Silver Nanocap Enabled Conversion and Tuning of ... - ACS Publications

Mar 10, 2017 - Yin YinJiawei WangXueyi LuQi HaoEhsan Saei Ghareh NazChuanfu ChengLibo MaOliver G. Schmidt. ACS Nano 2018 12 (4), 3726-3732...
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Letter

Silver Nanocap Enabled Conversion and Tuning of Hybrid Photon-Plasmon Modes in Microtubular Cavities Yin Yin, Yan Chen, Ehsan Saei Ghareh Naz, Xueyi Lu, Shilong Li, Vivienne Engemaier, Libo Ma, and Oliver G. Schmidt ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00990 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 12, 2017

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Silver Nanocap Enabled Conversion and Tuning of Hybrid Photon-Plasmon Modes in Microtubular Cavities Yin Yin,†,‡ Yan Chen,† Ehsan Saei Ghareh Naz,† Xueyi Lu,†,‡ Shilong Li,† Vivienne Engemaier,† Libo Ma,† Oliver G. Schmidt†,‡ †

Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstraße 20, 01069, Germany



Material Systems for Nanoelectronics, Technische Universität Chemnitz, 09111 Chemnitz,

Germany KEYWORDS: hybrid photon-plasmon mode, whispering gallery mode, surface plasmon, microcavity

ABSTRACT: Hybrid photon-plasmon modes are promising for the study of enhanced lightmatter interactions due to the formation of a unique plasmon-type evanescent field. Here, we demonstrate the tunability of photon-plasmon coupling enabled by a metal nanocap on a microtubular cavity. An angle-dependent tuning of the photon-plasmon hybridization is revealed, where the dominant polarization is transverse-magnetic (TM) polarized at the middle-top of the nanocap and gradually converts to be transverse-electric (TE) polarized at the sidewall of the microtube cavity. The intensity ratio of strongly hybridized TM and TE modes is extremely

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sensitive to nano-perturbations at the metal nanocap, thus providing a novel scheme for surface sensing. Theoretical calculations show that the sensitive intensity ratio change originates from the distinct tuning effect on the TM and TE polarized hybrid modes, which is particularly significant in thin-walled microtubular cavities. Our work reports photon-plasmon modes tuned by a metal nanostructure which are promising for the fundamental studies of enhanced lightmatter interactions and relevant applications.

Manipulation of light-matter interactions, which are relevant for both fundamental and applied studies, have gained increasingly more attention over the years.1 In particular, optical whispering-gallery mode (WGM) microcavities, capable of confining light in a small volume, have been reported as a promising platform for the study of enhanced light-matter interactions.2-4 To further increase the coupling strength, plasmonic nanostructures have been integrated into photonic cavities for the formation of hybrid photon-plasmon modes.5-15 Plasmon-type modes supported by noble metal nanostructures can be squeezed into the subwavelength scale, thus generating enhanced optical fields at metal surfaces.16-22 In previous reports, uniformly distributed hybrid photon-plasmon modes were explored, which in turn exhibit only monotonous fields with fixed polarization state in the optoplasmonic WGM cavities.6,8,11 However, to obtain flexible field strengths for the manipulation of enhanced light-matter interactions, which are useful for a broader range of applications, it is crucial to be able to tune the field intensity of the hybrid modes in a single optoplasmonic microcavity. In this Letter, we design and fabricate silver nanogaps with graded thicknesses coated on microtubular microcavities. Instead of pure photonic modes, hybrid photon-plasmon modes are formed in the opto-plasmonic microcavities because of the presence of surface plasmons excited

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by resonant light. In contrast to previous reports, Angle-dependent non-uniform hybrid modes were realized in our opto-plasmonic microcavities, where not only the coupling strength but also the optical field polarizations can be tuned depending on the azimuthal angle. As a result, the dominant optical field is transverse-magnetic (TM) polarized at the middle-top of the nanocap and gradually converts to be transverse-electric (TE) polarized at the sidewall of the microtube cavity. Moreover, the strongly hybridized modes, which comprised of traditional optical field located inside of the dielectric cavity wall and plasmon-type field localized at the metal layer surface, are shown to be extremely sensitive to surface modifications such as dielectric coatings. The surface modifications lead to a distinct intensity variation of both the transverse magnetic (TM) and transverse electric (TE) modes, which provide a novel scheme for manipulating lightmater interactions based on a versatile nanotechnology platform. The microtubular microcavities were fabricated by self-rolling prestrained SiOx nanomembranes (see Supporting Information).23-28 Afterwards, a 16 nm thick silver layer was deposited onto the top of the microtubes, resulting in a silver nanocap positioned on the convex surface of the tube. The silver layer structure is revealed in the cross-sectional image of the microtube measured by scanning electron microscopy (SEM), as shown in Fig. 1(a). The thickness of the silver layer gradually changes with azimuthal position on the tube, which plays a key role in the tuning of the hybridization between photons and surface plasmons. Theoretical calculations show that the efficient coupling of TM modes and surface plasmons results in strong photon-plasmon hybridization at the top region of the silver nanocap, as shown in the top panel of Fig. 1(b). In contrast, the TE modes are significantly damped by the metal layer due to the polarization mismatch (see Fig. 1(c)). This is consistent with the results from a previous report,

