Understanding the ultrafast carrier dynamics and the mechanism of two-dimensional (2D) transition metal dichalcogenides (TMDs) is key to their applications in the field of optoelectronic devices. In this work, a single pulse pump probe method is introduced to detect the layer-dependent ultrafast carrier dynamics of monolayer and few-layer WS2 excited by a femtosecond pulse. Results show that the ultrafast carrier dynamics of the layered WS2 films can be divided into three stages: the fast photoexcitation phase with the characteristic time of 2–4 ps, the fast decay phase with the characteristic time of 4–20 ps, and the slow decay phase lasting several hundred picoseconds. Moreover, the layer dependency of the characteristic time of each stage has been observed, and the corresponding mechanism of free carrier dynamics has been discussed. It has been observed as well that the monolayer WS2 exhibits a unique rising time of carriers after photoexcitation. The proposed method can be expected to be an effective approach for studying the dynamics of the photoexcited carriers in 2D TMDs. Our results provide a comprehensive understanding of the photoexcited carrier dynamics of layered WS2, which is essential for its application in optoelectronics and photovoltaic devices.

Applying an ultrafast vortex laser as the pump, optical parametric amplification can be used for spiral phase-contrast imaging with high gain, wide spatial bandwidth, and high imaging contrast. Our experiments show that this design has realized the 1064 nm spiral phase-contrast idler imaging of biological tissues (frog egg cells and onion epidermis) with a spatial resolution at several microns level and a superior imaging contrast to both the traditional bright- or dark-field imaging under a weak illumination of 7 nW/cm2. This work provides a powerful way for biological tissue imaging in the second near-infrared region.

We propose and numerically demonstrate a simple and background-free all-optical chiral spectroscopy technique for gas molecules. Our approach is based on high harmonic generation driven by a new type of laser beam that is produced by one linearly polarized single-color beam passing through a lens and a prism. It is shown that chiral and achiral signals are completely separated in frequency, indicating strong background-free and highly sensitive chirality detection. We believe this all-optical method can open new opportunities for ultrafast detection for chiral dynamics in the femtoseond to attosecond time scale.

The dephasing of molecular alignment can lead to the deformation of the alignment signal during its periodic revivals. Most studies are concentrated on the first few rotational revival periods of the molecular alignment and neglect the dephasing effect. However, study of the alignment dephasing is still of great significance for both the long-term dynamics of the molecular alignment and the dephasing itself. In this work, we theoretically demonstrate that the dephasing effect is correlated with both the rotational temperature and the rotational revival period of the molecules. The results present that the dephasing is especially significant for those molecules with long rotational revival period at high rotational temperatures. The physics behind it is explored by taking advantage of the coherence of the rotational quantum state population. This work deepens our understanding of rotational dynamics and rotational spectroscopy in molecular alignment.

Ultrafast lasers with high repetition rate, high energy, and ultrashort pulse duration have enabled numerous applications in science and technology. One efficient route to generate such pulses is postcompression of high-power Yb-doped lasers. Here, we report on the generation of 24.5 fs pulses with an output energy of 1.6 µJ and a repetition rate of 500 kHz. The pulses are obtained by using a hybrid cascaded nonlinear compression of the pulses delivered by a Yb-based fiber chirped pulse amplification (CPA) system. In the first stage, the initial 390 fs laser pulses are compressed to 100.7 fs based on spectral broadening in three fused silica plates. In the second stage, the pulses have been shortened to sub-30 fs by means of nonlinear compression in a hollow-core fiber. Overall, we could achieve ∼16 times temporal shortening with the proposed approach. The results show that our system can effectively generate few-cycle pulses at a relatively high repetition rate and high energy, which can benefit future possible applications.

Motivated by the major role funneling dynamics plays in light-harvesting processes, we built some laser control strategies inspired from basic mechanisms such as interference and kicks, and applied them to the case of pyrazine. We are studying the internal conversion between the two excited states, the highest and directly reachable from the initial ground state being considered as a donor and the lowest as an acceptor. The ultimate control objective is the maximum population deposit in the otherwise dark acceptor from a two-step process: radiative excitation of the donor, followed by a conical-intersection-mediated funneling towards the acceptor. The overall idea is to first obtain the control field parameters (individual pulses leading frequency and intensity, duration, and inter-pulse time delay) for tractable reduced dimensional models basically describing the conical intersection branching space. Once these parameters are optimized, they are fixed and used in full-dimensional dynamics describing the electronic population transfer. In the case of pyrazine, the reduced model is four-dimensional, whereas the full dynamics involves 24 vibrational modes. Within experimentally achievable electromagnetic field requirements, we obtain a robust control with about 60% of the ground state population deposited in the acceptor state, while about 16% remains in the donor. Moreover, we anticipate a possible transposition to the control of even larger molecular systems, for which only a small number of normal modes are active, among all the others acting as spectators in the dynamics.

The aggregation and photoinduced excited state dynamics of organic π-conjugated molecules play a vital role in solar energy conversion and applications. This work investigates how solvent polarity affects the aggregation behavior and the photophysical process of perylene diimide dimer (PDI-II). The results show that the conjugations between PDI intramolecular chromophores are more likely to generate excimer, and the conjugations between PDI intermolecular chromophores are more likely to experience symmetry-breaking charge separation. Our study can provide a reference for the design of high-efficiency solar energy conversion materials.

Two-dimensional Dion–Jacobson (D-J) phase perovskites are prospective photovoltaic and optoelectronic materials. To study their mechanical properties and carrier-lattice interactions, we conduct femtosecond spectroscopic experiments on the films of a D-J perovskite. After optical excitation, a ∼33 meV bandgap oscillation is observed in the film by transient absorption spectroscopy. With the help of transient reflection methods, we reveal that the oscillation originates from the transport of coherent longitudinal acoustic phonons through the film. Large bandgap oscillation indicates a strong coupling between carriers and lattice, and significant bandgap modulation by strains in D-J perovskites.