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In Situ Generation of Plasmonic Nanoparticles for Manipulating Photon−Plasmon Coupling in Microtube Cavities Yin Yin,*,† Jiawei Wang,† Xueyi Lu,† Qi Hao,† Ehsan Saei Ghareh Naz,† Chuanfu Cheng,‡ Libo Ma,*,† and Oliver G. Schmidt†,§ †

Institute for Integrative Nanosciences, IFW Dresden, 01069 Dresden, Germany School of Physics and Electronics, Shandong Normal University, 250014 Jinan, China § Material Systems for Nanoelectronics, Technische Universität Chemnitz, 09107 Chemnitz, Germany

ACS Nano 2018.12:3726-3732. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/16/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: In situ generation of silver nanoparticles for selective coupling between localized plasmonic resonances and whispering-gallery modes (WGMs) is investigated by spatially resolved laser dewetting on microtube cavities. The size and morphology of the silver nanoparticles are changed by adjusting the laser power and irradiation time, which in turn effectively tune the photon−plasmon coupling strength. Depending on the relative position of the plasmonic nanoparticles spot and resonant field distribution of WGMs, selective coupling between the localized surface plasmon resonances (LSPRs) and WGMs is experimentally demonstrated. Moreover, by creating multiple plasmonic-nanoparticle spots on the microtube cavity, the field distribution of optical axial modes is freely tuned due to multicoupling between LSPRs and WGMs. The multicoupling mechanism is theoretically investigated by a modified quasipotential model based on perturbation theory. This work provides an in situ fabrication of plasmonic nanoparticles on three-dimensional microtube cavities for manipulating photon-plasmon coupling which is of interest for optical tuning abilities and enhanced light-matter interactions. KEYWORDS: plasmonic nanoparticles, photon-plasmon coupling, optoplasmonics, whispering-gallery mode, microtube cavity (EF).26 In previous reports, metallic nanoparticles have been transferred onto photonic microcavities by employing atomic force microscope manipulation, chemical-assisted adhesion, or nozzle spraying.18,27,28 In this context, it is of high interest to explore in situ fabrication of plasmonic nanoparticles on photonic microcavities. The in situ fabrication of plasmonic nanoparticles is expected to provide mechanically more stable nanoparticles grown on optical microcavities in a reproducible and location-selective fashion, allowing for spatially resolved selective coupling to optical resonances. In this work, spatially resolved selective photon-plasmon coupling is realized by in situ generation of plasmonic nanoparticles on an optical microtube cavity. The formation of plasmonic nanoparticles is enabled by laser-induced dewetting of a thin silver film on the HfO2 surface of a microtube cavity. Silver is a widely used material in plasmonics because of the low absorption loss and operating wavelength in visible spectral range. The LSPRs supported by silver

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urface plasmons, which originate from the collective oscillations of free electrons on metal interfaces, can be coupled to electromagnetic waves, leading to the excitation of surface plasmon polaritons (SPPs) and localized surface plasmon resonances (LSPRs).1 The plasmon modes localized at metal surfaces provide strong light confinement and small mode volume in the subwavelength scale regardless of the diffraction limit, which are promising for enhanced light-matter interactions, subwavelength imaging, and optical communications.2−4 To overcome the huge absorption loss in metal materials, hybrid photonic−plasmonic systems, such as optoplasmonic microcavities, waveguides, and circuits, have been explored in recent years.5−12 The hybrid systems combine the advantages of both photonic and plasmonic elements, allowing for applications such as nanolasers, enhanced biosensors, and single-particle absorption spectrometers.13−17 Moreover, great efforts have been devoted to investigating the fundamental mechanism and underlying physics of the photon−plasmon coupling between photonic microcavities and plasmonic nanostructures which have significant size mismatch.5,18−25 In particular, metallic nanoparticles and/or nanogap supporting LSPRs are of high interest for the study of enhanced light− matter interactions due to the strongly localized electric field © 2018 American Chemical Society

Received: February 5, 2018 Accepted: April 9, 2018 Published: April 9, 2018 3726

DOI: 10.1021/acsnano.8b00957 ACS Nano 2018, 12, 3726−3732

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Cite This: ACS Nano 2018, 12, 3726−3732

