Mid-to-far infrared tunable perfect absorption by a sub - λ/100 nanofilm in a fractal phasor resonant cavity.
J. Toudert, R. Serna, M. García-Pardo, N. Ramos, R. J. Peláez and B. Maté.
Optics Express 26, 26, 34043-34059 (2018).
Integrating an absorbing thin film into a resonant cavity is the most practical way to achieve perfect absorption of light at a selected wavelength in the mid-to-far infrared, as required to target blackbody radiation or molecular fingerprints. The cavity is designed to resonate and enable perfect absorption in the film at the chosen wavelength λ. However, in current state-of-the-art designs, a still large absorbing film thickness (∼λ/50) is needed and tuning the perfect absorption wavelength over a broad range requires changing the cavity materials. Here, we introduce a new resonant cavity concept to achieve perfect absorption of infrared light in much thinner and thus, really nanoscale films, with a broad wavelength tenability by using a single set of cavity materials. It requires a nanofilm with giant refractive index and small extinction coefficient (found in emerging semi-metals, semi-conductors and topological insulators) backed by a transparent spacer and a metal mirror. The nanofilm acts both as absorber and multiple reflector for the internal cavity waves, which after escaping follow a fractal phasor trajectory. This enables a totally destructive optical interference for a nanofilm thickness more than 2 orders of magnitude smaller than λ. With this remarkable effect, we demonstrate angle-insensitive perfect absorption in sub – λ/100 bismuth nanofilms, at a wavelength tunable from 3 to 20 μm.
Optical spectroscopy study of nano- and microstructures fabricated by femtosecond laser pulses on ZnO based systems.
E. Prado, C. Florian, B. Sotillo, J. Siegel, J. Solis and P. Fernández.
CrystEngComm 20, 21, 2952-2960 (2018).
The formation of laser induced periodic surface structures (LIPSS) upon irradiation with ultrashort laser pulses at the surface of polycrystalline ZnO based samples, and the potential use of irradiated areas as growth patterns for the production of highly ordered nanostructures have been studied. In particular, pure ZnO, ZnO:Al (5 wt% Al2O3) and ZnO:Mg (5 wt% MgO) samples have been investigated. The surface morphology of the laser fabricated structures depends on the processing parameters, such as energy, number of pulses, repetition rate and laser polarization. Three main types of periodic structures have been formed: LSF (low spatial frequency) and HSF (high spatial frequency) LIPSS and pseudo-spikes (PS). After irradiation, the samples have been used as substrates for vapor–solid growth. It has been found that the growth of micro- and nanostructures in the irradiated regions is much faster than in the non-irradiated ones. Strong differences in the final morphology of the deposited regions are appreciated, not only between irradiated and non-irradiated areas, but also between areas irradiated under different conditions. Scanning electron microscopy (SEM) and cathodoluminescence (CL) measurements at room temperature have been used to monitor the morphology and luminescence properties of the generated structures. For all processed samples, the CL analysis shows a shift of the band edge emission towards higher energies and a lower relative intensity of the defect band in the areas irradiated at higher fluence. The first effect might be attributed to the incorporation of dopants, as already observed for Al and Mg. Yet, the redistribution of defects associated with shallow levels might also be responsible for the observed shift, playing a major role in undoped samples. The decrease in the relative intensity of the defect band can be ascribed to the partial recovery of defects caused by the deposited energy (annealing effect) during the irradiation process.
I. Camps, A. Mariscal, L. Calvo‐Barrio and R. Serna.
Physica Statuts solidi (a) 25, 19, 1800260 (2018).
SiAlON materials in thin film configuration are very attractive because their composition can be easily modified from that of silicon dioxide to that of silicon nitride to effectively tune their electrical and optical response, therefore making them suitable for a wide range of optoelectronic devices. In addition, rare‐earth doped SiAlON phosphors in powder configuration are excellent down‐converters to visible light when excited by ultraviolet‐blue photons from light emitting diodes. In this work, the direct excitation by electrons of amorphous nanostructured Eu‐doped SiAlON thin films is explored. XPS analysis shows that the Eu in the films is in its divalent oxidation state (Eu2+), with traces of Eu with a richer oxygen coordination at the film surface. The cathodoluminescence emission from the films is dominated by a wide emission band (FWHM 195 nm) in the visible associated with the Eu2+ 5d to 4f electronic transitions, with a small narrow peak contribution from Eu3+ intra‐4f electronic transitions. Assessment of the CIE color coordinates shows that the spectral emission is close to the pure white. These results show the excellent potential of the Eu‐doped SiAlON films for its implementation in CMOS structures as white light solid state emitters in field emission lamps and displays.
