Terahertz radiation

Terahertz radiation is a specific type of electromagnetic wave. It sits in a transition region between microwaves and far infrared light. This middle ground is often called the "terahertz gap." It is called a gap because the technology to create and control these waves is still developing.

The International Telecommunication Union designates this band between 0.1 and 10 terahertz (THz). Some sources consider the upper boundary to be as high as 30 THz. One terahertz is a massive number, equaling 1,000 gigahertz or 10 to the 12th power hertz. These waves are also known as submillimeter radiation because their wavelengths range from 3 mm down to 30 micrometers.

The way these waves interact with the world is very unique. Terahertz radiation travels in a straight line, known as line of sight. It is non-ionizing, which means it does not have enough energy to damage DNA or living cells. This makes it a safer alternative to X-rays for certain uses. Like microwaves, these waves can penetrate many non-conducting materials. This includes things like clothing, paper, cardboard, wood, plastic, and ceramics. However, their penetration depth is usually less than that of microwaves. They are also blocked by metal and liquid water.

Because of how these waves behave, they have specific uses in science and industry. They can pass through thin layers of material for inspection. This allows for material characterization and relief measurement. Scientists can use them to produce high-resolution images of the interior of solid objects. In medicine, they could potentially detect epithelial cancer. This is because they can detect differences in the density and water content of tissue. While they can penetrate some body tissue, they have lower resolution than X-rays. This means the resulting images often need to be enhanced to be useful.

Creating these waves has historically been a major technical challenge. Conventional electronic devices used for radio waves cannot easily generate these frequencies. Most current devices require very low temperatures to work. This is because the energy of THz photons is very small. Low temperatures are needed to suppress environmental noise. Researchers are currently using strategies like optomechanical meta-devices to improve operation temperatures. In 2008, engineers at Harvard University achieved a breakthrough. They used a semiconductor source to create coherent terahertz radiation at room temperature. They did this through the nonlinear mixing of two modes in a mid-infrared quantum cascade laser.

There have been many different ways to produce these waves. Some methods use vacuum electronic devices like the gyrotron or the free-electron laser. Other methods use solid-state sources like resonant-tunneling diodes, which can operate up to 1.98 THz. In 2007, scientists at Argonne National Laboratory announced a compact device using superconducting crystals. These crystals use the Josephson effect. This effect occurs when an external voltage causes an alternating current to flow across junctions. This current then induces an electromagnetic field. Even simple actions can create these waves. In 2009, scientists discovered that unpeeling adhesive tape generates terahertz radiation. This happens through tribocharging, which is the buildup and discharge of static electricity.

Terahertz radiation is also vital for studying the universe. It is emitted as part of black-body radiation from anything warmer than 2 kelvin. While this emission is weak, it is very important for astronomy. It helps scientists characterize cold cosmic dust in interstellar clouds within the Milky Way. It also helps study distant starburst galaxies.

Because Earth's atmosphere strongly absorbs these waves, telescopes must be placed in special locations. They are often located at very high altitudes or in space. Notable observatories include the James Clerk Maxwell Telescope and the Herschel Space Observatory. The Atacama Large Millimeter Array is another important tool for these observations.

Finally, the study of terahertz waves connects to the very building blocks of life. The frequencies of these waves are similar to the motion of biomolecular systems. For example, 1 THz is equivalent to a timescale of one picosecond. This range is comparable to the relaxation timescales of many biological molecules. This means that radiation in this range could potentially modulate biological or neurological functions. Because the energy is low, it can achieve these effects without causing significant heating or ionization. As technology improves, the "terahertz gap" will continue to close, opening new doors in medicine, communication, and space science.