Arijit Kumar De

Low-dimensional halide perovskites are promising materials because of their excellent optoelectronic and photovoltaic properties, rendering them a propitious choice for device applications. Here we elucidate the photophysics and ultrafast relaxation dynamics in two-dimensional Cs2AgBiBr6 double-perovskite nanoplatelets. At room temperature, a photoluminescence band higher in energy than that of the three-dimensional counterpart is observed Surprisingly, we observe that the emissive state can be directly photoexcited. Another interesting observation is that at 77 K, the photoluminescence spectrum shows a diminished higher-energy band and a prominent lower-energy band similar to that in three-dimensional systems. Based on these findings, we rationalize that the population from the initially photoexcited state branches into the ground state and a state responsible for the higher-energy band, and that the dynamics is governed by heterogeneity. Thus, the present work underscores the importance of understanding the microscopic processes governing the excited-state relaxation dynamics of two-dimensional perovskites.

We set out to resolve a long‐standing dilemma regarding the contradictory claims for the less or equal effectiveness of high‐repetition‐rate ultrashort pulsed excitation compared with continuous‐wave (CW) excitation in optical trapping of dielectric microparticles. We present a comparative analysis of the trap stiffness of polystyrene and silica microparticles under CW and femtosecond pulsed excitation. We investigate how optical nonlinearity influences trap stiffness and highlight the distinct responses exhibited by polystyrene and silica particles under different excitations. Our results explain previous confounding results of trap stiffness under pulsed excitation compared to CW excitation in optical trapping of microscopic particles.

Many light-driven chemical processes (for example, excited state proton transfer, twisted intramolecular charge transfer, etc.) involve excited state potential energy surfaces having multiple local minima, driving the course of photochemistry. To unveil ultrafast coherent dynamics in such systems, we theoretically explore the excited-state linear wavepacket interferometry (WPI) upon excitation by time-delayed ultrafast pulse-pairs, modelling the excited state potential as a symmetric double-well potential. The temporal as well as spatiotemporal oscillations in excited-state population resulting from interference between wavepackets are simulated over a long period of time (of several tens of picoseconds), capturing tunnelling and reflection, both at zero temperature and at finite temperature. The influences of tuning molecular and excitation parameters, i.e., height of the barrier separating the wells and interpulse phase-locking frequency, on these oscillations are also explored. The localisation of population in either the left well or the right well as a function of interpulse delay is examined and shown to be controlled by chirping of the pulses. Further, we simulate the differential Shannon entropy as a function of time, replicating the wavepacket dynamics. Finally, we show strategies of quantum control and establish a connection between WPI in a double-well and qubits. We also extend our study to the asymmetric double-well potential to shed light on dynamics in real physical systems. Therefore, our study underscores the importance of WPI in molecular systems, having prospective applications in quantum computation and quantum information.

Mimicking the efficiency of natural photosynthesis for multielectron CO ₂ photoreduction remains a central challenge in solar fuel research. Toward this goal, we developed herein a noble metal free supramolecular ion‐pair assembly comprising rhodamine B ( RB ), an underexplored organic photosensitizer, and a cobalt(II) tris(2‐pyridylmethyl)amine complex ( 1Co ), which self‐assembles into core–shell nano/micro‐assembly via counterion‐mediated contact ion pairing. This confined ion pair architecture promotes counterion mediated, light‐driven hopping electron transfer, suppressing geminate recombination and enabling efficient charge migration, as demonstrated by optical spectroscopy, femtosecond–nanosecond transient absorption, electrochemical impedance spectroscopy, and 2D mass spectrometry studies. Orthogonal dual‐light excitation activates radical anion excited states (RB ^•−* ) within the assembly, enabling a diffusion‐independent cascade electron transfer pathway. This unique approach achieves selective CO ₂ photoreduction to syngas under single‐light irradiation and switches product distribution toward methane generation under dual‐light conditions‐ a rare feat for photocatalytic CO ₂ reduction systems. This work introduces a generalizable supramolecular ion‐pairing strategy for engineering spatially preorganized donor–acceptor architectures that harness high‐energy radical excited states for sustainable solar fuel generation.

In a quest to find an optimal laser beam shape for the most stable optical trap, here we investigate relative trapping efficiencies for a set of concentric annular beams, comparing with the conventionally used Gaussian beam. Combining an intuitive theoretical model considering a focused Gaussian beam decomposed into concentric light cones with an experiment implementing laser beam-shaping, we show that contrary to the preconception that tighter focusing would lead to better trapping, the efficiency depends on a balancing act not only among gradient force and scattering force but aberration effects as well. Furthermore, we present a new technique for controlled axial manipulation of a trapped particle by utilizing the axial focus shifts that are intrinsic to each annular beam in conjunction with real-time switching between ring beams through the use of a spatial light modulator. This approach paves the way for practical applications requiring controlled three-dimensional particle manipulation by enabling stable axial positioning across long distances.

The core–shell type structure of metal-dielectric nanoparticles has great applications in the fields of nanophotonics and biomedical sciences due to the plasmonic properties of metals and field enhancements at metal-dielectric interfaces. The present study reveals critical insights into the influence of these properties on the optical trapping of core–shell nanoparticles, investigated using generalized Lorenz-Mie theory. Specifically, the presence of Fano resonance is discussed based on the size contribution of the core and shell and its influence on the directionality of the scattering force. Furthermore, the effects of optical nonlinearity on the appearance of Fano resonance and trapping efficiency are explored under femtosecond pulsed excitation.

Two different types of self-assemblies have been fabricated from 2,2'-bithiophene-5,5'-dicarboxaldehyde (BTDA) and 2,2':5',2''-terthiophene-5,5''-dicarboxaldehyde (TTDA) molecules through reprecipitation methods. Unlike the BTDA molecule, one thiophene unit remains out of plane with respect to the other two thiophene units in TTDA. This results in partial face-to-face intermolecular interactions during self-assembly, causing the formation of a defect-mediated emissive trap state at longer wavelengths. These trap states are charge-transfer types with a longer lifetime compared to the pure TTDA molecule. Detailed computational studies and femtosecond transient absorption spectroscopy support the findings. The appearance of the charge transfer type trap state in TTDA self-assembly facilitates the photoinduced charge separation and free carrier accumulation, which helps to boost the photocatalytic solar H₂ production. However, highly defined head-to-tail molecular arrangements in BTDA self-assembly hinder the formation of such an emissive trap state, resulting in a decrease in the photocatalytic efficiency.