Ranjan Singh

• PhD
• Professor (Full) at University of Notre Dame

A toroidal dipole in metasurfaces provides an alternate approach for the excitation of high-Q resonances. In contrast to conventional multipoles, toroidal dipole interaction strength depends on the time derivative of the surrounding electric field. A characteristic feature of a toroidal dipole is tightly confined loops of oscillating magnetic field...

Radiative losses are crucial in optimizing the performance of metamaterial-based devices across the electromagnetic spectrum. Introducing structural asymmetry in meta-atom design leads to the excitation of sharp Fano resonances with reduced radiative losses. However, at larger asymmetries, the Fano resonance becomes highly radiative which results i...

Metasurfaces have provided a novel route to control the local phase of electromagnetic radiation through subwavelength scatterers where the properties of each element remain passive. A passive metasurface design can only achieve a specific functionality as it is extremely challenging to reconfigure each element that contributes toward the control o...

Fano resonances offer exciting features in enhancing the non-linearity and sensing capabilities in metamaterial systems. An active photoswitching of Fano resonances in a terahertz metadevice at low optical pump powers is demonstrated, which signifies the extreme sensitivity of the high-quality-factor resonant electric field to the external light il...

Realizing strong plasmon–vibration interactions between infrared-active vibrational bands and resonating plasmonic metasurfaces opens up the possibilities for ultrasensitive label-free detection of chemical and biological agents. The key prerequisites for exploiting strong plasmon–vibration interactions in practical spectroscopy are structures, whi...

Topological photonic waveguides for lossless transport through edge states have enabled the design of cavities with sharp bends in a closed‐loop configuration, where the confinement of photons in topological cavities has typically been controlled by varying the cavity size. Here, a slow light topological cavity is presented, where the light confine...

Photonic crystal line‐defect waveguides featuring bandgap confined low loss waveguiding are hindered by high bending losses. Although vortex‐mediated topological modes can facilitate seamless propagation through sharp bends, studies of topological transport have primarily focused on interfacial waveguides. In this work, we unveil the prevalence of...

On‐chip photonic resonant cavity plays a critical role in widespread applications including lasing, sensing, and spectroscopy. However, the excitation of these cavities typically relies on evanescent coupling within sub‐wavelength distances, limiting flexible and precise chip integration. Here, an on‐chip supercoupled topological cavity is demonstr...

Chip‐scale non‐reciprocity is essential for advancing integrated photonics, particularly in realizing photonic circulators and isolators for data communication, signal modulation, and quantum computing. However, achieving a non‐reciprocal silicon chip with a small footprint, high isolation ratio, low loss, and active control remains a challenge. He...

Programmable on‐chip terahertz (THz) topological photonic devices are poised to address the rising need for high‐capacity data systems, offering broad bandwidth, minimal loss, and reconfigurability. However, current THz topological chips rely on volatile tuning mechanisms that require continuous power to function. Here, a nonvolatile, programmable...

The magnetic behaviour of type-II superconductors is explained by a quantum vortex with a supercurrent encircling a coherence-length-sized core. In a superconducting film with a thickness of t < λL, the vortex field decays slowly as 1/r², extending to the Pearl length \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepacka...

Waveguide interconnect coupling control is essential for enhancing the chip density of photonic integrated circuits to incorporate a growing number of components. However, a critical engineering challenge is to achieve both strong waveguide isolation and efficient long‐range coupling on a single chip. Here, a novel photonic supercoupling phenomenon...

The phase singularity, a sudden phase change occurring at the reflection zero is widely explored using various nanophotonic systems such as metamaterials and thin film cavities. Typically, these systems exhibit a single reflection zero with a phase singularity at a specific incident angle, particularly at larger angles of incidence (>50 degrees). H...

Harnessing confined light on a subwavelength scale within Fano metasurfaces enhances light‐matter interaction on a flexible, low‐loss substrate. This approach enables the integration of ultra‐thin semiconducting films for designing low‐power, ultrafast switchable terahertz, and optical meta devices. Here, an ultra‐thin, ∼λ/6000 germanium overlayers...

Terahertz (THz) wireless communication holds immense potential to revolutionize future 6G to XG networks with high capacity, low latency and extensive connectivity. Efficient THz beamformers are essential for energy-efficient connections, compensating path loss, optimizing resource usage and enhancing spectral efficiency. However, current beamforme...

Photonic transport facilitated by topological protection is a proposed advantage of photonic topological waveguides based on valley photonic crystals (VPCs). Although topological protection significantly suppresses backscattering in these waveguides, it is often desirable to achieve active control over the transmission characteristics. We utilize p...

