The coincident alignment of two galaxies on the sky can create the rare cosmic phenomenon
of strong gravitational lensing, in which light from the more distant galaxy is bent around the
foreground galaxy to create multiple, distorted, and magnified images. When the background
galaxy hosts a bright active galactic nucleus, a quasar, the system becomes a probe of
accretion disk physics, quasar-host galaxy relations, the Hubble constant, the stellar IMF,
smooth matter fractions, amongst many other applications.
It has been 40 years since the discovery of the first gravitationally lensed quasar, and
dedicated spectroscopic and imaging surveys have added over one hundred new systems
to this list. In recent years, the amount of available data across the whole-sky has grown
exponentially. Full-sky data from X-ray to radio wavelengths exist, and predictions suggest
there are many bright lensed quasars hidden in these datasets. This thesis presents several
new techniques to mine these rare systems from whole-sky photometric datasets.
We use the excellent resolving ability of Gaia, coupled with other wide-field surveys
such as the Dark Energy Survey (DES), Pan-STARRS, and WISE, and present spectroscopic
follow-up from the WHT, NTT, and Keck. By looking for multiple Gaia detections around
photometric quasar candidates, and single Gaia detections near morphological galaxies, we
have discovered 105 new lensed quasars. We also present a search based on significant offsets
in astrometry and flux between Gaia and SDSS for spectroscopic quasars, suggesting several
promising small-separation lens candidates. We characterise the confirmed systems based
on ground-based imaging and the spatially resolved spectra, and comment on the purity,
efficiency, and biases in our selection. DES data provides multi-epoch photometry over the
baseline of years at optical wavelengths, allowing a colour-independent selection of lensed
quasars by looking for nearby variable pairs. We create a parametric modelling pipeline of
the DES images to extract lightcurves of system components, and show that it is a highly
effective way to remove quasar and star projections before spectroscopic follow-up. We
demonstrate that future searches based on detecting variability in multiple images will be
biased towards four-image lensed quasars.STF