  The "brighter-fatter" (BF) effect is a phenomenon (originally discovered in
charge coupled devices) in which the size of the detector point spread function
(PSF) increases with brightness. We present, for the first time, laboratory
measurements demonstrating the existence of the effect in a Hawaii-2RG HgCdTe
near infrared (NIR) detector. We use the Precision Projector Laboratory, a JPL
facility for emulating astronomical observations with UV/VIS/NIR detectors, to
project about 17,000 point sources onto the detector to stimulate the effect.
After calibrating the detector for nonlinearity with flat-fields, we find
evidence that charge is nonlinearly shifted from bright pixels to neighboring
pixels during exposures of point sources, consistent with the existence of a
BF-type effect. The Wide Field Infrared Survey Telescope (WFIRST) by NASA will
use similar detectors to measure weak gravitational lensing from the shapes of
hundreds of million of galaxies in the NIR. The WFIRST PSF size must be
calibrated to approximately 0.1 percent to avoid biased inferences of dark
matter and dark energy parameters; therefore further study and calibration of
the BF effect in realistic images will be crucial.
