Share with Facebook
Tweet This
Post on reddit
Share with LinkedIn
Add to CiteULike
Add to Mendeley
Add to BibSonomyAbstract
Here we investigated the effect of substrate and film thickness on photoluminescence (PL) enhancement of wide bandgap semiconductor (i.e. ZnO) by dielectric microsphere array/luminescence film/substrate (MLS) sandwich structures. The PL enhancement channels in the sandwich structure were revealed, for the first time, including the focusing property of microsphere array (MSA) distinctly enhancing free-exciton recombination, anti-reflection effect of MSA increasing excitation cross-section area, MLS-supported TW-/SW-WGMs inducing ASE and Purcell’s effect, and optical directional antenna effect for high equivalent NA of objective lens as well as out-coupling efficiency. The enhancement ratio of intensity (ERI) for ZnO UV-PL from free-exciton recombination in the sandwich structure was found to be strongly dependent upon the refractive index of substrate and luminescence film thickness. In order to achieve high ERI for PL emission, the refractive index of substrate should differ from luminescence film and the film thickness needs to be chosen to support WGMs in the sandwich structure. The maximum ERI beyond one order of magnitude for ZnO UV-PL was therefore predicted theoretically and validated experimentally, where 11.25-fold UV PL enhancement ratio was achieved in ~650-nm-thick ZnO film grown on SiC substrate and capped with 5.06-μm-diameter MSA. The ERI could further be increased by improving above-mentioned enhancement channels. The present work provides a novel platform to manipulate light by low-loss dielectric microstructures for enhancing photon-matter interaction, which would be employed for other semiconductors achieving energy-saving luminescence and high-sensitivity photoelectric detection in future.
© 2017 Optical Society of America
1. Introduction
Wide bandgap semiconductors such as GaN, ZnO, and SiC are regarded as the 3rd-generation semiconductors after Si, GaAs and InP. They have been extensively investigated in past decades for energy harvesting [1–4], LEDs/LDs [5–7], photocatalytic [8–10], and quantum communication applications [11–14]. The interest in ZnO is due to its direct wide bandgap (~3.37 eV) at 300 K and large exciton binding energy (60 meV), making it promising for high-efficient optoelectronic devices such as light sources and lasing actions above room temperature [15,16]. It is well known that wurtzite-type ZnO possesses two luminescence bands from ultraviolet (UV) to visible regions. The UV band emission corresponds to free-exciton recombination from near band edge (NBE) at ~380 nm whereas the visible band emission at ~550 nm is attributed to various radiative combinations from deep defect levels (DL) [17–19]. In general, the intensity of DL emission is increased with the ZnO size reducing, owing to the higher level of surface and sub-surface oxygen vacancies on the smaller sized samples, where the deep-level defect results in radiative recombination of a photo-generated hole with an electron occupying the oxygen vacancy [19–21]. Therefore, the intensity of UV-band emission from NBE free-exciton recombination is suppressed and the performance of ZnO-based devices is deteriorated [22–24].
In order to improve the UV-band emission, surface plasmon polaritons (SPPs)/localized surface plasmon resonances (LSPRs) have been widely employed by combining noble metal film/nanostructures with ZnO. The decay rate of free-exciton recombination is therefore dramatically increased by resonant coupling of the spontaneous emission into surface plasmons, by which the NBE emission is enhanced whereas the DL emission is suppressed. Large enhancement (one order of magnitude) in UV emission has been observed from ZnO thin films capped by Ag, Al or AlxAg1-x metallic layers [25–27]. Compared with the SPPs, LSPRs confine electromagnetic (EM) oscillation in the near field of metallic nanostructures (e.g. nanoparticles, nanowires, and nanobelts, etc.), significantly enhancing the interaction between incident light with material in the vicinity of metallic nanostructures. Meanwhile, the DL emission photons can be absorbed by LSPRs to generate hot electrons in metals, which are then transferred to the conduction band of ZnO for further enhancement of UV emission. Au, Ag, Pt and Al nanostructures have been demonstrated one order of magnitude for UV emission enhancement in previous studies [28–35]. Other semimetals such as single-walled carbon nanotubes and graphene have also achieved similar PL enhancement via SPPs/LSPRs coupling with ZnO thin films [36–40]. Unfortunately, SPPs/LSPRs-mediated PL enhancement is generally suitable for ZnO thin films with thickness < 100 nm and low inherent quantum efficiency (i.e. a high level of defects resulting from low crystalline quality and massive surface states) [26,29,32,41]. For high-quality ZnO films, SPPs/LSPRs-mediated PL enhancement ratios are normally lower than 4 folds due to their high internal quantum efficiency of NBE exciton recombination for UV emission and low DL emission intensity. Moreover, the UV emission could be quenched by energy loss in SP modes and low transmission of noble metals capping on ZnO films (except 2D materials with high transmission) [42]. Therefore, the under-layer or embedded metallic nanostructures with a low refractive index of substrate are recommended for a higher PL enhancement ratio by eliminating the block of emission light from the luminescence film [30–32,43]. On the other hand, the under-layer metallic nanostructures, embedded nanoparticles and lattice mismatch to the substrate with a low refractive index (e.g. SiO2) would deteriorate crystalline quality of the grown ZnO film.
Dielectric structure is an alternative way to achieve ZnO UV emission enhancement according to its low absorption and high transmission with respect to the spectrum ranging from UV to NIR band. Dielectric micro/nanostructures have been widely studied to improve the out-coupling efficiency of light-emitting devices, e.g. microlenses, 2D photonic crystal, AAO nanohole arrays, and dielectric metasurfaces, etc [44–48]. Dielectric microspheres have drawn research interests for the last decade due to their unique properties in micro/nanophotonics, e.g. photonic nanojets [49], whispering gallery modes (WGMs) [50], and full spectra directional antenna [51], etc. Several applications of dielectric microspheres have been demonstrated in nanoparticle detection [52], single nanoparticle/molecule/ion sensing (including interactions) [53–55], enhanced fluorescence/Raman scattering [56–59], sub-diffraction-limited superresolution imaging [60–62] and nano-patterning [63]. Our pioneered work on dielectric microsphere monolayer capping on a planar ZnO thin film opened up new opportunities for giant UV-PL enhancement [64]. However, the enhancement channels by the dielectric microspheres have not been completely revealed, in which the effects of substrate and luminescence film thickness on intensity modulation of UV emission is unclear.