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clearly display the distribution of the optical filed in the lobe region. The higher order axial modes are derived from the measured resonant mode spectrum. A scanning electron microscopy (SEM) image of the microtube cavity from the top view is displayed in Figure 1b, and the edge of the lobe structure is indicted by the dashed line. To generate silver nanoparticles, first a thin silver layer (∼6 nm) was deposited on the microtube surface by electron-beam evaporation.41 Afterward, a 457 nm solid state laser was focused on the top-middle of the silver-coated microtube by a 100× objective, as sketched in Figure 1a. A laser power of 2 mW is sufficient to melt and dewet the silver layer without damaging the tube structure, leading to the in situ generation of silver nanoparticles.42 The generated silver nanoparticles show good stability, which provides a stable tuning on the optical resonances. After a laser exposure for 20 s, locally dewetted silver nanoparticles in a spot area of around 2 μm2 were formed in the top-middle of the microtube cavity, as shown in Figure 1b. The generated silver nanoparticles are clearly visible in the laser-irradiated spot in comparison to the area without laser irradiation, as shown in the bottom panels of Figure 1b. It is known that silver nanoparticles support LSPRs in the visible range accompanied by highly localized EF (also known as “hot spots”). Here, the LSPRs supported by the locally dewetted silver nanoparticles are coupled to the optical axial WGMs in the opto-plasmonic microtube cavity. To explore the photon−plasmon coupling strength, the plasmonic resonances were investigated by changing the morphology of the silver nanostructures under different laser irradiation time (varying from 5−20 s) with a fixed power of 2 mW. As shown in Figure 2a, the SEM images of the laserdewetted silver nanostructures formed on the microtube are displayed after different laser irradiation times. After a short laser irradiation time of 5 s, an increased roughness of the silver layer is observed but there are no isolated nanoparticles. By increasing the irradiation time to 10 and 15 s, aggregated silver nanoparticles are recognized, as shown in Figure 2a. As the laser irradiation time was further increased, the aggregated silver nanoparticles become more isolated in spacing and more regular in shape. The particle size distributions are determined for the analysis of LSPRs (see the Supporting Information). The corresponding LSPRs are simulated by a finite-difference time-domain (FDTD) method according to the size distribution of the silver nanoparticles, as shown in Figure 2a. The material permittivity is adopted from ref 43. The nanoparticle sizes and distributions were directly analyzed by the commercial software Lumerical FDTD Solutions according to the SEM images. Afterward, the corresponding boundary conditions and proper matching layers are set to simulate the field resonances. A far-field monitor was used to measure the resonances in the range of 200−1200 nm. It shows that the resonant peak of LSPRs continuously blue-shifts as the silver nanoparticles become more isolated and regular in shape. Moreover, the LSPR peak becomes narrower because the more isolated and regular silver nanoparticles provide an increased localization of the plasmonic resonance. The EF distribution around the silver nanoparticles after 20 s laser irradiation is shown in the inset of Figure 2a where the “hot spots” can be clearly identified. Due to the different EF distributions of each order axial mode, they can selectively couple to the LSPRs confined at the silver nanoparticles spot, depending on the effective/no spatial EF overlapping, as schematically illustrated in Figure 2b. Here,

nanoparticles can couple with optical whispering-gallery modes (WGMs) confined in the microtube cavity due to spatial overlapping. The morphology of the laser-dewetted plasmonic nanoparticles is adjusted by controlling the laser power and irradiation time. Consequently, the photon-plasmon coupling strength is tuned owing to the variation of LSPRs supported by the metallic nanoparticles. The selective coupling between optical WGMs and plasmonic resonance is observed by monitoring the resonance spectral shift of the coupled modes. More interestingly, the optical filed distributions of WGMs are readily tuned via simultaneous photon−plasmon coupling at multiple positions on the opto-plasmonic microcavity. The coupling mechanism is theoretically illustrated by a modified quasipotential model based on perturbation theory, which shows excellent agreement with the experimental results. Our work reports a simple but efficient way to manipulate photon-plasmon coupling in hybrid microcavities. And this strategy can also be applied to other opto-plasmonic systems for the study of enhanced light-matter interactions and related applications.

