Article pubs.acs.org/journal/apchd5
Defect Mode Passband Lasing in Self-Assembled Photonic Crystal Kuo Zhong,† Liwang Liu,‡ Xiaodong Xu,‡,§ Michael Hillen,† Atsushi Yamada,⊥ Xingping Zhou,∥ Niels Verellen,‡,¶ Kai Song,*,# Stijn Van Cleuvenbergen,† Renaud Vallée,*,⊥ and Koen Clays*,† †
Department of Chemistry and ‡Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Heverlee (Leuven), Belgium § Key Laboratory of Modern Acoustics and ∥School of Physics and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China ⊥ CNRS, University Bordeaux, CRPP, UPR 8641, 115 Avenue Schweitzer, 33600 Pessac, France # Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ¶ IMEC, Kapeldreef 75, B-3001 Leuven, Belgium S Supporting Information *
ABSTRACT: Up to now colloidal photonic crystal (CPC) lasers essentially relied on photonic band gap edge effects, as a consequence of the poor passband quality achieved by previously reported engineering methods. In this paper, we demonstrate lasing oscillation in CPCs based on the defect mode passband effect. As defect mode, we introduce a highquality monolayer of silica spheres internally functionalized with laser dyes in a sandwiched CPC structure. This defect layer contains the gain medium for lasing action and at the same time breaks the translational symmetry of the crystal, resulting in a pronounced passband within the photonic band gap. The CPC acts as an optical resonator, effectively ensuring the feedback mechanism. The spectroscopic measurements and theoretical simulations match well and reveal that the relatively low-threshold lasing exhibited by the structure can uniquely be attributed to the efficient coupling of the spontaneous emission of the dye to the defect mode of the CPC. Our work provides a new promising strategy toward applications of functionalized self-assembled CPCs with a planar defect in all-optical switching, optoelectronics, and energy-harvesting, and even in the future generation of electro-optical devices, such as lab-on-a-chip with allintegrated optical spectroscopy techniques. KEYWORDS: colloidal photonic crystals, laser, planar defect, spontaneous emission, self-assembly
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feedback effect in the PhC structures.15−23 The defect mode on the other hand introduces a defect or allowed passband in the otherwise forbidden PBG stop band, supporting a highly localized resonant mode. This is considered as one of the most promising strategies to fabricate low-threshold and even zerothreshold lasers.14,24−27 Recent successes in realizing PhC lasing are based predominately on PBG-edge or top-down fabricated defect mode lasers.14−28 However, the top-down method cannot be applied to realize lasing in the visible range due to the diffraction limit of the applied techniques. In contrast, bottom-up fabrication by self-assembly methods allows one to fabricate colloidal photonic crystals (CPCs) with a PBG in the visible range very straightforwardly, at large scale and low cost.11,29,30 Both inorganic and polymeric monodisperse colloidal spheres have been used as building
hotonic crystals (PhCs) are periodic nanostructures of two (or more) dielectric materials. The periodical contrast in electric permittivities introduced in the structure by the variously employed materials results in a photonic band gap (PBG): a frequency window for which photon propagation is forbidden or strongly suppressed.1 The PBG also significantly changes the photonic density of states (DOS) distribution of a material. This effect has been used extensively to modify spontaneous and stimulated emission processes strongly dependent on the interaction between the emitter and its local electromagnetic environment.2−8 In various optoelectronic devices, such as light-emitting diodes, solar cells, optical amplifiers, and low-threshold lasers, PBG effects allow improving overall efficiency.9−14 Ongoing effort is devoted to realizing lasing in a variety of PhC devices including one-, two-, and three-dimensional structures. PhC lasers can be divided into two types: PBG edge lasers and defect mode lasers. The former is utilizing the high density of states at the PBG edge, resulting in a distributed © XXXX American Chemical Society
Received: July 21, 2016 Published: November 22, 2016 A
DOI: 10.1021/acsphotonics.6b00511 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic illustration of the fabrication process of the sandwiched CPC, in which a monolayer of large spheres modified by RhB was embedded in a CPC, at the same time acting as a planar defect and gain medium. (b) SEM image of a monolayer of SiO2 spheres (∼380 nm) modified with RhB at the surface, used as a planar defect layer. The inset is a fluorescence image of the corresponding spheres’ monolayer. (c) Typical SEM cross-section view of the obtained sandwiched CPC270, in which a monolayer of spheres (∼380 nm) larger than the host spheres (∼270 nm) is embedded in the CPC, serving as a planar defect and gain medium as indicated by the green lines. (d) The experimental reflectance spectra of the CPC270 with (black solid line) and without (gray dashed line) a defect layer. The inset is an enlarged spectrum of the passband, with an fwhm of only 11.7 nm (experiment, black line; Gaussian fit, red line).
