Cu_(1.81)S纳米晶的空间自相位调制阈值厚度研究

Research on threshold thickness of spatial self-phase modulation based on Cu_(1.81)S nanocrystals

激光诱导空穴相干或电子相干机制会在数毫米厚度的二维材料离散体中产生空间自相位调制(SSPM)现象.该现象能否在更小厚度实现,对其在全光调制等的应用十分关键.采用热注入法制备了硫化铜纳米晶,控制Cu与S原子比为1.81:1,调制带隙为1.75 e V.利用低频飞秒脉冲激发,观测到多波长下的SSPM现象,表征Cu_(1.81)S的非线性折射率n_(2)为10^(-3)cm~2/W量级,并利用非局域空穴相干解释.聚焦于激发波长为800 nm(略小于带隙)的测试,逐步将离散体的厚度从10 mm降低到1 mm,并提出利用Fresnel-Kirchhoff衍射公式仿真拟合的新方法.实验和仿真模型均证明,在入射光强为25 W/cm~2时,尽管Cu_(1.81)S具有较大的n_(2),但其SSPM阈值厚度仍为5 mm左右;将入射光功率密度提高到1805 W/cm~2以上或改变聚焦条件,有望实现100μm厚度的SSPM效应.该研究提出了微米尺度SSPM光路改进方案,预测微纳尺度SSPM研究需要建立新的光场相互作用模型,有助于扩宽其在全光微纳器件领域的应用.

Laser-induced hole coherence or electron coherence mechanisms produce spatial self-phase modulation(SSPM)phenomena in dispersion of two-dimensional material with several millimeters in optical path.Whether this phenomenon can be achieved at smaller thicknesses is key to its application in all-optical modulation,and so on.Copper sulfide nanocrystals were prepared by the hot injection method,by controlling the atomic ratio between Cu and S to 1.81:1 and a modulation band gap of 1.75 eV.SSPM phenomena at multiple wavelengths are observed using low-frequency femtosecond pulse excitation,the nonlinear refractive index n_(2)of Cu_(1.81)S is characterized on the order of 10~(-3)cm~2/W and explained using nonlocal hole coherence.Focusing on the test at an excitation wavelength of 800 nm(slightly smaller than the band gap),the optical path of the dispersion is gradually reduced from the conventional 10 to 1 mm,a new method of simulation fitting using the Fresnel-Kirchhoff diffraction formula is proposed.Both experimental and simulation models demonstrate that when the incident light intensity is 25 W/cm~2,although Cu_(1.81)S has a large n_2,its SSPM threshold thickness is still about 5 mm.However,increasing the incident optical power density above 1805 W/cm~2or changing the focusing conditions is expected to achieve the SSPM effect from 100μm optical path.This research proposes a micron-scale SSPM optical path improvement scheme and predicts that new optical field interaction models are needed for micro-and nano-scale SSPM studies,which can help broaden the application of SSPM in the field of alloptical micro-and nano-devices.