Stelios Tzortzakis

• PhD, Ecole Polytechnique, France
• Professor at Foundation for Research and Technology Hellas

Optical information encoded in holograms is transferred by means of ultrashort laser filaments propagating in highly nonlinear and turbulent media. After propagation, the initial optical information is completely scrambled and cannot be retrieved by any experimental or physical modeling system. Yet, we demonstrate that neural networks trained on ex...

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 (PBs) were found to have a redu...

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 scin...

Ultrafast laser three-dimensional (3D) writing has made breakthrough in manufacturing processes with for instance the advent of super-resolution 3D printing and perennial optical memory technologies. However, similar precision applications were inaccessible for silicon and other semiconductor technologies due to a lack of in-chip controllability of...

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...

We propose an integrated methodology for the design and fabrication of 3D micromodels that are suitable for the pore-scale study of transport processes in macroporous materials. The micromodels, that bear the pore-scale characteristics of sandstone, such as porosity, mean pore size, etc, are designed following a stochastic reconstruction algorithm...

In this work we study in-depth the antireflection and filtering properties of ultrathin-metal-film-based transparent electrodes (MTEs) integrated in thin-film solar cells. Based on numerical optimization of the MTE design and the experimental characterization of thin-film perovskite solar cell (PSC) samples, we show that reflection in the visible s...

We introduce a new class of beams the “inverted pin beams”. We show that in atmospheric turbulence inverted pin beams outperform (have reduced scintillations) other classes of beams in moderate to strong fluctuation regimes.

We report on strong-field THz sources from optical rectification in organic crystals or two-color plasma filaments, driven by intense mid-IR pulses. Non-linear perturbation of semiconductor materials is demonstrated by the applied THz transients.

The increasing demand for functional nanodevices in sustainable energy applications necessitates the development of innovative approaches. In this study, we present the fabrication and characterization of three-dimensional (3D) structures coated with titanium dioxide (TiO2) nanorods (NRs). These novel devices are created through the integration of...

We introduce a new, to the best of our knowledge, 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 a width that gradually increases during propagation. We examine num...

Heating adversely affects solar cells performance and reliability. 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...

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, a chiral metamaterial (CMM) structure composed of pairs of vertical U‐shape res...

We demonstrate for the first time ultrafast laser welding in the silicon–metal and silicon–silicon configurations, with focused infrared picosecond pulses. This achievement relies on accurate characterizations of filamentation in silicon with nonlinear propagation imaging. In the silicon–metal configuration, precompensating for the nonlinear focal...

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 comp...

We report on record optical-to-THz conversion efficiencies in two-color plasma filaments and organic crystals, driven by mid-IR pulses. Generated strong THz fields permit nonlinear manipulation of optical properties in bulk semiconductors and colloidal quantum dots.

We report the spectral shaping of supercontinuum generation in liquids by employing properly engineered Bessel beams coupled with artificial neural networks. We demonstrate that given a custom spectrum, neural networks are capable of outputting the experimental parameters needed to generate it experimentally.

Three-dimensional (3D) geometries ensure large surface area interaction of photonic devices with the surrounding medium. Here, we demonstrate that metamaterials composed of 3D metallic Split Cube Resonator (SCR) elements assembled in various arrangements enable resonantly enhanced refractive index sensing. The proper arrangement of the SCR elements...

The development of a superhydrophobic and, even, water-repellent metal alloy surface is reported utilizing a simple, fast, and economical way that requires minimum demands on the necessary equipment and/or methods used. The procedure involves an initial irradiation of the metallic specimen using a femtosecond laser, which results in a randomly roug...

Solar cell technology requires materials that are efficient, lightweight, and stable. Organic–inorganic lead halide perovskite- and polymer-based bulk heterojunction solar cells have emerged as highly promising, ultralightweight, flexible, and highly efficient power sources. However, they suffer from limited stability, which is significantly affect...

We experimentally demonstrate that 2D Airy wave packets can produce intense curved two-color filaments that emit terahertz (THz) radiation with unique characteristics. Due to the curvature of the plasma channel, THz waves, emitted from different longitudinal regions of the plasma, propagate in different directions resulting in non-concentric THz co...

We demonstrate that thermal effects limiting femtosecond laser filamentation in gases can be addressed using a simple gas replenishment method. This enables high average power filamentation applications like THz sources.

We demonstrate controllable spectral shaping of supercontinuum radiation through intense femtosecond laser filamentation in ethanol. Neural networks are shown to predict robust results even under the presence of high-order nonlinearities and noise.

Back cover of Laser & Photonics Reviews Vol.15, No. 11, associated with the article entitled "In-Volume Laser Direct Writing of Silicon—Challenges and Opportunities" (https://doi.org/10.1002/lpor.202100140)

The development of in-volume laser direct writing of silicon as it exists for transparent materials for a quarter of a century remains a major challenge to tackle. In article number 2100140, Maxime Chambonneau and co-workers give a comprehensive review on the physical limitations to the excitation of bulk silicon with ultrashort laser pulses, the d...

In-volume ultrafast laser direct writing of silicon is generally limited by strong nonlinear propagation effects preventing the production of modifications. By using advantageous spectral, temporal, and spatial conditions, we demonstrate that modifications can be repeatably produced inside silicon. Our approach relies on irradiation at ≈2 μm wavele...

Laser direct writing is a widely employed technique for 3D, contactless, and fast functionalization of dielectrics. Its success mainly originates from the utilization of ultrashort laser pulses, offering an incomparable degree of control on the produced material modifications. However, challenges remain for devising an equivalent technique in cryst...

We evaluate the ultraviolet radiation impact on the temperature and efficiency of realistic photovoltaic modules. We perform this investigation for crystalline silicon-based photovoltaics that operate outdoors, by employing a thermal-electrical modeling approach, which takes into account all the major intrinsic processes affected by the temperature...

Ultra-fast modulation of 3D THz LC resonators is presented with a 280 GHz frequency shift obtained in 200 fs. The overall modulation cycle of the device takes 2 ps, yet convoluted by the probing technique.

Outdoor devices comprising materials with mid-IR emissions at the atmospheric window (8–13 μm) achieve passive heat dissipation to outer space (~ − 270 °C), besides the atmosphere, being suitable for cooling applications. Recent studies have shown that the micro-scale photonic patterning of such materials further enhances their spectral emissivity....

In-volume ultrafast laser direct writing of silicon is generally limited by strong nonlinear propagation effects preventing the initiation of modifications. By employing a triple-optimization procedure in the spectral, temporal and spatial domains, we demonstrate that modifications can be repeatably produced inside silicon. Our approach relies on i...

We present the experimental demonstration of a subpicosecond all-optical THz switch based on three-dimensional (3D) terahertz meta-atoms. Combining a special design of 3D meta-devices and the ultrafast dynamics of low temperature grown gallium arsenide, we can modulate the reflectance of the THz microcavities within 2.2 ps. The device enables a 280...

Ultrafast laser welding is a fast, clean, and contactless technique for joining a broad range of materials. Nevertheless, this technique cannot be applied for bonding semiconductors and metals. By investigating the nonlinear propagation of picosecond laser pulses in silicon, it is elucidated how the evolution of filaments during propagation prevent...

Inside front cover of Laser & Photonics Reviews, Vol.15, No. 2 associated with the article entitled "Taming Ultrafast Laser Filaments for Optimized Semiconductor–Metal Welding" (https://doi.org/10.1002/lpor.202000433)

In article number 2000433, Maxime Chambonneau, Qingfeng Li, Stefan Nolte, and co‐workers demonstrate semiconductor–metal ultrafast laser welding—an additive manufacturing technique limited to other material families so far. This achievement is made possible by the determination and the precompensation of the nonlinear focal shift in the filamentati...

We report on THz generation from curved filaments produced by 2D Airy wave packets. Due to the curvature of the plasma channel, non-concentric THz beams with different polarizations are generated.

We report on the impact of laser pulse repetition rate on two-color filamentation based terahertz sources. A 50% decrease on the terahertz energy is observed when the repetition rate increases from 0.6 to 6 kHz.

We demonstrate an ultrafast self-induced terahertz absorption modulator operating at 2.3 THz. A modulation of 50 dB is observed in the absorption when the THz field strength increases from 145 to 654 kV/cm.

We present a radiative cooling approach for photovoltaic cells’ temperature and efficiency evaluation. We derive the maximum temperature-drop requirements and apply the approach in a nano-micro-grating remarkably enhancing both thermal radiation emission and solar absorption.

Ultrafast laser welding is a fast, clean, and contactless technique for joining a broad range of materials. Nevertheless, this technique cannot be applied for bonding semiconductors and metals. By investigating the nonlinear propagation of picosecond laser pulses in silicon, it is elucidated how the evolution of filaments during propagation prevent...

This document is the Supporting Information associated with the article entitled "Taming Ultrafast Laser Filaments for Optimized Semiconductor–Metal Welding" (10.1002/lpor.202000433).

Graphical Abstract for the article entitled "Taming Ultrafast Laser Filaments for Optimized Semiconductor–Metal Welding" (https://doi.org/10.1002/lpor.202000433)

We experimentally demonstrate that the terahertz (THz) emission from two-color laser filaments in gases is strongly affected by the pulse repetition rate of the driving laser. We show that at repetition rates above 100 Hz, propagation of every next laser pulse in the pulse train is altered by gas density depressions produced by the preceding laser...

Strong terahertz (THz) electric and magnetic transients open up new horizons in science and applications. We review the most promising way of achieving sub-cycle THz pulses with extreme field strengths. During the nonlinear propagation of two-color mid-infrared and far-infrared ultrashort laser pulses, long, and thick plasma strings are produced, w...

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 considera...

