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Abstract
Polarization-sensitive anisotropic plasmonic interaction between gold nanorods (AuNRs) and quantum dots (QDs) encapsulated in an epoxy resin polymer has been experimentally investigated. The anisotropic plasmonic interaction utilized the polarization-dependent plasmonic properties of aligned AuNR in AuNR-QD composite. AuNRs were aligned by an external AC electric field of 3.5 ×105 Vm−1. The plasmonic interaction modified QD absorption and emission dependent on excitation light polarization and maximum enchantment of 10% and 59%, respectively. Moreover, anisotropic plasmonic interaction induced directional emission of QDs has improved emission decay rate by 20% and modulated emission polarization ratio of out-of-plane (vertical) and in-plane (horizontal) from 1 to 0.84.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The optical properties of optical emitters are highly function of the material composition, synthesis process, and local surrounding. It is highly desirable to be able to engineer the optical properties of an optical emitter without altering the material composition and synthesis process for advanced optical device applications [1–3]. This can be achieved modifying local surrounding by adding another interface such as metal nanostructures. Metal nanostructures can manipulate the local electromagnetic field (photon mode density) and hence the interaction between light and the optical emitter [4]. A metallic nanostructure, such as metal nanoparticles (MNPs), can significantly enhanced local electromagnetic field through localized surface plasmon resonance (SPR) phenomena [5–8]. The localized SPR is a unique optical phenomenon in MNPs that can confine and localize interacting optical field into the subwavelength range and as result it enhanced the local electromagnetic field intensity [9–14]. SPR phenomena is collective oscillations of loosely bounded conduction band electrons in MNPs. These oscillations are result of interacting optical field induced electronic polarization and dipole in MNPs. Moreover, the SPR enlarged the ratio of optical scattering to geometrical cross-section of MNPs. The SPR induced plasmonic properties are maximized at SPR frequency, which is strongly dictated by shape, size, and metals [10–15].
The plasmonic properties are omnidirectional and independent of excitation light polarization state in spherical MNPs due to their shape isotropy. However for anisotropic MNPs, such as spheroid and nanorods (NR), the optical excitation along the long and short axes is sensitive to the polarization state of excitation light [16,17]. They exhibit two spectrally distinct SPR, longitudinal and transverse (in and out-of-plane), due to surface curvature and symmetry as illustrated in Fig. 1(a). The strength of the longitudinal significantly dominates over that of the transverse SPR in NRs. Therefore, local enhancement of electromagnetic field is anisotropic for transverse and longitudinal SPR and similarly optical scattering cross-section in NRs [9,18]. In particular, gold nanorods (AuNR) are well-studied because of their shape-induced anisotropic plasmonic properties and broad tunability of the longitudinal SPR frequency [19–21]. Moreover, they are easy to orient and align in a particular direction by applied external electric field since their shape anisotropy induces dipolar polarizability that exert the aligning rotation torque on NRs [22–25].
Fig. 1. (a) Schematic diagram of the transverse and longitudinal SPR excitation in AuNR. Where EH and EV denote the horizontal and vertical components of excitation light, respectively (magnetic component is not shown). (b) Representation of the plasmonic interaction between AuNRs and QDs located at different space position in SPR enhanced local electric field plane.
When an optical emitter such as quantum dot (QD) is placed in the SPR enhanced local electromagnetic field, the QDs optical excitons interacts with SP of MNPs and is known as exciton-SP or plasmonic interaction [6–8,26–33]. The excitons in QDs are equivalent to SP in MNPs. Exciton-SP interaction is maximized for optical excitation frequencies that are in resonant with SP and exciton energies and precise space control between QDs and MNPs. These interactions can be broadly categorized into strong and weak plasmonic interaction. The former is considered when exciton-SP interaction modifies exciton wave function and SP modes that changes in exciton and SP energies [34–37]. The latter is when exciton wave function and electromagnetic modes of SP are considered unperturbed. At quantum level these exciton-SP interactions are often described by the interaction between exciton and radiating SP dipoles. This research focused on weak plasmonic interaction which has been used to explain the optical emitter enhanced absorption, emission enhancement and quenching [26,32,33,38–43], excitation and emission rate [44–46], modified radiative rate, fluorescence quantum yield and lifetime [39,43,47–49], exciton-plasmon energy transfer [30,33,41,42,47,50], emission wavelength shift [51], and angular distribution of emission [52–54]. The anisotropic exciton-SP interaction can modify the emission direction and polarization state [14–17] which depends on the exciton- SP coupling angle.
