In the last decade, the concept of Floquet engineering and Floquet theory has penetrated the fields of condensed-matter and statistical physics. The standard Floquet theory is applicable to periodically driven “isolated” systems and can describe only a short-time evolution of such driven systems. However, if we focus on laser-driven systems in solids, the effect of environments around the systems is often inevitable and hence it is expected to affect the Floquet engineering. Recently, we have developed the theory for Floquet engineering in periodically driven “open (dissipative)” systems coupled to environments [1,2] within the formalism of quantum master equation. In this talk, I will shortly review our Floquet theory for dissipative systems. Then I apply it to the inverse Fraday effect in two-dimensional spin-orbit coupled electron models [3].
[1] T. N. Ikeda and MS, Sci. Adv. 6, eabb4019 (2020).
[2] T. N. Ikeda, K. Chinzei and MS, SciPost Phys. Core 4, 033 (2021).
[3] M. Tanaka and MS, Phys Rev. B110, 045204 (2024).

Time-periodic laser field can provide a route to manipulate quantum materials, called Floquet engineering [1]. It was theoretically predicted that when 3D Dirac semimetals are irradiated by circularly polarized light (CPL), the time-periodic field of the CPL acts as an effectively time-independent chiral gauge field which split the Dirac cone into the pair of Weyl cones [2,3]. In this talk, I will present our experimental research of the CPL-induced anomalous Hall conductivity (AHC) of Co3Sn2S2, which is a paramagnetic 3D Dirac semimetal at room temperature. By performing the mid-infrared (MIR) pump-terahertz (THz) Faraday rotation probe spectroscopy, we observed the instantaneous and helicity-dependent AHC. The field-strength and driving-frequency dependence of the AHC is well accounted for by CPL-induced band splitting in Co3Sn2S2 predicted by the Floquet theory, which demonstrates a promising route toward the realization of Floquet-Weyl states from massive Dirac semimetals [4].

[1] T. Oka and S. Kitamura, Annu. Rev. Condens. Matter. Phys. 10, 387 (2019).
[2] R. Wang et al., EPL 105, 17004 (2014).
[3] S. Ebihara et al., Phys. Rev. B 93, 155107 (2016).
[4] N. Yoshikawa et al., arXiv:2209.11932

I will present our recent and ongoing theoretical work on Floquet engineering topological phases by coherent spatiotemporal modulations. In particular, I will discuss the phases of graphene structures irradiated by lasers with twists in both space and time. First, we find [1] that the band flatness and gaps in twisted bilayer graphene near the magic angle can be tuned optically and that these gapped flat bands can carry nonzero Chern numbers. Second, we find [2] that driving a single layer of graphene with twisted circularly-polarized laser carrying orbital angular momentum generates multiple Floquet topological phases in the same sample with tunable real-space degenerate states at their interfaces, suitable for quantum state manipulations.

[1] Floquet-engineered topological flat bands in irradiated twisted bilayer graphene. Y. Li, H. A. Fertig and B. Seradjeh, Physical Review Research 2, 043275 (2020).
[2] Multiple tunable real-space degeneracies in graphene irradiated by twisted light. S. Aich and B. Seradjeh, Physical Review B 110, 054314 (2024).

Half-filled Mott insulators have fundamental insulating states of strongly correlated electron systems and are known to show large third-order optical nonlinearity and ultrafast metallization induced by a visible light or a strong electric field [1-9]. Recently, the electronic responses of those materials to an intense mid-infrared (MIR) pulse, which can be obtained as an electromagnetic-field pulse of several cycles, have been pursued. In this talk, I will discuss new types of optical nonlinearity investigated using an MIR pulse and a terahertz radiation in 1D half-filled Mott insulators and related materials focusing on the following topics.
・Observation of a photon-dressed excitonic Floquet state in 1D Mott insulators [10,11].
・Phase controllable terahertz radiation using large third-order optical nonlinearity in 1D Mott insulators [12].
・Observation of a phonon-dressed Floquet state in 1D dimerized Mott insulators [13].
・Destabilization of a spin-Peierls phase via phonon-dressed Floquet states: a Floquet engineering [14].
This work has been done in collaboration with T. Miyamoto, R. Ikeda, T. Yamakawa, D. Sakai, N. Sono, N. Takamura, and T. Otaki.

