Mishra, Sobhan Subhra, Darren Yao Lim Chin, Aolong Li, Ietro Teng Chen Pang, Thomas CaiWei Tan, Baolong Zhang, Yogesh Kumar Srivastava, Faxian Xiu, Ranjan Singh "Ultrafast active 3D Dirac terahertz metadevice" APL Engineering Physics 1, 026101 (2026), Featured Article
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Cadmium arsenide (Cd3As2), a three-dimensional Dirac semimetal and analog of graphene, exhibits ultrafast optical modulation of its metallic and dielectric properties. Benefiting from the linear band dispersion and ultrahigh electronic mobility, Cd3As2 supports ultrafast carrier dynamics and tunable terahertz (THz) electromagnetic responses. In this work, ultrathin (130 nm) Cd3As2 films are patterned with a periodic hole array to engineer geometric resonances and realize an optically switchable ultrafast THz metadevice. Optical pump–THz probe spectroscopy reveals that small pump fluence induces significant changes in conductivity, enabling the emergence and active tuning of resonance features on picosecond timescales. The device demonstrates ultrafast carrier relaxation time of 25 ps, low power operation, and all-optical THz modulation with a switching speed of up to 40 GHz. These results highlight the advantages of Cd3As2 for next-generation THz optoelectronic devices and open pathways for scalable, high-speed metadevices based on topological 3D Dirac materials.
Mishra, Sobhan Subhra, Thomas CaiWei Tan, Faxian Xiu, and Ranjan Singh. "Electrically Tunable Terahertz Chirality from Quantum Geometry" arXiv:2604.27294 (2026) (Submitted).
Quantum geometry encoded in the momentum space structure of electronic wavefunctions, governs charge dynamics through Berry curvature, enabling unconventional transport and optical responses. In topological semimetals, this geometry is sampled over Fermi pockets, suggesting electrical control by Fermi surface tuning, yet such control has remained largely limited to DC transport. Here we show that electrostatic gating of the 3D Dirac semimetal Cd3As2 reshapes Fermi pockets surrounding photoinduced Floquet Weyl nodes, enabling electrical control of terahertz (THz) emission chirality. Gate tuning selectively modulates the Berry curvature driven linearly polarized THz component by up to 60% and 49% at positive and negative bias, respectively, while the orthogonal linearly polarized photon-drag component remains unchanged. With the two orthogonal fields intrinsically phase-locked at \sfrac{\pi}{2} by the excitation geometry, the selective gate-tuned amplitude control enables the polarization tuning across the Poincaré sphere, achieving near-circular polarization (\chi\approx-42°) at +10 V. These results establish Fermi surface tuning as a general route to programmable quantum geometric control of chiral terahertz emission.
Mishra, Sobhan Subhra, Ranjan Singh. "Non-Equilibrium Orbital Transport in Terahertz Optorbitronics." arXiv:2604.26717 (2026). (Submitted)
Modern information technologies rely on controlling the flow of electrons through their charge and spin. A rapidly emerging alternative is to use the orbital motion of electrons, the way they circulate around atomic sites as a new carrier of information. This orbital angular momentum (OAM) could enable more energy-efficient devices and reduce reliance on scarce heavy elements, but how orbital currents are generated and transported, especially on ultrafast timescales, remains largely unknown. In this review, we introduce terahertz optorbitronics, an approach that uses ultrafast femtosecond laser pulses and terahertz radiation to observe orbital transport in real time. On timescales of quadrillionth of a second, this technique allows us to track how orbital currents are launched, propagate, and convert into electrical signals in nanoscale thin-film materials. Surprisingly, recent experiments have revealed conflicting pictures such as orbital currents may travel over tens of nanometres like ballistic waves or instead decay within just a few atomic layers, highlighting a fundamental unresolved question in the field. We explain how these ultrafast measurements can disentangle orbital motion from conventional spin transport, and we highlight new materials from engineered graphene to altermagnets, that could act as tunable sources of orbital currents. We also discuss how light, electrical gating, strain, and interface design can be used to actively control orbital transport and improve its conversion into usable electronic signals. By revealing orbital transport as a dynamic, non-equilibrium process, terahertz optorbitronics opens a new direction for nanoscale science, the one that could lead to faster, more efficient technologies operating beyond the limits of conventional spin-based electronics.
Jana, Sambhu, Sobhan Subhra Mishra, James Lourembam, Ranjan Singh. "Ultrafast Control of Néel Vector in Collinear Antiferromagnet MnPt." Adv. Sci.13, no. 4 (2026) (2026).
Exchange-bias-coupled spintronic terahertz emitters (EBC-STEs) utilize an antiferromagnet (AFM) as both a passive pinning layer for ferromagnetic spins and a detector of spin currents. Conventionally, terahertz (THz) emission from spintronic terahertz emitters (STEs) is attributed solely to the ferromagnetic subsystem, with the ultrafast magnetization dynamics of AFM considered undetectable due to its zero net magnetic moment. Here, we demonstrate a novel experimental approach to probe and control ultrafast magnetization dynamics driven by laser-induced optical torque on the Néel vector in collinear AFM MnPt. The Néel vector induced transient magnetization is isolated from ferromagnetic magnetization dynamics by detecting the distinct THz emission from the canted magnetic moment, as confirmed in Pt/MnPt bilayers. We further investigate the stability of the EBC-STE compared to conventional STEs under optical excitation. This work provides direct evidence of ultrafast AFM magnetization dynamics in an EBC-STE, transforming it from a passive emitter into a sensitive probe of interfacial ultrafast magnetism and unlocking the potential of antiferromagnetic THz spintronics.
Jana, Sambhu, Baolong Zhang, Sobhan Subhra Mishra, Thomas CaiWei Tan, Apoorva Chaturvedi, Edwin Hang Tong Teo & Ranjan Singh. "Ferron-driven photoferroic hysteresis in van der Waals CuInP2S6." Nat Commun (2026). https://doi.org/10.1038/s41467-026-73771-0 .
Controlling ferroelectric polarization with light promises a frontier for ultrafast optoelectronics. Yet the fundamental mechanism by which transient photoexcitation produces persistent photocurrent retention has remained elusive. Here we demonstrate a quasiparticle state that mediates photoferroic retention in the van der Waals ferroelectric CuInP2S6. We identify ferrons, collective excitations of dipole-carrying lattice vibrations, as carriers of optically imprinted polarization states. Narrowband resonant ferron oscillations at 2.16 THz exhibit a butterfly-shaped hysteresis that mirrors the photocurrent hysteresis response, directly linking ferron dynamics to polarization-dependent photocurrent retention at room temperature. Above-bandgap photoexcitation drives ferron formation through displacive excitation of coherent lattice motion, enabling carrier-mediated reconfiguration of ferroelectric domains, while sub-bandgap excitation leaves the polarization unaltered. The analytical calculations quantitatively capture the symmetry-dependent photocurrent generation and the consequence of the ferron-mediated hysteresis. These results establish ferronics, the manipulation of ferroelectric order through ferron quasiparticles, as a distinct paradigm for optically controlled polarization and photocurrent retention in low-dimensional quantum materials.
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