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Nanocrescent antenna for nanofocusing of excitation radiation and generating asymmetric frequency upconverted emission Doyeon Bang, Eun-Jung Jo, SoonGweon Hong, JuYoung Byun, Jae Young Lee, Min-Gon Kim, and Luke P. Lee Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02327 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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Nanocrescent antenna for nanofocusing of excitation

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radiation and generating asymmetric frequency

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upconverted emission

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Doyeon Bang,† Eun-Jung Jo,‡ SoonGweon Hong,† Ju-Young Byun,†, ‡ Jae Young Lee,‡ Min-

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Gon Kim,*, ‡ and Luke P. Lee*,†, §

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†Berkeley Sensor and Actuator Center, Department of Bioengineering, University of California

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at Berkeley, California, 94720, USA.

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‡Department of Chemistry, School of Physics and Chemistry, Gwangju Institute of Science and

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Technology (GIST), Gwangju 500-712, Republic of Korea

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§Department of Electrical Engineering and Computer Science, Biophysics Graduate Program,

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University of California at Berkeley, California, 94720, USA.

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ABSTRACT

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Frequency upconversion activated with Lanthanide has attracted attention in various real-

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world applications, because it is far simpler and more efficient than traditional nonlinear

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susceptibility-based frequency upconversion, such as second harmonic generation. However, the

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quantum yield of frequency upconversion of Lanthanide-based upconversion nanoparticles

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remains inefficient for practical applications, and spatial control of upconverted emission is not

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yet developed. To overcome this limitation, we developed asymmetric hetero-plasmonic

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nanoparticles (AHPNs) consisting of plasmonic antennae in nanocrescent shapes on the

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Lanthanide-based upconversion nanoparticle (UC) for efficiently delivering excitation light to

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the UC core by nanofocusing of light and generating asymmetric frequency upconverted

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emission concentrated toward the tip region. AHPNs were fabricated by high-angle deposition

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(60º) of gold (Au) on the isolated upconversion nanoparticles supported by nanopillars then

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moved to refractive-index matched substrate for orientation-dependent upconversion

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luminescence analysis in single-nanoparticle scale. We studied shape-dependent nanofocusing

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efficiency of nanocrescent antennae as a function of the tip-to-tip distance by modulating the

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deposition angle. Generation of asymmetric frequency upconverted emission toward the tip

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region was simulated by the asymmetric far-field radiation pattern of dipoles in the nanocrescent

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antenna and experimentally demonstrated by the orientation-dependent photon intensity of

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frequency upconverted emission of an AHPN. This finding provides a new way to improve

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frequency upconversion using an antenna, which locally increases the excitation light and

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generates the radiation power to certain directions for various applications.

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KEYWORDS

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upconversion, plasmonics, Lanthanide, nanocrescent, antenna

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Nano Letters

TEXT

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Frequency conversion has been an important topic in various fields from optics to biophysics

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due to the possibility of revolutionizing multiple applications. Frequency upconversion, which is

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a multi-photon process emitting photons with higher frequencies than that of excitation light has

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drawn significant attention since frequency upconversion of near-infrared to visible or visible-

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infrared is promising for biomedical imaging and solar cell application. However, traditional

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frequency upconversion requires a complicated setup, such as symmetry and phase matching and

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a high power pulsed laser, because this nonlinear susceptibility-based frequency upconversion is

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involved with the virtual energy state.1 In contrast, frequency upconversion based on Lanthanide

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is not conducted by a single nonlinear process, but continuous processes carried out by real

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intermediate energy states.2, 3 Therefore, the upconversion process is much more efficient and

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can be performed using a simple experimental setup, such as a low power continuous-wave laser.

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In this system, frequency upconversion phosphors, such as Er3+ or Tm3+ ions are spread in a

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NaYF4 crystal structure and emit light with very sharp peaks located between ultraviolet (UV)

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and near-infrared spectroscopy (NIR) light upon excitation by 980 nm NIR light. Location of the

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peak is conveniently controlled by modulation of the composition of frequency upconversion

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phosphor or introduction of co-dopant ions, such as Yb3+.4 Therefore, Lanthanide-based

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frequency upconversion is ideal for the real-world application with a simple setup. However, due

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to the small absorption cross section of Lanthanide ions, quantum efficiency of frequency

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upconversion still remains quite limited.5

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To overcome these limitations, several studies have been conducted in an attempt to enhance

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the quantum yield of frequency upconversion by using a plasmonic antenna to manipulate light-

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matter interaction at the nanoscale.6 Metals in an approximate location with light affect the local


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distribution of the electromagnetic field due to excitation of surface plasmons, which have been

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widely utilized for ultrasensitive molecular detection, functional microscopy, and optical

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nanoscopic antenna. These plasmon resonance based optical nanoscopic antenna have gained

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significant attention due to their unique ability to focus light into a sub-wavelength volume

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beyond the diffraction limit. There were a number of studies on enhancing frequency

