Carbon nanotubes (CNTs) are structures of carbon atoms bonded together in hexagonal honeycomb shapes, with multi-walled CNTs having a very high thermal conductivity of 3000 W/m$\cdot$K and single-walled CNTs having a conductivity of 6000 W/m$\cdot$K. In this work, the transmittance and the thermal diffusivity of a multi-walled carbon nanotube dispersion with a concentration of 1.5 M were measured using a single beam method, a dual beam method, and the thermal lens effect. The nonlinear optical coefficients were obtained by using the z-scan method, which moved the sample in the direction of propagation of the single laser beam, propagation and the thermal diffusivity was measured using a double laser beam. As a pump beam, a diode-pumped solid state (DPSS) laser with a wavelength of 532 nm and an intensity of 100 mW was used. As the probe beam, a He-Ne laser having a wavelength of 633 nm and an intensity of 5 mW was used. The experimental result shows that when the concentrations of the sample were 9.99, 11.10, 16.65, and 19.98 mM, the nonlinear absorption coefficients were 0.046, 0.051, 0.136 and 0.169 m/W, respectively. Also, the nonlinear refractive indices were 0.20, 0.51, 1.25 and 1.32 $\times$ 10$^{-11}$ m$^2$/W, respectively, and the average thermal diffusivity was 1.33 $\times$ 10$^{-6}$ m$^2$/s.

Keywords: Thermal lens effect, Multi-walled carbon nanotube, Z-scan, Nonlinear optical coefficient, Thermal diffusivity

탄소 나노 튜브는 탄소 원자들이 육각형의 벌집모양으로 서로 연결된 고분자 탄소동소체로 다중벽일 경우 3000 W$\cdot$m$^{-1}\cdot$K$^{-1}$, 단일벽일 경우 6000 W$\cdot$m$^{-1}\cdot$K$^{-1}$ 정도로 매우 높은 열전도도를 보인다. 본 연구에서는 단일 빔과 이중 빔 방법으로 열렌즈 효과를 이용하여, 1.5 M 다중벽 탄소 나노튜브 분산액의 투과율과 열확산도를 측정하였다. 단일 레이저 빔의 진행방향으로 시료를 움직이는 z-scan 방법을 통해 비선형 광학계수들을 구하고, 이중 레이저 빔을 이용하여 열확산도를 측정하였다. 펌프 빔으로는 파장 532 nm이고 세기가 100 mW인 DPSS (Diode-pumped solid state, DPSS) 레이저를 사용하였고, 프로브 빔으로는 파장이 633 nm이며 세기가 5 mW인 He-Ne 레이저를 사용하였다. 실험 결과 농도가 9.99, 11.10, 16.65, 19.98 mM일 때 비선형 흡수계수는 각각 0.046, 0.051, 0.136, 0.169 m/W였다. 또한 비선형 굴절률은 0.20, 0.51, 1.25, 1.32 $\times$ 10$^{-11}$ m$^2$/W였고, 열확산도 평균치는 1.33 $\times$ 10$^{-6}$ m$^2$/s이었다.

Keywords: 열렌즈 효과, 다중벽 탄소 나노 튜브, Z-스캔, 비선형광학계수, 열확산도