Ashish Arora

Two-dimensional (2D) materials (Graphene, 2D transition metal dichalcogenides (MX ₂ ), hBN) have emerged as exciting materials for discovering novel physical phenomena, and have great potential for electronic and optoelectronic devices, as well as energy and catalysis applications. Further, materials such as Cr ₂ X ₂ Te ₆ (X = Si, Ge), VSe ₂ , Fe ₃ GeTe ₂ , CrX ₃ (X = Cl, Br, I) and MPX ₃ (M = Mn, Fe, V, Zn, Co, Ni, Cd, Mg; X = S, Se) have emerged as fascinating platforms for 2D magnetism. The heterostructures of these 2D materials, and the twist angle between layers are opening new possibilities of studying the effect of Moiré potential, exotic topological phases, magnetic-proximity effects, and strong correlations. At the same time, the research is leading towards many application-based directions such as in excitonics, valleytronics, straintronics, and quantum technologies. In this context, growth of novel 2D materials and heterostructures, as well as large scale growth becomes critically important. This roadmap aims to provide a comprehensive overview of 2D materials research in India and across the globe. The roadmap is broadly divided into six themes: 1. Topology in 2D Materials, Magnetism, Exotic Physics and Emergent Phenomena; 2. Moiré Physics in 2D Heterostructures; 3. Excitonic Physics and Optical properties, including Defects; 4. Transport Phenomena in 2D Materials; 5. 2D Materials Synthesis and Growth; and 6. 2D Materials Electrical Devices and Applications. This roadmap highlights the cutting edge-research being done by Indian and global researchers, emphasizes the challenges, and suggests possible technological solutions.

Controlling nonequilibrium responses in optically driven quantum materials is essential for advancing applications in energy conversion, ultrafast electronics, and quantum computation. Nonlinear optical spectroscopy serves as a powerful tool to investigate ultrafast electron and phonon dynamics in these systems; however, conventional nonlinear approaches often require intense laser pulses and typically encounter a strong background. Here, we introduce a phase‐sensitive nonlinear spectroscopic technique that operates at low pulse energies (~10 pJ) and enables real‐time monitoring and active control of coherent phonons in few‐layer (three to five layers thick) 2 H ‐MoTe ₂ . Upon excitation with ultrashort (~10 fs) pump pulses, we achieve displacive excitation of coherent phonons, which periodically modulate the Kerr nonlinearity of the material, leading to cross‐phase modulation (XPM) of a delayed probe pulse. This phase modulation induces spectral broadening and oscillations in the center of mass (COM) of the probe spectrum in time, enabling the detection of subtle nonlinear optical responses in a background‐free manner. The nonlinear response can be selectively amplified or attenuated by adjusting the strength of the pump pulse. A dual‐pump pulse scheme enables precise control of phonon oscillations, allowing selective activation or suppression of specific phonon modes and correspondingly the modulation of the Kerr nonlinearity.

Exciton-phonon interactions govern the optical response of semiconductors, yet disentangling multiple coupling channels in lead halide perovskites remains challenging. We investigate CsPbBr₃ microcrystals using photoluminescence, Raman, and reflectance spectroscopy at low temperature, revealing the simultaneous presence of high-energy and Rashba excitons, each accompanied by distinct phonon replica series. High-energy exciton replicas are uniquely spaced by approximately 9 meV, whereas Rashba exciton replicas exhibit a characteristic approximately 6 meV spacing, indicating the specificity of the exciton-phonon coupling. k-means clustering applied to a large low-temperature photoluminescence data set reveals these replica features are prevalent. With increasing temperature, replica features broaden and merge, evolving into a dominant longitudinal optical phonon coupling regime at room temperature. This work establishes direct spectroscopic evidence for concurrent, exciton-specific phonon coupling within a single material, offering pathways to engineer light-matter interactions for optoelectronic and phonon-photon-based quantum device applications.

