Anisotropic saturable absorption of single wall carbon nanotubes aligned in polyvinyl alcohol
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Anisotropic saturable absorption of single wall carbon nanotubes aligned in polyvinyl alcohol Aleksey G. Rozhin1, Youichi Sakakibara1, Hiromichi Kataura1, Shun Matsuzaki2, Kohtaro Ishida2, Yohji Achiba3, Madoka Tokumoto1,2 1 National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan 2 Faculty of Science and Technology, Tokyo University of Science, Noda 278-8510, Japan 3 Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami–Ohsawa, Hachioji, Tokyo 192-0397, JAPAN ABSTRACT We report anisotropic optical absorption and saturable absorption (SA) properties of aligned single wall carbon nanotubes (SWNT) embedded in the mechanically stretched polyvinyl alcohol (PVA) film. Polarized absorption spectra of the film showed a strong anisotropy for the near-infrared band gap transition of semiconductor SWNT with a dichroic ratio of 7.4, indicating the good alignment of the embedded SWNT. The SA property of the band gap transition measured using a polarized femtosecond fiber laser showed a strong polarization angle dependence for the stretched film. When the polarization of light is parallel to the stretching direction, the absorption saturation of the stretched film became more efficient than that of the unstretched film because of the apparently stronger transition moment. INTRODUCTION Recent research advances regarding the optical properties of single wall carbon nanotubes (SWNT), in particular the findings of the strong optical absorption and emission with ultrafast excited dynamics in the near IR spectral range, have revealed attractive novel applications for optoelectronics related to optical fiber communication technology [1-4]. Among these applications, those using saturable absorption (SA) of SWNT have opened up a series of promising applications, such as all-optical switches [2], mode-lockers in short pulse lasers [3] and as amplified spontaneous emission noise suppressors [4]. Saturable absorption is a phenomenon in which the absorption coefficient of a material decreases when illuminated with intense light. This phenomenon occurs through the process that the primary strong excitation causes the filling of excited states, and the absorption coefficient for the light that comes next is reduced during the period in which the primary excited states
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remains. In homogeneously broadened two level systems with continuous wave excitation, the dependence of absorption coefficient α(P) on the power P of the incident light is given by the following formula [5]:
α ( P) =
α (0) 1+ P
(1)
PS
where α(0) the linear absorption coefficient and Ps is a saturation power with a relationship to the transition moment µ of constituent atoms or molecules, an excitation lifetime T1, and a phase relaxation time T2
Ps ∝
h
4⋅ µ
2
⋅ T1 ⋅ T2
(2)
In many applications, it is desired that saturation occurs at lower power. In equation (2), the material is usually assumed to be isotropic and hence the transition probability of the each atom or m
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