Threshold measurements were collected on a custom setup, where a 447 nm laser was focused on the sample, and UCPL was collected at 90 degrees by the spectrometer with a 425 nm short pass filter. The power of the laser at the focal point was measured using an optical power detector (818-SL, Newport Corporation) threaded with an OD3 attenuator if necessary, and the photocurrent was reported by a Keithley 2400 sourcemeter. The image of the laser spot was captured by a CMOS scientific camera (CS165MU, ThorLabs, Inc.) and analyzed in ImageJ to determine the spot size.