RESULTS AND DISCUSSION The WGM microtube cavities were fabricated by rolling up prestrained 35 nm SiOx nanomembranes from a U-shape pattern, followed by coating a 30 nm HfO2 layer via atomiclayer deposition.29−32 This kind of WGM microcavity has received increasing attention due to its advantages such as small cavity size, ultrathin cavity wall, and hollow cavity core,33,34 showing photonic optical phenomena and applications including high sensitive opto-fluidic sensing, on-chip integration, and optical spin−orbit coupling.35−39 As shown in Figure 1, a lobe structure was designed in the middle of the microtube

Figure 1. (a) (Top) Sketch of spatially resolved laser dewetting on a silver-coated microtube. (Bottom) Calculated field distribution of higher order axial modes (E1−E6) for azimuthal mode m = 41, together with the corresponding resonant mode spectrum. The dotted line represents the lobe region. (b) SEM image shows the top view of a silver-coated microtube cavity. The blue curve denotes part of the lobe edge. The scale bar is 5 μm. Silver nanoparticles are clearly seen in the area dewetted by the laser (indicated by dotted yellow circle), in comparison to other areas (e.g., green dotted circle) without laser dewetting.

cavity, providing axial confinement.29 Because of the presence of an axial potential induced by the lobe, the WGMs oscillate along the tube axis within the potential well, leading to the formation of higher order axial resonances under 3D confinement.29,40 In the bottom panels of Figure 1a, the first six orders of axial modes (E1−E6) (with azimuthal mode m = 41 as an example) are calculated, where the antinodes of the axial modes 3727


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Figure 2. (a) SEM images show the laser-dewetted nanoparticles under irradiation times of 5, 10, 15, and 20 s, respectively, treated at the same power of 2 mW. The scale bar is 200 nm. The corresponding plasmonic resonant peaks of the laser-dewetted silver nanoparticles are simulated for each case. The bottom inset shows the EF distribution of the plasmonic resonances for silver nanoparticles after 20 s irradiation. (b) Schematic shows the relative axial spatial position between plasmonic “hot spots” and the antinodes distribution of each order axial mode. Here, the different order axial modes are displayed separately to show their antinode distribution. (c) Measured resonant-mode spectra after each laser irradiation. Selective coupling to odd order axial modes are observed after longer time laser irradiations.

Figure 3. Measured spatial distributions of the axial modes in a lobe region after a series of point-by-point laser irradiations on a silver-coated microtube as schematically shown in the top panels of (a−e). The yellow marked regions denote the laser irradiation site along the tube axial direction.

the nanoparticles spot was fabricated in the top-middle of the lobe on a microtube cavity. For the odd order axial modes (i.e., E1, E3, and E5), one of the antinodes is located in the middle of the lobe which can couple to the LSPRs, resulting in efficient photon−plasmon coupling. In contrast, the even order axial modes (i.e., E2, E4, and E6) have no antinodes effectively overlapping with the nanoparticle spot and cannot efficiently couple to the plasmonic “hot spots”. The resonant spectra of photon−plasmon coupling were measured after each laser irradiation. In the measurements, the same laser line of 457 nm was used but with a much lower laser power of 0.2 mW (see the Supporting Information). Such a low laser power does not lead to any morphology change of the

silver nanoparticles, but it is sufficient to excite photoluminescence (PL) from the SiOx cavity wall to feed in light for the WGM resonances. The axial mode spectrum before the laser-dewetting was also measured. The axial peaks E1−E6 are evenly distributed as shown in Figure 2c. The free spectral range (FSR) of the axial modes is around 3.01 meV. After the 5 s laser irradiation on the top-middle of the lobe, all of the axial modes slightly blue-shift, while there is no distinct selective coupling to the higher order axial modes due to the weak plasmonic resonance (see the blue curve in Figure 2a). When the laser irradiation time is further increased to 10 s, a significant blueshift of E1 is observed, as shown by the green curve in Figure 2c. The mode spacing between E1 and E2 3728


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Figure 4. (a−e) Calculated spatial distributions of the axial modes after the point-by-point laser irradiations in the lobe region of the microtube cavity. The dashed lines represent the modified quasipotentials, and the sharp deformations originate from the plasmonic resonances supported by the laser-dewetted spot of silver nanoparticles.

