The electromagnetic spectrum is a series of frequencies ranging from radio waves to microwaves, visible light, X-rays, and gamma rays. As the wavelength of the electromagnetic radiation shortens, the waves have a higher frequency—how quickly electromagnetic waves follow each other—and therefore more energy.
Ultraviolet (UV) is electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. Ultraviolet rays are invisible to all humans, although insects, birds, and some mammals can see near-UV.
UV radiation is present in sunlight constituting about 10% of the total light output of the Sun. It is also produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights. Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce. Consequently, the chemical and biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules.
UV light has been used as a disinfectant and kills more than 99.9% of airborne viruses – many hospitals & surgical rooms use UV lights. UVC rays penetrate the skin and the eyes and can cause significant damage in just a few seconds. Similarly, the waves are absorbed by microbes and quickly destroy the genetic material and protein-based outer covering, rendering them inactive. Scientists saw this weakness as an opportunity to use UVC light to kill viruses and bacteria and disinfect both objects and the air. UVC technologies have been used to disinfect and reuse N-95 masks during this Covid -19 pandemic.
There are a few barriers to putting UVC light to work disinfecting air and surfaces all over the world. The specific wavelength of UV light is very important. One set of viruses and bacteria can be resistant to one wavelength while another set is sanitized by a different wavelength. The wavelength best suited to disinfect SARS-CoV-2 is not yet known for sure.
The second challenge is that UVC light disinfects only what it sees. So if something is in shadow or some of the virus is hiding under a layer of dirt, it will not be neutralized without a significant exposure and a number of angles. Remember, at the microscopic level, even the fibers of an ordinary surgical mask can cast a shadow on a virus. For these reasons, it is difficult to know when a surface is clean. One scientist compared it to painting an invisible brush.
Finally, remember, there’s some safety concern. UV light, including UVA and UVB rays, which cause sun burns and aging, is considered a carcinogen. UV light can also be harmful to the cornea, which is part of the eye. The damage could cloud the lens or cause tissue to grow on the eye surface, which can limit vision. That’s why it can’t be used continually in most cases. And the area is only clean until a new person or object is introduced.
The Ultravoilet spectrum also have many military applications. A compact, efficient, and high-power UV laser will enable a wide range of applications including: remote detection of biological and chemical compounds; compact atomic clocks for precise timing and navigation; and point-of-need diagnostics.
Advancements in areas like biological agent and bomb-detection have been prodigious. U.S. Army Research Laboratory and private industry jumped in to develop such cost-effective solutions based on sensor technology as the Edgewood Chemical and Biological Center’s Tactical Biological Detector. TAC-BIO is a UV, LED-based biological agent device that, when compared to other detector solutions, is said to cost nearly 10 percent less, is 50 percent smaller, weighs 80 percent less, and has only 4 percent of the energy consumption. JKL Components CCFL lamps that provide the low-power illumination UV CCFLs used to trace bomb-making materials.
The AN/AAR-47 Missile Warning System is a Missile Approach Warning system used on slow moving aircraft such as helicopters and military transport aircraft to notify the pilot of threats and to trigger the aircraft’s countermeasures systems. The AN/AAR-47 passively detects missiles by their Ultraviolet signature, and uses algorithms to differentiate between incoming missiles and false alarms. Newer versions also have laser warning sensors and are capable of detecting a wider range of threats. he system’s algorithms include looking for temporal variations in a signal’s strength, such as the brightening of an incoming missile. It also evaluates the spectral bandpass of the threat to reduce false alarms and has software for detecting events, such as the launch of a surface-to-air missile.
UV Imaging and camouflage
Observation in the visual region, either by the eye, or by photography, remains the primary means of military surveillance and target acquisition. However, modern battlefield surveillance devices may operate in one or more wavebands of the electromagnetic spectrum, including the ultraviolet (UV), Near Infra Red (NIR), Far Infra Red (FIR). and millimetric or centimetric radar wavebands.
