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Engineering the Self-Assembly of DCM Dyes into Whispering-Gallery-Mode µ-Hemispheres and Fabry–Pèrot-Type µ-Rods for Visible–NIR (600–875 nm) Range Optical Microcavities Dasari Venkatakrishnarao and Rajadurai Chandrasekar* Bottom-up molecular self-assembly technique[1–3] has emerged as one of the powerful methods to produce miniaturized organic photonic structures,[4,5] for example optical wave guides,[6–10] lasers,[11,12] resonators,[13,14] filters,[15] modulators,[16] and circuits.[17] Amongst the nano/microscale photonic components, resonators or µ-cavities are useful for specific applications such as nanocoherent laser sources,[18] label-free nanobiosensors,[19] nanooptical mode filters,[20] all-optical switches,[21] and future quantum technology devices. In µ-cavities, the mirrorlike geometry allows them to tightly trap the light by repeated internal reflection at the interface, and thus behave as optical gain media exhibiting high quality factor (Q). Shape and size are the two most important determinants of the resonator properties of microscale photonic structures. Photonic structures with curved and plane parallel mirror like reflecting geometries act as whispering-gallery-mode (WGM) and Fabry–Pèrot (F–P)-type lasing resonators, respectively. Mostly, the resonating WGM effect is realized in amorphous glass,[22] polymer,[23–26] and liquid[27] materials, which are created in variety of shapes such as toroids,[22] disks,[23,25] hemispheres,[24,27] spheres,[26] rings,[17] etc. F–P type modes are observed in inorganic nanowire[28] µ-cavities and recently in organic materials.[13,14] In comparison to other photonic materials, the optical band gap of the organic materials is synthetically tunable and solution processable into a variety of shapes; hence, they are suitable for the creation of self-assembled optical resonating structures with tunable emission wavelengths ranging from UV to IR. Tuning the self-assembly of the molecular building blocks is another challenging strategy to achieve photonic solids having different shapes and thereby dissimilar optical characteristics.[6,13] For example, resonating organic solids with 0D, 1D, and 2D shapes, upon light–matter interaction, can act as WGM or F–P type optical µ-cavities depending upon their geometry. In this context, the use of nano/microscale photonic solids obtained from organic dye molecules is essential because they provide a range of FL emissions in the visible region (broad D. Venkatakrishnarao, Prof. R. Chandrasekar Functional Molecular Nano/Micro-Solids Laboratory School of Chemistry University of Hyderabad Prof. C. R. Rao Road Hyderabad 500046, India E-mail: [email protected]/[email protected]

DOI: 10.1002/adom.201500362

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gain bandwidth), high quantum yield, room temperature processing, and low fabrication cost. For example, Takazawa et al. have employed self-assembly technique to fabricate thiacyanine dye-based nanofiber optical wave guides[8] and microring resonators.[17] Recently, Zhao et al. have assembled polystyrene microdisks doped with 1,4-bis(α-cyano-4-diphenylaminostyryl)2,5-diphenylbenzene dye molecules by emulsion–solvent–evaporation method to obtain WGM lasing behavior.[25] To tune the resonator shape, Fu et al. have used protic and nonprotic solvents to produce crystalline µ-wires and µ-disks from a donor– acceptor molecule to achieve WGM and F–P type optical resonators, respectively.[13] Another interesting red laser dye 4-(dicyanomethylene)2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) is a donor–π–bridge–acceptor type molecule, which emits FL via intramolecular charge transfer mechanism[29] (Scheme 1). Unfortunately, DCM dye has a tendency to J-aggregate at high concentration due to its electric polarity and thus its FL emission (600–700 nm) is quenched. To enhance the solid-state FL of DCM, a thin film of dye was resonantly coupled in a critically coupled geometry to observe a 20-fold increase in the red FL intensity.[30] It should be mentioned here that any self-assembled resonating structures composed of DCM dye might possibly enhance the FL intensity upon optical excitation; sometimes the increase is high enough to observe the lasing action. However in our knowledge, so far, there are no reports available on the controlled self-assembly of DCM dye molecules into either WGM or/and F–P resonators. In this work, we describe a simple concentration-dependent self-assembly method to engineer two types of organic resonators from DCM dye, namely, WGM-type 0D µ-hemispheres and F–P-type 1D µ-rods. The detailed self-assembly mechanism of these intriguing photonic structures is discussed. We also present a comprehensive single-particle µ-FL spectroscopy studies to show remarkably enhanced resonance emission signals from these µ-resonators covering visible and near-infrared (NIR) regions (≈600–875 nm) with high Q values. For the self-assembly studies, an acetonitrile (HPLC grade) solution of DCM dye in two different concentrations, 1 mg/2 mL and 4 mg/2 mL were used to fabricate µ-hemispheres and elongated µ-rods, respectively. The DCM solutions were sonicated for 30 s and kept for 15 min without any disturbance. Latter, 3–5 drops of the DCM solution of desired concentration was drop-casted on a clean glass cover slip under slow evaporation condition to produce self-assembled hemispherical or elongated rods for single-particle photonic studies. Laser

