arXiv subjects

Tadas Paulauskas

Publications and source records attributed to Tadas Paulauskas.

4 recordsLinked to original sources

arXiv2026

Towards Site-Selective Fabrication of Near-Infrared Emitters in hBN

Spatial control of near-infrared (NIR) emission from hexagonal boron nitride (hBN) would facilitate coupling atomic-scale light sources to photonic structures, yet oxygen-related NIR emitters are generally formed at stochastic locations. Here, we combine single-shot femtosecond laser writing with annealing in an oxygen-rich environment to bias NIR activation toward predefined coordinates in exfoliated hBN. Spectra acquired with 532, 635, and 785 nm excitations show narrow and multipeak emission extending to a wavelength of 1 um. Among the spectra collected at written sites, over 83% under 785 nm excitation and 71% under 635 nm excitation contain at least one resolved peak above 810 nm. The emission intensity increases monotonically with writing-pulse energy, suggesting tunability and indicating that the optimum for the single-emitter regime may require lower energies. Band-pass-resolved measurements show zero-delay correlation dips with g2(0) values indicative of single-photon emission but also reveal contributions from the spectral background. Spectrally resolved time series under 785 nm excitation show persistent bands over the recorded intervals as well as intermittent emission above 900 nm. This approach demonstrates NIR-emitter activation at predefined sites while identifying residual off-site activation and spectral multiplicity as the principal targets for further optimization.

physics.optics↗

arXiv2026

Symmetry-guided modal control in elliptical femtosecond-laser-written photonic waveguides

Few-mode photonic circuits can increase functionality without multiplying waveguide paths, but bends and fabrication errors can mix their transverse modes. We investigate a strategy in which waveguide confinement and perturbation parity are engineered together in vertically elliptical, femtosecond-laser-written glass waveguides. The intended modal basis comprises the even $1S$ mode and the vertically odd $2P_y$ mode. No window-converged $2P_x$ state is resolved for a lower-confinement (LC) design, whereas a higher-confinement (HC) design guides $2P_x$, which must therefore be isolated by symmetry. In scalar beam-propagation calculations, the $1S$-$2P_y$ propagation-constant splitting predicts the optimized periods of a vertically modulated coherent modal splitter to within $1.0\%$. Horizontal S-bends remain parity-mismatched for $1S \leftrightarrow 2P_y$ coupling, while the symmetry-allowed HC $1S \rightarrow 2P_x$ transfer reaches only $0.6\%$ at the largest displacement. At a displacement of $150~μ\mathrm{m}$, the HC design retains approximately the same $2P_y$ power as the LC design retains at $40~μ\mathrm{m}$. Thermal and stochastic writing-error calculations reveal the resulting trade-off: stronger confinement improves modal-power retention, but writing jitter that breaks $x$-parity can populate the guided $2P_x$ mode. These results demonstrate how modal-basis engineering can shift part of the crosstalk-control burden from the trajectory to waveguide symmetry, supporting joint path--mode degrees of freedom in quantum photonic applications.

physics.optics↗

arXiv2026

Thermal Decoherence and Population Transfer of MeV Channeling Electrons in Diamond

Channeling radiation from MeV-regime electrons is governed by transitions between quantized transverse bound states, but experimental spectra are strongly modified by thermal diffuse scattering. To capture these open-system dynamics, a frozen-phonon multislice framework is combined with bound-state projection analysis to construct depth-dependent reduced density matrices in selected transverse manifolds. Beyond reproducing experimental channeling-radiation transition energies, this approach separates thermal population transfer, intra-manifold decoherence, and cross-manifold coherence loss. Applied to 16.9 MeV axial electron channeling in $\langle100\rangle$ diamond, the results show approximately exponential population decay from the initially occupied states, accompanied by strongly channel-dependent feeding among low-lying manifolds. Starting from a coherent superposition within the degenerate 2p manifold, stochastic symmetry breaking by thermal displacements drives the intra-manifold purity toward the maximally mixed limit, indicating rapid phase scrambling. Under 1s initialization, population transferred into the 2p and 3d manifolds remains internally close to maximally mixed, yet a weak residual 2p-3d cross-manifold coherence persists. This framework goes beyond static mean-field thermal broadening and provides a microscopic basis for evaluating population dynamics and coherence lifetimes in strongly quantized channeling-radiation systems.

cond-mat.mtrl-sci↗

arXiv2018

Decay of high-energy electron bound states in crystals

High-energy electrons that are used as a probe of specimens in transmission electron microscopy exhibit a complex and rich behavior due to multiple scattering. Among other things, understanding the dynamical effects is needed for a quantitative analysis of atomic-resolution images and spectroscopic data. In this study, state-correlation functions are computed within the multislice approach that allow to elucidate behaviors of transversely bound states in crystals. These states play an important role as a large fraction of current density can be coupled into them via focused electron probes. We show that bound states are generically unstable and decay monoexponentially with crystal depth. Their attenuation is accompanied by a resonant intensity transfer to Bessel-like wavefunctions that appear as Laue rings in the far-field diffraction patterns. Behaviors of bound states are also quantified when thermal effects are included, as well as point defects. This approach helps to bridge the Bloch wave and multisliced electron propagation pictures of dynamical scattering providing new insights into fundamental solutions of the wave equation, and may assist in developing quantitative STEM/TEM imaging techniques.

physics.comp-ph↗