Tikalon Header Blog Logo


August 17, 2020

I built a home laboratory while I was in the sixth grade of grammar school, and I did mostly electronics there through high school. Many of my projects were done for the annual science fairs we had in school. These were a great learning experience, and they've evolved into the STEM and robotics workshops and competitions that we have today.

In those days, before the Internet, electronic circuit projects could only be found in hobby electronics magazines, so I regularly bought issues of Popular Electronics, Radio-Electronics and Electronics World. One circuit that captured my attention was a radio receiver for VLF (very low frequency) reception. VLF frequencies range from 3-30 kHz, well within the frequency range of the primitive audio frequency transistors of that time.

This receiver wasn't intended to receive any broadcast stations. It was built to receive natural electromagnetic emissions called radio atmospherics. I wrote about these in a previous article (Very Low Frequencies, May 27, 2019). One interesting phenomenon is the "Dawn Chorus," a VLF signal heard at dawn that resembles dawn birdsong.

VLF is just a small portion of the radio spectrum. The International Telecommunication Union (ITU) has applied the two- and three-letter designations in the following table to bands in the radio spectrum. Cellphones first used the UHF band, but their frequencies are migrating to SHF to enable communication at higher data rates. Your Wi-Fi signals are at 2.4 and 5 gigahertz, near the boundary of the UHF and SHF bands.

Frequency Bands of the International Telecommunication Union (ITU).

TLF=Tremendously Low Frequencies (unofficial), ELF=Extremely Low Frequencies, SLF=Super Low Frequencies, ULF=Ultra Low Frequencies, VLF=Very Low Frequencies, LF=Low Frequencies, MF=Medium Frequencies, HF=High Frequencies, VHF=Very High Frequencies, UHF=Ultra High Frequencies, SHF=Super High Frequencies, EHF=Extremely High Frequencies, THF=Terahertz Frequencies.

ITU Band Number Abbreviation Frequency Wavelength
0 TLF <3 Hz >100,000 km
1 ELF 3-30 Hz 100,000-10,000 km
2 SLF 30-300 Hz 10,000-1,000 km
3 ULF 300-3,000 Hz 1,000-100 km
4 VLF 3-30 kHz 100-10 km
5 LF 30-300 kHz 10-1 km
6 MF 300-3,000 kHz 1,000-100 m
7 HF 3-30 MHz 100-10 m
8 VHF 30-300 MHz 10-1 m
9 UHF 300-3,000 MHz 1-0.1 m
10 SHF 3-30 GHz 100-10 mm
11 EHF 30-300 GHz 10-1 mm
12 THz 300-3,000 GHz 1-0.1 mm

As Richard Feynman (1918-1988) so famously stated, "There's plenty of room at the bottom." While he was talking about nanotechnology at the time, this statement is also true for radio waves. Most scientists and engineers will insist that direct current is a zero frequency signal. However, zero frequency corresponds to infinite wavelength according to the frequency-wavelength relationship,
λ = c/f,
where λ is the wavelength, c is the speed of light, and f is the frequency. Since the observable universe is finite, with a radius of 4.4 x 1026 meters, and the speed of light is about 3 x 108 m/sec, then the lowest natural frequency would be about 7 x 10-19 Hz. The photons carrying such a low frequency would have minuscule energy, so there might be some interesting physics in determining what the real lower limit of frequency might be.

Electromagnetic waves extend beyond these radio bands, and they include light, X-rays, and gamma rays. Ultra-high-energy gamma rays have been detected with energies greater than 100 TeV. These have a frequency greater than 2.42 x 1028 Hz and a wavelength smaller than 1.24 v 10−20 m. Taking this wavelength and the presumed highest wavelength of 7 x 10-19 Hz gives a range of wavelengths about 40 orders of magnitude. I note that in the context of the Dirac large numbers hypothesis, 1040 is nearly equal to the ratio of the electric force to the gravitational force between a proton and an electron.

Far above cellphone frequencies at about 1 GHz are the terahertz (THz) frequencies, a thousand times higher. While the transistors in your cellphone can operate at GHz frequencies, semiconductor devices have difficulty operating even below 100 GHz. As I wrote in a previous article (Bolometry, July 29, 2019), The terahertz spectrum has been relatively underutilized, since it's difficult to generate terahertz radiation and also difficult making detectors at those frequencies. Terahertz frequencies exist in a so-called "terahertz gap" between radio frequencies and visible light. The frequency of red light is 425 terahertz, and violet light is 750.

Visible spectrum with THz frequencies indicated

Visible spectrum with terahertz frequencies (THz) indicated. (Created using Inkscape, based on a Wikimedia Commons image by Gringer. Click for larger image.)

Electromagnetic radiation outside the visible spectrum is often used for imaging. infrared cameras are presently used as real-time temperature scanners of body temperature for coronavirus disease detection. When the first video infrared cameras came to market, one of our research labs bought one for its infrared laser research, but the first use of the camera wasn't for laser experiments, but for a lunchtime photo session of everyone for that wing. A true terahertz camera would be useful for such applications as body scanners that detect natural terahertz emissions, but also for imaging systems in which terahertz radiation illuminates an object, just as electrons are used for imaging in electron microscopes. Terahertz radiation penetrates thin material layers, so such a camera would be useful for non-destructive evaluation.

