1. Introduction

Light manipulation on the nanoscale using plasmonic nanostructures has been at the center of scientific interest for the past two decades [1]. Among the various applications of plasmonic particles, methods to modify the luminescence of coupled quantum emitters has been one of the areas of most interest, in particular methods to quench [2,3] or enhance [1,3], spectrally reshape [4,5] and modify the directionality of luminescence emission [6]. A variety of hybrid systems have been reported as ideal candidates for luminescence modification, including single nanoparticles of various shapes ranging from spheres [3], rod shaped [7], nano-shells [8] to even more complex morphologies [9,10] Additionally, nanoparticle assemblies, such as dimers [11], aggregates of randomly assembled nanoparticles [12] and well-ordered nanoparticles in arrays [13] also offer luminescence modification. In recent years, structures where the plasmonic response of hybrid nanoantenna can be directly tuned using phase change materials (PCMs) has seen increased interest with a view to applications in nanophotonic devices. Many hybrid systems consisting of phase change materials have been reported including systems that incorporate chalcogenide PCMs such as GST [14], switchable metal hydrides [15], gallium surface-mediated transitions [16] and transitions in correlated oxides such as VO₂, V₂O₃ and Ti₂O₃ [17]. The incorporation of PCMs into nano-plasmonic devices has allowed for development of tunable absorbers [18], modulators [19], dynamic structural color [20] and waveguide modes nano-gratings [21].

In general, plasmonic nanostructures modify luminescence though two mechanisms, modification of the excitation rate and the emission rate. Both mechanisms depend strongly on a number of factors including the spectral overlap of the quantum emitter’s absorption and emission with the plasmon modes of the nanostructure, the size and morphology of the nanostructures, the distance between the quantum emitter and the nanostructure, and the orientation of the molecular dipole with respect to the nanostructure. Plasmonic nanostructures can concentrate light in nanoscale regions close to their surface so that the local electric field intensity is substantially higher than the incident electric field intensity [22]. An emitter coupled to a plasmonic nanostructure will therefore experience a higher electric field and thus an excitation rate enhancement. Modification of the emission rate can be attributed to the effect of the nanoantenna. Since plasmonic nanostructures modify the local density of optical states [23], the presence of the nanostructure in the proximity of an emitting dipole can alter the radiative decay rate and provide new loss mechanisms as non-radiative decay pathways for the emitter, such as energy transfer.

Although surface enhanced phenomena with nanostructures, such as luminescence modification and surface enhanced Raman Scattering (SERS), are well studied, there are few methods to alter or tune such effects via plasmon resonance tuning post fabrication. Previously reported methods rely on changes in the relative position of the emitters with respect to the nanostructure; by conformational changes of ligands attached to both the emitter and the nanostructure [24], mechanically via manipulation with an AFM tip [25] or with the aid of elastic substrates [26]. However, such methods are not feasible for photoluminescence (PL) tuning in optoelectronic devices.

We propose a method that can both dynamically tune the luminescence enhancement and compensate for thermal quenching effects using noble metal nanostructures on top of vanadium dioxide (VO₂) thin films. VO₂ undergoes a transition from a semiconducting monoclinic (M1) phase to a metallic rutile (R) phase [27–29] which can be induced thermally [30], electrically [31] and optically [32]. VO₂ is a promising candidate for various applications due the thermally induced phase transition occurring at 68° C [30], remarkably close to room temperature and at a significantly lower temperature compared to other phase change materials such as GeSbTe (GST) [33] or AgInSbTe (AIST) [34]. Fabrication of VO₂ thin films is achievable through reactive physical vapour deposition methods such as pulsed laser deposition [35] and RF sputtering [36] and recent reports of VO₂ growth by chemical vapour deposition show large-scale VO₂ deposition [37]. Upon the vanadium dioxide phase transition a sharp change in optical properties can be seen, c.f. Figure 1(a) and other reports [27,38]. Although the largest changes in the dielectric function are in the NIR and IR spectral region (above 900 nm) [27,39], the changes in the dielectric function seen in the visible spectral region are pronounced enough to substantially shift the spectral position of nanoparticle plasmons [40]. Due to the smaller changes in the dielectric function below 900 nm there have been relatively few reports considering the use of VO₂ [41,42] for tunable systems in the visible. However, here it will be shown that significant increases in emitter emission can be achieved in plasmon-coupled VO₂ structures even at these lower wavelengths. The plasmon resonances of noble metal nanostructures on top of VO₂ films spectrally blue shift with the phase transition of VO₂ from the semiconducting to metallic phase [43].

