2019 Publications

Nanosecond laser-induced interference grating formation on silicon

R.J. Pelaez, E. Rebollar, R. Serna, C. Acosta-Zepeda, P. Saavedra, J. Bonse, E. Haro-Poniatowski
Journal of Physics D-Appl. Phys. 52, 225302 (2019).

Abstract

The formation of gratings on the surface of a silicon wafer by nanosecond laser irradiation through a phase mask using an ArF laser emitting at 193 nm is studied. The phase mask along with some focusing optics is capable to generate via interference a periodic intensity distribution, which can be used for surface patterning. The surface patterning strongly depends on the laser energy density and on the number of pulses, as revealed by atomic force microscopy (AFM). The results show that irradiation even with a single laser pulse produces periodic depth modulations on the surface. The spatial surface modulation is in the micrometer (1.7 µm) range while the depth modulation is in the nanometer regime (1–20 nm). With an increasing number of pulses (1–100), the depth modulation amplitude increases smoothly. Increasing the number of pulses further results in the progressive destruction of the grating, vanishing completely after ~5000 pulses. This evolution is also monitored in situ by measuring the intensity of the first order-diffracted probe beam and the behavior is in accordance with what is observed by AFM. Finally, we qualitatively explain the results invoking thermally induced effects in the melted Si: these physical processes involved are probably thermocapillary and/or Marangoni effects inducing material displacement as the surface melts.

Ellipsometric characterization of Bi and Al2O3 coatings for plasmon excitation in an optical fiber sensor

E. Rodríguez-Schwendtner, A. Álvarez-Herrero, A. Mariscal, R. Serna, A. González-Cano, M.-C. Navarrete, N. Díaz-Herrera.
J. Vac. Sci. Technol. B: Nanotechnology and Microelectronics 37, 062914 (2019).

Abstract

The authors present the results of the ellipsometric characterization of thin layers of bismuth and aluminum oxide deposited over the waist of a tapered optical fiber by pulsed laser deposition. The characteristics of the deposits are studied by spectroscopic ellipsometry. From the effective thicknesses determined by the ellipsometric characterization, it is shown by simulations that surface plasmon resonances (SPRs) can occur in the fiber device, and it is demonstrated experimentally. These results show the feasibility of employing bismuth as a plasmonic material in SPR fiber sensors based on doubly-deposited uniform-waist tapered optical fibers, which show excellent performance and versatility.

Tailoring metal-dielectric nanocomposite materials with ultrashort laser pulses for dichroic color control

N. Sharma, N. Destouches, C. Florian, R. Serna, J. Siegel.
Nanoscale 11, 18779-18789 (2019).

Abstract

Metal-dielectric nanocomposites are multiphase material systems containing nanostructures, whose size and arrangement determine the optical properties of the material, enabling the production of new materials with custom-designed response. In this paper, we exploit a femtosecond laser-based strategy to fabricate nanocomposites based on silver nanoparticles (Ag NPs) with tunable optical spectral response. We demonstrate how the spectral response, specifically color and dichroic response, is linked to Ag NPs growth and self-organization processes that are controlled locally by the choice of the laser irradiation parameters, such as scan speed and laser light polarization. When the scan speed increases, the Ag NPs are formed at larger depths below the film surface and give rise to the formation of embedded NPs gratings. As a result, the effective optical properties of the films are strongly modified enabling the display of a broad range of solid colors in the visible region. Furthermore, the choice of the laser light polarization allows to fabricate films either with iridescent or dichroic properties (linear polarization) or with non-diffractive and non-dichroic colors (circular polarization). Finally, the high spatial control over the transformed areas achieved with the laser processing, allows the building of hybrid nanostructures by means of interlacing structures with different optical responses. These results demonstrate the high potential of fs-laser technology to process Ag-based nanocomposites to fabricate coatings with a designed reflectivity, transmission, diffraction, as well as polarization anisotropy response. The Ag nanocomposites investigated in this work hold great promise for a broad range of applications especially for coloring, for enhanced visual effects, and for smart information encoding for security applications.

Femtosecond x-ray diffraction reveals a liquid–liquid phase transition in phase-change materials

P. Zalden, F. Quirin, M. Schumacher, J. Siegel, S. Wei, A. Koc, M. Nicoul, M. Trigo, P. Andreasson, H. Enquist, M. J. Shu, T. Pardini, M. Chollet, D. Zhu, H. Lemke, I. Ronneberger, J. Larsson, A. M. Lindenberg, H. E. Fischer, S. Hau-Riege, D. A. Reis, R. Mazzarello, M. Wuttig, and K. Sokolowski-Tinten
Science 364, 1062 – 1067 (2019).

Abstract

In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.

