Time-Resolved Fluorescence Spectroscopy

Time-resolved fluorescence spectroscopy provides information on molecular processes in the excited state. Several techniques allow measuring fluorescence dynamics at different time scales using the same experimental setup:

  • Kerr gate,
  • fluorescence upconversion,
  • time-correlated single-photon counting (TCSPC),
  • phosphorescence.

Kerr gate

The optical Kerr gate technique involves an ultrafast laser pulse (pump), a second pulse (gate), and a Kerr medium. The pump pulse is for triggering the molecular process of interest. The fluorescence is first passed through a linear polarizer. A second linear polarizer (analyzer) is positioned perpendicular to polarizer, which effectively blocks the transmitted emission signal. A Kerr medium is placed between the analyzer and polarizer. Gate pulse records spectroscopic changes induced by the pump pulse, it switches material birefringence “on” on 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 and the timing of the slicing is 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.

Fluorescence upconversion

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 tPump = 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 (ωupconversion) is directly proportional to the temporal product of the two interacting pulses. As we forward the gating pulse in time (tGate = tGate1, tGate2, tGate3, …), 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 is an electronic time-domain fluorescence measurement technique that relies on the probabilistic nature of fluorescence. For illustrative purposes, it can be envisioned as fluorescence event detection with a fast stopwatch. When a sample is excited by a short laser pulse at a relative t = 0, after a time period Δt (with some probability) it will emit a fluorescence photon. 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 we have gathered a statistically meaningful number of single-photon detection events, we can sort them out according to 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.

  • Femtosecond-to-microsecond measurements
  • Automated switching between fluorescence upconversion and TCSPC
  • Automated spectral scanning and calibration
  • Optional operation as a stand-alone unit
  • Market-leading sensitivity
  • 330 nm – 24 μm spectral range
  • Probe delay ranges from 2 to 8 ns
  • Pump pulse energies down to nJ
  • Cryostat and peristaltic pump support
  • Tunable pulse duration, 100 fs – 20 ps
  • Maximum pulse energy of up to 4 mJ
  • Down to < 100 fs right at the output
  • Pulse-on-demand and BiBurst for pulse control
  • Up to 5th harmonic or tunable extensions
  • CEP stabilization or repetition rate locking
  • Thermally-stabilized and sealed design
  • Tunable pulse duration, 190 fs – 20 ps
  • Maximum output of 120 W and 2 mJ
  • Single-shot – 2 MHz repetition rate
  • Pulse-on-demand and BiBurst for pulse control
  • Up to 5th harmonic or tunable extensions
  • Air-cooled model
  • Compact industrial-grade design
  • Continuous tunability from UV to MIR, 190 – 16000 nm
  • High energy and high power models for all needs
  • Single-shot – 2 MHz repetition rate
  • Up to 80 W pump power
  • Up to 2 mJ pump pulse energy

Optically Excited Lasing in a Cavity-Based, High-Current-Density Quantum Dot Electroluminescent Device

N. Ahn, Y. Park, C. Livache, J. Du, K. Gungor, J. Kim, and V. I. Klimov, Advanced Materials 9 (35), 2206613 (2023).

Dopamine Photochemical Behaviour under UV Irradiation

A. Falamaş, A. Petran, A. Hada, and A. Bende, International Journal of Molecular Sciences 10 (23), 5483 (2022).

Electron–Hole Binding Governs Carrier Transport in Halide Perovskite Nanocrystal Thin Films

M. F. Lichtenegger, J. Drewniok, A. Bornschlegl, C. Lampe, A. Singldinger, N. A. Henke, and A. S. Urban, ACS Nano (2022).

Intrachain photophysics of a donor–acceptor copolymer

H. Nho, W. Park, B. Lee, S. Kim, C. Yang, and O. Kwon, Physical Chemistry Chemical Physics 4 (24), 1982-1992 (2022).

Large π-Conjugated Metal–Organic Frameworks for Infrared-Light-Driven CO2 Reduction

J. Zeng, X. Wang, B. Xie, Q. Li, and X. Zhang, Journal of the American Chemical Society 3 (144), 1218-1231 (2022).

Novel Synthetic Dopamine Analogues: Carbon-13/Nitrogen-15 Isotopic Labeling and Fluorescence Properties

C. Lar, S. Radu, E. Gál, A. Fălămaş, J. Szücs‑Balázs, C. Filip, and A. Petran, Analytical Letters, 1-13 (2022).

Size-dependent spectroscopic insight into the steady-state and time-resolved optical properties of ZnO photocatalysts

A. Falamas, I. Marica, A. Popa, D. Toloman, S. Pruneanu, F. Pogacean, F. Nekvapil, T. D. Silipas, and M. Stefan, Materials Science in Semiconductor Processing 145, 106644 (2022).

Comparison of growth interruption and temperature variation impact on emission efficiency in blue InGaN/GaN MQWs

J. Mickevičius, K. Nomeika, M. Dmukauskas, A. Kadys, S. Nargelas, and R. Aleksiejūnas, Vacuum 183, 109871 (2021).

Double Charge Transfer Dominates in Carrier Localization in Low Bandgap Sites of Heterogeneous Lead Halide Perovskites

A. Fakharuddin, M. Franckevičius, A. Devižis, A. Gelžinis, J. Chmeliov, P. Heremans, and V. Gulbinas, Advanced Functional Materials 15 (31), 2010076 (2021).


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