@article{21382,
  abstract     = {The exceptional energy-harvesting efficiency of lead-halide perovskites arises from unusually long photocarrier diffusion lengths and recombination lifetimes that persist even in defect-rich, solution-grown samples. Paradoxically, perovskites are also known for having very short exciton decay times. Here, we resolve this apparent contradiction by showing that key optoelectronic properties of perovskites can be explained by localized flexoelectric polarization confined to interfaces between domains of spontaneous strain. Using birefringence imaging, electrochemical staining, and zero-bias photocurrent measurements, we visualize the domain structure and directly probe the associated internal fields in nominally cubic single crystals of methylammonium lead bromide. We demonstrate that localized flexoelectric fields spatially separate electrons and holes to opposite sides of domain walls, exponentially suppressing recombination. Domain walls thus act as efficient mesoscopic transport channels for long-lived photocarriers, microscopically linking structural heterogeneity to charge transport and offering mechanistically informed design principles for perovskite solar-energy technologies.},
  author       = {Rak, Dmytro and Lorenc, Dusan and Balazs, Daniel and Zhumekenov, Ayan A. and Bakr, Osman M. and Alpichshev, Zhanybek},
  issn         = {2041-1723},
  journal      = {Nature Communications},
  publisher    = {Springer Nature},
  title        = {{Flexoelectric domain walls enable charge separation and transport in cubic perovskites}},
  doi          = {10.1038/s41467-026-68660-5},
  volume       = {17},
  year         = {2026},
}

@article{21469,
  abstract     = {Terahertz (THz) spectroscopy is a powerful probe of low-energy excitations in complex materials. Extending it into the nonlinear regime broadens its scope and can provide valuable insight into interactions among these modes. However, interpreting nonlinear spectra is challenging because resonant features in this case do not always reflect intrinsic material dynamics. Here, we study nonlinear THz-induced Kerr effect in a generic material LaAlO3. After detailed analysis of temporal oscillations of the Kerr signal, we identify an 𝐸𝑔 Raman mode at 1.1 THz excited through a two-photon process, while two additional peaks (0.86 and 0.36 THz) arise from phase matching of the near-infrared probe beam with co- and counterpropagating THz pump fields, mediated by off-resonant electronic hyperpolarizability. These results demonstrate the crucial role of kinematic effects in shaping THz-induced Kerr response and establish a framework for interpreting nonlinear spectroscopies in complex materials.},
  author       = {Shen, Chao and Frenzel, Maximilian and Maehrlein, Sebastian F. and Alpichshev, Zhanybek},
  issn         = {1079-7114},
  journal      = {Physical Review Letters},
  number       = {10},
  publisher    = {American Physical Society},
  title        = {{Disentangling electronic and ionic nonlinear polarization effects in bulk THz Kerr response}},
  doi          = {10.1103/1c5k-9z82},
  volume       = {136},
  year         = {2026},
}

@article{20405,
  abstract     = {Dielectric breakdown of physical vacuum (Schwinger effect) is the textbook demonstration of compatibility of Relativity and Quantum theory. Although observing this effect is still practically unachievable, its analogue generalizations have been shown to be more readily attainable. This paper demonstrates that a gapped Dirac semiconductor, methylammonium lead-bromide perovskite (MAPbBr3), exhibits analogue dynamic Schwinger effect. Tunneling ionization under deep subgap mid-infrared irradiation leads to intense photoluminescence in the visible range, in full agreement with quasi-adiabatic theory. In addition to revealing a gapped extended system suitable for studying the analogue Schwinger effect, this observation holds great potential for nonperturbative field sensing, i.e., sensing electric fields through nonperturbative light-matter interactions. First, this paper illustrates this by measuring the local deviation from the nominally cubic phase of a perovskite single crystal, which can be interpreted in terms of frozen-in fields. Next, it is shown that analogue dynamic Schwinger effect can be used for nonperturbative amplification of nonparametric upconversion process in perovskites driven simultaneously by multiple optical fields. This discovery demonstrates the potential for material response beyond perturbation theory in the tunneling regime, offering extremely sensitive light detection and amplification across an ultrabroad spectral range not accessible by conventional devices.},
  author       = {Lorenc, Dusan and Volosniev, Artem and Zhumekenov, Ayan A. and Lee, Seungho and Ibáñez, Maria and Bakr, Osman M. and Lemeshko, Mikhail and Alpichshev, Zhanybek},
  issn         = {2330-4022},
  journal      = {ACS Photonics},
  number       = {9},
  pages        = {5220--5230},
  publisher    = {American Chemical Society},
  title        = {{Observation of analogue dynamic Schwinger effect and non-perturbative light sensing in lead halide perovskites}},
  doi          = {10.1021/acsphotonics.5c01360},
  volume       = {12},
  year         = {2025},
}

