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Using dipole approximation, a comparative study of trapping force/potential on bare, core/shell and core/shell/shell types of dielectric nanoparticles is presented. The optimal conditions for high-force or long confinement time are discussed.

Light harvesting dyads consisting of β-monofunctionalized mono- and distyryl-BODIPYs and perylenediimides show ultrafast charge separation and slower charge recombination probed by steady-state and transient absorption spectroscopy.

Charge-transfer excited state (CTES) defines the ability to split photon energy into work producing redox equivalents suitable for photocatalysis. Here, we report inter-net CTES formation within a two-fold catenated crystalline metal-organic framework (MOF), constructed with two linkers, N,N'-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNDI) and 2,6-dicarboxynaphthalene (NDC). The structural flexibility puts two complementary linkers from two nets in a proximal position to interact strongly. Supported by the electrochemical and steady-state electronic spectroscopic data, this ground-state interaction facilitates forming CTES that can be populated by direct excitation. We map the dynamics of the CTES which persists over a few nanoseconds and highlight the utilities of such relatively long-lived CTES as enhanced conductivity of the MOF under light over that measured in dark and as a proof-of-the-principle test, photo-reduction of methyl viologen under white light.

Singlet fission (SF) is a phenomenon for the generation of a pair of triplet excitons from anexcited molecule in singlet electronic state interacting with another adjacent molecule in its ground electronic state. By increasing the effective number of charge carriers and reducing thermal dissipation of excess energy, SF is promised to enhance light-harvesting efficiency for photovoltaic applications. While SF has been extensively studied in thin films and crystals, the same has not been explored much within a confined medium. Here, we report the ultrafast SF dynamics of triisopropylsilylethynyl pentacene (TIPS-Pn) in micellar nanocavity of varying sizes (prepared from TX-100, CTAB, and SDS surfactants). The nanoparticles with a smaller size contain weakly coupled chromophores which are shown to be more efficient for SF followed by triplet generation as compared to the nanoparticles of larger size which contain strongly coupled chromophores which are less efficient due to the presence of singlet exciton traps. Through these studies, we delineate how a subtle interplay between short-range and long-range interaction among chromophores confined within nanoparticles, fine-tuned by the curvature of the micellar interface but irrespective of the nature of the micelle (cationic or anionic or neutral), play a crucial role in SF through and generation of triplets.

Lead-free halide perovskites, as environment-friendly materials, have received critical interest in photovoltaic applications. In this regard, the bismuth halide perovskites demonstrate better stab...

Photocycle in wild-type green fluorescent protein (wt-GFP) involves generation of a bright fluorescent deprotonated chromophore from feebly fluorescent protonated form via excited-state proton transfer. In addition to this usual photocycle, wt-GFP is also known to exhibit irreversible photoconversion upon illumination with ultraviolet and visible radiation. However, a detailed understanding of photoconversion in enhanced GFP (EGFP: S65T/F64L mutant of wt-GFP), which predominantly exists in deprotonated form, is yet to be explored. Using 254 nm irradiation, we study how photoconversion proceeds in EGFP. The key findings are observation of spreading out of an isosbestic point and existence of an initial lag phase in spectral kinetics of absorbance, indicative of sequential photoconversion through an intermediate. Fluorescence kinetics of EGFP and its photoproduct are estimated by assigning two unique fluorescence lifetimes which is further complicated by the fact that their fluorescence are spectrally inseparable, as evident from global analysis of fluorescence lifetime. Time-resolved fluorescence anisotropy studies further suggest minimal structural changes in protein scaffold upon photoconversion. Based on these findings, an analytic model is developed to account for the overall decay in fluorescence (as photoconversion proceeds) that inherently incorporates the initial lag phase and a summary of energetics and processes involved is provided.

Among the all-inorganic lead halide perovskites, CsPbI3 has emerged as a competent photovoltaic material because of its enhanced stability and comparable efficiency to that of organic–inorganic hyb...

