Diffuse near-infrared imaging of tissue with picosecond time resolution.

Optical imaging of biological tissue in vivo at multiple wavelengths in the near-infrared (NIR) spectral range can be achieved with picosecond time resolution at high sensitivity by time-correlated single photon counting. Measuring and analyzing the distribution of times of flight of photons randomly propagated through the tissue has been applied for diffuse optical imaging and spectroscopy, e.g. of human breast tissue and of the brain. In this article, we review the main features and the potential of NIR multispectral imaging with picosecond time resolution and illustrate them by exemplar applications in these fields. In particular, we discuss the experimental methods developed at the Physikalisch-Technische Bundesanstalt (PTB) to record optical mammograms and to quantify the absorption and scattering properties from which hemoglobin concentration and oxygen saturation of healthy and diseased breast tissue have been derived by combining picosecond time-domain and spectral information. Furthermore, optical images of functional brain activation were obtained by a non-contact scanning device exploiting the null source-detector separation approach which takes advantage of the picosecond time resolution as well. The recorded time traces of changes in the oxy- and deoxyhemoglobin concentrations during a motor stimulation investigation show a localized response from the brain.

Spatial Localization of Excitons and Charge Carriers in Hybrid Perovskite Thin Films.

The fundamental photophysics underlying the remarkably high-power conversion efficiency of organic-inorganic hybrid perovskite-based solar cells has been increasingly studied using complementary spectroscopic techniques. However, the spatially heterogeneous polycrystalline morphology of the photoactive layers owing to the presence of distinct crystalline grains has been generally neglected in optical measurements; therefore, the reported results are typically averaged over hundreds or even thousands of such grains. Here we apply femtosecond transient absorption microscopy to spatially and temporally probe ultrafast electronic excited-state dynamics in pristine methylammonium lead tri-iodide (CH3NH3PbI3) thin films and composite structures. We found that the electronic excited-state relaxation kinetics are extremely sensitive to the sample location probed, which was manifested by position-dependent decay time scales and transient signals. Analysis of transient absorption kinetics acquired at distinct spatial positions enabled us to identify contributions of excitons and free charge carriers.