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 2D charge current through the inverse Rashba–Edelstein effect. Exploring and ass...

Dynamic terahertz devices are vital for the next generation of wireless communication, sensing, and non‐destructive imaging technologies. Metasurfaces have emerged as a paradigm‐shifting platform, offering varied functionalities, miniaturization, and simplified fabrication compared to their 3D counterparts. However, the presence of in‐plane mirror...

Collective spin arrangements manifest diverse spin textures, encompassing ferromagnetism, antiferromagnetism, and chiral vortices. However, mapping these spin textures in ultrathin magnetic multilayers has remained elusive. A reconfigurable chiral spintronic terahertz emission method is introduced through investigations on a model system of synthet...

Tunable thin-film coating-based reflective color displays have versatile applications including image sensors, camouflage devices, spatial light modulators, and intelligent windows. However, generating high-purity colors using such coatings have posed a challenge. Here, we reveal high-purity color generation using an ultralow-loss phase change mate...

Photonic diplexers are being widely investigated for high data transfer rates in on‐chip communication. However, dividing the available spectrum into nonoverlapping multicarrier frequency sub‐bands has remained a challenge in designing frequency‐selective time‐invariant channels. Here, an on‐chip topological diplexer is reported exhibiting terahert...

Topological slow light exhibits potential to achieve stopped light by virtue of its widely known robust and non-reciprocal behaviours. Conventional approach for achieving topological slow light often involves flat-band engineering without disentangling the underlying physical mechanism. Here, we unveil the presence of counter-propagating waves with...

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 topologica...

Thin-film coatings offer a scalable optical platform, as compared to nanopatterned films, for various applications including structural coloring, photovoltaics, and sensing. Recently, Fano resonant optical coatings (FROCs) have gained attention. FROCs consist of coupled thin film nanocavities composed of a broadband and a narrowband optical absorbe...

The sixth generation (6G) wireless network is envisioned to provide faster and more reliable communication by utilizing electromagnetic waves in the terahertz spectrum. However, terahertz waves suffer from high propagation loss and, hence, wavefront manipulation solutions to focus and redirect terahertz waves are of paramount importance. Reconfigur...

Surface enhanced Resonance Raman spectroscopy (SERRS) is a powerful technique for enhancing Raman spectra by matching the laser excitation wavelength with the plasmonic resonance and the absorption peak of biomolecules. Here, we propose a tunable Tamm plasmon polariton (TPP) cavity based on a metal on distributed Bragg reflector (DBR) as a scalable...

The topological phase revolutionized wave transport, enabling integrated photonic interconnects with sharp light bending on a chip. However, the persistent challenge of momentum mismatch during intermedium topological mode transitions due to material impedance inconsistency remains. We present a 100-Gbps topological wireless communication link usin...

Miniaturized photonic devices at the terahertz (THz) band are envisioned to bring significant enhancement to data transfer capacity and integration density for computing and future wireless communications. Broadband silicon waveguiding technology has continuously matured to advance low-loss platforms for integrated solutions. However, challenges ar...

Phase Change Materials (PCMs) have demonstrated tremendous potential as a platform for achieving diverse functionalities in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum, ranging from terahertz to visible frequencies. This comprehensive roadmap reviews the material and device aspects of PCMs, and their div...

Femtosecond laser‐induced photoexcitation of ferromagnet (FM)/heavy metal (HM) heterostructures has attracted attention by emitting broadband terahertz frequencies. The phenomenon relies on the formation of an ultrafast spin current, which is primarily attributed to the direct photoexcitation of the FM layer. However, during the process, the FM lay...

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. Expl...

Non-radiative bound states in the continuum (BICs) allow construction of resonant cavities with confined electromagnetic energy and high-quality (Q) factors. However, the sharp decay of the Q factor in the momentum space limits their usefulness for device applications. Here we demonstrate an approach to achieve sustainable ultrahigh Q factors by en...

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 development of terahertz integrated circuits is vital for realizing sixth-generation (6G) wireless communication, high-speed on-chip interconnects, high-resolution imaging, on-chip biosensors, and fingerprint chemical detection. Nonetheless, the existing terahertz on-chip devices suffer from reflection, and scattering losses at sharp bends or d...

The revolutionary 5G cellular systems represent a breakthrough in the communication network design to provide a single platform for enabling enhanced broadband communications, virtual reality, autonomous driving, and the internet of everything. However, the ongoing massive deployment of 5G networks has unveiled inherent limitations that have stimul...

