Ten-fold enhancement of ZnO thin film ultraviolet-luminescence by dielectric microsphere arrays Yinzhou Yan,1 Yong Zeng,1 Yan Wu,1 Yan Zhao,1 Lingfei Ji,1 Yijian Jiang,1,* and Lin Li2 2
1 Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China Laser Processing Research Centre, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester M13 9PL, UK * [email protected]
Abstract: Here we report strong enhancement in ultravioletphotoluminescence (UV-PL) of ZnO thin films (grown on a SiC substrate) covered by monolayer dielectric fused silica or polystyrene microspheres with diameters ranging from 0.5 to 7.5 μm. The excited light scatted in the film is collected by the microspheres to stimulate whispering gallery modes, by which the internal quantum efficiency of spontaneous emission is enhanced. Meanwhile, the microsphere monolayer efficiently couples emitted light energy from the luminescent film to the far-field for PL detection. A UV-PL enhancement up to 10-fold via a 5-µm-diameter microsphere monolayer is experimentally demonstrated in this work. The unique optical property of microsphere in photoluminescence (PL) enhancement makes them promising for high-sensitivity PL measurements as well as design of photoelectric devices with low loss and high efficiency. ©2014 Optical Society of America OCIS codes: (250.5230) Photoluminescence; (160.2540) Fluorescent and luminescent materials; (170.6280) Spectroscopy, fluorescence and luminescence; (240.6675) Surface photoemission and photoelectron spectroscopy.
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1. Introduction In the past decades, numerous efforts on harvesting of efficient radiation from luminescent materials have led to the development of strategies for enhancement of photoluminescence (PL) by coupling with metal plasmons. Surface plasmon (SP) mediated PL enhancement has been observed at metal layer/ZnO interfaces [1–5], by which 15-fold emission enhancement from a Ag layer/ZnO film structure was demonstrated [1]. The PL enhancement is attributed to the coupling of the emission to SP modes at the interface and then the scattering with granules and roughness on the film surface [6]. Localized surface plasmon (LSP) resonance has also been observed to increase radiation from luminescent films due to the significantly enhanced electromagnetic (EM) oscillation in the proximity of metal nanostructures (e.g. nanoparticles) [7–11]. The Au-ZnO nanostructured composites achieved 20-fold PL enhancement [12, 13]. Furthermore, metallic properties of single-walled carbon nanotubes and graphene have been found to enable similar SP-coupled PL enhancement by capping on luminescent films [14–16]. However, the inherent special coupling condition and the quenching loss in SP/LSP excitation are obstacles to photoemission enhancement [3, 11, 17]. Dielectric microspheres have been widely investigated in nanophotonics. Dielectric microspheres have been found to generate sub-wavelength focus regions, which are known as photonic nanojets [18–20]. Backscattering enhancement has been observed when metallic nanoparticles are placed inside the photonic nanojet region [21, 22]. It provides an alternative way for the detection of metallic nanoparticles with diameters smaller than 50 nm in the visible spectrum [18]. The photonic nanojets have also been employed to enhance the multiphoton-excited fluorescence/upconversion due to the high photon density in the focus area [23–25]. Moreover, the sub-wavelength focus region generated by a discrete dielectric transparent microsphere can achieve super-resolution optical imaging far beyond Abbe’s diffraction limit. Some nano-scale images in the visible spectrum have been demonstrated by coupling various microspheres to standard optical microscopes, such as microsphere-based white-light nanoscopy [26], submerged microsphere optical nanoscopy [27], microspherecoupled scanning laser confocal nanoscopy [28], microsphere-based fluorescence nanoscopy [29], and microsphere-based near-field Raman nanoscopy [30]. Furthermore, dielectric microsphere supporting whispering gallery modes (WGM) is an alternative method to enhance fluorescence, which was first observed by Garrett et al in 1961 [31]. Then dielectric microspheres with doping ions or quantum dots have been widely studied to achieve lasing in the microcavities [32–38]. However, the stimulated emissions
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23554
were only generated inside the doped microspheres. Actually, whispering gallery modes can also enhance radiation stimulated outside microspheres. Gerard et al demonstrated the strong fluorescence enhancement of single-molecule placed around a dielectric microsphere [39]. They found that the microsphere increased the excitation intensity up to 2.2 times and collected up to 60% of light emitting from the molecule. By combining the two effects, the fluorescence enhancement factor can be up to 5 [40]. In addition, it has been found that a high refractive index dielectric microsphere combining with a metallic dimer is a highly efficient optical antenna, in which the metallic nanoparticles permits large enhancements in both excitation strength and radiative decay rates while the dielectric microsphere efficiently collects light without spoiling the emitter quantum efficiency [41]. Such an optical antenna provides significant enhancement of the emitter radiative rate and efficient long-range transfer of emitted photons for detection [42, 43]. Unfortunately, most of these studies pay attention to molecule fluorescence rather than inorganic film luminescence. In this work, we present an approach to enhance PL by spreading dielectric microspheres onto luminescent films deposited on a non-metallic substrate. The strong PL enhancement was demonstrated from the microsphere monolayer/luminescent film hybrid structure. The mechanism of dielectric microsphere enhanced PL is also discussed. 