Publications

2025

1. Adv. Materials

   Monolithic Terahertz Topological Mach-Zehnder Interferometer

   Nikhil Navaratna, Yi Ji Tan, Wenhao Wang, and 1 more author

   Advanced Materials, 2025

   Abs DOI

   The pursuit of on-chip electromagnetic wave control for high throughput communication, spectroscopy, and quantum computing underlies the motivation for terahertz photonic integrated circuitry. Recent breakthroughs in topological photonics have enabled the development of chips that harness topologically resilient interface modes to achieve area and performance efficiency. However, the demonstration of a compact, monolithic topological Mach-Zehnder interferometer (MZI), remains a critical gap. In this work, a terahertz topological MZI is presented, exhibiting six interferometric fringes across a 17 GHz bandwidth, with splitter, combiner, and arms integrated on a single chip. The interferometric fringes exhibiting high on-off contrast with extinction ratios over 20 dB are achieved by utilizing tailored unit cells that facilitate interface-dependent out-of-plane radiation losses. Active tuning of the MZI response is also demonstrated through photoexcitation of the MZI arms. The presented approach is the first step toward realizing topological photonic modulators that leverage the phase degree of freedom for photonic integrated communication and quantum information processing.

2. Adv. Opt. Materials

   Topological Line Defect Waveguide

   Nikhil Navaratna, Yi Ji Tan, and Ranjan Singh

   Advanced Optical Materials, 2025

   Abs DOI

   Abstract 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 topological modes across diverse photonic crystal platforms by experimentally demonstrating topological transport in line-defect waveguides. Furthermore, this is achieved by employing interfacial waveguides as mode convertors to selectively excite latent topological states. Topological mode signatures, such as counterpropagating waves, are utilized to establish the existence of topological transport in line-defect waveguides designed using translational unit cells whose electromagnetic eigenstates exhibit phase vortices and non-zero Berry curvatures. It is envisioned that extending topological photonics beyond interfacial systems unlocks new avenues in applications ranging from non-Hermitian topological photonics to topological light-matter interactions.

3. Adv. Materials

   Photonic Supercoupling in Silicon Topological Waveguides

   Ridong Jia, Yi Ji Tan, Nikhil Navaratna, and 2 more authors

   Advanced Materials, 2025

   Abs DOI

   Abstract 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 is demonstrated for waveguide coupling over separation distances from a quarter to five wavelengths (λ), leveraging the tunable mode tails and the vortex energy flow in topological valley Hall system. A supercoupled integrated chip is developed, realizing a 91% coupling ratio and a −30 dB isolation over 2.8λ waveguide separations simultaneously. Supercoupled devices are further showcased including a waveguide-cavity system with 3.2λ excitation distance, and a waveguide directional supercoupler with a compact coupling area of nearly λ2/4, which outperform conventional devices. Supercoupling provides new degrees of freedom for optimizing coupling and isolation between photonic integrated components, facilitating new applications in on-chip sensing, lasing, and telecommunications.

2024

1. IMS 2024

   On-Chip Terahertz Topological Filter Antenna for 6G

   Sonu Kumar, Nikhil Navaratna, Arokiaswami Alphones, and 1 more author

   In 2024 IEEE/MTT-S International Microwave Symposium - IMS 2024, 2024

   DOI
2. APL

   Valley-Hall photonic crystal waveguides under non-Hermitian active defect

   Shrinivas Jayaram, Yi Ji Tan, Nikhil Navaratna, and 3 more authors

   Applied Physics Letters, Jul 2024

   Abs DOI

   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 photoexcited carriers in silicon to implement an active defect—a local, actively tunable, dissipative non-Hermitian perturbation in the path of a terahertz VPC waveguide—and systematically characterize the transport characteristics. We study waveguides constructed from different VPC interfaces (zigzag and bearded) and show that the high group index VPC waveguide modes are more strongly modulated by the phototunable defect. In both the waveguides, the faster modes exhibit approximately linear variation in transmission loss with increase in defect through enhanced photocarrier generation. However, for slower modes, the transmission loss varies nonlinearly, indicative of enhanced interaction with the active defect. We are able to model this behavior in terms of a group delay dependent loss. Our study not only highlights the superior performance of low index VPC waveguide modes but also paves the way for the systematic development of on-chip modulators based on active defects.