Physicists and electrical engineers from the Chinese University of Hong Kong (Hong Kong, China), Chongqing University (Chongqing, China), and the University of Warwick (Warwick, UK) have created a terahertz camera with 32x32 pixel resolution at eight frames per second using a single pixel detector, spatial light modulation, and laser technology.[1-2] Their research is presented in an open access paper at Nature Communications.[1]

Terahertz radiation is a non-ionizing radiation capable of imaging beneath layers of plastics and clothing,[1-2] Another single-pixel terahertz camera was able to image micrometer-sized circuit defects hidden by silicon.[1] Terahertz radiation is also highly sensitive to water, so it can detect small changes to the hydration state of biological matter.[2] This new terahertz camera is a hundred times faster than previous such cameras while maintaining sub-picosecond temporal resolution.[1-2] The camera operates at room temperature, and it can be produced for about $20,000.[2]

As study author, Emma Pickwell-MacPherson,[3] a professor at the Department of Physics of the University of Warwick, explains,
"We use what is called a single-pixel camera to obtain our images. In short, we spatially modulate the THz beam and shine this light onto an object. Then, using a single-element detector, we record the light that is transmitted (or reflected) through the object we want to image. We keep doing this for many different spatial patterns until we can mathematically reconstruct an image of our object."[2]

Terahertz camera system

Terahertz camera. An object is illuminated by a collimated terahertz pulse, and its image is projected onto the top surface of a silicon prism at 30°. The top surface of the prism has a two-dimensional conductive pattern imprinted onto to act as a spatial light modulator. The modulated light excites a single-pixel photoconductive antenna, and a series of spatial filter masks are computer processed to create the image. In this figure, much of the visible light optics have been left out for clarity. (University of Warwick image, licensed under a Creative CommonsAttribution 4.0 International License. Click for larger image.)

This terahertz camera builds upon previous research by this team, including terahertz modulators based on a total internal reflection geometry and improved signal processing techniques.[2] The spatial light modulator is realized by having the beam from a laser diode at 450 nm formed into a pattern by a digital micromirror device and having this pattern illuminate a silicon layer atop a prism.[1] This irradiation causes the low-conductivity silicon (>2000 Ω-cm, 500-μm-thick) to become conductive.[1] While the present device uses a laser for the spatial light modulation, the authors remark that the same effect could be produced electrically.[1] The online article includes two supplementary videos of the operation of this device in imaging a metal bar and a leaf from an Achyranthes aspera plant.[1]


  1. Rayko Ivanov Stantchev, Xiao Yu, Thierry Blu, and Emma Pickwell-MacPherson, "Real-time terahertz imaging with a single-pixel detector," Nature Communications, vol. 11, Article no. 2535, May 21, 2020, https://doi.org/10.1038/s41467-020-16370-x. This is an open access article with a PDF file here.
  2. T-ray camera speed boosted a hundred times over, University of Warwick Press Release. July 8, 2020.
  3. Emma MacPherson's Research Group Website.

Linked Keywords: laboratory; sixth grade; grammar school; electronics; high school; science fair; STEM fields; robotics; training workshop; competition; Internet; hobby; magazine; Popular Electronics; Radio-Electronics; Electronics World; electronic circuit; radio receiver; VLF (very low frequency); kHz; frequency; audio frequency; transistor; radio broadcasting; broadcast station; nature; natural; electromagnetic radiation; electromagnetic emission; radio atmospherics; phenomenon; Dawn chorus (electromagnetic); dawn; bird vocalization; birdsong; radio spectrum; International Telecommunication Union (ITU); Mobile phone; cellphone; Ultra high frequency; UHF; Super high frequency; SHF; bit rate; data rate; Wi-Fi; gigahertz; Extremely low frequency; Super Low Frequencies; Ultra Low Frequencies; Very Low Frequencies; Low Frequencies; Medium Frequencies; High Frequencies; Very High Frequencies; Ultra High Frequencies; Super High Frequencies; Extremely High Frequencies; Terahertz radiation; Terahertz Frequencies; ELF; SLF; ULF; VLF; LF; MF; HF; VHF; UHF; SHF; EHF; THz; Richard Feynman (1918-1988); There's plenty of room at the bottom; nanotechnology; scientist; engineer; direct current; signal electrical engineering; infinity; infinite; wavelength; equation; relationship; speed of light; observable universe; finite; radius; meter; nature; natural; photon; minuscule; energy; physics; electromagnetic wave; light; X-rays; gamma rays; ultra-high-energy gamma rays; electronvolt; TeV; orders of magnitude; Dirac large numbers hypothesis; electrostatics; electric force; gravitation; gravitational force; proton; electron; semiconductor device; detector; terahertz gap; radio frequency; visible light; red; violet (color); Inkscape; Wikimedia Commons image; Gringer; image sensor; imaging; infrared; digital camera; temperature; normal human body temperature; coronavirus disease; video; market (economics); research; laboratory; lab; laser; experiment; work meal break; lunchtime; wing (building); body scanner; electron microscope; non-destructive evaluation; physicist; electrical engineer; Chinese University of Hong Kong (Hong Kong, China); Chongqing University (Chongqing, China); University of Warwick (Warwick, UK); pixel; image resolution; frame rate; frames per second; spatial light modulation; open access paper; Nature Communications; non-ionizing radiation; plastic; clothing; micrometer-size; product defect; silicon; water; tissue hydration; hydration state; biological; picosecond; temporal resolution; room temperature; author; Emma Pickwell-MacPherson; professor; Department of Physics of the University of Warwick; mathematics; mathematically; terahertz camera system; collimated light; pulse (physics); projector; projected; surface; silicon; prism (optics); two-dimensional; electrical conductor; conductive; pattern; spatial light modulator; photoconductivity; photoconductive; antenna (radio); computer data processing; computer processed; optics; Creative CommonsAttribution 4.0 International License; total internal reflection; geometry; signal processing; light beam; laser diode; digital micromirror device; Internet; online; metal; cuboid; bar; leaf; Achyranthes aspera; plant.