These plasmonic interactions are complex phenomena and more specifically anisotropic plasmonic interaction between the QDs and AuNRs [28]. Which is governed by size and shape of AuNR and QD [55], spacing [42,56], spectral overlap [57,58], dipole orientation [57], encapsulating medium [59] and their interdependence. It is significantly challenging to control plasmonic interaction parameters in a AuNR-QD composite. This study focused on manipulating SP dipole orientation in the exciton-SP interaction in AuNR-QD composite encapsulated in polymer. The orientation of longitudinal and transverse SP dipoles changed through AuNRs orientation with respect to QDs. The orientation of AuNRs controlled by an applied external electric field. In the fixed orientation AuNRs, a polarization selective (sensitive) SP dipole can be excited using a particularly polarized excitation light [55]. The schematic of a polarization selective anisotropic plasmonic interaction between the exciton-SP is presented in Fig. 1(b). Polarization selective anisotropic plasmonic interaction in AuNR-QD composite can substantially change the angular distribution of emission [60–62], polarization state of emission [63,64], and shows sharp directional emission [47,48,59]. There have been few research papers reported on controlling the directional emission using plasmonic interaction in the photonic crystal structures and surface-plasmon–based devices [65]. Research is limited since to realize directional emission requires precise control of the orientation of AuNR with respect to QDs in addition to plasmonic interaction parameters such as spacing, and spectral overlap. Experimentally, it was realized in a combined structure fabricated using electron-lithography [61].
This research fabricated a homogenous polymer plasmonic composite of AuNR-QD. The AuNR orientation with respect to QD was manipulated by an applied external AC electric field. Plasmonic interaction parameters in the composite were controlled externally. The spacing between AuNR-QD in the composite was controlled through the doping concentration distribution using an earlier reported method [56]. The spectral overlap of QD emission and AuNR extinction spectra was optimised using an appropriate size AuNRs. These polymer composite optically investigated and series of polarization resolved optical characterizations performed; emission enhancement, polarization state of emission, and directional emission. Finally of these optical characterizations analysed to establish polarization sensitive anisotropic plasmonic interaction in the polymer AuNR-QD composite.
2. Materials and experimental methods
2.1 Materials
Spheroid gold nanorods (represented as AuNRs in the paper) with an aspect ratio of ≈1.85(D ∼13 nm and L ∼24 nm) were synthesised using an earlier reported method [66]. The polyvinyl pyrrolidone (PVP) protected AuNRs were extracted from the parent solvent by centrifuging, and re-dispersed in ethanol. Their extinction spectra are shown in Fig. 2(a) which shows the transverse and longitudinal surface plasmon resonance peaks at ≈610 and ≈520 nm, respectively. Core-shell CdSe/ZnS quantum dots (diameter of 4-6 nm, Plasma Chem, Germany) were used as the fluorescent material with absorption and emission peak wavelengths at 575 and 610 nm, respectively, as presented in Fig. 2(a).
Fig. 2. (a) Normalized extinction, absorption, and emission spectra of spheroid AuNRs and QDs. (b) Schematic block diagram to orient and align the AuNRs by an external AC electric field.
2.2 AuNRs alignment setup
The schematic block diagram shown in Fig. 2(b) was used to orient and align AuNRs by an external electric field. A transparent electrode mould cell of 45×25×2 mm was used to apply an external AC electric field to align AuNR in the liquid epoxy resin polymer. Moreover, it served as mould for curing the polymer composite containing AuNRs and QDs. The high transparency (≈ 80%) of ITO coated Polyethylene Terephthalate (PET) electrode allowed for simultaneous measurements, recording the change in absorption due to alignment of AuNRs in the epoxy resin polymer when the external AC electric field was applied. An external AC electric field of 0 to 3.5 ×105 Vm−1 was applied. The high voltage was applied by a voltage amplifier and a function generator (Trek). A fiber optic spectrometer (AvaSpec-2048) measured live absorption spectra as a function of electric field strength, and samples were illuminated by DH-2000-BAL light source (Ocean Optics). The spectrometer has a signal-to-noise ratio of 200:1 with resolution 1 nm and using a broad range of integration times from 2×10−3 to 2 seconds allowed detection of very low optical signals and changes.
