arXiv subjects

Emmanuel Stratakis

Publications and source records attributed to Emmanuel Stratakis.

At least 37 records · Page 2Linked to original sources

arXiv2023

Influence of Heat Accumulation on Morphology Debris Deposition and Wetting of LIPSS on Steel upon High Repetition Rate Femtosecond Pulses Irradiation

The fabrication of laser-induced periodic surface structures (LIPSS) over extended areas at high processing speeds requires the use of high repetition rate femtosecond lasers. It is known that industrially relevant materials such as steel experience heat accumulation when irradiated at repetition rates above some hundreds of kHz, and significant debris redeposition can take place. However, there are few studies on how the laser repetition rate influences both the debris deposition and the final LIPSS morphology. In this work, we present a study of fs laser-induced fabrication of low spatial frequency LIPSS (LSFL), with pulse repetition rates ranging from 10 kHz to 2 MHz on commercially available steel. The morphology of the laser-structured areas as well as the redeposited debris was characterized by scanning electron microscopy (SEM) and μ-Raman spectroscopy. To identify repetition rate ranges where heat accumulation is present during the irradiations, we developed a simple heat accumulation model that solves the heat equation in 1 dimension implementing a Forward differencing in Time and Central differencing in Space (FTCS) scheme. Contact angle measurements with water demonstrated the influence of heat accumulation and debris on the functional wetting behavior. The findings are directly relevant for the processing of metals using high repetition rate femtosecond lasers, enabling the identification of optimum conditions in terms of desired morphology, functionality, and throughput.

physics.app-ph↗

arXiv2023

Impact of plasmonic modes on the formation of self-organised nano-patterns in thin films

Formation of nanoscale laser-induced periodic surface structures on thin metal films (of the size of the optical penetration depth) is a yet unexplored area that is expected to open new routes for laser patterning and a wealth of exciting applications in optics, photonics, and sensing. In contrast to the common belief that excitation of Surface Plasmon Polaritons (SPPs) on the air/metal interface plays the dominant role in the features of the induced topographies, in this work, we demonstrate that the excitation of coupled SPPs in both air/metal and metal/substrate interfaces, along with other parameters such as the thickness of the material, the photon energy, and the substrate refractive index, dictate the spatial modulation of the absorbed energy. A detailed theoretical analysis of the excited plasmonic waves and a multiscale modelling of laser-induced physical phenomena manifests that depending on the laser conditions and thickness of the irradiated solid, topographies with periodic features of diverse sizes (ranging from $λ_L/3$ to $λ_L$, where $λ_L$ stands for the laser wavelength) and different orientation can be realized. The capability to control and tune the characteristics of the produced structures on thin films is expected to enable novel surface patterning approaches.

physics.optics↗

arXiv2023

Ionization dynamics and damage conditions for transparent materials irradiated with Mid-Infrared femtosecond pulses

The employment of ultrashort laser sources at the mid-IR spectral region for transparent materials is designed to open new routes for laser patterning and a wealth of exciting applications in optics and photonics. To elucidate the material response to irradiation with mid-IR laser sources, a consistent analysis of the interaction of long wavelength femtosecond pulses with dielectric materials is presented. The influence of the pulse duration is, in particularly, emphasized towards specifying the laser parameters for which photoionization and impact ionization are important. Simulations results using pulses at 2.2 mum, 3.2 mum and 5 mum are conducted to illustrate the optimum conditions for the onset of damage on the solid that is related to the occurrence of the optical breakdown. Results predict that the damage threshold scales as ~$τ_p^α$ (0.31<$α$<0.37) at all laser wavelengths. Given the significant effect of the induced excitation level on the excitation of Surface Plasmons (SP) which account for the formation of laser induced periodic structures (LIPSS) oriented perpendicular to the laser polarization, a correlation of the produced electron densities with SP and the threshold of SP excitation (~$τ_p^β$, 0.33<$β$<0.39) are also discussed in this yet unexplored spectral region. Results are expected to allow a novel surface patterning approach with strong mid-IR pulses for advanced applications.

physics.optics↗

arXiv2022

Probing the carrier dynamics of polymer composites with single and hybrid carbon nanotube fillers for improved thermoelectric performance

