We know that the luminescence of any light source is related to the motion state of the particles inside the material. When a particle at a low energy level absorbs external energy of an appropriate frequency and is excited to jump to a corresponding high energy level, it will always try to jump to a lower energy level, and at the same time release the excess energy in the form of photons. If light is released spontaneously without the action of external photons, the light released at this time is ordinary light (such as electric lamps, neon lights, etc.), which is characterized by inconsistent frequency, direction and phase of light. However, if the excess energy is released in the form of photons when transitioning from a high-energy level to a low-energy level under the direct action of external photons, the released photons are completely consistent with the external incident photons in terms of frequency, phase, and propagation direction. It means that the extraneous light has been strengthened, which we call light amplification. In this way, we can get a laser with good directionality, high brightness, good monochromaticity and strong coherence.
The color of light depends on the wavelength, and the wavelength of yellow light is around 570nm. However, the energy level difference of commonly used semiconductor laser crystals does not match the energy of the yellow light band, and it is not easy to generate yellow light band signals by frequency doubling and other methods. The output power for the 540nm-620nm band is very low, basically below the milliwatt level (‘- ωก̀ ). Due to the low output power of semiconductor lasers for yellow laser light, complex operations such as nonlinear frequency conversion are usually required to obtain yellow laser light with sufficient intensity. In addition, the application of yellow lasers is much smaller than that of green lasers and red lasers, and is usually limited to astronomical navigation and medicine. Therefore, we rarely see yellow lasers on the market (/ω＼).
Back to the question, there are actually blue-violet laser pointers. However, under the same transmission power, compared with red and green light, blue-violet light is less irritating to the human eye, and it is a bit laborious to use as an indicator, so laser pointers rarely use the blue-violet band.
