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The laser has revolutionized many areas of science and society, providing bright and versatile light sources that transform the ways we investigate science and enables trillions of dollars of commerce. In manufacturing, for example, robotic lasers have been programmable in a way that mechanical cutting tools had not been, with the same factory floor laser station capable of cutting, drilling, measuring, and in some cases welding and peening. Lasers are also becoming increasingly important for military owing to applications in communications, remote sensing, directed energy, and the production and diagnosis of materials in extreme environments.
There are mode-locked lasers emitting ultrashort pulses, i.e. light pulses with durations of femtoseconds or picoseconds: mostly below 100 ps, often even well below 100 fs. A more precise but less common term is actually ultrashort pulse lasers; such lasers utilize ultrafast processes and emit light with very fast changes of optical power. Typical pulse repetition rates of ultrafast lasers are of the order of 100 MHz, but it is also possible to have only a few megahertz or many gigahertz.
Attosecond Pulsed lasers
But physicists and engineers are interested in pushing these limits ever further. To understand the movements of electrons, and eventually those of subatomic particles, requires attaining the attosecond and ultimately zeptosecond (sextillionths of a second) range, Kaertner says. Achieving that requires pushing technology to produce pulses using higher-wavelength sources, and also producing pulses that encompass a wider range of frequencies — a more broadband source.
So far, Kaertner says, “the shortest pulse people have measured is 80 attoseconds.” But various groups are working to push the limits even further, he says, using several different methods, including large-scale electron accelerators such as the Stanford Linear Accelerator. The field of attosecond science was first enabled by nonlinear compression of intense laser pulses to a duration below two optical cycles. Twenty years later, creating such short pulses still requires state-of-the-art few-cycle laser amplifiers to most efficiently exploit “instantaneous” optical nonlinearities in noble gases for spectral broadening and parametric frequency conversion.
A further conceptual breakthrough in ultrafast optics was achieved with the discovery of high-harmonic generation (HHG) from noble gas atoms illuminated by ultrashort light pulses with a peak intensity of the order of 1013-1015 W/cm2 . The process of HHG can be intuitively understood in terms of the so-called “three-step model”
In general, attosecond pulses are generated using high harmonic generation, said ~Eric (Institute for the Frontier of Attosecond Science and Technology, UCF).
This process can generally be described by the “three step model”:
1) Ionization: An intense oscillating electric field (i.e. your driving laser) ionizes an electron from an atom (commonly a noble gas) and accelerates it away into free space.
2) Acceleration: When the electric field changes direction, the electron turns around and accelerates back towards the parent ion, gaining a substantial amount of kinetic energy in the process.
3) Recombination: If the electron recombines with the parent ion, the excess amount of kinetic energy is released as a single high-energy photon (usually in the VUV to soft x-ray region, depending on the laser). This burst possesses attosecond time signatures.
This process occurs every half-cycle of your driving laser, meaning a linearly-polarized, multi-cycle pulse will generate an attosecond pulse train, with the spectrum of this output showing discrete, odd-order high harmonics of the incident laser pulse energy (which makes sense if you think about the Fourier transform of a train of closely-separated ultrashort pulses).
Experimentally, it may be more useful to use isolated attosecond pulses. To generate only one attosecond pulse instead of a train of closely-separated pulses, one of several different types of optical gating can be used. In general, this can be done by modifying any of the steps in the “three step model” above:
1) Ionization: Under ionization gating, a single field maximum of the driving laser pulse is responsible for ionizing a substantial portion of the atoms in the interaction region. While these electrons can be accelerated and recombined to generate an attosecond pulse, there are either a) no more electrons for the following driving laser cycles to ionize because the target has been depleted or b) no longer appropriate phase matching conditions because of the plasma created by the pulse. This effectively results in the production of a single attosecond pulse.
2) Acceleration: Under amplitude gating, the strongest field maximum of the driving laser pulse accelerates the photoionized electrons the hardest, leading to the generation of the highest-energy photons out of all the laser cycles. By filtering out the spectral components below the cutoff generated by the next strongest field maximum, it is ensured that the remaining spectrum all originated from the same attosecond burst. This effectively results in the production of a single attosecond pulse.
