This means that different parts of the original light beam encounter the raindrop's curved surface at different angles and bend different amounts , even for a single color. The diagram above shows how each color gets bent into many angles. Although pure red is mostly bent by a raindrop into a Similarly, pure orange is mostly bent into the The color in a rainbow at The end result is that the colors in a rainbow tend to blur together and wash each other out.
The extended shape of the sun also sends light into the raindrop at slightly different angles and further blurs the colors together. A prism and a raindrop are in principle very similar. They both spread white light out into a span of colors through refraction.
The main difference though is that a prism has flat surfaces, leading to a pure spectrum, while a raindrop has a round surface, leading to an impure spectrum. Ooi and the team further tuned the emission wavelength using a simple thermal annealing process. This involved quickly heating the semiconductor up to degrees Celsuis for 30 seconds.
Each repetition of this process gradually reduced the emission wavelength, with five cycles shifting the as-grown wavelength of nanometers to nanometers. This happens because the annealing made atoms slowly diffuse from the barrier to the well and relaxed the grown-in strain in the well. With their material optimized, the researchers fabricated a one-millimeter long laser cavity and attached electrical contacts. Their device worked at room temperature and generated 46 milliwatts of orange light.
Part of the light falling on this water drop enters and is reflected from the back of the drop. This light is refracted and dispersed both as it enters and as it leaves the drop. Rainbows are produced by a combination of refraction and reflection.
You may have noticed that you see a rainbow only when you look away from the sun. Light enters a drop of water and is reflected from the back of the drop, as shown in Figure 4. The light is refracted both as it enters and as it leaves the drop. Since the index of refraction of water varies with wavelength, the light is dispersed, and a rainbow is observed, as shown in Figure 5a.
There is no dispersion caused by reflection at the back surface, since the law of reflection does not depend on wavelength. The effect is most spectacular when the background is dark, as in stormy weather, but can also be observed in waterfalls and lawn sprinklers. The arc of a rainbow comes from the need to be looking at a specific angle relative to the direction of the sun, as illustrated in Figure 5b.
This rare event produces an arc that lies above the primary rainbow arc—see Figure 5c. Figure 5. Dispersion may produce beautiful rainbows, but it can cause problems in optical systems. White light used to transmit messages in a fiber is dispersed, spreading out in time and eventually overlapping with other messages. Since a laser produces a nearly pure wavelength, its light experiences little dispersion, an advantage over white light for transmission of information.
In contrast, dispersion of electromagnetic waves coming to us from outer space can be used to determine the amount of matter they pass through. As with many phenomena, dispersion can be useful or a nuisance, depending on the situation and our human goals. How does a lens form an image? See how light rays are refracted by a lens. Watch how the image changes when you adjust the focal length of the lens, move the object, move the lens, or move the screen.
Now, a team of physicists from the University of Bath has found a way to use resonance to harness the energy of light more effectively inside structures called microresonators. For light, microresonators act as miniature racetracks, with photons zipping around the circle in loops.
Light consists of photons of different colors, with each color corresponding to waves oscillating at specific wavelengths and frequencies.
If the peaks of these waves reach the same point after a full loop is made around the resonator , then the energy storage capacity of the resonator hits a maximum when measured against frequency. In other words, the resonator and the light inside come to resonance. The ability of a resonator to store energy is characterized by the sharpness of the resonance, also called finesse.
Physicists are caught in a race to maximize the finesses of resonators, so as to store as much energy as possible in a single resonator. The reason for this is not just bragging rights. When high light energy is circulating in a resonator, it starts to reveal interesting properties. For instance, the resonator begins to produce photons of light with new frequencies and therefore of different colors.
A newly created rainbow of colors is known as a frequency comb. A comb's many useful properties led to researchers working on 'the optical frequency comb technique' winning the Nobel Prize in Physics. Unlike a sky rainbow, the one created in a resonator doesn't display a continuous spectrum of colors. Instead, it contains a regular and equally spaced pattern of colors, similar to the teeth on a comb.
The regularity of these teeth allows these combs to be used for ultra-precise measurements—for instance, of distances and time. The University of Bath study has found that boosting the strength of light matter interactions to make frequency combs is not the only reasons high-finesse microresonators are important.
If finesse is relatively small, then tuning a laser around one of the resonances causes a given comb tooth to adjust its color continuously. Reaching finesses of several thousands and into tens of thousands, however, starts to break this continuity.
When the continuity is broken, a laser tuned to generate a pair of photons with two specific colors will need to pass through the 'idle interval' before the next color becomes ignited. During this interval, there can be no conversion into new colors. In the language of resonance theory, the interval creation is called Arnold tongues. Arnold tongues is a phenomenon often found in networks of oscillators. The neurons in our brains work according to the rules of Arnold tongues to synchronize the transmission of signals.
The microresonator tongues reported in the Bath study represent a map of the narrow tongue-like structures that shows how laser parameters should be tuned to either generate or not generate new colors. The photon pair generation process is a key phenomenon underpinning the development of tunable light sources for various applications, and in particular for optical data processing and transmission.
0コメント