arXiv subjects

Sajal Dhara

Publications and source records attributed to Sajal Dhara.

At least 19 recordsLinked to original sources

arXiv2026

Observation of Intrinsic Anderson Localization in Few-Layer ReS$_2$

Electron localization phenomena are expected to play an important role in the transport properties of two-dimensional materials. Rhenium disulfide (ReS$_2$), with its narrow conduction bandwidth, is uniquely susceptible to this effect. However, extrinsic disorder caused by fabrication methods obscures inherent localization behavior arising from reduced dimensionality and degrades transport properties. We report intrinsic Anderson localization in few-layer ReS$_2$ by eliminating extrinsic fabrication-induced disorder through all-dry van der Waals assembly and suppressing interface charge trapping through a hexagonal boron nitride (hBN) gate dielectric. Temperature-dependent transport reveals a crossover from nearest-neighbor hopping to two-dimensional (2D) Mott variable-range hopping (VRH). The non-monotonic gate-voltage dependence of activation energy provides direct access to the energy-resolved band-tail density of states of the ReS$_2$ conduction band. 2D Mott VRH yields a localization length of (3.5 $\pm$ 0.1) nm, an order of magnitude larger than disorder-dominated devices, providing a quantitative characterization of intrinsic Anderson localization in ReS$_2$.

cond-mat.mes-hall↗

arXiv2026

Anisotropic exciton-polaritons reveal non-Hermitian topology in van der Waals materials

Topological band theory has expanded into various domains in applied physics, offering significant potential for future technologies. Recent developments indicate that unique bulk band topology perceived for electrons can be realized in a system of light-matter quasiparticles with reduced crystal symmetry by utilizing tunable light-matter interaction. In this work we realize topologically non-trivial energy band dispersion of exciton-polaritons confined in two-dimensional anisotropic materials inside an optical microcavity, and show the emergence of exceptional points (EPs) due to non-Hermitian topology arising from excitonic dipole oscillators with finite quasiparticle lifetime. Fourier-plane imaging reveals two pairs of EPs connected by bulk Fermi arcs for each of the transverse electric and magnetic polarized modes. An anisotropic Lorentz oscillator model captures the exact band dispersion observed in our experiment in two-dimensional momentum space. Our findings establish anisotropic two-dimensional materials as a platform for exploring non-Hermitian topological physics, with implications for polarization-controlled optical technologies.

physics.optics↗

arXiv2025

Giant Resonance Raman Scattering via Anisotropic Excitons in ReS2

Anisotropic two-dimensional (2D) semiconductors recently have emerged as a promising platform for polarization-controlled Raman amplification. In this study, we probe energy-dependent resonant Raman scattering in few layer ReS2 under different polarization configurations. We identify two distinct excitation regimes, each characterized by a resonance condition where either the pump or the Stokes photon energy aligns with an excitonic transition. A two-order-of-magnitude enhancement in Raman intensity is observed when the pump energy is tuned near the exciton resonance. Under Stokes-resonant conditions, additional Raman lines accompanied by excitonic photoluminescence are observed, suggesting the participation of non-Bloch intermediate states in the scattering process. These findings shed light into the influence of excitons in modulating nonlinear optical phenomena in anisotropic 2D materials, offering valuable insights for the design of tunable photonic and optoelectronic devices based on anisotropic layered materials.

physics.optics↗

arXiv2025

Zero-Threshold PT-Symmetric Polariton-Raman Laser

Anisotropy endows topological aspects in optical systems and furnishes a platform to explore non-Hermitian physics, which can be harnessed for the polarization-selective amplification of light. Here, we show a zero-threshold Raman laser can be achieved in an anisotropic optical microcavity via polarization-controlled optical pumping. A loss-gain mechanism between two polarized Stokes modes arises naturally via polarization-dependent stimulated scattering and anisotropic Raman gain of the active layered material inside the microcavity. A Parity-Time (PT) symmetric Hamiltonian has been proposed to explain the emergence of a single polarization mode, essential for achieving a zero-threshold lasing condition. Additionally, intensity correlation measurements of the Stokes modes validate the coherence properties of the emitted light. Our realization of the zero-threshold Raman laser in anisotropic microcavity can open up a new research direction exploring non-Hermitian and topological aspects of light in anisotropic two-dimensional materials.