The simplest method to induce the VO₂ phase transition is thermally, by heating the entire sample above the transition temperature with a heating stage. However, a 50-60 degree increase in temperature, from room temperature to a temperature above the transition temperature, will diminish the luminescence intensity of most quantum dots and dye molecules, an effect known as thermally induced luminescence quenching. Depending on the emitter, luminescence intensity decreases of between 50-80% can be expected. The PL intensity decrease for dye molecules, such as Alexa Fluor, is in the order of 0.2-2% per degree [44,45], while for quantum dots sometimes a decrease of up to 80% can be observed for a temperature difference of 60 degrees [46]. For many optoelectronic devices it’s necessary they perform efficiently at elevated temperatures. Devices which incorporate dyes and QDs frequently suffer from reduced performance in these environments or require power consuming cooling elements. We present structures that use the phase change of a VO₂ film to tune the plasmon resonance of Ag nanostructures so that the PL enhancement of coupled emitters in the high temperature phase counterbalances any thermally induced PL quenching effects.

2. Results and discussion

In this work, the FDTD solver of the commercially available Lumerical Device Suite software [47] is used to investigate the PL response of systems consisting of various Alexa dye quantum emitters coupled to Ag nanodiscs of diameter D, height H on a VO₂ layer of thickness t (Fig. 1(b)). The modified photoluminescence spectrum (PL) can be expressed as $\; PL(\lambda )= {\mathrm{\gamma }_{Exc\; }}{g_{em}}(\lambda ){f_0}(\lambda )$, the product of the intrinsic luminescence spectrum of the quantum emitters ${f_0}(\lambda )$, the modified excitation rate ${\mathrm{\gamma }_{Exc\; }}$ and modified emission rate ${g_{em}}(\lambda )$.

 

Fig. 1. (a) Real and imaginary parts of the dielectric function of a VO₂ thin film in the semiconducting (SC) and metallic (M) phases. (b) Schematic of Emitter – Nanostructure -VO₂ system with Ag nanodisc diameter D, height H and VO₂ thickness t. The schematic indicates the 15 emitters (red circles) positions chosen to emulate a uniform coating of Alexa dye molecules. 6 positions are on top of the nanodisc, so that the emitters are located 5 nm and 7.5 nm above the nanoparticle and are distributed along the nanodisc, i.e. at the center of the nanodisc, at a distance from the centre corresponding to half radius of the nanodisc, and at the upper edge of the nanodisc. The remaining 9 positions mimic a homogeneous distribution of emitters at the sides of the nanodisc. For simplicity we divide our system in 3 (z direction) by 3 (in x direction) side positions. In the radial (x) direction the positions are separated by 2.5nm and the first position corresponds to 5 nm away from the surface of the nanostructure. In the z direction the emitters are distributed at three different heights from the VO₂ thin film, i.e. 5 nm from the VO₂ film surface, a height corresponding to the half-height and a height corresponding to the height of the nanodisc.

