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We reported on the observation of extremely high-density ($>10^{14}$cm$^{-2}$) 2D electron gas in N-polar AlGaN/GaN heterostructures grown on sapphire substrates. Due to introducing the GaN/AlN superlattice (SL) back barrier between the GaN buffer layer and AlGaN barrier layer, we observed a giant enhancement of the 2D electron gas density at the GaN/AlGaN interface from $3\times10^{13}$cm$^{-2}$ (without SL) to $1.4\times10^{14}$cm$^{-2}$ (with SL back barrier) that is only one order of magnitude below the intrinsic crystal limit of $\approx10^{15}$cm$^{-2}$. We found that the changes of 2D electron gas density with SL correlated well with the changes of the wafer warp parameter which suggests that the strains are responsible for the 2D electron gas density enhancement (reduction of the piezoelectric polarization in the GaN channel). Nevertheless, this finding is probably insufficient to fully explain the observed high 2D electron gas density. Simultaneously, the room temperature electron mobility was 169 cm$^2$/Vs, which with the electron density of $1.4\times10^{14}$cm$^{-2}$ gives a low sheet resistance of 264 $\Omega$/sq (one of the lowest reported so far for the N-polar 2D electron gas channel). Finally, the possibility of application of such high-density 2D electron gas with low sheet resistance to transistors, emitters and detectors was discussed., Comment: Version 2.0

Intense ultrashort laser pulse irradiation of solid targets was systematically investigated at the first-principles level, both theoretically and computationally. In the method, the propagation of a pulsed light through a thin film is described by a one-dimensional Maxwell's equation, and the microscopic electronic motion at different positions in the film is described by employing first-principles time-dependent density functional theory (TDDFT). The method uses a coarse-graining approximation to couple light propagation and electronic motion, and is termed the multiscale Maxwell-TDDFT method. The reflectance, transmittance, and absorbance of pulsed light incident normally on thin films of 50-200 nm thickness were calculated for materials with different optical properties, such as aluminum (simple metal), graphite (semi-metal), silicon (small-gap dielectric), and quartz (wide-gap dielectric). Optical response transitions were explored as the light intensity shifted from the linear regime, represented by the dielectric function for weak light, to the extremely nonlinear regime, represented by plasma reflection under intense light conditions. Numerous mechanisms that depend on the laser pulse intensity and material type were found to contribute to these changes. These include multiphoton absorption, saturable absorption, sign change of the effective dielectric constant, and transition from quantum occupation to classical Boltzmann distribution. Thus, the calculations provide a unified understanding of the interaction of intense pulsed light with solids, occurring on an extremely short time scale., Comment: 17 pages, 6 figures

Surgical tool segmentation and action recognition are fundamental building blocks in many computer-assisted intervention applications, ranging from surgical skills assessment to decision support systems. Nowadays, learning-based action recognition and segmentation approaches outperform classical methods, relying, however, on large, annotated datasets. Furthermore, action recognition and tool segmentation algorithms are often trained and make predictions in isolation from each other, without exploiting potential cross-task relationships. With the EndoVis 2022 SAR-RARP50 challenge, we release the first multimodal, publicly available, in-vivo, dataset for surgical action recognition and semantic instrumentation segmentation, containing 50 suturing video segments of Robotic Assisted Radical Prostatectomy (RARP). The aim of the challenge is twofold. First, to enable researchers to leverage the scale of the provided dataset and develop robust and highly accurate single-task action recognition and tool segmentation approaches in the surgical domain. Second, to further explore the potential of multitask-based learning approaches and determine their comparative advantage against their single-task counterparts. A total of 12 teams participated in the challenge, contributing 7 action recognition methods, 9 instrument segmentation techniques, and 4 multitask approaches that integrated both action recognition and instrument segmentation. The complete SAR-RARP50 dataset is available at: https://rdr.ucl.ac.uk/projects/SARRARP50_Segmentation_of_surgical_instrumentation_and_Action_Recognition_on_Robot-Assisted_Radical_Prostatectomy_Challenge/191091

Linear and nonlinear optical response of WSe$_{2}$ monolayer is investigated by two-dimensional Maxwell plus time-dependent density functional theory with spin-orbit interaction. By applying the chiral resonant pulses, the electron dynamics along with high harmonic generation are examined at weak and strong laser fields. WSe$_{2}$ monolayer shows the linear optical response at the intensity I = 10$^{10}$~W/cm$^{2}$ while a complex nonlinear behavior is observed at I = 10$^{12}$~W/cm$^{2}$. The nonlinear response of WSe$_{2}$ monolayer in terms of saturable absorption is observed at strong laser field. By changing the chirality of the resonant light, a strong circular dichroic effect is observed in the excited state population. A relatively weak laser field shows effective valley polarization while strong field induces spin-polarized carrier peak between $K$($K'$) and $\mathit{\Gamma}$-point via nonlinear process. On the other hand, the strong laser field shows high harmonics up to the 11th order. Our results demonstrate that circularly polarized resonant pulse generate high harmonics in WSe$_{2}$ monolayer of order 3n$\pm1$., Comment: arXiv admin note: text overlap with arXiv:2110.06433

