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  • JILA Upgrades Terahertz Source
Jan 2011
GAITHERSBURG, Md., Jan. 24, 2011 — 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.

JILA is a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.

Close-up of the electron source. (Images: Zhang/JILA)

Terahertz 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 optical methods.

The JILA technology, described in the Jan. issue of 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 fs (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 radiation.

The 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 radio-frequency signal ensures that electrons generated by the light cannot react quickly enough to cancel the electric field.

This JILA instrument generates 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 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 V/cm for an input power of 160 mW) comparable to that of other methods.

While 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.

NIST 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 physicist.

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A solid with a structure that exhibits a basically symmetrical and geometrical arrangement. A crystal may already possess this structure, or it may acquire it through mechanical means. More than 50 chemical substances are important to the optical industry in crystal form. Large single crystals often are used because of their transparency in different spectral regions. However, as some single crystals are very brittle and liable to split under strain, attempts have been made to grind them very...
The addition of impurities to another substance, usually solid, in a controlled manner that produces desired properties. Silicon doping with small amounts of other semimetallic elements increases the number of electrical carriers.
1. In raster scan television, one of the two scans that are interlaced to make up a frame. 2. See field of view.
An instrument consisting essentially of a tube 160 mm long, with an objective lens at the distant end and an eyepiece at the near end. The objective forms a real aerial image of the object in the focal plane of the eyepiece where it is observed by the eye. The overall magnifying power is equal to the linear magnification of the objective multiplied by the magnifying power of the eyepiece. The eyepiece can be replaced by a film to photograph the primary image, or a positive or negative relay...
Pertaining to optics and the phenomena of light.
The emission and/or propagation of energy through space or through a medium in the form of either waves or corpuscular emission.
See optical spectrum; visible spectrum.
1. In optics, one of the exterior faces of an optical element. 2. The process of grinding or generating the face of an optical element.
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