References
[1] H. Kishida et al., Nature 405, 929 (2000).
[2] H. Okamoto et al., Phys. Rev Lett. 98, 037401 (2007).
[3] H. Okamoto et al., Phys. Rev B 83, 125102 (2011).
[4] H. Yamakawa et al., Nature Mater. 16, 1100 (2017).
[5] T. Miyamoto et al., Nature Commun. 9, 3948 (2018).
[6] T. Terashige et al., Science Adv. 5, eaav2187 (2019).
[7] T. Miyamoto et al., Commun. Phys. 2, 131 (2019).
[8] H. Yamakawa et al., Nature Commun. 12, 953 (2021).
[9] N. Takamura et al., Phys. Rev. B 107, 085147 (2023).
[10] T. Yamakawa et al., New J. Phys. 25, 093044 (2023).
[11] R. Ikeda et al., arXiv:2407.17759.
[12] T. Miyamoto et al., Nature Commun.14, 6229 (2023).
[13] N. Sono et al., Commun. Phys. 5, 72 (2022).
[14] D. Sakai et al., Commun. Phys. 7, 40 (2024)

When fermions are in equilibrium with a heat bath they occupy states according to the well-known Fermi-Dirac distribution. When the bath is very cold, this distribution displays a discontinuous jump defining the location of the Fermi surface. But how should fermions occupy states when they are driven away from equilibrium by a time dependent periodic force? would their occupation still have sharp jumps? or would the periodic drive simply heat them up and smear their fermi surfaces?
Recent investigations have revealed that the non-equilibrium steady states of periodically driven fermions can retain sharp fermi surfaces and remain much more quantum than previously anticipated. Interestingly, the non-equilibrium steady states of fermions can be very different in a grand-canonical setting where the system exchanges particles and energy with the bath (i.e. fermions coupled to a fermionic bath) and a canonical setting where the system only exchanges energy with the bath (i.e. fermions coupled to a bosonic bath). In the grand-canonical setting there is a non-equilibrium fermi-liquid-like steady state with an occupation that displays multiple jumps resembling a staircase shape, and therefore features multiple non-equilibrium fermi surfaces. In contrast, in the canonical setting there is a non-equilibrium non-fermi-liquid steady state where the occupation does not have jumps but rather multiple sharp kinks, which, remarkably, remain sharp even when the bath is at finite temperature. Some of the platforms and regimes to realize experimentally these states are readily accessible, and include ultra-clean and cold two-dimensional metallic systems such as Gallium arsenide hetero-structures or graphene irradiated with microwaves.

Investigating functional nonlinearities of antiferromagnets for spintronics requires driving spin systems into states far from equilibrium. It has been demonstrated that electric-field pulses, spanning from visible to terahertz frequencies, can rapidly switch between different spin states. In this study, we show that a multicycle terahertz magnetic-field pulse can induce non-thermal spin switching in antiferromagnets. When a strong pulse is applied to antiferromagnets, the magnetic order parameter is first displaced from the barrier between the two potential minima of the antiferromagnet and then crosses the barrier during the subsequent inertial motion in the opposite direction. Our analysis indicates that this initial motion is driven by a dynamic modification of the magnetic potential, which is further enhanced by coupling between the two magnon modes.

