Wide-band-gap semiconductors underpin high-electron-mobility transistors (HEMTs) and Schottky barrier diodes (SBDs) for high-power and high-frequency applications as well as emerging THz plasmonic devices. Precise control of surface and subsurface features is critical for device functionality and is conventionally achieved using photolithography-based processing. In this work we report on laser-based micro-processing technique proposed for device isolation and ohmic contacts formation in AlGaN/GaN HEMTs and SBDs. Femtosecond laser source is employed and directly compared with UV photolithography. Mask-less laser micro-machining enables localized material removal and controlled recessing below the barrier layers. Electrical characterization using current-voltage (I-V) measurements demonstrates the formation of effective device isolation and low-resistance ohmic contacts. The results indicate that laser-based micro-processing offers a viable alternative to conventional fabrication routes, reducing process complexity and material consumption while enabling advanced recessed contact architectures in GaN- as well as GaO-based devices.

Accurate measurements of light-matter interactions at subwavelength scales are critical for advancing nanophotonic and quantum optical technologies. In this paper, we present the far-field terahertz (THz) spectroscopy of a single planar meta-atom of subwavelength dimensions embedded within a square or circular aperture on a thin free-standing metal film. The meta-atom, composed of concentric disk and ring structures interconnected by narrow bridges, was fabricated by a mask-less direct laser ablation (DLA) technique to exhibit a pronounced transmission peak near a resonance frequency of 0.35 THz. We propose a novel spectral analysis framework that accounts for aperture-to-beam area mismatch suppressing non-resonant background contributions originating from edge diffraction and aperture discontinuities which are commonly encountered in subwavelength geometries. This technical analysis yields transmission spectra with improved accuracy providing good agreement with finite-difference time-domain (FDTD) simulations. A foundation for precise optical characterization of a single subwavelength size resonator is demonstrated paving the way for applications in quantum sensing, meta-surface design, and low-dimensional optoelectronic systems.

Resonance behavior of a single meta-atom composed of concentric disk and ring with inter-bridges embedded in a free-standing metal film was investigated experimentally and numerically. The meta-atom was integrated either in square or in circular aperture aiming to exhibit transparency at the resonant frequency of about 0.35 THz. Transmission of the samples were characterized in far-field considering diffraction effects caused by a sub-wavelength structure dimensions being much smaller than the beam diameter of incident radiation used. These findings have broader implications for understanding mutual coupling, anapole states, and other resonant phenomena in meta-surfaces, opening avenues for advanced applications in THz science and technology.

We present the experimental observation of electromagnetic mutual coupling in an array of ring-shaped resonators (meta-atoms) fabricated on a free-standing thin metal film using a maskless direct laser ablation technique. The transmission spectra of various resonator configurations were measured via terahertz time-domain spectroscopy and a vector network analysis. Numerical modeling of periodically arranged resonators, employing multipole decomposition, revealed a clear dependence of inter-element coupling on the number of meta-atoms in the array. Theoretical analysis of electric and magnetic dipoles and quadrupoles elucidates the nature of resonance peak splitting and broadening, resulting in a reduction in the quality factor as the number of meta-atoms increases. We anticipate that observed inter-element coupling behavior along with multipole mode analysis could advance the development of multi-pixel emitters, 2D plasmonic THz sources, sensors, electro-optical modulators, and resonators for subwavelength photonic and plasmonic applications.

Near-field imaging of the hybrid surface plasmon-phonon polaritons on the n-GaN semiconductor was performed using a scattering scanning near-field optical microscope at the selected frequencies of 920 cm-1 and 570 cm-1. The experimental measurements and numerical modeling data were in good agreement, revealing the large propagation distances on the n-GaN semiconductor and other insights which could be obtained by analyzing the dispersion characteristics of hybrid polaritons. In particular, the decay lengths of polaritons at the excitation frequency of 920 cm-1 were measured to be up to 25 and 30 µm in experiment and theory, respectively. In the case of excitation at the frequency of 570 cm-1, the surface plasmon-phonon polaritons' decay distances were 25 µm and 105 µm, respectively, noting the limitations of the near-field optical microscope setups used. Dispersion characteristics of the resonant frequency and the damping rate of hybrid polaritons were numerically modeled and compared with the analytical calculations, validating the need for further experiment improvements. The launch conditions for the near-field observation of extraordinary coherence of the surface plasmon-phonon polaritons were also discussed.

Nano- and micro-structuring of transparent glass for IR or visible spectrum applications is challenging due to low yield and slow processes, especially while maintaining high surface quality. The method proposed in this work involves the structuring and subsequent thermal oxidation of a thin metallic layer on a glass substrate. Annealing of the structured metal layer produces a transparent fully inorganic oxide surface with periodic patterns, avoiding the subtleties of glass structuring. A new method for the formation of sub-micrometer non-absorbing gratings with 750 nm periodicity is presented by direct laser interference patterning (DLIP) of thin tantalum layers on fused silica substrates followed by thermal oxidation by annealing. This method aims to improve the field of laser optics by providing a scalable and efficient method for the creation of precise metal-oxide photonic structures.

the diffractive optical elements based on planar high-resistivity silicon gratings are proposed to perform as a quarter waveplate as well as a half waveplate component in the selected band of THz frequencies. As proof of principle quarter waveplates we developed the Si-based quarter waveplate for 0.4 THz frequency demonstrating an operational bandwidth of up to 200 GHz. The finite difference time domain simulations were found in good agreement with THz time domain spectroscopy, frequency domain spectroscopy as well as vector network analyzer experiments measuring transmission amplitude and phase spectra in the range of 0.1-1.0 THz.

