Ultraviolet pulses close in on 1 fs regime

Deep ultraviolet (DUV) pulses with a duration of just 3.7 femtoseconds have been generated by researchers at the Max-Planck-Institute for Quantum Optics in Germany. The pulses are said to be the shortest ever observed at this wavelength range and could allow the team to influence the outcome of chemical reactions by controlling the motion of electrons in molecules. (Optics Express 16 18956)

Previously, the shortest pulses in the DUV (wavelengths shorter than 300 nm) had a duration of 8 femtoseconds. Much of the research effort focused on compressing the ultraviolet pulses after they had been generated, which is a formidable challenge as this requires precise dispersion control.

Here, Ulrich Graf and colleagues take a different approach. They upconvert 780 nm, 0.25 mJ pulses with a duration of 6 femtosecond pulses directly into the UV range by means of harmonic generation in a noble gas jet.

The resulting 3.7 femtosecond pulses are characterized using a dispersion-minimized SD-FROG approach and have energies in excess of 1.4µJ. The conversion efficiency is approximately 0.6%.

The group is now hoping to extend its approach to generate ultrashort pulses in the vacuum ultraviolet (VUV) spectral range (wavelengths less than 180 nm). "In the VUV, matter absorbs light even more and this spectral range will offer plenty of opportunities to explore and control the microcosm on an ultrafast scale," said Goulielmakis. "At the same time, we anticipate that these pulses will be substantially shorter and will approach the 1 femtosecond frontier."

Ultra-short Laser Pulse Produces Positrons

More than 100 billion particles of antimatter have been created by using a short-pulse, ultraintense laser to irradiate a gold sample the size of the head of a push pin. The antimatter, also known as positrons, shoots out of the target in a cone-shaped plasma "jet."

This new ability to create a large number of positrons in a small laboratory opens the door to several avenues of antimatter research, including an understanding of the physics underlying various astrophysical phenomena such as black holes and gamma ray bursts. Antimatter research also could reveal why more matter than antimatter survived the Big Bang at the start of the universe.

In the experiment, the laser ionizes and accelerates electrons, which are driven right through the gold target. On their way, the electrons interact with the gold nuclei, which serve as a catalyst to create positrons. The electrons give off packets of pure energy, which decays into matter and antimatter, following the predictions by Einstein's famous equation that relates matter and energy. By concentrating the energy in space and time, the laser produces positrons more rapidly and in greater density than ever before in the laboratory.

Particles of antimatter are almost immediately annihilated by contact with normal matter, and converted to pure energy (gamma rays). There is considerable speculation as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter, and what might be possible if antimatter could be harnessed. Normal matter and antimatter are thought to have been in balance in the very early universe, but due to an "asymmetry" the antimatter decayed or was annihilated, and today very little antimatter is seen.

Over the years, physicists have theorized about antimatter, but it wasn't confirmed to exist experimentally until 1932. High-energy cosmic rays impacting Earth's atmosphere produce minute quantities of antimatter in the resulting jets, and physicists have learned to produce modest amounts of antimatter using traditional particle accelerators. Antimatter similarly may be produced in regions like the center of the Milky Way and other galaxies, where very energetic celestial events occur.

The presence of the resulting antimatter is detectable by the gamma rays produced when positrons are destroyed when they come into contact with nearby matter. Laser production of antimatter isn't entirely new either. Livermore researchers detected antimatter about 10 years ago in experiments on the since-decommissioned Nova petawatt laser -- about 100 particles. But with a better target and a more sensitive detector, this year's experiments directly detected more than 1 million particles. From that sample, the scientists infer that around 100 billion positron particles were produced in total.

Short fibre creates ultrafast OPO

A fibre optical parametric oscillator (FOPO) based on a 4.2 cm length of microstructured fibre that emits 70 fs pulses has been unveiled by researchers at the US universities of Cornell and California Merced. The system is said to deliver the shortest optical pulses reported for any FOPO and is a significant step towards making the technology commercially viable. (Optics Express 16 18050)

"The majority of OPOs are not portable and occupy a large footprint on an optical table," Jay Sharping of Merced's School of Natural Sciences told optics.org. "My motivation is to generate tunable pulsed light of sufficient output power in a portable fibre platform. This result is a step in that direction as it explores the generation of ultrafast laser light with tens of mW of average power."

