光熱對流是在高光功率下使粒子能穩定的被光學鑷子捕捉的一個主要障礙。粒子被近場光學捕捉力捕捉後，往往會受到熱對流干擾使粒子隨熱對流消失於光學晶格中。在此我們示範一個方法，利用水在低溫時膨脹係數趨近於零的特性，此特性能夠有效地抑制電漿子光熱效應產生的光熱對流。
我們知道使用一個非完全對焦的高斯光束照射於一個簡單方形奈米電漿陣列時，會產生可捕捉粒子的二維光學晶格。我們觀察到這些螢光微粒在室溫下，會因為光熱效應所造成的熱對流運輸現象，粒子將隨著熱對流往液體表面移動，所以粒子一旦進入光晶格中就會消失於光晶格的表面。與之相反的，在低溫下，我們觀察到即使提高光功率，近場光學梯度力依然能夠穩定的使大小為1um的粒子被二維光學晶格捕捉，而不受到光熱對流的干擾。
因此這種低溫技術將可大大增加可用的光功率，進而提高光學鑷子捕捉微小粒子的能力。甚至可以運用在含蛋白質的生物上，避免光熱效應破壞，導致黏滯於晶片，使晶片壽命提升。

Photothermal convection has been a major obstacle for stable particle trapping in plasmonic optical tweezer at high optical power. Here we demonstrate a strategy to suppress the plasmonic photothermal convection by using near zero thermal expansion coefficient of water at low temperature. A simple square nanoplasmonic array is illuminated with a loosely Gaussian beam to produce two dimensional optical lattice for trapping of nanoparticle. We observe stable particle trapping due to near-field optical gradient forces at elevated optical power at low temperature. In contrast, at the sample optical power at room temperature, the particles will get expelled, and disappear once they enter into the optical lattice. This technique will greatly increase usable optical power and enhance the trapping capability of plasmonic optical tweezer.

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