JILA instrument for generating terahertz radiation. Ultrafast pulses of near-infrared laser light enter through the lens at left, striking a semiconductor wafer studded with electrodes (transparent square barely visible under the white box connected to orange wires) bathed in an oscillating electric field. The light dislodges electrons, which accelerate in the electric field and emit waves of terahertz radiation. The radiation bounces off the reflective dishes at the bottom and top of the photo and hits the circular detector on the right. Credit: Zhang/JILA
researchers have developed a laser-based source of terahertz radiation
that is unusually efficient and less prone to damage than similar
systems. The technology might be useful in applications such as
detecting trace gases or imaging weapons in security screening.
radiation—which falls between the radio and optical bands of the
electromagnetic spectrum—penetrates materials such as clothing and
plastic but can be used to detect many substances that have unique
absorption characteristics at these wavelengths. Terahertz systems are
challenging to build because they require a blend of electronic and
JILA technology, described in Optics Letters,* is a new twist on a
common terahertz source, a semiconductor surface patterned with metal
electrodes and excited by ultrafast laser pulses. An electric field is
applied across the semiconductor while near-infrared pulses lasting
about 70 femtoseconds (quadrillionths of a second), produced 89 million
times per second, dislodge electrons from the semiconductor. The
electrons accelerate in the electric field and emit waves of terahertz
JILA innovations eliminate two known problems with these devices.
Adding a layer of silicon oxide insulation between the gallium arsenide
semiconductor and the gold electrodes prevents electrons from becoming
trapped in semiconductor crystal defects and producing spikes in the
electric field. Making the electric field oscillate rapidly by applying a
radiofrequency signal ensures that electrons generated by the light
cannot react quickly enough to cancel the electric field.
result is a uniform electric field over a large area, enabling the use
of a large laser beam spot size and enhancing system efficiency.
Significantly, users can boost terahertz power by raising the optical
power without damaging the semiconductor. Sample damage was common with
previous systems, even at low power. Among other advantages, the new
technique does not require a microscopically patterned sample or
high-voltage electronics. The system produces a peak terahertz field (20
volts per centimeter for an input power of 160 milliwatts) comparable
to that of other methods.
there are a number of different ways to generate terahertz radiation,
systems using ultrafast lasers and semiconductors are commercially
important because they offer an unusual combination of broad frequency
range, high frequencies, and high intensity output.
At right is a close-up of the electron source. Credit: Zhang/JILA
has applied for a provisional patent on the new technology. The system
currently uses a large laser based on a titanium-doped sapphire crystal
but could be made more compact by use of a different semiconductor and a
smaller fiber laser, says senior author Steven Cundiff, a NIST
H. Zhang J.K. Wahlstrand, S.B. Choi and S.T. Cundiff. Contactless
photoconductive terahertz generation. Optics Letters, Jan. 15, 2011.