deposition method, and the others31,40 were not very stable. The optical properties of the CQD microplates are also measured. The measured absorption spectrum of the CQD microplate under the illumination of a tungsten-halogen lamp is depicted by the black line in Figure 2d. It is observed that the CQD microplate exhibits strong absorption at short wavelengths (λ < 500 nm). Besides, there are some absorption features at the long wavelengths (see Figure 2d), which are C

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importantly, it is observed that the four edges of the CQD microplate above the pump threshold (P = 310 μJ/cm2, 1.6Pth) are much brighter than that below the pump threshold (P = 140 μJ/cm2, 0.7Pth), indicating the transition from the weak spontaneous emission to the strong stimulated emission (lasing). The bright edges together with the dark body (Figure 3d) also confirm the in-plane lasing emission from the CQD microplate, exhibiting a characteristic of the planar microcavities.31,35−41 It should be pointed that the dark points in Figure 3c,d come from the CCD because the positions of the dark points change when the CCD is slightly moved, as shown in Figure 3c,d. As analyzed above, the PL emissions from the CQDs are well confined in the planar glass-CQDs-air waveguide, and nearly total internal reflection can occur at the end facets of the microplate, constructing an optical microcavity. To support this analysis, the CQD microplate is simulated by the finite element method (see Simulation section). First, the eigenmodes of the planar glass-CQDs-air waveguide are simulated (Figure S4 in Supporting Information). When the thickness of the CQD layer is t = 780 nm, four waveguide modes are supported by the planar waveguide. Since the TE0 and TM0 waveguide modes possess the largest effective refractive indexes (neffTE = 1.699 and neffTM = 1.694) and tightest field confinements, only the two waveguide modes are considered in the following. For simplification, TE0 (or TM0) is abbreviated to be TE (or TM). The field distributions (|E|2) of the TE and TM waveguide modes are shown in Figure 3e. It is observed that the fields of the TE and TM waveguide modes are well confined in the CQD layer. The confinement factors, which are defined as the fraction of the modal intensities inside the CQD layer,18,50 are as large as ΓTE = 97% and ΓTM = 96%, respectively. Although both of the effective refractive index (neffTM = 1.694) and the confinement factor (ΓTM = 96%) of the TM waveguide mode are a little smaller than that of the TE waveguide mode (neffTE = 1.699 and ΓTE = 97%), the experimental results show that the resonances in the microplate are dominated by the TM waveguide mode. This abnormal phenomenon will be discussed later in detail. Furthermore, an equivalent 2D simulation model36,38 is employed to calculate the field distributions (|E|2) of the resonant modes in the CQD microplate, and the refractive index of the microplate is set to be neffTM = 1.694. One of the simulated field distributions (|E|2) in the microplate at the resonant wavelength of λ = 626.2 nm is displayed in Figure 3f. As can be seen from Figure 3f, the resonant mode can only be coupled out from the cavity edges, suggesting a good mode confinement in the plane of the CQD microplate. This is also verified by the 3D model simulations (Figures S5− S7 in Supporting Information). Therefore, by utilizing the CQD microplate manufactured by the proposed method, the room-temperature planar CQD laser can be achieved in the experiment. On-Chip Integration between the CQD Microplate Laser and the Waveguide. The room-temperature CQD lasers based on the microplates are an ultrasmall planar laser source,31,35−41 which can be easily integrated with other photonic devices on chips.51,52 Herein, the easy on-chip integration between a CQD microplate laser and a CQD waveguide is experimentally demonstrated, as shown in Figure 4a−d. The CQD microplate laser and the coupling waveguide are shown by the bright-field optical image in Figure 4a. It is observed that the CQD microplate (area ≈ 40 × 40 μm2) has a narrow lug (width ≈ 10 μm), and the narrow lug is well

Figure 3. Multimode lasing from the square-like CQD microplate. (a) Emission spectra of the CQD microplate under different pump fluences. The inset is a full emission spectrum of the CQD microplate at P = 170 μJ/cm2. (b) Integrated intensities and line widths of the resonant peak at λ = 626.6 nm under different pump fluences. CCD images of the CQD microplate (c) below and (d) above the pump threshold. (e) Simulated field distributions (|E|2) of the fundamental TE and TM waveguide modes in the planar glass-CQDs-air waveguide. (f) Simulated field distribution (|E|2) of the resonant mode (λ = 626.2 nm) in the CQD microplate.

