A combined laser-AFM-Raman setup is applied to study nanoscale surface modifications in DLC (a-C:H) films caused by multipulse laser irradiation at low (sub-threshold) fuences. The beam of a pulsed Nd:YAG laser (wavelength lambda = 532 nm, pulse duration tau = 7 ns, pulse repetition rate f = 1-2.5 kHz) is introduced into the optical path of an atomic force microscope (AFM), matched with that of a Raman confocal microscope. Three laser-induced processes that determine nanoscale surface modifications are found to occur in laser spots: (i) annealing - resulting in subnanometer surface deepening at the spot edges, (ii) graphitization - leading to surface swelling of 5-10 nm height, and (iii) nanoablation (oxidation) - resulting in formation of microcraters of few nm to hundred nm depth in the spot center. Raman spectra confirm the surface graphitization in the spots during formation of nm-height hillocks and microcraters, evidencing a decrease of the graphitized layer thickness at lower fluences. The nanoablation rates of 4 x 10(-5)-4 x 10(-3) nm/pulse are determined for multipulse irradiation of a-C:H films at the fluences of E = 0.045-0.1 J/cm(2). The extremely low rates of laser ablation of the DLC surface are characteristic of pulsed laser-assisted oxidation with the activation energy of similar to 6.3 kcal/mol estimated from the Arrhenius plot of the ablation rate-on-fluence dependence.

A possibility of laser printing of graphene nanoribbon pixels while preserving the integrity of the structure and shape on the silicon substrate in accordance with the irradiated laser spot is demonstrated. To provide the transfer, a target consisting of a transparent sapphire plate and an absorbing thin titanium film (500 nm thick) coated with a film consisting of graphene nanoribbons is irradiated with a KrF excimer laser (λ=248 nm, τ=20 ns). Optimal conditions for laser irradiation are determined and a technique is developed for transferring a carbon nanomaterial from a growth nickel surface to a titanium film aimed at creating stronger bonds between the nanoribbons. Raman spectroscopy confirms the preservation of structural features of the synthesized atomically precise 7-atoms-wide graphene nanoribbons with an armchair edge during laser transfer.

Conductive graphitized grooves on the dielectric surface of diamond have been created by KrF excimer laser radiation. The advantages of such a circuit board in high-field applications is rather limited because the crystal surface has a relatively low electrical breakdown threshold. To increase the electrical strength, a method of encapsulating surface conductive graphitized structures by chemical vapor deposition of an epitaxial diamond layer has been proposed and realized. The quality of the growth diamond is proved by Raman spectroscopy. A comparative study of the electrical resistivity of graphitized wires and the breakdown fields between them before and after diamond growth was carried out. The proposed technique is crucial for diamond-based high-field electro-optical devices, such as THz photoconductive emitters.

The paper overviews experimental findings of the direct laser processing and surface microstructuring (texturing) of various diamond-like carbon films (a-C:H, ta-C, DLN, metal-doped DLN), aimed at improvements of their tribological and nanotribological properties. The nanosecond UV and femtosecond IR/visible pulsed lasers were applied in microprocessing of the films, focusing on high precision surface structuring with fs-laser pulses. The studies were concentrated on the following tasks: (i) surface graphitization in laser microstructuring of the films under different irradiation conditions, (ii) lubricated friction performance of DLN films micropatterned with UV ns and visible fs pulsed lasers, and (iii) nanoscale friction of laser-structured DLN and metal-doped DLN films examined with contact-mode atomic force microscopy. The important findings of our studies are related to fabrication of highly-precise microgroove/microcrater patterns on DLN films and improvements of frictional properties of the laser-structured films at the macro, micro and nanoscale. The surface microstructures improved the film properties under oil-lubricated sliding in dependence on their geometrical parameters (size, depth, period) and ambient temperature. The nanoscale friction behavior of laser-structured films was shown to be controlled by the surface graphitization, nanoscale roughness, capillary forces and wear of AFM tips during friction force imaging.

