arXiv subjects

Rajan Singh

Publications and source records attributed to Rajan Singh.

6 recordsLinked to original sources

arXiv2026

Tunable Fano Resonance and Frequency Locking in a Graphene-SiNx Hybrid Nanomechanical Resonator

Fano resonances, arising from the interference between discrete and continuum states, are observed across a wide range of quantum and classical systems. Here, we report the experimental observation of Fano resonances in a graphene SiNx hybrid nanomechanical system modeled as coupled oscillators. The broad, low quality factor graphene mode plays the role of the continuum, while the dense comb of sharp, high quality factor SiNx modes provides the discrete states. The inter-mode detuning is tunable via a DC gate voltage, enabling dynamic control of the Fano resonance: we demonstrate gate controlled switching of both the sign and the magnitude of the Fano asymmetry parameter $q$, in quantitative agreement with a coupled oscillator theory that predicts $q=-\cot\phi$, with $\phi$ the phase of the continuum response. At strong drive, the graphene mode enters the Duffing regime and its jump-down frequency locks to successive SiNx modes, producing a staircase of drive insensitive frequency plateaus; a weak seeding tone deterministically switches the resonator between adjacent locked states. The dense SiNx mode thus acts, in the linear regime, as the discrete states of a tunable Fano interferometer and, in the nonlinear regime, as a frequency ruler that stabilizes and quantizes the graphene oscillation. This platform offers a controllable mechanical realization of Fano interference and opens new avenues for high resolution hybrid resonant sensors and stable nanomechanical frequency references.

cond-mat.mes-hall↗

arXiv2026

Tunable Nonlinear Landscapes in Graphene Nanoelectromechanical Systems

Nonlinear nanomechanical resonators give convenient solid-state access to classical analogs of extreme nonlinear optics and to phononic signal processing. Here we report integer high-harmonic generation and phononic frequency combs in a suspended monolayer graphene drum. A gate voltage breaks the out-of-plane symmetry of the membrane and tunes its fundamental flexural mode onto a 1:2 internal resonance with a higher mode at twice the frequency, where the quadratic coupling between the two modes becomes large. A single drive tone then generates phase-locked integer harmonics in sequence, and at larger drive these fill in to a dense frequency comb. Raising the drive further, we find a reverse period-doubling transition: the comb spacing doubles, the line density halves, and energy flows back into the even-order comb lines. The measured spectra yield the quadratic ($\zeta$) and cubic ($\beta$) nonlinear coefficients of the membrane. These results show how the tunable nonlinear landscape of graphene supports distinct dynamical regimes on demand, allowing a single gated device to act in turn as a frequency multiplier, a broadband comb source, and a chaotic generator.

cond-mat.mes-hall↗

arXiv2025

Observation of tunable discrete time crystalline phases

Discrete time crystals (DTCs) are emergent non-equilibrium phases of periodically driven many-body systems, with potential applications ranging from quantum computing to sensing and metrology. There has been significant recent interest in understanding mechanisms leading to DTC formation and a search for novel DTC phases beyond subharmonic entrainment. Here, we report observation of multiple DTC phases in a nanoelectromechanical system (NEMS) based on coupled graphene and silicon nitride membranes. We confirm the time-crystalline nature of these symmetry broken phases by establishing their many-body characters, long-range time and spatial order, and rigidity against parameter fluctuation or noise. Furthermore, we employ controlled mechanical strain to drive the transitions between phases with different symmetries, thereby mapping the emergent time-crystalline phase diagram. Overall, our work takes a step towards establishing time crystals as a system with complexity rivaling that of solid state crystals.

cond-mat.mes-hall↗

arXiv2023

Polarization independent lattice-coupled terahertz toroidal excitations

The toroidal dipole excitation is an important field for metamaterial research because of their low-loss attribute. In this study, we demonstrate numerically and experimentally, a unique polarization independent terahertz metamaterial that modulates a broad resonance into a sharp mode by coupling the inherent toroidal dipole excitation to the lattice mode of the metasurface. The advantage of polarization independence in the metasurface enables the excitation of lattice-coupled toroidal mode for both the transverse electric and transverse magnetic modes of the incident terahertz radiation. The interaction of the two dark low-loss modes results in the significant enhancement of the quality factor of the metasurface at the point of resonance matching. Such a polarization independent lattice-matched toroidal excitation-based device has the potential to impact the development of low loss terahertz component for ultrasensitive sensors, low loss equipment, and slow light devices for enhanced light matter interaction.

physics.optics↗

arXiv2020

Giant Tunable Mechanical Nonlinearity in Graphene-Silicon Nitride Hybrid Resonator

High quality factor mechanical resonators have shown great promise in developing classical or quantum technologies. Simultaneously, progress has been made in developing controlled mechanical nonlinearity. Here we combine these two directions of progress in a single platform consisting of coupled Silicon Nitride (SiNx) and graphene mechanical resonators. We show that nonlinear response can be induced on a large area SiNx resonator mode and can be efficiently controlled by coupling it to a gate-tunable, freely suspended graphene mode. The induced nonlinear response of the hybrid modes, as measured on the SiNx resonator surface is giant, with one of the highest measured Duffing constants. We observe a novel phononic frequency comb which we use as an alternate validation of the measured values, along with numerical simulations which are in overall agreement with measurements.

cond-mat.mes-hall↗

arXiv2018

Motion transduction with thermo-mechanically squeezed graphene resonator modes

There is a recent surge of interest in amplification and detection of tiny motion in the growing field of opto and electro mechanics. Here, we demonstrate widely tunable, broad bandwidth and high gain all-mechanical motion amplifiers based on graphene/Silicon Nitride (SiNx) hybrids. In these devices, a tiny motion of a large-area SiNx membrane is transduced to a much larger motion in a graphene drum resonator coupled to SiNx. Furthermore, the thermal noise of graphene is reduced (squeezed) through parametric tension modulation. The parameters of the amplifier are measured by photothermally actuating SiNx and interferometrically detecting graphene displacement. We obtain displacement power gain of 38 dB and demonstrate 4.7 dB of squeezing resulting in a detection sensitivity of 3.8 fm per square root Hz, close to the thermal noise limit of SiNx.

cond-mat.mes-hall↗