The high-contrast-grating waveplates utilizing high contrast between silicon and air refractive indexes were developed in order to perform as a quarter wave and a half wave plate in the selected THz frequency range. The waveplates possessed anti-reflective properties due to the specific inclination of the walls both in parallel and in perpendicular direction to grating axis, efficiently suppressing the reflection losses caused by air-dielectric interface for both transverse magnetic and transverse electric polarizations. Moreover, significant reduction of the transmittance gap was achieved between both polarizations while mitigating overall Fabry-Perot effect. Validation of the concepts was carried out by measuring transmission amplitude and phase spectra of the fabricated samples in a broadband of THz time-domain spectroscopy and vector-network-analysis systems considering also some real applications.

Over recent years, numerous designs of metamaterials with diverse functionalities have emerged from various research groups, including negative-refractive-index materials, electromechanical metamaterials, chiral materials, photonic crystals, electromagnetic cloaks, and optical metamaterials [1], [2]. Based on the type of electrical conductivity involved, metamaterials can be categorized into those utilizing metals and those relying on the dielectric optical properties of materials. Ultrathin planar metamaterials composed of periodic subwavelength structures are meticulously arranged to show remarkable electromagnetic characteristics, where the anapole states are of particular interest seeking to realize nonradiating electromagnetic sources [3], [4].

The development of new efficient, economical, and safe methods for strengthening the working surfaces of parts is an important task in the field of improving the reliability and resourcefulness of critical equipment and structures. In the present paper, laser boronizing is investigated as an alternative method for improving the wear resistance of maraging steel parts manufactured by laser powder bed fusion (LPBF). After LPBF, the specimens’ surface was covered with an amorphous boron paste (0.03–0.6 mm) and laser processed with a continuous-wave fiber laser in melting mode (λ—1070 nm; power—300 W; spot Ø—1.0 mm) at 500–1500 mm/min laser beam scanning speeds. Scanning electron microscopy, X-ray microanalysis, Knoop hardness, and dry sliding wear tests were applied to investigate the geometry, microstructure, hardness and its distribution, heat-affected zones, wear resistance, and wear mechanism of the alloyed layers. The boronized layers of thickness ~280–520 µm with microstructure from hypoeutectic to borides’ mixture were obtained, whose hardness varied from ~490 to ~2200 HK0.2. With laser boronizing, the wear resistance was improved up to ~7.5 times as compared with aged LPBF samples. In further method development, the problem of thermal cracking and softening of the heat-affected zone should be solved.

We analyzed theoretically and experimentally the inter-element coupling behavior between the periodic structure's so-called "meta-atoms" by varying the number of meta-atoms arranged periodically in the array. For this reason a planar metamaterial consisting of ring-shaped subwavelength periodic structures on a thin metal film was developed to exhibits a resonant transparency at the frequency of about 0.35 THz. The cross-talk between the meta-atoms due to electromagnetic multipole interferences lead to a significant change in resonance bandwidth and the quality (Q) factor. We anticipate that found coupling behavior between inter-element with subwavelength dimensions can potentially be used for a variety of applications such as filters, multi-pixel emitter and detector arrays, etc. for broad THz frequencies.

The hybrid multiphase Fresnel lenses (H-MPFLs) were developed on silicon wafer at the selected frequency of 585 GHz. For this reason, the design of a standard MPFL was modified thoroughly in various outer zone areas in order to reduce the complexity and manufacturing time of the diffractive optical elements by employing the direct laser ablation processes. The phase offset was found by precise control over the phase shift of incoming radiation in steps of π/12, revealing its optimal value to be of +π/4 independently on the hybridization order of the lens, the focusing gain of which was found to be up to 10 % higher than that achieved with a standard design MPFL. The numerical modeling data were confirmed by experiments demonstrating the effective THz beam focusing with H-MPFL samples to the diffraction-limited spot size. The proposed development procedure can be further modified by scaling the hybrid lens design to other frequencies with different zone numbers and/or by employing other materials suitable for THz photonics integration on-chip with the semiconductor devices.

Phase profile of the multi-phase zone plate (MPZP) was optimized for efficient lens operation at 0.585 THz frequency. Different type of the MPZP samples with a focal length of about 13 mm and a focal number of 1 were designed of high-resistivity silicon. The depth and size of initial eight step phase profile were optimized in the most outer subzone areas. Successful optimization of the phase profile allowed us to reduce manufacturing complexity and achieve higher focusing gain values in comparison to respective classical design MPZP.

