arXiv subjects

Saikat Ghosh

Publications and source records attributed to Saikat Ghosh.

At least 19 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↗

arXiv2026

Investigation of Thick-GEM detectors fabricated in India for muography application

Muography, commonly known as muon tomography, is a passive, non-destructive imaging technique that utilizes naturally occurring cosmic-ray muons to visualize the internal density structures of large, static, or inaccessible objects. In course of developing a small prototype muography system for material identification that relies upon the multiple Coulomb scattering of muons in matter, we explored the possible use of Thick-GEM detector as muon tracking device. It is a 5-20 fold scaled-up version of traditional GEM technology, that has become increasingly popular in recent years, owing to its mechanical robustness, cost-effective production, and excellent position sensing capabilities. A few prototypes of this detector of dimension $40\,\mathrm{mm} \times 48\,\mathrm{mm}$ with variation in other design parameters, were manufactured from a local industry. Subsequent to conditioning, detailed characterization of the detectors was performed to validate their suitability in muography applications. To identify the optimal operating region, gain variation was studied under various voltage configurations for both single and double-stage configurations. Experimental measurement of muon detection efficiency across the entire operating range yielded a maximum efficiency of 99.5\% in both cases. Using a collimated Fe$^{55}$-source, the best spatial resolution was determined to be 30 $\mu$m for both single and double-stage operation.

physics.ins-det↗

arXiv2026

Cavity-Driven Multispectral Gain for High-Sensitivity NV Center Magnetometers

We report a cavity-enabled solid-state magnetometer based on an NV ensemble coupled with a dielectric cavity, achieving 12 pT/$\sqrt{\rm{Hz}}$ sensitivity and a nearly threefold gain from multispectral features. The features originate from cavity-induced splitting of the NV hyperfine levels and leverages robust quantum coherence in the doubly dressed states of the system to achieve high sensitivity. We project simulated near-term sensitivities approaching 100 fT/$\sqrt{\rm{Hz}}$, close to the Johnson-Nyquist limit. Our results establish frequency multiplexing as a new operational paradigm, offering a robust and scalable quantum resource for metrology under ambient conditions.

quant-ph↗

arXiv2025

Solving tricky quantum optics problems with assistance from (artificial) intelligence

The capabilities of modern artificial intelligence (AI) as a ``scientific collaborator'' are explored by engaging it with three nuanced problems in quantum optics: state populations in optical pumping, resonant transitions between decaying states (the Burshtein effect), and degenerate mirrorless lasing. Through iterative dialogue, the authors observe that AI models--when prompted and corrected--can reason through complex scenarios, refine their answers, and provide expert-level guidance, closely resembling the interaction with an adept colleague. The findings highlight that AI democratizes access to sophisticated modeling and analysis, shifting the focus in scientific practice from technical mastery to the generation and testing of ideas, and reducing the time for completing research tasks from days to minutes.

quant-ph↗

arXiv2023

Coherence as an indicator to discern electromagnetically induced transparency and Autler-Townes splitting

Electromagnetically induced transparency (EIT) and Autler-Townes splitting (ATS) are generally characterized and distinguished by the width of the transparency created in the absorption profile of a weak probe in presence of a strong control field. This often leads to ambiguities, as both phenomena yield similar spectroscopic signature. However, an objective method based on Akaike's Information Criterion (AIC) test offers a quantitative way to discern the two regimes when applied on the probe absorption profile. The obtained transition value of control field strength was found to be higher than the value given by pole analysis of the corresponding off-diagonal density matrix element. By contrast, we apply the test on ground state coherence and the measured coherence quantifier, which yields a distinct transition point around the predicted value even in presence of noise. Our test accurately captures the transition between two regimes, indicating that a proper measure of coherence is essential for making such distinctions.

