arXiv subjects

Ranjan Singh

Publications and source records attributed to Ranjan Singh.

At least 19 recordsLinked to original sources

arXiv2026

Cavity-enhanced superconductivity in the two-dimensional limit of NbSe2

Vacuum electromagnetic fluctuations have emerged as a means of controlling collective quantum phases without external driving. Cavity-induced modification of superconductivity has been widely predicted. What sets the size of the effect, and which microscopic channel carries it, remain open. Here we couple few-layer NbSe2 to a terahertz complementary split-ring resonator (CSRR) and show that the enhancement grows sharply on approaching the two-dimensional limit. In bilayer NbSe2 the superconducting transition temperature rises by 10%, from 3.02 K to 3.41 K, on a cavity resonant at 0.92 THz - roughly four times the shift measured in a ten-layer device at the same resonance. Within a single device the shift maps onto the simulated cavity field profile, falling from 0.39 K at the field maximum to zero outside the resonator, with the lower critical field following the same spatial ordering; because all regions are measured on one continuous flake in a single cooldown, sample-to-sample variation is excluded by construction. The frequency dependence is non-monotonic, with suppression below resonance and maximal enhancement near 0.96 THz. Quantum electrodynamical density functional theory calculations show that cavity coupling redistributes spectral weight in the Eliashberg function, weakening the total electron-phonon coupling while hardening the logarithmic average phonon frequency; competition between the two reproduces a sign change in Tc. These results identify dimensionality, local field amplitude and detuning as the control parameters of cavity-enhanced superconductivity, and point to electron-phonon reweighting as its microscopic origin.

cond-mat.supr-con↗

arXiv2026

Dispersion-Engineered Terahertz Silicon Interconnects Enabling Terabit-Scale Data Links

The rapid growth of artificial intelligence (AI) and data-centric computing is driving exabyte-scale data transfer, pushing conventional interconnect technologies toward fundamental bandwidth and energy limits. Although optical interconnects provide high-capacity and long-reach communication, their complexity and energy overhead limit scalability in short-reach chiplet-based and on-chip systems. Terahertz (THz) silicon interconnects offer a promising alternative by bridging electronics and photonics in compact, complementary metal-oxide-semiconductor (CMOS)-compatible platforms capable of high bandwidth and low latency. However, practical THz interconnects require simultaneous multi-band operation, dual-polarization support, low propagation loss, low group-velocity dispersion (GVD), and terabit-per-second throughput, while avoiding Bragg-induced stopbands and dispersion penalties at high frequencies. Here, we demonstrate a CMOS-compatible, centimetre-scale, multi-band on-chip THz data link achieving an aggregate throughput of 1.004 Tbps. The performance is enabled by suppressing Bragg-induced stopbands using dispersion-engineered, effective-medium-supported unclad silicon waveguides, resulting in flat transmission and low-ripple group delay across multiple THz bands. The waveguide platform operates from 220 to 500 GHz and supports both transverse-electric (TE) and transverse-magnetic (TM) polarizations with low path loss, low bending loss, and low GVD. Fourteen channels in a straight waveguide and twelve channels in a 90$^\circ$ bend achieve aggregate data rates of 1.004 Tbps and 0.895 Tbps, respectively, with GVD as low as 0.15 ps$^2$/mm over the full operating band. These results establish a scalable and energy-efficient THz interconnect platform for high-density on-chip and chip-to-chip communication fabrics targeting next-generation AI systems and emerging 6G technologies.

