arXiv subjects

Rabindra Biswas

Publications and source records attributed to Rabindra Biswas.

4 recordsLinked to original sources

arXiv2025

Observation of moir\'e 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 moir\'e 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-moir\'e-pocket to intra-moir\'e-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 moir\'e exciton and a stabilization of moir\'e 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 moir\'e pockets compared with a flat sample. This creates an interesting platform to explore interaction among moir\'e trapped excitons and higher order quasiparticles.

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↗

arXiv2022

Tuning exciton complexes in twisted bilayer WSe2 at intermediate misorientation

Twist angle modifies the band alignment, screening, and interlayer (IL) coupling in twisted bilayers (tBLs) of transition metal dichalcogenides. Intermediate misorientation (twist angles > 15 degrees) bilayers (BLs) offer a unique opportunity to tune excitonic behavior within these concurrent physical mechanisms but are seldom studied. In this paper, we measure many-body excitonic complexes in monolayer (ML), natural BL, and tBL WSe2. Neutral biexciton (XX) is observed in tBL, while being undetected in nonencapsulated ML and BL, demonstrating unique effects of disorder screening in tBLs. The XX as well as charged biexciton are robust to thermal dissociation and are controllable by electrostatic doping. Vanishing of momentum-indirect IL excitons with increasing electron doping is demonstrated in tBL, resulting from the near alignment of Q-K and K-K valleys. Intermediate misorientation samples offer a high degree of control of excitonic complexes while offering possibilities for studying exciton-phonon coupling, band alignment, and screening.

physics.optics↗

arXiv2019

Strong Single- and Two-Photon Luminescence Enhancement by Nonradiative Energy Transfer across Layered Heterostructure

The strong light-matter interaction in monolayer transition metal dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their direct band gap nature and the ultra-fast radiative decay of the strongly bound excitons these materials host. However, the impeded amount of light absorption imposed by the ultra-thin nature of the monolayers impairs their viability in photonic applications. Using a layered heterostructure of a monolayer TMD stacked on top of strongly absorbing, non-luminescent, multi-layer SnSe2, we show that both single-photon and two-photon luminescence from the TMD monolayer can be enhanced by a factor of 14 and 7.5, respectively. This is enabled through inter-layer dipole-dipole coupling induced non-radiative Forster resonance energy transfer (FRET) from SnSe2 underneath which acts as a scavenger of the light unabsorbed by the monolayer TMD. The design strategy exploits the near-resonance between the direct energy gap of SnSe2 and the excitonic gap of monolayer TMD, the smallest possible separation between donor and acceptor facilitated by van der Waals heterojunction, and the in-plane orientation of dipoles in these layered materials. The FRET driven uniform single- and twophoton luminescence enhancement over the entire junction area is advantageous over the local enhancement in quantum dot or plasmonic structure integrated 2D layers, and is promising for improving quantum efficiency in imaging, optoelectronic, and photonic applications.

cond-mat.mes-hall↗