Sub-cycle pulse revealed with carrier-envelope phase control of soliton self-compression in anti-resonant hollow-core fiber.

The influence of the carrier-envelope phase (CEP) of a pump pulse on the multioctave supercontinuum (SC) generation in a gas-filled anti-resonant hollow-core fiber (AR HCF) by soliton self-compression (SSC) has been explored. We have shown an octave-wide third harmonic generation (THG) in the visible-to-near-infrared range during the pulse compression down to a sub-cycle duration. The CEP of a multi-cycle pump pulse provides control of interference between the third harmonic (TH) and the SC that indicates the coherent synthesis of a sub-cycle pulse with a duration of about 0.4 optical cycles and a peak power of more than 2 GW at the fiber output.

Direct Observation of Incommensurate-Commensurate Transition in Graphene-hBN Heterostructures via Optical Second Harmonic Generation.

Commensurability effects play a crucial role in the formation of electronic properties of novel layered heterostructures. The interest in these moiré superstructures has increased tremendously since the recent observation of a superconducting state (Nature 2018, 556, 43-50) and metal-insulator transition (Nature 2018, 556, 80-84) in twisted bilayer graphene. In this regard, a straightforward and efficient experimental technique for detection of the alignment of layered materials is desired. In this work, we use optical second harmonic generation, which is sensitive to the inversion symmetry breaking, to investigate the alignment of graphene/hexagonal boron nitride heterostructures. To achieve that, we activate a commensurate-incommensurate phase transition by a thermal annealing of the sample. We find that this structural change in the system can be directly observed via a strong modification of a nonlinear optical signal. Unambiguous interpretation of obtained results reveals the potential of a second harmonic generation technique for probing of structural changes in layered systems.

Chirp-controlled high-harmonic and attosecond-pulse generation via coherent-wake plasma emission driven by mid-infrared laser pulses.

Coherent-wake plasma emission induced by ultrashort mid-infrared laser pulses on a solid target is shown to give rise to high-brightness, high-order harmonic radiation, offering a promising source of attosecond pulses and a probe for ultrafast subrelativistic plasma dynamics. With 80-fs, 0.2-TW pulses of 3.9-µm radiation used as a driver, optical harmonics up to the 34th order are detected, with their spectra stretching from the mid-infrared region to the extreme ultraviolet region. The harmonic spectrum is found to be highly sensitive to the chirp of the driver. Particle-in-cell analysis of this effect suggests, in agreement with the generic scenario of coherent-wake emission, that optical harmonics are radiated as trains of extremely short, attosecond ultraviolet pulses with a pulse-to-pulse interval varying over the pulse train. A positive chirp of the driver pulse can partially compensate for this variation in the interpulse separation, allowing harmonics of the highest orders to be generated in the plasma emission spectrum.

High-order harmonic analysis of anisotropic petahertz photocurrents in solids.

Polarization maps of high-order harmonics are shown to enable a full vectorial characterization of petahertz electron currents generated in a crystalline solid by an ultrashort laser driver. As a powerful resource of this methodology, analysis of energy-momentum dispersion landscapes, defined by the electron band structure, can help identify, as our analysis shows, special directions within the Brillouin zone that can provide a preferable basis for polarization-sensitive high-harmonic mapping of anisotropic petahertz photocurrents in solids.

Heisenberg-exchange-free nanoskyrmion mosaic.

Isotropic Heisenberg exchange naturally appears as the main interaction in magnetism, usually favouring long-range spin-ordered phases. The anisotropic Dzyaloshinskii-Moriya interaction arises from relativistic corrections and is a priori much weaker, even though it may sufficiently compete with the isotropic one to yield new spin textures. In this work, we challenge this well-established paradigm, and propose to explore a Heisenberg-exchange-free magnetic world. In this case, the Dzyaloshinskii-Moriya interaction induces magnetic frustration in two dimensions, from which the competition with an external magnetic field results in a new mechanism producing skyrmions of nanoscale size. A single nanoskyrmion can already be stabilized in a few-atom cluster, and may then be used as LEGO® block to build a large magnetic mosaic. The realization of such topological spin nanotextures in sp- and p -electron compounds or in ultracold atomic gases would open a new route toward robust and compact magnetic memories.

Effective Heisenberg Model and Exchange Interaction for Strongly Correlated Systems.

