Keldysh photoionization theory: through the barriers

The Keldysh photoionization theory is a conceptual cornerstone and a universal framework for the description of a broad class of fundamental effects in light–matter interaction. Here, we provide an overview of the Keldysh theory as a significant milestone in the development of modern optical physics and offer a historical perspective on the fundamental role of this theory, from the early pioneering work on quantum tunneling to the latest breakthroughs in laser optics, attosecond technologies, and ultrafast optics of high-intensity laser pulses. (reviews of topical problems)