passes a given threshold value, lasing oscillation is observed. Theoretical calculations of lasing oscillation are in excellent agreement with our experimental observations and confirm that the lasing must be attributed to the high quality of the passband (defect mode) within the PBG.
blocks to fabricate CPCs, resulting in differences in processing conditions. Polymeric spheres are easily etched away, by simple calcination. On the contrary, SiO2 spheres need HF etching, which is a more demanding process. For the fabrication of sandwiched structures by convective self-assembly, hydrophilicity of the subsequent layers is required.31,32 This is obtained in a straightforward manner using inorganic building blocks such as SiO2 spheres. For polymeric building blocks, a post-treatment is usually necessary. While lasing oscillation in self-assembled CPCs has been demonstrated, almost all successful reports are based on the PBG edge effect.23,28,33,34 Few experiments have demonstrated lasing in defect mode CPCs, since the quality of the defect mode created by bottomup methods is usually not good enough to allow lasing in the passband.35−38 Up until now and to the best of our knowledge, only one paper reported defect mode laser generation within a sandwiched CPC,33 where the middle layer embedded in between two colloidal crystals serves as both a defect layer and gain medium. However, although predicted by theoretical calculations, the quality of the passband created by the defect layer was too low to be observed experimentally. In this work, we employ a previously developed method to introduce a high-quality planar defect into sandwiched CPCs, observed as a pronounced dip in the PBG.39 We experimentally and theoretically demonstrate, for the first time, the realization of lasing oscillation in the defect mode (passband) of bottomup self-assembled colloidal photonic crystal structures. In our device, a highly regular monolayer of large silica colloidal spheres (guest spheres) modified by rhodamine B (RhB) molecules has been inserted into CPCs consisting of small silica colloidal spheres (host spheres) by self-assembly at the air/ water interface. The inserted layer acts at the same time as a defect layer and gain medium. The RhBs’ spontaneously emitted photons are coupled into the high-quality defect mode microcavity. When the intensity of the pump light reaches and
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RESULTS AND DISCUSSION 1. Experiments. A schematic illustration of the fabrication process for the sandwiched CPC is shown in Figure 1a. First, a monolayer of RhB-modified silica guest spheres with a diameter of ∼380 nm was prepared using self-assembly at the air/water interface. The obtained high-quality monolayer has hexagonal order, as confirmed by scanning electron microscopy (SEM) shown in Figure 1b. Subsequently, the floating monolayer of colloidal crystals on the water surface was transferred onto the top of a prepared CPC consisting of ∼270 nm silica host spheres (CPC270). Finally, the second CPC of host spheres was directly grown on the surface of the defect layer, resulting in a sandwiched structure (CPC270−380−270). Assuming a hexagonally close-packed face-centered cubic (fcc) structure, the filling factor in our structures is around 74%, in agreement with an estimation performed by the Bragg law.40 A SEM image of the obtained structure is shown in Figure 1c, where a monolayer of large spheres embedded in the CPC can be clearly observed (see the highlighted layer). Both the upper and bottom layers of the host CPC comprising the defect layer consist of ∼10 layers of host spheres and are grown under the same conditions, consistent with our previous work.39 The planar defect breaks the periodicity of the PhCs and introduces a well-defined allowed passband within the forbidden stop band.35,37,41−43 This appears as a pronounced dip in the Bragg peak of the reflectance spectrum of the sandwiched sample (Figure 1d, black solid line), compared to the CPC270 without a defect layer (Figure 1d, dashed gray line). Essentially the presence of the defect layer disturbs the interference between the multiple B
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reflections within the periodic structure.9,39 In comparing the reflectance spectra of CPC 270 and the sandwiched CPC270−380−270, a broadening of the stop band is observed. This phenomenon was found in previously published papers as well,37,39,44,45 although a concise explanation is lacking. We speculate that the defect layer unavoidably introduces some degree of disorder in the subsequently grown top layer (though small in our system), leading to the observed broadening of the PBG.46 The passband, located at 591 nm, has a full width at half-maximum (fwhm) of only 11.7 nm and a quality (Q) factor of 51, defined as the ratio of the peak wavelength of the passband to its fwhm (λ/Δλ). In our previous work we found that the width of the defect layer (i.e., the chosen size of the spheres’ diameters), breaking the local crystal lattice symmetry, strongly