Ryosuke Kamakura

Plasmonic arrays of titanium nitride nanoparticles fabricated from epitaxial thin films

We have fabricated two-dimensional periodic arrays of titanium nitride (TiN) nanoparticles from epitaxial thin films. The thin films of TiN, deposited on sapphire and single crystalline magnesium oxide substrates by a pulsed laser deposition, are metallic and show reasonably small optical loss in the visible and near infrared regions. The thin films prepared were structured to the arrays of nanoparticles with the pitch of 400 nm by the combination of nanoimprint lithography and reactive ion etching. Optical transmission indicates that the arrays support the collective plasmonic modes, where the localized surface plasmon polaritons in TiN nanoparticles are radiatively coupled through diffraction. Numerical simulation visualizes the intense fields accumulated both in the nanoparticles and in between the particles, confirming that the collective mode originates from the simultaneous excitation of localized surface plasmon polaritons and diffraction. This study experimentally verified that the processing of TiN thin films with the nanoimprint lithography and reactive ion etching is a powerful and versatile way of preparing plasmonic nanostructures.

Directional outcoupling of photoluminescence from Eu(III)-complex thin films by plasmonic array

A plasmonic array, consisting of metallic nanocylinders periodically arranged with a pitch comparable to the optical wavelength, is a system in which both the localized surface plasmon polaritons (SPPs) and diffraction in the plane of the array are simultaneously excitable. When combined with a phosphor film, the array acts as a photoluminescence (PL) director and enhancer. Since the array can modify both excitation and emission processes, the overall modification mechanism is generally complex and difficult to understand. Here, we examined the mechanism by simplifying the discussion using an emitter with a high quantum yield, large Stokes shift, and long PL lifetime. Directional PL enhancement as large as five-fold occurred, which is mainly caused by outcoupling, i.e., the PL trapped in the emitter film by total internal reflection is extracted into free space through the SPPs and diffraction. The present scheme is robust and applicable to arbitrary emitters, and it is useful for designing compact and ef...

Excitation of collective plasmonic modes and photoluminescence enhancement in the Al nanocylinder array

Periodic arrays of metallic nanocylinder combined with the luminous layer are the good platform to control the properties of photoluminescence (PL). The array structure is especially useful to directionally enhance PL from the emitters of high quantum yield, in sharp contrast to a vast majority of the plasmonic nanostructures which decrease the PL intensity from such good emitters. The trick is to use collective plasmonic mode, i.e., the radiative coupling of surface plasmon polaritons via the in-plane diffraction. In this study, we demonstrated the directional PL enhancement using the high quantum yield emitters such as Eu(III)-complex and rhodamine 6G on the periodic array of Al nanocylinders. The result is useful for designing the compact and efficient luminescence source with directional output.

Optical characterization and emission properties of periodic arrays of titanium nitride nanoparticles

Various metallic nanostructures have been proposed to effectively utilize surface plasmon polaritons (SPPs) to improve the performance of optical devices such as solar cells, antennas, sensors, and light emission devices. With the progress in the research on SPP-based devices, the requirements for better materials have been increasing. Conventional SPP materials such as silver and gold have problems in processing; they are incompatible with reactive ion etching and they cannot be processed at high temperatures. Transition metal nitrides are promising candidates for the complementary material for noble metals. Especially, titanium nitride (TiN) attracts attention because of the compatibility with nanofabrication processes and the plasmonic properties in the visible and the near-infrared regions. So far, the plasmonic properties of TiN have been investigated mainly in the near-infrared region. In this study, we have fabricated the periodic arrays of TiN nanoparticles by using nanoimprint lithography and reactive ion etching processes to investigate the optical response in the visible. The periodic arrays allow the simultaneous excitation of SPPs of TiN nanoparticles and the optical diffraction in the plane of the arrays, resulting in the collective plasmonic mode where the SPPs radiatively coupled via diffraction. Combining fluorescent dye with the TiN arrays, we demonstrate the directionally-enhanced emission in the visible as large as 3 times in intensity by coupling the emission to the collective mode.

Excitation of surface plasmon polaritons on titanium nitride thin films through energy transfer from dye molecules

We have fabricated titanium nitride (TiN) thin films on sapphire substrates by pulsed laser deposition (PLD) technique, and excited surface plasmon polaritons (SPPs) on TiN thin films by energy transfer from dye molecules. The system was fabricated by depositing a thin polymer layer containing rhodamine 6G (R6G) on a TiN thin film, separated by a thin spacer made of silica. The SPPs were excited by irradiating the dye layer with a blue laser. We successfully collected radiation from SPPs by converting them into far-field radiation using a prism coupling technique.

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.