C. Florian, E. Skoulas, D. Puerto, A. Mimidis, E. Stratakis, J. Solis and J. Siegel.
ACS Appl. Mater. Interfaces 10, 36564−36571 (2018).
The wettability of a material surface is an essential property that can define the range of applications it can be used for. In the particular case of steel, industrial applications are countless but sometimes limited because of the lack of control over its surface properties. Although different strategies have been proposed to tune the wetting behavior of metal surfaces, most of them require the use of processes such as coatings with different materials or plasma/chemical etching. In this work, we present two different laser-based direct-write strategies that allow tuning the wetting properties of 1.7131 steel over a wide range of contact angles using a high repetition rate femtosecond laser. The strategy consists in the writing of parallel and crossed lines with variable spacing. A detailed morphological analysis confirmed the formation of microstructures superimposed with nanofeatures, forming a hierarchical surface topography that influences the wetting properties of the material surface. Contact angle measurements with water confirm that this behavior is mostly dependent on the line-to-line spacing and the polarization-dependent orientation of the structures. Moreover, we demonstrate that the structures can be easily replicated in a polymer using a laser-fabricated steel master, which enables low-cost mass production. These findings provide a practical route for developing user-defined wetting control for new applications of steel and other materials functionalized by rapid laser structuring.
Biomimetic surface structures in steel fabricated with femtosecond laser pulses: influence of laser rescanning on morphology and wettability.
C. Florian, A. Mimidis, D. Puerto, E. Skoulas, E. Stratakis, J. Solis and J. Siegel
Beilstein J. Nanotechnol. 9, 2802–2812 (2018).
The replication of complex structures found in nature represents an enormous challenge even for advanced fabrication techniques, such as laser processing. For certain applications, not only the surface topography needs to be mimicked, but often also a specific function of the structure. An alternative approach to laser direct writing of complex structures is the generation of laser-induced periodic surface structures (LIPSS), which is based on directed self-organization of the material and allows fabrication of specific micro- and nanostructures over extended areas. In this work, we exploit this approach to fabricate complex biomimetic structures on the surface of steel 1.7131 formed upon irradiation with high repetition rate femtosecond laser pulses. In particular, the fabricated structures show similarities to the skin of certain reptiles and integument of insects. Different irradiation parameters are investigated to produce the desired structures, including laser repetition rate and laser fluence, paying special attention to the influence of the number of times the same area is rescanned with the laser. The latter parameter is identified to be crucial for controlling the morphology and size of specific structures. As an example for the functionality of the structures, we have chosen the surface wettability and studied its dependence on the laser processing parameters. Contact angle measurements of water drops placed on the surface reveal that a wide range of angles can be accessed by selecting the appropriate irradiation parameters, highlighting also here the prominent role of the number of scans.
S. Levcenko, E. Hajdeu-Chicarosh, E. Garcia-Llamas, R. Caballero, R. Serna, I. V Bodnar, I. A. Victorov, M. Guc, J. M. Merino, A. Pérez-Rodriguez, E. Arushanov and M. León
Appl. Phys. Lett. 112, 161901 (2018).
The linear optical properties of Cu2ZnSnS4 bulk poly-crystals have been investigated using spectroscopic ellipsometry in the range of 1.2–4.6 eV at room temperature. The characteristic features identified in the optical spectra are explained by using the Adachi analytical model for the interband transitions at the corresponding critical points in the Brillouin zone. The experimental data have been modeled over the entire spectral range taking into account the lowest E0 transition near the fundamental absorption edge and E1A and E1B higher energy interband transitions. In addition, the spectral dependences of the refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity values have been accurately determined and are provided since they are essential data for the design of Cu2ZnSnS4 based optoelectronic devices.
