arXiv subjects

Emmanuel Stratakis

Publications and source records attributed to Emmanuel Stratakis.

At least 19 recordsLinked to original sources

arXiv2026

Alloy engineering of excitonic properties in TMD monolayers

We investigate monolayer MoS$_{2x}$Se$_{2(1-x)}$ alloys across the full composition range using optical spectroscopy. We demonstrate continuous tuning of the optical gap over $\sim$0.35 eV, accompanied by a systematic reduction of the B--A exciton splitting, in agreement with density functional theory calculations. Temperature-dependent measurements reveal a progressive increase of the average phonon energy from Se-rich to S-rich alloys that follows a simple reduced-mass scaling model. Polarization-resolved spectroscopy further shows a monotonic increase of the circular polarization from nearly zero in MoSe$_2$ to $\sim$15\% in MoS$_2$ at 78 K. The observed evolution of the polarization is attributed to alloy-induced modifications of the electronic structure that modify bright--dark exciton mixing and the associated valley depolarization. These findings establish alloy engineering as an effective strategy for controlling excitonic properties in TMD monolayers.

cond-mat.mtrl-sci↗

arXiv2026

It's not the Language Model, it's the Tool: Deterministic Mediation for Scientific Workflows

Language models can produce convincing scientific analyses, but repeated generations on the same data do not guarantee the same result. A researcher may regenerate an identical query and receive a different fit, a different peak position or a different analysis procedure, without an obvious way to decide which output to trust. We propose typed mediation, a pattern in which the model orchestrates deterministic tools rather than generating analytical code. Each tool encodes one researcher's exact procedure for one instrument, ported through structured interviews. The model selects which tool to call and with what parameters. The tool produces the result. Regeneration does not change it. We evaluate this claim by running the same photoluminescence analysis on four platforms, including three commercial foundation models, four times each with the same prompt. The typed tool produces identical results across all runs. The commercial platforms either vary in numerical output and analytical methodology across runs, or fail to produce valid results on the task. We deploy this pattern on two instruments serving users over approximately six months, with very positive user feedback. Both cases are very challenging: they involve proprietary binary formats and per-seat licensed software, which force the tool to remain on local infrastructure alongside the data and the instrument it operates. We argue that deployment topology is not just a preference, but a structural requirement of scientific tool mediation. The result is a practical pattern for deploying language models in scientific workflows where reproducibility is mandatory, reducing analysis time from weeks to minutes while guaranteeing identical outputs across runs.

cs.AI↗

arXiv2025

Fabrication of highly uniform laser-induced periodic structures on polycarbonate via UV femtosecond pulses

In this work, we focus on the fabrication of highly uniform laser-induced periodic surface structures (LIPSS) on bulk polycarbonate (PC) using 258 nm femtosecond laser pulses. A systematic approach was pursued to investigate the influence of various laser parameters such as fluence, effective number of pulses, energy dose and polarisation on the features of the generated LIPSS. Experimental results showed that linearly polarized beams produce LIPSS with period comparable to the laser wavelength. Moreover, it was observed that the orientation of LIPSS is either parallel or perpendicular to the laser polarization, depending on the excitation level. These features are similar to the LIPSS formed on dielectrics, despite the considerably higher absorption and larger extinction coefficient (~10-2) of polycarbonate at 258 nm. The orientation and features of the patterns at different excitation levels are explained using Sipe's theory, while the excitation and induced carrier densities were quantified through the application of a theoretical physical model that describes ultrafast dynamics in polymers. Furthermore, observations revealed that low excitation levels with linearly polarized enhance the uniformity of LIPSS, whereas irradiating PC targets with circularly polarized beams leads to the formation of microscale topographies decorated with protruding nanosphered-like structures, with their size varying based on the laser conditions. A detailed analysis of the impact of the induced topographies on their wetting and optical properties demonstrated a steady increase in hydrophilicity over time, in contrast to a modest increase in optical absorbance. The results regarding the morphological features and properties of the induced topographies on PC are anticipated to support future efforts in creating patterned surfaces on polymeric materials for potential applications.

