Control of Magnetic Properties in Nano- and Micro-scales with Large Optical Nonlinearity: a New Approach for Development of Functional Micromachines

由于在1,3,5-三硫-2,4,6-三氮杂二烯基（TTTA）晶体中发现了在宽温度范围内具有宽热滞回线的大一阶磁相变，因此有机自由基晶体有望成为一种具有光致相变（PIPT）现象的新型强相关电子体系。在本研究中，我们探索了如何在具有大光学非线性的纳米和微区域控制TTTA的磁性，并展示了利用PIPT现象开发功能纳米和微机械的新方法。在不同光子密度和波长的双光子吸收条件下，对TTTA晶体的光致发光和光致反磁向顺磁相变进行了详细的研究。在驱动PIPT的阈值光子密度下，反磁相呈现出宽的发光带，具有较大的Stokes位移，其强度几乎服从激发光子密度的二次幂定律。另一方面，在阈值光子密度以上，反磁向顺磁相变有效发生，转化率大，响应陡，而没有发生光致发光，表明相变是由双光子吸收引起的，具有较大的光学非线性。利用PIPT的大非线性特性，可以在室温下控制TTTA的磁性能。为了开发具有功能的纳米和微机械，TTTA分布在UV固化树脂中，该树脂适用于双光子微立体光刻技术生产三维微机械。我们发现，在飞秒再生放大器的强激光脉冲照射下，TTTA可以在保持PIPT特性的情况下成功地分布到树脂中。

Since a large first order magnetic phase transition with a wide thermal hysteresis loop over wide temperature range was found in 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal, an organic radical crystal is expected to be a new type of strong correlated electronic systems showing photoinduced phase transition (PIPT) phenomena. In this research, we explore how to control the magnetic properties of TTTA in nano- and micro-regions with large optical nonlinearity, and demonstrate a new approach for the development of functional nano and micromachines by utilizing the PIPT phenomena.Photoluminescence as well as photoinduced diamagnetic to paramagnetic phase transition in TTTA crystal has been studied in detail under two-photon absorption with different photon densities and wavelengths. Below the threshold photon density to drive the PIPT, the diamagnetic phase shows a broad luminescence band with a large Stokes shift, whose intensity obeys almost second power law of the excitation photon density. Above the threshold photon density, on the other hand, the diamagnetic to paramagnetic phase transition effectively takes place with a large conversion yield and a steep response instead of an occurrence of the photoluminescence, indicating that the phase transition is optically induced by two-photon absorption with large optical nonlinearity. Using the PIPT behavior with the large nonlinearity, the magnetic properties of TTTA can be controlled at room temperature.In order to develop the functional nano and micromachines, TTTA is distributed into UV curable resins, which are suitable for the production of 3D micromachines by using two-photon microstereolithography. We found that TTTA can be successfully distributed into the resins with keeping the PIPT characteristics by irradiation of intense laser pulses from a femtosecond regenerative amplifier.