A. Mariscal, A. Quesada, A. Tarazaga Martín-Luengo, M.A. García, A. Bonanni, J.F. Fernández and R. Serna
Appl. Surf. Sci. 456,980-984(2018).
Nanocrystalline textured EuO thin films are prepared by an oxygen loss process from a pure Eu2O3 bulk ceramic target through pulsed laser deposition in vacuum at room temperature. X-ray diffraction spectra evidence a well-defined diffraction peak corresponding to the EuO phase textured along the (1 1 0) direction. Analysis of the XRD peak profile indicates that the films are nanocrystalline (average crystallite size of 11 nm) with a compressive residual strain. The formation of stoichiometric EuO is further confirmed by a strong signal from Eu2+ in the X-ray photoelectron spectra. The complex refractive index in the near infrared has been determined by spectroscopic ellipsometry and shows that the EuO films have a high transparency (k < 10−3) and a refractive index of 2.1. A band-gap shift of 0.25 eV is found with respect to the EuO bulk. These films, deposited by an accessible and efficient method, open a new route to produce EuO films with optical quality, suitable for NIR optoelectronic components.
Imaging Ellipsometry Determination of the Refractive Index Contrast and Dispersion of Channel Waveguides Inscribed by fs-Laser Induced Ion-Migration.
P. Moreno-Zarate, A. Gonzalez, S. Funke, A. Dias, B. Sotillo, J. del Hoyo, M. García-Pardo, R. Serna, P. Fernández and J. Solís.
Phys. Status Solidi(a) 215(19),1800258 (2018).
The measurement of the refractive index of optical waveguides is a difficult task that involves different methods, among which those based on the refracted near field determination (RNF) are likely the ones providing the best resolution. Still, most such methods lack spectral resolution, which impedes accessing the index dispersion of the waveguide, an essential parameter for many applications. In this work, the refractive index of channel waveguides produced by fs‐laser induced ion‐migration in a P2O5‐La2O3‐K2O‐based glass is measured by imaging ellipsometry. Along with EDX compositional maps and guiding performance, the dispersion and refractive index maps of several waveguides are measured. The results confirm that, in this glass, waveguides are formed due to an enrichment in La in the topmost part of the laser‐excited region which is accompanied by the cross migration of K toward the region underneath. Interestingly, the index contrast of the waveguides shows a wavelength‐independent behavior for wavelengths above ≈600 nm. This indicates that in the compositional range analyzed, La3+ ions linearly contribute to the glass polarizability due to the relatively large mass of La3+ ions and the relatively small size of the isolated La‐polyhedra accommodated in the phosphate glass network.
Memristive behaviour of Si-Al oxynitride thin films: The role of oxygen and nitrogen vacancies in the electroforming process.
O.Blázquez, G. Martín, I. Camps, A. Mariscal, J. López-Vidrier, J. Manel Ramirez, S. Hernández, S. Estrade, F. Peiro, R. Serna and Blas Garrido.
Nanotechnology 29, 235702 (2018).
The resistive switching properties of silicon-aluminium oxynitride (SiAlON) based devices have been studied. Electrical transport mechanisms in both resistance states were determined, exhibiting an ohmic behaviour at low resistance and a defect-related Poole−Frenkel mechanism at high resistance. Nevertheless, some features of the Al top-electrode are generated during the initial electroforming, suggesting some material modifications. An in-depth microscopic study at the nanoscale has been performed after the electroforming process, by acquiring scanning electron microscopy and transmission electron microscopy images. The direct observation of the devices confirmed features on the top electrode with bubble-like appearance, as well as some precipitates within the SiAlON. Chemical analysis by electron energy loss spectroscopy has demonstrated that there is an out-diffusion of oxygen and nitrogen ions from the SiAlON layer towards the electrode, thus forming silicon-rich paths within the dielectric layer and indicating vacancy change to be the main mechanism in the resistive switching.
J.Martín-Sánchez, A. Mariscal, M. De Luca, A. Tarazaga Martín-Luengo, G. Gramse, A. Halilovic, R. Serna, A. Bonanni, I. Zardo, R. Trotta and A. Rastelli
Nano Research 11 3,1399-1414 (2018).
