  High-contrast imaging of exoplanets and protoplanetary disks depends on
wavefront sensing and correction made by adaptive optics instruments.
Classically, wavefront sensing has been conducted at optical wavelengths, which
made high-contrast imaging of red targets such as M-type stars or extincted T
Tauri stars challenging. Keck/NIRC2 has combined near-infrared (NIR) detector
technology with the pyramid wavefront sensor (PWFS). With this new module we
observed SR~21, a young star that is brighter at NIR wavelengths than at
optical wavelengths. Compared with the archival data of SR~21 taken with the
optical wavefront sensing we achieved $\sim$20\% better Strehl ratio in similar
natural seeing conditions. Further post-processing utilizing angular
differential imaging and reference-star differential imaging confirmed the
spiral feature reported by the VLT/SPHERE polarimetric observation, which is
the first detection of the SR~21 spiral in total intensity at $L^\prime$ band.
We also compared the contrast limit of our result ($10^{-4}$ at $0\farcs4$ and
$2\times10^{-5}$ at $1\farcs0$) with the archival data that were taken with
optical wavefront sensing and confirmed the improvement, particularly at
$\leq0\farcs5$. Our observation demonstrates that the NIR PWFS improves AO
performance and will provide more opportunities for red targets in the future.