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down to several tens of nanometers. Atomic force microscopy (AFM) investigation of a representative hemisphere indicated that its height and diameter are ≈800 nm and ≈4 µm, respectively (Figure 1E,F and Figure S4, Supporting Information). On the other hand, LCM investigation of the sample prepared from a highly concentrated DCM solution (4 mg/2 mL) indicated the formation of 1D elongated structures of varying dimensions, mostly in the range of 4–41 µm. Further FESEM studies of the sample demonstrated rod-like nature of the assemblies with a rectangular cross-section. AFM images of the selected rod showed height and width profile of ≈650 nm and 3 µm, respectively. To probe the formation mechanism of hemispheres, the evaporation of drop-casted Scheme 1. Concentration-dependent self-assembly of DCM dye into hemispherical and rectansolution was directly monitored under congular rod microoptical cavities, displaying WGM and F–P resonances, respectively. focal microscope at 18 °C. The evaporation process of droplet-containing dye typically lasted for 3–4 min. To understand the microscopic formation confocal microscopy (LCM) investigation of the µ-hemisphere mechanism of two types of µ-hemispheres with flat area facing sample indicated the formation of several circular disk like up and down, both confocal microscopy and FESEM studies microstructures (Figure 1A). To further confirm the geomwere performed. Particularly, hemispheres with flat area facing etry of the circular microstructures, field emission scanning up (dome facing the substrate) are rather unusual. During the electron microscope (FESEM) investigation was performed at evaporative inward flow of solvent on the substrate, the foldifferent sample tilt angles (Figure 1B–D and Figure S2, Suplowing sequence of events occurred at the substrate–solution– porting Information). The top view of the electron microscopy air interfaces (Figure 2): the DCM J-aggregate formed a thin images showed the formation of disk-like structures of various film-like assembly within the evaporating concentrated solution heights (Figure 1B); however, when the sample was tilted, layer. This solution layer segregated into several microdropleta clear hemispherical feature of the structure was revealed containing DCM film-like deposits. Further solvent evapora(Figure 1C,D). Interestingly, the flat circular area of most of tion produced several semicircular film edges because of the the µ-hemispheres was facing up (and dome facing down) in tendency of the solvent to reduce its surface tension to attain different angles, thus making minimum surface contact. The minimum energy. Upon continuous rapid solvent evaporation, diameter of the µ-hemispheres has also varied from 15 µm

Figure 1. A,G) Confocal microscopy image of single hemisphere and rod, respectively. B,I) FESEM images displaying the top-view of the µ-hemispheres and rod, respectively. C/D,J) FESEM images displaying the side-view of the hemispheres and rod, respectively. E,F) AFM image of a single hemisphere with a flat surface facing the tip and its corresponding height and diameter profile plot. The inset shows the 3D projection of the hemisphere shown in panel (F). K,L) AFM image of a single rod and its corresponding 3D AFM projection of height and width profiles, respectively.

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COMMUNICATION Figure 2. Self-assembly mechanism of µ-hemispheres and µ-rods. A,C) Confocal microscopy images of solvent islands containing growing hemispheres. FESEM images: B,D) development of standing cylinder from semicylindrical thin film edges. E,F) Top and side views of sand-clock-like structures. G) Demonstration of steps involved in the growth of hemispheres. H) Illustration of the steps involved in the development of µ-hemispheres. I) Formation of nuclei during the solvent evaporation and their growth in 1D fashion to form µ-rods.

the semicircular film edges transformed into several standing microcylinders attached to the substrate. Later, due to the rapid evaporative solvent inward flow, these microcylinders undergo a series of developments such as periphery etching, neck formation (neck height dependent upon the solvent height), and creation of sand-clock-like structure. Finally, the complete etching of the sand-clock neck created two kinds of µ-hemispheres of

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different orientations with their flat surface facing up and down (Figure 2G). The duration from initial island formation and the ensuing processes leading to final hemispherical structures took just less than a second. The confocal microscope could clearly discriminate the two hemispherical structures, as the flat area facing the light looked much brighter than the ones facing down. The internal structure of mechanically cleaved

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and Figures S6 and S7, Supporting Information). These seed nuclei formed at the interface are most probably J-aggregated nanocube-like structures, which further grew up in an elongated fashion to reduce their surface energies. For single-particle (hemisphere) µ-FL experiments, a confocal microscope (reflection mode) equipped with a continuous wave 488 nm Ar+ laser was used (DCM solid state absorption: λmax ≈ 490 nm, see Figure S11 in the Supporting Information). In an earlier report, WGM lasing was observed from a hemispherical-shaped dye-doped polymer droplet, where the curved dome-like geometry (flat circular area facing the substrate) Figure 3. A) Single-particle µ-FL spectra of three hemispheres with varying diameters (D) show [24] In our present case, interdecreasing number of modes and increasing FSR upon decreasing D. The insets show the cor- was facing up. responding images. B) µ-FL spectra of a single tilted hemisphere showing pump laser excitation estingly, the DCM hemispherical particles were orientated at different angles since the position dependent TM/TE modes intensities. The insets show the corresponding images. dome-like geometry was facing the substrate in an unusual way. Hence, to study the effect of µ-hemisphere µ-hemispheres looked very much solid; but down to nanoscale, orientation on the FL spectrum, at first a µ-hemisphere with a it is probably composed of tightly packed flake-like nanoflat circular area pointing upward (dome area facing the substructures (Figure S3, Supporting Information). These nanostrate) was selected. Upon laser illumination on the flat circular flakes were clearly visible in some of the over-etched structures. edge of a hemisphere, a bright red FL emission from the area The formation mechanism of DCM µ-rods is quite straightforof illumination and formation of red FL rim was observed at ward (droplet evaporation process is ≈3–4 min), as it clearly the opposite circular edge (Figure 3A, insets). involved heterogeneous nucleation path at the substrate–soluThe corresponding FL spectra of particles of varying diamtion interface during the solvent evaporation process (Figure 2I eters (the height and orientation angle of the particles are unknown) comprised of a series of sharp peaks due to WGM effect. The modes in the spectra were arranged in pairs of two pronounced peaks of nearly equal intensities, corresponding to the transverse electric (TE) and transverse magnetic (TM) modes.[24] This was supported by the spectra collected from a single tilted hemisphere excited at different positions (Figure 3B). Laser excitation at the center of the particle showed spectacular bright red FL ring along the edges as a result of repetitive total internal reflection of light at the mirror-like curved surface of the cavity pointing out the WGM resonance (Figure S8d, Supporting Information). One of the important findings of this experiment is that the recorded out-coupled emission intensity from the µ-hemisphere was several orders of magnitude higher than the corresponding thin film (Figure S10, Supporting Information) covering both visible and NIR range (600–875 nm). This emission enhancement is due to tight confinement of photons within the mirror-like reflecting curved hemispherical structure. The free spectral range (FSR) or mode Figure 4. A) Single-particle µ-FL spectra of a hemisphere at different laser power. B) Bright and spacing (Δλm) is inversely related to cavity dark field images of hemisphere before and after laser excitation, respectively. C) FSR versus 1D plot with a linear fit. D) The plot of normalized intensity of the spectra as a function of laser diameter (D) by the relation FSR ≈ 1/D. In line with this relation, representative power with a linear fit. E) The plot of Q factor as a function of µ-hemisphere diameter.

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µ hemispheres of three different diameters, ≈13.1 µm, ≈2.7 µm, and ≈1.5 µm (D estimated from the confocal microscope images) showed decreasing number of resonance modes and increasing FSR values, 16.7 nm, 31.4 nm, and 34.1 nm, respectively (Figure 3A). The slight deviation of the best fit value (Figure 4C) can be attributed to the error in the estimated D and FSR values due to confocal resolution limitation and variation in the particle orientation/height, respectively. It is important to mention here that the observation of TE/TM modes and their intensities are highly sensitive to the orientation of the hemisphere toward the pumping laser beam. Upon decreasing of the diameter, the TM/TE mode intensities were not significantly changed indicating the sensitivity of the measurements. CW laser pump power dependent studies showed that upon increasing of power, the spectral intensity increased almost linearly (Figure 4A,B,D). Further, upon increasing the diameter of hemispheres, the Q factor (Q = λ/δλ; where δλ is the line width of a peak at FWHM and λ is the wavelength of the guided light) has also increased up to ≈350, which is a good value for a pure organic resonator obtained Figure 5. A) Representative µ-FL spectra of a “standing” hemisphere at different positions (a–g) corresponding to the map shown in (B). B) 2D FL intensity map of a standing hemisphere by the self-assembly of small dye molecules acting as µ-cavity. The top inset shows the CCD intensity counts. C) Optical microscopy image (Figure 4E). Hemispheres with the flat cir- of hemisphere used for imaging. D,E) 3D FL intensity maps of standing hemisphere exhibiting cular area facing the substrate did not display varying FL intensity distribution. any resonator behavior except broad red FL type parallel mirrors,[14] by reflecting the photons back and band; this is possibly due to their internal structural defects acquired during the self-assembly process. forth between the facets and creating standing wave optical Further, a single “standing” hemisphere was employed to fields. Additionally, upon varying the cavity dimension of three completely map the effect of field distribution on the FL intenrepresentative µ-rods with aspect ratio W:L = 12.1: 80; 8.2:60, sity within the particle. The resultant 2D and 3D FL intensity and 5.8:53, the spacing between the F–P modes have also plots of the standing resonator are presented in Figure 5. In the decreased in the order of 3.76 µm, 7.89 µm, and 12.16 µm, 2D map, from points a to e, the FL spectra displayed modes with respectively. Correspondingly the observed number of modes varying resolutions (Figure 5A,B). Clear well-resolved modes has also decreased in the following order ≈44, ≈25, and ≈17 were observed from sharp circular edges at c and e, respectively. (Figure 6A). To further establish the role of two lateral facets From the corner (a) and center (d) edges of the cavity, a rather in the cavity action, similar measurement must be performed poor resolution was obtained. The center and convex edge on a single rod, which is cut into different lengths so that the of the standing cavity displayed a broad low intensity feature lateral dimensions remain almost identical. This tricky experiwithout any modes. The 3D plot (Figure 5D,E) showed a very ment was performed in an inbuilt AFM–confocal microscope. high, moderate, and poor FL intensity from the circular (a, d, At first, a selected µ-rod was manipulated using an AFM e), convex edge (f), and center (g) of the cavity, respectively. The tip and carefully sliced into three pieces of different lengths observed maximum FL intensity distribution pattern along the (L1 = 150 µm; L2 = 110 µm; and L3 = 74 µm), so that the lateral flat circular face of the standing organic hemisphere indicated dimensions (W = 8 µm) will be identical (Figure 6B). Point the resonant optical field reached its maximum in this area. laser excitation of all of these three rods at one of the lateral During single-particle FL experiment of a µ-rod edges displayed nearly the same mode spacing (Δλ ≈ 10 nm) (W = 12.1 µm; L = 80 µm), point laser excitation at one of the despite their different lengths (Figure 6C). A slight varialateral edges showed propagation red FL to the opposite facets tion of the mode spacing (Δλ ≈ ±0.8 nm) is due to the minor (Figure 6A inset). Further, the envelope of the corresponding experimental inaccuracy in finding the exact lateral edge out-coupled emission spectrum is comprised of a sequence point during optical excitation, because the mode splitof sharp intensity modulated lines from visible to NIR range ting also varies to a great extent along the lateral direction (600–875 nm), indicating the optical wave resonating charac(see Figure 7C,D). Further fitting a plot of spacing of the teristics of the µ-rod. This result clearly demonstrated that the optical modes (Δλm) against inverse cavity diameter showed two opposite lateral facets of the µ-rods evidently act as F–P nearly a linear relationship within the experimental accuracy

