arXiv subjects

Stelios Tzortzakis

Publications and source records attributed to Stelios Tzortzakis.

17 recordsLinked to original sources

arXiv2025

Graphene-Gold THz Metasurfaces with Tailored Resonant Structure for Enhanced Nonlinear Response

Graphene's exceptional nonlinear optical properties combined with resonant photonic structures offer a promising pathway for efficient nonlinear applications at terahertz (THz) frequencies. In this work, we propose and demonstrate a fabrication-friendly hybrid nonlinear metasurface composed of gold patches integrated with uniform graphene, circumventing the need for complex graphene patterning. The structure supports strong localized resonances that enhance nonlinear interactions. By exploiting resonant enhancement at both the fundamental and third harmonic frequencies, we predict via simulations third-harmonic generation efficiencies as high as -15 dB (3.2%) under continuous-wave excitation at modest intensities (0.1 MW/cm$^2$). The metasurface is fabricated via electron-beam lithography and experimentally characterized using THz time-domain spectroscopy. Under pulsed excitation, we experimentally observe pronounced nonlinear frequency shifts up to 0.5~THz (12.5% fractional change), driven by self-phase modulation, consistent with simulation results. These findings highlight the potential of tailored graphene-based metasurfaces for efficient nonlinear THz photonic devices.

physics.optics↗

arXiv2025

The universality of filamentation-caused challenges of ultrafast laser energy deposition in semiconductors

Light propagation in semiconductors is the cornerstone of emerging disruptive technologies holding considerable potential to revolutionize telecommunications, sensors, quantum engineering, healthcare, and artificial intelligence. Sky-high optical nonlinearities make these materials ideal platforms for photonic integrated circuits. The fabrication of such complex devices could greatly benefit from in-volume ultrafast laser writing for monolithic and contactless integration. Ironically, as exemplified for Si, nonlinearities act as an efficient immune system self-protecting the material from internal permanent modifications that ultrashort laser pulses could potentially produce. While nonlinear propagation of high-intensity ultrashort laser pulses has been extensively investigated in Si, other semiconductors remain uncharted. In this work, we demonstrate that filamentation universally dictates ultrashort laser pulse propagation in various semiconductors. The effective key nonlinear parameters obtained strongly differ from standard measurements with low-intensity pulses. Furthermore, the temporal scaling laws for these key parameters are extracted. Temporal-spectral shaping is finally proposed to optimize energy deposition inside semiconductors. The whole set of results lays the foundations for future improvements, up to the point where semiconductors can be selectively tailored internally by ultrafast laser writing, thus leading to countless applications for in-chip processing and functionalization, and opening new markets in various sectors including technology, photonics, and semiconductors.

physics.optics↗

arXiv2024

Vortex inverted pin beams: Mitigation of scintillations in strong atmospheric turbulence

We recently introduced a new class of optical beams with a Bessel-like transverse profile and increasing beam width during propagation, akin to an "inverted pin". Owing to their specially engineered distribution, these beams have shown remarkable performance in atmospheric turbulence. Specifically, inverted pin beams were found to have reduced scintillation index as compared to collimated or focused Gaussian beams as well as other types of pin beams especially in moderate to strong turbulence. In this work, we demonstrate that inverted pin beams carrying orbital angular momentum can further suppress intensity scintillations in moderate to strong irradiance fluctuation conditions. Our results can be useful in improving the performance and link availability of free-space optical communication systems.

physics.optics↗

arXiv2024

Cactus-like Metamaterial Structures for Electromagnetically Induced Transparency at THz frequencies

THz metamaterials present unique opportunities for next generation technologies and applications, as they can fill the ``THz gap'' originating from the weak response of natural materials in this regime, providing a variety of novel or advanced electromagnetic wave control components and systems. Here, we propose a novel metamaterial design, made of three-dimensional, metallic, "cactus like" meta-atoms, showing electromagnetically induced transparency (EIT) and enhanced refractive index sensing performance at low THz frequencies. Following a detailed theoretical analysis, the structure is realized experimentally using multi-photon polymerization and electroless silver plating. The experimental characterization results obtained through THz time domain spectroscopy validate the corresponding numerical data, verifying the high potential of the proposed structure in slow light and sensing applications.

physics.optics↗

arXiv2023

Inverted pin beams for robust long-range propagation through atmospheric turbulence

We introduce a new class of optical beams, which feature a spatial profile akin to an ``inverted pin''. In particular, we asymptotically find that close to the axis the transverse amplitude profile of such beams takes the form of a Bessel function with width that gradually increases during propagation. We examine numerically the behavior of such inverted pin beams in turbulent environments as measured via the scintillation index, and show that they outperform Gaussian beams (collimated and focused) as well as Bessel beams and regular pin beams, which are all optimized, especially in the moderate and strong fluctuation regimes.

physics.optics↗

arXiv2023

Transparent Multispectral Photonic Electrode for All-Weather Stable and Efficient Perovskite Solar Cells