Floquet theory enables us to design quantum phases of matter with exotic properties, via nontrivial modulation to the effective static Hamiltonian due to the time-periodic driving. We extend the framework to correlated systems, and show that the d-wave superconductivity in the doped Hubbard model can be dynamically changed to the topological d+id one under the illumination of circularly-polarized light [1]. We demonstrate this by performing the strong coupling expansion combined with the Floquet theory, where the emergent many-body interaction breaks the with broken time-reversal symmetry modulates the pairing symmetry. We also numerically compute the time evolution of the gap function within the framework of the time-dependent Gutzwiller approximation, and show that the topological superconductivity is indeed induced over a wide range of driving frequencies [2].
[1] S. Kitamura and H. Aoki, Commun. Phys. 5, 174 (2022).
[2] T. Anan, T. Morimoto, and S. Kitamura, Commun. Phys. 7, 99 (2024).

We study Dirac fermions in the presence of a space-dependent chiral gauge field and thermodynamic gradients, establishing a connection to the inverse spin Hall effect. The chiral gauge field induces a chiral magnetic field, resulting in a surface Fermi arc state and a chiral Landau level state which, although is delocalized in the bulk, we show to be more robust against impurities. By applying chemical potential and temperature gradients, we achieve nonzero charge currents, with each gradient leading to distinct Fermi level dependencies, both of which have been observed in a recent experiment. Unlike the conventional mixed axial-gravitational anomaly, our currents require a noncollinear chiral magnetic field and thermodynamic gradient. We further derive low-energy transport formulas and demonstrate the importance of carefully treating the ultraviolet cutoff for understanding our lattice calculations.

Floquet engineering has emerged as a powerful tool for controlling quantum materials, offering new pathways to realize topological and correlated states of matter. In this talk, I explore Floquet-driven Dirac electrons under different dynamical regimes, focusing on propagating waves, AC-magnetic fields, and cavity QED analogs.
First, I discuss Dirac electrons in propagating waves, such as surface acoustic waves and plasmons, which introduce Floquet-Bloch states with unique band tilting effects [1]. The dynamics in this regime reveal intriguing Lorentz transformation properties, suggesting emergent Type-II Weyl points and near-adiabatic tunneling phenomena.
In the second part, I explore Dirac electrons under oscillating magnetic fields, where Floquet-Landau levels emerge, leading to a homodyne Hall effect [2]. This effect, tied to chiral anomaly physics, suggests a new dynamical quantum geometric response in driven Dirac systems.
Finally, I examine strong light-matter interactions in carbon nanotubes embedded in cavities, drawing parallels with 1+1D QED [3]. I discuss charge confinement mechanisms, exciton dynamics, and the possibility of a strongly correlated ground state, including scenarios of excitonic insulators or confinement phases.
By unifying these different approaches—Floquet physics, and QED-inspired effects—I propose new avenues for engineering nontrivial quantum states in condensed matter and driven systems.

[1] TO, arXiv:2407.21458
[2] Kitamura, TO, arXiv:2407.08115
[3] TO, J. Phys. A: 2022 (arXiv:1007.5393)

Weak localization is the quintessential quantum interference phenomenon that features prominently in disordered conductors. Since its discovery, the canonical diagnostic for electron weak localization has been the magnetoresistance, which works as a space-domain interferometry. In this talk, we propose an ultrafast diagnostic for electron weak localization based on the nonlinear optical response in the terahertz regime. Our analytical and numerical calculations reveal that, in orthogonal/symplectic class systems, two consecutive, phase coherent optical pulses generate an electric current echo that appears after the second pulse, and at a time equal to the pulse delay time. The current echo reflects the quantum interference between a self-intersecting electron path and its time reversal partner, and, therefore, provides a time-domain interferometry of weak localization. Our results can be potentially tested on disordered metal films by using terahertz two-dimensional coherent spectroscopy or ultrafast transport measurements.

