Is it possible to control the frequency of light?

Question

I mean, controlling directly in the source, not filtering the light or arranging density of the medium. For example, brightness can be controlled by current or voltage directly. However, when I came to frequency, I could not find a source-controlled "frequency control technique".

Thanks to comments another question is evolved: Is it possible to control the movement of electrons between the energy levels?

Answer 1

Of course you can.

1. Tunable lasers are a thing; you can modulate different aspects of the light generation process in different types of lasers.
2. You can excite a laser diode with a sinusoidal signal, which inherently means you've built a frequency synthesizer. That sinsusoidal signal can be electronically generated. Modulating laser diodes is commonly done in fiberoptical communication, and since electronic speeds limit modulation frequencies to a couple hundred gigahertz.
3. Same goes for modulating a Mach-Zehnder modulator with harmonic oscillations. All the remarks given to sinusoidal excitation apply here, too.
4. using a frequency comb and a nonlinear medium, you can generate a large set of intermodulation products; depending on how you control the comb, you can thus select from a grid of wavelengths.

Answer 2

I mean, controlling directly in the source, not filtering the light or arranging density of the medium.

This is essentially what a tunable laser does. Internally a laser is an excited piece of matter that has optical gain between two mirrors. That gain is highest at some (usually very small) number of wavelengths, so the laser output is nearly monochromatic. Tunable lasers further include a mechanism to move where that gain maximum is. RP Photonics has an article article on the many, many ways this can be done:

https://www.rp-photonics.com/wavelength_tuning.html

Rather than repeat it here, I will give one simple example. Many laser diodes can be thermally tuned, where you heat or cool them (possibly by driving more or less current into the diode) to change the wavelength they emit. This effect has several causes, including changes in the band structure of semiconductor from temperature/current as well as changes in refractive index due to thermal expansion of the material.

Thanks to comments another question is evolved: Is it possible to control the movement of electrons between the energy levels?

The actual comment that caused you to change your question is incorrect. The band structure of a material isn't completely fixed (see temperature/current tuning), you can emit wavelengths of light that do not correspond to a energy levels in a material, and you also don't necessarily need to change energy levels in order to change the wavelengths that are emitted. Many materials are available in which the band structure spans to tens or hundreds of nanometers worth of energy levels. For example, the Ti:S laser cavity can emit light at all wavelengths from about 680nm to more than 1100nm, a range of more than 400nm. Tunable light sources using a Ti:S crystal can emit any (or all) of these wavelengths.

Then there are devices like parametric oscillators. Since they do not depend on the energy levels of a material to generate new photons, a single device can tune between thousands of nanometers worth of wavelengths, sometimes covering the UV, VIS an NIR.

So in summary, it is absolutely possible to control the frequency/wavelength of light with the right equipment.

Answer 3

LEDs temperature-tune quite nicely, but it take liquid nitrogen to get a few ten of nanometres. I just tried one that's amber at room temperature but greenish-yellow at 77 K. They also get much brighter for constant current as you reduce the temperature, while the forward voltage drop increases dramatically. This changing brightness and my phone not allowing me to disable auto-exposure is why my attempt to film it for you failed miserably.

Little laser diodes show less shift. I had a red laser-pointer type diode here, and in LN it got brighter, then dimmer as it cooled, without appreciably changing colour

Answer 4

Use an incandescent bulb with a dimmer switch, the temperature of the emitter will change as you dim, and the blackbody curve will change accordingly. This of course only applies if you don't want a monochromatic or coherent source.

Answer 5

What you are asking about is just an antenna...but for optical frequencies rather than radio. They are called "optical antennas" and are still in development since visible light has very short wavelength which means very short antennas which presents manufacturing difficulties (they are nanoscale). On top of that the drive signals required are also very high frequency which also makes things tricky.

Answer 6

"the frequency of light" is a well-defined quantity only for a monochromatic source.

Good monochromatic visible-light sources are spectrum lines, and either Stark effect (electric field) or Zeeman effect (magnetic field) can change them. It just isn't very sensitive, give small modulation compared to typical line widths.

Various dodges, however, can get better effects; notably, photoacoustic gratings can be combined with laser-gain amplifiers to make tuned sources.

Easiest, though, is to use two LEDs of different colors, and only turn one on.

Answer 7

This is the kind of question someone asks when the do not understand homo-lumo transistion in molecular orbit theory and band gaps in semiconductor materials allow electrons to be excited and fall back to ground states to create emissions.

Once those phenomena are understood, the question becomes clear that cleaver use of light filtering and solicitation with mirrors and laser tuning must be used to select a specific wavelength from a tail on a Gaussian emission curve from the aforementioned emission mechanisms.

So, if you don't like those techniques and they are all disqualified by your criteria, the only possible answer thst accounts for all of your criteria is, no. No, light color cannot be tuned at the source according to your criteria.