[apo]

I was surprised by this explanation describing how the beam is focused:

Optical tweezers are capable of manipulating nanometer and micron-sized dielectric particles by exerting extremely small forces via a highly focused laser beam. The beam is typically focused by sending it through a microscope objective. ...

I imagined something more complex, although what exactly I'm not sure.

Are there any other ways of doing it? Are off-the-shelf microscope lenses used routinely, or are custom lenses the norm?

[carreau]

Note that usually you are also using the microscope to observe what you are trapping, so it' super convenient (but not easy), to use the same objective that the one you observe to send a laser through (in the opposite direction) and trap your sample. If you send your laser-beam quasi parallel it will focus and trap particula quasi-where your are observing.

(source: did my PhD with optical traps: http://matthiasphd.herokuapp.com/html/parts/part1.html#optic..., picture of trapped beed as viewd through microscope: http://matthiasphd.herokuapp.com/html/parts/part3.html#exper...)

[arctux]

Usually optical traps use 1064 nm NdYAG lasers, which is in the infrared. Most objectives aren't designed for infrared light, so the objectives used for optical trapping are usually customized.