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From the simulated structures we calculate ${\gamma _{Exc}}$, the modified excitation rate, and ${g_{em}}(\lambda )$, the modified emission rate of the system. The nanodisc material and geometric parameters are selected so that the plasmon resonance overlaps with the PL spectra of emitters in the wavelength range where the phase change of the underlying VO₂ layer allows significant shifts in a nanodisc plasmon resonance. The size and shape of the presented nanodiscs allows straightforward patterning on VO₂ through electron beam lithography and deposition of Ag using metal evaporation techniques. The Ag dielectric function used in all numerical calculations is from Palik [48] and the VO₂ dielectric function in semiconducting (SC) and metallic (M) phase used in our simulations was measured via ellipsometry (Fig. 1(a)). The thickness of the VO₂ film is initially set to 15 nm as increased thickness has a damping effect on nanoparticle plasmons, decreasing the scattering cross section. The influence of VO₂ thickness on the PL is further discussed later. Silica (n = 1.5) is used as the substrate in our VO₂ – nanodisc simulation model. The simulation space consists of a cube with side length of 1.5 µm and perfectly matched layers used as boundary conditions. A normally incident total-field scattered-field (TFSF) plane wave source is used as an excitation source for calculation of the scattering cross section and ${\gamma _{Exc}}$. To model the PL of the hybrid system, the system is excited by a dipole source, to emulate the response of a quantum emitter. While placing emitters at plasmonic hotspots would give large enhancements [49,50], to accurately model emission enhancement we consider dipole sources at discrete locations. To model a coating of dye molecules ${g_{em}}$ and ${\gamma _{Exc}}$ are calculated at 15 different positions (Fig. 1(b)) and for 3 dipole orientations, see Fig. S3. The modified emission rate ${g_{em}} = {\tau _{PL}}{g_{Det}}$ where ${g_{Det}}$ is defined as the detection rate, a factor determined by calculating the ratio of power transmitted along the detection path to the total power and ${\tau _{PL}}$ is the emission lifetime. Further discussion of the model used in our simulations is detailed in the supporting information. With the calculated modified PL, the total number of detected photons per second can be determined by the following equation

As the phase transition of VO₂ can be induced thermally, optically and electrically we do not include thermal quenching directly in our calculated PL emission. However, as thermally induced phase transition is one of the most common methods used, it is important to determine if the increases in emission can compensate for thermally induced luminescence quenching. We introduce a figure of merit R = ${I_{PL}}(\textrm{M} )/{I_{PL}}({SC} )$ where ${I_{PL}}(\textrm{M} )$ and ${I_{PL}}({SC} )$ are the PL intensities in the metallic and semiconducting phases respectively. Considering the PL quenching rate of dye molecules is in the range of 0.2-2% per degree, a value of R = 2 will provide sufficient enhancement of the PL intensity to compensate for thermal losses, and R>2 will correspond to an increase in the emission compared with the low temperature semiconducting phase of VO₂.

To determine the best candidates for dynamic PL modification we consider several emitters. Dyes from the Alexa Fluor family are suitable for implementation in our hybrid system due to their strong emission peaks in the visible and NIR spectral region. These are taken by way of example, but the results can be equally applied to quantum dot emitters. We investigate Alexa Fluor dyes with PL emission peaks at 673 nm (Alexa647), 670 nm (Alexa700), 780 nm (Alexa750) and 805 nm (Alexa790). The excitation wavelengths used within the model to calculate emitter excitation rates for these dyes are 632 nm, 670 nm, 705 nm and 730 nm, respectively. The optical properties of each dye emitter are given in Table S1 in the Supporting Information (SI). The impact of the phase change of VO₂ on the excitation and emission in the absence of the nanodiscs is shown in Fig. S1. Since there are no reports on PL modification in the near-infrared spectral range upon the phase transition of VO₂, a brief discussion on the impact of the VO₂ phase transition on the emission properties for each dipole orientation is included in the SI for the case of Alexa790 Dye with a 15 nm VO₂ film. In addition comparison of the emission after the introduction of the Ag nanodisc compared with the bare VO₂ film is also described in the SI Fig. S2, whereas herein we focus on the influence of the VO₂ phase transition for the Ag nanodisc on thin film VO_2.

For the hybrid system, we first consider the dependence on the diameter of the Ag nanodisc. A schematic of the structure is given in Fig. 1(b). From the spectra and graph shown in Fig. 2(a) and Fig. 2(b) respectively, it can be seen the optimal choice of emitter shifts to increased wavelengths with increasing disc diameter and redshifted plasmon resonance position. For a disc of 120 nm, Alexa700 has a maximum ratio of 2.54, while Alexa750 and Alexa790 have maximum ratios of 2.8 and 3.16 for nanodiscs with diameter of 140 nm and 160 nm, respectively. The increase in PL ratios can be attributed to the increased overlap of the nanodisc plasmon when the VO₂ layer is in the metallic phase. For Ag nanodiscs of diameters 120 nm, the PL emission of Alexa700 is spectrally on the blue side of the nanodisc plasmon resonance when the system is in the semiconducting phase and strongly overlaps the resonance for the metallic phase. The 140 nm diameter Ag nanodisc shows increased PL ratio for nearly all dyes. Again, in the semiconducting phase the dye emission is on the blue side of the plasmon resonance for the semiconducting phase and has a better overlap with the resonance peak in the metallic phase. This effect is increased even further for the 160 nm diameter Ag nanodisc with an increase in PL ratio for the longer wavelength emitters. However, it must be noted that the increased PL seen for discs with wider diameter is accompanied by a decrease in overall emission intensity. A full analysis for the 15 positions and three dipole orientations is presented in SI Fig. S3 and S4 for the example of Alexa790. The scattering cross section and PL spectra for all dyes, each nanodisc diameter and both phases of VO₂ are shown in Fig. S5 and maps showing electric field enhancement surrounding the Ag nandiscs for each diameter and both phases are shown in Fig. S6.