physics.optics↗

arXiv2025

Silicon nanoantennas for tailoring the optical properties of MoS2 monolayers

Silicon-based dielectric nanoantennas provide an effective platform for engineering light-matter interactions in van der Waals semiconductors. Here, we demonstrate near-field coupling between monolayer MoS2 and silicon nanoantennas arranged in hexagonal lattices with tunable geometric parameters, leading to a three-fold enhancement in photoluminescence and an excitation-wavelength-dependent emission that aligns with Mie-resonant modes. Raman spectroscopy reveals an up to 8-fold enhancement in the vibrational modes of MoS2, while second-harmonic generation exhibits a 20 to 30-fold increase in efficiency, closely correlating with the presence of the underlying nanoantennas. Our experiments and simulations quantify the tunable benefits of the near-field interactions, taking into account thin-film interference and strain-induced effects. Our findings present dielectric nanoantennas as a promising platform for tailoring linear and nonlinear optical properties in 2D materials, with potential applications in nanophotonic devices and integrated photonics.

physics.optics↗

arXiv2025

Shell-Core Structural Anisotropy in Starch Granules Probed by Polarization Third-Harmonic Generation Microscopy

Lately the non-linear optical third harmonic generation (THG) microscopy is starting to emerge as a laboratory standard for label-free studies in biological samples. In this study, the THG signals produced from corn starch granules are investigated. In particular, the polarization-dependent THG (P-THG) signals emerging from the outer layer (shell) of the starch granules are compared with the P-THG signals originating from their inner portion (core). By rotating the linear polarization of the excitation beam, two distinct P-THG modulation patterns are revealed within single granules, corresponding to their shells and to their structurally different cores. These patterns are analyzed using a theoretical framework, that describes THG from an orthorhombic crystal symmetry, characteristic of corn starch. This allows us to extract point-by-point in the granules the ratios of the \c{hi}^(3) susceptibility tensor elements and the average molecular orientations. Then, the anisotropy ratio (AR=\c{hi}_xxxx^((3) )/\c{hi}_yyyy^((3) ) is defined and used as a quantitative descriptor of the local molecular arrangements. Our results show that the shells and cores exhibit distinct AR values, probing the anisotropy in the molecular arrangements between the two regions. This study establishes P-THG as a powerful contrast mechanism for probing structural anisotropy in biological samples beyond conventional THG intensity-only microscopy.

physics.optics↗

arXiv2025

Deciphering Carrier Dynamics in Polycarbonate Following Excitation with Ultrashort Laser Pulses

Polymers exposed to ultrashort pulsed lasers (UPL) experience a range of physical and chemical changes that play a key role in applications ranging from material processing to advanced photonics, and biomedicine. To elucidate the interaction of UPL with polymeric materials, ultrafast phenomena such as carrier dynamics, recombination and relaxation are investigated assuming polycarbonate (PC) as a test material exposed to laser pulses of moderate energies. A theoretical model developed for dielectric materials is extended to describe the, previously, unexplored excitation and carrier dynamics for PC while femtosecond Transient Absorption Spectroscopy is used to elucidate the evolution of the materials response and ultrafast dynamics. Interpreting the experimental measurements using the theoretical model suggests the existence of an energy level that facilitates the formation of self-trapped exciton metastates between the conduction and valence bands (approximately 2.4-2.8 eV below the conduction band). It also predicts the electron-plasma lifetime (around 110-150 fs), the recombination time (about 34 ps), and the non-linear part of the refractive index due to Kerr effect (with n_2 values ranging from (1.1-1.5)x10^{-16} cm2/W). Furthermore, the dominant character of multi-photon assisted ionisation is emphasised while the optical breakdown threshold is also calculated and found to be equal to 2.55 x10^12 W/cm2. The results are expected to support future efforts aimed at elucidating how intense ultrashort laser pulses interact with polymeric materials which is crucial for optimizing the manufacturing processes of these materials for potential applications.

physics.optics↗

arXiv2024

Highly-stable, eco-friendly and selective Cs2AgBiBr6 perovskite-based ozone sensor

