In the last decade quantum cascade lasers (QCLs) have become the most widely used source of mid-infrared radiation, finding large scale applications because of their wide tunability and overall high ...performance. However far-infrared (terahertz) QCLs have lagged behind in terms of performance and impact due to the inability so far of achieving room temperature operation. Here we review recent research that has led to a new class of QCL light sources that has overcome these limitations leading to room temperature operation in the terahertz spectral range, with nearly 2 mW of optical power and significant tunability, opening up also this region of the spectrum to a wide range of applications.
Intersubband transitions in n-doped multi-quantum-well semiconductor heterostructures make it possible to engineer one of the largest known nonlinear optical responses in condensed matter ...systems--but this nonlinear response is limited to light with electric field polarized normal to the semiconductor layers. In a different context, plasmonic metasurfaces (thin conductor-dielectric composite materials) have been proposed as a way of strongly enhancing light-matter interaction and realizing ultrathin planarized devices with exotic wave properties. Here we propose and experimentally realize metasurfaces with a record-high nonlinear response based on the coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered electronic intersubband transitions in semiconductor heterostructures. We show that it is possible to engineer almost any element of the nonlinear susceptibility tensor of these structures, and we experimentally verify this concept by realizing a 400-nm-thick metasurface with nonlinear susceptibility of greater than 5 × 10(4) picometres per volt for second harmonic generation at a wavelength of about 8 micrometres under normal incidence. This susceptibility is many orders of magnitude larger than any second-order nonlinear response in optical metasurfaces measured so far. The proposed structures can act as ultrathin highly nonlinear optical elements that enable efficient frequency mixing with relaxed phase-matching conditions, ideal for realizing broadband frequency up- and down-conversions, phase conjugation and all-optical control and tunability over a surface.
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DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We report a simple technique that allows obtaining mid-infrared absorption spectra with nanoscale spatial resolution under low-power illumination from tunable quantum cascade lasers. Light absorption ...is detected by measuring associated sample thermal expansion with an atomic force microscope. To detect minute thermal expansion we tune the repetition frequency of laser pulses in resonance with the mechanical frequency of the atomic force microscope cantilever. Spatial resolution of better than 50 nm is experimentally demonstrated.
Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in ...three-dimensional (3D) space. While chiral optical properties are weak in most of naturally occurring materials, they can be engineered and significantly enhanced in synthetic optical media known as chiral metamaterials, where the spatial symmetry of their building blocks is broken on a nanoscale. Although originally discovered in 3D structures, circular dichroism can also emerge in a two-dimensional (2D) metasurface. The origin of the resulting circular dichroism is rather subtle, and is related to non-radiative (Ohmic) dissipation of the constituent metamolecules. Because such dissipation occurs on a nanoscale, this effect has never been experimentally probed and visualized. Using a suite of recently developed nanoscale-measurement tools, we establish that the circular dichroism in a nanostructured metasurface occurs due to handedness-dependent Ohmic heating.
Strong interaction of graphene with light accounts for one of its most remarkable properties: the ability to absorb 2.3% of the incident light’s energy within a single atomic layer. Free carrier ...injection via field-effect gating can dramatically vary the optical properties of graphene, thereby enabling fast graphene-based modulators of the light intensity. However, the very thinness of graphene makes it difficult to modulate the other fundamental property of the light wave: its optical phase. Here we demonstrate that considerable phase control can be achieved by integrating a single-layer graphene (SLG) with a resonant plasmonic metasurface that contains nanoscale gaps. By concentrating the light intensity inside of the nanogaps, the metasurface dramatically increases the coupling of light to the SLG and enables control of the phase of the reflected mid-infrared light by as much as 55° via field-effect gating. We experimentally demonstrate graphene-based phase modulators that maintain the amplitude of the reflected light essentially constant over most of the phase tuning range. Rapid nonmechanical phase modulation enables a new experimental technique, graphene-based laser interferometry, which we use to demonstrate motion detection with nanoscale precision. We also demonstrate that by the judicious choice of a strongly anisotropic metasurface the graphene-controlled phase shift of light can be rendered polarization-dependent. Using the experimentally measured phases for the two orthogonal polarizations, we demonstrate that the polarization state of the reflected light can be by modulated by carrier injection into the SLG. These results pave the way for novel high-speed graphene-based optical devices and sensors such as polarimeters, ellipsometers, and frequency modulators.
