Measuring eXtreme UltraViolet (XUV), femtosecond Laser pulses with slow, visible cameras

Shortly after the invention of the laser, researchers developed lasers which emit trains or sequences of extremely short bursts of light, where each laser pulse lasts for femtoseconds (1 quadrillionth of a second), or 10-15 s. Femtosecond laser pulses can serve as “strobe-lights”, freezing the motion of electrons, atoms or molecules, making them powerful tools for understanding the fundamental interactions that make up almost any kind of matter. Squeezing all of the energy of a continuous laser into short pulses also means that femtosecond laser pulses are extremely bright and they give rise to new kinds of light-matter interactions resulting in a host of applications in nonlinear optics (i.e. generating new colors of light), laser machining, plasma physics and optical control of chemical reactions or material properties. 
With the invention of this new kind of light source came a new dilemma. How do you measure the shortest man-made event which is 1000 times faster than the fastest electronic detectors or cameras? How can you use this light source, or even know that you made femtosecond pulses if you cannot measure them? For ultraviolet, visible or infrared femtosecond laser pulses, this challenge was solved in the 1980’s. While electronics are too slow, optical effects are not. Femtosecond laser pulses have the ability to instantaneously modify a material’s optical properties (i.e. absorption or refraction), creating an effectively instantaneous “optical shutters”. By measuring the resulting “nonlinear-optical” signal, it is possible to measure femtosecond pulses with standard, slow cameras.  
In the meantime, there has been a revolution in the generation of femtosecond laser pulses with new Free Electron Lasers (FEL), generating higher frequency ionizing radiation at eXtreme UltraViolet (XUV, wavelengths of 10-100 nm) and x-ray wavelengths (10 - 0.1 nm wavelengths). With these new light sources, again comes the dilemma of how to measure the pulses. Ionizing radiation interacts with matter differently, pulling electrons from atoms, and the concepts used for measuring longer wavelength femtosecond pulses no longer apply. 
In recent work done at the FERMI Free Electron laser at the DiProI end-station and published in Optica, it was shown that ionization itself can be used as a “femtosecond optical shutter” for measuring XUV laser pulses at 31 nm. Ionization changes the optical properties of a material on a nanosecond time scale, 10,000 times slower than the FEL pulse duration. Still, the duration of the rising edge of ionization is on a femtosecond time-scale, and is determined by how long it takes the electron to leave the atom. Using this concept, it was demonstrated that the electric field of an FEL pulse could be encoded in a visible laser pulse, so that a standard, slow visible camera could be used to measure individual FEL pulses. The experimental concept is drawn in Fig. 1.
This work has the potential to lead to a new online diagnostic for FEL’s, where the exact pulse shape of each light pulse can be determined, information that will help both the end-user and the accelerator scientists. This work also paves the way for measuring harder x-ray pulses or images.


Figure 1.  The mini-TIMER split and delay line at the DiProI end-station creates a pair of 31 nm beams. The XUV beams cross and interfere in a piece of glass, which results in a spatially modulated absorption pattern, or a transient diffraction grating, which lasts as long as the XUV absorption or nanoseconds. An optical femtosecond probe laser beam which overlaps in time with the XUV pulses, diffracts off of this grating which encoding the FEL pulse shape in the more-easily-measured 400 nm light. Measuring the spectrum of the diffracted 400 nm pulses reveals a spectrogram (like a musical score) of the FEL pulses which contains their amplitude and phase (i.e. arrival time of the colors in the 31 nm pulses) of individual pulses in the FEL pulse train. 


This research was conducted by the following research team:

William K. Peters,1Travis Jones,1,2Anatoly Efimov,1Emanuele Pedersoli,3Laura Foglia,3Riccardo Mincigrucci,3Ivaylo Nikolov,3Rick Trebino,2Miltcho B. Danailov,3Flavio Capotondi,3Filippo Bencivenga,3and Pamela Bowlan1

Los Alamos National Lab, Los Alamos, New Mexico, USA 
School of Physics, Georgia Institute of Technology, Atlanta, Georgia, USA 
Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy 

Contact persons:

Pamela Bowlan, email:


William K. Peters, Travis Jones, Anatoly Efimov, Emanuele Pedersoli, Laura Foglia, Riccardo Mincigrucci, Ivaylo Nikolov, Rick Trebino, Miltcho B. Danailov, Flavio Capotondi, Filippo Bencivenga, and Pamela Bowlan, All-optical single-shot complete electric field measurement of extreme ultraviolet free electron laser pulses”, Optica 8, 545, (2021),


Last Updated on Friday, 14 May 2021 09:03