The resolution of images made with a standard optical microscope is usually limited to about a wavelength of light. But what if the light source is smaller than that? Researchers have now demonstrated a technique that could produce images from the light emitted by microscopic particles acting as tiny lasers. By embedding a large number of these emitters in a sample such as a biological cell, the researchers say it should be possible to make an image with a resolution comparable to the size of the particles, which are smaller than a wavelength. The method could be particularly useful for deep imaging of biological tissues.
The phenomenon of diffraction means that light coming from two sources can’t generally be distinguished if they are closer than about half the light’s wavelength. Obtaining optical resolution smaller than the wavelength of the light is thus said to beat the diffraction limit. For visible light, this means a few hundred nanometers.
Various imaging techniques for beating the diffraction limit already exist. One of them, laser confocal microscopy, manipulates the light waves to illuminate, say, a sample of biological tissue, one subwavelength spot at a time, where it induces fluorescent emission from a dye that was previously added to the tissue. Scanning the cell in this way yields an image with subwavelength resolution, but unwanted emission from cell components other than the dye reduces the sensitivity of the method and makes it ineffective for thick samples.
The new approach reported by Seok Hyun Yun and co-workers at the Massachusetts General Hospital and Harvard Medical School in Cambridge, Massachusetts, overcomes these limitations. The researchers made tiny lasers in the form of short rods (nanowires) of a crystalline compound called lead iodide perovskite, just 3–7 micrometers