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Optics Letters

Radiative coupling between photonic paint layers

Jordi Martorell, R. M. Balachandran, N. M. Lawandy

Published Feb 15, 1996• 23 citations
Publisher

Abstract

Intense isotropic laser emission from an optically pumped scattering gain medium has been used to induce isotropic lasing in an adjacent scattering gain medium at a longer wavelength. The energy-transfer mechanism between the media involved has been studied with a simple model based on radiation-transfer theory. The applications of this effect for wavelength-domain coding are also discussed.

DOI
10.1364/ol.21.000239

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Intense isotropic laser emission from an optically pumped scattering gain medium has been used to induce isotropic lasing in an adjacent scattering gain medium at a longer wavelength. The energy-transfer mechanism between the media involved has been studied with a simple model based on radiation-transfer theory. The applications of this effect for wavelength-domain coding are also discussed.
In several recent experiments, isotropic lasing action was observed in paints that contain rhodamine 640 dye molecules in methanol solution as gain media and titania particles as optical scatterers. These so-called paint-on laser systems are extraordinary because they are highly disordered systems. The microscopic mechanism for laser activity and the coherence properties of light emission in this multiple-light-scattering medium have not yet been elucidated. In this paper we derive the emission intensity properties of a model dye system with excited singlet and triplet electronic energy levels, which is immersed in a multiple-scattering medium with transport mean free path l*. Using physically reasonable estimates for the absorption and emission cross section for the singlet and triplet manifolds, and the singlet-triplet intersystem crossing rate, we solve the nonlinear laser rate equations for the dye molecules. This leads to a diffusion equation for the light intensity in the medium with a nonlinear intensity-dependent gain coefficient. Using this model we are able to account for nearly all of the experimentally observed properties of laser paint reported so far when l*\ensuremath{\gg}${\ensuremath{\lambda}}_{0}$, the emission wavelength. This includes the dependence of the peak intensity of amplified emission on the mean free path l*, the dye concentration \ensuremath{\rho}, and the pump intensity characteristics. Our model recaptures the collapse of the emission linewidth at a specific threshold pump intensity and describes how this threshold intensity varies with l*. In addition, our model predicts a dramatic increase in the peak intensity and a further lowering of the lasing threshold for the strong scattering limit l*\ensuremath{\rightarrow}${\ensuremath{\lambda}}_{0}$. This suggests a striking enhancement of the characteristics of laser paint near the photon localization threshold in a disordered medium. \textcopyright{} 1996 The American Physical Society.
The narrow-linewidth laser emission from a scattering gain medium can be forced to oscillate in a narrower emission bandwidth with the introduction of a seed in much the same way as the injection-seeding technique used to produce single-mode oscillation from conventional lasers. The system exhibits complete spectral collapse at all wavelengths other than that of the seed and a large enhancement of the isotropic emission peak intensity. A theory explaining the emission characteristics and the minimum threshold seed energy required for locking is presented.
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.
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.
It is proposed that a feedback resulting from scattering be used to achieve lasing in a disperse reactive medium. The example of a simple two-level system shows that under advanced lasing conditions the quantum efficiency of the radiation emission approaches the quantum efficiency of the excitation of the upper level, and the emission spectrum becomes much narrower. Feasibility of chemical pumping of such a laser is estimated on the basis of calculations of heterophase burning of a drop of a fuel in an oxidizing atmosphere. The growth increment of light is calculated and the threshold conditions for the excitation of lasing are found. Examples are given to illustrate the feasibility of purely chemical pumping of a laser with a nonresonant feedback. It is shown that dense reactive media can be used in such lasers.