This talk will discuss collective modes in quantum materials with charge-density-wave (CDW) phases driven by strong charge-lattice coupling. We focus on the non-magnetic chiral chain lattice system (TaSe4)2I, which undergoes an incommensurate CDW phase transition near 260 K. Notably, the coexistence of the CDW wavevector and Weyl fermions suggests the possibility of axion-like quasiparticles governed by E⋅B dynamics, though this remains debated [1,2]. Despite decades of research, key details about CDW modes in (TaSe4)2I remain elusive.
We present advancements in detecting collective modes using ultrafast spectroscopy, including massive phase [3] and composite amplitude modes [4]. These experiments provide new insights and pave the way for future investigations of axion-like quasiparticles. Our use of femtosecond light pulses highlights the critical role of ultrafast spectroscopy in uncovering complex electronic and lattice structures in quantum materials.

[1] J. Gooth et al., Nature 575, 315 (2019).
[2] A. A. Sinchenko et al., Appl. Phys. Lett. 120, 063102 (2022).
[3] S. Kim et al., Nat. Mater. 22, 429 (2023).
[4] Q.L. Nguyen, R. Duncan et al., Phys. Rev. Lett. 131, 076901 (2023).

Time-periodic light-field can dress the electronic states of quantum materials, providing a fascinating controlling knob for transient modifications of the electronic structure with light-induced emergent phenomena [1]. In this talk, I will present our recent experimental progress on the Floquet engineering of quantum materials using time- and angle-resolved photoemission spectroscopy (TrARPES). In particular, experimental progress from black phosphorus upon resonance pumping [2] and below-gap (non-resonance) pumping [3] will be presented, from which experiments insights about Floquet engineering will be discussed. I will also present more recent experimental results to demonstrate how the symmetry of the photon-electron hybrid system can also be manipulated by the pumping light-field [4].

[1] Changhao Bao et al., Nat. Rev. Phys. 4, 33 (2022)
[2] Shaohua Zhou et al., Nature 614, 75 (2023)
[3] Shaohua Zhou et al., Phys. Rev. Lett. 131, 11640 (2023)
[4] Changhua Bao et al., Nat. Commun. 15, 10535 (2024)

Berry curvature pervades modern solid state physics. In transport, it gives an anomalous velocity to electrons inside an electric field, giving rise to the anomalous and/or spin Hall effect. In nonlinear optics, it determines the interband transition probability for circularly polarized light, giving rise to the circular photogalvanic effect. These two phenomena meet in circularly polarized light-induced anomalous Hall effect (LI-AHE). On one hand, either spin-polarized carriers or Floquet-Bloch bands generated by light field acquire a non-vanishing Berry curvature, leading to a net anomalous velocity. Band mixing by the bias electric field, on the other hand, generates another effective Berry curvature which causes the field-induced circular photogalvanic effect (FI-CPGE). Both effects should contribute to the transverse current simultaneously, though most previous studies have focused only on the former mechanism.
We developed an experimental technique to resolve this competition. Using a terahertz electromagnetic pulse as a time-varying bias field, we successfully disentangled the transverse currents generated by light- and field-induced Berry curvatures. In Cd3As2, a Dirac semimetal, sign of the light-induced anomalous Hall conductivity clearly identified FI-CPGE as the dominant origin of LI-AHE, against the expectation of a Floquet-Weyl semimetal phase [1]. In GaAs, a prototypical semiconductor, we also observed a prominent manifestation of FI-CPGE showing a resonant enhancement near the band gap [2]. Our theoretical analysis showed that this enhancement effectively unveils the band degeneracy at the valence band top, revealing a possibility of detecting monopole charges accompanying band degeneracy points. Inspection of theory also showed that not only the field-induced Berry curvature but also energy shift associated with the shift vector contributes to FI-CPGE, underlining the geometrical aspect of this phenomenon. Our results show universality of FI-CPGE and suggest Berry curvature engineering using terahertz electric field, a possible counterpart to Floquet engineering.

[1] Y. Murotani et al., Phys. Rev. Lett. 131, 096901 (2023).
[2] T. Fujimoto et al., arXiv:2411.00528 [cond-mat.mtrl-sci] (2024).

