Nanoscale Imaging

Nonlinear coherent spectroscopy is a powerful probe of the electronic and vibrational structure and dynamics of molecular systems.  However, these methods are ensemble measurements, thereby giving information on average quantities rather than distributions.  Recently, our group has developed a custom ultrafast microscope to map out spatial heterogeneities of materials such as lead halide perovskites and other semiconductor thin films.  This instrument is capable of sub-200 nm spatial resolution, sub-50 fs temporal resolution, and sub-10 cm-1 spectral resolution simultaneously.   The combined spatial, temporal, and spectral resolution capabilities of the instrument are critical in uncovering the basic photo-physics of carrier generation and relaxation in perovskites and other semi-conductor materials.  In addition, this instrument allows single-particle studies, which allow us to peer beneath the in homogeneously broadened line shapes of traditional pump-probe spectroscopies and to recover the distribution of carrier properties such as binding energies, carrier temperatures, and relaxation rates.  Such capabilities are especially important in polycrystalline materials where grain size and shape may play an important role in controlling transport behavior.

Spectrally-resolved transient absorption microscope setup. Near-transform limited pulses are delivered to the focal plane by means of a reflective objective (R.O.) and collected in transmission or reflection mode. The probe is spectrally dispersed on a CCD after passing through a spectrometer. Representative optical and transient absorption images of lead halide perovskite thin film are shown.

 

S. Nah, B. Spokoyny, C. Stoumpos, C. M. M. Soe, M. Kanatzidis, and E. Harel, Spatially segregated free-carrier and exciton populations in individual lead halide perovskite grains, Nat. Photonics., DOI: 10.1038/NPHOTON.2017.36 (2017). PDF