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

由于在1，3，5-三硫杂-2，4，6-三氮杂戊烯(TTTA)晶体中发现了较大的一阶磁性相变和较宽的热滞回线，有机自由基晶体有望成为一种新型的具有光致相变(PIPT)现象的强关联电子体系。在这项研究中，我们探索了如何在光学非线性较大的纳米和微米区域控制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途径。详细研究了TTTA晶体在不同光子密度和波长的双光子吸收下的光致发光以及光致反磁到顺磁相变。在驱动PIPT的阈值光子密度以下，反磁相发光带较宽，斯托克斯位移较大，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，反磁到顺磁的相变有效地发生，转换效率高，响应陡峭，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。利用具有大非线性的PIPT行为，TTTA的磁性可以在室温下控制。为了开发功能纳米和微机械，TTTA被分散到紫外光固化树脂中，利用双光子显微光固化技术制造适合于制造三维微机械的树脂。我们发现，在来自飞秒再生放大器的强激光脉冲的照射下，TTTA可以在保持PIPT特性的情况下成功地分散到树脂中。

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.