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upconversion efficiency obtained using the plasmonic interaction between upconversion

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nanoparticles and metallic antennae. However, geometrical constraints, for example, randomly

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formed plasmonic antenna geometry formed by dropcasting or well-ordered plasmonic antenna

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structure, but fixed on the substrate limits’ practical application of plasmon enhanced frequency

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upconversion.7-9

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In order to overcome the above problems, we propose a novel approach for developing

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asymmetric hetero-plasmonic nanoparticles (AHPNs) that consist of plasmonic antennae in a

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nanocrescent shape on the Lanthanide-based upconversion nanoparticle for nanofocusing of

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excitation light to the upconversion nanoparticle and concentrating upconverted photon emission

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into a specific direction toward the nanocrescent antenna tip. According to the analytical solution

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of nanocrescent structure solved by transformation optics, which utilizes the covariant property

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of Maxwell’s equation for the conversion of spatial coordinates of the known field solution to

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complicated geometry, a hotspot emerges around the singular point (tip region) of the

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nanocrescent structure due to the propagation of electromagnetic waves as a form of surface

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plasmon polariton (SPP) toward the tip region, where their group velocity vanished and energy

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accumulated.10 Therefore, our group reported the first experimental demonstration of the

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fabrication of the nanocrescent structures for the application of ultrasensitive biomolecular

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detection.11 Then, we demonstrated changing of fluorescence intensity of nanocrescent antenna


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attached fluorescent polystyrene nanosphere according to the direction of the singular point.12

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However, we did not explain the origin of the difference of orientation dependent fluorescence

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intensity and also did not address the enhancement of fluorescence due to the nanocrescent

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structure. In this paper, we demonstrated nanofocusing of excitation light by near-field

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simulation of electric field distribution and head-to-head comparison of upconversion

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luminescence of AHPNs with different nanocrescent geometry in a single nanoparticle

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resolution. Also, the asymmetric pattern of upconverted photon emission was demonstrated by

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the asymmetric far-field radiation pattern of dipoles in the nanocrescent antenna and orientation-

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dependent upconversion luminescence measurement.

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Figure 1 shows the design strategy of the asymmetric plasmonic antenna for the nanofocusing

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of excitation light and concentration on frequency upconverted emission. We designed

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asymmetric hetero-plasmonic nanoparticles (AHPNs), which consist of the asymmetric

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plasmonic antenna in a nanocrescent shape on the upconversion nanoparticle. Nanocrescent

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antennae act as receiving antennae, which are nanofocusing excitation light to enhance the

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excitation of the frequency upconversion nanoparticle (UC) (Figure 1c). As validated in various

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models, including transformation optics10, finite-difference-time-domain (FDTD)13 and finite

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element method (FEM)14 modeling, the plasmonic nanocrescent structure focusing excitation

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light on the nanoscale regime, especially a singular tip region, by excitation in the body of the

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crescent then propagating toward the tip region as SPP. Propagation of SPP toward the tip region

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of the nanocrescent is not dependent on the location of the excitation as demonstrated by incident

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angle dependent near-field electric field distribution simulation (Figure 1b). Therefore, the

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plasmonic nanocrescent structure has the potential to efficiently excite UC near the tip region.

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Moreover, in the meantime, nanocrescent antennae act as emitting antennae for frequency


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upconverted emission by modulating the far-field radiation profile (Figure 1d). Without the

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nanocrescent antennae, UC exhibits a symmetric emission pattern due to the anisotropic

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geometry of UC, but when it is assembled with nanocrescent antennae, the asymmetric emission

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is exhibited due to the interaction of radiated emission with the nanocrescent antenna.

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To understand the interaction of the nanocrescent antenna with excitation and emission

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radiation, we employed three-dimensional full-field FEM simulation of AHPN. First, we

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analyzed the near-field E-field norm distribution of AHPN, then compared it with bare dielectric

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and dielectric core, metal shell (core-shell) nanoparticles. Core-shell is one of the most

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frequently investigated plasmonic structures, due to its tunable and intensive plasmon resonance.

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Also, a broad range of nanofabrication techniques were developed to readily synthesize the core-

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shell plasmonic nanostructures. Figure 2a shows the near-field simulation of the electric field

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distribution around AHPN. The dimension of AHPN is matched with fabricated AHPN

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according to the TEM image depicted in the next section (DUC: 70 nm, DAu: 30 nm and Dgap: 32

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nm). The results showed that the electric field is highly localized around the tip region of

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nanocrescent antenna, and the magnitude of the enhanced electric field within the dielectric core

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is 181.2 times higher than that of the integral of the electric field within the dielectric core

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without the nanocrescent antenna (Depicted as a red dot in Figure 2h). However, 31.2 times

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enhancement of the integral of the electric field within the dielectric core was obtained from the

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similar dimension (DUC: 70 nm, DAu: 30nm) of core-shell nanoparticles (Figure 2c). This result