 

Fig. 2. (a) Scattering cross-sections and PL spectra for Ag nanodiscs (D = 160 nm, H = 40 nm) on VO₂ (t = 15 nm) in the semiconducting (SC) and metallic phase (M). In the case of the PL spectra the dotted and solid lines correspond to the SC and M phases, respectively. (b) ${I_{PL}}(M )/{I_{PL}}({SC} )$ for Alexa dyes coupled to Ag nanodisc-VO₂ system (D = 120 nm,140 nm,160 nm, H = 40 nm) on VO₂ (t = 15 nm). Ratios above 2 (indicated by the black horizontal line) are considered suitable for compensation of thermal quenching.

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As Ag nanostructures are known to suffer degradation over time due to oxidation, we consider the effect of thin silica shells on the Ag nanodiscs. The spectra for 140 nm Ag nanodisc with a 5 nm shell case are shown in Fig. 3(a). The PL ratios for increasing SiO₂ thickness are given in Fig. 3(b), with silica shells thickness of 5, 10 and 15 nm considered. Increasing silica shell thickness has a minor influence on the plasmon resonance, with a 10 nm redshift for every 5 nm increase in shell thickness. Additionally, increasing shell thickness increases separation of emitter from the plasmonic nanoparticle, and consequently, there is in general a decrease in both PL intensity and PL ratio as seen in Fig. 3(b). However, it is worth noting that for the Alexa790 dye the emission marginally increases for the 5 nm silica shell, making the 5 nm shell ideal for preventing oxidation of the Ag discs. The spectra for all dyes are shown in Fig. S6. Spectra showing the impact of attaching quantum dot (QD) emitters, QD800, to the outside of a 15 nm SiO₂ shelled structure and embedding the QDs within the 15 nm shelled are shown in the Supporting information (Fig. S7). Embedding has the effect of increasing the PL ratio while decreasing the emission in both phases.

 

Fig. 3. (a) Scattering cross sections and PL spectra for Ag nanodiscs (D = 120 nm, H = 40 nm) with 5 nm SiO₂ shell on VO₂ (t = 15 nm) in semiconducting (SC) and metallic phase (M) (b) ${I_{PL}}(\textrm{M} )/{I_{PL}}({SC} )$ for Alexa dyes coupled to Ag nanodisc-VO₂ system (D = 120, H = 40 nm) on VO₂ (t = 15 nm) with varying SiO₂ shell thicknesses (0 nm, 5 nm, 10 nm, 15 nm) (c) Scattering cross sections and PL spectra for Ag nanodiscs (D = 120 nm, H = 40 nm) on VO₂ (t = 30 nm) in semiconducting (SC) and metallic phase (M) (d) ${I_{PL}}(\textrm{M} )/{I_{PL}}({SC} )$ for Alexa dyes coupled to Ag nanodisc-VO₂ system (D = 120, H = 40 nm) on VO₂ (t = 15 nm, 30 nm, 60 nm). (e) Scattering cross sections and PL spectra for Ag nanodics (D = 120 nm, H = 40 nm) on VO₂ (t = 15 nm) in semiconducting (SC) and metallic (M) phase (f) ${I_{PL}}(\textrm{M} )/{I_{PL}}({SC} )$ for Alexa dyes coupled to Ag nanodisc-VO₂ system (D = 120, H = 20 nm, 40 nm, 60 nm) on VO₂ (t = 15 nm).

