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Optics and Spectroscopy

Obtaining a structureless broadband stimulated-emission spectrum of pulsed sources

Yu K Verevkin, A. D. Tertyshnik

Published Oct 1, 1987• 0 citations
OpenAlex

Abstract

The effect of the resonator parameters, the recording scheme and the pumping characteristics on the structure of the broadband stimulated-emission spectrum of solutions of organic dyes with pulsed pumping was investigated. It was shown that the fundamental causes of the appearance of random structure in the spectral distribution of the radiation of such sources are quantum fluctuations. On lasing multimode with respect to the transverse indices a structureless broadband spectral distribution of the radiation was obtained.

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The effect of the resonator parameters, the recording scheme and the pumping characteristics on the structure of the broadband stimulated-emission spectrum of solutions of organic dyes with pulsed pumping was investigated. It was shown that the fundamental causes of the appearance of random structure in the spectral distribution of the radiation of such sources are quantum fluctuations. On lasing multimode with respect to the transverse indices a structureless broadband spectral distribution of the radiation was obtained.
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The luminescence and laser characteristics of a synthetic opal matrix filled with organic dyes are studied upon excitation by nanosecond laser pulses. The appearance of stimulated emission in a partially ordered scattering medium is investigated. It is shown that if the luminescence spectrum of a dye (oxazine-17) is located far outside the photonic bandgap of the opal matrix, stimulated emission along a preferential direction in the (111) plane is observed when pumping exceeds a threshold even without an external optical cavity. The stimulated emission spectrum is considerably narrower than the luminescence spectrum and consists of several narrow lines located within the dye luminescence band. If the luminescence spectrum of a dye (rhodamine 6G) overlaps with the photonic bandgap of the opal matrix, a different picture is observed. The loss of radiation in the matrix leads to the red shift of the luminescence spectrum, while the stimulated emission as in the case of oxazine-17 lies is observed within the luminescence band.
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.
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 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.
We simulate the transition from amplified spontaneous emission (ASE) to lasing in random systems with varying degrees of mode overlap. This is accomplished by solving the stochastic Maxwell-Bloch equations with the finite-difference time-domain method. Below lasing threshold, the continuous emission spectra are narrowed by frequency-dependent amplification. Our simulations reproduce the stochastic emission spikes in the spectra. Well-defined peaks, corresponding to the system resonances, emerge at higher pumping and are narrowed by stimulated emission before lasing takes place. Noise tends to distribute pump energy over many modes, resulting in multimode operation. Well above the lasing threshold, the effects of noise lessen and results become similar to those without noise. By comparing systems of different scattering strength, we find that weaker scattering extends the transition region from ASE to lasing, where the effects of noise are most significant.