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originates from the fact that, although core-shell exhibits a large effective cross-section, its

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plasmon resonance does not produce intensive near-field hot spots as shown in the nanocrescent

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antenna, and the electric field is localized toward the outside of the shell, rather than toward the

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core region. To develop analytical insight into the unique nanofocusing effect of the


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nanocrescent structure, we further analyzed the z-component of the electric field distribution

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(number 4 in Figures 2a, b, and c). The z-component of the electric field distribution of AHPN

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demonstrates that SPPs are excited at the body of the nanocrescent antenna, then propagate along

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the nanocrescent surface and toward tip regions, where the group velocity becomes zero and

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energy is accumulated as explained by the transformational optics theory.10, 14 In this region, a

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highly localized electric field excites UC to release frequency upconverted luminescence with

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enhanced efficiency. To investigate the ultimate limit of the nanofocusing effect of AHPN, near-

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field simulation of AHPNs with a smaller gap (16, 8, 4 and 2 nm), although these were not

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experimentally realized in this paper, are depicted in Figures 2d. The electric field is localized in

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the tip region in all AHPNs as depicted in z-sectioned (number 1) and three-dimensional view

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(number 2) of electric field norm distribution, in which the color range is saturated for qualitative

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visualization. For quantitative investigation of the gap size-dependent nanofocusing effect, the

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integral of the electric field within the dielectric core of AHPNs with various gap widths

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normalized by the integral of the electric field within the dielectric core without the nanocrescent

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antenna is depicted in Figure 2e. Electric field enhancement is gradually increased as the gap size

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is decreased and is radically increased when the gap size is below 8 nm, as observed in a dimer

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of Au nanospheres or a metal-insulator-metal (MIM) structure.15-17 In the z-component of electric

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field distribution (number 3), as the gap width (Dgap) is decreased, the number of nodes of

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standing wave are increased, and its concentration at the end of the tip region is also increased

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due to efficient propagation of SPP as explained by the truncated slab model of the truncated

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crescent.18 To examine the interaction of nanocrescent antenna with frequency upconverted

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emission, the radiation pattern of an electric dipole emitter located in the dielectric core in

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AHPN, bare UC, and core-shell structure is depicted in Figures 2f to h. According to the formula


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detailed in the Experimental methods section in the Supporting Information, energy flux density

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obtained from the models (Single dipole in AHPN, UC and core-shell) were sectioned along the

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xy-plane (vertical pattern; number 1), xz-plane (horizontal pattern; number 2), and perspective

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view (number 3). The radiation pattern of a dipole in AHPN (Fig. 2f) exhibited asymmetric

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radiation, and a lobe facing the tip region has higher emission flux than a lobe facing

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nanocrescent antenna body. In contrast, a dipole in UC (Figure 2g) or core-shell nanoparticle

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(Figure 2h) exhibited symmetric radiation, in which both lobes exhibited equal emission flux.

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The angular distribution of the emitted power obtained in the xy-plane (vertical plane) is shown

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in Figure 2i. These results demonstrated the emission from a dipole is efficiently coupled to the

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nanocrescent antenna and result in the focusing of radiation toward the tip region as evidenced

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by the nanogap slit antenna.19,

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implement emitting antenna on a single nanoparticle scale, which has great importance in

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practical application.

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Notably, in our case, we demonstrated the capability to

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AHPNs were fabricated by angle deposition of the Au layer on the isolated UCs (Figure S1).

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Spherical UCs consisted of sodium yttrium fluoride (NaYF4) as a host matrix, which was doped

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with Yb3+ and Tm3+ using the thermal decomposition method. Synthesized UCs have a spherical

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shape with uniform size distribution (Average diameter: 70.4 nm), which exhibit a perfect

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hexagonal close-packed (HCP) structure depicted by a selected area electron diffraction (SAED)

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pattern and a single crystalline structure with a uniform lattice distance of 0.5 nm in high-

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resolution (HR) TEM image (Figure 3a). Since bare UC is not stably dissolved in the water, the

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surface of UC is coated by phospholipid-mPEG to minimize the production of aggregated UCs to

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produce isolated AHPNs for single particle microscopy. In order to prepare AHPNs with smaller

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gaps, UCs were drop-casted on the poly(methyl methacrylate) (PMMA) layer-coated substrate.


