We observed periodic optical pulses from an Er:YSO crystal during irradiating with a continuous-wave excitation laser. We refer to this phenomenon as “periodic superradiance.” This periodicity can be understood qualitatively by a simple model, in which a cyclic process of a continuous supply of population inversion and a sudden burst of superradiance is repeated. The excitation power dependences of peak interval and the pulse area can be interpreted with our simple model. In addition, the linewidth of superradiance is much narrower than an inhomogeneous broadening in a crystal. This result suggests that only Er3+ ions in a specific environment are involved in superradiance. Published by the American Physical Society 2024
We observed periodic optical pulses from an Er:YSO crystal during irradiating with a continuous-wave excitation laser. We refer to this phenomenon as “periodic superradiance.” This periodicity can be understood qualitatively by a simple model, in which a cyclic process of a continuous supply of population inversion and a sudden burst of superradiance is repeated. The excitation power dependences of peak interval and the pulse area can be interpreted with our simple model. In addition, the linewidth of superradiance is much narrower than an inhomogeneous broadening in a crystal. This result suggests that only Er3+ ions in a specific environment are involved in superradiance. Published by the American Physical Society 2024
We observed periodic optical pulses from an Er:YSO crystal during irradiating with an continuous-wave excitation laser. We refer to this new phenomenon as "periodic super-radiance". This periodicity can be understood qualitatively by a simple model, in which a cyclic process of a continuous supply of population inversion and a sudden burst of super-radiance is repeated. The excitation power dependences of peak interval and the pulse area can be interpreted with our simple model. In addition, the linewidth of super-radiance is much narrower than an inhomogeneous broadening in a crystal. This result suggests that only Er3+ ions in a specific environment are involved in super-radiance.
We conduct a theoretical study to understand the periodic superradiance observed in an Er:YSO crystal. First, we construct a model based on the Maxwell-Bloch equations for a reduced level system, a pair of superradiance states and a population reservoir state. Analysis of the eigenvalues of the linearized differential equations shows that periodic superradiance can be realized only for certain parameters. We also derive two-variable equations consisting of the coherence and population difference between the two superradiance states, which contain the essential feature of the periodic superradiance. The two-variable equations clarify a mathematical structure of this periodic phenomenon and give analytical forms of the period, pulse duration, and number of emitted photons. Our model successfully reproduces the periodic behavior, but the actual experimental parameters are found to be outside the parameter region for the periodic superradiance. This result implies that some other mechanism(s) is required. As one example, assuming that the field decay rate varies with the electric field, the periodic superradiance can be reproduced even under the actual experimental condition.
To achieve more controllable development of coherence in solids, we investigated the effect of a trigger laser tuned to the superradiance transition wavelength on periodic superradiance observed in an Er:YSO crystal. For period control, applying the trigger laser reduced both the superradiance period and its variance, demonstrating enhanced controllability of coherence development dynamics. As the trigger laser power increased, both the period and the number of emitted superradiance photons decreased while maintaining a proportional relationship. This behavior is explained by a reduced superradiance threshold under a constant excitation rate and is reproduced by numerical simulations based on the Maxwell-Bloch equations. For timing control, we found that superradiance could be triggered even when the excitation laser alone was insufficient. This enabled us to control the emission timing of superradiance using short trigger pulses and provided a device capable of generating superradiance at desired timing.
Spontaneous decay is a phenomenon that has been described for the first time by Dirac, Wigner
and Weisskopf, at the early ages of the development of quantum electrodynamics. The theory is
applicable to a multi-body system only when atoms don’t interact during their decay. Dicke introduces
in his seminal paper the case where this assumption doesn’t hold.
The results he obtains not only tell that the system emits pulses, but the pulse duration now depends
on 1/N, where N is the number of participating atoms to the process. The pulses temporal profile
is described by a squared hyperbolic secant function. The resulting emission intensity scales as N^2.
The atomic system radiates more intensively than the ordinary fluorescent emission, this is the reason
why the process is called superradiance, or by a generalization to extended systems, superfluorescence.
The superradiant emission has preferential escape directions, depending on the atomic sample shape.
Superradiance is a collective process and it takes place when the group of excited atoms is left unper-
turbed during the formation of of the macroscopic dipole. A fundamental difference compared to the
spontaneous and stimulated emission processes is that all atoms act together as a single entity and it
isn’t possible to identify single atoms involved.
This remarkable behaviour is manifest when the atomic coherence of the system, while influenced by
environmental interactions that cause dephasing in atoms, persists for a sufficient interval of time.
Typical dephasing mechanisms, such as collisions in gases or lattice phonon interactions in crystals,
make its experimental observation difficult. A possible solution to this problem is the use of optical
crystals doped with rare-earths that are characterized by long and narrow levels lifetimes.
We investigate superradiant lasing in 1D photonic crystal taking into account both homogeneous and inhomogeneous broadening in a two-level active medium. The latter may be based on, e.g., the quantum-well heterostructures and the fibers activated by color centers where Bragg structure can be fabricated artificially by means of a periodic modulation of the cladding layers. The truncated Maxwell-Bloch equations for the counter-propagating waves, the polarization and inversion of an inhomogeneously broadened two-level medium are used to analyze numerically the interplay between the back-scattering, amplification, dispersion, and de-phasing of waves. It is found that, if the photonic band gap is less than the coherent amplification bandwidth, there is a wide range of the pumping and relaxation parameters of a two-level medium where superradiant lasing exists and results in generation of a quasi-periodic and/or chaotic series of powerful and short pulses similar to those of Dicke superfluorescence. It is the cavity mode selection due to distributed feedback caused by the resonant Bragg structure which is responsible for the existence of superradiant lasing. Typical parameters of superradiant photonic-crystal lasers, including lengths, photonic band gaps and relaxation rates, are indicated.
Temporal sequences of different spectral components of the 4/11/2 → 4/13/2 manifold in YAlO3:Er have been analyzed in a laser operated at 300 K. Several lines can oscillate in a spiking train. Their position in the pulse is discussed in terms of population inversion, laser linewidth, and resonant ion-ion interaction. Erbium crystals with dopant concentrations of 10, 20, and 50% have been used in the experiments. The wavelengths of the 3-µm transitions range from 2.71 to 2.92 µm. They are strongly dependent on the crystal orientation with respect to the polarization of the laser light. The polarization dependence of the most prominent emission lines has been determined for laser crystals oriented with their axes along the crystallographic a and b directions. The bandwidth of the laser lines has been measured with a resolution of 0.12 nm using a grating spectrometer in combination with an image converter. The bandwidth ranges from 0.3 nm at 2.73µm up to 2.6 nm at 2.92 µm.