3) Recombination: Under polarization gating, the ellipticity of the laser field is caused to vary with time such that a single linearly-polarized field maximum is surrounded on both sides by field maxima with increasing ellipticity. Because these elliptically-polarized fields lead their associated electrons on trajectories away from the parents ions, only the electrons accelerated by the single linearly-polarized field maximum will experience recombination and yield high-energy photons. This effectively results in the production of a single attosecond pulse.
Some of these methods can be combined, allowing one to reap the benefits of each kind of gating. As an example, double optical gating and its variants combine polarization gating and two-color gating (an ionization-type gating) to relax the restrictions on the driving laser.
There are also wavefront-based gating methods (attosecond lighthouse and noncollinear optical gating) where, instead of generating only a single attosecond pulse, an attosecond pulse train is produced in which each pulse is angularly-resolvable from the other. Of course, each gating has its advantages and disadvantages (driving laser requirements on pulse duration or carrier-envelope phase stability; achievable spectrum; need for finding the right filters; etc.), but this should already be enough introductory material to help someone not familiar with the field.
Applications of attosecond pulses
Since the first experimental demonstration of the generation of attosecond pulses in 2001, such pulses have been used in time-resolved spectroscopy with extreme temporal resolution. The main target of attosecond science is to get direct access to the electronic dynamics in atoms, molecules, nanostructures and solids, and the possibility to directly control such ultrafast processes. A number of spectacular applications of attosecond pulses has been reported in the last 18 years, ranging from atomic physics to solid-state physics.
The application of attosecond techniques to atomic physics has proven to be crucial for example for the measurement of the delay in photoemission and for the analysis of the process of tunnel ionization. Application of attosecond pulses in molecular physics has been pioneered in 2010, with the investigation of the charge localization process in hydrogen molecules initiated by isolated attosecond pulses . Attosecond pulses have been used more recently to investigate the process of charge migration in aminoacids . Theoretical studies have pointed out that very efficient charge dynamics can be driven by purely electronic effects, which precede any rearrangement of the nuclear skeleton and which can evolve on a temporal scale ranging from few femtoseconds down to tens of attoseconds. This ultrafast charge dynamics, essentially driven by electron correlations, has been referred to as charge migration. The first experimental measurement of charge migration in the α-amino acid phenylalanine (C9H11NO2), after attosecond excitation was reported in 2014.
An α-amino acid consists of a central carbon atom (α carbon), linked to an amine (-NH2) group, a carboxyl acid (-COOH) group, a hydrogen atom and a side chain, which is specific to each amino acid. In particular, we investigated the aromatic amino acid phenylalanine, where the side chain is a methylene (-CH2-) group terminated by a phenyl ring. Phenylalanine plays a key role in the biosynthesis of other amino acids and is important in the structure and function of many proteins and enzymes.
The application of attosecond technology to the investigation of electron dynamics in biologically relevant molecules represents a multidisciplinary work, which can open new research frontiers: those in which few-femtosecond and even sub-femtosecond electron processes determine the fate of bio-molecules.
The extension of attosecond methods to solid-state physics is still limited, but important results have been already achieved. The experimental results reported so far demonstrate that the application of attosecond techniques to solids offers the possibility to investigate important physical processes, not accessible by using longer temporal resolution. The measured delays in photoemission from various solids are of the order of a few tens of attoseconds; the intra-band motion of charges leading to the Franz-Keldysh effect in dielectrics evolves on the attosecond time scale ; the measured upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in
the conduction band of silicon is of the order of a few hundreds of attoseconds. These results clearly demonstrate the advantages offered by the application of attosecond techniques.
Finally, we mention that an alternative way for generating intense ultrashort pulses in the VUV/X-ray energy region is provided by X-ray free-electron lasers (FELs): several techniques have been proposed in the last years to generate high-energy attosecond pulses with these lasers.