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After drop-casting of UCs, the sample was oxygen plasma treated to elevate UCs from the

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substrate to prevent connection between the Au layer on UCs and the Au layer on the substrate

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for high-angle deposition of the Au layer (Diagram 3 in figure S1). Then, the Au layer (30 nm)

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was deposited onto UCs by physical vapor deposition, tilted at certain angles (0, 20, 40, and 60˚)

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to assemble the nanocrescent antenna on the UCs. Without elevation of UCs using the PMMA

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layer, the connection of the Au layer on the UC to the Au layer on the substrate was observed

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upon deposition of the Au layer at 60˚ (red arrow in Figure S3a). In contrast, after the elevation

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of UCs using the PMMA layer, separation of the Au layer on the UCs from the Au layer on the

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substrate was observed (Figure S3b). AHPNs were released from the substrate by sonication,

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then drop-casted on the TEM grid (Figures 3b). According to TEM imaging, the AHPN consists

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of a plasmonic antenna in a nanocrescent shape on the spherical dielectric core (UC; diameter:

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70.4 nm) in a sub-wavelength size (λex: 980 nm). For frequency upconversion efficiency

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measurement of AHPN with single-particle resolution, AHPNs were selectively transferred to a

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new bare glass substrate, while the Au layer on the original substrate remains (Diagram 5 in

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Figure S1). During this step, the orientation of AHPNs is conserved, and directionality-

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dependent UCL emission measurement is enabled (Figure S4). Also, we prepared the sample to

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have a low density (less than 50 particles per 10 µm2) to ensure the presence of single AHPN

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within a diffraction limit of the microscope (White circles in Figure S4a). Then, the refractive

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index around the AHPN is matched by locating AHPNs in immersion oil between two cover

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glass substrates, which has a very similar refractive index (n = 1.5) to minimize the effect of

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discontinuity of dielectric constant for directionality-dependent UCL emission measurement

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(Diagram 6 in Figure S1). Finally, the side area of the sample is sealed using lacquer to minimize

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evaporation of the immersion oil (Diagram 7 in Figure S1). Since we fabricate AHPNs in a 1-D


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manner, the low fabrication yield (5.8 x 109 particles per 4-inch wafer) limits practical

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application. However, it is noteworthy that our group previously demonstrate an innovative way

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of synthesis of the colloidal nanocrescent in the solution.21 Therefore, we believe that this

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method would solve the low yield problem of the preparation of the AHPN. More technical

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details regarding sample preparation can be found in the Methods section.

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Correlated dark-field scattering images and upconversion luminescence images of AHPNs are

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depicted in Figures 3c and d. In order to obtain the correlated dark-field scattering and

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upconversion luminescence images, the substrate is located between the objective lens and the

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dark-field condenser. For acquiring dark-field scattering images, white light is illuminated

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through a dark-field condenser, and for acquiring upconversion luminescence image, a 980 nm

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laser was illuminated through an objective lens. Since metallic nanoparticles have a

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characteristic color in scattered images regarding the onset of localized surface plasmon

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resonance (LSPR), AHPNs are seen as orange dots in dark-field scattering images. In

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upconversion luminescence images, AHPNs are seen as blue dots, and these two images are

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overlapping. Then, we obtained the dark-field scattering spectrum from individual AHPNs

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(Number 1, blue dot) and UCs (Number 2, gray dot), which is related to the characteristic LSPR

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of the nanocrescent antenna (Figure 3f). The spectral position of LSPR of AHPN obtained from

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the dark-field scattering measurement (blue dot) is around 597 nm and well matched with the

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simulation result of scattering cross-section calculation (blue line). The position of LSPR can be

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modulated by controlling the size of the nanocrescent antenna by using UCs with different sizes.

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Although the location of LSPR is not matched to the excitation wavelength (980 nm), we chose a

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UC with a diameter of 70.4 nm, because this is the largest available spherical UC with a well-

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ordered hexagonal phase β-NaYF4 structure. Moreover, one set of maximum near-field


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enhancement is red-shifted from the spectral position of LSPR due to the damping.22, 23 The

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spectrum of upconversion luminescence of individual AHPNs (red line) and ensemble averaged

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AHPN clusters (blue line) are depicted in Figure 3g. To obtain the upconversion luminescence

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spectrum of individual AHPNs, excitation light (cw laser, λ = 980 nm) is focused on individually

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separated AHPNs using a 100x/1.3 NA objective lens. Then zeroth order diffraction image of

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individual AHPNs is obtained by spatially confining the region of interest (ROI) using a slit. To

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measure the ensemble averaged spectrum, excitation light is focused on the AHPN powder using

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a 10x/0.25 NA objective lens. Under 980 nm laser excitation, Tm3+ ions are excited by energy

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transfer through Yb3+ → Tm3+ and UC exhibited peaks at 450 nm (1D2 → 3F4) and 474 nm (1G4

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→ 3H6) corresponding to the relaxation of electrons in the Tm3+.4 The emission spectrum of the

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individual AHPN (red line) and ensemble averaged AHPN powder (red line) showed a high

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resemblance, but the full width at half maximum (FWHM) of the peaks in the ensemble averaged

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AHPN is wider. Since both spectra were obtained with the same detection system, this difference

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could not be originated to instrumental artifacts. This discrepancy is originated by the UC-UC

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interaction process, which is related to nonradiative and radiative energy transfer among adjacent

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Tm3+ ions.24, 25 Therefore, for quantitative analysis, single particle measurement is important,

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because it is not affected by inter-particle interactions.24, 26 Figure 3h shows the representative

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upconversion luminescence spectrum of single AHPN (Number 1, blue line) and UC (Number 2,

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gray dotted line). Upconversion luminescence of UC is enhanced by 16.1 times after assembled

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with the Au nanocrescent antenna. A detailed discussion regarding enhancement of upconversion

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luminescence can be found in the following section.