Crucial to effective optical trapping is the ability to precisely control the nature of force/potential to be attractive or repulsive. The nature of particles being trapped is as important as the role of laser parameters in determining the stability of the optical trap. In this context, hybrid particles comprising of both dielectric and metallic materials offer a wide range of new possibilities due to their tunable optical properties. On the other hand, femtosecond pulsed excitation is shown to provide additional advantages in tuning of trap stiffness through harnessing optical and thermal nonlinearity. Here we demonstrate that (metal/dielectric hybrid) core/shell type and hollow-core type nanoparticles experience more force than conventional core-type nanoparticles under both continuous-wave and, in particular, ultrafast pulsed excitation. Thus, for the first time, we show how tuning both materials properties as well as the nature of excitation can impart unprecedented control over nanoscale optical trapping and manipulation leading to a wide range of applications.

The usefulness of a chirped broadband probe and spectral dispersion to obtain Raman spectra under nonresonant/resonant impulsive excitation is revisited. A general methodology is presented that inherently takes care of phasing the time-domain low-frequency oscillations without probe pulse compression and retrieves the absolute phase of the oscillations. As test beds, neat solvents (CCl

The effect of micellar nanocavities on ultrafast singlet fission dynamics of TIPS-pentacene is explored. It is shown how short- and long-range interactions among chromophores affect the efficiency of singlet fission, thereby the yield of triplets.

We explore dielectric-controlled excited-state dynamics of a push-pull stilbene undergoing twisting and/or isomerization and demonstrate how global analysis of pump-probe data aids in elucidating involvement of interconverting conformers and/or isomers, often obscured in single-wavelength kinetics.

We investigate ultrafast dynamics of excited-state proton transfer coupled with cis-trans isomerization in the red fluorescent protein mKeima, elucidating the mechanism of “reverse protonation” and how it is fine-tuned by pH of the local environment.

Ultrafast electron and hole relaxation dynamics in microcrystals of a layered double-perovskite in two solvents is studied and found to proceed at different rates depending on the nature of charge carriers as well as solvents.

In this work, synergistic effects or hydrogen bond acceptor - donor (HBA - HBD) interactions in the binary mixture of deep eutectic solvents (DESs) with organic co-solvent, are investigated using impulsive stimulated Raman spectroscopy (ISRS).

Planar carbazole based hexaphyrin-like macrocycles with bis-coordinating cores and box-shaped cyclic BODIPYs were synthesized. Solution and solid-state structure analysis of the free macrocycles indicates an inversion of two pyrrole rings, resulting in a two-dipyrrin-like environment. The BF2 complexes show large Stokes shifts and exhibit excitonic coupling, fine-tuned by the meso-substituents.

Serendipitous observations offer newer insights into materials properties. Here we describe g-C3N4 nanosheets exhibiting remarkably blue-shifted photoluminescence within the 390–580 nm range centred at 425 nm which matches more closely with its ‘quantum-dots’, and yet is excitation-wavelength independent. Moreover, surprisingly, the luminescence decay becomes increasingly slow at higher emission wavelengths, by up to 10 times. The nanosheets absorb only a fraction of visible-light. To improve this, when we designed a nanocomposite of g-C3N4 and Au nanoparticles (NPs), it retained the PL characteristics and also exhibited excellent light-harvesting and photocatalytic efficiency for benzylamine (BA) oxidation. The surface plasmon resonance of the Au NPs is responsible for the high visible-light response and assisting the reaction by the ‘hot-electron’ injection mechanism, while an uncharacteristic, pronounced co-catalytic effect by them further improves the efficiency. The conversion is 98% with >99% selectivity over 1.5 h of natural sunlight and open-air, probably the highest efficiency for BA photo-oxidation reactions. Even though such a dual role by plasmonic NPs has remained unscrutinized, we argue that the charge-transfer processes for the ‘hot-carrier’ injection and co-catalytic events are well-separated in time due to a much slower photon incident frequency on a catalyst particle, enabling both to occur simultaneously in the same particle.

