{"id":3489,"date":"2025-10-08T14:51:40","date_gmt":"2025-10-08T12:51:40","guid":{"rendered":"https:\/\/www.swissrocketman.fr\/Wordpress\/?p=3489"},"modified":"2025-12-21T15:54:31","modified_gmt":"2025-12-21T14:54:31","slug":"autocorrelation-monocoup-par-fluorescence-a-deux-photons","status":"publish","type":"post","link":"https:\/\/www.swissrocketman.fr\/Wordpress\/?p=3489","title":{"rendered":"Autocorr\u00e9lation monocoup par fluorescence \u00e0 deux photons"},"content":{"rendered":"

Autocorr\u00e9lation monocoup par fluorescence \u00e0 deux photons<\/h2>\n

D\u00e8s 1967, Giordmaine a d\u00e9velopp\u00e9 un autocorr\u00e9lateur monocoup mettant en oeuvre
\nl\u2019absorption bi-photonique . Cette technique a longtemps \u00e9tait la seule qui permette de mesurer des impulsions picosecondes.<\/h5>\n<\/div><\/section>\n
<\/div><\/div><\/div><\/div>\n

Autocorr\u00e9lateur monocoup par fluorescence \u00e0 deux photons (Giordmaine, 1967)<\/h2>\n

1. Principe g\u00e9n\u00e9ral<\/strong><\/h4>\n

L\u2019id\u00e9e est de mesurer la dur\u00e9e d\u2019une impulsion ultracourte (picoseconde ou femtoseconde) en exploitant la\u00a0fluorescence induite par absorption bi-photonique<\/strong>\u00a0dans un milieu fluorescent.
Cette fluorescence ne se produit que l\u00e0 o\u00f9 l\u2019intensit\u00e9 optique est \u00e9lev\u00e9e, donc lorsque\u00a0deux impulsions laser se recouvrent spatialement et temporellement<\/strong>.<\/h4>\n

2. Montage exp\u00e9rimental<\/strong><\/h4>\n

Comme montr\u00e9 dans ton image :<\/h4>\n
    \n
  • \n

    Le\u00a0faisceau incident<\/strong>\u00a0est scind\u00e9 en deux demi-faisceaux par un syst\u00e8me de miroirs (souvent une lame semi-r\u00e9fl\u00e9chissante et des miroirs fixes).<\/h4>\n<\/li>\n
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    Ces deux faisceaux se\u00a0croisent sous un angle<\/strong>\u00a0dans une\u00a0cuve contenant un colorant fluorescent<\/strong>\u00a0(par exemple du Rhodamine 6G).<\/h4>\n<\/li>\n
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    Il n\u2019y a\u00a0aucune variation m\u00e9canique de retard<\/strong>\u00a0: les deux impulsions se propagent en\u00a0contre-sens<\/strong>, et le retard temporel entre elles varie\u00a0spatialement le long de la zone d\u2019intersection<\/strong>.<\/h4>\n<\/li>\n<\/ul>\n

    Ainsi :<\/h4>\n
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      Chaque point le long de l\u2019axe\u00a0zz<\/span>z<\/span><\/span><\/span><\/span>\u00a0correspond \u00e0 un certain\u00a0retard temporel\u00a0\u03c4\\tau<\/span>\u03c4<\/span><\/span><\/span><\/span><\/strong>\u00a0entre les deux faisceaux.<\/h4>\n<\/li>\n
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      L\u2019intensit\u00e9 de fluorescence \u00e9mise d\u00e9pend du\u00a0recouvrement temporel local<\/strong>\u00a0des deux impulsions.<\/h4>\n<\/li>\n
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      L\u2019ensemble de la zone \u00e9met une\u00a0trace d\u2019autocorr\u00e9lation spatiale<\/strong>.<\/h4>\n<\/li>\n<\/ul>\n

      3. D\u00e9tection<\/strong><\/h4>\n

      La fluorescence \u00e9mise dans la r\u00e9gion de croisement est recueillie :<\/h4>\n
        \n
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        \u00e0 travers un\u00a0filtre optique<\/strong>\u00a0(pour \u00e9liminer la lumi\u00e8re laser directe),<\/h4>\n<\/li>\n
      • \n

        puis enregistr\u00e9e par une\u00a0cam\u00e9ra CCD<\/strong>.<\/h4>\n<\/li>\n<\/ul>\n

        Cette cam\u00e9ra observe donc une\u00a0image de fluorescence<\/strong>\u00a0dont l\u2019intensit\u00e9 varie selon la position\u00a0zz<\/span>z<\/span><\/span><\/span><\/span>.
        Le profil d\u2019intensit\u00e9 correspond directement \u00e0 la\u00a0fonction d\u2019autocorr\u00e9lation de l\u2019impulsion<\/strong>.<\/h4>\n<\/div><\/section><\/div>\n
        <\/div><\/div><\/div><\/div>\n
        <\/div><\/div><\/div><\/div>\n