To get the pump threshold, the integrated intensities (626− 627 nm, black square symbols) and line widths (red circular symbols) at different pump fluences are measured, and the results are depicted in Figure 3b. The black lines are the linear fitting curves, where a kink is clearly observed, revealing a pump threshold only about Pth = 200 μJ/cm2. It is worth mentioning that this pump threshold by using the picosecond pump laser is smaller than that of the previous CQD lasers.4,6,9,12,15,17,22,23,25,26 Meanwhile, the line width of the resonant peak suddenly drops from about 50 to 0.5 nm above the pump threshold, as shown by the red circular symbols in Figure 3b. This obvious spectral narrowing further confirms the lasing behavior in the CQD microplate. A little increase in the line width above the pump threshold is also observed, as shown in Figure 3b, which results from the nonequilibrium carrier energy distributions when a microlaser is pumped by the short and intense pulses.47−49 The CCD images of the CQD microplate below and above the pump threshold are displayed in Figure 3c and d, respectively. It is noticed that the body of the CQD microplate remains very dark for both cases, as shown in Figure 3c and d. This reveals that the CQD emissions are well confined in the microplate. More D

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scattered spectrum is attributed to that a part of the coupling waveguide is illuminated by the pump beam, as shown by the green dash-dotted circle in Figure 4a. When the same pump beam (spot diameter ≈ 110 μm and pump fluence P ≈ 560 μJ/ cm2) only impinges the coupling waveguide, as shown by the cyan dashed circle in Figure 4a, no lasing emissions are observed at the positions P1 and P2. This further verifies that the scattered spectrum from P2 comes from the CQD microplate laser. Hence, the easy on-chip coupling and integration between the CQD microplate laser and the waveguide are demonstrated in the experiment. The integrated structure is easily found from the lots (∼1 × 105) of the CQD microplates. However, the on-chip integration between the CQD microplate laser and the waveguide is not as easy as the integrated structure to be found in the experiment because of the random assembly of the integrated structures. It should be pointed out that the CQD strip waveguide used here has an absorption at the lasing wavelengths (red line in Figure S3a in Supporting Information), so it is better to couple and integrate the planar CQD microplate laser to other low-loss (or lossless) waveguides. Single-Mode Lasing from the CQD Microplate. The single-mode lasers can also be realized in the CQD microplates manufactured by the proposed method. Due to the large number of the CQD microplates, a CQD microplate with a very irregular shape is utilized, as shown in Figure 5a, where the bright lasing emissions are observed at the microplate edges under the pump fluence about P = 470 μJ/cm2. The measured emission spectrum from the position I of the microplate (P = 400 μJ/cm2) is displayed by the red line in Figure 5b. Only one resonant peak is clearly observed, revealing the single-mode lasing in the irregular microplate. The pump threshold of the single-mode laser is measured to be Pth = 260 μJ/cm2. The realization of the single-mode laser is attributed to the irregular shape and the defect points of the CQD microplate (see Figure 5a), both of which reduce the number of the resonant modes in the CQD microplate.38,44−46 Polarization State of the Lasing Emission. It should be pointed out that the emission spectrum of the single-mode CQD laser (red line in Figure 5b) is measured after a linear polarizer, which allows the electric field vectors perpendicular to the x axis passing. By rotating the linear polarizer with an angle of 90°, the measured emission spectrum is depicted by the black line in Figure 5b. It is found that the peak value is considerably decreased as compared to the red line in Figure 5b, indicating that the lasing emission from the CQD microplate is linearly polarized. Moreover, the peak values of the single-mode emission spectra at different angles of the polarizer (θP) are measured, and the results are displayed in Figure 5c. Herein, θP is defined as the angle between the x axis and the axis of the polarizer. It is observed that the electric field vectors of the lasing emission from the single-mode microplate laser are nearly parallel to the z axis. Based on Figure 5a, the electric field vectors of the lasing emission from position I are perpendicular to the corresponding edge, of which the normal direction is depicted by the blue dashed line in Figure 5c. The degree of linear polarization (DOLP) is calculated to be about DOLP = 87%. This phenomenon indicates that the resonance of the TM waveguide mode happens in the microplate. For another position (position II) of the microplate, the measured peak values of the single-mode emission spectra at different θP are displayed in Figure 5d. Also, it is found that the electric field vectors of the lasing emission from the single-mode microplate

Figure 4. On-chip integration between the CQD microplate laser and the waveguide. (a) Bright-field optical image of the CQD microplate and the coupling waveguide. The green dash-dotted and the cyan dashed circles denote the positions and sizes of the pump beams that impinge the CQD microplate and the waveguide, respectively. (b) CCD image of the coupled microplate laser and waveguide above the pump threshold. The inset is the zoomed-in image of the position P2 in the coupling waveguide, of which the intensity is amplified by three times. (c) Emission spectrum from the position P1 in the CQD microplate laser (red line) and scattered spectrum from the position P2 in the waveguide (blue line). CCD images of the CQD microplate (d) below and (e) above the pump threshold.