In this work, ZnO UV-PL enhancement channels in the dielectric microsphere array (MSA)/luminescence film/substrate (MLS) sandwich structure were investigated sophisticatedly. The effects of dielectric microsphere, luminescence film thickness and substrate refractive index on the enhancement ratio of intensity (ERI) of PL were studied experimentally and theoretically. It would provide a novel structure without SPs/LSPRs to dramatically enhance light emission in semiconductors, quantum dots and organic luminescence films.
2. Materials and methods
The dielectric microsphere array/luminescence film/substrate (MLS) sandwich structure used in this work is shown in Fig. 1(a). The dielectric enhancer was a (5.06 ± 0.03)-μm-diameter fused silica microsphere array for the maximum PL enhancement ratio up to 10 folds according to our previous work [64]. The luminescence films were ZnO thin films grown on various substrates by pulsed laser deposition (PLD) using a 248-nm KrF excimer laser (Coherent LPXpro-305). In the PLD vacuum chamber, the growth temperature was set as 450 °C and oxygen pressure was ~40 Pa. The fluence of focused laser on the rotated ZnO ceramic target was 2.5-4 J/cm2 with the number of pulse (NOP) ranging from 5000 to 20000, in order to obtain the desired film thickness ranging from 250 to 1350 nm. The pulse repetition rate and pulse duration were fixed at 3 Hz and 20 ns, respectively. SiC, c-Al2O3, GaN, Si, SiO2 and ZnO wafers with the corresponding refractive indexes of 1.47-6.5 were selected to investigate the effect of substrate on UV-PL enhancement. The close-packed dielectric microsphere monolayer was self-assembled on the luminescence film by drop-coating [64]. Figure 1(b) demonstrates a typical morphology of a close-packed microsphere array on a ZnO thin film. The microstructure and topography of as-grown ZnO thin film are shown in Figs. 1(b) and 1(c) by SEM and AFM, respectively. It can be clearly observed that the high quality ZnO thin film with the crystal-grain size of ~200 nm was deposited onto the substrate. Under the NOP of 10000, the typical film thickness was ~600 nm. Figure 1(d) shows the XRD pattern of the ZnO thin film grown on SiC substrate, where the growth of wurtzite-type ZnO was highly directional along [0001].
Fig. 1 Experimental configuration of PL enhancement by MLS sandwich structure. (a) Schematic of PL spectrometer and experimental setup. (b) Surface morphology of microsphere monolayer capped on ZnO thin film. (c) Top and (d) cross-section view of the microstructure of as-grown ZnO thin film. (e) XRD pattern of a ZnO thin film grown on a SiC substrate.
In order to excite the PL spectra from ZnO thin films, a 325-nm-wavelength laser beam emitted from a He-Cd fiber-coupled laser system (Kimmon Koha IK3301R-G) was focused by a 14x objective lens with NA of 0.5. The focused laser spot with ~100 μm in diameter arrived onto the sandwich structure as indicated by the yellow circle in Fig. 1(b). PL emission from the sandwich structure was then collected by the same lens in backscattering configuration, where the excitation laser was blocked by a 350 nm edge filter. A monochromator (Princeton Instruments Acton SP2750) of 750 mm focal length with a 1200 lines/mm blazed grating was employed to acquire the PL signals with a spectral resolution of 0.023 nm. The integration time was fixed at 0.2 s. The ERI of PL was sampled from 5 different points for each MLS sandwich structure.
The numerical simulation in this work was performed by a FDTD algorithm using COMSOL Multiphysics (licensed by COMSOL Co. Ltd.). The EM fields in the MLS system, including light focusing and trapping, WGMs as well as directional antenna effects of MSA, were calculated to understand the channels of UV-PL emission enhancement by the sandwich structure.
3. Results and discussion
3.1 Experimental ERI of PL emission for MLS sandwich structure with various substrates and luminescence film thickness
Figure 2 shows the PL enhancement for MSA-capped ZnO thin film grown on various substrates. The maximum ERI for NBE emission (ERINBE) at UV band up to 11.25 was achieved on the SiC substrate. For the SiO2 substrate, the ERINBE was also close to one order of magnitude. The lowest ERINBE is from the GaN substrate, of which the ERINBE was only 6.73. The ERI for defect-levels (DL) emission (ERIDL) at visible band was found to be significantly lower than ERINBE, where it was fixed at 2-4 folds and independent on the substrate. The effect of ZnO film thickness on ERINBE and ERIDL is shown in Fig. 3. It can be seen that the ERINBE was greater than 10 folds for the ZnO film thickness in the range of 500-900 nm. Thinner or thicker films would reduce the ERINBE. The ERINBE converging to 8.14 folds (same as that of MSA-capped ZnO bulk) with ZnO film thickening indicated the film thickness greater than 1200 nm making PL enhancement independent from the substrate and close to ZnO bulk. Unlike ERINBE, the effect of ZnO film thickness on ERIDL was negligible, where the ERIDL for different film thicknesses approximated to a constant, i.e. 2.90 folds, same as that of MSA-capped ZnO bulk. According to the experimental results, the dielectric MSA demonstrated the capability of distinct PL enhancement for the NBE emission boosting more significantly than DL emission. It indicated different channels for ZnO PL enhancement at UV and visible bands. In order to understand the PL enhancement channels, the light manipulation in the MLS sandwich structure was individually studied.