The normal camera with a UV bandpass filter over the lens can have many counter camouflage properties. While the images taken in visible light, they might blend well with the foliage and shadows., however adding a few optical filters allows enemy to obtain a clear, real-time imaging in the near-ultraviolet. This piece of equipment is also not an expensive and readily available worldwide to any quasi-military or terrorist group. It is the availability of such inexpensive, real-time, UV imaging sensors that has made near-ultraviolet camouflage a field necessity for both personnel and strategic military equipment.
With development of the battlefield surveillance devices that operate in various waveband of electromagnetic spectrum, camouflaged munitions of soldiers should be protected not only in visible range but also should be concealed in wide spectral ranges, including ultra violet (UV), infrared (IR), radar, and so forth.
Ultraviolet camouflage is necessary because of the variability of ultraviolet albedo – the percentage of ultraviolet light reflected from the soldier’s immediate environment. Green foliage absorbs ultraviolet light, reflecting 7% percent or less, while the majority of the camouflage patterns, both for personnel and equipment, are significantly more reflective. Sandy turf, depending upon the silicates involved, may reflect as little as 3.3%, while many of the tans and grays used in camo are reflecting 50%+. For decades, NATO has mandated a highly ultraviolet-reflective white paint as ultraviolet camouflage for all vehicles used in snow conditions.
Ultraviolet communication technique and its application
UVC utilizes UV radiation to transmit signals which can be scattered and reflected by the particles and aerosols floating in the air. Its transmission range can be extended up to several kilometers regardless of the topographic features on the ground. Compared to traditional wireless communication, UVC has several unique features that render it a promising communication technique for future applications.
Traditional wireless communication demands an obstacle-free communication channel between transceivers, while UVC can easily bypass these obstacles through scattering and reflection. The UV radiation employed in communication is also called solar-blind radiation which is located between 200–280 nm in the DUV spectrum. Most solar-blind radiation is absorbed by ozone and oxygen when passing through the atmosphere, which leaves a low background-noise communication channel near the ground.
Furthermore, the power of DUV radiation drops exponentially with the transmission distance, which limits its propagation and makes it an ideal option for short-range communication. In radio-silent scenarios, UVC can work as an alternative to conventional wireless communication.
Recently, Alkhazragi et al. demonstrated a UVC system with a record-breaking data rate of 2.4 Gb/s. The proposed system was based on 279 nm LED and Si-based avalanche photodiode (APD), signaling that UVC has great potential in replacing traditional wireless communication in more and more scenarios.
Military Wireless Communications
Benefits of a compact, deep UV laser are by no means limited to providing a source for Raman spectroscopy. This device could also aid non-line-of-sight (NLOS) communication, which is enabled by special characteristics of our atmosphere. In its upper region, ozone absorbs nearly all the sun’s deep UV light. Consequently, light in this solar band is nearly non-existent on the earth’s surface. The result is a low noise environment for the deep UV radiation, where photodetectors can reach a quantum-limited level of photon-counting detection.
The U.S. military has been chasing ultraviolet (UV) communication for decades. Now researchers say radios that communicate using UV light are finally within reach. Working with the Army Research Lab (ARL) in Adelphi, Md., these researchers are mapping out the steps needed to commercialize UV radios. They’ve reached the last piece of the puzzle: untangling the poorly understood, extraordinarily complex way ultraviolet light scatters. If they can do that, they will have unlocked the secret to a new form of non-line-of-sight communication.
Optical wireless links in the solar-blind ultraviolet (SB-UV) band transmit data without the need of a line-of-sight through the use of atmospheric scattering. These links have a host of advantages including security, covertness and ease of deployment, however, they inherently suffer large path loss and delay spread due to the underlying atmospheric scattering process.
Another appealing feature of the NLOS communication technology is that close to ground level, molecules and aerosols produce strong angle-independent scattering of deep UV light. This creates numerous communication paths from the source to the receiver, enabling the transfer of information even when line-of-sight obstacles are in the way − such as buildings in an urban city.