2.3 Fabrication of plasmonic composites
Three types of plasmonic composite samples (referred as Plascomp) of AuNRs and QDs were prepared. Epoxy resin polymer (ABL Resin & Glass, UK) was used as the matrix medium for the plasmonic composites. The use of a polymeric matrix allowed the ability to tune the average AuNR-QD distance, to obtain uniform dispersion of QDs and AuNRs, and fix the position of the aligned Au NR. It contains two parts; resin and hardener, mixed in 100:42 ratio by weight for processing. The epoxy resin polymer has a high viscosity of 300 to 450 mPa which was lowered to 100 mPa s by diluting with analytical grade ethanol (Lennox, Ireland) of viscosity of 1.44 mPa s. The low viscosity helped to reduce the viscous forces of the medium on AuNRs. For the plasmonic composite, QDs and AuNRs were dispersed in the diluted epoxy resin polymer and the dispersion homogeneity was confirmed by optical characterization. Subsequently, a liquid plasmonic composite solution was cast in the transparent conducting mould and subjected to an external AC electric field of 3.5 ×105 Vm−1 for 3 hours to orient and align the AuNRs. The plasmonic composite was cured in a mould cell at room temperature for 48 hours after which solid Plascomp samples (20×20×2 mm) were removed from the mould cell as displayed in Fig. 3(a). QD and AuNR doping concentrations were 0.04% and 1 ppm, respectively. The table in Fig. 3(a) details the plasmonic composite samples.
Fig. 3. (a) Fabricated Plascomp sample along with details of sample compositions; a schematic of AuNRs aligned to normal of surface and QDs represented by red emission colour and; (b) Optical characterization setup to examine Plascomp samples.
2.3 Optical measurements setup
Plascomp samples were subjected to optical characterization to investigate the plasmonic interaction between QDs and AuNRs. Their absorption, emission, emission transport properties, and polarization state of emission were measured. Unpolarised, linearly polarized excitation light was used as presented in Fig. 3(b). A linear polarizer was placed between the Plascomp edge and detector to collect various plane polarized emission. Two optical fiber detectors were used; one positioned at the rear of the Plascomp for absorption, and the second at the edge for emission measurement.
3. Results and discussion
3.1 Aligned AuNRs in liquid epoxy resin polymer
AuNRs were aligned in liquid epoxy resin polymer using the setup described in Fig. 2(b) with vertically polarized excitation light. Extinction spectra of AuNRs as a function of the applied external electric field strength is shown in Fig. 4(a).
Fig. 4. Extinction spectra of AuNRs in the liquid epoxy resin polymer as function of applied external AC electric field, (a) extinction spectrum and, (b) Normalized extinction spectrum.