We consider the extended Hubbard model and introduce a corresponding Heisenberg-like problem written in terms of spin operators. The derived formalism is reminiscent of Anderson's idea of the effective exchange interaction and takes into account nonlocal correlation effects. The results for the exchange interaction and spin susceptibility in the magnetic phase are expressed in terms of single-particle quantities. This fact not only can be used for realistic calculations of multiband systems but also allows us to reconsider a general description of many-body effects in the most interesting physical regimes, where the physical properties of the system are dominated by collective (bosonic) fluctuations. In the strongly spin-polarized limit, when the local magnetic moment is well defined, the exchange interaction reduces to a standard expression of the density functional theory that has been successfully used in practical calculations of magnetic properties of real materials.

Excitonic Instability and Pseudogap Formation in Nodal Line Semimetal ZrSiS.

Electron correlation effects are studied in ZrSiS using a combination of first-principles and model approaches. We show that basic electronic properties of ZrSiS can be described within a two-dimensional lattice model of two nested square lattices. A high degree of electron-hole symmetry characteristic for ZrSiS is one of the key features of this model. Having determined model parameters from first-principles calculations, we then explicitly take electron-electron interactions into account and show that, at moderately low temperatures, ZrSiS exhibits excitonic instability, leading to the formation of a pseudogap in the electronic spectrum. The results can be understood in terms of Coulomb-interaction-assisted pairing of electrons and holes reminiscent of that of an excitonic insulator. Our finding allows us to provide a physical interpretation of the unusual mass enhancement of charge carriers in ZrSiS recently observed experimentally.

[DIC-syndrome-related mortality in acute necrotic pancreatitis]. / DVS-sindrom pri pankreonekroze.

AIM: To improve the results of acute necrotic pancreatitis management via an analysis of changes in the main causes of mortality in patients with this disease over the last decade. MATERIAL AND METHODS: We have analyzed medical records of patients with acute necrotic pancreatitis followed by lethal outcome for the period 2015-2016. In all cases autopsy was performed in order to determine morbidity and mortality. Complications of pancreatic necrosis significant for tanatogenesis, treatment and prevention strategy were compared with those for 2005-2006. RESULTS: It was shown that not only significantly decreased mortality in acute necrotic pancreatitis but also important changes in its structure occurred for the last ten years. At present time DIC-syndrome and its organ complications are the most significant causes of death. CONCLUSION: Prevention of DIC-syndrome and its complications is necessary to improve the outcomes. Endothelial protectors' administration in early period, timely diagnosis and adequate treatment of DIC-syndrome are advisable.

Mapping anomalous dispersion of air with ultrashort mid-infrared pulses.

We present experimental studies of long-distance transmission of ultrashort mid-infrared laser pulses through atmospheric air, probing air dispersion in the 3.6-4.2-µm wavelength range. Atmospheric air is still highly transparent to electromagnetic radiation in this spectral region, making it interesting for long-distance signal transmission. However, unlike most of the high-transmission regions in gas media, the group-velocity dispersion, as we show in this work, is anomalous at these wavelengths due to the nearby asymmetric-stretch rovibrational band of atmospheric carbon dioxide. The spectrograms of ultrashort mid-infrared laser pulses transmitted over a distance of 60 m in our experiments provide a map of air dispersion in this wavelength range, revealing clear signatures of anomalous dispersion, with anomalous group delays as long as 1.8 ps detected across the bandwidth covered by 80-fs laser pulses.

Dynamical and Reversible Control of Topological Spin Textures.

Recent observations of topological spin textures brought spintronics one step closer to new magnetic memories. Nevertheless, the existence of Skyrmions, as well as their stabilization, require very specific intrinsic magnetic properties which are usually fixed in magnets. Here we address the possibility to dynamically control their intrinsic magnetic interactions by varying the strength of a high-frequency laser field. It is shown that drastic changes can be induced in the antiferromagnetic exchange interactions and the latter can even be reversed to become ferromagnetic, provided the direct exchange is already non-negligible in equilibrium as predicted, for example, in Si doped with C, Sn, or Pb adatoms. In the presence of Dzyaloshinskii-Moriya interactions, this enables us to tune features of ferromagnetic Skyrmions such as their radius, making them easier to stabilize. Alternatively, such topological spin textures can occur in frustrated triangular lattices. Then, we demonstrate that a high-frequency laser field can induce dynamical frustration in antiferromagnets, where the degree of frustration can subsequently be tuned suitably to drive the material toward a Skyrmionic phase.

