Impact of α↿β Transition in the Ultrafast High-Order Nonlinear Optical Properties of Metal-Free Phthalocyanine Thin Films

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• 作者：K. V. Anil Kumar ; Samir Kumar ; S. M. Dharmaprakash ; Ritwick Das
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：March 31, 2016
• 年：2016
• 卷：120
• 期：12
• 页码：6733-6740
• 全文大小：431K
• ISSN：1932-7455

文摘

We present a study on the impact of phase transformation (α→ β) of tetra-tert-butylphthalocyanine (H₂Pc) thin films on their third-order as well as fifth-order nonlinear optical (NLO) properties. The α-phase of H₂Pc thin film is prepared by thermally evaporating the commercially available H₂Pc powder on a glass substrate which is further annealed at 473 K to obtain the more stable β-phase. The structural modifications in H₂Pc films have a distinct signature in UV–vis absorption spectra, powder X-ray diffraction, and Raman spectra which have been employed for identifying the transformation from α-phase to β-phase. The NLO characteristics which include nonlinear refractive indices such as n₂ and n₄, as well as nonlinear absorption coefficient (β_eff), were measured by using a single beam Z-scan technique. Using an ultrashort pulsed fiber laser at 1064 nm central wavelength in the Z-scan experiment, we obtained a smaller value of β_eff in α-phase as compared to β-phase due to a smaller value of saturation intensity (I_s) which, in turn, is a consequence of ground-state bleaching in thermally unstable amorphous state of the molecule. The impact of χ⁽⁵⁾ nonlinearity in the Z-scan transmittance of α-phase was apparent, and interestingly, the optical phase changes due to χ⁽³⁾ and χ⁽⁵⁾ nonlinearities bear opposite sign. In addition, they alter their signs as the laser intensity increases. A dominant fifth-order NLO contribution is mainly a consequence of inherently large χ⁽⁵⁾ for the films as well as two-photon-assisted nonlinear refraction as a result of excited state absorption.