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Optics Express

Spectro-temporal behavior of dye-based solid-state random lasers under picosecond pumping regime

Ignacio Iparraguirre, J. Azkargorta, Sara García‐Revilla, J. Fernández, Rolindes B. Balda

Published Feb 10, 2022• 7 citations
Publisher

Abstract

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.

DOI
10.1364/oe.451017

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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.
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.
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.
Solutions of the rate equations for a two-level system describing spectral properties in the presence of stimulated emission under picosecond-pulse pumping conditions are examined. The materials of interest are broad band tunable, vibronic laser crystals whose lasing properties under normal pumping conditions have been related to spectral properties through the McCumber theory1. The population inversion at threshold can then be determined by three methods: lifetime shortening, band narrowing, and peak gain measurements.
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.
We investigated random scattering of light in a disorder gain medium of ZnO powder using the pump-probe technique. Using a probe beam at (lambda) =390nm, the width ((theta) ) of the coherent backscattering peak from the ZnO powder is measured to be ~7.5 degree(s), thus the coherent scattering length l is approximately 1.2(lambda) ((theta) =(lambda) /2(pi) l) which is close to the strong scattering regime. When a pump beam ((lambda) =267nm) exceeds a certain excitation threshold, supernarrow emission peaks (bandwidth less than 1nm) emerged from the ZnO broad photoluminescence background. Concurrently, we also observed enhancement and sharpening of the coherent backscattering cone. Since light from the center of the backscatter cone experience the largest number of scatterings (i.e. longest gain length), this result is thus consistent with the random laser model that the supernarrow peak is due to amplification and stimulated emission of photon in the random gain medium. The time-resolved pump-probe measurement shows that the lifetime of the emission state above the lasing threshold is only a few picoseconds which is consistent with the interpretation that the supernarrow peaks are due to stimulated emission.
We investigated random scattering of light in a disorder gain medium of ZnO powder using the pump-probe technique. Using a probe beam at (lambda) =390nm, the width ((theta) ) of the coherent backscattering peak from the ZnO powder is measured to be ~7.5 degree(s), thus the coherent scattering length l is approximately 1.2(lambda) ((theta) =(lambda) /2(pi) l) which is close to the strong scattering regime. When a pump beam ((lambda) =267nm) exceeds a certain excitation threshold, supernarrow emission peaks (bandwidth less than 1nm) emerged from the ZnO broad photoluminescence background. Concurrently, we also observed enhancement and sharpening of the coherent backscattering cone. Since light from the center of the backscatter cone experience the largest number of scatterings (i.e. longest gain length), this result is thus consistent with the random laser model that the supernarrow peak is due to amplification and stimulated emission of photon in the random gain medium. The time-resolved pump-probe measurement shows that the lifetime of the emission state above the lasing threshold is only a few picoseconds which is consistent with the interpretation that the supernarrow peaks are due to stimulated emission.