The incorporation of carbon nanotubes (CNTs) within polymer hosts offers a great platform for the development of advanced thermoelectric (TE) composite materials. Over the years, several CNT/polymer composite formulations have been investigated on an effort to maximize the TE performance. Meanwhile, several studies focused on the decay dynamics of the charged excitons within CNTs itself and therefrom derived structures, aiming to investigate the lifetimes and the corresponding recombination processes of free charge carriers. The latter physical phenomena play a crucial role in the performance of various types of energy converting and scavenging materials. Nevertheless, up to this date, there is no systematic study on the combination of TE parameters and the critical charge carrier dynamics within CNT containing TE polymer composites. Herein, a variety of composites with single and hybrid CNT fillers based on polycarbonate (PC) and polyether ether ketone (PEEK) polymer matrices were prepared by melt-mixing in small scale. At the same loading, the addition of single fillers in PC results in higher Seebeck coefficients and similar conductivities when compared to the use of hybrid filler systems. In contrast, with hybrid filler systems in PEEK composites, higher power factors could be reached than in single filler composites. Moreover, the PC-based composites are studied using ultrafast laser time-resolved transient absorption spectroscopy (TAS), for the investigation of the exciton lifetimes and the physical origins of free charge carrier transport within the TE films. The findings of this study reveal interesting links between the TE parameters and the obtained charge carrier dynamics.

physics.app-ph↗

arXiv2022

Damage threshold evaluation of thin metallic films exposed to femtosecond laser pulses: the role of material thickness

The employment of femtosecond pulsed lasers has received significant attention due to its capability to facilitate fabrication of precise patterns at the micro- and nano- lengths scales. A key issue for efficient material processing is the accurate determination of the damage threshold that is associated with the laser peak fluence at which minimal damage occurs on the surface of the irradiated solid. Despite a wealth of previous reports that focused on the evaluation of the laser conditions that lead to the onset of damage, the investigation of both the optical and thermal response of thin films of sizes comparable to the optical penetration depth is still an unexplored area. In this report, a detailed theoretical analysis of the impact of various parameters such as the photon energies and material thickness on the damage threshold for various metals (Au, Ag, Cu, Al, Ni, Ti, Cr, Stainless Steel) is investigated. A multiscale physical model is used that correlates the energy absorption, electron excitation, relaxation processes and minimal surface modification which leads to the onset of material damage. The satisfactory agreement of the theoretical model with some experimental results indicates that the damage threshold evaluation method could represent a systematic approach towards designing efficient laser-based fabrication systems and optimizing the processing outcome for various applications.

cond-mat.mtrl-sci↗

arXiv2022

The impact of the substrate on the opto-thermal response of thin metallic targets following irradiation with femtosecond laser pulses

Femtosecond pulsed lasers have been widely used over the past decades due to their capability to fabricate precise patterns at the micro- and nano- lengths scales. A key issue for efficient material processing is the determination of the laser parameters used in the experimental set ups. Despite a systematic investigation that has been performed to highlight the impact of every parameter independently, little attention has been drawn on the role of the substrate material on which the irradiated solid is placed. In this work, the influence of the substrate is emphasised for films of various thicknesses which demonstrates that both the optical and thermophysical properties of the substrate affect the thermal fingerprint on the irradiated film while the impact is manifested to be higher at smaller film sizes. Two representative materials, silicon and fused silica have been selected as typical substrates for thin films of different optical and thermophysical behaviour (gold and nickel) and the thermal response and damage thresholds are evaluated for the irradiated solids. The pronounced influence of the substrate is aimed to pave the way for new and more optimised designs of laser-based fabrication set ups and processing schemes.

physics.optics↗

arXiv2022

The synergy of electromagnetic effects and thermophysical properties of metals in the formation of laser induced periodic surface structures

Femtosecond pulsed lasers have been widely used over the past decades for precise materials structuring at the micro- and nano- scales. In order, though, to realize efficient material processing and account for the formation of laser induced periodic surfaces structures (LIPSS), it is very important to understand the fundamental laser-matter interaction processes. A significant contribution to the LIPSS profile appears to originate from the electromagnetic fingerprint of the laser source. In this work, we follow a systematic approach to predict the pulse-by-pulse formation of LIPSS on metals due to the development of a spatially periodic energy deposition that results from the interference of electromagnetic far fields on a non-flat surface profile. On the other hand, we demonstrate that the induced electromagnetic effects, alone, are not sufficient to allow the LIPSS formation, therefore, we emphasize on the crucial role of electron diffusion and electron-phonon coupling on the formation of stable periodic structures. Gold and stainless Steel are considered as two materials to test the theoretical model while simulation results appear to confirm the experimental results that, unlike gold, fabrication of pronounced LIPSS on stainless Steel is feasible.