Solid-State Source of Subcycle Pulses in the Midinfrared.

We demonstrate a robust, all-solid-state approach for the generation of microjoule subcycle pulses in the midinfrared through a cascade of carefully optimized parametric-amplification, difference-frequency-generation, spectral-broadening, and chirp-compensation stages. This method of subcycle waveform generation becomes possible due to an unusual, ionization-assisted solid-state pulse self-compression dynamics, where highly efficient spectral broadening is enabled by ultrabroadband four-wave parametric amplification phase matched near the zero-group-velocity wavelength of the material.

Mid-infrared-to-mid-ultraviolet supercontinuum enhanced by third-to-fifteenth odd harmonics.

A high-energy supercontinuum spanning 4.7 octaves, from 250 to 6500 nm, is generated using a 0.3-TW, 3.9-µm output of a mid-infrared optical parametric chirped-pulse amplifier as a driver inducing a laser filament in the air. The high-frequency wing of the supercontinuum spectrum is enhanced by odd-order optical harmonics of the mid-infrared driver. Optical harmonics up to the 15th order are observed in supercontinuum spectra as overlapping, yet well-resolved peaks broadened, as verified by numerical modeling, due to spatially nonuniform ionization-induced blue shift.

Multioctave, 3-18 µm sub-two-cycle supercontinua from self-compressing, self-focusing soliton transients in a solid.

Strongly coupled nonlinear spatiotemporal dynamics of ultrashort mid-infrared pulses undergoing self-focusing simultaneously with soliton self-compression in an anomalously dispersive, highly nonlinear solid semiconductor is shown to enable the generation of multioctave supercontinua with spectra spanning the entire mid-infrared range and compressible to subcycle pulse widths. With 7.9 µm, 150 fs, 2 µJ, 1 kHz pulses used as a driver, 1.2 cycle pulses of mid-infrared supercontinuum radiation with a spectrum spanning the range of wavelengths from 3 to 18 µm were generated in a 5 mm GaAs plate. Further compression of these pulses to subcycle pulse widths is possible through compensation of the residual phase shift.

Mid-infrared laser filaments in the atmosphere.

Filamentation of ultrashort laser pulses in the atmosphere offers unique opportunities for long-range transmission of high-power laser radiation and standoff detection. With the critical power of self-focusing scaling as the laser wavelength squared, the quest for longer-wavelength drivers, which would radically increase the peak power and, hence, the laser energy in a single filament, has been ongoing over two decades, during which time the available laser sources limited filamentation experiments in the atmosphere to the near-infrared and visible ranges. Here, we demonstrate filamentation of ultrashort mid-infrared pulses in the atmosphere for the first time. We show that, with the spectrum of a femtosecond laser driver centered at 3.9 µm, right at the edge of the atmospheric transmission window, radiation energies above 20 mJ and peak powers in excess of 200 GW can be transmitted through the atmosphere in a single filament. Our studies reveal unique properties of mid-infrared filaments, where the generation of powerful mid-infrared supercontinuum is accompanied by unusual scenarios of optical harmonic generation, giving rise to remarkably broad radiation spectra, stretching from the visible to the mid-infrared.

Frequency-tunable sub-two-cycle 60-MW-peak-power free-space waveforms in the mid-infrared.

A physical scenario whereby freely propagating mid-infrared pulses can be compressed to pulse widths close to the field cycle is identified. Generation of tunable few-cycle pulses in the wavelength range from 4.2 to 6.8 µm is demonstrated at a 1-kHz repetition rate through self-focusing-assisted spectral broadening in a normally dispersive, highly nonlinear semiconductor material, followed by pulse compression in the regime of anomalous dispersion, where the dispersion-induced phase shift is finely tuned by adjusting the overall thickness of anomalously dispersive components. Sub-two-cycle pulses with a peak power up to 60 MW are generated in the range of central wavelengths tunable from 5.9 to 6.3 µm.

A pseudopotential model for Dirac electrons in graphene with line defects.

We consider electron transport in a planar fermion model containing various types of line defects modeled by δ-function pseudopotentials with different matrix coefficients. After determining the necessary boundary conditions, the transmission probability for electron transport through the defect line is obtained for various types of pseudopotentials. For the schematic model considered, which may describe a graphene structure with different types of linear defects, the valley polarization is obtained.

[Endoscopic retrograde cholangiopancreaticography for the treatment of necrotic pancreatitis].