Fig. 2 PL enhancement in MSA-capped ZnO thin films grown on (a) SiC, (b) Si, (c) SiO2, (d) Al2O3, (e) ZnO and (f) GaN substrates. The inserts are reflectance spectra of as-grown and MSA-capped ZnO thin films with respect to 325-nm-wavelength excitation laser.
Fig. 3 PL enhancement in MSA-capped ZnO thin films with different film thicknesses. (a) PL spectra for ZnO films with film thickness from 250 nm to 1350 nm. (b) Effect of film thickness on ERIs of PL at UV and visible bands.
3.2 MSA focusing property for distinct PL enhancement
The dielectric microsphere focusing property is one of most studied aspects in previous work on super-resolution imaging [60–62]. It is well acknowledged that the photonic nanojet can be generated at the shadow side of a micro-sized sphere via Mie scattering [60]. Considering the microsphere array directly contacting the ZnO thin film and the high absorption of ZnO with respect to 325 nm laser (beyond the bandgap), the distribution of normalized light intensity on the ZnO thin film was calculated as shown in Fig. 4(a), in which the laser spot was focused by the microsphere as a doughnut distribution prior to the photonic nanojet formation. The UV-PL of ZnO is generally assigned to the NBE free-exciton recombination and the corresponding peak intensity is increased superlinearly with increase of excitation power intensity, following a power law of [65]
where IPL is the integrated intensity of the PL peak and P is the excitation power. The fitted exponent γ for NBE emission by increasing excitation power from 0.1 mW to 8 mW was 1.194, showing a typical behavior of free-exciton emission, whereas the exponent γ = 0.686 for DL emission was distinctly different from the free-exciton emission, as Fig. 4(b). Further considering the periodic doughnut distribution of laser spots focused by the microsphere array, the ERI by MSA focusing (ERIf) can therefore be written aswhere rms is the radius of microsphere and |E(r)|2 is the intensity of electric field on the luminescence film surface normalized by the intensity without microsphere array. The difference of γ between free-exciton and deep-defect level emission resulted in distinct ERIs of PL for UV and visible bands. Under the same excitation power, the redistributed excitation laser intensity via microsphere array focusing would increase the PL intensity from NBE emission ~1.311 folds whereas reduce the DL emission intensity to ~0.760 times according to the numerical calculation from Eq. (2) and Fig. 4.Fig. 4 Effect of distribution of excitation laser intensity on PL enhancement in MLS sandwich structure. (a) Excitation laser focused by a dielectric microsphere in MSA and the corresponding distribution of light intensity. (b) PL spectra of ZnO film with excitation power increasing from 0.1 to 8 mW. The inset shows the integrated PL peak intensities at 380 nm and ~550 nm as a function of the excition power in a double logarithmic plot.
3.3 Anti-reflection effect of MSA for the excitation laser
Figure 5(a) shows the light scattering of 325-nm-wavelength excitation laser from the MLS system. It can be seen that the scattered light was partially trapped in the microsphere cavity by solid immersion lens effect and travelling via total internal reflection along the equator of the microsphere. Considering the narrow bandwidth of 325 nm laser, the microsphere should support travelling wave whispering gallery modes (TW-WGMs) rather than standing wave whispering gallery modes (SW-WGMs). The multiple interaction of TW-WGMs with ZnO thin film would increase the excitation cross-section area, by which the absorption of ZnO for the 325-nm excitation laser was increased and the PL intensity was thereby enhanced. As shown in the insets of Fig. 1, the reflection of MSA-capped ZnO films were reduced ~80% compared with bare ZnO ones grown on different substrates, by which the typical reflection of ZnO with respect to 325 nm wavelength is reduced from ~17.7% to 3.5% by capping MSA. The microsphere array is therefore a natural anti-reflection layer for high absorption at 325 nm wavelength to harvest UV-light energy. The polar distribution of scattered light intensity with/without MSA in the far field is also shown in Fig. 5(a). The reflection reduction (ΔR) for 325 nm wavelength light can be determined by
where θ is the angle with respect to the axis normal to the ZnO film surface. Ebare(θ) and EMSA(θ) are electric field vectors without and with MSA in the far-field, respectively. The integration is performed in the half of full solid angle. ΔR was numerically estimated by Eq. (3) to be 58.3%, of which the scattered light energy was trapped in MSA travelling along the equators of microspheres by total internal reflection, i.e. travelling-wave WGMs (TW-WGMs). The PL intensity can therefore be increased due to the trapped excitation light re-interaction with ZnO film at the contacting points. The MSA provides a new channel for harvesting incident light energy and enhancing interaction of light and material. Unfortunately, considering the high inherent absorption of bare ZnO film (82.3%) at 325 nm, the maximum PL enhancement ratio, ERIΔR, contributed by reflection reduction of 58.5% (i.e. absorption increased to 92.6%) is only ~1.12. However, such an anti-reflection effect of MSA would become significant for the materials with low absorption.Fig. 5 Scattered light manipulation by MLS sandwich structure. (a) Scattered 325 nm excitation light trapping in the microsphere, (b) travelling wave WGM (TW-WGM) of NBE emission at 380 nm in MLS sandwich structure, (c) TW-WGM of DL emission at 550 nm in MLS sandwich structure, and (d) standing wave WGM (SW-WGM) of NBE emission near 380 nm in MLS sandwich structure. Top panel is the corresponding polar plot of light scattering intensity with/without MSA in the far field.