physics.optics↗

arXiv2021

Impact of pre-patterned structures on features of Laser Induced Periodic Surface structures

The efficiency of light coupling to surface plasmon polariton (SPP) represents a very important issue in plasmonics and laser fabrication of topographies in various solids. To illustrate the role of pre-patterrned surfaces and impact of laser polsarisation in the excitation of electromagnetic modes and periodic pattern formation, Nickel surfaces are irradiated with femtosecond laser pulses of polarisation perpendicular or parallel to the orientation of the pre-pattern ridges. Experimental results indicate that for polarisation parallel to the ridges, laser induced periodic surface structures (LIPSS) are formed perpendicularly to the pre-pattern with a frequency that is independent of the distance between the ridges and periodicities close to the wavelength of the excited SPP. By contrast, for polarisation perpendicular to the pre-pattern, the periodicities of the LIPSS are closely correlated to the distance between the ridges for pre-pattern distance larger than the laser wavelength. The experimental observations are interpreted through a multi-scale physical model in which the impact of the interference of the electromagnetic modes is revealed.

cond-mat.mtrl-sci↗

arXiv2021

In-plane anisotropic quantum confinement effect in ultrasmall SnS sheets

Black phosphorus (BP) analogous tin(II) sulfide (SnS) has recently emerged as an attractive building block for electronic devices due to its highly anisotropic response. Two-dimensional (2D) SnS has shown to exhibit in-plane anisotropy in optical and electrical properties. However, the limitations in growing ultrasmall structures of SnS hinder the experimental exploration of anisotropic behavior in low dimension. Here, we present an elegant approach of synthesizing highly crystalline nanometer-sized SnS sheets. Ultrasmall SnS exhibits two distinct valleys along armchair and zig-zag directions due to in-plane structural anisotropy like bulk SnS. We show that in such SnS nanosheet dots, the band gaps corresponding to two valleys are increased due to quantum confinement effect. We particularly observe that SnS quantum dots (QDs) show excitation energy dependent photoluminescence (PL), which originates from the two nondegenerate valleys. Our work may open up an avenue to show the potential of SnS QDs for new functionalities in electronics and optoelectronics.

cond-mat.mtrl-sci↗

arXiv2021

Tailoring Sub-micrometer Periodic Surface Structures via Ultrashort Pulsed Direct Laser Interference Patterning

Direct laser Interference Patterning (DLIP) with ultrashort laser pulses (ULP) represents a precise and fast technique to produce tailored periodic sub-micrometer structures on various materials. In this work, an experimental and theoretical approach is presented to investigate the previously unexplored fundamental mechanisms for the formation of unprecedented laser-induced topographies on stainless steel following proper combinations of DLIP with ULP. DLIP is aimed to determine the initial conditions of the laser-matter interaction by defining an ablated region while double ULP are used to control the reorganisation of the self-assembled laser induced sub-micrometer sized structures by exploiting the interplay of different absorption and excitation levels coupled with the melt hydrodynamics induced by the first of the double pulses. A multiscale physical model is presented to correlate the interference period, polarization orientation and number of incident pulses with the induced morphologies. Special emphasis is given to electron excitation, relaxation processes and hydrodynamical effects that are crucial to the production of complex morphologies. Results are expected to derive new knowledge of laser-matter interaction in combined DLIP and ULP conditions and enable enhanced fabrication capabilities of complex hierarchical sub-micrometer sized structures for a variety of applications.

physics.app-ph↗

arXiv2020

Dispersion behavior of two dimensional monochalcogenides

Solution processable two-dimensional (2D) materials have provided an ideal platform for both fundamental studies and wearable electronic applications. Apart from graphene and 2D dichalcogenides, IVA-VI monochalcogenides (MMCs) has emerged recently as a promising candidate for next generation electronic applications. However, the dispersion behavior, which is crucial for the quality, solubility and stability of MMCs, has been quite unexplored. Here, the exfoliation and the dispersion behavior of Germanium (II) monosulfide (GeS) and Tin (II) monosulfide (SnS) nanosheets has been investigated in a wide range of organic solvents. Nine different organic solvents were examined and analyzed, considering the solvent polarity, surface tension, and Hansen solubility parameters. A significant yield of isolated GeS and SnS flakes, namely ~16.4 and ~23.08 μg/ml in 2-propanol and N-Methyl-2-pyrrolidone respectively were attained. The isolated flakes are few-layers nanosheets with lateral sizes over a few hundreds of nanometers. The MMCs colloids exhibit long-term stability, suggesting the MMCs applicability for scalable solution processable printed electronic device applications.