Two-dimensional transition metal dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact footprints. However, the high sensitivity of atomically thin materials to the surrounding dielectric media imposes severe limitations on their practical applicability. Hence, to enable the effective integration of these materials in devices, the development of reliable encapsulation procedures that preserve their physical properties is required. Here, the excitonic photoluminescence (at room temperature and 10 K) is assessed on mechanically exfoliated WSe2 monolayer flakes encapsulated with SiOx and AlxOy layers by means of chemical and physical deposition techniques. Conformal coating on untreated and non-functionalized flakes is successfully achieved by all the techniques examined, with the exception of atomic layer deposition, for which a cluster-like oxide coating is formed. No significant compositional or strain state changes in the flakes are detected upon encapsulation, independently of the technique adopted. Remarkably, our results show that the optical emission of the flakes is strongly influenced by the stoichiometry quality of the encapsulating oxide. When the encapsulation is carried out with slightly sub-stoichiometric oxides, two remarkable phenomena are observed. First, dominant trion (charged exciton) photoluminescence is detected at room temperature, revealing a clear electrical doping of the monolayers. Second, a strong decrease in the optical emission of the monolayers is observed, and attributed to non-radiative recombination processes and/or carrier transfer from the flake to the oxide. Power- and temperature-dependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation, opening a promising route for the development of integrated two-dimensional devices.
Evidencing early pyrochlore formation in rare-earth doped TiO2 nanocrystals: Structure sensing via VIS and NIR Er3+ light emission.
I.Camps, M. Borlaf, J. Toudert, A. de Andrés, M. T. Colomer, R. Moreno, and R. Serna.
J. Alloys and Compounds 735, 2267-2274 (2018).
Er3+ doping of TiO2 colloidal nanocrystals enhances their performance for photo-induced applications. Such doping is known to delay the anatase to rutile transformation under thermal treatment; nonetheless relevant information on the Er3+ light emission and location within the TiO2 structures is still incomplete. Er3+ photoluminescence emission both in the visible (upconverted) and infrared photoluminescence is used for the first time to probe the ions location within the different TiO2 structures. The results show that Er3+ ions in the as-prepared xerogels are not embedded in the anatase crystallites, and only upon thermal treatment Er3+ diffusion is induced into crystal interstitial positions to form a solid solution. At higher temperatures rutile is formed inducing Er3+ segregation and giving rise to the formation of pyrochlore (Er2Ti2O7), as shown by a distinct emission in the infrared spectrum due to the Er3+ ions located within the pyrochlore compound. Although pyrochlore is usually a high temperature phase, analysis of the photoluminescence allows its labeling at temperatures as low as 600–700 °C for small Er3+ concentrations (1 mol %). Increasing Er3+ concentration promotes its enrichment at the nanocrystallites surface accomplished by the anatase-to-rutile phase transformation, suggesting that Er3+ ions control the TiO2 nanocrystals surface properties.
Strain-tuning of the optical properties of semiconductor nanomaterials by integration onto piezoelectric actuators.
J.Martín-Sánchez, R. Trotta, A. Mariscal, R. Serna, G. Piredda, S. Stroj, J. Edlinger, C. Schimpf, J. Aberl, T. Lettner, J. Wildmann, H. Huang, X. Yuan, D. Ziss, J. Stangl and A. Rastelli.
