以強場雷射脈衝與固態靶材交互作用產生高強度兆赫茲脈衝

Abstract

兆赫茲波是頻率介在紅外光與微波之間的電磁波，波長範圍在30 μm到3 mm之間，許多分子的振動、轉動能階能量皆落在這個波段，使兆赫茲波光源成為研究物質的絕佳利器，在天文、化學、凝態物理、生物醫學皆更重要的應用。但是產生高強度的兆赫茲波並不容易，幸好，超短脈衝雷射的誔生帶來了曙光：光導天線、光整流、四波混頻、雷射與固態靶材交互作用等方法，不但產生了具時間解析能力的兆赫茲脈衝，並且將單發脈衝的兆赫茲波能量提升到 μJ的等級。 不過就應用的需求來說，μJ等級的能量仍是不足的，世界各地的團隊迄今仍致力於改良現更的產生方法，期能將光強度大幅提升。2005年，中國科學院物理研究所的盛政民教授等人針對以往的雷射與固態靶材交互作用的實驗結果，提出線性模式轉換的物理機制來解釋兆赫茲波產生。根據他們的理論，固態靶材表面預游離電漿的密度梯度大小對兆赫茲波產生更決定性的影響，只要適當控制電漿環境，可能可以產生高強度兆赫茲波。 本實驗便是以該理論為指引，多使用一道雷射預脈衝，更系統的控制主脈衝入射前的電漿環境，期能驗證盛政民等人的模型，並且提升現更的兆赫茲脈衝能量紀錄。實驗結果發現，電漿密度梯度比較平緩的環境，更利於兆赫茲波光源的產生，與盛政民等人的預測並不相違背；此外，我們一舉將單發兆赫茲脈衝能量的紀錄提升到近20 μJ，產生高強度的兆赫茲波脈衝光源。

Terahertz wave is electromagnetic wave ranging from 30 μm to 3 mm. Because a large portion of molecules have vibrational states and rotational states with energy at same order of Terahertz photon, Terahertz source becomes a powerful tool to investigate micro world. It has wide applications in fields such as astronomy, chemistry, condensed physics, and biomedical sciences. Though Thz wave can play an important role in science field, the lack of intense terahertz light source frustrated the progress of its application. Fortunately, the invention of ultrashort pulse laser provide a series of new methods to generate THz wave. These methods include: photoconductive antenna, optical rectification, four-wave mixing, and laser-plasma interaction on surface of solid target. These methods not only raises the energy record of Thz production, but also produces Thz wave in pulse form, which means it can be the source of a time-resolved spectroscopy. These methods can produce Thz pulse with energy as high as μJ level per shot, but it still doesn’t meet the need for applications. Many research groups are still working on the modification of these methods, wishing to raise the energy record. In 2005, group from Institute of Physics Chinese academy of sciences published their new result: Z.M. Sheng at el proposed a model to explain the Thz production in laser-plasma interaction on solid target. According to their theory, density scale length of preformed plasma plays an important role on THz generation. By controlling the plasma environment properly, intense THz pulse might be enhanced. Guided by above model, our experiment is the first using a laser prepulse to control the plasma environment in order to verify the model and attempt to raise current energy record. Experimental result shows that longer density scale length profits THz generation. This result is in agreement with the model. Moreover, Intense THz pulse is produced and has energy nearly 20 μJ per shot.