Controlling excitation energy transfer within self-assembled molecular aggregates has been a major challenge since such systems represent as prototypes for highly efficient light-harvesting pigment assemblies within photosynthetic organisms. Using time-resolved fluorescence depolarization studies, here we report complete reversal of trends in anisotropy decay traces up on increasing fluorophore concentration while traversing a series of solvent mixtures (water and glycerol) with increasing viscosity. For glycerol, in dilute solution depolarization results from rotational diffusion whereas Forster resonance energy transfer (FRET) is found to be the key mechanism in concentrated solution; in contrast, for water, rotational diffusion is the dominant reason for fluorescence depolarization at all concentrations. We further investigate how viscosity, rather than dielectric constant of the solvent, controls contribution from FRET towards depolarization by partially immobilizing the fluorophores. The results are explained within the framework of analytical models for fluorescence depolarization due to either rotational diffusion or FRET. We further examine the unusual ‘dip-and-rise’ behavior in anisotropy traces for intermediate range of concentration which is suggestive of presence of heterogeneity within homochromophoric system. These results show how one can fine-tune the explicit nature of solvent to facilitate FRET over other competing dynamical processes.

Under short pulsed excitation, depending on the nonlinear refractive index of the particle and the surrounding medium, optical nonlinearity plays a significant role in modulating optical forces on particles. Here, we explore the trapping forces experienced by dielectric nanoparticles in a highly nonlinear medium taking into account nonlinear propagation of Gaussian beam.

Using „spectrally dispersed’ impulsive stimulated Raman spectroscopy, vibrational spectra arising from motion of nuclear wavepackets in ground and excited electronic states of iodine in carbon tetrachloride are recorded isolating from solvent modes.

We present a comparative study on optical trapping dynamics for particles of different sizes. . The trapped particles’ confinement time is found to be nicely correlated with the axial trap stiffness.

Recent theoretical and experimental results have shown how the trapping force/potential can be dramatically modulated due to optical and thermal nonlinearity. Compared with dielectrics, metals show even more interesting behavior (for example, trap-splitting, enhanced forward scattering, etc.) owing to higher-order optical nonlinearities. Hence, we present a comparison study for dielectric and metallic nanoparticles using generalized Lorenz-Mie theory.

Recently, the effect of optical nonlinearity in laser trapping has been investigated under pulsed excitation, and it was observed that the inclusion of nonlinearity significantly modulates trapping potential for metallic nanoparticles using dipole approximation. In this paper, we present theoretical studies on nonlinear laser trapping for silver nanoparticles using generalized Lorenz-Mie theory. We observe a reversal in the direction of asymmetry of potential well and splitting of the potential well due to nonlinear effects which is further modulated with an increase in laser power.

Hybrid nanoparticles have gained intense attention in the field of optical trapping due to their potential wide-ranging applications, for example, in drug delivery. The utility of the optical Kerr effect in modulating trapping force and potential under high-repetition-rate ultrafast pulsed excitation has recently been realized for dielectric and metallic particles ranging from micron to nanometer. However, in the context of hybrid nanoparticles, the mechanism of trapping is yet to be fully explored. Here, we present a comparative study of trapping force and potential on conventional, hybrid, and hollow-core type nanoparticles using generalized Lorenz-Mie theory and dipole approximation incorporating third-order optical nonlinearity. We find an explicit advantage of using pulsed excitation over continous-wave excitation which can have potentially far-reaching practical applications.

