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Plasmon-based fluorescence modulation has led to important advances in various fields and has paved the way toward promising scientific research aimed at enabling new applications. However, the modulation of fluorescence properties based on both localized surface plasmon (LSP) and cavity modes of propagating surface plasmon polaritons (SPPs) are rarely reported. Here, we raster scanned a hybrid nanowire (HNW) with quantum dots (QDs) adsorbed onto a Ag nanowire (NW) and obtained two-photon fluorescence (TPF) maps of the intensity and decay rate. The spatial distributions of the intensity and decay rate strongly depend on the Fabry-Pérot (FP) cavity modes of the SPPs, the LSP mode launched by the incident laser and the excitation energy of the QDs. A double exponential decay process was observed, which is attributed to different decay channels through the LSP and cavity modes. The experimental results are explained using numerical simulations. This work shows that many physical parameters, such as the polarization of the incident beam and the geometry of the Ag NW, can modulate the fluorescence properties of the QDs, which has potential applications in many important fields.
© 2016 Optical Society of America
1. Introduction
Plasmon nanostructures (PNs) have attracted significant attention because they possess tremendous potential for various applications including optical information processing beyond the diffraction limit [1,2], optical antennas [3–5], and surface-enhanced Raman scattering [6,7]. The capabilities of PNs, such as squeezing electromagnetic waves into nanoscale regions near the metal surface and thus altering the properties of neighbor emitters, open the prospect of finely controlling and tailoring light–matter interactions at the nanoscale [8–15]. A variety of theoretical and experimental investigations have shown that PNs placed in close proximity to fluorescent particles can result in considerably modified fluorescence properties. To modify the fluorescence properties, one can take advantage of the localized surface plasmon (LSP) mode or propagating surface plasmon polariton (SPP) modes in the plasmonic cavity. Fluorescence intensity and lifetime modifications by LSPs have been observed in the coupled system of single PNs with one or numerous emitters [12–14,16–19], as well as in aggregates of PNs and emitters [11,20,21]. Furthermore, several phenomena related to the LSP mode, such as changes in the fluorescence spectra and polarization, have also been theoretically analyzed and experimentally demonstrated [22–24]. In addition, plasmonic cavities can effectively enhance or suppress spontaneous emission of emitters that are located around the cavities. Various methods, such as changing the topology of plasmonic cavities and the polarization and wavelength of the incident light, have been used to tailor fluorescence properties, including fluorescence intensity, lifetime and polarization [25–28]. In particular, fluorescence radiation patterns have been substantially tuned by exciting different orders of plasmonic cavity modes [29–31].
Ag nanowires (NWs) have proven to be key elements in subwavelength optical waveguides and are also used to modulate the florescence of nearby quantum dots (QDs). Single plasmon generation [32–34], local field imaging and exciton−plasmon−photon conversion have been demonstrated [35–40]. Fluorescence spectra and lifetimes have been shown to be modified by the Fabry-Pérot (FP) cavity modes of Ag NWs [38,40]. By tuning the relative positions of emitters along the longitudinal axis of a Ag NW, the spontaneous emission of QDs and images of the local density of states (LDOS) of Ag NWs have been obtained [41,42]. In a Ag NW, LSP and FP cavity modes can be launched by both the incident excitation beam and the excitation energy of the QDs, which provides abundant physical parameters to control the fluorescence. However, this has not been sufficiently discussed. Furthermore, two-photon luminescence tailored by the plasmonic cavity modes can reveal the LDOS of plasmonic cavities [43–45], and two-photon absorption of emitters can be significantly enhanced by PNs [46,47]. Compared to the single-photon excitation, the fluorescence induced by two-photon absorption is a nonlinear emission process. Thus, it is of great interest for bioimaging applications and three-dimensional display technologies [19,48,49]. The control of the spontaneous emission of two-photon fluorescence (TPF) via plasmonic cavity modes has not yet been reported.