cond-mat.mtrl-sci↗

arXiv2020

Prominent room temperature valley polarization in WS2/graphene heterostructures grown by chemical vapor deposition

We examine different cases of heterostructures consisting of WS2 monolayers grown by chemical vapor deposition (CVD) as the optically active material. We show that the degree of valley polarization of WS2 is considerably influenced by the material type used to form the heterostructure. Our results suggest the interaction between WS2 and graphene (WS2/Gr) has a strong effect on the temperature dependent depolarization (i.e. decrease of polarization with increasing temperature), with polarization degrees reaching 24% at room temperature under near-resonant excitation. This contrasts to hBN- encapsulated WS2, which exhibits a room temperature polarization degree of only 11%. The observed low depolarization rate in WS2/Gr heterostructure is attributed to the nearly temperature independent scattering rate due to phonons and fast charge and energy transfer processes from WS2 to graphene. Significant variations in the degree of polarization are also observed at 4K between the different heterostructure configurations. Intervalley hole scattering in the valence band proximity between the K and Γ points of WS2 is sensitive to the immediate environment, leading to the observed variations.

cond-mat.mtrl-sci↗

arXiv2020

Detecting ultra-low ozone and hydrogen concentrations with CsPbBr$_3$ microcrystals direct grown on electrodes

Highly stable ozone and hydrogen sensing elements were fabricated based on well-crystalline rounded cube-shaped CsPbBr$_3$ submicron crystals, synthesized by a facile solution process performed under ambient conditions. It is shown that such elements demonstrate enhanced room temperature gas sensing ability compared to the previously reported metal halide and oxide-based ones. Electrical measurements performed on these sensing components revealed high response to ultra-low ozone and hydrogen concentrations, namely 4 ppb and 5 ppm respectively, as well as an impressive repeatability of the sensing behavior even after a few months of storage in ambient conditions. Both ozone and hydrogen detecting sensors were self-powered, i.e. they do not require the use of UV or heating external stimuli, and exhibited fast detection and short restoration times. These attractive properties along with the simple synthesis conditions could provide an easy, efficient and low-cost potential technology for the realization of future gas sensing devices.

physics.app-ph↗

arXiv2020

Improved Charge Carrier Dynamics of CH$_3$NH$_3$PbI$_3$ Perovskite Films Synthesized by Means of Laser-Assisted Crystallization

Although it has been recently demonstrated that the laser-assisted (LA) crystallization process leads to the formation of perovskite absorber films of superior photovoltaic performance compared to conventional thermal annealing (TA), the physical origin behind this important discovery is missing. In this study, CH$_3$NH$_3$PbI$_3$ perovskite thin films have been synthesized by means of LA and TA crystallization on the surface of two hole transport layers (HTL) namely poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and poly(triarylamine) (PTAA). A systematic study of the effect of laser irradiation conditions on the crystalline quality and morphology of the perovskite films was performed via scanning electron microscopy, X-Ray diffraction and absorption spectroscopy. Meanwhile, time-resolved transient absorption spectroscopy under inert atmosphere conditions was used to evaluate the carrier transport dynamics. It is found that for the PEDOT:PSS/CH$_3$NH$_3$PbI$_3$, structures the LA process resulted to perovskite layers of larger grains, faster charge carrier extraction properties and slower bimolecular recombination, when compared to TA. On the contrary, the LA-assisted formation of the PTAA/CH$_3$NH$_3$PbI$_3$ heterostructures leads to extensive presence of residual PbI$_2$ and thus inferior performance and charge carrier dynamics.

physics.app-ph↗

arXiv2020

In-Situ Monitoring of the Charge Carrier Dynamics of CH$_3$NH$_3$PbI$_3$ Perovskite Crystallization Process

