arXiv subjects

Sudipta Saha

Publications and source records attributed to Sudipta Saha.

At least 19 recordsLinked to original sources

arXiv2026

Polarization-Selective Near-Perfect Absorption via Mie-Type Resonance in van der Waals Anisotropic ReS$_2$/$\alpha$-MoO$_3$/Au Heterostructure

We investigate polarization-selective absorption in a visible-wavelength heterostructure consisting of a ReS$_2$ stripe grating, an $\alpha$-MoO$_3$ spacer, and an Au back-reflector using finite-difference time-domain simulations. For an optimized geometry with a grating period of 500 nm, stripe width of 250 nm, and ReS$_2$ thickness of 80 nm, the structure exhibits near-unity absorption of 99.99\% at 650.5 nm under TE-polarized illumination. The resonant field is concentrated near the outer edges of the ReS$_2$ stripe, while absorption power density is localized in the same region, consistent with a localized edge mode. The absorption response depends strongly on polarization, producing a TE--TM resonance separation of 16.2 nm. Replacing either the biaxial ReS$_2$ layer or the anisotropic $\alpha$-MoO$_3$ spacer with isotropic equivalents substantially modifies the spectral response and reduces the polarization-dependent wavelength separation. In addition, rotating the crystal orientation of the ReS$_2$/$\alpha$-MoO$_3$ stack shifts both the resonance wavelength and peak absorption without changing the device geometry. The results show that the combination of anisotropic resonator and spacer layers provides an effective means of controlling resonant absorption and polarization selectivity in van der Waals photonic structures.

physics.optics↗

arXiv2026

Polarization-Multiplexed Spatial Differentiation and Filtering Driven by van der Waals Birefringence

We report that the biaxial birefringence of $\alpha$-MoO$_3$ can activate two spectrally distinct quasi-bound states in the continuum (quasi-BICs) within a single symmetric TiO$_2$ nanobar-pair metasurface, with each resonance governed by a different crystallographic axis pair of the van der Waals crystal. With a full 60\,nm $\alpha$-MoO$_3$ gap fill, a TE resonance at 883.9\,nm ($Q=92$, Fano $q=0.090$) and a TM resonance at 923.2\,nm ($Q=31$, Fano $q=0.393$) are obtained. The Q ratio follows the inverse-square permittivity contrast, $Q\propto(\Delta\varepsilon)^{-2}$, calibrated across both polarization channels. Oblique-incidence sweeps show that the TE channel acts as a dual-null spatial highpass filter with a broadband stopband ($|H|<0.13$ for $|k_x|\leq0.63\,\mu$m$^{-1}$, $T_\mathrm{bg}=0.962$), while the TM channel transfers as $|H|\propto|k_x|$ ($R^2=0.94$), consistent with first-order spatial differentiation. Both operations are verified on a USAF~1951 resolution chart processed in a simulated 4$f$ framework. Channel selection is purely by input polarization angle with no structural modification.

physics.optics↗

arXiv2026

van der Waals Crystal Anisotropy Controls Dual-Channel Refractive Index Sensing in a TiO$_{2}$/$\alpha$-MoO$_{3}$ Nanobar Metasurface

Filling the gap of a TiO$_2$ nanobar-pair metasurface with $\alpha$-MoO$_3$, a biaxial orthorhombic crystal, produces two high-$Q$ Fano resonances with asymmetric quality factors: $Q_{\mathrm{TE}}=87$ at 863.3 nm and $Q_{\mathrm{TM}}=31$ at 960.1 nm, separated by 97 nm. The same device with amorphous or crystalline Sb$_2$S$_3$, both isotropic, yields comparable quality factors in both channels, confirming that the $Q$-ratio asymmetry originates in the biaxial crystal symmetry of $\alpha$-MoO$_3$ rather than the index magnitude of the fill. The two inequivalent permittivity contrasts of the orthorhombic lattice ($\Delta\varepsilon_{\beta\gamma}=0.983$ for TE, $\Delta\varepsilon_{\alpha\gamma}=2.420$ for TM) place each channel at a different point on the $Q\propto(\Delta\varepsilon)^{-2}$ scaling curve, consistent with quasi-BIC mode character. The TE channel delivers sensitivity $S=155.3$ nm RIU$^{-1}$, figure of merit 15.71 RIU$^{-1}$, and limit of detection $6.44\times10^{-5}$ RIU. TM delivers $S=139.1$ nm RIU$^{-1}$, figure of merit 4.44 RIU$^{-1}$, and limit of detection $7.19\times10^{-5}$ RIU. Simultaneous readout produces a polarization fingerprint with isotropic slope 0.896, deviations from which encode analyte optical anisotropy.

