arXiv subjects

Kausik Majumdar

Publications and source records attributed to Kausik Majumdar.

At least 19 recordsLinked to original sources

arXiv2026

Supermoiré-trapped quadrupolar exciton

Moiré-trapped dipolar interlayer exciton in heterobilayers offers a rich platform to explore interaction-driven phenomena. Extending the number of layers to three and beyond leads to highly intriguing multipolar exciton - a superposition state of vertically aligned phase-coherent excitons. However, in experiments, unintentional twist-angle mismatch among layers may degrade the strength and homogeneity of the vertical Coulomb coupling. Here we propose that the supermoiré effect in a heterotrilayer comes to the rescue by creating periodic pockets of vertically aligned atomic registries that facilitate the formation of quadrupolar excitons trapped in such pockets. Using WS$_2$/WSe$_2$/WS$_2$ stack, we show interaction between multiple confined levels of the top and bottom moiré interfaces, creating electric field tunable multi-level hybridized bright (symmetric) and dark (anti-symmetric) quadrupolar states. Our work underscores the critical role of supermoiré effect in quadrupolar excitons. The discovery of reduced sensitivity on precise angle-alignment will ignite exploration of complex excitonic states in multi-layered heterostructures.

cond-mat.mes-hall↗

arXiv2026

Confined exciton polaron in MoS$_2$ on twisted-hBN

The simple electrostatic picture of a trion is that of an excess charge inducing an exciton polarization and binding closer (farther) to the hole (electron) side of it. Trion formation can be forbidden when such spontaneous rearrangement of charges is not allowed by the application of external perturbation, such as electric field. Here we test this hypothesis experimentally using a non-monotonic electric field. We realize this scenario by imprinting the ferroelectric domains at the AA-stacked twisted-hBN (t-hBN) interface onto a monolayer of MoS2 placed over it. The spatially varying in-plane electric field around the domain wall serves the dual purpose of (a) confining and polarizing the 2D exciton in the domain wall, and (b) depleting the free charge carriers from the domain wall. We observe a large quantized exciton splitting confirming strong exciton confinement in the domain wall. Forced by the confining potential, the electron side of the polarized exciton lies closer to the domain with accumulated free electrons, which should ideally prevent any trion formation. Contrary to the laid hypothesis, we observe signatures of quantized charged exciton emission, with an inter-level splitting that mimics the level-splitting of the quantized excitons. This paradox is explained using the many-body picture of exciton polaron, where a conduction band hole attractively binds the polarized exciton and the electron Fermi sea. The results provide a definitive way to unambiguously discern exciton polaron from trion.

cond-mat.mes-hall↗

arXiv2026

Low-dimensional platforms for single photon detection

A Single-Photon Detector (SPD) can detect extremely low intensity of electromagnetic wave - down to a single photon. Driven by the rapid developments in quantum information science and an increasing demand for ultra-low-light sensing across various domains, there is a need for transformative advancements in the design and development of SPDs. In this context, low-dimensional platforms, including quantum dots, superconducting nanowires and layered materials have emerged as crucial frontiers of research. This review explores the state-of-the-art of different low-dimensional SPD platforms, focusing on the engineering physics across their device architectures, performance parameters and application potential. By critically comparing the performance and addressing current challenges inherent to each low-dimensional platform, the review aims to outline future research directions to advance next-generation SPD technologies.

quant-ph↗

arXiv2026

The origin and promise of transition metal dichalcogenide hosted single photon emitters for quantum technologies

Single photon emitters (SPEs) are integral parts of several quantum technology implementations. Over the past decade or so, monolayers of transition metal dichalcogenides (TMDCs) have emerged as one of the promising candidates for SPE platforms with attractive characteristics. To move ahead, it is necessary to understand the atomistic origin of SPEs in TMDCs - a topic which is highly debated with contradicting proposals. In this paper, we critically review these existing proposals to elucidate their origin. Further, we perform a critical trend analysis for different figures of merit of TMDC-based SPEs, and propose a characterization methodology to streamline the reporting process. Finally, we review several quantum technology implementations where solid state SPEs are being used, and identify the advancements required in TMDC-based SPEs for their successful adoption in these technologies.

