I. Camps, A. Mariscal, and R. Serna, J. Lum.191, 97-101 (2017).
The optical performance of europium (Eu) doped SiAlON thin films and its preparation by pulsed laser deposition are studied. The undoped SiAlON films show a high refractive index (2.34) close to that of silicon nitride. After doping with Eu the oxygen content increases in the films and leads to the formation of an oxynitride with a refractive index of 1.87 and excellent transparency in the visible-near infrared. Upon excitation at 355 nm the films show a broad band (FWHM of 200 nm) emission in the visible range (400–750 nm) that is associated to the emission of the Eu in the 2+ oxidation state. Enhancement of the emission together with a blue spectral shift has been found when the films are annealed at low temperature (600 °C). These films show promising properties for the development of Si compatible thin films integrated emitters.
Multifunctional ZnO/Fe-O and graphene oxide nanocomposites: Enhancement of optical and magnetic properties
V. Fuertes, A. Mariscal, R. Serna, F.J. Mompeán, M. García-Hernández, J. F. Fernández, E. Enríquez, J. Eur.Ceram.Soc.37, 3747-3758 (2017).
ZnO is a semiconductor with a great interest, but it has several deficiencies which limit its use in technologic applications. One important limitation is having the band gap in the UV which reduces its use in optical devices. To solve this problem, in this work, composites based in ZnO with goethite and graphene oxide (GO) by sol-gel are prepared. The obtained samples (powders and thin films) were characterized microstructurally (DTA, XRD, micro-Raman, FE-SEM), optically (transmittance and photoluminescence) and magnetically (SQUID). The ZnO band gap of multifunctional composites shows a red-shift towards visible range (Eg ∼3.01 eV) with high transmittance ∼85% (thickness of 362 nm) over the visible wavelength range. A long-range magnetic order at room temperature appears in these nanocomposites (Ms = 1.60·10−2 emu/g). The combination of both dopants allows modifying the functional properties of ZnO, opening a great field of applications in ZnO composites, such as spintronic and optoelectronic devices.
Interband transitions in semi-metals, semiconductors, and topological insulators: A new driving force for plasmonics and nanophotonics
J. Toudert and R. Serna, Opt.Mat.Express 7, 2299-2325 (2017).
Plasmonic and dielectric Mie resonances in subwavelength nanostructures provide an efficient way to manipulate light below the diffraction limit that has fostered the growth of plasmonics and nanophotonics. Plasmonic resonances have been mainly related with the excitation of free charge carriers, initially in metals, and dielectric Mie resonances have been identified in Si nanostructures. Remarkably, although much less studied, semi-metals, semiconductors and topological insulators of the p-block enable plasmonic resonances without free charge carriers and dielectric Mie resonances with enhanced properties compared with Si. In this review, we explain how interband transitions in these materials show a major role in this duality. We evaluate the plasmonic and Mie performance of nanostructures made of relevant p-block elements and compounds, especially Bi, and discuss their promising potential for applications ranging from switchable plasmonics and nanophotonics to energy conversion, especially photocatalysis.
SiGe layer thickness effect on the structural and optical properties of well-organized SiGe/SiO2 multilayers
E.M. F Vieira, J. Toudert, A. G. Rolo, A Parisini, J.P. Leitão, M. R. Correia, N. Franco, E. Alves, A. Chahboun, J. Martín-Sánchez, R. Serna, M. J.M. Gomes, Nanotechnology 28, 345701 (2017).