Here, we measure maps of the intensities and decay rates of the TPF of CdSe/ZnS QDs adsorbed onto a Ag NW by raster scanning the focus of an excitation laser with a wavelength of 800 nm. Both the LSP mode and the FP cavity modes of the SPPs launched by the incident laser, as well as the polarization of the laser, are critical to the fluorescence properties of the QDs. The energy released by excitonic recombination in the QDs has two SPP generation channels: couplings between the QDs and the propagating SPPs or the LSP of the Ag NW, which show different intensity distributions and decay rates. The results present a new way to modulate the TPF properties based on both the FP cavity and LSP modes excited by the incident laser and the QDs.
2. The measurements of spectra and emission dynamics
Single crystalline Ag NWs with a diameter of approximately 200 nm and a length ranging from one to several tens of micrometers were synthesized using a modified polyol method as described previously [50]. The prepared Ag NWs were washed several times with ethanol to remove Ag nanoparticles and then dispersed in ethanol for silica (SiO2) coating [51]. Based on previous reports, commercially available stabilized CdSe/ZnS core/shell QDs with a central emission peak at 630 nm were diluted and then added dropwise with vigorous stirring to attach onto the SiO2 shell [11,39,46]. After being washed with tetrahydrofuran (THF) to remove the supernatant, which mainly contained unattached QDs, the obtained samples were dispersed in THF for subsequent experiments. Figure 1(a) shows a transmission electron microscope (TEM) image of a representative hybrid nanowire (HNW) at low magnification. Large quantities of QDs can clearly be seen adsorbed onto the SiO2 shell. The inner diameter of the Ag NW and the thickness of the SiO2 shell are approximately 200 nm and 20 nm, respectively. The high magnification TEM image in the inset of Fig. 1(a) shows the lattice fringes of the QDs. Figure 1(b) illustrates the schematic diagram of the experimental configuration based on an inverted microscope. The HNWs were deposited on a clean cover slip, which was fixed on a three-dimensional piezoelectric transition (PZT) stage with nanometer accuracy. The light source was a Ti:sapphire femtosecond laser (76 MHz repetition rate and 130 fs pulse width) with a wavelength of 800 nm, and the polarization was controlled by a half-wave plate. The laser beam was expanded and focused onto the HNW through an oil immersion objective (100 × , NA = 1.4). The average power of the incident laser beam was kept in the range 100-300 μW. The TPF of the HNW was collected using the same objective and sent to a CCD camera for imaging, a spectrometer for spectral measurements, or an avalanche photodiode for measurements of the TPF intensity and decay rate. In the measurement system, a shortpass filter (cutoff at 650 nm) was used to remove the excitation light. The TPF intensity and decay rate maps were acquired using a time correlated single photon counting instrument (PicoHarp 300) operating in the time-tagged−time-resolved mode. The PZT stage was used to raster scan an individual HNW in the y-z plane; the step size of the raster scanning was 50 nm. Each pixel value in the maps represents the intensity or radiative decay rate of the TPF from the entire HNW. The two-photon luminescence cannot only locate with subdiffraction resolution but also image the SPP LDOS inside the metallic nanostructures. More importantly, the TPF can probe all of the planar components of the LDOS rather than one component in a particular direction and does not produce an additional intrinsic response (like SNOM probes) that alters the study results [43].
Fig. 1 Sample, experimental setup and the measurements of spectra and emission dynamics. (a) TEM image of a representative HNW. The HNW with Ag NW core (200 nm in diameter) and SiO2 shell (20 nm in thickness) is clearly observed, and the black dots represent adsorbent CdSe/ZnO QDs. The inset displays high-magnification showing lattice fringes of the QDs. The scale bar in (a) and inset is 200 nm and 5 nm, respectively. (b) Schematic description of the experimental setup. (c) Normalized fluorescence spectra of the pure QDs (solid red line), an individual HNW (solid green line) and Ag NW (solid blue line). The inset shows the spectrum of laser light. (d) Normalized fluorescence decay of an individual HNW: the red dots and green line represent the experimental data and the fitting curve, respectively. Two components are identified in the decay including a fast component of lifetime τ1 = 12 ps and a slow component of lifetime τ2 = 0.219 ns. The inset shows the decay of pure QDs (lifetime τ0 = 6.43 ns). The red squares and blue line represent the experimental data and the fitting curve, respectively.
