arXiv subjects

Abhishek Shiva Kumar

Publications and source records attributed to Abhishek Shiva Kumar.

4 recordsLinked to original sources

arXiv2026

Alignment-free ultra-broadband parametric frequency conversion in lead-halide perovskites

Lead-halide perovskites were demonstrated to exhibit some of the largest known optical nonlinearities, yet their potential for frequency conversion remains largely untapped. Here we demonstrate ultra-broadband four-wave mixing of near- and mid-infrared femtosecond pulses in thick single-crystal LHPs, generating bright, coherent, and highly collimated emission across an exceptionally wide continuous tuning range without phase-matching engineering, angular alignment, or dispersion optimization. Time resolved measurements reveal that the emission originates near the crystal surfaces, where phase-matching constraints are relaxed, while the unusually large intrinsic $χ^{(3)}$ response preserves efficient and directional frequency conversion despite the strongly localized interaction volume. These results position LHPs as a powerful bulk platform for ultra-broadband nonlinear photonics, opening a pathway toward compact, alignment-free architectures for ultrafast frequency conversion.

cond-mat.mtrl-sci↗

arXiv2025

Massive Dirac-Pauli physics in lead-halide perovskites

In standard quantum electrodynamics (QED), the so-called non-minimal (Pauli) coupling is suppressed for elementary particles and has no physical implications. Here, we show that the Pauli term naturally appears in a known family of Dirac materials -- the lead-halide perovskites, suggesting a novel playground for the study of analogue QED effects. We outline measurable manifestations of the Pauli term in the phenomena pertaining to (i) relativistic corrections to bound states (ii) the Klein paradox, and (iii) spin effects in scattering. In particular, we demonstrate that (a) the binding energy of an electron in the vicinity of a positively charged defect is noticeably decreased due to the polarizability of lead ions and the appearance of a Darwin-like term, (b) strong spin-orbit coupling due to the Pauli term affects the exciton states, and (c) scattering of an electron off an energy barrier with broken mirror symmetry produces spin polarization in the outgoing current. Our study adds to understanding of quantum phenomena in lead-halide perovskites, and paves the way for tabletop simulations of analogue Dirac-Pauli equations.

cond-mat.mes-hall↗

arXiv2023

Effective model for studying optical properties of lead-halide perovskites

We use general symmetry-based arguments to construct an effective model suitable for studying optical properties of lead-halide perovskites. To build the model, we identify an atomic-level interaction between electromagnetic fields and the spin degree of freedom that should be added to a minimally-coupled $\mathbf{k\cdot p}$ Hamiltonian. As a first application, we study two basic optical characteristics of the material: the Verdet constant and the refractive index. Beyond these linear characteristics of the material the model is suitable for calculating non-linear effects such as the third-order optical susceptibility. Analysis of this quantity shows that the geometrical properties of the spin-electric term imply isotropic optical response of the system, and that optical anisotropy of lead-halide perovskites is a manifestation of hopping of charge carriers. To illustrate this, we discuss third-harmonic generation.

cond-mat.mes-hall↗

arXiv2023

Spin-Electric Coupling in Lead Halide Perovskites

Lead-halide perovskites enjoy a number of remarkable optoelectronic properties. To explain their origin, it is necessary to study how electromagnetic fields interact with these systems. We address this problem here by studying two classical quantities: Faraday rotation and the complex refractive index in a paradigmatic perovskite CH$_3$NH$_3$PbBr$_3$ in a broad wavelength range. We find that the minimal coupling of electromagnetic fields to the k$\cdot$p Hamiltonian is insufficient to describe the observed data even on the qualitative level. To amend this, we demonstrate that there exists a relevant atomic-level coupling between electromagnetic fields and the spin degree of freedom. This spin-electric coupling allows for quantitative description of a number of previous as well as present experimental data. In particular, we use it here to show that the Faraday effect in lead-halide perovskites is dominated by the Zeeman splitting of the energy levels, and has a substantial beyond-Becquerel contribution. Finally, we present general symmetry-based phenomenological arguments that in the low-energy limit our effective model includes all basis coupling terms to the electromagnetic field in the linear order.

cond-mat.other↗