In this research, the wetting property control of a stainless-steel surface, structured using parallel processing via an array of 64-femtosecond laser beams, is presented. The scanning of an 8 × 8-beam array over the sample was used to uniformly cover the large areas with LIPSS. The static water contact angle and the LIPSS period dependence on processing parameters were investigated. The wettability control of water droplets on laser-patterned stainless steel, ranging from contact angles of ~63°, similar to those of the plain surface, to the superhydrophobic surface with contact angles > 150°, was achieved. The relationship between the static water contact angle and the LIPSS parameters in the Fourier plane was investigated.

The laser surface microtexturing is increasingly used for the production of surfaces with desired wetting characteristics (i.e., superhydrophobicity). For this not only precise but also fast laser processing is needed. Microtextures not based entirely on laser induced material self-organization require relatively tight focusing of the laser beam which can not be achieved using fast beam scanning systems. Therefore, in this work, the possibility to use parallel laser processing by multi-beam array for control of the EN 1.4301 (AISI 304) stainless steel surface was investigated. High-energy picosecond laser pulses were used to pattern the polished steel surface using 8 x 8 beam array with two different patterning strategies: mesh and dot. The multi-beam patterning inevitably brings some irregularities due to variation of properties of the beams in the array. However, after the investigation of the influence of laser patterning parameters on the wetting properties of the steel surface, it was determined that multi-beam mesh patterning allows obtaining superhydrophobic surface properties through the right combination of texture depth, surface chemical properties and laser induced surface ripples. The dot patterning was even more promising and was found suitable for obtaining a broad spectrum of wetting states ranging from superhydrophobic to superhydrophilic, when appropriate patterning parameters were used, mostly by controlling selforganized surface structure shape by changing texture period (laser spot overlap).

We present investigations of the fin-shaped GaN/AlGaN field effect transistors with two lateral Schottky barrier gates exactly placed on the edges of the fin-shaped transistor channel. We call this kind of FinFET modification the EdgeFET. It allowed us to efficiently control the current flow in two-dimensional electron gas conduction channel. We present experimental data of sub-THz detection by EdgeFETs. Control of the side gates allows changing the width of two-dimensional electron gas and forming a wire, as we expect should be beneficial for observation of terahertz plasma wave resonances. This paves the way towards future terahertz optopair using high-quality factor plasma wave resonances, for which it is necessary to eliminate oblique modes. We report also on the high-voltage, noise, and radio frequency (RF) performances of aluminium gallium nitride/gallium nitride (AlGaN/GaN) on silicon carbide (SiC) devices without any GaN buffer. Such a GaN–SiC hybrid material was developed in order to improve thermal management and to reduce trapping effects should be beneficial for observation of resonant emission.

In this work transmission and reflection losses in silicon diffractive THz optical components fabricated by direct laser ablation are investigated. One of the possible sources of reflection/transmission losses in laser-fabricated optics is scattering due to the roughness of laser formed surface. Therefore, influence of laser processing parameters on the transmittance of laser processed silicon wafers was investigated in 0.1 –4.7 THz range. Transmittances of silicon samples ablated in ambient air and argon atmospheres were also compared. Using laser ablation technology MPFLs for 0.6 and 4.7 THz frequency radiation were fabricated and their performance was evaluated.

Sapphire and silicon carbide substrates are used for growth of the III-N group heterostructures to obtain the electronic devices for high power and high frequency applications. Laser micromachining of deep channels in the frontside of the transparent wafers followed by mechanical cleavage along the ablated trench is a useful method for partitioning of such substrates after the development of the electronics on a backside. However, in some cases damage to the component performance occurs. Therefore, the influence of various parameters of the laser processing, such as fluence in the spot size, substrate thickness, orientation, and the polarization of focused laser beam, to the formation of damage zones at both sides of the transparent substrate with thin coatings when ablating the trenches from one side was investigated. The vicinity effect of the ablated trenches on the performance of the electronics was also evaluated, confirming the laser micromachining suitability for the dicing of transparent wafers with high accuracy and flexibility.

In this work, we report about the fabrication of textured aluminium surfaces using Direct Laser Interference Patterning with picosecond (70 ps) and femtosecond (400 fs) laser pulses as well as their wetting properties.The structuring process was performed by varying the pulse numbers from 25 to 250, resulting in various depths ranging from 0.9 µm to 6.8 µm.The wetting analysis shows that the ps-patterned surfaces exhibit long-term superhydrophobic characteristics (at 21 °C and 16 % air humidity).Differently, for the fs-processed substrates, a hydrophobic character was firstly observed, which later (after 16 days) dropped to contact angles similar as the untreated material.

Two-dimensional (2D) plasmons in grating-gated AlGaN/GaN heterostructures were investigated by terahertz time domain spectroscopy observing the distinctive minima and inflection points in the transmission amplitude and phase spectra, respectively. The main features of 2D plasmons at temperature of 80 K were the fundamental mode position at 2.2 THz (grating period of 600 nm, filling 50%), the quality factor up to 8 and the modulation of transmission amplitude and phase up to 50% and 18 deg, respectively (period 1000 nm, filling 80%).