Taking a closer look at light

17 Jun 2002

European scientists have developed techniques to measure optical fields more accurately than ever before.

Courtesy of PhysicsWeb

Wolfgang Lange and colleagues at the Max Planck Institute for Quantum Optics in Germany have built a single-ion probe that can measure a standing light wave with a resolution of better than a wavelength. Meanwhile Niek van Hulst and co-workers at the University of Twente in the Netherlands have followed changes in the shape of an ultrashort laser pulse as it travels through a waveguide.

One of the problems that arises when measuring an optical field is that the measuring device can disturb the field being studied. The Munich team overcome this problem by using a single calcium ion in a radio-frequency trap to measure the intensity of a standing light wave inside a cavity (G Guthöhrlein et al 2001 Nature 414 49).

The standing wave causes the ion to fluoresce at a certain wavelength, and the intensity of this fluorescence is proportional to the strength of the optical field in the cavity.

By detecting the fluorescence from the ion when it is at different positions inside the cavity, it is possible to map out the intensity of the optical field in three dimensions. Lange's team achieved a resolution of about 60 nanometres in measurements of the standing wave produced by radiation with a wavelength of 397 nanometres.

'This approach takes all the probabilistic elements out of the atom-field interaction,' Lange told PhysicsWeb. The team plans to use the technique in fundamental tests of quantum theory where it is important to have maximum control over the position of single ions.

Meanwhile, van Hulst and co-workers have used measurements of the light fields on surfaces to track the progress of laser pulses in a silicon-based waveguide (M Balistreri et al 2001 Science 294 1080).

Laser pulses that last just femtoseconds - or 10^-15 seconds - are used in a wide range of optoelectronic and optical fibre systems. But these pulses get distorted as they travel through devices.

Existing methods compare the final shape of the pulse as it leaves a device with its original shape. But such 'black box' methods cannot pinpoint when or where the shape changes.

Now van Hulst and colleagues have found that surface light waves produced by the laser pulse as it travels through the waveguide have the same group and phase velocities as the pulse itself.

Using a fibre-optic probe to measure intensity variations in these surface fields, the Twente team was able to monitor changes in the shape of a laser pulse as it travelled through the waveguide. Van Hulst and colleagues believe that their method will allow physicists to see how nonlinear effects - which can be both helpful and destructive - emerge in different systems.