  Optical tweezers is a very well-established technique that has developed into
a standard tool for trapping and manipulating micron and submicron particles
with great success in the last decades. Although the nature of light enforces
restrictions on the minimum particle size that can be efficiently trapped due
to Abbe's diffraction limit, scientists have managed to overcome this problem
by engineering new devices that exploit near-field effects. Nowadays, metallic
nanostructures can be fabricated which, under laser illumination, produce a
secondary plasmonic field that does not suffer from the diffraction limit. This
advance offers a great improvement in nanoparticle trapping, as it relaxes the
trapping requirements compared to conventional optical tweezers. In this work,
we review the fundamentals of conventional optical tweezers, the so-called
plasmonic tweezers, and related phenomena. Starting from the conception of the
idea by Arthur Ashkin until recent improvements and applications, we present
some of the challenges faced by these techniques as well as their future
perspectives. Emphasis in this review is on the successive improvements of the
techniques and the innovative aspects that have been devised to overcome some
of the main challenges.