cond-mat.mes-hall↗

arXiv2025

Room Temperature Single Photon Detection at 1550 nm using van der Waals Heterojunction

Single-photon detectors (SPDs) are crucial in applications ranging from space, biological imaging to quantum communication and information processing. The SPDs that operate at room temperature are of particular interest to broader application space as the energy overhead introduced by the cryogenic cooling can be avoided. Although silicon-based single photon avalanche diodes (SPADs) are well matured and operate at room temperature, the bandgap limitation restricts their operation at telecommunication wavelength (1550 nm) and beyond. InGaAs-based SPADs, on the other hand, are sensitive to 1550 nm photons, but suffer from relatively lower efficiency, high dark count rate, afterpulsing probability, and pose hazards to the environment from the fabrication process. In this work, we demonstrate how we can leverage the properties of nanomaterials to address these challenges and realise a room temperature single-photon detector capable of operating at 1550 nm. We achieve this by coupling a low bandgap ($\sim 350~meV$) absorber (black phosphorus) to a sensitive van der Waals probe that is capable of detecting discrete electron fluctuation. We optimize the device for operation at $1550~nm$ and demonstrate an overall quantum efficiency of $21.4\%$ (estimated as $42.8\%$ for polarized light), and a minimum dark count of $\sim 720~Hz$ at room temperature.

quant-ph↗

arXiv2025

Observation of moiré trapped biexciton through sub-diffraction-limit probing using hetero-bilayer on nanopillar

The ability to tune the degree of interaction among particles at the nanoscale is highly intriguing. The spectroscopic signature of such interaction is often subtle and requires special probes to observe. To this end, inter-layer excitons trapped in the periodic potential wells of a moiré superlattice offer rich interaction physics, specifically due to the presence of both attractive and repulsive components in the interaction. Here we show that the Coulomb force between two inter-layer excitons switches from repulsive to attractive when the length scale reduces from inter-moiré-pocket to intra-moiré-pocket in a WS$_2$/WSe$_2$ hetero-bilayer - thanks to the complex competition between direct and exchange interaction. The finding is a departure from the usual notion of repelling inter-layer excitons due to layer polarization. This manifests as the simultaneous observation of an anomalous superlinear power-law of moiré exciton and a stabilization of moiré trapped biexciton. The experimental observation is facilitated by placing the hetero-bilayer on a polymer-nanopillar/gold-film stack which significantly reduces the inhomogeneous spectral broadening by selectively probing a smaller ensemble of moiré pockets compared with a flat sample. This creates an interesting platform to explore interaction among moiré trapped excitons and higher order quasiparticles.

cond-mat.mes-hall↗

arXiv2024

Simultaneously enhancing brightness and purity of WSe$_2$ single photon emitter using high-aspect-ratio nanopillar array on metal

Monolayer semiconductor transferred on nanopillar arrays provides site-controlled, on-chip single photon emission, which is a scalable light source platform for quantum technologies. However, the brightness of these emitters reported to date often falls short of the perceived requirement for such applications. Also, the single photon purity usually degrades as the brightness increases. Hence, there is a need for a design methodology to achieve enhanced emission rate while maintaining high single photon purity. Using WSe$_2$ on high-aspect-ratio ($\sim 3$ - at least two-fold higher than previous reports) nanopillar arrays, here we demonstrate $>10$ MHz single photon emission rate in the 770-800 nm band that is compatible with quantum memory and repeater networks (Rb-87-D1/D2 lines), and satellite quantum communication. The emitters exhibit excellent purity (even at high emission rates) and improved out-coupling due to the use of a gold back reflector that quenches the emission away from the nanopillar.

cond-mat.mes-hall↗

arXiv2024

Motion of 2D exciton in momentum space leads to pseudospin distribution narrowing on the Bloch Sphere

