Institute of Experimental Physics, Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02-093 Warsaw, Poland.
Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland.
Sci Rep. 2017 Feb 22;7:42889. doi: 10.1038/srep42889.
Although new optical materials continue to open up access to more and more wavelength bands where femtosecond laser pulses can be generated, light frequency conversion techniques are still indispensable in filling the gaps on the ultrafast spectral scale. With high repetition rate, low pulse energy laser sources (oscillators) tight focusing is necessary for a robust wave mixing and the efficiency of broadband nonlinear conversion is limited by diffraction as well as spatial and temporal walk-off. Here we demonstrate a miniature third harmonic generator (tripler) with conversion efficiency exceeding 30%, producing 246 fs UV pulses via cascaded second order processes within a single laser beam focus. Designing this highly efficient and ultra compact frequency converter was made possible by full 3-dimentional modelling of propagation of tightly focused, broadband light fields in nonlinear and birefringent media.
虽然新的光学材料不断开辟途径，使飞秒激光脉冲能够在越来越多的波长波段产生，但在超快光谱范围内填补空白仍然离不开光频转换技术。对于重复率高、脉冲能量低的激光源（振荡器），为了实现稳健的波混频，需要进行紧密聚焦，而宽带非线性转换的效率受到衍射以及空间和时间走离的限制。在这里，我们通过在单个激光束焦点内通过级联二阶过程，展示了转换效率超过 30%的微型三倍频器（三倍频器），产生了 246fs 的紫外脉冲。通过对非线性和双折射介质中紧密聚焦的宽带光场传播进行全三维建模，才使得这种高效、超紧凑的频率转换器的设计成为可能。
