Observing ultrafast processes requires techniques that are capable of simultaneously resolving spatial features at the nanometer scale and femtosecond time resolution. A time-resolved fluorescence microscopic method is developed by the authors of this Optics Letters
article that uses a Kerr-gated imaging system to observe the ultraviolet fluorescence at femtosecond time resolution. Their system allows the observation of the spatio-temporal dynamics of samples emitting in the ultraviolet region of the spectrum achieving sub 90 femtosecond resolution of time-gated signals. A nonlinear optical crystal is used as a Kerr medium in a wide field microscope to control the birefringence induced in a sample by a femtosecond laser pulse. The temporal characteristics of the fluorescence emission are controlled by another time-gating pulse. Delaying the gating pulse with respect to the excitation pulse, the temporal properties of a sample can be ascertained from its image sequences recorded at femtosecond resolution and fluorescence decay rates calculated. For example, the transient properties of zinc oxide nanowires were characterized from the traces obtained at different spatial locations along the nanowires, helping in the observation of ultrafast processes. This data can also be used to estimate carrier relaxation times and charge carrier mobilities in single nanowires. According to the authors, this technique may provide a “powerful tool capable of contactless ultrafast measurements of charge carrier dynamics in single nanoparticles.
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