Perovskite solar cells (PSCs) are the most promising technology for advancing current photovoltaic performance. However, the main challenge for their practical deployment and commercialization is their operational stability, affected by solar illumination and heating, as well as the electric field that is generated in the PV device by light exposure. Here, we propose a transparent multispectral photonic electrode placed on top of the glass substrate of solar cells, which simultaneously reduces the device solar heating and enhances its efficiency. Specifically, the proposed photonic electrode, composed of a low-resistivity metal and a conductive layer, simultaneously serves as a highly-efficient infrared filter and an ultra-thin transparent front contact, decreasing devices' solar heating and operating temperature. At the same time, it simultaneously serves as an anti-reflection coating, enhancing the efficiency. We additionally enhance the device cooling by coating the front glass substrate side with a visibly transparent film (PDMS), which maximizes substrate's thermal radiation. To determine the potential of our photonic approach and fully explore the cooling potential of PSCs, we first provide experimental characterizations of the absorption properties (in both visible and infrared wavelengths) of state-of-the-art PSCs among the most promising ones regarding the efficiency, stability, and cost. We then numerically show that applying our approach to promising PSCs can result in lower operating temperatures by over 9.0 oC and an absolute efficiency increase higher than 1.3%. These results are insensitive to varying environmental conditions. Our approach is simple and only requires modification of the substrate; it therefore points to a feasible photonic approach for advancing current photovoltaic performance with next-generation solar cell materials.

physics.optics↗

arXiv2023

Strong and Broadband Pure Optical Activity in 3D Printed THz Chiral Metamaterials

Optical activity (polarization rotation of light) is one of the most desired features of chiral media, as it is important for many polarization related applications. However, in the THz region, chiral media with strong optical activity are not available in nature. Here, we study theoretically, and experimentally a chiral metamaterial structure composed of pairs of vertical U-shape resonators of "twisted" arms, and we reveal that it demonstrates large pure optical activity (i.e. optical activity associated with negligible transmitted wave ellipticity) in the low THz regime. The experimental data show polarization rotation up to 25 (deg) for an unmatched bandwidth of 1 THz (relative bandwidth 80 %), from a 130 um-thickness structure, while theoretical optimizations show that the rotation can reach 45 (deg). The enhanced chiral response of the structure is analyzed through an equivalent RLC circuit model, which provides also simple optimization rules for the enhancement of its chiral response. The proposed chiral structures allow easy fabrication via direct laser writing and electroless metal plating, making them suitable candidates for polarization control applications.

physics.optics↗

arXiv2020

Ultraviolet radiation impact on the efficiency of commercial crystalline silicon-based photovoltaics: A theoretical thermal-electrical study in realistic device architectures

We investigate and evaluate the contribution of the ultraviolet radiation spectrum on the temperature and efficiency of commercial crystalline silicon-based photovoltaics (PVs) that operate outdoors. The investigation is performed by employing a comprehensive thermal-electrical modeling approach which takes into account all the major processes affected by the temperature variation in the photovoltaic devices. We show that effectively reflecting the ultraviolet radiation (i.e. up to a certain wavelength) results in a reduction of the overall operation temperature and enhancement of the PV cell's efficiency. In addition, blocking the high energy ultraviolet photons prolongs the life time of the PV and its performance on the long term.

physics.app-ph↗

arXiv2020

Passive radiative cooling impact on commercial crystalline silicon-based photovoltaics

The radiative cooling of objects during daytime under direct sunlight has recently been shown to be significantly enhanced by utilizing nanophotonic coatings. Multilayer thin film stacks, 2D photonic crystals, etc. as coating structures improved the thermal emission rate of a device in the infrared atmospheric transparency window reducing considerably devices' temperature. Due to the increased heating in photovoltaic (PV) devices, that has significant adverse consequences on both their efficiency and life-time, and inspired by the recent advances in daytime radiative cooling, we developed a coupled thermal-electrical modeling to examine the physical mechanisms on how a radiative cooler affects the overall efficiency of commercial photovoltaic modules. Employing this modeling, which takes into account all the major processes affected by the temperature variation in a PV device, we evaluated the relative impact of the main radiative cooling approaches proposed so far on the PV efficiency, and we established required conditions for optimized radiative cooling. Moreover, we identified the validity regimes of the currently existing PV-cooling models which treat the PV coolers as simple thermal emitters. Finally, we assessed some realistic photonic coolers from the literature, compatible with photovoltaics, to implement the radiative cooling requirements, and demonstrated their associated impact on the temperature reduction and PV efficiency. Providing the physical mechanisms and requirements for cooling radiatively solar cells, our study provides guidelines for utilizing suitable photonic structures as radiative coolers, enhancing the efficiency and the lifetime of PV devices.

physics.app-ph↗

arXiv2019

Ultrafast modulation in a THz graphene-based flat absorber through negative photoconductivity

We present the experimental and theoretical study of an ultrafast graphene based thin film absorption modulator for operation in the THz regime. The flat modulator is composed of a graphene sheet placed on a dielectric layer backed by a metallic back reflector. A near IR pulse induces the generation of hot carriers in the graphene sheet reducing effectively its conductivity. The system provides a platform with ultrafast modulation capability for flat optics and graphene based metasurfaces applications.