What’s more, deep UV light from the source is absorbed moderately by the atmosphere. This limits the distance that information may be transmitted, making this communication technology ideal for tactical applications
The US Defense Advanced Research Projects Agency (DARPA) launched two programs involving developing novel light sources and detectors for future military use in 2002 and 2007: the Semiconductor UV Optical Source (SUVOS) program and the Deep Ultraviolet Avalanche Photodiode (DUVAP) program. These two programs sponsored numerous pieces of research on DUV light sources and solar-blind detectors, which further boosted the development of UVC based on DUV light sources and solar-blind detectors. In 2006, a 24-unit array LED with wavelength of 274 nm and output power of 40 mW was demonstrated by Shaw et al.. They achieved a 2.4 Kb/s mixed-excitation linear predictive (MELP) vocoder link in full sunlight at a range of 11 m. Xu et al. and his team from the University of California (Riverside) conducted a series of experiments on system performance analysis, scattering model, modulate scheme and network communication since 2007. The research papers were also published continuously.
UVC research in China was first started by the Beijing Institute of Technology in the 1990s. Ni and his group built a short-distance voice communication UVC prototype using a low-pressure mercury lamp as the light source, and a PMT as the receiver. In 2007, Tang et al. put forward a single scatter model that can effectively budget and evaluate the transmission distance of the non-line-of-sight (NLOS) UVC system in the solar-blind range. The model was successfully verified by the experiment results. They further extended the application of this model to the evaluation of signal-noise ratio (SNR) and BER.
In 2009, researchers from Ben Gurion University in Israel developed an underwater UVC system based on 250 nm LED The system showed excellent communication performance. When transmission distance was over 170 m, the BER was lower than 10–4 and the data rate was 100 Mb/s. Researchers from Greece University conducted a series of experiments on path losses in the UVC system under different distances, elevation angles, and atmospheric conditions[
Secret military communication scheme from the 1960s is finally practical
Now researchers say radios that communicate using UV light are finally within reach. Working with the Army Research Lab (ARL) in Adelphi, Md., these researchers are mapping out the steps needed to commercialize UV radios. Proposed UV radios communicate in the so-called solar blind portion of the UV-C band–light having wavelengths from 200 to 280 nanometers–which, unlike the sun’s UV-A and UV-B rays, is almost completely blotted out by the atmosphere.
Near Earth’s surface, even a strong UV-C signal would die off within a few kilometers, as individual photons are picked off one by one by oxygen, ozone, and water molecules. But that attenuation also makes UV_C radiation ideal for short-range wireless links, such as in unattended ground sensors. The U.S. military is interested in such short-range communications because they can’t be intercepted or jammed outside their intended range. What’s more, within its limited range the UV-C band has an inherently high signal-to-noise ratio, enabling the use of very-low-power transmitters, according to ARL scientist Brian Sadler.
In contrast with other optical schemes, which rely on the transmitter sending a signal more or less directly to the receiver, a UV system can take advantage of the signal scattering in the atmosphere. The transmitter beams a modulated signal into the sky in the shape of a cone. The receiver is trained on the sky as well, at an overlapping angle. That positioning makes it ideal for sensors in dense urban environments where line-of-sight communication doesn’t work. ”This goes around corners, through forests, anyplace you can get light,” says Russell Dupuis, an electro-optics professor at Georgia Tech.
Early UV-C radio prototypes were far too clunky to make it out of the lab–the transmitter was a massive laser, and the receiver was a bulky vacuum-tube-based photodetector. But thanks to materials-science advances, the new transmitters are tiny, commercially available UV LEDs. The receivers have also shrunk to tiny, solid-state avalanche photodiodes–devices in which a single photon produces an avalanche of electrons. With currently available devices, and under typical operating conditions, a low-power UV-C system with the right overlap could transmit roughly 100 kilobits per second at 10 meters, dropping to less than 10 kb/s at 100 meters, still more than enough for good digital audio.
A major remaining challenge is modeling the behavior of the signal as it scatters randomly in the sky. ”It’s not like there’s a mirror up there,” Sadler says. At the University of California, Riverside, he and electrical engineering professor Zhengyuan Xu experimented with different transmission sources and receivers to characterize the angle at which the transmitter beam and the receiver’s field of view should cross. ”It’s not just about getting the overlap volume as large as possible,” Xu adds. ”When it’s narrower, the signal is sometimes more enhanced.”
