  Humankind has long endeavored to control the propagation direction of light.
Since time immemorial, shades, lenses and mirrors have been used to control the
flow of light. In modern society, with the rapid development of nanotechnology,
the control of light is moving toward devices at micrometer and even nanometer
scales. At such scales, traditional devices based on geometrical optics reach
their fundamental direction limits and cease to work. Nano photonics, on the
other hand, has attracted wide attention from researchers, especially in the
last decade, due to its ability to manipulate light at the nanoscale. This
review focuses on the nano photonics systems that aim to select light based on
its propagation direction. In the first half of this review, we survey the
literature and the current state of the art focused on enabling optical
broadband angular selectivity. The mechanisms we review can be classified into
three main categories: (i) microscale geometrical optics, (ii) multilayer
birefringent materials and (iii) Brewster modes in plasmonic systems, photonic
crystals and metamaterials. In the second half, we present two categories of
potential applications for broadband angularly selective systems. The first
category aims at enhancing the efficiency of solar energy harvesting, through
photovoltaic process or solar thermal process. The second category aims at
enhancing light extracting efficiency and detection sensitivity. Finally, we
discuss the most prominent challenges in broadband angular selectivity and some
prospects on how to solve these challenges.
