@article{14886, abstract = {It is a basic principle that an effect cannot come before the cause. Dispersive relations that follow from this fundamental fact have proven to be an indispensable tool in physics and engineering. They are most powerful in the domain of linear response where they are known as Kramers-Kronig relations. However, when it comes to nonlinear phenomena the implications of causality are much less explored, apart from several notable exceptions. Here in this paper we demonstrate how to apply the dispersive formalism to analyze the ultrafast nonlinear response in the context of the paradigmatic nonlinear Kerr effect. We find that the requirement of causality introduces a noticeable effect even under assumption that Kerr effect is mediated by quasi-instantaneous off-resonant electronic hyperpolarizability. We confirm this by experimentally measuring the time-resolved Kerr dynamics in GaAs by means of a hybrid pump-probe Mach-Zehnder interferometer and demonstrate the presence of an intrinsic lagging between amplitude and phase responses as predicted by dispersive analysis. Our results describe a general property of the time-resolved nonlinear processes thereby highlighting the importance of accounting for dispersive effects in the nonlinear optical processes involving ultrashort pulses.}, author = {Lorenc, Dusan and Alpichshev, Zhanybek}, issn = {2643-1564}, journal = {Physical Review Research}, number = {1}, publisher = {American Physical Society}, title = {{Dispersive effects in ultrafast nonlinear phenomena: The case of optical Kerr effect}}, doi = {10.1103/PhysRevResearch.6.013042}, volume = {6}, year = {2024}, } @article{14342, abstract = {We propose a simple method to measure nonlinear Kerr refractive index in mid-infrared frequency range that avoids using sophisticated infrared detectors. Our approach is based on using a near-infrared probe beam which interacts with a mid-IR beam via wavelength-non-degenerate cross-phase modulation (XPM). By carefully measuring XPM-induced spectral modifications in the probe beam and comparing the experimental data with simulation results, we extract the value for the non-degenerate Kerr index. Finally, in order to obtain the value of degenerate mid-IR Kerr index, we use the well-established two-band formalism of Sheik-Bahae et al., which is shown to become particularly simple in the limit of low frequencies. The proposed technique is complementary to the conventional techniques, such as z-scan, and has the advantage of not requiring any mid-infrared detectors.}, author = {Lorenc, Dusan and Alpichshev, Zhanybek}, issn = {0003-6951}, journal = {Applied Physics Letters}, number = {9}, publisher = {AIP Publishing}, title = {{Mid-infrared Kerr index evaluation via cross-phase modulation with a near-infrared probe beam}}, doi = {10.1063/5.0161713}, volume = {123}, year = {2023}, } @article{13251, abstract = {A rotating organic cation and a dynamically disordered soft inorganic cage are the hallmark features of organic-inorganic lead-halide perovskites. Understanding the interplay between these two subsystems is a challenging problem, but it is this coupling that is widely conjectured to be responsible for the unique behavior of photocarriers in these materials. In this work, we use the fact that the polarizability of the organic cation strongly depends on the ambient electrostatic environment to put the molecule forward as a sensitive probe of the local crystal fields inside the lattice cell. We measure the average polarizability of the C/N–H bond stretching mode by means of infrared spectroscopy, which allows us to deduce the character of the motion of the cation molecule, find the magnitude of the local crystal field, and place an estimate on the strength of the hydrogen bond between the hydrogen and halide atoms. Our results pave the way for understanding electric fields in lead-halide perovskites using infrared bond spectroscopy.