A multimodal approach for high-spatial-resolution, time-resolved spectroscopy

To date, methods have been developed to investigate the atomic structure or the mesostructure (nanoscale structure) of the materials: electron scanning microscopy, atomic force microscopy, near-field scanning microscopy, X-ray microscopy, fluorescence microscopy. But these methods are not without drawbacks and many of them do not have time resolution capabilities. In this work, we focus our attention on combining optical microscopy with time-resolved techniques, to have both spatial resolution and time resolution, which is useful to follow the dynamics in heterogeneous samples. In time-resolved ultrafast spectroscopy, the reaction is initiated by a femtosecond laser pump. By shifting the time of arrival of a probe pulse relative to the pump pulse, it is possible to monitor the changes in sample properties (e.g. absorption, reflection, fluorescence, etc.) stimulated by the pump, with a time resolution of the order of the duration of the laser pulses. To merge this with a microscope, we will create a unique scheme using ultrafast fiber lasers to implement the time-resolved experiments using the asynchronous optical sampling (ASOPS). In this scheme two ultrafast lasers delivering the pump and probe pulses are locked together at a tunable repetition rate difference. This method obviates the need for a mechanical delay line and allows high-speed scanning over several nanoseconds of time delay without moving parts. Since these microscopy/nanoscopy techniques come with novel requirements, new schemes to adapt the standard pump and probe techniques to the microscopy environment will be implemented, e.g. the scanning strategies to obtain a time-resolved spectum at each pixel of an image of the sample. To validate the pump-probe technique, the transient dynamics of thin films (e.g. graphene layers) will be pursued, with particular attention to the measurements of surface inhomogeneities. It is well known that an optical microscope objective cannot resolve features smaller than 250–300 nm under white lighting illumination. As described in the literature, using dielectric microspheres as additional optical elements it is possible to enhance a microscope resolution and focus light in subwavelenght beams that may act as signal/contrast enhancers for electromagnetic waves scattered from or located near a surface. We will try to use the microsphere-assisted microscopy as signal/contrast enhancers for the probe beam. As a preliminary study, we will use the microspheres in a Raman microscope as enhancers of the Raman signal. Atomic force microscopy techniques will complement the optical microscopy platform to characterize the mechanical/chemical aspects of the investigated physics. This technique will help us to characterize the topography of the samples. It will be important to pay attention to the data interpretation strategies, also given the data storage and management needed in experiments where a great deal of spectroscopic information is potentially available in each pixel of an image.