Chinmoy Biswas

High-resolution, momentum-resolved photoelectron emission microscopy is employed to investigate ejected electrons extending all the way from the directly emitted primary photoelectrons to the slow secondary electrons resulting from the ensuing collision cascade. Excitation with unpolarised radiation from a helium light source enables the valence band to be mapped across the entire first surface Brillouin zone. The measured quasi-particle bands show very good quantitative agreement with state-of-the-art density functional theory supplemented by many-body corrections within the GW approximation, establishing a reliable reference for the initial-state electronic structure. Extreme-ultraviolet light of controlled polarisation from a high-harmonic generation source is used to access high-lying conduction-band states. The polarisation-dependent momentum distributions of directly photoemitted electrons reveal the roles of initial-state symmetry and final conduction-band states. In the low-energy secondary-electron regime we map out, for the first time, the momentum-space distribution of the quasi-bound in-plane resonance in the conduction band which acts as a doorway state for secondary-electron emission through the X-peak. Six-fold-symmetric intensity patterns at characteristic kinetic energies above vacuum, independent of photon energy and light polarisation, indicate cascading secondary-electron emission through preferential conduction-band channels. These nearly non-dispersive features arise from hybridised, sp2-derived σ∗ conduction-band states. By disentangling contributions from the valence bands, conduction bands mediating direct photoemission, and low-energy resonances populated by the slowing-down electron cascade, our results provide unprecedented experimental access to the valence-band electronic structure and multiple emission channels of bulk graphite, relevant also to other layered quantum materials.

Dynamic interference is an elusive strong-field effect where photoelectrons from intense laser pulses interfere in time, forming rich kinetic energy patterns. Here, we present the first experimental demonstration of isolated dynamic interference using a novel two-color scheme: chirped laser-assisted dynamic interference. Isolation was achieved with a crossed-polarization setup combining an extreme ultraviolet harmonic field and an infrared pulse with tailored spectrotemporal properties. Beyond prior works, our approach enables precise control over interfering trajectories, yielding holographic interference patterns and advancing our understanding of strong-field phenomena.

Zinc porphyrins (ZnP) are an important class of small molecule donors that have gained significant attention in the domain of organic photovoltaics. In this work, we investigate and correlate various photophysical and third-order nonlinear optical (NLO) properties of three different zinc porphyrin molecules: a monomer – ZnP1 (ZnP-TEH-DPP) with two diketopyrrolopyrrole (DPP) units bridged to zinc porphyrins, its corresponding porphyrin dimer ZnP2 (ZnP2-DPP) with an additional porphyrin unit at the centre, and another porphyrin dimer ZnP3 (HDDPP-EHTOM) having two zinc porphyrin donors linked with DPP and endcapped with 3-ethylrhodanine. To investigate the excited state dynamics of the systems, a femtosecond transient absorption spectroscopy (fs-TAS) experiment was employed in the visible range by exciting the molecules at 2.53 eV. The estimated lifetime values from the global analysis were in the range of 230–560 fs, 10–27 ps, and 200–350 ps, which is attributed to the internal conversion, vibrational relaxation, and relaxation from the lowest excited singlet state, respectively. Furthermore, the NLO features of ZnP1, ZnP2, and ZnP3 were investigated using a femtosecond Z-scan technique. All the three samples exhibited saturable absorption (SA) at lower intensities and SA followed by reverse saturable absorption (RSA) at higher input intensities. The second hyperpolarizability (γ) from closed aperture Z-scan measurements was estimated to be 5.72 × 10⁻³¹, 7.02 × 10⁻³¹, and 10.6 × 10⁻³¹ esu for ZnP1, ZnP2, and ZnP3, respectively, which were consistent with the theoretically obtained ones using the DFT calculations. Zinc porphyrin dimers (ZnP2 and ZnP3) having extended π-conjugated systems exhibit better NLO properties in comparison to the monomer ZnP1. The observed properties of these samples revealed that these porphyrins can be useful for organic photovoltaic as well as other NLO applications.

With the increasing dependence on fluorescence bioimaging, luminogens with aggregation-induced emission (AIE) properties have gained significant attention due to their excellent photostabilization, minimal photobleaching, high reliability, and superior biocompatibility. Since mitochondria are crucial subcellular organelles in eukaryotic cells with important biological functions, organelle-specific AIE emitters with distinct functions have been highly sought after, but with limited success using simple synthetic methods. Here, we describe a strategy for synthesizing two triphenylamine (TPA) based acrylonitriles, tethered to different donor groups, TPA and phenothiazine (PTZ), respectively, with superior AIE properties using Suzuki coupling. We conducted a systematic and detailed experimental analysis of the structural characteristics of both AIE luminogens, which exhibited excellent photostability, a large Stokes shift, and bright solid-state emission. A cell viability study carried out with F1 and F2 dyes revealed that both luminogens exhibited excellent biocompatibility. Based on fluorescence experiments, F2 displayed excellent AIE characteristics, permeability, biocompatibility, and photostability compared to rhodamine 123, allowing it to selectively stain and track mitochondria in cancer cells over an extended period of time. The Pearson correlation coefficient of F2 and rhodamine 123 was estimated to have an r-value of 0.99. Our findings are expected to provide insight into the synthesis of an extensive archive of AIE-based acrylonitriles with fascinating properties for mitochondrial staining.

Thiophene Substituted Copper Phthalocyanines (ThPcCu), used as hole transport material in perovskite solar cell fabrication, is investigated for its ultrafast excited state dynamics. ThPcCu data shows long-lived GSB kinetics ≈ 4.5ns and long-lived triplets.

Ultrafast photo charge-generation kinetics are investigated in bulk heterojunction blend of zinc porphyrin and fullerene utilizing near infra-red femtosecond transient absorption spectroscopy. Data revealed charge formation < 100fs and long-lived charge kinetics.