Broadband high-contrast-grating-type waveplates for the terahertz range.

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.

Laser-induced spatially-selective tailoring of high-index dielectric metasurfaces.

Optically resonant high-index dielectric metasurfaces featuring Mie-type electric and magnetic resonances are usually fabricated by means of planar technologies, which limit the degrees of freedom in tunability and scalability of the fabricated systems. Therefore, we propose a complimentary post-processing technique based on ultrashort (≤ 10 ps) laser pulses. The process involves thermal effects: crystallization and reshaping, while the heat is localized by a high-precision positioning of the focused laser beam. Moreover, for the first time, the resonant behavior of dielectric metasurface elements is exploited to engineer a specific absorption profile, which leads to a spatially-selective heating and a customized modification. Such technique has the potential to reduce the complexity in the fabrication of non-uniform metasurface-based optical elements. Two distinct cases, a spatial pixelation of a large-scale metasurface and a height modification of metasurface elements, are explicitly demonstrated.

Laser-processed diffractive lenses for the frequency range of 4.7 THz.

The development of diffractive lenses for the upper terahertz (THz) frequency range above 1 THz was successfully demonstrated by employing a direct laser ablation (DLA) technology. Two types of samples such as the Soret zone plate lens and the multi-level phase-correcting Fresnel lens were fabricated of a metal foil and crystalline silicon, respectively. The focusing performance along the optical axis of a 4.745 THz quantum cascade laser beam with respect to the positioning angle of the sample was studied by using a real-time microbolometric camera. A binary-phase profile sample demonstrated the values of the focusing gain and focused beam size up to 25 dB and 0.15 mm (2.4λ), respectively. The increase of the phase quantization level to eight led to higher (up to 29 dB) focusing gain values without a measurable increase of optical losses. All the samples were tolerant to misalignment as large as 10 deg of oblique incidence with a focusing power drop no larger than 10%. The results pave the way for new applications of industry-ready DLA technology in the entire THz range.

Nanoscale thermal diffusion during the laser interference ablation using femto-, pico-, and nanosecond pulses in silicon.

Laser interference ablation in silicon using femto-, pico-, and nanosecond pulses was investigated. The experimental and computational results provide information about nanoscale thermal diffusion during the ultra-short laser-matter interaction. The temperature modulation depth was introduced as a parameter for quality assessment of laser interference ablation. Based on the experiments and calculations, a new semi-empirical formula which combines the interference period with the laser pulse duration, the thermal modulation depth and the thermal diffusivity of the material was derived. This equation is in excellent agreement with the experimental and modelling results of laser interference ablation. This new formula can be used for selecting the appropriate pulse duration for periodical structuring with the required resolution and quality.

Fiber Laser Alloying of Additively Manufactured 18Ni-300 Maraging Steel Part Surface: Effect of Processing Parameters on the Formation of Alloyed Surface Layer and Its Properties.

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.

Large-Area Fabrication of LIPSS for Wetting Control Using Multi-Parallel Femtosecond Laser Processing.

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.

Laser Processing of Transparent Wafers with a AlGaN/GaN Heterostructures and High-Electron Mobility Devices on a Backside.

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.

AlGaN/GaN on SiC Devices without a GaN Buffer Layer: Electrical and Noise Characteristics.

We report 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. Fabricated Schottky barrier diodes (SBDs) demonstrated an ideality factor n at approximately 1.7 and breakdown voltages (fields) up to 780 V (approximately 0.8 MV/cm). Hall measurements revealed a thermally stable electron density at N2DEG = 1 × 1013 cm-2 of two-dimensional electron gas in the range of 77-300 K, with mobilities µ = 1.7 × 103 cm2/V∙s and µ = 1.0 × 104 cm2/V∙s at 300 K and 77 K, respectively. The maximum drain current and the transconductance were demonstrated to be as high as 0.5 A/mm and 150 mS/mm, respectively, for the transistors with gate length LG = 5 µm. Low-frequency noise measurements demonstrated an effective trap density below 1019 cm-3 eV-1. RF analysis revealed fT and fmax values up to 1.3 GHz and 6.7 GHz, respectively, demonstrating figures of merit fT × LG up to 6.7 GHz × µm. These data further confirm the high potential of a GaN-SiC hybrid material for the development of thin high electron mobility transistors (HEMTs) and SBDs with improved thermal stability for high-frequency and high-power applications.

Direct and High-Throughput Fabrication of Mie-Resonant Metasurfaces via Single-Pulse Laser Interference.

High-index dielectric metasurfaces featuring Mie-type electric and magnetic resonances have been of great interest in a variety of applications such as imaging, sensing, photovoltaics, and others, which led to the necessity of an efficient large-scale fabrication technique. To address this, here we demonstrate the use of single-pulse laser interference for direct patterning of an amorphous silicon film into an array of Mie resonators a few hundred nanometers in diameter. The proposed technique is based on laser-interference-induced dewetting. A precise control of the laser pulse energy enables the fabrication of ordered dielectric metasurfaces in areas spanning tens of micrometers and consisting of thousands of hemispherical nanoparticles with a single laser shot. The fabricated nanoparticles exhibit a wavelength-dependent optical response with a strong electric dipole signature. Variation of the predeposited silicon film thickness allows tailoring of the resonances in the targeted visible and infrared spectral ranges. Such direct and high-throughput fabrication is a step toward a simple realization of spatially invariant metasurface-based devices.