quant-ph↗

arXiv2023

Geologic Disposal Safety Assessment (GDSA) Biosphere Model Development

The Spent Fuel and Waste Science and Technology (SFWST) Campaign of the U.S. Department of Energy Office of Nuclear Energy (DOE-NE), Office of Spent Fuel and Waste Disposition is conducting research and development (R&D) on geologic disposal of spent nuclear fuel (SNF) and high-level nuclear waste (HLW). This work includes the Geologic Disposal Safety Assessment (GDSA) program which is charged with development of generic deep geologic repository concepts and system performance assessment models. One part of the GDSA framework is the development of a biosphere model capable of assessing dose to potential receptors exposed to radionuclides released via groundwater from geologic disposal sites. This work includes the development of a biosphere model capable of estimating doses to potential receptors living in the biosphere and exposed to radionuclides released from a hypothetical geologic disposal site. The GDSA biosphere model is being developed so that it is compatible with the GDSA framework including PFLOTRAN, the massively parallel subsurface flow and reactive transport code. PFLOTRAN is a subsurface flow and reactive transport code that solves a system of partial differential equations for multiphase flow and transport of components in porous materials such as shale formation. PFLOTRAN simultaneously simulates energy and mass flow with fluid properties as function of pressure and temperature through equations of state. PFLOTRAN also solves the mass conservation and transport equations of multicomponent formulations of aqueous chemical species, gases, and minerals reactive transport. It contains a waste form process model that simulates the radionuclide inventory under potential failures in a geological repository. PFLOTRAN can thus calculate the source term of radionuclides in ground water which can then be used as the GDSA biosphere model input.

physics.geo-ph↗

arXiv2023

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↗

arXiv2022

TALISMAN: Targeted Active Learning for Object Detection with Rare Classes and Slices using Submodular Mutual Information

Deep neural networks based object detectors have shown great success in a variety of domains like autonomous vehicles, biomedical imaging, etc. It is known that their success depends on a large amount of data from the domain of interest. While deep models often perform well in terms of overall accuracy, they often struggle in performance on rare yet critical data slices. For example, data slices like "motorcycle at night" or "bicycle at night" are often rare but very critical slices for self-driving applications and false negatives on such rare slices could result in ill-fated failures and accidents. Active learning (AL) is a well-known paradigm to incrementally and adaptively build training datasets with a human in the loop. However, current AL based acquisition functions are not well-equipped to tackle real-world datasets with rare slices, since they are based on uncertainty scores or global descriptors of the image. We propose TALISMAN, a novel framework for Targeted Active Learning or object detectIon with rare slices using Submodular MutuAl iNformation. Our method uses the submodular mutual information functions instantiated using features of the region of interest (RoI) to efficiently target and acquire data points with rare slices. We evaluate our framework on the standard PASCAL VOC07+12 and BDD100K, a real-world self-driving dataset. We observe that TALISMAN outperforms other methods by in terms of average precision on rare slices, and in terms of mAP.

cs.CV↗

arXiv2022

Active Data Discovery: Mining Unknown Data using Submodular Information Measures

Active Learning is a very common yet powerful framework for iteratively and adaptively sampling subsets of the unlabeled sets with a human in the loop with the goal of achieving labeling efficiency. Most real world datasets have imbalance either in classes and slices, and correspondingly, parts of the dataset are rare. As a result, there has been a lot of work in designing active learning approaches for mining these rare data instances. Most approaches assume access to a seed set of instances which contain these rare data instances. However, in the event of more extreme rareness, it is reasonable to assume that these rare data instances (either classes or slices) may not even be present in the seed labeled set, and a critical need for the active learning paradigm is to efficiently discover these rare data instances. In this work, we provide an active data discovery framework which can mine unknown data slices and classes efficiently using the submodular conditional gain and submodular conditional mutual information functions. We provide a general algorithmic framework which works in a number of scenarios including image classification and object detection and works with both rare classes and rare slices present in the unlabeled set. We show significant accuracy and labeling efficiency gains with our approach compared to existing state-of-the-art active learning approaches for actively discovering these rare classes and slices.

cs.CV↗

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↗

arXiv2020

Observation of quantum phase synchronization in spin-1 atoms

With growing interest in quantum technologies, possibilities of synchronizing quantum systems has garnered significant recent attention. In experiments with dilute ensemble of laser cooled spin-1 $^{87}{Rb}$ atoms, we observe phase difference of spin coherences to synchronize with phases of external classical fields. An initial limit-cycle state of a spin-1 atom localizes in phase space due to dark-state-polaritons generated by classical two-photon tone fields. In particular, when the two couplings fields are out of phase, the limit-cycle state synchronizes only with two artificially engineered, anisotropic decay rates. Furthermore, we observe a blockade of synchronization due to quantum interference and emergence of Arnold tongue-like features. Such anisotropic decay induced synchronization of spin-1 systems with no classical analogue can provide insights in open quantum systems and find applications in synchronized quantum networks.

