Hybrid Mushroom Nanoantenna for Fluorescence Enhancement by

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Hybrid Mushroom Nanoantenna for Fluorescence Enhancement by Matching the Stokes Shift of the Emitter Song Sun, Ru Li, Mo Li, Qingguo Du, Ping Bai, and Ching Eng Png J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01978 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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The Journal of Physical Chemistry

Hybrid Mushroom Nanoantenna for Fluorescence Enhancement by Matching the Stokes Shift of the Emitter Song Sun,1,2*, Ru Li1,2, Mo Li,1,2, Qingguo Du3,4, Ching Eng Png,5 Ping Bai5* 1

Microsystem & Terahertz Research Center, China Academy of Engineering Physics,

No.596, Yinhe Road, Shuangliu, Chengdu, China 610200. 2

Insititute of Electronic Engineering, China Academy of Engineering Physics, Mianyang, China

621999. 3

School of Information Engineering, Wuhan University of Technology, 122 Luo Shi Road,

Wuhan, Hubei, China 430070. 4

Key Laboratory of Fiber Optic Sensing Technology and Information Processing (Wuhan

University of Technology), Ministry of Education, Wuhan, Hubei, China 430070. 5

A*STAR Institute of High Performance Computing, Electronics and Photonics Department, 1

Fusionopolis Way, #16-16 Connexis (North), Singapore 138632.

ACS Paragon Plus Environment

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ABSTRACT. Nanoantenna-enhanced fluorescence is a promising method in many emergent applications, such as single molecule detection. The excitation and emission wavelengths of emitters can be well separated depending on the corresponding Stokes shifts, preventing optimal fluorescence enhancement by a rudimentary nanoantenna. We illustrate a hybrid mushroom nanoantenna that can achieve overall enhancements (e.g., excitation rate, quantum yield, fluorescence enhancement) in fluorescence emission. The nanoantenna is made of a plasmonic metal stipe and a dielectric cap, and the resonances can be flexibly and independently controlled to match the Stokes shift of the emitter. By fully leveraging the advantages of the large field enhancement from the metal and the low loss feature from the dielectric, a fluorescence enhancement factor (far field intensity) twice (20 times) as high as that from a pure metallic antenna, accompanied by improved directivity. Approximately, 70% of the overall radiation was directed toward the mushroom cap via coupling to the dielectric resonance, which could benefit the collection efficiency. This hybrid concept introduces a way to build high-performance nanoantennas for fluorescence enhancement applications.


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1. INTRODUCTION Based on the unrivalled advantages of selective labelling and the ability to interact with objectives at a molecular or atomic scale, fluorescence spectroscopy and related detection techniques serve as some of the most enabling methods in sensing, imaging and tracking, demonstrating excellent reliability, fast responses and reproducible results.1,2 Driven by recent advances in emergent applications such as DNA sequencing,3,4 explosive gas monitoring,5 food safety,6 and early diagnosis,7 among others, the detection of targeted species at the single molecule level becomes extremely important, thereby challenging the capabilities of routinely used fluorescence assays, whose optical signals are generally too weak to be measured at low concentrations.8-10 As such, a reliable approach to magnify fluorescence emissions has fascinated the scientific community for decades. Originating from the near-field microscopy community, the development of nanoantennas offers a viable solution to manipulate and control the fluorescence emitter in the deep subwavelength regime.11-14 Due to excellent optical confinement, the nanoantenna can establish a strong local electromagnetic field at the resonance frequency in the vicinity of the antenna surface, which can be exploited to boost the excitation rate of a fluorescence emitter.15-17 Once excited, the emitter subsequently releases a photon that can in turn couple with the resonant mode of the nanoantenna to further reinforce the radiative decay rate, which would be reflected in the far-field intensity.18 Moreover, the presence of a nanoantenna naturally alters the surrounding environment of the emitter, thereby enabling the directivity of fluorescence emission to be engineered and improving the photon collection efficiency at the detector end.19 Combining these advantages, extraordinary fluorescence enhancements20-22 and highly directional far field radiation patterns23-25 have been achieved with various forms of nanoantennas.