3.4 Amplified spontaneous emission of UV PL by TW-WGMs in MLS sandwich structure
In addition to 325-nm excitation light, the UV-PL emission at 380 nm can also be trapped by the microspheres as TW-WGMs, as shown in Fig. 5(b). The 380-nm-wavelength light travelling in the microsphere cavity would interact with ZnO at the contacting point and increase the UV-PL intensity via the well-known amplified spontaneous emission (ASE) process, in which the focusing property and anti-reflection effect of MSA provided sufficient excitation intensity of 325-nm-wavelength causing population inversion at the contacting points. However, the linewidth of the UV emission cannot be significantly narrowed owing to the short length of gain medium in the microsphere cavity (i.e. lgain-media<<2πr where r is the radius of microsphere). It was assumed that and the corresponding enhancement ratios for various substrates and ZnO film thicknesses were calculated as Figs. 6(a) and 6(c). The strong dependence of ERIASE on the substrate and ZnO film thickness indicates the TW-WGMs propagating in the system of microsphere cavity and ZnO film as the refractive index of ZnO (~2.45) is higher than fused silica microsphere (~1.48), by which the TW-WGMs can be coupled into the ZnO film. In Fig. 6(a), the refractive index of substrate away from ZnO film (~2.45) increased the ERIASE. It is attributed to the reflection at the interface of ZnO film and substrate. According to the Snell’s law, the significant variation of refractive index between ZnO film and substrate enhances the reflection at the interface. The high reflection makes more light energy back to the microsphere cavity as TW-WGMs, as shown in Fig. 5(b). Oppositely, the similar refractive index increased the energy loss via the leaky mode from microsphere to substrate. As a result, the GaN/ZnO substrate generated the lowest ERIASE. The effect of ZnO film thickness on ERIASE is shown in Fig. 6(c), in which the substrate is SiC (n~2.80). It can be seen that the thickness in the range of 550-800 nm obtained the highest enhancement ratio. For thinner ZnO film thickness, the TM-WGMs can be directly coupled from microsphere into substrate as leaky modes via evanescent wave coupling. The thicker film thickness would increase the 380-nm light travelling length in the ZnO film and make the reflected light away from the microsphere bottom, by which the trapped light energy by the MSA is lowered and ERIASE is reduced. Compared with 380-nm scattered light, the TW-WGMs for DL emission at ~550 nm in the MSA were very weak, as shown in Fig. 5(c). It illustrates the TW-WGM-supported ASE process boosting the internal quantum efficiency of free-exciton recombination and increasing the NBE emission more significantly than DL emission. The maximum ERIASE for NBE emission at 380 nm was ~2.2 folds (as Fig. 6) whereas for DL emission at ~550 nm was negligible.
Fig. 6 Dependence of ERI for NBE emission on substrate refractive index and ZnO film thickness. (a) Effect of refractive index of substrate on ERIASE and Q factor by numerical simulation. (b) Theoretical and experimental ERI variation for refractive index of substrate. (c) Effect of ZnO film thickness on ERIASE and Q factor in numerical simulation. (d) Theoretical and experimental ERI variation with respect to ZnO film thickness.
3.5 Purcell’s effect for UV-PL enhancement by SW-WGMs in the MLS sandwich structure
Considering the possibility of phase match of light wave in the microsphere by free-space coupling [66], the standing-wave WGMs (SW-WGMs) of NBE emission at ~380 nm can be supported in the microsphere/thin film system. As the modal analysis shown in Fig. 5(d), the split of SW-WGMs occurred around the contacting point of microsphere and ZnO film due to the high refractive index of ZnO. It dramatically reduced the Q factor of the optical resonator from ~105 in general for a microsphere cavity to ~102 for the formed microsphere/thin film system as shown in Figs. 6(a) and 6(c). It should be noted that such a Q factor in the range of 100-300 indicates a cavity linewidth of ~1-4 nm significantly narrower than the linewidth of NBE (~10-15 nm). Considering the non-uniform size of the microspheres with diameters of 5.06 ± 0.03 μm, the light intensity, I(λ), in the MSA can be expressed as
where N is the supported azimuthal mode order in the Mth microsphere within the excitation laser spot area. The intensity distribution of SW-WGM in a single microsphere, IM,N, is satisfied with Lorentzian line function, where Γ is the cavity linewidth, λ is the wavelength, and λM,N is the central wavelength of the Nth azimuthal mode order in the Mth microsphere. The inhomogeneity of SW-WGM resonance caused by the non-uniform size of microspheres degenerated the discrete resonant peaks with narrow linewidths to a continuum. Further considering the PL enhancement at different wavelengths still governed by the individual microspheres with specific diameters and azimuthal mode orders, the corresponding ERIPurcell for the broadband NBE can therefore be approximated by the Purcell’s function as followingwhere Q and V are quality factor and mode volume of the MLS sandwich structure, respectively. In this work, only the radiative (curvature) loss was considered. λ is the light wavelength and n is the refractive index of the optical cavity. Figures 6(a) and 6(c) illustrate the Q factor depending on the substrate and ZnO film thickness. The variation of Q factor with substrate refractive index and ZnO film thickness was similar to that of ASE process by TW-WGMs as mentioned above. The lowest Q factor was from the ZnO/GaN substrate, of which the refractive index was close to ZnO thin film. For SiC substrate, the Q factor was about 3-fold higher than that of ZnO/GaN substrate. Based on Eq. (5) and Fig. 6(a), the ERIPurcell of SiC substrate should be ~3 folds higher than ZnO/GaN substrate. The greater difference of refractive index between substrate and ZnO film generated higher ERIPurcell. However, the enhancement ratio was reduced when extinction coefficient (k) of the substrate was considered. Figure 6 illustrated that both ERIASE and Q factor for Si substrate with k = 0.89 were lower than without k, validating the occurrence of TW-/SW-WGMs interaction with the substrate of MLS sandwich structure.3.6 Optical directional antenna effect and prediction of ERI by the sandwich structure
