Stimulated gain and spontaneous loss pump-probe microscopy

In this work, the pump-probe microscope was used to study the stimulated gain and spontaneous loss phenomenon. A pulsed diode laser, pu=635 nm as the pump (excitation) beam and a mode-locked Ti-sapphire laser, 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.