Liquid switchable radial polarization converters made of sculptured thin films

M. Oliva-Ramírez, V. J. Rico, J. Gil-Rostra, O. Arteaga, E. Bertran, R. Serna, A. R. González-Elipe, and F. Yubero.
Appl. Surf. Sci. 475, 230–236 (2019).

Abstract

A radial polarization converter is a super-structured optical retarder that converts a conventional linearly polarized light beam into a structured beam with radial or azimuthal polarization. We present a new type of these sophisticated optical elements, which is made of porous nanostructured sculptured single thin films or multilayers prepared by physical vapor deposition at an oblique angle. They are bestowed with an axisymmetric retardation activity (with the fast axis in a radial configuration). In particular, a Bragg microcavity multilayer that exhibits a tunable transmission peak in the visible range with a retardance of up to 0.35 rad has been fabricated using this methodology. Owing to the highly porous structure of this type of thin films and multilayers, their retardance could be switched off by liquid infiltration. These results prove the possibility of developing wavelength dependent (through multilayer optical design) and switchable (through vapor condensation or liquid infiltration within the pore structure) radial polarization converters by means of oblique angle physical vapor deposition.

Optical Properties of Bismuth Nanostructures Towards the Ultrathin Film Regime

Optical Properties of Bismuth Nanostructures Towards the Ultrathin Film Regime

J. Toudert, R. Serna, C. Deeb, and E. Rebollar.
Opt. Mater. Express 9, 2924–2936 (2019).

Abstract

Bulk bismuth presents outstanding optical properties, such as a giant infrared refractive index (n∼10) and a negative ultraviolet-visible permittivity induced by giant interband electronic transitions. Although such properties are very appealing for applications in nanophotonics, the dielectric function of bismuth nanostructures has been scarcely studied. Here, we determine by spectroscopic ellipsometry the far infrared-to-ultraviolet dielectric function of pulsed laser deposited bismuth thin films with nominal thickness tBi varied from near 10 nm to several tens of nm. For tBi > 15 nm, the films display a continuous structure and their dielectric function is comparable with that of bulk bismuth. For tBi < 15 nm, the film structure is discontinuous, and the dielectric function differs markedly from that of bulk bismuth. It is proposed from FDTD simulations that this marked difference arises mainly from effective medium effects induced by the discontinuous film structure, where quantum electronic confinement does not play a dominant role. This suggests that ultrathin and continuous bismuth films should present the same outstanding optical properties as bulk bismuth for high performance nanophotonic devices.

Influence of the Zn plasma kinetics on the structural and optical properties of ZnO thin films grown by PLD

J. A. Guerrero de León, A. Pérez-Centeno, G. Gómez-Rosas, A. Mariscal, R. Serna, M. A. Santana-Aranda, and J. G. Quiñones-Galván.
SN Appl. Sci. 1, (2019).

Abstract

We analyze the effect of the average kinetic energy of Zn ions on the crystalline orientation and quality, along with its effect on the optical response of ZnO thin films deposited by reactive pulsed laser deposition. With the use of laser attenuators, the Zn plasma density was kept constant while the mean kinetic ion energy was varied from 40 to 100 eV. The results show that the films are polycrystalline with a preferential orientation along the [101] direction. As the mean kinetic energy of the ions in the plasma is increased, the degree of crystallinity decreased. The preferred orientation was found to be a plasma-related effect. As deposited films show high transmittance in the visible range and an intense UV photoluminescence emission, which is associated to the excitonic recombination.

Conformal covering and optical response of pulsed laser deposited bidimensional Ag nanoparticle arrays

E. Soria, G. Baraldi, M. Martinez-Orts, J. Toudert, R. Serna, and J. Gonzalo.
Appl. Surf. Sci. 473, 442–448 (2019).

Abstract

We demonstrate successful nanometric conformal coverage by pulsed laser deposition (PLD) of Ag nanoparticle (NP) arrays with a transparent amorphous Al2O3 layer. The Ag NPs-arrays are formed by a bimodal distribution of well separated spheroidal Ag NPs with an average in-plane diameter of 40 or 90 nm and a maximum height of ∼70 nm. The cover layer induces no relevant alteration of the morphology and organization of the underlying Ag NPs-arrays. The conformal nature of the Al2O3 cover layer is confirmed by its surface topography that shows the presence of characteristic nanodomes, with sizes determined by those of the underlying Ag NPs. The distinctive tapered shape of the nanodomes is related to the angular dependence with respect to the NPs surface normal of the deposition rate and the self-sputtering induced by energetic Al+ ions present in the laser generated plasma. The presence of the amorphous Al2O3 cover layer red-shifts the characteristic dipolar and quadrupolar Surface Plasmon Resonances of Ag NPs and increases the total and diffuse reflectance of the NPs arrays.