At lower applied electric field strength, SPR peaks shape and intensity remained unchanged. However, it changed with increasing the electric field strength and transverse SPR peak intensity was higher compared to longitudinal SPR peak in Fig. 4(a). The change is more clearly pronounced in the normalized ratio of longitudinal to transverse SPR peak intensity in Fig. 4(b). These changes directly indicate that AuNRs were responding to the applied electric field as AuNRs are considered as polarizable elements and susceptible to electric polarization in an external electric field. The electric polarization induced dipole moment is given by [22];
Where ${\alpha _n}$ is the polarizability tensor, subscript $n = L,S$ labels the long and short axis of the AuNRs, and E is the electric field. The induced dipole generated a rotation moment (that is an external electric field exerted on the anisotropic particle) which depends on the contrast between the polarizability along the long (longitudinal) and short (transverse) axes of AuNR [23]. The rotation moment rotates and aligns AuNR along the long axis and in the direction of the applied electric field which lies in the incident plane of vertically polarized excitation light. Hence the vertically polarized excitation light can only excite the out-of-plane transverse SPR and intensity increased with electric field as the AuNRs rotate and align. It is clearly pronounced at and above the electric field strength of 2.0×105 vm−1 after that the longitudinal SPR peak intensity decreases in Figs. 4(a)-(b). The incremental change in SPR peak intensity illustrates AuNRs aligning is a gradual process [67,68]. To achieve a higher degree of alignment, an electric field was applied for a longer duration of 3 hours for Plascomp samples.Ideally the longitudinal SPR peak should have vanished. However, it was not due limited degree of alignment and background signal contributed by partially, randomly orientated, size, and shape dispersion of AuNR. The electro-optical response demonstrated that the epoxy polymer is a suitable dispersion medium to orient and align AuNR by application of an external AC electric field. Therefore it was chosen for Plascomp samples which contained AuNRs-QDs.
3.2 Plasmonic composite in the liquid epoxy resin polymer
The plasmonic composites containing 0.04wt % of QDs and 1 ppm of AuNRs in the liquid epoxy resin polymer was subjected to electric field of 0.0 to 2.0×105 Vm−1 and their recorded absorption and emission as function of electric field strength are shown in Fig. 5.
Fig. 5. (a) the absorption, (b) the emission and, (c) the percentage change of absorption and emission in the plasmonic composite as function of electric field strength. (The electric field increased in the step of 0.25×105 Vm−1 and applied for two minute in each step).
QD absorption and emission in the AuNR-QD composite changed with applied electric field strength. This indicated plasmonic interaction is controlled by the orientation of AuNRs in the AuNR-QD since the other plasmonic interaction parameters are constant. As established in pervious section that applied electric field oriented and aligned AuNRs in epoxy resin polymer and assuming it replicated for these samples since medium.is unchanged. The longitudinal and transverse SPR can be selectively excited in these aligned AuNRs by polarized excitation light. Therefore, the plasmonic interaction between QD exciton dipole, and radiating longitudinal and transverse SP dipole are also a function of AuNR orientation in AuNRs-QDs composite. Hence, the plasmonic interaction enhanced absorption and emission is function of applied electric field as demonstrated in Fig. 5. The degree of orientation is manipulated through aligning AuNRs by applying an external electric field in the composite of AuNR-QDs. The response can be divided in three regions of electric field strength; i) unresponsive region from 0 to 1×105 Vm−1 where the field strength is not enough to overcome random motion of AuNRs; ii) active region from 1 to 2.5×105 Vm−1 where the electric field strength started exerting enough rotation moment on AuNR to exceed thermal energy and resistive force of medium to orient and align them; iii) saturation region above from 2.5×105 Vm−1, where the degree of alignment of AuNRs is complete. The saturation electric field strength of 3.5×105 Vm−1 was chosen and applied for three hours for fabrication of Plascomp samples. Although the liquid epoxy polymer has a pot life time of 6 hours, it starts getting thicker after one hour and becomes viscous enough after three hours to hold the orientation and position of aligned AuNRs in the composite.
3.3 Plasmonic composite absorption profile
QD absorption in the Plascomp samples for vertically polarized and unpolarized light is shown in Fig. 6. The detector has a field of view of 180° to collect specular and diffuse transmission through Plascomp samples. Samples Plascomp2 and 3 have higher absorption than the reference QD only Plascomp1. For unpolarized light, plasmonic interaction increased QD absorption and independent of orientation of AuNR as shown in Fig. 6(a). Whereas for vertically polarized light, it is sensitive to orientation of AuNRs as demonstrated in Fig. 4(b).
