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Coherent interactions between light and matter

In a recent study, Dr. Ofer Firstenberg and Prof. Barak Dayan introduced a new platform to explore light/matter interactions—one which employs a nanofiber-guided mode with a super-extended evanescent field, characterized by low transit-time broadening. This unique mode is achieved using a single-mode fiber, tapered down to a quarter of a wavelength.

The scientists worked together to fabricate an extremely thin optical fiber that supports a super-extended mode with a diameter as large as 13 times the optical wavelength, residing almost entirely outside the fiber and guided over thousands of wavelengths (5 mm)—all this to couple guided light to warm atomic vapor. This unique configuration balances between strong confinement of the photons and long interaction times with the thermal atoms, thereby enabling fast and coherent interactions (see Figure 2).

The researchers demonstrated narrow coherent resonances (tens of MHz) of electromagnetically induced transparency for signals at the single-photon level, and long relaxation times (10 nanoseconds) of atoms excited by the guided mode. The resulting platform is particularly suitable for observing quantum nonlinear optics phenomena (Optica 2021).

(Image: Super-extended evanescent field of a nanofiber interacting with atomic vapor. (a) Illustration of the extent of an optical mode surrounding a thin optical fiber. (b) Mode field diameter as a function of fiber diameter. Blue and red circles mark the parameters of the two fibers shown in (a). Dashed black line marks the physical fiber dimensions. (c) Calculated Doppler-free absorption spectra for rubidium (Rb) vapor in the evanescent field silica fibers (n=1.45. The effect of transit-time broadening is apparent.)

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Next generation telescope

Dr. Sagi Ben Ami and Prof. Eran Ofek of the Department of Particle Physics and Astrophysics compared the potential of current telescope system designs to enable new celestial discoveries by looking at their cost-effectiveness, or the observed volume of the Universe, per unit time, per dollar. They concluded that recent technological advancements make it possible to construct a telescope using multiple small ‘off-the-shelf’ telescopes to scan an extremely wide swath of the Universe very quickly. Such a system will allow them to gather more data than what is currently possible using the world’s most powerful survey telescopes—all for a fraction of the cost. The game changer in this case is a new, full-frame complementary metal oxide semiconductor (CMOS) detector (camera) that uses small pixels with higher resolution and has a substantially lower price tag compared to the standard charge couple devices typically used in sky surveys.

Putting their findings into practice, the pair is building the Weizmann Astrophysical Observatory (WAO). Located near Kibbutz Neot Smadar in Israel’s Negev Desert, the WAO is based on a novel, modular approach in which dozens of telescopic components are linked together. The system’s unique potential stems from its modularity—which will allow a steady increase in power as new nodes are added—as well as from original algorithms that allow Weizmann astrophysicists to store and analyze the immense data sets generated by their observations.

Inside the WAO, Dr. Ben-Ami and Prof. Ofek are developing the Large Array Survey Telescope (LAST). The LAST design is an assembly of 48 telescopes on 12 mounts. Each telescope will be equipped with full-frame CMOS detectors, creating a system that supports large‑format, high-

performance distributed camera optimization. Along with its superior imaging capabilities, the LAST can view more volume of the Universe per dollar – by an order of magnitude—than any existing ground-based survey telescope. The researchers assert that, once it is ready—by the end of the year, according to current estimates—LAST will be the largest telescope in Israel and among the leading survey telescopes in the world.

LAST’s advanced capabilities are ideal for identifying signatures of hard-to-track celestial events. When observatory is fully operational, studies by multiple Weizmann research groups are expected to help answer the some of most pressing questions in physics and astrophysics, such as the nature of gravity, the origin of “stuff” of the Universe—from heavy elements like gold, to stars, solar systems, and galaxies, and how to identify massive asteroids that could crash into Earth.

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Laser focus on cancer

Prof. Victor Malka and team has begun to explore the use of LPAs to produce Very High Energy Electron (VHEE) beams as an alternative to X-ray treatment. Several million patients throughout the world receive X-ray-based radiation therapy cancer treatments. More energetic electron beams, such as those produced by laser plasma accelerators, could be used for RT and provide better clinical results.