3. Nature Comm.

   Slow light topological photonics with counter-propagating waves and its active control on a chip

   Abhishek Kumar, Yi Ji Tan, Nikhil Navaratna, and 3 more authors

   Nature Communications, Jan 2024

   Abs DOI

   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 within valley kink states as the distinctive hallmark of the slow light topological photonic waveguides. These counter-propagating waves, supported by topological vortices along glide-symmetric interface, provide significant flexibility for controlling the slowness of light. We tune the group velocity of light by changing the spatial separation between vortices adjacent to the glide-symmetric interface. We also dynamically control the group delay by introducing a non-Hermitian defect using photoexcitation to adjust the relative strength of the counter-propagating waves. This study introduces active slow light topological photonic device on a silicon chip, opening new horizons for topological photon transport through defects, topological light-matter interactions, nonlinear topological photonics, and topological quantum photonics.

2023

1. APL

   327 Gbps THz silicon photonic interconnect with sub-λ bends

   Manoj Gupta, Nikhil Navaratna, Pascal Szriftgiser, and 2 more authors

   Applied Physics Letters, Oct 2023

   Abs DOI

   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 are faced in realizing compact waveguiding platforms with different degrees of bends due to bend induced losses and mode distortion. Here, we demonstrate multiple bend incorporated photonic crystal waveguide platforms for multicarrier on-chip transmission. Our silicon interconnect device exhibits optimized bending radius to the free space wavelength ratio of 0.74, without signal distortion and transmission bandwidth of 90 GHz, representing 25.4% fractional bandwidth at 355 GHz. The broadband waveguide interconnect enables an aggregate data transfer rate of 327 Gbps by sending the complex modulated data over multiple carriers. This work augments the development of THz photonic integrated circuit for the future generations of on-chip high data rate interconnect and wireless communication, ranging from the sixth to X generation (6G to XG).

2. APL

   On-chip topological THz biosensors

   Nikhil Navaratna, Yi Ji Tan, Abhishek Kumar, and 2 more authors

   Applied Physics Letters, Jul 2023

   Abs DOI

   On-chip terahertz (THz) biosensors have enormous potential in advancing the development of integrable devices for real-time, label-free, and noninvasive detection of proteins, DNA, and cancerous tissue. However, high absorption of THz waves by water necessitates evanescent field-based biosensing. The conventional on-chip THz biosensors with small mode confinement volumes and scaling sensitivity to defects severely limit the interaction of analyte with the electromagnetic field. Here, we reveal a topological waveguide cavity system with topologically protected propagating interfacial modes, exhibiting evanescent waves with an out-of-plane field extent of 0.3 λ 0, where λ 0 is the wavelength corresponding to the cavity resonance frequency. Our experiments involving biomolecule detection and leaf-hydration monitoring show that the near-field of high-Q topological cavity resonances accurately detects minute frequency shifts over extended periods, facilitating real-time sensing and monitoring of biological matter. Implementation of topologically protected evanescent fields in waveguide-cavity systems will enhance on-chip THz biosensing.

2021

1. JMMM

   Faraday rotation in ligand capped Fe3O4 nanoparticles – Experimental investigations for single scattering model

   Hemanth Kumar Narsetti, Waseem Ahmad Wani, Nikhil Navaratna, and 3 more authors

   Journal of Magnetism and Magnetic Materials, Jul 2021

   Abs DOI

   Magnetite (Fe3O4) nanoparticles capped with oleic acid and oleylamine were prepared by wet-chemical method. Faraday rotation on the synthesized Fe3O4 nanoparticles were measured as a function of magnetic field strength and concentration. The magnitude of the three components of the dielectric tensor of the Fe3O4 nanoparticle were obtained based on single-scattering model. These are Im∊xy=5.2×10-4,Im∊xx/yy=-0.07, and Im∊zz=-0.32. Besides, we found that Faraday rotation data obtained show two distinct features when compared to the data reported in the literature. The distinct features are 1) saturation in Faraday rotation with magnetic field strength, and 2) negligible concentration dependence. More importantly, our samples did not follow chain formation like other samples reported in the literature. These observations can be attributed to a stable colloid without any aggregation in our samples as a result of sample preparation method. In samples reported in the literature although they are stable these suffer from aggregation which can lead to the formation of chains easily than in our samples.