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According to the results of electromagnetic field simulation of AHPNs in the previous section,

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the nanofocusing effect of the nanocrescent antenna is mainly dependent on the gap size (Dgap).


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Therefore, we optimized the structure of the nanocrescent antenna by modulating the deposition

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angle (θ) of the Au vapor to the UCs (Figure 3i). Under the lower deposition angle, AHPNs with

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wider gaps are produced, because Au vapor is deposited on the face of UCs facing the Au vapor.

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In contrast, under higher deposition angles, AHPNs with narrower gaps are produced because Au

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vapor can access the side region of UCs. However, practically, there is a limitation of the

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deposition angle because Au vapor has volume and also tends to form a grain when it is

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condensed to solid (grain size – 10 nm).27 This result forms the connection of Au layer on the

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AHPN to the Au layer on the surface of the substrate. Therefore, we fabricated AHPNs with θ =

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0, 20, 40, and 60˚, then measured the deposition angle-dependent gap size and frequency

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upconverted emission from an individual AHPN (Figures 3j and k). The gap size of AHPNs

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becomes narrower as the deposition angle is increased, and this tendency is matched with the

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geometrical calculation result (Figure 3l). In the geometrical calculation, we neglected the

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volume of Au vapor and the effect of the graining process during condensation and calculated

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the angle-dependent Dgap by using the geometrically derived equation depicted in the Figure S5.

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The gap width-dependent plot of the intensity of frequency upconverted emission from an

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individual AHPN is depicted in Figure 3m. The experimental result (red dot) is matched with the

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simulation result (blue dot), which exhibited an exponential increase of frequency upconverted

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emission from an AHPN. AHPNs with gap widths of 31.9 nm, which exhibited 16.1 times

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enhanced emission are the narrowest gaps experimentally attainable under our experimental

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procedure. This result is 1.9 times higher than resonance frequency matched core-shell

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nanoparticles, and this can be increased by fabricating AHPNs with smaller gaps.28

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Figure 4a depicts a schematic illustration of the experimental setup for the measurement of

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asymmetric frequency upconverted emission of AHPN. Tip-aligned AHPNs, whose tips are


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facing toward the bottom are attached on the bottom cover glass substrate (denoted as B) and

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covered by the top cover glass substrate (denoted as T) to contain the refractive index-matched

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immersion oil to minimize the effect of discontinuity of the dielectric constant. When emission

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toward the tip region of the nanocrescent antenna is collected (number 1), AHPNs exhibited 16.1

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times higher frequency upconverted emission per particle than UC without the nanocrescent

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antenna as depicted in Figure 4b. In contrast, when emission toward the body region of the

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nanocrescent antenna is collected (number 2), the intensity of emission is decreased from the

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bare UC by 46.4 %. Several studies have determined that when an emitter is located near a

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metallic surface, both enhancements of radiation due to local field enhancement (LFE) caused by

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the LSPR effect of the metallic nanostructure and quenching of radiation by non-radiative energy

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transfer (NRET) resulting from the Forster mechanism exist competitively.9, 29 According to the

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angle-dependent near-field electric distribution simulation depicted in Figure 1b, the maximum

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value of LFE is not dependent on the direction of the excitation electromagnetic wave and is

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formed on the tip region delivered as SPP. Therefore, the difference between intensity emission

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collected at the tip region (number 1) and body region (number 2) originated from the

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asymmetric far-field radiation pattern. Also, since Forster quenching is critically dependent on

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the distance between metallic surface and emitter source with the r-4 relation30 and the geometry

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of AHPN is fixed, the efficiency of Forster quenching is not significantly different between

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intensity emission collected at tip region (number 1) and that collected at the body region

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(number 2). In the control experiment, UC did not exhibit asymmetric frequency upconverted

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emission upon collecting from the bottom (number 3) or top (number 4). Since enhancement of

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frequency upconverted emission of AHPN is originated from LSPR of the nanocrescent antenna,

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the characteristic LSPR of AHPN and UC is depicted in dark-field microscopy images (Figures


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4c to f) and dark-field scattering spectrum images (Figures 4g to j). In the dark-field scattering

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microscopy images, AHPN exhibited different characteristic resonance spectrums dependent on

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the orientation. Upon illumination toward the tip region, AHPN exhibited dark-field scattering in

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orange color due to the plasmon resonance of 597 nm, whereas it exhibits red color (resonance

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632 nm) when the light is illuminated toward the body region. In contrast, UC did not exhibit

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orientation-dependent dark-field scattering spectrum, instead of following the Rayleigh-

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scattering pattern, which has λ-4 dependence.