In our previous work, the optical directional antenna effect of MSA on PL and Raman scattering enhancement has been discussed [59, 64]. The antenna effect of MSA significantly increased the equivalent NA of the objective lens and out-coupling efficiency. The ERI by antenna effect, ERIantenna, can be estimated by
where |E(θ)|2 can be acquired from polar distribution of electric field vectors in the far field as the top panels of Figs. 5(b) and 5(c). The numerical calculation indicated that the ERIantenna was independent from the ZnO film thickness and refractive index of substrate. Moreover, it was insensitive to the size variation of the microspheres within ± 0.03 μm in diameters. For the employed 5.06-μm-diameter fused silica MSA, the ERIantenna was ~3.90 for the NBE emission at 380 nm and ~2.07 for DL emission at ~550 nm by Eq. (6).Considering the above-mentioned enhancement channels, the ERI of PL via MLS sandwich structure can be determined by
where β is the percentage of trapped light energy for the ASE process by TW-WGMs and (1-β) is the coupling efficiency of SW-WGMs supporting Purcell’s effect. It was found that the numerical calculation for NBE emission enhancement was in good agreement with experimental results for various substrates and ZnO film thicknesses under the coupling efficiency (1-β) of 20%, as shown in Figs. 6(b) and 6(d). It should be noted that the ZnO and GaN substrates have similar refractive indexes. The PL enhancement ratio for ZnO substrate higher than GaN substrate was attributed to the extra UV-PL contribution from the ZnO substrate. The calculated ERI for DL emission was fixed at 3.04, in good agreement with the experimental results shown in Figs. 2 and 3. The offsets of ERIDL in experiments could be resulted from the inherent PL and surface scattering of the fused silica MSA.According to Eq. (7), the evolution of ERI of UV-PL for ZnO-based MSL sandwich structure related to the substrate and luminescence film thickness was predicted as Fig. 7. It can be seen that low substrate refractive index (n<2.45) with a thin luminescence film (<1200 nm) is beneficial to increase ERIASE, ERIPurcell and total ERI as shown in region I of Fig. 7. In this case, the substrate has a minor effect on TW-/SW-WGMs in MSA and ZnO film due to its low refractive index. For the substrate with n>2.45, the light energy trapped in the microsphere would be coupled into the substrate by evanescent waves as leaky modes for energy loss. Therefore, the film thickness should be increased to suppress the evanescent wave coupling. Meanwhile, several reflection peaks for travelling waves, e.g. 600-800 nm and 1100-1300 nm, existed due to the interference reflection enhancement and incident angle match to total internal reflection, as shown in region II of Fig. 7(a). It should be noted that the thicker film would reduce the ERI due to the high energy loss in the ZnO film open cavity. Considering the phase match condition, the SW-WGMs were more difficult to be supported than TW-WGMs. According to the numerical simulation in Fig. 7(b), it can be found that only the low refractive index of substrate (n<2.45) with limited film thickness (<400 nm) as region I and the high refractive index of substrate (n>2.45) with specified film thickness (~650 nm) as region II would achieve high ERIPurcell. The total ERIs estimated by Eq. (7) for various substrate refractive indexes and ZnO film thicknesses were therefore predicted as shown in Fig. 7(c), where two regions existed for high ERIs. The film thickness and substrate refractive index should be carefully chosen according to Fig. 7(c) for the maximum ERI of UV-PL from the ZnO-based MLS sandwich structure. The enhancement ratio in region I and II is beyond one order of magnitude.
Fig. 7 Evolution of ERI of UV-PL from ZnO film with substrate refractive index and film thickness. Dependence of refractive index of substrate and thickness of ZnO film on (a) ERIASE caused by TW-WGMs, (b) ERIPurecell resulted from SW-WGMs and (c) total ERI for UV PL from ZnO-based MLS sandwich structure.
4. Conclusions
In this work, we investigated the PL enhancement in dielectric microsphere array/luminescence film/substrate (MLS) sandwich structures. The UV-PL enhancement channels from ZnO film in the sandwich structure were revealed, for the first time. The focusing property of microsphere array (MSA) redistributes the light intensity on the luminescent film, by which the free-exciton recombination in the ZnO film (i.e. UV-PL emission intensity) is enhanced whereas the DL emission is suppressed. In addition, MSA is a natural anti-reflection layer, by which the excitation light can be trapped as TW-WGMs in microspheres and hence the excitation cross-section area is increased by harvesting excitation light energy. Then the excited 380 nm PL emission is trapped in the MSA as TW-/SW-WGMs boosting the UV-PL emission by ASE process and Purcell’s effect, respectively. Finally, the optical directional antenna effect of MSA further enhances the PL intensity by increasing the equivalent NA of objective lens and out-coupling efficiency. According to the experiment and numerical simulation, it has been observed that the ERI of PL from free-exciton recombination in MLS sandwich structure is strongly dependent upon the refractive index of substrate and luminescence film thickness. The refractive index of substrate should be different from luminescence film and the film thickness needs to be carefully chosen to support WGMs in the MLS sandwich structure. The maximum ERI beyond one order of magnitude for ZnO UV-PL was predicted theoretically and validated experimentally, where the 11.25-fold UV-PL enhancement ratio was achieved in ~650-nm-thick ZnO film grown on SiC substrate. According to the theoretical analysis, the enhancement ratio could further be increased by improving the above-mentioned channels. The present work provides a novel platform to manipulate light by low-loss dielectric microstructures for enhancing photon-matter interaction. The MLS sandwich structure would be applied to other semiconductors achieving energy-saving luminescence and high-sensitivity photoelectric detection in future.
Funding
National Natural Science Foundation of China (NFSC) (11504012, 11674018); Scientific Research General Program of Beijing Municipal Commission of Education (KM201510005013); Beijing Nova Program.
