Question

why is it difficult to achieve laser action (stimulated emission) at higher frequency range like x-rays?

Answer 1

X-ray laser action is difficult to achieve for several reasons:

1) The effectivity of light amplification decreases with the increase in the wavelength

2) Problems associated with construction of mirrors that could reflect X-rays

3) The emitted radiation, based on amplified spontaneous emission, has relatively low spatial coherence

4) The line is mostly Doppler broadened, which depends on the ions' temperature.