connected with one end facet (width ≈ 10 μm) of the CQD strip (length ≈ 150 μm) that serves as the coupling waveguide (Figure S8 in Supporting Information). As the pump beam (spot diameter ≈ 110 μm) impinges the CQD microplate, the similar pump threshold and spectral narrowing of the emission spectrum are observed, and the pump threshold is measured to be about Pth = 330 μJ/cm2. Figure 4b depicts the obvious lasing emissions from the edges of the microplate under the pump fluence of P ≈ 560 μJ/cm2. The position and size of the pump beam is denoted by the green dash-dotted circle in Figure 4a. The emission spectrum of the CQD microplate laser (measured at the position P1) is depicted by the red line in Figure 4c, where the resonant peaks are obviously observed. The CCD images of the CQD microplate below (P = 220 μJ/cm2, 0.7Pth) and above (P = 560 μJ/cm2, 1.7Pth) the pump threshold are displayed in Figure 4d and e, respectively. It is observed that the scattered light from the connecting facet of the narrow lug above the pump threshold (Figure 4e) is much brighter than that below the pump threshold (Figure 4d), further indicating the occurrence of lasing. The measured scattered spectrum from the position P2 (inset in Figure 4b) in the coupling waveguide (about 100 μm away from P1) is displayed by the blue line in Figure 4c. Herein, P2 is a defect in the coupling waveguide. Because of the CQD absorption at the lasing wavelengths (red line in Figure S3a in Supporting Information) and the scattering loss at the defect, the defect in front of the waveguide end (along the propagation direction) is brighter than the waveguide end, as shown in Figure 4b. This scattered spectrum from the position P2 resembles that from the position P1 in the microplate laser (red line in Figure 4c), indicating that the lasing emission from the microplate can be coupled to the nearby coupling waveguide. The nonzero background of the E

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Consequently, the TM mode has low radiation losses at the end facets of the microplates, and thus, the low-threshold lasing is expected from the resonances of the TM waveguide mode. It is known that the magnetic field vectors of the TM waveguide mode are mainly parallel to the x−z plane. For each resonant mode in the microplate, there are both the clockwise and the anticlockwise round trips, as shown in Figure 5f. Moreover, the clockwise and anticlockwise round trips interfere constructively at the edges of the microplate, resulting in the bright edges, as shown in Figure 5a. The resultant magnetic field vector (H) of the two magnetic field vectors from the clockwise (H2) and anticlockwise (H1) round trips is parallel to the edge of the microplate, as shown by the inset in Figure 5f. Hence, the electric field vectors of the lasing emission from the CQD microplate at the far field are perpendicular to the edge of the microplate, matching well with the experimental results (Figure 5c,d). Further simulations by using the 3D models also show that the resonances in the CQD microplates come from the TM waveguide mode, of which the electric field vectors (or magnetic field vectors) are mainly perpendicular (or parallel) to the x−z plane (Figures S5−S7 in Supporting Information). This agrees well with the experimental results and analysis.

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CONCLUSIONS In summary, the simple and low-cost method, which only comprised the drop-casting and water-dripping steps, was proposed to manufacture the CQD microplates. By carefully choosing the proper solvents and solvent ratio as well as environment temperature, we manufactured the high-quality CQD microplates, which were verified by both the measured surface morphologies and optical properties. Based on these high-quality CQD microplates, the room-temperature planar CQD lasers with low pump thresholds (Pth = 200 μJ/cm2) and narrow line widths (Δλ ≈ 0.5 nm) as well as the easy on-chip integration with the waveguide were experimentally demonstrated. Also, by choosing the CQD microplates with a very irregular shape, the single-mode CQD microplate lasers were also successfully realized. Moreover, the lasing emissions from the CQD microplates were measured to be linearly polarized, and the TM waveguide mode dominated the resonances in the CQD microplates although both of the effective refractive index (neffTM = 1.694) and the confinement factor (ΓTM = 96%) of the TM waveguide mode were a little smaller than that of the TE waveguide mode (neffTE = 1.699 and ΓTE = 97%). The underlying physics of this abnormal phenomenon was explained. The CQD microplates exhibited outstanding stability to environments, such as baking, water steeping, cracking, and substrate transferring in the ambient environment. Therefore, this water-dripping method to manufacture the CQD microplates opens a wide range of possible activities in the area of solid-state small lasers, which are important building blocks for the true integration of optoelectronic circuitry.