Through systematic experimental and theoretical studies of layer-thickness-dependent absorption in semiconducting MoSe$_2$ and WS$_2$ across the visible to near-infrared spectral range, we demonstrate a universal absorption behavior in solids at nanoscale thicknesses. With increasing thickness, a non-monotonic evolution of absorption integrated over the measured spectral region is revealed which is accompanied by pronounced oscillatory features. This strongly deviates from the expected Beer-Lambert law. The observed behavior has origins in the electromagnetic interference effects taking place between the two surfaces of the thin crystals. The present work on 2D semiconductors is extendable to all kinds of solids such as conventional semiconductors (e.g. Si, GaAs, GaN, InP), (semi)metals (e.g. Al, Ag, Au, c-HOPG) and 2D magnetic materials (e.g. CrSBr and NiPS$_3$). Our results provide fundamental insights into light-matter interactions in solids at the nanoscale and are vital for optimally designing the new generation of absorption-based flexible optoelectronic devices.

We report on the observation of an excited 2s state of a trion in a 4.2 nm wide doped GaAs/Al_{0.3}Ga_{0.7}As quantum well using magneto-optical Kerr effect (MOKE) spectroscopy under out-of-plane magnetic fields up to 6 T. This resonance appears slightly below the 2s exciton in energy. Strikingly, the 2s trion is found to be bound only for magnetic fields larger than 1 T. The signature of the 2s trion is absent in the magnetoreflectance spectra, while it is detectable in the MOKE spectra signifying the importance of the powerful technique. Similar to the 1s states, the 2s trion shows an opposite degree of magnetic-field-induced polarization compared to its exciton counterpart, in agreement with our theoretical calculations. This transfer of oscillator strength between the complexes establishes an optical fingerprint of the 2s excited trion.

We report on the observation of an excited 2s state of a trion in a 4.2 nm wide doped GaAs/Al(0.3)Ga(0.7)As quantum well (QW) using magneto-optical Kerr effect (MOKE) spectroscopy under out-of-plane magnetic fields up to 6 T. This resonance appears slightly below the 2s exciton in energy. Strikingly, the 2s trion is found to be bound only for magnetic fields larger than 1 T. The signature of the 2s trion is absent in the magneto-reflectance spectra, while it is detectable in the MOKE spectra signifying the importance of the powerful technique. Similar to the 1s states, the 2s trion shows an opposite degree of magnetic-field-induced polarization compared to its exciton counterpart, in agreement with our theoretical calculations. This transfer of oscillator strength between the complexes establishes an optical fingerprint of the 2s excited trion.

Faraday rotation is a fundamental effect in the magneto-optical response of solids, liquids and gases. Materials with a large Verdet constant find applications in optical modulators, sensors and non-reciprocal devices, such as optical isolators. Here, we demonstrate that the plane of polarization of light exhibits a giant Faraday rotation of several degrees around the A exciton transition in hBN-encapsulated monolayers of WSe₂ and MoSe₂ under moderate magnetic fields. This results in the highest known Verdet constant of -1.9 × 10⁷ deg T⁻¹ cm⁻¹ for any material in the visible regime. Additionally, interlayer excitons in hBN-encapsulated bilayer MoS₂ exhibit a large Verdet constant (V_IL ≈ +2 × 10⁵ deg T⁻¹ cm⁻²) of opposite sign compared to A excitons in monolayers. The giant Faraday rotation is due to the giant oscillator strength and high g-factor of the excitons in atomically thin semiconducting transition metal dichalcogenides. We deduce the complete in-plane complex dielectric tensor of hBN-encapsulated WSe₂ and MoSe₂ monolayers, which is vital for the prediction of Kerr, Faraday and magneto-circular dichroism spectra of 2D heterostructures. Our results pose a crucial advance in the potential usage of two-dimensional materials in ultrathin optical polarization devices.

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS₂, MoSe₂, and WSe₂ van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe₂/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.