Ultraviolet (UV) radiation has been identified as one of the most critical factors for the degradation of photovoltaics (PVs). Besides that, the UV spectral regime (∼0.28-0.4 µm) is less efficient for silicon-based PVs owing to the excess of the energy of the incident UV photons relative to the semiconductor’s bandgap; thus, a large part of the UV...

Ultraviolet (UV) radiation has been identified as one of the most critical factors for the degradation of photovoltaics (PVs). Besides that, the UV spectral regime (~0.28-0.4 μm) is less efficient for silicon-based PVs owing to the excess of the energy of the incident UV photons relative to the semiconductor’s bandgap; thus, a large part of the UV p...

Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here, we experimentally demonstrate that two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm all...

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 considera...

A three-dimensional (3D) holographic focal volume engineering method is proposed and employed for advanced multiphoton polymerization. A large number of foci are closely positioned in space according to a designed geometry, avoiding undesired interference effects by phase engineering. Through all-optical micro-displacements in space, the discrete f...

A review on the recent development of intense laser‐driven terahertz (THz) sources is provided here. The technologies discussed include various types of sources based on optical rectification (OR), spintronic emitters, and laser‐filament‐induced plasma. The emphasis is on OR using pump pulses with tilted intensity front. Illustrative examples of ne...

Based on the development of a μJ-class Thulium-doped fiber laser operating in the picosecond regime at 1970-nm wavelength, we introduce a solution for three-dimensional (3D) laser writing technologies inside silicon (Si). We reveal that the nonlinear effects preventing from bulk modification in Si with femtosecond pulses persist in the picosecond r...

We derive nonparaxial input conditions for simulations of tightly focused electromagnetic fields by means of unidirectional nonparaxial vectorial propagation equations. The derivation is based on the geometrical optics transfer of the incident electric field from significantly curved reflecting surfaces such as parabolic and conical mirrors to the...

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 ef...

The metrology of laser-induced damage usually finds a single transition from 0% to 100% damage probability when progressively increasing the laser energy in experiments. We observe that picosecond pulses at 2-µm wavelength focused inside silicon provide a response that strongly deviates from this. Supported by nonlinear propagation simulations and...

We report on the action of exposure time and peak Intensity on the growth of long-scale focal volumes in multiphoton polymerization. Using modified engineered Bessel beams, we explore the effects that rise during the voxel growth, while we present a counterintuitive action of the expected expansion of the polymerized volumes that is revealed for a...

We theoretically study the evolution of ring-Airy beams during harmonic generation with the focus on the regime of pump energy depletion. We demonstrate that in this regime, ring-Airy beams still preserve their abrupt autofocusing properties, while transforming to a multiple ring-Airy structure. A similar transformation is observed on the beam of t...

At first glance, the amount of water molecules naturally contained in humid air is negligibly small to affect filamentation of ultrashort laser pulses. However, here we show, both experimentally and numerically, that for ultraviolet laser pulses with 248 nm wavelength this is not true. We demonstrate that with increase of air humidity the plasma ch...

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 coi...

We present the experimental demonstration and interpretation of an ultrafast optically tunable, graphene-based thin film absorption modulator for operation in the THz regime. The graphene-based component consists of a uniform CVD-grown graphene sheet stacked on a SU-8 dielectric substrate which is grounded by a metallic ground plate. The structure...

An ultrafast, optically tunable graphene-based thin film absorption modulator for operation in the THz regime is studied theoretically and demonstrated experimentally. The modulator is composed of a graphene sheet on a dielectric layer (light gray) backed by a metallic back-reflector (yellow). An ultrafast near-IR pulse induces the generation of ho...

We report on THz generation driven by 3.9 µm pulses, via either optical rectification in organic crystals or in two-color plasma filaments. Outstanding THz conversion efficiency of more than 2% and bandwidth exceeding 15 THz are achieved.

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 μm, thus covering the whole range of existing and future powerful laser sources in the near, mid and far-infrared. We show how different parameters...

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 $\mu$m, thus covering the whole range of existing and future powerful laser sources in the near, mid and far-infrared. We show how different parame...

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 filam...

We demonstrate that paraxial ring-Airy beams can approach the wavelength limit, while observing a counterintuitive, strong enhancement of their focal peak intensity. Using numerical simulations, we show that this behavior is a result of the coherent constructive action of paraxial and nonparaxial energy flow. A simple theoretical model enables us t...

We demonstrate, strong THz emission from two-color mid-infrared (3.9 μm) femtosecond laser filaments. The conversion efficiency approaches the percent level, at least one order of magnitude higher than the previously reported for plasma-based THz sources.

We show numerically that two-color filamentation of mid-infrared 3.9 µm laser pulses allows one to generate THz pulses of multi-millijoule energies and extreme conversion efficiencies, with electric and magnetic fields of GV/cm and kT level, exceeding by far any available quasi-DC field source today.

We report on precise experimental measurements of the electron plasma density in plasma strings generated in air by 3.9μm laser filaments using holography. We discuss the origin and impact of the high electron densities observed.