laser irradiation at the first spot, there is only a slight shift to the right of the antinode of E1 along the tube axis, marked by the dashed yellow circle in Figure 3a. This effect can be explained by a LSPR-modified quasipotential which will be discussed in the following section. A large spatial shift of E1 to the right side is observed after the laser exposure at the second spot, as displayed by the white dashed circle in Figure 3b. When the laser irradiation is extended to the right (in this way generating a larger plasmonic spot) the axial modes are efficiently tuned and modified, as shown in Figure 3c. After further laser irradiation at the right side, more intensity changes, energy shifts, and spatial movements of the antinodes, especially for E1, are observed due to complex multicoupling between plasmonic resonances and optical WGMs, as shown in Figure 3d,e. The antinodes of the higher order modes become weaker due to larger scattering loss caused by the increased number of silver nanoparticles spots. It is also notable that by adjusting the thickness of silver layer as well as the laser power and irradiation time, the distribution of silver nanoparticles can get more scattered and sparse on cavity surface. Thus, in principle, it is also possible to investigate the photon−plasmon coupling of resonant light with a single plasmonic nanoparticle on the cavity surface. In fact, the hybrid single-plasmonic nanoparticlecoupled WGM microcavity system has gained increasing attention in cavity-related nonlinear optics, quantum optics, absorption spectrometers, and sensing applications.17,45−47 Mode shifts as well as EF redistributions induced by photonplasmon coupling can be well explained by a modified quasipotential model based on perturbation theory.21 First, the field distributions of the axial WGMs in microtube cavities are determined by solving the scalar Helmholtz equation which can be written in a form of the Schrödinger equation:29

decreases to 1.97 meV, which is much smaller than the initial axial FSR of 3.01 meV. The mode shift is ascribed to the selective coupling of the odd order axial mode to the plasmonic resonance, as discussed above. However, as an odd order axial mode, the spectral shift of mode E3 is as small as that of other even order modes. This effect is explained by the relatively weak field distribution in the middle of the lobe in comparison with that of E1 (see Figure 2b), which leads to a low photonplasmon coupling strength. As the laser irradiation time is increased to 15 and 20 s, the shift of odd order mode E3 becomes pronounced owing to the enhanced LSPRs at the nanoparticle spot which facilitates an increase in the photon− plasmon coupling strength. The mode spacing between E1−E2 and E3−E4 further decreases to 1.52 and 2.29 meV, respectively, with a longer dewetting time as a result of more efficient selective coupling. The shifts of the higher order modes such as E5 and E6 are not as distinct as the lower order modes because of the lower mode intensity at the middle of lobe, which reduces the photon−plasmon coupling efficiency. As a reference, the bare microtube cavities without silver coating were irradiated under the laser beam with the same power and time, and the spectra were also measured to examine the shift of the axial modes (see the Supporting Information). There are no uneven mode shifts as observed in Figure 2c, and only small blueshifts of all axial modes are found due to the desorption of water molecules from the microtube surface upon laser irradiation.44 Thus, it can be concluded that the selective mode shifts in Figure 2c are indeed induced by selective coupling between the LSPR confined at the laser-dewetted silver nanoparticles and the optical axial modes. In the following, we investigate the interaction of spatially varying nanoparticle spots along the tube axis with the EF distribution of the resonant modes. The axial WGMs confined in the lobe area of the microtube cavities resemble those of quantum mechanical particles oscillating in a potential well.29 In the following we record spatial mappings of the axial EF distributions along the lobe area to reveal the energy, potential, and state of the opto-plasmonic quasi-quantum system, as shown in Figure 3. In the lobe region, the laser beam irradiated point by point from the left to the right position, with an irradiation time of 20 s and a laser power of 2 mW to create multispots of silver nanoparticles. The axial location of each spot is indicated by the yellow region in Figure 3a−e. Apart from the uneven mode shifts as discussed in Figure 2c, a dramatic redistribution of the antinodes is observed due to multicoupling between plasmonic resonances supported by each nanoparticles spot and higher order axial modes. After the

−

1 ∂2 ψ (z) + kcirc 2(z)ψ (z) = kz 2(z)ψ (z) 2 2 n ∂z

Here, kcirc (z) denotes the quasipotential, kz(z) is the eigenenergy, and ψ(z) represents an eigenstate of the axial WGMs. The coupling between plasmonic resonances and axial optical modes results in a variation of the quasipotential kcirc(z) (= ω/c), which can be treated by perturbation theory:48 Δω = −