The experiment was performed on an HNW with a length of 2.8 μm. Two orthogonal polarizations of the excitation laser, parallel and vertical to the HNW, were used. When the laser beam was focused on the left end of the HNW with a vertical polarization, the TPF spectrum was obtained [Fig. 1(c)]. The cut off at 650 nm originates from the shortpass filter. For comparison, the TPF spectra of pure QDs and a Ag NW with the same excitation power are also shown in Fig. 1(c). The fluorescence of the HNW is obviously modified compared with that of the pure QDs due to the presence of the PNs. The fluorescence of the Ag NW is very weak and can be ignored. The normalized TPF decay curve of the HNW is depicted in Fig. 1(d), which shows a double exponential decay process. The fitting contains two components: a fast component (lifetime τ1 = 12 ps) and a slow component (lifetime τ2 = 0.219 ns). After the lifetime of the fast decay channel, the population of the excited state decreases to 1/e of its initial value at t = 0 through the fast decay channel without the slow decay channel. The fast decay channel cannot deplete all the QDs in a time that is much shorter than the lifetime of the slow decay channel. As a result, both the two decay channels contribute to the relaxation process. For comparison, the lifetime of the pure QDs is 6.43 ns, which is shown in the inset of Fig. 1(d). It is worthy to mention that the response function of the TCSPC system is 25ps, which is larger than the lifetime of the fast component. As a result, the real value of the lifetime of the fast decay channel should be smaller than the measured one.
3. Fluorescence intensity and decay rate mappings with parallel excitation polarization
We raster scanned the HNW to obtain the two-dimensional distributions of the TPF intensity and decay rate tailored by the Ag NW, and the results with parallel polarization of the excitation laser are shown in Fig. 2. The TPF from the whole HNW was collected, and its polarization was not specialized. The map of the TPF intensity is shown in Fig. 2(a). The distribution range of the pattern along the y direction is much larger than the diameter of the HNW, and a part of the pattern is arc-shaped. Such a pattern should not originate from the FP cavity modes of the propagating SPPs. The intensity distribution along the symmetrical axis of the HNW is plotted in Fig. 2(b). It is seen that the patterns near the two ends show different periods than that in the middle, which may originate from the strong near-field interaction between the focused incident beam and the sharp ends of the Ag NW. Here, we just fit the middle part of the curve in Fig. 2(b) using a sine function as the blue line and obtain a period of 340.8 nm.
Fig. 2 Fluorescence intensity and decay rate mappings with parallel excitation polarization. (a) The map of normalized fluorescence intensity with parallel excitation polarization. The white double-arrows at left corner indicate the excitation polarization. (b) Plot of fluorescence intensity as a function of position along the HNW. (c) The total energy () of the SPPs and light on the HNW resulting from the incident beam with respect to its excitation position calculated by FDTD. (d) and (e) Maps of the fast and slow decay rate 1/τ1 and 1/τ2, respectively. (f) Line profile along the symmetric axis of the HNW in (e). The red dots in (b) and (f) represent the experimental data along the HNW, and blue lines are the fitting curves.
To understand the origin of the TPF pattern, we calculated the total energy on the HNW resulting from the incident laser with respect to its focusing position using the finite difference time domain (FDTD) method. In the calculation, the light source was a Gaussian beam that was focused by an objective with an NA of 1.4. The period of the pattern is 320.5 nm, which agrees with the experimental result 340.8 nm in Fig. 2(b). The pattern also shows nonlocalized characteristics similar to the pattern of the TPF in Fig. 2(a). Because the incident laser has a parallel polarization, it launches propagating SPPs at the two ends of the Ag NW; therefore, the intensities depend on the distances between the incident point and the two ends. The propagating SPPs with a vacuum wavelength of 800 nm have two waveguide modes, TM0 and TM1, for which transverse mode patterns with effective indices of 1.48 and 1.07, respectively. The interference of the SPPs launched at the two ends determines the pattern of the total energy on the Ag NW, which has a period of half the effective wavelength of the SPPs. Considering that the TM1 mode has an effective wavelength of 747.7 nm, the intensity pattern mainly originates from the launching of the TM1 mode SPPs. The agreement between the excitation intensity pattern and the experimental TPF pattern, especially their nonlocalized characteristics, demonstrates that the latter mainly depends on the total excitation energy rather than the SPP LDOS of the HNW at the fluorescence wavelength. However, when the incident point is near the two ends, the period of the experimental TPF pattern is much larger than that of the excitation intensity pattern, and it should depend on the SPP LDOS of the Ag NW.