cond-mat.mes-hall↗

arXiv2024

Trion Engineered Multimodal Transistors in Two dimensional Bilayer Semiconductor Lateral Heterostructures

Multimodal device operations are essential to advancing the integration of 2D semiconductors in electronics, photonics, information and quantum technology. Precise control over carrier dynamics, particularly exciton generation and transport, is crucial for finetuning the functionality of optoelectronic devices based on 2D semiconductor heterostructure. However, the traditional exciton engineering methods in 2D semiconductors are mainly restricted to the artificially assembled vertical pn heterostructures with electrical or strain induced confinements. In this study, we utilized bilayer 2D lateral npn multijunction heterostructures with intrinsically spatially separated energy landscapes to achieve preferential exciton generation and manipulation without external confinement. In lateral npn FET geometry, we uncover unique and nontrivial properties, including dynamic tuning of channel photoresponsivity from positive to negative. The multimodal operation of these 2D FETs is achieved by carefully adjusting electrical bias and the impinging photon energy, enabling precise control over the trions generation and transport. Cryogenic photoluminescence measurement revealed the presence of trions in bilayer MoSe2 and intrinsic trap states in WSe2. Measurements in different FET device geometries show the multifunctionality of 2D lateral heterostructure phototransistors for efficient tuning and electrical manipulation of excitonic characteristics. Our findings pave the way for developing practical exciton-based transistors, sensors, multimodal optoelectronic and quantum technologies

physics.app-ph↗

arXiv2023

Anisotropic exciton polariton pairs as a platform for PT-symmetric non-Hermitian physics

Non-Hermitian systems with parity-time (PT) symmetry have been realized using optical constructs in the classical domain, leading to a plethora of non-intuitive phenomena. However, PT-symmetry in purely quantum non-Hermitian systems like microcavity exciton-polaritons has not been realized so far. Here we show how a pair of nearly orthogonal sets of anisotropic exciton-polaritons can offer a versatile platform for realizing multiple spectral degeneracies called Exceptional Points (EPs) and propose a roadmap to achieve a PT-symmetric system. Polarization-tunable coupling strength creates one class of EPs, while Voigt EPs are observed for specific orientations where splitting of polariton modes due to birefringence is compensated by Transverse Electric (TE) -Transverse Magnetic (TM) mode splitting. Thus, paired sets of polarized anisotropic microcavity exciton-polaritons can offer a promising platform not only for fundamental research in non-Hermitian quantum physics and topological polaritons, but also, we propose that it will be critical for realizing zero threshold lasers.

cond-mat.mes-hall↗

arXiv2023

Influence of the coherence of spectral domain interference of Fano resonance on the degree of polarization of light

We show an intriguing connection between the coherence of spectral domain interference of two electromagnetic modes in Fano resonance and the resulting degree of polarization of light. A theoretical treatment is developed by combining a general electromagnetic model of partially coherent interference of a spectrally narrow and a broad continuum mode leading to Fano resonance and the cross-spectral density matrix of the interfering polarized fields of light. The model suggests a characteristic variation of the degree of polarization across the region of spectral dip and the peak of Fano resonance as an exclusive signature of the connection between the degree of polarization and the coherence of the interfering modes. The predictions of the model is experimentally verified in the partially polarized Fano resonance spectra from metal Chalcogenides systems, which emerged due to the interference of a narrow excitonic mode with the background continuum of scattered light in the reflectance spectra from the system. The demonstrated connection between polarization and coherence in the spectral domain Fano-type interference of electromagnetic modes is fundamentally important in the context of a broad variety of non-trivial wave phenomena that originate from fine interference effects, which may also have useful practical implications.

