While the concept of using a standing wave and a semi-reflective surface is employed in both lasers and microwave ovens, it is not as simple as replacing a wall of a microwave oven with a semi-transparent surface to create a maser (microwave amplification by stimulated emission of radiation).
A maser is the microwave equivalent of a laser and operates on the same principle of stimulated emission. In a laser, stimulated emission is achieved by introducing a population inversion (more atoms or molecules in an excited state than in the ground state) within the gain medium, such as a crystal or a gas. This population inversion allows the amplification of light through stimulated emission.
In a maser, the same concept is applied to the microwave frequency range. However, creating and maintaining a population inversion in the microwave range is considerably more challenging than in the optical range. It requires specific techniques such as using specialized materials, cryogenic temperatures, and precise control of the microwave fields.
Merely replacing a wall of a microwave oven with a semi-transparent surface would not be sufficient to create a maser. The design and engineering complexities involved in achieving a population inversion and stimulating coherent microwave emission are much more intricate and specialized.
Maser technology does exist and has been developed for specific applications such as atomic clocks, amplification of weak signals, and scientific research. These devices require careful design and implementation using specialized components and techniques specific to maser operation.