The relaxation process of the TPF always shows a double exponential decay process with fast and slow decay rates 1/τ1 and 1/τ2, respectively. The fast component dominates the decay process, accounting for approximately 89% of the initial decrease in amplitude. Figures 2(d) and 2(e) show the maps of the fast and slow decay rates 1/τ1 and 1/τ2, respectively. No clear pattern can be found in the fast decay rate map [Fig. 2(d)]. The average decay rate of the fast component is 5.00 × 1010 s−1, corresponding to lifetimes of 20 ps. Compared to the decay rate of the pure QDs, the coupling between the Ag NW and the QDs results in decay rate enhancement with an enhancement factor of 322. The real fast decay rate should be faster than the measure value due to the limit of the response function of the TCSPC system. Thus, the enhancement factor of 322 is the lower limit. Figure 2(e) plots the map of the slow decay rate, which has a very similar pattern with that of the TPF intensity [Fig. 2(a)]. The middle part of the line profile along the symmetric axis of the HNW shows a period of 336.9 nm [the blue line in Fig. 2(f)], which is almost the same as that in Fig. 2(b). The sine fit of the lifetime of the slow component in the middle of the HNW along the axis shows an averaged lifetime of 0.86 ns with an amplitude of 0.31 ns.
Energy released by the excitonic recombination in the excited QDs can be converted and carried away by the LSPs and propagating SPPs. The field enhancement of the two modes benefits the TPF. We calculated the dipole resonant properties of the LSP and waveguide modes of the HNW with the same geometry in the experiment using the FDTD method. The patterns of the LSP mode at 630 nm and 800 nm are shown in Figs. 3(a) and 3(b), respectively. There are three SPP waveguide modes of the HNW for a vacuum wavelength of 630 nm, TM0, TM1, and TM2, for which transverse mode patterns are shown in Figs. 3(c)-3(e) with effective refractive indices of 1.53, 1.15, and 1.07, respectively. Because the lifetime of the TPF is much shorter than that of the pure QDs, the enhanced fluorescence signal should originate from the coupling between the QDs and the LSP and propagating SPP modes. The LSP mode has a much stronger confinement and field enhancement than the propagating SPP modes. Due to the Purcell effect, the LSP mode should result in a higher decay rate [52]. Thus, we attribute the fast and slow decay processes of the TPF to the decay channels of the LSP and propagating SPP modes, respectively. The fast decay process is the dominant decay channel because it has a higher decay rate. The reason that the fast decay process has no pattern is that the contribution of the LSP mode to the fluorescence enhancement is the same when the laser spot is incident upon different position on the z axis.
Fig. 3 Patterns of the dipole mode at 630 nm and 800 nm and transverse mode patterns of the HNW at the fluorescence wavelength. (a) and (b) Patterns of the dipole modes () at 630 nm and 800 nm, respectively. The light source in (a) and (b) is plane wave and the polarization is along the y axis. (c)-(e) Transverse mode patterns () of the TM0, TM1, and TM2 modes for a vacuum wavelength of 630 nm in the HNW, respectively. The light sources in (c)-(d) are TM0, TM1, and TM2 mode SPP source, respectively.