affects the position as well as the amplitude of the passband. We therefore attribute the pronounced passband in our system to both the carefully chosen spheres’ diameters and the highly regular obtained defect layer deposited on the target surface. This regularity also ensures the high quality of the subsequently grown second host CPC. Note that, to optimize the amplitude of the passband, the thickness of the top and bottom host CPC layers must also be matched, as is the case here.39 It is well known that the spontaneous emission can be controlled (suppressed or enhanced) by PhCs or optical microcavities because it is strongly dependent on the surrounding environment of the emitter (the DOS redistribution). In the spectral region of the passband the high DOS results from the allowed states formed within the stop band, which allows modifying the spontaneous emission of the embedded RhB molecules. To investigate the effect of the PBG and the passband on the spontaneous emission (SE) of RhB, we compared the emission spectra of RhB in the active (for which the stop band affects the emission of RhB molecules) CPC270 (without defect layer) and the sandwiched CPC270−380−270 (with defect layer) to various CPC reference samples for which the stop bands are shifted away from the emission of RhB, so that the gap’s effect does not alter the emission properties of RhB molecules. As reference samples, we used two CPCs without a defect layer, one consisting of 230 nm silica spheres (functionalized with RhB molecules), ensuring a blue-shifted stop band (CPC230, stop band around 507 nm) with respect to the emission of RhB molecules, the other consisting of 380 nm spheres (functionalized with RhB molecules), ensuring a red-shifted stop band (CPC380, stop band around 847 nm) with respect to the emission of RhB molecules. The relevant spectra for the reference samples can be found in the Supporting Information, SI. It is noteworthy that the emission spectra of RhB in both reference samples CPC230 and CPC380 (Figure 2a and b) are virtually identical. Since the stop bands of these structures are shifted away from the emission of RhB, these structures do not suppress or enhance its emission. On the contrary, for the active structures, CPC270 on one hand, the emission of RhB shows a clear dip matching the stop band of the structure around 589 nm (Figure 2a), compared to the reference samples. On the other hand, the fluorescence spectrum of RhB molecules in the CPC270−380−270 (Figure 2b) exhibits a significant modification, with a suppression of the signal within the forbidden stopband region and an enhancement in the allowed passband. This is attributed to the DOS redistribution caused by the introduction of the planar defect.
Figure 2. (a) Fluorescence emission (subthreshold, blue line, left axis) and reflectance spectra (gray shadow, right axis) of CPC270 without a defect layer, compared to reference sample CPC230, for which the stop band is blue-shifted away from the emission of RhB. (b) Fluorescence emission (subthreshold, blue line, left axis) and reflectance spectra of the sandwiched CPC270−380−270 (with defect layer), compared to reference sample CPC380, for which the stop band is red-shifted away from the emission of RhB. (c) Spectra of laser emission (abovethreshold, blue line, left axis) and reflection (gray shadow) obtained from sandwiched CPC270−380−270. The line width of the lasing peak (around 0.77 nm) is shown in the inset.
The obtained spectral narrowing of the emission observed in the sandwiched CPC270−380−270 provides a possibility to generate lasing oscillation in these structures. This is evidenced by the appearance of a very significant narrowing of the emission peak when the pump power surpasses a certain threshold, occurring at 590 nm, almost in the center of the passband (Figure 2c). The fwhm of the observed peak is reduced to 0.77 nm. We can thus determine a Q-factor for lasing in the same way as we did before to evaluate the quality of the passband (λ/Δλ), with a value of 766. The occurrence of a clear threshold together with the significant narrowing of the emission line width is strong evidence of a lasing action within the PBG passband of our sandwich CPC. In order to quantitatively evaluate the threshold for our device, emission spectra were measured for various excitation intensities, as shown in Figure 3a. During the measurements a pulse duration of 8 ns and the 300 μm spot size of the focused excitation beam were kept constant. When using low energy to excite the sample, a broad emission spectrum of RhB molecules was observed around a wavelength of 572 nm. While increasing the pump energy, a narrow peak (at 590 nm, matching the passband position in the reflectance spectrum of the C
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Figure 3. (a) Emission spectra obtained from the sandwich CPC with various excitation energy. (b) Dependence of the emission intensity (black squares, left axis) and the line width (red triangles, right axis) of the obtained laser spectra on the excitation energy.