Semiconductor Science and Technology 33, 013001 (2018). Review
The tailoring of the physical properties of semiconductor nanomaterials by strain has been gaining increasing attention over the last years for a wide range of applications such as electronics, optoelectronics and photonics. The ability to introduce deliberate strain fields with controlled magnitude and in a reversible manner is essential for fundamental studies of novel materials and may lead to the realization of advanced multi-functional devices. A prominent approach consists in the integration of active nanomaterials, in thin epitaxial films or embedded within carrier nanomembranes, onto Pb(Mg1/3Nb2/3)O3–PbTiO3-based piezoelectric actuators, which convert electrical signals into mechanical deformation (strain). In this review, we mainly focus on recent advances in strain-tunable properties of self-assembled InAs quantum dots (QDs) embedded in semiconductor nanomembranes and photonic structures. Additionally, recent works on other nanomaterials like rare-earth and metal-ion doped thin films, graphene and MoS2 or WSe2 semiconductor two-dimensional materials are also reviewed. For the sake of completeness, a comprehensive comparison between different procedures employed throughout the literature to fabricate such hybrid piezoelectric-semiconductor devices is presented. It is shown that unprocessed piezoelectric substrates (monolithic actuators) allow to obtain a certain degree of control over the nanomaterials’ emission properties such as their emission energy, fine-structure-splitting in self-assembled InAs QDs and semiconductor 2D materials, upconversion phenomena in BaTiO3 thin films or piezotronic effects in ZnS:Mn films and InAs QDs. Very recently, a novel class of micro-machined piezoelectric actuators have been demonstrated for a full control of in-plane stress fields in nanomembranes, which enables producing energy-tunable sources of polarization-entangled photons in arbitrary QDs. Future research directions and prospects are discussed.
J.Olea, A. del Prado, E. García‐Hemme, Rodrigo García‐Hernansanz, D. Montero, G. González‐Díaz, J. Gonzalo, J. Siegel and E. López
Progress in Photovoltaics 26, 214-222 (2018).
Photovoltaic solar cells based on the intermediate band (IB) concept could greatly enhance the efficiency of future devices. We have analyzed the electrical and photoconductivity properties of GaP supersaturated with Ti to assess its suitability for IB solar cells. GaP:Ti was obtained by ion implantation followed by pulsed‐laser melting (PLM) using an ArF excimer laser. It was found that PLM energy densities between 0.35 and 0.55 J/cm2 produced a good recovery of the crystalline structure of GaP (both unimplanted and implanted with Ti), as evidenced by high mobility measured values (close to the reference GaP). Outside this energy density window, the PLM failed to recover the crystalline structure producing a low mobility layer that is electrically isolated from the substrate. Spectral photoconductivity measurements were performed by using the van der Pauw set up. For GaP:Ti a significant enhancement of the conductivity was observed when illuminating the sample with photon energies below 2.26 eV, suggesting that this photoconductivity is related to the presence of Ti in a concentration high enough to form an IB within the GaP bandgap. The position of the IB was estimated to be around 1.1 eV from the conduction band or the valence band of GaP, which would lead to maximum theoretical efficiencies of 25% to 35% for a selective absorption coefficients scenario and higher for an overlapping scenario.
T. T. Fernandez, M. Sakakura, S. M. Eaton, B. Sotillo, J Siegel, J. Solis, Y. Shimotsuma and K. Miura.
Prog. Mater. Sci. 94, 68-113 (2018). Review
This Review provides an exhaustive and detailed description of ion migration phenomena which occur inside transparent dielectric media due to the interaction with intense ultrashort pulses. The paper differentiates various processes underlying the ion migration influenced by simultaneous heat accumulation and diffusion. The femtosecond laser induced temperature distribution, the major driving force of ions in dielectrics, is described in detail. This discussion is based on three meticulous analysis methods including the thermal modification of transparent dielectrics at various ambient temperatures, numerical simulations and comparison with direct observation of the light-matter interaction and micro-Raman spectroscopy. The ion migration phenomena studied have been triggered in four different configurations: at low repetition and high repetition rates, and observations perpendicular and parallel to the laser irradiation direction. Inspired by this research, potential applications are highlighted including space-selective phase separation, a laser-based ion exchange fabrication method and optical micropipetting by tailoring the plasma profile.
Self‐Assembled Nanostructured Photonic‐Plasmonic Metasurfaces for High‐Resolution Optical Thermometry.
G. Baraldi, M. García-Pardo, J. Gonzalo, R. Serna and J. Toudert.
Advanced Materials Interfaces 1800241 (2018).
