Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Can radio antennas emit visible light?

Category: Physics      Published: October 2, 2015

red laser
The laser is the most practical visible-light equivalent of a radio antenna. Public Domain Image, source: Christopher S. Baird.

Yes, radio antennas can emit visible light, but probably not in the way that you're thinking. If you pump enough energy into a radio antenna, you can heat it up until it glows and emits visible light through the process of thermal radiation. However, a regular radio antenna cannot emit visible light that carries information, similar to how it does with radio waves. There are, however, other devices that can do this.

As you may have learned, electromagnetic waves come in many different frequencies, from radio, infrared, visible, and ultraviolet to x-rays and gamma rays. The red light emitted by a glow stick is fundamentally the same as the radio wave emitted by your Wi-Fi router. Both are electromagnetic waves. The red light just has a much higher frequency than the radio wave (the frequency is a measure of how many cycles the wave completes every second). Because they are fundamentally the same, you could be tempted to conclude that you can get a radio antenna to emit controlled visible light by simply cranking up the frequency of the circuit that is driving the antenna. While this makes sense at first glance, the reality of the material properties of antennas gets in the way. A radio antenna works by using electric circuits to push electrons up and down the antenna, causing the electric fields of the electrons to wave up and down as well. These oscillating electric fields then travel away as electromagnetic radio waves. The frequency of the radio wave is equal to the frequency at which you push the electrons up and down the antenna.

A typical Wi-Fi router radio antenna emits radio waves that have a frequency of 2.4 GHz (2.4 billion cycles per second), which corresponds to a wavelength of 12.5 centimeters. In general, a radio antenna emits waves most efficiently when its length is equal to the wavelength of the radio wave, or to a half or a quarter of the wavelength. It therefore should not be surprising that the antennas on your Wi-Fi router are about 12.5 centimeters long. In contrast, the wavelength of blue light is about 470 nanometers. To give you an idea, this is a hundred times smaller than the smallest cell in your body. Blue light has a wavelength that is about 300,000 times smaller than that of a Wi-Fi radio wave. A normal-sized radio antenna is simply too large to efficiently emit visible light because of this size mismatch, even if we managed to overcome the material problems. You may think we could just chop down the size of the antenna to match the wavelength of visible light, but such an antenna would have to be only 1000 atoms long. Making such a small antenna is difficult, but not impossible. The emerging field of plasmonic nanoantennas accomplishes this very task, as I will discuss at the end of this article. Even if you successfully make such a small antenna, you still need to build an electronic circuit that can drive the electrons up and down the antenna at the right frequency. The frequency of blue light is about 640 THz (640 trillion cycles per second). Electronic circuits can only drive electric currents that oscillate at best in the hundreds of GHz (hundreds of billions of cycles per second). If you try to go higher, the electronic circuits stop working because the material properties of the circuit components change.

Even if you managed to make a radio antenna that is small enough to match the wavelength of blue light and managed to create a device that can drive electrons at the frequency of blue light, there is still one major problem that gets in the way: the atomic structure of the antenna material. For large-wavelength electron oscillations, the antenna material looks uniform and lacks significant resistance. In contrast, for nanoscale oscillations, the electrons are more likely to bump into atoms and lose their energy to the atoms before they have a chance to emit their energy as light. The ordered motion of the electrons is quickly transferred to a disordered motion of the atoms. Macroscopically, we say that when the frequency is too high, most of the electrical energy is converted to waste heat before it has a chance to be emitted as light.

The three main obstacles are therefore: the small size needed for the antenna, the difficulty in finding a way to drive the electrons at high frequency, and the tendency of high-frequency electrons to lose their energy to heat. These obstacles can be overcome to some extent using three different approaches: (1) lock the electrons down in small, localized atomic/molecular states where they can't bump into atoms as much and then drive the electron oscillations using the fact that they naturally oscillate when they transition between states, (2) shoot the electrons through a vacuum at high speed past magnets, and (3) build nanoscale, precisely shaped antennas and drive the electron oscillations using incident light.

The first method is exactly how a traditional laser works. Materials are chosen where certain electrons are locked into useful states. The electrons are excited to new states and then stimulated to fall back to their original states. Rather than oscillate back and forth between two points in space, the electrons in a traditional laser oscillate back and forth between two atomic/molecular states. This different kind of wiggling allows the frequency of oscillation to be high and helps prevent the electrons from bumping into atoms, thereby losing their energy to heat. The problem of electrons colliding with atoms is still a problem in lasers (scientists call this effect “phonon emission”), but it's not an insurmountable obstacle. Because lasers are controlled sources of visible light, they can be used to send information similar to how radio waves carry information. In fact, fiber optic cables contain information-carrying light beams that were created by lasers (although, most optical fibers use infrared light rather than visible light for efficiency reasons). Lasers can also be used to send information-carrying visible light through free space. This set-up is called optical wireless communication.

The second method is how a free electron laser works. In this case, electrons are shot through a vacuum at very high speed and then a series of magnets are applied to get the electrons to wiggle back and forth at high frequency, thereby emitting visible light. A free electron laser that is designed to force the electrons to wiggle at 640 THz will indeed emit blue light in a controlled way. Since free electron lasers need vacuum chambers and high-power electron accelerators to function, free electron lasers are used mostly in the laboratory setting.

The third method is how plasmonic nanoantennas work. Out of all of the devices that emit visible light in a controlled way, plasmonic nanoantennas are the closest to traditional radio antennas. A plasmonic nanoatenna is a nanoscale, precisely shaped metal antenna that has plasma resonances excited in it (bunched-up electron oscillations). Since plasmonic nanoantennas rely on electrons that slosh back and forth between one point in space and another just like traditional radio antennas, thermal loss is still a major problem when they operate at visible light frequencies. For this reason, optical plasmonic nanoantennas are still laboratory oddities and are not practical sources of controlled visible light. Since lasers are becoming increasingly cheap, small, and reliable, there isn't really a motivation to develop plasmonic nanoantennas to emit information-carrying visible light. Furthermore, since electronic circuits can't run at optical frequencies, plasmonic nanoantennas can't be excited by hooking them up to an electronic circuit. They have to be excited by being hit with incident light. In this way, plasmonic nanoantennas aren't like traditional antennas at all. They are more like scattering objects.

Note that there are many other ways to create visible light – fires, incandescent light bulbs, fluorescent light bulbs, gas discharge tubes, chemical reactions – but none of these ways create visible light in a controlled way (i.e. coherent visible light) such that a lot of information can be carried on the light waves, similar to as is done with radio waves.

In summary, if you want a radio antenna to emit visible light, just heat it up. If you want a traditional antenna to emit visible light in a controlled way such that you can transmit information, you are out of luck. Fortunately, there are other devices that can transmit information using visible light, the most practical of which is the laser.

Topics: antenna, free electron laser, laser, light, plasmonic, radio antenna