Ultrafast lasers and Attosecond Science

“LASER”, the immediate picture striking our mind after hearing this word is most likely a Sci-Fi movie weapon. In reality lasers serve a lot more than just to entertain us. In today’s world we encounter lasers either directly or indirectly so often that we do not even realize how much we are surrounded by lasers. They are a major tool used by the modern world for numerous industrial processes and helped to unravel fundamental concepts of science.

What is a LASER?

The word LASER abbreviates to Light Amplification by Stimulated Emission of Radiation. To simplify a LASER can be explained as a pair of mirrors (one highly reflective and one partially reflective) with a medium (also known as Active medium) to generate light which will bounce back and forth between the mirrors forming a laser cavity. Upon multiple round trips the light inside the cavity interacts multiple times with active medium intensifying the generated light. After several round trips a portion of light is transmitted through the partially reflective mirror as output. There are several kinds of lasers but pulsed lasers, in particular with ultrashort pulses (so called ultrafast lasers) have been of key interest for many scientific as well as industrial applications. An ultrafast laser pulse can be understood as a flash of light as short as a few femtoseconds (a millionth of a billionth of a second) and can be imagined as a very thin pancake of light much thinner than the width of a human hair. Using these kind of ultrafast lasers, scientists can break down the events happening at atomic or molecular level to map out their behavior at their natural time scales.

A Brief History

It was only one year after the invention of Lasers in 1961, that the first controlled pulse operation was demonstrated to achieve exceptional intensity. An estimated pulse duration of about 120 nanoseconds (a billionth of a second, abbreviated ns) with a peak power of few hundreds of kW intensity was demonstrated and named “Giant Optical Pulsation” [1]. The technique used to generate these pulses, known as Q-switching where Q represents the quality factor typically used to denote the energy storage inside the laser cavity.[2]. Q-switching can be achieved by simply putting a mechanical shutter which modulates the light storage inside the laser cavity. Q-Switching opened the door to pulse lasers, however, they are not considered as ultrafast lasers mainly due to the relatively longer durations (nanosecond timescales) and are mainly limited to mechanical shutter speeds.

The generation of even shorter pulses is possible by introducing a fast modulation process of the scale of picosecond or even femtosecond. This can be achieved by intrinsic property of the laser cavity and can be operated without any active light modulations. Light inside the laser cavity propagates differently creating so-called cavity modes of the laser which bounce back and forth between the cavity mirrors. The oscillating modes can be phase-locked at a single instant of time, leading to constructive interference of modes resulting in a very energetic burst of light that comes out through the partially reflected mirror. The more oscillating modes present inside the cavity the shorter the pulses. In 1964, the first mode-locking principle was demonstrated by W.E LAMB and L.E HARGROVE et al [3]. By implementing this technique nanosecond pulses were produced.

The next breakthrough was the development of femtosecond LASERs. The femtosecond light sources soon became the trademark of the LASER world and opened a whole new era of ultrafast science because of their high power and ultrashort time durations. This kind of lasers not only helped to understand the basic science of things but also took the industrial processes to new heights.

Snapshot of an ultrafast laser source. Photo by M.M. on Unsplash.

Going into the Attosecond World!

The next big leap for the ultrafast light sources was to overcome the time constraints of femtoseconds to attoseconds (billionth of a billionth of a second). But typical laser sources within the visible range or near infrared are limited to few femtoseconds because of optical cycle (1 optical cycle = Optical wavelength/Speed of light in vacuum). Going into attosecond time scales require secondary light sources by using nonlinear phenomenon of frequency conversion into higher frequency or shorter wavelength lasers. These lasers typically operate in deep Ultraviolet to extreme ultraviolet (XUV) range.

Attosecond science has opened new frontiers in the field of atomic/molecular sciences for understanding the behavior of atoms/molecules at their natural time scales. The Shortest isolated pulses ever generated uses the technique called high harmonics generation (HHG). Typically the duration of the laser pulse is limited by the optical cycle of the laser wavelength. To access durations shorter than fs time scales typically shorter wavelengths are required. Shorter wavelengths can be accessed by generating the harmonics of the fundamental laser. This scheme of high harmonic generation opened new frontiers to sub-fs and also attosecond regime [4]. Today, Pulses even shorter than 100 as have already been demonstrated [5]. Three conditions must be fulfilled to generate isolated attosecond pulses by this scheme [6]:

  • The peak power of the input pulse must be of the order 1013 to 1015 W/cm2 to initiate the nonlinear process.
  • Pulse duration should be in few-cycle regime to generate single attosecond pulse every shot.
  • The waveform of the input pulse must be controlled to control the temporal and spectral shape of the attosecond pulse.

By fullling these conditions one can generate not only a train of attosecond pulses but also an isolated attosecond pulse by making use of one of the gating techniques [7,8].

To sum up, ultrafast lasers and attosecond science has helped us understanding the fundamental concepts of life. However, this was not only relevant for Academia but also has been well-developed for industry applications. Most importantly, the field of ultrafast science with many different kinds of lasers from table-top light sources to big and bulky Free Electron Lasers (FEL) help us see and potentially control the atomic/molecular events happening at their natural time scales. These kinds of lasers are the reason that we can see in real time how the sub-atomic particles behave. However, there is still a long way to go to fully understand life around us and who knows in the future we will venture into the zeptosecond (trillionth of a billionth of a second) science.

References:

[1] FJ McClung and RW Hellwarth. “Giant optical pulsations from ruby”. In: Journal of Applied Physics 33.3 (1962), pp. 828-829.

[2] Robert W. Boyd. “The Electrooptic and Photorefractive Effects”. In: Nonlinear Optics. Elsevier, 2008, pp. 511-541. DOI: 10.1016/b978-0-12-369470-6.00011-3.

[3] L. E. Hargrove, R. L. Fork, and M. A. Pollack. “LOCKING OF He-NeLASER MODES INDUCED BY SYNCHRONOUS INTRACAVITY MODULATION”. In: Applied Physics Letters 5.1 (July 1964), pp. 4-5. DOI: 10.1063/1.1754025.

[4] P.B. Corkum. “Plasma perspective on strong field multiphoton ionization”. In: Physical Review Letters 71.13 (1993), p. 1994.

[5] Kun Zhao et al. “Tailoring a 67 attosecond pulse through advantageous phase-mismatch”. In: Optics Letters 37.18 (Sept. 2012), p. 3891. DOI: 10.1364/ol.37.003891.

[6] Ferenc Krausz and Misha Ivanov. “Attosecond physics”. In: Reviews of Modern Physics 81.1 (Feb. 2009), pp. 163-234. DOI:10.1103/revmodphys.81.163.

[7] Marcus Seidel. A new generation of high-power, waveform controlled, few-cycle light sources. Springer, 2019.

[8] Michael Chini, Kun Zhao, and Zenghu Chang. “The generation, characterization and applications of broadband isolated attosecond pulses”. In: Nature Photonics 8.3 (Feb. 2014), pp. 178-186. DOI: 10.1038/nphoton.2013.362.

Ammar Bin Wahid

PhD Candidate at Attosecond science Group DESY. Attosecond Charge dynamics in complex molecules

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