2. Dielectric microsphere monolayer/luminescent film hybrid structure and ultravioletphotoluminescence (UV-PL) enhancement Figure 1(a) shows the schematic of the dielectric microsphere monolayer/luminescent film hybrid structure, where the cross-section view and the top view of the hybrid structures are illustrated respectively. The luminescent film used in this work was a ZnO thin film of 300 nm thickness grown on a SiC single crystal substrate (as Fig. 1(b)) using pulsed laser deposition (PLD) with a 248-nm KrF excimer laser (Coherent LPXpro-305). The growth temperature was 450 °C with an oxygen pressure of 40 Pa. The spacing between the ZnO ceramic target and the SiC substrate was 5 cm. The laser fluence irradiating on ZnO ceramic target was 2.5 J/cm2 and the number of pulse was 10000, where the pulse repetition rate was set as 3 Hz with a fixed pulse duration of 20 ns. The grown ZnO films typically exhibit PL in two spectral bands at room temperature: the narrow ultraviolet emission band at ~380 nm related to the free-exciton emission, and the broad visible emission band at ~510 nm (i.e. green-band emission) attributed to the intrinsic defects (e.g. interstitial zinc or oxygen vacancies) in ZnO [44]. A closely-packed dielectric microsphere monolayer was spread on the luminescent films by self-assembly [45]. The microspheres (Bang Laboratories) used in this work were first diluted by deionized water. The microsphere density in the suspension was 104-106 µL−1. Then the suspension was deposited onto ZnO film surface by drop coating. The microspheres were then self-assembled and a hexagonal close-packed array was formed on the ZnO film surface during the drop drying. The formed microsphere close-packed structures were examined by an Olympus optical microscope (model: LEXT-OLS3100) fitted with a 50 × objective lens (MPLAPO). Figure 1(c) demonstrates a typical topography of a close-packed 5-μm-diameter microsphere monolayer on ZnO film. The microspheres used in this study are 0.5, 1.5, 2.5, 5.0 and 7.5-μm-diameter fused silica (FS) as well as polystyrene (PS) microspheres with a diameter of 5 µm. A 325-nm He-Cd fiber-coupled laser (Kimmon Koha IK3301R-G) was used as the PL excitation source with a normal incident angle onto the film surface. The excited laser was focused by a 14 × objective lens, of which the NA = 0.5. The backward scattering PL signal was collected by the same objective lens and filtered by an optical splitter (i.e. a multi-coating filter) for wavelength separation. A spectrograph (Princeton Instruments Acton SP2750 at 300-900 nm spectrum range) was employed to the PL signal analysis, as Fig. 1(d).
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23555
Fig. 1. Schematic of UV-PL enhancement by microsphere monolayer/luminescent film hybrid structure. (a) Schematic of dielectric microsphere monolayer/ZnO film hybrid structure. (b) The cross-section view of a ZnO film grown on a SiC substrate. (c) The micrograph of a close-packed dielectric microsphere monolayer covered on the ZnO film. (d) The experimental setup of PL spectrum detection.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23556
Fig. 2. The PL spectra of the as-grown ZnO film and the corresponding ones covered with (a) 0.5-μm-diameter FS microspheres, (b) 1.5-μm-diameter FS microspheres, (c) 2.5-μm-diameter FS microspheres, (d) 5-μm-diameter FS microspheres, (e) 7.5-μm-diameter FS microspheres, and (f) 5-μm-diameter PS microspheres. The insets are the micrographs of various closepacked microsphere monolayers.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23557
Figure 2 shows the PL spectra of as-grown and microsphere-covered ZnO films. It can be clearly seen that the UV-PL intensities are strongly enhanced by the microsphere monolayers at the UV band. The FS microspheres increase the PL intensity within the UV band by 2-10 times, in which the maximum enhancement factor (Er = 10.04) is achieved by the 5-μmdiameter microsphere monolayer, as shown in Fig. 2(d). Similarly, the 5-μm-diameter PS microsphere monolayer also demonstrates strong PL enhancement (as Fig. 2(f)), where the UV-PL enhancement factor is over 10. It should be noted that the unusual PL intensity enhancement at the green-band is due to the visible fluorescence of PS excited by the UV laser [46]. 3. Focusing property of dielectric microsphere monolayer The microspheres on the luminescent films focus the incident light when it penetrates the monolayer, as shown in Fig. 1(a). The focusing property of microsphere array affects the incident light energy distribution on the film surface. In order to reveal the focusing property of dielectric microsphere monolayer, a FDTD model was developed to simulate the energy distribution during light focusing by microspheres. As shown in Fig. 1(a), microspheres are self-assembled as a hexagonal close-packed structure. The primitive cell of the periodic structure can be divided as a rectangle that contains two microspheres, as dash lines indicated in Fig. 1(a). Figure 3(a) demonstrates the developed model based on the primitive cell, where the periodic boundary conditions are applied. The simulated light intensity distributions on luminescent films via various microspheres were shown in Figs. 3(b)–3(g). It can be found that the energy focusing localizes over 90% of incident laser energy within the area with a radius of (0.1~0.5)r, where r is the microsphere radius. The focused light intensity can hence be increased up to 4~100 times via the microsphere monolayer. However, ZnO thin films are considered to be uniform luminescent in this work. The equivalent power intensity per area (Ie) by various microspheres is thereby similar, as indicated in Fig. 3, i.e. the similar amount of photons arriving on ZnO films. The energy of light emission from ZnO films is constant under the same PL excitation laser fluence. Therefore the focusing property of microsphere monolayer has no significant contribution to PL enhancement in uniform luminescent thin films. Furthermore, it should be noted that the light coupling to WGM is inefficient without sophisticated methods (e.g. prism coupling, tapered fiber, angle polished fiber, etc.) due to the rotational symmetry of microsphere. According to our numerical simulation in Fig. 3, the WGM in microsphere coupled from the incident light are negligible.