Processes leading to coherence in lasers

Xiangming Hu, Zhizhan Xu

We study the statistical properties of a three-level laser where cascade coherent excitation as a two-step process is used as a coherent pump and a laser field is generated on the upper transition. It is shown that the laser intensity fluctuations are suppressed up to 50% below the shot noise limit and that the laser linewidth is reduced below the Schawlow-Townes formula by more than one order of magnitude of the laser intensity. By transforming to the picture dressed by the coherent field on the upper transition, we identify a transfer of the lower coherent excitation to be responsible for the quantum noise reduction. Cascade coherent excitation turns out to be a new way in which one can create a pathway for recycling of active electrons and regularization of laser excitation process. The conditions for the physical realization of the Raman scheme (Ritsch et al 1992 Europhys. Lett. 19 7) as a limiting case is discussed.

H. Hodara

Hermann Haken

Jiawei Li, Juntao Li, Shu Hu +4 more

Besides the scattering structures, the energy transfer (ET) process in the gain medium plays a significant role in the competition between coherent (comprising strongly coherent components) and incoherent (consisting of weakly coherent or "hidden" coherent components) modes of random lasers. In this study, bichromatic emission random lasers were successfully created using polydimethylsiloxane (PDMS) replicas with grooved structures that imitate the inner surface of abalone shells as scattering substrates. The influence mechanism of the ET process from the monomer to dimer in the Rhodamine 640 dye on the competition of random laser modes was thoroughly investigated from both spectral and temporal dimensions. It was confirmed that the ET process can reduce the gain of monomers while amplifying the gain of dimers. By considering the dominant high-efficiency ET processes, an energy transfer factor associated with the pump energy density was determined. Notably, for the first time, it was validated that the statistical distribution characteristics of the time sequence variations in the coherent random laser generated by dimers closely resemble a normal distribution. This finding demonstrates the feasibility of producing high-quality random number sequences.

Yongqiang Li, Cheng Gao, Wenpu Dong +2 more

We develop a large scale quantum master equation approach to describe dynamical processes of practical open quantum systems driven by both coherent and stochastic interactions by including more than one thousand true states of the systems, motivated by the development of highly bright and fully coherent lasers in the X-ray wavelength regime. The method combines the processes of coherent dynamics induced by the X-ray laser and incoherent relaxations due to spontaneous emissions, Auger decays, and electronic collisions. As examples, theoretical investigation of {\it real} coherent dynamics of inner-shell electrons of a neon gas, irradiated by a high-intensity X-ray laser with a full temporal coherence, is carried out with the approach. In contrast to the rate equation treatment, we find that coherence can suppress the multiphoton absorptions of a neon gas in the ultra-intense X-ray pulse, due to coherence-induced Rabi oscillations and power broadening effects. We study the influence of coherence on ionization processes of neon, and directly prove that sequential single-photon processes for both outer- and inner-shell electrons dominate the ionizations for the recently typical experiments with a laser intensity of $\approx10^{18}$ ${\rm W/cm^2}$. We discuss possible experimental implementations such as signatures for coherent evolution of inner-shell electrons via resonance fluorescence processes. The approach can also be applied to many different practical open quantum systems in atomic, quantum optical, and cold matter systems, which are treated qualitatively by a few-level master equation model before.

BERNARD SHORE