A transient grating (TG) is used as an optical gating element with sub-picosecond time resolution for luminescence measurements from a photo-detector array. The transient grating is formed in a gate medium by one or more pulsed gate beams. For photoluminescence measurements such as photoluminescence spectroscopy or imaging, a source is excited by a pulsed excitation beam, and the pulsed gate beams are synchronized to the pulsed excitation beam with an adjustable delay between the excitation of the source and the formation of the TG. Moreover, a source or its spectra can be imaged at two different regions of the photo-detector array at two different times spaced in time by a selected duration of time with sub-picosecond resolution over a range of a nanosecond or more. A beam from the source is deflected to the different regions by changing the frequency or geometry of the pulsed gate beams.