Sharping and colleagues start with a commercially available microstructured fibre that has been drawn down to a reduced core size in order to modify the fibre's dispersion profile. They place the 4.2 cm length of fibre in a Fabry-Perot cavity and pump it using a ytterbium-doped fibre laser emitting at 1032 nm. The end result is 70 fs, 0.4 nJ pulses at 880 nm with an output peak power for 5kW for a pump peak power of 22 kW.

Optical oscilloscope is fit for high-speed studies

Physics in the US have made an oscilloscope that can take snapshots of optical waveforms at a resolution fives times better than current devices. Based on an all-optical rather than an electronic design, the oscilloscope should be able to accurately profile modern telecommunications signals and various ultrafast chemical and physical phenomena.

Oscilloscopes are used to trace graphs of signals over time. Conventional models are based on microelectronics and, using photodetectors, can take snapshots of optical signals at as low as 30 ps resolution.

But as telecommunication data transmission gets faster and faster, and as scientists want to probe more high-speed systems, oscilloscopes based on microelectronics are being stretched to the limit. This is because they can only cope with a relatively narrow frequency spread or “bandwidth”, which holds back their resolution.

All-optical circuits, on the other hand, can process much wider bandwidths. Although optical techniques already exist — indeed, with resolutions going down to a few femtoseconds — these have only been able to take snapshots of small segments of waveforms, and take a long time to update.

A team led by Alexander Gaeta at Cornell University in New York has found a way to exploit the fine resolution of optical techniques for longer waveforms. The researchers make use of the fact that electromagnetic waves have a space–time duality, in that there is a link between their spatial and temporal wavefunctions. This means that the researchers can use a lens to convert the temporal profile of a dispersed snapshot into a detailed, spectral output via a so-called Fourier transformation.

In the Cornell team’s device, an input waveform enters an optical fibre and mixes with a pump laser pulse, which ensures the waveform matches the focal length of the lens. As the waveform travels through the fibre it stretches out or “disperses”. Then, at the end of the fibre the lens — a nano-scale silicon waveguide — converts the waveform into a spectrum that can be measured with a spectrometer (Nature 456 81).

The device can record an input waveform at a resolution of 220 fs over lengths greater than 100 ps, giving the largest length-to-resolution ratio (more than 450) of any snapshot oscilloscope technique. Moreover, the technique uses components that can easily be integrated on chips.

Watching Electrons with Lasers

(PhysOrg.com) -- A team of researchers from the Stanford PULSE Institute for Ultrafast Energy Science at SLAC National Accelerator Laboratory has recently moved a step closer to visualizing the motions of electrons in molecules using a technique called high harmonic generation, or HHG.

Understanding these movements may help scientists better understand the early stages of chemical reactions. Electrons fuel chemical reactions. When chemicals react, electrons move between the molecules, building and breaking the connections, or bonds, that link atoms.

But in the world of quantum mechanics, electrons aren't easy to pin down. Physicists and chemists create mathematical descriptions called orbitals to illustrate the chance of finding an electron at a specific location of a molecule. Representations of these orbitals look like balloons attached to an atom's nucleus, the center of the atom.

SLAC researcher Markus Guehr and the PULSE team used HHG to learn about the electron orbitals of nitrogen gas molecules. In an HHG experiment, the researchers use molecules as tiny accelerator light sources. A laser beam is focused onto a stream of cooled nitrogen gas. The electric field of the laser tears an electron from a nitrogen molecule. As the laser field oscillates, the electron is accelerated back into the molecule and recombines with its orbital. Once the electron returns to the molecule, its energy is converted into light in the extreme ultraviolet range.

The spectrum of the light emanating from the molecule depends on the nature of the orbital the electron hits. By analyzing the number of photons at particular energies produced by this molecular laser, the team can characterize a specific orbital in the molecule.

But to understand how electrons move within a molecule over time, physicists need to characterize multiple orbitals.

In a report published in Science Express on October 30, the PULSE team, which also included Brian McFarland, Joseph Farrell and PULSE director Philip Bucksbaum, described the first evidence of HHG light signals from two different orbitals. Before these experiments, scientists had observed only light generated from electrons colliding with an orbital called the highest occupied molecular orbital, or HOMO. This orbital is the highest energy orbital that contains an electron. Physicists had theorized that detecting other orbitals was possible, but no one had observed multiple signals in an experiment.