In this work, we report on the production of regular (SiGe/SiO2)20 multilayer structures by conventional RF-magnetron sputtering, at 350 °C. Transmission electron microscopy, scanning transmission electron microscopy, raman spectroscopy, and x-ray reflectometry measurements revealed that annealing at a temperature of 1000 °C leads to the formation of SiGe nanocrystals between SiO2 thin layers with good multilayer stability. Reducing the nominal SiGe layer thickness (t SiGe) from 3.5–2 nm results in a transition from continuous SiGe crystalline layer (t SiGe ~ 3.5 nm) to layers consisting of isolated nanocrystals (t SiGe ~ 2 nm). Namely, in the latter case, the presence of SiGe nanocrystals ~3–8 nm in size, is observed. Spectroscopic ellipsometry was applied to determine the evolution of the onset in the effective optical absorption, as well as the dielectric function, in SiGe multilayers as a function of the SiGe thickness. A clear blue-shift in the optical absorption is observed for t SiGe ~ 2 nm multilayer, as a consequence of the presence of isolated nanocrystals. Furthermore, the observed near infrared values of n = 2.8 and k = 1.5 are lower than those of bulk SiGe compounds, suggesting the presence of electronic confinement effects in the nanocrystals. The low temperature (70 K) photoluminescence measurements performed on annealed SiGe/SiO2 nanostructures show an emission band located between 0.7–0.9 eV associated with the development of interface states between the formed nanocrystals and surrounding amorphous matrix.
Unveiling the Far Infrared-to-Ultraviolet Optical Properties of Bismuth for Applications in Plasmonics and Nanophotonics
J. Toudert, R. Serna, I. Camps, J. Wojcik, P. Mascher, E. Rebollar, and T. A Ezquerra, J. Phys.C. 121, 3511-3521 (2017).
For years bismuth (Bi) has appealed to a broad community of scientists due to its peculiar electronic, optical, and more recently plasmonic and photocatalytic properties, which enable both the understanding of basic science phenomena and the development of a wide range of applications. In spite of this interest, a comprehensive spectral analysis of the dielectric function (ε = ε1 + jε2) of bulk Bi from the far infrared (IR) to the ultraviolet (UV) region is not available. So far, the data have been reported in limited spectral ranges and show a wide dispersion that is especially notorious for the IR region. In this work we report ε for Bi in a wide spectral range from 0.05 to 4.7 eV (24.8 to 0.3 μm, far IR to UV). ε is extracted from spectroscopic ellipsometry measurements of excellent quality (dense and smooth) Bi films by using the transfer matrix formalism and Kramers–Kronig consistent analysis. The higher quality and accuracy of the obtained ε compared with the literature data is demonstrated. The analysis and use of this reference bulk dielectric function provides crucial information for the exploration and understanding of the optical, plasmonic, and photocatalytic properties of Bi nanostructures. From its analysis, it is evidenced that the optical properties of Bi in the mid wave IR-to-UV are driven only by interband transitions, which are responsible for the dominant absorption band peaking at about 0.8 eV. Therefore, the plasmonic behavior and the photocatalytic performance of Bi nanostructures in the visible and UV are likely driven by these interband transitions that make ε1 turn negative in this region without the need of exciting free carriers. Furthermore, classical electrodynamic simulations using the obtained ε show a strong size dependence for the optical extinction of Bi nanospheres in the far IR-to-near IR with Mie-like resonances broadly tunable across this region.
R. Morea, T. T. Fernandez, J. Fernandez, R. Balda, J. Gonzalo, J. Mat. Sci. 28, 7000-7005 (2017)
This work presents a detailed study of the preparation and microstructure of transparent glass–ceramics obtained from a TeO2−ZnO−ZnF2 fluorotellurite glass doped with ErF3. The determination of nucleation and crystal growth rate-like curves allows establishing a narrow temperature range (340–350 °C) for a controlled crystallization. Thereafter, a crystalline phase was grown through a two-step heat treatment: 10–20 h at a temperature slightly above the glass transition temperature, followed by a 2.5–3 h treatment at 340 °C. Structural analysis showed the nucleation of ErF3 nanocrystals (NCs) with a typical size of ≈50 nm that were homogeneously distributed in the glass matrix. The resulting glass–ceramic material remains highly transparent, with a small crystalline to amorphous ratio; yet the presence of ErF3 NCs has a large impact on the photoluminescence response of Er3+ ions. Upconverted visible emission is analyzed under 980 nm excitation: red emission from the 4F9/2 level is dramatically enhanced with respect to green 2H11/2, 4S3/2 → 4I15/2 hypersensitive transitions. The observed behaviour is attributed to the presence of Er3+ ions in the NCs, which are sites with lower phonon energy than the glass matrix. Moreover, the shorter inter-ionic distance between Er3+ ions in the NCs eases energy transfer between them.
