Nhng tin b trong Quang hc, Quang ph vÃ ng dng VI ISSN 1859 - 4271

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Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271LASING MODES WITH COUPLED ATOM-PHOTON INTERACTION INRANDOM CAVITYPham Van Hoi, Duong Hai TrieuInstitute of Materials Science, VAST, 18 Hoang Quoc Viet Rd., Cau giay Dist., Hanoi Viet NamAbstract. We introduce a new model of coupled <strong>ph</strong>oton-atom interactions with amplification in therandom cavity that can explain the experimental results of random laser <strong>ph</strong>enomenon created by glass– air gap – polymer coated layer in the Er-doped fibers. The lasing emission at 537 nm from the Erdopedfiber pumped by laser diode of 976nm does not responded to radiative transitions 2 H 11/2 →4 I 15/2 and 4 S 3/2 → 4 I 15/2 in Er-ions, that give the emissions at 522 nm and 547 nm, respectively. Thiseffect can be seen as interaction between excited ions at two upper levels and resonant <strong>ph</strong>oton in theamplifying media in the random cavity. Sharp emission peaks are observed experimentally when theair-gap between glass and polymer is suitable to interfering condition of cavity-resonant wavelengththat has a value between two spontaneous emission peaks of atom.Keywords: Atom-<strong>ph</strong>oton interaction, random laser, Er-doped silica glasses.I. INTRODUCTIONThe lasing <strong>ph</strong>enomenon in random cavity was predicted theoretical by Letokhov [1] about 40years ago. Random lasing has been recognized in multiply scattered from disordered media suchas ZnO powder[2], solution of TiO 2 nanoparticles[3], Rhodamine dye inPolymethylmethhacrylate (PMMA) and in several polymer systems [4]. The <strong>ph</strong>enomenon isknown as coherent backscattering or weak localization. Random lasing with non-resonantfeedback is seen as the remarkable narrowing of the luminescence spectrum to a single peak ofwidth about several nanometers, while the coherent feedback lasing is identified as the series ofhigh and narrow peaks having width decreasing with increase of the pump power to at least thetenth nanometer scale[5]. In addition, more interference effects were recognized such as thespatial correlations in the intensity transmitted through random media [6]. These experimentswere performed on passive random media. Two different theoretical approaches to randomlasing can be dis<s<strong>trong</strong>>tin</s<strong>trong</strong>>guished. The analysis of average system response performed within thediffusion model possibly including coherent backscattering corrections, but it fails to predict thelasing threshold behavior for the laser action. The rare fluctuation is leading to the formation ofhigh quality random cavities responsible for lasing and the right method should be based on amicroscopic approach [4,5]. Recently, there is a surge of experimental studies of atom-<strong>ph</strong>otoncoupling in the context of the interaction between single or few atoms and resonant optical mediasuch as cavities or <strong>ph</strong>otonic crystal, which is a structured material for <strong>ph</strong>otonic gaps. Thisinterest is fundamental, leading to a better understanding of atom-field coupling, which can bevery efficient in Nano <strong>ph</strong>otonic applications such as single-<strong>ph</strong>oton and/or threshold less lasers[7].The goal of this paper is to study random laser in a first principle model based on experiment.The active medium is a high-concentration Er-doped silica fiber and the cavity randomly createdby glass fiber - air gap - coated polymer layer. We will show a model of random cavity laserwhich have thin air-gap between high index material layers and the light amplification issues bythe superposition of the excited states of atom and the state of incident <strong>ph</strong>oton in the cavity. The78
Những tiến bộ <strong>trong</strong> <strong>Quang</strong> học, <strong>Quang</strong> <strong>ph</strong>ổ và Ứng dụng VI ISSN 1859 - 4271comparison of theoretical study and experimental results will be made and some problem will bediscussed.II. FORMATION OF RESONANT WAVELENGTHS IN RANDOM CAVITYIn common case the up-conversion emission of the Er-doped silica fiber pumped by laserbeam at 976 nm is a broad-band fluorescence emission with two peaks at wavelengthsλ02~ 520nm and λ 01 ~ 547nm, which is corresponded to radiative transitions 2 H 11/2 → 4 I 15/2 and4 S 3/2 → 4 I 15/2 in Erbium ions, respectively. But when observing the emitted light in the directionalmost perpendicular (or may be perpendicular) to the fiber axes we find a very thin spectrumline of increased intensity with wavelength of λ 03 = 537 nm in experiment [8]. Remarkable, thatthe wavelength of 537 nm is not responded to any radiative transitions in Erbium ions. In [8] weproposed the scheme of the polaron interaction in the cavity which could be emit<s<strong>trong</strong>>tin</s<strong>trong</strong>>g the <strong>ph</strong>otonwith energy differed from radiative energy of the atoms. In this section we concentrate onstudying the <strong>ph</strong>oton-atom interaction in the cavity, which created randomly in the Er-dopedsilica fibers. The scheme of the random cavity in optical fiber is demonstrated in Figure 1. Theproposed random cavity has structure of silica glass-air gap-polymer layer that have the differen<s<strong>trong</strong>>tin</s<strong>trong</strong>>dices and light can reflect at the interface between them. The optical active medium is an Erdopedsilica glasses with the Er-concentration of 2500 – 4000 ppm.Er - doped areaPolymerSilicaFELight outputLight outputDCBAAir GapFig. 1. Scheme of random cavity created by silica-air gap –polymer in the Er-doped fiberUnder the pumped light at 976 nm from laser diode the Erbium atom from fiber center emitslight spontaneously, including the up-conversion emission on green light. Consider a beamtransmitted along the direction FA (Fig. 1). At C a part of the beam reflects with no change in<strong>ph</strong>ase. When the beam is transmitted in the air gap to B , a second part of the beam reflects wit<strong>hc</strong>hange in <strong>ph</strong>ase byπ . The reflected parts meet one another at C with difference in optical pathof (2BC- λ 03 /2 ), where λ 03 is the wavelength of light in vacuum (or, approximately, in air). Thereflected light has a significant intensity if optical path difference equals k λ03( k is an integer),then the λ 03 is called resonant wavelength in the cavity, or:2BCλ 03 = . (2.1)( k + (1/ 2))When the light beam is transmitted to A , it reflects at A with no change in <strong>ph</strong>ase. The lightreflected at B and the light reflected at A meet one another at B with difference in optical pathof 2ABn p − ( λ03/ 2). The reflected light has a significant intensity if this difference equals l λ03,where l is an integer and n p is the refractive index of the polymer layer, or:2ABn pλ 03 = . (2.2)( l + (1/ 2))79
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