One of the hallmarks of light-spin interaction in solids is the appearance of photocurrent that depends on the light helicity. Recent studies have shown that spin orbit interaction plays an essential role in setting the helicity dependent photocurrent. The effect is particularly evident in systems with strong spin orbit interaction and inversion symmetry breaking. For example, helicity dependent photocurrent has been found in topological insulators and semiconductors that include heavy elements, both of which possess spin-momentum locked states.
We have studied helicity dependent photocurrent in Dirac semimetals[1-3]. Here we use bismuth thin films as a prototype Dirac semimetal. As the crystal structure of bismuth possesses inversion symmetry, the system lacks spin-momentum locked states. Nevertheless we find clear signatures of helicity dependent photocurrent. A continuous wave (cw) semiconductor laser in the visible range is used as a light source. The photocurrent increases with increasing bismuth layer thickness and tends to saturate at a length scale significantly larger than the light penetration length. Interestingly, the photocurrent changes its direction as the Fermi level moves across the Dirac point. Based on these results, we infer that the photocurrent is caused by light induced spin current and the inverse spin Hall effect of bismuth. The mechanism of how such light induced spin current emerges is not identified. In the presentation, we discuss possible scenario together with experimental results.

References
[1] H. Hirose et al., Circular photogalvanic effect in Cu/Bi bilayers, Appl. Phys. Lett. 113 222404 (2018).
[2] Y. Hirai et al., Terahertz Emission from Bismuth Thin Films Induced by Excitation with Circularly Polarized Light, Phys. Rev. Appl. 14 064015 (2020).
[3] H. Hirose et al., Interface-enhanced helicity dependent photocurrent in metal/semimetal bilayers, Phys. Rev. B 103 174437 (2021).

The break-down of a Mott-insulator when subjected to intense laser fields is characterized by the formation of elementary charge excitations known as doublon-hole pairs. This break-down is furthermore evidenced by the production of high harmonics that can be experimentally measured. We investigate coupling a metal to the Mott insulator in an interface environment to study the effect this had on the production of these doublon-hole pairs and the resultant high harmonics at the interface. We find that interfacial coupling of a metal to the Mott insulator enhances high harmonic production inside the insulator and suggest an increase in the doublon-hole correlation length as the physical mechanism behind this lowered threshold for dielectric break-down.

Moving vortices and in-core states in type-II superconductors have long attracted interest because they are considered to cause energy dissipation and current-induced quenching in superconductors. Recently, it has been found that terahertz (THz) nonlinear spectroscopy of superconducting films can offer new insights into moving vortices, such as their inertial mass [1]. We observed terahertz second harmonic generation (THz-SHG) induced by asymmetric vortex motion under supercurrent injection in both a dirty-limit superconductor, NbN [1], and a clean superconductor, Fe(Se,Te) [2]. Contrary to expectations, the cleanliness of the superconductor has little effect on the ultrafast vortex motion, suggesting that the vortex core can move independently, leaving the in-core quasiparticles behind.
[1] Sachiko Nakamura, Kota Katsumi, Hirotaka Terai, and Ryo Shimano, Phys. Rev. Lett. 125, 097004 (2020).
[2] Sachiko Nakamura, Haruki Matsumoto, Hiroki Ogawa, Tomoki Kobayashi, Fuyuki Nabeshima, Atsutaka Maeda, and Ryo Shimano, Phys. Rev. Lett. 133, 036004 (2024).

Steady illumination of a non-centrosymmetric semiconductor results in a bulk photovoltaic current, which is contributed by real-space displacements ('shifts') of charged quasiparticles as they transit between Bloch states. The shift induced by interband excitation via absorption of photons has received the prevailing attention. However, this excitation-induced shift can be far outweighed by the shifts induced by intraband relaxation and interband recombination, owing to: (i) an anomalous shift of quasiparticles as they scatter with phonons, as well as to (ii) topological singularities of the interband Berry phase. Both (i) and (ii) make the photocurrent extraordinarily sensitive to the frequency and polarization of the light source, and potentially lead to large non-linear conductivities of order mA/V^2 without experimental fine-tuning. A case study of BiTeI showcases these effects in a realistic material.