References and links
1. A. B. Djurišić, A. M. C. Ng, and X. Y. Chen, “ZnO nanostructures for optoelectronics: material properties and device applications,” Prog. Quantum Electron. 34(4), 191–259 (2010). [CrossRef]
2. D. I. Son, B. W. Kwon, D. H. Park, W. S. Seo, Y. Yi, B. Angadi, C. L. Lee, and W. K. Choi, “Emissive ZnO-graphene quantum dots for white-light-emitting diodes,” Nat. Nanotechnol. 7(7), 465–471 (2012). [CrossRef] [PubMed]
3. J. M. Luther, J. Gao, M. T. Lloyd, O. E. Semonin, M. C. Beard, and A. J. Nozik, “Stability assessment on a 3% bilayer PbS/ZnO quantum dot heterojunction solar cell,” Adv. Mater. 22(33), 3704–3707 (2010). [CrossRef] [PubMed]
4. A. I. Hochbaum and P. Yang, “Semiconductor nanowires for energy conversion,” Chem. Rev. 110(1), 527–546 (2010). [CrossRef] [PubMed]
5. X. H. Huang, R. Chen, C. Zhang, J. W. Chai, S. J. Wang, D. Z. Chi, and S. J. Chua, “Ultrafast and robust UV luminescence from Cu-doped ZnO nanowires mediated by plasmonic hot electrons,” Adv. Opt. Mat. 4(6), 960–966 (2016). [CrossRef]
6. Z. L. Wang, “ZnO nanowire and nanobelt platform for nanotechnology,” Mater. Sci. Eng. Rep. 64(3–4), 33–71 (2009). [CrossRef]
7. P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, K. H. Ploog, and K. H. Ploog, “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes,” Nature 406(6798), 865–868 (2000). [CrossRef] [PubMed]
8. H. Zeng, W. Cai, P. Liu, X. Xu, H. Zhou, C. Klingshirn, and H. Kalt, “ZnO-based hollow nanoparticles by selective etching: elimination and reconstruction of metal-semiconductor interface, improvement of blue emission and photocatalysis,” ACS Nano 2(8), 1661–1670 (2008). [CrossRef] [PubMed]
9. O. Akhavan, “Graphene nanomesh by ZnO nanorod photocatalysts,” ACS Nano 4(7), 4174–4180 (2010). [CrossRef] [PubMed]
10. J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, and Y. Dai, “Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO,” ACS Appl. Mater. Interfaces 4(8), 4024–4030 (2012). [CrossRef] [PubMed]
11. H. Kind, H. Q. Yan, B. Messer, M. Law, and P. D. Yang, “Nanowire ultraviolet photodetectors and optical switches,” Adv. Mater. 14(2), 158–160 (2002). [CrossRef]
12. S. Castelletto, B. C. Johnson, V. Ivády, N. Stavrias, T. Umeda, A. Gali, and T. Ohshima, “A silicon carbide room-temperature single-photon source,” Nat. Mater. 13(2), 151–156 (2014). [CrossRef] [PubMed]
13. G. Almonacid, R. Martín-Rodríguez, C. Renero-Lecuna, J. Pellicer-Porres, S. Agouram, R. Valiente, J. González, F. Rodríguez, L. Nataf, D. R. Gamelin, and A. Segura, “Structural metastability and quantum confinement in Zn1–xCoxO nanoparticles,” Nano Lett. 16(8), 5204–5212 (2016). [CrossRef] [PubMed]
14. L. Zhang, C. H. Teng, P. C. Ku, and H. Deng, “Site-controlled InGaN/GaN single-photon-emitting diode,” Appl. Phys. Lett. 108(15), 153102 (2016). [CrossRef]
15. U. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho, and H. Morkoc, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98(4), 041301 (2005). [CrossRef]
16. A. Janotti and C. G. Van de Walle, “Fundamentals of zinc oxide as a semiconductor,” Rep. Prog. Phys. 72(12), 126501 (2009). [CrossRef]
17. L. Guo, S. Yang, C. Yang, P. Yu, J. Wang, W. Ge, and G. K. L. Wong, “Highly monodisperse polymer-capped ZnO nanoparticles: preparation and optical properties,” Appl. Phys. Lett. 76(20), 2901–2903 (2000). [CrossRef]
18. H. Zhou, H. Alves, D. M. Hofmann, W. Kriegseis, B. K. Meyer, G. Kaczmarczyk, and A. Hoffmann, “Behind the weak excitonic emission of ZnO quantum dots: ZnO/Zn(OH)2 core-shell structure,” Appl. Phys. Lett. 80(2), 210–212 (2002). [CrossRef]
19. P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, and H. J. Choi, “Controlled growth of ZnO nanowires and their optical properties,” Adv. Funct. Mater. 12(5), 323–331 (2002). [CrossRef]
20. M. H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, and P. Yang, “Catalytic growth of zinc oxide nanowires by vapor transport,” Adv. Mater. 13(2), 113–116 (2001). [CrossRef]
21. K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, and B. E. Gnade, “Mechanisms behind green photoluminescence in ZnO phosphor powders,” J. Appl. Phys. 79(10), 7983–7990 (1996). [CrossRef]
22. K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt, “Correlation between photoluminescence and oxygen vacancies in ZnO phosphors,” Appl. Phys. Lett. 68(3), 403–405 (1996). [CrossRef]
23. Z. Fu, W. Dong, B. Yang, Z. Wang, Y. Yang, H. Yan, S. Zhang, J. Zuo, M. Ma, and X. Liu, “Effect of MgO on the enhancement of ultraviolet photoluminescence in ZnO,” Solid State Commun. 138(4), 179–183 (2006). [CrossRef]
