Researchers Increase Spatiotemporal Resolution of Imaged Lightwaves

A team of physicists from the Max Planck Institute of Quantum Optics and the Ludwig Maximilian University of Munich, in cooperation with Stanford University researchers, have developed a technique to measure the electrical field of ultrashort laser pulses in time and space. According to the researchers, the measurement technique makes it possible to take “photographs” of lightwaves with a previously unachieved spatial and temporal resolution.

The development could in turn lead to an improved quality in spatially resolved spectroscopy, as well as drive applications in field-resolved microscopy.

The speed at which light oscillates — around one quadrillion oscillation per second — prevented methods to measure the temporal evolution of the light field directly until the start of the century. To move toward the ability to precisely steer and control light, scientists must achieve a high-precision temporal measurement, in combination with high spatial resolution. This is because a measurement of light must take place inside a laser focus, with the volume much less than the focus size.

This poses additional challenges: When light is focused onto a point, the resolution is the order of the focus size. Because light is diffracted, the theoretically achievable resolution is limited to about the size of the wavelength, which is around a few hundred nanometers. It is difficult to achieve this limit in typical applications. Further, the focus size is often a few micrometers.

Focused light alone cannot be used to examine effects on scales that are smaller than the focus size.

The collaborating researchers used a tiny metallic “nanotip,” sized at only a few nanometers, much smaller than the focus of light, and as opposed to conventional electrodes, to develop a method called “nanoTIPTOE.” The field enhancement is at the end of the tip, which the researchers described as similar in principle to a lightning rod. The conductivity of the tip material in turn enabled use of state-of-the-art electronic measurement methods, which made the approach simple to handle and precise.

A nanometric needle tip interacting with a few-cycle femtosecond laser pulse and a near-petahertz vortex field. The femtosecond pulse induces an ultrashort current of electrons that escape from the tip. The vortex field is probed by measuring the change in the electron current it induces. The localized field enhancement at the tip of the needle facilitates the spatial resolution of the helicoid wavefront of the vortex field within the laser focus. Courtesy of RMT.Bergues.

In the setup, the tip probed the field in one point in space. To obtain an overall image of the light field, the tip was scanned across the focus, so that each tip position corresponded to one pixel of the image. The scientists simultaneously measured the temporal evolution of the field in each pixel.

A short current pulse was generated when light hit the nanotip. The pulse flowed through the tip in a few hundred attoseconds. The laser field that the technique aimed to characterize modulated the induced current, which is subsequently measured. With these changes to the current, in the extremely short time interval, the physicists achieved the temporal resolution necessary to observe the light field.

In tests, the researchers measured the field of an optical vortex beam. Though the light frequency of the beam is many orders of magnitude higher than that which conventional electronics can detect, the achieved spatial resolution enabled the scientists to reconstruct the spatial and temporal field distribution of the optical vortex in the focus of the laser beam. They also observed the field amplitudes of the femtosecond vortex pulses rotating around the propagation axis.

“The field enhancement allowed the characterization of laser fields with moderate intensity — a major advance compared to techniques requiring high-power laser sources,” the team said in its paper. “As compared to previous approaches for electronic field detection with nanotips, which were often limited to the low terahertz region, nanoTiptoe increases the temporal resolution by nearly three orders of magnitude.”

The scientists said that their method is polarization sensitive, which makes it applicable to vectorial field reconstruction.

The research was published in Optica (www.doi.org/10.1364/OPTICA.459612).