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Optical magnetic recording

Increasing storage capacity is an ongoing challenge. Heat-assisted magnetic recording (HAMR) is considered the most promising future solution for the magnetic storage technology, as it is predicted to increase storage capacity, to accelerate the reading/writing speed and increase the data lifetime of future hard disk drives. This technology for hard drivers is based on the use of a small laser to heat the part of the disk that is being written to. The heat reduces the coercive field of the material, that is, it improves the ability of the material to react to external magnetic fields. After the heat dissipates, the magnetic field is switched off and the bit is “frozen” in its new magnetic state. The result is the increase of the quantity of information bits that can be stored on a given surface.

At the moment, industry concepts still fail to produce drives with sufficient lifetime. A study by Prof. Dan Oron from the Department of Physics of Complex Systems and his collaborators presents a method which may open an alternative route to high-durability HAMR.

Image (left): Arrays of magnetically recorded spots written using a 3-micron diameter heating beam (yellow circle) with varying pulse duration. The ability to write magnetic features significantly smaller than the HAMR heating laser is clearly demonstrated. [Credit:  Scheunert et al., Beilstein Journal of Nanotechnology, vol 8, 2017]

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Analyzing a nearby supernova

The supernova SN 2015bn, discovered in 2015, was one of the closest Type I superluminous supernova (SLSN) ever reported since their initial discovery in 2003. Prof. Avishai Gal-Yam and Prof. Eran Ofek, along with their colleagues with the Palomar Transient Observatory team, used the opportunity to collect the most extensive data set for any SLSN I to date. 

This included densely sampled spectroscopy and photometry, from the ultraviolet (UV) to the near infrared (NIR) spectrum, spanning from 50 to more than 250 days from the optical maximum. SN 2015bn faded slowly, but exhibited surprising undulations in the light curve on a timescale of 30 to 50 days, especially in the UV part of the spectrum.

This seems to indicate a complex density structure. The team was able to derive physical properties of the SN. The resulting data set provides a new benchmark for observations of SLSNe I. The scientists are continuing to follow the evolution of SN 2015bn as it fades, and to share data about this and other supernovae in the Weizmann Interactive Supernova data Repository (WISeREP).

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Higher energy wakefield accelerators

Plasma-based particle accelerators use charge separation between electrons and ions to create powerful electric fields. The charge separation is induced by a laser beam that acts as a driver in the technique known as laser wakefield acceleration (LWFA). During the past two decades this method has been used to accelerate electrons from rest to some tens of million electron-volts (MeV), to ever higher energies, reaching the giga (GeV)-scale and, recently, multi-GeV energies.

Plasma wakefield acceleration can be seen as a special type of resonance acceleration, whose accelerating structure is a plasma wave. In this kind of accelerator, a particle first goes through an injection process, whose primary challenge is to make the particles co-propagate with the wave.

However, the possible energy gain in laser wakefield accelerators is limited by dephasing between the driving laser pulse and the highly relativistic electrons in its wake. Most efforts to increase the power of these systems have concentrated on continuous phase-locking in the linear wakefield regime.

Prof. Victor Malka from the Department of Physics of Complex Systems examined an alternative scenario that uses rapid rephasing in sharp density transitions, and was able to demonstrate the possibility experimentally. He estimates that a unique phase reset can lead to a gain in energy in the order of 30 percent.

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ULTRASAT is a scientific mini-satellite carrying a telescope that is proposed by an Israeli/US collaboration to be constructed and launched to high-altitude (near geostationary graveyard) orbit by 2022.

The unprecedentedly wide field of view and the advanced UV detectors (210 squared degrees) will enable the discovery and monitoring of transient sources within a cosmic volume 300 times larger than that of the most powerful UV satellite to date, GALEX, thus revolutionizing our understanding of the transient UV universe.

The mission will enable scientists to identify the progenitor stars that explode as supernovae, to follow the disruption flares of hundreds of stars falling into super-massive black holes, and to search for the UV counterparts of tens of gravitational waves’ sources.

To the ULTRASAT website at Weizmann Institute of Science

Gamma ray bursts

Gamma-ray Bursts (GRBs) are the most powerful explosions in the universe, and include the highest redshift objects ever observed. The widely accepted interpretation of these cosmological sources is the so called “Fireball  model” (Goodman and Paczyński 1986).

Prof. Eli Waxman from the Department of Particle Physics and Astrophysics and his colleagues analyzed the gamma ray energy emitted by a sample of 91 gamma ray bursts. Using techniques developed by Prof. Waxman and his former student, Dr. Deborah Freedman, they were able to estimate the fireball energy based on X-ray afterglow observations from these bursts. 

They noted that the burst-to-burst variations in the efficiency of fireball energy conversion to gamma-rays are small; this implies that deviations from the standard fireball model description, if present, are small. However, one of the variations to the fireball theory is that the fireball is not a spherical burst, but rather jet-like. Their observations also imply that that if fireballs are indeed jets, then the jet opening angle is greater than or equal to 0.1 for most cases.

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Simplifying high-order harmonic spectroscopy

Probing electronic wave functions of polyatomic molecules is one of the major challenges in high-harmonic spectroscopy. The extremely nonlinear nature of the laser-molecule interaction and the multiple degrees of freedom of the probed system make this especially difficult.

Prof. Nirit Dudovich of the Department of Physics of Complex Systems and her group combined two-dimensional control of the electron trajectories and vibrational control of the molecules to disentangle the two main steps in high-harmonic generation: ionization and recombination.

In a study published in Physical Review Letters, they demonstrated a new measurement scheme, called frequency-resolved optomolecular gating, which resolves the temporal amplitude and phase of the harmonic emission from excited molecules. Focusing on the study of vibrational motion in dinitrogen tetroxide gas (N2O4), the team showed new ways to provide a unique insight into the structural and dynamical properties of the underlying mechanisms.

Quantum coded

Prof. Barak Dayan and his group from the Department of Chemical Physics investigate quantum optics by exploring interactions between single photons and single atoms.
This video explains the idea of quantum computers that might use photons.