"If you shine a laser on a nanoparticle then almost all of the energy absorbed will be released as heat to the surroundings," researcher, Laurent Cognet told Optics.org. "This so-called photothermal effect modifies the refractive index of the adjacent media, and can be used to pinpoint the nanoparticle."

The scientists originally came up with their particle-based imaging method simply to overcome the limitations of fluorescence microscopy, which include short observation times as a result of photobleaching.

"This gold nanoparticle technique is very important, because it means that we can now study non-fluorescent nano-objects," explained group leader, Brahim Lounis. "The resolution is limited by the optical set-up, but we have the sensitivity to detect particles that are as small as 2.5 nm."

Based on a closed-loop scanning platform, the Bordeaux technique involves the use of two lasers. The first, a time-modulated (100 kHz - 15 MHz) Nd:Yag laser (532 nm) is used to heat the nanoparticle. The second, a HeNe laser (633 nm) aimed at the sample and coupled to a fast photodiode detector, tracks the position of the nano-object by locking on to the characteristic beat frequency in the reflected signal.