Pump–Probe Microscopy: Spatially Resolved Carrier Dynamics in ZnO Rods and the Influence of Optical Cavity Resonator Modes

Abstract

Femtosecond pump-probe microscopy is used to investigate the charge recombination dynamics at different points within a single needle-shaped ZnO rod. Recombination in the tips of the rod occurs through an excitonic or electron-hole plasma state, taking place on a picosecond time scale. Photoexcitation in the larger diameter sections of the interior exhibit dramatically slower recombination that occurs primarily through defects sites, i.e., trap mediated recombination. Transient absorption imaging shows that the spatial variation in the dynamics is also influenced by the cavity resonances supported within the hexagonal cross section of the rod. Finite element simulations suggest that these optical resonator modes produce qualitatively different intensity patterns in the two different locations. Near the end of the rod, the intensity pattern has significant standing-wave character, which leads to the creation of photoexcited carriers in the core of the structure. The larger diameter regions, on the other hand, exhibit intensity distributions in which the whispering gallery (WG) mode character dominates. At these locations, the photoexcited carriers are produced in subsurface depletion zone, where the internal fields separate the electrons and holes and lead to a greater degree of trap recombination on longer time scales.

Super-diffusion of excited carriers in semiconductors

“…Yet, the ultrafast spatial dynamics of excited carriers is still challenging to characterize. For example, pump-probe microscopy with visible light is inherently limited by diffraction, and it remains a non-routine approach in spite of recent progress3. Studies of the spatiotemporal evolution of excited carriers have recently become possible using scanning ultrafast electron microscopy (SUEM)4567, a technique that combines the spatial resolution of electron microscopy and the time resolution of ultrafast lasers.…”

Imaging nano-objects by linear and nonlinear optical absorption microscopies

“…The fast 7 ps decay in this trace is attributed to internal relaxation, and the slower nanosecond decay that appears as an offset in these experiments is assigned to energy transfer from the particle to its surroundings (in this case the glass substrate). TAM experiments can also be used to study charge carrier relaxation in semiconductor nanostructures, [21,45,46,55,83,97,109,110,134,137,145,146,164] and conducting polymers. [52,[171][172][173] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 between different nanostructures, and also about spatial variations within a single nanostructure.…”

“…The past decade has seen tremendous activity in the development of optical 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 dynamics of the system being studied, such as the time scales for charge carrier trapping and diffusion, [21,45,46,49,55,62,72,97,109,110,145,171] and how the materials dissipate energy into the environment. [20,101,133,152,153,179] However, these experiments are difficult to implement and require complex ultrafast laser systems.…”

Correlative imaging across microscopy platforms using the fast and accurate relocation of microscopic experimental regions (FARMER) method

“…With this method, multiplatform imaging of nanostructures can be readily performed, allowing one to obtain detailed structural information (e.g., via electron microscopy) to complement optical information (including spectroscopic data). [19][20][21] Monitoring microfluidic reactions 22 and crystallization 23 would also benefit from this method.…”