Opt. Lett. 41, 1498–1501 (2016)

Transient absorption spectroscopy is widely used to study short-lived substances. The usual pump–probe and continuous-wave approaches for studying the absorption response of a sample, however, suffer from limited temporal regimes of operation and resolution. Tatsuo Nakagawa and collaborators from Unisoku and Nihon University in Japan have now devised a method based on a randomly interleaved pulse train (RIPT), thus combining continuous-wave and pulsed strategies. The scheme requires two asynchronous radiation sources, in this case a picosecond laser (producing the pump beam) with repetition frequency Rrep = 1 kHz and a supercontinuum light source (generating probe pulses) with Rrep = 20 MHz. Pump–probe delays are evaluated passively through repeated pumping cycles. The researchers determine pump–probe delays with better than 10-ps accuracy, and demonstrate measurement time windows ranging from less than a nanosecond up to the microsecond scale. The experimental transient absorption spectra for C60 shows that the RIPT method gives access to previously unobserved excited states for this substance; the researchers also acquire spectra over visible to near-infrared wavelengths. Furthermore, this technique makes it possible to correct transient absorption spectra for fluorescence contamination.