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23558
Fig. 3. Numerical simulation of excitation laser focusing through a close-packed microsphere monolayer. (a) The schematic of FDTD model that represents a primitive cell of the hexagonal periodic close-packed microsphere monolayer. (b)-(g) The profiles of focused light intensity on ZnO films via (b) 0.5-µm-diameter FS microspheres, (c) 1.5-µm-diameter FS microspheres, (d) 2.5-µm-diameter FS microspheres, (e) 5-µm-diameter FS microspheres, (f) 7.5-µm-diameter PS microspheres and (g) 5-µm-diameter PS microspheres. The insets are the g contours of power intensity focused by the corresponding close-packed microsphere monolayer.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23559
4. Mechanism of microsphere-enhanced luminescence When the focused light arrives on the ZnO film, some are scattered in the film and others are absorbed by the ZnO molecule for spontaneous emission. The scattered photons could be recollected by the microspheres placed on the film to stimulate WGM. A FDTD model is developed as represented in Fig. 4(a). The scattered light source at 325 nm wavelength in the film was according to the excited light intensity distribution as shown in Fig. 3. The periodic boundary conditions are applied to consider the effect of microsphere array on the electric field intensity distribution. Figures 4(b)–4(g) demonstrate the electric field intensity distributions near microspheres. It can be clearly seen that the WGM are stimulated when the microsphere diameter is greater than 2.5 μm. The optical microcavity is therefore formed. The modification of the optical density of states can enhance spontaneous emission of ZnO emitters in the vicinity of the microcavity due to the Purcell effect. The Q factor (Q) and mode volume (V) of dielectric microsphere is up to ~108 and ~5000 μm3 [47]. Therefore the Purcell factor (Fp) can be calculated by [48] 3
FP =
3 λC Q 4π 2 n V
(1)
Where (λC/n) is the wavelength in the film with a refractive index of n. The maximum Purcell factor for dielectric microspheres is ~15. The internal quantum efficiency of spontaneous emission in ZnO film is therefore enhanced. However, it should be noted that the WGM stimulated in this case give a limited contribution to spontaneous emission enhancement due to the low coupled energy. Most of scatted excited light energy is refracted by the microsphere and back to the far field as shown in Fig. 4. It is well known that light always tends to propagate in materials with high refractive index, e.g. optical fiber. The light emission from a luminescent film (i.e. ZnO where n≈2.1) is therefore always trapped inside the luminescent layer due to total internal reflection on the interface of film and ambient media, resulting in a low external efficiency of photoemission. When dielectric microspheres are placed onto the luminescent layer, the excited light trapped in the films could be coupled to the microspheres. In order to understand the mechanism of emitted light extraction from the luminescent film by the microsphere array, the developed FDTD model is modified for spontaneous emission at 380-nm wavelength, as shown in Fig. 5(a). The emitted light in the film was set by the excited light energy distribution as shown in Fig. 3, i.e. the emission only from the irradiated area. Meanwhile, the periodic boundary conditions are applied in order to simulate the microsphere array. Figures 5(b)–5(h) demonstrate the electric field intensity distributions near microspheres and the corresponding far-field angular distributions for an average over all orientations. It is apparent that light emitted from the ZnO film is largely redirected toward the optical axis when the microsphere diameter is greater than 2.5 μm, allowing efficient PL detection. Meanwhile, the evanescent fields are weakly coupled to microspheres to stimulate WGM at 380 nm wavelength as shown in Fig. 5.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23560
Fig. 4. Numerical simulation of electric field intensity distributions of the scatted excited light near microspheres. (a) The schematic of the FDTD model and the electric field intensity distributions by (b) a 0.5-µm-diameter FS microsphere, (c) a 1.5-µm-diameter FS microsphere, (d) a 2.5-µm-diameter FS microsphere, (e) a 5-µm-diameter FS microsphere, (f) a 7.5-µm-diameter FS microsphere, and (g) a 5-µm-diameter PS microsphere.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23561
Fig. 5. Numerical simulation of electric field intensity distributions of the PL emitted light near microspheres. (a) The schematic of the FDTD model. (b)-(g) The electric field intensity and angular distributions by (b) a 0.5-µm-diameter FS microsphere, (c) a 1.5-µm-diameter FS microsphere, (d) a 2.5-µm-diameter FS microsphere, (e) a 5-µm-diameter FS microsphere, (f) a 7.5-µm-diameter FS microsphere, and (g) a 5-µm-diameter PS microsphere.