The PULSE team reported detecting light from another orbital called the HOMO-1, which is one energy level lower than the HOMO. To detect light from the HOMO-1, the researchers had to align the nitrogen molecules perpendicular to the laser's electric field, to produce more efficient collisions between electrons and the orbital.

Science article: http://www.sciencemag.org/cgi/rapidpdf/1162780.pdf

Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks

(PhysOrg.com) -- “When a laser goes through a plasma,” John Cary tells PhysOrg.com, “it pushes electrons away. Then when it snaps back, it generates an electric wake behind the laser pulse, picking the electrons up and carrying them along.” Cary is a physics professor at the University of Colorado in Boulder, as well as the founder of Tech-X Corporation, a company that specializes in computational physics and simulation software. He is a member of a collaboration that wanted to see if it was possible to accelerate heavy ions with a laser.

“Accelerating electrons is easier, because they are light,” he says. “Instead, we wanted to see if there could be the possibility of doing this with protons and heavier nuclei.” The collaboration, a team from the Shanghai Institute of Optics and Fine Mechanics in China and Cary, produced a simulation outlining possibilities. The results of the simulation are reported in Physical Review Letters: “Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks.”

Cary says that the information found in the simulation may have a variety of applications. “But the most exciting application, and the one that many people are looking to use,” he points out, “is for use in cancer therapy.”

The simulation shows that for heavier ions, it is possible to accelerate them, as well as control what is known as the Bragg Peak. “When you have a small charge to mass ratio,” Cary explains, “as an ion beam travels through matter, it deposits energy. At the end, just before it comes to rest, there is a very sharp peak of energy deposition.” This Bragg Peak is used in proton therapy to concentrate the energy on cancerous tumors to destroy them.

But there can be a problem: “If the beam is not monoenergetic, the peak smears out, potentially overlapping healthy tissue, which can then be damaged,” Cary says. “Researchers are trying to narrow this peak so that it is more precise, destroying the tumor but not the surrounding healthy cells.” This new simulation implies that this could be possible: “We found that carbon may have what is needed. The configuration seems to have nice properties, with a small energy spread and a fair amount of beam.”

More information: Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks, Phys. Rev. Lett. 101, 164802 (2008)

Single laser traps and dissects cells

(Optics.org)--Researchers have shown for the first time that a single near-infrared laser can be used to both trap and dissect living cells.

A team of researchers from the Philippines and Japan has shown that a single laser can be switched between continuous wave (cw) and femtosecond pulsed mode to trap and penetrate yeast cells respectively. The researchers say that using a single laser with two operational modes simplifies the trapping system and allows complex and intricate manipulation of cell components (Review of scientific instruments 79 103705).

"As far as we know, this is the first time that the cw-mode of an ultrafast laser is functionally used in a system," Vincent Ricardo Daria, a researcher at the University of the Philippines, told optics.org. "While optical trapping and optical surgery is not new, it is the combination of both functionalities using a single laser that makes this work unique."

Optical trapping requires a low energy laser to avoid damaging the sample, while the opposite is required for optical surgery. Until now, two separate lasers were necessary to carry out each function - one for trapping and another for surgery. Daria and colleagues have now developed a single 780 nm Ti:Sa laser that when operated in cw mode enables non-invasive trapping of cells and by switching to femtosecond pulsed mode, precise surgery can be performed.

"When the laser is in cw mode using around 10 mW of power, we can perform the trapping part of the experiment," explained Daria. "By modulating the intensity of the incident laser at a repetition rate of 80 MHz, at the same power level, we can attain incision sizes smaller than the diffraction limit of light."

The team switched between cw and pulsed mode by adjusting the output of the pump laser. "Ultrafast lasers normally come with an electronic control module to trigger fs-pulse mode," explained Daria. "CW-mode is achieved by disturbing the fs-laser cavity by temporarily shutting-off the pump laser."

In the setup, the laser is introduced into an inverted laser-scanning microscope and brought to focus inside the sample using a water immersion objective lens. A dichroic mirror is used to direct the NIR laser to the objective lens while allowing images of cells at the focus of the same objective lens to be viewed on a CCD camera.

According to Daria the potential of such a setup goes beyond trapping and surgery of yeast cells alone. The combined system could also be used for yeast cell growth analysis, tissue and cell engineering and micro-manipulation for reproductive medicine.