Dual-Modulation in Broadband Ultrafast Fluorescence Spectroscopy: Frequency-Domain Suppression of Steady-State Fluorescence Leakage

Time-resolved photoluminescence (TRPL) spectroscopy with broadband detection and femtosecond temporal resolution is essential for probing early-time excited-state dynamics, yet the intense steady-state fluorescence background from long-lived emitters often overwhelms the weak gated signal. Here, a double-chopper modulation strategy is developed for fluorescence noncollinear optical parametric amplification-based TRPL spectroscopy to address this challenge. The excitation and gate beams are independently modulated at different frequencies, encoding the amplified transient fluorescence into sum- and difference-frequency channels in the Fourier domain, while the parametric and steady-state fluorescence backgrounds remain at the fundamental modulation frequencies. This frequency-domain separation enables selective extraction of the gated signal with efficient background rejection. In measurements on Rhodamine 6G, the steady-state fluorescence leakage is suppressed by a factor of 120 compared to conventional single-chopper detection, achieving a nearly 4-fold improvement in the signal-to-noise ratio. The method is further applied to InP/ZnS quantum dots with a 60 ns fluorescence lifetime, resolving ultrafast dynamics including a subpicosecond rise and picosecond decay components that are otherwise obscured by the steady-state background. This double-chopper approach is broadly applicable to other optical-gating techniques and provides a robust route for broadband ultrafast TRPL measurements on long-lived emissive systems.