No abstract

Random lasers (RLs) rely on obtaining laser emission in disordered systems with gain. The characterization of the RLs are performed by the observations of bandwidth narrowing, increasing in the slope efficiency, time shortening of the upper level of the RL transition, intensity and spectral fluctuations, and even photon statistics transitions when increasing the optical gain. However, the gain clamping, which is a known phenomenon in conventional lasers, i.e. lasers with well-defined cavity, was never investigated in RLs. Here we realize an experimental-theoretical investigation of the gain clamping in RLs, and demonstrate that it can be used as an alternative tool to characterize the transition from spontaneous emission to the RL regime.

In recent years, an unconventional excitation of trivalent neodymium ions ( N d 3 + ) at 1064 nm, not resonant with ground-state transitions, has been investigated with the unprecedented demonstration of a photon-avalanche-like (PA-like) mechanism, in which the temperature increase plays a fundamental role. As a proof-of-concept, N d A l 3 ( B O 3 ) 4 particles were used. A consequence of the PA-like mechanism is the absorption enhancement of excitation photons providing light emission at a broad range covering the visible and near-infrared spectra. In the first study, the temperature increase was due to intrinsic nonradiative relaxations from the N d 3 + and the PA-like mechanism ensued at a given excitation power threshold ( P t h ). Subsequently, an external heating source was used to trigger the PA-like mechanism while keeping the excitation power below P t h at room temperature. Here, we demonstrate the switching on of the PA-like mechanism by an auxiliary beam at 808 nm, which is in resonance with the N d 3 + ground-state transition 4 I 9 / 2 → { 4 F 5 / 2 , 2 H 9 / 2 } . It comprises the first, to the best of our knowledge, demonstration of an optical switched PA, and the underlying physical mechanism is the additional heating of the particles due to the phonon emissions from the N d 3 + relaxation pathways when exciting at 808 nm. The present results have potential applications in controlled heating and remote temperature sensing.

We demonstrate the feasibility of an unconventional excitation of Nd3+ at 1064 nm by a photon-avalanche-like mechanism, which involves an energy looping due to energy transfer among excited Nd3+ followed by resonant excited-state absorption.