UCF Researchers Generate Attosecond Light from Industrial Laser
The ultrafast measurement of the motion of electrons inside atoms, molecules and solids at their natural time scale is known as attosecond science and could have important implications in power generation, chemical- and biological-weapon detection, and medical diagnostics.
University of Central Florida researchers are making the cutting-edge field of attosecond science more accessible to researchers from all disciplines. Their method to help open up the field is detailed in a new study published today in the journal Science Advances in August 2020 An attosecond is one billionth of a billionth of a second, and the ability to make measurements with attosecond precision allows researchers to study the fast motion of electrons inside atoms and molecules at their natural time scale.
Measuring this fast motion can help researchers understand fundamental aspects of how light interacts with matter, which can inform efforts to harvest solar energy for power generation, detect chemical and biological weapons, perform medical diagnostics and more. UCF physics associate professor Michael Chini leads a team that is making attosecond science more accessible.
In the stretched-pulse soliton Kerr resonator developed by the lab of William Renninger, a single frequency laser enters a fiber ring cavity, generating a broad bandwidth comb of frequencies at the output that supports ultrashort femtosecond pulses. Inside the fiber cavity the pulses stretch and compress in time, reaching a minimum duration twice in the cavity near the center of each of the two fiber sections. The stretching and compressing temporal evolution is a salient characteristic of femtosecond stretched-pulse soliton Kerr resonators. (University of Rochester / Illustration by Michael Osadciw/)
“One of the main challenges of attosecond science is that it relies on world-class laser facilities,” says Michael Chini, an associate professor in UCF’s Department of Physics and the study’s principal investigator. “We are fortunate to have one here at UCF, and there are probably another dozen worldwide. But unfortunately, none of them are truly operated as ‘user facilities,’ where scientists from other fields can come in and use them for research.” This lack of access creates a barrier for chemists, biologists, materials scientists and others who could benefit from applying attosecond science techniques to their fields, Chini says.
“Our work is a big step in the direction of making attosecond pulses more broadly accessible,” Chini says. “We show that industrial-grade lasers, which can be purchased commercially from dozens of vendors with a price tag of around $100,000, can now be used to generate attosecond pulses.” Chini says the setup is simple and can work with a wide variety of lasers with different parameters.
Attosecond science works somewhat like sonar or 3D laser mapping, but at a much smaller scale. When an attosecond light pulse passes through a material, the interaction with electrons in the material distorts the pulse. Measuring these distortions allows researchers to construct images of the electrons and make movies of their motion. Typically, scientists have used complex laser systems, requiring large laboratory facilities and clean-room environments, as the driving lasers for attosecond science.
Producing the extremely short light pulses needed for attosecond research – essentially consisting of only a single oscillation cycle of an electromagnetic wave – has further required propagating the laser through tubes filled with noble gases, such as xenon or argon, to further compress the pulses in time. But Chini’s team has developed a way to get such few-cycle pulses out of more commonly available industrial-grade lasers, which previously could produce only much longer pulses.
They compress approximately 100-cycle pulses from the industrial-grade lasers by using molecular gases, such as nitrous oxide, in the tubes instead of noble gases and varying the length of the pulses they send through the gas. In their paper, they demonstrate compression to only 1.6 cycles, and single-cycle pulses are within reach of the technique, the researchers say. UCF physics doctoral student John Beetar is the lead author of a new study that details a method to make attosecond science more accessible to researchers from all scientific disciplines. The choice of gas and duration of the pulses are key, says John Beetar, a doctoral student in UCF’s Department of Physics and the study’s lead author. “If the tube is filled with a molecular gas, and in particular a gas of linear molecules, there can be an enhanced effect due to the tendency of the molecules to align with the laser field,” Beetar says.
“However, this alignment-caused enhancement is only present if the pulses are long enough to both induce the rotational alignment and experience the effect caused by it,” he says. “The choice of gas is important since the rotational alignment time is dependent on the inertia of the molecule, and to maximize the enhancement we want this to coincide with the duration of our laser pulses.” “The reduction in complexity associated with using a commercial, industrial-grade laser could make attosecond science more approachable and could enable interdisciplinary applications by scientists with little to no laser background,” Beetar says.
