THz Light, on the Boundary between Visible Light and Radio Waves
Terahertz (THz) light is a relatively unexplored portion of the electromagnetic spectrum, lying between light and radio waves. Its frequency is 1012 (1 trillion = 1 tera) Hz, which is the origin of the name. This light has been relatively unused for industrial purposes because of the lack of a good source of THz light and the lack of methods to detect it.
Nikon began work on industrial application of THz light in 2000, co-developing a real-time imaging system with the Communications Research Laboratory (formerly known as the Electrotechnical Laboratory) of the Ministry of Posts and Telecommunications. The RAYFACT-RIM-001EX introduced here is the world's first real-time imaging system.
This real-time imaging system can utilize terahertz light to perform a number of feats, for example, observing the inside of an object. The X-ray that we are all familiar with can do the same thing, but THz light can be used on a much wider range of materials. Many specimens which could not be observed with X-rays can be observed with THz light.
When materials are examined on the molecular level, molecules can be seen to be vibrating in a variety of modes. When illuminated with light at a specific frequency, light at the same frequency as the molecular vibration is absorbed. This absorption creates tonal differences on the resulting image and THz light has a much wider range of synchronous frequencies than X-rays. This is why THz light can be used to observe so many different types of materials and makes it possible to identify the specific characteristics of the specimen.
X-rays have extremely short wavelengths and therefore high in energy, which can be harmful to the human body, while the longer wavelength of THz light means they have low energy levels and have no effect on the human body.
Fig. 1The world's first motion imagery made with THz light (insectivorous plant)
Fig. 1 shows an insectivorous plant capturing an insect, the world's first realtime motion imagery made with THz light.
Fig. 2 is an illustration of the optical system used in the RAYFACT-RIM-001EX real-time imaging system. Laser light is emitted from the top (where it says "femtosecond optical pulse") to generate THz light. A wavelength of the laser light is 800 nm in the visible spectrum and is output as a pulse for the extremely short interval of only 100 fs (one fs is 1/1000th of a trillionth of a second).
The laser light is split by a half mirror: one beam used as the light source to generate the THz light and the other used to transfer the view seen through the object to the CCD camera.
A GaAs compound semiconductor is used as the light source. The wafer is cut into rectangular chips and mounted with electrodes to input voltage. When illuminated by laser light at 800 nm, current flows for that instant output a THz pulse. The pulse lasts for only about 1 ps (one trillionth of a second). This pulse of THz light passes through the specimen to reach the imaging plate (photoelectric crystal; see Fig. 3).
Next is the detector. THz light cannot be seen with the naked eye or acquired by CCD cameras.
When the THz light reaches the imaging plate after passing through the specimen, the refractive index is changed (some THz light making up the image is absorbed by the vibration of the specimen, and therefore doesn't reach the imaging plate). This minute change is detected by laser light (scan laser) and transferred to the CCD camera to visualize the THz light image. The CCD camera can capture up to 30 images per second.
Fig. 4 is an actual image, captured every 0.33 ps when the THz light reaches the imaging plate. This is the first image in the world to actually show the light wavefront.
A number of technical issues were involved in creating a system incorporating this THz light source and the detector.
One was the need for a larger imaging plate. The first imaging plates were only a few mm squares, but now they are available up to 80 mm square, providing ample space for THz imagery.
In addition, the light source size (spot diameter) can be easily changed to match the specimen. At present, the most common spot diameter is about 20 mm.
Technical innovation in signal processing made it possible to achieve precise synchronization between laser emission, THz light pulses, scan laser pulses and CCD camera image acquisition, making the THZ real-time imaging a reality.
The RAYFACT-RIM-001EX real-time imaging system, one of the first practical utilization for THz light, is pioneering exciting new applications in polymer analysis, non-destructive tests, and medical diagnostics such as examination of skin carcinoma.
Terahertz light, a relatively new and unexplored section of the electromagnetic spectrum, will give rise to a host of new applications and unexpected capabilities around the world.