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Further simulations investigate the dependence of the PL modification on the thickness of the underlying VO₂. Increasing VO₂ thickness has the effect of damping the plasmon resonance which is seen in the decrease in the scattering cross section. While it might be expected that a damping of the plasmon resonance would negatively impact the PL ratio, increasing the VO₂ thickness from 15 nm to 30 nm results in a dramatic increase in PL ratio for the Alexa790 dye emitters. The ∼4-fold PL enhancement is attributed to the increased shift of plasmon resonance upon the VO₂ phase transition. The spectral response for the 30 nm VO₂ case and the PL ratios for the Alexa dyes on a different VO₂ film thicknesses are shown in Fig. 3(c) and Fig. 3(d), respectively. The resonance shift from 865 nm in the semiconducting phase to 740 nm in metallic phase results in a total shift of 125 nm compared to the 75 nm shift seen for the 15 nm thick VO₂ case. A further increase in VO₂ thickness to 60 nm has no further impact on the magnitude of the plasmon resonance shift and, coupled with the further decrease in scattering cross section, results in a decreased PL ratio when compared to the 30 nm thick VO₂ film. 30 nm is therefore the optimal VO₂ layer thickness for an increased PL ratio for the longer wavelength emitters. The calculated scattering cross section and PL spectra for all dyes are shown in the Supporting Information (Fig. S9) along with maps showing the electric field enhancement surrounding the Ag nanodiscs on VO₂ of different thicknesses (Fig. S10).

Finally, we investigate the influence of the nanodisc height on the PL. The scattering cross section and PL spectra for Alexa750 and Alexa790 with a 40 nm nanodisc height are shown in Fig. 3(e). The PL ratios for a range of Ag nanodisc heights are presented in Fig. 3(f). As outlined in the SI, the magnitude of the non-radiative rate is dominated by the distance from the VO₂ film and the main contribution to the PL comes from the upper edge of the nanodiscs, Fig. S4. The total PL is enhanced for increasing disc height with a substantial increase in PL intensity seen when increasing the nanodisc height from 20 nm to 40 nm. This increase in PL with increasing Ag nanodisc height is attributed to the vertical plasmon near 400 nm, oscillating parallel to the surface normal. The increased PL is accompanied by a marginal increase in the PL ratio for Alexa647 and Alexa700, however for the dye emitting at the longest wavelength a slight decrease in enhancement factor is observed. This can be attributed to the blueshift in the plasmon resonance seen with increasing disc height, shifting the wavelength where the maximum excitation rate is seen. Further increase in disc height has no effect on the plasmon resonance, the PL intensities increase slightly, with a slight decrease in PL ratio for all dye emitters observed, except for Alexa790 which has a small increase. All spectra are shown in the Supporting Information with the electric field maps (Fig. S11, Fig. S12).

From the calculation of the PL ratios for various Ag nanodisc parameters and applying a criterium that the ratio of the PL intensity between the metallic phase and semiconducting phase should be in excess of 2, R = $\frac{{{I_{PL}}(\textrm{M} )}}{{{I_{PL}}({SC} )}} > 2$, it is observed that this criterium can be achieved for emitters at wavelength of 670 nm or higher using Ag discs with diameters between 120 nm and 160 nm. Different Alexa dye emitters can be selectively optimally enhanced, with a 160 nm disc optimal for Alexa790. A 30 nm VO₂ film is ideally suited to tunable PL enhancement due to the large plasmon shift without a large decrease in the scattering cross section. Further details on the comparison of the PL enhancement due to the Ag nanodisc on the VO₂ film at room temperature, ${I_{PL}}({\textrm{NP}} )/{I_{PL}}({Film} )$ and those due to the phase transition in the Ag nanodisc-VO₂ structure are shown in Fig. S13.

Calculation of PL ratios for intermediary temperatures can be achieved by simulation of the Ag nanodisc structure on VO₂ with dielectric functions calculated using the Maxwell Garnett equation [51],

The dielectric functions used for calculation of PL enhancement for three volume fractions (0.2,0.5,0.8) of metallic VO₂ in semiconducting VO₂ are shown in Fig. 4(a). Figure 4(b) shows the PL enhancement for Alexa790 on a silver disc of diameter 120 nm and height 40 nm on 30 nm VO₂ using intermediary dielectric functions of VO₂. It is seen that PL enhancement is achieved for the range of volume fractions corresponding to temperatures where the VO₂ is transitioning. No temperature is assigned to these intermediary phases as the volume fraction is strongly dependent on the hysteresis and transition temperature of the VO₂ film. Through strain [52] and doping with high valence metal ions such as W⁶⁺ or Nb⁵⁺ [53], the VO₂ transition temperature can be tuned to much closer to room temperature allowing for even greater exploitation of the enhanced PL before the onset of effects of significant thermal quenching.