Based on the above numerical simulation, the schematic of light emission out coupled of luminescent films with and without dielectric microspheres can be represented in Figs. 6(a)
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23562
and 6(b), respectively. It can be seen that only the propagating waves (k’p) can be detected in the far field without microspheres and the evanescent waves (k’e) caused by total internal reflection are confined in the near field. When a microsphere monolayer is placed onto the film surface, the propagating waves are redirected toward the optical axis (k”p) due to the focal effect of the microsphere. It increases the equivalent NA of the objective lens to improve detection efficiency. At the same time, some evanescent waves with low wave vectors can be converted into propagating waves due to the solid immersion lens effect of the microsphere and collectable in the far field [49]; the other evanescent waves stimulate WGM within the microsphere (as Fig. 5). Considering the light scattering on the microsphere rough surface, the WGM would be leaked out of the microsphere (k”e). The three effects of dielectric microsphere monolayer contribute the PL enhancement.
Fig. 6. Schematics of (a) propagation in wave vectors in the luminescent film without microsphere coupling and (b) wave vectors coupled by microsphere array. (c) Variations of PL enhancement factor for various microspheres covered on ZnO thin films in experiment and numerical calculation
In order to validate above hypothesis on PL enhancement by microspheres, the PL enhancement factors by various microspheres were calculated. For the film without the microsphere monolayer, the solid angle of wave vectors that can be detected is dominated by the refractive index of luminescent film (n) and the NA of objective lens. Microspheres
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23563
couple more wave vectors and redirect them toward the optical axis, as illustrated by numerical simulation as Fig. 5. The PL enhancement factor (A) can therefore be expressed as E′ / E A= m m = E0′ / E0
sin −1 NA
0
sin −1 NA
0
I m (ϕ ) sin ϕ dϕ I 0 (ϕ ) sin ϕ dϕ
π
2 0
π
2 0
I m (ϕ ) sin ϕ dϕ I 0 (ϕ ) sin ϕ dϕ
(2)
where E’m/Em is the ratio of collected light energy by objective lens (E’m) to overall light energy (Em) with microsphere coupling and E’0/E0 is the energy ratio without microsphere coupling. Im(φ) and I0(φ) are the light intensities (i.e. electric field intensities) with respect to the angle to optical axis (φ). Figure 6(c) shows that the numerical calculation is in a good agreement with the experimental result. The experimental PL enhancement factors slightly higher than the calculations are attributed to the WGM caused Purcell effect in the vicinity of microspheres, resulting in the enhancement of internal quantum efficiency. This effect was ignored in the numerical simulation of light collection. According to the experimental and simulated results, it can be found that the PL enhancement factor is determined by the refractive index and diameter of the dielectric microsphere. The 5-μm-diameter FS and PS microspheres couple the most energy from luminescent films to the far field. 5. Conclusions The present work experimentally demonstrated that dielectric microspheres strongly enhance PL of uniformed luminescent films deposited on a non-metallic substrate, for the first time. The excited light penetrates the microsphere monolayer and forms a hexagonal array of focal points exciting the luminescence in the ZnO film. Meanwhile, the excited light would be scatted in the film and recollected by the microspheres, in which the WGM is stimulated and thereby the internal quantum efficiency of spontaneous emission is enhanced by Purcell effect. When the luminescence is excited, a large amount of light energy is trapped inside the luminescent layer and scatted with a large angle that cannot be detected. The microsphere monolayer has been found to efficiently couple emitted light energy from the luminescent film, by which the propagating waves are redirected toward the optical axis and the evanescent waves are conversed into propagating ones via the effect solid immersion lens and leaked WGM in microspheres. The emitted light energy collected in the far field is therefore significantly increased. With 0.5-7.5 μm diameter FS and PS microspheres, the PL enhancement factor can be increased over 10. Such a PL enhancement ratio is competitive to the existing approach via SP/LSP resonance excitation near a noble metal layer. Most importantly, the inherent low energy loss makes dielectric microspheres perfect candidates for photoemission enhancement. The microsphere monolayer/luminescent layer hybrid structure could be used for design of novel high-efficiency photoelectric devices. This work opens up new opportunities to improve luminescent device performance and PL measurement sensitivity by dielectric microspheres. Acknowledgments The authors acknowledge the support of National Science Foundation of China (51005005), Scientific Research Key Program of Beijing Municipal Commission of Education (KZ201210005004) and Doctoral Research Startup Fund of Beijing University of Technology.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23564
1 Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China Laser Processing Research Centre, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester M13 9PL, UK * [email protected]
Abstract: Here we report strong enhancement in ultravioletphotoluminescence (UV-PL) of ZnO thin films (grown on a SiC substrate) covered by monolayer dielectric fused silica or polystyrene microspheres with diameters ranging from 0.5 to 7.5 μm. The excited light scatted in the film is collected by the microspheres to stimulate whispering gallery modes, by which the internal quantum efficiency of spontaneous emission is enhanced. Meanwhile, the microsphere monolayer efficiently couples emitted light energy from the luminescent film to the far-field for PL detection. A UV-PL enhancement up to 10-fold via a 5-µm-diameter microsphere monolayer is experimentally demonstrated in this work. The unique optical property of microsphere in photoluminescence (PL) enhancement makes them promising for high-sensitivity PL measurements as well as design of photoelectric devices with low loss and high efficiency. ©2014 Optical Society of America OCIS codes: (250.5230) Photoluminescence; (160.2540) Fluorescent and luminescent materials; (170.6280) Spectroscopy, fluorescence and luminescence; (240.6675) Surface photoemission and photoelectron spectroscopy.