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 $\xi$, 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...

Exponential growth in data rate demands has driven efforts to develop novel beamforming techniques for realizing massive multiple-input and multiple-output (MIMO) systems in sixth-generation (6G) terabits per second wireless communications. Existing beamforming techniques rely on conventional optimization algorithms that are too computationally exp...

Energy-efficient spintronic technology holds tremendous potential for the design of next-generation processors to operate at terahertz frequencies. Femtosecond photoexcitation of spintronic materials generates sub-picosecond spin currents and emission of terahertz radiation with broad bandwidth. However, terahertz spintronic emitters lack an active...

Realizing perfect light absorption in stacked thin films of dielectrics and metals through critical light coupling has recently received intensive research attention. In addition, realizing ultra‐thin perfect absorber and tunable perfect absorber in the visible spectrum is essential for novel optoelectronics applications. However, the existing thin...

Rapid scaling of semiconductor devices has led to an increase in the number of processor cores and integrated functionalities onto a single chip to support the growing demands of high‐speed and large‐volume consumer electronics. To meet this burgeoning demand require an improved interconnect capacities in terms of bandwidth density and active tunab...

A ferromagnetic metal consists of localized electrons and conduction electrons coupled through strong exchange interaction. Together, these localized electrons contribute to the magnetization of the system, while conduction electrons lead to the formation of spin and charge current. Femtosecond out of equilibrium photoexcitation of ferromagnetic th...

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 metamateri...

Identification of spectroscopically silent heavy‐metal ions in environmental and bio‐analytical samples has gained immense importance. Lowering the limit of detection of these ions at low concentrations is crucial for biomedicine and food safety applications. Optical chemical sensors have shown great potential in improving the sensing solutions for...

Nonradiating charge‐current configurations have attracted attention in photonics for the efficient localization of the electromagnetic field. Anapole mode is a unique nonradiating state of light induced by the interference of electric and toroidal dipole that possesses rich physics with potential applications in micro‐nanophotonics. Active control...

Tunable terahertz (THz) transmission filters are essential for various key applications including miniaturized spectrometers, hyperspectral imagers, and channel selectors in high‐speed wireless communication systems. However, tunable THz transmission filters have remained elusive, so far. Here, an electromechanically reconfigurable microcantilevers...

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 function...

The sixth generation (6G) communication standard is expected to include support for very-high data rates (over 100Gbit/s) and device electronics will require processors with on-chip communications able to support such high bandwidths. Although the terahertz band possesses ample bandwidth,conventional THz waveguides suffer from high bending losses a...

In article number 2100463, Ranjan Singh, Weili Zhang, Xiaojun Wu, and co‐workers successfully fabricate novel nonlinear THz metasurfaces with nanometer capacitive gaps and large size and experimentally realized with precise frequency tuning. The gigantic resonant electric field enhancement in nanogaps induces carrier multiplication in silicon subst...

A bound state in the continuum (BIC) is a non-radiating state of light embedded in the continuum of propagating modes providing drastic enhancement of the electromagnetic field and its localization at micro-nanoscale. However, access to such modes in the farfield requires symmetry breaking. Here, we demonstrate that a nanometric dielectric or semic...

Toroidal excitation manifests as currents flowing on the surface of a torus along its meridians that are known as poloidal currents. Several intriguing phenomena such as nonradiating anapole excitation, dynamic Aharonov–Bohm effect, and vector potential in the absence of electromagnetic fields occur in the presence of toroidal excitations. Toroidal...

The realization of electrically tunable plasmonic resonances in the ultraviolet (UV) to visible spectral band is particularly important for active nanophotonic device applications. However, the plasmonic resonances in the UV to visible wavelength range cannot be tuned due to the lack of tunable plasmonic materials. Here, we experimentally demonstra...

The change of the phase of light under the evolution of a nanomaterial with time is a promising new research direction. A phenomenon directly related to the sudden phase change of light is the Goos–Hänchen (G–H) shift, which describes the lateral beam displacement of the reflected light from the interface of two media when the angles of incidence a...

The Dirac semimetal cadmium arsenide (Cd3As2), a 3D electronic analog of graphene, has sparked renewed research interests for its novel topological phases and excellent optoelectronic properties. The gapless nature of its 3D electronic band facilitates strong optical nonlinearity and supports Dirac plasmons that are of particular interest to realiz...