It has been shown recently that a photoconductive antenna (PCA) based on a nitrogen-doped diamond can be effectively excited by the second harmonic of a Ti:sapphire laser (λ = 400 nm). The THz emission performance of the PCA can be significantly increased if a much stronger electric field is created between the close-located electrodes. To produce a homogeneous electric field over the entire excited diamond volume, the laser fabrication of deep-buried graphite electrodes inside the diamond crystal was proposed. Several electrodes consisting of the arrays of buried pillars connected by the surface graphite stripes were produced inside an HPHT diamond crystal using femtosecond and nanosecond laser pulses. Combining different pairs of the electrodes, a series of PCAs with various electrode interspaces was formed. The THz emission of the PCAs equipped with the buried electrodes was measured at different values of excitation fluence and bias voltage (DC and pulsed) and compared with the emission of the same diamond crystal when the bias voltage was applied to the surface electrodes on the opposite faces. All examined PCAs have demonstrated the square-law dependencies of the THz fluence on the field strength, while the saturation fluence fluctuated in the range of 1200–1600 µJ/cm2. The THz emission performance was found to be approximately the same for the PCAs with the surface electrodes and with the buried electrodes spaced at a distance of 1.4–3.5 mm. However, it noticeably decreased when the distance between the buried electrodes was reduced to 0.5 mm.

For the first time, the ability of semiconducting diamond to convert near-IR laser radiation into terahertz radiation has been demonstrated. A set of photoconductive antennas based on single-crystal diamonds doped with boron (~1 ppm) was assembled and tested under conditions of pumping with ultrashort (τ opt ≈ 150 fs) radiation pulses with a wavelength of 800 nm and a pulsed voltage (τ E ≈ 10 ns, E bias ≈ 10 kV/cm). The characteristics of the boron-doped emitters were compared with recently implemented nitrogen-doped diamond antennas pumped by 400-nm-wavelength radiation pulses, since substituting nitrogen requires a much higher quantum energy for single-photon excitation of carriers. The results obtained are another step towards the use of diamond as a material for high-performance photoconductive antennas.

Chemical vapor deposition synthesis of graphene on polycrystalline copper substrates from methane is a promising technique for industrial production and application. However, the quality of grown graphene can be improved by using single-crystal copper (111). In this paper, we propose to synthesize graphene on epitaxial single-crystal Cu film deposited and recrystallized on a basal-plane sapphire substrate. The effect of film thickness, temperature, and time of annealing on the size of copper grains and their orientation is demonstrated. Under optimized conditions, the copper grains with the (111) orientation and a record size of several millimeters are obtained, and the single-crystal graphene is grown over their entire area. The high quality of synthesized graphene has been confirmed by Raman spectroscopy, scanning electron microscopy, and the sheet resistance measurements by the four point probe method.

The novel design of a terahertz large aperture photoconductive antenna (LAPCA) is reported. It features a longitudinal orientation of the bias electric field within the photoconductive substrate, and has the advantage of a small interelectrode gap, resulting in a higher field for the same applied voltage. The proposed LAPCA configuration has been tested with a nitrogen-doped (∼10 ppm) synthetic monocrystalline diamond, which is a promising material for high-intensity and high-power terahertz sources. Two antennas with different high-voltage electrode realizations were assembled, pumped by a 400 nm femtosecond laser, and tested for THz emitter function. The experimental data are found to be in good correlation with the numerical simulation results. The performance of antennas with the conventional transverse E-field configuration and the novel longitudinal configuration is compared and discussed.

The transfer of synthesized graphene nanoribbons with the initial characteristics obtained on the growth surface is an urgent and complex problem. Laser methods proved themselves well as a delicate and selective tool for the transfer of carbon nanomaterials. The simplicity of implementation of laser methods reduces the number of intermediate manipulations with the transferred material, increasing the safety of its structure. Here, we studied and implemented laser-induced blister forward transfer of high-quality graphene nanoribbons from a metal surface to a SiO2/Si substrate. We also studied the effect of the growth parameters and the transfer method on the structure of transferred carbon sites. The retention of the initial crystal structure of the transferred atomically precise graphene nanoribbons was confirmed by Raman spectroscopy.