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where TM polarized modes efficiently couple to surface plasmons at the metal layer surface while the TE polarized modes do not due to the polarization mismatch.13 The thickness of the silver nanocap layer gradually decreases from 16 nm (at θ=0°) to zero (at θ=±90°). In this range, the exterior field intensity of the TM polarized hybrid mode continuously decreases as θ increases, as the metal layer becomes too thin to separate and support the plasmon-type field from the inner photonic mode (see left panel of Fig. 1(b)). In contrast, the exterior field of the hybrid TE mode becomes stronger when θ increases, as the metal layer becomes too thin to shield the TE polarized field (see left panel of Fig. 1(c)). The variations of the evanescent field intensities for TM and TE modes can be recorded by monitoring the intensity changes of the resonant mode spectra measured at different azimuthal positions of the nanocap structure. Because a more intense/weak evanescent field leads to a stronger/weaker far-field emission, the emission intensity of the TM mode decreases while that of the TE mode increases when measuring at a larger θ position, as schematically illustrated in Fig. 1(d). In the experimental measurements, a confocal laser excitation setup was utilized to measure the optical resonant spectra of the microcavities (see SI). The silver-nanocap-coated microcavity was mounted on a rotational stage for azimuthal-angle-dependent measurements, as schematically shown in Fig. 2(a). The resonant spectra for both TM and TE modes measured at different θ are displayed in Fig. 2(b). The emission peaks of TM and TE modes change reversely which is consistent with the theoretical analysis discussed in Fig. 1(b) and (c). The intensity variations of the resonant modes are shown in Fig. 2(c) as a function of measured azimuthal angle. The relative intensities of the TM and TE modes, which represent the degree of the corresponding photon-plasmon hybridization, smoothly vary along the graded silver nanocap

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structure. In the middle-top position (θ=0°) of the silver nanocap the TM polarized light plays the key role to interact with the surrounding medium while in the lateral position (θ=±90°) the TE polarized light dominates the interactions, as indicated by the intensity changes in Fig. 2(b). In the range between 0° and ±90°, the competition between TM and TE polarization is present depending on the azimuthal angle. This angle-dependent tuning of TM and TE mode hybridization constitutes a novel and versatile platform for light-matter interactions in a single photonic device, where not only the coupling strength but also the optical field polarization can be tuned depending on the azimuthal angle. To further investigate the tuning effect on the hybrid photon-plasmon modes, an additional oxide layer was coated onto the metal nanocap. This additional oxide layer effectively changes the refractive index at the silver layer surface. Oxide layers have been utilized to study the tuning effect on mode shifts and polarization states of pure optical resonances in microtubular cavities.29, 30 Here, however, the hybrid photon-plasmon modes tuned by the oxide layer exhibit distinct angle and polarization dependencies owing to the metal nanocap structure. As shown in Fig. 3(a), an Al2O3 layer was homogenously deposited onto the silver cap by uniformly rotating the sample holder during deposition. The relative intensities of TM and TE modes are determined by polarization mappings measured at the top (θ = 0°) and side position (e.g. at 75°) after each oxide layer deposition, as shown in Fig. 3(b). Optical redshifts of both TM and TE modes are observed due to the presence of the oxide layers on the cavity surface. Similar mode shifts have been reported previously when microtubular cavities were coated by nano-layers.29,31 More interestingly, distinct changes in mode intensities are observed due to the tuning of the photon-plasmon hybridization at the middle-top of the nanocap. As the oxide layer thickness increases, the TM mode becomes weaker while the TE mode becomes stronger. Thus

