Why two level laser is not possible?

A two-level laser is a hypothetical laser system that consists of two energy levels and would only require a single photon to achieve stimulated emission. However, a two-level laser is not possible in reality due to several reasons:

  1. Non-radiative transitions: In a two-level laser, the transition between the two energy levels should only involve the emission of photons. However, in real-world materials, there are often other non-radiative transitions that can occur, such as the transfer of energy to other molecules or vibrations within the material. These non-radiative transitions would prevent the efficient operation of a two-level laser.
  2. Broadening of the emission line: The emission of a laser should be at a single frequency or wavelength. However, in real-world materials, the emission line is broadened due to factors such as Doppler broadening, pressure broadening, and collisional broadening. These broadening effects would make it difficult to achieve a sharp emission line in a two-level laser.
  3. Absorption and re-absorption: Another issue with a two-level laser is that the emitted photons could be re-absorbed by the material before they could escape the laser cavity. This would lead to inefficient operation and would prevent the laser from achieving a high level of output power.

As a result, most practical laser systems have multiple energy levels, and the transitions between these levels involve the absorption and emission of multiple photons. This allows for more efficient operation and a sharper emission line. While a two-level laser is not possible, laser researchers are constantly exploring new materials and laser designs to create more efficient and powerful laser systems.