Tomoya Miyanishi

Near field properties in the vicinity of gold nanoparticles placed on various substrates for precise nanostructuring

We present theoretical results on the near field distribution in the vicinity of gold particles excited by laser radiation and placed on various substrate materials. The study is directed towards the precise nanostructuring of different material surfaces. The calculations are performed on the basis of the FDTD simulation technique. Metal (Au), semiconductor (Si) and dielectric (SiO2) substrate materials are investigated. Experimental results are shown to confirm the validity of the obtained FDTD simulation results. The results show that the field in the vicinity of the point of contact is enhanced, and the enhancement factor depends on the substrate material, particle size and the wavelength of the incident optical radiation. The characteristic size of the field enhanced area is found to be several times smaller than the gold particle size: for the case of the smallest particle diameter of 40 nm it is about 10 nm. The enhancement factor of the electric field on the substrate surface is highest when the substrate is gold metal and it decreases about two orders of magnitude when dielectric SiO2 substrate is used. The characteristic penetration depth of the enhanced field is within several tens of nanometres and the depth weakly depends on the substrate material and the laser excitation wavelength. The dependence of the gold particle size on the field enhancement factor is investigated as well. The present results can be used to predict and design precise nanostructuring on different materials with gold nanoparticles excited by a femtosecond laser pulse.

Near field distribution in two dimensionally arrayed gold nanoparticles on platinum substrate

Theoretical and experimental results for near field properties in the vicinity of two dimensionally aligned gold nanoparticles are presented. The numerical analysis is based on finite difference time domain simulation code. The simulated system consists of gold particles with a radius of 100nm, deposited on platinum substrate. The near field distribution on the substrate surface and its magnitude are found to depend on the interparticle distance. The experimental results obtained confirm the theoretical findings and demonstrate that the produced near field can result in a permanent substrate surface nanomodification and selective nanoparticle removal.

Plasmonic and Mie scattering control of far-field interference for regular ripple formation on various material substrates

We present experimental and theoretical results on plasmonic control of far-field interference for regular ripple formation on semiconductor and metal. Experimental observation of interference ripple pattern on Si substrate originating from the gold nanosphere irradiated by femtosecond laser is presented. Gold nanosphere is found to be an origin for ripple formation. Arbitrary intensity ripple patterns are theoretically controllable by depositing desired plasmonic and Mie scattering far-field pattern generators. The plasmonic far-field generation is demonstrated not only by metallic nanostructures but also by the controlled surface structures such as ridge and trench structures on various material substrates.

Nano-dimple processing of silicon surfaces by femtosecond laser irradiation with dielectric particle templates in the Mie scattering domain

Dielectric particles sized comparable to the wavelength of light mounted on silicon substrates are irradiated with 800 nm femtosecond laser pulses. From this template of dielectric particles, a novel and interesting optical intensity distribution of the femtosecond laser irradiation is obtained. A result of this optical intensity distribution is a distinct pattern on the silicon substrate, which stems from the micro-lens and Mie scattering mechanism by the dielectric particles. In this paper, we investigated the dependence of the particle size, determined by the equation for size parameter α = 2πr/λ where r is the radius of the dielectric particle and λ is the incident laser wavelength, on the optical intensity distribution using the finite differential time domain method. A change in the size parameter induces a significant change in the optical intensity distribution in the vicinity of the particle. The distribution of the near-field intensity is analysed by its fingerprint on a substrate where the particle is deposited and irradiated by the femtosecond laser pulse. Using this method, we define the boundary between the lens effect and the contribution from Mie scattering. The experimental results indicate that the generated near-optical intensity, mediated by the dielectric particles, can produce a nano-hole with a size that overcomes the diffraction limit. Specifically, given certain boundary conditions, the processed nano-hole features have a characteristic shape governed by the incident light polarization, which has an ellipsoidal shape with the long axis perpendicular to the polarization of the incident light. In the case of using dielectric particles smaller than the incident wavelength, the contribution of the lens effect diminishes and the optical field intensity distribution is determined predominantly by the Mie scattering mechanism.

Substrate nanomodification based on heating and near field properties of gold nanoparticles

Theoretical and experimental results on nanosized modifications of substrates placed in the near field zone of metal nanoparticles are demonstrated. Gold nanoparticles with diameter of 200 nm are deposited on different substrates (dielectric, semiconductor, and metal) and irradiated by ultrashort laser pulses at wavelength of 800 nm. Formation of surface modification under the nanoparticles is observed at laser fluences below the modification threshold of the bulk substrate. The mechanisms of the surface modifications are explained by the heating of the nanoparticles and by the local field enhancement in their vicinity. The heating of the nanoparticle is described by two-temperature diffusion model as the input optical properties of the nanoparticles are evaluated on the basis of the Mie theory. The near field distribution is obtained by finite difference time domain (FDTD) simulation. The influence of the incident irradiation properties and the dielectric properties of the substrates on the modification characteristics is discussed.