Although methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$) perovskite has attracted enormous scientific attention over the last decade or so, important information on the charge extraction dynamics and recombination processes in perovskite devices is still missing. Herein we present a novel approach to evaluate the quality of CH$_3$NH$_3$PbI$_3$ layers, via in-situ monitoring of the perovskite layer charge carrier dynamics during the thermal annealing crystallization process, by means of time-resolved femtosecond transient absorption spectroscopy (TAS). In particular, CH$_3$NH$_3$PbI$_3$ films were deposited on two types of polymeric hole transport layers (HTL), poly(3,4-ethylenedioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS) and poly-(triarylamine) (PTAA), that are known to provide different carrier transport characteristics in perovskite solar cells. In order to monitor the evolution of the perovskite charge carrier dynamics during the crystallization process, the so-formed CH$_3$NH$_3$PbI$_3$/HTL architectures were studied in-situ by TAS at three different annealing temperatures, i.e. 90, 100 and 110 oC. It is revealed that the annealing time period required in order to achieve the optimum perovskite film quality in terms of the decay dynamics strongly depends on the annealing temperature, as well as, on the employed HTL. For both HTLs, the required period decreases as higher annealing temperature is used, while, for the more hydrophobic PTAA polymer, longer annealing periods were required in order to obtain the optimum charge carrier dynamics. The correlation of the TAS finding with the structural and morphological features of the perovskite films is analysed and provides useful insights on the charge extraction dynamics and recombination processes in perovskite optoelectronic devices.

physics.app-ph↗

arXiv2020

Recent Advances in Two-Dimensional Metal Monochalcogenides

The family of emerging low-symmetry and structural in-plane anisotropic 2D materials have been expanding rapidly in recent years. As an important emerging anisotropic 2D material, the black phosphorene (BP) analog group IVA-VI metal monochalcogenides (MMCs) have been surged recently due to their distinctive crystalline symmetries, exotic in-plane anisotropic electronic and optical response, earth abundant and environmentally friendly characteristics. In this article, we review the recent research advancements in the field of anisotropic 2D MMCs. At first, the unique wavy crystal structures together with the optical and electronic properties of such materials are discussed. The review continues with the various methods adopted for the synthesis of layered MMCs including micromechanical and liquid phase exfoliation as well as physical vapor deposition. The last part of the article focuses on the application of the structural anisotropic response of 2D MMCs in field effect transistors, photovoltaic cells nonlinear optics and valleytronic devices. Besides presenting the significant research in the field of this emerging class of 2D materials, this review also delineates the existing limitations and discusses emerging possibilities and future prospects.

cond-mat.mtrl-sci↗

arXiv2020

Polarization dependent spike formation on black silicon via ultrafast laser structuring

A comparative experimental and theoretical investigation is presented that centres on the effects of structuring black silicon surfaces with linearly, circularly and azimuthally polarized laser pulses under SF6 ambient atmosphere. It is shown that the asymmetric elliptical micro-cone formations induced by linearly polarized beams result in variable light absorption due to their spatial asymmetry. By contrast, the use of azimuthally polarized beams leads to an omni-directionality of the elliptical cone orientation which is dependent on the local electric field during laser scanning. The locally variant electric field state that is azimuthally polarized leads to a selective conical orientation for the induced structures. The omni-directional conical distribution induced by azimuthal polarization produces similar, angle-independent absorption in the visible spectrum with the symmetrical conical structures that could only be realized with circularly polarized beams.

cond-mat.mtrl-sci↗

arXiv2020

Biomimetic Omnidirectional Anti-reflective Glass via Direct Ultrafast Laser Nanostructuring

We report on a single-step, biomimetic approach for the realization of omnidirectional transparent antireflective glass. In particular, it is shown that circularly polarized ultrashort laser pulses produce self-organized nanopillar structures on fused silica (SiO2). The laser induced nanostructures are selectively textured on the glass surface in order to mimic the spatial randomness, pillar-like morphology, as well as the remarkable anti-reflection properties found on the wings of the glasswing butterfly, Greta oto[1] and various Cicada species[2]. The artificial structures exhibit impressive anti-reflective properties, both in the visible and infrared frequency range. Accordingly, the laser-processed glass surfaces show reflectivity smaller than 1% for various angles of incidence in the visible spectrum for S-P linearly polarized configurations. While, in the near infrared spectrum, the laser-textured glass shows higher transmittance compared to the pristine. It is envisaged that our current results will revolutionize the technology of anti-reflective transparent surfaces and impact numerous applications from glass displays to optoelectronic devices.

physics.optics↗