ω ⟨E(r , φ)|Δε(r , φ)|E(r , φ)⟩ 2 ⟨E(r , φ)|ε(r , φ)|E(r , φ)⟩

Here ω, E(r,φ), and ε(r,φ) are the angular frequency, the azimuthal optical field, and the permittivity, respectively. It is known that LSPRs result in “hot spots” which possess electric field intensities |ELSP|2 in excess of |Er,φ|2, leading to a variation 3729


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ACS Nano of ω which consequently redetermines the quasipotential kcirc(z).16,49,50 |ELSP|2 was simulated by a FDTD method as mentioned in Figure 2a. The calculated eigenenergies, quasipotential and eigenstates are displayed in Figure 4. The sharp deformations of the quasipotential reflect the multiplasmonic-resonances supported by the spots of the laserdewetted nanoparticles. Both, the energy shifts and spatial movements of the modes agree well with the experimental results shown in Figure 3.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b00957. Sketch of spatial distribution of optical axial modes in the lobe region (Figure S1); SEM images of microtubes under laser irradiation with different power (Figure S2); particle size distributions of the laser-dewetted nanoparticles under different laser irradiation time (Figure S3); measured axial WGMs of a microtube cavity without silver coating after the laser irradiation (Figure S4) (PDF)

CONCLUSION In conclusion, in situ generation of plasmonic nanoparticles on a 3D confined microtube cavity is explored for investigating spectrally and spatially resolved photon−plasmon coupling. The photon−plasmon coupling is tuned by both the size and morphology of metallic nanoparticle spots and the relative location of the nanoparticles on the tube cavity. By monitoring uneven energy shifts of certain optical modes, spatially selective coupling between plasmonic nanoparticles and the field distributions of different order axial modes was experimentally demonstrated. Moreover, by creating multicoupling scenarios between LSPRs and optical WGMs, the spatial distribution of the optical field can be freely tuned. The coupling mechanism was well explained by a modified quasipotential model based on perturbation theory. Our work provides a convenient way to in situ manipulate the photon−plasmon coupling in hybrid microcavities which are promising for improving and understanding light−matter interactions.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yin Yin: 0000-0002-5318-0020 Jiawei Wang: 0000-0001-6323-8081 Xueyi Lu: 0000-0002-2867-4707 Qi Hao: 0000-0002-5525-4417 Libo Ma: 0000-0001-9850-2292 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Y. Chen, R. Engelhard S. Harazim, B. Eichler, and S. Baunack for technical support. This work was supported by the German Research Foundation DFG (Grant Nos. FOR 1713 and SCHM 1298/22-1).

METHODS Microtube Cavity Fabrication. First, a photoresist layer (ARP3510, Allresist GmbH) was spin-coated on a precleaned silicon wafer as sacrificial layer. Then the photoresist layer was patterned with Ushapes by standard lithography (MJB4, SÜ SS Microtec). A 35 nm SiOx layer was then deposited onto the photoresist layer using angled electron beam evaporation (Edwards Auto500 e-beam), and a start line for the following rolling process was introduced. During the evaporation, the deposition rate was gradually changed from 6 to 0.5 Å/s to create differential strained layers which can largely facilitate the rolling process. Next, acetone was used to etch away the photoresist layer, resulting in the rolling of nanomembranes. A critical point dryer (931 GL, Tousimis CPD) was applied to dry the microtubes for avoiding structural collapse. In addition, a 30 nm HfO2 layer was grown by atomic layer deposition on the microtube surfaces to mechanically strengthen the structure and to optically enhance the light confinement. Finally, a 6 nm silver layer was deposited on the SiOx/HfO2 microtubes before the laser dewetting process. Laser Dewetting. A 457 nm solid-state laser was focused by a 100× objective on the top-middle of the silver-coated microtube to melt and dewet the thin film to nanoparticles. The laser power was adjusted in the range of 2−20 mW by changing the neutral density filters to investigate the structural variations of silver-coated microtubes (see the Supporting Information). Optical Measurements. Optical resonances were characterized by measuring the light emission using a laser confocal microphotoluminescence (μ-PL). An excitation source with 457 nm laser line was utilized to excite the photoluminescence of defects51 in the amorphous silica tube, which served as light source to pump optical resonances. A 50× objective was used to focus the laser beam onto the microtube cavity and collect the light emission. The resonant light was guided to the spectrometer with 600 blz/mm and an electrically cooled charge coupled device (CCD) camera.

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