The Ag NW acts as an FP cavity for the propagating SPP modes, and the intensity distributions of the FP cavity modes for the TM0, TM1, and TM2 modes are also calculated. At the same time, the patterns of the FP cavity modes at the excitation wavelength also alter the TPF. In order to obtain the total QDs decay rates (Fig. 4), we calculated mode-dependent radiative decay rate enhancement factors at the fluorescence wavelength using the FDTD method for the HNW with the same geometry as the experiment. The mode-dependent radiative decay rate enhancement factor was obtained by multiplying the excitation enhancement factor at the excitation wavelength and the radiative decay rate enhancement factor at the fluorescence wavelength for each mode. The excitation enhancement factor is |E|4/|E0|4 for two-photon absorption, where |E| and |E0| are the electric field amplitudes with and without the Ag NW, respectively. The distributions of |E|2 of the FP cavity modes at the fluorescence wavelength are considered as the radiative decay rate enhancement factors of the QDs around the HNW. By multiplying the excitation enhancement factor and the radiative decay rate enhancement factor at each pixel in the calculated area, the excitation-weighted fluorescence enhancement as a spatially weighted average at each incident position was obtained. Figure 4(a)-4(c) show the maps of the normalized mode-dependent radiative decay rate enhancement for the TM0, TM1 and TM2 modes. Here, we assume that the three waveguide modes at the fluorescence wavelength have the same contribution to the total fluorescence enhancement, and their average is shown in Fig. 4(d), which is very similar to Fig. 2(e). Figure 4(e) is the line profile of Fig. 4(d) along the symmetric axis of the HNW, which shows a period of 330.6 nm in the middle part. It almost has the same period as that seen in the experiment [340.8 and 336.9 nm in Figs. 2(b) and 2(f), respectively]. The agreement between the experimental results and the simulation demonstrates that the launching of SPPs by the incident beam and their FP modes, as well as the LDOS of the Ag NW cavity at the florescence wavelength, dominate the patterns of the slow component of the decay process and the total TPF intensity. The incident position dependence of the decay rate reveals a new way to control fluorescence using different SPP cavity modes launched by the excitation light.
Fig. 4 FDTD calculated mode-dependent radiative decay rate enhancements and excitation-weighted fluorescence enhancement. (a)-(c) Normalized mode-dependent radiative decay rate enhancements of the TM0, TM1 and TM2 modes, respectively. (d) Excitation-weighted fluorescence enhancement. (e) Intensity distribution along the symmetrical axis of (d).
4. Fluorescence intensity and decay rate mappings with vertical excitation polarization
When the polarization of the incident beam is vertical to the HNW, the LSP mode is launched at 800 nm [Fig. 3(b)], which dominates the excitation of the QDs. The raster scan map of the TPF intensity is shown in Fig. 5(a). The white dashed line indicates the position of the HNW. The pattern is localized on the HNW and shows periodicity along the HNW. The intensity distribution along the symmetrical axis of the HNW is plotted in Fig. 5(b). The middle part of the line can be fitted by a sine function with a period of 299.6 nm, as shown by the blue line in Fig. 5(b). The period corresponds to an effective refractive index of 1.05 for a vacuum wavelength of 630 nm, which is in accordance with that of the TM2 mode [Fig. 3(e)] at 630 nm. The result shows that the FP cavity mode of the TM2 mode dominates the pattern of the TPF, although the three propagating SPP modes can be excited simultaneously. The intensity distribution of the LSP mode launched by the excitation laser overlaps very well with that of the TM2 mode at 630 nm [see Fig. 3(b) and 3(e)]. As a result, the TM2 mode is excited preferentially and the intensity pattern in Fig. 5(a) depends on the FP cavity mode of the TM2 waveguide mode. When the laser beam is incident near the two ends of the HNW, propagating SPPs at 800 nm can also be launched. Both the LSP and propagating SPP modes excite the TPF and make the intensity pattern of the TPF complicated. This is the reason why the intensity distributions of the TPF near the two ends of the HNW have a different period from that in the middle of the HNW. The temporal response of the TPF of the HNW also shows a double exponential decay process, and the fast component dominates the decay process, accounting for approximately 86% of the initial decrease in amplitude.
Fig. 5 Fluorescence intensity and decay rate mappings with vertical excitation polarization. (a) Map of normalized fluorescence intensity with vertical excitation polarization. The white double-arrows at left corner indicate the excitation polarization. (b) Plot of the fluorescence intensity as a function of position along the axis of the HNW. The red dots in (b) represent the experimental data, and blue line in (b) is the fitting curve. (c) and (d) are the maps of the fast decay rate 1/τ1 and slow decay rate 1/τ2, respectively. The black dashed line indicates approximate size and location of the HNW.