24. R. Huang, S. Xu, X. Wang, W. Guo, C. Song, J. Song, K. Ming Ho, S. Du, and N. Wang, “Effective control of photoluminescence from ZnO nanowires by a-SiNx:H decoration,” Opt. Lett. 37(2), 211–213 (2012). [CrossRef] [PubMed]
25. C. W. Lai, J. An, and H. C. Ong, “Surface-plasmon-mediated emission from metal-capped ZnO thin films,” Appl. Phys. Lett. 86(25), 251105 (2005). [CrossRef]
26. W. H. Ni, J. An, C. W. Lai, H. C. Ong, and J. B. Xu, “Emission enhancement from metallodielectric-capped ZnO films,” J. Appl. Phys. 100(2), 026103 (2006). [CrossRef]
27. D. Y. Lei, J. Li, and H. C. Ong, “Tunable surface plasmon mediated emission from semiconductors by using metal alloys,” Appl. Phys. Lett. 91(2), 021112 (2007). [CrossRef]
28. P. Cheng, D. Li, Z. Yuan, P. Chen, and D. Yang, “Enhancement of ZnO light emission via coupling with localized surface plasmon of Ag island film,” Appl. Phys. Lett. 92(4), 041119 (2008). [CrossRef]
29. P. Cheng, D. Li, X. Li, T. Liu, and D. Yang, “Localized surface plasmon enhanced photoluminescence from ZnO films: extraction direction and emitting layer thickness,” J. Appl. Phys. 106(6), 063120 (2009). [CrossRef]
30. B. J. Lawrie, R. F. Haglund Jr, and R. Mu, “Enhancement of ZnO photoluminescence by localized and propagating surface plasmons,” Opt. Express 17(4), 2565–2572 (2009). [CrossRef] [PubMed]
31. M. Liu, S. W. Qu, W. W. Yu, S. Y. Bao, C. Y. Ma, Q. Y. Zhang, J. He, J. C. Jiang, E. I. Meletis, and C. L. Chen, “Photoluminescence and extinction enhancement from ZnO films embedded with Ag nanoparticles,” Appl. Phys. Lett. 97(23), 231906 (2010). [CrossRef]
32. M. E. Koleva, A. O. Dikovska, N. N. Nedyalkov, P. A. Atanasov, and I. A. Bliznakova, “Enhancement of ZnO photoluminescence by laser nanostructuring of Ag underlayer,” Appl. Surf. Sci. 258(23), 9181–9185 (2012). [CrossRef]
33. R. Khan, P. Uthirakumar, K. B. Bae, S. J. Leem, and I. H. Lee, “Localized surface plasmon enhanced photoluminescence of ZnO nanosheets by Au nanoparticles,” Mater. Lett. 163, 8–11 (2016). [CrossRef]
34. J. Lu, J. Li, C. Xu, Y. Li, J. Dai, Y. Wang, Y. Lin, and S. Wang, “Direct resonant coupling of Al surface plasmon for ultraviolet photoluminescence enhancement of ZnO microrods,” ACS Appl. Mater. Interfaces 6(20), 18301–18305 (2014). [CrossRef] [PubMed]
35. W. F. Yang, Y. N. Xie, R. Y. Liao, J. Sun, Z. Y. Wu, L. M. Wong, S. J. Wang, C. F. Wang, A. Y. S. Lee, and H. Gong, “Enhancement of bandgap emission of Pt-capped MgZnO films: Important role of light extraction versus exciton-plasmon coupling,” Opt. Express 20(13), 14556–14563 (2012). [CrossRef] [PubMed]
36. S. Kim, D. H. Shin, C. O. Kim, S. Won Hwang, S. H. Choi, S. Ji, and J. Y. Koo, “Enhanced ultraviolet emission from hybrid structures of single-walled carbon nanotubes/ZnO films,” Appl. Phys. Lett. 94(21), 213113 (2009). [CrossRef]
37. S. W. Hwang, D. H. Shin, C. O. Kim, S. H. Hong, M. C. Kim, J. Kim, K. Y. Lim, S. Kim, S. H. Choi, K. J. Ahn, G. Kim, S. H. Sim, and B. H. Hong, “Plasmon-enhanced ultraviolet photoluminescence from hybrid structures of graphene/ZnO films,” Phys. Rev. Lett. 105(12), 127403 (2010). [CrossRef] [PubMed]
38. K. Kim, S. M. Lee, Y. S. Do, S. H. Ahn, and K. C. Choi, “Enhanced photoluminescence from zinc oxide by plasmonic resonance of reduced graphene oxide,” J. Appl. Phys. 114(7), 074903 (2013). [CrossRef]
39. R. Liu, X. W. Fu, J. Meng, Y. Q. Bie, D. P. Yu, and Z. M. Liao, “Graphene plasmon enhanced photoluminescence in ZnO microwires,” Nanoscale 5(12), 5294–5298 (2013). [CrossRef] [PubMed]
40. F. Han, S. Yang, W. Jing, K. Jiang, Z. Jiang, H. Liu, and L. Li, “Surface plasmon enhanced photoluminescence of ZnO nanorods by capping reduced graphene oxide sheets,” Opt. Express 22(10), 11436–11445 (2014). [CrossRef] [PubMed]
41. E. J. Guidelli, O. Baffa, and D. R. Clarke, “Enhanced UV emission from silver/ZnO and gold/ZnO core-shell nanoparticles: photoluminescence, radioluminescence, and optically stimulated luminescence,” Sci. Rep. 5, 14004 (2015). [CrossRef] [PubMed]
42. Y. Zeng, Y. Zhao, and Y. Jiang, “Investigation of the photoluminescence properties of Au/ZnO/sapphire and ZnO/Au/sapphire films by experimental study and electromagnetic simulation,” J. Alloys Compd. 625, 175–181 (2015). [CrossRef]
43. P. Cheng, D. Li, and D. Yang, “Influence of substrates in ZnO devices on the surface plasmon enhanced light emission,” Opt. Express 16(12), 8896–8901 (2008). [CrossRef] [PubMed]
44. S. Möller and S. R. Forrest, “Improved light out-coupling in organic light emitting diodes employing ordered microlens arrays,” J. Appl. Phys. 91(5), 3324–3327 (2002). [CrossRef]
45. Y. D. Do, Y. C. Kim, Y. W. Song, C. O. Cho, H. Jeon, Y. J. Lee, S. H. Kim, and Y. H. Lee, “Enhanced light extraction from organic light-emitting diodes with 2D SiO2/SiNx photonic crystals,” Adv. Mater. 15(14), 1214–1218 (2003). [CrossRef]
