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We have developed a femtosecond time-resolved luminescence spectroscopy by the optical Kerr-gate ͑OKG͒ 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 ϳ250 fs, respectively. By employing the OKG method, we revealed the internal conversion from S 2 to S 1 state of ␤-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.

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Time-resolved luminescence was measured on a colorless anatase single crystal under pulsed-laser excitation. The time evolution of luminescence is composed of fast and slow components with time constants of 10 À6 and 10 À5 s, respectively. The fast component corresponds to a direct formation of STE. Some traps near the conduction band give a retardation effect on the slow component. The traps are occupied by conduction electrons at low temperatures and the trapped electron can be excited thermally at temperatures higher than 100 K. They compete with non-radiative recombination process. A possible model for the relaxation process is proposed. r