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Figure 6. A) Single-particle µ-FL spectra of three µ-rods of varying length (L) and width (W) profiles. The insets show their corresponding bright and dark field images before and after laser excitation, respectively. B) AFM manipulation and cutting of a long µ-rod into three different lengths. C) Singleparticle FL spectra of AFM cut µ-rods with three different length (L) and same width (W) profiles. The insets show the corresponding bright and dark field images of µ-rods before and after laser excitation, respectively. D) A plot of Δλm as a function of 1/W. The red line shows the linear fit. E) A plot of group refractive index (ng) as a function of wavelength for the three µ-rods presented in (A). F) A plot of Q factor as a function of µ-rod width.

(Figure 6D). Here the slight deviation of data from the fit may be attributed to the error in estimating the W values and also finding the specific lateral optical excitation points as mentioned earlier. The value of Δλm at different wavelengths is related to real part of the complex refractive index by the relation Δλ = λ2/2Wng, where ng is group refractive index. This equation was employed to evaluate ng(λ) (Figure 6E) for the µ-rods of varying lengths. The plot clearly showed increase of the refractive index value of up to ≈7.5 upon decreasing the wavelength for all the µ-rods, point out a strong optical confinement within the µ-cavities. The Q factor also changed linearly with respect to the width of the µ-rods reaching a maximum around ≈720 (Figure 6F). Further, upon increasing the laser pump power, the intensity of the F–P cavity spectra increased together with the resolution of the spectrum (Figure 7A,B). Additionally, FL imaging was performed on a selected rod to explain the lateral direction of the F–P cavity (Figure 7C). The corresponding spectra of µ-rods showed apparent variation of the mode spacing in the lateral direction at three representative

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points (see labels a, b, and c) (Figure 7C,), while the number of modes and mode spacing (Δλ ≈ 16 nm) remain almost similar in the lateral direction (see labels c, d, and e in Figure 7C,D). This experiment further established the role of lateral edges in the cavity action. In conclusion, we demonstrated a simple concentrationdependent self-assembly method to produce DCM dye based 0D µ-hemispheres and 1D µ-rods acting as WGM and F–P optical cavities, respectively. Interestingly, the formation of hemispheres with a flat surface area facing up is quite rare; hence, they assemble at the surface at different angles with a minimum surface contact. Single-particle µ-FL emissions from these cavities displayed a multimodal resonance in the vis–NIR range with high Q factors upon continuous wave optical pumping. Further, the DCM µ-cavities displayed improved emission band (composed of sharp resonance modes) due to efficient optical-field enhancement compared to its thin-film state. The number of multimodal resonance lines from these resonators decreased quite sharply upon reducing the cavity size, confirming the resonator character-

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COMMUNICATION Figure 7. A) Single-particle µ-FL spectra of a µ-rod at different laser pump power. The left insets show the bright and dark field images of the µ-rod before and after laser excitation, respectively. B) Plots of FL intensity versus laser power with a linear fit. C) 2D FL intensity map of a µ-rod acting as a µ-cavity. The top inset shows the CCD counts. D) The corresponding selected FL spectra of a µ-rod shown in C. Representative spectra collected at different positions are labeled from a–e.

istics. Although the presented results only support the photonic WGM and F–P modes, further experiments are currently underway to probe the lasing action and exciton–polariton nature (strong coupling between exciton and cavity photons) of these µ-cavities.[31,32] Finally, we expect that the presented simple commercial dye-based µ-resonators will serve as miniaturized functional nanophotonic devices, sensors, and integrated photonic structures.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by UGC-UPE 2 and SERB (EMR/2015/000186). D.V. thanks CSIR-New Delhi for a SRF. Received: June 30, 2015 Revised: August 27, 2015 Published online:

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