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IJS, KILJ, NUK, PNG, UL, UM
Currently, work is under way to create new thermionic reactor-converters. However, when designing an electrogenerating channel, difficulties arise in the complex description of the physical processes ...occurring in it. Moreover, empirical models of current–voltage characteristics do not always agree with the obtained results in the analysis of reactor experiments of electrogenerating channels. This is due to the fact that the empirical current–voltage characteristics are based on the experimental results of a specific electrode pair in a given mode of operation of a thermionic converter. In this work, we propose a complex mathematical model of a thermionic converter in a nuclear reactor. The mathematical model is based on solving the equations of charged particle transfer, which take into account the features of all operating modes of thermionic converter.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
IMPORTANCE: Recent concerns regarding repetitive use of general anesthesia in children younger than 3 years have placed greater importance on the controversy surrounding the timing of the initiation ...of port-wine stain (PWS) laser treatment. OBJECTIVE: To evaluate the use of PWS treatments at the age of 1 year or younger in the office setting without general anesthesia. DESIGN, SETTING, AND PARTICIPANTS: Retrospective cohort study based on medical record reviews at a single, high-volume laser center for children who started pulsed dye laser treatment at the age of 1 year or younger for their PWS between 2000 and 2017. The data cutoff was at 1 year after the initial treatment to have comparable data points. MAIN OUTCOMES AND MEASURES: The primary outcome was improvement of PWSs using before and after photographs, which were reviewed by 4 physicians independently and graded using the following 5-point visual analog scale (VAS): poor (grade 1: 0%-25% improvement), fair (grade 2: 26%-50% improvement), good (grade 3: 51%-75% improvement), excellent (grade 4: 76%-99% improvement), and complete (grade 5: 100% improvement) clearance. RESULTS: Of the 197 patients (73 37.1% boys; 124 62.9% girls), most (149 75.6%) had facial lesions. The mean age at the time of first treatment was 3.38 months (range, 5-355 days) and the mean number of treatments was 9.8 (range, 2-23; median, 10). Per the mean physician VAS grading of 197 patients, 51 patients (25.9%) showed 100% clearance (mean range VAS score of 4.78 4.5 – 5); 81 patients (41.1%) showed 76 to 99% improvement (mean range VAS score of 3.91 3.5 to <4.5); 44 patients (22.3%) showed 51% to 75% improvement (mean range VAS score of 2.86 2.5 to <3.5); 13 patients (6.6%) showed 26% to 50% improvement (mean range VAS score of 2.12 1.5 to <2.5); and 8 patients (4.1%) showed 0 to 25% improvement (mean range VAS score of 0.78 0 to <1.5). The presence of a V1 (first branch of the trigeminal nerve ophthalmic nerve) lesion was associated with a statistically significantly higher clearance rate by a VAS grade of 0.55 (95% CI, 0.25-0.84; P < .001). The mean (SD) VAS grade for all patients was 3.65 (1.26), corresponding to excellent clearance. None of the patients experienced scarring or permanent pigmentary change. CONCLUSIONS AND RELEVANCE: In this study, treatment of PWSs in infancy was both safe and effective. Early intervention allows for treatment without general anesthesia, maximizing the chance to achieve clearance before school age and thereby minimizing the negative outcome of PWSs for both the patient and the family.