Fig. 6. QD absorption changes in the Plascomp samples due to plasmonic interaction, (a) unpolarized and, (b) vertically polarized, (c) integrated absorption, d) QDs total (specular and scattered) absorption for Plascomp3. (Where Plascomp3 and 4 have similar absorption profiles)
The plasmonic interaction modified QD absorption in the AuNR-QD composite can be attributed to the SPR enhanced optical scattering cross-section of AuNRs compared to their geometric cross-section. This resulted in a high extinction coefficient for AuNRs and it maximized at SPR peak wavelength. The overall extinction is dominated by scattering compared to absorption for AuNRs. The SPR enhanced scattering cross-section amplified the magnitude of local electromagnetic field which is the sum of the incident light plus SPR scattered light, moreover, lengthening the optical path-length. Therefore, QDs in the composite of AuNR-QD experience stronger electromagnetic field and that improved absorption rate which is directly proportional to the local electromagnetic field intensity. However the enhanced local electromagnetic field is different for unpolarized and vertically polarized light for aligned and randomly oriented AuNRs in the composite so the absorption rate. Since the vertically polarized light excited out-of-plane SPR (transverse SPR) in the aligned AuNRs hence sensitive to its orientation. Whereas, unpolarized light efficiently excited transverse and longitudinal SPR in single AuNR, irrespective of their orientation. Therefore, the overall scattering cross-section (transverse plus longitudinal SPR) is higher for unpolarized light hence the locally enhanced electromagnetic field and consequently the increased QD absorption and it is the same for Plascomp2 and 3 as shown in Figs. 6(a)-(c). Integrated QD absorption showed the maximum increase of ≈10% in Plascomp2 and 3 for unpolarized light presented in Fig. 5(c). Vertically polarized light can excite transverse and longitudinal SPR for a randomly oriented AuNR with equal probability. Thus the overall scattering cross-section remained lower than unpolarized light and higher than the aligned AuNRs. Therefore it has been observed that Plascomp2 have higher QD absorption for vertically polarized excitation light and lower than the unpolarized excitation light in Figs. 6(b)-(c). It can be concluded that QDs in the composite AuNR-QD is highly sensitive to polarization state of excitation light.
The difference in total and specular absorption in Fig. 6(d) manifest the SPR scattering cross-section induced lengthening of optical path-length. The SPR scattering deviates the path trajectory of light as it travelled a longer distance and exited at higher solid angles from the Plascomp samples. The specular transmission excluded scattered light, hence, had lower transmission compared to total. That implied, higher absorption for specular and lower for total.
3.3 Emission profile of plasmonic composites
QD emission in the Plascomp samples was examined using the setup shown in Fig. 7(a). Their emission profile for unpolarized and vertically polarized excitation light is presented in Figs. 7(b)-(e). Emission enhanced in Plascomp samples that contained plasmonic composite AuNR-QD, moreover, it is higher in the AuNRs aligned Plascomp for both unpolarized and vertically polarized excitation light. The emission enhancement can be attributed to the increased absorption (excitation rate), radiative rate, and directional emission of the QDs in the AuNR-QD composite. As it has been discussed in section 3.2, anisotropic plasmonic interaction modified QD absorption is sensitive to polarization of excitation light and resulted in enhanced QD excitation rate.
Fig. 7. (a) Schematic diagram for QDs emission measurement in the Plascomp samples. QD emission profile; (b) &(c) for unpolarized excitation light and; (d) & (e) for vertically polarized excitation light. (f) Proposed schematic illustration of anisotropic plasmonic interaction between the emission and radiating dipoles of QDs and AuNRs.
Absorption increased by ≈10%, whereas emission enhancement was ≈59% and ≈54% in plasmonic composite Plascomp3 for vertically and unpolarized excitation light, respectively, as presented in Figs. 7(c)-(e). This manifests the role of other plasmonic interactions phenomena mediated QDs emission enhancement; enhanced radiative rate and directional emission in the AuNR-QD that is strongly a function of spacing, energy resonance, and dipole orientation radiating dipole of AuNRs and emission dipole of QDs. The plasmonic interaction spacing in AuNR-QD is optimized through the doping concentration of 1ppm AuNRs and 400 ppm QDs in the polymer composite using the method described in reported literature [56]. Higher doping concentrations led to reduced emission due to emission quenching effects. The enhanced excitation and radiative rate is maximized when SPR wavelength resonating to the absorption (400-600 nm) and emission (610 nm) wavelength of QDs, respectively. AuNRs transverse (≈530) and longitudinal (≈610 nm) SPR have different wavelength ranges and their excitation is sensitive to polarization. The plasmonic interaction modified excitation and radiative decay rate. Simultaneously, QDs emission spectrum excited the longitudinal SPR in the AuNR-QD due to spectral overlap in emission and extinction. This emission and longitudinal SPR energy overlap also promoted the dipole-dipole interaction between the longitudinal SP radiating dipole and QD emission dipole.
