Light is beautiful. It can probe matter in a multitude of ways, but there are limitations when we try to use it to investigate the revolution in nanotechnology that is currently taking place. The first problem with light in the nanometre domain is diffraction. Light cannot be focused to a point smaller than half its wavelength – this is the famous Rayleigh criterion of optical resolution. The second problem is out-of-focus light. In essence, light that passes through a lens illuminates the regions before and after the focal point, as well as the focal spot itself.

An approach known as spectral confinement may be capable of restricting light to within a few nanometres along certain directions. Spectral confinement occurs when we consider how the electrons and atoms in a molecular or a solid-state system interact with the large electric field that is produced by a laser.

The combination of near-field optics (with its spatial restrictions on light) and nonlinear optics (with its spectral confinement of light), is a marriage made in heaven. And in the quest to probe optical properties at the resolutions associated with the nanoworld, the combined technique is the star on the horizon.

Now Jeffrey Guest and co-workers at the University of Michigan and the Naval Research Laboratory in Washington have taken a step on this road by employing both near-field optics and nonlinear optics to probe the nanoworld of a quantum-dot system (J Guest et al. 2001 Science 293 2224).

In the December issue of Physics World, Aaron Lewis of The Hebrew University of Jerusalem, Israel, shows how the new approach opens a window on the analysis of nanoscale semiconductor systems.