Photophysical properties of tricarbocyanine dyes in various solvents have been widely investigated using a variety of spectroscopic tools. However, the presence of several ground-state isomers and interconversion between these isomers on an ultrafast timescale upon photoexcitation render unambiguous assignment of spectral features quite difficult. In this work, ultrafast excited-state dynamics of two tricarbocyanine dyes in two solvents, DNTTCI and IR140, in ethanol and ethylene glycol, are studied by two-dimensional electronic spectroscopy (2DES). We present a detailed discussion on design and calibration of the 2DES apparatus and on the method for data processing by phase-cycling. For DNTTCI we report a method to obtain solvation correlation function, the nature of which is found to be strongly dependent on the excitation frequencies; a blue-shifted spectrum at early time is observed and explained based on preferential emission from a subset among various isomers having overlapping spectral features. For IR140 in ethanol, four isomers with distinct spectral features are identified, and most importantly, three of these isomers were found to interconvert upon photoexcitation which completes within 100 fs and is explained based on a kinetic model of consecutive chemical reaction. Density functional theory calculations show the presence of several ground-state isomers for both these dyes. Through this work we demonstrate how 2DES can help us to decipher distinct excited-state photophysics in two carbocyanine dyes, polar solvation and photoisomerization, by resolving spectral congestion without sacrificing time resolution.

The increasing energy demand and resulting decay in the conventional energy resources indicate to find viable and affordable alternative resources. The cell efficiency and properties of absorber/window layers concerned can be tailored by the different post-treatments and therefore an evolution of post-treatment atmosphere conditions is undertaken in this work to enhance the performance of vapor proceed CdZnTe solar cells. Firstly, the microstructural and optical properties of absorber CdZnTe layer were optimized where films have zinc blende cubic structure with maximum crystallinity for Ar + O2 atmosphere while the energy band gap is influenced by the annealing in different atmospheres. Secondly, CdZnTe solar cells were fabricated using vapor proceed technique with device structure ITO/CdS/CdZnTe/Au and performance analysis is done by varying the atmosphere of post-annealing treatment. The device treated at 400 °C in the Ar + O2 atmosphere has maximum cell efficiency of 8.49% and more trap-states crossed the Fermi level as confirmed by Mott-Schottky plots. The dopant density and built-in potential are also evaluated. The findings of this work reveal that the efficiency of CdZnTe solar cells can be enhanced by the post-annealing treatment in different atmospheres.

We present a detailed theoretical study on choosing optimum excitation parameters for maximizing the fluorescence yield. Using a model system, we show how the time-averaged emission (fluorescence) is modulated as the excitation conditions are changed from continuous wave excitation to pulsed excitation with various combinations of pulse widths and pulse repetition-rates for the same time-averaged excitation intensity. We conclude that depending on the excitation intensity, different pulse parameters are required for generating maximum fluorescence output. Our method can be implemented for other more sophisticated models without much difficulty.

Xanthene dyes have wide ranging applications as fluorescent probes in analytical, biochemical and medical contexts. Being cationic/anionic in nature, the solvation dynamics of xanthene dyes confined within a negatively/positively charged interface are very interesting. Unfortunately, the floppy structure and small Stokes shift render any xanthene dye unsuitable for use as a solvation probe. Using di-sodium fluorescein, we present our work on the picosecond solvation dynamics of bulk and confined water (at pH = 9.2). We also propose a new methodology for studying picosecond solvation dynamics using any fluorescent dye with a small Stokes shift. We discuss how scattering contributions can be effectively removed, and propose an alternative way of defining zero time of solvation. Finally, we demonstrate the tuning location of the probe within confinement.

Fluorescent proteins exhibit interesting excited state photochemistry, leading to bright fluorescence emission that renders their versatile biological role and wide use as biomarkers. A molecular-level mechanism of the excited state dynamics is desirable to pinpoint the origin of the bright fluorescence of these proteins. Here we present studies on a yellow fluorescent protein variant, Venus, and investigate the photophysics behind the dual fluorescence emission upon UV excitation. Based on our studies, we propose that the unique nature of the potential energy surface is responsible for the observation of minor fluorescence in Venus which is not seen in wild type GFP.

Using femtosecond pump-probe spectroscopy, we explore the ultrafast excited state relaxation dynamics of two types of supramolecular assemblies (rod-shaped and spherical micelles) of chlorophyll-a, formed at different ratios of ACN/water.