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In summary, we have reported a novel nanostructure based on asymmetric hetero-plasmonic

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nanoparticles consisting of the plasmonic antenna in nanocrescent shapes on the Lanthanide-

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based upconversion nanoparticle (UC) for efficiently delivering excitation light to the UC core

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by nanofocusing of light and generating asymmetric frequency upconverted emission

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concentrated toward the tip region. According to directionality-dependent frequency upconverted

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emission measurement from single particles, AHPN generates a 16.1 times higher photon

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emission toward the nanocrescent antenna tip region, whereas a 46.4% decreased photon

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emission is exhibited toward the nanocrescent antenna body. It should be noted that, until

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recently, generating asymmetric far-field radiation patterns in a 2D substrate had been widely

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studied, but have not been available in a nanoparticle. However, as described in this work, the

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nanocrescent antenna enables not only nanofocusing of excitation light but also generating

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asymmetric emission, which may provide insights into various nanoparticle-based applications.

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For example, by the addition of a magnetic layer in the nanocrescent antenna, local modulation

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of the beam path of frequency upconverted emission light can be enabled.12 If the nanocrescent

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antenna is assembled with UV emitting frequency upconverting nanoparticles and reactive

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oxygen species (ROS) generating Porphyrin sensitizer, efficient photodynamic therapy can be


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achieved by the release of ROS toward specific targets, such as cell walls controlled by an

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external magnetic field.31

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FIGURES

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Figure 1. The design strategy of the asymmetric plasmonic antenna. (a) Schematic illustration of

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nanofocusing of excitation light and concentrating upconversion emission of asymmetric hetero-

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plasmonic nanoparticles (AHPNs), which consist of nanocrescent-shaped asymmetric plasmonic antennae

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on the upconversion nanoparticle (UC). (b) Schematic diagram for incident angle-dependent near-field

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electric field distribution around an AHPN. The direction of the propagating electromagnetic field (k) is

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fixed at 0˚ and location of the tip of the nanocrescent is rotated by angle θ from 0˚. (Left lane) Near-field

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distribution of z-component (Ez) and (Right lane) norm (|E|/|E0|) of the electric field in the vicinity of an

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AHPN with various incident angles (θ = 0˚, 45˚, 90˚, 135˚, and 180˚). (c) Schematic diagram and

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representative simulation of nanofocusing of excitation light of AHPNs with (left) and without (right)

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nanocrescent antenna. In the simulation, electric field norm distribution under the background TE


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polarized plane wave propagating along the direction of the k-vector in the diagram is depicted. (d)

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Schematic diagram and representative simulation of asymmetric emission of AHPNs with and without

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nanocrescent antennae. In the simulation, the far-field radiation pattern (energy flux density) of a z-

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polarized point dipole without (left) and with (right) nanocrescent antenna is depicted.

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Figure 2. Simulation of AHPNs. (a-c) Schematic diagram describing simulation sectioned in xy-plane

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(number 1), norm (|E|/|E0|) of electric field sectioned in xy-plane (number 2), perspective view of surface

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profile of norm (|E|/|E0|) of electric field (number 3) and z-component (Ez) of electric field sectioned in

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xy-plane (number 4) of an (a) AHPN, (b) UC, and (c) core-shell nanoparticle (Scale bar: 50 nm). (d) Gap

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(Dgap)-dependent norm (|E|/|E0|) of electric field sectioned in xy-plane (number 1), perspective view of

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surface profile of norm (|E|/|E0|) of electric field (number 2) and z-component (Ez) of electric field

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sectioned in xy-plane (number 3) of an AHPN with Dgap = 16 nm, 8 nm, 4 nm and 2 nm (Scale bar: 50

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nm). Inset in Ez plot (number 3) is enlarged Ez distribution around the tip region of the nanocrescent

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antenna. In the simulation, the norm or z-component of the electric field under the background TE

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polarized plane wave propagating along the direction of the k-vector in the diagram is depicted. (e) Dgap-

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dependent distribution of normalized integral of the electric field norm within the dielectric core of an

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AHPN (EAHPN/EUC). Integral of the electric field norm (EAHPN) is normalized by that of the dielectric core

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without the nanocrescent (EUC). (f-h) Schematic diagram of the geometry describing simulation of the far-

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field radiation patterns of (f) AHPN, (g) UC, and (h) core-shell nanoparticles. A point dipole oscillating


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parallel to the antenna tip direction is located at the center of the dielectric core as an emitting source.

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Numbers noted in each colored plane (1: xy-plane, 2: xz-plane and 3: perspective view) describe the

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sectioning planes of the far-field radiation pattern, which is calculated based on the energy flux density in

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the unit voxel (high: red, low: blue). (i) The angular distribution of the radiated power of a dipole in an

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AHPN (number 1), UC (number 2), and core-shell nanoparticle (number 3) sectioned in xy-plane.