physics.optics↗

arXiv2026

Active Control of Topological Exceptional Points in Non-Hermitian Metasurfaces

Active control and ultrafast switching of non-Hermitian photonic systems are essential for next-generation reconfigurable optical technologies. Here, we demonstrate dynamic temporal manipulation of EPs in the terahertz (THz) regime using optically excited germanium (Ge) as an active medium. By exploiting pump-probe delay as a continuous tuning parameter, we achieve sub-picosecond eigenmode switching (~0.5 ps) and realize a complete time-resolved EP encirclement within ~2 ps, enabling direct observation of topological phase accumulation. At EP, the metasurface exhibits highly asymmetric transmission for circularly polarized light, characteristic of chiral mode response. Furthermore, we observe ultrafast eigenmode switching and topological phase evolution within ~1 ps, achieving >99% cross-polarization modulation depth. The measured results show strong agreement with theoretical modeling, with a high Petermann factor of approximately 10^3, confirming the effectiveness of the design. Our work establishes pump-probe delay time as a dynamical control parameter for EP topology, introducing a new regime of ultrafast non-Hermitian photonics for high-speed switching, enhanced sensitivity, and tunable polarization control in the THz domain.

physics.optics↗

arXiv2026

Electrically Tunable Terahertz Chirality from Quantum Geometry

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 \sfracπ{2} by the excitation geometry, the selective gate-tuned amplitude control enables the polarization tuning across the Poincaré sphere, achieving near-circular polarization (χ\approx-42°) at +10 V. These results establish Fermi surface tuning as a general route to programmable quantum geometric control of chiral terahertz emission.

cond-mat.mes-hall↗

arXiv2026

Non-Equilibrium Orbital Transport in Terahertz Optorbitronics

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.

cond-mat.mes-hall↗

arXiv2025

Complex Refractive Index Extraction for Spintronic Terahertz Emitter Analysis

Spintronic terahertz emitters (STEs) generate broadband terahertz (THz) radiation, which is essential for spectroscopy, imaging, and communication. The performances and the essential physical parameters of STE devices are linked to the dielectric properties of the constituent materials. Terahertz time-domain spectroscopy (THz-TDS) is an effective tool to measure these properties, but conventional analysis struggles with thin or complex multilayered systems due to simplifying approximations or complex transfer functions. In this work, we present a practical method to extract dielectric properties of STE multilayers using the Transfer Matrix Method (TMM). By comparing the THz pulse calculated using the Transfer Matrix Method (TMM) with the experimentally measured pulse transmitted through the sample, we can extract the dielectric properties of STEs, enhancing THz-TDS analysis and facilitating STE design and optimization. This method avoids constructing complex transfer functions, accommodates diverse sample geometries, and is designed to be accessible, with a publicly available codebase, making it a useful tool for STE research.

physics.optics↗

arXiv2025

Electric-field Control of Giant Ferronics

Ferrons are quantum excitations of electric polarization in ferroelectrics and electric analogues of magnons but have lacked direct experimental verification at room temperature. We harness the coupling of soft phonons and ferroelectric order in layered NbOX2 (X = I, Br, Cl) to generate, detect, and control giant ferrons, creating a new class of ultralow-power, chip-scale terahertz (THz) sources. Multiple ferron modes produce intense, narrowband THz emission with quality factors up to 228 and radiation efficiencies up to five orders of magnitude greater than state of the art semiconductor emitters. Resonant excitation of a high-Q ferron mode achieves efficiencies two orders of magnitude higher than intense lithium niobate THz sources. We further demonstrate direct, non-volatile electric-field control of ferron oscillations. These findings provide evidence for multiple ferrons and establish Ferronics as a foundational platform for light- and field-driven control of quantum order, with broad impact on ultrafast electronics, photonics, quantum technologies, and next-generation wireless communication.

