  Cubic boron arsenide (BAs) is attracting greater attention due to the recent
experimental demonstration of ultrahigh thermal conductivity \k{appa} above
1000 W/mK. However, its bandgap has not been settled and a simple yet effective
method to probe its crystal quality is missing. Furthermore, traditional
\k{appa} measurement methods are destructive and time consuming, thus they
cannot meet the urgent demand for fast screening of high \k{appa} materials.
After we experimentally established 1.82 eV as the indirect bandgap of BAs and
observed room-temperature band-edge photoluminescence, we developed two new
optical techniques that can provide rapid and non-destructive characterization
of \k{appa} with little sample preparation: photoluminescence mapping
(PL-mapping) and time-domain thermo-photoluminescence (TDTP). PL-mapping
provides nearly real-time image of crystal quality and \k{appa} over mm-sized
crystal surfaces; while TDTP allows us to pick up any spot on the sample
surface and measure its \k{appa} using nanosecond laser pulses. These new
techniques reveal that the apparent single crystals are not only non-uniform in
\k{appa}, but also are made of domains of very distinct \k{appa}. Because
PL-mapping and TDTP are based on the band-edge PL and its dependence on
temperature, they can be applied to other semiconductors, thus paving the way
for rapid identification and development of high-\k{appa} semiconducting
materials.