 

Fig. 4. (a) Dielectric functions of intermediary VO₂ phases calculated from Maxwell Garnett effective media theory. (b) ${I_{PL}}(X )/{I_{PL}}({SC} )$ for Alexa790 dye coupled to Ag nanodisc-VO₂ system (D = 120 nm, H = 40 nm) on VO₂ (t = 30 nm). The ratios shown are for VO₂ effective media containing 0.2, 0.5, 0.8 volume fraction metallic VO₂ as well as the fully metallic case.

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Further analysis of the contributions to the PL enhancement for Alexa790 emitters around a Ag nanodisc (Diameter = 140 nm, Height = 40 nm) on VO₂ (Thickness = 15 nm) are presented in Fig. 5. Figure 5(b) shows the ratio of the excitation rate modification in metallic state VO₂ relative to the semiconducting state. For almost all emitter positions an enhancement in the modified rate can be seen with the highest enhancement factor corresponding to emitters placed at the upper edge of the Ag nanodisc. Averaged over the 15 positions an enhancement factor of 2.16 can be seen.

 

Fig. 5. (a) Numbered schematic of emitters positions around Ag nanodisc (b) Ratio ${\gamma _{Exc}}(M )/{\gamma _{Exc}}({SC} )$ for an Ag nanodisc (Diameter = 140 nm, Height = 40 nm) on VO₂ (Thickness = 15 nm). The black line indicates a mean ${\gamma _{Exc}}(M )/{\gamma _{Exc}}({SC} )$ of 2.16 over the 15 emitter positions (c) Ratio ${g_{em}}(M )/{g_{em}}({SC} )$ for Alexa790 emitters coupled to Ag nanodisc (Diameter = 140 nm, Height = 40 nm) on VO₂ (Thickness = 15 nm). The black line indicates the mean ${g_{em}}(M )/{g_{em}}({SC} )$ of 1.04 over the 15 emitter positions.

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In contrast the ratio of the modified emission rates, shown in Fig. 5(b) demonstrates quenching or enhancement depending on the dipole position. The dipoles closest to the VO₂ film are most strongly quenched, however those at least 5 nm from the surface of the Ag nanodisc and VO₂ film experience an increase by a factor of approximately 1.4. The average ratio over all dipole positions is approximately 1. Comparison of Figs. 5(b) and 5(c) clearly demonstrating that the main contribution to the PL emission enhancement arises from the increased excitation rate for metallic phase VO₂.

3. Conclusion

In this work, we have considered controlling the emission of quantum emitters in the wavelength range of ∼650–850 nm by tuning the plasmon resonance of noble metal nanostructures using the phase transition of thin film VO₂. In particular, we consider a range of dyes, taking account of their excitation and emission properties. In our system we observe that PL ratios of up to 4 can be achieved for Ag nanodiscs on thin film VO₂. It is noted that these enhancements are sufficient to compensate for thermal luminescent quenching and to yield an increase in PL even if a thermally induced VO₂ phase transition is employed. Through optimization of the structure parameters, the proposed system can compensate for up to 70% temperature induced PL intensity decrease. This can also be relevant for applications in the field of dye synthesized solar cells and solar concentrators where elevated temperatures can significantly reduce efficiencies. Ag nanoparticles of different diameters allow dye molecules with different emission wavelengths to be optimally enhanced, with Ag nanodiscs on 30 nm VO₂ films a promising candidate. Additional tuning of the VO₂ critical temperature through strain or doping with high valence metal ions, for example, can allow for large observed emission enhancement at lower temperatures, where the impact of thermal quenching is lessened. Our systems are also of interest in applications where the VO₂ phase change is induced electrically or optically. In such applications, if ohmic heating effects are negligible, the large range for dynamic tuning of PL enhancement can be exploited.