Within the last decade, powerful scanning microscopy techniques which can achieve atomic resolution have been developed. These techniques include several types of Scanning Probe Microscopy (SPM) such as Scanning Tunneling Microscopy (STM), Scanning Force Microscopy (SFM) and Near-field Scanning Optical Microscopy (NSOM). In these SPM techniques, an atomically sharp probe (or a very small aperture in the case of NSOM) of a scanning probe microscope is scanned very close to or in contact with the surface of a sample. Typically, the probe is disposed, for example, a few angstroms from the surface of the sample. Due to the close proximity of the probe to the sample, different interactions or coupling mechanisms can occur between the probe and the sample. These interactions or coupling mechanisms include the generation of tunneling current between the sample and the probe, forces acting on the probe and the sample, and evanescent (or propagating) light wave coupling. The strength of these interactions is a very nonlinear function of the distance of the probe from the surface of the sample. Thus, this nonlinearity provides the means for atomic-scale control of the distance of the probe from the sample. Additionally, in SPM the nonlinearity allows for high spatial resolution of the sample limited by the sharpness of the probe of the scanning probe microscope.
As an example of such SPM techniques, in STM an image of the surface of the sample is achieved by collecting the tunneling current between the probe and the sample while scanning the probe over the surface of the sample. The tunneling current reflects the local density of states of the electrons at the Fermi level close to the surface of the sample. This information is then used to "map-out" the surface of the sample. In the case of SFM, an image of the surface of the sample is obtained by measuring the minute deflection of a cantilever on which the probe is mounted. That is, as the probe is scanned over the surface of the sample, forces such as electrostatic and magnetic forces, inter-atomic forces, and Van der Waals forces in turn exert forces onto the probe and the cantilever. The forces exerted on the probe cause minute deflection of the cantilever. These deflections are measured and used to map the surface of the sample. An NSOM image of the sample, on the other hand, is obtained by evanescent wave coupling to a sub-wavelength sized aperture of the scanning probe microscope.
Although these techniques have revolutionized the field of surface science, by providing spatial resolution on the atomic level, (or below the optical wavelength for NSOM) the time or temporal resolution of SPM techniques is limited by the scanning rate or speed of the scanning probe microscope and by the data acquisition electronics used in the scanning probe microscope. As a result, SPM techniques are generally limited to temporal resolution on the order of milliseconds for point measurements, and on the order of seconds for imaging.
Ultrafast time-resolved laser microscopy techniques have also been developed which can provide information about the surface of a sample with temporal resolution limited by the duration, or pulse width, of a short laser pulse. However, although such Ultrafast laser microscopy systems can provide temporal resolution on the order of a few femtoseconds, the spatial resolution of such systems is limited by the diffraction limit of the laser light. As a result, Ultrafast laser microscopy systems are limited to spatial resolution on the order of about a few microns for visible light. Furthermore, these techniques are not directly surface sensitive. The depth resolution of these techniques is limited to the smaller of the diffraction limit or the absorption length for light in the material under investigation.
However, in order to investigate the phenomena that govern the physics of certain mesoscopic and atomic systems and for characterizing the operation of submicron electronic and optoelectronic devices, it is necessary to simultaneously have high spatial resolution and high temporal resolution. For example, in processes such as carrier transport in mesoscopic structures, electric field and voltage wavefront propagation at metal semiconductor interfaces, and light emission in quantum confined structures, variations of interest occur over length scales much smaller than a few microns. Furthermore, due to high propagation velocities, excitations in materials to be observed may occur on a time scale well into the sub-picosecond domain. For example, electronic velocity in semiconductors and metals is on the order of approximately 1-10 angstroms per femtosecond, while voltage wave fronts propagate on high speed transmission lines at velocities on the order of approximately 1000 angstroms per femtosecond. Therefore, the spatial resolution of Ultrafast laser microscopy techniques is inadequate for studying such mesoscopic phenomena, and the temporal resolution of SPM techniques are inadequate for observing such high speed phenomena.
Consequently, as the technological demands for microscopy techniques increase, a need exists for a new microscopy technique to produce high spatial resolution and high temporal resolution simultaneously.