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1. Introduction In the past decades, numerous efforts on harvesting of efficient radiation from luminescent materials have led to the development of strategies for enhancement of photoluminescence (PL) by coupling with metal plasmons. Surface plasmon (SP) mediated PL enhancement has been observed at metal layer/ZnO interfaces [1–5], by which 15-fold emission enhancement from a Ag layer/ZnO film structure was demonstrated [1]. The PL enhancement is attributed to the coupling of the emission to SP modes at the interface and then the scattering with granules and roughness on the film surface [6]. Localized surface plasmon (LSP) resonance has also been observed to increase radiation from luminescent films due to the significantly enhanced electromagnetic (EM) oscillation in the proximity of metal nanostructures (e.g. nanoparticles) [7–11]. The Au-ZnO nanostructured composites achieved 20-fold PL enhancement [12, 13]. Furthermore, metallic properties of single-walled carbon nanotubes and graphene have been found to enable similar SP-coupled PL enhancement by capping on luminescent films [14–16]. However, the inherent special coupling condition and the quenching loss in SP/LSP excitation are obstacles to photoemission enhancement [3, 11, 17]. Dielectric microspheres have been widely investigated in nanophotonics. Dielectric microspheres have been found to generate sub-wavelength focus regions, which are known as photonic nanojets [18–20]. Backscattering enhancement has been observed when metallic nanoparticles are placed inside the photonic nanojet region [21, 22]. It provides an alternative way for the detection of metallic nanoparticles with diameters smaller than 50 nm in the visible spectrum [18]. The photonic nanojets have also been employed to enhance the multiphoton-excited fluorescence/upconversion due to the high photon density in the focus area [23–25]. Moreover, the sub-wavelength focus region generated by a discrete dielectric transparent microsphere can achieve super-resolution optical imaging far beyond Abbe’s diffraction limit. Some nano-scale images in the visible spectrum have been demonstrated by coupling various microspheres to standard optical microscopes, such as microsphere-based white-light nanoscopy [26], submerged microsphere optical nanoscopy [27], microspherecoupled scanning laser confocal nanoscopy [28], microsphere-based fluorescence nanoscopy [29], and microsphere-based near-field Raman nanoscopy [30]. Furthermore, dielectric microsphere supporting whispering gallery modes (WGM) is an alternative method to enhance fluorescence, which was first observed by Garrett et al in 1961 [31]. Then dielectric microspheres with doping ions or quantum dots have been widely studied to achieve lasing in the microcavities [32–38]. However, the stimulated emissions
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23554
were only generated inside the doped microspheres. Actually, whispering gallery modes can also enhance radiation stimulated outside microspheres. Gerard et al demonstrated the strong fluorescence enhancement of single-molecule placed around a dielectric microsphere [39]. They found that the microsphere increased the excitation intensity up to 2.2 times and collected up to 60% of light emitting from the molecule. By combining the two effects, the fluorescence enhancement factor can be up to 5 [40]. In addition, it has been found that a high refractive index dielectric microsphere combining with a metallic dimer is a highly efficient optical antenna, in which the metallic nanoparticles permits large enhancements in both excitation strength and radiative decay rates while the dielectric microsphere efficiently collects light without spoiling the emitter quantum efficiency [41]. Such an optical antenna provides significant enhancement of the emitter radiative rate and efficient long-range transfer of emitted photons for detection [42, 43]. Unfortunately, most of these studies pay attention to molecule fluorescence rather than inorganic film luminescence. In this work, we present an approach to enhance PL by spreading dielectric microspheres onto luminescent films deposited on a non-metallic substrate. The strong PL enhancement was demonstrated from the microsphere monolayer/luminescent film hybrid structure. The mechanism of dielectric microsphere enhanced PL is also discussed. 2. Dielectric microsphere monolayer/luminescent film hybrid structure and ultravioletphotoluminescence (UV-PL) enhancement Figure 1(a) shows the schematic of the dielectric microsphere monolayer/luminescent film hybrid structure, where the cross-section view and the top view of the hybrid structures are illustrated respectively. The luminescent film used in this work was a ZnO thin film of 300 nm thickness grown on a SiC single crystal substrate (as Fig. 1(b)) using pulsed laser deposition (PLD) with a 248-nm KrF excimer laser (Coherent LPXpro-305). The growth temperature was 450 °C with an oxygen pressure of 40 Pa. The spacing between the ZnO ceramic target and the SiC substrate was 5 cm. The laser fluence irradiating on ZnO ceramic target was 2.5 J/cm2 and the number of pulse was 10000, where the pulse