  Many brown dwarfs are on ultra-short period and tidally-locked orbits around
white dwarf hosts. Because of these small orbital separations, the brown dwarfs
are irradiated at levels similar to hot Jupiters. Yet, they are easier to
observe than hot Jupiters because white dwarfs are fainter than main-sequence
stars at near-infrared wavelengths. Irradiated brown dwarfs are, therefore,
ideal hot Jupiter analogs for studying the atmospheric response under strong
irradiation and fast rotation. We present the 1.1--1.67$\rm \mu m$
spectroscopic phase curve of the irradiated brown dwarf (SDSS1411-B) in the
SDSS J141126.20+200911.1 brown-dwarf white-dwarf binary with the near-infrared
G141 grism of Hubble Space Telescope Wide Field Camera 3. SDSS1411-B is a
$50~\rm M_{\rm Jup}$ brown dwarf with an irradiation temperature of 1300K and
has an orbital period of 2.02864 hours. Our best-fit model suggests a
phase-curve amplitude of 1.4$\%$ and places an upper limit of 11 degrees for
the phase offset from the secondary eclipse. After fitting the white-dwarf
spectrum, we extract the phase-resolved brown-dwarf emission spectra. We report
a highly wavelength-dependent day-night spectral variation, with the water-band
flux variation of about $360\pm70\% $ and a comparatively small J-band flux
variation of $37\pm2\%$. By combining the atmospheric modeling results and the
day-night brightness-temperature variations, we derive a pressure-dependent
temperature contrast. We discuss the difference in the spectral features of
SDSS1411-B and hot Jupiter WASP-43b, and the lower-than-predicted day-night
temperature contrast of J4111-BD. Our study provides the high-precision
observational constraints on the atmospheric structures of an irradiated brown
dwarf at different orbital phases.
