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 S2 to S1 state of b-carotene with a low fluorescence quantum yield and an ultrafast fluorescence decay time, and the lifetime of the S2 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.