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13 Laser Focus World www.laserfocusworld.com March 2016
To create an attosecond optical
pulse, a few-femtosecond
continuum pulse is spectrally divided
into four bands (represented here by
colors), each of which is separately
compressed using dispersive
mirrors. The four pulses are then
combined precisely in space and
time to produce the final pulse
(represented here by white, with the
half-cycle temporal pulse shape
shown at bottom right). (Courtesy of
See page 14
Visible-light pulses are
only 380 attoseconds long
Scientists have for years been doing productive
research using ultrafast light pulses with durations
down to 2 or 3 fs. Now, researchers at the Max Planck
Institute for Quantum Optics and Ludwig-Maximilians
University (both in Garching, Germany), Texas A&M
University (College Station, TX), and M.V. Lomono-sov Moscow State University (Moscow, Russia) have
pushed visible-light pulses beyond the femtosecond
region into the attosecond (as) regime, creating pulses
in the visible-light region only 380 as long.
1 The group
is headed by Eleftherios Goulielmakis, who is already
well-known for his research into attosecond technology and science.
The new technique is in contrast to the more-usual
production of attosecond pulses using high-harmonic
generation (HHG), in which light is created over a very
broad spectrum into the x-ray region, with the wide
spectrum allowing the production of pulses under
100 as in duration (Goulielmakis and his lab are leaders
in this area, too).
The optical attosecond pulses will enable not only
science, as they for the first time provide direct access
to the nonlinear response of bound electrons; they may
also lead to photonic devices operating at subfemto-second time scales and at petahertz (1015 Hz) rates.
The group created optical attosecond pulses using a
light-field synthesizer that can manipulate the properties of visible light along with nearby infrared (IR) and
ultraviolet (UV) frequencies—about 1. 1 to 4. 6 eV, or on the
order of 1130 to 270 nm. The synthesizer managed broadband
and almost dispersion-free spectral equalization of the synthesized pulses, producing pulses with twice the initial bandwidth.
The resulting 380 as light pulses are so short that they are barely
more than a half-oscillation of a light field in duration, making
them the fastest pulses of visible light ever created.
First, broadband ( 1. 1–4. 6 eV) light pulses are generated via
nonlinear broadening of laser pulses with 22 fs duration, 1 mJ
energy, and 790 nm wavelength
achieved by passing them through
a hollow-core fiber filled with neon
gas; the supercontinuum pulses have
energies of about 550 µJ each.
Each of these pulses is then spec-
trally divided into four spectral
bands: near-infrared (NIR; 1130–710
nm), visible (710–500 nm), visible-
UV (500–350 nm), and deep-UV
(350–270 nm). Each of the four
resulting pulses is individually com-
pressed via dispersive mirrors to durations of 8. 5 fs for the NIR
pulse, 7 fs for the visible, 6. 5 fs for the visible-UV, and 6. 5 fs for
the deep-UV pulse. The four pulses are then recombined spa-
tially and temporally to create the final attosecond pulse, which
has an energy of about 320 µJ (see figure).
The physical overlaying of the four spectrally divided pulses
to create the attosecond pulse must be done very precisely.
In addition, the intensity of each of the four pulses is carefully controlled to achieve the proper spectral-band power in