When is a laser a real laser?

Pulsed lasers are intense and coherent light sources, and the latest category is that of Free Electron Lasers, such as FERMI. First order coherence is a familiar phenomenon, and is manifested for example in diffraction phenomena. This represents the correlation between the amplitudesof a wave at different points in space (transverse coherence) or time (longitudinal coherence.) However, a high degree of first order coherence is not enough to define a laser, according to the Nobel laureate Roy Glauber, who stated that a laser can be defined as a source that is coherent in all orders. The higher order correlations are between intensityat different points in time and space. How are these correlations measured? For this one has to look at the statistics of the photons.
Glauber’s work was inspired by the famous Hanbury Brown and Twiss experiment, in which coincidences of photons (i.e. correlations) were measured of photons coming from distant stars. By varying the distance between two detectors, they were able to determine the degree of coherence of the star, and extract other information. This is the key to measuring the second order coherence of a light source: the intensity of light at different points is measured in coincidence, and statistical analysis is made. This experiment is considered by many as initiating the whole field of quantum optics. Now a team led by Ivan Vartaniants (DESY, Hamburg, and the National Research Nuclear University, Moscow) has performed a Hanbury Brown and Twiss experiment at FERMI. Instead of the two discrete photodetectors used originally, a CCD detector was used. Since all of the photons arrive in less than 100 fs, there is no need to use coincidence methods: the signal is naturally synchronised.

>Read more on the FERMI at Elettra Sincrotrone Trieste website

Figure 1.  Difference between chaotic and coherent light sources. (a) photon correlation map for FERMI operated in seeded mode. (b) corresponding spectrum. (c) correlation map for FERMI operated in Self Amplified Stimulated Emission mode (the mode of operation of most Free Electron Lasers). (d) corresponding spectrum.
Credit: Reprinted from O. Yu. Gorobtsov et al, Nature Communications 9 (2018) 4498. (Copyright Nature Publishing Group)

Writing and deleting magnets with lasers

Scientists * have found a way to write and delete magnets in an alloy using a laser beam – a surprising effect.

* at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia in Charlottesville, USA

The reversibility of the process opens up new possibilities in the fields of material processing, optical technology, and data storage.
Researchers of the HZDR, an independent German research laboratory, studied an alloy of iron and aluminum. It is interesting as a prototype material because subtle changes to its atomic arrangement can completely transform its magnetic behavior. “The alloy possesses a highly ordered structure, with layers of iron atoms that are separated by aluminum atomic layers. When a laser beam destroys this order, the iron atoms are brought closer together and begin to behave like magnets,” says HZDR physicist Rantej Bali.

Bali and his team prepared a thin film of the alloy on top of transparent magnesia through which a laser beam was shone on the film. When they, together with researchers of the HZB, directed a well-focused laser beam with a pulse of 100 femtoseconds (a femtosecond is a millionth of a billionth of a second) at the alloy, a ferromagnetic area was formed. Shooting laser pulses at the same area again – this time at reduced laser intensity – was then used to delete the magnet.

>Read more on the Bessy II at HZB website

Image: Laser light for writing and erasing information – a strong laser pulse disrupts the arrangement of atoms in an alloy and creates magnetic structures (left). A second, weaker, laser pulse allows the atoms to return to their original lattice sites (right). (Find the entire image here)
Credit: Sander Münster / HZDR

Shedding new light on laser additive manufacturing

Additive manufacturing (AM, also known as 3D printing) allows us to create incredibly complex shapes, which would not be possible using traditional manufacturing techniques. However, objects created using AM have different properties from traditional manufacturing routes, which is sometimes a disadvantage.

Laser additive manufacturing (LAM) uses a laser to fuse together metallic, ceramic or other powders into complex 3D shapes, layer by layer. The cooling rates are extremely rapid, and since they are unlike conventional processes we don’t know the optimal conditions to obtain the best properties, delaying the uptake of LAM in the production of safety-critical engineering structures, such as turbine blades, energy storage and biomedical devices. We need a method to see inside the process of LAM to better understand and optimise the laser-matter interaction and powder consolidation mechanisms.

Based in the Research Complex at Harwell, a team of researchers have worked with scientists at I12, the Joint Engineering Environment Processing (JEEP) beamline and the Central Laser Facility to build a laser additive manufacturing machine which operates on a beamline, allowing you to see into the heart of the process, revealing the underlying physical phenomena during LAM.

>Read more on the Diamond Light Source website

Picture: The Additive Manufacturing Team from the Research Complex at Harwell on the Joint Engineering Environment Processing (JEEP, I12) beamline. The Laser Additive Manufacturing Process Replicator (or LAMPR) on the right is used to reveal the underlying physical phenomena during LAM.

Berkeley Lab delivers injector that will drive X-Ray laser upgrade

Unique device will create bunches of electrons to stimulate million-per-second X-ray pulses

 

Every powerful X-ray pulse produced for experiments at a next-generation laser project, now under construction, will start with a “spark” – a burst of electrons emitted when a pulse of ultraviolet light strikes a 1-millimeter-wide spot on a specially coated surface.

A team at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) designed and built a unique version of a device, called an injector gun, that can produce a steady stream of these electron bunches that will ultimately be used to produce brilliant X-ray laser pulses at a rapid-fire rate of up to 1 million per second.

The injector arrived Jan. 22 at SLAC National Accelerator Laboratory (SLAC) in Menlo Park, California, the site of the Linac Coherent Light Source II (LCLS-II), an X-ray free-electron laser project.

Getting up to speed

The injector will be one of the first operating pieces of the new X-ray laser. Initial testing of the injector will begin shortly after its installation.

The injector will feed electron bunches into a superconducting particle accelerator that must be supercooled to extremely low temperatures to conduct electricity with nearly zero loss. The accelerated electron bunches will then be used to produce X-ray laser pulses.

>Read more on the Advanced Light Source website

 Image: Joe Wallig, left, a mechanical engineering associate, and Brian Reynolds, a mechanical technician, work on the final assembly of the LCLS-II injector gun in a specially designed clean room at Berkeley Lab in August.
Credit: Marilyn Chung/Berkeley Lab

Superconducting X-Ray laser takes shape in Silicon Valley

The first cryomodule has arrived at SLAC

Linked together and chilled to nearly absolute zero, 37 of these segments will accelerate electrons to almost the speed of light and power an upgrade to the nation’s only X-ray free-electron laser facility.

An area known for high-tech gadgets and innovation will soon be home to an advanced superconducting X-ray laser that stretches 3 miles in length, built by a collaboration of national laboratories. On January 19, the first section of the machine’s new accelerator arrived by truck at SLAC National Accelerator Laboratory in Menlo Park after a cross-country journey that began in Batavia, Illinois, at Fermi National Accelerator Laboratory.

These 40-foot-long sections, called cryomodules, are building blocks for a major upgrade called LCLS-II that will amplify the performance of the lab’s X-ray free-electron laser, the Linac Coherent Light Source (LCLS).

 

>Read more on the Linac Coherent Light Source website

Photo credit: Fermilab / Jefferson Lab