PITTSBURGH - Many of the communication tools of today rely on the function of
light or, more specifically, on applying information to a light wave. Up
until now, studies on electronic and optical devices with materials that are
the foundations of modern electronics-such as radio, TV, and computers-have
generally relied on nonlinear optical effects, producing devices whose
bandwidth has been limited to the gigahertz (GHz) frequency region. (Hertz
stands for cycles per second of a periodic phenomenon, in this case 1billion
cycles). Thanks to research performed at the University of Pittsburgh, a
physical basis for terahertz bandwidth (THz, or 1 trillion cycles per
second)-the portion of the electromagnetic spectrum between infrared and
microwave light-has now been demonstrated.
In a paper published March 4 in Nature Photonics, Hrvoje Petek, a professor
of physics and chemistry in Pitt's Kenneth P. Dietrich School of Arts and
Sciences, and his colleague Muneaki Hase, a professor of applied physics at
the University of Tsukuba in Japan and a visiting scientist in Petek's lab,
detail their success in generating a frequency comb-dividing a single color
of light into a series of evenly spaced spectral lines for a variety of
uses-that spans a more than 100 terahertz bandwidth by exciting a coherent
collective of atomic motions in a semiconductor silicon crystal.
"The ability to modulate light with such a bandwidth could increase the
amount of information carried by more than 1,000 times when compared to the
volume carried with today's technologies," says Petek. "Needless to say,
this has been a long-awaited discovery in the field."
To investigate the optical properties of a silicon crystal, Petek and his
team investigated the change in reflectivity after excitation with an
intense laser pulse. Following the excitation, the team observed that the
amount of reflected light oscillates at 15.6 THz, the highest mechanical
frequency of atoms within a silicon lattice. This oscillation caused
additional change in the absorption and reflection of light, multiplying the
fundamental oscillation frequency by up to seven times to generate the comb
of frequencies extending beyond 100 THz. Petek and his team were able to
observe the production of such a comb of frequencies from a crystalline
solid for the first time.
"Although we expected to see the oscillation at 15.6 THz, we did not realize
that its excitation could change the properties of silicon in such dramatic
fashion," says Petek. "The discovery was both the result of developing
unique instrumentation and incisive analysis by the team members."
Petek notes the team's achievements are the result of developing
experimental and theoretical tools to better understand how electrons and
atoms interact in solids under intense optical excitation and of the
invested interest by Pitt's Dietrich School in advanced instrumentation and
laboratory infrastructure.
The team is currently investigating the coherent oscillation of electrons,
which could further extend the ability of harnessing light-matter
interactions from the terahertz- to the petahertz-frequency range. Petahertz
is a unit of measure for very fast frequencies (1 quadrillion hertz).
This research was funded by a grant from the National Science Foundation.
For more information on Petek's research, visit: www.ultrafast.phyast.pitt.edu/Home.html.