Macroscopic quantum tunneling is a fundamental phenomenon of quantum mechanics related to the actively debated topic of quantum-to-classical transition. The ability to realize macroscopic quantum ...tunneling affects implementation of qubit-based quantum computing schemes and their protection against decoherence. Decoherence in qubits can be reduced by means of topological protection, e.g., by exploiting various parity effects. In particular, paired phase slips can provide such protection for superconducting qubits. Here, we report on the direct observation of quantum paired phase slips in thin-wire superconducting loops. We show that in addition to conventional single phase slips that change the superconducting order parameter phase by 2π , there are quantum transitions that change the phase by 4π . Quantum paired phase slips represent a synchronized occurrence of two macroscopic quantum tunneling events, i.e., cotunneling. We demonstrate the existence of a remarkable regime in which paired phase slips are exponentially more probable than single ones.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UL, UM, UPUK
Room temperature, broadly tunable, electrically pumped semiconductor sources in the terahertz spectral range, similar in operation simplicity to diode lasers, are highly desired for applications. An ...emerging technology in this area are sources based on intracavity difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers. Here we report terahertz quantum cascade laser sources based on an optimized non-collinear Cherenkov difference-frequency generation scheme that demonstrates dramatic improvements in performance. Devices emitting at 4 THz display a mid-infrared-to-terahertz conversion efficiency in excess of 0.6 mW W(-2) and provide nearly 0.12 mW of peak power output. Devices emitting at 2 and 3 THz fabricated on the same chip display 0.09 and 0.4 mW W(-2) conversion efficiencies at room temperature, respectively. High terahertz-generation efficiency and relaxed phase-matching conditions offered by the Cherenkov scheme allowed us to demonstrate, for the first time, an external-cavity terahertz quantum cascade laser source tunable between 1.70 and 5.25 THz.
Evaluation of the effect of pharmacological modulation of the rehabilitation process with the drug mexidol as an adjuvant component of the rehabilitation treatment of cognitive-emotional disorders in ...patients who have suffered acute cerebral insufficiency (ACI) due to acute cerebrovascular accident or traumatic brain injury.
The study was conducted as a randomized interventional prospective study and consisted of 5 visits. Patients were divided into 2 groups: main (
=30, standard therapy + Mexidol IV 500 mg per day for 10 days, followed by Mexidol FORTE 250 orally, 1 tablet 3 times a day for 8 weeks) and control (
=30, standard therapy for 66 days).
The study randomized 60 patients who underwent ACN and received rehabilitation treatment in accordance with regional routing. In the main group, there was an improvement in cognitive functions comparable to the control group (
<0.001, in both groups there was an improvement in the Schulte test «work efficiency» and «total execution time», according to the MoCA scale (visit 5 - 23.8±2.6 vs 22.9±31,
=0.227). A significant superiority of the main group over the control group was shown in such indicators as a decrease in anxiety (according to the HADS scale) (visit 4 - 2.6±2.4 vs 4.4±2.4,
=0.004), a decrease in the severity of depression (according to the Beck scale) (visit 3 - 7.5±4.5 vs 11.4±5.6,
=0.005). There was a tendency for the main group to be superior in terms of muscle strength (according to the MRC scale (visit 4 - 3.3±5.1 vs 2.1±2.2,
=0.051), level of vital activity (according to the ShRM - visit 5 - 2.9±0.7 vs 3.3±0.6,
=0.053). A statistically significant increase in the level of mobility of patients in the group using the drug Mexidol was proven compared to the control group (the difference in the Rivermead index at the 5th visit was 10.3±2.8 and 8.0±2.8, respectively,
=0.006), the average increase in the Rivermead index by visit 5 (5.4±2.1 vs 3.4±1.6,
<0.001). A decrease in intensive care aftereffects syndrome (ITS) scores was detected in both groups; a statistically significant decrease in the severity of ITS in relation to the previous visit was detected only in the group using the drug Mexidol (
<0.001). In the main group, the best indicators of the dynamics of systolic cerebral blood flow velocity and overshoot coefficient were also determined, compared to the control group. There were no adverse events recorded in the study.
A positive modulating effect of Mexidol has been demonstrated in terms of accelerating the restoration of tolerance to cognitive loads, improving the psycho-emotional background by reducing symptoms of anxiety and depression, and secondary improving the results of motor rehabilitation in the early recovery period in patients who have undergone ACI, including those with manifestations of PIT syndrome. During the study, no adverse events were recorded, as well as significant differences in vital functions in the study groups, which indicates comparable safety of therapy in the control and main groups.
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