The characteristics of high-power vortex Bessel beams in the terahertz range (λ=141 μm) obtained with the use of diffractive axicons (DAs) illuminated by a Gaussian beam of the Novosibirsk free-electron laser were studied. Two of the three possible types of DA recently described in our previous paper, namely, binary spiral silicon axicons (BAs), forming beams with a topological charge l equal to 0–4 and 9, and a diamond “holographic” axicon (HA), forming a beam with l=9, were used in the experiments. These axicons formed beams whose cross sections in the region of inner Bessel rings were close to those of ideal Bessel beams, but their intensities varied in azimuth with a frequency of l and 2l for the BAs and HA, respectively. However, in the case of the BAs, the beams had a pronounced helical structure at the periphery, whereas for the HA, the beam was axisymmetric. By focusing these beams with a lens, we studied the structure of the so-called “perfect” beams (PBs). While an ideal Bessel beam exhibits a PB as a thin ring, in the case of the BAs, we observed a broadened ring structure consisting of 2l short spirals, and for the HA, we observed a narrow ring with 2l maxima in azimuth. A comparison of the numerical calculations and experiments showed that the observed azimuthal intensity variations can be attributed to inaccuracies in the preparation of the axicon relief and/or discrepancies between the calculated and actual wavelengths, within a few percent. The results of this work enable the establishment of quality requirements for axicon manufacture and the appropriate selection of the axicon type in accordance with the requirements for the beam.

The generation of terahertz radiation in a photoconductive emitter based on nitrogen-doped single-crystal diamond was realized for the first time. Under 400 nm femtosecond laser pumping, the performance of diamond antennas with different dopant levels was investigated and compared with a reference ZnSe antenna. Terahertz waveforms and corresponding spectra were measured. A low saturation level for high-nitrogen-containing diamond substrate was revealed. The results indicate the prospects of doped diamond as a material for high-efficiency large-aperture photoconductive antennas.

A new approach to the fabrication of graphene field emitters on a variety of substrates at room temperature and in an ambient environment is demonstrated. The required shape and orientation of the graphene flakes along the field are created by the blister-based laser-induced forward transfer of CVD high-quality single-layer graphene. The proposed technique allows the formation of emitting crumpled graphene patterns without losing the quality of the initially synthesized graphene, as shown by Raman spectroscopy. The electron field emission properties of crumpled graphene imprints 1 × 1 mm2 in size were studied. The transferred graphene flakes demonstrated good adhesion and emission characteristics.

In this issue, the Editorial Board of Laser Physics is happy to offer to our readers a special series of articles dedicated to the memory of Professor Pavel P Pashinin and his successful endeavor in the field of laser physics. Pavel P Pashinin (1935–2020), a Corresponding Member of the Russian Academy of Sciences, was a distinguished member of the founding team of laser physicists in Russia, led by a winner of the 1964 Nobel Prize in physics, Alexander M Prokhorov. For many years, Prof. Pashinin headed the ‘Interaction of Coherent Radiation with Matter’ Department at the Institute of General Physics in Moscow. The pioneering works of Prof. Pashinin on lasers and their applications and the research results of his department are widely known and recognized by the international scientific community. In 2002, after the death of AMProkhorov, Prof. Pashinin became the Editor-in-Chief of the International Journal ‘Laser Physics’ (LP). Since 2004 Prof. Pashinin has also undertaken chief editorship of the newly created journal ‘Laser Physics Letters’ (LPL). In addition to his scientific and editorial work, Prof. Pashinin had actively participated in organizing and running the Annual International Laser Physics Workshop (LPHYS). The workshop had been closely associated with both journals, LP and LPL. This world-renown conference started in 1992 and, since then, has taken place every consecutive year, except for 2020, when it was postponed due to the world covid pandemic. From 2003 to 2019, all LPHYS Workshops were chaired by Prof. Pashinin. Under his leadership, both the LP and LPL journals and the LPHYS Workshop became widely recognized and accepted worldwide. The Workshops were held annually in many countries in Europe, Asia, and South and North America. The following is the list of issue-related articles.

