Curved light bends the rules

Everyone knows that light travels in a straight line — right? A couple of years back, however, physicists discovered something very different for certain laser pulses that have one intense peak next to a series of smaller peaks. The brightest part of these lopsided "Airy" pulses, they found, appear to follow a curved trajectory.

Researchers in the US have now found that sufficiently intense Airy pulses can ionize the surrounding air molecules and create curved filaments of plasma. What's more, Airy pulses interact with air such that the pulses are continually focused and so can travel long distances without being dispersed.

The bright white light given off by the plasma filaments could be used make remote spectroscopic measurements of the atmosphere — and the bending effect itself could be exploited in new kinds of waveguide.

The bendy behaviour of Airy pulses was first discovered in 2007 by Demetrios Christodoulides and colleagues at the University of Florida. Interference between the peaks causes the intense peak to veer off in one direction, while the other peaks move in the opposite direction. Although the total momentum of the pulse travels in a straight line, its brightest part appears to follow a curved path.

Christodoulides and his colleagues have now teamed up with Pavel Polynkin and others at the University of Arizona to create curved “filaments” of plasma using Airy pulses. The key to their success, according to Jerome Kasparian of the University of Geneva who was not part of the group, is their ability to — for the first time — create Airy pulses of extremely high intensity.

The team began with an intense infrared laser pulse that is about 35 fs in duration. The initially pancake-shaped pulse, which is symmetric around its direction of propagation, is then passed through a “phase mask” and then a lens, giving it a chevron shape with an intense peak at the vertex (see figure). This Airy pulse then travels about 1 m through air to a fluorescent screen where the light is detected.

As well as confirming that extremely intense Airy pulses appear to curve, the pulses also produced curved filaments of plasma by ionizing nearby molecules in the air.

Although physicists have long known that symmetric laser pulses can create such filaments, the process has proved very difficult to study. This is because symmetric laser pulses travel in the same direction as the white light given off by the plasmas they create, which means that any device that attempts to detect this light is dazzled or even destroyed by the pulse.

With Airy pulses, however, Polynkin, Christodoulides and colleagues discovered that the plasma light travels in straight lines tangentially to the curvature of the bright peak. The plasma light can therefore be detected — and perhaps even be used as a source of white light for spectroscopy.

Firing intense and long-range pulses into the air, for example, could allow researchers to make remote spectroscopic measurements of the atmosphere.

Polynkin also speculates that intense pulses could be fired into thunderclouds to create filaments that "guide" lightning to safe locations on the ground.

Studying the plasma light itself could even help physicists gain a better understanding of the complicated non-linear optics that define how intense laser beams travel through air. These include a “self-healing” effect whereby the beam is continually refocused by the plasma — rather than being dispersed — allowing intense pulses to travel very long distances.

The team are now studying the creation of curved filaments in water rather than air.

Ultrashort pulses create ultrabroad source

By sending laser pulses with a duration of just 5 femtoseconds through a helium cell held at high pressure, researchers have created a coherent supercontinuum with near-uniform spectral intensity spanning the range 270 to 1000 nm. The result relies on a process known as self-channeling and gives the team a new tool with which to explore electron motion inside atoms (Optics Letters 33 1407).

"Our ultimate goal is to generate coherent continuum light that spans several optical octaves," researcher Eleftherios Goulielmakis from the Max-Planck Institute for Quantum Optics in Garching, Germany, told optics.org. "In attosecond physics, we aim to steer the electron motion on atomic scales of space and time. To do this, we require fields that can be precisely controlled and shaped with sub-cycle (attosecond) accuracy and that are intense enough to enable nonlinear interactions with matter."

High and near-uniform efficiency are the prerequisites for generating light fields on a sub-cycle scale. While supercontinuum generation has been at the forefront of ultrafast research for several years, with groups using nonlinear propagation in photonic crystal fibres and solids, this prerequisite combination has remained elusive.

"Using few-cycle pulses dramatically improves the situation," explained Goulielmakis. "Once the duration of the pulse approaches the oscillating period (around 2.5 fs) of the light wave, phenomena like ionization-induced blue shift and shockwave effects result in a dramatic enhancement of the generation of light in the blue wing of the spectrum. We have been able to generate light that extends into the UV part of the spectrum at nearly uniform intensity."

The team focused 5 fs pulses with a central wavelength of 750 nm into a gas cell filled with helium. Self-channeling sets in at a pressure of around 25 bar, which results in a 5 cm long channel and a substantial reduction of the beam divergence in the far field. A intensity-calibrated fibre spectrometer monitored the emerging supercontinuum.

With this impressive result under its belt, the team now has several new experiments in the pipeline. "We plan to extend the supercontinuum source into the VUV by means of quasi-monocyle (~ 1.5 cycles of the field) laser pulses recently realized in out laboratories," said Goulielmakis.

A second follow-on experiment will see Goulielmakis and colleagues split the supercontinuum into narrower bands. The plan is to control properties such as the duration, phase and amplitude of these narrower bands separately before recombining them to synthesize intense light waveforms with a desired shape. "We plan to use these waveforms to control the generation of intense attosecond soft x-ray pulses from atoms," said Goulielmakis.

Other partners in the team come from the Technical University of Vienna, Austria; the Lomonosov Moscow State University, Russia; and the Ludwig-Maximilians University, also in Garching, Germany.

Source: optics.org