Optical property measurements of 235 mm large-scale Ti:Sapphire crystal
A Ti:Sapphire crystal with a diameter of 235 mm and a thickness of 72 mm was grown by heat exchange method
The emergence of ultra-intense and ultrashort laser system makes it possible to attain the extreme conditions in the laboratory like ultrastrong electric field, ultrahigh magnetic field, ultrahigh energy density, and gigantic light pressure, which are otherwise found only in the interior of a star or the edge of a black hole. It has largely promoted the development of a number of basic and frontier interdisciplinary subjects including laser science, attosecond optics, and plasma physics. Ultra-intense ultrashort laser applications have made some progress in studying antimatter, proton imaging, and driving the wake field electron acceleration.
At present, the main technology of constructing ultra-intense and ultrashort laser facility in the world is chirped pulse amplification technology(CPA). Ti sapphire crystal is usually chosen as terminal amplifier in the laser facility based on CPA technology. In order to further improve the peak power of laser pulse, a larger Ti:Sapphire crystal with high quality is required.
The research team led by Yin Hangfrom Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, used heat exchange method (HEM) to grow a Ti:Sapphire crystal with a diameter of 235 mm and a thickness of 72 mm.(the largest Ti:Sapphire crystal ever reported for a petawatt femtosecond-class laser system). Optical and laser properties were measured. The measurement results show that the crystal has good absorbability for 527 nm green light. Figure of merit (FOM) value is high. Internal quality of the crystal is better. And small signal gain value with high uniformity is obtained. The results have been published in Chinese Optics Letters, Vol 16, Issue 7, 2018 (H. Cao et al., Optical property measurements of 235 mm large-scale Ti:Sapphire crystal).
Huang Pei, a member of the research team, thinks that the study demonstrates that the large Ti:Sapphire crystal has excellent optical properties and laser performances, which can further support higher peak power of the laser pulse output.
Based on the above research results, the next step will be to increase the input energy and test the gain under large input energy.
COL封面故事：235 mm大型钛宝石晶体的光学性能测试
用热交换法生长的直径为235 mm、厚度为72 mm的钛宝石晶体
超强超短激光的诞生使得在实验室内就能产生只有在恒星内部或是黑洞边缘才能找到的极端物理条件，如超强电场、超高磁场、超高能量密度、巨大光压等，这大大加速了一些基础与前沿交叉学科的发展，如激光科学、阿秒光学和等离子体物理学等。超强超短激光在研究反物质、质子成像、驱动尾波场电子加速等领域已经取得了重要的研究成果。
目前世界上搭建超强超短激光实验装置的主要技术为啁啾脉冲放大（CPA）技术，基于CPA技术的实验装置一般选用钛宝石晶体作为终端放大器，为了进一步提高激光脉冲峰值功率需要更大口径的优质钛宝石晶体。
中科院上海光机所杭寅研究团队利用热交换法生长出了一块直径235 mm、厚度72 mm的大口径钛宝石晶体（已报道的用于超强超短激光系统中的最大口径的钛宝石晶体）并对其进行了光学性能以及激光性能的测试，结果表明该晶体对527 nm绿光具有良好的吸收性，品质因子（FOM）较高，晶体内部质量较好，并具有均匀性较好的小信号增益数值。该成果发表在Chinese Optics Letters 2018年第7期上（H. Cao et al., Optical property measurements of 235 mm large-scale Ti:Sapphire crystal）。
该研究团队的成员黄培认为，该研究成果证明了这块大口径钛宝石晶体具有优良的光学性能以及激光性能，可以进一步支持更高峰值功率的激光脉冲输出。
基于上述研究成果，下一步将提高注入能量，测试大能量注入下的增益情况。