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We study the role of many-body correlations in the ultrafast dynamics of quantum well magnetoexcitons. In linear response, inter-Landau level transitions are suppressed if magnetic field is stronger than magnetoexciton binding energy. However, in nonlinear response, two-electron Auger processes remain unsuppressed even at very strong field. We show that Auger scattering plays a dominant role in the coherent exciton dynamics in strong magnetic field. We perform numerical calculations of four-wave-mixing (FWM) polarization which incorporate Auger processes nonperturbatively, and find a strong amplitude enhancement of the FWM signal accompanied with polarization oscillations at negative time delays. We identify these oscillations as quantum beats between dark two-exciton states related to each other via Auger processes.

Manganese silicide (MnSi${}_{1.7}$) semiconductor thin films with a thickness of 80 nm are prepared by magnetron sputtering deposition technology. The reflectivity change of thin film samples is studied by femtosecond (fs) pump–probe technique under different pump pulse energies and the laser pulse width is 120 fs. The results show that the transient reflectivity increases within a time-scale of about 100 fs. Then, a fast decay of reflectivity occurs in 0.6 picosecond (ps), and it is mainly due to the carrier–carrier scattering. Next, a slower decay of the reflectivity on a time-scale of dozens of ps is detected, and the Auger recombination and diffusion are the main processes. The effective mass of electron and hole in MnSi${}_{1.7}$ film are calculated by using the pseudo-potential plane wave method on first-principles methods. The effective mass of electron is 0.25$m_0$ ($m_0$ is the electron mass), while 0.13$m_0$ for hole. Experimental results are explained with the results of theoretical simulation.

Making smaller and faster functional devices has led to an increasing demand for a microscopic technique that allows the investigation of carrier and phonon dynamics with both high spatial and temporal resolutions. Traditional optical pump–probe methods can achieve femtosecond temporal resolution but fall short in the spatial resolution due to the diffraction limit. Scanning tunneling microscopy (STM), on the contrary, has realized atomic-scale spatial resolution relying on the high sensitivity of the tunneling current to the tip-sample distance. However, limited by the electronics bandwidth, STM can only push the temporal resolution to the microseconds scale, restricting its applications to probe various ultrafast dynamic processes. The combination of these two methods takes advantages of optical pump–probe techniques and highly localized tunneling currents of STM, providing one viable solution to track atomic-scale ultrafast dynamics in single molecules and low-dimensional materials. In this review, we will focus on several ultrafast time-resolved STM methods by coupling the tunneling junctions with pulsed electric waves, THz, near-infrared and visible laser. Their applications to probe the carrier dynamics, spin dynamics, and molecular motion will be highlighted. In the end, we will present an outlook on the challenges and new opportunities in this field.

Ultrafast reverse saturable absorption (RSA) in chromophore-based borondipyrro- methenes (BODIPYs) derivatives, 1,7-Diphenyl-3,5-bis(9,9-dimethyl-9H-fluoren-2-yl)- boron-diuoride-azadipyrromethene (ZL-61) and 1,7-Diphenyl-3,5-bis(4-(1,2,2-triphenyl- vinyl)phenyl)-boron-diuoride azadipyrromethene (ZL-22), has been theoretically analyzed with femtosecond (fs) laser pulses at 800 nm and 850 nm. The results are in good agreement with the reported experimental results. The effect of input pulse intensity, pulse width, nonlinear absorption (NLA) coefficients, sample thickness and laser pulse shaping on transmittance has been studied and optimized for high contrast and low-power operation. All-optical fs NOT and the universal NOR and NAND Boolean logic gates have been designed with ZL-61 and ZL-22. The logic gates with top-hat input pulse profile have a sharp switch ON/OFF time compared to the Gaussian input laser profile. Ultrafast operation at relatively low pump intensities opens up exciting prospects for the applicability of BODIPY derivatives for ultrafast low-power all-optical computing and information processing.

Excited-state (ES) hydrogen-bonding dynamics and excited-state intramolecular proton transfer (ESIPT) have been extensively implicated in many important physical, chemical and biological phenomena. These events are coupled, resulting in the superior fluorescence properties of luminous molecules. We summarize our recent works, focusing on the ES hydrogen-bonding dynamics of coumarin derivatives and the ESIPT of white lighting materials. The insights into the ultrafast hydrogen-bonding events at the ESs in this chapter are expected to uncover the link between the microstructural dynamics and fluorescence properties of the luminous molecules.

Light harvesting in photosynthetic purple bacteria is reviewed in detail based on recent advances in structure determination and ultrafast laser spectroscopy. Knowledge obtained from photosynthesis research forms a solid ground for studies of various artificial light harvesting systems. We present our recent research of excitation transfer and charge separation in conjugated polymers. We continue by summarizing our studies of ultrafast photoinduced processes in dye-sensitized nanocrystalline large band-gap semiconductor films – a key part of the Grätzel solar cell. Finally, our recent studies of energy transfer in transition metal supramolecular complexes, a kind of artificial antenna, are presented.

We study the role of many-body correlations in the ultrafast dynamics of quantum well magnetoexcitons. In linear response, inter-Landau level transitions are suppressed if magnetic field is stronger than magnetoexciton binding energy. However, in nonlinear response, two-electron Auger processes remain unsuppressed even at very strong field. We show that Auger scattering plays a dominant role in the coherent exciton dynamics in strong magnetic field. We perform numerical calculations of four-wave-mixing (FWM) polarization which incorporate Auger processes nonperturbatively, and find a strong amplitude enhancement of the FWM signal accompanied with polarization oscillations at negative time delays. We identify these oscillations as quantum beats between dark two-exciton states related to each other via Auger processes.