Simultaneous time-space resolved reflectivity and interferometric measurements of dielectrics excited with femtosecond laser pulses
M. Garcia-Lechuga, L. Haahr-Lillevang, J.Siegel, P. Balling, S. Guizard, and J. Solis, Phys. Rev.B 95 211114, (2017).
Simultaneous time-and-space resolved reflectivity and interferometric measurements over a temporal span of 300 ps have been performed in fused silica and sapphire samples excited with 800 nm, 120 fs laser pulses at energies slightly and well above the ablation threshold. The experimental results have been simulated in the frame of a multiple-rate equation model including light propagation. The comparison of the temporal evolution of the reflectivity and the interferometric measurements at 400 nm clearly shows that the two techniques interrogate different material volumes during the course of the process. While the former is sensitive to the evolution of the plasma density in a very thin ablating layer at the surface, the second yields an averaged plasma density over a larger volume. It is shown that self-trapped excitons do not appreciably contribute to carrier relaxation in fused silica at fluences above the ablation threshold, most likely due to Coulomb screening effects at large excited carrier densities. For both materials, at fluences well above the ablation threshold, the maximum measured plasma reflectivity shows a saturation behavior consistent with a scattering rate proportional to the plasma density in this fluence regime. Moreover, for both materials and for pulse energies above the ablation threshold and delays in the few tens of picoseconds range, a simultaneous “low reflectivity” and “low transmission” behavior is observed. Although this behavior has been identified in the past as a signature of femtosecond laser-induced ablation, its origin is alternatively discussed in terms of the optical properties of a material undergoing strong isochoric heating, before having time to substantially expand or exchange energy with the surrounding media.
Z. Liu, J. Siegel, M. Garcia-Lechuga, T. Epicier, Y.Lefkir, S. Reynaud, M. Bugnet, F. Vocanson, J. Solis, G.Vitrant, N. Destouches, ACS Nano 11, 5031–5040, (2017).
Controlling plasmonic systems with nanometer resolution in transparent films and their colors over large nonplanar areas is a key issue for spreading their use in various industrial fields. Using light to direct self-organization mechanisms provides high-speed and flexible processes to meet this challenge. Here, we describe a route for the laser-induced self-organization of metallic nanostructures in 3D. Going beyond the production of planar nanopatterns, we demonstrate that ultrafast laser-induced excitation combined with nonlinear feedback mechanisms in a nanocomposite thin film can lead to 3D self-organized nanostructured films. The process, which can be extended to complex layered composite systems, produces highly uniform large-area nanopatterns. We show that 3D self-organization originates from the simultaneous excitation of independent optical modes at different depths in the film and is activated by the plasmon-induced charge separation and thermally induced NP growth mechanisms. This laser color marking technique enables multiplexed optical image encoding and the generated nanostructured Ag NPs:TiO2 films offer great promise for applications in solar energy harvesting, photocatalysis, or photochromic devices.
Y. Fuentes-Edfuf, M. Garcia-Lechuga, D. Puerto, C. Florian, A. Garcia-Leis, S. Sanchez-Cortes, J. Solis, and J. Siegel, Appl. Phys. Lett 110, 211602 (2017).
