Copyright OSA 2006

In recent years conventional transmission microscopy has been left behind in the wake of a dazzling range of sophisticated new imaging techniques — scanning near-field optical microscopy, tip-enhanced Raman microscopy and fluorescence life-time imaging, to name a few. Many of these techniques, although ingenious, are tricky to implement and they are often limited by the sample’s characteristics — such as its fluorescence or whether its Raman tensor aligns with the direction in which the electric field is enhanced. However, they have a common lure: they are all capable of imaging with a resolution below the diffraction limit. Now, with the aid of a clever modification, Vainrub et al. have halved the resolution previously obtainable with a transmission microscope and shown that it, too, is capable of subdiffraction-limit imaging.

The authors replaced the circular condenser used in a commercial optical microscope with a homebuilt illumination system; the key component of which is a cardioid annular condenser. Whereas a circular condenser fills all of the back of a microscope objective, an annular condenser illuminates into a hollow cone, which is known to narrow the point-spread function of the incident light. The cardioid annular condenser has the extra benefit of being free from both chromatic and spherical aberrations. These combined attributes provide coherent illumination, so that 90 nm resolution could be achieved. With the power to resolve detail that is less than a fifth of the illumination wavelength, the traditional transmission optical microscope is no longer limited by the diffraction limit.