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the intensity ratio η = ITM/ITE can be used for characterizing the relative intensity variation, as shown in Fig. 3(b). The intensity ratio η as a function of oxide layer thickness is displayed by the orange line (θ = 0°) in Fig. 3(c). As the deposited oxide layer increases, the intensity ratio η decreases due to the suppressed TM modes and enhanced TE modes. In contrast, when measured at θ = 75°, TM and TE modes show almost no changes in intensity ratio (see Fig. 3(b)), resulting in a nearly constant η as shown by the blue curve in Fig. 3(c). These phenomena are further explored by theoretical calculations which will be discussed below. The evanescent field distributions before and after oxide layer deposition are calculated to verify the variations of TM and TE modes. The intensity of the evanescent field, i.e. plasmontype field, of the TM mode drops when an oxide layer is deposited on the metal layer surface, as shown in Fig. 4(a) and (b). Because this additional oxide layer confines the TM field to the added dielectric medium, the formation of strong evanescent fields at the cavity outer surface is inhibited. On the contrary, the confinement of the TE field in the additional oxide layer leads to an increased evanescent field at the dielectric layer surface compared to that on the silver layer surface. Thus the measured intensity of TM mode decreases while that of TE mode increases after the oxide layer deposition. At θ = 75°, the hybrid TM modes have a weaker plasmon-type evanescent field as discussed in Fig. 1(b), which in turn are less prone to the perturbation of the added oxide layer, as shown in Fig. 4(c) and (d). Due to the weak hybridization, the plasmontype fields for both TM and TE remain nearly constant after oxide layer deposition. As such, the additional oxide coating layers can efficiently tune the strongly hybridized photon-plasmon modes at θ=0°, while they have almost no influence on the weakly hybridized modes (e.g. at θ=75°). The efficient tuning on strongly hybridized modes provides a novel sensing methodology. The sensing scheme uses the intensity variation rather than the conventional

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method of monitoring the mode shifts for surface sensing. Thus, this proposed sensing scheme does not rely on the quality (Q) factor of the resonant modes which is often required in conventional methods for surface nano-sensing.32-35 In the above we have shown that the polarization intensity ratio η is extremely sensitive to surface perturbations (i.e. oxide layers) at the nanoscale. To further clarify the behavior of the intensity ratio variation, systematic investigations of the intensity ratio η were theoretically carried out as a function of the permittivity variation ∆ε of the metal layer coated by dielectric (Al2O3) nanomembranes and the cavity wall thickness T. The dielectric nanomembrane deposition leads to a permittivity change of △ε of the metal/dielectric-nanomembrane, which in turn results in the variation of η. In addition, the intensity ratio changes at the top side for microcavities with different wall thicknesses were shown in Fig. 5. As reported previously, the thin cavity wall (around 200 nm) coated by the metal layer supports strongly hybridized photonplasmon modes, exhibiting intense exterior fields at the metal surface.13,14 When the cavity wall becomes thicker, the exterior field decreases due to the increased light confinement in the thick cavity wall. In agreement with those reports, the largest variation of η is observed in the thinwalled cavity with a thickness of around T = 175 nm. This wall thickness is close to the thinnest possible in optical microcavities as it will be too thin to confine any photonic modes if it further decreases. A sensitivity, defined as S=∆η/∆ε, where ∆η denotes the variations of intensity ration and ∆ε is the change of permittivity, is calculated as 0.573 per permittivity unit. The detection limit (DL) of sensing is defined as DL=R/S, where R is the sensor resolution. In conventional sensing applications based on the spectral shifts, 1/50 of the full width at half maximum of the resonant peak is used as the spectral resolution R. Here, R is defined as 1/50 of the peak intensity. As such, a DL of 0.068 is obtained which is higher than that for metal surface sensing