Motional narrowing implies narrowing induced by motion, for example, in nuclear resonance, the thermally induced random motion of the nuclei in an inhomogeneous environment leads to counter-intuitive narrowing of the resonance line. Similarly, the excitons in monolayer semiconductors experience magnetic inhomogeneity: the electron-hole spin-exchange interaction manifests as an in-plane pseudo-magnetic field with a periodically varying orientation inside the exciton band. The excitons undergo random momentum scattering and pseudospin precession repeatedly in this inhomogeneous magnetic environment - typically resulting in fast exciton depolarization. On the contrary, we show that such magnetic inhomogeneity averages out at high scattering rate due to motional narrowing. Physically, a faster exciton scattering leads to a narrower pseudospin distribution on the Bloch sphere, implying a nontrivial improvement in exciton polarization. The in-plane nature of the pseudo-magnetic field enforces a contrasting scattering dependence between the circularly and linearly polarized excitons - providing a spectroscopic way to gauge the sample quality.

cond-mat.mes-hall↗

arXiv2023

Tensile strain induced brightening of momentum forbidden dark exciton in WS$_2$

Transition-metal dichalcogenides (TMDs) host tightly bound quasi-particles called excitons. Based on spin and momentum selection rules, these excitons can be either optically bright or dark. In tungsten-based TMDs, momentum-forbidden dark exciton is the energy ground state and therefore it strongly affect the emission properties. In this work, we brighten the momentum forbidden dark exciton by placing WS$_2$ on top of nanotextured substrates which put the WS$_2$ layer under tensile strain, modifying electronic bandstructure. This enables phonon assisted scattering of exciton between momentum valleys, thereby brightening momentum forbidden dark excitons. Our results will pave the way to design ultrasensitive strain sensing devices based on TMDs.

cond-mat.mes-hall↗

arXiv2023

Polarized and narrow excitonic emission from graphene-capped monolayer WS$_2$ through resonant phonon relaxation

The broadening and polarization of excitonic luminescence in monolayer TMDs largely suffer from inhomogeneity and temperature - an unresolved problem to date. In this work, through few-layer-graphene encapsulation of monolayer WS$_2$, we reduce the inter-excitonic energy separation, which then can have a narrow resonance with a specific phonon mode of our choice. The resulting single-step exciton relaxation with the resonating phonon mode significantly suppresses the inhomogeneous broadening, allowing us to achieve the narrowest exciton linewidth of 1.06 meV (which translates to 0.19 meV after deconvolution with the excitation laser linewidth). The single-phonon resonance helps to achieve a high quantum efficiency despite graphene encapsulation. The technique is powerful in tuning the exciton polarization during relaxation by choosing a specific resonating phonon mode. For example, the valley coherence (polarization) improves from $\sim$68% ($\sim$40%) to $\sim$90% ($\sim$75%) on resonance with 2$A_1$' and $A_1$' modes respectively. We further demonstrate a strong polarization reversal on resonance with a chiral phonon mode. Strikingly, the above features remain robust against temperature (up to 200 K) and sample age (few months in ambient condition). The findings will lead to clean excitonic measurements without requiring cryogenic cooling.

cond-mat.mes-hall↗

arXiv2023

Harmonic to anharmonic tuning of moiré potential leading to unconventional Stark effect and giant dipolar repulsion in WS$_2$/WSe$_2$ heterobilayer

Excitonic states trapped in harmonic moiré wells of twisted heterobilayers is an intriguing testbed. However, the moiré potential is primarily governed by the twist angle, and its dynamic tuning remains a challenge. Here we demonstrate anharmonic tuning of moiré potential in a WS$_2$/WSe$_2$ heterobilayer through gate voltage and optical power. A gate voltage can result in a local in-plane perturbing field with odd parity around the high-symmetry points. This allows us to simultaneously observe the first (linear) and second (parabolic) order Stark shift for the ground state and first excited state, respectively, of the moiré trapped exciton - an effect opposite to conventional quantum-confined Stark shift. Depending on the degree of confinement, these excitons exhibit up to twenty-fold gate-tunability in the lifetime ($100$ to $5$ ns). Also, exciton localization dependent dipolar repulsion leads to an optical power-induced blueshift of $\sim$1 meV/$μ$W - a five-fold enhancement over previous reports.