@article{20705,
  abstract     = {Optical tweezers are widely used as a highly sensitive tool to measure forces on micron-scale particles. One such application is the measurement of the electric charge of a particle, which can be done with high precision in liquids, air, or vacuum. We experimentally investigate how the trapping laser itself can electrically charge such a particle, in our case a ∼1  μ⁢m SiO2 sphere in air. We model the charging mechanism as a two-photon process which reproduces the experimental data with high fidelity.},
  author       = {Stöllner, Andrea and Lenton, Isaac C and Volosniev, Artem and Millen, James and Shibuya, Renjiro and Ishii, Hisao and Rak, Dmytro and Alpichshev, Zhanybek and David, Grégory and Signorell, Ruth and Muller, Caroline J and Waitukaitis, Scott R},
  issn         = {1079-7114},
  journal      = {Physical Review Letters},
  number       = {21},
  publisher    = {American Physical Society},
  title        = {{Using optical tweezers to simultaneously trap, charge, and measure the charge of a microparticle in air}},
  doi          = {10.1103/5xd9-4tjj},
  volume       = {135},
  year         = {2025},
}

@article{19531,
  abstract     = {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 analog 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 the understanding of quantum phenomena in lead-halide perovskites and paves the way for tabletop simulations of analog Dirac-Pauli equations.},
  author       = {Shiva Kumar, Abhishek and Maslov, Mikhail and Lemeshko, Mikhail and Volosniev, Artem and Alpichshev, Zhanybek},
  issn         = {2397-4648},
  journal      = {npj Quantum Materials},
  publisher    = {Springer Nature},
  title        = {{Massive Dirac-Pauli physics in lead-halide perovskites}},
  doi          = {10.1038/s41535-025-00754-7},
  volume       = {10},
  year         = {2025},
}

@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{15406,
  abstract     = {We report on dynamic Shubnikov–de Haas (SdH) oscillations that are measured in the optical response, subterahertz transmittance of two-dimensional systems, and reveal two distinct types of oscillation nodes: “universal” nodes at integer ratios of radiation and cyclotron frequencies and “tunable” nodes at positions sensitive to all parameters of the structure. The nodes in both real and imaginary parts of the measured complex transmittance are analyzed using a dynamic version of the static Lifshitz-Kosevich formula. These results demonstrate that the node structure of the dynamic SdH oscillations provides an all-optical access to quantization- and interaction-induced renormalization effects, in addition to parameters one can obtain from the static SdH oscillations.},
  author       = {Savchenko, M. L. and Gospodarič, J. and Shuvaev, A. and Dmitriev, I. A. and Dziom, Vlad and Dobretsova, A. A. and Mikhailov, N. N. and Kvon, Z. D. and Pimenov, A.},
  issn         = {2643-1564},
  journal      = {Physical Review Research},
  number       = {2},
  publisher    = {American Physical Society},
  title        = {{Optical Shubnikov-de Haas oscillations in two-dimensional electron systems}},
  doi          = {10.1103/PhysRevResearch.6.L022027},
  volume       = {6},
  year         = {2024},
}

@article{17476,
  abstract     = {Lead halide perovskites have recently been reported to demonstrate an exceptionally high nonlinear (Kerr) refractive index n2 of up to 10−8cm2/W in CH3⁢NH3⁢PbBr3. Other researchers, however, observe different, substantially more conservative numbers. In order to resolve this disagreement, the nonlinear Kerr index of a bulk sample of lead halide perovskite was measured directly by means of an interferometer. This approach has many advantages as compared to the more standard z-scan technique. In particular, this method allows studying the induced changes to the refractive index in a time-resolved manner, thus enabling to separate the different contributions to 𝑛2. The extracted 𝑛2 values for CsPbBr3 and MAPbBr3 at 𝜆≈1µ⁢m are 𝑛2=+2.1×10−14cm2/W and 𝑛2=+6×10−15cm2/W, respectively. Hence, these values are substantially lower than what has been indicated in most of the previous reports, implying the latter one should be regarded with great care.},
  author       = {Lorenc, Dusan and Zhumekenov, Ayan and Bakr, Osman M. and Alpichshev, Zhanybek},
  issn         = {2475-9953},
  journal      = {Physical Review Materials},
  number       = {8},
  publisher    = {American Physical Society},
  title        = {{No extraordinary χ(3) in lead-halide perovskites: Placing an upper bound on Kerr nonlinearity by means of time-resolved interferometry}},
  doi          = {10.1103/PhysRevMaterials.8.085403},
  volume       = {8},
  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{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{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{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},
}