40 patients with the acute necrotic pancreatitis and verified biliary hypertension were included in the study. During the endoscopic retrograde cholangiopancreaticography we avoided the main pancreatic duct contrast enhancement. The canulation during papillosphincterotomy (PST) was not used if the concrement was infringed in the ampulla. The use of these methods led to fast relief of the biliary hypertension and truthworthy positive dynamics of the disease course. The negative consequences were usually limited to transitory amilasemia and the pancreonecrosis progression was registered in only 4±4% cases. In comparison with cholecystostomy, the ERCP with PST showed better prognosis considering the frequency of septic complications and terms of hospital stay. The statistical analysis proved the the ERCP with PST are preferable to cholecystostomy in cases of pancreonecrosis with choledochlithiasis and biliary hypertension and the overall severity of pancreonecrosis is not higher than 11 by the physiological condition scale.

[Artificial esophagus in children]. / Iskusstvennyi pishchevid u detei.

The aims of the study were: 1) to formulate up-to-date principles of coloesophagoplasty (CEP) and to compare methods of CEP; 2) to determine the optimal age for esophagoplasty in children with esophageal atresia; 3) to evaluate quality of life and long term results after esophagoplasty performed in childhood; 4) to present results of surgical treatment of pharynx stenosis in children. Over the last 50 years more than 2 thousand surgeries on the esophagus in children were performed including more than 600 operations of artificial esophagus creation. From 1995 in some patients we carried out extirpation of the esophagus with one-stage plastic operation coloesophagoplasty taken the transplant in the posterior mediastinum. From 1992 on the pharynx with free revascularized intestinal segment was performed in isolated stenosis of the pharynx. Case histories of 54 children aged from 3 months to 15 years were analyzed to compare methods of esophagoplasty when the esophagus was taken behind the sternum and in posterior mediastinum. Case histories of 27 children with atresia of the esophagus were analyzed for determination of optimal age for coloesophagoplasty. Responses to questionnaires were analyzed to evaluate quality of life. From 1980 ive hase operated 28 patients with scarry stenosis of the pharynx (isolated or in combination with esophageal stenosis). Current policy of surgical treatment of pharyngeal stenosis which is considered as surgery of choice-pharyngoplasty with a free intestinal transplant--is described. Development and clinical appliance of up-to-date principles of CEP permitted to reduce the number of complications and to decrease lethality from 3 to 0.5%. After introduction of antireflux cologastroanastomosis the number of complications due to reflux reduced. Extirpation of the esophagus may be valid in peptic stenosis, Barrett's metaplasia, tumors, portal hypertension or angiodisplasia, and in the majority of cases of scarry stenosis of the esophagus.

[Laparoscopic surgery in children--current possibilities and perspectives]. / Laparoskopicheskaia khirurgiia u detei--sovremennye vozmozhnosti i perspektivy.

Experience in use of different laparoscopic methods in 6046 children who needed urgent and elective surgeries are analyzed. Urgent surgeries were performed in 3292 children for acute appendicitis and it complications, acute adhesive intestinal obstruction, invagination, trauma of abdominal organs, pathological changes of Meckel diverticulum, urgent inflammatory and non-inflammatory gynecological diseases. In this group endosurgery was successful in 3120 (94.8%) patients, conversion to open surgery was necessary in 5.2% cases (172 patients). Laparoscopic cholecystectomy, splenectomy, nephrectomy, surgeries for cysts of parenchymatous organs, benign cysts and tumors of abdominal cavity and retroperitoneal space, varicocele, syndrome of unpalpable testes and abdominal cryptorchism were performed as elective surgeries in 2754 patients. Endosurgical methods to minimized number of complications (1.1% or 29 patients in the whole group), conversion to open surgery was necessary in 10 (0.4%) cases. Laparoscopic methods in many cases are the "gold standard" in elective and urgent surgery in children.

[Thoracoscopic and video-assisted surgeries on thoracic organs of children]. / Torakoskopicheskie i videoassistirovannye operatsii na organakh grudnoi kletki u detei.

Forty-seven video-surgical thoracic operations performed in children are analyzed. Age of children ranged from 4 days to 14.5 years, body weight--from 700 g to 54 kg. Two types of surgeries are singled out--thoracoscopic and video-assisted (when minithoracotomy was performed in addition to thoracoscopic technique). There were no severe complications during surgery. One child died due to non-diagnosed penumothorax on the 4th day after surgery. Problems of anesthesia and surgical technique in children are considered.