Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science, Seoul 02841, South Korea.
J Chem Phys. 2018 Mar 21;148(11):114201. doi: 10.1063/1.5022092.
Two-beam stimulated Raman scattering (SRS) has been used in diverse label-free spectroscopy and imaging applications of live cells, biological tissues, and functional materials. Recently, we developed a theoretical framework for the three-beam double SRS processes that involve pump, Stokes, and depletion beams, where the pump-Stokes and pump-depletion SRS processes compete with each other. It was shown that the net Stokes gain signal can be suppressed by increasing the depletion beam intensity. The theoretical prediction has been experimentally confirmed recently. In the previous scheme for a selective suppression of one SRS by making it compete with another SRS, the two SRS processes occur in a parallel manner. However, there is another possibility of three-beam double SRS scheme that can be of use to suppress either Raman gain of the Stokes beam or Raman loss of the pump beam by depleting the Stokes photons with yet another SRS process induced by the pair of Stokes and another (second) Stokes beam. This three-beam double SRS process resembles a cascading energy transfer process from the pump beam to the first Stokes beam (SRS-1) and subsequently from the first Stokes beam to the second Stokes beam (SRS-2). Here, the two stimulated Raman gain-loss processes are associated with two different Raman-active vibrational modes of solute molecule. In the present theory, both the radiation and the molecules are treated quantum mechanically. We then show that the cascading-type three-beam double SRS can be described by coupled differential equations for the photon numbers of the pump and Stokes beams. From the approximate solutions as well as exact numerical calculation results for the coupled differential equations, a possibility of efficiently suppressing the stimulated Raman loss of the pump beam by increasing the second Stokes beam intensity is shown and discussed. To further prove a potential use of this scheme for developing a super-resolution SRS microscopy, we present a theoretical expression and numerical simulation results for the full-width-at-half-maximum of SRS imaging point spread function, assuming that the pump and Stokes beam profiles are Gaussian and the second Stokes beam has a doughnut-shaped spatial profile. It is clear that the spatial resolution with the present 3-beam cascading SRS method can be enhanced well beyond the diffraction limit. We anticipate that the present work will provide a theoretical framework for a super-resolution stimulated Raman scattering microscopy that is currently under investigation.
双光束受激拉曼散射（SRS）已被广泛应用于活细胞、生物组织和功能材料的无标记光谱和成像应用中。最近，我们提出了一个三光束双 SRS 过程的理论框架，其中涉及泵浦光、斯托克斯光和耗尽光，泵浦-斯托克斯和泵浦-耗尽 SRS 过程相互竞争。结果表明，通过增加耗尽光束的强度，可以抑制斯托克斯增益信号。最近的实验证实了这一理论预测。在之前的方案中，通过使一个 SRS 过程与另一个 SRS 过程竞争来选择性地抑制一个 SRS 过程，两个 SRS 过程以并行的方式发生。然而，还有另一种三光束双 SRS 方案的可能性，可以通过用另一个 SRS 过程耗尽斯托克斯光子来抑制斯托克斯光束的拉曼增益或泵浦光束的拉曼损耗，该 SRS 过程由斯托克斯光和另一（第二）斯托克斯光对诱导。这个三光束双 SRS 过程类似于从泵浦光束到第一斯托克斯光束（SRS-1）的级联能量转移过程，然后从第一斯托克斯光束到第二斯托克斯光束（SRS-2）。这里，两个受激拉曼增益-损耗过程与溶质分子的两个不同的拉曼活性振动模式相关联。在本理论中，辐射和分子都被量子力学地处理。然后，我们表明，级联型三光束双 SRS 可以用泵浦和斯托克斯光束的光子数的耦合微分方程来描述。从耦合微分方程的近似解和精确数值计算结果中，可以看出通过增加第二斯托克斯光束的强度来有效抑制泵浦光束的受激拉曼损耗的可能性，并对此进行了讨论。为了进一步证明该方案在开发超分辨率 SRS 显微镜方面的潜在用途，我们给出了 SRS 成像点扩散函数的半峰全宽的理论表达式和数值模拟结果，假设泵浦和斯托克斯光束的轮廓为高斯形状，第二斯托克斯光束具有环形空间轮廓。很明显，使用本 3 光束级联 SRS 方法可以大大提高空间分辨率，超过衍射极限。我们预计，本工作将为当前正在研究的超分辨率受激拉曼散射显微镜提供一个理论框架。
