Suman Chatterjee

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.

Van der Waals layered materials are emerging as promising nanophotonic platforms for realizing linear and nonlinear optical devices. In particular, the high refractive indices of transition metal dichalcogenides, combined with their strong layer dependent optical properties offer interesting possibilities for designing photonic functionalities. In this study, the resonant enhancement of second harmonic generation (SHG) using molybdenum disulfide (MoS ₂ ) disks on a silicon substrate with an intermediate silicon dioxide (SiO ₂ ) layer is demonstrated. The resonant enhancement observed here is attributed to the combined interaction of the anapole scattering modes supported within the disk and Fabry‐Perot (FP) modes within the multilayer stack. For a fixed MoS ₂ disk dimensions, an optimized SiO ₂ layer thickness results in constructive interference between the anapole and FP modes, thereby enhancing the fundamental field within the disk. Utilizing this resonance mechanism, SHG enhancements as high as 160 and 85‐times are experimentally demonstrated when compared with unpatterned multilayer and monolayer MoS ₂ flakes respectively. These are the highest reported enhancement values for the widely used 2H‐polytype MoS ₂ . A significant enhancement in SHG is observed here for 2H‐MoS ₂ despite the alternating sheet dipoles canceling the far‐field emission owing to the strong electric field gradient setup within the disk at the anapole resonance.

Transition-metal dichalcogenides (TMDs) host tightly bound electron-hole pairs—excitons—which can be either optically bright or dark based on spin and momentum selection rules. In tungsten-based TMDs, a momentum-forbidden dark exciton is the energy ground state, and therefore, it strongly affects the emission properties. In this work, we brighten the momentum-forbidden dark exciton by placing monolayer tungsten disulfide on top of nanotextured substrates, which imparts tensile strain, modifying its electronic band structure. This enables phonon-assisted exciton scattering between momentum valleys, thereby brightening momentum-forbidden dark excitons. In addition to offering a tuning knob for light-matter interactions in two-dimensional materials, our results pave the way for designing ultrasensitive strain-sensing devices based on TMDs.

Excitonic states trapped in harmonic moiré wells of twisted heterobilayers is an intriguing testbed for exploring many-body physics. 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 WS₂/WSe₂ 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 ~ 1 meV/μW - a five-fold enhancement over previous reports.

Strong Coulomb interactions in monolayer semiconductors allow 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-WS₂ 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 time scale 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 interlayer charge transfer, exemplifies the promise of manipulating many-body effects in monolayers toward viable optoelectronic applications.

The monolayers of semiconducting transition metal dichalcogenides host strongly bound excitonic complexes and are an excellent platform for exploring many-body physics. Here we demonstrate a controlled kinetic manipulation of the five-particle excitonic complex, the charged biexciton, through a systematic dependence of the biexciton peak on excitation power, gate voltage, and temperature using steady-state and time-resolved photoluminescence. With the help of a combination of the experimental data and a rate equation model, we argue that the binding energy of the charged biexciton is less than the spectral separation of its peak from the neutral exciton. We also note that while the momentum-direct radiative recombination of the neutral exciton is restricted within the light cone, such restriction is relaxed for a charged biexciton recombination due to the presence of near-parallel excited and final states in the momentum space.

The strong light-matter interaction in monolayer transition metal dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their direct bang 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 Förster 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 two-photon 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.