Sensing devices for environment, safety, healthcare, and optoelectronic applications require an accurate and noninvasive monitoring of their temperature, because its variations markedly affect the overall response of the device. Monitoring the optical response of temperature‐sensitive integrated photonic elements, such as microresonators or microinterferometers, is an appealing solution in this context. However, achieving high‐resolution optical thermometry with such elements is unpractical and costly as this requires lithography processing, highly monochromatic laser sources, complex light coupling strategies. Here, a photonic‐plasmonic metasurface design that enables practical optical thermometry with a sub‐10−3 °C resolution is proposed. It is based on a self‐assembled nanostructured material implemented with a lithography‐free process. The optical response of the temperature‐sensitive metasurface is probed using a white light source and by monitoring the optical phase in a standard reflectance configuration. This facile, yet powerful, sensing scheme stands on the effective response of the metasurface, which involves the hybridization of thin film interference and low‐quality‐factor plasmon resonances to enable a quasi‐darkness response with a sharp spectral variation (jump) of the optical phase. Such jump is equivalent with a high‐quality‐factor resonator that yields a high sensor responsivity and thus enables high‐resolution optical thermometry.
Key stages of material expansion in dielectrics upon femtosecond laser ablation revealed by double-color illumination time-resolved microscopy.
M. Garcia-Lechuga, J. Solis and J. Siegel.
Applied Physics A 124:221 (2018).
The physical origin of material removal in dielectrics upon femtosecond laser pulse irradiation (800 nm, 120 fs pulse duration) has been investigated at fluences slightly above ablation threshold. Making use of a versatile pump–probe microscopy setup, the dynamics and different key stages of the ablation process in lithium niobate have been monitored. The use of two different illumination wavelengths, 400 and 800 nm, and a rigorous image analysis combined with theoretical modelling, enables drawing a clear picture of the material excitation and expansion stages. Immediately after excitation, a dense electron plasma is generated. Few picoseconds later, direct evidence of a rarefaction wave propagating into the bulk is obtained, with an estimated speed of 3650 m/s. This process marks the onset of material expansion, which is confirmed by the appearance of transient Newton rings, which dynamically change during the expansion up to approximately 1 ns. Exploring delays up to 15 ns, a second dynamic Newton ring pattern is observed, consistent with the formation of a second ablation front propagating five times slower than the first one.
Femtosecond laser writing of photonic devices in borate glasses compositionally designed to be laser writable.
A. Dias, F. Muñoz, A. Alvarez, P. Moreno-Zárate, J. Atienzar, A. Urbieta, P. Fernandez, M. García-Pardo, R. Serna and J. Solis.
Optics letters 43 11, 2523-2526 (2018).
The design and performance of borate glass samples compositionally pre-designed to be femtosecond laser writable via laser-induced ion migration is reported in this Letter. It is demonstrated that borate glasses modified on purpose with small amounts of La2O3 and Na2O can be straightforwardly used to produce high-index contrast (Δ) waveguides by femtosecond-laser-assisted ion migration. The positive Δ of the waveguides is caused by the local enrichment of La2O3 in the guiding region with a slope of 8·10−3(mol.%)−1. The value of this is consistent with numerical aperture measurements of the waveguides and local compositional measurements at the guiding region performed by energy-dispersive x-ray micro-analysis. The maximum achievable Δ values can be controlled through the initial La2O3 content of the glass. Maximum Δ values >10−2 for samples with just 5.5 mol. % of La2O3 have been produced. This compositional design approach could be potentially used to produce highly efficient femtosecond laser writeable glasses in other glass families.
Two-dimensional carbon nanostructures obtained by laser ablation in liquid: effect of an ultrasonic field.
L. Escobar-Alarcón, ME. Espinosa-Pesqueira, DA. Solis-Casados, J. Gonzalo, J. Solis, M. Martinez-Orts and E. Haro-Poniatowski.
Applied Physics A 124 2, 141 (2018).
The ablation of a carbon target immersed in deionized water, in absence and in presence of ultrasonic waves is reported, and the differences investigated. The obtained nanostructures are characterized by transmission electron microscopy, Raman spectroscopy and photoluminescence. Transmission electron images reveal that the produced carbon nanostructures, with and without ultrasonic excitation, are graphene-like sheets with improved quality in the first case. Samples prepared with ultrasounds show graphene layers with large sizes (several microns) and regular shapes, whereas the samples prepared without ultrasounds show smaller sizes and irregular shapes; additionally, some dispersed quasi-spherical nanoparticles are observed in the samples prepared without ultrasound. Photoluminescence measurements of the obtained nanostructures reveal emission in the blue spectral region.