Abstract The valley degree of freedom in two-dimensional materials provides an opportunity to extend the functionalities of valleytronics devices. Very short valley lifetimes demand the ultrafast control of valley pseudospin. Here, we theoretically demonstrate the control of valley pseudospin in WSe2 monolayer by single-cycle linearly polarized laser pulse. We use the asymmetric electric field controlled by the carrier-envelope phase (CEP) to make the valley polarization between K and K'-point in the Brillouin zone (BZ). Time-dependent density functional theory with spin-orbit interaction reveals that no valley asymmetry and its CEP dependence is observed within the linear-optical limit. In the nonlinear-optical regime, linearly polarized pulse induces a high degree of valley polarization and this polarization is robust against the field strength. Valley polarization strongly depends and oscillates as a function of CEP. The carrier density distribution forms nodes as the laser intensity increases, our results indicate that the position of the carrier density in the BZ can be controlled by the laser intensity. From the analysis by the massive Dirac Hamiltonian model, the nodes of the carrier density can be attributed to the Landau-Zener-St\"uckelberg interference of wave packets of the electron wave function.

We study high order harmonics generation (HHG) in crystalline silicon and diamond subjected to near and mid-infrared laser pulses. We employ time-dependent density functional theory and solve the time-dependent Kohn-Sham equation in the single-cell geometry. We demonstrate that clear and clean HHG spectra can be generated with careful selection of the pulse duration. In addition, we simulate dephasing effects in a large silicon super-cell through displacement of atomic positions prepared by a molecular dynamics simulation. We compare our results with the previous calculations by Floss et al. [arXiv:1705.10707] [Phys. Rev. A 97, 011401(R) (2018)] on Diamond at 800 nm and by Tancogne-Dejean et al. [arXiv:1609.09298] [Phys. Rev. Lett. 118, 087403 (2017)] on Si at 3000 nm., Comment: Accepted to Phys. Rev. B

Modulation of probe signal in pump-probe measurements of coherent phonons in dielectrics, with and without spectral resolution, are investigated theoretically taking diamond as an example. Analytical investigation as well as first-principles calculations based on time-dependent density functional theory is utilized to clarify the mechanism of the modulation of the probe signals. Boundary and bulk effects are investigated systematically, putting emphasis on the phase relation between the modulation and the atomic motion of the coherent phonon. They are summarized as follows: Modulation by the boundary effect is in phase with the coherent phonon amplitude, while that by the bulk effect shows $\pi/2$ phase difference. Strong frequency dependence appears in the modulation by the bulk effect, while no frequency dependence by the boundary effect. First-principles calculations support the reliability of the analytical result.

We have developed a novel multiscale computational scheme to describe coupled dynamics of light electromagnetic field with electrons and atoms in crystalline solids, where first-principles molecular dynamics based on time-dependent density functional theory is used to describe the microscopic dynamics. The method is applicable to wide phenomena in nonlinear and ultrafast optics. To show usefulness of the method, we apply it to a pump-probe measurement of coherent phonon in diamond where a Raman amplification takes place during the propagation of the probe pulse.

Energy transfer processes from a high-intensity ultrashort laser pulse to electrons in simple dielectrics, silicon, diamond, and $\alpha$-quartz are theoretically investigated by first-principles calculations based on time-dependent density functional theory (TDDFT). Dependences on frequency as well as intensity of the laser pulse are examined in detail, making a comparison with the Keldysh theory. Although the Keldysh theory reliably reproduces the main features of the TDDFT calculation, we find some deviations between results by the two theories. The origin of the differences is examined in detail.

SALMON (Scalable Ab-initio Light-Matter simulator for Optics and Nanoscience, http://salmon-tddft.jp) is a software package for the simulation of electron dynamics and optical properties of molecules, nanostructures, and crystalline solids based on first-principles time-dependent density functional theory. The core part of the software is the real-time, real-space calculation of the electron dynamics induced in molecules and solids by an external electric field solving the time-dependent Kohn-Sham equation. Using a weak instantaneous perturbing field, linear response properties such as polarizabilities and photoabsorptions in isolated systems and dielectric functions in periodic systems are determined. Using an optical laser pulse, the ultrafast electronic response that may be highly nonlinear in the field strength is investigated in time domain. The propagation of the laser pulse in bulk solids and thin films can also be included in the simulation via coupling the electron dynamics in many microscopic unit cells using Maxwell's equations describing the time evolution of the electromagnetic fields. The code is efficiently parallelized so that it may describe the electron dynamics in large systems including up to a few thousand atoms. The present paper provides an overview of the capabilities of the software package showing several sample calculations.

The possible emergence of a spin liquid phase in the half-filled Hubbard model on the honeycomb lattice; a simple model of graphene, is studied using the variational cluster approximation. We found that the critical interaction strength of a magnetic transition is slightly lower than that of the non-magnetic metal-to-insulator transition. Thus a semi-metallic state becomes a magnetic insulator as the interaction strength increases and a non-magnetic insulating state does not occur. Therefore a spin liquid state characterized by an insulating state without long range order, is not realized in this system., Comment: 4 pages, 3 figures, typo corrected