Laser looks into cells cascade_B_x220 Cap:The gold bars on a quantum-cascade laser are each 1.2-µm long. The bars act as antennae, focusing mid-infrared light to a spot size equivalent to the gap between them, 100 nanometers. Assisting on the project are Harvard’s Nanfang Yu, Ertugrul Cubukcu, and Federico Capasso.

Harvard University engineers have built a laser that may let researchers peer into cells with ultrahigh resolution and watch cellular events as they happen. Adding nano antennae to infrared lasers have let researchers focus its light so tightly the devices could lead to imaging with 100 times greater resolution, or below 12 µm.

Microscope resolution needed to examine the chemical composition of tissues has been constrained by light's diffraction limit. For instance, traditional lenses only focus into beams half a wavelength wide. If a microscope uses mid-infrared light with a wavelength of 24 µm, it can only be focused onto a spot 12 µm wide. Considering the size of animal cells (10 µm), bacteria (1 µm), and viruses (tens of nanometers), 12 µm is too large.

A solution to the wavelength limit, quantum-cascade lasers, was developed by researchers at Bell Labs in 1994. These compact lasers can emit light at any wavelength throughout the mid-infrared spectrum. This light, from 3 to 24 micrometers, is useful for identifying different chemicals because it makes molecules resonate at identifiable frequencies. Quantum-cascade lasers are also useful for sensing small amounts of gases at levels as low as one part per billion.

Optical physicists Kenneth Crozier and Federico Capasso created a sharper focus for quantum-cascade lasers by forming two tiny gold bars from a thin layer of gold to serve as rectangular antennae. Each is about one micrometer across. When hit by laser light, an intense electric field forms in the gap between the antennae, which concentrates the light into a beam the same width as the gap, about 100 nanometers. A microscope using such an apparatus would also have a resolution of about 100 nanometers.

“Quantum-cascade lasers are not yet used in high-resolution imaging,” says Claire Gmachl, an electrical engineer at Princeton University who was involved in the development of such lasers at Bell Labs. Gmachl says the technique shows the most promise for biological imaging at the cellular level. Microscopes using the new lasers should be able to detect, for example, changes in individual proteins on the surfaces of cells. Crozier says the spot size of laser light is limited only by the gap between the gold bars. As nanofabrication techniques improve, it should be possible to make even higher-resolution optical microscopes.