Therefore, the excitation rate enhancement is solely a contribution of the transverse SPR enhanced absorption. Whereas the longitudinal SPR modified the radiative rate and directional emission through SP radiating dipole-QD emission dipole. It is controlled by SP radiating dipole orientation since emission dipole is omnidirectional in addition to energy resonance. A schematic is illustrated in Fig. 7(f), of the parallel and orthogonal orientation of emission and radiating dipole that led to radiative rate enhancement (contribution) and suppression (cancel), moreover, the emission direction. The directional emission in combination with enhanced radiative rate can improve the overall emission transportation by minimizing the optical scattering associated with optical losses. These two effects acted constructively in the Plascomp 3, since it has a more orderly aligned radiating-emission dipole in comparison to Plascomp2. Therefore, there is higher emission compared to Plascomp2 for vertically and unpolarized excitation light. Although unpolarized light excited both transverse and longitudinal SPR in Plascomp2, however, the emission was lower than Plascomp2 because the optical losses compensated the emission due to randomly distributed pairs of SP radiating–emission dipole.
3.4 Polarized emission
The proposed polarization sensitive anisotropic plasmonic interaction between AuNR-QDs is further corroborated through the analysing polarization state of emission in the Plascomp samples. The polarization state of emission was investigated by placing a polarizer between the Plascomp edge and detector in the setup described in Fig. 7(a). The emission collection and incident excitation light planes were perpendicular. The vertically (0°) and horizontally (90°) polarized emission profile and their corresponding ratio remained unchanged for QD only Plascomp1, it was slightly varied for Plascomp2 (randomly oriented AuNR-QDs), and decreased from 1 to 0.84 in the aligned AuNR-QD Plascomp3 & 4 in Figs. 8(a)-(b).
Fig. 8. Polarization of QD emission in the Plascomp samples; (a) vertically (0°) and horizontally (90°) polarized emission for vertically polarized excitation light; (b) polarization ratio $({\mathrm{\eta }_\textrm{p}})$ of various planes of polarized emission.(${\mathrm{\eta }_\textrm{p}}\textrm{ = }\frac{{{\textrm{I}_\textrm{0}}}}{{{\textrm{I}_\textrm{i}}}}\; ,{\textrm{I}_{\textrm{i}\; \;}}\textrm{ = }{\textrm{0}^\textrm{0}}\textrm{,3}{\textrm{0}^\textrm{0}}\textrm{,6}{\textrm{0}^\textrm{0}}\textrm{,9}{\textrm{0}^\textrm{0}}$, where 0° and 90° represents vertically and horizontally polarized emission, respectively)
The varying ratio of vertically to horizontally polarized emission components in the aligned AuNR-QD samples proved that anisotropic plasmonic interaction between radiating-emission dipole manipulated the emission profile. The longitudinal and transverse radiating dipole in the aligned AuNR-QD lies in and out of excitation and emission collection plane. The strong longitudinal SP radiating-emission dipole interaction polarized emission in-plane (horizontally), therefore, high component of horizontally polarized emission. The polarization ratio is limited as the polarized emission undergoes multiple absorption and SPR scattering events while traveling to the edge in the sample which mitigated the polarization of emission. Therefore, the emission at the edge has both vertical and horizontal components.