finite difference time domain (FDTD) calculations (details found in the SI) based on a custom-made code. In order to simulate the lasing process, we consider a pump−probe mechanism developed in a semiclassical way similar to ref 47. The overall framework consists in a classical electromagnetic field propagation (FDTD) through the periodically varying dielectric constant 3D structure, combined with a quantum framework where the dye molecules, embedded in the defect layer, obey Bloch equations for four-level systems (SI, Figure S4). The chosen gain medium is composed of rhodamine 6G molecules, which are structurally sufficiently close to RhB in order to nicely account for their optical and lasing properties.48,49 All parameters and details about the Bloch equations are given in the SI. In a first step, we calculated, using traditional classical FDTD methods, the transmittance, reflectance, and absorption spectra of the CPC with a defect mode. These static results are shown in Figure 4 and indicate that the proposed structure indeed
sandwiched CPC270) suddenly appears at the long-wavelength shoulder of the broad fluorescence spectrum when the excitation energy reached 14.3 μJ. Upon further increasing the excitation energy, the line width of the emission spectrum dramatically narrows while the amplitude is simultaneously enhanced (see Figure 3a). The dependence of the emission intensity and line width (fwhm) on excitation energy per pulse is plotted in Figure 3b. The line width of the emission spectra is dramatically decreased from 38 nm to 0.77 nm (see Figure 3b, right axis) when increasing the excitation energy. An excitation threshold pulse energy of ∼12.7 μJ/pulse is found, which corresponds to a peak power of ∼1.9 MW/cm2. The threshold for our device is larger than previously reported results.33 However, while the quality of the passband is better in our system, the dye concentration in the defect layer is much lower. To further reduce the threshold, the dye concentration should be increased, for instance by using mesoporous silica spheres to increase the active surface concentration of RhB. Other strategies should be directed at optimizing the optical confinement, e.g., by using thicker top and bottom layers to narrow the PBG and increase its amplitude or by fine-tuning the ratio of the diameters of guest and host spheres to even better optimize the Q-factor of the passband. To compare with the reference (CPC270), we also investigated the intensitydependent emission in CPC270 consisting of the same host spheres with RhB modification but without a defect layer (SI, Figure S3a and c). The emission spectra in the CPC270 without a defect mode were clearly suppressed at that PBG wavelength, and no lasing action could be observed, even at excitation energies well above the threshold found for the sandwiched CPC270 with a defect (SI, Figure S3b). This is expected since the emission of RhB falls almost completely within the forbidden stopband of the CPC270, for which an extremely low DOS acts as an inhibitor of emissive transitions. In addition, the lasing oscillation is not observed in the reference samples (either CPC230 or CPC380) when the excitation energy exceeded the threshold found in the sandwiched CPC270 (see SI, Figures S1c and S2c). Hence, this further confirms that the lasing in our sample is attributed to the defect mode induced passband in the sandwiched CPC270−380−270, creating a highly localized state that strongly enhances the emission of RhB. 2. Simulations. To gain insight into the lasing oscillation in the cavity mode of the sandwich CPCs270−380−270, the responses of the system corresponding to either reflection/transmission/ absorption spectra or lasing characteristics were simulated by
Figure 4. Simulated transmission (blue), reflection (red), and absorption (green) spectra normalized by respective spectra (reference spectra in a vacuum) for 0 [M] dye density system. The typical lasing spectrum (black) for a 0.2 [M] dye density system after the pump/ probe process while pumping at 800 kV/cm. The latter is normalized with respect to the spectrum obtained without pumping.
exhibits a clear passband in the middle of the nondefect CPC photonic stopband. This figure clearly reveals that the defect mode has been nicely designed as a donor defect by careful examination of the refractive index, size, and position of the defect layer within the nondefect CPC, as was performed experimentally. The matching between Figures 4 and 2 is D
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Figure 5. (a) Excited-state population N2 (top part: average excited-state population vs incoming pump electric field amplitude) and (b) electric field intensity profiles on a xz-cross-section of the CPC obtained at 800 kV/cm pump amplitude. The experimental and simulation structures are illustrated for easier understanding on the right part of the figure.