physics.optics↗

arXiv2022

A steady state approach for studying valley relaxation using optical vortex beam

Spin-valley coupling in monolayer transition metal dichalcogenides gives rise to valley polarization and coherence effect, limited by intervalley scattering caused by exciton-phonon, exciton-impurity, and electron-hole exchange interaction (EHEI). We explore an approach to tune the EHEI by controlling excitons center of mass momentum (COM) utilizing the photon distribution of higher-order optical vortex beam. By virtue of this, we have observed excitons COM-dependent valley depolarization and decoherence which gives us the ability to measure the timescale associated with valley dynamics in the steady-state measurement. Our steady-state technique to probe the valley dynamics can open up a new paradigm to explore the physics of excitons in two-dimensional systems.

cond-mat.mes-hall↗

arXiv2021

Interfacial anisotropic exciton-polariton manifolds in ReS$_2$

Light-matter coupling in van der Waal's materials holds significant promise in realizing Bosonic condensation and superfluidity. The underlying semiconductor's crystal asymmetry, if any, can be utilized to form anisotropic half-light half-matter quasiparticles. We demonstrate generation of such highly anisotropic exciton-polaritons at the interface of a biaxial layered semiconductor, stacked on top of a distributed Bragg reflector. The spatially confined photonic mode in this geometry couples with polarized excitons and their Rydberg states, creating a system of highly anisotropic polariton manifolds, displaying Rabi splitting of up to 68 meV. Rotation of the incident beam polarization is used to tune coupling strength and smoothly switch regimes from weak to strong coupling, while also enabling transition from one three-body coupled oscillator system to another. Light-matter coupling is further tunable by varying the number of weakly coupled optically active layers. Our work provides a versatile method of engineering devices for applications in polarization-controlled polaritonics and optoelectronics.

cond-mat.mes-hall↗

arXiv2020

Additional excitonic features and momentum-dark states in ReS2

Unidirectional in-plane structural anisotropy in Rhenium-based transition metal dichalcogenides (TMDs) introduces a new class of 2-D materials, exhibiting anisotropic optical properties. In this work, we perform temperature dependent, polarization-resolved photoluminescence and reflectance measurements on several-layer ReS$_{2}$. We discover two additional excitonic resonances (X$_{3}$ and X$_{4}$), which can be attributed to splitting of spin degenerate states. Strong in-plane oscillator strength of exciton species X$_{1}$ and X$_{2}$ are accompanied by weaker counterparts X$_{3}$ and X$_{4}$ with similar polarization orientations. The in-plane anisotropic dielectric function has been obtained for ReS$_{2}$ which is essential for engineering light matter coupling for polarization sensitive optoelectronic devices. Furthermore, our temperature dependent study revealed the existence of low-lying momentum-forbidden dark states causing an anomalous PL intensity variation at 30 K, which has been elucidated using a rate equation model involving phonon scattering from these states. Our findings of the additional excitonic features and the momentum-dark states can shed light on the true nature of the electronic band structure of ReS$_{2}$.

cond-mat.mes-hall↗

arXiv2015

Carrier Transport in High Mobility InAs Nanowire Junctionless Transistors

Ability to understand and model the performance limits of nanowire transistors is the key to design of next generation devices. Here, we report studies on high-mobility junction-less gate-all-around nanowire field effect transistor with carrier mobility reaching 2000 cm2/V.s at room temperature. Temperature-dependent transport measurements reveal activated transport at low temperatures due to surface donors, while at room temperature the transport shows a diffusive behavior. From the conductivity data, the extracted value of sound velocity in InAs nanowires is found to be an order less than the bulk. This low sound velocity is attributed to the extended crystal defects that ubiquitously appear in these nanowires. Analyzing the temperature-dependent mobility data, we identify the key scattering mechanisms limiting the carrier transport in these nanowires. Finally, using these scattering models, we perform drift-diffusion based transport simulations of a nanowire field-effect transistor and compare the device performances with experimental measurements. Our device modeling provides insight into performance limits of InAs nanowire transistors and can be used as a predictive methodology for nanowire-based integrated circuits.

cond-mat.mes-hall↗

arXiv2011

High Q electromechanics with InAs nanowire quantum dots

In this report, we study electromechanical properties of a suspended InAs nanowire (NW) resonator. At low temperatures, the NW acts as the island of a single electron transistor (SET) and we observe a strong coupling between electrons and mechanical modes at resonance; the rate of electron tunneling is approximately 10 times the resonant frequency. Above and below the mechanical resonance, the magnitude of Coulomb peaks is different and we observe Fano resonance in conductance due to the interference between two contributions to potential of the SET. The quality factor ($Q$) of these devices is observed $\sim10^5$ at 100 mK.

