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Optics Express

Spectro-temporal behavior of dye-based solid-state random lasers under a picosecond pumping regime: Part III

Ignacio Iparraguirre, Sara García‐Revilla, Jon Azkargorta, Joaquin R. Fernandez, Rolindes B. Balda

Published Jan 12, 2023• 1 citation
Publisher

Abstract

In two recent works, the authors have investigated the spectro-temporal properties of the random laser emission from solid state dye-doped powders in picosecond pumping regime. Each emission pulse consists, both above and below threshold, in a collection of narrow peaks of a spectro-temporal width at the theoretical limit (ΔωΔt≅1). The distribution of path lengths traveled inside the diffusive active medium by photons that can be amplified by stimulated emission explains this behavior, as demonstrated by a simple theoretical model developed by the authors. The aim of the present work is, first, to develop an implemented model that does not depend on fitting parameters, and which is compatible with the energetic and spectro-temporal properties of the material; and second, to get knowledge about the spatial properties of the emission. The transverse coherence size of each emitted photon packet has been measured; and in addition, we have shown the existence of spatial fluctuations of the emission of these materials, as our model predicts.

DOI
10.1364/oe.482086

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•Jan 1, 2004•0 citations
In two recent works, the authors have investigated the spectro-temporal properties of the random laser emission from solid state dye-doped powders in picosecond pumping regime. Each emission pulse consists, both above and below threshold, in a collection of narrow peaks of a spectro-temporal width at the theoretical limit (ΔωΔt≅1). The distribution of path lengths traveled inside the diffusive active medium by photons that can be amplified by stimulated emission explains this behavior, as demonstrated by a simple theoretical model developed by the authors. The aim of the present work is, first, to develop an implemented model that does not depend on fitting parameters, and which is compatible with the energetic and spectro-temporal properties of the material; and second, to get knowledge about the spatial properties of the emission. The transverse coherence size of each emitted photon packet has been measured; and in addition, we have shown the existence of spatial fluctuations of the emission of these materials, as our model predicts.
In this work, the spectral and temporal properties of the random laser emission from dye-doped solid state powders are investigated in picosecond pumping regime. Ultrafast time-resolved spectroscopy achieved with a streak-camera has been used to perform a detailed study of the temporal evolution of the spectrum of their single pulses. Under conditions of low population inversion density, it is observed that the detected radiation occurs as isolated peaks with a very narrow spectro-temporally spread (ΔωΔt≅1). This behavior remains under conditions of high population inversion density, suggesting that the underlying physical mechanism that produces the emission is the same whatever the pumping conditions. Measurements carried out by varying the numerical aperture of the detection system show that each detected peak within a single pulse is associated with a photon pack emitted in a random direction and wavelength. The relationship between the distribution of paths lengths done by photons inside the active medium, and the gain explains the observed behavior.
The spatial and spectral evolution of emission from gain media with homogeneously distributed scatterers is modeled using a Monte Carlo simulation of the random walk of pump and emitted photons. We find a sharp transition to ultrafast, narrow-linewidth emission for a 10-ps incident pump pulse and a rapid approach to steady state for longer pulses. The threshold for this transition is raised significantly when the transverse diffusion of photons out of the excited region is substantial. The quantitative agreement of the results with recent observations of emission from random gain media lends further support to the interpretation of ``random lasers'' in terms of amplified spontaneous emission within a multiply-scattering medium.
The dynamics of stimulated emission in random lasers is studied using the system of rate equations for population inversion and density of emitted photons. In order to model the behavior of random lasers with nonresonant feedback, no coherence effects are intentionally taken into account. It has been shown that the feedback in the system is necessary for the realization of the regime of relaxation oscillations and spatial confinement of the stimulated emission to the interior regions of the pumped volume. The model also predicts the possibility of localization of stimulated emission in several spatially separated subvolumes of the random laser medium. Finally, it has been demonstrated that two adjacent random laser volumes can strongly enhance the stimulated emission in each other (the ``critical mass'' effect in random lasers).
We study the dynamics of lasing from photonic paints excited by short, localized, optical pulses, using a time-dependent diffusion model for light propagating in the medium containing active atoms. The full time-dependent, nonadiabatic nonlinear response of the atomic system to the local optical field intensity is described using the Einstein rate equations for absorption and emission of light. Solving the time-dependent diffusion equation for the light intensity in the medium with nonlinear gain and loss, we derive detailed information on the spectral, spatial, and temporal properties of the emitted laser light. Our model recaptures the effects of scatterers to narrow the emission spectral linewidth and to narrow the emitted pulse duration, at a specific threshold pump intensity. Our model also describes how this threshold pump intensity decreases with scatterer density and excitation spot diameter, in excellent agreement with experimental results.
The coherent properties of the temporally and spectrally narrowed emission of laser-induced fluorescence of organic dyes hosted inside artificial scattering matrices (random lasers) were investigated. The excitation source was a frequency-doubled 200-fs pulsed laser emitting at 400 nm. Spectral and temporal features were simultaneously recorded with a spectrograph and a streak camera operating in photon-counting mode. Photon-number distributions were thus created. The temporal coherence of the laserlike emission above and below the excitation energy threshold was investigated from the photon-number distribution that was obtained.
In the first part of this thesis, a detailed analysis of lasing in random multiple-light-scattering media with gain is presented. Random laser emission is analyzed using a time-dependent diffusion model for light propagating in the medium containing active atoms. We demonstrate the effects of scatterers to narrow the emission spectral linewidth and to shorten the emitted pulse duration at a specific threshold pump intensity. This threshold pump intensity decreases with scatterer density and excitation spot diameter, in excellent agreement with experimental results. The coherence properties of the random laser are studied using a generalized master equation. The random laser medium is treated as a collection of low quality-factor cavities, coupled by random photon diffusion. Laser-like coherence, on average, is demonstrated above a specific pumping threshold. We demonstrate that with stronger scattering, the pumping threshold for the transition from chaotic to isotropic coherent light emission decreases and enhanced optical coherence for the emitted light is achieved above threshold. The second part of this thesis presents a study of lasing in photonic crystals (PCs). The emission from an incoherently pumped atomic system in interaction with the electro-magnetic reservoir of a PC is analyzed using a set of generalized semiclassical Maxwell-Bloch equations. We demonstrate that the photonic band edge facilitates the enhancement of stimulated emission and the reduction of internal losses, leading to an important lowering of the laser threshold. In addition, an increase of the laser output at a photonic band edge is demonstrated. We next develop a detailed quantum theory of a coherently pumped two-level atom in a photonic band gap material, coupled to both a multi-mode wave-guide channel and a high-quality micro-cavity embedded within the PC. The cavity field characteristics are highly distinct from that of a corresponding high-Q cavity in ordinary vacuum. We demonstrate enhanced, inversionless, and nearly coherent light generation when the photon density of states (DOS) jump between the Mollow spectral components of atomic resonance fluorescence is large. In the case of a vanishing photon DOS on the lower Mollow sideband and no dipolar dephasing, the emitted photon statistics is Poissonian and the cavity field exhibits quadrature coherence.