Spectral interference technique in stimulated parametric emission microscopy

摘要

We recently proposed a novel 3D microscopy technique called stimulated parametric emission (SPE) microscopy. Stimulated parametric emission microscopy shares the four-wave-mixing properties with CARS (coherent anti-stokes Raman scattering) microscopy. The SPE microscope maps the distributions of the third-order nonlinear susceptibility, or information about the nonlinear refractive index and two-photon absorption coefficient of a certain electronic-excitation level of materials such as unstained living cells. Wide-band or equivalently ultra-short pulses from a mode-locked Ti:sapphire laser and an optical parametric oscillator were used for the pump and dump pulses with different central optical frequencies (ω_1 and ω_2) for the SPE process. The pulses are then focused into the sample and the third frequency (ω_3 = 2ω_2-ω_1) component is generated in the sample due to the nonlinear optical susceptibility. This signal pulse having the third frequency and the reference pulse generated by using a standard material are superposed on an array sensor. If we introduce a proper time delay into the signal and reference pulses, the mixing process produces a fringed pattern on the array sensor. Simple calculation enables us to obtain the stronger signal levels of the third-order nonlinear susceptibility that are separated into the real and imaginary parts. Spatial distributions of the complex susceptibility at the several bands can in principle be obtained. We will show the feasibility of this method and a 3D image of the complex susceptibility of a living cell.