Abstract: Optical tweezer methods and apparatus that operate in reflection mode use polarization selection to isolate an input laser signal from optical signals associated with trapping. A trapping aperture or apertures can be defined in a conductive film such as a gold film have a polarization-dependent reflectance due to asymmetry of the aperture, and a reflected beam is polarized differently than an incident beam. Polarization of scattered light from the aperture can thus be distinguished from polarization of the incident beam. In some cases, an aperture is not used but instead polarizing nanoparticles, such as nanorods, are detected based on either transmitted or reflected optical radiation in a different state of polarization than that of the incident beam.