repetition rate was set as 3 Hz with a fixed pulse duration of 20 ns. The grown ZnO films typically exhibit PL in two spectral bands at room temperature: the narrow ultraviolet emission band at ~380 nm related to the free-exciton emission, and the broad visible emission band at ~510 nm (i.e. green-band emission) attributed to the intrinsic defects (e.g. interstitial zinc or oxygen vacancies) in ZnO [44]. A closely-packed dielectric microsphere monolayer was spread on the luminescent films by self-assembly [45]. The microspheres (Bang Laboratories) used in this work were first diluted by deionized water. The microsphere density in the suspension was 104-106 µL−1. Then the suspension was deposited onto ZnO film surface by drop coating. The microspheres were then self-assembled and a hexagonal close-packed array was formed on the ZnO film surface during the drop drying. The formed microsphere close-packed structures were examined by an Olympus optical microscope (model: LEXT-OLS3100) fitted with a 50 × objective lens (MPLAPO). Figure 1(c) demonstrates a typical topography of a close-packed 5-μm-diameter microsphere monolayer on ZnO film. The microspheres used in this study are 0.5, 1.5, 2.5, 5.0 and 7.5-μm-diameter fused silica (FS) as well as polystyrene (PS) microspheres with a diameter of 5 µm. A 325-nm He-Cd fiber-coupled laser (Kimmon Koha IK3301R-G) was used as the PL excitation source with a normal incident angle onto the film surface. The excited laser was focused by a 14 × objective lens, of which the NA = 0.5. The backward scattering PL signal was collected by the same objective lens and filtered by an optical splitter (i.e. a multi-coating filter) for wavelength separation. A spectrograph (Princeton Instruments Acton SP2750 at 300-900 nm spectrum range) was employed to the PL signal analysis, as Fig. 1(d).
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23555
Fig. 1. Schematic of UV-PL enhancement by microsphere monolayer/luminescent film hybrid structure. (a) Schematic of dielectric microsphere monolayer/ZnO film hybrid structure. (b) The cross-section view of a ZnO film grown on a SiC substrate. (c) The micrograph of a close-packed dielectric microsphere monolayer covered on the ZnO film. (d) The experimental setup of PL spectrum detection.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23556
Fig. 2. The PL spectra of the as-grown ZnO film and the corresponding ones covered with (a) 0.5-μm-diameter FS microspheres, (b) 1.5-μm-diameter FS microspheres, (c) 2.5-μm-diameter FS microspheres, (d) 5-μm-diameter FS microspheres, (e) 7.5-μm-diameter FS microspheres, and (f) 5-μm-diameter PS microspheres. The insets are the micrographs of various closepacked microsphere monolayers.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23557
Figure 2 shows the PL spectra of as-grown and microsphere-covered ZnO films. It can be clearly seen that the UV-PL intensities are strongly enhanced by the microsphere monolayers at the UV band. The FS microspheres increase the PL intensity within the UV band by 2-10 times, in which the maximum enhancement factor (Er = 10.04) is achieved by the 5-μmdiameter microsphere monolayer, as shown in Fig. 2(d). Similarly, the 5-μm-diameter PS microsphere monolayer also demonstrates strong PL enhancement (as Fig. 2(f)), where the UV-PL enhancement factor is over 10. It should be noted that the unusual PL intensity enhancement at the green-band is due to the visible fluorescence of PS excited by the UV laser [46]. 3. Focusing property of dielectric microsphere monolayer The microspheres on the luminescent films focus the incident light when it penetrates the monolayer, as shown in Fig. 1(a). The focusing property of microsphere array affects the incident light energy distribution on the film surface. In order to reveal the focusing property of dielectric microsphere monolayer, a FDTD model was developed to simulate the energy distribution during light focusing by microspheres. As shown in Fig. 1(a), microspheres are self-assembled as a hexagonal close-packed structure. The primitive cell of the periodic structure can be divided as a rectangle that contains two microspheres, as dash lines indicated in Fig. 1(a). Figure 3(a) demonstrates the developed model based on the primitive cell, where the periodic boundary conditions are applied. The simulated light intensity distributions on luminescent films via various microspheres were shown in Figs. 3(b)–3(g). It can be found that the energy focusing localizes over 90% of incident laser energy within the area with a radius of (0.1~0.5)r, where r is the microsphere radius. The focused light intensity can hence be increased up to 4~100 times via the microsphere monolayer. However, ZnO thin films are considered to be uniform luminescent in this work. The equivalent power intensity per area (Ie) by various microspheres is thereby similar, as indicated in Fig. 3, i.e. the similar amount of photons arriving on ZnO films. The energy of light emission from ZnO films is constant under the same PL excitation laser fluence. Therefore the focusing property of microsphere monolayer has no significant contribution to PL enhancement in uniform luminescent thin films. Furthermore, it should be noted that the light coupling to WGM is inefficient without sophisticated methods (e.g. prism coupling, tapered fiber, angle polished fiber, etc.) due to the rotational symmetry of microsphere. According to our numerical simulation in Fig. 3, the WGM in microsphere coupled from the incident light are negligible.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23558
