arXiv subjects

Pawan Ratra

Publications and source records attributed to Pawan Ratra.

4 recordsLinked to original sources

arXiv2026

Ultrafast programmable Bragg reflection in photonic integrated circuits

Distributed Bragg reflectors (DBRs) are foundational building blocks of classical and quantum photonic technologies. However, their optical responses are typically fixed upon fabrication, limiting circuit robustness, reconfigurability, and functionality in applications from high-speed communications to quantum computing. Here, we demonstrate photonic chip-based programmable DBRs at telecommunications wavelengths, which are formed by electro-optically inducing refractive index contrast between periodic ferroelectric domains in thin-film lithium niobate waveguides. We achieve voltage-controlled Bragg reflection from zero to near-unity, and gigahertz-speed reflectivity modulation. Our results bring DBRs into the ultrafast programmable regime, opening new opportunities in topological photonics, cavity quantum electrodynamics, integrated lasers, and optical interconnects. The interplay between nanoscale ferroelectric domain engineering and strong electro-optic nonlinearity establishes a new design strategy for nanophotonic devices, otherwise inaccessible in bulk media.

physics.optics↗

arXiv2026

Raman suppression in nanophotonics enabled by multimode spectral filtering

Miniaturized photonic cavities generating nonlinear optical states of light are central to telecommunications and metrology applications. The emergence of such states is primarily underpinned by the ubiquitous Kerr nonlinearity that is present in all media. However, stimulated Raman scattering (SRS), an additional process inherent to many materials, has been shown to critically hinder the states' formation, imposing fundamental constraints on the choice of photonic platforms. Here, we introduce a novel strategy for the suppression of SRS in nanophotonic devices, adaptable to diverse Raman spectral responses. This is achieved by controlling the coupling and loss among multiple transverse spatial modes of the system, tailored across ultrabroad spectral bandwidths. Specifically, we combine nanometrically-corrugated Bragg gratings and tapered waveguides that, together enable co-directional multimode coupling and mode-selective filtering. We use lithium niobate as an exemplary Raman-active material to realize the concept, and we demonstrate the robust generation of two distinct Kerr nonlinear states (corresponding to coherent optical frequency combs) using the fabricated devices. The simplicity and generality of the concept suggest wide applicability to classical and quantum light generation on many technologically-relevant platforms nominally plagued by SRS (e.g., silicon and diamond photonics). More broadly, our multimode spectral shaping and filtering concept opens a path forward for highly-structured, wavelength-specific losses in nanophotonic waveguides and cavities, with potential applications in ultrafast and nonlinear integrated photonics.

physics.optics↗

arXiv2024

Lasing on hybridized soliton frequency combs

Coupling is an essential mechanism that drives complexity in natural systems, transforming single, non-interacting elements into intricate networks with rich physical properties. Here, we demonstrate a chip-scale coupled laser system that exhibits complex optical states impossible to achieve in an uncoupled system. We show that a pair of coupled semiconductor ring lasers spontaneously forms a frequency comb consisting of the hybridized modes of its coupled cavity, exhibiting a large number of phase-locked tones that anticross with one another. Experimental coherent waveform reconstruction reveals that the hybridized frequency comb manifests itself as pairs of bright and dark picosecond-long solitons circulating simultaneously. The dark and bright solitons exit the coupled cavity at the same time, leading to breathing bright solitons temporally overlapped with their dark soliton counterparts - a state inaccessible for a single, free-running laser. Our results demonstrate that the rules that govern allowable states of light can be broken by simply coupling elements together, paving the way for the design of more complex networks of coupled on-chip lasers.

physics.optics↗

arXiv2024

Driven bright solitons on a mid-infrared laser chip

Despite the ongoing progress in integrated optical frequency comb technology, compact sources of short bright pulses in the mid-infrared wavelength range from 3 {\mu}m to 12 {\mu}m so far remained beyond reach. The state-of-the-art ultrafast pulse emitters in the mid-infrared are complex, bulky, and inefficient systems based on the downconversion of near-infrared or visible pulsed laser sources. Here we show a purely DC-driven semiconductor laser chip that generates one picosecond solitons at the center wavelength of 8.3 {\mu}m at GHz repetition rates. The soliton generation scheme is akin to that of passive nonlinear Kerr resonators. It relies on a fast bistability in active nonlinear laser resonators, unlike traditional passive mode-locking which relies on saturable absorbers or active mode-locking by gain modulation in semiconductor lasers. Monolithic integration of all components - drive laser, active ring resonator, coupler, and pump filter - enables turnkey generation of bright solitons that remain robust for hours of continuous operation without active stabilization. Such devices can be readily produced at industrial laser foundries using standard fabrication protocols. Our work unifies the physics of active and passive microresonator frequency combs, while simultaneously establishing a technology for nonlinear integrated photonics in the mid-infrared.

physics.optics↗