2D compositional self-patterning in magnetron sputtered thin films

2D compositional self-patterning in magnetron sputtered thin films

A. García-Valenzuela, R. Alvarez, V. Rico, J. P. Espinos, M. C. López-Santos, J. Solís, J. Siegel, A. del Campo, A. Palmero, A. R. González-Elipe
Applied Surface Science, 480, 115-121 (2019)

Abstract

Unlike topography patterning, widely used for numerous applications and produced by means of different technologies, there are no simple procedures to achieve surface compositional patterning at nanometric scales. In this work we have developed a simple method for 2D patterning the composition of thin films. The method relies on the magnetron sputtering deposition at oblique angles onto patterned substrates made by laser induced periodic surface structures (LIPSS). The method feasibility has been demonstrated by depositing SiOx thin films onto LIPSS structures generated in Cr layers. A heterogeneous and aligned distribution of O/Si ratios (and different Sin+ chemical states) along the LIPSS structure in length scales of some hundreds nm’s has been proven by angle resolved X-ray photoelectron spectroscopy and a patterned arrangement of composition monitored by atomic force microscopy-Raman analysis. The obtained results are explained by the predictions of a Monte Carlo simulation of this deposition process and open the way for the tailored one-step fabrication of surface devices with patterned compositions.

Surface Plasmon Polaritons on Rough Metal Surfaces: Role in the Formation of Laser-Induced Periodic Surface Structures

Surface Plasmon Polaritons on Rough Metal Surfaces: Role in the Formation of Laser-Induced Periodic Surface Structures

Y. Fuentes-Edfuf, J. A. Sánchez-Gil, C. Florian, V. Giannini, J. Solis, and J. Siegel
ACS Omega 4, 6939-6946 (2019).

Abstract

The formation of self-organized laser-induced periodic surface structures (LIPSS) in metals, semiconductors, and dielectrics upon pulsed laser irradiation is a well-known phenomenon, receiving increased attention because of their huge technological potential. For the case of metals, a major role in this process is played by surface plasmon polaritons (SPPs) propagating at the interface of the metal with the medium of incidence. Yet, simple and advanced models based on SPP propagation sometimes fail to explain experimental results, even of basic features such as the LIPSS period. We experimentally demonstrate, for the particular case of LIPSS on Cu, that significant deviations of the structure period from the predictions of the simple plasmonic model are observed, which are very pronounced for elevated angles of laser incidence. In order to explain this deviation, we introduce a model based on the propagation of SPPs on a rough surface that takes into account the influence of the specific roughness properties on the SPP wave vector. Good agreement of the modeling results with the experimental data is observed, which highlights the potential of this model for the general understanding of LIPSS in other metals.

Tuning the period of femtosecond laser induced surface structures in steel: From angled incidence to quill writing

Tuning the period of femtosecond laser induced surface structures in steel: From angled incidence to quill writing

Y. Fuentes-Edfuf, J. A. Sánchez-Gil, M. Garcia-Pardo, R. Serna, G. D. Tsibidis, V. Giannini, J. Solis, and J. Siegel
Appl. Surf. Sci. 493, 948–955 (2019).

Abstract

Exposure of metal surfaces to multiple ultrashort laser pulses under certain conditions leads to the formation of well-defined periodic surface structures. We show how the period of such structures in steel can be tuned over a wide range by controlling the complex interaction mechanisms triggered in the material. Amongst the different irradiation parameters that influence the properties of the induced structures, the angle of incidence of the laser beam occupies a prominent role. We present an experimental and theoretical investigation of this angle dependence in steel upon irradiation with laser pulses of 120 fs duration and 800 nm wavelength, while moving the sample at constant speed. Our findings can be grouped into two blocks. First, we observe the spatial coexistence of two different ripple periods at the steel surface, both featuring inverse scaling upon angle increase, which are related to forward and backward propagation of surface plasmon polaritons. To understand the underlying physical phenomena, we extend a recently developed model that takes into account quantitative properties of the surface roughness to the case of absorbing metals (large imaginary part of the dielectric function), and obtain an excellent match with the experimentally observed angle dependence. As second important finding, we observe a quill writing effect, also termed non-reciprocal writing, in form of a significant change of the ripple period upon reversing the sample movement direction. This remarkable phenomenon has been observed so far only inside dielectric materials and our work underlines its importance also in laser surface processing. We attribute the origin of symmetry breaking to the non-symmetric micro- and nanoscale roughness induced upon static multiple pulse irradiation, leading to non-symmetric modification of the wavevector of the coupled surface plasmon polariton.

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