cond-mat.mtrl-sci↗

arXiv2025

Electrically tunable Floquet Weyl photon emission from Dirac semimetal Cd3As2

The ability to optically engineer the Dirac band and electrically control the Fermi level in two-dimensional (2D) Dirac systems, such as graphene, has significantly advanced quantum technologies. However, similar tunability has remained elusive in three-dimensional Dirac systems. In this work, we demonstrate both optical and electrical tunability of the band structure in the 3D Dirac semimetal Cd3As2. Photoexcitation with circularly polarized light breaks time-reversal symmetry, lifting the degeneracy at Dirac points to transform the material into a Floquet Weyl semimetal with chiral Weyl nodes. This transition induces nonzero Berry curvature, giving rise to helicity-dependent transverse anomalous photocurrents, detectable through terahertz emission at normal incidence. Furthermore, applying an external electric field displaces the Fermi level away from the Dirac point, enlarging the Dirac cone projection leading to a reduced density of states of Fermi arcs. As a result, we achieve precise electrical control over Floquet band engineering, resulting in a large modulation of THz emission. Moreover, at oblique incidence, the circular photon-drag effect induces helicity-dependent longitudinal photocurrents. Simultaneous generation and manipulation of both longitudinal and transverse photocurrents enable precise control of the helicity of emitted THz pulses. These results pave the way for electric field-controlled Floquet Weyl THz sources, offering significant potential for applications in quantum computing and low-power electronics.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz Cavity Phonon Polaritons in Lead Telluride in the Deep-Strong Coupling Regime

Lead telluride is an important thermoelectric material due to its large Seebeck coefficient combined with its unusually low thermal conductivity that is related to the strong anharmonicity of phonons in this material. Here, we have studied the resonant and nonperturbative coupling of transverse optical phonons in lead telluride with cavity photons inside small-mode-volume metallic metasurface cavities that have photonic modes with terahertz frequencies. We observed a giant vacuum Rabi splitting on the order of the bare phonon and cavity frequencies. Through terahertz time-domain spectroscopy experiments, we systematically studied the vacuum Rabi splitting as a function of sample thickness, temperature, and cavity length. Under the strongest light-matter coupling conditions, the strength of coupling exceeded the bare phonon and cavity frequencies, putting the system into the deep-strong coupling regime. These results demonstrate that this uniquely tunable platform is promising for realizing and understanding predicted cavity-vacuum-induced ferroelectric instabilities and exploring applications of light-matter coupling in the ultrastrong and deep-strong coupling regimes in quantum technology.

physics.optics↗

arXiv2024

Photonic Supercoupling in Silicon Topological Waveguides

Electromagnetic wave coupling between photonic systems relies on the evanescent field typically confined within a single wavelength. Extending evanescent coupling distance requires low refractive index contrast and perfect momentum matching for achieving a large coupling ratio. Here, we report the discovery of photonic supercoupling in a topological valley Hall pair of waveguides, showing a substantial improvement in coupling efficiency across multiple wavelengths. Experimentally, we realize ultra-high coupling ratios between waveguides through valley-conserved vortex flow of electromagnetic energy, attaining 95% coupling efficiency for separations of up to three wavelengths. This demonstration of photonic supercoupling in topological systems significantly extends the coupling distance between on-chip waveguides and components, paving the path for the development of supercoupled photonic integrated devices, optical sensing, and telecommunications.

physics.optics↗

arXiv2024

Active Control of Ballistic Orbital Transport

Orbital current, defined as the orbital character of Bloch states in solids, can ballistically travel with larger coherence length through a broader range of materials than its spin counterpart, facilitating a robust, higher density and energy efficient information transmission. Hence, active control of orbital transport plays a pivotal role in propelling the progress of the evolving field of quantum information technology. Unlike spin angular momentum, orbital angular momentum (OAM), couples to phonon angular momentum (PAM) efficiently via orbital-crystal momentum (L-k) coupling, giving us the opportunity to control orbital transport through crystal field potential mediated angular momentum transfer. Here, leveraging the orbital dependant efficient L-k coupling, we have experimentally demonstrated the active control of orbital current velocity using THz emission spectroscopy. Our findings include the identification of a critical energy density required to overcome collisions in orbital transport, enabling a swifter flow of orbital current. The capability to actively control the ballistic orbital transport lays the groundwork for the development of ultrafast devices capable of efficiently transmitting information over extended distance.