A new approach to the fabrication of efficient heat sinks for GaN-based transistors is demonstrated. A key feature of this work is the growth of polycrystalline diamond coating on the functional silicon layer of SOI wafers followed by etching of a thick silicon substrate and a thin thermal oxide. As a result, composite epi-ready substrates consisting of a thin (410 nm) monocrystalline silicon functional layer on top of the 150 mu m-thick polycrystalline diamond heat sink were fabricated. GaN heterostructures were grown on top of the silicon layer, which resulted in an effective thermal contact between CVD diamond and GaN structure. The packaged ungated transistors were made to analyze the efficiency of the developed heat sink. Improved heat removal structures showed the decrease in surface temperature by more than 50 degrees C at base temperature of T-b =85 degrees C and dissipation power of P-d(i)ss=6.9 W/mm compared to conventional GaN-on-SiC technology and by more than 20 degrees C at T-b=25 degrees C, P-diss=6.9 W/mm compared to up-to-date GaN-on-Diamond equivalent transistors reported by other groups. New substrate fabrication technology positively impacts GaN-based device output characteristics and reliability, which is important in improving communication systems, radars, and secondary power supply systems. (C) 2021 The Authors. Published by Elsevier Ltd.

The laser polishing of rough (roughness of 5 μ m) diamond plate using femtosecond and nanosecond pulses was realized for the first time. The effect of the angle of incidence, the number of passes, scanning speed, and laser fluence on the surface roughness was investigated and the optimal conditions were found. The initial roughness of the diamond plate was reduced by 5 times from 5 to 1 μ m for both femtosecond and nanosecond laser sources. The results indicate the prospects of laser polishing of rough polycrystalline diamond samples.

We are reporting on laser microstructuring of thin nanocrystalline diamond membranes, for the first time. To demonstrate the possibility of microstructuring, we fabricated a diamond membrane, of 9 μm thickness, with a two-dimensional periodic array of closely located chiral elements. We describe the fabrication technique and present the results of the measurements of the infrared transmission spectra of the fabricated membrane. We theoretically studied the reflection, transmission, and absorption spectra of a model structure that approximates the fabricated chiral metamembrane. We show that the metamembrane supports quasiguided modes, which appear in the optical spectra due to grating-assisted diffraction of the guided modes to the far field. Due to the C4 symmetry, the structure demonstrates circular dichroism in transmission. The developed technique can find applications in infrared photonics since diamond is transparent at wavelengths >6 μm and has record values of hardness. It paves the way for creation of new-generation infrared filters for circular polarization.

The efficiency of the generation of terahertz radiation from nitrogen-doped (∼0.1–100 ppm) diamonds was investigated. The synthetic polycrystalline and monocrystalline diamond substrates were pumped by a 400 nm femtosecond laser and tested for the photoconductive emitter operation. The dependency of the emitted THz power on the intensity of the optical excitation was measured. The nitrogen concentrations of the diamonds involved were measured from the optical absorbance, which was found to crucially depend on the synthesis technique. The observed correlation between the doping level and the level of the performance of diamond-based antennas demonstrates the prospects of doped diamond as a material for highly efficient large-aperture photoconductive antennas.

Comparative studies of characteristics of Bessel and "perfect" vortex beams with a topological charge 9, created using a binary silicon axicon and a "holographic" diamond axicon with continu-ous profile at a wavelength of 141 μm, are carried out. Beams with linear and radial polarization are investigated. An example of the use of a perfect radially polarized beam for the excitation of vortex plasmon-polaritons on a cylindrical conductor is given.

Chemical vapor deposition synthesis of graphene on copper foil from methane is the most promising technology for industrial production. However, an important problem of the formation of the additional graphene layers during synthesis arises due to the strong roughness of the initial copper foil. In this paper, various approaches are demonstrated to form a smooth copper surface before graphene synthesis to reduce the amount of few layer graphene islands. Six methods of surface processing of copper foils are studied and the decrease of the roughness from 250 to as low as 80 nm is achieved. The correlation between foil roughness and the formation of the additional layer is demonstrated. Under optimized conditions of surface treatment, the content of the additional graphene layer drops from 9 to 2.1%. The quality and the number of layers of synthesized graphene are analyzed by Raman spectroscopy, scanning electron microscopy and measurements of charge mobility.

A possibility of laser printing of single diamond nanoparticles with luminescent SiV centers is demonstrated. To provide the transfer, a target consisting of a transparent sapphire plate and an absorbing thin titanium film (600 nm thick) coated with nanoparticles was irradiated with a KrF excimer laser ("lambda"=248 nm, "tau"=20 ns). Optimal values of the laser fluence have been determined and a technique for applying labels for the targeted transfer of nanoparticles has been developed. Luminescence mapping of the donor and the receiving substrates confirmed the transfer of diamond nanoparticles with SiV centers.