The occurrence of nonspherical oscillations (or surface modes) of coated microbubbles, used as ultrasound contrast agents in medical imaging, is investigated using ultra-high-speed optical imaging. Optical tweezers designed to micromanipulate single bubbles in 3-D are used to trap the bubbles far from any boundary, enabling a controlled study of the nonspherical oscillations of free-floating bubbles. Nonspherical oscillations appear as a parametric instability and display subharmonic behavior: they oscillate at half the forcing frequency, which was fixed at 1.7 MHz in this study. Surface modes are shown to preferentially develop for a bubble radius near the resonance of radial oscillations. In the studied range of acoustic pressures, the growth of surface modes saturates at a level far below bubble breakage. With the definition of a single, dimensionless deformation parameter, the amplitude of nonspherical deformation is quantified as a function of the bubble radius (between 1.5 and 5 microm) and of the acoustic pressure (up to 200 kPa).