A quantum clock is a type of atomic clock with laser cooled single ions confined together in an electromagnetic ion trap. Both the aluminium-based quantum clock and the mercury-based optical atomic clock track time by the ion vibration at an optical frequency using a UV laser, that is 100,000 times higher than the microwave frequencies used in NIST-F1 and other similar time standards around the world. Quantum clocks like this are able to be far more precise than microwave standards and current optical clocks
UV semiconductor laser
The most promising class of materials for making a compact, reliable, low-cost, and efficient deep UV semiconductor laser is the III-nitrides. Its attributes include high chemical and mechanical toughness and a very suitable range of bandgaps − the wavelength of the band edge can be as short as 200 nm.
Providing further motivation for the development of a deep UV III-nitride laser is the success of this material system in blue LEDs and laser diodes. But replicating performance at shorter wavelengths is far from easy. As of today, the wall-plug efficiency of most commercial deep UV LEDs is still in the low single-digit range, and there is yet to be a demonstration of an accompanying laser diode.
One of the biggest challenges associated with developing a deep UV laser is that the highly mature, blue-emitting InGaN system is not up to the task, due to its smaller bandgap. Stretching emission from the blue to UV demands the addition of aluminium to GaN, and this increases the in-plane lattice mismatch between the III-nitride and the most common substrate, sapphire. Material quality degrades, with imperfections dragging down the quantum efficiency.
On top of this, it is difficult to design a deep UV laser diode. One challenge is to develop a structure that provides sufficient optical confinement. Judged in these terms AlGaN is not ideal, with changes in the aluminium content producing small variations in refractive index.
The next milestone in deep UV laser development came in 2011, when a partnership between Palo Alto Research Centre and the US Army Research Laboratory announced the demonstration of an optically pumped 267 nm laser with a threshold power density of just 126 kW/cm2. One of the big differences between this laser and that produced at Kohgakuin University is the choice of substrate – it had been switched from SiC to bulk AlN. Reasons for preferring the latter include a low dislocation density – it is around 104 /cm2 – and a similar lattice constant and thermal expansion coefficient to that of aluminium-rich AlGaN. Thanks to these merits, there is a low dislocation density in the AlGaN heterostructures.
During the last few years, more groups have had success with AlN substrates. Low thresholds have been obtained by optical pumping while pushing the lasing wavelength ever shorter, to reach to 237 nm. These accomplishments have highlighted the AlN substrate as a technically promising platform for demonstrating and developing the deep UV laser diode.
Ultraviolet (UV) detectors denote those devices showing spectral response with wavelength short than 400 nm, which can be widely applied in various fields such as flame sensing, medial phototherapy, missile warning, radiation detection, astronomical studies, optical communications and electronic industry.
Compared to UV-enhanced Si photodetectors, a new generation of wide bandgap semiconductors, such as (Al, In) GaN, diamond, and SiC, have the advantages of high responsivity, high thermal stability, robust radiation hardness and high response speed.
Wide-bandgap semiconductors such as III nitrides, SiC, and diamond have been emerging as the advanced materials for the UV detection due to their wide band gaps. These semiconductors exhibit intrinsic visible-blind or solar-blind features as UV detectors. Due to the high optical absorption coefficient or the high-quality single crystal nature of these semiconductors, thin films with submicron thickness were sufficient to achieve the high performance photoresponse properties.
Recently, low-dimensional nanostructured wide-bandgap semiconductors are attracting growing attention for UV detectors due to their high crystal quality, large surface-to-volume ratio, and low-cost synthesis method. Researchers from the U.K. and Kuwait have created a hydrothermal technique to make on-chip ultraviolet (UV) detectors out of zinc oxide nanowires that are 10,000 times more sensitive than their traditional flat-film counterparts. UV detectors can help prevent sun overexposure and can sense radiation from fires; therefore, they must be fast and sensitive to be effective. The authors say that in addition to greatly improving detector response time and sensitivity, their flexible nanowire arrays can also fit into many different environments and are relatively low-cost compared to film detectors.