}, author = {Wei, Yujing and Volosniev, Artem and Lorenc, Dusan and Zhumekenov, Ayan A. and Bakr, Osman M. and Lemeshko, Mikhail and Alpichshev, Zhanybek}, issn = {1948-7185}, journal = {The Journal of Physical Chemistry Letters}, keywords = {General Materials Science, Physical and Theoretical Chemistry}, number = {27}, pages = {6309--6314}, publisher = {American Chemical Society}, title = {{Bond polarizability as a probe of local crystal fields in hybrid lead-halide perovskites}}, doi = {10.1021/acs.jpclett.3c01158}, volume = {14}, year = {2023}, } @article{12723, abstract = {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 CH3NH3PbBr3 in a broad wavelength range. We find that the minimal coupling of electromagnetic fields to the k⋅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.}, author = {Volosniev, Artem and Shiva Kumar, Abhishek and Lorenc, Dusan and Ashourishokri, Younes and Zhumekenov, Ayan A. and Bakr, Osman M. and Lemeshko, Mikhail and Alpichshev, Zhanybek}, issn = {1079-7114}, journal = {Physical Review Letters}, keywords = {General Physics and Astronomy}, number = {10}, publisher = {American Physical Society}, title = {{Spin-electric coupling in lead halide perovskites}}, doi = {10.1103/physrevlett.130.106901}, volume = {130}, year = {2023}, } @article{12724, abstract = {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 k⋅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 nonlinear 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.}, author = {Volosniev, Artem and Shiva Kumar, Abhishek and Lorenc, Dusan and Ashourishokri, Younes and Zhumekenov, Ayan and Bakr, Osman M. and Lemeshko, Mikhail and Alpichshev, Zhanybek}, issn = {2469-9969}, journal = {Physical Review B}, number = {12}, publisher = {American Physical Society}, title = {{Effective model for studying optical properties of lead halide perovskites}}, doi = {10.1103/physrevb.107.125201}, volume = {107}, year = {2023}, } @article{11737, abstract = {Spin-orbit coupling in thin HgTe quantum wells results in a relativistic-like electron band structure, making it a versatile solid state platform to observe and control nontrivial electrodynamic phenomena. Here we report an observation of universal terahertz (THz) transparency determined by fine-structure constant α≈1/137 in 6.5-nm-thick HgTe layer, close to the critical thickness separating phases with topologically different electronic band structure. Using THz spectroscopy in a magnetic field we obtain direct evidence of asymmetric spin splitting of the Dirac cone. This particle-hole asymmetry facilitates optical control of edge spin currents in the quantum wells.}, author = {Dziom, Uladzislau and Shuvaev, A. and Gospodarič, J. and Novik, E. G. and Dobretsova, A. A. and Mikhailov, N. N. and Kvon, Z. D. and Alpichshev, Zhanybek and Pimenov, A.}, issn = {2469-9969}, journal = {Physical Review B}, number = {4}, publisher = {American Physical Society}, title = {{Universal transparency and asymmetric spin splitting near the Dirac point in HgTe quantum wells}}, doi = {10.1103/PhysRevB.106.045302}, volume = {106}, year = {2022}, } @article{12278, abstract = {Mercury telluride (HgTe) thin films with a critical thickness of 6.5 nm are predicted to possess a gapless Dirac-like band structure. We report a comprehensive study on gated and optically doped samples by magnetooptical spectroscopy in the THz range. The quasi-classical analysis of the cyclotron resonance allowed the mapping of the band dispersion of Dirac charge carriers in a broad range of electron and hole doping. A smooth transition through the charge neutrality point between Dirac holes and electrons was observed. An additional peak coming from a second type of holes with an almost density-independent mass of around 0.04m0 was detected in the hole-doping range and attributed to an asymmetric spin splitting of the Dirac cone. Spectroscopic evidence for disorder-induced band energy fluctuations could not be detected in present cyclotron resonance experiments.}, author = {Shuvaev, Alexey and Dziom, Uladzislau and Gospodarič, Jan and Novik, Elena G. and Dobretsova, Alena A. and Mikhailov, Nikolay N. and Kvon, Ze Don and Pimenov, Andrei}, issn = {2079-4991}, journal = {Nanomaterials}, keywords = {General Materials Science, General Chemical Engineering}, number = {14}, publisher = {MDPI}, title = {{Band structure near the Dirac Point in HgTe quantum wells with critical thickness}}, doi = {10.3390/nano12142492}, volume = {12}, year = {2022}, }