physics.optics↗

arXiv2026

Single-Device VOC Fingerprinting via Polarization-Selective Anisotropic BeS-Clad Silicon Microring Resonator

A silicon microring resonator with an anisotropic beryllium sulfide (BeS) cladding is proposed for polarization-selective detection of exhaled-breath volatile organic compound biomarkers. The anisotropic dielectric response of BeS enables the transverse-electric (TE) and transverse-magnetic (TM) modes to probe orthogonal components of the cladding permittivity tensor, generating two independent optical observables from a single device. Five clinically relevant biomarkers are investigated: acetone, isoprene, 4-hydroxyhexenal, 2-propenal, and benzene. First-principles optical constants are incorporated into three-dimensional finite-difference time-domain simulations to evaluate the sensing response. The TE mode exhibits a uniform resonance shift of 0.263 nm across all analytes and serves as a concentration reference channel, while the TM mode produces analyte-specific shifts ranging from 0.200 to 0.426 nm. A unique TM amplitude inversion is observed for benzene, enabling additional discrimination. The resulting dual-polarization response forms a two-dimensional optical fingerprint that distinguishes all five biomarkers without requiring a sensor array or multiple functionalized resonators. The device achieves quality factors of 4520 and 3151 for the TE and TM modes, respectively, with sensitivities up to 6.5 nm/RIU, figures of merit up to 14.9 RIU^-1, and detection limits as low as 1.5 mRIU. Cross-sensitivity analysis further shows that CO2 and H2O produce negative TM resonance shifts, separating interferents from target biomarkers in the fingerprint plane. The proposed platform demonstrates a compact route toward array-free photonic breath analysis using intrinsic cladding anisotropy.

physics.optics↗

arXiv2026

Inverse Design Validated Optimization of Lead-Free Cs$_3$Cu$_2$Cl$_5$ Visible-Light Microring Resonators Using a Coupled DFT-FDTD Framework

Microring resonators (MRRs) are indispensable for wavelength filtering, sensing, and on-chip signal routing in photonic integrated circuits, yet visible-wavelength implementations using environmentally benign materials remain scarce. We report a numerical design study of add-drop MRRs employing Cs$_3$Cu$_2$Cl$_5$, a lead-free all-inorganic halide with favorable optical properties in the visible spectral range. Wavelength-resolved refractive index (n) and extinction coefficient (k) of Cs$_3$Cu$_2$Cl$_5$, calculated using density functional theory (DFT), are used as direct inputs to three-dimensional finite-difference time-domain (FDTD) simulations. Independent parametric sweeps are performed over ring waveguide width (500-900 nm), coupling gap (150-300 nm), and bend radius (5-20 um). At the balanced operating point of 600 nm ring width, 200 nm gap, and 10 um radius, the device achieves a loaded quality factor Q approx 5386, a free spectral range of 11.3 nm, a drop-port extinction ratio of 32.2 dB, and a finesse of 95.8. The coupling-gap sweep reveals the full transition from over-coupled through critically coupled to under-coupled operation, with the critical point occurring near 200 nm. A pronounced bending-loss threshold is observed between 5 and 10 um, below which all performance metrics degrade rapidly. These results provide the first systematic geometry-performance map for Cs$_3$Cu$_2$Cl$_5$ based microring resonators. Cross-platform validation using Tidy3D reproduces the spectral characteristics of the optimized device, and inverse design of the bus coupling region yields an additional 3 percent improvement in drop-port power transfer.

