Optical Responses of Localized and Extended Modes in a Mesoporous Layer on Plasmonic Array to Isopropanol Vapor

Mesoporous silica features open and accessible pores that can intake substances from the outside. The combination of mesoporous silica with plasmonic nanostructures represents an interesting platform for an optical sensor based on the dependence of plasmonic modes on the refractive index of the medium in which metallic nanoparticles are embedded. However, so far only a limited number of plasmonic nanostructures are combined with mesoporous silica, including random dispersion of metallic nanoparticles and fl at metallic thin fi lms. In this study, we make a mesoporous silica layer on an aluminum nanocylinder array. Such plasmonic arrangements support both localized surface plasmon resonances (LSPRs) and extended modes which are the result of the hybridization of LSPRs and photonic modes extending into the mesoporous layer. We investigate in situ optical re fl ectance of this system under controlled pressure of isopropanol vapor. Upon exposure, the capillary condensation in the mesopores results in a gradual spectral shift of the re fl ectance. Our analysis demonstrates that such shifts depend largely on the nature of the modes; that is, the extended modes show larger shifts compared to localized ones. Our materials represent a useful platform for the fi eld of environmental sensingEspaña MINECO grant MAT2017-88584-R

貴金属を用いないプラズモニックアレイの作製と光物性

京都大学0048新制・課程博士博士(工学)甲第21113号工博第4477号新制||工||1696(附属図書館)京都大学大学院工学研究科材料化学専攻(主査)教授 田中 勝久, 教授 三浦 清貴, 教授 作花 哲夫学位規則第4条第1項該当Doctor of Philosophy (Engineering)Kyoto UniversityDGA

Plasmonic mesostructures with aligned hotspots on highly oriented mesoporous silica films

Gold mesostructures are fabricated by oblique angle deposition on highly oriented mesoporous silica (MPS) thin films utlizing the periodic surface corrugation on the scale of 10 nm as a prepattern for controlled deposition. Scanning electron microscopy analysis clarified that the mesostructures comprised an array of interconnected gold nanorods oriented in plane to form meso gratings. We measured the surface enhanced Raman scattering (SERS) to demonstrate that the nanosized gaps between the rods act as hotspots. Although the sample was as thin as 8.0 nm, large SERS signals appeared because of the very narrow gaps (< 2 nm). The spatial mapping confirmed the uniform distribution of hotspots over the sample

Enhanced Photoluminescence from Organic Dyes Coupled to Periodic Array of Zirconium Nitride Nanoparticles

Noble metals, particularly gold, have been conventionally used for their suitable optical properties in the field of plasmonics. However, gold has a relatively low melting temperature, especially when nanosized, and the abundance of gold in the earth’s crust is low. These material-related limitations hinder the exploration of the use of plasmonics in several application areas. Transition metal nitrides are promising material alternatives because of their high mechanical and thermal stabilities, in addition to their acceptable plasmonic properties in the visible spectral region. Zirconium nitride (ZrN) is one such promising alternative owing to a higher carrier density than that of titanium nitride (TiN), which has been the most studied complementary material to gold. In this study, we have fabricated periodic arrays of ZrN nanoparticles and found that the ZrN array enhances the photoluminescence from an organic dye on the array; the photoluminescence intensity is increased by as much as 9.7× in the visible region. This result experimentally verifies that ZrN is useful as an alternative material to gold, to further develop plasmonics, and mitigate the conventional material-related limitations

Enhanced photoluminescence and directional white-light generation by plasmonic array

次世代の指向性白色光源の開発に成功 --ナノアンテナで明日を照らす--. 京都大学プレスリリース. 2018-12-06.White light-emitting diodes (LEDs), light sources that combine blue LEDs and yellow phosphors, are equipped with bulky optics such as lenses, mirrors, and/or reflectors to shape the light into the required directions. The presence of bulky optics causes optical loss and limits the design. Here, a periodic array of metallic nanocylinders, which exhibits a high scattering efficiency owing to the excitation of localized surface plasmon resonance, is proposed as an alternative means of achieving a directional output without the limitations of bulky optics. A prototype of a directional light emitter is fabricated consisting of an Al nanocylinder array on a yellow phosphor plate and a blue laser. The array shapes the yellow luminescence into the forward direction and generates directional quasi-white light (correlated color temperature of 4900 K). The intensity enhancement reaches a factor of five in the forward direction and is further improved up to a factor of seven by the deposition of a multilayer dichroic mirror on the back side of the phosphor plate, resulting in conversion efficiencies as high as 90 lm/W. Our results pave the way toward the development of efficient and compact directional white-light-source devices without any bulky optics

Surface-Enhanced Infrared Absorption for the Periodic Array of Indium Tin Oxide and Gold Microdiscs: Effect of in-Plane Light Diffraction

Surface-enhanced infrared absorption (SEIRA) is an important phenomenon to achieve nondestructive, simplified, and <i>in situ</i> high-sensitivity infrared (IR) sensors. Conventionally, metal structures with nanogaps are employed to realize the high sensitivity owing to the extremely strong field enhancement in the hot spot. Although a library of surface modifiers has been developed, the manipulation of nanogaps and immobilization of target molecules in the hot spot are still complicated. In addition, target molecules immobilized at the positions other than the hot spot have relatively low sensitivity. A periodic array with pitch comparable to the wavelength of interest is an alternative structure in which the coupling of the plasmonic mode to in-plane light diffraction provides the hybrid mode accompanied by an enhanced electric field. Although the field enhancement by the hybrid mode depends on matching between localized surface plasmon resonance (LSPR) and diffraction, the contribution of the matching to SEIRA enhancement has never been clarified. In this work, we fabricated periodic arrays of indium tin oxide (ITO) and Au microdiscs (pitch: 3 μm, diameter: 2 μm) to analyze the contribution of the hybrid mode through varied LSPR and diffraction conditions. As a result, the ITO and Au arrays demonstrate a similar plasmonic–photonic hybrid mode in the mid-IR region despite the different excitation frequency of LSPR. To estimate the effect of the hybrid mode on SEIRA enhancement, the incident angular profiles of IR spectra of the polymer layer on the ITO and Au arrays were measured. The SEIRA enhancement factors for ITO and Au arrays are comparable in the IR measurement region (2200–1400 cm^–1). Our results verify that the plasmonic–photonic hybrid mode is very efficient for SEIRA enhancement, and the periodic array of microdiscs is very suitable for this application

Plasmonic–Photonic Hybrid Modes Excited on a Titanium Nitride Nanoparticle Array in the Visible Region

Conventionally used plasmonic materials generally have low thermal stability, low chemical durability (except gold), and are incompatible with complementary metal–oxide semiconductor processes. However, titanium nitride (TiN), an emerging plasmonic material, possesses gold-like optical properties, but displays relatively large ohmic losses. We fabricated a periodic array of TiN nanoparticles to effectively reduce these losses by coupling the localized surface plasmon resonance with light diffraction. The height of the nanoparticle and the periodicity of the array were designed to match the excitation conditions of both the localized surface plasmon resonance and light diffraction. As a result, the array supported a plasmonic–photonic hybrid mode in the visible region. For the loss mitigation effect to be assessed, photoluminescence (PL) from the light emitting layer on the array was measured. The PL intensity was larger than that from the same layer on a TiN thin film, demonstrating reduced loss. The angular and spectral profiles of the PL could be controlled by the hybrid mode. Our results thus pave the way toward plasmonic devices that can be fabricated using traditional complementary metal–oxide semiconductor processes