Properties of two-photon response in a [111]-cut nearly-intrinsic Si hemisphere photodetector are studied.The measured photocurrent of the photodetector responding to the 1.32 μm continuous wave laser shows a quadratic dependence on the coupled optical power and is saturated with the bias voltage.Also,the photocurrent is independent of polarization.Such properties are in good agreement with the theory of two-photon absorption.The isotropic photocurrent generated from the [111]-cut Si hemisphere is compared to the anisotropic one induced in the [110]-cut Si sample and the ratio of Xxxxx /Xxxyy for silicon performing at 1.32μm is calculated to be 2.4via the fitted function of the anisotropic photocurrent from the [110]-cut sample.Silicon materials have a variety of applications in microelectronics and silicon optoelectronics and are still attractive to relevant researchers.Commercial Si photodetectors are largely designed based on singlephoton absorption (SPA).However,nonlinear characteristics have been exhibited in silicon devices.Specifically,two-photon absorption (TPA) has attracted much attention in such devices of Si p-n and p-i-n photodiodes,Si waveguides and Si avalanche diodes,etc.for the autocorrelation measurements of ultrashort laser pulses.[1-4] TPA is a kind of third-order nonlinear optical effect,which typically occurs in semiconductors when the incident photon energy is below the band gap but greater than the half of the band gap.Ling and Lu[5] reported 230cm/MW at 300 K and 360cm/MW at 77K for the simultaneous TPA coefficients at 800nm in Si.Nevertheless,the TPA coefficients near the telecommunication wavelengths in silicon waveguides[2] and bulk crystalline silicon[6]are small,which hinders the TPA-based silicon devices from being efficient at telecommunication wavelengths.