physics.optics↗

arXiv2026

Composition-Dependent Plasmon-Enhanced Emission in Lead-Free Cs$_3$Cu$_2$X$_5$ Halides: A DFT--FDTD Study

Lead-free Cs$_3$Cu$_2$X$_5$ (X = Cl, Br, I) halides exhibit high photoluminescence quantum yields and excellent ambient stability, yet light-emitting devices based on these materials remain limited by poor optical outcoupling. In this work, we develop an integrated density functional theory (DFT) and finite-difference time-domain (FDTD) framework to establish quantitative links between halide composition, wavelength-dependent optical constants, and plasmonic enhancement. First-principles calculations are used to obtain composition-specific refractive index (n) and extinction coefficient (k) spectra, which are directly implemented into three-dimensional FDTD simulations of a complete PeLED stack incorporating Ag/SiO$_2$ core--shell nanostructures. Among the investigated compositions, Cs$_3$Cu$_2$Cl$_5$ demonstrates the strongest plasmonic response, achieving a 4.4$\times$ Purcell enhancement and 30\% light extraction efficiency (LEE) using optimized nanorods. The superior performance originates from its lower refractive index, which reduces dielectric screening and improves near-field coupling. Cs$_3$Cu$_2$Br$_5$ exhibits the highest spectral overlap ($J_{\mathrm{cos}} = 0.955$) but yields moderate extraction (26%) due to increased optical confinement. Cs$_3$Cu$_2$I$_5$ requires a nanosphere geometry and shows limited enhancement, with LEE restricted to 10%. Distance-ependent analysis reveals composition-specific optimal emitter--plasmon separations, ranging from 8--12 nm for Cs$_3$Cu$_2$Br$_5$ to approximately 15 nm for Cs$_3$Cu$_2$Cl$_5$. These results provide composition-dependent design guidelines for plasmon-enhanced lead-free PeLEDs and highlight the critical role of accurate optical constants in predictive device optimization.

physics.optics↗

arXiv2025

Segmentation-Engineered Ge4Sb6Te7 Switch on SOI Platform for Multilevel Non-Volatile Photonic Neural Inference

Phase-change materials (PCMs) have emerged as key enablers of non-volatile, ultra-compact photonic switches for energy-efficient deep neural network (DNN) applications. In this work, we investigate the recently discovered $\mathrm{Ge_{4}Sb_{6}Te_{7}}$ (GST-467) as a high-contrast optical PCM and demonstrate its suitability for multi-level photonic computing. The complex refractive indices of amorphous and crystalline GST-467 were experimentally extracted and used to propose a segmented silicon-on-insulator photonic switch optimized at 1550 nm. Three-dimensional FDTD simulations reveal that segmentation significantly enhances the extinction ratio while maintaining low insertion loss, resulting in a more than seven times higher design figure of merit than an unsegmented design. Laser-induced thermo-optical simulations further establish efficient, reversible switching with sub-nJ energy requirements for crystallization and amorphization. Compared with established GST, GSST, and GSS compositions, GST-467 provides the largest transmission contrast and supports up to 48 resolvable optical states. When deployed as multi-level weights in photonic DNN architectures, the GST-467 switch achieves superior classification accuracy on EMNIST and Fashion-MNIST benchmarks. These results position GST-467 as a highly promising PCM for scalable, low-energy photonic computing and neuromorphic hardware.

physics.optics↗

arXiv2025

An Integrated DFT-FDTD Design of Plasmon-Enhanced Lead-Free $CsSn$$_x$$Ge$$_{1-x}$$I$$_3$ Perovskite LEDs

