Rb optical resonance inside a random porous medium

Abstract

We studied resonant laser interaction with Rb atoms confined to the interstitial cavities of a random porous glass. Due to diffusive light propagation, the effect of atomic absorption on the light scattered by the sample is almost entirely compensated by atomic fluorescence at low atomic densities. For higher densities, radiation trapping increases the probability of non-radiative decay via atom-wall collisions. A simple connection of the fluorescence/absorption yield to the sample porosity is given. c 2018 Optical Society of America OCIS codes: 300.6210, 290.4210, 300.6500.Recent advances in the miniaturization and integration of nonlinear optical devices, have attracted considerable attention on atoms confined to micrometric or nanometric-size volumes such as hollow-core optical fibers [1,2], wave-guides [3], thin cells [4][5][6][7][8] or interstitial cavities in nano-structures such as opals [9]. Positronium atoms were created and observed in porous silica [10]. Here, the first spectroscopic study of alkali atoms confined to the interstitial cavities of a random porous medium is presented. Porous media are strong light scatterers. Mean optical paths of several meters have been measured for cm-size porous ceramics used as compact gas cells for the detection of O 2 [11, 12]. Our sample is made of ground glass with 50 − 100 µm grain size. The scattering mean free path is ∼ 1 mm. Strong light scattering results in high randomization of the photon trajectories. As a consequence, the scattered light reaching an external detector tends to be proportional to the total light present in the medium no matter whether it originated from the laser or from atomic fluorescence. We call this the "integratingsphere effect" (ISE) by analogy with an extended detector wrapping the full solid angle around the sample. A second influence of the medium porosity regards the laser-atom dynamics. Due to the smallness of the interstitial cavities, the laser-atom evolution has a significant probability to be interrupted by a collision with the pore wall [1,10,11,13,14]. Excited atoms may collide with the walls and relax their energy non-radiatively .We have studied the optical resonance of Rb atoms contained inside porous glass.In the linear regime, on resonance with an atomic transition, the steady state detected light power after the porous sample I ′ is: I ′ = I (1 − α + β) where I is the offresonance signal. Here α represents the relative light absorption and β the light emission by excited atoms. The contributions of α and β can be separated if one assumes that the absorption rapidly follows any change in the light intensity while the emission has a slower evolution due to the finite lifetime of the atomic excited state [15]. If I is switched at t = 0 from I 1 to I 2 , the transmitted light power is:Here θ is the decay time of the slowly varying term. Thus, the transmitted light transient evolution allows the determination of θ and β, then α is obtained from the steady state transmission.At high atomic densities, the characteristic d...

Nonlinear atomic spectroscopy inside a random porous medium

“…We interpret these facts as owing to the frequency redistribution of light due to multiple absorption and reemission cycles. Indeed, we know from previous work that photon trapping is significant at the vapor density used in the experiments [20]. The frequency redistribution would result in an effective spectral broadening of the exciting light and consequently of the nonlinear signal line.…”

“…Based on our previous study [20], we believe the first mechanism to be dominant. In any case, the two mechanisms are proportional to the excited-state population.…”

“…In a recent work [20], we have reported a study of the resonance spectroscopy of Rb inside a random porous glass medium. In such a medium, the atomic motion is limited by the pore dimensions, resulting in reduced light-atom interaction times due to collisions with the pore walls.…”

Experimental progress in positronium laser physics

“…Ps is a unique system for such studies [403]: spectroscopy of "normal" atoms can be used (e.g. [404,405]), but Ps has some advantages, namely: (1) the atoms can be produced inside isolated voids following positron bombardment, (2) Ps annihilation radiation can be used as a signal and is easily detected outside the material being studied, and (3) Ps atoms are light, and so may have a de Broglie wavelength comparable to the internal spaces being investigated. This leads to stronger interactions with the walls, and may therefore be more useful for studying the dynamics of interactions with the confining surfaces.…”

Two-photon excitation of rubidium atoms inside porous glass

“…In addition, the atomic response to the light is affected, under suitable conditions, by the spatial confinement of the atomic motion imposed by the random material. Such confinement effects can have spectroscopic consequences which in turn can be used to characterize the random medium [8].…”