In the implementation of CARS nanoscopy, signal strength decreases with focal volume size decreasing. A crucial problem that remains to be solved is whether the reduced signal generated in the suppressed focal volume can be detected.Here reported is a theoretical analysis of detection limit(DL) to time-resolved CARS(T-CARS) nanoscopy based on our proposed additional probe-beam-induced phonon depletion(APIPD) method for the low concentration samples. In order to acquire a detailed shot-noise limited signal-to-noise(SNR) and the involved parameters to evaluate DL, the T-CARS process is described with full quantum theory to estimate the extreme power density levels of the pump and Stokes beams determined by saturation behavior of coherent phonons, which are both actually on the order of ～ 109W/cm2. When the pump and Stokes intensities reach such values and the total intensity of the excitation beams arrives at a maximum tolerable by most biological samples in a certain suppressed focal volume(40-nm suppressed focal scale in APIPD method), the DL correspondingly varies with exposure time, for example, DL values are 103 and 102when exposure times are 20 ms and200 ms respectively.
本文提出一种突破衍射极限的红外显微成像方法,该方法基于抽运-探测模式,采用了环形而非高斯型强度分布的抽运光,由于样品在环形光强度峰值附近区域达到吸收饱和,因此当高斯分布的探测光随后到达样品时,只有环形光的中心区域才能吸收探测光的能量,而且吸收区域随着环形光的强度增加而减小.这意味着,如果以被吸收的探测光能量作为该成像系统的信号,本文提出的方法可以使系统的分辨率超越衍射极限的限制.本文模拟了不同环形光能量下成像系统的空间分辨率,结果表明:当环形光能量为100 n J、探测光能量为0.1 n J时,该方法的理论分辨率在236 nm,比传统红外显微成像系统分辨率提高了约14倍.
