Laser-trapped microsphere becomes surface-measuring probe

March 1, 2009
A surface probe tipped with a microsphere can be scanned across the surface of a microlens or other microscale component to measure its 3-D coordinates, determining heights to nanometer-scale accuracies.

A surface probe tipped with a microsphere can be scanned across the surface of a microlens or other microscale component to measure its 3-D coordinates, determining heights to nanometer-scale accuracies. However, the radius of the microsphere is large enough to cause height errors when it is scanned across a sloped surface. Scientists at Osaka University (Osaka, Japan) have developed a completely different way to manipulate a microsphere that also allows the slope of the surface to be measured.

An 8-µm-diameter sphere, made of fused silica, is held in position, not by a mechanical probe, but instead is laser-trapped via the single-beam-gradient technique. The trapping laser, a 300 mW TEM00 Nd:YAG laser, is circularly polarized; a second laser, a 628 nm HeNe laser, is used to measure the probe position in combination with a position-sensitive detector. The focal point of the trapping laser is driven in a circle by an acousto-optic deflector, causing the microsphere to orbit circularly in free space. When brought near a surface that is perpendicular to the focal plane, the orbit of the microsphere becomes ellipsoidal with its major axis parallel to the surface; in this way, both surface position and slope can be determined. Contact Masaki Michihata at [email protected].

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