The classical description of laser field buildup, based on time-averaged photon statistics of Class A lasers, rests on a statistical mixture of coherent and incoherent photons. Here, applying multiple analysis techniques to temporal streams of data acquired in the threshold region of a Class B mesoscale laser, we conclusively show that new physics is involved in the transition: the lasing buildup is controlled by large dynamical spikes, whose number increases as the pump is raised, evolving into an average coherent field, modulated by population dynamics, and eventually relaxing to a steady state for sufficiently large photon numbers. These results explain inconsistencies observed in small scale devices. Implications for nanolaser coherence properties, threshold identification and regimes of operation, including new potential applications, are discussed.

Subharmonic bifurcations, generalized multistability, and chaotic behavior were found experimentally in a Q-switched C0 2 laser operating at 10.6 /um. Jumps between two strange attractors lead to a low-frequency (1// type) divergence in the power spectrum. This is the first experimental evidence of these phenomena in a quantum-optical molecular system. A theoretical model is also presented whose results are in good agreement with the experimental data.

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A Stochastic Simulator (SS) is proposed, based on a semiclassical description of the radiation-matter interaction, to obtain an efficient description of the lasing transition for devices ranging from the nanolaser to the traditional "macroscopic" laser. Steady-state predictions obtained with the SS agree both with more traditional laser modeling and with the description of phase transitions in small-sized systems, and provide additional information on fluctuations. Dynamical information can easily be obtained, with good computing time efficiency, which convincingly highlights the role of fluctuations at threshold.

We consider a laser with an injected signal, in which the polarization can be adiabatically eliminated, we study the stability of the steady-state solutions, and we discuss the time-dependent solutions. For the laser alone, the only possible solution is constant intensity. However, the introduction of an external field, with an amplitude that does not satisfy the injection-locking condition, destabilizes the system. In such a case, numerical results show the existence of a self-Q-switching process, which induces relaxation oscillations. The frequency of the giant pulses is directly related to the amplitude of the external field, whereas the frequency of the relaxation oscillations depends on the damping rates. We show also that, depending on the value assigned to control parameters, the interaction between these frequencies leads to a chaotic behavior through intermittency or period-doubling bifurcations. Finally, topological equivalence between our laser system and a unidimensional circle map is shown for some values of control parameters.