The field identity of the long-range surface plasmon (LRSP) mode in an asymmetric metal dielectric structure is elucidated and it is shown that it can be pictured as having a zero crossing of the longitudinal electric field at the middle of the metal film. A parametric dependence between the metal and the dielectric layer thicknesses leading to a LRSP mode in an asymmetric structure is given. The sensitivity of an asymmetric four layer structure supporting a grating excited LRSP mode regarding sensing objectives has been investigated. It is compared with the sensitivity of a usual plasmon mode propagating along a metal–dielectric interface. The existence of an anomalous increase of the reflection coefficient in the case of the LRSP is observed theoretically and experimentally. The comparative study is made on the basis of analytical expressions which reveal that the LRSP does not bring a decisive advantage over the standard plasmon for sensor application but its specific features can be advantageously used once well understood.
The capacity to synthesize and design highly intricated nanoscale objects of different sizes, surfaces, and shapes dramatically conditions the development of multifunctional nanomaterials. Ultrafast laser technology holds great promise as a contactless process able to rationally and rapidly manufacture complex nanostructures bringing innovative surface functions. The most critical challenge in controlling the growth of laser-induced structures below the light diffraction limit is the absence of external order associated to the inherent local interaction due to the self-organizing nature of the phenomenon. Here high aspect-ratio nanopatterns driven by near-field surface coupling and architectured by timely-controlled polarization pulse shaping are reported. Electromagnetic coupled with hydrodynamic simulations reveal why this unique optical manipulation allows peaks generation by inhomogeneous local absorption sustained by nanoscale convection. The obtained high aspect-ratio surface nanotopography is expected to prevent bacterial proliferation, and have great potential for catalysis, vacuum to deep UV photonics and sensing.
We report advances in understanding how femtosecond laser ripples formation in metal works. The surface topology features are discussed in terms of periodicity and contrast of the pattern formation, related to chronological laser-induced events. Resonant excitation of Surface Plasmons (SP) in metallic gratings show that SP wave excited during the femtosecond laser pulse can initiate the observed patterning and play a key role. Metals behavior under nonequilibrium conditions on the picosecond timescale is then investigated to correlate the amount of material experiencing solid-to-liquid transitions and the subsequent ripples contrast. With the de-rived observation, a calculation of the transient nonequilibrium thermodynamic characteristics of excited nickel is also performed allowing defining characteristic timescales of thermocapillary proc-ess which may occur under multi- pulse irradiation.
Polarization filtering in a laser mirror is achieved by means of a corrugation grating defined in the last high index layer of multilayer mirror based on a metal or metallized substrate. The grating couples the undesired TE polarization to a high order propagation mode of the metal based multilayer stack. The resulting dip in the TE spectral reflection curve is wide enough to cover the gain bandwidth of a Nd:YAG active medium. A technology trimming scheme is designed and demonstrated to shift the TE reflection dip to 1064 nm.
Ultrafast light coupling with solid surfaces shows strong potential for nanostructuring applications, relying on the capacity to localize energy on a wide range of materials and to design periodic patterns involving transient matter transformations. Laser-Induced Surface Structures (LIPSS) consisting of regular periodic self-organized surface nanopatterns are often supposed to result from surface plasmon excitation, while the onset of periodic self-arrangement of matter is also observed in non-plasmonic materials. To shed a new light on this unsettling discrepancy, we have conducted a comprehension study devoted to define the excitation mechanisms from two perspectives, one related to the electromagnetic origin of spatial optical enhancement and one to the transient optical and thermo-mechanical response of the irradiated material. To identify the electromagnetic origin of localized photoexcitation arrangements, 3-dimensional Finite-Difference Time-Domain calculations have been carried out on semiconductors and metals [1]. An initially random distributed rough surface reveals that LIPSS can be initiated by the coherent superposition of far-field and near-field scattered waves and the refracted waves, potentially involving collective excitation on plasmonic materials. This indicates that a complex electromagnetic origin, mainly based on the coherence of the laser field, predicts a spatially-ordered energy deposition. The electromagnetic structured field highlights the role of the evolving surface topology on the pulse-to-pulse development of the pattern [2]. The transient optical response is then a second crucial question to identify the mechanisms responsible for spatial optical resonances. Under conditions of electron-phonon nonequilibrium [3], we have investigated by both ab initio calculations and pump-probe experiments the optical behavior of tungsten material, which is a typically non-plasmonic metal in infrared regime, potentially switching to a surface plasmonic state during ultrafast irradiation. Subtle effects on the electronic structure suggest that transient variation of optical properties can be large, impacting the surface response and fostering the collective electron charge excitation. Finally, the localized sub-surface region experiencing high photoexcitation has shown to undergo phase transformation and thermo-mechanical change at the nanoscale [4]. Dedicated electron backscatter diffraction experiments and transmission electron microscopy reveal that lattice defects are distributed inhomogeneously, depending on the surface crystal orientation. These are a signature of the resulting transient thermodynamic states following ultrafast laser energy absorption, with practical relevance on the feedback effects dependent on the different relaxation ways, i.e. defects formation or surface growing.
A sub-micron grating can be the key miniaturized planar element in Micro-opto-electromechanical systems where it performs a number of optical functions such as light routing, beam splitting, spectral-analysis, polarization filtering, beam recombination, spatial resonance excitation, in the domains of displacement measurement, biochemical sensors, environment monitoring, laser emission control, WDM communications, to quote a few current applications. This paper will describe the main stream technology of sub-micron gratings and illustrate its application potential.
The present work relates to the design, the fabrication and the spectroscopic characterisation of an efficient grating polarising structure which uses a quasi extra-cavity resonant grating in association with a multilayer to dictate the polarisation emitted by a semiconductor pumped solid-state Nd:YAG laser at 1.064 μm wavelength.