Lead halide perovskites have attracted considerable attention as potential gas sensing elements due to their unique ability to detect and respond to an external stimulus with measurable electrical or optical signals. The distinctive characteristics of these materials lie in their ability to operate upon gas exposure at room temperature. However, the presence of lead (Pb) poses a serious challenge for their widespread application and commercialization, owing to its toxicity. To address this issue, lead-free Cs2AgBiBr6 double perovskite is explored as a promising alternative sensing element for room temperature gas detection. This sensor can be regarded as eco-friendly for the following reasons: it does not contain Pb; it is synthesized at room temperature without using strong organic solvents; and it operates and recovers at room temperature without heating or UV irradiation and with a very low input voltage (0.1V), thus reducing overall energy consumption. The influence of the perovskite morphology on the ozone (O3) sensing performance is investigated, as well as the sensing stability under varying conditions of time, humidity, and temperature. Notably, the sensors' exceptional selectivity for O3 over other gases and the interaction between the gases and the perovskite surface is confirmed through experimental and first-principles calculations.

physics.app-ph↗

arXiv2024

Wettability and Color Change of Copper by Controlling Area Fraction of Laser Ablated Surface

In this research, wettability control by area fraction of laser ablated surface of copper is presented. The functional surfaces with full wettability control from highly hydrophilic to super-hydrophobic were created on copper by nanosecond (ns) and picosecond (ps) laser irradiation. The area fraction and color change were evaluated by digital image processing of microscopic images of the laser-ablated copper surface. The control of the wetting angle from almost 0 degrees to 132 degrees was achieved for both ps and ns pulses by controlling the area fraction of the laser-ablated surface. Cassie, Cassie-Baxter, and Wenzel models were adopted to explain the experimental results. For the first time, the wettability and color of copper were controlled by controlling the area fraction of the laser-ablated surface. It is expected that the current results make an impact on the heat exchanger technology of water heat sinks, cooling units, atmospheric water generators, and fog harvesting and impact numerous applications from power plants to solar thermal water systems devices where highly-hydrophilic to super hydrophobic copper can be applied.

physics.optics↗

arXiv2024

Towards the optimization of a perovskite-based room temperature ozone sensor: A multifaceted approach in pursuit of sensitivity, stability, and understanding of mechanism

Metal halide perovskites (MHPs) have attracted significant attention owing to their simple manufacturing process and unique optoelectronic properties. Their reversible electrical or optical properties changes in response to oxidizing or reducing environments make them prospective materials for gas detection technologies. Despite advancements in perovskite-based sensor research, the mechanisms behind perovskite-gas interactions, vital for sensor performance, are still unexclusive. This work presents the first evaluation of the sensing performance and long-term stability of MHPs, considering factors such as halide composition variation and Mn doping levels. The research reveals a clear correlation between halide composition and sensing behavior, with Br-rich sensors displaying a p-type response to O3 gas, while Cl-based counterparts exhibit an n-type sensing behavior. Notably, Mn-doping significantly enhances the O3 sensing performance by facilitating the gas adsorption process, as supported by both atomistic simulations and experimental evidence. Long-term evaluation of the sensors provides valuable insights into evolving sensing behaviors, highlighting the impact of dynamic instabilities over time. Overall, this research offers insights into optimal halide combination and Mn-doping levels, representing a significant step forward in engineering room temperature perovskite-based gas sensors that are not only low-cost and high-performing but also durable, marking a new era in sensor technology.

physics.chem-ph↗

arXiv2024

Anisotropic Third Harmonic Generation in Two-Dimensional Tin Sulfide

The in-plane anisotropic properties of two-dimensional (2D) group IV monochalcogenides provide an additional degree of freedom which can be used in future optoelectronic devices. Here, it is shown that the third harmonic generation (THG) signal produced by ultrathin tin (II) sulfide (SnS) is in-plane anisotropic with respect to the incident linear polarization of the laser field. We fit the experimental polarization-resolved THG (P-THG) measurements with a nonlinear optics model, which accounts for the orthorhombic crystal structure of 2D SnS. We calculate the relative magnitudes of the \{chi}^(3) tensor components by recording and simultaneously fitting both orthogonal components of the P-THG intensity. Furthermore, we introduce a THG anisotropy ratio, whose calculated values compare the total THG intensity when the excitation linear polarization is along the armchair crystallographic direction with the case when it is along the zigzag direction. Our results provide quantitative information on the anisotropic nature of the THG process in SnS, paving the way to a better understanding of anisotropic nonlinear light-matter interactions, and the development of polarization-sensitive nonlinear optical devices.

