The optical Kerr gate technique involves an ultrafast laser pulse (pump), a second pulse (gate), and a Kerr medium. The pump pulse triggers the molecular process of interest. The fluorescence first passes through a linear polarizer. A second linear polarizer (analyzer) is positioned perpendicular to the first polarizer, effectively blocking the transmitted emission signal. A Kerr medium is placed between the analyzer and polarizer. The gate pulse records spectroscopic changes induced by the pump pulse, it switches the material birefringence “on” in the Kerr medium, which is typically an isotropic material. This laser-induced anisotropy lasts for a few hundred femtoseconds. During this short temporal window, the linearly polarized fluorescence beam will get phase-shifted after passing through the Kerr medium, resulting in the depolarization of the original fluorescence beam, and, therefore, its partial transmittance through a cross-polarized optical system. 

Fluorescence spectra on the femtosecond time scale are frequently measured using an optical gate. The optical gate works by slicing through the fluorescence decay, with the timing of the slicing determined by a delay stage. This delay stage sets the interval between the moment of excitation by the pump pulse and the gating. By repeating the experiment with different delay values, the decay trace or profile can be obtained.

The fluorescence upconversion experiment (FU) offers better temporal resolution but is more complex than the Kerr gate. In this experiment, the pump pulse promotes the molecular system to a singlet excited state S₁ at t_Pump = 0. Fluorescence from the excited state typically persists for tens or hundreds of nanoseconds. This relatively long signal is collected from the emissive sample and focused into a non-linear medium χ⁽²⁾ along with a much shorter gate pulse. This results in sum-frequency mixing of the two waves (i.e., upconversion). The intensity of the upconverted radiation (ω_upconversion) is directly proportional to the temporal product of the two interacting pulses. As we forward the gating pulse in time (t_Gate = t_Gate1, t_Gate2, t_Gate3, …), these temporally-progressed gating pulses interact with different segments of the fluorescence pulse envelope. Since the temporal duration of the laser pulse is much shorter that the fluorescence signal, the gating pulse effectively acts as a strobe that “slices up” the much longer fluorescence pulse, creating upconverted pulses of varying intensity. The generated sum-frequency can then be detected in a spectral device using a “slow” detector.

TCSPC (Time-correlated single-photon counting) is an electronic time-domain fluorescence measurement technique that relies on the probabilistic nature of fluorescence. For illustrative purposes, it can be envisioned as using a fast stopwatch to detect fluorescence events. When a sample is excited by a short laser pulse at a relative t = 0, it emits a fluorescence photon after a time period Δt (with some probability). If the excitation-to-emission time Δt can be measured several times using many laser pulses, the obtained values will be distributed according to the emission probability, i.e., the fluorescence decay curve will be obtained. This process is illustrated in the picture below. After gathering a statistically meaningful number of single-photon detection events, they are sorted by photon detection times. This produces a histogram that represents the probability of emitting a photon from the excited state, i.e., the fluorescence decay probability.

The HARPIA-TF is a time-resolved fluorescence measurement module for HARPIA-TA spectroscopy system that combines Kerr gate or fluorescence upconversion with TCSPC techniques. With the use of a high repetition rate PHAROS or CARBIDE femtosecond laser, the fluorescence dynamics are measured while exciting the samples with pulse energies down to several nanojoules.