CsSn$_x$Ge$_{1-x}$I$_3$ as lead-free perovskites are promising for next generation NIR emitting perovskite LEDs due to their tunable bandgaps and stability. However, they suffer from poor light extraction efficiency, and accurate composition-specific optical data for these materials remain scarce. This study presents a DFT-FDTD framework to optimize light extraction via compositional tuning and plasmonic enhancement. First, DFT calculations were performed to obtain composition-specific complex refractive index and extinction coefficient values for $x = 0, 0.25, 0.5, 0.75$, and $1$. Results show bandgap increased from 1.331 eV for CsSnI$_3$ to 1.927 eV for CsGeI$_3$ with increasing Ge content, while refractive index ranges from 2.2 to 2.6 across compositions. These optical constants were then used as inputs for FDTD simulations of a PeLED structure with optimized Au/SiO$_2$ core-shell nanorods for plasmonic enhancement. A 12.1-fold Purcell enhancement was achieved for CsSn$_{0.25}$Ge$_{0.75}$I$_3$, while light extraction efficiency reached 25% for CsSn$_{0.5}$Ge$_{0.5}$I$_3$. LEE enhancement of 36% was obtained for CsSnI$_3$, and spectral overlap between emitter and plasmon resonance reached 96% for Sn-rich compositions. Design guidelines indicate CsSn$_{0.5}$Ge$_{0.5}$I$_3$ offers optimal balance of extraction efficiency (25%), Purcell enhancement (5.3$\times$), spectral overlap (93%), and oxidation stability for wearable and flexible optoelectronic applications, while CsSn$_{0.25}$Ge$_{0.75}$I$_3$ is recommended for applications prioritizing spontaneous emission rate.

physics.optics↗

arXiv2025

Exploring the Interaction of BeS Monolayer and Lung Disease Biomarkers: Potential Material for Biosensing Applications

Considerable attention has been directed towards the prognosis of lung diseases primarily due to their high prevalence. Despite advancements in detection technologies, current methods such as computed tomography, chest radiographs, bold proteomic patterns, nuclear magnetic resonance, and positron emission tomography still face limitations in detecting diseases related to the lungs. Consequently, there is a need for swift, non-invasive and economically feasible detection methods. Our study explores the interaction between BeS monolayer and breathe biomarkers related to lung disease utilizing the density functional theory (DFT) method. Through comprehensive DFT analysis, including electronic properties analysis, charge transfer evaluations, work function, optical properties assessment and recovery times, the feasibility and efficiency of BeS as a VOC (volatile organic compound) detection are investigated. Findings reveal significant changes in bandgap upon VOC adsorption, with notable alteration in work function for selective compounds. Optical property analyses demonstrate the potential for selective detection of biomarkers within specific wavelength ranges. Moreover, the study evaluates the impact of electric fields and strain on VOC-2D BeS interaction. Furthermore, the desorption of these VOCs from the BeS surface can be achieved through a heating process or under the illumination of UV light. This feature enables the reusability of the 2D material for biosensing applications. These findings highlight the potential of the BeS monolayer as a promising material for the sensitive and selective detection of breath biomarkers related to lung disease.

cond-mat.mtrl-sci↗

arXiv2025

Investigating the Optical and Thermodynamic Properties of 2D MoGe2P4 : Potential Material for Photothermal Therapy

In this study, we analyzed the optical, thermodynamic and electronic properties of 2D MoGe2P4 from the first principle calculation. 2D MoGe2P4 demonstrates superior optical absorption in the NIR-I biological window (750 nm ~ 1000 nm) with a peak near 808 nm and excellent thermal conductivity (63 Wm-1K-1). Finite-difference time-domain (FDTD) simulations and Heat simulations demonstrate that 2D MoGe2P4 possesses efficient photothermal conversion under low laser power (0.5 W/cm2) which is operated in 808nm. Theoretical investigation demonstrates, rapid temperature elevation ({\Delta}T = 24.8 {\deg}C) of the 2D MoGe2P4 within two minutes and photothermal stability over multiple laser cycles, achieving temperatures suitable for effective photothermal therapeutic applications. Photothermal therapy (PTT) is an emerging tumor treatment technique that utilizes photothermal agents (PTAs) to convert near-infrared (NIR) light into localized heat for tumor ablation. To enhance biocompatibility, we analyzed the PEGylation of 2D MoGe2P4 nanosheets through molecular dynamics simulation. PEGylation at human body temperature was stable which signifies 2D MoGe2P4's prospect in therapeutic applications. This research highlights the potential of 2D MoGe2P4 as an emerging material for PTA, establishing a foundation for experimental and clinical trials.