Optical waveguides leveraging photonic topological edge states have gathered significant attention due to their capability to enable robust light guiding. Among the structures that realize such topological waveguides, valley photonic crystals (VPhCs) stand out as a promising platform because of their easy realization. VPhC waveguides demonstrate immunity to certain types of structural disorders and sharp waveguide bends, rendering them appealing for semiconductor integrated photonics. In this presentation, we will provide a brief introduction to VPhC waveguides, followed by an exploration of their application as slow-light waveguides. Unlike conventional slow-light waveguides, chish suffer from considerable bending losses, VPhC slow-light waveguides exhibit substantial suppression of such losses. We also introduce VPhC heterostructure waveguides. These heterostructures enable the realization of single-mode semiconductor waveguides with a wide core region, which cannot be implemented in conventional semiconductor waveguides.

In solid state physics, the notion of geometry and topology for Bloch electrons in solids has provided a new insight into the light-matter interaction in this decade. One such example is the bulk photovoltaic effect called “shift current” that arises from a geometrical (Berry) phase of a Bloch wave function and has a close relationship to the modern theory of electric polarization [1]. While most previous studies of the bulk photovoltaic effects have focused on the photocurrent due to the above band gap excitations of noninteracting electrons, systems of interacting electrons have a potential to support a novel nonlinear functionality. In this talk, I will present novel bulk photovoltaic effects induced by shift current responses of quasiparticles in interacting electron systems, especially those below the electronic band gap. We present several examples of such generalized shift current originating from quasiparticle excitations, including excitons in semiconductors [2], magnetic excitations in multiferroic materials [3], and phonon excitations in electron-phonon coupled systems [4].

[1] T. Morimoto, and N. Nagaosa, Sci. Adv. 2, e1501524 (2016).
[2] T. Morimoto, and N. Nagaosa, Phys. Rev. B 94, 035117 (2016).
[3] T. Morimoto, and N. Nagaosa, Phys. Rev. B 100, 235138 (2019).; T. Morimoto, S. Kitamura, S. Okumura, Phys. Rev. B 104, 075139 (2021).
[4] T. Morimoto, and N. Nagaosa, Phys. Rev. B 110, 045129 (2024).

Elementary excitations in crystalline solids, such as phonons and magnons, are often observed through the light-matter interaction in the terahertz region far below the electronic band-gap, giving rise to the various optical phenomena. Since the spontaneous symmetry breaking, which are essentially important for the nonlinear optical effects, are generally associated with the low energy elementary excitations, the resonance of these excitations potentially show the dramatically enhanced optical effects. The photo-creation of elementary excitation has been demonstrated to show the photovoltaic effect through the extended shift current mechanism recently [1], although only the interband optical transition above the band gap has been believed to cause the bulk photovoltaic effect so far. The usual shift current mechanism is established to explain the bulk photovoltaic effect of interband transition, where the photocurrent is described by the difference of Berry connection of valence band and conduction band. The extended shift current mechanism, on the other hand, explains the existence of photocurrent generation through the photo-creation of elementary excitations without any charge carriers through coupling of electronic polarization and elementary excitations.
Here we demonstrate the bulk photovoltaic effect for terahertz light through the photo-creation of phonon in ferroelectrics and of electromagnon in multiferroics [1, 2]. In type-I multiferroics, the formation of a specific spin order leads to the breaking of space-inversion symmetry, leading to ferroelectricity and second order nonlinear optical effects including the bulk photovoltaic effect. The elementary excitation associated with the spin-driven ferroelectricity is electromagnon, collective spin excitation endowed with electric transition dipole, giving rise to the terahertz optical responses. Since the electronic band gap is as large as 1.6 eV, the terahertz photon (1 THz ~ 4.2 meV) cannot create the electron-hole pair. However, it is revealed that the creation of charge neutral electromagnon induces the dc photocurrent. The observed terahertz photocurrent is reasonably explained by the extended shift current model.