46. S. Jeon, J. W. Kang, H. D. Park, J. J. Kim, J. R. Youn, J. Shim, J. Jeong, D. G. Choi, K. D. Kim, A. O. Altun, S. H. Kim, and Y. H. Lee, “Ultraviolet nanoimprinted polymer nanostructure for organic light emitting diode application,” Appl. Phys. Lett. 92(22), 223307 (2008). [CrossRef]
47. K. Endo and C. Adachi, “Enhanced out-coupling efficiency of organic light-emitting diodes using a nanostructure imprinted by an alumina nanohole array,” Appl. Phys. Lett. 104(12), 121102 (2014). [CrossRef]
48. I. Staude, V. V. Khardikov, N. T. Fofang, S. Liu, M. Decker, D. N. Neshev, T. S. Luk, I. Brener, and Y. S. Kivshar, “Shaping photoluminescence spectra with magnetoelectric resonances in all-dielectric nanoparticles,” ACS Photonics 2(2), 172–177 (2015). [CrossRef]
49. Z. Chen, A. Taflove, and V. Backman, “Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique,” Opt. Express 12(7), 1214–1220 (2004). [CrossRef] [PubMed]
50. C. G. B. Garrett, W. Kaiser, and W. L. Bond, “Stimulated emission into optical whispering modes of spheres,” Phys. Rev. 124(6), 1807–1809 (1961). [CrossRef]
51. A. Devilez, B. Stout, and N. Bonod, “Compact metallo-dielectric optical antenna for ultra directional and enhanced radiative emission,” ACS Nano 4(6), 3390–3396 (2010). [CrossRef] [PubMed]
52. X. Li, Z. Chen, A. Taflove, and V. Backman, “Optical analysis of nanoparticles via enhanced backscattering facilitated by 3-D photonic nanojets,” Opt. Express 13(2), 526–533 (2005). [CrossRef] [PubMed]
53. B. B. Li, W. R. Clements, X. C. Yu, K. Shi, Q. Gong, and Y. F. Xiao, “Single nanoparticle detection using split-mode microcavity Raman lasers,” Proc. Natl. Acad. Sci. U.S.A. 111(41), 14657–14662 (2014). [CrossRef] [PubMed]
54. M. D. Baaske, M. R. Foreman, and F. Vollmer, “Single-molecule nucleic acid interactions monitored on a label-free microcavity biosensor platform,” Nat. Nanotechnol. 9(11), 933–939 (2014). [CrossRef] [PubMed]
55. M. D. Basske and F. Vollmer, “Optical observation of single atomic ions interacting with plasmonic nanorods in aqueous solution,” Nat. Photonics 10(11), 733–739 (2016). [CrossRef]
56. S. C. Hill, V. Boutou, J. Yu, S. Ramstein, J. P. Wolf, Y. Pan, S. Holler, and R. K. Chang, “Enhanced backward-directed multiphoton-excited fluorescence from dielectric microcavities,” Phys. Rev. Lett. 85(1), 54–57 (2000). [CrossRef] [PubMed]
57. D. Gérard, J. Wenger, A. Devilez, D. Gachet, B. Stout, N. Bonod, E. Popov, and H. Rigneault, “Strong electromagnetic confinement near dielectric microspheres to enhance single-molecule fluorescence,” Opt. Express 16(19), 15297–15303 (2008). [CrossRef] [PubMed]
58. K. J. Yi, H. Wang, Y. F. Lu, and Z. Y. Yang, “Enhanced Raman scattering by self-assembled silica spherical microparticles,” J. Appl. Phys. 101(6), 063528 (2007). [CrossRef]
59. Y. Yan, C. Xing, Y. Jia, Y. Zeng, Y. Zhao, and Y. Jiang, “Self-assembled dielectric microsphere array enhanced Raman scattering for large-area and ultra-long working distance confocal detection,” Opt. Express 23(20), 25854–25865 (2015). [CrossRef] [PubMed]
60. Z. Wang, W. Guo, L. Li, B. Luk’yanchuk, A. Khan, Z. Liu, Z. Chen, and M. Hong, “Optical virtual imaging at 50 nm lateral resolution with a white-light nanoscope,” Nat. Commun. 2, 218 (2011). [CrossRef] [PubMed]
61. L. Li, W. Guo, Y. Yan, S. Lee, and T. Wang, “Label-free super-resolution imaging of adenoviruses by submerged microsphere optical nanoscopy,” Light Sci. Appl. 2(9), e104 (2013). [CrossRef]
62. Y. Yan, L. Li, C. Feng, W. Guo, S. Lee, and M. Hong, “Microsphere-coupled scanning laser confocal nanoscope for sub-diffraction-limited imaging at 25 nm lateral resolution in the visible spectrum,” ACS Nano 8(2), 1809–1816 (2014). [CrossRef] [PubMed]
63. E. McLeod and C. B. Arnold, “Subwavelength direct-write nanopatterning using optically trapped microspheres,” Nat. Nanotechnol. 3(7), 413–417 (2008). [CrossRef] [PubMed]
64. Y. Yan, Y. Zeng, Y. Wu, Y. Zhao, L. Ji, Y. Jiang, and L. Li, “Ten-fold enhancement of ZnO thin film ultraviolet-luminescence by dielectric microsphere arrays,” Opt. Express 22(19), 23552–23564 (2014). [CrossRef] [PubMed]
65. H. M. Cheng, H. C. Hsu, Y. K. Tseng, L. J. Lin, and W. F. Hsieh, “Raman scattering and efficient UV photoluminescence from well-aligned ZnO nanowires epitaxially grown on GaN buffer layer,” J. Phys. Chem. B 109(18), 8749–8754 (2005). [CrossRef] [PubMed]
66. Y. C. Liu, Y. F. Xiao, X. F. Jiang, B. B. Li, Y. Li, and Q. Gong, “Cavity-QED treatment of scattering-induced free-space excitation and collection in high-Q whispering-gallery microcavities,” Phys. Rev. A 85(1), 013843 (2012). [CrossRef]