cond-mat.mtrl-sci↗

arXiv2023

Real Time Vehicle Identification: A Synchronous-Transmission Based Approach

Identification of the vehicles passing over the roads is a very important component of traffic monitoring/surveillance. There have been many attempts to design and develop efficient strategies to carry out the job. However, from the point of view of practical usefulness and real-time operation, most of them do not score well. In the current work, we perceive the problem as efficient real-time communication and data-sharing between the units in charge of recording the identities of the vehicles, i.e., Vehicle Recorders (VR), and the Vehicles (VE). We propose a strategy to address the issue with the help of Synchronous-Transmission (ST), which is a newer paradigm of communication compared to the traditional paradigm based on Asynchronous-Transmission (AT). First, we theoretically show that the presence of the physical layer phenomena called Capture-Effect in ST brings a significant benefit. Next, we also implement the strategy in a well-known IoT-Operating System Contiki, and compare its performance with the existing best-known strategy.

cs.NI↗

arXiv2023

Microscopic analysis of spin-momentum locking on a geometric phase metasurface

We revisit spin-orbit coupling in a plasmonic Berry metasurface comprised of rotated nanoapertures, which is known to imprint a robust far-field polarization response. We present a scattering formalism that shows how that spin-momentum locking emerges from the geometry of the unit cell without requiring global rotation symmetries. We find and confirm with Mueller polarimetry measurements that spin-momentum locking is an approximate symmetry. The symmetry breakdown is ascribed to the elliptical projection of circularly polarized light into the planar surface. This breakdown is maximal when surface waves are excited, and a new set of spin-momentum locking rules is presented for this case.

physics.optics↗

arXiv2022

Consensus-based Fast and Energy-Efficient Multi-Robot Task Allocation

In a multi-robot system, the appropriate allocation of the tasks to the individual robots is a very significant component. The availability of a centralized infrastructure can guarantee an optimal allocation of the tasks. However, in many important scenarios such as search and rescue, exploration, disaster-management, war-field, etc., on-the-fly allocation of the dynamic tasks to the robots in a decentralized fashion is the only possible option. Efficient communication among the robots plays a crucial role in any such decentralized setting. Existing works on distributed Multi-Robot Task Allocation (MRTA) either assume that the network is available or a naive communication paradigm is used. On the contrary, in most of these scenarios, the network infrastructure is either unstable or unavailable and ad-hoc networking is the only resort. Recent developments in synchronous-transmission (ST) based wireless communication protocols are shown to be more efficient than the traditional asynchronous transmission-based protocols in ad hoc networks such as Wireless Sensor Network (WSN)/Internet of Things (IoT) applications. The current work is the first effort that utilizes ST for MRTA. Specifically, we propose an algorithm that efficiently adapts ST-based many-to-many interaction and minimizes the information exchange to reach a consensus for task allocation. We showcase the efficacy of the proposed algorithm through an extensive simulation-based study of its latency and energy-efficiency under different settings.

cs.RO↗

arXiv2022

LiPI: Lightweight Privacy-Preserving Data Aggregation in IoT

In the modern digital world, a user of a smart system remains surrounded with as well as observed by a number of tiny IoT devices round the clock almost everywhere. Unfortunately, the ability of these devices to sense and share various physical parameters, although play a key role in these smart systems but also causes the threat of breach of the privacy of the users. Existing solutions for privacy-preserving computation for decentralized systems either use too complex cryptographic techniques or exploit an extremely high degree of message passing and hence, are not suitable for the resource-constrained IoT devices that constitute a significant fraction of a smart system. In this work, we propose a novel lightweight strategy LiPI for Privacy-Preserving Data Aggregation in low-power IoT systems. The design of the strategy is based on decentralized and collaborative data obfuscation and does not exploit any dependency on any trusted third party. In addition, besides minimizing the communication requirements, we make appropriate use of the recent advances in Synchronous-Transmission (ST)-based protocols in our design to accomplish the goal efficiently. Extensive evaluation based on comprehensive experiments in both simulation platforms and publicly available WSN/IoT testbeds demonstrates that our strategy works up to at least 51.7% faster and consumes 50.5% lesser energy compared to the existing state-of-the-art strategies.

