We present a novel experimental scheme for two-dimensional fluorescence-detected coherent spectroscopy (2D-FDCS) using a non-collinear beam geometry with the aid of "confocal imaging" of dynamic (population) grating and 27-step phase-cycling to extract the signal. This arrangement obviates the need for distinct experimental designs for previously developed transmission detected non-collinear two-dimensional coherent spectroscopy (2D-CS) and collinear 2D-FDCS. We also describe a novel method for absolute phasing of the 2D spectrum. We apply this method to record 2D spectra of a fluorescent dye in solution at room temperature and observe "spectral diffusion."

Here we report how ultrafast pulsed illumination at low average power results in a stable three-dimensional (3D) optical trap holding latex nanoparticles which is otherwise not possible with continuous wave lasers at the same power level. The gigantic peak power of a femtosecond pulse exerts a huge instantaneous gradient force that has been predicted theoretically earlier and implemented for microsecond pulses in a different context by others. In addition, the resulting two-photon fluorescence allows direct observation of trapping events by providing intrinsic 3D resolution.

The use of low-power high-repetition-rate ultrafast pulsed excitation in stable optical trapping of dielectric nanoparticles has been demonstrated in the recent past; the high peak power of each pulse leads to instantaneous trapping of a nanoparticle with fast inertial response and the high repetition-rate ensures repetitive trapping by successive pulses However, with such high peak power pulsed excitation under a tight focusing condition, nonlinear optical effects on trapping efficiency also become significant and cannot be ignored. Thus, in addition to the above mentioned repetitive instantaneous trapping, trapping efficiency under pulsed excitation is also influenced by the optical Kerr effect, which we theoretically investigate here. Using dipole approximation we show that with an increase in laser power the radial component of the trapping potential becomes progressively more stable but the axial component is dramatically modulated due to increased Kerr nonlinearity. We justify that the relevant parameter to quantify the trapping efficiency is not the absolute depth of the highly asymmetric axial trapping potential but the height of the potential barrier along the beam propagation direction. We also discuss the optimal excitation parameters leading to the most stable dipole trap. Our results show excellent agreement with previous experiments.

An effective z-scan setup with a minimum thermal effect is shown for intensity-dependent measure of two-photon absorption (TPA) with high-repetition rate lasers. Use of an additional intensity modulation with an optical chopper provides enough blanking time for a high-repetition rate laser to yield equally accurate results in TPA measurements compared to a low repetition laser. Extension of this method of thermal effect management with an optical chopper to emission studies also results in good correspondence for two-photon cross-section measurements from either z-scan or two-photon fluorescence techniques. The method also significantly enhances two-photon fluorescence, which could be promising for multiphoton microscopy.

We briefly review the coherent quantum beats observed in recent two-dimensional electronic spectroscopy experiments in a photosynthetic-light-harvesting antenna. We emphasize that the decay of the quantum beats in these experiments is limited by ensemble averaging. The in vivo dynamics of energy transport depends upon the local fluctuations of a single photosynthetic complex during the energy transfer time (a few picoseconds). Recent analyses suggest that it remains possible that the quantumcoherent motion may be robust under individual realizations of the environmentinduced fluctuations contrary to intuition obtained from condensed phase spectroscopic measurements and reduced density matrices. This result indicates that the decay of the observed quantum coherence can be understood as ensemble dephasing. We propose a fluorescence-detected single-molecule experiment with phase-locked excitation pulses to investigate the coherent dynamics at the level of a single molecule without hindrance by ensemble averaging. We discuss the advantages and limitations of this method. We report our initial results on bulk fluorescence-detected coherent spectroscopy of the Fenna-Mathews-Olson complex.

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 (<1 ps) dynamics.

Lead-free halide perovskites, as environment-friendly materials, have received critical interest in photovoltaic applications. In this regard, the bismuth halide perovskites demonstrate better stability under ambient conditions than lead halide perovskites and consequently remain one of the critical areas for the development of lead-free absorber materials. The steady-state optical properties are widely investigated in these bismuth halide perovskites, but excited-state charge carrier dynamics such as hot carrier relaxation remain elusive. However, it is crucial to investigate the rapid relaxation of above band gap “hot” carriers as it restricts the fundamental efficiency limit in the perovskite solar cells. Here, we demonstrate the cation-dependent hot carrier cooling in the lead-free A3Bi2I9 [A = FA (formamidinium), MA (methylammonium), and Cs (cesium)] perovskite by using femtosecond transient absorption spectroscopy. These lead-free perovskites were fabricated from gamma-butyrolactone (γ-GBL) solvent to ensure uniformity and continuity of the as-grown film and were well characterized by XRD, SEM, and steady-state absorption and photoluminescence spectroscopy. With varying A-cations, we observe that the hot-hole relaxation is slowest in the all-inorganic perovskite Cs3Bi2I9 (12.83 ps) and hot electron relaxation is slowest in the hybrid MA3Bi2I9 perovskite (6.42 ps) at the same excitation energy. The observed strong dependence of carrier cooling on cation composition is explained by the interaction between the different organic cations (A = FA, MA, and Cs) with the Pb–Br frameworks. Our study provides an opportunity to understand the effect of cations on the excited-state carrier dynamics, especially the hot carrier relaxation in the bismuth halide perovskites. This will pave the way for designing hot carrier-based high-efficient lead-free perovskite photovoltaic devices.

We report a large Stokes shift and broad emission band in a Mn-based organic–inorganic hybrid halide, (guanidinium)6Mn3Br12 [GuMBr], consisting of trimeric units of distorted MnBr6 octahedra representing a zero-dimensional compound with a liquid like crystalline lattice. Analysis of the photoluminescence (PL) line width and Raman spectra reveals the effects of electron–phonon coupling, suggestive of the formation of Frenkel-like bound excitons. These bound excitons, regarded as the self-trapped excitons (STEs), account for the large Stokes shift and broad emission band. The excited-state dynamics was studied using femtosecond transient absorption spectroscopy, which confirms the STE emission. Further, this compound is highly emissive with a PL quantum yield of ∼50%. With chloride ion incorporation, we observe enhancement of the emissive properties and attribute it to the effects of intrinsic quantum confinement. Localized electronic states in flat bands lining the gap and their strong coupling with phonons are confirmed with first-principles calculations.