Fig. 3. Numerical simulation of excitation laser focusing through a close-packed microsphere monolayer. (a) The schematic of FDTD model that represents a primitive cell of the hexagonal periodic close-packed microsphere monolayer. (b)-(g) The profiles of focused light intensity on ZnO films via (b) 0.5-µm-diameter FS microspheres, (c) 1.5-µm-diameter FS microspheres, (d) 2.5-µm-diameter FS microspheres, (e) 5-µm-diameter FS microspheres, (f) 7.5-µm-diameter PS microspheres and (g) 5-µm-diameter PS microspheres. The insets are the g contours of power intensity focused by the corresponding close-packed microsphere monolayer.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23559
4. Mechanism of microsphere-enhanced luminescence When the focused light arrives on the ZnO film, some are scattered in the film and others are absorbed by the ZnO molecule for spontaneous emission. The scattered photons could be recollected by the microspheres placed on the film to stimulate WGM. A FDTD model is developed as represented in Fig. 4(a). The scattered light source at 325 nm wavelength in the film was according to the excited light intensity distribution as shown in Fig. 3. The periodic boundary conditions are applied to consider the effect of microsphere array on the electric field intensity distribution. Figures 4(b)–4(g) demonstrate the electric field intensity distributions near microspheres. It can be clearly seen that the WGM are stimulated when the microsphere diameter is greater than 2.5 μm. The optical microcavity is therefore formed. The modification of the optical density of states can enhance spontaneous emission of ZnO emitters in the vicinity of the microcavity due to the Purcell effect. The Q factor (Q) and mode volume (V) of dielectric microsphere is up to ~108 and ~5000 μm3 [47]. Therefore the Purcell factor (Fp) can be calculated by [48] 3
FP =
3 λC Q 4π 2 n V
(1)
Where (λC/n) is the wavelength in the film with a refractive index of n. The maximum Purcell factor for dielectric microspheres is ~15. The internal quantum efficiency of spontaneous emission in ZnO film is therefore enhanced. However, it should be noted that the WGM stimulated in this case give a limited contribution to spontaneous emission enhancement due to the low coupled energy. Most of scatted excited light energy is refracted by the microsphere and back to the far field as shown in Fig. 4. It is well known that light always tends to propagate in materials with high refractive index, e.g. optical fiber. The light emission from a luminescent film (i.e. ZnO where n≈2.1) is therefore always trapped inside the luminescent layer due to total internal reflection on the interface of film and ambient media, resulting in a low external efficiency of photoemission. When dielectric microspheres are placed onto the luminescent layer, the excited light trapped in the films could be coupled to the microspheres. In order to understand the mechanism of emitted light extraction from the luminescent film by the microsphere array, the developed FDTD model is modified for spontaneous emission at 380-nm wavelength, as shown in Fig. 5(a). The emitted light in the film was set by the excited light energy distribution as shown in Fig. 3, i.e. the emission only from the irradiated area. Meanwhile, the periodic boundary conditions are applied in order to simulate the microsphere array. Figures 5(b)–5(h) demonstrate the electric field intensity distributions near microspheres and the corresponding far-field angular distributions for an average over all orientations. It is apparent that light emitted from the ZnO film is largely redirected toward the optical axis when the microsphere diameter is greater than 2.5 μm, allowing efficient PL detection. Meanwhile, the evanescent fields are weakly coupled to microspheres to stimulate WGM at 380 nm wavelength as shown in Fig. 5.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23560
Fig. 4. Numerical simulation of electric field intensity distributions of the scatted excited light near microspheres. (a) The schematic of the FDTD model and the electric field intensity distributions by (b) a 0.5-µm-diameter FS microsphere, (c) a 1.5-µm-diameter FS microsphere, (d) a 2.5-µm-diameter FS microsphere, (e) a 5-µm-diameter FS microsphere, (f) a 7.5-µm-diameter FS microsphere, and (g) a 5-µm-diameter PS microsphere.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23561
Fig. 5. Numerical simulation of electric field intensity distributions of the PL emitted light near microspheres. (a) The schematic of the FDTD model. (b)-(g) The electric field intensity and angular distributions by (b) a 0.5-µm-diameter FS microsphere, (c) a 1.5-µm-diameter FS microsphere, (d) a 2.5-µm-diameter FS microsphere, (e) a 5-µm-diameter FS microsphere, (f) a 7.5-µm-diameter FS microsphere, and (g) a 5-µm-diameter PS microsphere.