Dissipationless and scattering‐free spin‐based terahertz electronics is the futuristic technology for energy‐efficient information processing. Femtosecond light pulse provides an ideal pathway for exciting the ferromagnet (FM) out‐of‐equilibrium, causing ultrafast demagnetization and superdiffusive spin transport at sub‐picosecond timescale, giving...

Phase change materials provide unique reconfigurable properties for photonic applications that mainly arise from their exotic characteristic to reversibly switch between the amorphous and crystalline nonvolatile phases. Optical pulse based reversible switching of nonvolatile phases is exploited in various nanophotonic devices. However, large area r...

Extreme terahertz (THz) science and technologies, the next disruptive frontier in nonlinear optics, provide multifaceted capabilities for exploring strong light‐matter interactions in a variety of physical systems. However, current techniques involve the need for an extremely high‐field free space THz source that is difficult to generate and has li...

Bismuth-based double perovskite Cs 2 AgBiBr 6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10 ⁻¹¹ cm ³ s...

Excitation of the non‐radiative eigenmodes in photonics and metamaterials above their light cone, that is, the bound states in the continuum (BICs), has drawn fundamental and technological interest. However, the current studies on photonic BICs are mainly restricted to the characterization of the eigenmodes with fixed point group. Here, a fourfold...

High speed inter-chip communications are required to operate despite multiple waveguide bends. Valley photonic crystal waveguides have a great potential in this area as bends can be implemented with near-zero losses. Here we demonstrate a 50 Gbit/s terahertz communication using QAM-16 modulation in the 300 GHz band. The experiment employed a uni-tr...

Active polarization switching devices are important to control the polarization state of light including terahertz (THz) waves that are technologically challenging to manipulate. Here, we propose and numerically demonstrate a hyperbolic metamaterial-based (HMM-based) active Brewster polarization switch for the intensity and phase modulation of THz...

Dielectric metasurfaces composed of subwavelength resonators are widely employed for manipulating electromagnetic waves over a broad frequency spectrum ranging from microwaves to optics. Here, a novel type of metasurfaces, created by a periodic lattice of elliptical holes fabricated in a thin dielectric membrane, is studied both theoretically and e...

The realization of integrated, low-cost and efficient solutions for high-speed, on-chip communication requires terahertz-frequency waveguides and has great potential for information and communication technologies, including sixth-generation (6G) wireless communication, terahertz integrated circuits, and interconnects for intrachip and interchip com...

The Josephson junction (JJ) is the corner stone of superconducting electronics and quantum information processing. While the technology for fabricating low Tc JJ is mature and delivers quantum circuits able to reach the “quantum supremacy”, the fabrication of reproducible and low-noise high-Tc JJ is still a challenge to be taken up. Here we report...

Next‐generation devices for low‐latency and seamless communication are envisioned to revolutionize information processing, which would directly impact human lives, technologies, and societies. The ever‐increasing demand for wireless data traffic can be fulfilled by the terahertz band, which has received tremendous attention as the final frontier of...

Confinement of electromagnetic radiation in a subwavelength cavity is an important platform for strong light–matter interaction as it enables efficient design of photonic switches, modulators, and ultrasensitive sensors. Metallic metasurfaces consist of an array of planar cavities that allow easy access to confined electromagnetic modes on the surf...

Spatiotemporal manipulation of electromagnetic waves has recently enabled a plethora of exotic optical functionalities, such as non‐reciprocity, dynamic wavefront control, unidirectional transmission, linear frequency conversion, and electromagnetic Doppler cloak. Here, an additional dimension is introduced for advanced manipulation of terahertz wa...

Recent breakthroughs in the field of terahertz science and technology have inspired novel explorations and innovations through variety of phenomena in spectroscopy, photonics, biomedical imaging, nonlinearity, communication technologies and many more. The editorial review on this special issue (see article number 1901984) provides the flavor on som...

In recent years, two-dimensional (2D) Ruddlesden-Popper perovskites have emerged as promising candidates for environmentally stable solar cells, highly efficient light-emitting diodes, and resistive memory devices. The remarkable existence of self-assembled quantum well (QW) structures in solution-processed 2D perovskites offers a diverse range of...

Planar metamaterials are extensively studied in recent years due to their potential applications in design of flat optical components, ultrasensitive sensors, lasing spasers, and nonlinear devices. Recent studies have reported dynamic control of photoactive material–based metamaterials through optical excitation. However, most of the previous demon...

We introduce Mie-resonant dielectric membrane metasurfaces for THz frequencies. We design silicon membrane metasurfaces with the 2π phase coverage and high transmission efficiencies to provide novel opportunities for an efficient wavefront control and multifunctional operations.