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Despite these promising features, the full potential of nanoantennas is yet to be realized. Two sources of hindrance are consistently encountered. 1) Choice of materials.18 Noble metals can produce large field enhancements to excite fluorescence emitters due to strong electron oscillation so called surface plasmon resonances (SPR). However, high intrinsic dissipation leads to an inevitably high non-radiative decay rate that compromises the ultimate fluorescence enhancement.26-28 Alternatively, the high permittivity low dissipation dielectric naturally suppresses non-radiative decay channels. Nevertheless, it possesses a relatively weaker field enhancement compared with the noble metal.29-32 Such a paradox is potentially resolvable with the recently proposed hybrid nanoantennas, which combine the advantages of both the metal and dielectric.33-35 2) Ability to simultaneously enhance the excitation rate, quantum yield, and directivity of the emitter.24 As a benchmark toward which to strive in both theory and experiment, this criterion essentially requires the nanoantenna to resonantly couple with the emitter at both excitation and emission wavelengths, and it is particularly crucial for an emitter with a large Stokes shift (excitation wavelength and emission wavelength are well separated).1 The degree of complexity is further increased once the selective-coupling property of the emitter is considered, because the nature (e.g., electric or magnetic) of the resonance modes of the nanoantenna can also influence the fluorescence intensity.18,36,37 To our best knowledge, a delicately designed nanoantenna that can simultaneously satisfy a high excitation rate, low dissipation, and good collection efficiency still has not been reported in the literature. In this contribution, we proposed a novel hybrid mushroom nanoantenna that was judiciously designed to simultaneously enhance the excitation, emission and directivity of the emitter. The mushroom nanoantenna was made of a plasmonic metal stipe and a low loss dielectric cap. The former was designed to resonate at the excitation wavelength of the emitter to produce a large


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field enhancement, while the latter was controlled to resonate at the emission wavelength of the emitter to enhance the radiative decay rate while maintaining high quantum yield. The unique mushroom structure also improved the directivity of the emission. The natures of the resonant modes were rigorously analyzed with the multipole decomposition theory,38-41 which was exploited to examine the selective-coupling between the emitter and nanoantenna. The far-field radiation pattern was carefully studied not only for a single fluorescence emitter to explore the physical insight, but also for an assembly of random-oriented incoherent emitters via the reciprocity principle42-44 to better mimic realistic situations. The resultant excitation rate, quantum yield, radiative/non-radiative decay rate, fluorescence enhancement factor and collection efficiency were thoroughly investigated to evaluate the performance of the hybrid mushroom nanoantenna.

2. METHODS Nanoantenna enhanced fluorescence. Under a weak coupling assumption (e.g., donor and acceptor are distinguishable and energy backtransfer is unlikely to occur),45 the overall fluorescence enhancement ηem/η0em from a nanoantenna can be legitimately approximated as the product of the excitation rate enhancement γext/γ0ext and quantum yield enhancement q/q0:18,27,28

= ∙

(1)

where the superscript “0” represents the respective quantity in a homogeneous medium. The fluorescence emitter is routinely modeled as a two-level system and mathematically expressed as an oscillating dipole. Below saturation, the excitation rate can be neatly expressed as ∝ | ∙ |, where is the dipole momentum and E(λext) is the nanoantenna enhanced local electric field at the position of the emitter illuminated by a plane wave at the excitation


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wavelength λext. Subsequently after excitation, the quantum yield q is used to monitor the competition between the radiative decay rate Γr and non-radiative decay rate Γnr of the dipole emitter at the emission wavelength λem, thereby giving the probability that an emitted photon can actually be collected at the far field. Within the validity of Fermi’s golden rule, q is determined by the weighted sum of all possible decay channels as: =

⁄ ⁄ ⁄ ⁄

(2)

where q0 is the intrinsic quantum yield of the emitter, which is assumed to equal 1 to objectively evaluate the merit of the nanoantenna regardless of the choice of the emitter.46 Note that within the weak coupling regime, the excitation rate and quantum yield are treated independently since there is no coherence between the two processes. The classical framework adopted here gives the same results as that of the full quantum electrodynamic (QED) model, and the fidelity has been well demonstrated in experiments.11,24,27 Multipole decomposition for an arbitrary shape nanostructure. In a study of the interactions between an emitter and nanoantenna, it is rather important to understand the nature of the resonances in the nanostructure. Depending on the orientation, the emitter selectively couples to a certain mode of the nanoantenna.18,31,36 For a simple spherical nanoantenna, Mie theory can provide an adequate analytical interpretation on the nature of each resonant peak observed in the extinction spectrum.47,48 However in a more general case of arbitrary shape nanoantenna, the electromagnetic field cannot be perfectly decomposed into spherical harmonics, thereby compromising the applicability of Mie theory. To solve this problem, we implemented the multipole decomposition method which provides multipole expansion coefficients by introducing a set of electric current multipoles.38,39 Any type of resonance (e.g., electric/magnetic


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dipole, quadrupole) can thus be rigorously described as a linear superposition of these current multipoles. The lth-order current multipole moment is expressed as: !

()* … & 234 = !# $ %& &&

(3)

+, ./&01

where M(l) is a tensor of rank l, and ω is the angular frequency. %& = −iω89 [8; & − 8< ]4 is the scattering current density, where ε0, εr(r) and εm are the permittivities of vacuum, nanoantenna and environment medium respectively. E(r) is the total electric field inside the nanostructure generated by an incident plane wave. The resultant extinction cross-sections (CS) of various resonances can be obtained straightforwardly as: ?

> = − A ∑NM ∑

Hybrid Mushroom Nanoantenna for Fluorescence Enhancement by

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