[1] Y. Okamura, et al., Proc. Natl. Acad. Sci. USA, 119 No. 14 e2122313119 (2022).
[2] M. Ogino, et al., Nature Communications 15: 4699 (2024).

Photocurrent generation via nonlinear light-matter interaction has recently garnered significant interest from both fundamental physics and applicational perspectives. A notable example is the shift current, a steady-state photocurrent that arises in noncentrosymmetric compounds. Unlike conventional photocurrent driven by drift and diffusion of photoexcited free carriers, shift current is induced by the change in the geometric phase upon the optical transition. This unique mechanism confers high robustness against carrier localization1 and ultrafast responsivity to pulsed light2. Since shift current does not rely on free carriers, it can also be generated by charge-neutral quasiparticles such as excitons, which typically do not contribute to electric current. While the exciton-induced shift current has been theoretically predicted3, direct experimental evidence for this phenomenon has remained elusive.
Recently, we have unambiguously demonstrated exciton-induced shift current in MBE-grown epitaxial thin films of cuprous iodide (CuI), a wide-gap semiconductor with a noncentrosymmetric zinc-blende structure4. We observe a significant enhancement of shift current responses at exciton resonance energies. It appears not only at the lowest exciton energies but also at higher Rydberg exciton states, providing more concrete evidence for the exciton shift current. First-principles calculations reveal that minute strain in the thin films critically affects both the sign and magnitude of the exciton shift current. Our findings highlight the importance of the exciton effect in understanding and enhancing the shift current response.
[1] H. Hatada, M. Nakamura et al., PNAS 117, 20411 (2020).
[2] M. Sotome, M. Nakamura. et al., PNAS 116, 1929 (2019).
[3] T. Morimoto and N. Nagaosa, Phys. Rev. B 94, 035117 (2016).
[4] M. Nakamura et al., Nat. Commun. 15, 9672 (2024).

Symmetries of two-dimensional van der Waals materials can be controlled by several physical methods such as thinning, making heterointerfaces, applying an external field such as an electric field, magnetic field, or strain etc. In such symmetry-controlled two-dimensional materials, novel transport/optical properties reflecting the symmetry breaking can emerge. In this presentation, I will report the nonlinear optical responses (second harmonic generation and spontaneous photovoltaic effect) in such symmetry-controlled two-dimensional materials.
Structural/magnetic transitions have been successfully detected by the second harmonic generation (SHG) in two-dimensional multiferroics [1]. In the magnetic phase, SHG shows the large enhancement under the magnetic field, reflecting the magnetoelectric effect (magnetic-field-induced electric polarization). Thickness dependent symmetries and resultant modulation of the magnetoelectric effect has been also studied.
In symmetry-controlled two-dimensional materials represented by WS2 nanotube, van der Waals heterointerfaces of two-dimensional materials with different symmetries, uniaxial strained 3R-MoS2, spontaneous photovoltaic effect reflecting the emergent polarization has been observed [2-5]. Observed characteristic behaviors of the photocurrent response including the directional dependences, polarization dependences, wavelength dependences can be explained by the microscopic mechanisms related with the quantum geometrical nature of the wave function (shift current and injection current).
These results indicate that nonlinear optical responses are powerful probe of exotic electronic/magnetic states in symmetry-controlled two-dimensional materials as well as novel functionalities in nanomaterials.
References:
[1] S. Aoki et al., Advanced Materials 36, 2312781 (2024).
[2] Y. J. Zhang et al. Nature 570 349-353 (2019).
[3] T. Akamatsu et al., Science 372, 68 (2021).
[4] Y. Dong et al., Nature Nanotechnology 18, 36 (2023).
[5] S. Duan et al., Nature Nanotechnology 18, 867 (2023).