physics.app-ph↗

arXiv2019

Phase memory preserving harmonics from abruptly autofocusing beams

We demonstrate both theoretically and experimentally that the harmonics from abruptly auto-focusing ring-Airy beams present a surprising property, they preserve the phase distribution of the fundamental beam. Consequently, this "phase memory" inherits to the harmonics the abrupt autofocusing behavior, while, under certain conditions, their foci coincide in space with the one of the fundamental. Experiments agree well with our theoretical estimates and detailed numerical calculations. Our findings open the way for the use of such beams and their harmonics in strong field science.

physics.optics↗

arXiv2018

Generation of intense terahertz radiation by two-color filamentation of laser pulses with different wavelengths

We theoretically study the generation of terahertz (THz) radiation by two-color filamentation of ultrashort laser pulses with different wavelengths. We consider wavelengths in the range from 0.6 to 10.6 $μ$m, thus covering the whole range of existing and future powerful laser sources in the near, mid and far-infrared. We show how different parameters of two-color filaments and generated THz pulses depend on the laser wavelength. We demonstrate that there is an optimal laser wavelength for two-color filamentation that provides the highest THz conversion efficiency and results in generation of extremely intense single cycle THz fields.

physics.optics↗

arXiv2017

Extreme THz fields from two-color filamentation of mid-infrared laser pulses

Nonlinear THz photonics is probably the last frontier of nonlinear optics. The strength of both the electric and the magnetic fields of these ultrashort low frequency light bunches opens the way to exciting science and applications. Progress in the field though is slow because of the deficiency in suitable sources. Here we show that two-color filamentation of mid-infrared 3.9 um laser pulses allows one to generate single cycle THz pulses with multi-millijoule energies and extreme conversion efficiencies. Moreover, the focused THz peak electric and magnetic fields reach values of GV/cm and kT, respectively, exceeding by far any available quasi-DC field source today. These fields enable extreme field science, including into other, relativistic phenomena. Besides, we elucidate the origin of this high efficiency, which is made up of several factors, including a novel mechanism where the harmonics produced by the mid-infrared pulses strongly contribute to the field symmetry breaking and enhance the THz generation.

physics.optics↗

arXiv2017

Non-diffracting states in one-dimensional Floquet photonic topological insulators

One dimensional laser-written modulated photonic lattices are known to be particularly suitable for diffraction management purposes. Here, we address the connection between discrete non-diffracting states and topological properties in such devices through the experimental observation and identification of three classes of non-diffracting state. The first one corresponds to topologically protected edge states, recently predicted in Floquet topological insulators, while the second and third are both bulk modes. One of them testifies of a topological transition, although presenting topological features different from those of the edge states, whether the other one result from specific band structure engineering.

cond-mat.mes-hall↗

arXiv2016

Janus Waves

We show the existence of a family of waves that share a common interesting property affecting the way they propagate and focus. These waves are a superposition of twin waves, which are conjugate to each other under inversion of the propagation direction. In analogy to holography, these twin "real" and "virtual" waves are related respectively to the converging and the diverging part of the beam and can be clearly visualized in real space at two distinct foci under the action of a focusing lens. Analytic formulas for the intensity distribution after focusing are derived, while numerical and experimental demonstrations are given for some of the most interesting members of this family, the accelerating Airy and ring-Airy beams

physics.optics↗

arXiv2016

Wavefront shaping for opaque cylindrical lenses

Wavefront shaping has revolutionized the concepts of optical imaging and focusing. Contrary to what was believed, strong scattering in the optical paths can be exploited in favor of light focusing through turbid media and ultimately improve optical imaging and light manipulation capabilities. The use of light shapers and appropriately engineered scattering structures, i.e. opaque lenses enables the production of nano-scale confined foci and of extended fields of view. Exploiting this concept we fabricate configurable scattering structures by direct femtosecond laser writing. The properly shaped light trespassing the customized structure, a photonic lattice of parallel rods, forms a light-sheet at user defined positions. We demonstrate that our technique enables light-sheets with sub-micron resolution and extended depth of focus, a significant advantage when compared to the existing free space systems. Moreover, our approach permits to focus light of different wavelengths onto the same defined position without moving any optical element and correcting for chromatic aberrations. Furthermore, we demonstrate how an opaque cylindrical lens can be created using a biological structure, hence unveiling the potential advantages for optical microscopic and mesoscopic imaging.

physics.optics↗

arXiv2012

A Faithful Communication Hamiltonian in Photonic Lattices

Faithful communication is a necessary precondition for large scale all-optical networking and quantum information processing. Related theoretical investigations in different areas of physics have led to various proposals in which finite discrete lattices are used as channels for short-distance communication tasks. Here, in the framework of femtosecond-laser-written waveguide arrays, we present the first experimental realization of such a channel with judiciously engineered couplings.

quant-ph↗