quant-ph↗

arXiv2019

Terahertz modulated optical sideband generation in graphene

Exploration of optical non-linear response of graphene predominantly relies on ultra-short time domain measurements. Here we propose an alternate technique that uses frequency modulated continuous wavefront optical fields, thereby probing graphene's steady state non-linear response. We predict frequency sideband generation in the reflected field that originates from coherent electron dynamics of the photo-excited carriers. The corresponding threshold in input intensity for optimal sideband generation provides a direct measure of the third order optical non-linearity in graphene. Our formulation yields analytic forms for the generated sideband intensity, is applicable to generic two-band systems and suggests a range of applications that include switching of frequency sidebands using non-linear phase shifts and generation of frequency combs.

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↗

arXiv2018

Probing, Quantifying and Freezing Coherence in a Thermal Ensemble of Atoms

Creating stable superposed states of matter is one of the most intriguing aspects of quantum physics, leading to a variety of counter-intuitive scenarios along with a possibility of restructuring the way we understand, process and communicate information. Accordingly, there has been a major research thrust in understanding and quantifying such coherent superposed states. Here we propose and experimentally explore a quantifier that captures effective coherent superposition of states in an atomic ensemble at room-temperature. The quantifier provides a direct measure of ground state coherence for electromagnetically induced transparency (EIT) along with distinct signature of transition from EIT to Autler-Townes splitting (ATS) regime in the ensemble. Using the quantifier as an indicator, we further demonstrate a mechanism to coherently control and freeze coherence by introducing an active decay compensation channel. In the growing pursuit of quantum systems at room-temperature, our results provide a unique way to phenomenologically quantify and coherently control coherence in atom-like systems.

physics.atom-ph↗

arXiv2018

Nonlinear and anisotropic polarization rotation in two dimensional Dirac materials

We predict nonlinear optical polarization rotation in two dimensional massless Dirac systems including graphene and 8-$Pmmn$ borophene. When illuminated, a continuous wave optical field leads to a nonlinear steady state of photo-excited carriers in the medium. The photo-excited population inversion and the inter-band coherence gives rise to a finite transverse optical conductivity, $σ_{xy} (ω)$. This in turn leads to definitive signatures in associated Kerr and Faraday polarization rotation, which are measurable in a realistic experimental scenario.

cond-mat.mes-hall↗

arXiv2017

Nonlinear, anisotropic and giant photoconductivity in intrinsic and doped graphene

We present a framework to calculate the anisotropic and non-linear photoconductivity for two band systems with application to graphene. In contrast to the usual perturbative (second order in the optical field strength) techniques, we calculate photoconductivity to all orders in the optical field strength. In particular, for graphene, we find the photoresponse to be giant (at large optical field strengths) and anisotropic. The anisotropic photoresponse in graphene is correlated with polarization of the incident field, with the response being similar to that of a half-wave plate. We predict that the anisotropy in the simultaneous measurement of longitudinal ($σ_{xx}$) and transverse $(σ_{yx})$ photoconductivity, with four probes, offers a unique experimental signature of the photo-voltaic response, distinguishing it from the thermal-Seebeck and bolometric effects in photoresponse.

cond-mat.mes-hall↗

arXiv2016

Nonlinear optical conductivity of a generic two band systems, with application to doped and gapped graphene

We present a general formulation to calculate the dynamic optical conductivity, beyond the linear response regime, of any electronic system whose quasiparticle dispersion is described by a two band model. Our phenomenological model is based on the optical Bloch equations. In the steady state regime it yields an analytic solution for the population inversion and the interband coherence, which are nonlinear in the optical field intensity, including finite doping and temperature effects. We explicitly show that the optical nonlinearities are controlled by a single dimensionless parameter which is directly proportional to the incident field strength and inversely proportional to the optical frequency. This identification leads to a unified way to study the dynamical conductivity and the differential transmission spectrum across a wide range of optical frequencies, and optical field strength. We use our formalism to analytically calculate the nonlinear optical conductivity of doped and gapped graphene, deriving the well known universal ac conductivity of $σ_0={e^2}/4\hbar$ in the linear response regime of low optical intensities (or equivalently high frequencies) and non-linear deviations from it which appear at high laser intensities (or low frequencies) including the impact of finite doping and band-gap opening.

cond-mat.mes-hall↗