3.5 Directional emission improved decay profile
The polarization sensitive anisotropic plasmonic interaction enhanced directional and enhanced emission were investigated using QD emission decay profile in Plascomp samples. Plascomp samples were excited at various positions from the detector to vary the distance over which the emission travelled using the setup in Fig. 7(a). For unpolarized excitation light, the integrated and normalized emission decay rate is plotted in Figs. 9(a) and (b). Integrated emission for Plascomp3 and 4 is higher than the Plascom1 and 2 in Fig. 9(a) which is consistent with the total emission trend in Figs. 7(b)-(e). The integrated emission is higher for short distances of 1-10 mm and indicated that polarization sensitive anisotropic plasmonic interaction enhanced emission is localized and more dominant at shorter distance. However, enhancement is compensated by the emission losses due to multiple scattering and re-absorption when it travel over longer distances. Similarly for vertically polarized excitation light, Plascomp3 has a higher integrated emission compared to Plascomp1 and 2 in Fig. 9(c). However, for shorter ranges of 1-6 mm, Plascomp2 is higher than Plascomp1 as seen in Fig. 9(c), which is contrary to the unpolarized excitation light profile in Fig. 9(a).
Fig. 9. Integrated and normalized QD emission decay profile as function distance for Plascomp samples; (a) & (b) for unpolarized excitation light and; (c) & (d) for vertically polarized excitation light.
The normalized emission decay profile quantified emission decay rate in Figs. 9(b)–9(d). For unpolarized excitation light, at 4 mm, the emission decay was 30 and 50% in Plascomp1 and 3, respectively. Plascomp3 showed ≈ 20% less decay compared to Plascomp1 and 2 for shorter distance ranges of 1-6 mm and difference in decay rate is diminished for longer distance. Whereas for vertically polarized excitation light, decay difference ≈ 15%, moreover, remained constant over large distance of 1-15 mm in Fig. 9(d). Overall, Plascomp3 that contained aligned AuNR-QD demonstrated less decay compared to other Plascomp samples, moreover, even less decay for vertically polarized excitation light over a longer distance due to more number of orderly aligned SP radiating-emission dipole interaction. The strong longitudinal SP radiating-emission dipole interaction forced the transition emission dipole to emit the light in a narrow angular distribution that led to very sharp directional emission which improved the decay rate. Directional emission minimizes the optical transportation losses such as scattering and re-absorption, hence, the emission decay is lower over longer distances and higher emission in Plascomp3.
4. Conclusions
In conclusion, this research has successfully demonstrated polarization sensitive anisotropic plasmonic interaction modified absorption, emission, polarization state of emission, and directional emission in the plasmonic composite of AuNR-QD. Homogenous QDs only reference and AuNR-QD plasmonic composites of 20×20×2 mm in the epoxy resin polymer were fabricated. The orientation of AuNRs in the AuNR-QD composite was manipulated using an external AC electric field of 3.5×105 Vm−1. The polarization sensitive anisotropic plasmonic properties of transverse and longitudinal SPR of AuNRs has led to anisotropic plasmonic interaction in the AuNR-QD. The plasmonic interaction increased QD absorption in the AuNR-QD was varied for polarized and unpolarized excitation light. Maximum increase of ≈10% was observed for unpolarized excitation light. The emission enhancement was highly sensitive to polarization of excitation light in the AuNR-QD composite. It was maximum for aligned AuNR-QD composite with 59% for polarized excitation light. The strong longitudinal SP radiating-emission dipole interaction modulated the polarization state of emission in the AuNR-QD. The polarization ratio of out-of-plane (vertical) and in-plane (horizontal) emission changed from 1 to 0.84. The longitudinal SP radiating-emission dipole interaction in the AuNR-QD induced directional emission has improved the optical transportation properties in the plasmonic composite. That resulted a ≈ 20% less emission decay in aligned AuNRs-QDs composite compared to the reference.
Funding
European Research Council (PEDAL Project 639760); Horizon 2020 Framework Programme (IDEAS Project 815271).
Acknowledgments
This research work was funded by European Research Council under the project Plasmonic Enhancement and Directionality of Emission for Advanced Luminescent Solar Devices (PEDAL Project: 639760) and European Union funded Horizon 2020 project Novel building Integration Designs for increased Efficiencies in Advanced climatically tunable renewable energy Systems (IDEAS Project: 815271). Science Foundation Ireland (SFI) for their support under the SFI ERC support fund for research infrastructure.
Disclosures
The authors declare no conflicts of interest.
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