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based on the spectral shifts in cavities.36 In fact, various metal materials (e.g. Pd, Pt, Au, Al) can be used to provide alternative properties for enhanced surface sensing, such as Pd based hydrogen sensing, plasmonic enhanced photocatalysis, etc.37-39 As the cavity wall becomes thicker, the variation of η becomes less pronounced because of the decreased exterior field of the hybrid modes. Therefore, it can be concluded that the strongly hybridized modes are more sensitive to surface perturbations than the weakly hybridized modes in thick-walled cavities, which agrees well with the above observed experiment results. In summary, a metal nanocap structure was fabricated on rolled-up microtubular cavities for photon-plasmon coupling. An angle-dependent tuning of hybrid photon-plasmon modes was demonstrated in the optoplasmonic microcavities, where varying coupling strength and shielding effect are observed. As a result, the dominant polarization gradually converts from TM polarized at the middle-top of the nanocap to TE polarized at the sidewall of the microtube cavity. The intensity ratio of the strongly hybridized TM and TE modes is extremely sensitive to surface perturbations at the nanoscale, which provides a novel scheme for sensing applications. Theoretical calculations show that the intensity ratio variation originates from the distinct tunability of the TM and TE polarized hybrid modes, which is more pronounced in thin-walled microtubular optoplasmonic cavities. Our work demonstrates that a metal nanocap is capable to tune photon-plasmon coupling in optoplasmonic microcavities which is interesting for further fundamental studies of enhanced light-matter interactions and future relevant sensing applications.

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FIGURES

Figure 1. (a) Cross-sectional SEM images of a silver nanocapped microtubular cavity. The hybrid layers are shown in the top and left panels to highlight the nanocap structure. (b) and (c) show the optical field distributions in the silver nanocapped microtubular cavity for the TM and TE polarized modes, respectively. Distinct evanescent field distributions at two azimuthal positions are shown in the top and left panels. (d) Schematic diagram of the far-field mode intensity variations as the azimuthal angle changes.

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Figure 2. (a) Schematic shows the silver nanocapped microcavity being measured at different azimuthal angles θ. (b) Measured resonant peaks of both TM and TE modes in a silver nanocapped microcavity. (c) Intensity variations are plotted in the polar diagram as a function of the angle θ.

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Figure 3. (a) Schematic shows an oxide layer coated on a silver nanocapped microcavity. (b) Polarization mapping of TM and TE modes measured with different oxide layer thicknesses L. Distinct intensity variations are observed in addition to the mode shifts at θ=0° while no apparent intensity variation is observed at large angle of θ=75°. (c) Mode intensity ratios (η) and mode shifts of the TM and TE modes plotted as a function of the oxide layer thickness L.

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Figure 4. Calculated evanescent field of TM (top panel) and TE (bottom panel) at the azimuthal angle of θ=0° ((a) and (b)) and θ=75° ((c) and (d)) before and after deposition of an oxide nanolayer (10 nm thick).

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Figure 5. Intensity ratio (η) of the TM and TE modes calculated as a function of the permittivity variation △ε at the metal surface and the thickness of the microcavity wall. Inset shows the angle position where the modes were calculated.

ASSOCIATED CONTENT Supporting Information. Information on sample fabrication and spectral characterizations, theoretical modelling and calculation, measured mode intensity ratio η at various rotation angles in Figures S1-S7. AUTHOR INFORMATION Corresponding Author *Email: [email protected] ; *Email: [email protected]

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank R. Engelhard, S. Harazim, B. Eichler and S. Baunack for technical support. This work was supported by the DFG research group No. FOR 1713. Y.Y. acknowledges support by China Scholarship Council under file No. 201206090008.

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chemical sensors self-assembled from prestrained nanomembranes. Adv. Mater. 2013, 25, 23572361. 37. Tittl, A.; Mai, P.; Taubert, R.; Dregely, D.; Liu, N.; Giessen, H. Palladium-based plasmonic perfect absorber in the visible wavelength range and its application to hydrogen sensing. Nano Lett. 2011, 11, 4366-4369. 38. Tittl, A.; Giessen, H.; Liu, N. Plasmonic gas and chemical sensing. Nanophotonics 2014, 3, 157-180. 39. Mesch, M.; Zhang, C.; Braun, P. V.; Giessen, H. Functionalized hydrogel on plasmonic nanoantennas for noninvasive glucose sensing. ACS Photonics 2015, 2, 475-480.

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Silver Nanocap Enabled Conversion and Tuning of Hybrid Photon-Plasmon Modes in Microtubular Cavities Yin Yin,†,‡ Yan Chen,† Ehsan Saei Ghareh Naz,† Xueyi Lu,†,‡ Shilong Li,† Vivienne Engemaier,† Libo Ma,† Oliver G. Schmidt†,‡ †

Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstraße 20, 01069, Germany



Material Systems for Nanoelectronics, Technische Universität Chemnitz, 09111 Chemnitz,

Germany

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Angle-dependent tuning and conversion of TM and TE polarized modes is enabled by a metal nanocap coated on a microtube cavity.

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