cond-mat.mtrl-sci↗

arXiv2023

On-chip Amorphous THz Topological Photonic Interconnects

Valley Hall photonic crystals (VPCs) offer the potential to create topological waveguides capable of guiding light through sharp bends on a chip. They can seamlessly integrate with functional components while occupying minimal space, making them a promising technology for terahertz (THz) topological photonic integrated circuits. However, a significant limitation for THz-scale integrated VPC-based devices has been the absence of arbitrary bend interconnects. Due to the crystalline symmetry, the traditional VPC designs restrict waveguides to the principal lattice axes (i.e., only 0-, 60- or 120- degree orientations). Here, we present an on-chip, all silicon implementation of deformed VPCs enabling topological waveguides with a variety of shapes and bends. Although the lattice is amorphous and lacks long-range periodicity, the topological protection of the waveguides is sustained by short-range order. We experimentally demonstrate the robust on-chip transmission of THz waves through waveguides of complicated shapes and arbitrary bends. We implement an amorphous lattice that serves as a frequency-dependent router capable of splitting the input signal into two perpendicular output ports, which cannot be achieved with an undeformed VPC. In addition, we showcase on-chip THz communication through 90 degree and P-shaped VPC waveguides, achieving data rates of 72 Gbit/s and 32 Gbit/s, respectively. Our findings demonstrate that the amorphous topological photonic crystals significantly enhance the adaptability of on-chip interconnections while preserving the performance of the topological waveguides.

physics.optics↗

arXiv2023

Interfacial spintronic THz emission

The broken inversion symmetry at the ferromagnet (FM)/heavy-metal (HM) interface leads to spin-dependent degeneracy of the energy band, forming spin-polarized surface states. As a result, the interface serves as an effective medium for converting spin accumulation into two-dimensional charge current through the inverse Rashba-Edelstein effect. Exploring and assessing this spin-to-charge conversion (SCC) phenomenon at the FM/HM interface could offer a promising avenue to surpass the presumed limits of SCC in bulk HM layers. We utilize spintronic heterostructures as a platform to measure the spin-to-charge conversion (SCC) experienced by photoexcited spin currents. These heterostructures emit terahertz electric field when illuminated by femtosecond laser pulses, enabling us to quantitatively assess the ultrafast SCC process. Our results demonstrate a robust interfacial spin-to-charge conversion (iSCC) within a synthetic antiferromagnetic heterostructure, specifically for the NiFe/Ru/NiFe configuration, by isolating the SCC contribution originating from the interface itself, separate from the bulk heavy-metal (HM) region. Moreover, the iSCC at the NiFe/Ru interface is discovered to be approximately 27% of the strength observed in the highest spin-Hall conducting heavy-metal, Pt. Our results thus highlight the significance of interfacial engineering as a promising pathway for achieving efficient ultrafast spintronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Secondary spin current driven efficient THz spintronic emitters

Femtosecond laser-induced photoexcitation of ferromagnet (FM)/heavy metal (HM) heterostructures have attracted attention by emitting broadband terahertz frequencies. The phenomenon relies on the formation of ultrafast spin current, which is largely attributed to the direct photoexcitation of the FM layer. However, we reveal that during the process, the FM layer also experiences a secondary excitation led by the hot electrons from the HM layer that travel across the FM/HM interface and transfer additional energy in the FM. Thus, the generated secondary spins enhance the total spin current formation and lead to amplified spintronic terahertz emission. The results also emphasize the significance of the secondary spin current, which even exceeds the primary spin currents when FM/HM heterostructures with thicker HM are used. An analytical model is developed to provide deeper insights into the microscopic processes within the individual layers, underlining the generalized ultrafast superdiffusive spin-transport mechanism.

