Scientists from the Max Planck Institute of Quantum Optics, Ludwig Maximilian University of Munich, and Stanford University have successfully recorded a helical light field on the shortest time and length scales using their newly devised “nanoTIPTOE” method.
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 wave front of the vortex field within the laser focus. Image Credit: RMT.Bergues
Since the end of the 19th century, scientists have understood that light is an electromagnetic wave whose frequency affects its hue. Light oscillates at a rate of around one quadrillion times per second, making it impossible to directly quantify the temporal development of its field until the first decade of the 21st century.
Since then, several mysteries of light have been uncovered.
A new method, the “nanoTIPTOE” technique, has been developed by physicists from the Ultrafast Electronics and Nanophotonics group. Headed by Dr. Boris Bergues and Prof. Matthias Kling from the attoworld team at the Ludwig-Maximilians-Universität (LMU) and the Max Planck Institute for Quantum Optics (MPQ), the electrical field of ultrashort laser pulses could be measured in both time and space.
As a result, it is now feasible to capture light waves in “photographs” with unprecedented levels of spatial and temporal detail.
The speed of light oscillations is astounding. The current electronic technology’s frequency range is far inferior to light. To compare, modern computers operate at speeds of 1 to 10 GHz, which is a million times slower than the speed of light.
Computers would run much more efficiently if it was possible to control them with light. Learning the specific controls for steering and controlling light is a starting step in this approach.
As measurements must take place inside a laser focus, in a volume considerably smaller than the focus size, high spatial resolution and accurate temporal measurements must be established.
Since the resolution is proportional to focus size when light is concentrated on a point (like sunlight with a magnifying glass), this presents new complications for physics. As light is diffracted, the theoretically possible resolution is restricted to a few hundred nanometers, or around the size of the wavelength.
However, in typical applications, it is challenging to reach this limit, and the focus size is frequently in around a few micrometers. As a result, concentrated light alone cannot be utilized to study effects on scales smaller than the focus size.
Now, this issue has been ingeniously resolved by physicists from the ultrafast electronics and nanophotonics section of the team, led by Dr. Boris Bergues and Prof. Matthias Kling. They employed a tiny metallic nanotip for their measurements, far smaller than the light’s focus.
This allows the measurement to be limited to the minuscule end of the tip due to the field augmentation at its edge, which functions similarly to a lightning rod. Modern electronic measuring techniques can be used due to the conductivity of the tip material, which makes the procedure simple to use while still being accurate.
The tip itself measures just a few nanometers and is used to measure the field at a specific location in space. The tip is moved across the focus to get a broad picture of the light field. Thus, each tip location represents a single image pixel. The temporal development of the field in each pixel can also be measured simultaneously by the physicists.
The needle tip produces a brief current pulse when light strikes it. In a few hundred attoseconds, the latter passes through the tip (an attosecond is one billionth of a billionth of a second).
The induced current that is then measured is modulated by the laser field that has to be characterized. The physicists can see the light field because of these current changes occurring in such a little amount of time.
The researchers’ method, which they named “nanoTIPTOE,” pioneered a novel method for measuring lightwaves.
Researchers Johannes Blöchl and Dr. Johannes Schötz from the Ultrafast Electronics and Nanophotonics department examined the field of an optical vortex beam, a specifically structured type of laser field that resembles a spiral of light, as a first application.
The beam’s light frequency is far greater than what can be detected by ordinary electronics.
The spatial resolution that was attained allowed researchers to reconstruct the spatial and temporal field distribution of the optical vortex in the laser beam’s focus. They could also measure the field amplitudes of femtosecond vortex pulses revolving around the propagation axis (a femtosecond is one-millionth of a billionth of a second).
With our new methodology based on current measurements, we can achieve a new quality in spatially resolved spectroscopy and thus also drive applications in field-resolved scanning microscopy.
Johannes Blöchl, Ultrafast Electronics and Nanophotonics, Attoworld, Max Planck Institute of Quantum Optics
Journal Reference:
Blöchl, J., et al. (2022) Spatiotemporal sampling of near-petahertz vortex fields. Optica. doi:10.1364/OPTICA.459612.