cond-mat.mes-hall↗

arXiv2023

Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time

In monolayer transition metal dichalcogenide semiconductors, valley coherence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manipulation of exciton a highly challenging task. Using monolayer MoS2 sandwiched between top and bottom graphene, here we demonstrate fully valley coherent excitons by observing ~100% degree of linear polarization in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a combination of calculation (solutions of Bethe-Salpeter and Maialle-Silva-Sham equations) and a careful choice of design of experiments using four different stacks with systematic variation of screening and exciton lifetime. To the best of our knowledge, this is the first report in which the excitons are found to be valley coherent in the entire lifetime in monolayer semiconductors, allowing optical readout of valley coherence possible.

cond-mat.mes-hall↗

arXiv2022

Optical manipulation of layer-valley coherence via strong exciton-photon coupling in microcavities

Coherent control and manipulation of quantum degrees of freedom such as spins forms the basis of emerging quantum technologies. In this context, the robust valley degree of freedom and the associated valley pseudospin found in two-dimensional transition metal dichalcogenides is a highly attractive platform. Valley polarization and coherent superposition of valley states have been observed in these systems even up to room temperature. Control of valley coherence is an important building block for the implementation of valley qubit. Large magnetic fields or high-power lasers have been used in the past to demonstrate the control (initialization and rotation) of the valley coherent states. Here we demonstrate control of layer-valley coherence via strong coupling of valley excitons in bilayer WS2 to microcavity photons by exploiting the pseudomagnetic field arising in optical cavities owing to the TE-TM splitting. The use of photonic structures to generate pseudomagnetic fields which can be used to manipulate exciton-polaritons presents an attractive approach to control optical responses without the need for large magnets or high intensity optical pump powers.

cond-mat.mes-hall↗

arXiv2022

Integration of 3-level MoS$_2$ multi-bridge channel FET with 2D layered contact and gate dielectric

Multi-bridge channel field effect transistor (MBCFET) provides several advantages over FinFET technology and is an attractive solution for sub-5 nm technology nodes. MBCFET is a natural choice for devices that use semiconducting layered materials (such as, MoS$_2$) as the channel due to their dangling-bond-free ultra-thin nature and the possibility of layer-by-layer transfer. MoS$_2$-based MBCFET is thus an attractive proposition for drive current boost without compromising on the electrostatics and footprint. Here we demonstrate a 3-level MoS$_2$ MBCFET, where each vertically stacked channel is dual-gated to achieve a saturation current of 174.9 $μ$A (which translates to \txc{90 $μ$A per $μ$m footprint width (@2.7 $μ$m channel length), a near-ideal sub-threshold slope of 63 mV/dec, and an on-off ratio $>$$10^8$. This work sets the benchmark for layer-material-based MBCFET in terms of the number of parallel channels integrated, simultaneously providing high drive current and excellent electrostatic control.

cond-mat.mes-hall↗

arXiv2022

Charge-Density Wave Driven Giant Thermionic-Current Switching in 1T-TaS$_{2}$/2H-TaSe$_{2}$/2H-MoS$_{2}$ Heterostructure

1T-TaS$_2$ exhibits several resistivity phases due to the modulation of charge density wave (CDW). The fact that such phase transition can be driven electrically has attracted a lot of attention in the recent past towards \emph{active-metal} based electronics. However, the bias-driven resistivity switching is not very large ($<$ 5 fold), and an enhancement in the same will highly impact such phase transition devices. One aspect that is often overlooked is that such phase transition is also accompanied by a significant change in the local temperature due to the low thermal conductivity of 1T-TaS$_2$. In this work, we exploit such electrically driven phase transition induced temperature change to promote carriers over a thermionic barrier in a 1T-TaS$_{2}$/2H-TaSe$_{2}$/2H-MoS$_{2}$ T-Junction, achieving a $964$-fold abrupt switching in the current through the MoS$_2$ channel. The device is highly reconfigurable and exhibits an abrupt reduction in current as well when the biasing configuration changes. The results are promising for several electronic applications, including neuromorphic chips, switching, nonlinear devices, and industrial electronics such as current and temperature sensing.