We demonstrate a simple way to fabricate amorphous micro-rings in crystalline silicon using direct laser writing. This method is based on the fact that the phase of a thin surface layer can be changed into the amorphous phase by irradiation with a few ultrashort laser pulses (800 nm wavelength and 100 fs duration). Surface-depressed amorphous rings with a central crystalline disk can be fabricated without the need for beam shaping, featuring attractive optical, topographical, and electrical properties. The underlying formation mechanism and phase change pathway have been investigated by means of fs-resolved microscopy, identifying fluence-dependent melting and solidification dynamics of the material as the responsible mechanism. We demonstrate that the lateral dimensions of the rings can be scaled and that the rings can be stitched together, forming extended arrays of structures not limited to annular shapes. This technique and the resulting structures may find applications in a variety of fields such as optics, nanoelectronics, and mechatronics.
This work has been supported by the LiNaBioFluid Project (H2020-FETOPEN-2014–2015RIA, Grant No. 665337) of the European Commission as well as the research grant (TEC2014–52642-C2–1-R) from the Spanish Ministry of Economy and Competitiveness. M.G.-L. thanks the Spanish Ministry of Education for a FPU fellowship. The authors are grateful to C. Dorronsoro for pointing out a Matlab script for flattening the background of the data shown in Fig. 4 and helping with its implementation.
High efficiency waveguide optical amplifiers and lasers via fs-laser induced local modification of the glass composition
J. Hoyo, P. Moreno-Zárate, G. Escalante, J.A. Vallés, P. Fernández, J. Solis, J. Lightwave Tech. 35, 2955 – 2959, (2017).
FS-laser irradiation at high repetition pulse rate enables producing high-performance passive waveguides in different glasses through the local modification of the glass composition. In this paper, we show that this mechanism can similarly be used to produce high-performance waveguide amplifiers and lasers. Furthermore, we show the feasibility of producing active waveguides with different optical gains in the same phosphate glass sample by changing the laser writing parameters. The lanthanides present in the glass composition (Er 3+ , Yb 3+ , and La 3+ , the refractive index carrying element) experience similar local concentration changes upon fs-laser writing, enabling to determine the concentration of La 3+ and thus the refractive index contrast of the guiding region by measuring its absorption at 1534 nm. The produced waveguide lasers show slope efficiencies (respect to the absorbed pump power) above 38%, which could reach up to 42% by further optimization of the waveguide laser cavity configuration. The active waveguides produced are thermally stable for temperatures up to at least 450 °C.
J. Martínez, A. Ródenas, A. Stake, M. Traveria, M. Aguiló, J. Solis, R. Osellame, T. Tanaka, B. Berton, S. Kimura, N. Rehfeld, and F. Díaz, Materials Technol. 2, 1700085 (2017).
State‐of‐the‐art ultrahigh‐sensitivity photonic sensing schemes rely on exposing the evanescent field of tightly confined light to the environment. Yet, this renders an inherent fragility to the device, and since adding a protective layer disables light exposure, there exists a technology gap for highly sensitive harsh‐environment‐resistant surface photonic sensors. Here, a novel type of mid‐infrared waveguide sensors is reported which exploit vibrational resonance‐driven directional coupling effects besides absorption, with optical sensing elements that can be buried (≈1–10 µm) and resist systematic exposure to industrial environments without failure. A harsh‐environment‐resistant, fiber‐coupled, surface sensor for monitoring the structural phase of water (liquid‐supercooled‐solid), as well as the type of ice microstructure (clear rime), is shown. It is demonstrated how this type of sensor can be designed to detect ice layers with nanometric (≈100 nm) to microscopic (≈30 µm or higher) thicknesses, and the first experimental tests both in optical laboratory and in icing wind tunnel inflight aircraft simulation tests are reported.
Coherent scatter-controlled phase-change grating structures in silicon using femtosecond laser pulses
Y. Fuentes-Edfuf, M. Garcia-Lechuga, D. Puerto, C. Florian, A.Garcia-Leis, S. Sanchez-Cortes, J. Solis, and J. Siegel, Scientific Reports 7, 4594 (2017).