Based on the above numerical simulation, the schematic of light emission out coupled of luminescent films with and without dielectric microspheres can be represented in Figs. 6(a)
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23562
and 6(b), respectively. It can be seen that only the propagating waves (k’p) can be detected in the far field without microspheres and the evanescent waves (k’e) caused by total internal reflection are confined in the near field. When a microsphere monolayer is placed onto the film surface, the propagating waves are redirected toward the optical axis (k”p) due to the focal effect of the microsphere. It increases the equivalent NA of the objective lens to improve detection efficiency. At the same time, some evanescent waves with low wave vectors can be converted into propagating waves due to the solid immersion lens effect of the microsphere and collectable in the far field [49]; the other evanescent waves stimulate WGM within the microsphere (as Fig. 5). Considering the light scattering on the microsphere rough surface, the WGM would be leaked out of the microsphere (k”e). The three effects of dielectric microsphere monolayer contribute the PL enhancement.
Fig. 6. Schematics of (a) propagation in wave vectors in the luminescent film without microsphere coupling and (b) wave vectors coupled by microsphere array. (c) Variations of PL enhancement factor for various microspheres covered on ZnO thin films in experiment and numerical calculation
In order to validate above hypothesis on PL enhancement by microspheres, the PL enhancement factors by various microspheres were calculated. For the film without the microsphere monolayer, the solid angle of wave vectors that can be detected is dominated by the refractive index of luminescent film (n) and the NA of objective lens. Microspheres
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23563
couple more wave vectors and redirect them toward the optical axis, as illustrated by numerical simulation as Fig. 5. The PL enhancement factor (A) can therefore be expressed as E′ / E A= m m = E0′ / E0
sin −1 NA
0
sin −1 NA
0
I m (ϕ ) sin ϕ dϕ I 0 (ϕ ) sin ϕ dϕ
π
2 0
π
2 0
I m (ϕ ) sin ϕ dϕ I 0 (ϕ ) sin ϕ dϕ
(2)
where E’m/Em is the ratio of collected light energy by objective lens (E’m) to overall light energy (Em) with microsphere coupling and E’0/E0 is the energy ratio without microsphere coupling. Im(φ) and I0(φ) are the light intensities (i.e. electric field intensities) with respect to the angle to optical axis (φ). Figure 6(c) shows that the numerical calculation is in a good agreement with the experimental result. The experimental PL enhancement factors slightly higher than the calculations are attributed to the WGM caused Purcell effect in the vicinity of microspheres, resulting in the enhancement of internal quantum efficiency. This effect was ignored in the numerical simulation of light collection. According to the experimental and simulated results, it can be found that the PL enhancement factor is determined by the refractive index and diameter of the dielectric microsphere. The 5-μm-diameter FS and PS microspheres couple the most energy from luminescent films to the far field. 5. Conclusions The present work experimentally demonstrated that dielectric microspheres strongly enhance PL of uniformed luminescent films deposited on a non-metallic substrate, for the first time. The excited light penetrates the microsphere monolayer and forms a hexagonal array of focal points exciting the luminescence in the ZnO film. Meanwhile, the excited light would be scatted in the film and recollected by the microspheres, in which the WGM is stimulated and thereby the internal quantum efficiency of spontaneous emission is enhanced by Purcell effect. When the luminescence is excited, a large amount of light energy is trapped inside the luminescent layer and scatted with a large angle that cannot be detected. The microsphere monolayer has been found to efficiently couple emitted light energy from the luminescent film, by which the propagating waves are redirected toward the optical axis and the evanescent waves are conversed into propagating ones via the effect solid immersion lens and leaked WGM in microspheres. The emitted light energy collected in the far field is therefore significantly increased. With 0.5-7.5 μm diameter FS and PS microspheres, the PL enhancement factor can be increased over 10. Such a PL enhancement ratio is competitive to the existing approach via SP/LSP resonance excitation near a noble metal layer. Most importantly, the inherent low energy loss makes dielectric microspheres perfect candidates for photoemission enhancement. The microsphere monolayer/luminescent layer hybrid structure could be used for design of novel high-efficiency photoelectric devices. This work opens up new opportunities to improve luminescent device performance and PL measurement sensitivity by dielectric microspheres. Acknowledgments The authors acknowledge the support of National Science Foundation of China (51005005), Scientific Research Key Program of Beijing Municipal Commission of Education (KZ201210005004) and Doctoral Research Startup Fund of Beijing University of Technology.
#216900 - $15.00 USD Received 15 Jul 2014; revised 31 Aug 2014; accepted 2 Sep 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023552 | OPTICS EXPRESS 23564
