  We study the rest-frame optical properties of 74 luminous (L_bol=10^46.2-48.2
erg/s), 1.5<z<3.5 broad-line quasars with near-IR (JHK) slit spectroscopy.
Systemic redshifts based on the peak of the [OIII]5007 line reveal that
redshift estimates from the rest-frame UV broad emission lines (mostly MgII)
are intrinsically uncertain by ~ 200 km/s (measurement errors accounted for).
The overall full-width-at-half-maximum of the narrow [OIII] line is ~ 1000 km/s
on average. A significant fraction of the total [OIII] flux (~ 40%) is in a
blueshifted wing component with a median velocity offset of ~ 700 km/s,
indicative of ionized outflows within a few kpc from the nucleus; we do not
find evidence of significant [OIII] flux beyond ~ 10 kpc in our slit
spectroscopy. The [OIII] line is noticeably more asymmetric and weaker than
that in typical less luminous low-z quasars. However, when matched in quasar
continuum luminosity, low-z quasars have similar [OIII] profiles and strengths
as these high-z systems. Therefore the exceptionally large width and
blueshifted wing, and the relatively weak strength of [OIII] in high-z luminous
quasars are mostly a luminosity effect rather than redshift evolution. The
Hbeta-[OIII] region of these high-z quasars displays a similar spectral
diversity and Eigenvector 1 correlations with anti-correlated [OIII] and
optical FeII strengths, as seen in low-z quasars; but the average broad Hbeta
width is larger by 25% than typical low-z quasars, indicating more massive
black holes in these high-z systems. These results highlight the importance of
understanding [OIII] emission in the general context of quasar parameter space
in order to understand quasar feedback in the form of [OIII] outflows. The
calibrated one-dimensional near-IR spectra are made publicly available, along
with a composite spectrum.