        Dans l\u2019exp\u00e9rience que je vais r\u00e9aliser, le laser femtoseconde sera le ORIGAMI 10PL\u00a0 permettant de faire des pulses \u00e0 1030nm et 200 femtosecondes \u00e0 40 MHz. Le colorant choisi est la Rhodamine 6G.<\/em><\/h4>\n
        Pour la visualisation il faudra utilis\u00e9 un binoculaire de 50-100 fois car la largeur de fluorescence sera de 80 microns pour un pulse de 200 fs.<\/h5>\n<\/div><\/section><\/div>\n
        <\/div><\/div><\/div><\/div>\n
        <\/div><\/div><\/div><\/div>\n

        Cette technique consiste tout d\u2019abord \u00e0 d\u00e9doubler l\u2019impulsion \u00e0 analyser \u00e0 l\u2019aide d\u2019un
        \ninterf\u00e9rom\u00e8tre de type Sagnac. Une cuve contenant une solution alcoolique fluorescente est
        \nplac\u00e9e au centre de cet interf\u00e9rom\u00e8tre. Cette solution fluoresce par absorption \u00e0 deux photons. Les deux impulsions contra-propagatives produites par l\u2019interf\u00e9rom\u00e8tre se croisent dans la cuve. Dans la zone de recouvrement des impulsions (au voisinage de z=0 et t=0 ), la trace d\u2019autocorr\u00e9lation de l\u2019impulsion s\u2019affiche alors spatialement le long de l\u2019axe de propagation. Il suffit alors d\u2019enregistrer transversalement l\u2019image de la zone fluorescente. On obtient un maximum de signal de fluorescence \u00e0 deux photons au centre de la cuve, l\u00e0 o\u00f9 les deux impulsions se recouvrent totalement.<\/h4>\n
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        Excellent r\u00e9flexe\u00a0\u00a0\u2014 oui,\u00a0la fr\u00e9quence de r\u00e9p\u00e9tition change \u00e9norm\u00e9ment les choses<\/strong>, car elle agit sur la\u00a0puissance moyenne<\/strong>\u00a0et sur le\u00a0nombre de photons excit\u00e9s par seconde<\/strong>, m\u00eame si\u00a0l\u2019intensit\u00e9 cr\u00eate d\u2019une impulsion ne change pas<\/strong>\u00a0(et c\u2019est elle qui d\u00e9termine l\u2019efficacit\u00e9 de l\u2019absorption bi-photonique).<\/h4>\n

        \u00a0Donn\u00e9es mises \u00e0 jour<\/h4>\n
        \n\n\n\n\n\n\n\n\n\n\n\n
        Param\u00e8tre<\/th>\nSymbole<\/th>\nValeur<\/th>\n<\/tr>\n<\/thead>\n
        Longueur d\u2019onde<\/td>\n\u03bb\\lambda<\/span>\u03bb<\/span><\/span><\/span><\/span><\/td>\n1030 nm<\/td>\n<\/tr>\n
        Dur\u00e9e d\u2019impulsion<\/td>\n\u03c4p\\tau_p<\/span>\u03c4<\/span>p<\/span><\/span><\/span>\u200b<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/td>\n200 fs =\u00a02.0\u00d710\u2212132.0\\times10^{-13}<\/span>2.0<\/span>\u00d7<\/span><\/span>1<\/span>0\u221213<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span>\u00a0s<\/td>\n<\/tr>\n
        \u00c9nergie par pulse<\/td>\nEpE_p<\/span>E<\/span>p<\/span><\/span><\/span>\u200b<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/td>\n2.2 nJ =\u00a02.2\u00d710\u221292.2\\times10^{-9}<\/span>2.2<\/span>\u00d7<\/span><\/span>1<\/span>0\u22129<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span>\u00a0J<\/td>\n<\/tr>\n
        Fr\u00e9quence de r\u00e9p\u00e9tition<\/td>\nfrepf_{\\text{rep}}<\/span>f<\/span>rep<\/span><\/span><\/span><\/span>\u200b<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/td>\n40 MHz =\u00a04.0\u00d71074.0\\times10^{7}<\/span>4.0<\/span>\u00d7<\/span><\/span>1<\/span>07<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span>\u00a0Hz<\/td>\n<\/tr>\n
        Diam\u00e8tre du faisceau<\/td>\ndd<\/span>d<\/span><\/span><\/span><\/span><\/td>\n0.8 mm \u2192 rayon\u00a0r=4.0\u00d710\u22124 r = 4.0\\times10^{-4}<\/span>r<\/span>=<\/span><\/span>4.0<\/span>\u00d7<\/span><\/span>1<\/span>0\u22124<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span>\u00a0m<\/td>\n<\/tr>\n
        Colorant<\/td>\nRhodamine 6G, 1 mM<\/td>\n<\/td>\n<\/tr>\n
        Mode<\/td>\nTEM\u2080\u2080, monomode<\/td>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/article>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div><\/section><\/div>\n
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