cond-mat.mtrl-sci↗

arXiv2022

The Elusive High-Tc Superinductor

Ginzburg-Landau (GL) parameters formed the basis for Abrikosov discovery of the quantum vortex of a supercurrent in type-II superconductor with a normal core of size $ξ$, the superconductor coherence length and circulating supercurrent induced magnetic field diverging as $log(1/r)$ from the core with a decay length of the London penetration depth, $λ_L.$ In 1964, J. Pearl predicted the slowly decaying $(1/r^2)$ field around a vortex spreading out to Pearl length, $P_L=2λ_L^2/t$, in a superconductor film of thickness $t < λ_L$. However, his quintessential theory failed to predict the existence of giant kinetic inductance (GKI) that arises from the enlarged screening currents of the vortex. Here, we discover giant kinetic inductance in a $high-T_c$ metasurface due to the 1400% expansion of the vortex screening supercurrent from $λ_L$ to 14$λ_L$ in ultrathin film meta-atom of $λ_L/7$ thickness, which leads to the emergence of terahertz superinductance possessing quantum impedance exceeding the resistance quantum limit of $R_Q=h/(2e)^2 =6.47 kΩ$ by 33%. Our discovery presents a new class of $high-T_c$ superconductor electronic, photonic, and quantum devices enabled through metasurface designed at the Pearl length scales, providing novel applications in quantum circuitry, metrology, and single photon kinetic inductance detectors.

cond-mat.supr-con↗

arXiv2022

Terahertz sensing of highly absorptive water-methanol mixtures with multiple resonances in metamaterials

Ultrasensitive terahertz sensing of highly absorptive aqueous solutions remains challenging due to strong absorption of water in the terahertz regime. Here, we experimentally demonstrate a cost-effective metamaterial-based sensor integrated with terahertz time-domain spectroscopy for highly absorptive water-methanol mixture sensing. This metamaterial has simple asymmetric wire structures that support multiple resonances including a fundamental Fano resonance and higher order dipolar resonance in the terahertz regime. Both the resonance modes have strong intensity in the transmission spectra which we exploit for detection of the highly absorptive water-methanol mixtures. The experimentally characterized sensitivities of the Fano and dipole resonances for the water-methanol mixtures are found to be 160 and 305 GHz/RIU, respectively. This method provides a route for readily available metamaterial-assisted terahertz spectroscopy for ultrasensitive sensing of highly absorptive chemical and biochemical materials with multiple resonances and high accuracy.

physics.optics↗

arXiv2022

Ultra-high terahertz index in deep subwavelength coupled bi-layer free-standing flexible metamaterials

We report extensive enhancement in the refractive index of artificially designed metamaterials by exploiting the deep subwavelength coupling in a free-standing, thin-film metal-dielectric-metal checkboard structure. A record high refractive index of 77.02+43.22i is obtained at terahertz frequencies. The detailed investigations reveal that the enhancement of the effective refractive index of the structure via deep subwavelength coupling in a bilayer design is governed by a power law, which is an effective and simpler approach to design high index metamaterial. The approach relies on deep subwavelength coupling to obtain extremely high refractive index values that can lead to many practical applications in the field of imaging, lithography, design of delay lines and interferometers.

physics.optics↗

arXiv2021

Temporal Loss Boundary Engineered Photonic Cavity

Losses are ubiquitous and unavoidable in nature inhibiting the performance of most optical processes. Manipulating losses to adjust the dissipation of photons is analogous to braking a running car that is as important as populating photons via a gain medium. Here, we introduce the transient loss boundary into a photon populated cavity that functions as a photon brake and probe photon dynamics by engineering the brake timing and brake strength. Coupled cavity photons can be distinguished by stripping one photonic mode through controlling the loss boundary, which enables the transition from a coupled to an uncoupled state. We interpret the transient boundary as a perturbation by considering both real and imaginary parts of permittivity, and the dynamic process is modelled with a temporal two-dipole oscillator, one with the natural resonant polarization and the other with a frequency-shift polarization. The model unravels the underlying mechanism of concomitant coherent spectral oscillations and generation of tone-tuning cavity photons in the braking process. By synthesizing the temporal loss boundary into a photon populated cavity, a plethora of interesting phenomena and applications are envisioned such as the observation of quantum squeezed states, low-loss nonreciprocal waveguides and ultrafast beam scanning devices.

physics.optics↗