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Efficient upper-excited state fluorescence in an organic hyperbolic metamaterial Yufei Shen, Yixin Yan, Alyssa N. Brigeman, Hoyeon Kim, and Noel C. Giebink Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04738 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018
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Nano Letters
Efficient upper-excited state fluorescence in an organic hyperbolic metamaterial Yufei Shen1,2, Yixin Yan1, Alyssa N. Brigeman1, Hoyeon Kim1, and Noel. C. Giebink1* 1. School of Electrical Engineering and Computer Science, The Pennsylvania State University, University Park, PA 16802 2. Department of Physics, The Pennsylvania State University, University Park, PA 16802 *email:
[email protected] Abstract Upper-excited state emission is not usually observed from molecules owing to competition with much faster non-radiative relaxation pathways; however, it can be made more efficient by modifying the photonic density of states to enhance the radiative decay rate. Here, we show that embedding the small molecule zinc tetraphenylporphyrin (ZnTPP) in a hyperbolic metamaterial enables an ~18-fold increase in fluorescence intensity from the second singlet excited state ( ) relative to that from the lowest singlet excited state (). By varying the number of periods in the HMM stack, we are able to systematically tune the ZnTPP fluorescence spectrum from red (dominated by emission from ) to blue (dominated by emission from ) with an instrumentlimited decay lifetime 1,000-fold) Purcell enhancements for molecular and quantum dot emitters.1-5 While most work to date has focused on modifying emission from the lowest excited state of a system (typically the lowest quantum dot exciton or the lowest singlet excited state, , in molecules), it is also possible to accelerate radiative transitions from upper-excited or 'hot' states as well.6-8 Upper-excited state emission is not usually observed owing to competition with much faster non-radiative relaxation pathways (a result commonly referred to as Kasha's rule);9, 10 however, by exploiting large Purcell enhancements, emission from these short-lived (~ps) states can be made efficient. Approached from this perspective, the challenge of achieving ultrafast luminescence shifts from making an efficient (lowest excited state) emitter faster to making an intrinsically fast (upper-excited state) emitter more efficient, which may constitute a more favorable trade-off in applications such as visible light communication that prioritize high bandwidth11, 12. Here, we show that embedding the organic small molecule zinc tetraphenylporphyrin (ZnTPP) in a metal-dielectric multilayer HMM enables an ~18-fold increase in emission from the second singlet excited state ( ) relative to that from the lowest singlet excited state (). By varying the number of periods in the HMM stack, we are able to systematically tune the ZnTPP fluorescence spectrum from red (dominated by emission from ) to blue (dominated by
emission from ) with an instrument-limited decay lifetime 370 nm and the structure exhibits hyperbolic dispersion over both the and ZnTPP transitions.
The Purcell enhancement experienced by an emitter in this structure is rigorously analyzed using a classical transfer matrix model for dipolar power dissipation, where the overall decay rate enhancement, %, for an emitter with quantum yield, , is given by18, 19: -
% = 1 − ! + Re ( ) *, , ,!, .
(1)
Here, ) is the wavevector-resolved local photonic density of states (WLDOS) for a dipole located at position * within a given ZnTPP:PMMA layer and , = / ⁄/ is the in-plane
component of the wavevector normalized to its magnitude, / , in the emitting layer. The
expression for ) follows from Ref. [19] and implicitly depends on the s- and p-polarized Fresnel
reflection coefficients of each layer interface in the HMM stack as well as the orientation of the emitting dipole. Figure 2(a) shows the WLDOS calculated for a randomly oriented emitter located in the middle of the uppermost ZnTPP:PMMA layer in a six period HMM stack with = 25 and
= 30 nm (see Fig. 1(b)). The six bright bands correspond to propagating modes within the HMM that result from hybridization of the individual surface plasmon polariton (SPP) modes associated with each metal-dielectric interface. Owing to their high in-plane momentum (i.e. relative to that in free space, / ⁄/ > 1), all of these modes are evanescent and must be
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Nano Letters
scattered out for observation in the far field. The range of in-plane momenta that they span increases with decreasing HMM layer thickness and the number of modes scales directly with the number of periods in the stack.18 In addition to propagating HMM modes, dipoles located very close to the Ag layers may couple to high-k lossy surface waves, which leads to strong quenching. This is evident in the WLDOS of Fig. 2(b) as a broad feature that emerges at high wavevector (, > 10) for dipole positions within ~1 nm of a Ag layer. Quenching is therefore insignificant for the majority of emitters in each ZnTPP:PMMA layer and their average Purcell factor (1 , represented by the WLDOS integral in Eqn. (1)) shown in Fig. 2(c) is dominated by emission into the propagating HMM modes. There, 1 is presented separately for the horizontal and vertical dipole basis orientations (i.e. in-plane and perpendicular to the layers, respectively) that make up the isotropic
average, 1234 = 16 7 + 1847 , describing a randomly oriented dipole. 5 5 The peak at ~365 nm for both vertical and horizontal dipole orientations (corresponding to the bright horizontal band in Fig. 2(a)) marks the surface plasmon frequency of the Ag-PMMA interface, : = : ⁄; + 1, where : is the bulk plasma frequency of Ag. A broadband Purcell enhancement is observed at longer wavelengths in the hyperbolic regime, where 1, ~32 and 1, ~35 are predicted for and emission from randomlyoriented ZnTPP molecules, respectively. The Purcell enhancement is much larger for vertical relative to horizontally-oriented dipoles because the former radiate exclusively into p-polarized modes, which dominate the HMM modal spectrum (all of the HMM-hybridized SPP modes are p-polarized), whereas the latter couple more strongly to s-polarized modes.