physics.optics↗

arXiv2024

Strain distribution in WS2 monolayers detected through Polarization-resolved Second Harmonic Generation

Two-dimensional (2D) graphene and graphene-related materials (GRMs) show great promise for future electronic devices. Nevertheless, GRMs result distinct properties under the influence of the substrate that serves as support through uneven compression/ elongation of GRMs surface atoms. Strain in GRM monolayers is the most common feature that alters the interatomic distances, band structure, providing a new degree of freedom that allows regulation of their electronic properties and introducing the field of straintronics. Having an all-optical detection, a minimally invasive tool that rapidly probes strain in large areas of GRM monolayers, would be of great importance in the research and development of novel 2D devices. Here, we use Polarization-resolved Second Harmonic Generation (P-SHG) optical imaging to identify strain distribution, induced in a single layer of WS2 placed on a pre-patterned Si/SiO2 substrate with cylindrical wells. By fitting the P-SHG data pixel-by-pixel, we produce spatially resolved images of the crystal armchair direction. In regions where the WS2 monolayer conforms to the pattern topography, a distinct cross-shaped pattern is evident in the armchair image owing to strain. The presence of strain in these regions is independently confirmed using a combination of atomic force microscopy and Raman mapping.

physics.app-ph↗

arXiv2024

Formation of Highly Tunable Periodic Plasmonic Structures on Gold Films Using Direct Laser Writing

Direct laser writing method is a promising technique for the large-scale and cost-effective fabrication of periodic nanostructure arrays exciting hybrid lattice plasmons. This type of electromagnetic mode manifests a narrow and deep resonance peak with a high dispersion whose precise controllability is crucial for practical applications in photonic devices. Here, the formation of differently shaped gold nanostructures using the direct laser writing method on Au layers of different thicknesses is presented. The resonance peak is demonstrated to be highly dependent on the shape of the structures in the array, thus its position in the spectra, as well as the quality, can be additionally modulated by changing the morphology. The shape of the structure and the resonance itself pertain not only on the laser pulse energy but also on the grating period. This overlapping effect occurring at distances smaller than the diameter of the focused laser beam is studied in detail. By taking advantage of the highly controllable plasmonic resonance, the fabricated gratings open up new opportunities for applications in sensing.

cond-mat.mtrl-sci↗

arXiv2023

Control of the damage threshold of Si via a SiO2 coating upon irradiation with Mid-IR femtosecond laser pulses

A key issue in the use of high-power mid-infrared (Mid-IR) laser sources for a plethora of applications is the investigation of the exciting laser driven physical phenomena taking place in materials coated with dielectric films. Here, we present a theoretical investigation of the ultrafast processes and thermal response upon excitation of two-layered complexes consisting of fused silica thin films placed on silicon substrates with ultrashort pulsed lasers in the Mid-IR spectral regime. Through the development of a theoretical model, we demonstrate that the control of the underlying ultrafast phenomena and the damage threshold (DT) of the substrate are achieved via an appropriate modulation of the thickness of the SiO2 film. It is shown that a decrease of DT by up to 27% compared to the absence of coating is feasible emphasising the impact of coatings of a lower refractive index than the substrate. Besides this, it is demonstrated that no absorption of energy occurs within the SiO2 layer, therefore the dynamics of the reflectivity of the two-layered complex is directly associated with the electron excitation in the substrate. These remarkable predictions can be employed for the development of new optical coatings and components for nonlinear optics and photonics for a large range of Mid-IR laser-based applications.