Directionally controlled plasmon excitation in gold nanoparticles for near-field nanopatterning by femtosecond laser

We present theoretical and experimental results of nanohole fabrication on a silicon substrate by plasmonic near field around multiple gold nanoparticles excited by oblique incidence of femtosecond laser. Using the enhanced near field around a gold nanoparticle, nanohole can be fabricated on the substrate surface even at the near-infrared laser excitation. The formation of nanoholes with the near-infrared incident wavelength will open up smart applications for new optical device fabrication in air. However, the plasmons inside gold nanoparticles are affected by the plasmons of neighboring gold particles, resulting in an alteration of near-field distribution and the shift of resonant wavelength of plasmons inside the gold particles. These cause inhomogeneous shape of nanoholes and decrease the near-field intensity on silicon substrate. Therefore it is necessary to control the direction of plasmon for precise nanohole fabrication. We propose a new method to reduce plasmon interaction between particles by using p-polarized irradiation at oblique incidence to the substrate surface. Experimental and theoretical study demonstrated that uniformed nanoholes can be achieved by restraining effects from neighboring plasmon when p-polarized beam is irradiated to gold nanoparticle arrays with optimal oblique incidence.

Nanostructure processing by near-field with femtosecond laser excitation: process switching and SERS application

This paper describes two topics. (1): Nano-processing by near-field optics can fabricate nano-scale structures even with near-infrared 800 nmTi:saphire laser. New phenomena using particles, leading to a new nano-processing technique via plasmonics, even with the use of dielectric particles is reported. The physics of nano-hole fabrication process is switchable simply by the laser fluence. (2): ZnO nanorod arrays on Si (100) substrate were grown by pulsed laser deposition (PLD) method, and then coated with Au. Two samples of Au-coated nanorod arrays with different average diameters of 150 nm and 400 nm were prepared to investigate the size dependence of the surface enhanced Raman scattering (SERS). The diameter of the nanorods was well controllable by the substrate position during PLD. High SERS enhancement was observed from both Au-coated ZnO nanorod arrays. The Raman spectra of Rhodamine 6G (R6G) as low as 1 nM were measured with average diameter of 400 nm at an excitation wavelength of 532 nm.

Uniform near-field nanopatterning due to the field distribution control by oblique femtosecond laser irradiation to silicon and gold nanoparticles

We present near-field optical properties around silicon and gold nanoparticles aligned on a silicon substrate excited by oblique incidence femtosecond laser for nanohole processing. Near-field nanofabrication will open up smart applications for new optical devices with high-throughput processing. The near field around silicon and gold particles is explained by Mie scattering theory, while the near field around gold nanoparticles is explained by plasmon polaritons inside nanoparticles. With gold nanoparticles, theoretical study revealed that the incident laser energy is concentrated into the contact point between the particle and the substrate due to the image charge inside the substrate at any incident angles. With particles with a dielectric constant as high as silicon, the polarized charge shows a similar effect to the plasmon charge. Therefore the distribution of the concentrated energy provided with silicon nanoparticles is similar to that of gold nanoparticle.

Nano-surface patterning by femtosecond laser for plasmonic surface optical applications

For plasmonic surface optical applications, localized optical field distribution properties in the vicinity of gold particles on a silicon substrate by backward and forward irradiation are presented. It is technically difficult to fabricate nanostructure on the surface by conventional forward laser incidence to the substrate because gold nanoparticles easily aggregate to form double-layered particle arrays. We calculated enhanced optical field properties in order to pattern the substrate surface only with a template of the bottom-layered particle arrays in case that the backward irradiation of femtosecond laser is used in the system of aggregated double-layered gold nanoparticle arrays. With the backward irradiation, the optical field intensity in the substrate for the double-layered hexagonal arrays is found to be only 30% lower than the mono-layered system. Moreover, near-field cannot be generated with the forward irradiation. As a result, only the backward irradiation scheme is found to be effective for uniform surface nanopatterning at enhanced plasmonic near-field zones.

Nanoprocessing of silicon substrate using surface plasmon polaritons of gold particle and polystyrene particle excited by femtosecond laser

Nanohole processing of silicon substrate using surface plasmon polaritons of nano gold excited by femtosecond laser is described in comparison with the nanohole processing with transparent polystyrene (PS) nanoparticle template. Gold particles with diameters of 40, 80, or 200 nm are spin coated on the substrate, and a 100 fs, 820 nm laser pulse is used to irradiate the samples. The produced holes are analyzed by scanning electron microscopy and atomic force microscopy. A theoretical analysis of the experimental results is conducted by FDTD (Finite Difference Time Domain) simulation. The dependence of the laser fluence and particle size on the nanohole properties is studied. The nanohole profiles correspond to the field distributions on the Si substrate at low fluence region. A highest electric field enhancement factor of about 26 is obtained for gold particles with a diameter of 200 nm.