T-Waves
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 10
26 meters, and the speed of light is about 3 x 10
8 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 10
28 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, 10
40 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 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. 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]
References:
- 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.
- T-ray camera speed boosted a hundred times over, University of Warwick Press Release. July 8, 2020.
- 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.