Periodic structures of alternating amorphous-crystalline fringes have been fabricated in silicon using repetitive femtosecond laser exposure (800 nm wavelength and 120 fs duration). The method is based on the interference of the incident laser light with far- and near-field scattered light, leading to local melting at the interference maxima, as demonstrated by femtosecond microscopy. Exploiting this strategy, lines of highly regular amorphous fringes can be written. The fringes have been characterized in detail using optical microscopy combined modelling, which enables a determination of the three-dimensional shape of individual fringes. 2D micro-Raman spectroscopy reveals that the space between amorphous fringes remains crystalline. We demonstrate that the fringe period can be tuned over a range of 410 nm – 13 µm by changing the angle of incidence and inverting the beam scan direction. Fine control over the lateral dimensions, thickness, surface depression and optical contrast of the fringes is obtained via adjustment of pulse number, fluence and spot size. Large-area, highly homogeneous gratings composed of amorphous fringes with micrometer width and millimeter length can readily be fabricated. The here presented fabrication technique is expected to have applications in the fields of optics, nanoelectronics, and mechatronics and should be applicable to other materials.
Francisco Muñoz, Pedro Moreno-Zárate, Javier Solis, Non. Cryst. Solids 473, (2017).
Glasses of the system of composition K2O-La2O3-Al2O3-SiO2-P2O5 have recently gained attention due to their response to femtosecond (fs) laser irradiation, and the fact that high repetition rate fs-laser writing allows for the production of very efficient waveguides. When doped with rare-earth ions, the glasses can also be used in amplifiers or lasers and, in this respect, a control of the water content in the phosphate-based glasses is of very high importance. Thus, in the present work we have studied the influence of the melting conditions on the final water content of glasses of the above system and on their structure as studied by Raman and Nuclear Magnetic Resonance spectroscopies. Increasing temperature and melting times conducts to the enrichment in SiO2 and a depletion of K2O and P2O5 contents in parallel with a reduction of the water content as measured by FTIR. On the other hand, the structural study showed that the chemical environment of phosphorous and silicon are not much affected by the changes in composition and that aluminium moves from being 6-fold coordinated to 4-fold. Finally, it has also been observed that SiO2 enters in the glasses as fully polymerized, which is consistent with the reduction of water directly on the ratio between oxygen and phosphorus in the glasses.
Mimicking bug-like surface structures and their fluid transport produced by ultrashort laser pulse irradiation of steel
V. Kirner, U. Hermens, A. Mimidis, E. Skoulas, C. Florian, F. Hischen, C. Plamadeala, W. Baumgartner, K. Winands, H. Mescheder, J. Krüger, J. Solis, J. Siegel, E. Stratakis, J. Bonse, Applied Phys. A, 123, 754 (2017).
Ultrashort laser pulses with durations in the fs-to-ps range were used for large area surface processing of steel aimed at mimicking the morphology and extraordinary wetting behaviour of bark bugs (Aradidae) found in nature. The processing was performed by scanning the laser beam over the surface of polished flat sample surfaces. A systematic variation of the laser processing parameters (peak fluence and effective number of pulses per spot diameter) allowed the identification of different regimes associated with characteristic surface morphologies (laser-induced periodic surface structures, i.e., LIPSS, grooves, spikes, etc.). Moreover, different laser processing strategies, varying laser wavelength, pulse duration, angle of incidence, irradiation atmosphere, and repetition rates, allowed to achieve a range of morphologies that resemble specific structures found on bark bugs. For identifying the ideal combination of parameters for mimicking bug-like structures, the surfaces were inspected by scanning electron microscopy. In particular, tilted micrometre-sized spikes are the best match for the structure found on bark bugs. Complementary to the morphology study, the wetting behaviour of the surface structures for water and oil was examined in terms of philic/phobic nature and fluid transport. These results point out a route towards reproducing complex surface structures inspired by nature and their functional response in technologically relevant materials.