Figure 5(c) and 5(d) show the maps of the fast and slow decay rates 1/τ1 and 1/τ2, respectively. The average decay rates of the fast and slow components are 5.26 × 1010 and 4.26 × 109 s−1, corresponding to lifetimes of 19 ps and 0.235 ns, respectively. There is no clear pattern found in the maps. Here, we also attribute the fast and slow components to the couplings between the QDs and the LSP and propagating SPP modes of the HNW, respectively. Unlike the map of the total TPF intensity, which shows a clear pattern, a clear spatial dependence of the decay rate was not observed in the experiment. To understand the origin of the unclear TPF decay rate pattern, we also calculated excitation-weighted fluorescence enhancement. Figure 6(a) shows the total energy distribution along the symmetrical axis of the HNW at the excitation wavelength, which shows a period of 380.9 nm. Considering that the effective wavelength of the TM1 waveguide mode at the excitation wavelength is 747.7 nm, the total energy distribution on the HNW is modulated by the FP cavity mode of the TM1 waveguide mode. We multiplied the excitation enhancement factor and the decay rate enhancement factor and obtained the mode-dependent decay rate enhancements for the TM0, TM1 and TM2 modes [Figs. 6(b)-6(d)]. The calculated fluorescence enhancement of the TM1 mode in Fig. 6(c) shows a very small fluctuation in the middle of the HNW. Considering that the averaged lifetime of the slow component is much smaller than that of the parallel polarization case, the variation of the lifetime based on the LDOS of the TM1 mode may be smaller than the response function of the TCSPC system. As a result, no clear pattern was observed in the experiment.
Fig. 6 (a) Normalized total energy distribution on the HNW when the incident point is on the symmetrical axis of the HNW. (b)-(d) are the normalized mode-dependent radiative decay rate enhancement factors for the TM0, TM1 and TM2 mode, respectively, when the incident point is on the symmetrical axis of the HNW.
When the incident laser has a vertical polarization, the QDs are mainly excited by the launched LSPs with a strong confinement. On the contrary, the QDs are excited by the propagating SPPs for the parallel polarization case. The different confinement properties of the excitation sources result in the large difference of the lifetimes of the slow components between the two different polarizations. No significant difference has been observed in the decay rates of the fast components for the vertical and parallel polarizations, and both the lifetimes are beyond the response function of the TCSPC system. The real decay rates of the fast components may be much faster than the measured value. Although the incident laser can excite TPF and the energy of the QDs can be converted into free-space photons directly without the involvement of the plasmon excitation process, these two processes were not clearly observed in the experiment. Excitation and relaxation of the QDs were dominated by a primary process of photon-plasmon-exciton-plasmon-photon conversion due to the strong enhancement of the TPF resulted by the Ag NW. The ability to modulate TPF properties based on different incident polarization provides an important method for controlling and optimizing light–matter interactions between QDs and plasmons.
5. Conclusion
In conclusion, we have measured the TPF intensity and decay rate maps of CdSe/ZnS QDs attached to a Ag NW excited by an 800 nm femtosecond laser beam. The launching of both the LSP and propagating SPP modes, as well as the FP cavity mode, by the excitation laser results in unique features of the TPF, which strongly depend on the polarization of the laser. The relaxation process of the TPF shows a double exponential decay process, in which the slow and fast components are attributed to the energy transfer between the QDs and the propagating SPP and LSP modes, respectively. When the excitation laser has a parallel polarization, it excites propagating SPPs, which makes TPF intensity and decay rate patterns that are different from those obtained with a vertical polarization. The incident polarization and position dependence of the intensity and decay rate of the TPF presents a very convenient way to control the TPF. As a result, other SPP cavities can also be designed to modulate the fluorescence properties and the energy transfer process between QDs or fluorescent molecules and SPP cavity modes. This method has promising applications in many fields, such as nanophotonic and plasmonic devices, plasmon rulers, and highly sensitive sensors.
Funding
Ministry of Science and Technology of China (Grant 2016YFA0301300); the National Natural Science Foundation of China (NSFC) (Grants 11574011, 61377050, 11374041, 11574035, 11074312 and 61575225); and by the State Key Laboratory of Information Photonics and Optical Communications.
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