VHEE beams can deliver a super high dose of high energy/low radiation electrons narrowly focused on a specific target, and offer a deeper penetration than conventional X-rays. The high speed can be especially advantageous for precise dose delivery to challenging indications of moving tumors, such as lung and liver, for which radiotherapy outcomes are still poor.

Prostate cancer is one of the most challenging cancers to treat without causing collateral damage to sensitive surrounding regions of the body—often leading to incontinence and/or sexual dysfunction. Using computer modeling, Prof. Malka’s group compared the advantages of photon, proton, and VHEE intensity-modulated radiation therapy (IMRT) for treating prostate cancer. Their results indicated that, compared with conventional X-ray treatment, VHEE would likely be better at sparing the surrounding normal tissue while still delivering the required dose level to the target cancer.

In a clinically approved treatment for prostate cancer, Prof. Malka has already shown that such beams are well-suited for delivering the required dose of radiation, precisely concentrated, and with a deep penetration into tissues, resulting in less damage to collateral and healthy tissues.

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Addressing imperfect spin models

Recently, there has been growing interest in using physical systems, such as lasers, as a kind of mental short cut for optimizing classical spin Hamiltonians. Unfortunately, such physical systems inherently suffer from an inexact mapping between variances in the network (oscillations) and the gradual loss of energy. And that makes the laser models woefully inaccurate.

Prof. Nir Davidson, Dr. Oren Raz, and their colleagues have devised a scheme for addressing this difficulty, which they analyzed theoretically and demonstrated experimentally in the October 2020 issue of Nanophotonics. The scheme involves controlling the laser oscillator amplitude through modification of individual laser oscillator loss.

The researchers demonstrated this approach in a classical XY model simulator, based on a digital degenerate cavity laser (the theoretical analysis). They proved that for each XY model energy minimum, there exists a unique, corresponding set of laser loss values that coincide with it, in terms of both oscillation amplitudes and phase values. The researchers also demonstrated an eight-fold improvement in the deviation from the minimal XY energy by employing their proposed solution scheme (the experimental demonstration—see Figure 1). The results are an important step towards perfecting the use of lasers as spin models.

(Image: Schematic illustration of the experimental setup used to support a laser oscillator network and to measure its state. (a) Folded ring degenerate cavity laser supporting the oscillator network. (b) Interferometer for laser network intensity and phase measurement. (c) The detected interference fringes for a house laser network with equal amplitudes. Abbreviations: SLM – spatial light modulator; OC – output coupler; PBS – polarizing beam splitter; RR1 and RR2 – retro reflecting mirrors; λ/2λ/2 – half wave plate; BS – beam splitter; RM – reflector mirrors.)

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Investigating high energy phenomena

Along with his ongoing research in gravitational waves detailed in last year’s report, Dr. Barak Zackay is beginning to apply his algorithms and analysis to a fascinating problem: the origin of fast radio bursts (FRBs), which are extremely short bursts of energy emitted from different parts of the universe.

Fast radio bursts are powerful pulses of energy at gigaHertz (GHz) frequencies that last only a few milliseconds. Astronomers and astrophysicists have found many fast radio bursts since they were first discovered in 2007. These bursts of radio-frequency appear to be coming from somewhere beyond our own galaxy and are a tantalizing mystery. While no galactic source has yet been documented to emit such a burst, astrophysicists estimate that FRBs may be appearing at a rate of perhaps 10,000 per day if one could watch the whole sky at once. More intriguingly, they have found a number of them that repeat the bursts from the same point in the sky, but researchers cannot pin down a regular period in the pattern of when the signals arrive.

One of the most fascinating aspects of FRBs is their possible relationship with other high energy phenomena found in the universe, such as super novae, pulsars, quasars, rotating radio transients (RRATs), and gamma ray bursts. Dr. Zackay is developing mathematical tools to test whether any of the repeating patterns could be due to the binary orbits of a pair of neutron stars. He is also creating tools for detecting gamma-ray pulsars in data from NASA’s Fermi Gamma-ray Space Telescope and comparing them to known FRBs. The current methods require tens of thousands of hours of computer time (such as provided by the Einstein@home network of thousands of volunteers who allow the project to use their computer’s idle time to search for weak astrophysical signals from pulsars). Dr. Zackay has ideas for algebraic short cuts for solving for the pulsar parameters and sky positions of a gamma-ray pulsar that could be computed on a laptop in a few seconds.

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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.