Figure 6. (a) Transmission spectra evolution with a pump power for 0.2 [M] dye density system and (b) laser characteristic: the transmission peak height and width vs pump intensity. The numbers indicated on the red curve designate the field amplitude in kV/cm. The dashed lines are guides for the eyes drawn in order to determine the crossing values of the curves, e.g., the lasing threshold.
striking and validates the use of our simulation model to further perform a dynamic analysis of the lasing process. Owing to both Bloch and Maxwell equations, combined and discretized in the region of the location of the dye, we could investigate the excitation and emission properties of the dye molecules during and after the pump/probe processes. Figure 5a shows a profile of the population inversion reached after pumping in the defect layer (xz cut), where the gain medium has been inserted. As expected, for a large enough (800 kV/m in this case) input amplitude of the electric field, the population inversion is large and its normalization with respect to the overall population density of the system ranges between 0.5 and 0.9. The evolution of the average (on the whole defect layer volume) population inversion is also shown in Figure 5a as a function of the pump amplitude, showing a nicely growing S-shape. Note that, in Figure 5, only one (or three) layer(s) of beads surrounds the defect layer on both sides of it, while in both experiments and simulations nine layers are effectively present. Note also that there are no finitesize effects in our simulations as compared to experiments, since periodic boundary conditions are applied on the lateral sides of the unit cell and the same number of layers as in the experiment is used longitudinally, in the direction of the exciting beam (Figure 5, right part). A similar simulation performed on a similar system but excluding the host CPCs around the defect layer also generates a significant population inversion (average population of 0.7), showing that the presence of these host CPCs does not tremendously affect the initial population inversion. Figure 5b then nicely shows the electric field intensity generated at the maximum of emission of the dyes (581 nm). In such conditions, the probe process,
generating the population relaxation of the dyes, leads to the observation of a sharp line with maximum intensity at 581 nm, as shown in Figure 6a. This very sharp line is of course very reminiscent of a lasing peak occurring at the exact maximum transmission of the defect mode. In order to clearly show the stimulated emission origin of this peak, we performed a whole set of pump/probe simulations at various excitation powers while keeping the probe process very weak (5 kV/cm). Figure 6 shows the spectra obtained while changing the pump amplitude from 0 to 1500 kV/cm (a) and the corresponding lasing characteristics (b). The peak intensity as a function of input intensity clearly shows a threshold at around 1500 MW/cm2 with a change in the slope of the curves at that location. Similarly, at around this threshold value, the peak width, after having decreased rather rapidly, reaches a saturation limit of about half a nanometer. Both curves thus indicate unambiguously the occurrence of a lasing process with well-defined characteristics. Note again that, for a similar simulation performed on a similar system excluding the host CPCs surrounding the defect layer, the lasing effect and characteristics could not be observed (Figure S5). Thus, the comparison between Figures 6 and S5 clearly shows that the role of the host CPCs is to significantly amplify the fields. Furthermore, although the simulation excellently reproduced the lasing character with a clear threshold and thus the appearance of amplified stimulated emission, spontaneous emission has not been included in our simulation’s framework. The latter would involve noise-induced variations of the lasing threshold and lead to a considerably larger narrowing when going from the spontaneous-emission-dominated regime to the lasing regime. The different lasing threshold observed between E
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simulated (1500 MW/cm2) and experimental ∼1.9 MW/cm2 results partly originates from a difference in the concentration of laser dyes present in the defect layers, which, while perfectly determined in the simulations, is difficult to estimate on the experimental side. Furthermore, the fact that spontaneous emission is not taken into account in the simulations makes a fair quantitative comparison rather difficult between simulations and experiments. In order to account for the effects of quantum noise and amplified spontaneous emission, a Langevin approach must be used,50 which is beyond the scope of this study. 3. Conclusion. We have successfully demonstrated lasing oscillation in CPC with a planar defect in experiment and theory. The device was fabricated by introducing a monolayer of colloidal spheres modified with RhB molecules between two CPCs by self-assembly at the air/water interface. This monolayer simultaneously serves as a defect layer and gain medium in the resulting sandwiched CPCs. This high-quality planar defect effectively breaks the translational symmetry of the crystals, resulting in a pronounced dip (fwhm of 11.7 nm) within the reflectance spectrum. For excitation energies over the threshold of 1.9 MW/cm2, a very narrow and single lasing peak with a 0.77 nm fwhm at 590 nm was observed, which matches the dip in the PBG of the reflectance spectrum. The theoretical predictions are in excellent agreement with the experimental observations, further demonstrating that the pronounced passband within the PBG of our sample indeed provides a strong feedback for laser action, coupling efficiently the laser dyes’ emission to the defect mode. The low-cost selfassembly method for preparing a CPC with a high-quality planar defect and subsequent emission modulation and lasing provides a promising strategy toward future applications of the CPC in integrated electro-optical or optoelectronic devices, such as optical spectroscopy based lab-on-a-chip.