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Figure 3. Optimization of nanofocusing of excitation light. (a) Transmission electron microscopy

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(TEM) image of upconversion nanoparticles (UCs; NaYF4:Yb3+, Tm3) (Scale bar: 100 nm). Inset is a

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high-resolution TEM image of a UC (Scale bar: 10 nm) with a selected area electron diffraction (SAED)

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pattern. (b) TEM image of AHPNs in low magnification (Scale bar: 1 µm) and high magnification (inset;

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scale bar: 50 nm). (c-d) Correlated image of (c) dark-field scattering and (d) frequency upconverted

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emission of AHPNs (Scale bar: 10 µm). (e) Schematic diagram for optical microscopy setup for

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correlated measurement of dark-field scattering and frequency upconverted emission. (f) Simulated

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scattering cross-section (line) and dark-field scattered spectrum of an AHPN (number 1, blue) and UC

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(number 2, gray). (g) Frequency upconverted emission spectrum of single AHPN (red) and ensemble

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averaged AHPN powder (blue). The inset shows CCD images obtained from each sample (Scale bar at

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left: 10 µm, right: 1 µm). (h) Representative frequency upconverted emission spectrum of an AHPN

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(number 1) and UC (number 2). (i) Schematic diagram of the modulation of gap width (Dgap) of


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nanocrescent antennae by changing deposition angle (θ). (j) TEM and (k) image of frequency

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upconverted emission of AHPNs with various deposition angles (θ = 0˚, 20˚, 40˚, and 60˚). (l)

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Geometrically calculated (blue) and experimentally obtained (red) θ -dependent Dgap of an AHPN. (m)

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The integral of the electric field norm within the dielectric region normalized by that of without

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nanocrescent antenna (EAHPN/EUC) (blue) and experimentally measured the intensity of frequency

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upconverted emission (red) of AHPNs fabricated with various deposition angles (θ= 0˚, 20˚, 40˚, and

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60˚). All standard deviations were calculated from five independent measurements.

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Figure 4. Asymmetric frequency upconverted emission of AHPN. (a) Schematic illustration of the

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experimental setup for the measurement of asymmetric emission of AHPNs. Tip-aligned AHPNs with

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tips are facing toward the bottom are attached on the bottom cover glass substrate (denoted as B) and

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covered by the top cover glass substrate (denoted as T) with the filling of refractive index matching oil.

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For the measurement of emission toward the tip region, the sample was mounted in the normal direction

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(number 1 and 3), and for the measurement of emission toward the body region, the sample was mounted

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in an inverted direction (number 2 and 4). (b) Frequency upconverted emission of an AHPN toward the

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tip region (number 1), an AHPN toward the body region (number 2), a UC toward the bottom (number 3),

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and a UC toward the top (number 4). (c-f) Dark-field scattering image of (c) AHPN toward the tip region,


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(d) AHPN toward the body region, (e) UC toward the bottom and (f) UC toward the top (Scale bar: 1

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µm). (g-j) Dark-field scattering spectrum (dot) and simulated scattering cross-section (line) of (g) AHPN

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toward the tip region, (h) AHPN toward the body region, (i) UC toward the bottom, and (j) UC toward the

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top. Each spectrum was averaged from 5 different measurements. Inset image depicts geometry for

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scattering

cross-section

simulation.


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ASSOCIATED CONTENT Supporting Information. Supporting Information includes experimental methods, schematic illustration of the fabrication of tip-aligned AHPN sample, TEM image and emission spectrum of bare UC, SEM image depicting the importance of PMMA pillar for high angle deposition, orientation and distribution control of isolated AHPNs, geometrical calculation of angle dependent gap width and Electric field distribution of AHPNs in figure 3. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was financially supported by grants from the Global Research Lab (GRL) Program (NRF-2013K1A1A2A02050616) funded by the Ministry of Science, ICT and Future Planning and the Air Force Office of Scientific Research Grants AFOSR FA2386-13-1-4120. REFERENCES (1) Svoboda, K.; Yasuda, R. Neuron 2006, 50, 823-839.