physics.optics↗

arXiv2023

Cell response on laser-patterned Ti/Zr/Ti and Ti/Cu/Ti multi-layer systems

Arranged patterns obtained by ultrafast laser processing on the surface of Ti/Cu/Ti/Si and Ti/Zr/Ti/Si thin film systems are reported. Two differently designed multilayer thin films, Ti/Cu/Ti/Si and Ti/Zr/Ti/Si, were deposited on silicon by the ion sputtering method. The bioactive surfaces on these systems involve the formation of laser-induced periodic surface structures (LIPSS) in each of the laser-written lines of mesh patterns on 5x5 mm areas. The formation of nano- and micro-patterns with an ultra-thin oxide film on the surfaces was used to observe the effects of morphology and proliferation of the MRC-5 cell culture line. To determine whether Ti-based thin films have a toxic effect on living cells, an MTT assay was performed. The relative cytotoxic effect as a percentage of surviving cells showed that there was no difference in cell number between the Ti-based thin films and the control cells. There was also no difference in the viability of the MRC-5 cells, except for the Ti/Cu/Ti/Si system, where there was a slight 10% decrease in cell viability.

cond-mat.mtrl-sci↗

arXiv2023

Influence of Antireflection Si coatings on the Damage Threshold of fused silica upon irradiation with Mid-IR femtosecond laser pulses

Recent progress in the development of high-power mid-IR laser sources and the exciting laser driven physical phenomena associated with the irradiation of solids via ultrashort laser pulses in that spectral region are aimed to potentially create novel capabilities for material processing. In particularly, the investigation of the underlying physical processes and the evaluation of the optical breakdown threshold (OBT) following irradiation of bulk dielectric materials with Mid-IR femtosecond (fs) pulses has been recently presented. In this report, we will explore the conditions that generate sufficient carrier excitation levels which leads to damage upon irradiated a dielectric material (SiO2) coated with antireflection (AR) semiconducting films (Si) of variable thickness with fs pulses. Simulation results demonstrate that the reflectivity and transmissivity of the Si/SiO2 are thickness-dependent which can be employed to modulate the damage threshold of the substrate. The study is to provide innovative routes for selecting material sizes that can be used for antireflection coatings and applications in the Mid-IR region.

physics.optics↗

arXiv2023

Laser-induced metal halide perovskite-rGO nanoconjugates as anode material in Zn-air supercapacitors

This work reveals that the direct conjugation of metal halide perovskite nanocrystals on rGO sheets can provide high performance and stable electrodes for Zn-air supercapacitors. In particular, we provide a simple, rapid and room temperature laser-triggered method to anchor CsPbBr3 nanocrystals on rGO sheets without affecting the initial morphology and crystal structure of the components. The flexible and high surface area of the rGO enables the conjugation of individual metal halide perovskite nanocrystals with the emergence of new synergetic functionalities from the combination of the two counterparts. These synergetic functionalities lead to the 178- and 152-times improvement of the specific capacity of the conjugated-based electrodes compared to the single rGO and perovskite electrodes respectively.

physics.app-ph↗

arXiv2023

Two-dimensional metal halide perovskites and their heterostructures: from synthesis to applications

Size- and shape- dependent unique properties of the metal halide perovskite nanocrystals make them promising building blocks for constructing various electronic and optoelectronic devices. These unique properties together with their easy colloidal synthesis render them efficient nanoscale functional components for multiple applications ranging from light emission devices to energy conversion and storage devices. Recently, two-dimensional (2D) metal halide perovskites in the form of nanosheets (NSs) or nanoplatelets (NPls) are being intensively studied due to their promising 2D geometry which is more compatible with the conventional electronic and optoelectronic device structures where film-like components are employed. In particular, 2D perovskites exhibit unique thickness-dependent properties due to the strong quantum confinement effect, while enabling the bandgap tuning in a wide spectral range. In this review the synthesis procedures of 2D perovskite nanostructures will be summarized, while the application-related properties together with the corresponding applications will be extensively discussed. In addition, perovskite nanocrystals/2D material heterostructures will be reviewed in detail. Finally, the wide application range of the 2D perovskite-based structures developed to date, including pure perovskites and their heterostructures, will be presented while the improved synergetic properties of the multifunctional materials will be discussed in a comprehensive way.

cond-mat.mtrl-sci↗

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 {\mu}-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↗