cond-mat.mtrl-sci↗

arXiv2022

Highly nonlinear biexcitonic photocurrent from ultrafast inter-layer charge transfer

Strong Coulomb interaction in monolayer semiconductors allows them to host optically active large many-body states, such as the five-particle state, charged biexciton. Strong nonlinear light absorption by the charged biexciton under spectral resonance, coupled with its charged nature, makes it intriguing for nonlinear photodetection - an area that is hitherto unexplored. Using the high built-in vertical electric field in an asymmetrically designed few-layer graphene encapsulated 1L-WS$_2$ heterostructure, here we report a large, highly nonlinear photocurrent arising from the strong absorption by two charged biexciton species under zero external bias (self-powered mode). Time-resolved measurement reveals that the generated charged biexcitons transfer to the few-layer graphene in a timescale of sub-5 ps, indicating an ultrafast intrinsic limit of the photoresponse. By using single- and two-color photoluminescence excitation spectroscopy, we show that the two biexcitonic peaks originate from bright-dark and bright-bright exciton-trion combinations. Such innate nonlinearity in the photocurrent due to its biexcitonic origin, coupled with the ultrafast response due to swift inter-layer charge transfer, exemplifies the promise of manipulating many-body effects in monolayers towards viable optoelectronic applications.

cond-mat.mes-hall↗

arXiv2022

A High-Quality Entropy Source Using van der Waals Heterojunction for True Random Number Generation

Generators of random sequences used in high-end applications such as cryptography rely on entropy sources for their indeterminism. Physical processes governed by the laws of quantum mechanics are excellent sources of entropy available in nature. However, extracting enough entropy from such systems for generating truly random sequences is challenging while maintaining the feasibility of the extraction procedure for real-world applications. Here, we present a compact and an all-electronic van der Waals (vdW) heterostructure-based device capable of detecting discrete charge fluctuations for extracting entropy from physical processes and use it for the generation of independent and identically distributed (IID) true random sequences. We extract a record high value ($>0.98~bits/bit$) of min-entropy using the proposed scheme. We demonstrate an entropy generation rate tunable over multiple orders of magnitude and show the persistence of the underlying physical process for temperatures ranging from cryogenic to ambient conditions. We verify the random nature of the generated sequences using tests such as NIST SP 800-90B standard and other statistical measures and verify the suitability our random sequence for cryptographic applications using the NIST SP 800-22 standard. The generated random sequences are then used in implementing various randomized algorithms without any preconditioning steps.

quant-ph↗

arXiv2022

Trion-trion annihilation in monolayer WS$_2$

Strong Coulomb interaction in monolayer transition metal dichalcogenides can facilitate nontrivial many-body effects among excitonic complexes. Many-body effects like exciton-exciton annihilation (EEA) have been widely explored in this material system. However, a similar effect for charged excitons (or trions), that is, trion-trion annihilation (TTA), is expected to be relatively suppressed due to repulsive like-charges, and has not been hitherto observed in such layered semiconductors. By a gate-dependent tuning of the spectral overlap between the trion and the charged biexciton through an "anti-crossing"-like behaviour in monolayer WS$_2$, here we present an experimental observation of an anomalous suppression of the trion emission intensity with an increase in gate voltage. The results strongly correlate with time-resolved measurements, and are inferred as a direct evidence of a nontrivial TTA resulting from non-radiative Auger recombination of a bright trion, and the corresponding energy resonantly promoting a dark trion to a charged biexciton state. The extracted Auger coefficient for the process is found to be tunable ten-fold through a gate-dependent tuning of the spectral overlap.

cond-mat.mes-hall↗