cond-mat.mes-hall↗

arXiv2011

Field-effect modulation of conductance in VO2 nanobeam transistors with HfO2 as the gate dielectric

We study field-effect transistors realized from VO2 nanobeams with HfO2 as the gate dielectric. When heated up from low to high temperatures, VO2 undergoes an insulator-to-metal transition. We observe a change in conductance (~ 6 percent) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system and we discuss its possible origins.

cond-mat.str-el↗

arXiv2011

Tunable thermal conductivity in defect engineered nanowires at low temperatures

We measure the thermal conductivity ($κ$) of individual InAs nanowires (NWs), and find that it is 3 orders of magnitude smaller than the bulk value in the temperature range of 10 to 50 K. We argue that the low $κ$ arises from the strong localization of phonons in the random superlattice of twin-defects oriented perpendicular to the axis of the NW. We observe significant electronic contribution arising from the surface accumulation layer which gives rise to tunability of $κ$ with the application of electrostatic gate and magnetic field. Our devices and measurements of $κ$ at different carrier concentrations and magnetic field without introducing structural defects, offer a means to study new aspects of nanoscale thermal transport.

cond-mat.mes-hall↗

arXiv2011

Facile fabrication of lateral nanowire wrap-gate devices with improved performance

We present a simple fabrication technique for lateral nanowire wrap-gate devices with high capacitive coupling and field-effect mobility. Our process uses e-beam lithography with a single resist-spinning step, and does not require chemical etching. We measure, in the temperature range 1.5-250 K, a subthreshold slope of 5-54 mV/decade and mobility of 2800-2500 $cm^2/Vs$ -- significantly larger than previously reported lateral wrap-gate devices. At depletion, the barrier height due to the gated region is proportional to applied wrap-gate voltage.

cond-mat.mes-hall↗

arXiv2010

Nanoscale electromechanical resonators as probes of the charge density wave transition in NbSe$_2$

We study nanoelectromechanical resonators fabricated from suspended flakes of NbSe$_2$ (thickness$\sim$30-50 nm) to probe charge density wave (CDW) physics at nanoscale. Variation of elastic and electronic properties accompanying the CDW phase transition (around 30 K) are investigated simultaneously using the devices as self-sensing heterodyne mixers. Elastic modulus is observed to change by 10 per cent, an amount significantly larger than what had been reported earlier in the case of bulk crystals. We also study the modulation of conductance by electric field effect, and examine its relation to the order parameter and the CDW energy gap at the Fermi surface.

cond-mat.mes-hall↗

arXiv2010

Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene NEMS resonators

We use suspended graphene electromechanical resonators to study the variation of resonant frequency as a function of temperature. Measuring the change in frequency resulting from a change in tension, from 300 K to 30 K, allows us to extract information about the thermal expansion of monolayer graphene as a function of temperature, which is critical for strain engineering applications. We find that thermal expansion of graphene is negative for all temperatures between 300K and 30K. We also study the dispersion, the variation of resonant frequency with DC gate voltage, of the electromechanical modes and find considerable tunability of resonant frequency, desirable for applications like mass sensing and RF signal processing at room temperature. With lowering of temperature, we find that the positively dispersing electromechanical modes evolve to negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature compensated sensors.

cond-mat.mes-hall↗

arXiv2010

Tuning mechanical modes and influence of charge screening in nanowire resonators

We probe electro-mechanical properties of InAs nanowire (diameter ~ 100 nm) resonators where the suspended nanowire (NW) is also the active channel of a field effect transistor (FET). We observe and explain the non-monotonic dispersion of the resonant frequency with DC gate voltage (VgDC). The effect of electronic screening on the properties of the resonator can be seen in the amplitude. We observe the mixing of mechanical modes with VgDC. We also experimentally probe and quantitatively explain the hysteretic non-linear properties, as a function of VgDC, of the resonator using the Duffing equation.

cond-mat.mes-hall↗