The illusive nature of optical trapping dynamics under high repetition-rate femtosecond pulsed excitation has recently been theoretically explained based on nonlinear nature of force and potential arising from the optical Kerr effect. Here we present experimental results of trapping of 1 µm polystyrene beads probed by analyzing back-scattered signal from real-time video microscopy which is helpful in studying the time control dynamics of micron-sized objects suitable for biological applications.

Recently, the role of ultrafast pulsed excitation in laser trapping of dielectric nanoparticles has been explored [1–4] and it was observed that optical Kerr effect (OKE) plays an important role in determining the stability of the trap [2–4]. Here, we theoretically investigate trapping behaviour of metallic (silver) nanoparticles and studied the effect of OKE (up to sixth order) under high repetition-rate femtosecond pulsed excitation. We observe that the trapping potential well splits into two wells along axial direction, enabling the study of long-range interaction between the metal nanoparticles.

Recently, the role of ultrafast pulsed excitation in laser trapping of dielectric nanoparticles has been explored [1–4] and it was observed that optical Kerr effect (OKE) plays an important role in determining the stability of the trap [2–4]. Using Generalized Lorentz Mie Theory (localized approximation) [5], here we theoretically investigate how optical trapping force/potential on a hollow-core polystyrene nanoparticle becomes repulsive to attractive in nature under high repetition-rate femtosecond pulsed excitation.

There is presently an increased research activity in understanding the nature of optical force when ultrashort pulsed excitation is used to trap and manipulate objects with sizes ranging from micrometers down to nanometers. Such a nonlinear laser tweezer is peculiarly promising because the nature of the force can be dramatically tuned owing to optical nonlinearity. However, accurate measurement of pulse-width at the sample position, force calibration, and high price, not encountered with a conventional laser tweezer, limits its practical use. Here, we describe design principles and force calibration of a nonlinear laser tweezer which is inexpensive, completely home-built, easy to be moved, and has a multitude of detection modalities. We also show how pulse-width at the sample position can be measured in a facile way and how trap stiffness varies with sample planes and, most importantly, with the nature of excitation (continuous-wave vs pulsed).

The 2018 Nobel Prize in Physics was awarded for the invention of optical trapping and generation of ultrashort pulses, which revolutionized many areas of modern science and technology. However, physics of optical trapping under ultra-short pulsed excitation has not been explored much. The nonlinear nature of optical trapping force/potential under ultra-short pulsed excitation was theoretically investigated, however, without any direct experimental demonstration and development of any generalized theory independent of the particle size. In this work, we present a methodology to numerically estimate trapping force/potential including optical as well as thermal nonlinearity under ultra-short pulsed excitation and implement a variety of detection modalities to capture the particle's real-time trajectories. We show how highly asymmetric nonlinear axial potential, created by a femtosecond pulse-train, can be mapped from the dynamics of the trapped particle. Considering fine-tuning of trap-stiffness by changing nonlinearity, we envision far-reaching applications of using ultra-short pulsed excitation in laser trapping and manipulation.

The dynamic role of solvent in influencing the rates of physico-chemical processes (for example, polar solvation and electron transfer) has been extensively studied using time-resolved fluorescence spectroscopy. Here we study ultrafast excited state relaxation dynamics of three different fluorescent probes (DNTTCI, IR-140 and IR-144) in two polar solvents, ethanol and ethylene glycol, using spectrally resolved degenerate pump-probe spectroscopy. We discuss how time-resolved emission spectra can be directly used for constructing relaxation correlation function, obviating spectral reconstruction and estimation of time-zero spectrum in non-polar solvents. We show that depending on the specific probe used, the relaxation dynamics is governed either by intramolecular vibrational relaxation (for IR140) or by intermolecular solvation (for DNTTCI) or by both (for IR144). We further show (using DNTTCI as a probe) that major differences in solvation by ethanol and ethylene glycol is contributed by early time (