Single-crystalline α-Fe2O3 hollow nanocrystals from nanotube to nanoring have been successfully synthesized by a facile hydrothermal method. Size and shape control of the hollow nanocrystals is achieved by simple adjustments of reactants’ concentration and molar ratio without any surfactant assistance. The steady absorption spectra indicate a size-dependent blue shift of these hollow nanocrystals. The femtosecond optically heterodyne optical Kerr effect measurements show that the sample that has the smallest diameter possesses the largest third-order nonlinear optical susceptibility. This is ascribed to the remarkably enhanced local electric field in the small particle in accord with finite-difference time-domain simulations. These results reveal that both size and shape of these hollow nanocrystals have significant contributions to their optical response.

Urban subsurface monitoring requires high temporal‐spatial resolution, low maintenance cost, and minimal intrusion to nearby life. Distributed acoustic sensing (DAS), in contrast to conventional station‐based sensing technology, has the potential to provide a passive seismic solution to urban monitoring requirements. Based on data recorded by the Stanford Fiber Optic Seismic Observatory, we demonstrate that near‐surface velocity changes induced by the excavation of a basement construction can be monitored using existing fiber optic infrastructure in a noisy urban environment. To achieve satisfactory results, careful signal processing comprising of noise removal and source signature normalization are applied to raw DAS recordings. Repeated blast signals from quarry sites provide free, unidirectional, and near‐impulsive sources for periodic urban seismic monitoring, which are essential for increasing the temporal resolution of passive seismic methods. Our study suggests that DAS will likely play an important role in urban subsurface monitoring.

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