Ultrafast Transient Absorption and Photoluminescence
The excitation source for both Transient absorption and photoluminescence experiments is the Coherent Libra amplified Ti:Sapphire laser, which generates 50 fs pulses of 800 nm light at 1 kHz. Using one of two Optical Parametric Amplifiers (OPAs), this light can be converted to any wavelength between 350 nm and 2600 nm, while retaining a pulse-width of less than 100 fs.
Transient absorption experiments are performed using the Helios and Eos units from Ultrafast Systems. In the Helios, a white light continuum probe pulse is generated by focusing part of the 800 nm beam from the Libra in a calcium fluoride or aluminum oxide crystal. The probe can be detected in any of three ranges: Visible (420-800 nm), NIR (800-1600 nm) and SWIR (1600-2400 nm). The delay between the pump and probe pulses is then modulated by a mechanical delay line up to 5 ns. For experiments requiring longer pump-probe delays, the Eos unit generates a white light probe pulse with an external q-switched Nd:YAG laser, with an electronically controlled delay up to 1 ms.
For photoluminescence experiments, emitted light is detected by a streak camera, which allows simultaneous collection of spectral and temporal information in a 2D image with time on one axis and wavelength on the other. This instrument has high sensitivity to PL from 350 nm to 850 nm. There are two sweep units that generate the time resolution: a “slow sweep” unit, which allows detection in time ranges from 1 ns to 1 ms with an ultimate time resolution of 20 ps. For faster time resolution (down to 2 ps), the “synchroscan” unit can be used, which triggers off of the 80 MHz oscillator. Because this unit sweeps every 12.5 ns, samples with long lifetimes will see pre-pulse intensity from repeated sweeping before complete decay.
Funding for this instrument was partially provided by the Clean Energy Institute and the ONR DURIP program.
Case study: CdSe/CdS dot-in-rod nanocrystals.
In these nanocrystals, a small spherical CdSe core (~3 nm) is embedded within a CdS nano-rod (~5 x 25 nm). When excited with a pulse of 365 nm light, most of the photons are absorbed in the shell material due to its larger fraction of the total volume, and therefore higher absorption cross-section. Shortly thereafter, the photo-generated electron-hole pair localizes the hole in the CdSe core due to its higher-energy valence band. This results in strongly red-shifted long-lived luminescence from a quasi-type-II transition at ~575 nm corresponding to the recombination of the delocalized electron and localized hole. Using ultrafast transient absorption, we can probe the kinetics of the initial localization process. This is shown in the figures below: