Photonic crystals will allow you to change the frequency of the light wave. Light waves How does the frequency of a light wave change?
In modern scientific journals It’s rare to read about “amazing discoveries” and “incredible physical phenomena,” but these are the terms used to describe the results of experiments on light waves conducted at the Massachusetts Institute of Technology.
The point, in fact, is this: one of the pioneers in the field of photonic crystals, John Joannopoulos, discovered very strange properties exhibited by such crystals when exposed to a shock wave.
Thanks to these properties, you can do anything with a beam of light passed through these crystals - for example, change the frequency of the light wave (that is, color). The degree of control of the process is approaching 100%, which, in fact, is what amazes scientists most of all.
So, what are photonic crystals?
This is not a very successful, but already quite common translation of the term Photonic Crystals. The term was introduced in the late 1980s to designate, so to speak, the optical analogue of semiconductors.
Professor John Ioannopoulos.
These are artificial crystals made of a translucent dielectric, in which air “holes” are created in an orderly manner, so that a ray of light passing through such a crystal enters media either with a high reflectivity or with a low one.
Due to this, the photon in the crystal finds itself in approximately the same conditions as the electron in the semiconductor, and accordingly, “allowed” and “forbidden” photonic bands are formed (Photonic Band Gap), so that the crystal blocks light with a wavelength corresponding to the forbidden photon zone, while light with other wavelengths will propagate unhindered.
The first photonic crystal was created in the early 1990s by Bell Labs employee Eli Yablonovitch, now at the University of California. Upon learning of Ioannopoulos's experiments, he called the degree of control achieved over light waves "shocking."
Through computer simulations, Ioannopoulos' team discovered that when a crystal is exposed to a shock wave, it physical properties change dramatically. For example, a crystal that transmitted red light and reflected green light suddenly became transparent to green light and opaque to the red part of the spectrum.
A small trick with shock waves made it possible to completely “stop” the light inside the crystal: the light wave began to “beat” between the “compressed” and “uncompressed” parts of the crystal - a kind of mirror room effect was obtained.
Scheme of the processes occurring in a photonic crystal when a shock wave passes through it.
As the shock wave passes through the crystal, the light wave undergoes a Doppler shift every time it comes into contact with the shock pulse.
If the shock wave moves in the opposite direction to the light wave, the frequency of the light becomes higher with each collision.
If the shock wave travels in the same direction as the light, its frequency drops.
After 10 thousand reflections, occurring in approximately 0.1 nanoseconds, the frequency of the light pulse changes very significantly, so that red light can turn blue. The frequency can even go beyond the visible part of the spectrum - into the infrared or ultraviolet region.
By changing the structure of the crystal, you can achieve complete control over which frequencies will enter the crystal and which will go out.
But Ioannopoulos and his colleagues are just about to begin practical tests - because, as already said, their results are based on computer simulations.
A still from a video sequence of a computer simulation conducted by Ioannopoulos and his colleagues.
Negotiations are currently underway with the Lawrence Livermore National Laboratory about “real” experiments: first, the crystals will be shot with bullets, and then, probably, with sound pulses, which are less destructive to the crystals themselves.
At the end of the 17th century, two scientific hypotheses about the nature of light arose - corpuscular And wave.
According to the corpuscular theory, light is a stream of tiny light particles (corpuscles) that fly from enormous speed. Newton believed that the movement of light corpuscles obeys the laws of mechanics. Thus, the reflection of light was understood as similar to the reflection of an elastic ball from a plane. The refraction of light was explained by the change in the speed of particles when moving from one medium to another.
The wave theory viewed light as a wave process similar to mechanical waves.
According to modern ideas, light has a dual nature, i.e. it is simultaneously characterized by both corpuscular and wave properties. In phenomena such as interference and diffraction, the wave properties of light come to the fore, and in the phenomenon of the photoelectric effect, the corpuscular ones.
Light as electromagnetic waves
In optics, light means electromagnetic waves fairly narrow range. Often, light is understood not only as visible light, but also in the broad spectrum regions adjacent to it. Historically, the term “invisible light” appeared - ultraviolet light, infrared light, radio waves. Visible light wavelengths range from 380 to 760 nanometers.
One of the characteristics of light is its color, which is determined by the frequency of the light wave. White light is a mixture of waves of different frequencies. It can be decomposed into colored waves, each of which is characterized certain frequency. Such waves are called monochromatic.
Speed of light
According to the newest measurements, the speed of light in a vacuum
Measurements of the speed of light in various transparent substances have shown that it is always less than in a vacuum. For example, in water the speed of light decreases by 4/3 times.
Light waves are electromagnetic waves that include the infrared, visible and ultraviolet parts of the spectrum. The wavelengths of light in a vacuum corresponding to the primary colors of the visible spectrum are shown in the table below. The wavelength is given in nanometers, .
Table
Light waves have the same properties as electromagnetic waves.
1. Light waves are transverse.
2. The vectors and oscillate in a light wave.
Experience shows that all types of influences (physiological, photochemical, photoelectric, etc.) are caused by oscillations of the electric vector. He is called light vector . The light wave equation has the following form
Amplitude of the light vector E m is often denoted by the letter A and instead of equation (3.30), equation (3.24) is used.
3. Speed of light in vacuum 
.
The speed of a light wave in a medium is determined by formula (3.29). But for transparent media (glass, water) usually, therefore.
For light waves, the concept of absolute refractive index is introduced.
Absolute refractive index is the ratio of the speed of light in a vacuum to the speed of light in a given medium
From (3.29), taking into account the fact that for transparent media, we can write the equality .
For vacuum ε = 1 and n= 1. For any physical environment n> 1. For example, for water n= 1.33, for glass. A medium with a higher refractive index is called optically denser. The ratio of absolute refractive indices is called relative refractive index:
4. The frequency of light waves is very high. For example, for red light with wavelength 
.
When light passes from one medium to another, the frequency of the light does not change, but the speed and wavelength change.
For vacuum - ; for environment - , then
.
Hence the wavelength of light in the medium is equal to the ratio of the wavelength of light in vacuum to the refractive index
5. Because the frequency of light waves is very high 
, then the observer’s eye does not distinguish individual vibrations, but perceives average energy flows. This introduces the concept of intensity.
Intensity called relation average energy, carried by the wave, to a period of time and to the area of the site perpendicular to the direction of propagation of the wave:
Since the wave energy is proportional to the square of the amplitude (see formula (3.25)), the intensity is proportional to the average value of the square of the amplitude
The characteristic of light intensity, taking into account its ability to cause visual sensations, is luminous flux - F .
6. The wave nature of light manifests itself, for example, in phenomena such as interference and diffraction.
Electrodynamics and optics. Changes in physical quantities in processes
The assignment relates to basic level difficulties. For correct execution you will receive 2 points.
It takes approximately 3 -5 minutes.
To complete task 17 in physics you need to know:
- Electrodynamics (change in physical quantities in processes)