Plasmonic and Mie scattering control of far-field interference for regular ripple formation on various material substrates.
ABSTRACT 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.
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ABSTRACT: We analyze both experimentally and theoretically the physical mechanisms that determine the optical transmission through deep sub-wavelength bull's eye structures (concentric annular grooves surrounding a circular hole). Our analysis focus on the transmission resonance as a function of the distance between the central hole and its nearest groove. We find that, for that resonance, each groove behaves almost independently, acting as an optical cavity that couples to incident radiation, and reflecting the surface plasmons radiated by the other side of the same cavity. It is the constructive contribution at the central hole of these standing waves emitted by independent grooves which ends up enhancing transmission. Also for each groove the coupling and reflection coefficients for surface plasmons are incorporated into a phenomenological Huygens-Fresnel model that gathers the main mechanisms to enhance transmission. Additionally, it is shown that the system presents a collective resonance in the electric field that does not lead to resonant transmission, because the fields radiated by the grooves do not interfere constructively at the central hole.Optics Express 05/2011; 19(11):10429-42. · 3.55 Impact Factor
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ABSTRACT: The effects of optical resonance and near field in the interaction of transparent particles on a substrate with laser light have been examined experimentally and theoretically. It is found that pits can be created at the contacting point between the particle and the metallic surface by laser irradiation (KrF,λ=248 nm) with a single pulse. The influence of the particle size and the laser fluence on the structuring of the surface has been investigated. The size of the particle ranges from 1.0 μm to 140 nm in diameter. The morphologies of the holes created have been characterized by an atomic force microscope and a scanning electron microscope. For constant laser fluence, the created hole is sensitive to the particle size. For higher-laser fluence, the corresponding hole becomes larger and deeper. With a low fluence of 300 mJ/cm2 and for 140 nm particles, the lateral dimensions of created pits can be down to 30 nm. With a high fluence of 750 mJ/cm2 and 1.0 μm particles, the diameter and the depth of created holes are about 350 and 100 nm, respectively. Theoretical calculations and an accurate solution of a boundary problem indicate that incident light could excite some resonance modes inside the particle and produce enhanced light intensities on the contacting area (substrate surface). The light intensity on the contacting area is nonuniform and sensitive to the particle size parameter. Experimental results are explained and are very consistent with those of theoretical calculations. The experimental results also provide direct evidence of the optical resonance and near-field effects in the interaction of transparent particles on the substrate with laser light. © 2002 American Institute of Physics.Journal of Applied Physics 08/2002; 92(5):2495-2500. · 2.21 Impact Factor
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ABSTRACT: e-mail: email@example.com Published online: 18 December 2005; doi:10.1038/nphys191 S hort-pulse laser ablation is promising owing to the low threshold for material removal from surfaces. In the laser-ablation process, solid material transforms into a volatile phase initiated by a rapid deposition of energy. Explosive boiling can be one of the mechanisms in which matter is heated close to the critical point. Other pathways of non-thermal excitation will be present for very short laser pulses 1 . Here we observe a different channel of ablation from gold nanoparticles, which takes place below the particle melting point. This process is induced by the optical near-field, a subwavelength field enhancement close to curved surfaces, in particular. Using picosecond X-ray scattering, we can track the temporal and energetic structural dynamics during material ejection from the nanoparticles. This effect will limit any high-power laser manipulation of nanostructured surfaces, such as surface-enhanced Raman measurements 2 or plasmonics with femtosecond pulses. Ablation can be characterized as a thermal process, where the material is allowed to thermalize to form a superheated liquid, before ejection sets in 3,4 . The limit of superheating, which determines the ablation threshold, is related to the spinodal temperature, at which no barrier for vaporization persists. Thermalization can be reached within times exceeding the electron–phonon and phonon–phonon scattering times 5 . For laser pulses shorter than this limit, the explosive boiling will be independent of the pulse length. In addition to the thermal regime, other channels of non-thermal structure modification exist. These will become important for femtosecond excitation. A well-studied example is plasma formation, an important mechanism for transparent media 1 . The so-called dielectric breakdown is caused by multiphoton absorption over the bandgap of dielectrics. Other non-thermal processes are observed in highly excited semiconductors 6 . Both pathways of material disintegration and ablation will be established at threshold laser fluences considerably above the limits of reversible interaction and material melting (0.4–1.4 J cm −2 for 100 fs to 1 ns pulses 1 , 0.25 J cm −2 from theoretical calculations 7 , both ablation thresholds on gold films; or 2 J cm −2 for dielectric breakdown in fused silica 1). The comparison to values of bulk gold is possible in the present case owing to the relative insensitivity of the interband absorption to geometry. The deposited energy is typically an order of magnitude above the threshold for the melting phase transition in gold. On the other hand, ablation processes have been observed in the vicinity of small metallic structures on a surface, and explained through the optical near-field effect 8 . At edges with high curvature, the evanescent field of the excited structures can reach enhancement factors of several orders of magnitude in comparison with the incident field. Surface-enhanced Raman spectroscopy takes advantage of this effect by amplifying the very weak Raman signal of molecules near corrugated metallic surfaces 2 . Understanding the fundamental interactions in an atomistic description requires the development of techniques that resolve the dynamics temporally as well as spatially. Ultrafast spectroscopy is a tool to resolve electronic and structural changes accompanied by the laser excitation. However, the spectral fingerprints of electron excitation, altered dielectric environment, particle size change or emerging clusters all tend to overlap in the visible light, making the separation of effects extremely difficult. Work on ultrafast X-ray (and electron) scattering on photo-excited matter has led to the discovery of new modes of phase transitions 6,9 such as non-thermal melting of densely excited semiconductors. Scattering experiments resolved the molecular rearrangements of photo-induced phase transitions wit angström precision, as in the case of bistable charge-transfer compounds 10 . Picosecond photo-electron spectroscopy has given insight into the structure of ejected material from a silicon surface 11 . This study presents an observation of ablation from 38-nm gold nanoparticles suspended in water. The energy transduction for gold nanoparticles as model probes for electron dynamics is very well understood (homogeneous electron excitation, followed by electron–phonon coupling, transfer of heat to the environment 12,13). Sparser data are available on the ionic dynamics; however, it is commonly agreed that most of the structural dynamics observed for particles larger than 5–10 nm are well within the continuum description 14–16 . A combination of time-resolved X-ray techniques allows us to render a complete picture of the structural rearrangement of the particle crystalline structure and shape, and the interaction with the water phase. The techniques are explained in more detail in the Supplementary Information.Nature Physics 01/2005; 2(1). · 19.35 Impact Factor