Figure 5. Single-mode lasing from the irregular CQD microplate. (a) CCD image of the CQD microplate above the pump threshold. (b) Emission spectra of the single-mode CQD microplate laser from the position I after a linear polarizer, which allows the electric field vectors perpendicular (red line) or parallel (black line) to the x axis passing. Lasing intensities at different angles of the polarizer from (c) position I and (d) position II at different edges of the microplate. The blue lines in (c) and (d) indicate the normal directions of the corresponding edges. (e) Reflectances of the TE- and TM-polarized plane waves at an infinite end facet of the microplate. The blue area indicates the incident angles below the critical angle for total internal reflection (θc). (f) Schematic illustration of the polarization state for the lasing emission from the microplate edge.

laser are perpendicular to the corresponding edge of the CQD microplate, which further verifies that the TM waveguide mode dominates the resonance in the CQD microplate. The similar phenomena are also observed in other microplates (Figure S9 in Supporting Information). It is worth mentioning that the polarization states of the pump beam have no influence on the CQD microplate laser (Figure S9 in Supporting Information). The reason is that the CQDs have a degenerate emission transition dipole, which is oriented isotropically in planes.53,54 To understand the polarization characteristics of the lasing emission from the CQD microplate, a further analysis is made. Although both the effective refractive index and the confinement factor of the TE waveguide mode (in the planar glassCQDs-air waveguide) are a little greater than that of the TM waveguide mode (Figure S4 in Supporting Information), the reflectance of the TE mode (RTE) is smaller than that of the TM mode (RTM) at the end facet of the microplate for θ < θc ≈ 36°, as shown by the solid red and dashed black lines in the blue area of Figure 5e. Here, θ is the incident angle, and the plane-wave reflections off an infinite end facet of the CQD microplate are calculated for simplification. For the confined waveguide modes, the range of the incident angle (RTE < RTM) becomes much wider (Figure S10 in Supporting Information).

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METHODS Fabrication. The commercially available CdSe/ZnS core/ shell CQDs are dispersed in hexane and the concentration is about 10 mg/mL. The diameter of the CdSe core and the thickness of the ZnS shell in the CdSe/ZnS core/shell CQDs are 6.7 ± 0.2 nm and 1.5 ± 0.3 nm, respectively. The surface ligand of the CQDs is cis-9-octadecenoic acid, and the quantum yields of the CQDs in hexane are measured to be about 62%. To obtain the high-quality CQD film, octane is added in the F

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CQD solution, and the volume ratio of octane to hexane is 1:5. Then, the mixed solvent of octane and hexane containing CQDs is drop-casted on the glass substrate, which is ultrasonically cleaned by toluene, acetone, and ethanol for 7 min, respectively. The thickness of the CQD film can be easily adjusted by varying the CQD concentration or the mixture volume. The environment temperature is about 25 °C. At last, the drop-casted CQD film is baked at 80 °C for 10 min to fully evaporate the mixed solvent. The CQD microplates in the water drop are redispersed on the water surface in a beaker. Since the CQD microplates are free-floating on the surface of the water, it is very convenient to transfer the CQD microplates to the target substrates by the drop-casting, spin- and dip-coating methods. Here, using the dip-coating method, a target glass substrate is immersed in the water, and the free-floating CQD microplates are adhered on the substrate surface accompanying with the slow withdrawal of the substrate. At last, the CQD microplates on the glass substrate are dried in the ambient environment for further characterization and utilization. Measurement. All the optical measurements are performed by the homemade microscope system in the ambient environment. The pump source for the lasing measurements is a picosecond laser, of which the wavelength, repetition rate, and pulse width are 430 nm, 1 kHz, and 20 ps, respectively. The pump pulses (electric field vectors along the z axis) are focused by a long-working-distance objective (Mitutoyo 20×, NA 0.4) from the back side of the substrate at room temperature, and the CQD emissions are collected by another long-workingdistance objective (Mitutoyo 50×, NA 0.42) from the other side of the substrate. The collected CQD emissions after a long-pass filter are divided into two parts, which are received by a CCD and a coupled fiber (connected to the spectrograph), respectively. The images of the CQD microplates (∼6 mm) are much larger than the diameter of the coupled fiber (∼1 mm), and thus, the results from positions I and II can be measured separately. Moreover, we can also use a pinhole on the image plane to achieve the spatial resolution power. To measure the polarization state of the lasing emission from the CQD microplate, a linear polarizer is inserted in front of the coupled fiber. Simulation. The simulations are performed by using the finite element method (Comsol Multiphysics). In the simulation, the refractive indexes of the CQD microplate, glass substrate, and air are n = 1.73, 1.5, and 1.0, respectively. In the 3D simulation, the eigenfrequency analysis of the RF module and the scattering boundary condition are used.

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reflections off an infinite end facet of the CQD microplate (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jianjun Chen: 0000-0002-1050-1189 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2013CB328704 and 2016YFA0203500) and the National Natural Science Foundation of China (11674014, 11204018, 61475005, and 11134001).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00363. CQD microplates with various shapes, thickness and surface roughness of the CQD microplate, absorption spectrum of the CQD microplate, simulation of the planar glass−CQDs−air waveguide, simulation of the resonant modes in the square, rectangular, and irregular microplates, simulation of the waveguide mode supported by the coupling waveguide, polarization state of the lasing emission from the square-like microplate, G

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