基于受激增益与自发衰减的激发-探测显微技术

In this work, the pump-probe microscope was used to study the stimulated gain and spontaneous loss phenomenon. A pulsed diode laser, λ_pu=635 nm as the pump (excitation) beam and a mode-locked Tisapphire laser, λ_pr=780 nm, as the probe (stimulation) beam were applied. For stimulated gain, the pump beam was modulated at a frequency, f₁, and the probe beam was demodulated accordingly to extract the signal in the transmission direction with a photodiode as the detector (PDA 36A, Thorlabs). For spontaneous loss, the probe beam was modulated at frequency, f₂, the spontaneous loss signal was then demodulated from the fluorescence detected in the reflection mode by a PMT. In all cases, a high performance lock-in amplifier (HF2LI, Zurich Instruments) was used. The output signal of the lock-in amplifier was then fed to the A/D channel of the scanning unit for image reconstruction. The scan rate was set at a frequency 500 Hz, to match the time constant (1.99 ms) of the lock-in amplifier. By demodulating fluorescence signal, the fluorescence lifetime and optical section images can be obtained with greatly reduced background, in which shot noise was attributed. Additionally, the signal-to-noise ratio was improve and penetration depth like multiphoton microscopy was enhanced, without expansive femtosecond lasers.