Time-resolved luminescence spectroscopy by the optical Kerr-gate method applicable to ultrafast relaxation processes

We have developed a femtosecond time-resolved luminescence spectroscopy by the optical Kerr-gate (OKG) method to investigate ultrafast carrier dynamics and relaxation processes of materials. Solid glasses with a high nonlinear refractive index were used as the Kerr media to obtain a subpicosecond time resolution. When a quartz plate was used as the Kerr medium, the Kerr efficiency and the instrumental response time of our spectroscopic system were 5–10 % and ∼250 fs, respectively. By employing the OKG method, we revealed the internal conversion from $S_2$ to $S_1$ state of β-carotene with a low fluorescence quantum yield and an ultrafast fluorescence decay time, and the lifetime of the $S_2$ state was determined to be 210 fs. An advantage of the OKG method relative to the conventional up-conversion technique is its ability to directly obtain time-resolved luminescence spectra, and thus the OKG method might be superior to the up-conversion technique to investigate ultrafast carrier dynamics and relaxation processes of materials.