Optimizing immersion media refractive index improves optical trapping by compensating spherical aberrations.

The efficiency of an optical trap is limited by its axial strength. Light focused by oil-immersion objectives provides stronger traps but suffers from spherical aberrations, thus restricting the axial stability and working distance. By changing the refractive index of the immersion media we compensate spherical aberrations and measure axial trapping strengths at least twice as large as previously reported. Moreover, the spherical aberrations can be compensated at any desired depth. The improved trapping efficiency implies significantly less heating of the particles, thus diminishing previously published concerns about using gold nanoparticles as handles for optical manipulation.