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Figure 3(a) depicts the experimental setup used to characterize the HMM samples, where emission is collected using an inverted microscope with an NA=0.3 objective and the = 355 nm excitation beam is incident obliquely on the sample surface to avoid autofluorescence from the microscope optical train. Figure 3(b) shows the fluorescence spectra collected for a series of samples with a varying number of HMM periods and a top Ag layer with naturally-occurring nanoscale surface roughness to scatter out high-k modes as discussed below. Relative to a bare 30 nm thick ZnTPP:PMMA reference film on Si, the ratio of to emission intensity systematically increases with the number of HMM periods, reaching an 18-fold enhancement for the six-period HMM and shifting the fluorescence hue from red to blue in the process. Figure 3(c) and 3(d) respectively show the fluorescence transient decay of and
emission recorded using a streak camera with ~20 ps ultraviolet excitation pulses ( = 310 nm, ~75 µJ/cm2 fluence). Relative to the decay of emission from the bare ZnTPP:PMMA film,
that from the six period HMM is accelerated by a factor of two (% = 2) at early times and is nonmonoexponential owing to the distribution of dipole positions within the HMM stack. By contrast, the emission from in Fig. 3(d) is ultrafast, with an instrument-limited lifetime
of excitation (B ), the radiative rate of the state (A7 ), its overall lifetime (?D E>?D ) of the HMM compared to that of the bare reference film (>?D,E>?D,) is therefore:
F=
>?D ⁄>?D
>?D,E>?D,
A7D, < , B4C ≈ G D, H G D H G H G H . < A7 B4C A7
A7
(2)
In Eqn. (2), < ⁄< , ≈ 1 since the lifetime of ZnTPP is strongly dominated by non-radiative
decay (AI7 ~660A7 based on a natural fluorescence quantum yield of ≈ 0.0015
determined above) and thus will not be significantly affected by the ~35x Purcell radiative rate enhancement predicted in Fig. 2(c). For the same reason, the HMM environment is not expected to appreciably alter the relative fraction of and states that are populated initially following
, , photon absorption (i.e. B B EB B ≈ 1). Finally, the outcoupling fractions for and
emission from the bare film are expected to be roughly equivalent, as verified by transfer matrix modeling. Using Eqn. (1) together with the fluorescence lifetimes extracted in Fig. 3(c) and the
fluorescence quantum yield determined above (D ≈ 0.03), we find a Purcell factor,
1 ≡ A1 EA1,0 ≈ 30, that agrees well with the prediction in Fig. 2(c). Plugging these results r r
into Eqn. (2) along with F = 18 for the six period HMM (determined from Fig. 3(b)) then leads , D ⁄ D !, to 1 ≡ A ≈ 270 B4C B4C which implies that blue emission must be outcoupled 7 EA7
roughly eight times more efficiently than red emission from the HMM if agreement with
1 ~35 from Fig. 2(c) is to hold.
Figure 4 rationalizes this outcoupling difference based on the rough Ag top surface of the HMM displayed in the atomic force micrograph of Fig. 4(a). There, the nucleation characteristics of the Ag lead to nanoscale roughness with lateral feature sizes on the order of 10-50 nm (see Fig.
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4(b)) that may plasmonically-enhance scattering (out of high-k HMM modes) in the 350 <
