Fischer Black published a paper shortly before his death in 1995 considering interest rates as having embedded options when considering the "shadow real interest rate" and the real interest rate. From the paper:
The nominal short rate is the "shadow real interest rate" (as defined by the investment opportunity set) plus the inflation rate, or zero, whichever is greater. Thus the nominal short rate is an option.
The paper is very short, but was published without incorporating details from a referee review.
Can someone flesh out more the shadow real interest rate and how it is used (as proposed) in quantitative models of interest rates? Has this line of research been picked up on in the last twenty five years?