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(2) Joubert, M.-F. Opt. Mater. 1999, 11, 181-203. (3) Haase, M.; Schafer, H. Angew. Chem. Int. Ed. Engl. 2011, 50, 5808-5829. (4) Gnach, A.; Bednarkiewicz, A. Nano Today 2012, 7, 532-563. (5) Park, W.; Lu, D.; Ahn, S. Chem. Soc. Rev. 2015, 44, 2940-2962. (6) Fischer, S.; Kumar, D.; Hallermann, F.; von Plessen, G.; Goldschmidt, J. C. Opt. Express 2016, 24, A460-475. (7) Saboktakin, M.; Ye, X.; Oh, S. J.; Hong, S. H.; Fafarman, A. T.; Chettiar, U. K.; Engheta, N.; Murray, C. B.; Kagan, C. R. ACS Nano 2012, 6, 8758-8766. (8) Wang, Y. L.; Mohammadi Estakhri, N.; Johnson, A.; Li, H. Y.; Xu, L. X.; Zhang, Z.; Alu, A.; Wang, Q. Q.; Shih, C. K. Sci Rep 2015, 5, 10196. (9) Feng, A. L.; You, M. L.; Tian, L.; Singamaneni, S.; Liu, M.; Duan, Z.; Lu, T. J.; Xu, F.; Lin, M. Sci Rep 2015, 5, 7779. (10) Fernandez-Dominguez, A. I.; Luo, Y.; Wiener, A.; Pendry, J. B.; Maier, S. A. Nano Lett. 2012, 12, 5946-5953. (11) Lu, Y.; Liu, G. L.; Kim, J.; Mejia, Y. X.; Lee, L. P. Nano Lett. 2005, 5, 119-124. (12) Liu, G. L.; Lu, Y.; Kim, J.; Doll, J. C.; Lee, L. P. Adv. Mater. 2005, 17, 2683-2688. (13) Wang, Y.; Zhou, W.; Liu, A.; Chen, W.; Fu, F.; Yan, X.; Jiang, B.; Xue, Q.; Zheng, W. Opt. Express 2011, 19, 8303-8311. (14) Wu, H.-W.; Deng, Y.-Q.; Zhou, Y.; Dong, Y.-Q.; Fan, R.-H. AIP Advances 2014, 4, 117102. (15) Lin, H. Y.; Huang, C. H.; Chang, C. H.; Lan, Y. C.; Chui, H. C. Opt. Express 2010, 18, 165-172.


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(16) Ciraci, C.; Hill, R. T.; Mock, J. J.; Urzhumov, Y.; Fernandez-Dominguez, A. I.; Maier, S. A.; Pendry, J. B.; Chilkoti, A.; Smith, D. R. Science 2012, 337, 1072-1074. (17) Ding, S.-Y.; Yi, J.; Li, J.-F.; Ren, B.; Wu, D.-Y.; Panneerselvam, R.; Tian, Z.-Q. Nature Reviews Materials 2016, 1, 16021. (18) Luo, Y.; Lei, D. Y.; Maier, S. A.; Pendry, J. B. Phys. Rev. Lett. 2012, 108, 023901. (19) Aouani, H.; Mahboub, O.; Bonod, N.; Devaux, E.; Popov, E.; Rigneault, H.; Ebbesen, T. W.; Wenger, J. Nano Lett. 2011, 11, 637-644. (20) Jun, Y. C.; Huang, K. C.; Brongersma, M. L. Nat Commun 2011, 2, 283. (21) Jeong, E.; Kim, K.; Choi, I.; Jeong, S.; Park, Y.; Lee, H.; Kim, S. H.; Lee, L. P.; Choi, Y.; Kang, T. Nano Lett. 2012, 12, 2436-2440. (22) Kleinman, S. L.; Sharma, B.; Blaber, M. G.; Henry, A. I.; Valley, N.; Freeman, R. G.; Natan, M. J.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2013, 135, 301-308. (23) Zuloaga, J.; Nordlander, P. Nano Lett. 2011, 11, 1280-1283. (24) Sarkar, S.; Meesaragandla, B.; Hazra, C.; Mahalingam, V. Adv. Mater. 2013, 25, 856-860. (25) Rodriguez-Sevilla, P.; Rodriguez-Rodriguez, H.; Pedroni, M.; Speghini, A.; Bettinelli, M.; Sole, J. G.; Jaque, D.; Haro-Gonzalez, P. Nano Lett. 2015, 15, 5068-5074. (26) Nadort, A.; Sreenivasan, V. K.; Song, Z.; Grebenik, E. A.; Nechaev, A. V.; Semchishen, V. A.; Panchenko, V. Y.; Zvyagin, A. V. PLoS One 2013, 8, e63292. (27) Gaspar, D.; Pimentel, A. C.; Mateus, T.; Leitao, J. P.; Soares, J.; Falcao, B. P.; Araujo, A.; Vicente, A.; Filonovich, S. A.; Aguas, H.; Martins, R.; Ferreira, I. Sci Rep 2013, 3, 1469. (28) Priyam, A.; Idris, N. M.; Zhang, Y. J. Mater. Chem. 2012, 22, 960-965. (29) Anger, P.; Bharadwaj, P.; Novotny, L. Phys. Rev. Lett. 2006, 96, 113002.


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(30) Ge, W.; Zhang, X. R.; Liu, M.; Lei, Z. W.; Knize, R. J.; Lu, Y. Theranostics 2013, 3, 282288. (31) Idris, N. M.; Gnanasammandhan, M. K.; Zhang, J.; Ho, P. C.; Mahendran, R.; Zhang, Y. Nat. Med. 2012, 18, 1580-1585.


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