cs.CR↗

arXiv2022

Real Time Vehicle Identification

Identification of the vehicles passing over the roads is a very important component of an Intelligent Transportation System. However, due to the presence of multiple vehicles together and their velocity, it gets hard to accurately identify and record them in real-time. Solutions based on Computer-vision use heavyweight equipment making them quiet inflexible, costly and hence unsuitable for wide-area coverage. Solutions based on RFID, although are lightweight and cost-effective, lack of fast and efficient communication protocol pertains to their inability to record multiple moving vehicles at the same time. We propose an IoT-assisted solution that leverages Synchronous-Transmission based communication to bridge these gaps. Through extensive experiments we demonstrate that our strategy can consistently record upto an average of 40 vehicles running at speed range 30-90 Km/h with at least 97.5% accuracy.

cs.DC↗

arXiv2022

Collaborative Load Management in Smart Home Area Network

An efficient Home Area Network (HAN) acts as a base of an Advanced Metering Infrastructure (AMI). A HAN not only facilitates AMI with efficient real-time monitoring of the electricity consumption but also manages the load profile of the whole system. However, the existing works on implementing HAN are mostly centralized and suffer from well-known problems. In this work, we propose an IoT-based efficient decentralized strategy using synchronous transmission to practically realize HAN. An inter-device coordination strategy is proposed to minimize the peak load as well as reduce the sudden changes in the overall system without compromising the users requirements. Through experiments over IoT-testbeds, we demonstrate that the proposed strategy can reduce the peak load upto 50% and reduce the load variations upto 58% for even a high and random rate of requests for execution of power-hungry house appliances.

cs.DC↗

arXiv2022

Decentralized Load Management in HAN: An IoT-Assisted Approach

A Home Area Network (HAN) is considered to be a significant component of Advanced Metering Infrastructure (AMI) and has been studied well in many works. It binds all the electrical components installed in a defined premise together for their close monitoring and management. However, HAN has been realized so far mostly as a centralized system. Therefore, like any other centralized system, the traditional realization of HAN also suffers from various well-known problems, such as single-point-of-failure, susceptibility to attacks, requirement of specialized infrastructure, inflexibility to easy expansion, etc. To address these issues, in this work, we propose a decentralized design of HAN. In particular, we propose an IoT based design where instead of a central controller, the overall system operation is controlled and managed through decentralized coordination among the the electrical appliances. We leverage Synchronous-Transmission (ST) based data-sharing protocols in IoT to accomplish our goal. To demonstrate the efficacy of the proposed decentralized framework, we also design a real-time intra-HAN load-management strategy and implement it in real IoT-devices. Evaluation of the same over emulation platforms and IoT testbeds show upto 62% reduction of peak load over a wide variety of load profiles.

cs.DC↗

arXiv2022

Multi-Party Computation in IoT for Privacy-Preservation

Preservation of privacy has been a serious concern with the increasing use of IoT-assisted smart systems and their ubiquitous smart sensors. To solve the issue, the smart systems are being trained to depend more on aggregated data instead of directly using raw data. However, most of the existing strategies for privacy-preserving data aggregation, either depend on computation-intensive Homomorphic Encryption based operations or communication-intensive collaborative mechanisms. Unfortunately, none of the approaches are directly suitable for a resource-constrained IoT system. In this work, we leverage the concurrent-transmission-based communication technology to efficiently realize a Multi-Party Computation (MPC) based strategy, the well-known Shamir's Secret Sharing (SSS), and optimize the same to make it suitable for real-world IoT systems.

cs.CR↗