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inserted into the water and lifted under a small angle to transfer the monolayer onto the colloidal crystal substrate. The SiO2 CPC slab with a monolayer of modified SiO2 guest spheres on top was dried under an angle of approximately 45°. In the third step, a sandwich CPC embedding a light-emitting layer can be obtained after growing a second SiO2 CPC slab on top of the defect layer using the same conditions as in the first convective self-assembly. Characterization. Colloids and colloidal crystals were imaged by SEM (Philips XL 30 ESEM FEG). The reflectance spectra of the samples were recorded using an Ocean Optics USB 4000 spectrometer connected to a fiber-optic reflectance probe (Avantes, FCR-7xx200-2BX/ME). The emission was recorded by a homemade optical system, in which a Nd:YAG laser with 532 nm wavelength obtained via a BBO frequency-doubling crystal, with 8 ns pulse width and repetition frequency of 10 Hz, was employed as a pump source. The excitation light was focused onto the sample by a lens with a focal length of 150 mm to a spot with a diameter of ∼300 μm, and the tilt angle between the incident light and the normal direction to the (111) planes of the photonic crystals was set to 10 degrees. The intensity of the excitation light was controlled by a neutral-density filter. The fluorescence emission from the samples was collected by a lens ( f = 70 mm) and then guided into a spectrometer (Ocean Optics, USB 4000). Simulations. The reflectance, transmittance, and emission spectra within the sandwiched CPC, as well as the lasing characteristics, were simulated using a custom-made code that makes use of the FDTD method,53 for which Maxwell equations were solved in regions free of dye molecules and Maxwell−Bloch equations were used in the defect layer containing the laser dyes. Further details can be found in the SI.
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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.6b00511. Additional information (PDF)
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EXPERIMENTAL SECTION
AUTHOR INFORMATION
Corresponding Authors
Preparation of RhB-Modified Silica Spheres. First, the silica spheres with various diameters (230, 270, and 380 nm) were respectively modified by 3-aminopropyltriethoxysilane (APTES).51 Briefly, a 0.084 g silica sphere was added into 20 mL of ethanol with 100 μL of APTES at room temperature for 12 h. After removal of the excess or unreacted APTES, the APTES-modified silica spheres were redispersed in 50 mL of ethanol solution consisting of 0.115 g of RhB, N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (EDC, 0.062 g), and diisopropylethylamine (0.2 mL) at room temperature for 24 h to graft RhB on the surface of the APTESmodified spheres according to the amidation reaction.52 Finally, the excess or unreacted reactants were removed by repeated centrifugation. Fabrication of CPC with a Light-Emitting Planar Defect. The CPC with a planar defect was fabricated using a method developed by our group.39 In the first step, a convective self-assembly method was employed to fabricate a SiO2 multilayer CPC.40 A cleaned substrate was placed vertically in a vial containing SiO2 particles with diameter of ∼270 nm in ethanol (5 mL) at a volume fraction of 0.3%. The convective self-assembly proceeded for 3 days at 34 °C. In the second step, a monolayer of silica spheres (∼380 nm), which were modified by RhB, was prepared using a direct assembly method at the air/water interface. Typically, 12 μL of the RhB-modified silica suspension (4 wt %) was dropped on a hydrophilic glass slide and then added to the air/ water interface within a dish (Φ = 6 cm) by tilting the slide by an angle of approximately 45° (shown in Figure 1a). After spreading, the patches of colloidal monolayer were floating at the interface. A 2 μL amount of a SDS (2 wt %) solution was added to the water surface to compress the particles closer together, hence serving as a soft barrier to help monolayer crystallization. Next, the prepared SiO2 CPC slab was
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[email protected]. ORCID
Renaud Vallée: 0000-0002-6950-2637 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.Z. warmly acknowledges the support provided by the China Scholarship Council (CSC) of the Ministry of Education, P. R. China. A.Y. is grateful for a postdoctoral fellowship provided by the French State, managed by the French National Research Agency (ANR) in the frame of the “Investments for the Future” Programme IdEx Bordeaux−LAPHIA (ANR-10-IDEX-03-02). N.V. acknowledges the Fonds Wetenschappelijk Onderzoek Vlaanderen (FWO)-Flanders for financial support.
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REFERENCES
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DOI: 10.1021/acsphotonics.6b00511 ACS Photonics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsphotonics.6b00511 ACS Photonics XXXX, XXX, XXX−XXX
