In this work we present a method to quantitatively measure the optical absorption of single nanowires that can be applied over a wide range of temperatures and with a high enough sensitivity to enable the measurement of below-band-gap absorption (as well as the absorption of single molecules). The method is based on accurately measuring the heat flow coming from a nanowire when it is illuminated by a laser beam. We experimentally verify this method by measuring the absorption of both a zincblende and a wurtzite GaAs, a wurtzite GaP, and a superlattice Zn3P2 nanowire. Furthermore, we find that the Zn3P2 nanowires have the largest absorption of all these materials. We analyze the advantages and disadvantages of the method and study its range of applicability.

I.Z. has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 756365) and Swiss National Science Foundation research grant (Project Grant No. 200021 165784). A.F.M. acknowledges funding from the Swiss National Science Foundation research grant (Project Grant No. BSCGI0-157705) and H2020 program through grant INDEED. H.D. acknowledges financial support by the Ministry of Education, Youth and Sports of the Czech Republic under the project CEITEC 2020 (LQ1601). M.D.L. acknowledges support from the Swiss National Science Foundation under the Ambizione grant (Grant No. PZ00P2 179801).