He worked as assistant professor at the Institute for Nuclear Research of the same university and defended his PhD in 1962.

After the PhD defense, he stayed for a year at MIT, and between 1964 and 1971 worked at Bell Labs on semiconductor lasers.

In 1970, Hayashi helped develop the first room-temperature continuous-wave semiconductor injection laser with double heterostructure.

[2] In 1971 he joined the Research Laboratories of NEC where he continued his studies of semiconductor lasers, aiming to improve their reliability and lifetime.

Between 1982 and 1987 he was a head scientist at NEC, and in 1987–1994 became director of the Optoelectronics Technology Research Laboratory in Tsukuba.