Missatge de l'editorial: "This paper was published in Optical letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-27-1-31. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law."
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机译：Missatge de l'editorial：“本文已以光学信函的形式出版，并经OSA许可以电子转载的形式提供。可在OSA网站的以下URL上找到该论文：http：//www.opticsinfobase。 org / ol / abstract.cfm？uri = ol-27-1-31。禁止通过电子或其他方式系统或多次复制或分发到多个位置，并受到法律的制裁。”
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