Saturday, September 23, 2023

Tuning to Terahertz Electronics

Dr S.S. Verma is a professor at Department of Physics, Sand Longowal Institute of Engineering & Technology, Sangrur, Punjab

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In 2008, engineers announced they had built a room-temperature semiconductor source of coherent THz radiation. Until then, sources had required cryogenic cooling, greatly limiting their use in everyday applications.

Many other THz-source technologies have been investigated in the past four decades. Numerous groups worldwide are producing tuneable CW THz radiation using photo-mixing of near-IR lasers. Direct multiplied (DM) sources take millimetre-wave sources and directly multiply their output up to THz frequencies. DM sources with frequencies up to a little more than 1THz and approximately 1µW of output have been used as local oscillators for heterodyne receivers in select applications, most of which are in radio astronomy.

However, these can produce substantially more output power at lower frequencies, and are often well-suited to applications requiring frequencies of less than 500GHz. In addition, physicists have recently demonstrated quantum-cascade semiconductor lasers operating at wavelengths in the 4.4THz regime. These lasers are made from 1500 alternating layers (or stages) of gallium-arsenide and aluminium-gallium-arsenide and have produced 2mW of peak power (20nW average power), and advances in output power and operating wavelength continue at a rapid pace.

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Recently, researchers at JILA (formerly known as Joint Institute for Laboratory Astrophysics), jointly operated by University of Colorado and National Institute of Standards and Technology, USA, have developed a laser based source of THz radiation that is unusually efficient and less prone to damage than similar systems. JILA instruments for generating THz radiation make use of ultra-fast pulses of near-IR laser light that enter through the lens on left, striking a semiconductor wafer studded with electrodes (transparent square that is 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 THz radiation. Fig. 3 shows the close-up of the electron source.

General applications
THz radiations are non-ionising, and therefore safe to humans. These penetrate a wide variety of non-conducting materials, including clothing, paper, plastics and ceramics, and can also penetrate fog and clouds, but are strongly absorbed by metal and water. Until recently, researchers did not extensively explore the material interactions occurring in the THz-spectral region because they lacked reliable sources of THz radiation. However, pressure to develop new THz sources arose from two dramatically different groups—ultra-fast time-domain spectroscopists who wanted to work with longer wavelengths, and long-wavelength radio astronomers who wanted to work with shorter wavelengths. Today, with continuous wave (CW) and pulsed sources readily available, investigators are pursuing potential THz-wavelength applications in many fields.

Fig. 4: Terahertz transistor
Fig. 4: Terahertz transistor

Companies are planning to exploit the commercial applications of TE. THz applications span the physical (security imaging), biological (cell formation) and medical (cancerous-tumour detection) sciences with a growing interest in the application of THz frequencies from security imaging through clothing in airport scanners to non-destructive pharmaceutical and manufacturing inspection through multilayered or opaque surfaces. The unique properties of THz radiation also include high-frequency radars to produce high-resolution images of objects through cloud, fog and dust storms to support aircraft landing in harsh environments.

Much of the recent interest in THz radiation stems from its ability to penetrate deep into many organic materials without the damage associated with ionising radiation such as X-rays. Also, because THz radiation is readily absorbed by water, it can be used to distinguish between materials with varying water content, for example, fat versus lean meat. These properties lend themselves to applications in process and quality control as well as biomedical imaging. Tests are currently underway to determine whether THz tomographic imaging can augment or replace mammography, and some people have proposed THz imaging as a method of screening passengers for explosives at airports as well as for detecting the presence of cancerous cells in humans. However, all these applications are still in the research phase.

THz radiation can also help scientists understand the complex dynamics involved in condensed-matter physics and processes such as molecular recognition and protein folding. CW THz technology has long interested astronomers because approximately one-half of the total luminosity and 98 per cent of the photons emitted since the Big Bang fall into the sub-millimetre and far-IR, and CW THz sources can be used to help study these photons. One type of CW THz source is the optically pumped THz laser (OPTL). These lasers are in use around the world, primarily for astronomy, environmental monitoring and plasma diagnostics. The emerging field of time-domain spectroscopy (TDS) typically relies on a broadband short-pulse THz source. A split antenna is fabricated on a semiconductor substrate to create a switch. A DC bias is placed across the antenna, and an ultra-short pump-laser pulse (<100fs) is focused in the gap in the antenna. The bias-laser pulse combination allows electrons to rapidly jump the gap, and the resulting current in the antenna produces a THz electromagnetic wave. This radiation is collected and collimated with an appropriate optical system to produce a beam.

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